Monodisperse Au Nanoparticles for Selective Electrocatalytic Reduction of CO₂ to CO

We report selective electrocatalytic reduction of carbon dioxide to carbon monoxide on gold nanoparticles (NPs) in 0.5 M KHCO3 at 25 °C. Among monodisperse 4, 6, 8, and 10 nm NPs tested, the 8 nm Au NPs show the maximum Faradaic efficiency (FE) (up to 90% at −0.67 V vs reversible hydrogen electrode, RHE). Density functional theory calculations suggest that more edge sites (active for CO evolution) than corner sites (active for the competitive H2 evolution reaction) on the Au NP surface facilitates the stabilization of the reduction intermediates, such as COOH*, and the formation of CO. This mechanism is further supported by the fact that Au NPs embedded in a matrix of butyl-3-methylimidazolium hexafluorophosphate for more efficient COOH* stabilization exhibit even higher reaction activity (3 A/g mass activity) and selectivity (97% FE) at −0.52 V (vs RHE). The work demonstrates the great potentials of using monodisperse Au NPs to optimize the available reaction intermediate binding sites for efficient and selective electrocatalytic reduction of CO2 to CO. T ever-increasing worldwide consumption of fossil fuels has accelerated the depletion of these finite natural resources and led to overproduction of the greenhouse gas carbon dioxide. To meet the fuel and chemical demands in a sustainable way, the overly produced CO2 must be converted into reusable carbon forms. Among many different approaches developed thus far for CO2 reactivation, electrochemical reduction of CO2 is considered a potentially “clean” method as the reduction proceeds at the expense of a sustainable supply of electric energy. Theoretically, CO2 can be reduced in an aqueous solution (pH 7, 1 M electrolyte at 25 °C and 1 atm CO2) to form carbon monoxide, formic acid, methane or other hydrocarbons at potentials around +0.2 to −0.2 V (vs reversible hydrogen electrode (RHE); all potentials reported in this paper are with respect to RHE). Experimentally, however, very negative potentials must be applied to initiate CO2 reduction. 4 These large overpotentials not only consume more electrical energy but also promote the uncontrolled formation of competitive reduction products, such as H2, causing low energetic efficiencies and poor selectivity. To succeed in CO2 reduction and conversion, highly efficient catalysts must be developed to lower the CO2 reduction overpotentials and to control the energy pathways of reaction intermediates. Various metal electrocatalysts have been screened exper imenta l l y and ana lyzed computationally to rationalize their activity and selectivity for CO2 reduction. Recent advances in the synthesis of nanoparticles (NPs) allow for testing of potentially increased reaction kinetics due to the controlled surface area and surface morphology achieved. This is demonstrated by electrochemical reduction of CO2 into hydrocarbons on Cu NPs, 16,17 or into CO on gold-based NPs/clusters. Recently, a new form of Au nanostructured catalyst made by anodization and electroreduction of an Au electrode was demonstrated to show high selectivity for catalyzing CO2 reduction to CO: its Faradaic efficiency (FE) was ∼96% at −0.35 V with current densities between 2 and 4 mA/cm2. It is suggested that the increased stabilization of a reduced CO2 adsorbate or the adsorbed reaction intermediate COOH as well as the weakened CO binding on the Au surface contribute to this selective reduction of CO2 to CO. 18,20 However, the structure feature of the catalyst surface is difficult to characterize, which complicates further catalyst optimization. Considering the size effect commonly observed in NPs and the promising results demonstrated from nanostructured Au, we chose to study monodisperse Au NPs as catalysts for electrochemical reduction of CO2 in 0.5 M KHCO3 (pH 7.3) at room temperature. We screened 4, 6, 8, and 10 nm Au NPs and found that the 8 nm Au NPs were especially active for CO2 reduction into CO. Using density functional theory (DFT) calculations, we rationalized this enhanced activity and selectivity with the presence of dominant edge sites on the 8 nm NP surface, which facilitates the adsorption/stabilization of key reaction intermediates (such as COOH*) for the CO2 reduction into CO and inhibits the hydrogen evolution reaction (HER). This reaction model was further supported experimentally as Au NPs embedded in a matrix of butyl-3methylimidazolium hexafluorophosphate (BMIM-PF6), a more efficient COOH* stabilizer, were indeed more active and selective for CO2 reduction into CO. The composite catalyst containing 8 nm NPs exhibited up to 97% FE toward CO and a mass activity of 3 A/g at −0.52 V. Our work demonstrates the great potential of tuning electrocatalysis of Au NPs by creating optimal edge sides on their surface for effective CO2 reduction into CO. Received: September 12, 2013 Published: October 24, 2013 Communication