Formaldehyde Oxidation over Co@N-Doped Carbon at Room Temperature: Tunable Co Size and Intensified Surface Electron Density

Developing transition-metal nanocatalysts with precious-metal-like ability is essential for complete mineralization of formaldehyde (HCHO) at room temperature. Herein, a series of N-doped carbon encased metallic Co (Co@NC-x) nanocatalysts have been fabricated to explore the effect of the Co particle size on HCHO oxidation for the first time. High-angle annular dark-field scanning transmission electron microscopy displayed the small-sized and highly dispersed Co nanoparticles that were formed in Co@NC-0.25, which exhibited the highest specific catalytic activity (0.594 μmol_CO2·g_Co⁻¹·s⁻¹) compared with its counterparts. Zero-valence cobalt was dominant in Co nanoparticles as revealed by X-ray photoelectron spectroscopy. Density functional theory calculations demonstrated an evident charge transfer occurring from the metallic Co core to the carbon layer. Moreover, O₂ binds to the carbon site adjacent to the nitrogen atom in an end-on fashion, and the length of the O−O bond is elongated. These results revealed that the special Co@NC structure could supply an electron-rich surface, facilitating adsorption and activation of O₂. Based on the identification of reaction intermediates and the microstructure of Co@NC-x, the conversion mechanism of HCHO over the as-prepared Co@NC-x was proposed. This study provides a new insight into indoor HCHO purification using transition-metal nanocatalysts with a similar efficiency as precious metals.