<i>In Situ</i> Ambient Pressure X‑ray
Photoelectron Spectroscopy Studies of Methanol Oxidation on Pt(111)
and Pt–Re Alloys
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Abstract
For methanol oxidation reactions,
Pt–Re alloy surfaces are
found to have better selectivity for CO<sub>2</sub> 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<sub>2</sub>/100 mTorr of methanol at temperatures ranging from
300 to 550 K. The activities of all of the surfaces studied are similar
in that CO<sub>2</sub> and H<sub>2</sub>O 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<sub>2</sub> over CO<sub>2</sub> 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<sub>2</sub> at 450 K before methanol oxidation, and
consequently this surface has the highest CO<sub>2</sub> production
at temperatures below that required for Re diffusion during methanol
reaction. Although the oxidized Re film also exhibits high selectivity
for CO<sub>2</sub> production and minimal carbon deposition, this
surface is unstable due to the sublimation of Re<sub>2</sub>O<sub>7</sub>; in contrast, the Pt–Re alloy is more resistant to
Re sublimation since the majority of Re resides in the subsurface
region