First-principles investigation of the dissociation and coupling of methane on small copper clusters: Interplay of collision dynamics and geometric and electronic effects

Abstract

Small metal clusters exhibit unique size and morphology dependent catalytic activity. The search for alternate minimum energy pathways and catalysts to transform methane to more useful chemicals and carbon nanomaterials led us to investigate collision induced dissociation of methane on small Cu clusters. We report here for the first time, the free energy barriers for the collision induced activation, dissociation, and coupling of methane on small Cu clusters (Cun where n = 2-12) using ab initio molecular dynamics and metadynamics simulations. The collision induced activation of the stretching and bending vibrations of methane significantly reduces the free energy barrier for its dissociation. Increase in the cluster size reduces the barrier for dissociation of methane due to the corresponding increase in delocalisation of electron density within the cluster, as demonstrated using the electron localisation function topology analysis. This enables higher probability of favourable alignment of the C-H stretching vibration of methane towards regions of high electron density within the cluster and makes higher number of sites available for the chemisorption of CH3 and H upon dissociation. These characteristics contribute in lowering the barrier for dissociation of methane. Distortion and reorganisation of cluster geometry due to high temperature collision dynamics disturb electron delocalisation within them and increase the barrier for dissociation. Coupling reactions of CHx (x = 1-3) species and recombination of H with CHx have free energy barriers significantly lower than complete dehydrogenation of methane to carbon. Thus, competition favours the former reactions at high hydrogen saturation on the clusters.

Sub-Surface Boron-Doped Copper for Methane Activation and Coupling: First-Principles Investigation of the Structure, Activity, and Selectivity of the Catalyst

“…Similar to the trend observed for the first C–H activation, the barrier to activate the C–H bond in the CH 3 * fragment on the Cu 2 u site of B–Cu is higher, with the value of 138 kJ/mol (Supporting Information, Figure S6). It is worth noting that C–H activation in the adsorbed CH 3 * would exhibit significantly lower entropic contribution (loss of entropy) than the initial CH activation of methane; however, the first activation is associated with collision (that cannot be captured in DFT calculations and would require ab initio molecular dynamics) and these collision effects would compensate the increase in the free energy of activation due to the loss of entropy …”

“…It is worth noting that C−H activation in the adsorbed CH 3 * would exhibit significantly lower entropic contribution (loss of entropy) than the initial CH activation of methane; however, the first activation is associated with collision (that cannot be captured in DFT calculations and would require ab initio molecular dynamics) and these collision effects would compensate the increase in the free energy of activation due to the loss of entropy. 41 The subsequent C−H activation in CH 2 * on the Cu 4 u site has a much higher barrier of 161 kJ/mol, as shown in Figure 4b (it is even higher on a Cu 2 u site), making it kinetically difficult. Due to the high activation barrier to further dehydrogenate the CH 2 * moiety, it could be possible for the CH 2 * fragments to couple to form C 2 hydrocarbons, as was also observed for the single Fe site catalyst synthesized by Bao et al 14 and the PdS catalyst of Neurock et al 15 It is also important to point out that subsequent dehydrogenation of CH* forming surface carbon also has a very high barrier of 197 kJ/mol (Figure 4c).…”

“…Therefore, there is a strong incentive to find cheaper and more abundant catalysts. Although being a commercial catalyst in the synthesis of methanol from syngas, in hydrolysis of fatty esters or in the fabrication of graphene by chemical vapor deposition due to the cheap price and abundance, the high activation barrier of copper (Cu) in dissociating the C–H bond of methane hinders its application in methane utilization. , Since Cu has a full d-shell, it performs poorly in methane C–H activation . However, similar to that of Pd, Ni, and Ru, the presence of boron is suspected to alter characteristics of Cu-based catalysts.…”

Insights into the synergistic role of metal–lattice oxygen site pairs in four-centered C–H bond activation of methane: the case of CuO

“…C 2 H 5 has been suggested to be an important intermediate in the formation of large hydrocarbons from methane on copper surface 61 and clusters. 62 Surface reactions of CH 3 chemisorbed on the lattice oxygen may generate oxygen-containing species, although an exhaustive investigation of the different pathways is beyond the scope of this article.…”

Mechanistic insights into the catalytic elimination of tar and the promotional effect of boron on it: first-principles study using toluene as a model compound

“…2,11,12 The catalyst for tar decomposition has to facilitate dehydrogenation reactions, cracking of C-C bonds, and efficient activation of water (in tar steam reforming) to produce syngas, and it should be mechanically stable and regenerative. Transition metals and their oxides are well-known conventional catalysts for these purposes, 5,8,[18][19][20][21][22][23][24] and hence they are popularly used as catalysts for tar elimination. Due to their high activity and cost-effectiveness, the most widely studied catalysts for tar elimination are the Ni-based ones.…”