Amorphous metal oxides find use in many fields ranging from electronics to heterogeneous catalysis. Herein, we study the surface reactivity of amorphous Al(1-X)MXOY (M=Fe, Mn) oxide thin films towards methanol oxidation. We use AP-XPS to identify and compare adsorbates at varying methanol (CH3OH): oxygen (O2) ratios and temperatures on Al8Fe2OY (Al8Fe2), Al4Fe6OY (Al4Fe6), and Al6Mn4OY (Al6Mn4). The spectra reveal that the transition metal sites are the active sites for methanol oxidation and that Al4Fe6 has a higher number of active sites than the other two films. Methoxy and hydroxyl species are quantified on the surface, consistent with their role as reaction intermediates in the literature, and the subsequent conversion of these adsorbates to adsorbed and gas-phase products sheds light on the nature of the active sites. The decrease in intensity of the hydroxyl and methoxy intermediates with increase in temperature indicates the conversion of methanol which is the highest for Al4Fe6 films. This conversion can be further confirmed by the chemical shifts in binding energy of the adsorbates. Formate is formed on the Al4Fe6 film with increase in temperature for oxidizing conditions (CH3OH:O2=1:2) by the dehydrogenation of methoxy groups on the surface. In contrast, the Al6Mn4 film shows a tendency to form water and carbon dioxide in the complete oxidation of methanol, consistent with the presence of basic sites on the surface. These findings illustrate the effectiveness of Fe-substituted films towards partial methanol oxidation and provide guidance on the design of amorphous oxide layers with tailored chemical reactivity for a given application.

Amorphous oxide thin films for methanol oxidation: An ambient pressure X-ray photoelectron spectroscopy (AP-XPS) study

ACS Spring Meeting 2021

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