Spot the difference: hydrogen adsorption and dissociation on unsupported platinum and platinum-coated transition metal carbides

Abstract

Hydrogenation reactions are involved in several processes in heterogeneous catalysis. Platinum is the best-known catalyst; however, there are limitations to its practical use. Therefore, it is necessary to explore alternative materials and transition metal carbides (TMCs) have emerged as potential candidates. We explore the possibility of using cheap TMCs as supports for a Pt monolayer, aiming to reduce the amount of the noble metal in the catalyst without a significant loss of its activity towards H₂ dissociation. Hence, analyzing H₂ dissociation from a fundamental point of view is a necessary step towards a further practical catalyst. By means of periodic DFT calculations, we analyze H₂ adsorption and dissociation on Pt/β-Mo₂C and Pt/α-WC surfaces, as a function of hydrogen surface coverage (Θ_H), resembling a more realistic model of a catalyst. H₂ dissociation rates were analyzed as a function of the reaction temperature. The results show that Pt/C-WC and Pt/Mo-Mo₂C have a Pt-like behavior for H₂ dissociation at Θ_H > 1/2 ML. At a particular temperature of 298 K, Pt/C-WC and Pt/Mo-Mo2C have low energy barriers for H₂* → 2H* (0.13 and 0.11 eV, respectively), close to the value of Pt (0.06 eV). For the highest coverage, i.e. Θ_H = 1, Pt/C-WC has a lower activation energy and a higher reaction rate than Pt. Finally, the H₂ dissociation rate is higher in Pt/Mo-Mo₂C than in Pt when increasing the temperature above 298 K. Our results put Pt/C-WC and Pt/Mo-Mo₂C under the spotlight as potential catalysts for H₂ dissociation, with a similar performance to Pt, paving the way for further experimental and/or theoretical studies, addressing the capability of Pt/TMC as practical catalysts in hydrogenation reactions.