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.

中文翻译：

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室温下Co @ N掺杂碳上的甲醛氧化：可调节的Co尺寸和增强的表面电子密度

开发具有类贵金属能力的过渡金属纳米催化剂对于室温下甲醛（HCHO）的完全矿化至关重要。本文中，首次制备了一系列的N掺杂碳包裹的金属Co（Co @ NC- x）纳米催化剂，以探讨Co粒径对HCHO氧化的影响。高角环形暗场扫描透射电子显微镜显示形成于Co@NC-0.25该小型且高度分散的钴纳米颗粒，其表现出最高的特定的催化活性（0.594微摩尔_{CO 2} ·克_钴^-1 ·s的^–1）与同类产品进行比较。X射线光电子能谱显示，零价钴在Co纳米颗粒中占主导地位。密度泛函理论计算表明，从金属Co核到碳层发生了明显的电荷转移。此外，O ₂以末端连接的方式结合到与氮原子相邻的碳原子上，并且O-O键的长度延长。这些结果表明，特殊的Co @ NC结构可以提供富电子的表面，从而促进O _2的吸附和活化。基于反应中间体的鉴定和Co的微观结构@ NC- X，HCHO的在所制备的钴转换机构@ NC- X被提出。这项研究为使用过渡金属纳米催化剂的室内HCHO提纯提供了新的见解，其效率与贵金属相似。