Operando characterisation of alumina-supported bimetallic Pd–Pt catalysts during methane oxidation in dry and wet conditions

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.

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

“…By in situ XAFS, the evolution of a bimetallic PtPd catalyst was followed, assessing its higher stability against sintering compared to a Pd-only sample thanks to the formation of a core-shell structure 25 . A recent near ambient pressure X-ray photoelectron spectroscopy (AP-XPS) study has shown that, under lean conditions, Pd tends to be fully oxidized making less clear the contribution of platinum on the catalytic activity in comparison to the behavior under stoichiometric conditions, where the bimetallic catalyst shows lower light-off temperature ascribed to the coexistence of Pd 2+ and Pd 0 promoted by the presence of Pt 26 . Previously, by in situ XPS, it was determined that the surface fraction of Pd 2+ depended on Pt content 27 .…”

In situ surface analysis of palladium–platinum alloys in methane oxidation conditions

“…The ratios were selected to match previously tested nanoparticle catalysts of the same compositions. 12 Measurements in multiple positions (with spot size of 0.1 mm  0.1 mm) on individual samples suggest the composition was uniform across the samples both pre-and post-testing. 35 Samples were cut, with 5  3 mm (AE1 mm) pieces used for the experiments described below.…”

“…Nanoparticles for these reactions from our previous work typically had diameters of 1–10 nm, and were supported on alumina grains which are around 100 μm. 12 The lack of support material could also limit the extent to the deactivation seen in wet conditions, due to prior studies showing migration of hydroxyls from the support to the metal. 33…”

“…10 Prior work in our research group focused on alumina-supported palladium and palladium-platinum nanoparticle catalysts. 11,12 For complete oxidation of methane, the addition of platinum to palladium catalysts showed benets under select reaction conditions. A key issue with palladium catalysts in methane oxidation is deactivation due to water, which is a core problem due to catalysts both operating in wet conditions, and producing water when active.…”

“…Nanoparticles for these reactions from our previous work typically had diameters of 1-10 nm, and were supported on alumina grains which are around 100 mm. 12 The lack of support material could also limit the extent to the deactivation seen in wet conditions, due to prior studies showing migration of hydroxyls from the support to the metal. 33 Despite the noted differences, this route still provides understanding of the behaviour of Pd and Pd-Pt systems under reaction conditions in a way that was not achievable using more realistic samples.…”