Reactivity and characterization of S-poisoning on Ru/Al2O3 catalysts for CO2 methanation

In this study, ex-situ and in-situ poisoning techniques were employed to elucidate the impact of sulphur on a Ru/Al2O3 catalyst for CO2 methanation reaction. Activity tests and characterizations were performed on ex-situ poisoned samples with sulphur loadings in range 0 – 5.5 mgS/gcat prepared by impregnation with a (NH4)2S solution or on in-situ poisoned sample obtained by cofeeding 10 ppm of H2S together with the reactant mixture. Regardless of the poisoning technique, the activity tests showed the same gradual decrease in CO2 conversion and increase in CO selectivity with increasing the S content. Indeed, in-situ XPS measurements indicated the presence of Ru sulphides in reducing environment, and as a result the H2 and CO chemisorption experiments showed a linearly decreasing trend of the amount adsorbed with increasing sulphur content, in particular for H2. This leads to a strong inhibition of the hydrogenation capability of the S-poisoned catalysts. The CO adsorption experiments, performed in DRIFTS experiments at room temperature, showed no major differences in the nature of the adsorbed species between poisoned and clean samples. However, the presence of sulphur strongly affected their stability, as evidenced by the easier CO desorption observed for S-containing samples. The effect of sulphur on the reaction mechanism was further investigated by operando DRIFTS, which pointed out that at reaction conditions the decrease of CO2 conversion is mainly due to the inhibition of hydrogen activation. Additionally, the observed effects were rationalized with a kinetic model as a function of the sulphur content, which provided a reliable description of the activity loss.