Impact and Detailed Action of Sulfur in Syngas on Methane Synthesis on Ni/γ-Al₂O₃ Catalyst

Stability and deactivation phenomena are of utmost importance for metal nanocatalysts from both fundamental and industrial points of view. The presence of small amounts of sulfur at ppm and ppb levels in the synthesis gas produced from fossil and renewable sources (e.g., biomass, coal) is a major reason for deactivation of nickel catalysts for carbon monoxide hydrogenation. This paper addresses reaction pathways and deactivation mechanisms of alumina-supported nickel catalysts for methane synthesis from pure syngas and syngas containing small amounts of sulfur. A combination of SSITKA and operando FTIR is indicative of both reversible molecular and irreversible dissociative carbon monoxide adsorption on nickel nanoparticles under the reaction conditions. Methanation reaction involves irreversible carbon monoxide adsorption, dissociation, and hydrogenation on nanoparticle steps and edges. Hydrogenation of adsorbed carbon species leading to methane seems to be the reaction kinetically relevant step. Molecular forms of carbon monoxide reversibly adsorbed on nickel terraces are likely not to be involved in carbon monoxide hydrogenation. The results suggest a competition between sulfur and carbon monoxide for nickel surface sites. During methanation, sulfur preferentially adsorbs on the sites of reversible molecular carbon monoxide adsorption, whereas the low-coordinated nickel sites responsible for carbon monoxide dissociation and hydrogenation are affected to a lesser extent by sulfur poisoning. The active sites of carbon monoxide hydrogenation are poisoned much more rapidly by sulfur, when the catalyst has been exposed to small amounts of H 2 S in the absence of methanation.