Operando X-ray Absorption Spectroscopy Study of SnO 2 Nanoparticles for Electrochemical Reduction of CO 2 to Formate.

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO 2 to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO 2 nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO 2 /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 SnO 2 NPs due to the increase of SnO 2 loading. The catalytic performance of SnO 2 has proved the size-dependent effect during the CO 2 reduction reaction process. The as-prepared SnO 2 /C-1 displayed the maximum Faradic efficiency of formate (FE HCOO- ) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO 2 /C-2 and SnO 2 /C-3 with larger particle sizes achieved lower maximum FE HCOO- 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 2 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 2 to SnO x and metallic Sn.