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      Tracking a Common Surface-Bound Intermediate during CO 2-to-Fuels Catalysis

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          Abstract

          Rational design of selective CO 2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO 2-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 CO 2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO 2, 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.

          Abstract

          The dynamic nature of CO binding to Cu as a function of pH, CO concentration, and applied potential serves to establish a pool of this key intermediate primed for higher order fuel synthesis from CO 2.

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          Most cited references46

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          Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide.

          The electrochemical reduction of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and molecular electrocatalysts for the reduction of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to determine the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 reduction on other metals and molecular complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
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            Activity Descriptors for CO2Electroreduction to Methane on Transition-Metal Catalysts

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              Particle size effects in the catalytic electroreduction of CO₂ on Cu nanoparticles.

              A study of particle size effects during the catalytic CO2 electroreduction on size-controlled Cu nanoparticles (NPs) is presented. Cu NP catalysts in the 2-15 nm mean size range were prepared, and their catalytic activity and selectivity during CO2 electroreduction were analyzed and compared to a bulk Cu electrode. A dramatic increase in the catalytic activity and selectivity for H2 and CO was observed with decreasing Cu particle size, in particular, for NPs below 5 nm. Hydrocarbon (methane and ethylene) selectivity was increasingly suppressed for nanoscale Cu surfaces. The size dependence of the surface atomic coordination of model spherical Cu particles was used to rationalize the experimental results. Changes in the population of low-coordinated surface sites and their stronger chemisorption were linked to surging H2 and CO selectivities, higher catalytic activity, and smaller hydrocarbon selectivity. The presented activity-selectivity-size relations provide novel insights in the CO2 electroreduction reaction on nanoscale surfaces. Our smallest nanoparticles (~2 nm) enter the ab initio computationally accessible size regime, and therefore, the results obtained lend themselves well to density functional theory (DFT) evaluation and reaction mechanism verification.
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                Author and article information

                Journal
                ACS Cent Sci
                ACS Cent Sci
                oc
                acscii
                ACS Central Science
                American Chemical Society
                2374-7943
                2374-7951
                08 August 2016
                24 August 2016
                : 2
                : 8
                : 522-528
                Affiliations
                []Department of Chemistry, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States
                []Institute for Catalysis, Hokkaido University , Sapporo 001-0021, Japan
                [§ ]Graduate School of Environmental Science, Hokkaido University , Sapporo 060-0810, Japan
                Author notes
                [* ]E-mail: yogi@ 123456mit.edu .
                Article
                10.1021/acscentsci.6b00155
                4999975
                27610413
                5a904c6c-ef4f-489f-b0d0-60d6a330d0b8
                Copyright © 2016 American Chemical Society

                This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

                History
                : 25 May 2016
                Categories
                Research Article
                Custom metadata
                oc6b00155
                oc-2016-00155u

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