Phosphorous addition to Rh-based catalyst to improve Sulphur tolerance during dry reforming

The dry reforming (DR) of methane has been studied over Rh-based catalysts modified by phosphorous addition in order to investigate the possibility to enhance their resistance to sulphur poisoning. In particular, three catalysts have been prepared: i) by dispersing phosphorous with rhodium on La-stabilized ?-Al2O3; ii) by stabilizing ?-Al2O3 with phosphorous and then dispersing rhodium; iii) by dispersing rhodium on an amorphous AlPO4 support. Rh supported on La-stabilized ?-Al2O3 has been used as a reference catalyst. Fresh and used catalysts and their corresponding supports have been characterized by ICP-MS, XRD, BET, H2-TPR, CO chemisorption, CO2-TPD, TG analysis and Raman spectroscopy. Phosphorous addition to the supports increases their surface acidity and inhibits CO2-activation, thus depressing both activity and stability of the corresponding supported Rh catalysts during methane DR. On the contrary catalysts supported on basic La-promoted alumina provide a stable methane conversion approaching equilibrium at 750-800°C. Small amounts of phosphorous co-impregnated with rhodium, although doubles the noble metal dispersion, do not significantly impact on the catalyst activity. Transient and steady state S-poisoning experiments during methane DR suggest that sulphur directly attacks and bonds to Rh active sites, causing a rapid drop of syn-gas production even at low S-contents and a corresponding increase in the temperature level. A secondary poisoning effect is related to the transient formation of coke, almost absent under S-free conditions on the reference Rh catalyst.