Identification of structure–activity relationships and active sites is essential for the rational design of heterogeneous catalysts. For metal nanoparticle catalysts, numerous efforts have been made to control the particle size, shape, and composition, thus to tailor the exposed surface sites such as facets, steps, edges, and corner sites. These efforts have led to important progresses in heterogeneous catalysis, but it is unclear if the accessible structure space is sufficient for finding optimal catalysts. Recently, we discovered that bulk defects such as grain boundaries (GBs) in metal catalysts, can stabilize surface sites that are uniquely active for electrochemical catalysis. Through a careful design of metal nanoparticle catalysts with different GB densities, extensive TEM characterization, and electrochemical measurements, we found that the catalytic activities for CO<sub>2</sub> reduction on Au and CO reduction on Cu are linearly correlated with the GB densities in the catalysts. The quantitative GB-activity relationship implicates GB surface terminations as new active sites for electrocatalysis, and the under-coordinated surface sites stabilized by GBs may provide unique binding sites for reaction intermediates. Accordingly, GB-rich metal catalysts were developed with high activity and stability for the electroreduction of CO<sub>2</sub> to liquid fuels such as ethanol and acetate. Our study may add GBs as a new dimension to the structure space for rational design of heterogeneous electrocatalysts.

Grain-boundary-supported active sites in heterogeneous electrocatalysis

2019 Spring National Meeting

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