Gregory S. Herman, College of Engineering, Chemical, Biological, and Environmental Engineering Oregon State University

Reaction of 2-Propanol on SnO2(110) Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy

Tin dioxide (SnO2) has a wide range of applications, including gas sensors, transparent conductors, and oxidation catalysts. The surface chemistries for each of these applications can be strongly influenced by the surface structure and cation oxidation states. The oxidation of volatile organic compounds (VOC) has recently been demonstrated using SnO2, where 2-propanol was used as the probe molecule.

More recently it was observed that the surface Sn2+/Sn4+ratio strongly influenced the activity of carbon monoxide oxidation. In this study, we have used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to characterize the surface chemistry of 2-propanol on well- defined SnO2(110) surfaces. We have prepared stoichiometric and reduced surfaces which were characterized with both AP-XPS and low energy electron diffraction. AP-XPS was performed on these surfaces for 2-propanol pressures up to 1 mbar, various 2-propanol/O2ratios, and a range of temperatures.

These studies allowed us to evaluate the chemical states of 2-propanol on the SnO2(110) surface under a wide range of experimental conditions. The effect of surface preparation, 2-propanol/O2 ratios, and sample temperature was evaluated using AP-XPS and mass spectrometry. Using valence-band spectra, we have found that the surface was reduced from Sn4+ to Sn2+ when the sample was heated in 2-propanol and that the main reaction product in the gas phase was acetone. This suggests that the reaction occurs through a mechanism where bridging oxygens are hydroxylated upon adsorption of 2- propanol. These bridging hydroxyl groups can react and result in water desorption. This process leads to the reduction of the SnO2(110) surface. We have found that the low temperature AP-XPS spectra (300-400 K) was nearly identical for 2-propanol and 2-propanol/O2 mixtures. After running the reactions at higher temperatures we found that the surface remained oxidized. Several oxidation products were also observed in the gas phase. Based on the experimental results we find that the surface was inactive for the oxidation of 2-propanol for temperatures below 500 K. With 2-propanol/O2 mixtures the reactivity increased substantially at lower temperatures. Furthermore, we propose that in 2-propanol/O2 mixtures the reaction occurs through a Mars−van Krevelen mechanism.

Short bio:
GREGORY S. HERMAN received his B.S. degree in Chemistry at the University of Wisconsin-Parkside in 1985 and his PhD. in Physical Chemistry at the University of Hawaii at Manoa in 1992. Gregory has had positions at Pacific Northwest National Laboratory, Hewlett-Packard Corporation, and Sharp Laboratories of America.  In 2009 he joined the School of Chemical, Biological and Environmental Engineering at Oregon State University (OSU) as an Associate Professor and is currently Professor, Head, and James and Shirley Kuse Chair in Chemical Engineering.

Host: Associate Professor Tobias Weidner, iNANO & Dept. of Chemistry, Aarhus University