Highly selective electrocatalytic reduction of carbon dioxide to methanol

On December 13, 2022, Beijing time, researcher Huang Fuqiang and researcher Wang Jiacheng of Shanghai Institute of Ceramics, Chinese Academy of Sciences, and the team of Professor Zheng Gengfeng of Fudan University published an article entitled “Delocalization State-Induced Selective Bond Breaking for Efficient Methanol” in the journal Nature Catalysis Electrosynthesis from CO2″. Inspired by the theory of soft and hard acids and bases, this work proposes to use the electron cloud delocalization enhancement effect of Cu catalytic sites in cuprous cyanamide (Cu2NCN) materials to induce selective bond breaking of key intermediate products in the electrocatalytic CO2 reduction process, and realize the highly selective electrochemical reduction of CO2 to methanol in aqueous electrolytes.

The corresponding authors of the paper are Huang Fuqiang, Zheng Gengfeng, and Wang Jiacheng; The co-first authors are Kong Shuyi and Lu Ximeng.

Methanol (CH3OH) is in high demand in the chemical industry as an important clean fuel and key precursor to synthetic chemistry. Traditional industrial methanol production mainly relies on Fischer-Tropsch synthesis process, which has harsh reaction conditions (50−100 bar, 200−300 °C) and causes large carbon emissions (more than 30 kg CO2 per ton of methanol produced). In order to reduce energy consumption and pollution in methanol production, new low-carbon technologies are urgently needed. Electrocatalytic carbon dioxide reduction reaction (CO2RR) driven by renewable electricity is a promising renewable fuel production technology that converts CO2 into gaseous or liquid fuel molecules at near-zero carbon emissions. At present, the most commonly used copper-based catalysts can catalyze CO2RR to produce a variety of high value-added products of one carbon (C1) and multiple carbon (C2+). However, the preparation of CH3OH from electrochemical CO2RR in aqueous solutions has been reported relatively little, and the selectivity and yield are generally much lower than methane (CH4) products in the gas phase (Figure 1a).

In this research work, the collaborative team first designed, synthesized and resolved the cuprous cyanoamide (Cu2NCN) compound. The results of crystal structure analysis and theoretical calculation showed that the long-chain cyanamide root [NCN]The introduction of 2− widens the distance between adjacent Cu atoms, making them relatively isolated active sites. During electrocatalytic CO2RR, isolated Cu(I) sites inhibit C−C coupling, thereby reducing the formation of C2+ products (Figure 2). On the other hand, in the strong coordination position [NCN]2−Under the polarization of the anion, the electron cloud around Cu(I) is dispersed, and the effective electron mass at the bottom of the conduction band is much lower than that of the metal Cu, showing a high degree of delocalized state characteristics, that is, a softer cationic acid site is formed (Figure 1c). Based on the theory of soft and hard acids, soft acidic bits weaken the bonding strength of hard bases such as *O, *OR, R = H, or alkyls. Therefore, this electron delocalization state of Cu(I) weakens the Cu−O bond strength, making Cu−O−CH3, a key reaction intermediate more inclined to Cu−O bond breakage and eventual hydrogenation to methanol (Figures 1b, 3 and 5). Cu2NCN electrocatalyst exhibits excellent electrocatalytic reduction of carbon dioxide to CH3OH selectivity and activity in aqueous solution (Figure 4). In situ Raman spectroscopy also confirmed the formation of key reaction intermediates during this reaction.

Figure 1: Schematic diagram of the selective effect of electron cloud delocalization states at Cu catalytic sites on products (methane or methanol).

Figure 2: Cu2NCN morphology, XRD refinement, Cu-Cu radial distribution function calculation, and synchrotron radiation characterization.

Figure 3: Crystal structure, delocalization, and bond enthalpy calculation for key intermediate*OCH3 of Cu2NCN.

Figure 4: Electrocatalytic CO2RR performance of Cu2NCN

Figure 5: Theoretical calculation of the associated electrocatalytic CO2RR pathway

In this study, a new idea is proposed to solve the challenge of low selectivity and activity of electrochemical CO2 reduction to CH3OH in aqueous electrolyte. By enhancing the electron delocalization state of Cu sites on cuprous cyanoamine catalysts, the relative strength of different bond-breaking sites of key intermediates was regulated, and highly selective methanol products were prepared. The research work was supported by the National Key Research and Development Program of the Ministry of Science and Technology (2018YFA0209401) and the National Natural Science Foundation of China (22025502, 21975051, 92163117, 52072389). (Source: Science Network)

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