Effect of partial substitution of Rh catalysts with Pt or Pd during the partial oxidation of methane in the presence of sulphur

The effect of the partial substitution of Rh/La–Al2O3 monolith catalysts with either Pt or Pd during the catalytic partial oxidation of methane in the presence of H2S under self-sustained high-temperature conditions was investigated. The catalysts were fully characterized by BET, SEM-EDS, SO2-TPD and in situ DRIFT spectroscopy of adsorbed CO at room temperature, which was used to study changes on the surface state of Rh before and after exposures to sulphur species at temperatures and conditions close to those expected under actual CPO of methane. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles and low-temperature light-off phase. The partial substitution of Rh with Pt was shown to reduce the detrimental effect of S, which strongly inhibits the steam reforming reaction on monometallic Rh catalyst. DRIFT experiments indicate that sulphur acts as a selective poison by preferentially adsorbing on smaller well-dispersed Rh crystallites while larger metallic Rh sites are mostly unaffected; the presence of Pt or Pd does not directly influence the way S adsorbs on highly dispersed Rh sites.