| AVS 63rd International Symposium & Exhibition | |
| In-Situ and Operando Spectroscopy and Microscopy for Catalysts, Surfaces, & Materials Focus Topic | Wednesday Sessions |
| Session IS+HC-WeA |
| Session: | Ambient Pressure XPS Studies of Surface and Chemistry of Catalysts |
| Presenter: | David Mullins, Oak Ridge National Laboratory |
| Authors: | D.R. Mullins, Oak Ridge National Laboratory T.Z. Ward, Oak Ridge National Laboratory S.H. Overbury, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
Perovskite materials that are characterized by the composition ABO3 can be formed by a wide variety of A and B cations. This enables the catalytic properties to be altered by selectively choosing the constituents while maintaining nominally the same structure. The adsorption and oxidation of simple alcohols such as methanol and ethanol have been identified as probe reactions to characterize and compare the catalytic properties of different oxide surfaces.
Methanol and ethanol oxidation on doped La0.7Sr0.3MnO3(001) have been studied using ambient pressure x-ray photoelectron spectroscopy (APXPS). La0.7Sr0.3MnO3(001) was grown on single crystal Nb-doped SrTiO3(001) by pulse laser deposition. The growth of this film has been extensively characterized as a function of temperature, oxygen pressure and laser fluence in order to produce near ideal crystallinity and morphology in the film. The APXPS experiments were performed on the end station at the recently commissioned Beamline CSX-2 at NSLS II. In order to investigate the so-called “pressure gap” that may occur between reactions studied under vacuum conditions and at pressure approaching atmospheric pressure, experiments were conducted at nominally 10-5 torr and at 0.1 torr between 250° C and 350° C.
Methanol forms methoxy when adsorbed on the perovskite surface at 250° C. The surface coverage was four times greater at 0.1 torr compared to 10-5 torr. Methoxy was the only C-containing surface species observed at 10-5 torr with or without O2. Methoxy was also the dominant surface species at 0.1 torr in the absence of O2. However, small amounts of formate and atomic C were also evident. At the higher pressure the Mn 2p spectra indicated that the alcohol partially reduced Mn3+ to Mn2+. There was also an indication in the O 1s spectra that O was removed from the surface.
When O was present at 0.1 torr formate became the dominant surface species with only trace amounts of methoxy and C also evident. Gas phase CO2 and H2O products were also detected in the C 1s and O 1s spectra.
Results using ethanol rather than methanol as the reactant were generally the same, i.e. only ethoxy on the surface at lower pressures, a mixture of ethoxy and acetate at higher pressure in the absence of O2, and exclusively acetate on the surface when O2 was present. The only significant difference between methanol and ethanol was a greater tendency for ethanol to form the carboxylate in the absence of O2.
Operando experiments are planned to monitor the products with a mass spectrometer to determine whether the different pressures, and resulting surface species, lead to different products.