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