Adsorbate coverages and surface reactivity in methanol oxidation over Cu(110): An in situ photoelectron spectroscopy study

Abstract

The adsorbate species present during partial oxidation of methanol on a Cu(110) surface have been investigated in the 10 -5 mbar range with in situ x-ray photoelectron spectroscopy (XPS) and rate measurements. Two reaction intermediates were identified, methoxy with a C 1s binding energy (BE) of 285.4 eV and formate with a C 1s BE of 287.7 eV. The c(2x2) overlayer formed under reaction conditions is assigned to formate. Two states of adsorbed oxygen were found characterized by O 1s BE's of 529.6 and 528.9 eV, respectively. On the inactive surface present at low T around 300 -350 K formate dominates while methoxy is almost absent. Ignition of the reaction correlates with a decreasing formate coverage. A large hysteresis of ≈ 200 K width occurs in T-cycling experiments whose correlation with adsorbate species was studied with varying oxygen and methanol partial pressures. The two branches of the hysteresis differ mainly in the amount of adsorbed oxygen, the methoxy species and a carbonaceous species. Methoxy covers only a minor part of the catalytic surface reaching at most 20%. Above 650 K the surface is largely adsorbate free.

Ambient pressure photoelectron spectroscopy: A new tool for surface science and nanotechnology

“…In the energy range where the high-pressure XPS reveals the presence of subsurface oxygen, several other species were detected in the low pressure and low temperature experiments. There is agreement that a peak found at 529.6 eV corresponds to adsorbed oxygen [77,88,89] . Whereas one study qualifies this oxygen as non-reactive [77] another assigns it to high reactivity.…”

“…This assignment should be contrasted with the low pressure studies carried out in-situ [77] , which also summarize the extensive work of static experiments [86,87] in the Cu/O/MeOH system. There is good agreement that at all pressures studied, copper oxide islands at metal surfaces are non-reactive and exhibit a binding energy of the oxygen species of 530.5 eV.…”

“…It is thus of interest to know how the surface of a metallic catalyst responds with different surface phases according to the oxidizing or reducing nature of the gas environment, given by the partial pressures of O 2 and CH 3 OH. It can also be expected that an oscillatory change between oxide and metal, as observed at high pressures [78] and in forced modes at low pressures [77] , may give rise to a spatially inhomogeneous distribution of oxidation states, which could not coexist in thermodynamic equilibrium. In such a situation the usual practice of performing the reaction in a reactor chamber and later transferring, after cooling and evacuation, into the UHV chamber for analysis will not show the correct surface state but rather the equilibrated state [42] produced during evacuation.…”

“…Whereas one study qualifies this oxygen as non-reactive [77] another assigns it to high reactivity. Reactive oxygen should be found at 528.9 eV according to the in-situ low pressure study [77] .…”

“…Copper with its facile redox chemistry between oxide and metal, accessible at low and intermediate pressures [77] , is a suitable system for mechanistic studies of selectivity control. The fact that extensive kinetic oscillations in methanol oxidation were reported is an indication [78] that active Cu may exist both as an oxide and as a metal.…”