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Perspective 2

Opportunities and Challenges in CO Reduction by Gold and Silver-based Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters Shuo Zhao, Renxi Jin, and Rongchao Jin ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01104 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Energy Letters

Opportunities and Challenges in CO2 Reduction by Gold and Silverbased Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters Shuo Zhao, Renxi Jin, and Rongchao Jin* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States Corresponding Author [email protected]

ABSTRACT. To tackle the excessive emission of greenhouse gas CO2, electrocatalytic reduction has been recognized as a promising way. Given the multi-electron, multi-product nature of the CO2 reduction process, an ideal catalyst should be capable of converting CO2 with high rates as well as high selectivity to either gas phase (e.g., CO, CH4) or liquid phase products (e.g., HCOOH, CH3OH, etc). Gold and silver-based materials have been extensively investigated as CO2 reduction catalysts for the formation of CO. This Perspective article focuses on the advances of gold and silver-based electrocatalysts for CO2 reduction in terms of catalyst design as well as some insights from theoretical investigations. In particular, a special emphasis is placed on the newly emerging, atomically precise metal nanoclusters for CO2 electroreduction. The strong quantum confinement effect and molecular purity, as well as the crystallographically solved atomic structures of nanoclusters make this new class of catalysts quite promising in fundamental studies, and valuable mechanistic insights for CO2 electroreduction at the atomic scale can be obtained. We hope that this Perspective highlights the opportunities and yet challenges in the exploration of the emerging nanomaterials.

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The heavy reliance on fossil fuels has led to a significant rise in atmospheric CO2, which is generally considered as the culprit for global climate change.1-4 To remediate CO2 emission, one of the strategies is to utilize CO2 as a feedstock for conversion into valuable chemicals and fuels. Despite the intense research efforts over the past few decades on catalytic conversion of CO2, the inertness of CO2 molecules still remains to be the bottleneck for industrial utilization of CO2 as a direct carbon feedstock. Thus, recent years have witnessed wide research interest in the design of more effective and efficient conversion approaches. Among the approaches, electrocatalytic CO2 conversion has received particular attention owing to the advantages such as ambient conditions of the reactions and controllable product formation at different voltages; thus, CO2 electrocatalytic conversion has been investigated to a great extent.5-8 As shown in Table 1, the CO2 electroreduction is a multi-proton and multi-electron process with several products formed at various conditions.9 In particular, the first step of CO2 reduction involves the formation of the key intermediate species, CO2•‒, which requires a large energy input with a high reduction potential of -1.90 V (vs NHE, normal hydrogen electrode) in aqueous solutions. Hydrogen evolution reaction (HER) is also present as a competing side-reaction in aqueous solutions. In order to facilitate efficient CO2 conversion, several major challenges remain to be solved, including the optimization of catalytic systems and electrolytes, etc. One of the most critical challenges in electrocatalytic CO2 conversion is the design of effective electrocatalysts with as small overpotentials as possible, large current densities, as well as high selectivity for the desired products. Table 1. Reduction potentials (vs. NHE) of selected reactions in CO2 reduction in aqueous solutions at pH 7, 25 oC and 1 atm. Adapted from ref 9. Copyright 2017 by CRC Press. Reactions

E (V vs. NHE), pH = 7 -0.41

2H+ + 2e‒ → H2 CO2 + e‒ → CO2•‒

-1.90

CO2 + 2H+ + 2e‒ → HCOOH

-0.61

CO2 + 2H+ + 2e‒ → CO + H2O

-0.53

CO2 + 4H+ + 4e‒ → HCHO + H2O

-0.48

CO2 + 6H+ + 6e‒ → CH3OH + H2O

-0.38

CO2 + 8H+ + 8e‒ → CH4 + 2H2O

-0.24



Metals have a quite long history in being used as catalysts for electrocatalytic CO2 reduction with good activity, and different metals show distinct selectivities of products. In 1985, Hori et al.10 performed electrocatalytic CO2 conversion using various bulk metals and four different groups of metals were classified according to the obtained products of HCOOH, CH4, CO, and H2, with the last group generating H2 only (with no CO2 reduction). Since then, metalbased materials have been utilized extensively as electrocatalysts for CO2 reduction, especially when new chemical synthetic advances are achieved, including the well-controlled nanoparticles and very recently the atomically precise nanoclusters. Among the metals, gold and silver show unique properties with high selectivity for the formation of CO. In addition, by tailoring the gold and silver-based catalysts and/or electrocatalytic conditions, syn-gas (a mixture of CO and H2) can be readily obtained in CO2 electroreduction and such a syn-gas can serve as a feedstock for industrial conversion to CH3OH, gasoline and diesel via the Fischer-Tropsch synthesis. In the past decade, various gold and silver-based materials have been explored extensively as effective catalysts for electrocatalytic reduction of CO2 to CO. In this Perspective, we summarize the experimental and theoretical developments of various gold and silver-based electrocatalysts for CO2 reduction, with a focus on how the surface engineering, morphology control, composition manipulation and support effect can tune the catalytic performance. To further enhance the efficiency of current gold and silver-based materials and pave the way for future design of more advanced catalysts, mechanistic understanding of CO2 reduction on gold and silver-based materials, especially at the atomic scale, is highly desirable and of critical importance. Hence, we particularly highlight the utilizations of a novel class of materials, atomically precise nanoclusters, for CO2 reduction to CO and propose our perspectives on this newly-emerging research area. Compared to regular nanoparticles⎯which are more or less polydispersed, atomically precise nanoclusters are molecularly pure, ultrasmall nanoparticles (from subnanometer to 3 nm) and exhibit strong quantum confinement effects due to ultrasmall sizes. Their molecular purity, as well as the crystallographically solved atomic structures, makes nanoclusters a well-defined platform for fundamental investigations of CO2 reduction, which may provide valuable mechanistic understandings for CO2 electroreduction at the atomic scale. Indeed, such nanoclusters have been reported to exhibit excellent performance in CO2 reduction. In the last section of the Perspective, we provide our thoughts on the potential opportunities and yet challenges in the utilization of 4 Environment ACS Paragon Plus

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ACS Energy Letters

gold and silver-based materials, especially novel nanoclusters, for CO2 electroreduction. More future work is to be carried out in order to further push the frontier of this research field. Metal electrodes. The utilization of gold and silver for CO2 electroreduction began a few decades ago when bulk gold and silver metal electrodes were systematically investigated by Hori et al in 1985.10 The activity and selectivity of bulk gold and silver electrodes for CO formation motivated early research efforts in modifying gold and silver electrodes for improving the CO2 electroreduction performance.11-15 Sputtering was applied to prepare gold electrode with different surface geometrical structures and areas for CO2 reduction catalysis.11 The gold-polymer electrode exhibited higher selectivity for CO than the gold plate electrode.12 The crystal facets of metal electrode can also influence the catalytic performance. For example, the Ag electrode with (110) facet showed higher current density compared to Ag(111) and Ag(100).13 Overall, gold and silver electrodes were the first catalysts for the electroreduction of CO2 to CO with high selectivity. However, the bulk form of metals remains to be improved for better activity. Nanostructures of gold and silver. With the prosperity of nanotechnology and advances obtained since the late 1990s, nanostructures have emerged as a new type of materials with unique advantages, such as large surface areas and high tunability in morphology as well as compositions. Various gold and silver-based nanostructures were synthesized and demonstrated as effective CO2 electroreduction catalysts. Surface engineering, morphology control, and composition manipulation to form alloy or coreshell nanostructures as well as the support utilization to build hybrids or composites can greatly tune the catalytic properties of nanostructures. Below we briefly discuss such efforts. 1). Surface engineering. The surface chemistry of catalysts is critical in heterogeneous catalysis. Tremendous research efforts have been focused on the surface engineering of catalysts to improve the catalytic performance by increasing surface area and introducing catalytic active sites.16-26 For instance, the flat surfaces of the bulk gold and silver electrodes show a limited activity in CO2 reduction, but surface modification to form nanostructures on the surface of the electrodes through reducing oxidized gold and silver electrodes is an effective strategy.16, 17 As shown in Figure 1a and 1b, the gold and silver electrodes consist of nanoparticles on the surfaces with some porosity after the reduction treatment, giving rise to the largely increased surface area and active sites. The catalytic performance of oxide-derived electrodes was compared to their 5 Environment ACS Paragon Plus


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polycrystalline bulk counterparts. Figure 1c shows the Faradaic efficiencies (FEs) of CO and HCOO‒ over oxide-derived and polycrystalline gold electrodes, where smaller overpotential and higher FE for CO were observed over oxide-derived gold catalysts. Similarly, the overall FE for CO on oxide-derived silver positively shifted by more than 0.4V compared to that of polycrystalline silver electrode, as shown in Figure 1d. Smaller Tafel slopes were obtained for both oxide-derived gold and silver electrodes compared to polycrystalline bulk electrodes, indicating a fast initial electron transfer to form the important intermediate CO2•‒ as represented in Figure 1e. Therefore, the surface engineering by oxide-reduction treatment not only improved the catalytic performance but also changed the reaction pathways. An oxygen plasma treatment can also introduce catalytically active defects and thus modify the flat surfaces of bulk gold and silver electrodes.18, 19 In addition, the grain boundary engineering on the surfaces of gold and silver catalysts has been demonstrated as an effective way of improving the activity as well as selectivity for CO2 reduction.20-22 A linear correlation between the grain boundary surface density and surface-area-normalized activity was observed experimentally.20 Theoretical analysis via density functional theory (DFT) calculations attributed the enhanced CO2 reduction features to the stabilization of COOH intermediates near the grain boundaries.21, 22 Besides the strategy of forming nanostructures on the surface of electrodes, molecular functionalization with monolayers of thiol-tethered ligands can also influence the CO2 reduction over gold electrodes.23 For nanomaterials, the surface can be modified by changing the protecting ligands. A molecular level surface ligand exchange from oleylamine to N-heterocyclic carbine changed the mechanistic pathways of CO2 reduction over gold nanoparticles and improved the FE for CO from 53% to 83%.24 Such an enhancement was attributed to the electron rich gold surfaces after ligand engineering with carbenes.

Despite different working principles of the techniques discussed above, the ultimate aim of surface engineering is to increase surface area and introduce active sites into the system for CO2 reduction with higher catalytic efficiency.


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ACS Energy Letters

Figure 1. SEM images of oxide-derived Au (a) and Ag (b) nanoparticles. (c) Catalytic performance of oxide Au catalysts and polycrystalline Au electrode at various potentials in 0.5 M NaHCO3 (pH = 7.2). (d) Catalytic performance of oxide Ag catalysts and polycrystalline Ag electrode at various potentials in 0.1 M KHCO3 (pH = 6.8). (e) Proposed mechanisms for CO2 reduction to CO on polycrystalline and oxide-derived Au catalysts. Adapted from refs 16 and 17. Copyright 2012 by the American Chemical Society and 2016 by John Wiley & Sons, Inc.

2). Morphology control. Surface low-coordinated atoms can serve as active sites in heterogeneous catalysis. Gold and silver nanostructures of different morphology contain different type of surface atoms in terms of coordination environments, which can greatly influence their catalytic features. Sun’s group reported monodispersed gold nanoparticles as effective catalysts for CO2 reduction.27 They proposed that the edges sites of gold nanoparticles can facilitate the formation of CO while the corner sites are active sites for hydrogen evolution. As the size of gold nanoparticles decreases, more low-coordinated gold atoms are present on the surface, which can promote H2 formation. Similar size dependency was also reported by the Cuenya group,28 where smaller gold nanoparticles showed products of decreased CO/H2 ratio. 7 Environment ACS Paragon Plus


Ag nanoparticles were reported as effective CO2 reduction catalysts, with 5 nm as the optimal size.29 The high tunability in morphology of nanomaterials provides diverse nanostructures with unique surface atom compositions. Nam’s group synthesized concave, rhombic dodecahedral gold nanocrystals (as shown in Figure 2a) and compared the CO2 reduction catalytic activity with gold nanocrystals of different morphology, such as cube and rhombic dodecahedron.30 As shown in Figure 2b, concave rhombic dodecahedral gold nanocrystals exhibit the highest FE for CO from -0.27 V to -0.97 V, which is ascribed to the existence of multiple high-index facets due to the unique morphology. Nanostructures of high dimensional morphology (i.e., 1D and 2D) can also catalyze the reduction of CO2.31-33 Sun’s group fabricated ultrathin gold nanowires which showed excellent CO2 reduction performance.31 Such 1D nanowires possessed a large ratio of edge atoms which can serve as the catalytically active sites for CO formation supported by DFT simulations. As displayed in Figure 2c and 2d, two dimensional triangular silver nanoplates were successfully synthesized and exhibit higher FE for CO compared to similar-sized nanoparticles, which is owing to the optimized ratio of edge-to-corner atoms as well as the existence of Ag(100) facets.32 Besides the tunable structures and properties, high dimensional nanostructures such as 1D nanowires31 and 3D porous framworks33 are self-supported, while zero dimensional nanoparticles usually require carbon or metal oxides as supports due to the dispersion issue. The surface edge atoms are demonstrated as active sites for CO2 reduction to CO by experimental and theoretical results on multiple gold and silver nanostructures.27,31,32 Therefore, manipulation of the ratio of surface edge atoms by morphology control can effectively tune the catalytic performance of gold and silver nanostructures for CO2 reduction.


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Figure 2. (a) SEM image of concave rhombic dodecahedral gold nanocrystals. (b) FE for CO of concave rhombic dodecahedral, rhombic dodecahedral, cubic gold nanocrystals and gold films. (c) TEM image triangular silver nanoplates. (d) FE for CO of triangular silver nanoplates (blue), similar sized silver nanoparticles (green), bulk silver electrode (red), carbon black (pink) and glass carbon electrode (black). Adapted from ref 30 and ref 32. Copyright 2015 and 2017 by the American Chemical Society.

3). Alloy nanostructures. Alloying has been recognized as another widely used method to tune the catalytic performance of nanomaterials. The addition of foreign atoms can modify the geometric and electronic structures of parent nanostructures and induce synergistic effects. As shown by Hori et al., different metals exhibited distinct selectivity for CO2 reduction and four groups of metals were classified.10 Alloying between metals from different groups may contribute to novel catalytic behavior.34-41 Yang’s group reported that the uniform Au/Cu nanoparticles showed composition-dependent selectivity in CO2 reduction (Figure 3).34 The addition of copper atoms into the gold nanoparticles increased the distribution of the obtained products, which is owing to the different binding strength of hydrogen and stabilization of the intermediates on the surface of the nanoparticles. Figure 3g shows the proposed mechanism for CO2 reduction over bimetallic Au/Cu nanoparticles with different compositions. The atomic packing modes can also greatly affect the catalytic performance of Au/Cu alloy nanoparticles. It was shown that atomic ordering transformation can further improve the Faradaic efficiency, which is ascribed to the formation of compressively strained, three-atoms-thick gold layers over the intermetallic core.35 Such a disorder-to-order transformation of individual Au/Cu 9 Environment ACS Paragon Plus


nanoparticles demonstrates the importance of atomic configurations in nanomaterials which can be used to tune the corresponding catalytic performance. Palladium (Pd) was also utilized to form Au/Pd alloy36, 37 and Au/Pd core@shell nanoparticles38 as well as trimetallic nanostructures such as AgPd-nanodendrite-modified Au nanoprism.39 The catalytic activity and selectivity for CO2 reduction can be tuned by controlling the shell thickness, composition as well as morphology.

Figure 3. (a) Total current density of Au-Cu bimetallic nanoparticles. FE for obtained products of Au (b), Au3Cu (c), AuCu (d), AuCu3 (e) and Cu (f). (g) Proposed mechanism for CO2 reduction on the surface of Au-Cu catalysts. Color code: C (grey), O (red), H (white). The relative intermediate binding strength is indicated by the stroke weight (on the top right corner). Additional binding between the COOH and the catalyst surface is presented as a dotted line. Arrows between the adsorbed COOH and adsorbed CO is to show the difference in probability of having COOH adsorbed on different types of surfaces. Colored arrows indicate the pathway to each product: red for CO, blue for formate and green for hydrocarbons. Larger arrows indicate higher turnover. Adapted from ref 34. Copyright 2014 by the Nature Publishing Group. 10 Environment ACS Paragon Plus

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ACS Energy Letters

Morphology control allows one to tailor the catalytically active site structures. Alloying is a universal method of tuning the geometric and electronic properties of parent nanostructures. The synergistic effect between different metals endows the alloy catalysts with novel catalytic features.

4). Supported nanocatalysts. For a heterogeneous catalytic reaction, good dispersion of catalysts is crucial for retaining the active sites. The introduction of supports can not only contribute to better dispersion and stabilization of the loaded nanostructures, but also create active interfaces and synergistic interaction between supports and gold/silver catalysts.20, 42-48 Carbon materials and metal oxides are popular support materials. Gold nanoparticles supported on carbon nanotubes20 as well as functional graphene nanoribbons42 were employed in CO2 reduction. The interaction between gold nanoparticles and nanoribbons improved the catalytic activity and longterm stability. The interface between gold and CeOx can facilitate the CO2 reduction to CO by stabilizing the important intermediates (*COOH).43 Such an interfacial effect was also manifested in the Ag/TiO2 system, indicating a universal way of tuning the catalytic performance.44, 45

Nanoclusters. Despite the excellent performance of various, conventional gold/silver nanostructures as electrocatalysts, the understanding of the CO2 reduction, especially at the atomic scale remains to be the bottleneck for further improving the catalytic performance. For such a reaction with multiple chemical processes and products, mechanistic understanding is very critical and yet challenging. Conventional nanostructures may be quite uniform at the nanoscale but are certainly polydispersed at the atomic scale, since no two nanoparticles contain the same number of atoms with the same structure. The slightly different structures, especially the surface environment, can affect the heterogeneous catalysis to a great extent. It has been recently demonstrated that even a single-atom difference in nanoclusters can induce significant changes in packing structures and accordingly the catalytic properties.49 The polydispersed nature of conventional gold and silver nanostructures makes the mechanistic investigation on electrocatalytic reduction of CO2 quite challenging and complicated. Thus, it is highly desirable 11 Environment ACS Paragon Plus


to have a more well-defined platform for achieving atomic level understandings on the catalytic mechanism, which would inspire chemists to further tune the composition and structure precisely with more controllable synthetic techniques. Recently, atomically precise thiolated-protected gold and silver nanoclusters have emerged as a new class of nanomaterials with promising applications in catalysis, sensing, optics, etc.50-53 Nanoclusters lie in the transition regime between few-atom clusters and plasmonic nanoparticles. Owing to the strong quantum confinement effects, nanoclusters exhibit unique properties such as the discrete electronic structure, different valence states of surface and core atoms, etc.50 The molecular purity and crystallographically solved atomic structures make them a very promising platform for achieving mechanistic understandings on the electrocatalytic CO2 reduction. Using the Au25(SR)18 nanocluster (SR=SCH2CH2Ph) as an example, this nanocluster consists precisely 25 gold atoms and 18 protecting thiolate ligands and was the first gold nanocluster utilized as catalysts for electroreduction of CO2.54 A reversible interaction between Au25(SR)18 nanocluster and CO2 was identified, which induced charge redistribution within the nanocluster upon interaction with CO2. Before the evaluation on their electrocatalytic CO2 reduction performance, gold nanoclusters were loaded on a carbon black support, which was achieved by stirring nanoclusters and carbon black in dichloromethane for 2 hours followed by drying over flowing nitrogen. The successful loading of nanoclusters onto carbon black support was verified by the clear supernatant. Under aqueous conditions, the FE for CO achieved approximately 100 % at the potential of -1.0 V (vs RHE) with a formation rate 7-700 times higher than conventional plasmonic Au nanoparticles and bulk gold electrodes as shown in Figure 4. In addition, the Au25(SR)18 nanocluster exhibited an overpotential as small as ~90 mV for the reduction of CO2, which outperforms that of plasmonic Au nanoparticles. The excellent catalytic performance of atomically precise Au25(SR)18 nanocluster provides exciting opportunities for the electrocatalysis of CO2 reduction over gold-based bulk or plasmonic nanoparticle materials. The Au25(SR)18 nanocluster exhibits multiple charge states, in which the negatively charged [Au25(SR)18]− can lose electrons to form oxidized species. The [Au25(SR)18]q species (q = -1, 0, +1) can serve as a unique platform to investigate the charge state dependent catalytic performance, which is apparently not possible for conventional nanostructures. Specifically, the negatively charged [Au25(SR)18]q nanocluster (q = -1) showed the best catalytic activity in CO2 reduction, followed by the neutral and positively charged counterparts.55 In 12 Environment ACS Paragon Plus

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ACS Energy Letters

practical processes, electrochemical CO2 reduction systems need to be integrated with renewable sources. As a proof of concept, currently commercially available renewable energy sources have been utilized to drive the reduction of CO2 catalyzed by Au25(SR)18 nanoclusters.56 Besides Au25(SR)18, Au137(SR)56 also showed CO2 reduction activity with a 6-fold increase in current in CO2 saturated solution compared to the case of N2-saturated solution.57 Such examples indicate the promise of gold nanoclusters for applications in CO2 reduction.

Figure 4. (a) UV-Vis and PL spectra of Au25(SR)18 nanoclusters in N2 and CO2 saturated DMF, respectively. (b) Difference spectra in (a). (c) X-ray structure of the Au25(SR)18 nanocluster with stable CO2 adsorption by DFT simulation. (d) Potential-dependent FE for CO and H2 over Au25(SR)18 nanoclusters. (e) Potential-dependent FE for CO over Au25(SR)18 nanoclusters, gold nanoparticles of 2 and 5 nm as well as bulk gold electrode. Adapted from ref 54. Copyright 2012 by the American Chemical Society. The stability of nanocluster catalysts is also important for practical applications. The Aun(SR)m nanoclusters are indeed thermodynamically stable sizes/structures that are produced via the harsh “size-focusing” synthesis.50 Kauffman et al.56 evaluated the long-term CO2 reduction performance of Au25(SR)18 nanoclusters. Figure 5 shows the catalytic performance over 6 days (6 h electrolysis each day). As shown in Figure 5a, 5b and 5c, Au25(SR)18 nanoclusters exhibit relatively durable performance within 6 days of electrolysis experiments. 13 Environment ACS Paragon Plus


After the long-term evaluation, TEM was utilized to examine the stability of Au25(SR)18 nanoclusters. Figure 5d and 5e show the TEM images of cluster catalysts loaded on a carbon black support. Larger sintered gold nanoparticles and individual Au25(SR)18 nanoclusters coexist on the support, which indicates that the majority of clusters can still retain the size/morphology after long-term electrolysis.

Figure 5. Day-to-day (a) product formation rates, (b) cumulative turnover number (TON, mol of CO/(mol of Au25)), and (c) Faradaic efficiency during a 36 h CO2 reduction experiment over Au25 nanocluster catalysts. (d) and (e) TEM images of Au25/CB after a 36 h CO2 reduction experiment. Adapted from ref 56. Copyright 2015 by the American Chemical Society.

Based on the crystallographically solved atomic structures of nanoclusters, theoretical simulations can reveal important mechanistic insights into the CO2 reduction over gold/silver nanoclusters. Alfonso et al. investigated the active sites of Au25(SR)18 nanoclusters for CO2 electroreduction using the DFT method.58 They compared free energy diagram of the fullyprotected Au25(SR)18 nanoclusters as well as singly dethiolated Au25(SR)17 nanoclusters for CO2 electroreduction to CO. Figure 6a and 6b compare the energy required for the formation of 14 Environment ACS Paragon Plus

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ACS Energy Letters

important intermediate COOH species over Au25 nanoclusters, where the fully protected Au25 nanoclusters show a larger energy barrier (i.e., 2.04 eV) than that of singly dethiolated species (0.34 eV). DFT calculations suggest that the reduction of CO2 to CO is more favorable over singly dethiolated Au25(SR)17 nanoclusters due to the better stabilization of the intermediate COOH species. Figure 6c and 6d are the proposed mechanism of CO2 reduction to CO over the fully-protected Au25(SR)18 as well as singly dethiolated Au25(SR)17 nanoclusters. Overall, nanoclusters can serve as unique electrocatalysts for CO2 reduction with excellent performance as demonstrated in experiments. Such materials also serve as excellent models for theoretical studies. On a note, research on gold nanocluster catalysts thus far only shows the CO product from CO2 electroreduction without the formation of other hydrogenated products. However, we expect that future work on Au-M (where, M represents a second metal) bimetal nanoclusters may lead to different products of CO2 electroreduction. Furthermore, such bimetal nanocluster systems will offer new opportunities for mechanistic understanding on the synergism for controlling the product selectivity.

The identification of the active sites for CO2 reduction is a grand challenge. However, without well-defined nanocatalysts, it would not be possible to pinpoint the active sites. With the advent of atomically precise and structurally characterized nanocluster catalysts, it offers exciting opportunities for investigating the catalytic properties at the single atom, single electron level.



Figure 6. Free energy diagram for CO2 electroreduction to CO over fully-protected Au25(SR)18 nanoclusters (a) and singly dethiolated Au25(SR)17 nanoclusters (b) at T=298 K, pH=0 and zero applied potential. Proposed mechanism for CO2 electroreduction to CO over fully-protected Au25(SR)18 nanoclusters (c) and singly dethiolated Au25(SR)17 nanoclusters (d). Adapted from ref 58. Copyright 2016 by the American Institute of Physics.

Opportunities and challenges. Ultrasmall, atomically precise metal nanoclusters of gold and silver are attracting intense research interest recently,50,53 and the application of these novel materials in electrocatalysis is a newly emerging research area.50,54-58 Molecular-like properties and atomic precision as well as the unique structures endow nanoclusters with high activity and selectivity for electrocatalytic reduction of CO2. The surface ligand can protect the nanoclusters from aggregation during electrocatalysis as demonstrated by the long-term activity.56 Recent 16 Environment ACS Paragon Plus

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ACS Energy Letters

prosperity in the synthesis of gold and silver nanoclusters has led to a library of nanoclusters with different sizes, atomic structures, surface motifs, and compositions (e.g., alloys and intermetallics).50 With the X-ray crystallographically solved atomic structures as well as high tunability in terms of structure and composition, metal nanoclusters are expected to serve as well-defined platforms for experimental and theoretical investigations of the dependence of various important parameters in CO2 electroreduction. Eventually, it is hoped to achieve better mechanistic understanding at the single-atom and single-electron level. All these exciting opportunities for CO2 electroreduction are rooted in the unique nanoclusters which lie in the transition regime between small molecules and regular nanoparticles. 1). Atomic-level surface engineering and morphology control: The surface engineering and morphology control over nanoclusters can be obtained with unprecedented precision, which provides exciting opportunities for CO2 reduction catalysis. For example, the surface ligand of the nanoclusters can be tuned experimentally. Nanoclusters with the same structure but different ligands can be utilized in future work to study the effect of surface ligand on catalytic performance. Nanocluster isomerization is also an exciting topic and makes it possible to investigate the influence of atomic structures.59 A series of gold nanoclusters of different sizes protected by the same ligand can help further reveal the precise size-dependent performance.60 Morphology can be also tuned at the atomic level for the first time, such as the Au25 nanosphere and nanorod clusters.50 The versatile nature of the nanocluster family with atomic precision and well-defined structures makes atomic-level surface engineering and morphology control possible, which will greatly benefit the electrocatalysis of CO2 reduction over gold and silver-based catalysts. 2). A well-defined platform for investigating the synergistic effects: The polydispersed nature of conventional nanomaterials severely hampers further investigation on the synergistic effect in alloy and supported nanostructures, especially due to the lack of information at the atomic scale. Thus, many important questions or issues remain unclear, for example, why can similar Au/Cu alloy nanoparticles exhibit distinctly different selectivity in CO2 reduction?34, 35 How does the packing structure of atoms (ordered vs. disordered) greatly affect the catalytic features? How can the interface facilitate the catalysis? Addressing such and many other fundamental questions requires atomically precise systems such as nanoclusters as well-defined platforms to understand the synergistic interactions between different metal atoms and between support and 17 Environment ACS Paragon Plus


nanostructures. Research progress has created a library of doped nanoclusters, such as MxAu25x(SR)18,

M=Pd, Pt, Cu, Ag, Cd, Hg, etc.50,61-64 In general, nanoclusters are very sensitive to the

dopants. The introduction of foreign metal atoms can bring significant changes to the parent nanoclusters in terms of electronic structure and catalytic reactivity. Bimetallic nanoclusters usually exhibit distinct absorption features different from their parent homogold nanoclusters. Foreign atoms may also change the charge state of the nanoclusters. As demonstrated in the case of plasmonic Au/Cu nanoparticles, the addition of Cu into gold nanoclusters is expected to greatly tune the selectivity of the products.34 With Cu atoms incorporated, the nanoclusters may facilitate the formation of hydrogen-containing products such as formic acid. The chemical doping with Pd atoms leads to the formation of the PdAu24(SR)18 nanocluster with a single Pd atom in the center of the cluster, and such a system deserves to be investigated in the future work. Considering the similar surfaces between the PdAu24(SR)18 and Au25(SR)18 nanoclusters, any difference in CO2 reduction performance would be correlated to the influence of the central atom and hence provides insights into the synergistic mechanism. Therefore, with such well controlled bimetal nanocluster systems, the alloying and synergistic effects can be studied precisely in future work. 3). Realistic simulations for mechanistic understanding: Previous theoretical investigations were based on assumed models such as face-centered cubic (fcc) structures.27, 31, 65-67 However, the realistic atomic packing modes may not be fcc, especially when the size of the nanostructures shrinks to the transition regime between few-atom clusters and plasmonic nanoparticles. Indeed, several novel crystalline structures in gold nanoclusters have been discovered, such as hexagonal close packed and body-centered cubic structures,50 as opposed to the exclusive fcc structure in bulk gold and regular nanoparticles. Therefore, revealing the atomic structure can further improve the accuracy of theoretical analysis with more realistic representations of the atomic packing modes in the simulations. For example, in the DFT calculations of Au25 nanoclusters for CO2 reduction, the total structure of Au25, which consists of an icosahedral Au13 core protected by six dimeric Au2(SR)3 motifs, was adopted rather than the fcc structure.54,58 The experimentally solved structures can provide precise atomic packing modes, especially the surface environment which is critical for heterogeneous catalysis. Therefore, the availability of atomic structures of metal nanoclusters and heteroatom distribution in bimetal nanoclusters will


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significantly contribute to realistic simulations and thus better understanding of CO2 electroreduction can be achieved in future work. Challenges are certainly present in this newly-emerging field of nanocluster catalyzed CO2 electroreduction. Crystallographically solving the total structures of large nanocluster is still quite challenging.68 Easy synthetic routes with high yield are highly desirable for practical applications. In addition, introducing cheap metals into gold nanoclusters to form alloy nanoclusters also deserves future efforts, which can decrease the amount of noble metal consumption yet without losing much of the catalytic activity. Overall, atomically precise nanoclusters hold great promise in fundamental studies of catalysis and future work is expected to further push the development and prosperity of this newly emerging research area.

AUTHOR INFORMATION [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT: R.J. acknowledges funding support from the Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154) and the National Science Foundation (DMREF-0903225).

Biographies Shuo Zhao is a Ph.D. candidate in chemistry under the supervision of Prof. Rongchao Jin at Carnegie Mellon University. He obtained his B.S. in chemistry from University of Science and Technology of China (USTC, Hefei, China) in 2013. His research is focused on the electrocatalysis with precious metal nanoclusters. Renxi Jin was a visiting graduate student in the Jin group at Carnegie Mellon University. He obtained his B.S. from Changchun University of Science and Technology in 2011 and then



pursued his Ph.D. (2011-2016) at Northeast Normal University under the guidance of Prof. Yan Xing. Rongchao Jin is Professor of Chemistry at Carnegie Mellon University. He received his B.S. in chemical physics from University of Science and Technology of China (USTC, Hefei, China) in 1995; his M.S. in physical chemistry/catalysis in 1998 from Dalian Institute of Chemical Physics, Chinese Academy of Sciences; and his Ph.D. in chemistry from Northwestern University (Evanston, IL) in 2003. He then performed postdoctoral research at the University of Chicago. He joined the chemistry faculty of Carnegie Mellon University in 2006 and was promoted to Associate Professor in 2012 and Full Professor in 2015. His current research interests include atomically precise nanoparticles, optics of nanoparticles, and catalysis.

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