Exploring CO2 hydrogenation to methanol at a CuZn–ZrO2 interface via DFT calculations

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 3.0 https://creativecommons.org/licenses/by/3.0/ Exploring CO2 hydrogenation to methanol at a CuZn–ZrO2 interface via DFT calculations © 2023 Royal Society of Chemistry Published version Lempelto, Aku; Gell, Lars; Kiljunen, Toni; Honkala, Karoliina Lempelto, A., Gell, L., Kiljunen, T., & Honkala, K. (2023). Exploring CO2 hydrogenation to methanol at a CuZn–ZrO2 interface via DFT calculations. Catalysis Science and Technology, 13(15), 4387-4399. https://doi.org/10.1039/d3cy00549f 2023 Catalysis Science & Technology PAPER Cite this: DOI: 10.1039/d3cy00549f Received 20th April 2023, Accepted 20th June 2023 DOI: 10.1039/d3cy00549f rsc.li/catalysis Exploring CO 2 hydrogenation to methanol at a CuZn–ZrO 2 interface via DFT calculations† Aku Lempelto, Lars Gell, Toni Kiljunen and Karoliina Honkala * Multi-component heterogeneous catalysts are among the top candidates for converting greenhouse gases into valuable compounds. Combinations of Cu, Zn, and ZrO 2 (CZZ) have emerged as promisingly efficient catalysts for CO 2 hydrogenation to methanol. To explore the catalytic mechanism, density functional theory (DFT) calculations and the energetic span model (ESM) were used to study CO 2 conversion routes to methanol on CuZn–ZrO 2 interfaces with a varying Zn content. Our results demonstrate that the presence of Zn sites at the interface improves CO 2 binding. However, the adsorption and activation energies are insensitive to Zn concentration. The calculations also show that the hydrogenation of adsorbate oxygen atoms at the interface is kinetically more favourable and requires hydrogen spillover from the metal to the zirconia. This leads to barriers that are lower than those reported on interface or metalonly sites in previous literature. While DFT calculations alone are unable to identify which one of the competing pathways is more favourable, the ESM model predicts that the carboxyl pathway has a higher turnover frequency than the formate route. Our findings also show the importance of considering effects such as hydrogen spillover which take place at a metal-oxide interface when modelling complex catalytic environments. 1 Introduction Transforming CO 2 into value-added products such as methanol, hydrocarbon fuels, and other platform chemicals has gained attention as an attractive approach to reduce the negative impact carbon dioxide has on the climate. 1–4 Using renewable energy and sustainable hydrogen would allow us to establish a circular economy based on carbon recycling. 1,2,5–8 However, due to the stable nature of CO 2 , highly active, selective, and deactivation-resistant catalysts are required to make large-scale adaptation feasible. 5 Various catalytic systems with different combinations of active metal and supporting metal oxide have been prepared and characterised to maximise catalytic activity and selectivity for CO 2 conversion to methanol (CTM): 3,9–18 CO 2 (g) + 3H 2 (g) →CH 3 OH(g) + H 2 O(g). (R1) Oxide-supported Cu nanoparticles have been extensively studied for catalysing CTM due to their promising methanol selectivity, which can be upwards of 60%. 3,19 Several experimental 3,20–24 and computational 3,21,23–26 studies have associated the catalytic activity with active sites at the metaloxide interface. The type of oxide support can substantially influence the activity and selectivity of the catalyst. 3,4,24 Common choices for suitable oxides include zinc oxide ZnO, zirconia ZrO 2 , titania TiO 2 , and alumina Al 2 O 3 . For example, the ternary system of Cu/ZnO/Al 2 O 3 is already an industrially established CTM catalyst, but it displays relatively low conversion, typically below 20%, leading to efforts to develop more selective catalysts in addition to the continuous search for increased activity. 3,6,9,27 To this end, zirconia (ZrO 2 ) has been suggested as an alternative oxide support due to increased turnover rates and selectivity of ZrO 2 -supported Cu. 3,24 Additionally, zirconia has been reported to offer enhanced thermal and mechanical stability and to prevent Cu particles from sintering and thus to hinder the deactivation of the catalyst. 3,24,28–33 Adding ZnO as another oxide into Cu/ZrO 2 has demonstrated higher methanol production rates and total conversion percentages than either Cu/ZnO or Cu/ZrO 2 , often reaching a 20% conversion whereas a lower 5 to 10 percent conversion is typical for binary systems. 3,4,30,34 The precise function of ZnO in Cu/ZnO/ZrO 2 (CZZ) is still under debate. 33 The ZnO component is suggested to be able to temporarily store hydrogen, 33 and it may also act as an additional structural modifier, enhancing Cu dispersion and Catal. Sci. Technol.This journal is © The Royal Society of Chemistry 2023 Department of Chemistry, University of Jyväskylä, Nanoscience Center, P.O. Box 35, FI-40014, Finland. E-mail: karoliina[email protected] †Electronic supplementary information (ESI) available: See ESI for the structures and energies of all adsorbed intermediates, potential energy diagrams, and details of the ESM analysis. See DOI: https://doi.org/10.1039/d3cy00549f Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal Catal. Sci. Technol. This journal is © The Royal Society of Chemistry 2023 increasing its surface area. 30 The actual oxidation state of Zn and the mechanism of catalytic promoting remain somewhat controversial, 3,9–11,13,20,22,23,35–39 and the question of whether or not a CuZn-alloy is involved as the active phase remains open. It is known that the strong interaction between the components may lead to partial ZnO coverage of a Cu surface. 11,20 Under certain conditions the ZnO component can partially reduce into metallic Zn which has two possible outcomes: either forming oxygen vacancies or creating surface alloys of Cu and Zn. 9,10,13,40,41 The CuZn alloys formed at defect and edge sites of metal particles can then be partially reduced by the adsorbates and serve as the active sites of Cu/ZnO catalysts. 9 Based on a combination of in situ analyses, it has been determined that the extent of the alloying varies largely based on the exact temperatures, the presence of CO 2 or other gaseous species i.e. the reducing quality of the conditions. 13,22,23,41,42 Therefore the significance of the alloying remains controversial. A bulk alloy may oxidise and separate into Cu and oxidised Zn, 22 or simply lose its ability to act as an efficient catalyst. 42 Instead, an oxidised phase of Zn in the form of Zn formates has been suggested to be the active intermediate species that lead to methanol formation. 22,36 While the discussion is often centered around bulk alloying, the presence of metallic Zn impurities or dilute alloys at the metal-oxide interface acting as the active sites demand investigation. In computational studies, a pristine Cu facet such as Cu(111) or Cu(211) has typically been chosen as the model to represent the Cu-containing catalyst system. 9,27,43 While simple to work with, these models obviously omit the effects of the supporting oxide. For example, when CO 2 is electronically activated upon adsorption, it takes a bent shape with an O–C– O angle of 123°,asifsp 2 hybridised. However, both at and stepped Cu surfaces bind CO 2 only in a linear fashion even though physisorption energies as large as −0.56 eV and −0.71 eV have been reported on Cu(111) 9,27,43,44 and Cu(211), 9,45,46 respectively. On the other hand, the copper-oxide interface has been reported to activate CO 2 , which bends upon adsorption and the adsorption energy varies in the range of −0.4–1.8 eV, depending on the specific structure of the interface. 25,44,47 Therefore, binary systems where a metal cluster or nanorod is supported on an oxide slab are a common alternative to a purely metallic model. 25,44,47,48 In a recent study combining experiments and calculations, a single Cu atom catalyst on ZrO 2 was found to promote CTM with near 100% selectivity whereas additional active sites at larger Cu clusters and particles were shown to diminish this efficiency. 14 As the precise role of the Zn promoter in CTM is unsettled, 35,41 the choice of how it should be included in computational models has varied. The density functional theory (DFT) studies on CZZ catalysts conducted so far have only considered one or two of the three components. For example, a CZZ system was recently modelled 21 by depositing a small ZrO 2 cluster on a ZnO(112 ¯0) surface to better understand the catalytic behavior on oxide interfaces. Very recently, an inverse ZrZn 2 O 3 /Cu(111) system was used to model ZrO 2 /Cu and ZnO/Cu interfaces. 49 Purely metallic Cu or CuZn models have also been used to mimic active sites at facets and edges of nanoparticles. 9,23,27,46,50 Even though a CuZn(211) surface is unable to activate CO 2 and the intermediates bound to it are thermodynamically less stable than the gas-phase reagents, 9 the hydrogenation intermediates are nevertheless more strongly bound to an alloyed CuZn surface than to a pure Cu surface. 9 While the reaction mechanism of CTM has been studied for a variety of catalyst compositions, the views are not unanimous on the identity of the key intermediates, the main reaction pathway, and an active catalyst domain. 3,19,24 Generally, the plausible pathways have been narrowed down to two competing ones: one converting CO 2 to a formate (HCOO) which then reacts onwards to methanol, and the other, where CO 2 first breaks down to carbon monoxide (CO) through a reverse water–gas shift (RWGS) reaction and then hydrogenates to methanol via a formaldehyde intermediate. Different interpretations of computational and experimental results have led to differing opinions on the dominant reaction route. For example, a formate species bound to the zirconia surface has been both computationally and experimentally determined to be highly stable and could be considered a strong thermodynamic sink. 25,28,44,48 Therefore, formates have been suggested to accumulate at the zirconia and poison the active sites rather than acting as key intermediates in CTM. However, the barrier for HCOO conversion to dioxymethylene (H 2 COO) has been computed to be only 0.66 eV on a ZrO 2 /ZnO interface. 21 This finding is supported by in situ diffuse reflectance infrared spectroscopy measurements, which have shown that the CZZ system can quickly convert formate to a methoxy (CH 3 O) species. 21,25 Another key feature for a hydrogenation catalyst is the ability to efficiently dissociate molecular hydrogen, which, in a Cu-based system, is assumed to be take place on the metal component, where hydrogen readily adsorbs dissociatively. 4,9,21,51–53 Hydrogen spillover from the metal to the ZrO 2 surface may take place and promote a zirconiabound hydrogen to participate in the CTM reactions. 51,54,55 However, the efficiency and mechanism of the spillover on irreducible oxides, such as zirconia, has been a subject of debate 56–58 and the importance of the kinetic control of the spillover step is unclear. In this work, we employ DFT calculations to examine the intermediate and transition states for the branched reaction network of CO 2 CTM by first discussing the reactant properties at the interface using supported CuZn nanorod models, then outlining the formate and RWGS route specifics, combining them to form the products, and finally evaluating the kinetic aspects of the catalytic cycle. We model the active interface by constructing mixed CuZn structures with varying concentration of Zn atoms at the Cu-monoclinic ZrO 2 (1 ¯11) interface mimicking a diluted interface nanoalloy. The energetic span model is used to identify rate-controlling intermediates and to draw a simplified comparison between the optimal kinetics of competing mechanisms. Catalysis Science & TechnologyPaper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol.This journal is © The Royal Society of Chemistry 2023 2 Computational methods DFT calculations were carried out using the BEEF-vdW exchange–correlation functional 59 in the projector-augmented wave (PAW) 60 formalism as implemented in the GPAW 61 package. The core electrons of all elements were described in the frozen-core approximation. A maximum spacing of 0.20 Å was used for the real-space grid basis, and the reciprocal space was sampled at the Γpoint. A Hubbard Ucorrection 62 of 2.0 eV, determined using a self-consistent linear response method detailed in ref. 63, was applied to the d-orbitals of the zirconium atoms. A higher value of 4 eV is common in literature 64–67 but the difference is not unusual as the value of the Uparameter is very sensitive to the specific DFT implementation used. The geometry optimisations were performed using the fast inertial relaxation engine (FIRE) algorithm as implemented in the atomic simulation environment (ASE). 68,69 During optimisations, the bottom layer of the ZrO 2 slab was kept frozen in its initial bulk geometry. All other atoms were allowed to relax until the maximum residual force was reduced below 0.02 eV Å −1 . The transition state searches were carried out using the climbing image nudged elastic band (CI-NEB) 70,71 method where the maximum residual force was set to 0.05 eV Å −1 which keeps the computational cost feasible. Hydrogenation reactions were carried out such that the H atom was initially placed either on the metal side of the interface or on the oxide side, close to the molecule to be hydrogenated. The transition states were confirmed by calculating the vibrational modes using the Frederiksen method 72 and verifying that only one mode with an imaginary frequency exists along the reaction coordinate. Partial charges on atoms were analysed with the Bader method 73 using code written by Tang et al. 74 For the metal-oxide interface, we utilise the oxidesupported metal nanorod concept consisting of a Cu rod with some edge Cu atoms replaced with Zn atoms at different concentrations (see Fig. 1 for the structures and denominations). The Cu–(m-ZrO 2 (1 ¯11)) interface model was adopted from our previous study, 47 where the length of the nanorod is eight atoms and the thickness three atomic layers. A (111) plane is exposed towards the interface. The zirconia surface is described by a two-layer-thick slab model, built from a 3 ×2 m-ZrO 2 (1 ¯11) supercell with periodic boundary conditions in the horizontal directions. While a Cu rod was found to be more stable on a tetragonal zirconia surface, 47 the monoclinic variety is thermodynamically favoured under reaction conditions 75 and has shown activity towards CO 2 activation and CTM in experiments. 34,76 Three different Zn concentrations were examined by replacing every fourth Cu atom, 3/4 of Cu atoms, or all Cu atoms on the nanorod edge with Zn. These models were named the Zn-dilute, the Znrich, and the Zn interface, respectively. This represents situations where Zn centres are dispersed to a varying degree, such as the case where atomic layer deposition has been used to prepare the catalyst. 34 In this instance we have chosen to place the Zn centers along the active interface instead of a more homogeneous alloying with Cu, allowing the use of a pure Cu interface model as a reference, assisting interpretation. The simulation cell measures 20.67 ×14.79 ×24.0 in Å with angles of 90°/90°/116.5°. This unit cell size results in a minor compressive strain of −1.02% for the Cu atoms along the direction of the nanorod. 47 For the Zn interface, the strain is −4.2% as a result of the longer, 2.69 Å, Zn–Zn bulk distance. The strains experienced by the other interfaces are less clearly defined but are likely between the −1.02% of Cu and −4.2% of the Zn interface. A strain of this magnitude at the Cu interface 47 was found to enhance CO 2 binding by up to 0.4 eV. On the other hand, a computed lattice constant of 2.91 Å has been reported for a bulk β-CuZn, 77 corresponding to a shorter Cu–Zn distance of 2.52 Å. This could mean that the compressive strain of the Zn-dilute interface is slightly lower than that of the Cu interface. All three doped interfaces have a Zn atom at the site that binds CO 2 and many further reaction intermediates the strongest, so that they all measure the impact of Zn against the performance of the pure Cu interface. The relative strength of the adsorption at this site is likely due to a lower coordination caused by the interaction between the nanoparticle and the oxide. The models represent cases where the Zn atoms exist as single atom centers dispersed at the metallic interface or as more conjugated assemblies that span several Zn centers. The energy ΔE def by which CO 2 adsorption deforms the interface was calculated as ΔEdef ¼E* CuZn=ZrO2−ECuZn=ZrO2;(1) where E* CuZn=ZrO2is the energy of a CuZn/ZrO 2 configuration after removal of CO 2 from an optimised adsorption geometry and E CuZn/ZrO 2 is the energy of the optimal interface without any adsorbate. A graph theory-based energetic span model 78,79 (ESM) for complex reaction networks was utilised to assess the catalytic cycles. By summing together pairs of intermediate and Fig. 1 The four metal/ZrO 2 interface systems with varying amounts of Zn in the Cu nanorod. A: Cu interface, B: Zn-dilute interface, C: Znrich interface, D: Zn interface. Red: O, white: H, dark grey: C, purple: Zn, turquoise: Zr, orange: Cu. Catalysis Science & Technology Paper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol. This journal is © The Royal Society of Chemistry 2023 transition state energies, it is possible to estimate turnover frequency of each mechanism, TOFn¼kBT h 1−eΔGr=RT Pi;jeTi−IjþδGi;j ðÞ =RT (2) where k B is the Boltzmann constant, Tis the temperature, h is the Planck constant, ΔG r is the Gibbs energy of reaction, and T i and I j are the Gibbs energies of a given transition state and intermediate, respectively. Furthermore, a summation over all npossible mechanisms in the network gives an estimate of the total TOF. 79 TOF ¼kBT hX n 1−eΔGr=RT Pi∈cyclen;jeTi−IjþδGi;j ðÞ =RT (3) The delta term δG i,j is defined as: 79 δGi;j¼0ifi>j;i:e:TS follows intermediate ΔGrif i≤j;i:e:TS precedes intermediate (4) The degree of turnover frequency control X TOF is defined for intermediates and transition states according to eqn (5) and (6) (see ESI†section S3). XTOF;Ti;n¼PjeTi−IjþδGi;j ðÞ =RT Pi∈cyclen;jeTi−IjþδGi;j ðÞ =RT (5) XTOF;Ij;n¼Pi∈cycleneTi−IjþδGi;j ðÞ =RT Pi∈cyclen;jeTi−Ij−δGi;j ðÞ =RT (6) The concept is similar to that of the degree of rate control. A value closer to 1 indicates that changes in the state's energy will affect the TOF more than the energies of states with X TOF values close to 0. However, while the degree of rate control is often determined for elementary steps, here we define the degrees of TOF control separately for intermediates and transition states. This way the values of X TOF can be maximal for two states that are not part of the same elementary step. We can show the direct influence of the energy of each intermediate of transition state as: Effect of Tion TOF ¼∂TOF ∂Ti (7) Effect of Ijon TOF ¼∂TOF ∂Ij (8) which was estimated by using ∂T i =∂I j =−0.001 eV. The DFT- computed adsorption and transition state energies were used as input. While the difference between free energies and DFT energies for gas-phase species can be in the order of 0.6 eV, we assume that the catalytic turnover frequencies (TOF) and the degree of TOF control values (X TOF ) can be compared between different reaction pathways that mainly take place on the surface, even though no free energy corrections were included. The TOF values were computed at a temperature of 500 K, which corresponds to experimental reaction conditions. 80 The ESM analysis was originally developed for studying homogeneous catalysis 78 but it has also been successfully applied for heterogeneous systems including CO 2 hydrogenation to methanol on Cu(111). 81 The basic assumptions of the ESM include that: (i) Eyring's transition state theory is valid, (ii) a steady-state regime is applicable, and (iii) the intermediates undergo fast relaxation to the thermodynamic equilibrium described by the Boltzmann distribution. 78 While the reaction kinetics for heterogeneous catalyst systems is often studied via microkinetic modelling or kinetic Monte Carlo simulations, 23,26–28,48,49,82 the ESM offers a simplified way to estimate which pathway is optimal. We used the gTOFfee software, 81,83 which was slightly modified to improve the performance for the present reaction network. Additionally, an extension was made to the code for calculating the degrees of TOF control, see ESI†for details. 3 Results and discussion 3.1 CO 2 and H 2 activation The CO 2 adsorption properties of the interfaces were determined first since the CO 2 reduction pathways start with the adsorption of the reactants on the catalyst surface. All the interface atoms were first considered as potential sites for the CO 2 adsorption. Subsequent hydrogenation steps are then performed for the most stable CO 2 adsorption geometry, which is similar to the one for the Cu interface. 47 CO 2 binds to the CuZn–ZrO 2 interface in a conformation where the carbon atom resides on top of a Zn atom (C–Zn bond length 2.1 Å) and the two oxygen atoms bind on two Zr cations close to the interface (see Fig. S6†). Upon adsorption, CO 2 takes a bent shape, which resembles a carbonate anion 25,47 and indicates the activation of the molecule with a partial charge of 1.3|e|. 47 The interaction of CO 2 with the CuZn–ZrO 2 interface leads to a local deformation of the rod such that the metal atom in contact with the C atom is pulled out from the (111) plane. The CO 2 adsorption is exothermic by −1.17 eV, −1.13 eV, and −1.30 eV at the Zn-dilute, Zn-rich and Zn interfaces, respectively. The Cu interface exhibits significantly weaker binding with an adsorption energy of −0.64 eV. 47 The difference can be rationalised by examining the energy penalty of deformation ΔE def , calculated using eqn (1), which is +1.7 eV for the Cu interface and +1.1 eV for the Zn-dilute interface. The ΔE def is consistent with the difference in adsorption energies: the stronger binding at the Zncontaining interface is due to smaller deformation energy. No energy penalty is seen for the Zn-rich and Zn interfaces, as their intrinsic strain already favours a deformed structure. CO 2 adsorption at the Zn-rich interface is a kinetically activated process with a barrier of 0.16 eV, which is 0.22 eV lower than the barrier computed for the Cu interface. 47 Similarly to the more negative adsorption energies, the lower barrier is likely due to the increased mobility of the Zn. Adsorption of CO 2 on the low-coordinated oxygen atoms of the ZrO 2 surface, without the involvement of the interface, is Catalysis Science & TechnologyPaper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol.This journal is © The Royal Society of Chemistry 2023 also possible in a trigonal carbonate-like geometry where the carbon atom of the CO 2 binds the lattice oxygen and one of its oxygens rests on a Zr cation. With an adsorption energy of −0.59 eV, this binding is likely in competition with the adsorption at the interface. In addition to adsorption, we considered dissociative adsorption of CO 2 into CO and O, as studied previously. 84–86 However, the reaction is endothermic with a barrier of 1.7 eV, see ESI†section 1.1 for details, and therefore this pathway was omitted from further evaluation. The dissociative adsorption of H 2 was considered at the CuZn–ZrO 2 interface at various positions. The dissociation can be homolytic, where both H atoms adsorb on Cu/Zn sites on the rod and have similar small charges of ca. −0.3 e, regardless of if they are in the vicinity of a Zn site. Alternatively, it can happen across the interface either homolytically, where one H binds on the surface of the rod and the other on top of a Zr, or heterolytically, where a hydride binds to the surface of the metal rod and a proton to an oxide anion. These sites are illustrated in Fig. S7.† Hydrides and protons on the zirconia have charges of −0.6 e and +0.6 e, respectively. The heterolytic dissociation is endothermic by ca. +1 eV and has a kinetic barrier of 1.4 eV, while the homolytic path on the rod is endothermic only by +0.4 eV with a slightly smaller activation energy of 1.1 eV. The spillover of hydrogen from the metal to the oxide surface is endothermic by ca. +0.6 eV depending slightly on the interface site. The kinetic barrier for hydride migration from the metal to the Zr on-top site of the oxide is 0.8 eV. However, the presence of CO 2 or further reaction intermediates can stabilise the oxide-bound H, up to the point where it becomes thermodynamically favourable compared to the metal-bound H atom. 3.2 The formate and RWGS routes The two most commonly proposed CTM reaction mechanisms are considered here: the formate pathway with intermediates labeled F i , and the RWGS pathway with labels R i , Fig. 2 displaying the detailed reaction network. The elementary steps along the formate and RWGS pathways were examined at both the Zn-dilute and Zn-rich interfaces, as well as examining selected steps at the Zn interface. We highlight the differences between the systems when they are relevant but in many cases they behave very similarly. In these situations, we use the Zn-dilute interface as an example. The computed adsorption and activation energies for all interfaces are summarised in the ESI†in Table S2 and the corresponding atomic structures are shown in Fig. S8–S13.† CO 2 to HCOO. Formate, HCOO, is formed across the interface from co-adsorbed CO 2 and H. The CO 2 is initially in its most stable geometry and the hydrogen is positioned on the metal, close to the reaction site. The diffusion of the hydrogen from its optimal geometry on surface of the Cu particle is not included in the calculations. Formate (F 01 ) binds on the ZrO 2 via O–Zr bonds and, unlike the CO 2 , interacts only with the support as demonstrated by the long Zn–C distance of 3.8 Å. The reaction is exothermic by −1.2 eV and the activation energy is +1.2 eV. The energies are similar across the interface models (see Table S2†). Fig. 3 shows the atomic geometries for the initial, final, and transition states. At the transition state (TS 01 ), the hydrogen atom has migrated from the Cu–Cu bridge geometry to a Zn-top position while the C atom is still in contact with the Zn atom with a mildly elongated C–Zn bond length of 2.2 Å. The activation and reaction energies at CuZn interfaces are not significantly different compared to a Cu interface (Table S2†). The slightly lower activation energy of +1.0 eV at the Cu interface is likely due to the Cu–H interaction at the transition state being stronger than that between Zn and H. The binding geometries of CO 2 and HCOO as well as reaction energies for formate formation are similar to those reported in literature for a variety of different interfaces, i.e.,Cu clusters or rods on zirconia, 25,44 zirconia clusters on Cu(111), 28 and other metal–zirconia interfaces. 48 The differences can be rationalised by the structural similarities of the interfaces. In all cases, the reaction energy varies only from −0.6 to −0.7 eV. Previously reported activation energies are in the order of +0.7 eV (ref. 25 and 48) with the exception Fig. 2 The CO 2 the two most common reaction routes for the CTM reaction network. The formate route is given in yellow whereas the RWGS route is displayed in pink. Solid lines imply hydrogenation and dashed lines stand for dissociation or ad-/desorption of the intermediate. Less favourable intermediates/paths are more faintly coloured. Fig. 3 Initial, transition and final state geometries for formate formation at the Zn-dilute interface. Light red: lattice O, dark red: adsorbate O, white: H, dark grey: C, purple: Zn, turquoise: Zr, orange: Cu. Catalysis Science & Technology Paper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol. This journal is © The Royal Society of Chemistry 2023 of the inverse ZrO 2 cluster on Cu(111) model 28 where the barrier was reported to be only +0.14 eV. HCOO to H 2 COO. The next reaction step in the formate route is formate hydrogenation to a dioxymethylene species (H 2 COO). In this step, the hydrogen again reacts with the carbon from the metal side. To this end, the HCOO has to still be relatively close to the interface. This reaction is shown in Fig. 4. The reaction is endothermic by +0.2 eV with activation energies around +0.5 eV. In the transition state (TS 02 ), the C–H distance is around 1.5 Å, which is shorter compared to the 2.0 Å observed during formate formation (TS 01 ). The activation energies are comparable to the value of +0.5 eV that was previously computed at a Cu– ZrO 2 (2 ¯12) interface. 26 The transition state geometry is also similar with a C–H distance of 1.58 Å. A study of a zirconia-supported Cu 38 cluster reported a slightly higher barrier of ca. +0.7 eV (ref. 25) still suggesting a fast interconversion between the HCOO and H 2 COO. Interestingly, at a model ZrO 2 /ZnO interface, 21 the reaction energy for H 2 COO formation is exothermic by −1.27 eV, but the activation energy of +0.66 eV is close to our values. Low activation energies reported for Cu/ZnO, Cu/ZrO 2 , and ZnO/ZrO 2 interfaces are in strong contrast to the high activation energies on bare Cu surfaces. For example, the barrier on a Cu(111) surface has been reported to be 1.59 eV 27 or 0.97 eV. 43 On a stepped Cu(533) surface, the activation energy was determined to be 1.42 eV. Consequently, several computational studies 9,23,27,28,45,46 on metal surfaces and inverse oxide-on-metal models have suggested that HCOO hydrogenation leads to formic acid (HCOOH) instead. In these cases, an oxygen atom of the adsorbate and the reacting H are connected to the same component, e.g. the metal surface. Thus it is sensible that the formation of an O–H bond to create formic acid is more facile than the reaction with the carbon atom of the formate, which points away from the surface. We find, however, that on CuZn/ZrO 2 the reaction to HCOOH is thermodynamically and kinetically significantly less favourable than the reaction to H 2 COO with an activation energy that is over 1 eV higher, as apparent in Fig. S2 and S4 (see ESI†section 1.2 for the full discussion). The difference can be explained simply by the fact that the structure of a metal-oxide interface is able to bring the reacting H and the carbon centre of the HCOO species closer together to create a more favourable pathway. H 2 COO reduction to H 2 COOH and splitting into H 2 CO. During the conversion of H 2 COO to a hydroxymethoxy species (H 2 COOH), both the H 2 COO and the H 2 COOH intermediates stay on the zirconia, attached by their oxygens to Zr top positions but disconnected from the interface. The reacting H atom is initially bound to a zirconia lattice oxygen near the dioxymethylene intermediate. The reaction of H 2 COO to H 2 COOH is exothermic in the range of −0.1 eV to −0.5 eV with moderate activation energies of ca. 0.5 eV at the mixed interfaces. Previous computational studies 21,25 on cluster systems (ZrO 2 on ZnO, Cu 38 on ZrO 2 ) have found comparable activation energies for this step but reported the reaction to be slightly endothermic, likely due to a stabilising effect of the zirconia surface. The next step on is the dissociation of H 2 COOH into hydroxyl (OH) and formaldehyde (H 2 CO). This reaction shows significant energetic variation depending on the interface, being practically thermoneutral at the Zn-dilute and Zn interfaces, and slightly endothermic by +0.2 eV at the Zn-rich interface. At the Cu interface, the reaction is more endothermic by ∼+0.4 eV, due to the weaker adsorption of the formaldehyde. The activation barriers are in the order of +0.2 to +0.5 eV. A previous study 80 found the reaction and activation energies on a Zn-decorated Cu(211) surface to be similarly slightly endothermic with a low barrier. When the C–O bond is broken, the OH part remains bound to a Zr-top site whereas the H 2 CO (F 06 ), while still attached to an oxide cation via its oxygen, tilts toward the nearby interface and binds to a Zn via its C atom. The resulting C–Zn distance of 2.17 Å is similar to that of activated CO 2 . The Zn atom is again slightly pulled out from the ideal position and there is no significant energy difference regarding which of the two neighbouring Zr atoms the oxygen atom binds to. Alternatively, H 2 COOH may hydrogenate to methanediol H 2 - COHOH (F 07 ). However, we found the activation energy of +0.9 eV to be clearly higher than that of the dissociation. Therefore, the pathway was not considered further. The full potential energy diagram of CTM through the formate mechanism is shown in Fig. 5. The final steps from H 2 CO to methanol are shared between both the formate and the RWGS route. CO 2 to COOH. The RWGS route begins with the formation and subsequent dissociation of a carboxyl intermediate (COOH) at the metal-oxide interface. Starting from the adsorbed CO 2 and dissociated H, the reacting H atom must be spilled from the metal to the oxide surface, from where it reacts with an oxygen atom of the CO 2 molecule (see Fig. 6). Attempts to make the H react directly from the metallic component to the CO 2 oxygen were not successful. The resulting COOH intermediate (R 01 ) binds to the metal via the C atom and to the oxide via both O atoms the same way as CO 2 does. This is in contrast to the formate, which detaches from the interface. The formation of COOH at the CuZn interfaces is endothermic by approximately 0.4 eV, and has a moderate +0.7 eV barrier (see ESI,†Table S2, TS 11 onward). In contrast, the reaction barrier is 1.8 eV for a CuZn-bound H, Fig. 4 Initial, transition and final state geometries for HCOO hydrogenation to H 2 COO, at the Zn-dilute interface. Light red: lattice O, dark red: adsorbate O, white: H, dark grey: C, purple: Zn, turquoise: Zr, orange: Cu. Catalysis Science & TechnologyPaper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol.This journal is © The Royal Society of Chemistry 2023 which would severely hinder the RWGS mechanism. Employing the oxide-bound H also minimises the distance that the H atom needs to move to form the new O–H bond. The COOH is thermodynamically less stable by ca. 1.5 eV compared to the HCOO intermediate but the activation energy for COOH formation is 0.5 eV lower. A similar 1.4 eV difference in adsorption was found at a Cu 38 /ZrO 2 interface. 25 However, in that case the H reacted directly from the Cu 38 cluster which corresponds to an activation energy of around +2 eV which is 1.2 eV higher than that of the formate formation. These activation energies and their differences are in line with our results but the case where H reacts from the oxide was not previously included. COOH dissociation. The COOH species dissociates into carbon monoxide (CO) and an OH group that end up bound to adjacent Zr atoms (see R 02 in Fig. S9–S13†). In the transition state, the CO is bound to a Zr cation via its O atom while having a 2.2 Å C–Zn distance and a 1.8 Å C–OH distance. After the reaction is complete, the CO can stay physisorbed on the zirconia surface at a C–Zr distance of 2.8 Å or diffuse to bind at the interface. In both cases the CO adsorption energy is very similar and in the order of −0.5 eV. Bader analysis gives the OH group a charge of ≃0.7 e, pointing to it having an anionic character, while the CO adsorbed at the interface is neutral. The COOH splitting reaction is exothermic by ≃0.1 eV and its activation energy is ca. 0.2 eV at the mixed interfaces. At the Cu interface, the reaction energy increases to −0.4 eV due to the CO binding more strongly to Cu than to Zn. A previous study on Cu(111) 27 shows a +0.42 eV activation energy for the dissociation of COOH, while the reaction energy remains slightly exothermic by −0.14 eV. CO to HCO. To produce a formyl (HCO) intermediate, a H atom moves in from the metal component to the C atom of a CO adsorbed at the interface (TS 13 and R 03 in Fig. S9–S13†). During the reaction, the C–Zn distance shortens to ∼2.1 Å, from the initial values ranging between 2.5 Å at the Zn-rich interface and 4.1 Å at the Zn-dilute. The reaction is exothermic by −0.4 eV, on average, and is accompanied by barriers ranging from 0.32 eV at the Zn-rich interface to 0.67 eV at the Zn interface. We note that these results were obtained in the presence of a bystander OH. We explored COH formation as an alternative but found it endothermic by +0.65 eV—that is—more than 1 eV less stable compared to HCO. Therefore this option was not explored further. In some previous studies, the formation of HCO has had a different character compared to our CuZn/ZrO 2 results. Fig. 5 Potential energy diagrams of the formate route for CO 2 hydrogenation to methanol. Dissociative adsorption steps of hydrogens are not included as they are assumed to be available. Fig. 6 CO 2 hydrogenation to COOH at the Zn-dilute interface. Light red: lattice O, dark red: adsorbate O, white: H, dark grey: C, purple: Zn, turquoise: Zr, orange: Cu. Catalysis Science & Technology Paper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Catal. Sci. Technol. This journal is © The Royal Society of Chemistry 2023 Namely, on a Cu(111) surface, 27 the reaction is endothermic by +0.78 eV and has an activation energy of almost +1 eV. Furthermore, on an inverse ZnO/Cu model, the reaction is endothermic by +0.39 eV and the barrier is +0.88 eV. 23 These differences point to HCO adsorbing less strongly to Cu surfaces, and to the stabilising effect of the interface. HCO to H 2 CO. We start the reaction of HCO hydrogenation to H 2 CO from a position where the molecule is connected to the interface via aC–Zn bond its oxygen rests on a Zr cation. Once again, the H preferably reacts from the metal side of the interface to the C atom rather than from the oxide. Both the intermediates and the transition state (TS 14 ) keep contact to the metal via aC–Zn bond while the OH group, if kept in the vicinity, remains a spectator. The reaction is exothermic by −0.9 eV to −1.2 eV on CuZn interfaces. At the Cu interface, the reaction energy is −0.7 eV, due to the slightly weaker binding of HCO compared to the CuZn interfaces. The reaction has an activation energy of +0.3–0.5 eV. This is close to the inverse ZnO/Cu model, 23 where the activation energy is +0.25 eV. The other reaction that HCO could participate in is the formation of hydroxymethylene (HCOH, R 06 ), which we found to be endothermic with a 0.9 eV kinetic barrier (visible in Fig. S5†), similar to the results on Cu(111). 27 See section 1.3 in ESI†for full discussion. The full potential energy diagram of the RWGS route for the Zn-dilute interface is shown in Fig. 7. 3.3 Formaldehyde hydrogenation to methanol The formate and RWGS pathways discussed above merge after the formation of formaldehyde (H 2 CO). This species is stable, as the adsorption energy of H 2 CO relative to formaldehyde in the gas phase is −1.4 eV. Next, we address two different ways to progress from H 2 CO onward by considering the options of removing and keeping the ZrO 2 - bound OH group produced during the previous reaction steps. This is done to investigate the influence of OH on reaction energetics, as it could have a co-catalysing effect via hydrogen bonds or due to its Lewis acidity. The removal of the OH from the zirconia can take place via H 2 O formation and desorption, which is endothermic by +0.7 eV on average. A previous study 48 showed no kinetic barriers for the dissociative adsorption of water on ZrO 2 and therefore its desorption likely does not have a kinetic barrier either. For brevity, these two routes are shown in the PES diagrams of the previously discussed formate and RWGS pathways. The case where OH has been removed is shown in the diagram of the formate route (Fig. 5) whereas the OH is kept on the surface at the end of the RWGS path in Fig. 7. H 2 CO to H 3 CO. Formaldehyde preferably adsorbs at the interface with its carbon attached to the Zn site and the oxygen on a Zr top site. The hydrogens of the H 2 CO are bent away from the interface leaving the carbon with an sp 3 -like geometry which shows that the formaldehyde is electronically activated. The reacting H is again sitting on the metal component. Upon hydrogenation to methoxy (H 3 CO), the bond between the product and the metal at the interface is broken and the methoxy binds solely to ZrO 2 via its O atom. The reaction is exothermic by −1 eV and requires an activation energy of 0.4–0.6 eV. In this case, the influence of the ZrO 2 - bound OH is rather insignificant as the reaction and Fig. 7 Potential energy diagrams of the RWGS path. States from H 2 CO forward include an OH on the zirconia surface. Catalysis Science & TechnologyPaper Open Access Article. Published on 21 June 2023. Downloaded on 6/29/2023 6:45:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online