Selective electroreduction of CO2 to acetone by single copper atoms anchored on N-doped porous carbon

Efficient electroreduction of CO2 to multi-carbon products is a challenging reaction because of the high energy barriers for CO2 activation and C–C coupling, which can be tuned by designing the metal centers and coordination environments of catalysts. Here, we design single atom copper encapsulated on N-doped porous carbon (Cu-SA/NPC) catalysts for reducing CO2 to multi-carbon products. Acetone is identified as the major product with a Faradaic efficiency of 36.7% and a production rate of 336.1 μg h−1. Density functional theory (DFT) calculations reveal that the coordination of Cu with four pyrrole-N atoms is the main active site and reduces the reaction free energies required for CO2 activation and C–C coupling. The energetically favorable pathways for CH3COCH3 production from CO2 reduction are proposed and the origin of selective acetone formation on Cu-SA/NPC is clarified. This work provides insight into the rational design of efficient electrocatalysts for reducing CO2 to multi-carbon products.


Supplementary Note 1. Impact of Zn on acetone production
According to the ICP-AES test (Supplementary Table 1), Zn impurity existed in the NPC and Cu-SA/NPC. The Zn content of NPC was 0.90 wt %, which was 12 times higher than that of Cu-SA/NPC (0.07 wt %). However, the liquid products of CO2 reduction on NPC were HCOOH and CH3COOH (Supplementary Fig. 13), demonstrating that Zn should not be responsible for the formation of CH3OH, C2H5OH and CH3COCH3. The Zn content of Cu-SA/NPC was only 0.07 wt %, and such low level of Zn on the catalysts could not combine with Cu nor change the selectivity toward CO2 reduction 1 . In this work, the Cu atom was identified to be coordinated with N atom as measured by the EXAFS (Fig.2b), demonstrating that Zn was not coordinated with  Tables 1, 5 and Figs. 19, 20) of Cu-SA/NPC and Cu-SA/NPCAr catalysts were also similar, therefore, the disparate CO2 reduction activity of these two catalysts may be resulted from the content and type of N species. Furthermore, the influence of Zn impurity on acetone production from CO2 reduction on Cu-SA/NPC catalyst was also investigated by DFT calculations.

Supplementary Note 2. Other C-C coupling pathways on Cu-pyrrolic-N 4 by DFT
Other possible C-C coupling pathways were also examined on the Cu-pyrrolic-N4 site of Cu-SA/NPC, including CO*-CHO* and CO*-COH* coupling, and were compared with the CO*-CO* coupling route. As illustrated in Supplementary Fig. 23, the formation of COH* from CO* reduction was 0.98 eV endothermic in free energy change and thus subsequent CO*-COH* coupling was not a preferred path despite that this C-C coupling was substantially exothermic (-2.27 eV). Meanwhile, the CO* reduction to CHO* (ΔG of -0.50 eV) was less favorable as compared to the direct coupling of two CO* species with a ΔG of -1.23 eV. Moreover, the formed COCHO* species was much less stable than the COCOH* species (Supplementary Fig. 23).
Therefore, the route going through CO*-CO* coupling followed by the formation of a COCOH* intermediate should be the most plausible pathway for acetone production from CO2 reduction and was reported in the main text. without adding Cu showed no acetone formation from CO2 reduction, which suggested that the oxidized N should not be the active site for acetone production on the Cu-SA/NPC catalyst. To further confirm this, DFT calculations of energetic pathways for CO2 reduction to acetone on two types of oxidized N sites, including pyridinic-and pyrrolic-N=O sites, were further performed. The results showed that the pyridinic-N=O site was not active for CO2 reduction to acetone since several steps involved in the conversion were highly uphill in ΔG, such as CO*-CO* coupling and CO*-COCH3* coupling steps with ΔG values of 1.05 and 1.62 eV, respectively, at 0 V (see Supplementary Fig. 26a). On the pyrrolic-N=O site, although the CO2 reduction to COOH* step became facile comparing to that occurred on the Cu-pyrrolic-N4 site, some elementary steps involved in acetone formation had large ΔG values, such as COCH2* reduction to COCH3* and CO*-COCH3* coupling steps with ΔG of ~0.8 eV at 0 V (see Supplementary Fig. 26b). Since the C-C coupling reaction could not be facilitated by the applied potential, the acetone formation should not prefer to occur on the pyrrolic-  Supplementary Figs. 27 and 28). Therefore, the oxidized N sites were not the active sites for acetone formation.

Supplementary Note 4. Zn-involved sites examined by DFT
To further investigate whether the Zn species had an impact on acetone production from CO2 reduction on Cu-SA/NPC, DFT calculations were performed on two types of catalyst models including Zn-pyrrolic-N4 and (Cu-Zn)-pyrrolic-N3 sites ( Supplementary Figs. 29 and 30). The Gibbs free energy change for several key elementary steps were calculated including CO2 reduction to COOH* and then to CO*, together with two non-electrochemical C-C coupling reactions, and compared with that obtained on Cu-pyrrolic-N4. As shown in Supplementary Fig. 29, on Cu-pyrrolic-N4 site without Zn, the first eletroreduction step of CO2 to COOH* had a ΔG value of 1.42 eV, and was identified to be the rate-limiting step for acetone formation according to on Zn-pyrrolic-N4 without Cu, the first electroreduction step of CO2 to COOH* was energetically more endothermic with a ΔG value of 1.53 eV. In addition, the C-C coupling of two CO* species had a ΔG value of 0.96 eV and this reaction could not be facilitated by the electrode potential. Therefore, the Zn-pyrrolic-N4 was not active for acetone production. Although the (Cu-Zn)-pyrrolic-N3 facilitated CO2 reduction to COOH* by reducing the ΔG value to 0.73 eV, the non-electrochemical C-C coupling of two CO* was highly endothermic with a ΔG value of 1.92 eV, as illustrated in Supplementary Fig. 29. These DFT results revealed that the Zn species was not responsible for acetone production from CO2 reduction, confirming the experimental results that the trace amount of Zn impurity had a negligible effect on CO2 reduction on Cu-SA/NPC.

Synthesis of porous carbon
The obtained ZIF-8 powder was placed into a tube furnace and carbonized at 1000 ℃ for 4 h under nitrogen (N2) atmosphere. The heating rate was set to 5 ℃ min -1 . The sample was denoted as NPC.

Preparation of working electrode
The working electrode was synthesized as follow. 10 mg of obtained powder was added into 3 mL mixture solution, which contained 2.95 mL of H2O and 0.05 mL of Nafion (5%). After ultrasound for 30 min, the catalyst ink was coated on the carbon paper with geometric area of 6 cm 2 . The catalyst ink was dried at room temperature and then heated at 120 ℃ for 4 h to remove volatile compound. For CV tests, the working electrode was glassy carbon electrode with geometric area of 0.07 cm 2 covered by 51 coating 10 μL of the above catalyst ink.

Analytical method
The gas products were analyzed by gas chromatograph (Shimadzu, GC-14C). Liquid products were identified by mean of 1 H nuclear magnetic resonance with a 500 MHz spectrometer (Rruker, AVANCE III 500). The solvent presaturation method was used to suppress the water peak. Quantization of formic acid and acetic acid was measured by using an ion chromatography (Shimadzu, SCL-10ASP). Methanol, ethanol and acetone were quantified by gas chromatography equipped a flame ionization detector

XPS analysis
The XPSPEAK41 software was used to analyze the obtained XPS spectra.
Component fitting for N element was based on Gaussian-Lorentzian product function with a 20% Lorentzian-Gaussian value using Shirley background. The C 1s at 284.5 eV was used as the reference for charge correction. The percentage of different N species was determined by the ratio of peak area. The binging energies (BE) and full width at half maximum (FWHM) was fixed at constant.