Binding and activation of ethylene on tungsten carbide and platinum surfaces

Abstract

Density functional calculations were used to evaluate the ability of cubic and hexagonal phases of tungsten carbide to bind ethylene, as a model compound of unsaturated hydrocarbons, since its adsorption is the first step in important catalytic processes. The calculations give the following trend in stability: α-WC(0001)-C > α-WC(0001)-W > Pt(111) > γ-WC(001), with the binding energy varying in the range of −0.72 to −2.91 eV. The sub-surface layers play a crucial role in the binding, favoring a charge reorganization at extended ranges (above 6 Å) from bulk towards the surface, however, the electronic structure of the surface was modified only in the topmost layer. The surface sites for geometric C₂H₄ activation were identified, leading to a surface distortion due to an upwards shifting of surface atoms in the range 0.13–0.61 Å observed in Pt(111), α-WC(0001)-C, and γ-WC(001), with distortion energies of 0.13, 0.15 and 0.61 eV, respectively. The activation of C₂H₄ on tungsten carbides was compared with other transition metal carbide surfaces, which leads to a general classification of the elongation of carbon–carbon bond into a set of only three groups. If the interest is to activate ethylene CC bond, the surface sites and the binding modes should be those of the groups II and III. The infrared spectra show mainly four useful signals as a fingerprint to support and complement future experiments. The results of this work indicate that the α-WC-W surface could be directly responsible for the catalytic performance, while the binding of olefins on α-WC-C could cause surface poisoning. The metastable γ-WC(001) surface could be a promising system as compared to the known α-WC(0001) surface, but challenges arise regarding its synthesis, stability and catalytic performance. These results pave the way to address further experimental and theoretical studies focused on the hydrogenation of ethylene and more complex unsaturated hydrocarbons.