Mechanistic Connections between CO2 and CO Hydrogenation on Dispersed Ruthenium Nanoparticles.

Abstract

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.