The Electrophilicity of Surface Carbon Species in the Redox Reactions of CuO‐CeO2 Catalysts

Abstract Electronic metal–support interactions (EMSI) describe the electron flow between metal sites and a metal oxide support. It is generally used to follow the mechanism of redox reactions. In this study of CuO‐CeO2 redox, an additional flow of electrons from metallic Cu to surface carbon species is observed via a combination of operando X‐ray absorption spectroscopy, synchrotron X‐ray powder diffraction, near ambient pressure near edge X‐ray absorption fine structure spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. An electronic metal–support–carbon interaction (EMSCI) is proposed to explain the reaction pathway of CO oxidation. The EMSCI provides a complete picture of the mass and electron flow, which will help predict and improve the catalytic performance in the selective activation of CO2, carbonate, or carbonyl species in C1 chemistry.


Introduction
Theinteraction between metals and supports plays avital role in modulating the catalytic performance of active sites. Concepts such as SMSI [1] and EMSI [2] are well-established to describe the geometric, [3] electronic [2b-d, 4] and bifunctional [5] modification of active sites from the support. With respect to the electronic interaction, there is af low of electrons either from am etallic centre to an oxidative support or from ar eductive support to an oxidative metal centre.T he net electron flow leads to modified electronic structures of the active centre and its surroundings,a nd thus to different behaviours in the adsorption and activation of reaction molecules compared to its unperturbed state.I na ddition to the oxide support, the destination of the electron flow can also be the surface carbon species resulting from the decomposition and deposition from carbonaceous reactants.C arbon atoms,w ith nine oxidation states and an electronegativity of 2.55, can accept electrons from metals.C arbon materials are widely used as electron acceptors in lithium-ion batteries [6] and organic photovoltaics. [7] There are some representative studies in catalysis find electrons are transferred from metal to carbon, [8] especially in electrocatalysis. [9] Reduction of carbon can form metal-carbon species in Fischer-Tropsch synthesis [10] and alkynes hydrogenation. [11] ACO 2 dÀ species is formed when the 2p u orbital of surface CO 2 accepts electrons from metallic Cu. [12] Theoretical calculations also predict the modification of the Cu electronic structure via surface carbons. [13] Cu has moderate adsorption strength for carbonaceous intermediates, [14] and is widely used for C1 chemistry,including CO oxidation, [15] water-gas shift, [16] steam reforming, [17] methanol synthesis, [18] and electrochemical reduction of CO 2 . [14,19] Thewell-documented Ce 4+ /Ce 3+ redox pair enables the transfer of oxygen atoms and electrons between Cu 2+ / Cu + /Cu 0 active sites and the CeO 2 support. [20] Va lidation of this catalytic cycle still requires precise quantification of their oxidation states to match the balance of electron transfer. [21] Here we report the full picture of electron transfer in this system by considering the electrophilicity of surface carbon species,w hich are in situ deposited from CO molecules.A t 453 K, electrons are initially enriched on metallic Cu via CO reduction (CO stage)tobuild up the chemical potential for an electron flow within the catalyst. Thee lectrophilicity of surface carbon species is then studied in an inert atmosphere (He stage)toexclude electron transfer from or to the gaseous molecules.F inally,O 2 is used to extract electrons that are originally injected into the catalyst from CO (O 2 stage). The CO oxidation activity of the catalyst at individual stage is compared below 353 Kt oe lucidate the impact of initial oxidation states of Cu and Ce as well as the surface carbon species.
In af ull cycle of CO oxidation, electrons are transferred from CO to Cu, then to Ce and carbon ( Figure 1) and finally to O 2 .W et herefore extend the concept of EMSI to "electronic metal-support-carbon interactions" (EMSCI) in order to address this interplay among Cu, Ce and carbon species.

Results and Discussion
Highly Dispersed CuO Clusters on CeO 2 Fine powder of highly dispersed 20 wt %CuO clusters on CeO 2 with 71 m 2 g À1 specific surface area were prepared via flame spray pyrolysis. [20b,22] Theh igh-resolution aberrationcorrected high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) images show crystalline CeO 2 particles with an average size of around 6nm (Figure S1f,g). Thec orresponding energy-dispersive X-ray spectroscopy (EDS) maps identify the presence of Cu in the form of ca. 5nmparticles surrounded by CeO 2 (Figure S1a-e). TheX -ray photoelectron spectroscopy (XPS) shows that ca. 18 %o ft he Ce is Ce 3+ ,i ndicating ac onsiderable number of oxygen vacancies in the lattice ( Figure S2 and Table S1). [23] Thes urface Cu/(Cu + Ce) ratio determined by XPS is 0.36, which is close to the theoretical bulk Cu/(Cu + Ce) ratio (0.35) for 20 wt %CuO-CeO 2 .T his indicates good dispersion of the Cu species over CeO 2 ,leading to large Cu/Ce interface. In comparison, Cu species are usually enriched on CeO 2 surface using conventional synthesis methods,s howing much higher surface Cu content compared to the bulk composition (Table S2). [24] Laboratory X-ray diffraction (XRD) pattern shows broadened CeO 2 diffraction peaks ( Figure S3a), whereas weak CuO diffraction peaks can be recognised by synchrotron X-ray powder diffraction (SXPD;F igure S3b). Therefore,the initial catalyst structure of 5nmCuO and 6nm CeO 2 is confirmed with 18 %Ce 3+ present. This 20 wt %CuO-CeO 2 catalyst has been applied for the preferential CO oxidation in the presence of 50 %H 2 , achieving aw ide temperature window from 377 to 388 K with 99 %conversion and selectivity of CO oxidation. [20b] Our previous in situ study used as equence of CO/N 2 /O 2 flows at 453 Kt op robe the change of Cu oxidation state and the corresponding gas profile. [20b] Tw oi nteresting phenomena were observed:1)under inert N 2 ,Cu 0 was slowly reoxidised; 2) when N 2 was changed to O 2 ,CO 2 was released without an external CO feed. In addition, the near ambient pressurenear-edge X-ray absorption fine structure (NAP-NEXAFS) spectra of as imilar system reveals the conversion from oxidised carbon species (288.3 eV) to reduced carbon species (284.9 eV) in ultra-high vacuum ( Figure S4). We hypothesise that carbon species will deposit on the surface of CuO-CeO 2 from gas-phase CO.S uch carbon species are reduced under inert conditions,w hereas Cu 0 is oxidised simultaneously. When O 2 is introduced, the carbon species are oxidised to CO 2 .T his hypothesis of EMSCI is verified via acombination of operando X-ray absorption fine structure (XAFS), SXPD, NAP-NEXAFS,d iffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and gas component quantification at the CO,Heand O 2 stages,aspresented below.

CO Stage:Electron Transfer from CO to Cu
Introducing CO at 453 Ktothe catalyst reduces Cu 2+ and Ce 4+ ( Figure 2). Most of the Cu reduction follows the sequence of Cu 2+ !Cu + !Cu 0 ,asi ndicated in the Cu K-edge X-ray absorption near edge structure (XANES) and SXPD (Figure 2a,b). TheCe 3+ content increases from 20 %to24% according to the peak fitting results of the Ce L 3 -edge XANES (Figure 2c,note that the initial 20 %Ce 3+ content is slightly different from the 18 %C e 3+ obtained from XPS study).
DRIFTS shows the absorption peaks of Cu + -carbonyl species (Cu + -CO) in the range of 2160-2080 cm À1 , [25] suggesting the presence of Cu + on the surface of metallic Cu ( Figure 2d). NAP-NEXAFS at the Cu L 3 -edge validates that Cu + dominates the surface layer after the reduction of CuO ( Figure S10). Such surface Cu + species on Cu 0 clusters over CeO 2 have also been reported in the water gas shift reaction. [16] In general, the surface CO absorption band undergoes aslight blue shift from 2098 to 2111 cm À1 ,and then ar ed shift to 2100 cm À1 with as ignificant intensity decrease. Theinitial blue shift indicates the increase of CO coverage [25] on Cu + along with the release of CO 2 detected at 2364 and 2337 cm À1 .Anew shoulder peak at 2138 cm À1 appears when the CO coverage is maximised. Thep eak is similar to the reported band at 2135 cm À1 of CO on partially reduced CuO x . [25b] Thei ncrease of ab road feature between 1700-1200 cm À1 (Figure S11 from blue to dark red) that corresponds to surface carbonate species is observed. [16b, 26] The formation of the carbonate stems from the oxidation of surface carbonyl species,aprocess that is reported in the literature. [16b] These carbonates can increase the electrophilicity of neighbour Cu + , [25a] leading to abroad absorption band of Cu + -CO at 2173 cm À1 when the reduction of Cu is complete as reflected by the disappearance of CO 2 peaks (Figure 2d). Thesecond stage red-shift to the steady and broadened band centred at 2100 cm À1 corresponds to ac ombination of Cu + -CO and Cu 0 -CO bands at 2160-2080 cm À1 and 2090-
TheC Or eduction of 20 wt %C uO-CeO 2 releases 2.99 mmol g À1 of CO 2 at 453 K( Figure 2e). Theoretically, the complete reduction from Cu 2+ to Cu 0 will generate 2.52 mmol g À1 of CO 2 .F urthermore,t here is a4%r eduction of Ce 4+ to Ce 3+ (according to XANES fitting) results in 0.09 mmol g À1 of CO 2 formation. These results indicate an additional 0.38 mmol g À1 CO 2 generation that is not related to the Cu and Ce redox. According to the literature,t his additional CO 2 may come from the disproportionation of CO. [27] Avery recent work on CO 2 methanation also finds that the presence of Ce 3+ can also help dissociate CO and CO 2 , leading to diverse carbon species. [28] Carbonyls and carbonates are detected by DRIFTS,suggesting the high complexity of those surface carbon species. ( Figure 2d and Figure S11).
Thec ombination of operando XAFS of the Cu and Ce oxidation states,S XPD of small crystalline clusters and DRIFTS of surface carbon species reveals the change of Cu, Ce and Cd uring the CO stage (Figure 2f). Them ajority of CuO is fast reduced to metallic Cu with at race amount of surface Cu + ,w hereas 4% Ce 4+ is reduced to Ce 3+ .S urface carbon species (i.e., carbonyls and carbonates) are deposited from gaseous CO.
He Stage:Electron Transfer from Cu 0 to Ce 4+ and Surface Carbon Thei nert He flow enables observation of the internal reaction among Cu 0 ,C e 3+ /Ce 4+ and surface carbon species. Cu 0 clusters are immediately oxidised to Cu + once CO is replaced by He (Figure 3a,b). Simultaneously,9 %C e 4+ is reduced to Ce 3+ (Figure 3c). These results suggest afast redox reaction [Eq. (1)].
To the best of our knowledge,t he observation of Cu oxidation and Ce reduction here is the first direct evidence for this reaction, proving the EMSI between Cu and Ce.The Cu + state is stable in the He flow (Figure 3a), whereas the crystalline Cu 2 Of eature gradually decreases with time at 453 K, suggesting the amorphization of Cu 2 O ( Figure 3b).  Figure S7. c) Contour map of the Ce L 3 -edge first derivative XANES spectra, showing the increase of Ce 3+ content after reduction.X ANES spectra are shown in Figure S8 and their fitting results are given in Table S3. d) DRIFTS spectra of the surface carbonyl species from CO adsorption. The initial spectrum obtained in O 2 at 453 Ki slabelled as 0min (blue curve) and the progress in CO at 453 Ki sc olour coded from light pink to black. e) Exhaust gas profile of the CO reduction.T he 80 mg sample was pre-oxidised in 20 %O 2 then purged with N 2 at 453 K. f) Simplified models of the structural evolution from CuO-CeO 2 to Cu 0 -CeO 2 .

Forschungsartikel
Theoxidation of all Cu 0 to Cu + would require 54 %Ce 4+ to be reduced to Ce 3+ ,which is much higher than the 9%formation of Ce 3+ .Within the system, the only destination to receive the additional electrons from Cu 0 are surface carbon species with their abundant valence states,g iven that both carbonyl species and carbonate species are present on Cu + -CeO 2 at the He stage (Figure 3d,F igure S15). Theb lue shift from 2100 cm À1 to 2127 cm À1 suggests the increased electrophilicity of Cu due to its oxidation. Thes urface carbon species as electron acceptors have also been reported for CO 2 activation on metallic Cu, where CO 2 dÀ species were formed via electron transfer from Cu 0 into the 2p u orbital of surface CO 2 . [12] Such CO 2 dÀ species were identified by surface enhanced Raman spectroscopy [29] and XPS. [30] In addition, the decomposition of these carbon species can generate surface Oa toms to form Cu À Ob ond. [26,31] Theflow of electrons from Cu 0 to Ce 4+ and surface carbon species under inert atmosphere proves the EMSCI concept (Figure 3f). Te mperature programmed desorption (TPD) is applied to support the presence of those surface carbon species at 453 Ki na ddition to the DRIFTS evidence.T o obtain asimilar metallic Cu and CeO 2 surface,the catalyst is reduced first with CO at 453 Ka nd then cooled to 298 K under the CO atmosphere.Subsequent heating in He leads to CO 2 desorption. Ther eleased CO 2 may come from the decomposition of surface carbonates [26] or the desorption of CO 2 formed during CO reduction. [31] 0.20 mmol g À1 and 0.39 mmol g À1 of CO 2 release are observed below and above 453 K, respectively (Figure 3e). Thelatter is stable under He in the in situ study and is partially responsible for the oxidation of Cu (Figure 3a,b). Theremaining surface carbon species that cannot be desorbed by heating in He may also accept electrons and oxidise Cu. Density functional theory (DFT) calculations with Bader charge analysis are performed to evaluate the electron transfer between Cu 0 and adsorbed carbon species.E ach Cu atom that adsorbs CO has extra + 0.11e and + 0.12e at low and high CO coverages,r espectively whereas each CO molecule gains À0.13e from Cu (Figure S16, S17 and Table S4). When the coverage of carbon species is further increased, the adsorbed carbon atoms,t he number of which equals to half of the Cu atoms,gain À16.58e from Cu, resulting in + 0.20e to + 0.70e on individual Cu atom  Figure S12. b) Contour map of the SXPD patterns,s howing the conversion of metallic Cu to Cu 2 Oand the following amorphization of Cu 2 O. SXPD patterns are shown in Figure S13. c) Contour map of the Ce L 3 -edge first derivative XANES spectra, showing the further increase of Ce 3+ content by accepting electrons from Cu 0 . XANES spectra are shown in Figure S14 and their fitting results are given in Table S3. d) DRIFTS spectra of the surface carbonyl species after CO desorption. The initial spectrum obtained in CO at 453 Kisl abelled as 0min (black curve) and the progress in He at 453 Kisc olour coded from light violet to purple. e) Exhaust gas profile during TPD from 300 Kt o673 K, then holding at 673 Kf or 30 min indicated by the pink-shaded region. The CO releasedd uring TPD is negligible. f) Simplified models of the structuralevolution from Cu 0 -CeO 2 to Cu + -CeO 2 .

Angewandte
Chemie Forschungsartikel ( Figure S18). Thetheoretical calculations results validate the electrophilicity of surface carbon species and its impact on reducing the electron density of Cu. An O 2 stage is then carried out to study those residual carbon and the further oxidation of Cu + /Ce 3+ . O 2 Stage:Electron Transfer from Ce 4+ ,Surface Carbon, and Cu + to O 2 When switching to O 2 at 453 K, the Ce 3+ content recovers to the initial level, whereas only as light increase of Cu 2+ is found (Figure 4a,c). No significant change is found in SXPD, suggesting Cu + remains amorphous (Figure 4b). While the majority of Cu remains in Cu + state,afull reoxidation to Cu 2+ is found on the surface as indicated in the NAP-NEXAFS ( Figure S22). Ap ossible explanation is that ad ense layer of CuO is formed over the Cu 2 Os urface,p reventing further oxidation of Cu + .W eh ypothesise that when the size of the Cu 2 Od ecreases,t he curvature of the surface CuO layer increases and more Cu can be oxidised into Cu 2+ .Anextreme case is the atomic Cu sites,which can be completely oxidised back to Cu 2+ even under He flow. [22] Ther esidual surface carbonyl species with ab and at 2137 cm À1 are removed via oxidation with simultaneously released CO 2 (Figure 4d,f), whereas the carbonates only slightly decrease under O 2 at 453 K( Figure S23). In addition to the 0.59 mmol g À1 CO 2 released during TPD in He up to 673 K ( Figure 3e), temperature programmed oxidation (TPO) of the residual surface carbon can further generate 0.28 mmol g À1 CO 2 (Figure 4e). Therefore,a tl east 0.87 mmol g À1 of carbon species can be deposited and 0.67 mmol g À1 of them can contribute to the oxidation of Cu 0 via EMSCI. Thea mount of carbon deposited on the surface may be directly proportional to the CO pressure (P CO ). Thedifferent P CO in each experiment may lead to small inconsistencies in the carbon deposition as well as the time required for achieving steady states (Table S5). Nevertheless, the general picture on the direction of the electron flow via EMSCI is valid.  Figure S19. b) Contour map of the SXPD patterns,s howing the crystalline Cu 2 Opreserved during oxidation. SXPD patterns are shown in Figure S20. c) Contour map of the Ce L 3 edge first derivative XANES spectra, showing the recovery of Ce 3+ content after oxidation. XANES spectra are shown in Figure S21 and their fitting results are given in Table S3. d) DRIFTS spectra of the surface carbonyl species as they are oxidised and releaseda sCO 2 .T he initial spectrum obtained in He at 453 Kisl abelled in purple and the progress in O 2 at 453 Ki scolour coded from light blue to blue. e) Exhaust gas profile shows the released CO 2 during TPO from 300 Kt o673 K, then holding at 673 Kfor 30 min indicated by the pink-shaded region. f) Simplified models of the structurale volution from Cu + -CeO 2 to Cu + /Cu 2+ -CeO 2 .

Forschungsartikel CO Oxidation Kinetics at Four Stages of Electron Flow
EMSCI describes the flow of electrons in the sequence of 1) CO;2)Cu 0 ;3)carbon + Ce 3+ ;4)O 2 .Atthese four stages, the Cu oxidation states are Cu 2+ ,C u 0 ,C u + and Cu + /Cu 2+ , respectively.T he CO oxidation kinetics is then studied at these four stages ( Figure S24) to understand the influence of surface carbon and Cu species in catalysis.
Ther eaction temperature is controlled below 353 Kt o preserve the initial states of the catalysts.T he distinct CO conversion profiles (Figure 5a)indicate the different catalytic behaviours at each stage.Ingeneral, the CO oxidation activity of the initial Cu species on CeO 2 follows the order Cu 2+ > Cu + > Cu 0 .T he fully oxidised CuO-CeO 2 shows the highest turnover frequency (TOF) at 323 Ka nd lowest apparent activation energy (E a ) ( Figure 5b,c blue) whereas the Cu 0 -CeO 2 shows the lowest activity and increased E a (Figure 5b,c red). TheC u 0 -CeO 2 catalysts separately reduced by CO and H 2 show similar TOF and E a in the kinetic region (Figure 5b,c red and wine). However,t he CO-reduced catalyst with considerable amount of surface carbon shows much higher activity in the high conversion region compared to the H 2reduced catalyst which is free of carbon (Figure 5a red and wine). It suggests that surface carbon on Cu 0 species is irrelevant to the activity in the kinetic region but can significantly promote the reaction in the high conversion region. After the CO-reduced Cu 0 -CeO 2 is annealed in He, the obtained Cu + with reduced surface carbon shows increased TOF( Figure 5). Theo btained Cu + cannot be fully oxidised to the initial Cu 2+ in 20 %O 2 /He at 473 K, thus the activity cannot fully recover although its E a significantly decreases (Figure 5a,c navy with open symbol).
TheC e 3+ content is generally accepted to be inversely proportional to thermodynamic oxygen vacancy formation energy (E vac ) [23] which is regarded as ad escriptor for CO oxidation activity. [32] Unfortunately,there is no clear relationship between the Ce 3+ content and CO oxidation performance can be correlated in this work. It suggests that influence of Ce 3+ content towards CO oxidation is not as obvious as that of the Cu oxidation state.
In conclusion, the lower oxidation state of Cu species on CeO 2 leads to inferior CO oxidation activity.Surface carbon is irrelevant to the activity in the kinetic region but can significantly promote the reaction in the high conversion region.

Conclusion
Thec oncept of EMSCI is established to describe the electron flow for the CuO-CeO 2 system during CO oxidation. More specifically,t he electrons flow from CO to Cu, then to Ce and surface carbon species,and finally to O 2 .T he EMSCI concept sheds light on the catalytic cycle of other CO and CO 2 involved reactions promoted by polyvalent metal oxides.The probability of transition metals transferring the d electrons to the p*orbitals of surface carbon species will govern how CO 2 or CO can be activated over the catalysts.
MG22776, MG22572, MG20643 and MG17559). We acknowledge the B18 and I20-EDE beamline of DLS for XAFS experiment (Proposal No.S P17377, SP20629 and SP20939). We acknowledge the I11 beamline of DLS for SXPD experiment (Proposal No.N T15763). We acknowledge the B07-C beamline of DLS for NAP-NEXAFS experiment (Proposal No.S I24197). We thank the DLS for the award of beamtime at B18 beamline as part of the UK Catalysis Hub Block Allocation Group (Proposal No.SP15151) and Energy Materials Block Allocation Group (Proposal No.S P14239). TheU KC atalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC (grants EP/K014706/ 2, EP/K014668/1, EP/K014854/1, EP/K014714/1 and EP/ M013219/1). Thea uthors would like to thank the Research Complex for access and support to these facilities and equipment. XPS data collection was performed at the EPSRC National Facility for XPS ('HarwellXPS'), operated by Cardiff University and University College London, under contract No.P R16195. Thep resent work is dedicated to the memory of Prof.D angsheng Su (01.07.1961-24.06.2019) who had worked on carbocatalysis for his whole life.

Conflict of interest
Theauthors declare no conflict of interest.