In-Situ Infared Study of Catalytic Ignition of Methane on Rh/Al2O3

Catalytic partial oxidation (CPO) of methane to syngas has received considerably interest recently as a way to utilize remote natural gas resources. Despite this interest, the mechanism of methane CPO is not completely understood. For example, there is disagreement in the literature if syngas is formed directly from oxidation or whether an indirect mechanism is responsible. Investigation of the catalytic ignition of methane CPO can provide insights on the mechanism of CPO, particularly on the role of the chemical and physical state of the noble metal catalyst. In this work, ignition of methane CPO on Rh/Al2O3 catalysts was studied using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The ignition temperature was found to change with the catalyst state, and is lowest on freshly reduced catalysts. Ignition temperature decreased with an increase in oxygen concentration in the reactant mixture, which is the opposite trend noted for Pt/Al2O3. To investigate this phenomenon, the oxidation state of rhodium was investigated by using CO as a probe molecule. Prior to ignition, ppm of CO were adsorbed on the working catalyst, and the DRIFTS signal was obtained. From this signal, the oxidation state of rhodium could be inferred. As the reactant mixture flowed over the catalyst and the temperature is raised towards the ignition temperature, the oxidation state of the catalyst changes significantly. An oxidized rhodium state, Rh, progressively forms as temperature is increased while Rh and Rh decrease. In addition, a greater amount of Rh is found when the oxygen concentration in the feed is higher. From these results, it is hypothesized that ignition of methane CPO on Rh/Al2O3 is related to the accumulation of the Rh state. * Corresponding author: Tel: 785 532 4315: Fax: 785 532 7372: Email: [email protected] (K. L. Hohn)