Rare Earth/Transition Metal Oxides for Syngas Tar Reforming: A Model Compound Study

A major problem in biomass or coal gasification is removal of syngas byproducts such as H2S, NH3, and tars (heavy hydrocarbons) that cause catalyst deactivation and clogging problems downstream. Rare earth oxides (REOs) doped with transition metals (TMs) are promising catalysts for tar reforming. With propane as a model compound, we compared such catalysts to a typical supported Ni catalyst, and also to recent density functional theory (DFT) results modeling these systems. The REO/TM catalysts are active over the range 920-1000 K, with no significant deactivation in non-sulfur containing feeds. In particular, a Mn/CeO2 catalyst showed good reforming activity with low carbon, CO2, and CH4 yields. This catalyst also maintained some activity in the presence of 40 ppm H2S. Kinetics calculations showed that most such catalysts have near zero order kinetics with respect to water, making them usable with a variety of gasifier effluents. Characterization of used catalysts by multiple techniques suggests that the metal-doped REOs do not undergo much (if any) phase separation in extended use under tar reforming conditions, with Mn- and La-doped CeO2 being especially stable.