Effect of Pd/Pt ratio on the reactivity of methane catalytic combustion in bimetallic Pd-Pt catalyst

Abstract

Effect of Pd/Pt ratio to methane catalytic combustion over Pd-Pt bimetallic sample system is investigated by using kinetic method in a fixed-bed reactor. Via changing Pd/Pt ratio in methane oxidation at high oxygen pressure, the catalyst composition exhibits highly catalytic performance for methane. Pd species is highly reactive at oxidized state, while Pt catalyzes methane oxidation to a higher level at metallic state. Based on these properties, we put forward a novelty energy utilization and catalyst synthesis method for methane combustion. In bimetallic catalyst system, at high oxygen pressure, Pd mainly contribute to the catalytic performance, and Pt species help to improve catalytic ability of Pd. However, reaction results, such as turnover rate normalized on surface Pd sites, show that catalytic activity of Pd do not continuously increase with increasing Pt loading. The reason about this problem is due to the oxygen migration from Pd to Pt, which cause the reduction of PdO to metallic Pd. Therefore, a small quantity of Pt in bimetallic catalyst is the optimal choice for methane combustion at high oxygen pressure. In the meantime, based on the correlation of Pd cluster diameter to reactivity, we also introduce a novelty method to differentiate surface Pd and Pt sites in bimetallic system. Migration of Pd and Pt elements in bimetallic catalyst is discussed, which shows Pd species tend to move to surface forming PdO shell, and Pt species, because of high surface free energy, prefer to migrate to the inner part of the bulk of alloy crystals, resulting the low dispersion of surface Pt/total Pt ratio. Compared the activation barrier of pure Pd catalyst with bimetallic Pd-Pt sample, it can be obtained that Pt helps to lower the activation barrier of Pd.