Mechanistic Descriptions and Kinetic Trends in CO ₂ Hydrogenation on Ru, Co, and Ni Nanoparticles

Abstract

Dispersed Ru, Co, and Ni nanoparticles are the preferred catalysts for converting the CO 2 –H 2 reactants to CO and CH 4 . This study shows that they exhibit similar kinetic effects of CO 2, H 2, and CO pressures on the CO 2 conversion turnover rates and CH 4 selectivities, reflecting the common identity and kinetic relevance of the elementary steps and intermediates on all three metals. CO 2 conversion proceeds via quasi-equilibrated direct dissociation into chemisorbed CO (CO*) and O* and the kinetically-relevant H-assisted removal of O*. CH 4 forms through H-assisted activation of CO*, which binds strongly on all three metals, leading to high CO* coverages and strong inhibition of both CO and CH 4 formation rates. CO adsorption–desorption remains quasi-equilibrated, with the fluid-phase CO pressure set by the relative rates of CO formation and conversion at each axial position along the catalyst bed. The rate equations derived from these elementary steps, embedded within convection-reaction mole balances that account for axial CO concentration gradients, accurately describe measured rates and selectivities on Ru, Co, and Ni catalysts over broad and practical ranges of temperature (483–573 K), pressure (4–1400 kPa CO 2, 2–700 kPa H 2 ), support (SiO 2, Al 2 O 3, TiO 2 ), and nanoparticle size (1–30 nm mean diameter). The similar functional form of these rate equations shows that these most competent metals for CO 2 –H 2 reactions are less mechanistically diverse than typically considered in the search for more active or selective catalysts through compositional or structural complexity. The proposed elementary steps shown to describe the CO 2 –H 2 reaction rates and selectivities on Ru, Co, and Ni nanoparticles provide a robust mechanistic framework that avoids ad hoc mechanistic proposals tailored to specific catalyst compositions, structures, or reaction conditions.

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