Density functional theory study of CH4 dissociation and C[sbnd]C coupling on W-terminated WC(0001) surface

Abstract

To explore the catalytic activity of tungsten monocarbide (WC), the methane (CH4) dehydrogenation and further CC coupling process on WC surface have been studied systematically by performing density functional theory (DFT) calculations. The scaling relationship of the adsorption energies of CHx intermediates on W-terminated WC(0001) surface has been summarized together with those cases on pure metal surfaces. By the way, we also studied the thermal stabilities of low Miller index surfaces of WC to obtain the Wulff construction of WC nanocrystal. The W-terminated WC(0001), W-terminated WC(112̄1), and WC-terminated WC(112̄0) facets constitute more than 96% of the total exposed surface in the WC Wulff construction. The van der Waals (vdW) interaction between adsorbate and W-terminated WC(0001) surface is corrected by the BEEF-vdW functional. Our calculations show that W-terminated WC(0001) surface is the most favorably exposed surface with a lower surface energy. Based on the data collected from literature, the linear relationship between the CHx adsorption energies and the x in CHx on metal (e.g., Fe, Co, Ni, Cu, Pt, Ir, Rh, Ru, Pd, Ag, and Au) surfaces has been analyzed. More interestingly, we found that the Co(111), Ni(111), and W-terminated WC(0001) surfaces possess very close slope factors in such a linear relationship, suggesting the similar catalytic property of methane dehydrogenation on these surfaces. More importantly, the rate-limiting step of CH4 dissociation on W-terminated WC(0001) surface corresponds to CH∗→ C∗+H∗. The energy barrier [i.e., 1.46 eV (1.63 eV) with (without) zero point energy (ZPE) correction] for this reaction step is slightly larger than the one on Ni(111) surface and smaller than the one on Cu(111) surface. Furthermore, we have studied the CC coupling through the CH∗ intermediates on W-terminated WC(0001) surface and found that the formation of C2H2 is kinetically favorable. Our results provides useful information for the development of tungsten-carbide-based catalyst materials.