In situ inf[r]ared studies of catalytic partial oxidation

by Cao, Chundi

Catalytic partial oxidation (CPO) has received considerable interest recently both as a way to utilize remote natural gas resources and to provide H[subscript]2 for a fuel cell. Studies on the reactions at lower temperatures and transient conditions were performed, which can provide insights on the mechanism of CPO at high reactions, particularly on the role of the chemical and physical state of the noble metal catalyst. In this work, ignition of methane CPO on Pt/Al[subscript]2O[subscript]3 and Rh/Al[subscript]2O[subscript]3 catalysts and methanol CPO on Pt/Al[subscript]2O[subscript]3 catalysts were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

The ignition mechanism study of CH4 on Pt/Al[subscript]2O[subscript]3 showed that oxygen mainly covers the surface until ignition. Competition between the two reactants is assumed. The heat of adsorption of oxygen is a key factor for ignition of the methane partial oxidation reaction on Pt/Al[subscript]2O[subscript]3. The ignition mechanism on Rh/Al[subscript]2O[subscript]3 was found to be different from Pt/Al[subscript]2O[subscript]3. The oxidation state of the catalyst changed significantly as the temperature was raised towards the ignition. An oxidized rhodium state, Rh[superscript]n+, progressively formed as the temperature was increased while Rh[superscript]0 decreased. In addition, a greater amount of Rh[supercript]n+ was found when the oxygen concentration in the feed was higher. From these results, it is hypothesized that ignition of methane CPO on Rh/Al[subscript]2O[subscript]3 is related to the accumulation of the Rh[superscript]n+ state.

Dissociation adsorption of methanol occurs on both Al2O3 and Pt/Al[subscript]2O[subscript]3. It is suggested that formate was one of the important intermediates in the reaction pathway. Oxygen species play a key role in the formation of formate on the catalysts, and it also affects the product composition. Formate mainly decomposed into CO, which is the dominant source for CO[subscript]2 production in the reactions at higher temperatures.