posted on 2021-07-21, 20:33authored byHaefa Mansour, Enrique Iglesia
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 CO bonds in chemisorbed CO,
formed as an intermediate in both reactions, via hydrogen-assisted
activation pathways. The CO 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 CO bonds in CO2 but instead reflect the intermediate formation of CO molecules,
which contain stronger CO 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.