Abstract
Near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) was used to study the chemical states of a range of alumina-supported monometallic Pd and bimetallic Pd–Pt nanocatalysts, under methane oxidation conditions. It has been suggested before that for optimal complete methane oxidation, palladium needs to be in an oxidised state. These experiments, combining NAP-XPS with a broad range of characterisation techniques, demonstrate a clear link between Pt presence, Pd oxidation, and catalyst activity under stoichiometric reaction conditions. Under oxygen-rich conditions this behaviour is less clear, as all of the palladium tends to be oxidised, but there are still benefits to the addition of Pt in place of Pd for complete oxidation of methane.
Highlights
Natural gas engines continue to offer a viable green alternative to traditional petrol and diesel engines, as they produce significantly lower quantities of carbon dioxide
The corresponding atom % are shown in table 1, and show that even for a 2.5Pd–2.5Pt catalyst, the atomic ratio of Pd:Pt is 1.83:1, i.e. for all bimetallic catalysts presented and discussed in detail, Pd is the major Platinum group metals (PGM) component
There is a clear benefit to the addition of platinum under wet conditions, in both stoichiometric and oxygen-rich conditions
Summary
Natural gas engines continue to offer a viable green alternative to traditional petrol and diesel engines, as they produce significantly lower quantities of carbon dioxide. Studies conducted to better understand the active state of Pd methane oxidation catalysts generally agree that a mixture of metal and oxide is necessary for optimum performance [6,7,8,9] For both metals a strong particle size effect has been demonstrated [10]. The same group had previously reported how the optimal palladium particle size varied with different supports [21] Another suggestion for limiting the water deactivation effects was to use bimetallic Pd–Pt catalysts instead of pure palladium [22, 23]. Our group used NAP-XPS to study the surface composition of Pd/Al2O3 catalysts under partial methane oxidation conditions [35]. By performing NAP-XPS studies of a series of Pd–Pt/Al2O3 catalysts with different Pd:Pt ratios, we observe significant changes in the oxidation state of palladium depending on the reaction conditions and the concentration of platinum
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