CO Dissociation Mechanism on Pd-Doped Fe(100): Comparison with Cu/Fe(100)

• Published in: Journal of physical chemistry. C Vol. 121; no. 12; pp. 6820 - 6834
• Main Authors: Wang, Wei, Wang, Ye, Wang, Gui-Chang
• Format: Journal Article
• Language: English
• Published: American Chemical Society 30.03.2017
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.7b00903

Spin-polarized density functional theory computations have been used to investigate the CO dissociation mechanisms and the different catalytic activities of the reaction on Fe(100) surfaces with different Pd coverages. CO can dissociate on Pd/Fe surfaces via three different mechanisms: direct and H-assisted mechanisms via HCO intermediate or COH intermediate. In our calculation, it was found that the activation barriers of direct CO and COH dissociation mechanisms on pure and Pd-doped Fe(100) surfaces were higher than that of the HCO dissociation mechanism. Besides, energy barriers for the identical reaction pathway on Fe-rich Fe(100) surfaces were lower than those on Pd-rich Fe(100) surfaces, namely, CO dissociation mainly occurs via the HCO intermediate pathway and the catalytic activity becomes lower with Pd coverage increasing toward CO dissociation in both direct CO and H-assisted CO dissociation mechanisms. As a result, CO dissociation mainly occurs on Fe-rich Pd/Fe surfaces, leading to the formation of CH x , and Pd-rich Pd/Fe surfaces can stabilize CO, which may afford the high selectivity to oxygenate. The bimetallic catalysts will provide two different active sites that are synergetic for the formation of higher alcohols. Moreover, the difference between Pd-doped and Cu-doped Fe(100) systems was compared and analyzed based on the d-band model, and it was found that the d-bandwidth of Cu/Fe(100) was more narrow compared to that of Pd/Fe(100); this was agreement with the calculation results that the energy barrier for C–O bond scission on Cu/Fe(100) was lower than that on Pd/Fe(100). We predicted that methane content decreases and methanol content increases with Pd coverage increases on Pd/Fe(100), and the selectivity of methanol on Pd/Fe(100) is higher than that on Cu/Fe(100). Importantly, a typical “ volcano curve” between ethanol synthesis and the HCO dissociation barrier was gained, in which the selectivity for the ethanol synthesis is highest on the Fe2Cu2/Fe­(100) system among these studied bimetallic model catalysts due to its moderate catalytic activity for HCO dissociation.