以理論計算探討奈米鐵銠團簇催化劑調控費托反應的產物選擇性研究

Abstract

本篇藉由密度泛函理論探討奈米複合材料中鐵銠參雜比例對於費托合成反應選擇性的影響。我們以 FenRh(13-n), n=1, 2, 6, 11, 12 奈米團簇為模型分別探討二氧化碳於團簇上的碳-氧鍵斷鍵反應與一氧化碳於團簇上的氫化還原和碳-氧鍵斷鍵反應。我們發現Fe6Rh7、Fe11Rh2 和 Fe12Rh1 奈米團簇不僅對二氧化碳的碳-氧鍵斷鍵具有良好的催化效果，當一氧化碳氫化還原成 CH2O 之後，其對 CH2O 的碳-氧鍵斷鍵也有良好的催化能力。藉由分析 CH2O 的吸附結構，我們發現 CH2O 的氧原子吸附於 Fe6Rh7、Fe11Rh2 和 Fe12Rh1 奈米團簇中三鐵構型的橋位。自然鍵軌域 (natural bond orbital, NBO) 電荷分析與鍵長分析顯示當 CH2O 的氧原子接於三鐵構型的橋位時，奈米團簇有較多電子移轉至 CH2O 的碳原子和氧原子上，導致 CH2O 分子的碳-氧鍵鍵長變長而有預斷鍵的情形，進而造成 C-O 斷鍵活化能下降。另外，CH2O 分子解離而生成 CH2 和 O 後，氧原子會落入三鐵構型的中心並藉由鐵-氧之間的作用而被穩定，使得 CH2 和 O 的生成能下降。Density functional theory is employed in this study to explore the effect of Fe-Rh composition ratio on the selectivity of Fischer-Tropsch synthesis. We investigate the carbon-oxygen bond breaking reaction of carbon dioxide, and the hydrogenation reduction and carbon-oxygen bond breaking reaction of carbon monoxide on the FenRh (13-n) (n = 1, 2, 6, 11, and 12) nanoclusters. We found that Fe6Rh7, Fe11Rh2 and Fe12Rh1 nanoclusters not only have a good catalytic effect on the carbon-oxygen bond breaking of carbon dioxide, but also have good catalytic ability to break the carbon-oxygen bonds of CH2O when carbon monoxide is reduced to CH2O. By analyzing the adsorption structure of CH2O, we also find that the oxygen atoms of CH2O are adsorbed at the bridge sites of the three-iron configuration in the Fe6Rh7, Fe11Rh2, and Fe12Rh1 nanoclusters. Natural bond orbital (NBO) charge analysis shows that when the oxygen atom of CH2O is at the bridge site of the three-iron configuration, more electrons are transferred to the carbon and oxygen atoms of the CH2O cluster. This causes an increase in the C-O bond length in the CH2O, and a decrease in the C-O bond breaking activation energy. In addition, after the CH2O dissociates to CH2 and O atom, the oxygen atoms will fall into the center of the thi-iron local structure. The formation of the CH2 and the O fragments are greatly stabilized due to the interaction between the dissociated O atom and the iron atoms.