In Situ Infrared Spectroscopic Evidence of Enhanced Electrochemical CO2 Reduction and C-C Coupling on Oxide-Derived Copper.

The reaction mechanism of CO2 electroreduction on oxide-derived copper has not yet been unraveled even though high C2+ Faradaic efficiencies are commonly observed on these surfaces. In this study, we aim to explore the effects of copper anodization on the adsorption of various CO2RR intermediates using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) on metallic and mildly anodized copper thin films. The in situ SEIRAS results show that the preoxidation process can significantly improve the overall CO2 reduction activity by (1) enhancing CO2 activation, (2) increasing CO uptake, and (3) promoting C-C coupling. First, the strong *COO- redshift indicates that the preoxidation process significantly enhances the first elementary step of CO2 adsorption and activation. The rapid uptake of adsorbed *COatop also illustrates how a high *CO coverage can be achieved in oxide-derived copper electrocatalysts. Finally, for the first time, we observed the formation of the *COCHO dimer on the anodized copper thin film. Using DFT calculations, we show how the presence of subsurface oxygen within the Cu lattice can improve the thermodynamics of C2 product formation via the coupling of adsorbed *CO and *CHO intermediates. This study advances our understanding of the role of surface and subsurface conditions in improving the catalytic reaction kinetics and product selectivity of CO2 reduction.

Slowly Removing Surface Ligand by Aging Enhances the Stability of Pd Nanosheets toward Electron Beam Irradiation and Electrocatalysis.

Surface ligands play an important role in shape-controlled growth and stabilization of colloidal nanocrystals. Their quick removal tends to cause structural deformation and/or aggregation to the nanocrystals. Herein, we demonstrate that the surface ligand based on poly(vinylpyrrolidone) (PVP) can be slowly removed from Pd nanosheets (NSs, 0.93±0.17 nm in thickness) by simply aging the colloidal suspension. The aged Pd NSs show well-preserved morphology, together with significantly enhanced stability toward both e-beam irradiation and electrocatalysis (e.g., ethanol oxidation). It is revealed that the slow desorption of PVP during aging forces the re-exposed Pd atoms to reorganize, facilitating the surface to transform from being nearly perfect to defect-rich. The resultant Pd NSs with abundant defects no longer rely on surface ligand to stabilize the atomic arrangement and thus show excellent structural and electrochemical stability. This work provides a facile and effective method to maintain the integrity of colloidal nanocrystals by slowly removing the surface ligand.

Kinetically Controlled Synthesis of Pd-Cu Janus Nanocrystals with Enriched Surface Structures and Enhanced Catalytic Activities toward CO2 Reduction.

Bimetallic nanocrystals often outperform their monometallic counterparts in catalysis as a result of the electronic coupling and geometric effect arising from two different metals. Here we report a facile synthesis of Pd-Cu Janus nanocrystals with controlled shapes through site-selected growth by reducing the Cu(II) precursor with glucose in the presence of hexadecylamine and Pd icosahedral seeds. Specifically, at a slow reduction rate, the Cu atoms nucleate and grow from one vertex of the icosahedral seed to form a penta-twinned Janus nanocrystal in the shape of a pentagonal bipyramid or decahedron. At a fast reduction rate, in contrast, the Cu atoms can directly nucleate from or diffuse to the edge of the icosahedral seed for the generation of a singly twinned Janus nanocrystal in the shape of a truncated bitetrahedron. The segregation of two elements and the presence of twin boundaries on the surface make the Pd-Cu Janus nanocrystals effective catalysts for the electrochemical reduction of CO2. An onset potential as low as -0.7 VRHE (RHE: reversible hydrogen electrode) was achieved for C2+ products in 0.5 M KHCO3 solution, together with a faradaic efficiency approaching 51.0% at -1.0 VRHE. Density functional theory and Pourbaix phase diagram studies demonstrated that the high CO coverage on the Pd sites (either metallic or hydride form) during electrocatalysis enabled the spillover of CO to the Cu sites toward subsequent C-C coupling, promoting the formation of C2+ species. This work offers insights for the rational fabrication of bimetallic nanocrystals featuring desired compositions, shapes, and twin structures for catalytic applications.

Recent Advances in Catalyst Structure and Composition Engineering Strategies for Regulating CO2 Electrochemical Reduction.

Electrochemical CO2 reduction has been recognized as a promising solution in tackling energy- and environment-related challenges of human society. In the past few years, the rapid development of advanced electrocatalysts has significantly improved the efficiency of this reaction and accelerated the practical applications of this technology. Herein, representative catalyst structures and composition engineering strategies in regulating the CO2 reduction selectivity and activity toward various products including carbon monoxide, formate, methane, methanol, ethylene, and ethanol are summarized. An overview of in situ/operando characterizations and advanced computational modeling in deepening the understanding of the reaction mechanisms and accelerating catalyst design are also provided. To conclude, future challenges and opportunities in this research field are discussed.