Ultrahigh surface density of Co-N2C single-atom-sites for boosting photocatalytic CO2 reduction to methanol

https://doi.org/10.1016/j.apcatb.2021.120695Get rights and content

Highlights

  • The two-step calcination process induced bulk cobalt nitrate vaporization to avoid the aggregation of Co atoms during the pyrolysis process.

  • The g-C3N4 nanosheets as the substrate was proven to afford rich exposed N sites for anchoring single Co atoms.

  • Formed ultrahigh density Co-N2C single dispersed active sites boost the photoreduction CO2 to methanol by localization of photogenerated electrons.

Abstract

Cobalt species as active sites for photocatalytic reduction of CO2 to valuable products such as methanol have received increasing attention, however, it remains a huge challenge to achieve the high activity. Herein, a pyrolysis-induced-vaporization strategy was successfully employed to fabricate Co/g-C3N4 single-atom catalysts (Co/g-C3N4 SACs) with surface Co atom loading up to 24.6 wt%. Systematic investigation of Co/g-C3N4 SACs formation process disclosed that concentrated-H2SO4 exfoliation of g-C3N4 nanosheets (g-C3N4 NSs) as the substrate followed by a two-step calcination process is essential to achieve ultrahigh metal loading. It was found that the ultrahigh-density of Co single-atom sites were anchored on the g-C3N4 substrate surface and coordinated with two nitrogen and one carbon atoms (Co-N2C). These single dispersed Co-N2C sites on the g-C3N4 surface were found to act not only as electron gathering centers but also as the sites of CO2 adsorption and activation, subsequently, boosting the photocatalytic methanol generation during light irradiation. As a result, the methanol formation rate at 4 h (941.9 μmol g−1) over Co/g-C3N4-0.2 SAC with 24.6 wt% surface Co loading was 13.4 and 2.2 times higher than those of g-C3N4 (17.7 μmol g−1) and aggregated CoOx/g-C3N4-0.2 (423.9 μmol g−1), respectively. Simultaneously, H2 (18.9 μmol g−1 h−1), CO (2.9 μmol g−1 h−1), CH4 (3.4 μmol g−1 h−1), C2H4 (1.1 μmol g−1 h−1), C3H6 (1.4 μmol g−1 h−1), and CH3OCH3 (3.3 μmol g−1 h−1) products were detected over Co/g-C3N4-0.2 SAC. Besides, the photocatalytic activity of the Co/g-C3N4-0.2 SAC for the reduction of CO2 to methanol was stable within 12-cycle experiments (~48 h). This work paves a strategy to boost the photoreduction CO2 activity via loading ultrahigh surface density single atomically dispersed cobalt active sites.

Introduction

Photocatalytic CO2 reduction with H2O into methanol is identified as a desirable strategy to achieve carbon neutrality [1], [2]. Because the methanol can act as the raw materials of platform chemical engineering for a vast range of the synthesis of chemicals [3]. Additionally, the cheap and accessible water as an electron and proton donor can drastically cut methanol production costs. Generally, the CH3OH generation involves a 6-electron process, hence gathering a large number of electrons on photocatalyst surface is favorable to trigger high-performance CH3OH formation [4]. Among the reported photocatalysts, using earth-abundant cobalt-based catalysts is beneficial to convert CO2 [5]. Because cobalt is believed to capture CO2 efficiently due to the strong hybridization between 2p orbitals of carbon/oxygen atoms and the 3d orbitals of cobalt atoms [6], [7]. Furthermore, cobalt atoms bonded with N/C atoms can act as active centers to gather electrons under light irradiation for enhancing CO2 activation and reduction to CH3OH [5], [8], [9]. However, bulk cobalt photocatalysts with limited surface-active sites of low atom-utilization efficiency are unfavorable to transfer and gather the photogenerated electrons at CO2 photoreduction sites during light irradiation [10], leading to the limited efficiency of CO2 reduction to CH3OH.

Recently, single-atom catalysts with monodispersed Co-NxC4−x (x =0–4) coordination active motifs (Co-NxC4−x/substrate SACs) possess the merits of maximum atom-utilization efficiency, tunable electronic environments, high efficiency of charge transfer, and high electron mobility, which were reported to be beneficial for CO2 reduction [11]. More importantly, the single dispersed Co-NxC4−x coordination moiety usually results in an enrichment of photogenerated electrons on catalysts surface under light irradiation [2], [12], [13]. It can facilitate the multielectron process in the photoreduction of CO2 to CH3OH [3], [14], [15]. However, most reported Co-NxC4−x/substrate SACs of low Co loadings with a limited number of active sites are unfavorable to achieve high photocatalytic CO2 reduction activity. Generally, increasing the cobalt loading is considered an effective solution strategy but cobalt atoms with high loadings are highly prone to migration and aggregation to CoOx species with much low activity for photocatalytic CO2 reduction [16]. Therefore, fabrication of highly-loaded Co-NxC4−x/substrate SACs (>10 wt%) to achieve high photocatalytic activity is still extremely difficult [9], [17], [18].

Up to date, although several strategies to fabricate high metal loading of Co-NxC4−x/substrate SACs are achieved via direct pyrolysis of a nitrogenous organic compound and metal salt precursors [19], [20], most of the Co-NxC4−x single-atom sites exist in the bulk supports [9], [17], [21]. These single metal atoms in the support matrix can be only used as electron transport channels, but not as electrons gathering centers for photocatalytic CO2 reaction on the surface [22], [23]. Subsequently, the CH3OH production of photocatalytic CO2 reduction showed a relatively low yield due to the limited surface-active sites. Therefore, designing and developing a strategy to fabricate ultrahigh surface loading cobalt single-atom catalyst with Co-NxC4−x coordination moiety is highly demanded but of great challenge for CO2 photoreduction to CH3OH.

Herein, we report a novel pyrolysis-induced-vaporization strategy for fabricating cobalt single-atom catalysts (Co SACs) with Co-N2C coordination moiety and surface cobalt loading as high as 24.6 wt%, which is a record value of all reported Co-NxC4−x/substrate SACs up to now. The as-synthesized Co/g-C3N4 SACs surface has extremely high monodispersed Co-N2C active sites. As a result, it showed much higher activity for photocatalytic CO2 reduction to CH3OH in the absence of both sacrificial reagent and photosensitizer. This study provides new insights into the fabrication of high surface density of Co-NxC4−x/substrate SACs for highly photocatalytic CO2 reduction activity to solar fuels.

Section snippets

Materials

All used chemicals were analytical-grade reagents without any further purification before the experiment. Co(NO3)2·6H2O and dicyandiamide were purchased from Aladdin Reagent Corp. Anhydrous H2SO4 (98%), absolute ethanol (95%), and silica sand (70–150 mesh) were purchased from Chengdu Kelong Chemical Reagent Corp. 99% 13C enriched 13CO2 was provided by Chengdu Keyuan Gas Corp. Dialysis bags were purchased from Viskase Corp (MD44, MW:3500).

Fabrication of g-C3N4 NSs catalyst

The bulk g-C3N4 was prepared using a thermal

Structure and morphology of Co/g-C3N4

The as-prepared g-C3N4 without Co loading shows two representative diffraction peaks at around 12.6° and 27.9° corresponding to (100) and (002) planes, respectively (Fig. 1a) [24]. Introduction of Co to g-C3N4 caused a decrease in crystallinity, however, no peak shifts were observed in all samples, indicating that Co atoms were not doped into the g-C3N4 matrix (Fig. 1a) [36]. All diffraction peaks disappeared when the desired Co to g-C3N4 weight ratio was higher than 0.1, implying that the

Conclusion

In summary, we have proposed a novel pyrolysis-induced-vaporization strategy for the fabrication of Co/g-C3N4 SACs with ultrahigh surface metal loading of 24.6 wt%. Such high surface loading of single Co atoms was achieved using g-C3N4 NSs as the substrate, because it can offer numerous uniform coordination N2C sites for Co-anchoring, followed by the two-step calcination to suppress the Co aggregation during the calcination process. The formation of the ultrahigh density of single dispersed Co-N

CRediT authorship contribution statement

Minzhi Ma , Zeai Huang and Ying Zhou designed the study. Zeai Huang, Wenjun Fa, Dmitry E. Doronkin, and Ying Zhou provided the support on experimental feasibility. Minzhi Ma, Zhiqiang Rao, Yanzhao Zou, Rui Wang, Yunqian Zhong, Yuehan Cao, Ruiyang Zhang performed characterization and analysis under the supervision of Zeai Huang and Ying Zhou. Minzhi Ma performed the MS calculation. Minzhi Ma wrote the paper. Zeai Huang, Dmitry E. Doronkin and Ying Zhou revised the paper. All authors contributed

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was financially supported by the Sichuan Provincial International Cooperation Project, China (2019YFH0164; 2021YFH0055). Y. Z. thanks for the Cheung Kong Scholars Program of China, China. We would like to thank the Institute for Beam Physics and Technology (IBPT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA). We acknowledge the KIT light source for the provision of instruments at the CAT-ACT beamline of the Institute of Catalysis Research and

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