Reaction pathways of phenol steam reforming over Rh and Ni-Co based catalysts supported on -Al₂O₃

Phenol steam reforming is an attractive method for the sustainable production of hydrogen. Phenol can be found in wastewaters from the textiles and pharmaceutical industries and is further contained at high concentrations in bio-oils and tars derived from biomass pyrolysis and gasification. Despite a range of studies reporting on optimal catalysts for the reaction, respective reaction pathways have been comparatively discussed in less detail. In the current work we present a detailed kinetic study of the steam reforming of phenol over Rh and Ni-Co catalysts supported on gamma-Al2O3. The effect of temperature, partial pressure of reactants, and contact time is studied to propose a reaction mechanism over the catalysts. Results suggest that initial reaction pathways are affected by the oxophilicity of the metal. Over Rh, phenoxy formation via O-H bond cleavage is prominent, while over Ni-Co, C-O bond scission appears to be dominant due to the presence of Co and its affinity to oxygen. Due to these kinetic features, the selectivity to benzene is pronounced over Ni-Co in contrast to Rh, while CO is found to be a secondary product on the bimetallic catalyst unlike the noble metal one. Across the range of conditions studied, Rh achieved higher conversion, hydrogen yield and higher stability as a result of significantly lower carbon formation as demonstrated via time-on-stream experiments and analysis of coke deposits on spent catalyst samples.