CO 2 Reduction on Supported Ru/Al 2 O 3 Catalysts: Cluster Size Dependence of Product Selectivity

The catalytic performance of a series of Ru/Al 2 O 3 catalysts with Ru content in the 0.1–5% range was examined in the reduction of CO 2 with H 2 . At low Ru loadings (≤0.5%) where the active metal phase is highly dispersed (mostly atomically) on the alumina support, CO is formed with high selectivity. With increasing metal loading, the selectivity toward CH 4 formation increases, while that for CO production decreases. In the 0.1% Ru/Al 2 O 3 catalyst, Ru is mostly present in atomic dispersion, as scanning transmission electron microscopy (STEM) images obtained from the fresh sample prior to catalytic testing reveal. STEM images recorded from this same sample, following the temperature programmed reaction test, clearly show the agglomeration of small metal particles (and atoms) into 3D clusters. The clustering of the highly dispersed metal phase is responsible for the observed dramatic selectivity change during elevated temperature tests: dramatic decrease in CO and large increase in CH 4 selectivity. Apparent activation energies, estimated from the slopes of Arrhenius plots, of 82 and 62 kJ/mol for CO and CH 4 formation were determined, respectively, regardless of Ru loading. These results suggest that the formation of CO and CH 4 follow different reaction pathways or proceed on active centers of a different nature. Reactions with CO 2 /H 2 and CO/H 2 mixtures (under otherwise identical reaction conditions) reveal that the onset temperature of CO 2 reduction is about 150 °C lower than of CO reduction.