Aggregation and Significant Difference in Reactivity Therein: Blocking the CO2-to-CH3OH Reaction

Xiaoyu Chen,Dongli Wei,Mårten S G Ahlquist

doi:10.1021/acs.organomet.1c00431

Xiaoyu Chen, Dongli Wei + Show 1 more

Open Access

https://doi.org/10.1021/acs.organomet.1c00431

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Abstract

A CoPc/CNT system has been only recently reported to transform CO2 to methanol via electrochemical reductions, despite the fact that catalyst has been studied extensively since the 1980s. The explanation of high methanol selectivity lies behind the fact that in the new report CoPc exists mainly as a monomer, while in earlier works aggregates dominate. Here, we have studied the reactivity of monomeric and dimeric CoPc by DFT. The mechanism involves rate-limiting CO2 association, with the C–O cleavage step having very similar activation free energy. Once the Co–CO– intermediate is formed, the reaction bifurcates with two possible paths: (1) CO dissociation or (2) one additional reduction follows a protonation to give the Co–CHO– intermediate, which then leads to methanol by further reactions. For the monomeric species at low reduction potentials, CO dissociation is favored, but the formation of Co–CHO– becomes competitive at more negative applied potentials. For the dimer, the CO dissociation is always favored, and the reduction needed to form the C–H bond is negative enough for it not to be observed. The more difficult reduction stems from repulsive interactions between the CoPc units and lower solvent stabilization of the charge in the aggregate.

Highlights

The conversion of CO2 into a value-added product such as CO1 or methanol[2] is gaining more attention from society as a result of the worsening environmental situation associated with high atmospheric CO2 concentration[3] as well as the increasing demand for energy globally.[4]
Geometry optimizations were performed at the B3LYP-D3/LACVP** level of theory with a water solvation model, and final electronic energies were calculated at the M06/LACV3P**++ level with thermochemical corrections added to get Gibbs energies
We first calculated the reduction potentials for the cobalt phthalocyanine (CoPc)/CoPc− (−0.30 V vs SHE) and CoPc−/CoPc2−

Summary

■ INTRODUCTION

The conversion of CO2 into a value-added product such as CO1 or methanol[2] is gaining more attention from society as a result of the worsening environmental situation associated with high atmospheric CO2 concentration[3] as well as the increasing demand for energy globally.[4]. It exhibits superior reactivity, good stability, and low overpotential when compared to other hybrid catalysts It is the first transition-metal complex reported that enables CO2-tomethanol reduction with good yields.[12]. Hybrid molecular catalyst/carbon support materials rely on the π−π interactions between the catalyst and a carbonsupporting material Such binding is by definition weak and low stability can limit a larger-scale production.[9,10] Our previous work on a Co(TPP)/carbon nantotube (CNT). CO dissociation pathway, which can limit further reduction to methanol

■ RESULTS AND DISCUSSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES