Mechanistic Connections between CO₂ and CO Hydrogenation on Dispersed Ruthenium Nanoparticles

Catalytic routes for upgrading CO₂ to CO and hydrocarbons have been studied for decades, and yet the mechanistic details and structure-function relationships that control catalytic performance have remained unresolved. This study elucidates the elementary steps that mediate these reactions and examines them within the context of the established mechanism for CO hydrogenation to resolve the persistent discrepancies and to demonstrate inextricable links between CO₂ and CO hydrogenation on dispersed Ru nanoparticles (6-12 nm mean diameter, 573 K). The formation of CH₄ from both CO₂-H₂ and CO-H₂ reactants requires the cleavage of strong C≡O bonds in chemisorbed CO, formed as an intermediate in both reactions, via hydrogen-assisted activation pathways. The C═O bonds in CO₂ are cleaved via direct interactions with exposed Ru atoms in elementary steps that are shown to be facile by fast isotopic scrambling of C¹⁶O₂-C¹⁸O₂-H₂ mixtures. Such CO₂ activation steps form bound CO molecules and O atoms; the latter are removed via H-addition steps to form H₂O. The kinetic hurdles in forming CH₄ from CO₂ do not reflect the inertness of C═O bonds in CO₂ but instead reflect the intermediate formation of CO molecules, which contain stronger C≡O bonds than CO₂ and are present at near-saturation coverages during CO₂ and CO hydrogenation catalysis. The conclusions presented herein are informed by a combination of spectroscopic, isotopic, and kinetic measurements coupled with the use of analysis methods that account for strong rate inhibition by chemisorbed CO. Such methods enable the assessment of intrinsic reaction rates and are essential to accurately determine the effects of nanoparticle structure and composition on reactivity and selectivity for CO₂-H₂ reactions.