Hydrogenation of Inorganic Metal Carbonates: A Review on Its Potential for Carbon Dioxide Utilization and Emission Reduction

Abstract Carbonaceous minerals represent a valuable and abundant resource. Their exploitation is based on decarboxylation at elevated temperature and under oxidizing conditions, which inevitably release carbon dioxide into the atmosphere. Hydrogenation of inorganic metal carbonates opens up a new pathway for processing several metal carbonates. Preliminary experimental studies revealed significant advantages over conventional isolation technologies. Under a reducing hydrogen atmosphere, the temperature of decarboxylation is significantly lower. Carbon dioxide is not directly released into the atmosphere, but may be reduced to carbon monoxide, methane, and higher hydrocarbons, which adds value to the overall process. Apart from metal oxides in different oxidation states, metals in their elemental form may also be obtained if transition‐metal carbonates are processed under a hydrogen atmosphere. This review summarizes the most important findings and fields of the application of metal carbonate hydrogenation to elucidate the need for a detailed investigation into optimized process conditions for large‐scale applications.


Introduction
Carbon dioxide (CO 2 )i sa na bundant chemical species. On earth, it is present in all three aggregate states:i ng aseous form in the atmosphere (approximately 2.5 10 12 tons), in the dissolveds tate in the hydrosphere (approximately 10 16 tons), and in the solid state fixed in carbonate rocks (approximately 10 14 tons). [1] Hence, more (by af actor of 10 4 )t errestrial carbon is fixed in carbonate rocks in the earth's crust than is actually presenti nt he gaseous atmosphere.C arbonate rocks have been known as av aluable and abundant resourcef or al ong time. The history of inorganic metal carbonate chemistry dates back to the early ages of solid-state chemistry.T he thermal decomposition, meaningd ecarboxylation reactions of metal carbonates in an oxidizing atmosphere,a lso knowna sc alcination, roasting, or burning, depending on the metal species involved, represent processes that were developed first on experience, later on research,a nd finally on technological optimization in industrialp roduction plants.C ato, for instance, mentioned the burningo fl imestone (calcite, CaCO 3 )i nk ilns in 184 B.C. In the 1700s, Joseph Black gave the first systematic technical explanation of the calcination of limestone, also including the evolution of gaseous carbon dioxide. [2] Lime (CaO) is one of the cheapest and most widely used alkalizing chemicals and the main constituent of cementp roduction.T he production of magnesium oxide (magnesia, MgO) through calcination of magnesite (MgCO 3 )f or refractorym anufacture was established over 100 years ago in Austria. [3] Ther oastingo fi ron carbonate (siderite, FeCO 3 )i su sed for iron and steel production in areas with vast mineral iron carbonate reserves, such as Austria [4] and China. [5,6] The cement industry,iron production, andmanufacture of magnesia sinter make use of large quantities of the respective metal carbonates. Further industrial applications of metal carbonates of, for instance, nickel, cobalt, manganese, and zinc, or more precisely of their respective oxides or the metals in their elemental form, include the production of catalysts (catalytic material and porous support), pigmentsa nd glass, the ceramics industries, and the electronics industries. [3,7,8] During the decarboxylation of metal carbonates to yield the respective metal oxide (MeO),i nevitably one mole of CO 2 and/ or carbon monoxide (CO) evolve per mole of metal oxide formed. The metal oxide,i nt urn, gives access to retransformation into the corresponding carbonatet hrough CO 2 uptake. This characteristic provides the basis for carbon capturea nd storage( CCS) technologies, referred to as mineral carbonation, in which CO 2 is fixed to and stored as carbonate minerals, mainly in the form of calcium and magnesium carbonate. [9] Whereas the decarboxylation products of main-group elements( e.g.,a lkaline-earth-metal carbonates)a re the correspondingm etal oxidesa nd CO 2 [Eq. (1)],t he decomposition of transition-metal carbonates followsamore complex reaction pathway [Eq. (2)] because redoxp rocesses may takep lace. The redox behavior of the transitions metals allows the reduction of CO 2 to CO by means of thermodynamic fundamentals. The solid products can be metal oxidesa nd mixtures of metal oxidest hat adopt different oxidation states. [10] MeCO 3 $ MeO þ CO 2 ð1Þ In metal carbonate decarboxylation,t he reaction conditions, especially the nature of the gas atmosphere,p lay ac rucial role in the courseo ft he reaction. If carried out in ar educing atmosphere with hydrogen (H 2 ), af ascinating reactionn etwork is observed. Equation (3) shows the reaction for the hydrogenation of alkaline-earth-metal carbonates,w hereas Equation (4) gives ap otential reaction pathway for the hydrogenation of transition-metal carbonates.A part from metal oxides and mixtures of metal oxides in different oxidation states,m etals in their elemental form can be formed as solidp roducts from transition-metal carbonates. Gaseous products may include methane (CH 4 )i na ddition to or in place of CO 2 and CO. [10] Some citations even report the formation of higher hydrocarbons (C 1 -C x ). [11][12][13] In this context, it is noteworthy that an admixture of iron oxidese nablest he direct conversion of calcium carbonate into C 1 -C 3 hydrocarbons. [13] Carbonaceous mineralsr epresent av aluablea nd abundant resource.T heir exploitation is based on decarboxylation at elevated temperature andu nder oxidizing conditions, which inevitably release carbon dioxide into the atmosphere.H ydrogenation of inorganic metal carbonates opens up an ew pathway for processing several metal carbonates. Preliminary experimental studies revealed significant advantages over conventional isolation technologies. Under ar educingh ydrogen atmosphere, the temperature of decarboxylation is significantly lower.C arbon dioxide is not directly released into the atmos-phere, but may be reduced to carbon monoxide,m ethane, and higherh ydrocarbons, whicha dds value to the overall process. Apart from metal oxides in different oxidations tates, metals in their elemental form maya lso be obtained if transition-metal carbonates are processed under ah ydrogen atmosphere. This review summarizes the most importantf indings and fields of the application of metal carbonate hydrogenation to elucidate the need for ad etailed investigation into optimized process conditions for large-scale applications.
n CO þð2 n þ 1Þ H 2 $ C n H 2 nþ2 þ n H 2 O ð5Þ In addition to different gaseous products that are released from metal carbonates under reducing conditions, changing the gaseous atmosphere from inert to reducing also has an effect on the morphology of the solid products. [22] In most cases, the reducing agent in reductive metal carbonate decarboxylation is hydrogen. Until now,t he productiono fh ydrogen on an industrials cale has mainly (> 95 %) been based on fossil fuels, for instance,t hrough steam reformingo fm ethane or gasification of coal and hydrocarbons, which, apart from hydrogen,g enerates CO 2 . [23,24] Apart from conventional production routes, hydrogen can also be produced renewably and sustainably from various sources( e.g.,w ater electrolysis or water splitting through photocatalysis, [25,26] solar thermal production, [27] photosynthesis by algae, [28] reformingo fb iomass [29] ). Hydrogen storagei sc hallenging, however,a nd requiresacompletely new distribution system. [23] Whereas the decomposition of variousm etal carbonates in a vacuum or under an oxidizing or inert atmosphere has been well investigated and reported (e.g.,f or FeCO 3 in vacuum, [30] oxygen, [31][32][33] and nitrogen [34][35][36] ), information on metal carbonate hydrogenation is scarce. Reduction of carbonates in aqueous solution is also well described in the literature and is not covered herein. [37][38][39][40][41][42] Metal carbonate hydrogenationi sn ot only challenging from ar eactionm echanismp oint of view,b ut also features some characteristics that render it promising in terms of carbon capturea nd utilization (CCU),h ydrogen storage, and novel productiont echnologiesinm etallurgy.T he purpose herein is to provide ac omprehensive literature review on metal carbonate hydrogenation and to discuss potential applications.A fter describing their natural occurrence (Section 2), thermodynamics of metal carbonate hydrogenationa re specified (Section 3). Experimental studies that have been conducted so far are listed in Section4.A fter summingu pt he main benefits of metal carbonate hydrogenation (Section 5), feasible process options are illustrated and evaluated (Section6). Finally,Section7identifies future needs for detailed research.

Natural Occurrence
Inorganic carbonates,c haracterizedb yp lanar [CO 3 ] 2À complexes with metal ions, form one of the most important mineral groups. Ta xonomyi sb ased on cationsa nd/ora nions outside of the [CO 3 ] 2À complexes. Water-free mineral carbonates may feature calcite-(trigonal, scalenohedric), dolomite-(trigonal, rhombohedral), and aragonite-type (orthorhombic) structures. Carbonates with additional anions may contain OH À anionso r water. Mineral lithium carbonate (Li 2 CO 3 ,z abuyelite) occurs in the lithosphere as an ore companion, mainly imbedded in halite in rock salt and in saline lakes. Sodiumc arbonate (Na 2 CO 3 ,natrite, also known as soda) appears in saline lakes andm inerals such as trona Na 3 (CO 3 )(HCO 3 )·2 H 2 Oa nd natronN a 2 CO 3 ·10 H 2 O. It undergoes ar apid change superficially in air to thermonatrite (Na 2 CO·H 2 O). Potassium carbonate (K 2 CO 3 )i sc alled potash because it was historically produced by treating wood ashw ith water in ap ot. Therea re no K 2 CO 3 -containingo res known that would be worth mining. It is generally produced from electrolyticallyp roduced KOH and CO 2 .R ubidium (Rb 2 CO 3 )a nd cesium (Cs 2 CO 3 )c arbonate are naturally found together in low concentrations accompanying sodium-and potassium-containing ores. Similar to the other alkaline metals, they also collect in saline lakes. Francium isotopes (Fr 2 CO 3 )a re radioactive and only found in traces in the lithosphere. [43,44]  Mining of these minerals is the main source for calcium carbonate. Three different naturallyo ccurring crystal structures of calcium carbonate exist:c alcite, aragonite, and vaterite. In calcite, the central Ca 2 + ion is coordinated by six oxygen atoms, whereas in aragonite nine oxygen atoms coordinate the central Ca 2 + ion. Strontium carbonate (SrCO 3 )i sf ound in the naturally occurring mineral strontianite. Barium carbonate (BaCO 3 ) forms in the lithosphere as witherite. [43,44]

Transition metals
Manganese is present in the lithosphere nearly as frequently as carbon or phosphorus. Manganese carbonate( MnCO 3 )i sf ound in the mineral rhodochrosite anda sa no re companion of iron. Iron carbonate (FeCO 3 )i sf ound in the lithosphere as siderite and ankerite (Ca(Mg,Fe)[CO 3 ] 2 ), for example, at the Erzbergi n Styria, Austria,a nd in China. Cobalt occurs in diversef orms in the lithosphere, often as an ore companion. Nevertheless, no major cobalt carbonate (CoCO 3 )-containing ore is known. Cobalt carbonate can be produced by precipitation of water-soluble cobalt(II)salts with alkaline-earthc arbonates.N ickel exists in diverse forms in the lithosphere, but no nickel carbonate ores are known.T he industrially most important nickel carbonatei sc austic nickel carbonate, 2NiCO 3 ·3Ni(OH) 2 ·4H 2 O, produced by precipitation of aqueous nickels ulfate with sodium carbonate. Caustic nickel carbonatec an be dehydrated to give anhydrous nickel carbonate or the hexahydrate NiCO 3 ·6H 2 O. Copper carbonate (CuCO 3 )o ccurs naturally as azurite 2Cu-CO 3 ·Cu(OH) 2 (blue) and malachite CuCO 3 ·Cu(OH) 2 (green). Silver carbonate (Ag 2 (CO 3 ) 3 )i sn ot found in minerals, but precipitates from water by using soluble silver species and alkaline-metal carbonates, such as soda. Zinc carbonate (ZnCO 3 )e xists as smithsonite. Cadmium carbonate (CdCO 3 )i sm ostly found as an ore companion of smithsonite. [43,44] 3. ThermodynamicsofMetal Carbonate Hydrogenation Thermodynamic analysis of alkaline, alkaline-earth, and transition-metal carbonates between 400 and 1200 Ka ta mbient pressure shows increasing standardf ree energies of reaction, DG R 0 ,f or methane formation with increasing temperature. Due to strongly positive DG R 0 values (> 60 kJ mol À1 ), methane formation is not possible through hydrogenation of alkalinemetal carbonates (Figure 1a). Amonga lkaline-earth-metal carbonates, only hydrogenation of MgCO 3 features an egative DG R 0 ,f avoring CH 4 formation ( Figure 1b). Hydrogenation of the transition-metal carbonates MnCO 3 ,F eCO 3 ,C oCO 3 ,N iCO 3 , CuCO 3 ,a nd ZnCO 3 to their respective bivalent oxides, CH 4 ,a nd H 2 Oi sf avorable (Figure 1c). Hydrogenation of the transition-metal carbonates and MgCO 3 to yield CH 4 exhibit pronounced exothermic behavior (DH R 0 < À50 kJ mol À1 ).

Experimental Studies
Experimental studies on the hydrogenation of metal carbonates are scarce. Although first reports date back to the late 1960s, tot he best of our knowledge, there are only 20 scientific publications from six research groups. Interestingly,h ardly any cross reference exists betweent he studies. Giardini and Salotti from the University of Georgia, USA, were the first to report on reactions between mineral calcite, dolomite, and siderite with pressurized hydrogen and the concomitantf ormation of hydrocarbons. [11,45,46] The primary purpose of their study was to address geological issues, such as the formation of hydrocarbons from inorganic sources occurring in the earth's crust. Ap atentw as filed on this topic. [47] The experimental apparatus consisted of a2 5cm 3 externally heated "cold seal"-type vessel, in which the carbonate charge was enclosed in ap latinum foil, and left unscaled ands us-pended in the heated part of the vessel. Before usage, the mineralsw ere handpicked for impurities, crushed, and heated in 30 %h ydrogen. The natural mineral deposit and the composition of the minerals were not stated.
From 1987, Reller and co-workers from the University of Zürich, Switzerland, investigated the thermal reactivity of pure alkaline-earth-metal carbonates, 3d transition-metal carbonates, and metal-doped alkaline-earth-metal carbonates in pure and dilute hydrogen by thermogravimetric( TG)e xperiments. [10,48,49] Twog roups from Japan-one from the To kyo Institute of Te chnology; [50] the other from Kobe University [51] -investigated metal carbonate hydrogenation in the 1990s. Tsuneto et al. used NiCO 3 and CoCO 3 without additional catalysts anda series of metal carbonates doped with catalytically active metals,a nd hydrogenated them in af ixed-bed flow reactor at atmosphericp ressure. [50] Formationo fg aseous products was reported in mmol h À1 at at otal gas flow of 9.6 cm 3 min À1 and feed samples of 2gafter 0.5-1 h. To estimate the reaction rate, we calculated the hourly conversion of the respective carbonate to CH 4 (CtM in %h À1 ), as depicted from Equation (7). Data listed in the table coincided only partially with the data given in the main text of their paper. [50] In case of discrepancies, data listed tabularly were used herein.
Yoshida et al. focusedo nC H 4 formation from metal-catalyzed CaCO 3 hydrogenation, in comparison to hydrogenation of pure CaCO 3 . [51] Experimentsw ere carried out by meanso f the temperature-programmed hydrogenation technique, and for isothermal kineticr uns two apparatuses were applied:a Cahn electrobalance andaclosed circulation apparatus.
In 2009 and 2013, Jagadeesan et al. from the Jawaharlal Nehru Centre for Advanced Scientific Research, India, investigated CaCO 3 and mixed-metal/CaCO 3 hydrogenation promoted with catalytically active metallic nanoparticlesi nacontinuousflow,p acked-bed, stainless-steel reactora nd managed to directly convert the inorganic carbonates into C 1 -C 3 hydrocarbons. [13,52] The effect of reaction temperature, pressure, and gas atmosphere on the reaction kinetics of mineral FeCO 3 ,M gCO 3 ,a nd CaMg(CO 3 ) 2 hydrogenation has recently become subject of detailed investigation by Baldauf-Sommerbauere tal. from Graz University of Te chnology,A ustria. [12,53,54] Experiments were carried out in at hermobalance and at ubular reactor setup.  Ad etailed list of the abovementioned studies, starting with the most recent ones, is given in Ta ble 1. The investigations can be dividedi nto two groups:1 )hydrogenation of singlemetal carbonates without additional catalysts, applied either as mineral oreo ri ns ynthetic form;2 )mixed-metal/metal carbonates, mainly synthetic, in which one of the metals,m ostly transition metals, acts as an internal catalyst. In general,s ynthetic carbonates and mixed-metal/metal carbonates were produced by precipitation and coprecipitation from aqueous solution of NaHCO 3 .

Single-metal carbonates
Hydrogenation of single-metalc arbonates includes the alkaline-earth-metal carbonates MgCO 3 ,C aCO 3 ,S rCO 3 ,B aCO 3 ,a nd CaMg(CO 3 ) 2 and the 3d transition-metal carbonates MnCO 3 , FeCO 3 ,C oCO 3 ,N iCO 3 ,a nd ZnCO 3 .D ecomposition of the respectivec arbonates in ar educing hydrogen atmosphere occurs at lower temperatures than that of decomposition under inert or oxidizing conditions. Reller et al. [48] and Padeste [55] performed TG measurements with finely ground MgCO 3 and CaCO 3 mineral,a nd synthetic SrCO 3 and BaCO 3 at atmosphericp ressure. Equal amounts of CO 2 and CO, together with H 2 O, were formed as gaseous products from MgCO 3 .I nt he case of CaCO 3 ,t he main volatile compound was CO (CO/CO 2 % 10:1) from which ac hange in the degradation mechanism was concluded. The concomitantf ormation of CO, in addition to CO 2 and H 2 O, was dedicated to the reduction of CO 2 through the reverse water-gas shift reaction [Eq. (6)].T he amount of CO increases with increasing atomic mass of the alkaline-earth-metal cation due to higher decarboxylation temperatures( T MgCO 3 < 800 K, T CaCO 3 % 900 K) and the endothermic nature of the reversew ater-gas shift reaction, which shifts the equilibrium composition towards the product CO at highert emperatures. The reactiont emperature for decarboxylation in hydrogen was lowered by at least 150 K compared with the analogous reaction in an itrogen atmosphere. In ah ydrogen atmosphere, all four alkaline-earth-metal carbonates fully degraded below temperatures of 1200 Ki nto their respective oxidesM gO, CaO, SrO, and BaO. They formed as solid conglomerates of microcrystalline domains with diameters of 10-20 nm and showed ap ronounced reactivity towards the respective hydroxides and towards recarbonation. [48] If synthetic CaCO 3 crystals were degraded in nitrogen and hydrogen atmospheres, distinct destruction in nitrogen occurred, which suggested that diffusion of H 2 into the carbonate, where it directly reacted with fixed CO 2 ,a nd reversed iffusion of formed CO and H 2 Op roceeded faster than that of CO 2 diffusion. [10,48] Kinetic studies with CaCO 3 were performed by Yoshida et al. in ac losedc irculation apparatus at af ixed H 2 pressure of 0.13 10 5 Pa and 748 K. [51] Ther eaction was of half-order with respectt oH 2 with an activation energy of 236 kJ mol À1 .I nitial reactionr ates did not differ for varying CaCO 3 amounts, which was explained by decomposition of CaCO 3 and hydrogenation of released CO 2 to CO. The lowest temperature at which hydrogenationo ccurred was 700 K. At temperatures of 700-773 K, only CO formed. Above 773 K, CO and CO 2 formed.T he formation of the two gaseous products differed over the course of time. Whereas CO pressure steadilyi ncreased with time, CO 2 pressurea bruptlyr eached am aximum value of 22.7 Pa and did not change subsequently.
Low temperature and elevated pressure facilitated CH 4 formation. Moderate to high temperature and low pressure facilitated CO formation.T he CH 4 yield was 38.6 %a fter 20 % MgCO 3 conversion at 748 Ka nd 1.2 MPa overpressure. With respect to the reaction mechanism, decreasing CH 4 formation with increasing magnesite conversion indicated ad ependency on the amounto fM gCO 3 .B aldauf-Sommerbauer et al. related the increase of CO concentration with rising MgCO 3 conversion to the amount of MgO. To scrutinize this interpretation, they examined reductively calcined MgO for its catalytic properties for CO 2 conversion withH 2 .A ta mbient pressure and 0.3 MPa overpressure, only CO formed. At 0.8 MPa overpressure, traces of CH 4 occurred in addition to the major product CO. These findings revealed significant reverse water-gas shift activity of reductively calcined MgO, but no CH 4 formation. CH 4 formation during reductive calcination of magnesite seemingly proceeds through adifferent mechanism.
Giardini and Salotti reported the formation of CH 4 and ethane( C 2 H 6 )t hrough heating (693-1243 K) of calcite and dolomite under ap ressurized hydrogen atmosphere (0.7-80 MPa H 2 ); however, it was not comprehensible whether the pure mineralsw ere used or if the reaction was catalytically accelerated. [11,[45][46][47] In their main publication, [11] they stated that metallic Ni, Pt, Cu, Ti,M g, and Fe;c ommercial mixtures of 0.5% Pd, Pt, Rh on alumina and dried silica gel;a ctivated alumina;h ematite, magnetite;chromic oxide;c hromium trioxide;and Kieselguhr mixtures were added, but no precise information was given on the type of catalystf or individuale xperimental data. The catalysta dmixture wasn either mentioned in the first publication [46] nor in the patenta pplication. [47] Apparently,n one of the catalytically active materials had ad iscernible effect on the rate of reaction. The reactionk inetics were thus expected to fit the uncatalyzed hydrogenation of calcite. Due to high initial hydrogenc oncentrationsr elative to calcite, the reaction kinetics simplified to pseudo-first-order kinetics,w ith an activation energy of 75 kJ mol À1 at 14 MPa. [45] Hydrogenation of calcite starteda t7 73 K. It was primarily dependento nt emperature and secondarily dependent on pressure and time andproceeded in ac rystallographicallya nisotropic manner.A ss olid products, CaO, Ca(OH) 2 ,g raphite (C), and ab lack residue, possibly solid hydrocarbons, formed. At higher temperatures, CaO was the principal solid product. CH 4 and H 2 Ow ere ubiquitousg aseous products,w hereas C 2 H 6 and CO appeared under certain specific conditions. CO 2 was never detected in remarkable amounts( > 0.01 %). CO 2 ,i fformed,i mmediately converted into CH 4 4 and yielded MgO as the final solid product. [56] CaMg(CO 3 ) 2 The effect of reaction temperature( 793 K-1108 K) and initial hydrogen pressure (14-34 MPa) on the hydrogenation of dolomite (40-60 mesh)w as investigated by Giardini and Salotti. [46] As for calcite, there was no cleari nformation on the catalyst admixture. We assume that the main findings apply to the hydrogenation of dolomite without additional catalysts. Solid products included CaCO 3 ,C a(OH) 2 ,C aO, noncrystalline Mg(OH) 2 ,graphite, and a"soot-like" material; gaseous products CH 4 ,C 2 H 6 ,C O, and CO 2 .H 2 Ow as the oxygenated product in all experiments. At wo-step reaction mechanism was proposed in which noncrystalline Mg(OH) 2 or MgO formed [Eq. (9)].A t 34 MPa, the reaction starteda t793 K.
Reller et al. found that the decarboxylationt emperature of dolomite and the ratio of the gaseous products CO and CO 2 lay between the corresponding values of the two pure carbonates CaCO 3 andM gCO 3 . [48] Baldauf-Sommerbauer et al. suggestedt he hydrogenation of mixed magnesite/dolomite (1:1 mol/mol) for the synthesis of CO. [54] TG measurements showed two decomposition steps. Compared with the reactioni nn itrogen, ah ydrogen atmosphere leads to ad ecrease of the decomposition temperature of 60 Kf or the first step and 100 Kf or the second step. In the first step, the decompositiono fc oncomitantM gCO 3 ,F eCO 3 , and MnCO 3 to the respective bivalent oxidesa nd CO 2 was observed. Then CaMg(CO 3 ) 2 decomposed into CaO, MgO, and CO 2 .R eductivec alcination experiments in at ubular reactor in- dicatedasequential mechanism of calcination followed by hydrogenation of CO 2 .C H 4 was only formed in traces, even at elevated pressure. It is assumed that CH 4 formation was kinetically hindered. AC Oy ield of 61-73 %w as achieved for partial reductivec alcination of the magnesite content below 813 K. An increase of pressure did not affect the formation of CO, but caused as light retardation of the reaction.

Transition-metal carbonates
The mineralsM nCO 3 (rhodochrosite) and FeCO 3 (siderite), and synthesized CoCO 3 ,N iCO 3 ,a nd ZnCO 3 were investigated. Synthesis requires hydrothermal conditions at high pressure. According to the size of their cations, the 3d transition-metal carbonates crystallize in at rigonal calcite-type structure. [22,57] As postulated by Emmenegger [22] from hydrogenation experiments in pure and dilute hydrogen (5 %H 2 in Ar), the partial hydrogen pressure decisively influences the hydrogenation reaction. Different solid products-transition-metal oxides and elemental transition metalsa rise-depending on the selected atmosphere,a nd morphological features may be controlled. In hydrogen, decarboxylation temperatures drop, in comparison to the respective reaction under an inert atmosphere.D ilute hydrogen resultsi nalower temperature drop. The degradation temperature drops with decreasing radii of the transitionmetal cations: The ratio of gaseous products CO 2 ,C O, CH 4 ,h igher hydrocarbons, and H 2 Ov aries, depending on the transition-metal species. According to Reller et al.,C O 2 is released duringt ransition-metal carbonate hydrogenation, and furtherc atalytically hydrogenated. [10] In the course of decarboxylation,t he catalysts Me or Me/MeO form in situ. Fe, Co, and Ni act as efficient hydrogenation catalysts.

MnCO 3
In pure and dilute (5 %H 2 in Ar) hydrogen, the mineral MnCO 3 decarboxylated to MnO without changing its oxidation number at 643 K. As gaseous products,t races of unconverted CO 2 ,C O, and H 2 Of ormed. Consequently,m anganese efficiently catalyzed the reverse water-gas shift reaction.I nd ilute hydrogen, this reactionw as retarded. After ap eriod of induction, released CO 2 was further converted into CO and H 2 O. Compared with pure hydrogen, the chemical equilibrium barely lay on the side of the product. [22] FeCO 3 Salotti and Giardini first studied the hydrogenation of siderite (40-60 mesh) at reaction temperatures of 618 to 878 Ka nd initial partial hydrogen pressures of 1.4 to 34 MPa (H 2 ). [46] Because there was no precise information on the catalyst admixtures reported, we assumed that the main findings appliedt os iderite only.A tt emperatures above 728 K, the solid productsc onsisted of elemental Fe and wüstite (FeO). At lower temperatures (673 Ka nd 14 MPa H 2 ), magnetite (Fe 3 O 4 )f ormed. From minute and rare flecks,G iardinia nd Salotti assumed that also graphite was formed. [11] Wüstite was the primary alteration product [Eq. (10)].F urther reductiono fw üstite to elemental Fe [Eq. (11)] or oxidation of wüstite to magnetite [Eq. (12)] depended on the reaction temperature and the ultimate H 2 O/H 2 ratio. Al ow reaction temperature and dry hydrogen need to be provided to effect reduction.
CH 4 and H 2 Ow ere ubiquitous gaseous products,w hereas CO, CO 2 ,a nd the higherh ydrocarbons ethane (C 2 H 6 ), propane (C 3 H 6 ), andb utane (C 4 H 10 )w ere present over al imited temperature and pressure range. An inverse relationship existed between the temperature and the length of the hydrocarbon chain,m eaning that al ower initial reaction temperature results in am ore complex hydrocarbons pecies. At 673 K( 14 MPa H 2 ), the gaseous product on ad ry basis contained4 . 45 [11] The reactiont emperature fors iderite hydrogenation is lowe nough to ensure thermals tability of ethane, propane, and butane. They concluded that higher hydrocarbons formed directly throughar eaction on the mineral surface, rather than in as ubsequentr eaction between released gases and hydrogen.
This conclusion contradicts the findings of Reller et al.,w ho explained the formation of CO and CH 4 by in situ formation of catalytically active transition-metals pecies. [10] Emmenegger compared the hydrogenation of siderite in pure and dilute hydrogen (5 %H 2 in Ar). [22] In pure hydrogen, the formation of mainlye lemental Fe together with FeO as solid products was observed. At elevated temperature above 823 K, decomposition was slow and not yet finished at 973 K. The gaseous products CH 4 ,H 2 O, and CO were formed. Higher hydrocarbons were not found. CO 2 was also released. The reactionp athways in Equations (13) and (14) were postulated for siderite hydrogenation. Because Fe was not the main product, according to Equation (14) reduction occurred only partially.T he reverse water-gas shift reaction for the reduction of CO 2 to CO was reported.
In dilute hydrogen, CH 4 did not form. The major amount of CO 2 released froms iderite [Eq. (13)] was not converted, but formed the main product of the product gas.R eductiono fi ntermediate FeO was incomplete ando nly minor amounts of Fe were formed. In addition to the distinct difference in product composition between conversion in pure and dilute hydrogen atmospheres, solid products also differed in morphology.I n pure hydrogen, the solid product contained ah igh amount of crystalline parts, whereas the product in dilute hydrogen featured ah igher amount of fine particles. The conversion temperatures ignificantly reduced in pure hydrogen (603 K), with respectt oa ni nert helium atmosphere (653 K). In dilute hydrogen, decarboxylation started at 623 K.
Baldauf-Sommerbauer et al. suggestedt he hydrogenation of siderite as am eans for sustainable iron production. [12] They performed kinetic computations to study the reaction kinetics of iron formation, and considered the concomitantd ecomposition of the accessory matrix carbonates of calcium, magnesium, and manganese The original mineral consisted of three main carbonate components of siderite FeCO 3 with partial Mg and Mn substitution,a nkerite (Ca a Fe b Mg c Mn d )CO 3 ,a nd dolomite CaMg(CO 3 ) 2 .P otassium,a luminum, and silicon existed in the form of muscovite KAl 2 (AlSi 3 O 10 )(OH) 2 ,w hereas am ajor part of the silicon wasq uartz (SiO 2 ). Ac oncentrateds iderite specimen (size fraction 100-200 mm) was used for kinetic analysis in at hermobalance. During conversion under ah ydrogen atmosphere,t he FeCO 3 contento fm ineral siderite was converted into elemental Fe [(79 AE 2) wt %; Eq. (15)].F rom calcium, magnesium, and manganese carbonates,t he respective oxides formed [Eq. (16)].

CoCO 3 and NiCO 3
Hydrogenation of synthetic CoCO 3 and NiCO 3 was investigated by Emmenegger [22] and Ts uneto et al. [50] Ts unetoe tal. hydrogenatedc ommercial transition-metal powders of CoCO 3 and NiCO 3 ·Ni(OH) 2 ·4 H 2 Oi naf ixed-bed flow reactor at atmospheric pressure and 473 K. [50] In both cases, CH 4 formed after ap eriod of induction, in which only CO 2 evolved throught hermald ecomposition. In the case of CoCO 3 ,amaximum methane formation rate of 97 mmol h À1 was achieved after 55 h. With NiCO 3 ,t he methane formation rate reached am aximum value (72 mmol h À1 )a fter 7h and hydrogen (12 mmol h À1 )w as con-sumedc ompletely.A t5 23 K, CH 4 and CO 2 formed promptly. The formation of metallicC oa nd Ni, together with the period of induction for methanef ormation,s uggests that Ni and Co act as hydrogenation catalysts for CO 2 .
Emmenegger performedT Ge xperiments with synthetic CoCO 3 and NiCO 3 in helium and pure andd ilute hydrogen (5 % H 2 in Ar). [22] The temperature at which carbonate degradation startedd ropped from 603 Ki nh elium to 543 Ki np ure hydrogen for CoCO 3 .F or NiCO 3 ,asimilart emperature dependency of conversion,w ith 623 Ki nh elium, 548 Ki nd iluteh ydrogen, and 513 Ki np ure hydrogen, was observed. Under ah ydrogen atmosphere (pure and dilute), elemental Co and Ni formed as solid products. Similarb ehavior between CoCO 3 and NiCO 3 was also visible, in terms of the gaseous product stream. The main gaseous products CH 4 and H 2 Of ormed. Theb yproducts CO 2 andb arely any CO were detected. The formation of gaseous products proceeded simultaneously.C O 2 fully converted after ac ertain period of induction, which indicated that cobalt and nickel catalyzed the hydrogenation of CO 2 into CH 4 .C onversion differed with respect to the rate of reactioni nd ilute hydrogen:a tt he beginning, the conversion of NiCO 3 was slower than that of CoCO 3 .I nb oth cases, the catalytic activity for CO 2 hydrogenation dropped drastically and the main gaseous products were CO 2 and H 2 O[ Eqs. (13) and ( 14) for CoCO 3 and NiCO 3 ]a nd only minor amounts of CO. These reactions seemed to occur simultaneously because CO 2 and H 2 Of ormed concurrently.F rom CoCO 3 conversion, barely any CH 4 formed, but still significant amountso fC H 4 arose from NiCO 3 .C onsequently,N iw as confirmed to be am ore efficient catalyst in a dilute hydrogen atmosphere than Co.

ZnCO 3
Contraryt oo ther 3d transition-metal carbonates, Emmenegger found that decomposition temperatures of ZnCO 3 increased from 583 Ki np ure hydrogen to 628 Ki nd iluteh ydrogen (5 % H 2 in Ar). [22] Due to al ower hydrogen concentration, the equilibriumc omposition of the reverse water-gas shift reaction preferably consists of H 2 and CO 2 .Z inc did not showa ny catalytic activity for the reduction of evolved CO 2 .T he composition of the gaseous product mixture (CO 2 ,C O, and H 2 O) resembled that expected for the water-gas equilibrium.

Caustic 3d transition-metal carbonates
Emmenegger investigated the hydrogenation of caustic 3d transition-metal carbonates of the type Me y + x/2 (OH) x (CO 3 ) y ·z H 2 O (Me = Cu, Ni, Zn), combinations thereof (Cu/Ni,C u/Co, Cu/Zn, Ni/Zn), and mixed caustic transition-metal/Mg carbonates (MeMg(CO 3 ) 2 with Me = Fe, Co, Ni, Cu, Zn;M e 1 Me 2 Mg(CO 3 ) 2 with Me 1 Me 2 = CuZn,N i/Zn,C u/Ni). [22] Decomposition temperatures were lower than those of the respective neutral carbonates.L oss of crystal water and condensation of OH À with caustic carbonates was observed. Higher water concentration shifts the equilibrium compositiono fp otential CO 2 hydrogenation to the reactants ide. Consequently,a ll caustic metal carbonates show low catalytic hydrogenation activity compared with that of the respective neutral carbonates.M ixed transition-metal/Mg carbonate systems show pronounced catalytic activity,w hich highlightst he effect of the noncatalytic support materialM gO. MgO forms fine particlest hat give access to high dispersion of the transition metals or alloys. [22]

Main-group metal carbonates combinedw ith transition metals
An admixture of transition metals to main-group metal carbonates opens up an ew pathway in metal carbonate hydrogenation:b ecause many transition metals catalyze hydrogenation reactions, CO 2 evolved from the carbonate is convertedi nto CO, CH 4 ,o rh igher hydrocarbons C x H y and C x H y O z .T he catalytically active transition-metal speciesisf ormed in situ during the decomposition reaction. Aw ider ange of transition metals (Fe, Ni, Co, Cu from the 3d group;R u, Rh, Pd,I r, Ag from the 4d group) was used for doping of variousm ain-group metal carbonates, such as Li 2 CO 3 ,N a 2 CO 3 ,K 2 CO 3 ,C aCO 3 ,M gCO 3 ,S rCO 3 , 4MgCO 3 ·Mg(OH) 2 ·5 H 2 O, and BaCO 3 .M ixed alkaline-earthmetal/transition-metal carbonateso ft he type MeCa(CO 3 ) 2 and Me 1 Me 2 (CO 3 ) 3 were also investigated.
In general,t wo promisinge ffects take place. First, the decarboxylation temperature of the alkaline and alkaline-earth-metal carbonates drops. Second, differentg aseous compounds evolve during decomposition due to the catalytic activity of the transition-metal species. The product compositiondepends on the transition metal in the carbonate.
They assumedt hat CH 4 ,a nd its higher homologues, if thermally stable under the reactionc onditions, formed directly through methanation of calcite, rather than through reactions betweenH 2 ,C O 2 ,a nd CO. CO 2 and CO were only detected at low pressure (1.4 MPa) and high reactiont emperature (973 K).
Reller et al. investigated the effects of Co, Ni, and Cu doping (10 %) on the hydrogenation of MgCO 3 and CaCO 3 . [48] Mixed alkaline-earth-metal/transition-metal carbonates, MgÀMe and CaÀMe carbonates, were prepared by coprecipitationw ith sodium carbonate from the respective nitrate solutions. In a second study,t he effect of coprecipitated Ni, Ru, and Rh was reported. [10] The decarboxylation temperatures dropped in the range of 200 Kt o, at most, 400 Ki nt he case of Ni, compared with the decarboxylation of the pure carbonates in an onreducinga tmosphere.M ixtures of CO and CO 2 werer eleased if Cu was used as ac oprecipitate. In the case of Co, predominantly CH 4 formed togetherw ith minor amounts of CO. The formation of CO 2 was negligible. With Ni on MgCO 3 and CaCO 3 ,m ore than 90 %o ft he gaseous product was CH 4 .T he formation of C x H y O z species from CaCO 3 in H 2 wasnot detected at atmosphericp ressure (p = 0.1 MPa). The solid products consisted of am ixture of microcrystalline alkaline-earth-metal oxides and elementalt ransition metals. Reller et al. confirmed that the formed solid products acted as effective catalysts for the partial reduction of CO 2 to CO or direct conversion of CO 2 into CH 4 .F or NiÀCa carbonate systems, the activity of the system was explained by the high dispersion of the catalytically active transition metal in the CaCO 3 matrix. [48] Consequently, the combination of transition-metal carbonates with alkalineearth-metal carbonates improves the catalytic activity of the in situ formedt ransition-metal speciesi fa na ppropriate dispersion or active surface area is generated during decarboxylation;a ne ffect that cannotb ea ccomplished with pure transition-metal carbonates only.
Padeste et al. published ad etailed study comparing the influence of the 3d transition metalsF e, Ni, Co, and Cu and the 4d transition metals Ru, Rh, Pd, andA go nt he thermal decompositiono fC aCO 3 in hydrogen. [49] The mixed-metal carbonates were produced by coprecipitation. Whereas 3d metal carbonates, except for FeCO 3 ,n ormally precipitate as caustic carbonates (hydroxocarbonates)f rom aqueous solutions, ions of the 4d metalsR h, Ru, and Pd precipitate as oxides or oxide hydrates and Ag predominantly forms the simple carbonate Ag 2 CO 3 .S amples from NiCO 3 /CaCO 3 and CoCO 3 /CaCO 3 showed that two-phase systems formed, even at transition-metal car- bonatec oncentrationso f5 %, rather than replacingl arge amountso fC a 2 + by transition metals with similari onicr adii. TG measurements showed that thermal decomposition in hydrogen proceeded in two steps:F irst, decomposition and reductiono ft he transition-metal carbonate below temperatures of 600-700K with H 2 Oe volution. Second, CaCO 3 decomposition at temperatures above 600-700K.H 2 Oe volution might result from loss of coprecipitated water,d ecomposition of hydroxidest oo xides [Eq. (20)],r eduction of CO 2 [Eqs. (21) and (22)],and reduction of the metal oxide formed [Eq. (23)].
Because the rate of CH 4 formation was greatert han that of CO 2 evolution, Ts uneto et al. concluded that direct CaCO 3 hydrogenation occurredo nt he CaCO 3 surface by hydrogen spillover from aN is urface, rather than thermal decomposition of CaCO 3 followed by hydrogenation of released CO 2 . [50] Yoshida et al. studied methane formation through the hydrogenationo fC aCO 3 catalyzed by Pd and Ir (5 wt %) over at emperaturer ange of 573-698 K. [51] Temperature-programmed hydrogenation data revealed high-temperature tails and lower temperatures for the start of decomposition. These findings were explained by an increaseo fa vailabler eactive hydrogen at low temperature, probably duet oa dsorbed Ha toms.A n admixture of transition metals that are ablet od issociate H 2 molecules lower the startingt emperature of metal oxide reduction. [62] Due to the low equilibrium decomposition pressure of CaCO 3 at 573 K( 0.0015 Pa), it wasa ssumed that, with transition-metal catalysts, hydrogenation occurred through direct interaction of CaCO 3 and Ha toms to form CH 4 .I ti sp ossible that reactioni ntermediates formed in the CaCO 3 surface layer,b ut this assumption has not been validated. Activation energies derived from experimentsi na ne lectrobalance were 105 and 111 kJ mol À1 for the Ir-and Pd-catalyzed reactions, respectively. The reaction rate increased steadilyw ith increasing H 2 pressure and remained constant at sufficiently high pressures. Yoshida et al. dedicated this tendency to at ransition in reaction kinetics from slightly higher than first order at low pressurest oz ero order at highp ressures. From reactionk inetics, it was suggested that the rate of dissociative adsorption of H 2 on the metal surfacewas fast relative to the overall reactionrate. [51] Jagadeesan et al. extensively studied hydrocarbon formation from various mixed inorganic carbonates and made remarkable conclusions about hydrocarbon selectivity for C 1 -C 3 chain lengths. [13,52] In the first study,J agadeesan et al. examined methane formationf rom mixed alkaline-earth-metal/transitionmetal carbonates at 823 K. [52] Theo perating pressure was not stated and the assumption could thus be made that the work was carried out at atmosphericp ressure. In as econd paper by Jagadeesan et al.,h owever,i tw as stated that the earlier study was performed at 0.3-0.5 MPa. [13] The mixed carbonates were prepared by precipitation form aqueous solutionso fN aHCO 3 ChemSusChem 2018, 11,3357 -3375 www.chemsuschem.org 2018 The Authors. Publishedb yWiley-VCH Verlag GmbH &Co. KGaA, Weinheim and had ac omposition of MeCa(CO 3 ) 2 ,i nw hich Me was Co, Ni, or Fe in aM e/Ca ratio of 1:1, and Me 1 Me 2 Ca(CO 3 ) 3 ,i nw hich Me 1 Me 2 was CoNi, NiFe, or FeCo in ar atio Me 1 /Me 2 /Ca of 1:1:2. Solid decomposition products consisted of nanoparticles of metal dispersed on metal oxide. Hydrogenationo fC oCa(CO 3 ) 2 gave CO 2 and CH 4 as major gaseous products.A st he reaction proceeded, transition-metal nanoparticles formed on the carbonates urface, which catalyzed the subsequentc onversion of CO 2 to CH 4 .A ni ncreasing amounto fH 2 facilitated the formation of transition-metal nanoparticles through the reduction of metal ions. The carbonates completely decomposed. For CoCa(CO 3 ) 2 at 823 K, the optimal H 2 flow rate for maximum conversion and CH 4 selectivity was 8cm 3 min À1 for 5h.T hese conditions appliedt oa ll mixed carbonates.A gain, the type of transition metal hadaleading role on product gas composition. With Co, complete carbonate conversion occurred with a selectivity to CH 4 of 80 %( 20 %C O 2 ). All other transition metals and transition-metal combinationsy ielded1 00 %C H 4 selectivity at reduced conversion. CO, H 2 O, and coke were not found. Carbonate conversion dropped in the order of NiCa (81 %) > CoNi (77 %) > FeCo (76 %) > NiFe (16 %) > Fe (4 %). Fe exhibited poor conversion. However,i nt he presence of Fe (Fe, NiFe, FeCo), traces of higher hydrocarbons up to C 3 formed. The introduction of Pt or Ki nto FeCa(CO 3 ) 2 did not increase the formation of higher hydrocarbons. Because reduced transitionmetal particles appeared to be essential for high CH 4 selectivity,c atalyst nanoparticles were prepared separately by heating freshly prepared transition-metalc arbonates in H 2 .T hey contained nanoparticles of transition metals, bivalent metal oxide, and CaOa nd wereh ighly efficient in catalyzingt he conversion of CO 2 to CH 4 .W ith CoCa(CO 3 ) 2 ,i fm ixed in a5 0:50 weight ratio with metal-metal oxide nanoparticles, the amount of CH 4 formed during the first 2h was four times higher.S tudies on the effect of transition-metal-metal oxide nanoparticles on hydrogenation of mixed carbonates indicated ac hange in reaction kinetics. In the presence of Co/CaO/CoO catalysts, H 2 efficiency improved to yield 100 %C H 4 selectivity for CoCa(CO 3 ) 2 , in contrast to 80 %i nt he absence of the catalyst. The effect of Co on the methanationo fc arbonates was higher than that of Ni and Fe, and combinations thereof. The catalyst nanoparticles were also capable of decomposingt he natural minerals calcite and dolomite. Complete conversiono fM gCO 3 and CaCO 3 with 100 %C H 4 selectivity was achieved with Co/CaO/ CoO. The type of catalyst significantly influenced the ability to convert CaCO 3 into CH 4 .N ia nd Co, for instance, workedw ell individually,b ut werel ess active upon combination. Fe was not very active itself, but effectively catalyzed CaCO 3 decomposition in combination with Ni and even more effectively with Co. CaCO 3 conversion decreased in the order of Co (100 %) > Ni (80 %) > Fe (18 %) for single transition metals and FeCo (89 %) > NiFe (40 %) > CoNi (34 %) for combinations thereof.
In as econd study,J agadeesan et al. directly focusedo nt he formation of C 1 -C 3 hydrocarbons from CaCO 3 through iron oxides. [13] The startingc arbonate, denominated FeCaCO, consisted of CaCO 3 and Fe oxides with Fe/Ca molar ratios, x,o f0 -5. Thec arbonate contained Fe in the form of Ca 1Ày Fe y CO 3 in the calcites tructure (for x < 2). Excess Fe was presenta sF e 2 O 3 and increased with increasing x. The effect of reaction temperature (573-873K)w as investigated at ambient pressure and a H 2 flow rate of 3cm 3 min À1 in ac ontinuous-flowp acked-bed reactor. The reactiont ime was 2h,a fter which time no further decomposition of the carbonate occurred. In any case, carbonate conversion was not complete. At 673 K, the carbonate conversion and yield of carbohydrates (23 %a tx = 5) were highest. Further gaseous products were CO 2 and CO. Iron metal (a-Fe), iron oxides (Fe 3 O 4 , g-Fe 2 O 3 , a-FeOOH, CaFe 2 O 4 ), and carbide( d-Fe 3 C, c-Fe 5 C 2 )p articles formed as solid residues supported on Ca-rich oxides. The level of conversion and yield of hydrocarbons, suggesting higher hydrocarbon selectivity,i ncreased with increasing molar ratios of Fe/Ca.A tx = 5, the yield of the gaseous products CH 4 ,C 2 H 4 ,C 2 H 6 ,C 3 H 8 ,C O, and CO 2 was 5, 7, 4, 4, 6, and 21 %, respectively.I nt he absence of Fe, mainly CO 2 formed witht races of CO. Consequently,F en ot only improved carbonate decomposition, but also the subsequenth ydrogenation of released CO 2 to higher hydrocarbons. For all molar ratios, the relative C 2 H 4 yield was highest among all hydrocarbons. They ield of C 3 H 8 increased with increasing amounts of Fe, which suggested that higherc oncentrations of Fe favored CÀCc oupling, rather than dehydrogenation. The total hydrocarbon yield from Fe-catalyzedC aCO 3 hydrogenation was comparable to that in the FT synthesis. Ar eaction mechanism based on carbonate decomposition to CO 2 ,w hich underwent furtherr eduction to CO andh ydrocarbons, was stated. According to Jagadeesan et al.,t he formation of CO was crucial because CO and H 2 were adsorbed on the catalysts urface and gave rise to hydrocarbon formation.T hey speculated that particle size played an important role in the selectivity of the reaction. From FT synthesis, it is known that smaller particles show lower H 2 chemisorption potential, which favors the formation of olefins insteado fC ÀCcoupling. [63]

Main Benefits
The main benefits of metal carbonate hydrogenation may be summed up as outlined in the following sections.

CO 2 emission reduction
CO 2 is the primary greenhouse gas emittedt hrough humana ctivities. In 2015, the global CO 2 concentration in the atmosphere reached an average value of (399.4 AE 0.1) ppm. [64] At present,t he industrial sector is responsible for approximately one-third of the total anthropogenicC O 2 equivalent (CO 2 e )e missions. [65] Decarboxylation of metal carbonates under reducing conditions may contribute to as ubstantial decrease of CO 2 emissions, especially in high-emission industrial sectors, such as the iron ands teel industry,i nw hich carbonaceous ores are used. [12] As opposed to conventionald ecarboxylationp rocesses, CO 2 is not released into the atmosphere,b ut reduced to CO, CH 4 ,a nd higherh ydrocarbons. The formation of reduced carbon species in the gas atmosphere adds value to the overall process compared with state of the art decarboxylation under oxidizingc onditions.

Renewable production of chemicals and fuels
At present, the three major carbon feedstocksa re still petroleum, coal, and biomass. Because most of earth's carbon (> 99.9 %) exists as carbonates, carbonaceousm ineralsm ay provide ap otentialc arbon source for hydrocarbon synthesis in the future. The conversion of carbonaceous inorganic rocks (e.g.,c alcite,m agnesite, dolomite) to organic compounds may help to fulfil future energyr equirements and providearenewable and nearly inexhaustible resource for the production of chemicals. [66] Because methane naturallyo ccurs in biogas, power generation is still its main purpose. The conversion of biogast oe lectricity is standard technology.A part from the production of synthesis gas (syngas) andi ts utilization, furtheru ses of methane include catalytic andn oncatalytic oxidative coupling (OCM) to C 2 + hydrocarbons, direct oxidation to methanol or formaldehyde, oxidative methylation of hydrocarbonsb y methane, and oxidative carboxylationo fm ethane by CO to acetic acid. [67] The generation of H 2 from CH 4 is industrially accomplished through steam methane reforming, but the conversion can also be achieved by pyrolysis. [68][69][70] From syngas, which is the feed material for aF Tp rocess, a synthetic crude oil (syncrude) is obtained. The syncrudec onsists of am ultiphase mixture of hydrocarbons, oxygenates,a nd water.R efiningo ft he syncrude yields products that are traditionally produced from conventionalc rude oil, such as transportation fuels and chemicals. [21,71]

Hydrogen storage
Hydrogenation of metal carbonates can also be seen as a means of chemical hydrogen storagei nt he form of CH 4 [72] or fuels produced from syngas. [73,74] This is similar to the powerto-methane (PtM) concept that converts electrical into chemical energyb yu sing capturedC O 2 and H 2 from water electrolysis. [75,76] The main advantage of these alternative hydrogenstoraget echnologiesi st he availability of storage and distribution systems prepared for natural gasa nd liquid hydrocarbon fuels. In regions where an atural gas infrastructure exists, both concepts provide ap romising option to absorb and exploit surplus renewable energy.

Catalyst preparation
Ta ilor-made solid products,r egarding compositiona nd morphology, may be achievedt hrough adjusting the process conditions, especiallyt he gas atmosphere.M orphology plays a crucial role if the solid products represent catalytically active materials, both catalytically active material and support material. [49] Ah ydrogen atmosphere opens up an ew pathway for transition-metalc arbonate transformation into finely dispersed, active catalystsf or in situ or ex situ use. [10] Although initial morphological studies seem promising, the catalytic activity of reductively calcined material has not yet been tested. To the best of our knowledge,o nly reductively calcined MgO was studied for its catalytic properties forC O 2 conversion with H 2 , revealing reversew ater-gas shift activity. [53] Long-term stability was not considered.

Chemical solar energy storage
Experiments with energetic light, for instanceV is or UV radiation, indicate that irradiation affects the course of metal carbonated ecarboxylation. [10] The mechanistic course could depend on the wavelength of the radiation source, giving accesst on ew pathwaysf or the application of solare nergy in solar furnaces. In principle, solar energy can be stored and transformed by means of cyclic inorganic processes. Primary electron spectroscopy for chemical analysis (ESCA)e xperiments revealed the difference between thermalh ydrogenation and hydrogenation induced by irradiation. [22] The results confirmed an effect of the type of energy( UV/Vis radiation) on the mechanism andk inetics of decomposition, but no precise conclusion was feasible and no further reports were made. [77] 6. PotentialF ields of Application Twof ields of application were identified and analyzed regarding potential, limitations, and domainsr equiring further research.B oth elucidatet he need for further studies in industrial-sized reactors, allowing appropriate process conditions to be stated for industrial application.
Due to the lower energy state of inorganic carbonatesr elative to CO 2 ,the reactionise xothermic. Therefore, in theory,t he process does not requirea ny energyi nput, butp roduces heat. Unfortunately,e xtensive preparation of the solid reactants (including mining, transportation, grinding, and activation, if necessary); the use, recycling,a nd loss of additives and catalysts; and disposal of carbonatesa nd byproducts render the overall process energy intensive and requiree xternal high-grade energys ources. [84] Due to its thermodynamics, carbonate formationi sf avored at low temperatures. High temperatures favor the reverse reaction, namely,d ecarbonation, whichi s generally referredt oa sc alcination.
Appropriate carbonaceousf eedstocks ources include abundant silicate rocks that involvel aborious mininga nd alkaline industrial residues that are readily available, but only on a small scale (e.g.,s lag from steel production or fly ash). [85] Pure calciuma nd magnesium oxides and hydroxides providet he ideal source materialb ecause they are more readily carbonat- ed than that of the corresponding silicates.H owever,d ue to their high reactivity,they are scarce in nature. [78,84] There are severals ingle (direct) or multistep (indirect) dry or wet process routes. In aqueous environment, carbonation is faster,b ut, duet oh igher dilution and lower reaction temperatures, the heat of reaction is difficult to retrieve.T he dry process is as impler approach that brings gaseous CO 2 into contact with particulate metal oxide bearing materials. Easy recovery of the heat of reaction is beneficial for this method,b ut the bottleneck is the slow rate of reactiona ts uitable temperature levels.I ti so nly feasible at elevated pressures for refined, rare materials, such as the oxides and hydroxides of calcium and magnesium. [9,84] The generated carbonates (CaCO 3 ,M gCO 3 ), if not disposed of, are used formine reclamation or in construction.
It is expected that alkaline-earth-metal carbonates will give access to reversible thermal decarbonation/recarbonation cycles if decarbonation is carried out under ar educing hydrogen atmosphere. [55] In H 2 ,C O 2 is not released into the flue gas, but furtherr educed to CO. [86] Reductively calcined MgO, CaO, SrO, andB aO were found to be constituted of solidc onglomerates of microcrystalline domainsf eaturing pronouncedr eactivity towards recarbonation;afact that renders them promising as potentialC O 2 -trapping systems. [48] Repeated carbonation/recarbonation cycleso mit excessive measures for reactant preparation, makeup of additives andc atalysts, and product disposal. Consequently,t he net energy input requiredi sp otentially lower.O ncep repared, refined, small particles of metal oxides can be repeatedly used;t his poses ap ronounceda dvantagef or the reaction of dry CO 2 gas with solid oxides not only as far as labor input is concerned, but also for the rate of reaction. It is well knownt hat small particle sizes facilitateh igh reactionr ates. [78] In conventionalm ineral carbonation of CO 2 , most of the energy required is neededf or grinding of the feedstock to particles of 100 mm. During carbonation,t he formation of silica and carbonatel ayers on the mineral surface hinderst he reactiona nd limits conversion.C arbonate layers are barely prevented, but silica layers do not form because pure metal oxidesare applied. Conventional CO 2 mineralization is criticized for its tremendouse nvironmental impact associated with large-scale miningd irectly leadingt ol and clearing and product disposal;a ni ssue that does not need any consideration if the metal oxides are repeatedly used.
Through transition-metal doping of the solidr eactant, a composite system mayb eg enerated, in which CO 2 is trapped on metal oxides (carbonation/recarbonation step) and subsequentlyt ransformed into higher organic species through hydrogenation of the metal carbonate (decarbonation step; Figure 3). [55] The technology for mineral carbonization is still immature. The concept of ac losed CO 2 circuit based on decarbonation through hydrogenation followed by recarbonation is ap romising concept. Nevertheless, it is merely based on primary laboratory-scale experiments andf urther research is required for a feasible economic analysis.
For an ew 600 MW e coal-fueled powerp lant with an annual CO 2 emission of 4Mtyear À1 ,atotal energy requiremento f 580 kWh for CO 2 capture and carbonation has been estimated. [9] Because most of the energy is neededf or grinding of the feedstock material( 280 kW h) and this step is omitted for repeatedd ecarbonation/recarbonation cycles,p ower plant efficiencies may increase from 23.6 to 33 %a nd CO 2 avoidance rates from 72.5 to 82 %f or the concept of ac losed CO 2 circuit. [9] In the literature,reported cost estimations of various carbonation routes differ significantly.A tp resent,d irect aqueous technologiess eem to be the mostr ealistic ones with costs rangingf rom E 60 to 100 t À1 CO 2 fixed. [87] Additional CO 2 emissions associatedw ith the energy required for the carbonation process will boost the costs to E 80 to 130 t À1 CO 2 fixed. Further taking into account the costs for capturing CO 2 from a powerp lant yields total costs of E 150 t À1 CO 2 avoided for a full CCS system with mineral carbonation. [9] At this point, composite systemsd erived from transitionmetal doping that may not only trap CO 2 ,b ut also transform it into higher organic species are not considered. Furthere nergy, and consequently,c ost savings are expected due to increasing reactionr ates of carbonation based on the use of refined metal oxide reactants with small particles izes. Metal doping also allows for lower reactiont emperatures for the carbonation step and decreasing activation energies. Furthermore, dry carbonation will allow for easy accessibility of the heato fr eaction of carbonation.C urrently,c ost estimation is not feasible because the type of catalyst, hydrocarbon species generated, and hydrocarbon selectivity still need to be established. Initial studies highlight its potentiala nd elucidate the need for further researchi nto process conditions and the long-term stability of the system, which are crucial factorsf or its economic viability.

Direct reduction of mineraliron carbonate
Austria and China have major siderite reserves for iron and steel production.S iderite beneficiationi sc hallengingb ecause of the low iron content of the ore compared with magnetite and hematite ores. The industrialp ractice is to blend siderite with other high-grade ores in the sinter plant. During the sintering process, siderite is converted into hematite through roastingi na ir.T he sinter product is fed to the blast furnace (BF), in which it is preferably reduced with coke via CO, pro- ducing at least 1.5 mol CO 2 per mole of iron due to the stoichiometry of reaction. Consequently,a tl east 2.5 mol CO 2 are emitted during the production of 1mol iron from iron carbonate.
Direct hydrogen reduction of the mineral iron carbonate represents an ovel process concept for sustainable pig iron production.I ti sahigh-potential approach for significant energy savings and CO 2 emission reduction, especially if coupled with catalytic CO 2 hydrogenation (e.g.,m ethanation) to furtherc onvert inevitably released CO 2 (Figure 4). [12] Due to the debate about as ustainable energy supply,r esearch into methanation has increased tremendously in recent years and is readily available. [88][89][90][91][92][93][94][95] 6.2.1. Proof of concept TG studies revealed that mineral iron carbonate was directly reduced to elemental iron under ah ydrogen atmosphere.I ron carbonate reduction was represented by ad istinct mass loss below 723 K, which was followed by as mall relative mass loss spanning over ab road temperature range (723-923 K) allocated to the concomitantd ecomposition of manganese, magnesium, and calcium carbonate to the respective oxides. [12] In the ideal case of complete carbonate conversion, elemental iron is formed together with CO 2 ,C O, CH 4 ,a nd potentially even higherh ydrocarbons (C x H y ). Baldauf-Sommerbauer et al. investigated the effect of temperature (Figure5)a nd pressure on the composition of the product gas that consisted of CO 2 , CO, and CH 4 . [96] Elevated pressurea nd low temperature increasedt he yield of CH 4 .C Of ormation was preferred at low pressure and highertemperatures. CO 2 emission savings of at least 60 %a re possible because, at most, 1mol CO 2 is released per mole of elemental iron. If hematite is reduced with hydrogen, 1.5 mol hydrogen is required per mole of elemental iron.C onsequently,u pt o3 3% less reducing agent is neededi fd irect siderite reduction is ap-plied, due to circumventing the hematite route. Direct siderite reduction can be run at relatively low temperatures( 673-773 K) compared with other metallurgical iron carbonate beneficiationp rocesses, such as the Midrex process for direct iron oxide reduction with natural gas (1053-1073 K) [97] or the classical BF process (1773 K). [98]

Case studies
Ac omparison of four different case studies with the state-ofthe art BF process highlights the potential of the concept of direct hydrogen reduction of the mineral iron carbonate.
To tal CO 2 emissions and the total energy demandf or all four cases are comparedw ith the benchmark BF process in Table 3. The results quantify the capability of CO 2 emission reduction. The classical BF process releases 2212kgCO 2 t À1 pig iron. 100 % CO formation (Red1)s aves 64 %o ft he CO 2 emittedi nt he benchmark process.NoCO 2 is released if full conversion to CH 4 is hypothesized (Red2). This scenarioe xhibits the highest energy demando f5 267 kW h( 111% comparedt ot he BF process) and is not aspired to from an economic point of view. Scenariosw ith CO and CH 4 (Red3 and Red4) show excellent CO 2 emissionr eduction (82 and 74 %) and decreased energy demand (11a nd 28 %, although ah ydrogen supply from water electrolysis was chosen), which underlines the highp otential of the proposed concept.
Direct iron carbonate reduction is ah igh-potential candidate to open up an ew route for environmentally benign pig iron production.T he findings are based on TG experiments with siderite [12] and tests in at ubular reactor setup; [96] thus direct conclusions for application in large-scale reactorsa nd optimized process conditions (e.g.,p article size, temperature) cannotb ed rawn yet. Iron separation from the unconverted siderite matrix and gangue through magnetic separation was suggested in the literature,b ut still needs verification.N evertheless, the presented case studies highlight the potentialo f reductivec alcination of siderite and the need for ongoing research in this field.

Summary and Outlook
Various aspects render metal carbonate hydrogenation ap owerful means for direct and indirectC O 2 emissionr eduction, CO 2 utilization,and metal carbonate exploitation.
Under ah ydrogen atmosphere, the decarboxylation temperature is significantly lower than that of the respective reaction under inert conditions. Doping with transition metals further lowerst he temperature level. The combination of decarboxylation and CO 2 reduction with the renewable energy carrierh ydrogen transforms the conventional endothermicp rocess into an overall exothermic process, which allows for significant energy savings.
In reductivem etal carbonate decarboxylation, CO 2 is not (or only partially) released, but reduced to CO, CH 4 ,a nd higher hydrocarbons. The compositiono ft he gaseous reaction product strongly depends on the gas atmosphere (pure or dilute hydrogen); the presenceo ft ransition-metal speciesa cting as in situ catalysts;a nd the reaction temperature, pressure, and residence time. Apart from metal oxidesi nv arious oxidation states, elemental metals are obtaineda ss olid reactionp roducts from transition-metal carbonates. Tailormade products,i n terms of composition and morphology, would give access to novel production routes for catalysts. Until now,p reliminarys tudies focusing on feasibility and chemismh ave mainly been made with metal carbonates in small-scale apparatus, lacking transferability to industrial scale. Additionally,d isagreement exists concerning the reaction mechanisms. Whereas some researchers propose the directr eaction of hydrogen with fixed CO 2 in the carbonate,o thers assume that hydrogen reacts with released CO 2 .D egradation studies in hydrogen and nitrogen revealed differences in morphologyt hat indicate the direct reaction of H 2 with CO 2 ;h owever,s everala spects requirec loser examination, especially when it comes to optimized process conditions for industrial applications. Once clarified,m etal carbonate hydrogenation could provide aq uantum leap in high-emission industrial sectors, such as the iron and steel industry,i farenewable hydrogen supply is accomplished. Furtherp otential fields of applications include the renewable production of chemicals andc atalyst preparation.