Impact of sulphur on dry reforming of biogas over a Rh/Al2O3 catalyst

Catalytic dry reforming (DR) is a potential option to valorize and upgrade biogas [1,2], a mixture of methane and carbon dioxide produced from the anaerobic microbial digestion of biomass. The syn-gas obtained by DR has a low H2/CO ratio (<=1), which is suitable for further processing in the synthesis of valuable liquid fuels and oxygenated chemicals. Literature reports agree that Rh-based catalysts can provide outstanding syn-gas yields with carbon-free operation [3,4], but, quite surprisingly, only few studies have reported on the effect of sulphur during the dry reforming of methane on Rh, where the situation is further complicated by the process tendency to give carbon deposition, and they show contradictory results. Therefore, in this work we set out to investigate the impact of sulphur poisoning during dry reforming (DR) of methane with CO2 over a Rh/?- Al2O3 catalyst by adding up to 30 ppmv of SO2 or H2S to the feed (CH4/CO2/N2=1/1/2) at reaction temperatures in the range 800 - 900 °C. As low as 1 ppmv of sulphur in the feed adversely affected the H2 and CO yields, and S-poisoning reached a saturation level for contents >= 10 ppmv, independently from the type of S-bearing compound. The impact of S addition on the RWGS reaction, occurring simultaneously to DR, was also investigated under steady state operation. Transient poisoning experiments showed a rapid drop of syn-gas production and a corresponding increase in the temperature level, suggesting that sulphur directly bonded to Rh active sites. An increase of coke formation on the catalyst was detected with respect to S-free conditions, and this effect was studied by Raman spectroscopy and thermogravimetric analysis. Sulphur inhibition was reversible and the Rh-based catalyst slowly recovered its initial activity after the removal of sulphur from the feed