Co-Pd催化剂上CH_4/CO_2合成乙酸反应中CO_2与表面金属物种作用的密度泛函理论研究

Density Functional Theory Study on Interaction of CO_2 with Metal Surface Carbon Species in Synthesis of Acetic Acid from CH_4/CO_2 on Co-Pd Catalysts

采用广义梯度近似(GGA)的密度泛函理论(DFT)(DFT-GGA)对Co-Pd催化剂上CH4/CO2两步法合成乙酸反应中CO2与金属表面物种M—H(M=Co,Pd)和Pd—CH3的插入反应机理进行了研究,给出了CO2与M—H和Pd—CH3的插入反应机理.计算结果表明,在CO2与M—H和Pd—CH3相互作用的4个反应路径中,反应以CO2与Co—H作用生成产物HCOO—Co为动力学优先路径,但由于HCOO以双齿形式与金属Co结合,其结合能较大,导致HCOO在金属表面不易脱附,故较难形成甲酸;反应生成H3CCOO—Pd产物路径次之,H3CCOO和Pd之间结合能较小,H3CCOO容易脱附形成主产物乙酸;生成H3COOC—Pd反应为动力学最不利路径,故甲酸甲酯为动力学禁阻产物;计算结果与实验结果吻合得很好.

The insertion reactions of CO2 with M--H（ M = Co, Pd） and Pd--CH3 were systematically investi- gated by the first-principle DFT-GGA calculations. The mechanisms of CO2 reacting with M--H and Pd--CH3 were obtained for the direct synthesis of acetic acid from CH4/CO2 by a two-step reaction sequence on Co--Pd catalysts. The calculation result showed that the pathway that C02 inserts into Co--H bonds forming HCOO--Co was the most advantageous in dynamics for all four designed reaction paths. However, HCOO was linked with Co in bi-dentate form, whose binding energy was very large, so the desorpfion of HCOO hardly taken place and formic acid was little produced. Meanwhile, the pathway that CO2 inserts into Pd--CH3 bonds forming H3 CCOO--Pd was secondly preferential pathway favored in dynamics. The binding energy between H3CCOO and Pd was small, and desorption of H3CCOO easily occurred, so the main product was acetic acid. While methyl formate was inhibited by dynamics. The calculated result was in accordance with the experimental facts.