In situ infrared study of the catalytic ignition of methane on Pt/Al2O3

The ignition process of the partial oxidation of methane over Pt/Al2O3 catalysts has been investigated at atmospheric pressure using in situ DRIFT spectroscopy. Contacting methane with the catalyst shows that the dissociative adsorption of methane, evidenced by the appearance of the linearly adsorbed CO species, is delayed to higher temperatures for an oxygen-covered surface compared to an oxygen-free surface. Carbonate species were progressively obtained on the surface before the ignition temperature when interacting CH4/O-2/He mixtures with the catalyst. They result from the interaction of produced carbon dioxide with the support. The temperature at which the carbonate species appears is lower for lower concentration of oxygen in the mixture. It has been shown that oxygen is mainly covering the surface until the ignition temperature. Competition between the two reactants is therefore assumed. It has been found that the state of platinum has a large effect on the ignition temperature, with the highest ignition temperature found for oxidized sample (platinum particle diameter, D-Pt = 1.9-2.2 nm), the next highest for freshly reduced sample (D-Pt = 1.7-1.9 nm), and the lowest on aged sample (D-Pt = 4-5 nm). All of these results linked to microcalorimetry measurements and literature data suggest that the heat of adsorption of oxygen is a key factor for ignition of the surface reaction. (c) 2006 Elsevier B.V. All rights reserved.