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 O2/100 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 H2 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 O2 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.
The nucleation, growth, and CO-induced changes in composition for Co–Au bimetallic clusters deposited on TiO2(110) have been studied by scanning tunneling microscopy (STM), low energy ion scattering ...(LEIS), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. STM experiments show that the mobility of Co atoms on TiO2(110) is significantly lower than of Au atoms; for equivalent or lower coverages of Co, the number of clusters is higher and the average cluster height is smaller than for Au deposition. Consequently, bimetallic clusters are formed by first depositing the less mobile Co atoms, followed by the addition of the more mobile Au atoms. Furthermore, the reverse deposition of Au followed by Co results in clusters of pure Co coexisting with clusters that are Au-rich. For clusters with a total coverage of 0.25 ML, the cluster density increases and average cluster height decreases as the fraction of Co is increased. Annealing to 800 K results in cluster sintering and an increase of ∼3–5 Å in average height for all compositions. LEIS experiments indicate that the surfaces of the bimetallic clusters are 80–100% Au for bulk Au fractions greater than 50%, but Co and Au coexist at the surfaces when there are not enough Au atoms available to completely cover the surfaces of the clusters. After heating to 800 K, pure Co clusters become partially encapsulated by titania, and for bimetallic clusters, the Co is selectively encapsulated at the cluster surface. The desorption of CO from the bimetallic clusters demonstrates that the presence of the CO adsorbate induces diffusion of Co to the cluster surface, but the extent of this diffusion is less than what is observed in the Ni–Au and Pt–Au systems. Density functional theory calculations confirm that for a 50% Co/50% Au bimetallic structure: the surface is predominantly Au in the absence of CO; CO induces diffusion of Co to the cluster surface; and this CO-induced diffusion is less extensive on Co–Au than on the Ni–Au and Pt–Au surfaces.
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