Influence of Carbon and Oxygen Chemical Potentials on the Hydrogen Donor Identity During Methanation on Ni, Co, and Ni‐Co Clusters

• Published in: ChemCatChem Vol. 11; no. 4; pp. 1244 - 1255
• Main Authors: Lachkov, Petar T., Chin, Ya‐Huei (Cathy)
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 20.02.2019
• Subjects: Adatoms; Binding energy; Carbon; Carbon dioxide; Carbon monoxide; Chemical reactions; chemisorbed oxygen; Hydrogen; hydrogen addition; Metal clusters; Methanation; Nickel; Operating ratios; Organic chemistry; Oxygen; periodic trend; transition metal cluster; Transition metals
• ISSN: 1867-3880; 1867-3899
• DOI: 10.1002/cctc.201801545

Rate measurements in the kinetically controlled regime and equilibrium carbon and oxygen chemical titrations show two distinct mechanistic paths during COx methanation reactions on first‐row transition metal clusters. On Ni and, for a limiting set of conditions, Ni−Co clusters, the reaction occurs via the addition of a hydrogen adatom into CH3* intermediates on clusters partially covered with carbon. On Co and, in a subset of conditions, Ni−Co clusters, it occurs via the donation of hydrogen from OH* to CH3* on clusters partially covered with reactive oxygen adatoms (O*). The [CO]2‐to‐[CO2] and [CO2]‐to‐[CO] operating ratios are the surrogates of carbon and oxygen chemical potentials, respectively, as a consequence of water‐gas shift equilibration. These ratios, together with the carbon and oxygen binding energies, determine the relative surface coverages of carbon and oxygen, the involvement of H* vs. OH* in the kinetically‐relevant step, and in turn, the rate dependencies. Stronger carbon and oxygen binding energies lead to more stabilized transition states of the kinetically relevant steps and larger methanation rates. Same destination, different paths: The carbon‐to‐oxygen chemical potentials and the difference in C* and O* binding energies influence the abundances of C* and O* and the identity of the hydrogen donor for methanation. Methanation occurs via H addition from H* into CH3* on Ni and Ni−Co cluster surfaces partially covered with C* and free of O* and from OH* into CH3* on Ni−Co and Co cluster surfaces partially covered with O* and free of C*.