A Mechanism-Based Strategy for Controlling CH₄ and CO Selectivities in CO₂–H₂ Reactions on Dispersed Ru, Co, and Ni Nanoparticles

The formation of CO as an intermediate in the conversion of CO₂-H₂ to CH₄ and its strong binding on dispersed Ru, Co, and Ni nanoparticles inhibit rates of CO and CH₄ formation but to different extents. CO₂ conversion rates decrease and CH₄ selectivities increase as CO concentration gradients evolve axially along the catalyst bed and radially within diffusion-limited porous aggregates. These trends and their interpretation in terms of the identity and kinetic relevance of surface-catalyzed elementary steps enable mechanism-based strategies for selectivity control through the purposeful introduction of CO pressures into inlet CO₂-H₂ streams. This strategy exploits the stronger CO inhibition of its formation (from CO₂) than its conversion (to CH₄), which causes the selective inhibition of CO₂ conversion relative to CH₄ formation. The presence of CO at levels accurately prescribed by the mechanism-derived rate equations, similar in functional form on Ru, Co, and Ni nanoparticles, and by diffusion-convection-reaction models that account for CO gradients at the bed and aggregate scales led to the exclusive formation of CH₄ and to the elimination of CO gradients at both scales, as evident from measured rates and selectivities for CO₂-H₂ reactions on Ru, Co, and Ni nanoparticles over a broad and practical range of temperature (483-573 K), reactant pressures (4-1100 kPa CO₂; 8-820 kPa H₂), and nanoparticle diameter (2-30 nm). This mechanism-based strategy enables the exclusive formation of CH₄ from CO₂-H₂ reactants, irrespective of reaction conditions or nanoparticle composition (Ru, Co, Ni) and size, without requiring complex catalyst architectures or intricate synthesis protocols.