Operando X-ray Absorption Spectroscopy Study of SnO₂ Nanoparticles for Electrochemical Reduction of CO₂ to Formate

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO2 to formate selectively. Understanding the size–property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO2 nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO2/C-n, n = 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 SnO2 NPs due to the increase of SnO2 loading. The catalytic performance of SnO2 has proved the size-dependent effect during the CO2 reduction reaction process. The as-prepared SnO2/C-1 displayed the maximum Faradic efficiency of formate (FEHCOO–) of 82.7% at −1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO2/C-2 and SnO2/C-3 with larger particle sizes achieved lower maximum FEHCOO– and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO2 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 SnO2 to SnOx and metallic Sn.