Operando characterisation of oxide-supportedbimetallic Pd-Pt nanocatalysts for methaneoxidation

Abstract

Near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) was utilised to understand the relationship between the surface chemistry and reactivity of catalysts for the complete oxidation of methane. This reaction is of interest for removing unburnt methane in vehicle exhausts, due to the high global warming potential of methane. A wide range of palladium, platinum, and palladium-platinum catalysts - both real powder catalysts and model systems - were studied for complete methane oxidation. Powder catalyst studies showed that palladium was significantly better than platinum in all relevant tested conditions. Exchanging palladium for platinum showed some benefits, most importantly decreasing the deactivation suffered in wet conditions - a key negative of palladium catalysts. NAP-XPS was utilised to investigate whether understanding the surface states could aid the development of catalysts. This showed some correlations between activity and palladium oxidation state for alumina-supported samples, with benefits seen to retaining significant amounts of Pd(0) on the surface. Pd-Pt catalysts consistently had a higher presence of reduced palladium, compared to Pd catalysts. An increased platinum presence ensures more Pd(0) is present on the catalyst, partially explaining the increased activity of bimetallic catalysts under select conditions. Support effect studies compared silica, alumina and silica-alumina supported catalysts of equivalent loading. A mixed silica-alumina support (90Al2O3-10SiO2) typically performed best, though the margins between this, silica and alumina were often very small for a given condition. NAP-XPS showed a less direct correlation between oxidation of palladium and catalytic activity, with other effects of support variation (grain size, particle size) having greater impacts. Issues with charging and signal in NAP-XPS experiments prompted the development of thin film model catalysts, utilising thin layers of alumina with nanoparticles of palladium deposited onto the surface using a cluster source. This method allowed for the production of samples with similar surface loadings, which were superior in regards to sample charging for XPS. Additionally, they were shown to behave in similar ways under methane oxidation conditions by NAP-XPS, with dynamic reduction and oxidation of palladium. Temperature-programmed NAP-XPS work on palladium foils under reaction conditions showed that a lower oxygen concentration or higher water concentration in the gas feed caused the oxidation of palladium to be inhibited. Pd-Pt foils were studied, with even greater inhibition of Pd oxidation seen with higher Pt loading. Where palladium was oxidised, platinum was typically migrating into the bulk of the foil. Platinum migrating to the bulk of Pd-Pt systems was also seen in thin film studies. Combined with the powder results, this is a clear sign that platinum presence aids palladium retaining a reduced state.