Operando\nX‑ray\nAbsorption Spectroscopy Study\nof SnO₂ Nanoparticles for Electrochemical Reduction of\nCO₂ to Formate

Abstract

Tin-based electrocatalysts exhibit\na remarkable ability\nto catalyze\nCO₂ to formate selectively. Understanding the size–property\nrelationships and exploring the evolution of the active size still\nlack complete understanding. Herein, we prepared SnO₂ nanoparticles\n(NPs) with a controllable size supported on commercial carbon spheres\n(SnO₂/C-<i>n</i>, <i>n</i> = 1, 2,\nand 3) by a simple low-temperature annealing method. The transmission\nelectron microscopy/scanning transmission electron microscopy images\nand fitting results of the small-angle X-ray scattering profile confirm\nthe increased size of SnO₂ NPs due to the increase of SnO₂ loading. The catalytic performance of SnO₂ has\nproved the size-dependent effect during the CO₂ reduction\nreaction process. The as-prepared SnO₂/C-1 displayed the\nmaximum Faradic efficiency of formate (FE_HCOO–)\nof 82.7% at −1.0 V versus reversible hydrogen electrode (RHE).\nIn contrast, SnO₂/C-2 and SnO₂/C-3 with larger\nparticle sizes achieved lower maximum FE_HCOO– and\nlarger overpotential. Moreover, we employed operando X-ray absorption\nspectroscopy to study the evolution of the oxidation state and local\ncoordination environment of SnO₂ under working conditions.\nIn addition to the observed shifts of the rising edge of Sn K-edge\nX-ray absorption near-edge structure spectra to a lower energy side\nas the applied voltage decreases, the decreased coordination number\nof Sn in the Sn–O scattering path and the presence of Sn metal\ncontribution in the extended X-ray absorption fine structure spectra\nverify the reduction of SnO₂ to SnO_<i>x</i> and metallic Sn.