Lateral Adsorbate Interactions Inhibit HCOO- While Promoting CO Selectivity for CO2 Electrocatalysis on Ag

The analysis presented in this manuscript helps bridge an important fundamental discrepancy between the existing theoretical and experimental knowledge regarding the performance of Ag catalysts for CO 2 electrochemical reduction (CO 2 ER). The results demonstrate how the intermediate species *OCHO is formed readily en-route the HCOO – pathway and plays a decisive role in determining selectivity of a predominantly CO producing catalyst such as Ag. Our theoretical and experimental approach develops a better understanding of the nature of competition as well as the complex interactions between the reaction intermediates leading to CO, HCOO – and H 2 during CO 2 ER. Details of computational and experimental methods are present in the Supporting Information provided. Abstract: Elucidating the pathways involved in the electrochemical reduction of carbon dioxide (CO 2 ER) is crucial for the advancement of sustainable chemical and fuel manufactur-ing processes. Ag is an appealing CO 2 ER catalyst due to its promising performance for carbon monoxide (CO) production and relatively low cost. In this report, we study the role of the formate (HCOO – ) intermediate ∗ OCHO, aiming to resolve the discrepancy between the current theoretical understanding and experimental performance of Ag catalysts. We demonstrate that the ﬁrst coupled proton-electron transfer (CPET) step in the pathway for CO production to form ∗ COOH competes with the Volmer step for formation of ∗ H, whereas this Volmer step is a prerequisite for the formation of the ∗ OCHO species. We show that ∗ OCHO should form readily on the Ag surface owing to substantial stabilization of the transition state due to solvation and favorable binding strength. In addition, we use in-situ surface-enhanced Raman spectroscopy (SERS) experiments to provide preliminary evidence of the presence of O-bound bidentate species on polycrystalline Ag catalyst during CO 2 ER at low overpotentials which we attribute to ∗ OCHO. Lateral adsorbate-adsorbate interactions in the presence of ∗ OCHO have a signiﬁcant inﬂuence on the

The electrochemical reduction of CO 2 is a very promising approach providing a means to manage intermittent renewable electricity production by converting it to a chemically valuable form, while recycling climate change-inducing CO 2 . 1 Understanding the pathways for the (electro)chemical transformations involved in CO 2 ER is critical to advance its technological utilization. The 2 proton-electron transfer products of CO 2 ER, namely CO and HCOOare highly attractive due to relatively low overpotentials needed to drive their production, and high achievable Faradaic efficiencies. 2,3 The need to balance the performance with low cost electrodes has led to an increased interest in using Ag as a CO 2 reduction catalyst, which has a high selectivity to CO while being 100 times cheaper than the alternative Au. 2,4,5 It is widely accepted that the formation of CO from CO 2 on transition metal catalysts with an aqueous electrolyte proceeds via the * COOH species, whereas the formation of HCOOproceeds via the bidentate O-bound * OCHO species, both forming after a single CPET step. 4,6-10 Consideration of the relative limiting potentials (UL) alone dictates that the formation of H 2 is most thermodynamically feasible on Ag(110), followed by HCOOand then CO (see Figure S2; Figure S1 depicts the limiting potentials with varying surface facets of Ag). Interestingly, this conclusion does not reconcile with the experimental observations of the formation of CO as the major product of CO 2 ER on Ag, H 2 as a by-product (through the hydrogen evolution reaction, HER), with the detection of only trace amounts of HCOOfor intermediate applied potentials of ca. -0.9 V to -1.3 V. 4 For applied potentials more or less negative of this range, the Faradaic efficiency for H 2 dominates that for CO. However, irrespective of the applied potential, the experimentally observed Faradaic efficiency for the formation of HCOOremains significantly lower (<10%) relative to CO and H 2 . The reasons for this discrepancy and the role of the stable species * OCHO in the catalytic performance of Ag is not well understood.
Here we report our findings of the mechanistic differences in the formation of * OCHO and * COOH and their respective interactions with the H 2 production pathways on an Ag(110) surface. We show that there are two major bifurcations in the reaction mechanism before and after formation of * H that control the selectivity between CO, HCOOand H 2 . We present reaction barrier calculations to show that the formation of * OCHO has a significantly lower kinetic barrier relative to * COOH on Ag(110) and that solvation of the transition state by the surrounding water molecules plays an important role in determining this barrier. Finally, we demonstrate that the influence of lateral adsorbate-adsorbate interactions resulting from the presence of * OCHO on the surface promotes CO production while inhibiting itself and the formation of HCOOas a consequence.
The lowest unoccupied molecular orbital (LUMO) of a bent CO 2 is highly localized at the C, whereas the highest occupied molecular orbital (HOMO) is highly localized at the O, making them strongly susceptible to interactions with nucleophiles and electrophiles respectively. 6 We use Bader charge analysis 11 to quantify this susceptibility and to chart the reaction path for the first CPET step for CO 2 reduction, as depicted in Figure 1. In order for the C−H bond of * OCHO to form, * H with a partial negative charge (δ − ) Figure 1. A cartoon depicting the first reduction step for CO 2 ER pathways to form * COOH (CO pathway) and * OCHO (HCOOpathway) on an Ag(110) surface (violet box above) along with the two H 2 formation pathways: Volmer-Tafel and Volmer-Heyrovsky (peach box below). The excess partial charges for the relevant chemical species is mentioned in blue for a excess positive charge and in red for excess negative charge.
acts as a nucleophile for the C δ + of CO 2 (top panel in Figure  1). Both Volmer-Tafel and Volmer-Heyrovsky mechanisms for the formation of H 2 also share the first CPET Volmer step of * H δ − formation (bottom panel in Figure 1). This implies that the formation of * COOH competes with the Volmer step which is in-turn a prerequisite for the formation of * OCHO. Following the Volmer step, the * H can either participate in a Tafel or Heyrovsky step to form H 2 , or react with the CO 2 to form * OCHO. According to our analysis, there are therefore two reaction bifurcations before and after formation of * H that generally control the selectivity for CO 2 ER. An analogous finding has been recently reported for CO 2 ER on Cu(100) surfaces 12 and as well as for the selectivity of CO 2 ER on metalloporphyrins. 13 This approach can be further extended to the formation of higher CPET products from CO 2 ER such as methanol (CH 3 OH) and methane (CH 4 ) (see extended reaction scheme in Figure S8). The thermodynamically most feasible routes for the formation of CH 3 OH and CH 4 go via * COOH followed by the * CHO species, as becomes clear from the UL for the various reaction pathways shown in Figures S9 and S10. Extending the Bader charge analysis to these pathways, it is expected that the formation of * CHO from * CO forms through a surface bound * H interacting with the C δ+ . θ * H is therefore expected to play an important role in the formation of higher electron reduction products of CO 2 ER and this analysis can potentially be useful for studying catalysts such as Cu where these reaction steps become prominent. Interestingly, very recent experimental findings for CO 2 ER on Cu catalysts draw similar conclusions and indicate an important role of * H in the formation of CH 4 . 14 Climbing image nudged elastic band (CI-NEB) calculations were performed to estimate the height of the activation barriers based on the reaction path analysis presented in Figure 1 (see Computational Details section in SI). As can be seen in Figure 2, the activation barriers after correction for solvation for formation of * COOH and H 2 via Heyrovsky and Tafel steps are 0.93 eV, 0.79 eV and 0.77 eV respectively, with H 2 as the most thermodynamically favorable product. Interestingly however, there is no kinetic barrier for the formation of * OCHO involving the direct nucleophilic attack by * H (see Figures S3-S6 for uncorrected activation barriers, atomic configurations and excess partial charges of the initial, transition and final states). The high solvation energy of the transition state relative to the initial state for * OCHO (Table S1, Figure S7) plays an important role in diminishing the activation barrier for its formation. This analysis highlights the importance of the consideration of solvation in theoretical mechanistic studies for CO 2 ER; a conclusion which is in agreement with what has been shown for other (electro)catalytic systems. 15 The presence of cationic and anionic species in the electrolyte may also influence the binding energies and activation barriers for the formation of certain reaction intermediates and there are ongoing efforts to include these effects in computational studies pertaining to CO 2 ER. 16 Based on the results in Figure 2, we show that there is a significant energy barrier for the formation of the CO pathway intermediate * COOH, whereas * OCHO is expected to form readily on the Ag(110) surface in the presence of * H.
In order to validate our theoretical findings, preliminary in-situ electrochemical SERS measurements were performed to probe polycrystalline Ag catalyst surface during CO 2 ER in 0.05 M Li 2 B 4 O 7 17 saturated with CO 2 , with a bulk pH of 6.1 (Experimental details in SI). We observe a double-band shape at 1436 and 1469 cm −1 (Figure 3) corresponding to an O-bound bidentate intermediate on the Ag surface, which we believe to be the * OCHO species based on similar evidence obtained in literature for carboxylate species on silver hydrosols. 18 The interactions of O-bound species with the Ag surface appear at relatively low overpotentials (-0.52 V vs RHE), in agreement with the obtained theoretical results.
We also observe a correlation in the appearance of a δC−H vibration band at 1298 cm −1 19 with the bidentate signal merging into a broader band at more cathodic potentials of -1.12 V vs RHE as can be seen in Figure 3. The same correlation is observed using a lithium borate buffer solution with a bulk pH of 6.9 albeit at higher overpotentials ( Figure S23). These experiments suggest that pH plays an important role in CO 2 ER at lower overpotentials due to its implications on the formation of * H on the Ag electrode. Adsorbate-adsorbate interactions are known to play an important role in determining the energetics of surface reactions including CO 2 ER. 20,21 Considering the high likelihood of the presence of * OCHO species on the Ag catalyst surface, we investigate its influence on the adsorption energies of reaction intermediates involved in the 2-electron reduction processes during CO 2 ER. The free energy diagram in Figure  4 shows that the presence of * OCHO weakens the * H binding significantly (green pathway in Figure 4 and Figure S12) whereas the effect of its presence on the binding energies of the * COOH, *OCHO and * CO species is relatively smaller. As a result, the UL for the formation of H 2 and HCOO -(both proceeding via the Volmer step), become much less favorable and comparable to the UL for the formation of CO. Figure S11 shows the effect of the coverage of * H (θ * H ) on the binding energy of * COOH. It is clear that as θ * H increases, the binding of * COOH on the catalyst surface becomes increasingly thermodynamically unfavorable. The presence of * OCHO therefore has two important consequences for the selectivity of Ag CO 2 ER catalysts: firstly, * OCHO weakens the binding strength of * H with the catalyst surface, bringing the CO formation pathway to a levelplaying field thermodynamically with the H 2 and HCOOpathways. Secondly, the lower θ * H as a consequence of the weaker * H binding enables the formation of * COOH, thereby improving the selectivity of the catalyst towards CO. This analysis highlights the importance of the consideration of lateral adsorbate-adsorbate interactions to bridge the discrepancy between theoretical predictions and the experimental observations for Ag as well as other CO 2 ER catalysts.  Table S1).
From our theoretical analysis, we come to the conclusion that the O-bound formate precursor species * OCHO, which is typically considered irrelevant for the CO producing catalyst Ag, should not only be present on the surface at low overpotentials during CO 2 ER, but is also likely playing an active role in promoting the selectivity of Ag towards CO production. In addition, the results strongly indicate that factors such as θ * H and solvation by surrounding water molecules will play an important role in controlling selectivity between the various CO 2 ER products. Calculation of activation barriers at constant potential and adjusted for activities of reactant and electrolyte species in order to simulate the conditions during electrocatalysis remains a challenge and there is a need for further development of computational methodologies for this purpose. Our approach demonstrates a constructive interplay between theory and experiments to advance the understanding of a complex system of high practical significance. The results highlight the need to study the catalyst surface in its operational steady state, both theoretically and experimentally, to understand the synergistic and competitive effects between the reaction species that ultimately result in the observed performance.
Acknowledgement The authors thank NWO for support via the VIDI grant (WAS and DB). Dr G. Li acknowledges the financial support from NWO for her personal VENI grant (no. 016.Veni.172.034).

Supporting Information Available
The following files are available free of charge.
• Supporting_Information.pdf: Computational and experimental details along with auxiliary computational results and in-situ SERS results.