Mechanistic Connections between CO and CO Hydrogenation on Dispersed Ruthenium Nanoparticles.

Catalytic routes for upgrading CO 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 CO and CO hydrogenation on dispersed Ru nanoparticles (6-12 nm mean diameter, 573 K). The formation of CH from both CO-H and CO-H 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 CO are cleaved via direct interactions with exposed Ru atoms in elementary steps that are shown to be facile by fast isotopic scrambling of CO-CO-H mixtures. Such CO activation steps form bound CO molecules and O atoms; the latter are removed via H-addition steps to form HO. The kinetic hurdles in forming CH from CO do not reflect the inertness of C═O bonds in CO but instead reflect the intermediate formation of CO molecules, which contain stronger C≡O bonds than CO and are present at near-saturation coverages during CO 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 CO-H reactions.