Elsevier

Journal of Catalysis

Volume 387, July 2020, Pages 95-101
Journal of Catalysis

The critical role of hydride (H) ligands in electrocatalytic CO2 reduction to HCOOH by [Cu25H22(PH3)12]Cl nanocluster

https://doi.org/10.1016/j.jcat.2020.04.011Get rights and content

Highlights

  • A new mechanism was unveiled for CO2 reduction by [Cu25H22(PH3)12]Cl nanocluster.

  • The presence of hydride ligands selectively promotes CO2 reduction to HCOOH.

  • The surface Cu is catalytically very reactive for HCOO* binding and HCOOH formation.

  • The competitive HER is suppressed as compared to the more favorable HCOOH formation.

Abstract

Copper is the prototype metal to produce hydrocarbons from CO2 electroreduction, however, rationale of the catalytic mechanism by copper nanoparticles has been severely impeded due to structural polydispersity. The recent efforts in the synthesis of nanosized copper clusters open up new opportunities for atomic-level understanding of the catalytic mechanism. Herein, we considered [Cu25H22(PH3)12]Cl cluster, bearing the highest ratio of hydride ligands, as a test model to examine its activity for CO2 electroreduction. Our DFT calculations showed that both the surface Cu and the hydride ligands are actively participated in the hydrogenation reactions and selectively reduce CO2 to HCOOH via the proton-reduction channel and the hydride-proton channel, as opposed to the commonly produced CO on copper surface/nanoparticles. We also evaluated the competitive HER activity, and found that H2 evolution requires higher barrier than HCOOH. Our studies highlight the important role of hydrides in directing the activity and selectivity for CO2 electroreduction.

Introduction

The increasing emission of CO2 into atmosphere causes lots of issues to our earth planet and brings us huge challenges such as urgent energy crisis. A promising way is to recycle CO2 waste into carbonaceous fuels while reducing carbon emissions [1], [2]. Among all those methods developed for CO2 reduction, the electrochemical reduction represents a promising path since the reaction is operated under ambient conditions, and the reduction product can be effectively tuned by the electrochemical potential [3], [4], [5].

For transition-metal based electrocatalysts, copper is very special since it is the only metal that can produce significant amounts of hydrocarbons [6]. Although the catalytic efficiency of copper is not so satisfactory, copper is still seen by far as the prototype electrocatalyst to validate the mechanism of CO2 conversion [7]. This has triggered a lot of research interests to study the catalytic behavior of CO2 reduction on copper nanoparticles [8], [9], [10], [11], [12], [13], [14], [15]. The morphology, geometry, surface orientation and particle size effect have been demonstrated to greatly affect the catalytic activity and selectivity [16], [17], [18], [19]. However, the detailed mechanistic understanding towards CO2 conversion by copper nanoparticles are still lacking, owing to their structural polydispersity.

Recently, the atomically precise metal clusters, which bridge the gap between metal-complexes and large nanoparticles, have attracted a great deal of attentions for screening and developing homogeneous or heterogeneous sub-nanometer cluster electrocatalysts [20]. Different from the bulk metal surfaces and larger nanoparticles, the ligand-protected atomically precise metal clusters exhibit size-dependent compositions, electronic, chemical and catalytic properties. Particularly, their structural nature with atomic precision allows for the establishment of structure-property relationships and developing roles for the design of advanced nanocatalysts of energetic importance. For example, electrochemical reduction of CO2 to CO by using [Au25(SR)18] clusters has been reported by Jin et al. [21] showing that [Au25(SR)18] demonstrated a remarkable improvement in activity over larger Au nanoparticles and bulk Au. More recently, Zheng and co-workers reported that thiolated copper-hydride nanocluster [Cu25H10(SPhCl2)18]3− can readily catalyze the hydrogenation of ketones to alcohols at room temperature [22], which is contrary to the traditional assumption that the stabilizing ligands would block the active site and poison the catalyst. We should note that although the copper-hydrides have demonstrated exciting catalytic opportunities for hydrogenation reactions, the catalytic applications of the well-defined copper nanoclusters for electrocatalytic CO2 reduction have been rarely pursued. Given by the experimental advances in the synthesis of a rich number of nano-sized copper-hydride clusters [23], [24], e.g., [Cu14H12(phen)6(PPh3)4][X]2 [25], [Cu18H7{1,2-S(C6H4)PPh2}10(I)] [26], [Cu20H11{S2P(OiPr)2}9] [27], [Cu25H22(PPh3)12]Cl [28], [Cu28H15(S2CNR)12]PF6 [29], and [Cu25H10(SPhCl2)18][PPh4]3 [22], it is of great fundamental and technological interest to investigate the catalytic behaviors of these sub-nanometer copper clusters. This has motivated us to perform molecular-level mechanistic studies on the electrocatalytic activity of CO2 reduction by the atomically precise copper nanoclusters.

In this work, we select one representative copper nanocluster, [Cu25H22(PPh3)12]Cl (the experimental PPh3 group was simplified as PH3 for computational feasibility) [28], as the test model to investigate its electrocatalytic activity for CO2 reduction via density functional theory (DFT) calculations. The [Cu25H22(PH3)12]Cl is a 2e super-atomic cluster, which has the largest number of hydrides reported so far for the structurally resolved copper clusters. For the electrocatalytic application of [Cu25H22(PPh3)12]Cl cluster, there are several interesting questions to address. Since hydride is relevantly important for hydrogenation reactions, does it also play a role during electrochemical CO2 conversion? The presence of capping ligands is usually a barrier for surface catalysis, will the surface Cu still be active and contribute to the reaction mechanism? Moreover, the hydrogen evolution (HER) is a competing reaction with CO2 electroreduction (CO2RR), how is the selectivity of CO2RR vs. HER by [Cu25H22(PPh3)12]Cl?

Section snippets

Computational methods

The spin-polarized DFT calculations were performed to investigate the electrocatalytic activity of [Cu25H22(PH3)12]Cl nanocluster for CO2 reduction by using the Vienna ab initio simulation package (VASP) [30]. The ion-electron interaction is described with the projector augmented wave (PAW) method [31]. Electron exchange-correlation is represented by the functional of Perdew, Burke and Ernzerhof (PBE) of generalized gradient approximation (GGA) [32]. A cutoff energy of 400 eV was used for the

Structure and Bader charge analysis of [Cu25H22(PH3)12]Cl nanocluster

The DFT optimized atomic structure of [Cu25H22(PH3)12]Cl was shown in Fig. 1a (the structure bond lengths are shown in Fig. S1), with the hydride ligands in different coordination modes colored in green, dark purple, and blue (the Cl counterion is omitted in Fig. 1). The 25 Cu atoms can be viewed as a Cu13-icosahedra core capped by four Cu3 triangles located at the tetrahedral sites. The 22 hydrides are all located at the surface capping sites, which can be clustered into 12 tri-coordinated μ3

Conclusion

In summary, we performed DFT studies to investigate the mechanism of CO2 electroreduction on a atomically precise copper-hydride nanocluster, [Cu25H22(PH3)12]Cl. We found that this cluster produces HCOOH as the main reduction product, where the surface Cu and hydride ligand play a crucial role in stabilizing the HCOO* intermediate. The HCOOH is formed via the proton-reduction mechanism or hydride-proton mechanism, the two routes have comparable thermodynamic and kinetic feasibility. In

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.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (No.21903008) and Chongqing human resources and Social Security Bureau (cx2019141).

References (40)

  • J. Fang et al.

    Coord. Chem. Rev.

    (2016)
  • X. Liu et al.

    Coord. Chem. Rev.

    (2018)
  • G.A. Olah et al.

    J. Am. Chem. Soc.

    (2011)
  • J.F. Huang et al.

    Nat. Commun.

    (2018)
  • E.E. Benson et al.

    Chem. Soc. Rev.

    (2009)
  • D.T. Whipple et al.

    J. Phys. Chem. Lett.

    (2010)
  • R. Francke et al.

    Chem. Rev.

    (2018)
  • Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc., Faraday Trans. I 85 (1989)...
  • D. Raciti et al.

    ACS Energy Lett.

    (2018)
  • A.A. Peterson et al.

    Energy Environ. Sci.

    (2010)
  • K.P. Kuhl et al.

    Energy Environ. Sci.

    (2012)
  • K.J.P. Schouten et al.

    J. Am. Chem. Soc.

    (2012)
  • A. Loiudice et al.

    Angew. Chem. Int. Ed.

    (2016)
  • K.D. Yang et al.

    Angew. Chem. Int. Ed.

    (2017)
  • Y. Huang et al.

    ACS Catal.

    (2017)
  • C. Hahn et al.

    Proc. Natl. Acad. Sci. USA

    (2017)
  • H. Zhang et al.

    Nat. Commun.

    (2019)
  • O.A. Baturina et al.

    ACS Catal.

    (2014)
  • R. Reske et al.

    J. Am. Chem. Soc.

    (2014)
  • K. Manthiram et al.

    J. Am. Chem. Soc.

    (2014)
  • Cited by (0)

    View full text