Mechanistic Studies of the Catalytic Reduction of CO 2 to CO: Efficient CO-Releasing Si- and Ge-Based Catalysts.

Besides its significant challenges, efficient catalytic conversion of CO 2 to value-added chemicals is highly desired. Herein, we report efficient silicon- and germanium-based catalysts for CO 2 activation and its reduction to CO studied using B3LYP-GD3/6-31++G(d,p)/tetrahydrofuran (THF) and M06-2X/6-311++g(d,p)/THF density functional theory methods. The catalysts were systematically designed based on the previously reported silicon- and germanium-based compounds. The germanium-based catalysts are reported for the first time in this study. The calculated transition state energy barriers (5.7-15.8 kcal/mol) indicate that all the catalysts can easily activate CO 2 . Among all the B3LYP-GD3-calculated transition-state energy barriers, the highest energy barrier found (27.2-28.3 kcal/mol) is for the protonation of the carboxylic acid group of the silacarboxylic and germacarboxylic acids. Once the silacarboxylic and germacarboxylic acids are protonated, the water molecule can easily dehydrate and leave the catalysts with CO. The electrochemical reduction of the M-CO (M = Si and Ge) complexes further enhances the complexes to easily release CO, with all transition state energy barriers being lower than 10 kcal/mol. The results show that both CO 2 activation and its reduction to CO using the studied catalysts are thermodynamically and kinetically favorable. This work provides an important insight for CO 2 activation and its reduction to CO using earth-abundant and nontoxic main group element-based catalysts.