CO 2 reduction in aqueous electrolytes suffers efficiency losses because of the simultaneous reduction of water to H 2 . We combine in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical kinetic studies to probe the mechanistic basis for kinetic bifurcation between H 2 and CO production on polycrystalline Au electrodes. Under the conditions of CO 2 reduction catalysis, electrogenerated CO species are irreversibly bound to Au in a bridging mode at a surface coverage of ∼0.2 and act as kinetically inert spectators. Electrokinetic data are consistent with a mechanism of CO production involving rate-limiting, single-electron transfer to CO 2 with concomitant adsorption to surface active sites followed by rapid one-electron, two-proton transfer and CO liberation from the surface. In contrast, the data suggest an H 2 evolution mechanism involving rate-limiting, single-electron transfer coupled with proton transfer from bicarbonate, hydronium, and/or carbonic acid to form adsorbed H species followed by rapid one-electron, one-proton, or H recombination reactions. The disparate proton coupling requirements for CO and H 2 production establish a mechanistic basis for reaction selectivity in electrocatalytic fuel formation, and the high population of spectator CO species highlights the complex heterogeneity of electrode surfaces under conditions of fuel-forming electrocatalysis.carbon dioxide reduction | catalyst selectivity | in situ spectroscopy | proton-coupled electron transfer P roduct selectivity is a principal design consideration for the development of practical catalysts. Catalyst selectivity is dictated by (i) the relative free energy barriers for progress along competing reaction pathways and (ii) the relative rates of reactant delivery to active sites (1). Enzymes fine tune these parameters with exquisite precision to achieve selectivity (2). Nature augments the coordination environment of metallocofactor active sites to optimize the binding strengths of reaction partners and preorganizes reaction participants toward low-barrier pathways (3). Additionally, many active sites reside at the terminus of molecular channels that gate the coordinated delivery of substrates (4, 5) required for selective transformations. Efforts to prepare artificial catalysts with product selectivities rivaling that of nature require a detailed understanding of these factors.Currently, our understanding of how to systematically modulate selectivity in heterogeneous catalysts remains poor (6-8). Unlike (bio)molecular catalysts, which ideally consist of a uniform ensemble of active sites, heterogeneous catalysts consist of a nonuniform distribution of surface sites (9), requiring an understanding of which are active and which are dormant. The surface site distribution is strongly dependent on the surface nanostructure, oxidation state, and degree of restructuring (7). Superimposed on this distribution are the rate-limiting elementary reaction steps that dictate kinetic branching ratios at surface active sites (1...

Gold inverse opal (Au--IO) thin films are active for CO 2 reduction to CO with high efficiency at modest over--potentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by ten fold with increasing porous film thickness while CO evo--lution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au--IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal meso--porosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish elec--trode mesostructuring as a complementary method for tun--ing selectivity in CO 2 --to--fuels catalysis.The electroreduction of carbon dioxide is a promising meth--od for storing intermittent renewable electricity in energy dense carbonaceous fuels. 1--4 However, the high cost and low efficiency of electrochemical CO 2 reduction (CDR) has pre--vented this technology from reaching economic viability. 4 CDR is most practically achieved in aqueous electrolytes, in which the more kinetically facile reduction of protons to H 2 often outcompetes CO 2 reduction, eroding reaction selectivi--ty. Indeed, the paucity of general materials design principles for selectively inhibiting the hydrogen evolution reaction (HER) impedes the systematic development of improved CDR catalysts. 1 Recently, numerous nanostructured metals have been shown to catalyze CO 2 reduction with improved selectivity relative to planar polycrystalline foils. For example gold, copper, and lead films prepared by electrochemical reduction of copper, gold, and lead oxides, respectively, display high CDR selectiv--ity at low overpotentials. 5--7Likewise, de--alloyed porous Ag films 8 and carbon--supported Au nanoparticle 9 --11 and nan--owire electrodes 12 have been shown to catalyze the reduction of CO 2 to CO with high selectivity. This enhanced selectivity may arise from increases in the specific (surface area normal--ized) activity for CDR and/or from a decrease in specific ac--tivity for HER. For oxide--derived gold, evidence points to both effects, 13 whereas for oxide--derived Cu and Pb, specific HER activity have been shown to diminish more dramatically than CDR activity, giving rise to enhanced selectivity for the latter. 5,7 In general, selectivity differences have been attribut--ed to the intrinsic selectivity of the active sites in the materi--al. However, observations of thickness--dependent product selectivity for electrodeposited porous copper thin films 14 suggest that mass transport effects may also play a role in determining product selectivity. For example, when consid--ering CO 2 reduction catalyzed by Au, which generates CO and H 2 predominantly, both the desired reaction (eq. 1) and H 2 evolution (eq. 2) consume protons,necessitating the formation of a pH gradient at the electrode surface irrespective of the product...

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate. Therefore, the kinetics and thermodynamics of CO adsorption to Cu play a central role in determining fuel-formation selectivity and efficiency, highlighting the need for direct observation of CO surface binding equilibria under catalytic conditions. Here, we synthesize nanostructured Cu films adhered to IR-transparent Si prisms, and we find that these Cu surfaces enhance IR absorption of bound molecules. Using these films as electrodes, we examine Cu-catalyzed CO2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO2, beginning at −0.60 V vs RHE with increasing surface population at more negative potentials. Adsorbed CO is in dynamic equilibrium with dissolved 13CO and exchanges rapidly under catalytic conditions. The CO adsorption profiles are pH independent, but adsorbed CO species undergo a reversible transformation on the surface in modestly alkaline electrolytes. These studies establish the potential, concentration, and pH dependencies of the CO surface population on Cu, which serve to maintain a pool of this vital intermediate primed for further reduction to higher order fuel products.

Herein, we show that group 11 CO 2 reduction catalysts are rapidly poisoned by progressive deposition of trace metal ion impurities existent in high purity electrolytes. Metal impurity deposition was characterized by XPS and in situ stripping voltammetry and is coincident with loss of catalytic activity and selectivity for CO 2 reduction, favoring hydrogen evolution on poisoned surfaces. Metal deposition can be suppressed by complexing trace metal-ion impurities with ethylenediaminetetraacetic acid or solid supported iminodiacetate resins. Metal ion complexation allows for reproducible, sustained catalytic activity and selectivity for CO 2 reduction on Au, Ag, and Cu electrodes. Together, this study establishes the principle mode by which group 11 CO 2 reduction catalysts are poisoned and lays out a general approach for extending the lifetime of electrocatalysts subject to impurity metal deposition.

We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO-saturated aqueous electrolytes.