Operando X-ray Absorption Spectroscopy Study of SnO Nanoparticles for Electrochemical Reduction of CO to Formate.

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO/C-, = 1, 2, and 3) by a simple low-temperature annealing method. The transmission electron microscopy/scanning transmission electron microscopy images and fitting results of the small-angle X-ray scattering profile confirm the increased size of SnO NPs due to the increase of SnO loading. The catalytic performance of SnO has proved the size-dependent effect during the CO reduction reaction process. The as-prepared SnO/C-1 displayed the maximum Faradic efficiency of formate (FE) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO/C-2 and SnO/C-3 with larger particle sizes achieved lower maximum FE and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO under working conditions. In addition to the observed shifts of the rising edge of Sn K-edge X-ray absorption near-edge structure spectra to a lower energy side as the applied voltage decreases, the decreased coordination number of Sn in the Sn-O scattering path and the presence of Sn metal contribution in the extended X-ray absorption fine structure spectra verify the reduction of SnO to SnO and metallic Sn.