Can a Single Valence Electron Alter the Electrocatalytic Activity and Selectivity for CO₂ Reduction on the Subnanometer Scale?

Abstract

Electrocatalytic reduction of CO2 (CO2RR) is an excellent strategy for addressing both the issue of everincreasing anthropogenic CO2 emissions as well as the rapid diminishing of non-renewable fossil reserves. Recently, significant attention has focussed on the development of size-selected subnanometre nanocatalysts because of the unique electronic, geometric and catalytic properties of these clusters, which often exhibit enhanced catalytic activities and selectivities compared to bulk metal catalysts and larger nanoparticles. In this paper, we investigate in detail the electrocatalytic activity of size-selected Cun clusters (n=3-6) employing the Computational Hydrogen Electrode (CHE) model. We have found a striking similarity between CO2RR activity of Cu3 and Cu5 and between Cu4 and Cu6 nanoclusters. The reaction proceeds through * + CO2 → COOH* → CO* + H2O → CHO* → CH2O* → CH3O* → O* + CH4 → OH* → * + H2O as in the case of copper surface on all the Cu clusters. The rate-limiting potential on Cu4 and Cu6 clusters is the proton-electron (H + + e-) transfer to CO* to form the CHO* adsorbed species, which is also the rate-limiting step on Cu surfaces, whilst on Cu3 and Cu5 clusters it is the removal of the adsorbed OH* from the cluster surface (OH* → * + H2O). Most importantly, we have identified a general trend in the exergonicity and endergonicity of each step with the spin-state of the nanocluster. In general, electrochemical steps corresponding to an odd total number of (H + + e-) pair transfers, leading to the formation of the doublet adsorbed species on Cu4 and Cu6 clusters, are highly endergonic uphill processes relative to the same steps on Cu3 and Cu5 clusters. However, steps corresponding to an even total number of proton-electron pair transfers, leading to the formation of the singlet adsorbed species on Cu4 and Cu6 clusters, are highly exergonic downhill process relative to the same steps on Cu3 and Cu5. We have also found that the competing hydrogen evolution reaction (HER) is more hindered on Cu3 and Cu5 compared to Cu4 and Cu6 clusters. There is also a general qualitative relationship between the exer/endergonicity of an electrochemical step and the HOMO-LUMO gap of the various cluster-adsorbate complexes. We have found that an increase or decrease of a single valence electron can significantly alter the electrocatalytic activity and reactivity at the subnanometre level and this has great implications in the design and development of size-selected nanoclusters for CO2RR and similar reactions.

DFT-Based Global Optimization of Sub-nanometer Ni–Pd Clusters

“…To elucidate the multiplicity of all of the MEGA-DFT global minima obtained with VASP, spin-polarized reoptimizations are carried out with Gaussian09 using the ωB97XD exchange–correlation functional , and def2TZVPP , basis set for all of the reminimizations and single-point energy calculations. The ωB97XD/def2TZVPP approach has been employed to find out the optimal spin state for their compromising between accuracy and computational resources, as evident by Raju et al for such metallic systems.…”

Screening of transition metal doped copper clusters for CO2 activation

“…The ratedetermining step is the cleavage of the adsorbed OH in the first case, while it is the proton-electron transfer to the adsorbed CO for the latter one. 67 The redox behaviour of Cu 5 and Cu 20 , and the effect of CO 2 also has been investigated. 68 CO 2 adsorbs more strongly on Cu 5 than on Cu 20 and larger nanoparticles, and forms bicarbonate in solution.…”

Recent Advances in Carbon Dioxide Conversion: A Circular Bioeconomy Perspective

“…The rate-limiting potential of Cu 4 and Cu 6 clusters in CO2R is the proton-electron (H + + e − ) transfer to CO* (species adsorbed on clusters) to form CHO*, which is also the rate-limiting step on Cu surfaces. On the other hand, with respect to Cu 3 and Cu 5 clusters, removing OH* from the cluster surface (OH* → S + OH) is the rate-limiting step in CO2R [63]. The above simulation unambiguously implies the role of surface defects, in addition to bulk electrocatalysts, in regulating CO2R reaction pathways.…”

“…The results of computational hydrogen electrode model simulation reveal a striking similarity in CO2R electrocatalytic activity for the Cu 3 vs. Cu 5 and Cu 4 vs. Cu 6 sizeselected clusters [63]. The rate-limiting potential of Cu 4 and Cu 6 clusters in CO2R is the proton-electron (H + + e − ) transfer to CO* (species adsorbed on clusters) to form CHO*, which is also the rate-limiting step on Cu surfaces.…”