Chemical Batteries with CO2

Abstract Efforts to obtain raw materials from CO2 by catalytic reduction as a means of combating greenhouse gas emissions are pushing the boundaries of the chemical industry. The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO2 does not yet exist.


Introduction
Current Research considers the hydrogenation of CO 2 from three perspectives.T here are few chemical reactions that concern so many people outside of chemistry as does the future processing of CO 2 .Inreference to these efforts,weuse the abbreviation "CCU" (Carbon Capture and Use). Theuse of CO 2 is first considered in the context of sustainable energy regimes.H ere,t he focus is directly on the storage and transport of sustainably produced hydrogen in the form of "synthetic fuels." Then otion should not be entertained here that hydrogen is am eans of removing CO 2 from the atmosphere.R ather,C O 2 is ac omponent of ac ircular economy for renewable energy.T his concept will be expressed more thoroughly in the introduction to chemical batteries.A sw ith electrical batteries,t here is ac hemical accumulator (or rechargeable battery) that can be repeatedly discharged and charged in aclosed cycle.T here is also atype of electrical battery that cannot be recharged. This device corresponds to the single-use binding of CO 2 to renewable hydrogen in synthetic fuels,w hich is referred to in the literature as "linear CCU" in contrast to the use of repeatedly (through biological or technical processes) collected CO 2 that leads to "circular CCU".
Thec oncept of the battery is not only aq uaint formulation, but also an attempt to draw the attention of energy regime regulators to the fact that CO 2 prices and taxes must be divided into three categories if asuccessful incentive is to be achieved. These categories are:noutilization (max price), linear use (discounted price), and circular use (no price). For each case,different definitions and rules apply.
As ignificant aspect-and one that is often treated in acursory fashion-is the use of CO 2 as araw material in the chemical industry.H ere,m any synthetic routes and concepts for novel production chains exist. They are of importance for todaysc hemical industry and will continue to be so in the future.
Al arge part of this Review is devoted to understanding the reaction of CO 2 with hydrogen, whereby copper is used as athermochemical or electrochemical catalyst. Beginning with anetwork of reactions,itwill be shown that the term "copper metal" is too simple aterm to describe the active component in the catalyst if the goal is to understand the different effects of nominally the same copper catalyst in the reaction network. Figure 1s hows as chematic "work program."I ti sg eared toward different target audiences of chemists and policy makers active in the energy sector.T his facilitation of information must take place while recognizing that the challenge of restructuring the energy regime can only be successful as acommunal effort and with asolid grasp of the tasks and possibilities.Researchers may see that the description of the underlying processes of their phenomenological research is am otivating factor for their work. Policym akers should come to understand the high accuracy with which we can already describe these central processes-but also be Efforts to obtain raw materials from CO 2 by catalytic reduction as ameans of combating greenhouse gas emissions are pushing the boundaries of the chemical industry.T he dimensions of modern energy regimes,onthe one hand, and the necessary transport and trade of globally produced renewable energy,o nthe other,w ill require the use of chemical batteries in conjunction with the local production of renewable electricity.The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary,r obust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO 2 does not yet exist.

Chemical Batteries
Batteries are devices that store energy.T he name comes from military speech and refers to the strengthening of an effect or action by organizing single units into groups.I n electrochemical batteries, [1] as eries of redox reactions are carried out within electrically connected cells.F ree electrons produced during the course of the reactions can then perform work in an external electric circuit. Thenecessary energy for this process is stored in the electrodes of the battery.T he interconversion between chemical and electrical energy takes place with only small losses because the oxidation state of the storage medium-the ion-can easily be changed. Fors ome combinations of redox reactions,t he storage process is reversible and the application of external electrical work can reverse the process with, again, only small losses. [2] In these cases,o ne speaks of accumulators or rechargeable batteries.
One disadvantage of both rechargeable and nonrechargeable batteries is that the energy is stored in the device-specifically in the solid electrodes.T he alternative redox-flow battery [3] avoids this disadvantage by including liquids as the storage medium for the ions,which can be held externally from the charge conversion device.T his form of accumulator is,h owever,n ot yet fully developed and cannot store large quantities of energy.Ifthe goal is to store electrical energy in quantities on the order of magnitude of the demand of entire countries,t hen chemical batteries are essential to make them globally transportable,f or example,o rt od efossilize applications and processes requiring high energy densities.S uch batteries consist of molecules containing energy stored in chemical bonds.F or example,h ydrogen, methane,orother alkanes,are often used for this purpose and are generally well-known today as fuels.Inchemical batteries, the processes of storing and recovering the energy is separated from the storage form itself.F or this reason, there is no limitation on the amount of energy that can be stored or the duration for which it can be stored. This advantage of almost unlimited storage capacity is,h owever, accompanied by the disadvantage that further reaction partners are required for the energy storage and recovery processes themselves.T he following reactions illustrate the difference between electrochemical batteries and chemical batteries with free electrons [Eq. (1,2)] and chemical bonds [Eq. (4,5)], respectively,a stheir storage medium.
Electrochemical battery: Storage: Recovery: Equations (1) and (2) are simplified to illustrate the principle of storage using ions.E quation (1) implies that, in real batteries and accumulators,c omplex reactions [4] take place between the storage ion and the electrode material. For ac hemical battery,afurther process [Eq. (3)] is necessary in addition to the reactions for storage [Eq. (4)] and recovery [Eq. (5)].
Chemical battery: Primary conversion: Storage:  The topics of the current Review.T he selected subjects are, taken together,important for asuccessful energy transition. This transitionc an only be realized collectively by the stakeholders from science and decision-making standing behind the elements shown in Figure 1. It would be highly desirable if all actions were based upon the fundamental insights existing today. Recovery: Chemical batteries require acircular economy of storage molecules to enable ac onstant supply of energy;t hese molecules are ah allmark of as ustainable energy regime. Water, oxygen, and nitrogen molecules are present in such large quantities on Earth that no closed cycles are necessary. Thep urpose of the present Review is,h owever,t oc onsider the suitability of CO 2 as the storage molecule.F or this material, ac losed cyclei sc ertainly necessary because of its multiple other functions in the atmosphere (greenhouse gas). Thei ndustrial revolution and all the resulting developments have depended on the high energy density of fuels and the resulting large amount of CO 2 released through the use of these fuels.Concepts for carbon cycles were first suggested by Asinger and Olah. [5] At af irst glance,t he lengthy process chain makes it appear that chemical batteries should perhaps be limited to hydrogen. Theconcept of ahydrogen economy [6] is based on this perception. However,the unfavorable storage characteristics of hydrogen (low density,high energy requirements for liquification) in addition to technical hurdles in the process chain of hydrogen production, transport, and storage as well as conversion back into electricity by fuel cell technologies are,t aken together, very demanding.T he current expectation [7] is,therefore,that acarbon-based circular economy will run parallel to ah ydrogen economy and will always have ap articular role to play (materials,a ircraft). Against the backdrop of the urgencyfor defossilizing energy regimes,the efficiency losses in ac arbon-based circular economy-which are largely understood technologically and can be welldescribed economically-are acceptable when compared to the challenges of ah ydrogen economy.F urthermore,t he possibility of ashrewd combination of reactions (3) and (4) is evident. In this case,the losses associated with creating an HÀ Hb ond may be avoided because the bond is broken in the subsequent process.T he price of bypassing this step requires the challenge of carrying out the liquid/gas (water/CO 2 ) reaction in an electrochemical reactor, [8] in which both phases react with one another at the electrode.T he gas-diffusion electrode offers av iable possibility.H owever,t his is significantly more complex (at large scale) than the combination of conventional electrolysis with as olid-gas reactor.
There will be many application scenarios for ac ircular economy of energy carriers and will include centralized as well as decentralized solutions. [9] Therefore,i tisadvisable to investigate and develop all viable pathways for achieving technical maturity.I nt his way,t he user of at echnology will have ar ange of possibilities from which the best systemic solution can be chosen. It is,therefore,necessary to exploit all possible gains in efficiency [10] in chemical batteries.F or these improvements to take place,ascience-based and robust understanding of the fundamentals of the materials and processes is aprerequisite.

CO 2 as aRaw Material with Value
Thec hemistry of CO 2 has long been the subject of research. [11] Themotivation for these investigations has been triggered by the wide availability of the reactant [12] as well as the desire for the construction of acircular economy [13] based on carbon. Foro thers,h owever, the thought of ac hemical exploitation of CO 2 is an atrocity or even a"thermodynamic crime". [14] Their arguments contend that the "love for the conversion of CO 2 " [13a] is aw aste of energy because CO 2 lies so far down the energy scale.Those opposed to the thought of aC O 2 cycle suggest deposition and confinement of the material as the only scalable means of combatting the greenhouse effect. [15] Nature itself exhibits many carbon cycles and exploits these opportunities to store energy in different ways. [16] CO 2 plays an important role in these cases. Fort his reason alone,i tis advisable to thoroughly study the chemistry of this molecule and its applications as ac hemical battery.
To assess the thermodynamic arguments,several standard enthalpies of formation will be considered. In no way do such considerations substitute for ac omplete analysis of chemical energy storage processes.H owever,s uch analyses contain so many process-specific values that the results lead to af ew general conclusions.D etailed examinations of this kind can be found in the literature. [11d, 17] Thes tandard enthalpy of formation of CO 2 is,atavalue of À393 kJ mol À1 ,318 kJ mol À1 more negative than the value of methane,which will be used here as ag eneral, or standard, energy carrier.I fC O 2 is allowed to react with atypical base such as Ca(OH) 2 to form the corresponding carbonate,1 207À393 = 814 kJ mol À1 are gained-a significant value for as upposedly low-energy [18] molecule.T his value also illustrates the care required for underground storage of large quantities of CO 2 .A lthough mineralization processes chemically bind the CO 2 ,t hey may also have alarge number of effects on the bedrock. Table 1p rovides some orientation for the energetic relationships during "charging" and "discharging" of the chemical battery CO 2 .Itshould be remembered here that the initial energy investment required for the synthesis of one mole of storage molecules (charging) is accompanied by ar elease of enthalpy during the reaction. Thed ifference in these values is equal to the enthalpy which remains stored in the system. Thee nergy source for the charging step is hydrogen, which must be obtained from sources and processes which release no CO 2 . Table 1also gives an impression of the "value" of selected molecules produced in the storage and transport of renewable energy.T he rule of thumb for the storage of renewable electricity in organic storage media is that approximately half the energy is lost. This value represents aconservative lower limit and can be markedly improved by an astute selection of storage material and processing techniques (for example, utilization of the enthalpy released during the reaction). For the energy investment listed in Table 1, the storage molecules can be produced with intermittent renewable energy.T hey can be stored, transported, and converted using existing infrastructure and can, therefore,b ec onsidered in their functionality to be "green oil and green gas". [19] Although the value of CO 2 hydrogenation is still being debated, the atmospheric concentration of CO 2 has climbed to 408 ppm and is growing at 2.17 ppm each year. [20] The question must be asked, what chemistry can do to reverse this trend, or at least stop it. Many answers have emerged [21] and can be found in ad iverse array of activities reported in the literature.A na nalysis of these answers [22] must display the urgencyo ft he task at hand as well as the complementary quantitative and temporal scalability of the chemical options. It should be clearly understood that the exploitation of CO 2 as ar aw material in the chemical industry,o ften viewed as asource of motivation in the literature,offers little in the way of greenhouse gas reduction when compared to the use of CO 2 as an energy carrier.I nG ermany,8 34 PJ equivalents of fossil resources were consumed by the chemical industry in 2018. This usage corresponded to 6.3 %ofGermanysprimary fossil energy requirement of 13 106 PJ.I ft he supply of raw materials for the chemical industry were shifted to CO 2 , aconsiderable increase in renewable electrical energy would be necessary to chemically reduce it. On average,t hree molecules of water would be needed to split one molecule of CO 2 ,i na ddition to the energy required for the conversion processes themselves.T he DECHEMA study "Roadmap Chemie 2050 [23] "estimates an energy investment of 550 TWh for the chemical reduction of CO 2 to hydrocarbon feedstock, which corresponds approximately to Germanyse ntire electricity consumption.
Thep roducts of the chemical industry then should be manufactured from CO 2 if this results in simplified synthesis processes,orifwaste and CO 2 emissions can be reduced. This change is only sensible if the necessary energy (and hydrogen) are supplied solely [11d] by renewable sources.B efore this happens,c hemical research should be tasked with finding processes and catalysts that reduce CO 2 emission on ap erproduct basis.Chemistry can also establish new,less-intensive CO 2 pathways for the supply of essential compounds.T his is, in fact, ac lassic area of research in chemistry [13a, 24] and has continuously been am otivating factor in the search for new catalysts.Atthe same time,however, the main motivation has always been supplied by the minimization of raw material precursors and the avoidance of waste with the corresponding monetary savings.T oday,c hemistrysc olossal task is to defossilize the resources that serve as the basis for the chemical industry.
Many products of the chemical industry are burned at the end of their lifecycle,orafter recycling, whereby the CO 2 still contained in them is set free.Ifthese practices were made to complete ac yclic process that used the burned waste as as ource of CO 2 ,t he cycle would remain closed. If control over the chemical products is lost, for example through landfilling, the integrity of the closed cycle cannot be guaranteed and the reduction of CO 2 emissions [25] is limited to 50 %. This maximum value corresponds to the reduction of fossil raw materials for the synthesis of chemical products and can only be reached [25] if all necessary energy supplies are drawn from sources emitting no CO 2 .
Chemical research can also improve existing processes in the energy industry (coal, oil, gas). This relationship is old [24b] and has contributed to the current high capacity and productivity of this industry.O ne fast and effective way to reduce CO 2 emissions in this area would be the application of hydrogen, which is used in large quantities in the petrochemical industry,f rom fossil-free sources (electrolysis,m ethane cracking [27] )a nd not from the more economical steam reforming of methane.A nother option is to replace fossil energy carriers used for heat generation by electricity from renewable sources (or green hydrogen). Here,c ollaborative research between chemical engineering and material science is needed to find the best way for electrical energy to be introduced in chemical processes.
Ac entral role for chemistry will be the storage and transport of renewable energy,w hich is initially produced as electricity,sothat this energy can become aglobally available traded commodity. [28] This operation will require "green oil and green gas [30] "a nd depend to as ignificant extent on chemical batteries.M ethane,m ethanol, [30] LOHC, [31] and ammonia [32] are currently crucial substances for this purpose. Thet ask of replacing fossil energy carriers will be difficult without the chemical reduction of CO 2 .I ti sf or this reason that the chemically simple,b ut urgently needed products arising from the reduction of CO 2 to "solar fuels" are the most valuable products [33] in the defossilization of the energy industry despite their low specific economic added value when compared to chemically complex molecules from the chemical industry.
Finally,t he direct replication of nature and the energy storage cycle( also based on the reduction of CO 2 following photochemical water splitting [34] )could be one of chemistrys central contributions to the energy supply of the future.T he natural photosynthesis of hydrocarbons occurs without the conversion of light into free electrons and the subsequent storage of the energy in chemical bonds.However,the process requires an extremely complex series of reactions which, at this time,c annot be imitated by technology." Artificial photosynthesis" has been, and is still currently,t he subject of intensive research. [16b,35] However,i tw ill not be discussed further here because the results of the research [30,36] have not yet contributed significantly to the supply of chemical products at the focus of this text.
This author is of the opinion that all these possibilities are important and should continue to be pursued in chemical research. Evidently,t his is currently the case and the results will provide aportfolio of options in the future.T he urgency to act immediately to convert and store quantities of energy on the scale of todaysoil and gas industry means that priority must now be given to the processes and materials needed for the restructuring of the present non-sustainable energy systems,e ven if they are not optimal in terms of process efficiency.T his perspective applies equally to process fundamentals as well as the mechanistic understanding and the identification of optimal functional materials.
Central questions surrounding the incorporation of CO 2 reduction into energy regimes involve the production and purification [37] of CO 2 (catalyst poisons), running facilities on sources of intermittent energy [38] (dynamic process management), and the question of facility size and complexity [9,39] (centralized versus decentralized). Diverse scientific questions resulting from this situation have received only secondary priority, [40] in part because they require experimental capabilities beyond the reach of typical academic research groups.
Thep rocesses of photosynthesis,b iomineralization, and also technical mineralization [41] of building materials show clearly that CO 2 is areactive molecule.The carbon contained in these materials may not be able to make at ransition to ah igher oxidation state,b ut the molecule is still reactive in many ways.I tc an, for example,g enerate different forms of organic and inorganic carbonates. [42] CO 2 can form adsorbates with metal surfaces,which have already been investigated for some time. [43] Thea dsorbates then form the basis for the heterogeneous catalytic reduction of CO 2 with hydrogen.

Valuable Reactions
Thequestion considered here will be which reactions lead to valuable products through the reduction of CO 2 .T o provide an answer, an overview of the current literature will first be provided and discussed. Along the way,t he meaning of the word "valuable" should be kept in mind. Thev iew is widespread [13a] that every molecule can be considered valuable when the oxidation state of carbon is less than it is in CO 2 .I norganic carbonates, [44] such as molecules [45] in which CO 2 plays the role of ab uilding block during synthesis,a re also considered valuable.The view is also held that CO 2 could be an important raw material [24b] for the chemical industry when the current stock of oil and gas either become too expensive or are no longer viable due to defossilization. This motivation has led to the development of novel reactions which form complex microstructures and polymers with the CO 2 building block. Them olecular chemistry of CO 2 has already been reviewed many times [11a,b, 13, 45, 46] and is treated only peripherally here.Interface catalysis [21,22,28,47] of the CO 2 reduction can be described in as imilar way.T he current Review uses the insight provided by these studies and makes an attempt at ac ritical analysis. This goal seems justified in light of the fact that the long history of research on the reduction of CO 2 has led to ac onstriction of research foci and makes as atisfactory overview difficult. Thef ormation of methanol will be considered here as an example of this situation.
Acarbon cycle [48] for the transport of renewable energy would certainly be the largest application of the hydrogenation of CO 2 .T he size of todayso il and gas industry provides an impression of the required dimensions.One application of CO 2 hydro-genation can be found in the production of synthetic fuels. [21,49] At this time,a lthough the opinion slowly abates that mobility should be completely electrified, it is especially important to intensively investigate the molecular structures which are in fact promising for use as fuels.F inding optimal pathways for their synthesis is also part of this search. The most valuable aspect of the chemical reduction of CO 2 is not only its economical nature,b ut also pertains to the defossilization of the energy regime.I ti sn oted here that this viewpoint is not shared by all [25,51] -especially not by those who point to efficiency arguments about the conversion chain. Furthermore,the "leakage" of synthetic fuels in acycle using CO 2 as the energy carrier has also been criticized [15] on the grounds that mobile sources burning carbon-containing fuels emit CO 2 and that the origin of the CO 2 (from the air or biomass (green), or from fossil sources (black)) plays ar ole. [7a, 29] In the more recent literature,t his criticism has led to increased scrutiny of life-cycle considerations for CO 2based processes.Afundamental examination of the hydrogenation of CO 2 has been reported by Bardow et al. [11d] Thes election of ar eaction for ac hemical battery is currently in no way straightforward. High-level discussions surrounding the structures of sustainable energy regimes currently include critical considerations of the use of chemical batteries.U nfortunately,i tm ust be stated that none of the reactions discussed above have been tested in the form of achemical battery for the storage of hydrogen with charging, transformation, and discharging on the scale required for ag lobal technology.I nT able 2, several viable molecules are listed along with parameters for their suitability as chemical batteries.
In addition to the simple stoichiometric parameters based on CO 2 ,t he central quantity for battery applications-the storage capacity-is given with an egative weighting to take into account the loss of hydrogen in the form of water molecules during the conversion of CO 2 .C onsideration of these weighted capacities is quite sobering for all the CO 2based processes.T he comparatively favorable values for ammonia (Table 2, entry 7) underscore this fact.
As mentioned at the outset, however, it cannot be concluded that CO 2 chemistry is unsuitable for chemical batteries.Inaddition to the data shown in Table 2, acomprehensive evaluation of chemical batteries also includes numerous technical and economic factors as well as,p erhaps most importantly,s ystem service of chemical batteries within an energy regime.T he central factor is the use of the energy supplied by the chemical battery and, with it, the total benefit to the system. In this case,d espite the unfavorable storage characteristics for hydrogen, carbon-based storage systems are often preferred due to their high energy density as well as their management and use,both of which are technologically well-understood. With regard to the technological maturity of the processes in Table 2, it is apparent that the storage of hydrogen (molecule synthesis) is being intensively investigated. The following detailed section on methanol molecules will illustrate these activities.Incomparison, the discharge process of the chemical battery,w ith the goal of recovering pure hydrogen (dehydrogenation), has enjoyed significantly less research and is not as well understood. This notable discrepancya rises from the lack of insight into the function and use of chemical batteries in acircular economy of energy carriers. An example is the LOHC process ( Table 2, entry 5). At the other end of the spectrum is the reforming of methanol, which has been investigated intensively in the context of fuel-cell automobiles. Figure 2s ummarizes key parameters from al iterature analysis with the keyword "hydrogenation of CO 2 ." Te nyears ago,a bout one publication on the subject appeared per working day.T oday,t hat number is af actor of eight higher and is growing at an exponential rate.Ifthe studies are sorted according to the reaction products,80% of all papers contain the four molecules methanol (53 %), methane (17 %), higher alcohols (16 %), and alkanes (14 %). Theanalysis underscores the view that the hydrogenation of CO 2 is specifically geared toward the preparation of synthetic fuels.

Status of the Literature
If the analysis is performed according to the country in which the research was performed, it can be seen in Figure 2 that China maintains the dominant position. Thec ountries traditionally strong in chemical research follow.I nE urope, Germany is the leader,w hile the topicality of the subject in Switzerland is notable.I ft he classification of research fields from Webo fS cience is used ( Figure 2C), the topic is,o f course,d eeply rooted in chemistry,a lthough aw ide disciplinary range is found when carrying out this search. Furthermore,i tc an be seen that contributions to research on the hydrogenation of CO 2 to generate commodities now come from fields not originally involved in finding solutions to these challenges.
In summary,C O 2 hydrogenation is ac urrent field of research and enormously dynamic,w ith the significant dominance of Asia being one of the main drivers.The subject is also investigated and treated in an interdisciplinary fashion. These conclusions are not surprising considering the substantial effort by China and Japan to reduce their reliance on imported oil. Further contemplation of the subjects to which the largest number of studies are dedicated reveals the notable difficulties remaining after al ong time of investigation-the easy insights have already been gained. Fort his reason, the interdisciplinary approach is also of no surprise.
These considerations skip over many novel approaches stemming mainly from molecular chemistry and catalysis.The task of producing something chemically valuable from carbon dioxide has resulted in much creativity [11a] and has led to arich synthetic chemistry of CO 2 .H ow these results will mature into new approaches for the energy question cannot be answered at present. Then ecessary focus on the technologically significant questions of the day pertaining to the synthesis of known solar fuels is accompanied by the dangerous tendency for an arrowing of the "research pipeline" needed for the solutions of tomorrow as well as for innovative developments in the synthesis of intermediates and fine chemicals.T hus,t he author emphatically supports pursuing novel approaches in the chemistry of CO 2 . [13,45] Even if this goal, in contrast to some statements in the introductions of publications,d oes not lead to relevant contributions to the climate problem, such studies provide urgently needed indepth and fundamental insight about the reactivity of this important molecule.T he result will be al ibrary of synthetic options,w hich could be decisive for future material family trees in the chemical industry that are structured differently from our present reliance on petrochemical raw materials. This assessment seems also valid for the energy supply in the mobility sector. Theimmense size of this application [7a] makes it difficult to implement novel concepts at al arge scale for improved combinations [49f,53] of fuels and motors.H owever, the large scale should not inhibit the search for better concepts.I fr esearch is allowed to cast this wide net in close coordination with the sciences involved, significant advances in the efficiency of the use of synthetic fuels [7a,50] should be expected along with areduction in the carbon leakage of the corresponding cycles.T oh ope for universal e-mobility is likely short-sighted in view of the extensive use of mobility in our societies.

Scale Effects and Urgency
In nearly every publication, the desire to contribute to the energy transition is given as as ource of motivation for the study.Several basic principles are,however, often overlooked in describing this impetus.The reason for the disconnect with real needs results from the fact that the defossilization of the energy regimes of the world is of extreme urgency and requires chemical conversion on an enormous scale.F igure 3 summarizes several quantitative arguments to this end. In the main graph, the development of global energy consumption is shown along with the fraction of renewable energy.Itshould be kept in mind that the vast majority of the latter stems from biomass (firewood) and hydropower.T he "new energies", wind and solar,which are the ones with the potential to supply the world, contribute to approximately half of all renewable energy.I ti squite worrisome,a si ss hown in the inset of Figure 3, that the annual growth in fossil energy consumption is still significantly higher than the growth of the contribution from renewable energy.
Thed imension of this growth is,a ts everal hundred million tons of oil equivalent, certainly formidable,especially when considering that this number, if multiplied by three,i s the number of supertankers required to transport this amount of energy around the world. Theu rgencyw ith which chemistry must react is evident from these dimensions. Without the complementary use of renewables to produce as olar fuel energy cycle,i twill hardly be possible to replace fossil fuels in the mandatory short time frame.With respect to the question of CO 2 hydrogenation, only materials and processes should be considered which can be upscaled to an order of magnitude of 100 Mt/annum CO 2 uptake and which rely on resources available in sufficient quantities.Ahuge number of large-scale production facilities will be required to process such dimensions of CO 2 ,t he construction of which will take years.S olutions will have to be found in these facilities for fundamental problems resulting from the combination of intermittent green energy and the use of novel material cycles. [54] Considering these factors,itisevident that one focus of research must be on the adaptation of existing processes to new challenges.Completely new approaches are important and provide more security for our future,but they should not be motivated by possibly contributing to the achievement of the goals of the Paris Climate Agreement. In general, it would help to orient the reader if the authors of studies dealing with these issues provided estimates on the scaling of their work with respect to material availability, robustness of synthesis processes,and feasibility.S ubsequent studies that wish to deal with the actual implementation of proposed processes will then be better equipped to differentiate between urgent applications and solutions which will be relevant in the distant future.
This argument can be elucidated with an example.Inthe project C2C, [54] an emissions reduction process has been realized during steel production by using CO 2 hydrogenation with green hydrogen. Thee nergy usage of the facility (Thyssenkrupp steel mill, Duisburg) is approximately 54 TWh/annum, or about 10 %ofGermanystotal consumption of electricity.I nt his project, only currently existing technologies can be employed to at am inimum partially redirect emissions into chemical applications in the next 10 years.N evertheless,s ignificant challenges appear during these operations in connection with the realities of fluctuating primary renewable energy sources.T hese challenges lead, with respect to opportune economical solutions,t ofundamental questions about the dynamic operation of methanol synthesis or the configuration of minimal gas purification methods which do not harm the subsequent catalytic processes and also leave the products free of unwanted trace substances.T he argument that other processes may also be involved in the reduction of emissions in heavy industry is taken into account in the C2C project by diversification of the product portfolio to include urea, higher alcohols,a nd reactive intermediates from CO 2 as well as in using unavoid- able CO 2 sources such as lime production and waste incineration.

The Reaction Network
Thea ctivation [11b] of CO 2 begins with the transfer of negative charge to the initially linear CO 2 molecule.A s aresult, the molecule becomes bent [43i] and the stability of its electronic structure is lowered. Now,r eduction products can be formed through reactions with hydrogen, or carbonates can be formed by reactions with ab ase.F igure 4s hows that further products are also possible through individual reactions.
It is apparent that the conversion of CO 2 with hydrogen results in the accessibility of al arge number of molecules which are both valuable for chemistry and suitable for renewable energy storage.T he energy is delivered via hydrogen. Acommon complaint in the discussion on the production of resources from CO 2 is that the processes are too "energy hungry." All reduction reactions produce water, with the exception of the formal addition of hydrogen to CO 2 to form formic acid (3 in Figure 4). Thew ater, with its high enthalpy of formation, provides the driving force for the endothermic target reaction. This process can be described as the charging of the "chemical battery" CO 2 ,which can then be discharged in the subsequent oxidation reaction. It would, therefore,b e ideal if-parallel to the necessary production of water-as much of the reaction enthalpy as possible could be stored in the target molecule (see also Table 1). In the case of formic acid (3), no water is produced. The"storage effect" is smaller in this case-but the energetic losses are also lower. Fort his reason, the formic acid molecule is also av aluable storage medium [11e,55] and optimization of charging and discharging processes with suitable catalysts-if possible,without the use of noble metals-is adesirable avenue of research.
From Figure 4i tc an be seen that the activation of CO 2 initially involves as eries of acid-base reactions.C arboxylate (a)c an either dimerize [43i, 56] (a reaction often missed in the literature) to produce oxalate (c)o ri tc an react with aB rønstedt or Lewis base (BO x )t op roduce carbonate (b). If water is added, carbonic acid is produced, which, if it is not esterified, will decompose into CO 2 and water. Thep rocess can proceed spontaneously or with metal catalysts which poorly bind atomic oxygen. [56] If an oxophilic metal such as Cu is used, oxalate can decompose into carbonate and CO (6). If hydrogen is present, H-CO (d)iseasily formed and can, via n, react to produce methoxy compounds (g)and methanol. The reaction need not proceed along the "formate," or "reverse water-gas shift" (RWGS) route (j, k). This fact is important, because it means am echanistic pathway to methanol is available,which is consistent with the stepwise hydrogenation of CO and yet contains the carbon atom from the CO 2 molecule.I sotope marker experiments designed to explain the reaction pathway of methanol synthesis are,t herefore, ambiguous.Aprecondition for this reaction pathway is that Figure 4. As ection of the reaction network for the reduction of CO 2 to simple products. The red arrows denote reduction reactions, the black arrows are redox reactions. The blue arrow shows the water-gas shift reaction. Stable products are labeled with numbers (blue), intermediates with lower case letters (red). the generated carbonate decomposes back to CO 2 to close the catalytic cycle.This is the case for the catalyst system Cu/ZnO under the reaction conditions,a sw as shown through in situ investigations on the formation of this catalyst. [59] Ther ole played by water and protons [60] in the formation of the intermediary carbonic acid is not currently known. One possibility is the formation of formaldehyde (2)f rom the intermediate (n). This process does not take place under normal reaction conditions because the additional energy of the subsequent reaction to generate methoxy compounds (g) is significantly higher than the energy maximum [47i, 58, 61] marked by free formaldehyde.T he blue arrow in Figure 4 denotes the water-gas reaction. There are different mechanistic views of this reaction that describe how lattice defects in the catalyst play an essential role in the redox reactions.F or clarity,these reasons are not given here.
Ther eaction pathway [21, 47i, 60, 62] to methanol is usually described as proceeding from carboxylate (a)t of ormate (f). From this point, the process splits and results in the hydrogenation of either the carbon (j)oroxygen (k); the latter can lead to the production of formic acid (3). Theintermediates j and m are identical, which makes experimental differentiation problematic.Ithas been observed that both formate (f), when used as ap recursor for the synthesis of the Cu/ZnO catalyst, and formate co-adsorbed with oxygen on copper decompose at 473 K. [63] This leads to the presumption that the important formate-oxygen intermediate [60] appears on the catalyst surface for only ashort time before reacting to form more stable products.A fter the reaction begins,i ti st he dominant intermediate on the catalyst surface and can be readily identified as long as low reaction pressures are used. [64] However,o ther adsorbates appear under pressures common in technical reactions. [65] Under the conditions of gas-phase catalysis,methanol (1) is formed from intermediate g,which, in the presence of acid sites,reacts further to generate dimethyl ether.Selectivity for methane (5)h as not been observed under normal conditions with ac opper catalyst. Under the conditions for electroreduction, [8a] however, methane can indeed be formed, although the reaction is usually unwanted. Mechanistically, it is unclear whether methane is formed from intermediate g or via intermediates n and h.I nc ontrast, the reactions between intermediates h and methanol to generate higher alcohols [66] are very much desired. Alternatively,h igher alcohols can also be formed along with hydrocarbons from intermediate e.T oobserve these reaction products on copper, on the other hand, co-doping with at ransition metal (oxide) or electroreduction is needed. Thei nnumerable resulting reactions,w hen starting from CO or methanol, [45] will not be covered as they are beyond the scope of this work. However, the chemistry of the Fischer-Tropsch synthesis [67] must be mentioned in the context of CO and chemical energy conversion. This process results in the availability of diverse products and mixtures which are important as fuels.T he industrial facilities for gas-to-liquid (GTL) processes make use of this chemistry in amodern form. Methanol itself [68] can be used as afuel, although it possesses unfavorable characteristics. [69] In the future,t herefore,t he acid-catalyzed oligomerization of methanol (methanol-to-olefins (MTO) process,or methanol-to-gasoline (MTG) process) will likely be used for fuels,o rm ethanol will be etherized with formaldehyde to form oxymethylene ethers (OME) [70] which are well-suited to replace fossil diesel fuel. Methanol itself would be ac heap fuel as its synthesis pathway in asingle step from CO 2 is highly efficient;itwould require,however, the design of adedicated combustion process to avoid its blending with other molecules,w hich diminishes the synthetic advantage.T he combustion of pure methanol can be very clean and minimize local emissions.

Methanol Synthesis
From the large number of possible reactions for the hydrogenation of CO 2 ,t he methanol synthesis reaction will now be considered in greater depth. In addition to the great significance of this reaction, it also serves as an illustration of the insight and uncertainties which exist today with respect to the function of ah ighly successful catalyst. Ther eaction network in Figure 4p rovides the basic principles for kinetic models that precisely describe the technical synthesis of methanol. An assessment of these mechanisms [71] has determined that the nature of the catalyst has little effect on the speed of methanol formation as long as the Cu/ZnO system is maintained. With this consolidated information, it should be possible to estimate the likelihood of finding anew catalyst [72] or an ovel process in this field which can be upscaled to the needs of energy applications.
Today,m ethanol is produced industrially from synthesis gas and hydrogen. Since the proposals of Asinger [5a] and Olah, [5b] the basic idea for defossilizing the energy system has been to replace the synthesis gas with CO 2 driven in ac ycle (the discharged state of the chemical battery) and to "charge" the battery with hydrogen from renewable sources ("green hydrogen"). Concern with respect to this concept has been expressed [73] that the efficiency of technical catalysts based on Cu/ZnO/X would suffer if CO 2 were chosen as the source of carbon instead of synthesis gas.
However,the situation is more complex if the reaction is made to proceed in aw ay that achieves am aximum spacetime yield. In this case,t he catalyst operates near an equilibrium line which is defined through an etwork of reactions [Eqs.(6)- (8)].
Thus,under the conditions of high CO 2 conversion, there is always also ap artial pressure of CO and water in the catalyst bed. If pure CO is used as the initial molecule,v ery harsh conditions are required (BASF process) to produce methanol. [74] Todayscommon Cu/ZnO/X catalyst was developed for the purpose of producing methanol [74,75] from CO 2 . In this process,t he multifunctionality of the ZnO components [76] play the different roles of carrier,mineral stabilizer of nanostructures, [77] and co-catalyst [78] on copper.The formation of aC u-Zn surface alloy [47h, 79] as the active phase has also been the subject of speculation and attempts have been made to verify it experimentally.F urthermore,t he Cu/ZnO/X catalyst is well-researched in terms of the structural dynamics [47j] of the active phase.T he resulting classification of structures and functions has turned out to be complex, because different active forms of the catalyst exist under the distinct conditions of any particular investigation.
If the fact is considered that the conversion into methanol is restricted under practical, relevant conditions, [74] the product gas must be recycled over the catalyst after water and methanol have been separated out. This procedure ensures that the catalyst is exposed to all three equilibrium reactions [Eqs.(6)- (8)],e ven if ap ure CO 2 /H 2 input gas is used. In technical processes,t he synthesis gas can be chosen such that only small amounts of water are produced to protect against possible decomposition of the delicate nanostructures of technical catalysts.I ts hould be noted that ac atalyst decomposition through reduction and resulting in brass [59] is also harmful and explains why ac ertain amount of water is required [80] to stabilize the system. Current studies (still in progress [81] )s how,h owever,t hat the harmful effect of water depends significantly on the absolute pressure as well as other contaminants (notably traces of oxygen) in the input gas present during the reaction.
Thep erformance of technical Cu/ZnO catalysts during the reduction of CO 2 to methanol has been determined as af unction of temperature.F igure 5s hows that technical catalysts can indeed achieve ad irect hydrogenation of CO 2 with arelevant space-time yield. Figure 5a lso demonstrates that other experimental catalytic systems [83] are not discernibly superior to the Cu/ZnO system in terms of their productivity.F urther reports detail that the Cu/ZnO/ZrO 2 system can exhibit noticeably increased productivities of up to 1.2 gg À1 hi ft he space velocity is increased by about af actor of 10 compared to normal values.
Ther elatively marginal sensitivity of the reaction to the nature of the catalytic system [71] is ac onsequence of the multiple equilibrium reactions which ensure that CO and CO 2 ,along with water, are present along most of the length of the catalyst bed. It is,t herefore,n ot strictly necessary [47b,58, 71] to develop an ew catalyst for the reduction of CO 2 to methanol because the performance and stability of the Cu/ ZnO system approach the reference values under synthesis gas conditions.Itseems possible to influence how completely the equilibrium is reached through modification of the composition of the Cu/ZnO system. Systems with large amounts of ZnO [85] result in high methanol yields and considerably less CO than would be expected from the position of the thermodynamic equilibrium. Hence,t he possibility may be considered that the selectivity of acatalyst toward the normally unwanted CO can be influenced to some extent while the productivity remains constant. Ther eason for this is presumably [74] that effective water-gas reactions at the defects of ZnO results in ah igher conversion than the reverse reaction at the copper sites.The copper is also present on the catalyst surface,b ut is expected to exhibit al ower reactivity than the ZnO.Such arguments tacitly suppose that ad istribution of active sites exist on aw orking catalyst, thus allowing the possibility to manipulate the extent to which the pathways shown in Figure 4a re followed after activation of the duct molecules.
As ac onsequence of the position of the chemical equilibria between CO,C O 2 ,a nd hydrogen, it would be desirable to increase the catalyst activity so that methanol synthesis can take place at temperatures below 473 K, where methanol is in aliquid state (slurry). [86] Forthis,the development of acatalyst completely different from the copper-based catalyst is likely required. It has been shown for the Cu/ZnO system that an adsorbate layer of water, -OH groups,aswell as formate and methoxy intermediates blocks the active centers [87] during methanol synthesis as soon as the temperature falls below 483 K. Thel ower limit of the working temperature of this system is,t herefore,d etermined by the reaction products preventing access to the active centers and not by an intrinsic,i nsufficient activity.T his shortcoming could be rectified if it were possible to find an active catalyst that could perform without the synergy [77a, 88] of copper and zinc oxide.One source of motivation for this search could be the use of catalysts which can produce methanol from CO under mild conditions.T he MoP system has recently been introduced [89] and may provide aclue to the correct approach. Furthermore,i ti si mportant for any successful application that no methane is generated as ab y-product. TheC u-based systems [66c,d, 90] display these characteristics in the gas-phase hydrogenation of CO 2 and CO.

Copper as aC atalyst
Theelement copper has achieved adistinguished position in the catalytic reduction of CO 2 due,among other things,to its ability to catalyze ag as-phase reaction with ah igh selectivity for the production of methanol. If solid acids are present, the reaction can also produce dimethyl ether. [83,91] In Figure 5. Space-time yields for methanol over atechnical Cu/ZnO catalyst as afunction of temperature (yellow squares). Experimental conditions: 30 bar,1:3 CO 2 /H 2 mixture [82] (the Zn/Zr system was measured at 50 bar). The individual data points for selected reference systems were taken from aliterature review. [83] particular,t he process does not seem to produce any CÀC bonds,a lthough this is certainly possible according to the reaction network shown in Figure 4. This behavior is often ascribed to the electronic structure of copper (see discussion in Ref. [89]) and it is expected that catalysts with ap artially occupied metallic dband will be required to produce higher hydrocarbons or their oxo derivatives from CO 2 (CO). It is certainly possible to modify copper with co-catalysts to "toggle" it into catalyzing the formation of C À Cb onds. [92] However,itisunclear whether this is atandem reaction of CO production followed by the conventional hydrogenation of dissociated CO on ac o-catalyst, or methanol generation followed by carbonylization with the simultaneous production of CO or, finally,t he direct formation of aC ÀCb ond during the reduction of CO 2 . [33b,93] These kinds of considerations are not only pertinent to copper, but also,for example, to rhodium-based systems. [94] Here,h owever, the necessary CO must be produced in apreceding reaction and, in addition to oxygenates,m ethane and other hydrocarbons are coproduced.
Theo bservation [8a] that the electrochemical reduction of CO 2 with copper can lead to an entire series of hydrocarbons is,therefore,even more striking. [47e,95] This result is also found with other transition metals, [96] although they can also generate such products during gas-phase catalysis.Anoxidative pretreatment of copper also seems advantageous [47e,97] for the production of higher hydrocarbons and their oxygenates.
It can be concluded from these considerations that the branching of the reaction network in Figure 4i sd etermined by the state of the surface (chemistry and morphology) of the catalyst. Thee xcellent selectivity of Cu for the formation of methanol under high-pressure conditions with hydrogen-rich reaction gases,only small amounts of water, and temperatures between 473 and 573 Ki se vidently due to the formation of au nique chemical state of metallic copper ("methanol copper"). Another state of copper is formed if conditions are shifted to those of electroreduction in an alkaline electrolyte at 300 Ka nd ar eaction potential capable of reducing Cu-oxide mixtures,which are stable when not in the presence of an electric current. Common to both states is that they consist overwhelmingly of metallic copper. Thediffering reactivities show,h owever, that the chemical and structural states are apparently not identical. Theimportance of the prehistory of an activated Cu surface illustrates that the states do not correspond to thermodynamically stable phases and that chemical dynamics [98] dictate the exact nature of each state. Theelectrolyte also seems to play asignificant role [95b] in the process.
Thed ifferent chemical states of the copper metal can be understood by remembering that electrochemical reduction takes place at room temperature and diffusion of oxygen atoms is slow.R apid and deep diffusion is,h owever,v ery much ap ossibility [99] during the activation of the gas-phase catalyst. [59] Furthermore,t he reactivity of hydrogen is different along with its redox potential with respect to oxygen in CO 2 and to the intermediates in Figure 4. Although atomic hydrogen is assumed in gas-phase catalysis," nascent hydrogen" with ahydride-like electron configuration and asignificantly stronger hydrogenating effect can arise by the electro-chemical reduction of water. In addition, the residence time of the intermediates on the electrocatalyst may be longer than on the gas-phase catalyst. Fort his reason, complex reaction sequences have ahigher probability in electrocatalysis.
Comprehensive studies on the selectivity of Cu metal in differing preparations as well as Cu oxides and alloys are described in an exhaustive review on the electroreduction of CO 2 with molecular catalysts at interfaces. [46a] Unfortunately, most of the systems described therein are extremely complex in their interface chemistry,and trends are difficult to identify. Thed ifferent reaction pathways shown in Figure 4a re certainly all represented. With improved [8a] measurement precision, the various crystal orientations in an electrode can be differentiated electroanalytically and in situ. [100] The effects described above of the impact on product selectivity through different electrochemical pretreatments of ac opper surface can, according to rigorous spectroscopic investigation, [95b] be linked to ac ombination of the presence of asurface copper oxide [97d] and morphological effects ("roughening"). It seems that the production of C 2 compounds can be traced to the surface oxide, [101] whereas copper modified at the surface by oxygen without oxidation [102] is beneficial for the production of C 1 compounds.
This hypothesis differs from an assumption in gas-phase catalysis that adsorbed atomic oxygen is necessary as an oxidant for CO in the synthesis gas to enable formate intermediates.P ulse kinetic measurements have clearly shown [43h] that no appreciable concentration of reactive atomic oxygen is present under the conditions for gas-phase methanol synthesis.Incontrast to this,ithas been known for some time that ac opper electrode binds oxygen at its surface [103] at all pH values,a lthough its protonation to generate OH groups depends on the pH value and applied potential. Thet hickness of am odified termination layer on such an electrode has been estimated to be 10 monolayers. [103] Theelectrochemical oxidation of copper results in alikewise thin passive layer [104] consisting of am ixture of CuO and Cu 2 O.
Theq uestion of the chemical nature of the active copper has long been the subject of investigation. [47c,h, 61, 77a, 79, 83, 97a,b, 105] It is indisputable that all activated copper catalysts and electrodes overwhelmingly contain metallic copper in the bulk phase. [106] Less clear,h owever, is whether this copper is pure. [99a] Here," pure" means that the copper, when active, contains neither zinc as an alloy [107] nor residues of oxygen from preceding states [106b] (either as an oxide or dissolved in the bulk [108] ). Furthermore,pure means that the active copper is not involved in a" strong oxide-metal interaction" with asurface phase of ZnO.There is evidence for the formation of as trong metal-support interaction (SMSI) surface layer. [78a, 105e,109] Other studies (under ETEM conditions) cannot identify the layer with certainty. [110] In the latter case, however, different catalysts were investigated (ratio of carrier/Cu and microstructure/carrier). It has been shown [88a, 106a] that the interaction energy between Cu and ZnO differs depending on the degree of reduction of the ZnO.This interaction can modify the morphology of ap ure copper cluster.Defective ZnO 1Àx can "creep" onto and around aCu cluster that has been, for example,c ontaminated with and roughened by dissolved oxygen. [61] It is questionable whether the functionality of copper in the electroreduction of CO 2 can be further clarified if another element, [111] such as Co [66b,c,e] or Ni, is added to the copperbased catalyst because they change the reactivity in acomplex way.T he chemistry of the support is likewise critical. Avery different reactivity of copper was demonstrated with respect to hydrogen depending on whether diamond or oxygencontaining sp 2 -hybridized carbon was used. With respect to the latter, ah eterolytic H 2 dissociation seems to occur. [112] This conclusion was also supported by electron microscopy studies on Cu nanostructures, [112] which identified ad istinct corrosive action of hydrogen on carbon carriers of differing quality.T he behavior was clarified by a" spill over" of hydrogen on graphene structures.Ondiamond carriers,onthe other hand, hydrogen only reduced oxidic copper without corroding the carbon.

Methanol Copper:S tructure and Dynamics
Thei nteraction of copper and its carrier material is dependent on the chemical potential of the environment. This ensures that the Cu-X system exhibits ad istinct structural dynamic: [88,113] the structure and wetting of the copper is changed reversibly as af unction of chemical potential. [110] Originally,i twas even postulated that the active form of the catalyst contained copper dissolved in ZnO. [114] Ap eripheral discussion concerns the question of whether active copper is affected by ac arbonate phase [59,115] that may act as ab inder phase between the metal and its oxidic supports.Inthe case of electrolytic copper,t here is the additional issue of the presence of OH or components of the electrolyte when the electrode is not thermally treated in an adequate manner after electrochemical synthesis. [116] Furthermore,t he molecular structure of the copper, that is,the type and number of lattice defects,plays asignificant role in the reactivity [61,117] which is not reflected in the statement "copper metal is the catalyst."It has long been known [118] that completely pure copper surfaces interact very weakly with adsorbates,w hile surfaces with coadsorbed oxygen exhibit as trong interaction. This function can also be taken over by dissociated water. Fort his reason, the separate branches of the reaction network for the gasphase reaction and for the electroreduction of CO 2 (see Figure 4) may actually be determined by adifferent extent of hydroxylation [119] of the metal surface with either water from the elelctrolyte or water from the reduction of CO 2 .T he statement that "the catalyst consists of metallic copper with local, additional structural and chemical modifications" will, therefore,describe the reality of aworking high-performance catalyst much better than just stating that methanol copper is ametal.
To give an impression of the function lying at the heart of the active structure of aC u/ZnO catalyst, the variety of morphologies will now be briefly discussed. Thes ystems described were produced by co-precipitation. Tw ok inds of catalysts,b oth with the same chemical composition, are obtained by controlling the kinetics of the precipitation and the subsequent work up.Inone case,copper nanoparticles are supported on ap orous mesh of ZnO needles.T he nanoparticles partially coated the ZnO in acomplex manner,while they themselves are partially coated by ar educed form of graphitic ZnO. [121] This resulting structure is shown in Figure 6 at different scales.
In the second case,t he copper was embedded as nanoparticles in aZ nO matrix and resulted in ac ompact agglomerate of platelets.A lthough the exposed copper surface shown in Figure 6i sa ctually larger than that of the platelets,t he latter result in an approximately 50 %h igher activity per copper surface [122] than the exposed particles. Figure 7a,b shows TEM images of both forms of copper.   (a, b) of fresh samples:a)from discontinuousprecipitation and b) from continuous overflow precipitation. c) Typical image of the catalyst in (a) after methanol synthesis at 10 bar in aCO/CO 2 mixture at 503 K. The particle size distribution in (d) was generated from the samples in (a; narrow distribution) and (c;b road distribution). 5000 particles were analyzed for each distribution. Data taken from Ref. [122].
High-performance catalysts are composed of significantly more copper than zinc oxide,asis evident in Figures 6and 7. This fact should be kept in mind during discussions of model systems,which often consist of only small amounts of copper particles on the carrier oxide.Inparticular, the ability of Cu to dynamically adapt its morphology to the reaction environment in high-performance catalysts is markedly limited due to the high density of particles or to their fixed embedded position in the carrier oxide.Therefore,the dynamic adaption of the carrier oxides themselves plays an important role in these systems. [77b,123] From diffraction images and corresponding powder X-ray data, it follows that the lattice constant of the exposed particles is,a t0 .3617 nm, very similar to that of pure copper (0.3615 nm). In contrast, the embedding of copper in ZnO creates lattice defects in the copper, thereby resulting in as ignificantly higher lattice constant of 0.3625 nm. [122] Figure 7c shows au sed catalyst of the type shown in Figure 6. After 400 hofreaction time at 10 bar it has aged and lost 20 % of its initial activity.Many twinned sinter particles of Cu plus platelets and amorphous fractions of ZnO can be seen. Figure 7d shows the changes in the distribution of volumeweighted particle size resulting from the aging process.T his behavior has been reported many times in the literature and attributed to the harmful effect of pure CO 2 as an input gas. However,i mproved synthesis methods lead to ab etter coating of the Cu active particles with ZnO so that the aging process is slowed and activities are mainly reduced through crystallization of the ZnO components [123] resulting in them losing their protective function as well as their possible cocatayltic function by aloss of interface area. Theanalysis and optimization of the catalyst is complicated by the role of lattice defects in ZnO. [110a] Thed efects are caused by promotors and are responsible for the crystalss tructural function [124] as well as its activation of CO 2 through electron transfer.
Arichly featured nanostructure and astrong dependence of the reactivity on copper particles have been identified as important characteristics [47e,97b,105c] for controlling catalytic function during electroreduction. Ac omplete and artifactfree structural analysis is still ongoing. [125] Re-activating ag iven electrode surface structure back to its initial state by pulse voltammetry after short operating times was traced back to the restoration of ap articular facet termination. [125] Whether this interpretation is actually unambiguous is not fully clear when considering the many forms of atomic oxygen which also take part in the process.
Theastonishing diversity of the answers to the seemingly simple question about the nature of the catalyst surely means that there are different forms of methanol copper. The reaction environment may be partially responsible for the variety of active states.I np articular, the presence of oxygen in the many manifestations of activated catalysts plays an important role along with the non-translational structure of the copper. These two factors are linked, because oxygen causes stress and strain in copper which results in aroughening of the copper surface.A dditionally,a ne ffect of the initial state of the oxide on the formation of the roughened surface is also relevant. Theinitial state (oxidation state,crystal form) is dictated by the activity of oxygen during formation of the preceding compound. Forexample,smooth metallic particles in all orientations can be formed from Cu 2 Oc rystallites by topotactic reduction. Thes tructural motif of CuO,o nt he other hand, requires an on-topotactic conversion where the roughening is aconsequence of the formation of polycrystalline particles [126] (crackling core and shell model). The demand to achieve al arge active surface does not allow the normal chemical synthesis procedure to choose sufficiently radical reaction conditions to obtain ap roduct at the thermodynamic minimum, for example,s ingle-phase,p ure copper. In catalysis,very mild methods of precipitation, [127] or impregnation [128] and subsequent activation are used, which result in materials which are difficult to characterize.I nt he case of copper,t here is the additional problem that the element, due to its pronounced affinity for oxygen, habitually tends to form solid solutions with oxygen, sub-oxides, [129] and surface oxides. [97d] Thes ituation is further complicated through the indirect effect of foreign particles on the internal stress and strain in the copper. [130] This geometric defect state is consequential for catalysis because it changes the electronic structure of the surface (d-band shift, step formation). The debate in the literature on the nature of the active phase is ambiguous,b ecause it is unclear exactly which form of methanol copper was used in the different studies.E ven rigorous measurements on single crystal copper do not prove that it is only metallic copper that acts as an effective catalyst. This ambiguity persists,d espite the fact that single crystal copper actuates methanol synthesis and that the effect can be matched [62] with data from as ophisticated kinetic model of ac omplex technical catalyst. Thei mplication that all other forms of the catalysts are,i nt he best case," contaminated" must be treated with great caution. This prudent approach is strengthened by descriptions of the material state of the activated state of copper, such as the analyses of surfaces with electron microscopy [59,132] and state-selective vibrational spectroscopy using CO as aprobe molecule. [109a, 133] Thef ollowing overall picture of methanol copper during gas-phase catalysis arises:i na ll cases,m etallic copper forms the matrix phase-all analytical methods show this to be the quantitatively predominant phase.T he simplest case is metallic copper with ar oughened surface.T he roughness is likely caused by internal forces of stress and strain; [126] the roughened regions are stabilized by oxygen atoms and contain electron-poor [99b] Cu species but do not correspond to acrystalline oxide.Oxygen atoms (and/or OH groups) are rigidly bound at or in the surface,although they do not act as oxidants for CO.I ft he copper is supported by an oxygencontaining substrate (oxides,c arbon), or if it is partially embedded within an oxide matrix [134] (ZnO,Z rO 2 ,C eO 2 ), perimeter states [132,135] will be formed on which copper can interact with the oxygen of the substrate,t hereby forming electron-poor Cu species.Adirect image of such perimeter states,which activate CO 2 on an Au/MgO model system, has been published by the Freund group. [56] If ZnO is present, it can be chemically reduced through very dry conditions or ah igh chemical potential of CO.T he result is the local generation of abrass crystal structure. [47h, 107] Under the conditions of afinite conversion into methanol and water, the ZnO,a sadefective (partially reduced) oxide phase, [110a, 124, 136] likely takes the form of ad ynamic thin film [109b] that is sensitive to the local chemical potential and covers the Cu roughened by internal stress and strain and by embedded oxygen atoms. [61,126] Ap reformed brass phase would quickly be oxidized back to ad efect-containing ZnO phase [105i] by water and/or active oxygen from the CO 2 activation. Oxygen atoms are required for the partially reduced and porous [137] defect ZnO film [135b] to adhere to the copper at all. Theo xygen facilitates the fixation of the zinc oxide layer (possibly with additional carbonate) especially well if it segregates from the bulk of the metal to the surface and forms asurface oxide. [105i] ZnO is multifunctional-as the carrier for the Cu nanoparticles,asthe active co-catalyst, or as matrix phase for embedded Cu particles.T he exact nontranslational structure of the ZnO,asastoichiometric mineral separator, as amatrix phase,orasadefect interface layer (or as acombination of all these;see,for example, Figure 7c), has been determined [59, 110a, 136] both by crystallization [138] from the precursor carbonate compound (needles,p latelets,o rientation) and by contact with the reactants (hydrogen and/or CO). All Cu/ZnO systems,t herefore,c ontain ap erimeter line at which Cu and ZnO interact. Thes trength of this interaction depends on the exact redox state of both phases,w hich can vary according to the morphological orientation and the local chemical potential. It is probable that the perimeter line corresponds to the geometrical location of the active centers for CO 2 reduction.
Theexistence of methanol copper can be verified, beyond the controversial discussion summarized here,b ym icrocalorimetric experiments with CO and CO 2 as probe molecules.F or such an experiment, nanostructured Cu was synthesized by using ZnO doped with Al or Mg (3 %b y weight in each case) using the conventional co-precipitation method. As acomparison, Cu nanopowder was also produced through precipitation and activation. [78a] Using the results obtained by Muhler and co-workers [105e] through calorimetry, the effect of ZnO as acarrier,which can be reduced easily (Al doped) or only with difficulty (Mg doped), on the chemisorption behavior of Cu will now be examined.
It is evident from Figure 8t hat "pure" nanostructured copper adsorbs CO in atypical manner.The observed heat of adsorption agrees well with data from model systems. [139] At ac overage of approximately 40 %, the adsorbate-adsorbate interaction begins to weaken the bond to the metal until, at full coverage,t he approximate heat of condensation is reached. Thec ase is completely different for catalysts on ZnO carriers.T he adsorbate-catalyst interaction is significantly stronger than at the pure metal and suggests asubstantially modified electronic structure of the "methanol copper." Thea dsorption energy,w hich grows with the degree of surface coverage,c an be explained through the structural dynamics [110] of the copper and the accompanying development of new active centers.Although Muhler and co-workers observed this phenomenon too, [105e] they interpreted it as an artifact. Asimilar observation was also reported by Parris and Klier,[105h] who explained it as the interaction of CO with electron-poor copper or ZnO.I ft he adsorbate-adsorbate interaction increases to ac overage of approximately 0.4, an ew phenomenon appears.T he adsorbate begins to react with ZnO and causes the interaction energy to increase.T his behavior can be seen clearly in Figure 8B.Acomparison of   Figure 8A and the activating effect of Al on the reduction of ZnO in Figure 8B.T he adsorption of CO 2 on the Cu catalyst with inhibited ZnO reduction is shown in Figure 8C.Toexclude interference from the potential formation of MgCO 3 ,pure nanocrystalline MgO was measured as acomparison. Thebehavior of CO 2 and CO are largely the same with regards to the dynamic exposure of the adsorption sites.Inthe case of CO 2 ,however, the absence of the reductive interaction between CO 2 and ZnO results in the more expected behavior of gradually weakening adsorption.

Copper and Oxygen:AUnique Relationship
Molecular catalysts with copper show adiverse chemistry of interaction with the element oxygen depending on the different geometries and oxidation states. [140] Thediversity of the Cu-O interaction is also present at the Cu surface.T he geometrical environment (smooth, rough), the presence of oxygen below the surface,a nd the formation of oxide-metal interfaces in the bulk and on the surface are responsible for the distinct chemical forms and dynamics [129b,c] and can be identified using spectroscopic methods.I fapossible codoping with Zn or another metal is included along with the other chemical possibilities,the result is awide array of local electronic structures similar to the molecular systems and their complex ligand systems.T he resulting diverse local electronic configurations govern the reactivity of the adsorbed CO 2 and its products during ac atalytic reaction. The quantitative determination of the reactive surface of the Cu/ ZnO system provides an example.U nspecific adsorption of nitrogen generates the geometrical surface.Using other probe molecules with specific chemisorption results in significantly smaller "active surfaces" which can, as expected, be differentiated through selection of the probe molecule.T able 3 provides an indication of these effects.
It is unlikely that pure copper is the only active catalyst, because both samples in Table 3e xhibit the same content of copper and the same nanostructure (particle size of 5-10 nm). Thec oncept, that the catalyst has only as mall effect on the formal kinetics of methanol synthesis [71] is,h owever, understandable considering the dynamic control of the active surface by the chemical potential of the reactant mix. An assessment of the efficacy of the two catalysts according to the concept of as tatic turnover frequency [142] is,i nt he best case, relevant with respect to the order of magnitude.
Theability of copper to stabilize multiple chemical states of oxygen is significant because the reduction of CO 2 represents ac ase of redox chemistry.I nt he formal simple reaction [Eq. (9)],two electrons are transferred in addition to the oxygen atom. It is clear that aredox catalyst such as Cu is required. Thea ssumption that electrons are transferred through each adsorption center means that am etallic center having the ability to bind oxygen as an oxyl group would be advantageous.T he formation and reactivity of the oxyl intermediate would then have to be explicitly considered in the context of actual microkinetics. [143] Thef act that no reactive oxygen was observed in pulse experiments [43h] does not mean, according to the dynamic concept of the system, that ad irect oxidation of CO cannot take place under the reaction conditions.
Afurther reaction (see Figure 4, intermediate f to k)isshown in Equation (10), whereby an adsorption site is exchanged for ahydrogen atom. If both adsorption sites in f were identical, it would be difficult to understand why an oxygen atom would remain bound, especially when the large number of active hydrogen atoms present under the reaction conditions is considered. If,o nt he other hand, as tep exists on the metal surface or an impurity atom (a ZnO site for example) is next to aC ua dsorption site,t he likelihood of reaction (10) in Figure 4i se asier to understand, because the two adsorption sites in f are no longer identical.
An analogous consideration holds for the form of the reacting hydrogen. It can be transferred as an Ha tom (radical), as ahydride (on ametal), or as aproton (on an oxo group) plus an electron. In each case,the participation of the catalyst is necessary,asit, at the very least, exchanges charge with the adsorbates.These processes must also be investigated in the context of am icrokinetic description to clarify,f or example,w hether some of the processes are simultaneously active.I llustrating the fine differences in the reactivity of metal-hydrogen species,studies [144] have shown that CO 2 can be activated by insertion into the metalÀHbond of adihydrido complex to form formate.T he local electronic structure is decisive for this process;ananalogous monohydrido complex does not show the same reactivity,b ut is electronically indistinguishable from the dihydrido analogue in NMR spectra. Both Zn and Cu are able to form hydrides as ac rystalline substance if they come from ap recursor compound of low valency (M 2 O) and are converted with atomic hydrogen. This situation occurs during electroreduction. Such hydrides cannot, however, be produced though synthesis from the elements.I ti sn ot known whether hydrides are formed during the reduction of Cu x Ounder ahigh hydrogen pressure. In this context it is interesting to note that the polymeric substance "CuH" exhibits the same red-brown coloration as Cu 2 Oand even contains embedded residual oxygen. Thus,the [a] The conversion from adsorbed material quantities to surfaces should, for chemisorption, be viewed critically.Nevertheless, they were chosen here to give an impression of the relative orders of magnitude.
idea that electron-poor copper is important for the reduction of CO 2 benefits from an ew aspect resulting from the so far largely overlooked possibility of CuH formation (along with its hydride adduct CuH 4 3À )a st he active form of the copper MeOH catalyst.
Ther eactions in Figure 4c an be assumed to be highly sensitive to the local electronic structures and the local morphological conditions. [83] These two properties are,e ven with the analytical methods used today,hardly distinguishable from each other when based on am atrix of Cu metal and ac arrier phase.Asurface-sensitive structural analysis under the reaction conditions [110b] is needed which does not influence or change atermination layer that dynamically responds to its surrounding chemical potential. If the system is extracted from the reaction environment, an unambiguous surface state [47h] can indeed be found. However,t his state is not necessarily the most reactive one. [136] Thus,i tb ecomes evident that arigorous description of afunctioning catalyst is, even today,still aproblem without asolution in sight. [145] That the current state of research can be confusing and seemingly contradictory to the outside observer is due to the fact that many studies are carried out with different systems that are not properly distinguished (although they may all be active). Furthermore,these studies are not always critical enough with respect to the methodological sensitivities [88b,113b,146] to unambiguously detect as tructure present as af raction of am onolayer which may not exist under standard conditions.M any conclusions in the literature lose their contradictory character if the boundary conditions,w hich are often not discussed when making av ery explicit claim, are considered.
Under mild reaction conditions,d ifferent surface structures co-exist and will, therefore,g enerate an umber of reaction products from CO 2 .F or example,t his happens in electroreduction, with its low thermal excitation for restructuring at the electrode.I nsitu XAS measurements on the deposition of Cu on ag old electrode showed [97a, 116] that, during the process,mixtures of Cu + and some Cu 0 arise from the disproportionation of Cu + into Cu 2+ and Cu 0 instead of only metallic copper, which makes up the main phase.Ifthis mixture is used under electrochemical reduction conditions for CO 2 in an alkaline electrolyte and as trong negative potential, the resulting material may not only be copper, but rather am etal modified with oxygen (OH). [102a, 147] This material may,t herefore,h ave different catalytic characteristics than pure copper.I ti sa lso plausible that the selectivity [47e,97a] of copper can be controlled using ad eliberately chosen pretreatment with oxygen. As witching between formate and CO generation can be observed depending on the pH of the electrolyte and thus the surface concentration of H 3 O + . [148] Thedecisive value is the local pH value,which can deviate significantly from the pH of the electrolyte as aresult of diffusion processes at the electrode. [8a] Thelocal pH value affects the degree to which Cu oxides-the thermodynamically stable forms of the electrode in the non-acid environment-are generated. Thec oncept of ap artially inhibited ("frustrated") phase change [98] between the metal and oxide brought on by an applied reductive potential likely describes well the state of active electrocatalysts in the hydrogenation of CO 2 .
Under harsh conditions,o nt he other hand, only states such as "alloy," "surface oxide," or "perimeter oxide" will exist and, therefore,o nly as mall number of products will be generated. An example of this is the high-pressure synthesis of methanol. If,h owever, stable co-catalysts such as transition-metal atoms and their low valencyoxo compounds are present, [66d,e] higher alcohols can, in addition to methanol, also be generated in relevant quantities under extreme conditions. This result makes it clear that the reaction network shown in Figure 4c an be realized in many different and parallel pathways with reactive copper.T he monotony of the highly selective methanol synthesis is determined by the singular synergy of copper and zinc leading to the uniformly modified copper metal "methanol copper." Thes ynergy is ac onsequence of the long process of material optimization of the catalyst and not adistinctive characteristic of the copper-CO 2 -H 2 system.

Epilogue
Thereduction of CO 2 leads to awide array of products.As fuels,some of the products will play astrategic role in future energy regimes along with acircular economy and the storage of fluctuating renewable electricity.T he products make possible ac oncept such as the chemical battery,w ith which nearly an unlimited supply of renewable energy can be stored and transported. Only thanks to the chemical battery concept will ag lobal trade of renewable energy be possible and replace the trade of fossil energy carriers.F urthermore,t he chemical battery enables the use of renewable energy in the mobility sector, where,m ost notably,h igh-performance applications with electric batteries are difficult to implement. Thes imilarities of the physicochemical material characteristics of chemical batteries and fossil fuels allow the further use of contemporary converters (motors and turbines). The chemical battery "hydrogen" is only able to fill this role in alimited way and in addition requires new infrastructure for transport to the user.
Other products resulting from the hydrogenation of CO 2 will change parts of the resource infrastructure of the chemical industry.Insome cases,the use of CO 2 as abuilding block for chemical synthesis will lead to new methods for the production of complex chemical products.Such processes will contribute to areduction of CO 2 emission from the chemical industry by simplifying synthetic routes and thus saving fossil raw materials and process energy.All of these applications are based on the chemistry of CO 2 in different reaction environments,which, at af undamental level, is well-understood.
Theo bjective of the hydrogenation of CO 2 is to use it as an energy carrier in the carbon cycle economy.I ts use as as cavenger for fossil emissions would be way too energyintensive and costly and could thus not contribute to the massreduction problem of reducing emissions (Gt per annum). Theprime objective must be to stop the emission of CO 2 from fossil sources,and this can only happen if we replace fossil by renewable energy carriers on ag lobal scale.T his in turn requires the concept of chemical batteries as abasis and with it the application of well-understood catalysis technologies. Scientific research into CO 2 chemistry has devoted much attention to as mall number of molecules such as methanol and olefins.Awide range of other possible sophisticated reactions has received much less focus in research. This choice can be justified by the challenge of both the urgencyand the scale of the processes based on CO 2 as araw material. In this field, as well as with the production and purification of CO 2 (from air,f or example), there are am ultitude of scientific problems still to be solved. At the same time,t hough, many synthetic and catalytic studies in CO 2 chemistry do not identify any pathways to scalable processes for the next two decades.O ne suggestion to overcome this is that, in the future,a uthors should either realistically evaluate their prospects for contributing to climate protection and reflect on these considerations during their experiments,o rt he generation of knowledge should be expressed as the motivation of the work.
This applies also to the literature on methanol synthesis and higher alcohols reviewed here.C omprehensive and highly detailed studies with seemingly contradictory conclusions describe ar eaction network for the hydrogenation of CO 2 ,w hich is,d epending on the conditions and catalyst, branched in different ways.T he multiple chemical states of the central element copper have only now begun to be recognized with respect to the molecular geometry and the presence of modifying atoms such as oxygen and structural dynamics.S ome controversies in the literature have arisen from the interpretation of experimental results that have been reported without adequate critical reflection on the rich and dynamic structural details of the copper. Them ethods were not surface-sensitive enough, did not contain enough evidence to support the conclusions,o rd id not sufficiently consider gaps in chemical potential connected with the dynamic reaction of an active catalyst. These insights have been possible thanks to many challenging operando experiments.Theory has also been responsible for supplying critical motivation for the atomic-level understanding of reaction pathways and the structural peculiarities that lie at their core. Nevertheless,t oday,acomprehensive description of the fundamental aspects of ar eacting system consisting of ac atalyst, carrier,a nd the reaction conditions is still not available.
However,t he analysis provided here concludes that the reduction of CO 2 to methanol proceeds by the formate/ RWGS route (see Figure 4) at bifunctional centers which are likely situated along the perimeter line between copper metal and partially reduced ZnO.Oxalate can also disproportionate on this line as dimerized active CO 2 and produce HCO which, at metallic centers,can then produce either methanol or lead to higher alcohols depending on the local electronic structure. To differentiate the pathway taken, the chemical potential of the active hydrogen is likely decisive.T he potential can, particularly in electrochemical reduction, be varied over awide range by means of an applied voltage and the pH value of the electrolyte.I nt his way,anumber of higher hydrocarbons can be produced which cannot be formed in gasphase chemistry using the Cu/ZnO system. Thea ssumption cannot be substantiated here [81] that contemporary methods for the industrial hydrogenation of CO 2 to methanol are not of adequate stability.I ti sc orrect that the productivity of modern systems is only technically satisfactory if the feed gases are recycled, something that places particular demands on controlling the impurity levels of gases and incurs cost to enable recirculation under pressure.H owever, there has been no description of as ystem in the academic literature that has even ac hance at technical realization when employing less or no recirculation. It seems,t hen, that this challenge must be met by afundamentally new approach, beginning, perhaps,with CO as the input gas (which can be readily produced from CO 2 )to produce am ixture of methanol and higher alcohols.A n opportune use of the resulting products could be fuels [49f, 53a] to avoid excessive separation and cracking efforts of the raw mixtures.T he CO 2 hydrogenation chemistry may then approach the well-known FT chemistry.
If,a si st he case here,t he fundamentals of ac omplex reaction sequence in an etwork of processes are unclear, an effective recourse is the employment of model systems (copper single crystals,f or example). Complexity can be reduced in this way to gather detailed information [145,149] on elementary processes and their dependence on the structure of the model catalyst. This strategy has provided much critical insight into the mechanisms of CO 2 reduction also used here for discussion. Theintended reduction of complexity excludes the detection of significant aspects of the reactivity under high-performance conditions that depend on the chemical constitution and dynamics of the metal surface of the working catalyst. This shortcoming results in the well-known "science gaps" in catalysis research. To overcome this challenge,t he application of artificial intelligence and its diverse methods may result in decisive contributions leading to structurefunction correlations that can bridge the gaps.
Amajor impediment in this respect is the incomplete and poorly structured presentation of comprehensive and highquality results.A ttempts at collecting all the descriptions of CO 2 reduction at interfaces using modern methods of digital catalysis research would quickly fail for this reason. Thus,one point of urgencyi nt he current Review is to declare that as tandardization of methods and descriptions in future theoretical and experimental studies on the hydrogenation of CO 2 should be introduced as ap recedent for the execution and documentation of future research on catalytic processes. Ther esult would be a" handbook" defining standards and general research guidelines for synthesis,t esting,a nd functional characterization as well as documentation in the framework of ac ertified metrology.I nt his way,aminimal standard would be established for future work without restricting the creative developments of tomorrowsresearch. All research carried out according to these norms would offer af ount of data [150] on which robust (complex) structurefunction relationships and extrapolations in the space of materials and the corresponding reaction conditions would be possible.The importance of the hydrogenation of CO 2 for our future justifies this undertaking without hesitation.