Consequences of Surface Oxophilicity of Ni, Ni-Co, and Co Clusters on Methane Activation

Abstract

This study describes a new C-H bond activation pathway during CH-CO reactions on oxophilic Ni-Co and Co clusters, unlike those established previously on Ni clusters. The initial C-H bond activation remains as the sole kinetically relevant step on Ni-Co, Ni, and Co clusters, but their specific reaction paths vary. On Ni clusters, C-H bond activation occurs via an oxidative addition step that involves a three-center (HC···*···H) transition state, during which a Ni-atom inserts into the C-H bond and donates its electron density into the C-H bond's antibonding orbital. Ni-Co clusters are more oxophilic than Ni; thus, their surfaces are covered with oxygen adatoms. An oxygen adatom and a vicinal Co-atom form a metal-oxygen site-pair that cleaves the C-H bond via a σ bond metathesis reaction, during which the Co inserts into the C-H bond while the oxygen abstracts the leaving H-atom in a concerted, four-center (HC···*···H···O*) transition state. Similarly, Co clusters also catalyze the σ bond metathesis step, but much less effectively because of their higher oxophilicities, much stronger binding to oxygen, and less effective hydrogen abstraction than Ni-Co clusters. On Ni-Co and Co clusters, the pseudo-first-order rate coefficients are single-valued functions of the CO-to-CO ratio (or HO-to-H ratio), because this ratio prescribes the oxygen chemical potentials and the relative abundances of metal-oxygen site-pairs through the water-gas shift equilibrium. The direct involvement of reactive oxygen in the kinetically relevant step leads to more effective CH turnovers and complete elimination of coke deposition on Ni-Co bimetallic clusters.

High‐Performance Bimetallic Catalysts for Low‐Temperature Carbon Dioxide Reforming of Methane

“…Several researchers reported on progresses in the application of bimetallic Ni‐based catalysts in the presence of transition metals, such as Co and Fe. Tu et al presented both experimental data and density functional theory (DFT) simulation on methane activation by using Ni, Co, and Ni‐Co clusters. Based on the findings, the active sites involved different mechanisms during the C–H bond activation in CH 4 as the sole kinetically relevant step.…”

Hierarchical oxygen reduction reaction electrocatalysts based on FeSn0.5 species embedded in carbon nitride-graphene based supports

“…The SSA of "Pt-free" ECs is mostly modulated by the "active metal". In particular, SSA is raised as the "active metal" is changed in the order Ni → Co → Fe, following the increase in oxophilicity of the metals [55,73,74]. The latter feature promotes the first approach of oxygen to the EC surface, that is crucial in modulating the first, rate-determining electron transfer process [70].…”

Design of stable Ni/ZrO2 catalysts for dry reforming of methane

“…The individual rates and in consequence the OAI will depend on a particular metal and rate at which the support provides labile oxygen atoms. Thus, the rate and the stability will be higher on metals that are better in dissociating CO2 or able to retain oxygen [12] and will be less stable for systems that dissociate CH4 with a high rate and have a low tendency to retain oxygen. Similarly, the ability of activating CO2 and the chemical potential of the oxygen produced with oxides other than ZrO2 may change the numeric values of OAI.…”

“…Recently, the use of oxophilic metals has been proposed for this purpose. With this strategy, the metal component such as Co [12] or Fe [13] is added to the metal primarily active in methane dissociation, e.g., Ni, in order to provide an oxygen reservoir that can be used to oxidize surface carbon. While this is an approach based on thermodynamics, it is conceptually also possible to provide labile oxygen in the reducing atmosphere of the reforming reactor by activating and dissociating CO2 at the boundary between the perimeter of the metal particles and the support [14].…”