Evaluating the Effect of a Strong Metal-Support Interaction on the Activity of Molybdenum Carbide Supported Platinum Water-Gas Shift Catalysts.

Abstract

The goal of this research was to understand an apparent strong interaction between platinum and molybdenum carbide (Mo2C), and its effect on the water-gas shift (WGS) activity of Pt/Mo2C catalysts. The state of the art catalyst for WGS is Cu/Zn/Al2O3, but several oxide supported Pt catalysts have also been shown to be active for WGS. Previous studies have shown that the activity of Mo2C and Pt/Mo2C catalysts can rival that of Cu/Zn/Al2O3, but the reason for this high activity is unclear. This works uses a combined theoretical and experimental approach to understand the fundamental mechanisms that contribute to the observed, enhanced activity. The strong interaction between Pt and Mo2C manifests itself through several defining phenomenon. First, the interaction affects the Pt loading mechanism. This interaction is observed through the ability of the Mo2C surface to directly reduce the Pt precursor in solution. This mechanism results in a highly dispersed Pt phase. Additionally, the interaction affects the Pt particle morphology. In this study, the particles were characterized using x-ray absorption spectroscopy and are hypothesized to be a mixture of small, raft-like particles and atomically dispersed Pt, again demonstrating the high dispersion of the Pt phase. Mechanistic studies suggest that the catalyst is bifunctional, supporting the WGS red-ox mechanism. The Mo2C support provides sites for water dissociation and hydrogen evolution. It was proposed that a suitable descriptor for this site is oxygen binding energy, which is within an optimal range on Mo2C resulting in an enhanced WGS rate. The Pt phase provides binding sites for CO, which undergoes oxidation at the metal-support interface by oxygen on the Mo2C surface. The strong metal-support interaction affects the chemistry of the atomically dispersed Pt phase. Charge transfer from the surface molybdenum atoms to the sp-states of Pt causes CO to bind less strongly, which is also beneficial towards the overall rate. In general, carbide supported catalysts exhibit interesting characteristics which can vary from those observed from traditional oxide supported catalysts and warrant further investigation.