Activity and selectivity for CO2 methanation of clusters and nanoplates of ruthenium dispersed on ceria: In-situ studies with XAFS and DRIFTS

Abstract

The performance of Ru/CeO₂ catalysts under CO₂ hydrogenation conditions was studied using in situ X-ray absorption fine structure (XAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to understand the structural evolution and chemical nature of each component under reaction conditions. The catalysts were prepared using a reverse microemulsion method that maximized the dispersion of RuO_x particles on ceria. A RuO_x → Ru transformation was observed upon exposure of the RuO_x/CeO₂ systems to H₂ at 250 °C. For a sample with 5 % molar Ru, two-dimensional clusters (2–4 atoms) of Ru formed on top of the ceria support. An increase in the loading of ruthenium to 20 % molar led to the formation of nanoplates of the metal 2–3 at. layers thick. Both Ru structures were highly dispersed on ceria. They were oxidized after exposure to CO₂ at 250 °C. However, under CO₂/H₂ mixture, they remained in a metallic state as two-dimensional clusters and nanoplates. In situ DRIFTS studies, the 5 % and 20 % Ru/CeO₂ catalysts showed distinct reaction intermediates when exposed to CO₂ or CO₂/H₂ (1/4) reaction mixtures. Normalized to the Ru molar percentage, the 5 % Ru/CeO₂ system was the most active catalyst exhibiting a selectivity of ∼ 60 % CH₄ and 40 % CO at 250 °C. On the other hand, under the same reaction conditions, the 20 % Ru/CeO₂ system was less active but had a CH₄ selectivity close to 80 %. These results highlight the importance of the structure of the metallic Ru particles as a factor that determines the catalytic performance.