Effect of Rh in Ni-based catalysts on sulfur impurities during methane reforming

The addition of Rh, in low concentrations (< 1 wt%), to Ni-based catalysts was investigated during steam-dry reforming of a gas mixture, which simulates the effluent of a biomass gasifier, after removal of tar compounds, but in presence of H2S at ppm level. Four Ni-Rh catalysts supported on MgAl2O4, with Ni:Rh molar ratio varying from 18 to 82, were characterized using N-2-BET, Inductively Coupled Plasma Atomic Emission Spectroscopy and X-ray Diffraction, while the evolution of the catalyst structure during temperature program reduction and oxidation was examined using time-resolved in-situ XRD. During reduction of the Ni-Rh catalyst, two types of Ni-Rh alloy were formed. The Rh-rich Ni alloy remained stable up to 1123 K under CO2 oxidation. The catalyst with a Ni:Rh molar ratio of 41 showed the best performance in terms of both activity and stability, in presence of 7 ppm H2S as a contaminant, at 1173 K and total pressure of 111.3 kPa, reaching 0.24 mol(CH4)center dot s(- 1).kg(metals)(-1) after 42.5 h time-on-stream. Regeneration of the catalysts was performed by removing H2S from the feed stream. The catalyst regeneration ability depended on the formation of a Ni-Rh alloy and hence on the Ni:Rh molar ratio. According to density functional theory calculations on the adsorption and dissociation of H2S on Ni and NiRh (111) surfaces, the Ni-Rh alloy inhibited H2S decomposition in contrast to monometallic Ni.