(367f) Ignition of Catalytic Partial Oxidation on Platinum and Rhodium Catalysts

Catalytic partial oxidation has significant potential for converting hydrocarbons and alcohols to more valuable products such as syngas, alkenes, and hydrogen for fuel cells. Catalytic partial oxidation of methane to syngas, in particular, has received much research interest due to its potential for utilization of remote natural gas reserves. Despite the extensive interest in this reaction, there is significant disagreement in the literature over the mechanism for catalytic partial oxidation, as some researcher suggest a direct route to syngas while others suggest an indirect route where methane is first combusted and then reformed to syngas.

Our laboratory has been studying the mechanism of methane catalytic partial oxidation by using in-situ infrared spectroscopy. Specifically, we have been studying the ignition of methane catalytic partial oxidation over platinum and rhodium catalysts. Through DRIFT (diffuse reflectance infrared Fourier transform) spectroscopy, the surface species can be probed as the catalyst temperature is raised. In addition, the state of the catalyst can be studied by using CO as a probe molecule. The effect of reactant composition and of catalyst state (reduced, oxidized, and aged) on the ignition temperature has been studied.

Results on platinum have been explained by the heat of adsorption of oxygen on platinum. The stronger the heat of adsorption of oxygen, the higher the ignition temperature. This is because methane and oxygen compete for surface sites. At low temperatures, the surface is nearly completely covered with oxygen. As temperature is raised, the coverage of oxygen decreases and the coverage of methane increases. The low the oxygen heat of adsorption, the easier it is to desorb oxygen and obtain a high enough methane coverage to achieve catalytic ignition. Aged catalysts ignite the easiest due to large platinum particles that have lower oxygen heat of adsorption than fresh catalysts.

Rhodium, however, is much different from platinum. For instance, higher oxygen composition in the reactants leads to a lower ignition temperature on rhodium, while it led to a higher ignition temperature on platinum. The simple theory of competitive adsorption of methane and oxygen can not explain the results on rhodium; instead, the state of rhodium must be considered. CO was adsorbed on rhodium catalysts at different temperatures as temperature was raised to the ignition temperature to probe the state of rhodium. As temperature was raised, peaks for gem dicarbonyl species and linearly adsorbed CO decreased, while peaks for oxidized rhodium increased. In addition, the more oxygen in the reactant mixture, the larger the peak for oxidized rhodium. Since more oxygen in the feed leads to lower ignition temperatures, it is hypothesized that oxidized rhodium species are critical for catalytic ignition, possibly helping activate methane.