Effect of phosphorous addition to Rh-supported catalysts for the dry reforming of methane

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 sulfur 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 resistance to coke formation of the corresponding supported Rh catalysts during methane DR. On the contrary, catalysts supported on basic La-promoted alumina provide a stable syngas production approaching equilibrium at 750-800 °C. Small amounts of phosphorous co-impregnated with rhodium increase the noble metal dispersion, but do not significantly impact on the catalyst activity. Transient and steady state S-poisoning experiments during methane DR suggest that sulfur directly attacks and bonds to Rh active sites, causing a rapid drop of syngas production even at low S-contents. A secondary poisoning effect is induced by sulfur that causes the rapid formation of some amorphous coke, which is almost absent under S-free operation on the reference Rh catalyst.