In situ IR investigation of activation and catalytic ignition of methane over Rh/Al2O3 catalysts

Catalytic partial oxidation (CPO) of methane to syngas has received considerable interest recently as a way to utilize remote natural gas resources. Despite this interest, the mechanism of methane CPO is not completely understood. Investigation of the catalytic ignition of methane CPO can provide insight into 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 and methane activation on Rh/Al2O3 catalysts was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).Different catalysts were studied in this research: fresh, aged, sintered and oxidized catalysts. The ignition temperature when the CH4 to O2 ratio was two is lowest on reduced fresh catalysts. The ignition temperature decreased with an increase in oxygen concentration in the reactant mixture, which is the opposite trend previously noted for Pt/Al2O3. To explain these results, methane activation on different catalysts was explored and the oxidation state of rhodium was characterized by using CO as a probe molecule. Methane activation was found to occur at different temperatures depending on the catalyst state. As the reactant mixture flowed over the catalyst and the temperature was raised towards the ignition temperature, the oxidation state of the catalyst changed and an oxidized rhodium state, Rhn+ (1 ≤ n ≤ 3), progressively formed. In addition, a greater amount of Rhn+ was found when the oxygen concentration in the feed was higher. From these results, it is hypothesized that ignition of methane CPO on Rh/Al2O3 is potentially related to the amount of the higher oxidation state Rh.Different Rh sites play different roles in the heating process during methane activation. Rh0 sites covered by O2 are responsible for C2 species formation at low temperatures while bare Rh0 sites are preferred for CO production. Rhn+ sites are also presumed active for CH4 dissociation. They are then progressively reduced into lower oxidation state of Rh under CH4/He environment with increasing temperature.

Graphical abstractIgnition of methane catalytic partial oxidation and methane activation on Rh/Al2O3 catalysts were studied using in situ DRIFTS. The ignition temperature decreased with increasing oxygen concentration and is lowest on reduced fresh catalysts. It is hypothesized that ignition of methane catalytic partial oxidation on Rh/Al2O3 is related to the amount of the higher oxidation state Rh.Figure optionsDownload full-size imageDownload as PowerPoint slide