Structure and Activity Transition from Oxidized to Metallic Tungsten for Catalytic Hydrogenation: A Density Functional Theory Study

Abstract

Oxide-based hydrogenation catalysts have attracted intensive interest, but the relationships between their composition, structure, and reaction mechanism are still ambiguous. Here, we conducted density functional theory (DFT) calculation on ethylene hydrogenation over WO 3 , WO 2.72 , WO 2 , and W catalysts to explore how the structure and catalytic activity change with the composition and explain why neither WO 3 nor metallic W is a good hydrogenation catalyst but WO 3−x is. Calculations on the geometric and electronic structures show a transition from semiconductor to metal-like with more W−W metal bonds appearing from the corresponding oxidized to metallic tungsten. Correspondingly, the H 2 dissociation mechanism changes from heterotypic on the semiconductor WO 3 and WO 2.72 to homotypic on the metal-like WO 2 and metallic W, and the adsorption strength of the dissociation product (2H) enhances from oxidized to metallic W. Calculations on the stepwise hydrogenation indicate that H 2 dissociation is the rate-limiting step (RLS) for WO 3 and WO 2.72 , whereas the following hydrogenation step (C 2 H 5 + H → C 2 H 6 ) is the RLS for WO 2 and W. By linear fitting the binding energies of H 2 , 2H, H + C 2 H 5 , and C 2 H 6 adsorptions, a perfect correlation between (H + C 2 H 5 ) and (2H) adsorptions is observed; thus, the catalytic activity and mechanism can be evaluated using 2H adsorption behavior as the descriptor. On the basis of the calculation results, we predict that the best ethylene hydrogenation activity will show at 2 < x < 2.72 for WO x . This work suggests that controlling the composition between the stoichiometric oxide and the metal may be an effective way to develop an active hydrogenation catalyst.

Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

“…Most notably, Mattheiss et al [18] provided the first theoretical study investigating the Fermi surface of tungsten using a nonrelativistic approach, followed by Christensen et al, who expanded on this study by calculating the band structure using a relativistic augmentedplane-wave (APW) method coupling the theory results with experimentally obtained photoelectron spectra [27,28]. More recently, theoretical investigations have transitioned from calculating tungsten's band structure to more applicationdriven investigations into the interaction of molecules with tungsten surfaces, point defect studies, or the study of nanostructures [29][30][31][32][33][34]. Several approaches have been used to calculate the projected density of states (PDOS) of tungsten, but these results have not yet been directly compared (with photoionization cross section and broadening corrections) to high-resolution valence band spectra [16,20,22,26,27].…”

Study on mechanism of hydrogen adsorption on WO3, W20O58, and W18O49

“…If there is a full coverage of hydrogen, tungsten oxide would be sequentially reduced to metal tungsten . As for the effect of coverage of H on adsorption, Jiajia Song et al found that the lower the coverage of H is, the more positive the binding before H is saturated . The main issue for hydrogen reduction would be the diffusion of hydrogen, which will be shown in our later work.…”

Lifetime effects and satellites in the photoelectron spectrum of tungsten metal

“…[27,28] More recently, theoretical investigations have transitioned from calculating tungsten's band structure to more application-driven investigations into the interaction of molecules with tungsten surfaces, point defect studies, or the study of nanostructures. [29][30][31][32][33][34] Several approaches have been used to calculate the projected density of states (PDOS) of tungsten, but these results have not yet been directly compared (with photoionisation cross section and broadening corrections) to high-resolution valence band spectra. [16,20,22,26,27] Beyond band structure features, the photoelectron spectrum also contains satellite peaks which arise from the additional excitation of a plasmon or electron-hole pair.…”