In Situ Ambient Pressure X-ray Photoelectron Spectroscopy Studies of Methanol Oxidation on Pt(111) and Pt-Re Alloys

For methanol oxidation reactions, Pt-Re alloy surfaces are found to have better selectivity for CO2 production and less accumulation of surface carbon compared to pure Pt surfaces. The unique activity of the Pt-Re surface is attributed to the increased ability of Re to dissociate oxygen compared to Pt and the ability of Re to diffuse gradually to the surface under reaction conditions. In this work, the oxidation of methanol was studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and mass spectrometry on Pt(111), a Pt-Re surface alloy, and a Re film on Pt(111) as well as Pt(111) and Pt-Re alloy surfaces that were preoxidized before reaction. Methanol oxidation conditions consisted of 200 mTorr of O-2/100 mTorr of methanol at temperatures ranging from 300 to 550 K. The activities of all of the surfaces studied are similar in that CO2 and H2O are the main oxidation products, along with formaldehyde, which is produced below 450 K. For reaction on Pt(111), there is a change in selectivity that favors CO and H-2 over CO2 at 500 K and above. This shift in selectivity is not as pronounced on the Pt-Re alloy surface and is completely absent on the oxidized Pt-Re alloy surfaces and oxidized Re film. AP-XPS results demonstrate that Pt(111) is more susceptible to poisoning by carbonaceous surface species than any of the Re-containing surfaces. Oxygen-induced diffusion of Re to the surface is believed to occur at elevated temperatures under reaction conditions, based on the increase in the Re/Pt ratio upon heating; density function theory (DFT) calculations confirm that there is a thermodynamic driving force for Re atoms to diffuse to the surface in the presence of oxygen. Furthermore, Re diffuses to the surface when the Pt-Re alloy is exposed to O-2 at 450 K before methanol oxidation, and consequently this surface has the highest CO2 production at temperatures below that required for Re diffusion during methanol reaction. Although the oxidized Re film also exhibits high selectivity for CO2 production and minimal carbon deposition, this surface is unstable due to the sublimation of Re2O7; in contrast, the Pt-Re alloy is more resistant to Re sublimation since the majority of Re resides in the subsurface region.