Recent advances in the synthesis and catalytic applications of ligand-protected, atomically precise metal nanoclusters

Accepted Manuscript Title: Recent advances in the synthesis and catalytic applications of ligandprotected, atomically precise metal nanoclusters Author: Jun Fang, Bin Zhang, Qiaofeng Yao, Yang Yang, Jianping Xie, Ning Yan PII: DOI: Reference:

S0010-8545(16)30006-6 http://dx.doi.org/doi: 10.1016/j.ccr.2016.05.003 CCR 112253

To appear in:

Coordination Chemistry Reviews

Received date: Accepted date:

5-1-2016 13-5-2016

Please cite this article as: Jun Fang, Bin Zhang, Qiaofeng Yao, Yang Yang, Jianping Xie, Ning Yan, Recent advances in the synthesis and catalytic applications of ligand-protected, atomically precise metal nanoclusters, Coordination Chemistry Reviews (2016), http://dx.doi.org/doi: 10.1016/j.ccr.2016.05.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Recent advances in the synthesis and catalytic applications of ligand-protected, atomically precise metal nanoclusters Jun Fanga,b,1, Bin Zhangb,1, Qiaofeng Yaob, Yang Yang,a Jianping Xieb,* and Ning Yanb,* a

State Key Laboratory of Materials-Oriented Chemical Engineering, College of

Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009 (P.R. China) b

Department of Chemical and Biomolecular Engineering, National University of

Singapore, 4 Engineering Drive 4, 117585, Singapore 1

These authors contributed equally to this review.

*Corresponding Author,

E-mail:

[email protected] (N. Y.) [email protected] (J. X.)

Tel.: +65 65162886 Fax: +65 67791936

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Highlights: 

Catalytic applications of ligand protected metal nanoclusters are summarized;



Structure-catalytic activity correlations are discussed;



Most recent advances in the field of atomically precise are introduced—with ca. 70 research articles in the years between 2014 and 2015;



Personal perspectives on future challenges and directions are also provided.

Graphical Abstract

Abstract Due to their excellent catalytic properties under mild reaction conditions, well-defined, nanosized noble metal catalysts have potential applications in the manufacture of fine chemicals, pharmaceuticals, and food additives. Ligand stabilized MnLm NCs (M: noble metal; n: number of metal atoms, n < 150; L: ligand; m: number of ligands) have a long history in catalysis, but recent advances in synthetic strategies and instrumental characterization have led to a renaissance in the catalytic applications of MnLm NCs. Thus, NCs can serve as model catalysts for understanding fundamental

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aspects of catalysis, but they also exhibit interesting properties in practical applications. MnLm NCs such as the recently investigated thiolated capped Au NCs have unique geometric structures with ultra-small size and strong quantum confinement, both of which are lacking in larger noble metal nanoparticles (>2 nm). The correlations among the catalytic performance of MnLm NCs with the size, structure, and composition of individual NCs at the atomic level can be demonstrated in realistic ambient reaction conditions, thereby contributing to the rational design of highly active catalysts with novel properties. Our current understanding of these newly emerging catalytic NCs is still in its infancy, but some studies have shown their potential for promoting new types of reactions. This review summarizes recent exciting advances in this field (since 2010), especially the catalytic properties of noble metal NCs in the presence of the ligand shell and after removing the ligand.

Keywords: catalysis; ligand; nanocluster; support; structure-activity correlation. Abbreviations:

1

O2,

singlet

N,N-4-methyldiphenylamidinate; biicosahedral-structural

oxygen; ATR,

AC,

activated

attenuated

total

[Au25(PPh3)10(SC12H25)5Cl2]2+;

carbon;

ArNC(H)Nar,

reflectance; Au25-i,

Au25-bi,

icosahedral

[Au25(SCH2CH2Ph)18]–; BN, boron nitride; BSA, bovine serum albumin; C12SH, 1-dodecanethiol; CB, carbon black; CJ, "Crown-Jewel"; CNT, carbon nanotube; DFT, density functional theory; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; dppe, 1,2-bis(diphenylphosphanyl)ethane; dppm, bis(diphenylphosphanyl)methane; DRIFT-IR,

diffuse

reflectance

infrared

fourier

transform;

EDTA, 3

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ethylenediaminetetraacetic acid; EDX, energy-dispersive X-ray spectroscopy; ESI-MS, electrospray ionization mass spectrometry; FT-EXAFS, fourier transform-extended X-ray absorption fine structure; FTIR, fourier transform infrared spectroscopy; GMC, mesoporous carbon; GO, graphene; GSH, glutathione; H2-C3H8 SCR, H2-assisted selective catalytic reduction of NO with propane; HAADF, high angle annular dark field; HAP, hydroxyapatite; Hdppa, N,N-bis(diphenylphosphino)amine; HMS, hexagonal mesoporous; HPCSs, hierarchically porous carbon nanosheets; HRTEM, high-resolution transmission electron microscopy; ICRMs, interfacially cross-linked reverse micelles; L-H, Langmuir−Hinshelwood; LSVs, linear sweep voltammograms; LUMO, lowest unoccupied molecular orbital; MAA, mercaptoacetic acid; MALDI-TOF-MS, matrix-assisted laser desorption/ionization-mass spectrometry; MCF, mesostructured cellular foam; MFI, mordenite framework inverted; MHA, 6-mercaptohexanoic acid; mSiO2, mesoporous silica; MvK, Mars-van Krevelen; NCs, nanoclusters; NPs, nanoparticles; NP-TNTAs, nanoporous layer-covered TiO2 nanotube arrays; ORR, O2 reduction reaction; P25, P25 typed TiO2; PAGE, polyacrylamide gel electrophoresis; PAMAM, poly(amidoamine); PAMAM-OH, PAMAM dendrimer with hydroxyl surface groups ; PEC, photoelectrochemical; p-MBA,

p-mercaptobenzoic

acid;

tris[2-(diphenylphosphino)ethyl]phosphine;

PNP, PPh3,

4-nitrophenol;

PP3,

triphenylphosphine;

PPhpy2,

bis(2-pyridyl)-phenylphosphine; PtCl4, platinum(IV) chloride; PVA, polyvinyl alcohol; PVP,

poly(N-vinyl-2-pyrrolidone);

SAdm,

adamantanethiol;

SC2H4Ph,

phenylethylthiol; SC4H9, tert-butyl thiol; S-c-C6H11, 1-cyclohexanethiol; S-Eind, 4

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1,1,3,3,5,5,7,7-octaethyl-s-hydrindacene-4-thiol; SH, thiol group; SHE, standard hydrogen electrode; SPh, thiophenol; SPh-t-Bu, 2-methyl-2-propanethiol; SPO, secondary phosphine oxide; S-t-Bu, 2-methyl-2-propanethiol; STEM, scanning transmission electron microscopy; TBBT, 4-tert-butylbenzenthiol; TBHP, tert-butyl hydroperoxide; TEG, triethylene glycol; TEM, transmission electron microscopy; TGA, thermogravimetric analysis; THF, tetrahydrofuran; TOA, tetraoctylammonium; TPM G4 dendrimer, tetraphenylmethane-core phenylazomethine dendrimer G4; UV-Vis, ultraviolet-visible; XANES, X-ray absorption near edge structure; XPS, X-ray

photoelectron

spectroscopy;

XRD,

X-ray

diffraction;

xy-xantphos,

4,5-bis[bis(3,5-dimethylphenyl)phosphino]-9,9-dimethylxanthene.

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Contents 1. Introduction .................................................................................................................................7 2. Physicochemical properties of MnLmNCs: a brief overview ..................................................10 3. Synthesis of metal cluster catalysts ..........................................................................................12 3.1. Synthesis of ligand-protected metal NCs catalysts ..........................................................12 3.1.1. Phosphine-protected MnLm NCs. ..................................................................................12 3.1.2. Thiolate-protected MnLm NCs. .....................................................................................15 3.1.3. Dendrimer-protected MnLm NCs. .................................................................................18 3.1.4. Other ligand-protected Mn NCs. ...................................................................................20 3.2 Synthesis of ligand (partially) removed metal NC catalysts ............................................21 4. Catalysis by MnLm NCs.............................................................................................................25 4.1. Catalysis by ligand-protected MnLm NCs .........................................................................26 4.1.1. Oxidation reactions ......................................................................................................26 4.1.2. Hydrogenation ..............................................................................................................34 4.1.3. Electrocatalysis and photocatalysis .............................................................................40 4.1.4. Other reactions .............................................................................................................46 4.2. Catalysis by ligand removed/partially removed MnLm NCs ...........................................49 4.2.1. Oxidation reactions ......................................................................................................50 4.2.2. Selective hydrogenation ...............................................................................................59 4.2.3. Photocatalysis ...............................................................................................................61 5. Outlook .......................................................................................................................................65 Acknowledgments .........................................................................................................................67 References ......................................................................................................................................69

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1. Introduction One of the most important discoveries in catalysis is the excellent catalytic properties of noble metals when their size is reduced to nanoscale, i.e., NPs. This has led to the rapid development of well-defined noble metal catalysts with various applications in the last two decades. A landmark event was the discovery of the superior activity of a supported nano-Au catalyst for CO oxidation at low temperature [1]. Subsequently, metal NPs were studied extensively in many reactions, which confirmed their excellent size-dependent catalytic performance [2-6]. In general, NPs have superior properties compared with traditional bulk materials because their significantly enlarged surface-to-volume ratio yields more active sites and they have modified surface geometric/electronic properties at nanoscale [7, 8]. By engineering the size and morphology [9-17], robust nano-catalysts can be obtained with excellent activity and selectivity. Nevertheless, it is still difficult to obtain a fundamental understanding of catalysis by NPs, partly due to the limitations of existing protocols for noble metal NP synthesis. Conventional NPs [18] (e.g., samples prepared by the impregnation method) often have nonuniform particle size distributions, so it is very difficult to relate the overall catalytic performance with the intrinsic properties of individual NPs. Therefore, designing novel nanocatalysts with precisely controlled particle size and structure would be highly desirable to overcome these obstacles. In the late 1970s and early 1980s, ligand (mainly CO and phosphines) protected, atomically precise noble metal NCs (Ir, Ru, etc.) attracted intense research interest due to their well-defined structures (often referred to as Mx(CO)y where M: metal, CO:

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carbonyl group, x,y: number of atoms). For instance, carbonyl complexes exhibit catalytic activity in several hydrogenation reactions [19-24]. However, the catalytic activity is often poor due to the inhibitory effect of CO. Various chemical events are observed

during

catalysis,

including

cluster

decomposition

[25],

cluster

transformation [26], and decomposition induced aggregation [27], thereby making the identification of catalytically active species challenging. In fact, it is more appropriate to refer to these complexes as precatalysts because the CO ligand must be removed in many cases, at least partially, prior to catalysis. In recent years, the rapid development of atomically precise noble metal NCs (e.g., Au, Ag, Cu, and Pt) stabilized mainly by thiolate or phosphate ligands has yielded a library of new model precatalysts with specific and uniform nanostructures. In general, noble metal NCs are defined as particles with a diameter less than 2 nm (excluding the ligand shell), or a metal core of <150 atoms [28, 29]. NCs are expected to have appealing catalytic properties due to their unique geometric and electronic structure. First, the number of atoms in one NC is far lower than that in a conventional NP, and the atomic packing structure is completely different from that in NPs and bulk materials, thereby exposing a large surface area and enriching the abundance of low-coordination atoms at the surface. Second, the ultra-small sized NCs have quantized energy levels, so the reactant can be activated as the electrons are excited from or to the quantized energy levels, which cannot be achieved by NPs due to their metallic property [30]. Third, the NCs exhibit dynamic structural features under working conditions [31], i.e., the geometric and electronic properties of NCs are 8

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readily changed by the activation of NCs during the reactions, thereby resulting in specific reaction pathways. This flexibility of NCs facilitates the exploration of novel catalytic processes. Previous reviews have summarized major studies of noble metal NCs stabilized by various ligands such as CO, phosphate, and phenanthroline [32, 33]. Many fundamental issues have been discussed in detail, including NC stabilization and stabilizer ranking criteria. In recent years, several excellent reviews have considered newly emerged noble metal NCs, especially thiolated stabilized Au NCs, but they focused on the synthesis and various applications of NCs, whereas catalysis was not discussed in detail, or only limited examples were included [29, 34, 35]. Thus, the present review summarizes recent advances in the synthesis of ligand-protected MnLm (M: metal atom, n: number of metal atoms per NC, L: ligand, m: number of ligands per cluster) NCs, and more importantly, the catalytic applications of MnLm NCs (Fig. 1). Other properties of NCs, such as chirality, [36-38] magnetism [39, 40], and luminescence [41, 42], have been discussed in other reviews [29, 34], and they are outside the scope of the present review. The uniform nanostructure of NCs facilitates the precise correlation of structure with catalytic properties. The study of atomically precise NCs protected by various ligands in catalysis is still in the early stage, but a current hypothesis is that these recently obtained NCs will be highly beneficial for catalysis because they are atomically precise and can be structurally characterized well at the atomic level [35]. These well-defined nanocatalysts are expected to provide exciting opportunities for 9

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fundamental catalysis research and they have great potential in real applications. Nevertheless, it should noted that NCs might not necessarily be the actual catalysts, but instead they may only be precatalysts because studies of Mx(CO)y demonstrated that these clusters often decompose/deform during reactions [25, 26].

2. Physicochemical properties of MnLmNCs: a brief overview Elucidating the crystal structures of MnLm NCs is of great importance for exploring the unique properties of various catalysts, although crystallizing NCs to determine their overall structure remains a major challenge [43-49]. X-ray crystallography and computational simulation studies have clarified the overall structures of many atomically precise MnLm NCs. However, unlike the conventional metal NPs, which are face-centered cubic packed, most of the known NCs possess a core-shell structure [43-47, 50-58]. The very first crystal structure determined by Kornberg et al. was Au102(p-MBA)44 [43], which comprises a central Marks decahedron core made of 49 Au atoms, two 20-atom caps with C5 symmetry on opposite poles, and a 13-atom equatorial band. Each p-MBA ligand is reported to bind two Au atoms and the resulting bridge conformation forms a layer that stabilizes the cluster. Later, Zhu et al. [46] and Heaven et al. [47] independently reported the X-ray structure of the best studied NC, Au25(SR)18 (SR refers to a thiol ligand, Fig. 2A–B), which possesses an icosahedral Au13 core capped by six dimeric Au2(SR)3 staple-like motifs anchored on 12/20 Au3 facets of the icosahedral Au13 core. In 2010, Qian et al. reported the X-ray crystal structure of Au38(SC2H4Ph)24 [59], which comprises a 10

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face-fused Au23 bi-icosahedral core, with three monomeric Au(SR)2 staples capped at the waist of the Au23 rod and six dimeric Au2(SR)3 staples capped on the opposite poles. Currently, the overall X-ray crystal structures of MnLm NCs are still limited to several Au and Ag NCs due to the difficulty crystallizing NCs [29, 34]. Some of the newly reported Au NCs are presented in Table 1, which shows the composition, characterization techniques used to determine the composition, and the UV-Vis absorption features of the NCs.

The ultrasmall (< 2 nm) MnLm NCs exhibit discrete energy levels, thereby yielding unique optical properties such as molecule-like absorption and strong luminescence [46, 84, 110-113]. Figure 2C shows the typical absorption spectrum of Au25(SCH2CH2Ph)18, where its main absorption peak at ca. 670 nm is attributed to the Au13 core of the NCs, whereas the typical surface plasmonic resonance peak at ~520 nm is absent. With the same Au25 core [114], the intensity of the NCs follows the order: [Au25(SC6H13)18]– < [Au25(SC12H25)18]– < [Au25(SC2H4Ph)18]– (TOA+ is the counter ion), which clearly demonstrates that the fluorescence intensity is strongly related to the surface ligand because the charge donation from the ligand to the metal core greatly enhances the fluorescence. Similar to Au NCs, the fluorescence behavior of Ag NCs has been studied in several cases, such as Ag7(H2MSA)7 and Ag8(H2MSA)8 [42]. In addition to their optical spectra, MnLm NCs also exhibit distinct properties in terms of chirality [36-38] and magnetism [39, 40], but these features are outside the scope of the present review, although interested readers may refer to a previous review 11

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[115].

3. Synthesis of metal cluster catalysts 3.1. Synthesis of ligand-protected metal NC catalysts Due to their ultrasmall size, the chemical properties of “naked” metal NCs are very active, which necessitates the use of ligands to stabilize and protect the metal NCs. During recent decades, significant efforts have been paid to develop the size-controlled synthesis of NCs because MnLm NCs with different core sizes and ligands would greatly facilitate the study of novel chemical and physical properties. Several ligands have been utilized successfully in the synthesis of atomically precise MnLm NCs, such as phosphines, thiolates, or dendrimers. The following discussion of the methods for synthesizing MnLm NCs categorizes them based on the ligand employed. 3.1.1. Phosphine-protected MnLm NCs Au NCs stabilized with phosphine ligands have been studied experimentally since the 1960s [116]. For atomically precise phosphine ligand-protected Aun NCs, reports of the synthesis of NCs with smaller metal cores (n < 10) have appeared in the last decade. For instance, [Au7(PPh3)7]C60•THF and [Au8(PPh3)8](C60)2 were synthesized in 2008 via ligand exchange between KC60 and [Au8(PPh3)8](NO3)2 starting from Au(PPh3)NO3 [64]. For the Aun clusters where n >10, the undecagold Au11 NCs are the most widely studied phosphine-stabilized systems. The synthesis of these NCs often involves the reduction of Au-phosphine compounds in a controlled 12

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manner to produce Au11 NCs (Scheme 1). For example, Au11(PPh)8Cl3 can be obtained by the reduction of AuPPhCl in NaBH4 solution [69]. Phosphine ligand-protected Au11NCs with similar molecular structures, such as Au11(PPh)8(CNO)2NO3, Au11(PPh3)7X3 (X = SCN, I, S-4-NC5H4) and Au11(PMePh2)10X3, may be obtained with a similar protocol [117]. All of the ligands bond radially to the Au atoms and no bridging motifs exist for chloride or thiolate ligands. In addition, [Au11(dppe)6]Cl3 [68] can be obtained by NaBH4 reduction of an Au(I)-phosphine precursor Au2(dppe)Cl2 in ethanol.

The synthesis and structure of [Au13(PMe2Ph)10Cl2]Cl3 were reported by Briantet et al. [118]. Ti(η-C7H8) is added to toluene solution containing AuCl(PMe2Ph), which allows the high yield production of [Au13(PMe2Ph)10Cl2]Cl3 with an icosahedral core. [Au14(PPh3)8(NO3)4]·(MeOH)6 [73], Au20(PPh3)4 [79], [Au20(PPhpy2)10Cl4]Cl2 [81], and [Au20(PP3)4]Cl4 [83] can also be synthesized via the reduction of an Au(I)-precursor by NaBH4, where the precursors employed are Au(PPh3)NO3, Au(PPh3)Cl, Au(PPhpy2)Cl, and PP3Au4Cl4, respectively. Shichibu et al. synthesized Au25(PPh3)10(SCnH2n+1)5Cl2(SbF6)2 (n = 2, 8, 10, 12, 14, 16, 18) [119] via the chemical reaction between [Au11(PPh3)8Cl2]SbF6 and n-alkanethiol CnH2n+1SH. Only the crystal structure of [Au25(PPh3)10(SC2H5)5Cl2](SbF6)2 has been determined by single-crystal X-ray structural analysis, thereby demonstrating that the Au25 core is constructed by bridging two icosahedral Au13NCs with thiolates that share a vertex atom.

In

2012,

Das

et

al.

presented

the

crystal

structure

of 13

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[Au24(PPh3)10(SCH2CH2Ph)5X]X2 (X = Cl, Br) [120], which is similar to that of Au25(PPh3)10(SCnH2n+1)5Cl2(SbF)2 but the shared vertex Au atom is missing. It was obtained by the reaction between [Au25(PPh3)10(SCnH2n+1)5Cl2]Cl2 and excessive PPh3. The structure comprises two incomplete (i.e., one vertex missing) icosahedral Au12 units joined by five thiolate linkages. Teo et al. [121] reported the production of [Au39(PPh3)14Cl6]Cl2 from the reduction of HAuCl4 mixed with PPh3 by NaBH4. It was suggested that the NC was built by a layer-by-layer mechanism and it had an idealized D3(32) point group symmetry. The NC structure might be described as seven layers (1:9:9:1:9:9:1) of Au atoms. The synthesis of the first Au55 NCs stabilized by PPh3 occurred in 1981 [122]. Ph3PAuCl is dissolved in benzene or toluene and treated with gaseous B2H6 at 60C to form Au55(PPh3)12Cl6 with a yield of ~30%. It was proposed that the NC possesses a shell of 42 Au atoms, which fully cap the Au13 core. This full-shell configuration explains the high stability of Au55(PPh3)12Cl6. Similar to Au NCs, Ag NCs have been synthesized using phosphine as the stabilizing ligand. Yang et al. [123] reported the preparation of Ag NCs by using a combination of thiolate and diphosphine as the stabilizing ligands. The Ag NCs, such as Ag16(DPPE)4(SC6H3F2)14, were obtained by the reduction of an Ag precursor in the presence of the thiolate and diphosphine ligand mixtures with NaBH4 at 0C in a dichloromethane/methanol mixture. According to single crystal X-ray analysis, the Ag NCs have a core-shell structure with a multinuclear Ag unit encapsulated in a shell containing Ag(I)–thiolate–diphosphine complex. The same group also reported the synthesis of Ag14(SC6H3F2)12(PPh3)8, with slight modifications. The Ag NC also 14

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maintains a core-shell configuration [124], where they exhibit a strong luminescent signal in the emission spectra at ca. 536 nm under excitation at 360 nm due to its unique crystal structure. The development of preparation methods is still ongoing for phosphine-protected NCs. In some cases, common synthetic strategies are applied, such as using an Au (I)-phosphine complex as the precursor and NaBH4 as the reducing agent. Future research should focus on developing generalized protocols for the synthesis of phosphine-protected Au NCs. Due to the much weaker coordination capability of phosphine toward Au compared with thiol, the phosphine ligand is removed more easily from the Au surface after NC preparation, which may be crucial for catalytic applications.

3.1.2. Thiolate-protected MnLm NCs

In recent years, remarkable advances have been made in the solution-based synthesis of atomically precise thiolate-protected MnLm NCs [35, 76, 93, 125-130]. Initially, thiolate was used extensively in the synthesis of small sized Au NPs due to the strong interaction between Au and S, which inhibits the rapid growth of the Au core during the early stage. Later, a synthetic method was established for atomically precise NCs, which allows the synthesis of a number of Au NCs, such as Au25(SR)18 [46, 125-127, 131, 132], Au38(SR)24 [59, 133-135], Au144(SR)60 [108, 136], and Au28(SR)20 [92]. All of them can be synthesized based on a modified Brust–Schiffrin 15

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method, which often involves the reduction of Au(I)-SR complexes (formed by mixing HAuCl4 with thiols) by NaBH4 in solution [137]. After determining the polarity of the thiol ligands and solvent used in the synthesis, the hydrophilic or hydrophobic property of the Aun NCs is determined accordingly. For example, Aun(SCH2CH2Ph)m is synthesized in toluene [77], whereas Aun(MHA)m is synthesized in water [127] (Considering the polarity of the ligands and solvents used in each condition, we may deduce that Aun(SCH2CH2Ph)m is lipophilic whereas Aun(MHA)m is hydrophilic. Hydrophilic NPs are sometimes preferred for preparing supported precatalysts because the immobilization of NCs on solid supports can be accomplished in aqueous solution. In the typical Brust–Schiffrin method, the reagents are accommodated in two immiscible phases (i.e., HAuCl4 in water and thiol ligands in the organic phase). The phase transfer of HAuCl4 from the aqueous to organic phase by a phase-transfer agent (e.g., cetyltrimethylammonium bromide) is required to initiate the reaction. After mixing HAuCl4 and the thiol ligands, a strong reductant (e.g., NaBH4) is added, which promotes the formation of Au NCs or small Au NPs in the organic phase. Subsequently, the conventional Brust–Schiffrin method was modified to synthesize thiolate protected NCs with different thiol ligands in water. Luo et al. used GSH as a reducing-cum-protecting agent, and produced a family of Au(0)@Au(I)-GSH NCs with Au(0) as the core and Au(I)-GSH as the shell (core size of Au29-43) [111], which exhibited ultra-bright luminescence via a unique aggregation-induced emission mechanism [138, 139]. Negishi et al. [140] reported the synthesis of Pd NCs by 16

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reacting n-alkanethiols (RSH, R=n-C18H37) with PdCl2 in toluene, where no external reducing reagent was added. The resulting Pd NCs measured ~1.0 nm in size. Negishi et al. also employed dithiols (e.g., dimercaptosuccinic acid) to synthesize Au10–13 NCs [141]. Recently Yang et al. obtained water soluble Au10 NCs using histidine as both the reducing reagent and protecting ligand [67]. Other mild reducing agents, such as CO [71, 84, 128, 142, 143], borane-tert-butylamine complex [77, 144], and sodium cyanoborohydride [145], have also been employed for the synthesis of metal NCs. Many other synthetic routes in the solution phase have been developed for preparing MnLm NCs, including photoreduction [146], a micro-emulsion method [147], microwave-assisted synthesis [148, 149], and an electrochemical method [150, 151]. In addition to these wet chemistry routes, a new protocol based on solid state chemistry provides an additional strategy for synthesizing MnLm NCs with diverse properties. For example, Ag9 [152], Ag32 [153], Ag44 [154], and Ag152 [155] NCs have been synthesized via a solid state route (Fig. 3). The metal ions, ligand precursors, and reductant are ground, and then reacted in solid form in air, where the resulting MnLm NCs are extracted using a solvent. Compared with traditional wet chemistry routes, this method has a distinct advantage because the reaction kinetics are more controllable in the solid state than solution. Large NPs may be generated in the solution phase due to rapid reduction by a strong reducing agent, whereas the reduction rate is greatly retarded in the solid state due to mass transfer limitations.

The metal atoms in NCs can originate from NPs or larger NCs via ligand 17

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mediated etching. Yuan et al. [156] reported a facile and scalable method for the synthesis of metal NCs (e.g., Au, Ag, Pt, and Cu) by a phase transfer-assisted mild etching process (Fig. 4) [157-159]. The surface chemistry of the resulting MnLm NCs is changed easily, thereby yielding NCs with reversible solubility in water and toluene. Recently, Qian et al. synthesized monodispersed Au38 NCs via a two phase ligand exchange process [134], where polydispersed, GSH-stabilized Au NCs (Aux(GSH)y) were mixed with neat C12SH to induce a ligand exchange process and mild etching of the NC surface atoms, thereby obtaining monodispersed C12SH-protected Au38 NCs.

3.1.3. Dendrimer-protected MnLm NCs Strictly speaking, when dendrimers are employed as NC stabilizers, it is not easy to determine the exact metal atom numbers, so the atom numbers in NCs are often estimated based on the size of the NCs. Encouragingly, in recent years, the ESI-TOF-MS technique has facilitated the determination of the precise metal atom numbers for dendrimer stabilized metal NCs. Therefore, this series of NCs is also introduced to provide a broader view of recent developments. In the past decade, dendrimers have been utilized widely as templates for synthesizing sub-nano-sized metal NCs. This template-based protocol possesses features that are distinct from other synthetic methods. Dendrimers act as a predetermined environment for NC generation, where the NCs can grow into desirable shapes and sizes. The pioneering study of dendrimer capped MnLm NCs was reported by Crooks et al. [160], where a PAMAM starburst dendrimer was used as the template 18

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to synthesize Cu NCs. Cu(II) ions are chelated in PAMAM (G4-OH) and excess NaBH4 helps to reduce Cu(II) into Cu NCs with well-defined size and conformation. Due to their capacity for chelating M ions from aqueous solution, OH-terminated dendrimers PAMAM such as G4-OH and G2-OH can also be used as templates for the synthesis of other MnLm NCs in aqueous solution. Zheng et al. reported the synthesis of Ag NCs comprising 3–8 Ag atoms capped by PAMAM [161]. PAMAM serves as the ideal template for the synthesis and stabilization of Ag NCs, where the inner core of PAMAM is well coordinated with Ag ions and the outer shell of the dendrimer prevents the successive growth of Ag NCs into NPs. Recently, a spherical macromolecular template (phenylazomethine dendrimer) was used to prepare atomically precise Pt NCs (Fig. 5) [162], where dispersed Pt12 (0.9 ± 0.1 nm), Pt28 (1.0 ± 0.1 nm), and Pt60 (1.2 ± 0.1 nm) NCs were obtained, and the cluster size could be controlled in a rational manner via the stepwise complexation of PtCl4 with the dendrimer. The atomic numbers in the Pt5(MAM)8 NCs have been determined by ESI-TOF-MS analysis. In addition to a phenylazomethine dendrimer, a fourth generation PAMAM (G4-OH) has been employed to produce size-focused water soluble Pt NCs [163]. It should be noted that the resulting G4-OH capped Pt NCs can be transformed into MAA-protected Pt NCs (i.e., Pt5(MAA)8) via a ligand exchange process with MAA. A “click” dendrimer comprising TEG termini and a 1,2,3-triazole ligand has also been utilized for synthesizing ultrasmall metal NCs [164]. The labile interaction between triazole ligand and the Pd precursors helps to form Pd (II) intradendritic complexes with exact sites, which has been confirmed by UV-Vis 19

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spectroscopy. By contrast, the hydrophilic TEG termini are involved in the formation of interdendritic species, thereby contributing to the stabilizing effect of Pd NCs [165].

3.1.4. Other ligand-protected Mn NCs Proteins [166], DNA [167], and other molecules have also been employed as templates for the preparation of atomically precise NCs. In 2008, Xie et al. reported the synthesis of Au25 NCs using biocompatible BSA as the template (Fig. 6) [166], where the numbers of atoms in the Au NCs were determined based on their MALDI-TOF-MS spectrum. A peak centered at ~66 kDa was observed for the BSA molecule and a 5-kDa increase was detected for the Au-BSA NCs, thereby indicating the existence of an Au25 core. Recently, iron carbonyl has emerged as an inorganic ligand to protect atomically precise MnLm NCs. For example, the preparation of [Au21{Fe(CO)4}10][NEt4]6Cl and [Au22{Fe(CO)4}12][NEt4]6·(CH3)2CO·0.5C6H14 can be achieved in acetone solution, where the yield reaches 20% on an Au atom basis [168]. The exact composition was confirmed by X-ray diffraction studies, which provided the crystal structures of these two NCs.

The development of synthetic strategies for MnLm NCs has provided an enriched library for catalysis research. These NCs can be utilized directly as precatalysts (see Section 4) as well as being further de-protected from the ligands before catalytic applications (see the following section). 20

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3.2 Synthesis of (partially) deligated metal NC catalysts It is essential that the surface of a catalyst is accessible to the reactants. However, in a typical ligand-protected MnLm NCs, the surfaces of the metal NCs are passivated with a number of ligand molecules, which limits the accessibility of metal NCs to the reacting substrates. These ligands are indispensable in the solution phase for preventing aggregation and agglomeration by the metal core. Thus, the ligands capped on the NCs can only be removed after loading the NC onto a solid support, where the NCs can be stabilized without the presence of ligands. Figure 7 depicts three general processes for ligand removal. In early studies, Muetterties et al. reported the synthesis of Ir or Ru clusters capped by CO [19, 169], where the CO ligand was then removed by heating in a helium or hydrogen flow to form Ir or Pt clusters. In addition, metastable metal NCs/NPs precatalysts have been obtained, including Ir(0) NCs/NPs stabilized by “weakly ligated/labile ligands” (e.g., 1,5-cyclooctadiene, BF4–, and P2W15Nb3O62) and “solvent-only” Ir(0)n NPs stabilized via anion coordination and electrostatic stabilization [170-172]. Due to the relatively weak interaction, the ligands of these precatalysts can be removed easily by mild hydrogen reduction. However, these methods might not be directly applicable to the removal of thiolate and phosphine ligands from NCs because the latter ligands exhibit strong bonding to the metal as well as non-volatility.

A conventional method for removing the capping ligand from NCs is calcination. The NCs on supports are thermally treated in vacuo to remove the organic monolayer 21

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and expose the metal core [173]. The loading process often follows an impregnation method, during which the NCs are adsorbed onto the surface of the support due to the strong interaction between the functional groups of the ligands and those on the support. Thus, the NCs are loaded rapidly onto the supports for further thermal treatment. An apparent disadvantage of calcination for ligand removal is catalyst sintering, where the aggregation of discrete NCs into NPs often occurs after the complete removal of capping ligands during heat treatment. To avoid this issue, a low loading of NCs should be maintained to minimize surface migration and contact with neighboring NCs. An improved thermal treatment method was established to obtain ligand removed metal NCs on a support without affecting the NC size, where the key step is the preparation of a transition metal oxide-modified SiO2 support. By incorporating CuO or Co3O4 onto mSiO2, Au144(SR)60 or Au25(SR)18 NCs are loaded onto the as-prepared support and calcined in vacuo [174]. Calcination removes the thiol ligands from Au144 NCs, without causing notable changes in the NC size. It has been suggested that transition-metal oxides have strong interactions with silica. Thus, they are isolated on the pore surface of mSiO2 to form ‘‘spacers’’ that hinder the movement of NCs, thereby avoiding sintering. The supported, deligated Au25 precatalysts can convert 65% of the CO at 20C, with total CO conversion at 70C. The use of calcination to remove the capping ligand is a so-called hard treatment method. Recently, several new strategies regarded as soft treatment methods have been developed for ligand removal from NCs under milder conditions, including 22

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reduction [175], oxidation, ozone treatment [176], and solvent flush treatments [177]. Compared with thermal treatment, the soft treatment methods are based mainly on chemical stripping, which is much milder (in ambient conditions), and they do not require extensive heating. These new strategies allow finer control over the removal of ligands from NCs, but without significantly affecting the size of the resulting deligated NCs. Das et al. reported a reduction method for ligand removal [175], where Au144(SCH2CH2Ph)60 NCs are loaded onto thiol-modified mesoporous silica (SBA-15) by impregnation. The Au144-SBA-15 composite is then treated with aqueous solutions of NaBH4 at different concentrations. After reduction, the thiolate ligands are stripped and the Au144-SBA-15 composites exhibit excellent activity during the catalytic oxidation of styrene. The catalysts exhibit high selectivity and yield benzaldehyde, thereby indicating the importance of ligand removal for enhancing the yield of the target product. Menard et al. reported an ozone treatment method for MnLm NCs ligand removal [176], where Au13[PPh3]4[S(CH2)11CH3]4 is deposited onto commercial anatase TiO2 dispersed in toluene and the composite is then placed into an ozone generator chamber. The capping ligands are effectively removed by ozone whereas the NC size remains unchanged. Recently, we suggested that the thiolate ligand on the surface of NCs might be oxidized by H2O2 or TBHP to cleave the metal-S bond and remove ligands from NCs. The as-prepared Au25(SR)18-HAP catalysts were treated with 0.1 M H2O2 or TBHP in aqueous solution [178] and after 36 h, the ligands were effectively stripped from the composite catalysts. Fine control 23

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over the partial removal of ligands from this catalyst could be achieved by adjusting the concentrations of H2O2 or TBHP, the reaction time, and temperature. Thus, a series of samples were obtained to study the ligand effects in specific catalytic reactions, thereby enriching our mechanistic understanding of the process. In addition, the ligands can be readily removed by using a simple solvent extraction method in some cases. Hutchings et al. [177] loaded Au NPs stabilized with PVA on TiO2, where this hybrid material was treated with water to extract the PVA stabilizer. Continued water extraction removed PVA from the immobilized Au NPs, but without changing the size and morphology of the Au NPs. The deligated catalysts exhibited excellent activity during CO oxidation. This also confirms that ligand removal is a crucial step for improving the direct contact between the reacting substances and metal NCs to obtain better catalytic performance. According to the examples discussed above, soft treatment methods for removing ligands from supported NCs have the following advantages. (1) The extent of ligand removal can be more finely controlled. Ligands play specific positive or negative roles in catalytic reactions, and thus the partial removal of ligands may facilitate detailed investigations of the effects of ligands on specific catalytic reactions. (2) The soft treatment methods provide a new approach for eliminating the mass transfer barrier between the metal surface and reaction substrate, but without compromising the catalytic activity by maintaining the cluster size. Therefore, soft treatments may be a promising strategy for preparing heterogeneous, weakly ligated NCs supported on various solid materials with high performance in catalytic applications. 24

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Finally, we should note that in all of the studies discussed in section 3.2, there was a general lack of solid evidence to support the assumption that the number of metal atoms in the NCs is well preserved after ligand removal. In fact, determining the number of NC atoms on a solid support remains a great technical challenge, especially after removing the ligands. Moreover, earlier research shows that completely removing ligands from NCs is always problematic, and thus only the ligands with residues that benefit specific catalytic reactions are suitable for use [179-181]. Somorjai et al. conducted comprehensive studies of ligand residues on NPs and indicated their positive effects on catalytic reactions [182, 183], as well as giving useful insights into NC ligands and ligand removal.

4. Catalysis by MnLm NCs Noble metal NPs have been applied extensively as catalysts to produce value-added chemicals, but understanding their behavior at the molecular level is critically important. However, establishing unambiguous correlations between structures and catalytic properties in conventional NP catalysts remains a major challenge because NPs often have non-uniform active site distributions. Advances in atomically precise, ligand-protected noble metal NCs provide a good opportunity for preparing rationally designed, well defined precatalysts, which can help to obtain fundamental insights into structure-catalytic property relationships. NC precatalysts also provide routes to new types of catalytic reactions that cannot be promoted readily 25

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by conventional catalysts. However, caution should be taken when probing the state of NCs after reactions because they may decompose or agglomerate, thereby yielding fragmented species or NPs as the true catalyst [25-27].

4.1. Catalysis by ligand-protected MnLm NCs During conventional noble metal NP catalysis, the ligands used to protect and stabilize small chemically reactive NPs are often considered to be poisons, which passivate the surface of the catalyst and hinder direct contact between the reactants and the metal surface [184, 185], thereby negatively affecting the catalytic activity. Some “weak/labile ligands” such as 1,5-cyclooctadiene and BF4– stabilized NCs have also been investigated, and their catalytic performance was reported to be negatively correlated with the stabilizing effect of their ligands [32, 33, 170-172]. However, interestingly, some ligand-protected MnLm NCs, especially the weakly ligated Au50(SPO)30−33, appear to be catalytically active and selective in several reactions in gas or liquid phases in the presence of ligands, where they are distinct from conventional ligand-protected metal NPs [186-188].

4.1.1. Oxidation reactions 4.1.1.1. CO oxidation CO oxidation is usually employed as a probe reaction for investigating structure-catalytic property relationships. CO oxidation is known to be highly sensitive to the size, charge state, and support employed for the metal catalyst [18]. 26

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Nevertheless, recent studies of CO oxidation by Aun(SR)m NCs have provided different views of how the ligand affects the reaction. CO oxidation by supported Au NPs has been studied extensively [18], where high reactivity was obtained by loading Au NPs onto an active support such as TiO2, but this is not necessarily the case for Aun NCs. Nie et al. [189] found that unlike a conventional Au NPs/TiO2 catalyst, which often exhibits an excellent CO oxidation activity at low temperature, Au25(SR)18/TiO2 has no catalytic activity, even when the temperature is increased to 200C. This indicates that there are remarkable differences in the CO oxidation reactivity of the traditional Au NPs/TiO2 catalyst and Au25(SR)18/TiO2 catalysts with thiol ligand poisoning. In addition, pretreatment of Au25(SR)18/metal oxide catalysts significantly affects the catalytic activity during CO oxidation. For instance, after treatment in an O2 flow for 1.5–2 h at 150C, there is a dramatic increase in the oxidation activity of Au25(SR)18/CeO2, where the onset of conversion occurs at room temperature, with complete CO conversion at 100C. This enhancement effect is not observed in the oxidation process with Au25(SR)18/TiO2. Furthermore, no enhancement effect is observed after changing the pretreatment atmosphere from O2 to N2. The thiol ligand cannot be removed by heating until the temperature reaches 250–300C, so it is unclear whether the ligand will remain on the Au NCs if the catalyst is pretreated in an O2 flow at 150C. It has been proposed that the active sites of this Au25(SR)18/metal oxide catalyst are located at the interface between the Au NCs and the CeO2 support at the perimeter sites. The O2 is converted to active oxygen species during the pretreatment and then activated further as the 27

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hydroperoxide at the interface on the CeO2 support. These phenomena highlight some interesting issues, such as the inertness of the Au25(SR)18/TiO2 compared with the traditional Au NPs/TiO2 catalyst, the nature of the active O species, and most importantly, the interaction (e.g., charge transfer) between Au NCs and CeO2 at the interfaces during catalysis. Based on preliminary results obtained with a Au25(SR)18/CeO2 catalyst, Nie et al. [190] investigated CO oxidation over CeO2-supported Au38(SR)24NCs. Similar to the Au25(SR)18/CeO2 catalyst, enhanced catalytic activity is observed after pretreatment in an O2 flow for 2 h at different temperatures, where the optimal activity is achieved with a pretreatment temperature of 175C. At a higher temperature, the thiol ligands are partially removed from the Au NCs and the catalytic activity deteriorates. Pretreatment in N2 flow does not enhance the activity of Au38(SR)24/CeO2. When water vapor is added to the system, the water vapor has distinct effects on CO oxidation over catalysts pretreated below and above 175C. The water vapor promotes CO oxidation over a catalyst pretreated below 175C, whereas it inhibits the reaction over a catalyst pretreated above 175C, which may be attributable to the partial removal of thiolate ligands. Thus, it appears that Au38(SR)24/CeO2 is more suitable for CO oxidation than the traditional Au/metal oxide catalyst in the presence of water vapor. This also demonstrates that weakly ligated, labile-ligand NCs are more suitable for promoting CO oxidation with higher activity. The adsorption of O2 and activation are considered to occur during the pretreatment process, where O2 is readily adsorbed onto Aun NCs and electrons are 28

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transferred from Au to O2 molecules to form peroxo species. The unique triangular Au3 sites present in Au38 NCs are considered to be the active sites for CO oxidation after the co-adsorption of CO and O2 onto the Aun NCs (Scheme 3). Moreover, the CeO2 support does not play a dominant role in CO oxidation because the plain CeO2 has no catalytic activity. Therefore, the onset of CO oxidation activity depends on the O2 pretreatment conditions rather than thiolate ligand removal. The nature of the oxide support (CeO2 vs. TiO2) appears to be crucial for active CO oxidation by Au NCs/MOx. However, the mechanism of CO oxidation over CeO2 supported Aun(SR)m NCs remains poorly understood and it requires further study.

4.1.1.2. Alcohol oxidation Similar to CO oxidation, the noble metal NCs are highly effective in promoting the aerobic oxidation of alcohols, even if the surface of the NC is still capped by ligands. Tsunoyama et al. [191] synthesized Au NCs stabilized by PVP measuring less than 1.5 nm, which exhibited superior activity during the aerobic oxidation of alcohols compared with larger NCs stabilized by poly(allylamine). The absorption of CO onto the Au:PVP NCs monitored by FTIR indicated that the smaller NCs tended to donate more electronic charges to CO than the larger NCs, probably because of the increased electric charge in the unoccupied orbitals with decreased size. The charge transfer from Au NCs to the adsorbate is crucial for the activation of O2 during the aerobic oxidation of alcohol. Analyses based on XPS, XANES, and the red-shift of absorbed CO in the FTIR spectrum show that the Au NCs are more electron enriched due to the 29

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coordination of PVP, where a plausible mechanism for this is illustrated in Scheme 4. The electrons are transferred from the anionic Au cores to the LUMO of molecular O2 to form superoxo-or peroxo-like species during the reaction, which subsequently promote the oxidation of alcohols. Due to the increase of the PVP:Au ratio from 40 to 100, the number of coordinated CO molecules decreases remarkably, thereby making a smaller surface area available for catalysis. However, the catalytic activity remains the same for NCs with different PVP:Au ratios, which can be rationalized by the donation of the electronic charge from PVP to Au, thereby increasing the activity per unit surface site. These Au NCs are also effective in the α-hydroxylation of benzylic ketones [192], N-formylation of amines [193, 194], oxidation of organoboron compounds [195, 196], and Lewis acidic assisted reactions, where the latter include the intramolecular addition of alcohols to alkenes [197], intramolecular addition of toluene sulfonamide [198], and intramolecular addition of primary amines [199].

A different mechanism was reported by Conte et al. [200] for the aerobic oxidation of alcohols using O2 as the oxidant over Au NCs incarcerated in styrene-based copolymers. It was proposed that the Au-H bond is formed between the surface Au atoms and the alcohol, instead of following the metal alkoxide pathway. Subsequently, the C-H bond on the Au surface is cleaved to produce ketones, where the function of O2 is only to remove the hydrogen from the Au-H bond and to restore the catalytic activity. Benzyl alcohol oxidation has also been investigated over Au25(MHA)18/HAP (Au loading: 1 wt%) with different TBHP treatments [178]. The 30

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precatalyst was first treated with TBHP solution to remove the thiolate ligands. Samples treated for 36 h and 48 h both lacked S 2p peaks in their XPS spectra, thereby indicating the removal of the surface ligands from the Au NCs. However, the size of the sample treated for 48 h increased to 1.5 nm whereas the other remained at ~1.2 nm. Thus, the differences between the two post-treated samples (36 h and 48 h) can be regarded as the differences between HAP-supported Au NCs and Au NPs. Under identical conditions, higher conversion of benzyl alcohol was observed with Au NPs (45.9%) compared with Au NCs (23.6%). The ligand residues were found to be sulfonate, which could poison the Au NCs. Zhang et al. [201] developed an alcohol reduction method for preparing so-called Pd147 NCs stabilized by PVP, followed by a galvanic replacement process to prepare CJ structured CJ-Au/Pd NCs for aerobic glucose oxidation. The number of Pd atoms in one cluster was estimated using HRTEM and HAADF-STEM by combining the mean size of the cluster (1.8 nm) with its ideal facet-oriented structure in STEM images. However, the real atom number in the “Pd147” cluster may actually vary by ± 25 to 50 or more Pd atoms. The CJ NCs had much higher activities than their counterparts (pure Au or Pd NCs, and Au/Pd alloys) based on control experiments, where the TOF was 20–30 times higher than that for Au and Pd clusters, and 8–10 times higher than that for the Au/Pd alloy. Electron donation from Pd to Au was observed, which was similar to that in the core-shell Au-Pd NPs in ionic liquids [202], and the anionic charge on the top Au atoms was proposed to be responsible for the high reactivity [201]. The electron transfer from the anionic top Au atoms to O2 is 31

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proposed to generate a hydroperoxo-like species, which plays a crucial role in the oxidation of glucose [201]. 4.1.1.3. Styrene oxidation The styrene oxidation reaction produces styrene oxide, which is a value-added fine chemical [203]. However, the conventional industrial route for styrene oxidation involves a problematic peracid as the oxidant [204]. Thus, there is a high demand for developing an efficient epoxidation catalyst, but especially for olefins rather than ethylene. Au NCs measuring in the sub-nano-range are known to be excellent precatalyst candidates. Zhu et al. [28] reported the preparation of three types of Au NCs, i.e., Au25(SCH2CH2Ph)18, Au38(SC12H25)24 and Au144(SC12H25)60, which were subsequently evaluated in styrene oxidation and benzalacetone hydrogenation reactions. The oxidation process was conducted using Au25(SR)18 in toluene at ~100C in 1 bar O2. After 24 h, a mixture comprising benzaldehyde (~70%), styrene oxide (~25%), and acetophenone (5%) was identified in the product mixture. Larger NCs such as Au38(SR)24 and Au144(SR)60 were less active than Au25(SR)18. The extraordinary activity of Au25(SR)18 was attributed to its unique electronic properties, where the small NCs had an appreciable HOMO-LUMO gap (Eg(Au25) ~1.3 eV), thereby exhibiting strong quantum confinement effects when activating styrene and molecular oxygen. Another important factor may be related to the low coordination numbers of surface Au atoms and the high curvatures of the smaller NCs, which facilitate the adsorption and activation of the reactant molecules. Au25(SR)18 NCs supported on silica yielded activity comparable to that of the unsupported catalyst. No 32

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apparent deterioration in catalytic performance was observed after six cycles [186].

Recently, Fang et al. [173] prepared Au25(MHA)18 catalysts supported on various solid materials, such as HAP, P25, AC, GO and SiO2. The supported Au25(MHA)18 NCs with a ligand attached exhibited similar activity during styrene oxidation using TBHP compared with the supported Au NC precatalysts, where their thiol ligands were removed prior to catalysis. TBHP provided a strong oxidation environment under the conditions employed, and thus the MHA ligands were oxidized and detached from the Au NCs during the reaction. Hence, this study demonstrated the similar activity of both the Au NCs with and without ligand capping. Qian et al. [205] reported single Pt atom-doped Au NCs with the NC composition of Pt1Au24(SR)18. A combination of experimental results and DFT calculations indicated that the Pt atom is located in the center of the NC. The enhanced stability of Pt1Au24(SR)18 was attributed to the stronger interaction between the central Pt atom and the Au12 icosahedral cage. When the doped NCs were loaded onto TiO2, the catalytic activity during styrene oxidation was better than that of pure Au25 NCs, where the conversion rate of styrene was 90.8% compared with only 54.0% over the pure Au25 NCs. The mechanism responsible for this enhancement effect remains unclear at present. Further investigations of the catalytic behavior during the catalysis of styrene oxidation using a wider range of doped NCs would be highly interesting. It should be noted that the NCs may be decomposed during styrene oxidation. Drerier et al. discovered that Au25(SR)18 NCs were not stable under oxidizing 33

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conditions when TBHP was employed as the oxidant [206]. They compared the catalytic activities of Au25(SR)18 NC, the Au–thiol NC precursor, and the NC decomposition products, and showed that all were equally effective at styrene oxidation. Based on poisoning experiments and XPS results, it was proposed that Au(I) thiolates are likely to be the active species.

4.1.2. Hydrogenation The hydrogenation of C=C and C=O bonds is an important area of hydrogenation catalysis. In a recent study, cyclohexene was employed to study C=C hydrogenation over mononuclear Rh1, Rh4 NCs, and Rh NPs to determine the actual active species. Analyses based on XAFS, 1H NMR, ex situ MS, and UV-Vis in a series of poisoning studies showed that homogeneous Rh1 played a more significant role than Rh NCs [207]. Typically, Au is not a good catalyst for hydrogenation because of its limited capacity for dissociating hydrogen. Nevertheless, Au NCs may be more active in the hydrogenation of C=O bond than the C=C bond, so they have the potential for the selective hydrogenation of α,β-unsaturated aldehydes to their related alcohols. For instance, the hydrogenation of benzalacetone over a mixture of Aun(SR)m NCs is preferred at the C=O bond rather than the C=C bond, thereby forming an unsaturated alcohol with 76% selectivity, with an additional 14% of unsaturated ketone and 10% saturated alcohol [28]. To further improve the selectivity, the reaction can be conducted in the presence of pure Au25(SR)18 NCs in a mixture of toluene and ethanol

34

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at 0C under 1 bar H2 [186], where 100% selectivity for unsaturated alcohol is obtained, although the conversion is moderate (22%) (Table 2, Entry 4). The integrity of the NCs used in tests is found to be preserved well according to the UV-Vis spectra and ESI-MS spectra after the catalytic reaction. The mechanism shown in Scheme 6 illustrates the proposed reaction process. The icosahedron Au13 cores are thought to favor the adsorption of the C=O bond via the interaction with the Au atom and the O in carbonyl group, with electron transfer from the electron-rich Au13 to the C=O bond. Next, the hydrogen attacks the activated, slightly nucleophilic C=O group, thereby leading to the production of unsaturated alcohols. The low coordination number of the surface Au atoms in the NCs is considered to facilitate the dissociation of H2 adsorbed onto the Au shell, which promotes the hydrogenation of the C=O bond. This analysis was extended to a series of substituted α,β-unsaturated ketones and aldehydes, where 100% selectivity for unsaturated alcohols was always achieved, except for crotonadehyde. alcohols. dN.M.: not mentioned. eHMF: 2-hydroxymethyl-5-furfural.

In addition to thiolate-protected Au NCs, other ligand-protected Au NCs can be used

to

catalyze

hydrogenation

reactions.

For

example,

a

SPO,

tert-butyl(naphthalen-1-yl)phosphine oxide, can be used to support and stabilize Au NCs (1.24 ± 0.16 nm) via a simple capping procedure. This new NC is an exceptional catalyst during the hydrogenation of α,β-unsaturated aldehydes to produce unsaturated 35

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alcohols [188]. Multiple techniques such as TGA, EDX, UV-Vis spectroscopy, solid state NMR, and XPS suggest that phosphine oxides can stabilize the Au NC core, with a composition of Au50(SPO)30−33. Isotopic labeling using

18

O to substitute

16

O in the

phosphine oxides in ATR-FTIR experiments also confirmed that Au-P(O)R2 bonding is the dominant coordination mode in Au-SPO NCs. Using the optimized reaction parameters for cinnamaldehyde hydrogenation, a maximum turnover number of 1064 and a maximum TOF of 283 h−1 were achieved, which is the best reported catalytic activity for unsupported Au NPs [3, 211, 212]. By contrast, to the best of our knowledge, the highest TOF over unsupported Au NPs is ~16 h–1 for cinnamyl alcohol, where the NPs were stabilized by an N-containing imidazolium-based ionic polymer (diameter: 1.8–2.8 nm) [213]. This reaction has also been examined using thiolate ligated Au NCs, where Au25(SCH2CH2Ph)18 exhibited the best performance with a TOF of 94 h–1 [186]. Thus, it appears that N-containing and S-containing ligands with much greater affinity for Au inhibit the catalytic performance, while the weakly absorbed SPO ligand promotes the exposure of more Au sites. The hydrogenation of a wide range of α,β-unsaturated aldehydes with different functional groups, some of which are important intermediates in the production of perfumes and fragrances, has also been tested with Au NC precatalytsts, where high conversion of the substrates and perfect chemoselectivity for the α,β-unsaturated alcohols were obtained in all cases. A mechanism was proposed where the H2 dissociation occurs in a cooperative manner between Au and the capping ligand, similar to that on supported Au NPs (Fig. 8). SPO is a capping agent, but it 36

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also participates in H2 activation. This catalytic hydrogen activation may facilitate the development of NC catalysts where the ligands can help to achieve specific catalytic selectivity and activity.

The reduction of PNP to 4-aminophenol by NaBH4 can be catalyzed by Aun(SR)m NCs. Kawasaki et al. [214] reported that both Au25(GSH)18 NCs and DMF-stabilized Au NCs (unidentified size) are capable of serving as precatalysts for PNP reduction. The activity of Au25 NCs was superior to that of DMF-stabilized Au NCs due to less steric hindrance from the surface ligands. Scott et al. [215] recently demonstrated the structural integrity of Au25(SR)18NCs after PNP reduction by NaBH4, where the high stability under the reaction conditions enabled the recycling of these NCs. Jin et al. [216] reported a new NC with atomic number precision, Au44(SC2H4Ph)32, which was prepared via Cu2+-induced transformation from Au25(SC2H4Ph)18 with a yield of 75%. The catalyst derived from the Au44(SR)32 precatalyst exhibited higher catalytic activity during the reduction of 4-nitrophenol compared with Au25(SC2H4Ph)18 and other Au NCs with different atomic numbers. It was even active at low temperature (e.g., 97% conversion at 0C), whereas Au25 no longer catalyzed the reaction. The crystal structure of Au44(SC2H4Ph)32 was not obtained due to technical difficulties, but it was demonstrated that the catalytic properties of NCs involved with nitro group reduction were highly sensitive to their size. Recently, Au25(SePh)18 NCs with similar structure 37

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to that of Au25(SC2H4Ph)18 were also crystallized and they exhibited activity during the reduction of 4-nitrophenolate by NaBH3CN [217].

Recently, Li et al. [187] synthesized Au99(SPh)42 via a two-step synthesis strategy to achieve the size-focused formation of Au99 NCs in the solution phase. This new Au NC was loaded onto several solid supports for the selective hydrogenation of nitrobenzaldehyde to nitrobenzyl alcohol in water using H2 as the reductant, where its performance differed remarkably compared with the traditional Au catalysts that favor the reduction of the nitro group. The free Au99(SPh)42 achieved 8.7% conversion of nitrobenzaldehyde with 100% selectivity for nitrobenzyl alcohol (Table 3, Entry 7), thereby indicating its potential role in challenging selective hydrogenation reactions. More importantly, after it was loaded onto SiO2, TiO2, and CeO2 supports, there was a dramatic enhancement in the conversion of nitrobenzaldehyde, with an optimal conversion rate of 93.4% over Au99/CeO2. The selectivity remained at 100% with all of the supported catalysts, thereby achieving pure C=O bond hydrogenation without affecting the nitro group. The Au NC was applicable to a range of primary nitrobenzaldehydes with excellent selectivity for nitrobenzyl alcohols. The effect of the support was attributed to the acid/base properties, i.e., CeO2 is basic and this favors the hydrogenation reaction. Based on initial DFT calculations, the key step related

to

this

excellent

selectivity

is

the

unique

adsorption

mode

of

nitrobenzaldehyde where the O=C(H) group preferentially binds to the exposed Au sites on the staple motifs of the Aun NCs instead of the nitro group (Table 3, Entry 9). 38

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Moreover, the composition of the Au99 clusters remained the same before and after the hydrogenation

reaction

according

to

the

UV-Vis

absorption

spectra

and

MALDI-TOF-MS analysis, thereby suggesting that Au99 NCs are stable under the reaction conditions. However, this cannot be considered indisputable evidence that Au99 NCs are the actual catalytic species, and this question requires further investigation using this interesting system.

Li et al. [220] reported the highly selective conversion of propargylic acetates into α,β-unsaturated ketones or aldehydes using Au25(SR)18 NCs prepared in the presence of TOA+. TOA+ is considered to protect the Au25(SR)18– from being oxidized by the neutral NC [46, 108]. Each NC bears one negative charge according to NMR and X-ray single-crystal analysis, which showed that an equal molar amount of the TOA+ cation always accompanied the NCs. Although they have almost the same crystal structure, the negatively charged NCs differ from the neutral NCs in three ways, where the highest isolated yield of (E)-chalcone was 76% from propargylic acetate in DMSO in an alkaline environment for the negative NC: 1) the reaction does not occur under neutral or acidic conditions; 2) water is indispensable, and 3) strong bases are not beneficial for the desired reaction. Two possible reaction pathways have been proposed, but an SN2′ addition pathway with OH– as the nucleophile appears to be the most plausible. It should be noted that Au25(SR)18– is recoverable and recyclable, thereby making this catalyst highly advantageous. The reaction is also solvent sensitive, as shown by Scheme 7, where DMSO is regarded as a linker that connects 39

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the NC anion and OH– together with the substrate. Similar to the Au99 NCs in the previous example, the UV-Vis absorption spectra and MALDI-MS analysis results both demonstrated that Au25(SR)18– is stable during the reaction. Maity et al. [210] reported the synthesis of Cu30 NCs encapsulated in PAMAM−OH, which can serve as an excellent hydrogenation precatalyst for carbonyl and olefin reduction in water. For example, the hydrogenation of nitrobenzene and analogues achieved a conversion rate of 90% and the selectivity for related alcohols was up to 100%. The activity increased monotonically with dendrimer generation and it depended on the NC size.

4.1.3. Electrocatalysis and photocatalysis Au NCs with capping ligands can also be utilized in photocatalysis and electrocatalysis, where they have distinct properties such as high stability and quantized capacitance charging [29]. Yu et al. [221] reported an Au25(SR)18/TiO2 composite with enhanced visible light-induced photocatalytic activity during the degradation of methyl orange. The Au25(SR)18 NCs were absorbed onto TiO2 and then treated at 120C in vacuo. There were no differences in the activity of the bare TiO2 and Au25(SR)18/TiO2 under UV light irradiation, thereby indicating that the ligand-coated Au NCs cannot promote the photo-reaction under UV light. By contrast, Au25(SR)18/TiO2 exhibited superior activity compared with the bare TiO2 under visible light irradiation. Two mechanisms might explain the considerably enhanced photocatlytic activity of Au25(SR)18/TiO2 under visible light (Fig. 9). Thus, the 40

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promotional effect of the photocatalytic performance may be ascribed to quantum confinement within the Au25(SR)18 NC with a band gap of Eg = 1.3 eV, similar to a semiconductor, which could be effectively excited by visible light inducing photo-electron injection from Au25 NC to the conduction band of TiO2. The photo-electron is subsequently involved in the formation of hydroxyl radicals, which further oxidize the dye. 1O2 is also produced on Au25 under visible light and 1O2 is also involved in the degradation of methyl orange. However, a limitation of this study is the lack of direct evidence to support the role of Au25SR18 as the actual catalyst.

Xiao et al. [222] reported hybrid photocatalysts where the building blocks comprised GSH-capped Au NCs and highly ordered NP-TNTAs. The heterogeneous Aux/NP-TNTAs were utilized in various photocatalytic reactions, including photooxidation of the organic pollutant methyl orange, the photocatalytic reduction of aromatic

nitro

compounds

(4-nitroaniline,

4-phenylenediamine,

and

1-chloro-4-nitrobenzene), and PEC water splitting under simulated solar light irradiation. A synergistic interaction occurs between the ligand-protected Aux NCs and NP-TNTAs,

which

in

combination

with

the

heterogeneous

structures

of

Aux/NP-TNTAs enhances light absorption by the catalyst, especially in the visible region, thereby contributing to the significantly enhanced photocatalytic and PEC water-splitting performance. The proposed mechanisms for the photocatalytic and PEC water-splitting activities are presented in Fig. 10. Figure 10A indicates that Na2S is the electron-donor, which quenches the photoexcited holes and the electrons react 41

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with H2O. Figure 10B suggests the formation of large quantities of reactive oxygen species.

Tatsuma et al. [223] found that Au25(GSH)18 is effectively adsorbed onto a TiO2 electrode because the carboxylic group in –GSH readily binds to the TiO2 surface. Under visible light irradiation, the photogenerated electrons from the –GSH group are injected rapidly into the TiO2 conduction band with an internal quantum yield of ~60%. Thus, the photoelectrode has been utilized in various visible light-induced photocatalytic reactions, including the photooxidation of phenol derivates and ferrocyanide, and the reduction of Ag+, Cu2+, and dissolved oxygen. Useful catalytic activities were observed, which indicate that these Aun NCs possess suitable HOMO-LUMO levels for these photocatalytic reactions [224, 225]. These Au NCs have been used as photo-sensitizers to extend the absorption spectra of hybrid NCs/semiconductor photocatalysts into the visible light region due to the quantized energy levels of NCs and the facile electron transfer from excited NCs to the conduction band of the semiconductor. Chen et al. reported the modification of mesoporous TiO2 films by thiolate-protected Au NCs [226], which yielded a stable photocurrent of 3.96 mA cm–2 and a photo energy conversion efficiency of 2.3% under AM 1.5 illumination. The performance of solar cells based on these Au NCs/TiO2 films is comparable to that of CdS quantum dot-sensitized solar cells. The composite material was also utilized for the photocatalytic H2 generation in both a PEC cell and a photocatalytic slurry reactor [227]. In the PEC system, the 42

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incident photon-to-charge carrier efficiency using Au NCs/TiO2 films as the working electrode reached 2.9% and it increased further to 4.8% when 0.1 M EDTA was present in the electrolyte as the electron donor. In the photocatalytic slurry reactor, Au NCs were used to sensitize Pt/TiO2 NPs, where the H2 generation rate using this hybrid photocatalyst reached 0.3 mmol of hydrogen/(h·g) with the Au NCs under visible light irradiation. The H2 generation rate was significantly enhanced by five times when EDTA was added as a sacrificial reagent. These applications of Au NCs as photo-sensitizers are based on their quantized electronic energy levels and suitable energy level positions. Further investigations have shown that the electron transfer properties and excited states of these Au NCs are size dependent [228]. Thus, the reduction activity decreases as the Au NC size increases (Au25 < Au18 < Au15 < Au10−12). Therefore, the intrinsic electronic structures of different Au NCs with various core sizes are proposed to be responsible for the distinct activities based on a strong correlation between the ligand-to-metal charge-transfer excited state lifetime and photocatalytic activity. Au NCs with different atomic compositions are deposited onto a glass carbon electrode before the electrochemical reduction of O2. Chen et al. [229] found that the activity of the ORR is highly dependent on the size of the Au NCs loaded onto the electrode, where smaller Au NCs always exhibit higher activity, according to the onset potential and peak current density. The activities of these ligand-protected Au NCs have been compared with those of the deligated NCs and appreciable voltammetric currents were detected, thereby indicating that the O2 is readily accessible to the Au 43

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NCs without blocking by the protecting ligands.

Recently, Lu et al. [230] reported the selective electrochemical reduction of O2 to produce H2O2 using Au25(SC12H25)18 NCs as the precatalyst. The influence of the charging state of Au25 NCs (+1, 0, –1) on the reactivity was studied, which showed that the maximum H2O2 production rate (~90%) was obtained from the negatively charged NCs (Au25–). The characterization of Au25 NCs with different charging states combined with DFT calculations suggests that the high yield of H2O2 on Au25– is due to electron transfer from the anionic Au25 core into the LUMO (π*) of O2, thereby resulting in the release of a peroxo-like species. Based on the energy diagram for Au25(GSH)18 depicted in Fig. 5, the potential of the conduction band was estimated to be less negative than that of the LUMO level.

The steady adsorption of CO2 onto Au25(SCH2CH2Ph)18 NCs in DMF was observed based on changes in the optical absorbance of Au NCs after CO2 introduction, where DFT calculations demonstrated the strong interaction between CO2 and the S atoms in the shell of the Au NCs [231]. This unique property does not exist in conventional Au NPs, so the Au25 NCs were deposited onto carbon black for the electroreduction of CO2. The Au NCs exhibited promising activity, where the formation of CO was within 90 mV of the conventional CO2-CO formal potential (Fig. 12). Unsurprisingly, the NCs had a superior capacity for reduction compared with larger Au NPs on a per accessible gold atom basis. 44

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The Au NCs have been investigated in electrooxidation processes as well as electroreduction reactions. Thus, Kumar et al. [232] prepared an Au25(GSH)18 modified electrode for the oxidation of various compounds, such as ascorbic acid and dopamine, where the catalytic oxidation activity was excellent over a wide linear range from 0.71 to 44.4 μM. In addition, a pH-dependent electrocatalytic activity was observed and an amperometric method was developed for sensing various compounds based on this feature (Fig. 13). It has been suggested that the NCs initially in the form of Au25– are oxidized and the resulting Au250 then oxidizes the reduced analyte to complete a catalytic cycle (Figure 13).

Chen et al. [233] developed a one-pot procedure for synthesizing Cun NCs (n < 9) protected by 2-mercapto-5-n-propylpyrimidine via a wet chemical reduction. ESI-MS analysis showed that the molecular composition of the NC was CunL4 (L = C7H9N2S). The NCs exhibited apparent photoluminescence with dual emissions at 425 and 593 nm, where the quantum yields were 3.5% and 0.9%, respectively. The NCs were employed in electrocatalytic oxygen reduction reactions. The onset potential of O2 reduction (–0.07 V) was comparable to that of Au11 NCs (–0.08 V) and some commercial Pt catalysts; hence, this low-cost electrocatalyst merits further attention as a cathode material in alkaline fuel cells. Atomically precise Pt NCs are also effective electrochemical O2 reduction catalysts. Yamamoto et al. [162] prepared Pt NCs with different atomic numbers, i.e., 12, 28, 60, using PAMAM dendrimer as the template. The as-prepared Pt NCs 45

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catalysts were utilized in the ORR, where the Pt NCs exhibited much higher catalytic activity compared with a conventional Pt/C catalyst on a per surface Pt atom basis, thereby suggesting that the ORR was not inhibited by the dendrimer shell. It was concluded that the turn-over rates were enhanced by reducing the particle size to the subnanometer scale. Kim et al. [234] designed and synthesized a highly durable Ptn/genomic double-stranded

DNA–GO

(Ptn/gDNA–GO)-based

ORR

electrocatalyst.

The

gDNA-GO support was first synthesized to load Ptn NCs via a NaBH4 reduction process under ambient conditions. The interaction between gDNA and GO provided an easy means of positioning Ptn NCs on GO from Pt ions. The as-synthesized Ptn/gDNA–GO composite exhibited a greater ORR onset potential, ORR half-wave potential, specific activity, and mass activity compared with Pt NPs/GO and Pt/C catalysts. The superior stability and anti-corrosion capability make this a promising method for the effective synthesis of other noble metal NCs on GO supports with excellent ORR performance.

4.1.4. Other reactions Recently, a newly developed Au NCs was utilized in the hydroamination of 1-octyne with aniline at room temperature in much milder conditions compared with that required by an Au NP precatalyst (100C) [235]. The preparation of Au NCs involves the encapsulation of NPs in ICRMs [236] and Scheme 8 depicts the reaction pathway. The size of the NCs can be controlled by the water/surfactant and the 46

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aurate/surfactant ratio employed during synthesis. Starting from a series of alkynes, the Au NC catalyst provides a moderate yield of the corresponding aldehydes when used as-prepared, but the catalyst also becomes much more reactive when the ICRM core is incorporated into metal oxides via the sol-gel method.

Shimizu et al. [237] reported the H2-C3H8 SCR using MFI zeolite-supported Ag NCs. The supported NCs were prepared by the ion exchange method using AgNO3 as the Ag precursor. The overall reaction mechanism is shown in Scheme 9, which indicates that the addition of H2 accelerates C3H8 oxidation to acetate and NO oxidation to NO2. The reaction of acetate and NO2 forms CH3NO2 and NCO– species as intermediates, which are further converted into NH4– and finally into N2. The Ag4+ clusters obtained from the H2 reduction of the Ag ion promote the reduction of O2 to the superoxide ion (based on a gyy value of 2.009 and gzz value of 2.030 in the ESR spectra), thereby oxidizing C3H8 and NO.

Vilar-Vidal et al. [238] reported the effective reduction of methylene blue by size-selected Cun NCs (2 ≤ n ≤ 20) in the presence of hydrazine. Cu NCs with three sizes (Cu5, Cu13, and Cu20) and stabilized by tetrabutylammonium nitrate were prepared by the electrochemical reduction of Cu ion on the Pt cathode from the Cu anode in the electrolyte. The NCs were very stable compared with Cu NPs due to their large HOMO-LOMO band-gap (Fig. 14). The LUMO bands of Cu5 and Cu13 are 47

Page 47 of 104

located above the redox potential of methylene blue, but below that of hydrazine, so these two NCs exhibit catalytic activity during methylene blue reduction by hydrazine. The reduction capacity was highly dependent on the size of the NCs, where Cu5 had the optimal activity and Cu20 did not catalyze the reaction. The Cu13 catalyst exhibited extreme stability after performing the reaction cycle up to 42 times, which indicates the possible application of these NCs to other reduction reactions. In another study, Cu NCs efficiently catalyzed methylene blue reduction, thereby suggesting their potential applications in biosensing [239].

Atomically precise Pt12 NCs were synthesized by Takahashi et al. [240] using a TPM G4 dendrimer. Next, the Pt12 NCs were loaded onto GMC to prepare the heterogeneous catalyst Pt12@TPM G4/GMC, which was utilized in the catalytic reductive amination of aldehydes with amines. Pt12@TPM G4/GMC had a much higher activity than conventional Pt NPs on a per Pt basis. For instance, cyclohexylamine was quantitatively converted into the related imine with 100% selectivity over a Pt12@TPM G4/GMC catalyst, whereas the conversion rate for cyclohexylamine only reached 62% when Pt NPs was employed. In addition, the Pt NCs had greater tolerance of surface poisons in the absence of protic acids. Atomically precise metal NCs have not yet been applied to C-C coupling reactions, but these reactions have been examined extensively over click dendrimer-stabilized Pd NCs with estimated atom numbers [241, 242]. These Pd NPs 48

Page 48 of 104

are stable and they retain their catalytic activities in the Suzuki-Miyaura reaction for months [243]. Pd NCs stabilized by dendrimers containing 1,2,3-triazolyl ligands were synthesized by Orlenas et al., who found that the terminal group on the dendrimer significantly affected the catalytic performance of the Pd NCs [244]. It should be noted that the integrity of ligand-protected NCs after catalytic reactions has not been considered often, except for the hydrogenation reactions over Au25(SR)18 and Au99(SPh)42. Thus, future studies should monitor the integrity of NCs and determine whether they are the actual catalysts or precatalysts [245-247]. 4.2. Catalysis by ligand removed/partially removed MnLm NCs Ligands that function as stabilizers are crucial for the preparation of ultrasmall MnLm NCs, but clean, unpoisoned surfaces on the MnLm NCs are generally considered to be necessary to activate the reactants and obtain high catalytic activities. Thus, the development of ligand removed or partially removed NCs as catalysts has also attracted significant interest. Studies based on Ir NCs have shown that starting from an Ir metal-organic framework as the precursor, “weakly ligated/labile ligand” Ir(0) NCs or “solvent-only” stabilized Ir(0)n NCs can be obtained after the hydrogen reduction process, as discussed in Section 3.2. These Ir(0)n NCs have been used as prototype catalysts in reduction reactions, where their activities were superior to those of both NP catalysts and precatalysts without ligand removal [170-172]. However, it is often difficult to remove ligands from noble metal clusters such as Aum(SR)n, as discussed earlier. Somorjai et al. demonstrated that the ligand residues from NPs plays important roles in catalytic reactions [182, 183]. A similar effect may 49

Page 49 of 104

also hold in NC catalysis and previous studies have provided several examples, as follows. 4.2.1. Oxidation reactions 4.2.1.1. CO oxidation CO oxidation has been investigated widely using Au NCs [248, 249]. Recently, Wu et al. [250] investigated CO oxidation on CeO2 nanorod-supported Au25(SCH2CH2Ph)18 NCs based on experiments and DFT calculations, where they found that the partial removal of the thiolate ligand by calcination greatly enhanced the CO oxidation activity at room temperature. The Au25/CeO2 nanorod catalysts were calcined at different temperatures between 423 K and 673 K. All of the catalysts pretreated at a temperature above 423 K exhibited enhanced activity in CO oxidation, i.e., 39 times higher compared with the untreated sample, because of the partial removal of the ligands capped on the Au NCs. The results obtained using IR and EXAFS techniques suggest that the active sites for CO activation are temperature dependent, where partially positively charged Auδ+ (0 < δ < 1) species are the active sites at low temperature, whereas Au+ and Auδ– (0 < δ < 1) are the active species above room temperature. Isotopic labeling was conducted using

18

O2 or

16

O2 as the

oxidant for CO oxidation. As shown in Fig. 15, CO activated on the interface of Au sites reacted with the lattice oxygen on the CeO2 nanorod rather than the O2 molecule adsorbed on the catalyst. A MvK channel was proposed as the main mechanism for CO oxidation on the CeO2-supported Au25 NCs rather than a L−H mechanism. However, there was experimental evidence for both reaction channels during the 50

Page 50 of 104

oxidation process when the ligands were removed more thoroughly, as demonstrated by the smaller Au-S shell according to the in-situ EXAFS spectrum for Au25/CeO2 nanorod catalysts pretreated at 523 K. However, it is unclear whether these observations can be generalized to Au NCs with different sizes.

Au144(SR)60 NCs supported on transition-metal-oxide modified ordered mesoporous silica (EP-FDU-12) were also investigated as precatalysts for CO oxidation [174]. The Au144(SR)60 loading concentration was at a much higher level (1.6%) compared with previous studies (usually below 0.2% to prevent particle sintering during the thermal treatment). The Au144/MOx-mSiO2 sample exhibited impressive particle sintering resistance after thermal treatment at 300C compared with Au144/mSiO2 without the MOx modification. The strong interaction between the transition metal oxide and mSiO2-stabilized Au NCs is a major factor that prevents aggregation after removing the thiolates. The mesoporous SiO2 also has a confinement effect on the transition metal oxide, which improves the stabilization of Au NCs. Without calcination, these catalysts are inactive during CO oxidation because the thiol groups cover the Au NC surface (Fig. 16). After calcination at 300C in air, it was found that both Au144/CuO-mSiO2 and Au144/Co3O4-mSiO2 effectively promoted CO oxidation at low temperature with 1.3% and 2% Au144NC loadings. By contrast, Au144/ mSiO2-300C with a similar loading (1.6%) had negligible activity below 200C due to the aggregation of the Au NCs during calcination. This is a new strategy for preparing 51

Page 51 of 104

heterogeneous catalysts with high NC loadings on supports but without appreciable size increases during thermal treatment, specifically for using Aun/MOx-mSiO2 catalysts.

4.2.1.2. Epoxidation Another reaction that distinguishes the unique role of Au NCs from traditional supported Au NPs larger than 2 nm is the selective epoxidation of alkenes, which is known to be an important transformation process during fine chemical production [251]. Styrene has been used widely as a substrate for evaluating the performance of supported Au NCs using either O2 or a peroxide (such as TBHP) as the oxidant. For example, Turner et al. [252] prepared Au55(PPh)12Cl6 NCs loaded onto BN and SiO2 with a loading of 0.6 wt%, before applying thermal treatment to remove the ligands. They also prepared other Au/SiO2 samples using the traditional PVP stabilization method, microemulsion method, and incipient wetness method. The 0.6 wt% Au55/BN or 0.6 wt% Au55/SiO2 catalysts obtained retained monodispersed Au55 NCs on the supports with a mean diameter of 1.4 nm, and they achieved a similar styrene conversion rate (~20%), as well as exhibiting similar selectivity for benzaldehyde (~80%), styrene oxide (~15%), and acetophenone (~5%). However, other supported Au NP samples with mean Au diameters of ~4, ~17, and ~30 nm were completely inactive (Table 4). It was suggested that the decrease in the size of Au NPs is associated with an increase in the d-electron density of the Au atoms and the onset of reactivity toward oxygen in air. Styrene is assumed to be adsorbed weakly on the 52

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surfaces of Au NCs but without strongly perturbing their electronic structures, thereby avoiding complete oxidation into CO2 and H2O. Thus, it has been suggested [252] that the co-adsorption of styrene and O2, as well as the activation of O2 to dissociate into active O atoms, allows selective styrene oxidation, although other possible mechanisms [253, 254] cannot be excluded.

Liu et al. [255] tested an Au25(SR)18/HAP precatalyst during styrene oxidation using TBHP as an oxidant. The thiol ligands were removed by calcination of the catalyst at 300C in vacuo. The uniformly dispersed Au NCs on HAP exhibited a flattened morphology after the ligands were removed (Fig. 17). The reaction was conducted in toluene at 80C using anhydrous TBHP as the oxidant. After 12 h, the conversion of styrene reached 100% with a high yield of styrene oxide (92–95%), which was much higher than that using conventional Au nanocatalysts (50–60%) with the same loading concentration (0.5%) based on the same amount of gold. The high conversion rate and excellent selectivity were attributed to the combined effects of the solvent (toluene) and oxidant (anhydrous TBHP). Zhu et al. [256] studied three systems using Aun(SR)m/SiO2 and Aun(SR)m/HAP under different oxidation condition: 1) TBHP as the oxidant, 2) TBHP as the initiator and O2 as the oxidant, and 3) O2 as the oxidant. As expected, the conversion rate of styrene in these three systems followed the order of: 1) > 2) > 3). After thermal pretreatment, the catalysts exhibited greater activity than the precatalysts without

53

Page 53 of 104

calcination. The conversion of styrene over Au NCs with different atomic compositions and sizes followed the order: Au25 > Au38 > Au144, thereby highlighting the dependence of the catalytic activity on the particle size. It should be noted that these NCs all possessed non-metallic electronic structures that differed from conventional Au NPs (>2 nm), where they exhibited collective electronic excitation (i.e., plasmon excitation), which indicated a quantum confinement effect. Aun(SR)m/HAP always exhibited higher activity than Aun(SR)m/SiO2 under identical reaction conditions, thereby suggesting a critical role for the SiO2 support. HAP had no activity so the major role of HAP is to stabilize the Au NCs and to promote the catalytic activity indirectly. The proposed mechanism for the selective oxidation of styrene catalyzed by Au25(SR)18 is illustrated in Fig. 18, which shows the three reaction pathways and the formation of the selective product. These pathways are differentiated according to induction of the reactivity of the oxidants and the production of the Au25-O2(ad) intermediate (species D).

Liu et al. [257] correlated the atomic and electronic structures of Au NCs with their catalytic properties during styrene oxidation using Au25-bi (counter ion Cl–) and Au25-i (counter ion TOA+) NCs supported on SiO2. Based on XAFS and ultraviolet photoelectron spectroscopy, they concluded that due to the higher d-band depletion of Au sites in Au25-bi compared with those in Au25-i, more d-band electrons are transferred from Au to the ligands in Au25-bi, thereby making the Au sites in Au25-bi more electropositive. Thus, uncalcined Au25-bi NCs produced more oxidized 54

Page 54 of 104

benzaldehyde over styrene oxide, whereas uncalcined Au25-i exhibited comparable selectivity between these two products. It is possible that the direct contact between TBHP and Au25-i NCs promotes the hybridization of TBHP molecular states with the Au25-i d-band states, thereby facilitating the formation of benzaldehyde from styrene oxidation. A mechanism similar to that proposed by Jin’s group (Fig. 18) was suggested [225]. Thermal treatment for the removal of surface ligands is an aggressive method and it sometimes leads to sintering of the Au NCs even at a low loading concentration. Das et al. [175] used SH-grafted SBA-15 as the support to prepare heterogeneous precatalysts using Au25(SCH2CH2Ph)18 and Au144(SCH2CH2Ph)60 NCs. The thiol ligands were effectively removed after the NaBH4 treatment, but without comprising the small size of the Au NCs, where the conversion and selectivity for benzaldehyde during styrene oxidation were increased dramatically on supported, bare Au NCs compared with the thiol ligand-protected Au NC catalyst (24 h, 100% conversion and 100% selectivity for benzaldehyde vs 30% conversion and 100% selectivity for benzaldehyde over the thiol ligand-protected Au NC catalyst) (Fig. 19) [175]. This study also obtained well-defined Aun catalysts for use in mechanistic studies of selective styrene oxidation over supported, deligated Au NC catalysts.

Lei et al. [261] studied direct propylene epoxidation by Ag NCs with various sizes (from Ag3 to 3.5 nm Ag NPs). The Ag3 NCs were prepared with a mass-selected 55

Page 55 of 104

technique using a quadrupole mass filter and deposited on the support with 2.2% atomic monolayer coverage. The Ag3 NCs and the silver aggregates exhibited the highest activity reported to date for propylene oxide formation in O2 at 110C, with a turnover rate of ~1 s–1 per surface Ag atom, whereas the turnover rate of previously reported Ag based catalysts is no higher than 10–2 s–1 [262, 263]. In addition, the turnover rate for the Au6-10 NC catalyst was only ~0.2 s–1 under identical reaction conditions [264], which is only one-fifth of that for Ag3 NCs with comparable selectivity for propylene oxide. Based on in-situ XPS analysis, the Ag3 NCs had a 0.2–0.4 eV lower binding energy in the Ag 3d core level compared with that of small silver NCs with similar size, thereby indicating that the Ag3 is partially oxidized during the reaction. DFT calculations confirmed that Ag3O is the key intermediate species in this reaction.

4.2.1.3. Aerobic oxidation Triphenylphosphine-protected Au11 NCs were immobilized onto mesoporous silica (SBA-15, MCF, and HMS) in a mixture of CH2Cl2 and C2H5OH, followed by careful removal of the ligands by heating at 200°C for 2 h in vacuo [265]. The resulting Aun/SBA catalysts had a polydispersed size of 0.8 ± 0.3 nm. Using H2O2 as the oxidant under microwave-assisted conditions, Au~11/SBA catalyzed the oxidation of benzyl alcohol with 100% conversion and over 90% selectivity for benzoic acid in 1 h. This catalytic system was generalized to other primary benzylic alcohols, where excellent conversion (≥ 85%) and selectivity (≥ 80%) were achieved. Au NPs larger 56

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than 2 nm prepared using conventional methods exhibited much lower activity and the support effect was excluded because silica is known to be inert. Therefore, the superior catalytic activity was attributed to the size effect for these ultrasmall Aun NCs.

Xie et al. [266] synthesized Au25/CNT and Pd1Au24/CNT nanocomposites by depositing Au25(SC12H25)18 and Pd1Au24(SC12H25)18 onto CNTs, which was followed by calcination to remove the ligands (Fig. 20). Aggregation of the NCs was not observed according to TEM analysis. The prepared precatalysts were then employed during the aerobic oxidation of benzyl alcohol with O2 as the oxidant under ambient conditions. After 6 h, the conversion was 74% over Pd1Au24/CNT, which was much higher than that with the pure Au25/CNT composite (22%). Two possible mechanisms may explain the enhanced catalytic performance of the Pd1Au24/CNT catalyst: doping with the Pd atom could create a highly active reaction site on the surface of Au24 (a so-called ensemble effect); or the Pd atom may activate the Au sites by a ligand effect, where electrons are transferred from Pd to Au via the interaction between these two hetero atoms. Yaskamtorn et al. [268] prepared supported Au25 NCs via the calcination of Au25(SC12H25)18 on HPCSs in vacuo at different temperatures to gradually remove the capping thiol ligands. Catalysis using these Au NCs was evaluated during the aerobic oxidation of benzyl alcohol. Interestingly, the selectivity for benzaldehyde improved remarkably as the amount of residual thiolates on the Au25 increased, but the activity 57

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was reduced, as shown in Fig. 21. TEM observations confirmed that the size did not change after calcination and EXAFS results showed that Au25 NCs with residual thiolates were more positively charged than the bare Au25 due to electron withdrawal by the thiolates. Therefore, the suppression of further oxidation from benzaldehyde to benzoic acid was attributed to the more positively charged Au25 NCs, whereas the inhibited formation of the ester was attributed to a site isolation effect caused by the capped surface dodecanethiolate. Overall, these results demonstrate that the catalytic performance of small metal NCs can be controlled by modulating their electronic structure and the steric environment via chemical modification.

Cyclohexanol and cyclohexanone are very important materials in the nylon-related chemical industry. Liu et al. [270] explored a green conversion route from cyclohexane to cyclohexanol and cyclohexanone via oxidation over supported Au NCs. The Aun NCs were prepared with different sizes (n = 10, 18, 25, 39) and loaded onto HAP. The Aun NCs retained their size without dramatic sintering after calcination according to TEM due to the strong interactions between Aun NCs and the HAP support. However, it was not shown whether the NCs retained exactly the same number of atoms after treatment by heating. These Aun/HAP catalysts were utilized in the aerobic oxidation of cyclohexane with 1 bar O2 and 150C. The selectivities for the primary products of cyclohexanol and cyclohexanone were both ca. 50%, without the formation of adipic acid. The TOF values for these catalysts exhibited a 58

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volcano-shape with an optimal NC size of Au39. However, the nature of the reactive species, the reaction mechanism, and the origin of the strong size-dependent property remain unclear. Similarly, toluene oxidation was investigated over CNT-supported Pt NCs. In addition to its high efficiency, the metallic state of the Pt species was stable against O2 even when the temperature increased to 200C, which was ascribed to the encapsulation and restriction by CNT channels [271]. Zhang et al. [272] investigated the distinct interactions of thiolate-protected Au38(SC2H4Ph)24 with Al2O3 and CeO2. The removal of the thiolate ligands was studied under different atmospheres (N2 and air) during calcination. The ligand removal process yielded metallic Au in the case of Al2O3 and cationic Au with CeO2. The pretreated catalysts were more effective than the untreated catalysts during the aerobic oxidation of cyclohexane due to the exposure of more active sites after ligand removal. The NCs supported on CeO2 performed better than those on Al2O3 due to the larger proportion of cationic Au. As expected, cyclohexanethiol was observed, which indicates that the thiol ligand blocked the active sites on the catalytic surface but it also participated directly in the catalytic cycle.

4.2.2. Selective hydrogenation Li et al. [273] reported the synthesis of two Au NCs, Au25(SC2H4Ph)18 and rod-shaped Au25(PPh3)10(C≡CPh)5X2 (X = Br, Cl), which were both supported on TiO2 and they exhibited very high catalytic activity during the semihydrogenation of 59

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terminal alkynes into alkenes, with a conversion rate >99% and ∼100% selectivity for alkenes. A new activation mode by ligand-on Au25 NCs was discovered, i.e., the deprotonation activation pathway via an R'—C≡C—[AunLm] (where L represents the protecting ligands on the cluster) intermediate, which differs greatly from the activation mechanism based on conventional Au NPs. As shown in Fig. 22, the open triangular Au3 facet on the spherical Au25(SR)18 NCs and the waist sites of the rod-shaped Au25(PPh3)10(C≡CPh)5X2 (X = Br/Cl) NCs are the proposed active sites during semihydrogenation. However, using deligated catalysts, the internal alkynes could undergo semihydrogenation to yield Z-alkenes in a similar manner to conventional Au NP catalysts.

Fang et al. [173] reported the preparation of heterogeneous Au25(MHA)18 precatalysts supported on various solid materials, such as HAP, P25, AC, GO, and SiO2. The hybrid catalysts were utilized in the selective hydrogenation of nitrobenzene to aniline. The as-prepared, ligand-on Au25 catalysts had no catalytic activity, which indicates that the active sites on the surfaces of the Au NCs are blocked before ligand removal. After calcination, Au25 NCs supported on HAP and P25 exhibited excellent activity, thereby facilitating the quantitative conversion of nitrobenzene to aniline. The high reactivity was attributed to the retention of the Au size after calcination as well as the strong interaction between the Au NCs and the HAP support.

60

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4.2.3. Photocatalysis Noble metals such as Au and Pt are utilized widely as co-catalysts (promoters) in combination with typical semiconductors (such as TiO2 and ZnO) to achieve efficient solar energy conversion. In general, the noble metal is in the form of NPs, which are prepared by photodeposition or impregnation onto the semiconductors. The use of smaller NPs can in principle obtain a higher photo-activity, but conventional methods cannot control the size of the NPs down to the atomic level. Thus, metal NCs provide a

new

opportunity

for

improving

the

photocatalytic

performance

of

metal/semiconductor photocatalysts due to their ultra-small size and unique electronic structures.

New

protocols

have

been

developed

for

synthesizing

these

Au/semiconductor nanocomposites as photocatalysts with heterostructures. In particular, Lee et al. prepared GSH-coated monolayer-protected Au NCs and anchored them onto TiO2 as the co-catalyst for use in the photo-degradation of Uniblue [274]. The photocatalysts were treated at different temperatures, e.g., 250C and 400C, in air to activate the photocatalytic activity. The activities were quite similar (low) before and after loading the Au NCs on TiO2 without calcination, thereby indicating that Au NCs with heavy ligand shielding are not photocatalytic co-catalysts. However, after partial ligand removal via thermal treatment at 250C, the photodegradation of Uniblue by the Au NCs-TiO2 catalyst was enhanced remarkably, which was attributed to the enhanced charge separation in the calcined composites, where the partial removal of the ligands greatly facilitated the rapid electron transfer from TiO2 to Au to achieve effective charge separation (Fig. 23). After further 61

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increasing the heating temperature to 400C to allow the complete removal of the ligands, the photocatalytic activity of the catalyst declined dramatically. This result may also be explained by the ligand effect, where the sulfur in the thiol ligands might react with TiO2 when the organic spacers between the sulfur and TiO2 are removed at 400C, and the sulfur residues on the TiO2 surface (i.e., TiSx) can then act as recombination centers for the photo-electrons and -holes, thereby attenuating the photocatalytic activity. The molecular composition (specific atomic number) of the Au NC was not identified but this should be considered in future research.

A hot topic in photocatalysis is the development of catalysts that can operate in the visible light range because only 5% of the photo-energy in the solar spectrum comes from the UV region. Au NCs/semiconductor materials have potential applications in H2 production under sunlight irradiation. Thus, Negishi et al. [275] developed a new photocatalyst by loading Au25(GSH)18 onto BaLa4Ti4O15, followed by the thermal removal (300 C, 2 h, in vacuo) of the ligands. The nanocomposite was utilized during photocatalytic water splitting to produce H2 and O2 simultaneously. After the heating treatment, the thiol ligands were removed almost completely from all of the samples because the S 2p peak was not detected by XPS analysis (Fig. 24). When the loading concentration was reduced to 0.1 wt%, NC agglomeration was not found after the heating treatment. This catalyst exhibited better performance in terms of H2 and O2 generation compared with other samples with higher Au loadings because a gradual 62

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increase in the Au particle size was observed in the latter samples. Remarkably, the 0.1 wt% Aun/BaLa4Ti4O15 catalyst produced 2.6 times more H2 and O2 than 0.5 wt% AuNP/BaLa4Ti4O15,

which

was

the

most

active

among

the

traditional

AuNP/BaLa4Ti4O15 catalysts. The superior activity was again attributed to the size and electronic effects in the ultrasmall Au NCs. The same quantity of active sites could be introduced by loading lower quantities of Au into Aun/BaLa4Ti4O15 compared with conventional methods. The Aun NCs exhibited unique electronic structures, which promoted the water splitting reaction and suppressed the reverse reaction [276]. Nevertheless, further experimental and theoretical studies are needed to elucidate the source of the enhanced photocatalytic activity obtained after incorporating Aun NCs and semiconductors.

Subsequently, the same group [277] loaded a series of Aun(GSH)m NCs (n = 10, 15, 18, 22, 25, 29, 33, 39) onto BaLa4Ti4O15, where the supported Au NCs photocatalysts were categorized into two groups: group 1 (n = 10, 15, 18, 25, 39) and group 2 (n = 22, 29, 33). After ligand removal via thermal treatment (300C, 2 h, in vacuo), the size distribution of the Au NCs from group 1 remained narrow (≤ ±25%) with only a slight increase in size (from 0.1 to 0.5 nm) due to the change in structure caused by the loss of ligands after calcination (no direct evidence was provided for the exact structure after ligand removal). By contrast, the average particle size of the group 2 NCs increased by ca. 1 nm (estimated from TEM images) after calcination. The thermal stability of the NCs appeared to be closely related to their stability 63

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against degradation in solution because the group 1 Au NCs had greater stability than those in group 2 in the solution phase. The sizes of the group 1 Au NCs were retained better and they were utilized in photocatalytic water-splitting, which showed that they were at least two times more active than the conventional Au NPs examined with the same Au loading. The improved activity caused by the ultra-miniaturization of the co-catalysts was attributed to the increased number of surface Au atoms, which overcame the reduced activity. Furthermore, the catalytic activities of the ligated and deligated Au25/BaLa4Ti4O15 were compared, where the activity of the deligated catalyst was four times higher than that of the ligated catalyst, which suggests either that the efficiency of the electron relay between Au NCs and BaLa4Ti4O15 decreased, or that the water reduction performance of the individual surface Au atom decreased when the ligands were present. In each scenario, the removal of the ligands is essential for the Aun/BaLa4Ti4O15 catalysts to achieve efficient photocatalytic water splitting. It should be noted that there is still a lack of effective strategies for determining the actual amounts and compositions of ligand residues after ligand removal, although the exact amounts of ligands or residues may be crucial for promoting several NC catalyzed reactions [35]. Importantly, EXAFS and FTIR techniques have been employed to probe the existence and nature of the S-containing residues in Au NCs [173, 178]. EXAFS can detect the existence of Au-S interactions, although with low sensitivity, and FTIR can characterize the nature of the S-containing functional groups. However, quantitative and qualitative analysis, as well as clarifying the roles of ligand residues in catalytic reactions, will be of fundamental importance in future research to 64

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improve NC catalysis after ligand removal. The stability of supported NCs after ligand removal is another issue that needs to be examined in much greater detail. For example, the Au-Au bond energy in NCs is 2.43 eV [278], ca. 234 kJ mol–1, and the related Hvaporization is ca. 368 kJ mol–1 [279]. These numbers are low for the stabilization of bare Au NCs, and thus the metal-support interactions play a critical role in stabilizing NCs. Moreover, Klabunde et al. found that a thiol ligand could etch polydispersed Au NPs in the solution phase to undergo digestive ripening, thereby converging them into monodispersed Au NPs [280-282]. It is not clear whether a similar etching process occurs during the ligand removal process with supported NCs in the solid state. Thus, a key challenge is the lack of effective techniques for characterizing the actual number of atoms in NCs after loading them onto solid supports, especially when the ligands were removed. At present, the usual strategy is to compare the size and the size distribution of NCs based on HRTEM images before and after ligand removal, and/or before and after the catalytic reaction. This indicates whether the size of the NCs can be preserved, but it does not show the number of atoms in a single cluster. We hope that the development of HRTEM and other advanced instrumentation techniques will help to solve this issue, thereby clarifying whether we are dealing with supported atomically precise NCs or poly-dispersed NPs, as well as more clearly distinguishing the different catalytic activities of NCs and NPs [241].

5. Outlook 65

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The atomically precise synthesis of metal NCs can facilitate direct observations of the catalytic performance of single component NCs, thereby providing a fundamental understanding of structure-catalytic activity correlations. NCs inherently have a very large surface-to-volume ratio and their surfaces are enriched with low-coordination atoms. The atom configurations of the NCs are often distinct from those of the bulk and conventional NPs. The ultrasmall size of NCs yields discrete energy levels, thereby inducing strong electron energy quantization. All of these features significantly influence the behavior of NCs during catalytic reactions, which can explain their specific performance characteristics (sometimes they are superior but others are not). Thus, NCs can be used as novel model catalysts for mechanistic research, but they also have the potential to obtain the selectivity desired in challenging reactions. Ligand-protected NCs have been used directly in catalysis, where the capping ligands on the NCs have distinct effects in different reactions, i.e., they sometimes promote the reaction by enhancing the activity and/or selectivity, but in many other cases, they act as a poison by preventing contact between the reactant and the surface metal atoms. The exact roles of the ligands during NC catalysis remain unclear, and thus more extensive, systematic, and quantitative studies of ligand effects in NC catalysis are highly desirable in the future. It would also be useful to systematically and carefully compare the stability, activity, and selectivity of strong ligand-capped, atomically precise NCs with those of weakly ligated NCs of similar size. In addition, most recent studies of atomically precise NCs have focused on thiolate-capped Au 66

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NCs. Thus, future research should consider more diverse combinations of ligands and metals, as well as clusters bearing more readily removed ligands under desirable reaction conditions, such as weakly ligated NPs [170-172, 186-188]. Supported MnLm NCs catalysts, where the ligands are either thoroughly or partially removed, also have considerable potential in a number of reactions. Ligand removal could result in the exposure of more metal atoms to the reactants, thereby leading to higher catalytic activity. However, the partial removal of the ligand is critical for exceptionally selective reactions in many cases. Therefore, new strategies should be developed to allow the removal or partial removal of ligands from MnLm NCs at high loading concentrations, but without inducing aggregation. Moreover, identifying the precise number of atoms in the NC after ligand removal remains a major challenge, which needs to be addressed via advances in instrumentation and analysis. Finally, we should note that the structure of NCs is dynamic. Therefore, elucidating the structural evolution of supported MnLm NCs, especially under working condition, is another important and fundamental advance that is required to obtain a better understanding of reaction mechanisms. In-situ or in operando [247, 283-285], techniques, such as DRIFT-IR, XPS, and EXAFS, should be employed more intensively in these studies.

Acknowledgments N.Y. sincerely acknowledge the NUS Young Investigator Award (WBS: R-279-000-464-133) for financial support. J.F. and Y.Y. express their gratitude to the 67

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Project for Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and an open research grant from the State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201410). We thank the editor and the anonymous referees, particularly referee #2, for extensive help, including critical comments, valuable suggestions, and language editing.

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[269] C. Lavenn, A. Demessence, A. Tuel, J. Catal., 322 (2015) 130-138. [270] Y. Liu, H. Tsunoyama, T. Akita, S. Xie, T. Tsukuda, ACS Catal., 1 (2011) 2-6. [271] F. Zhang, F. Jiao, X. Pan, K. Gao, J. Xiao, S. Zhang, X. Bao, ACS Catal., 5 (2015) 1381-1385. [272] B. Zhang, S. Kaziz, H. Li, M.G. Hevia, D. Wodka, C. Mazet, T. Bürgi, N. Barrabés, J. Phys. Chem. C, 119 (2015) 11193-11199. [273] G. Li, R. Jin, J. Am. Chem. Soc., 136 (2014) 11347-11354. [274] M. Lee, P. Amaratunga, J. Kim, D. Lee, J. Phys. Chem. C, 114 (2010) 18366-18371. [275] Y. Negishi, M. Mizuno, M. Hirayama, M. Omatoi, T. Takayama, A. Iwase, A. Kudo, Nanoscale, 5 (2013) 7188-7192. [276] A. Iwase, H. Kato, A. Kudo, Appl. Catal. B: Environ., 136–137 (2013) 89-93. [277] Y. Negishi, Y. Matsuura, R. Tomizawa, W. Kurashige, Y. Niihori, T. Takayama, A. Iwase, A. Kudo, J. Phys. Chem. C, 119 (2015) 11224-11232. [278] J. Wang, G. Wang, J. Zhao, Phys. Rev. B: Condens. Matter, 66 (2002) 035418. [279] A. Takami, H. Kurita, S. Koda, J. Phys. Chem. B, 103 (1999) 1226-1232. [280] B.L.V. Prasad, S.I. Stoeva, C.M. Sorensen, K.J. Klabunde, Langmuir, 18 (2002) 7515-7520. [281] B.L.V. Prasad, S.I. Stoeva, C.M. Sorensen, K.J. Klabunde, Chem. Mater., 15 (2003) 935-942. [282] S. Stoeva, K.J. Klabunde, C.M. Sorensen, I. Dragieva, J. Am. Chem. Soc., 124 (2002) 2305-2311. [283] S.J. Tinnemans, J.G. Mesu, K. Kervinen, T. Visser, T.A. Nijhuis, A.M. Beale, D.E. Keller, A.M.J. van der Eerden, B.M. Weckhuysen, Catal. Today, 113 (2006) 3-15. [284] H. Topsøe, J. Catal., 216 (2003) 155-164. [285] P.J. Ellis, I.J.S. Fairlamb, S.F.J. Hackett, K. Wilson, A.F. Lee, Angew. Chem., 122 (2010) 1864-1868.

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Fig. 1. Overview of metal NCs and their catalytic applications.

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Fig. 2. (A) Space-filling model of Au25(SCH2CH2Ph)18 NCs (yellow, red, and green represent gold, sulfur, and ligand carbon atoms, respectively). (B) Simulated FT-EXAFS from Au25. (C) UV-Vis spectra of Au25(SCH2CH2Ph)18 and pure Au25NCs. (D) Mass spectrometry analysis of Au25(SCH2CH2Ph)18NCs. The figure has been reproduced from Ref. [34], Ref. [60], and Ref. [61], with permission from the American Chemical Society (Copyright 2011); and the Royal Society of Chemistry (Copyright 2009).

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Fig. 3. (A–D) Images illustrating the changes during Ag9(H2MSA)7NC synthesis. (E) PAGE of the crude mixture showing the presence of two bands in visible light. (F) UV-Vis profile of the pure Ag9(H2MSA)7NC. (G) Luminescence spectrum of Ag9(H2MSA)7 NC. Insets (a) and (b) show Ag9(H2MSA)7 NC under white and UV light, respectively, at 5°C. (F) TEM image of the PAGE-separated NC. The inset shows a schematic of the prepared Ag9(H2MSA)7 NC. The figure has been reproduced from Ref. [152], with permission from the American Chemical Society (Copyright 2010).

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Fig. 4. Schematic illustration showing the synthesis of highly luminescent metal (Au, Ag, Pt, or Cu) NCs via a phase transfer-assisted mild etching process. The figure has been reproduced from Ref. [156], with permission from Wiley-VCH (Copyright 2011).

Fig. 5. Schematic representation of platinum NC synthesis using phenylazomethine dendrimer as the template. (A) Stepwise complexation of PtCl4. (B) Pt12, (C) Pt28, and

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(D) Pt60 NCs encapsulated in the spherical macromolecular dendrimer shell. The figure has been reproduced from Ref. [162], with permission from Nature Publishing Group (Copyright 2009).

Fig. 6. Schematic representation of BSA-assisted Au cluster synthesis. The figure has been reproduced from Ref. [166], with permission from the American Chemical Society (Copyright 2009).

Fig. 7. Diagram illustrating the synthesis of supported NCs where the ligand is fully or partially removed.

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Fig. 8. Proposed heterolytic cleavage of hydrogen by Au50(SPO)30–33 NCs (right: ligand facilitated H2 activation) compared with the heterolytic cleavage of dihydrogen at the edges of Au NPs supported on oxide (e.g., TiO2) surfaces (left). No direct evidence has been provided for this tentative mechanistic scheme; hence, it still needs to be tested experimentally. The figure has been reproduced from Ref. [188], with permission from the American Chemical Society (Copyright 2014).

Fig. 9. Photocatalytic mechanisms involved in the degradation of methyl orange by Au25(SR)18/TiO2 under visible light irradiation. The figure has been modified from Ref. [221], with permission from the American Chemical Society (Copyright 2013).

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Fig. 10. Schematic depicting the proposed mechanisms of the reactions activated by the Aux/NP-TNTA heterostructure under simulated solar light irradiation: A) PEC; B) photocatalytic degradation of methyl orange, and C) photocatalytic reduction of aromatic nitro compounds. The figure has been modified from Ref. [222], with permission from Wiley-VCH (Copyright 2015).

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Fig. 11. Energy diagram for Au25(GSH)18, Au38(GSH)24 (Au CL-M), and larger examples (Au CL-L). The potential values are shown with respect to SHE. The figure has been modified from Ref. [225], with permission from the Royal Society of Chemistry (Copyright 2012).

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Fig. 12. Electrochemical reduction of CO2 in 0.1 M KHCO3 by a CB-supported Au25(SR)18 precatalyst. (a) LSVs of the CB-supported Au25(SR)18 precatalyst in N2-purged (pH = 9) and CO2-saturated (pH = 7) 0.1 M KHCO3. (b) Potential dependencies of the H2 and CO formation rates on Au25(SR)18/CB. (c) LSVs of various Au precatalysts in CO2-saturated 0.1 M KHCO3. (d) Potential dependencies of the CO formation rates on various Au precatalyst. The figure has been modified from Ref. [231], with permission from the American Chemical Society (Copyright 2012).

Fig. 13. Entrapped Au25-mediated electrocatalytic oxidation of an analyte and the ensuing electron transport across the sol-gel network attached to an Au electrode. Only one glutathione ligand is shown for clarity. The figure has been modified from Ref. [232], with permission from Wiley-VCH (Copyright 2011).

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Fig. 14. Schematic energy diagram showing the catalytic activity of different Cu NCs (Cu5, Cu13, and Cu20) used for methylene blue reduction by N2H4. The figure has been modified from Ref. [238], with permission from the American Chemical Society (Copyright 2012).

Fig. 15. Proposed CO oxidation mechanism on intact, partially, and fully dethiolated Au25(SR)18/CeO2 rod catalysts. The figure has been modified from Ref. [250], with permission from the American Chemical Society (Copyright 2014).

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Fig. 16. (A) Element mapping for Au NCs on CuO-EP-FDU-12; (B) Light-off curves for the oxidation of CO as a function of temperature over supported Au NC catalysts. The figure has been reproduced from Ref. [174], with permission from the Royal Society of Chemistry (Copyright 2012).

Fig. 17. (A) Representative HAADF-STEM image of 0.5Au25-HAP and (B) size distribution of the Au clusters. The bar in panel (A) represents 10 nm. Note that the size is 1.4±0.6 nm, i.e., 0.8–2.0 nm, which corresponds to a polydispersed Au~10 for Au~200. The figure has been modified from Ref. [255], with permission from the Royal Society of Chemistry (Copyright 2010).

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Fig. 18. Proposed mechanism for the selective oxidation of styrene catalyzed by Aun(SR)m clusters as the putative catalyst. The thiolate ligands are not shown for clarity. Magenta: Au atoms in the core, blue: Au atoms in the shell. The figure has been reproduced from Ref. [225], with permission from the Royal Society of Chemistry (Copyright 2012).

Fig. 19. Enhanced catalytic activity during selective oxidation with supported, ultrasmall organothiolate-protected Au NP catalysts produced by mild chemical stripping of their surface ligands. The figure has been reproduced from Ref. [175], with permission from Wiley-VCH (Copyright 2014).

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Fig. 20. Au25/CNT and Pd1Au24/CNT catalysts for benzyl alcohol oxidation. The figure has been reproduced from Ref. [266], with permission from the American Chemical Society (Copyright 2012).

Fig. 21. Selectivity and yield during benzyl alcohol oxidation over various supported Au25 NCs. The figure has been reproduced from Ref. [268], with permission from the American Chemical Society (Copyright 2014).

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Fig. 22. Schematic of the proposed mechanism for the semihydrogenation of alkynes into the corresponding alkenes by a spherical Au25(SR)18 NC and the rod-shaped Au25(PPh3)10(C≡CPh)5X2 NC. The figure has been reproduced from Ref. [273], with permission from the American Chemical Society (Copyright 2014).

Fig. 23. Schematic (not to scale) showing the photoexcitation, photoelectron transfer, and photocatalytic reduction of Uniblue using the Au NCs/TiO2 composite. The figure has been reproduced from Ref. [274], with permission from the American Chemical Society (Copyright 2010).

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Fig. 24. (A) TEM image of the Au25(GSH)18–BaLa4Ti4O15 photo-precatalyst; (B) size distribution (from TEM data) of the adsorbed Au25(GSH)18 NCs with an initial diameter of 1.1 nm, and with sizes from 0.9 to 1.5 nm, where the Au NCs range from Au~20 to Au~100; (C) S 2p X-ray photoelectron spectrum; and (D) Au 4f X-ray photoelectron spectrum of the photocatalyst prepared by the calcination of Au25(GSH)18–BaLa4Ti4O15 (0.1 wt% Au). The figure has been reproduced from Ref. [275], with permission from the Royal Society of Chemistry (Copyright 2013).

Scheme. 1. Typical reaction pathways employed to produce Au11(PPh)8X3 NCs [117].

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Scheme 2. Two-phase Brust–Schiffrin Au NCs synthesis, where X donates the halogen ligand.

Scheme 3. Adsorption and activation of O2 on the surface of Au38(SR)24NCs [190].

Scheme 4. Possible (although untested) mechanism for the activation of molecular oxygen by Au:PVP NCs. The figure has been reproduced from Ref. [191], with permission from American Chemical Society (Copyright 2009).

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Scheme 5. Selective oxidation of styrene by Au NCs catalysts.

Scheme 6. Proposed mechanism for the chemoselective hydrogenation of α,β-unsaturated ketones to unsaturated alcohols over Au25(SR)18. The thiolate ligands are not shown for clarity. Purple: Au atoms in the core; blue: Au atoms in the shell. The figure has been reproduced from Ref. [186], with permission from Wiley-VCH (Copyright 2010).

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Scheme 7. Proposed mechanism for formation of α,β-unsaturated ketones from propargylic acetates catalyzed by Au25(SR)18– clusters. The figure has been reproduced from Ref. [220], with permission from the Royal Society of Chemistry (Copyright 2014).

Scheme. 8. Reaction pathway of the Aun@ICRM catalyzed hydroamination of 1-octyne with aniline. The figure has been modified from Ref. [236], with permission from the American Chemical Society (Copyright 2014).

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Scheme 9. Proposed mechanism for the H2-assisted selective catalytic reduction of C3H8–SCR by Ag-MFI.

Table 1. Characteristics of AunLm NCs reported to date. No.

Year

Cluster*

Synthesis method

Structural derivation techniques

Peaks in UV-Vis absorption (nm)

1

2007

Au4[ArNC(H)NAr]4[62]

Reaction of

Single crystal

-

amidinate salt with

XRD

Au(I) chloride in THF 2

2009

[Au6(xy-xantphos)3]Cl

[63]

Addition

of

Single crystal

PhMe2SiH

to

XRD

-

AuCl(xy-xantphos), recrystallization 3

2008

[64]

[Au7(PPh3)7]C60·THF

NaBH4 reduction of

Single crystal

Au(PPh3)NO3,

XRD

followed by adding PPh3 and ligand exchange with KC60 4

2008

[64]

[Au8(PPh3)8](C60)2

Same as for 3

Single crystal

-

XRD 5

2008

6

2006

[65]

[Au9(PPh3)8](NO3)3

[Au10(AsnPr)4(dppe)4]Cl2[66]

NaBH4 reduction of

Single crystal

Au(PPh3)NO3

XRD

Mix [(AuCl)2dppe]

Single crystal

with

XRD and

450, 380, 350

-

96

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7

2006

[66]

[Au10(AsPh)4(dppe)4]Cl2

As(nPr)(SiMe3)2

ESI-MS

Mix [(AuCl)2dppe]

Single crystal

with As(Ph)(SiMe3)2

XRD and

-

ESI-MS 8

2011

Au10@Histidine

[67]

Mix HAuCl4 with

ESI-MS

Below 300

Single crystal

390, 460, 663

histidine in water 9

2012

10

2013

[68]

[Au11(dppe)6]Cl3

NaBH4 reduction of

Au11(PPh3)8Cl3[69]

Au2(dppe)Cl2

XRD

NaBH4 reduction of

Single crystal

Au(PPh3)Cl

XRD and

300, 420

ESI-MS 11

2008

[70]

Au12L6

L: bicarbodithiolate ligand

12

2013

[71]

Au10−12(GSH)10−12

Mix (THT)AuCl

MALDI-TOF-

362, 408

with salts of ligands

MS and Single

in CHCl3

crystal XRD

CO reduction of the

ESI-MS

330, 370

NaBH4 reduction of

ESI-MS and

360

Au2(dppe)Cl2 in

Single crystal

CH2Cl2 ethanol

XRD

NaBH4 reduction of

Single crystal

Au(PPh3)NO3

XRD

Core etching of

ESI-MS

mixture of HAuCl4 and GSH 13

14

15

2010

2013

[72]

[Au13(dppe)5Cl2]Cl3

[73]

[Au14(PPh3)8(NO3)4]·(MeOH)6

2011

Au15@cyclodextrin

2013

[71]

[74]

larger clusters 16

Au15(GSH)13

CO reduction of the

-

~370 ~420

ESI-MS

375, 415

mixture of HAuCl4 and GSH 17

18

2006

2005

[Au17(AsnPr)6(As2nPr2)(dppm)6]

Mix [(AuCl)2dppm]

Single crystal

Cl3[66]

with

XRD and

As(nPr)(SiMe3)2

ESI-MS

a: NaBH4 reduction

ESI-MS

560, 620

Brust method#

ESI-MS

330, 370

NaBH4 reduction of

Single crystal

-

PhC≡CAu and

XRD and

HdppaAu2(SbF6)2

ESI-MS

NaBH4 reduction of

ESI-MS

[71, 75, 76]

Au18(GSH)14

of HAuCl4 and GSH b: Brust method# c: CO reduction of the mixture of HAuCl4 and GSH 19 20

2011 2015

[77]

Au19(SR)13

[Au19(PhC≡C)9(Hdppa)3](SbF6)2 78]

21

2004

[79]

Au20(PPh3)4

[

Au(PPh3)Cl 22

2009

Au20(SR)16

[80]

Brust method#

ESI-TOF-MS

350, 420, 485

97

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23

2012

[Au20(PPhpy2)10Cl4]Cl2[81]

NaBH4 reduction of

Single crystal

260, 344,

Au(PPhpy2)Cl

XRD and

493

ESI-MS 24

2014

[82]

Au20(TBBT)16

Reacting

Single crystal -

[Au25(SC2H4Ph)18]

-

XRD

with TBBT 25

2014

[83]

[Au20(PP3)4]Cl4

NaBH4 reduction of

Single crystal

PP3Au4Cl4

XRD and

370, 495, 550

ESI-MS 26

2014

[84]

Au22(GSH)18

CO reduction of the

ESI-MS

450, 515

ESI-MS and

570

mixture of HAuCl4 and GSH 27

2013

[85]

[Au23(SR)16](TOA)

Brust method#

Single crystal XRD 28

2014

29

2014

[86]

Au23(SC4H9)16

Au23(SAdm)16 [87]

Au25(SAdm)16

Brust method

#

MALDI-MS

566

NaBH4 reduction of

Single crystal

-

HAuCl4 in THF/H2O

XRD and ESI-MS

30

2014

[87]

Au24(SAdm)16

NaBH4 reduction of

Single crystal

HAuCl4 in THF/H2O

XRD and

495, 580, 690

ESI-MS 31

2013

Au20(SR)16

Brust method

#

ESI-MS

495 for Au20

Single crystal

383, 560

[88]

Au24(SR)20 32

2012

[Au24(PPh3)10(SC2H4Ph)5X2]X[89]

Brust method#

XRD and ESI-MS 33

2010

[71, 90, 91]

Au25L18

a: Brust method

#

Single crystal

b: ligand exchange

XRD and

c: NaBH4 reduction

ESI-MS

670, 785

of Au precursor d: CO reduction of Au precursor 34

35

2013

2013

[92]

Au28(TBBT)20

[93]

Au30(S-t-Bu)18

Reacting

Single crystal

Au25(PET)18 with

XRD and

excess TBBT

ESI-MS

NaBH4 reduction of

MALDI-TOF-

HAuCl4 via one-pot

MS

365, 480, 580

545, 620

THF method 36

37

2014

2011

Au30S(S-t-Bu)18[55]

Au36(SPh)23

[94]

NaBH4 reduction of

Single crystal

HAuCl4 via one-pot

XRD and

THF method

ESI-MS

Etching of large

MALDI-TOF-

545, 700

430, 370, 566

98

Page 98 of 104

38

2012

[95]

Au36(SR)24

clusters

MS

Etching of large

Single crystal

375,

clusters

XRD and

570

ESI-MS 39

40

2009

2010

[59]

Au38(SR)24

[96, 97]

Au40(SR)24

Etching of large

MALDI-TOF-

490, 520, 560,

clusters

MS and Single

620, 745,

crystal XRD

1050

Etching of large

ESI-MS and

-

clusters

MALDI-TOFMS

41

42

2012

2014

[98]

Au41(S-Eind)12, Au43(S-Eind)10 [99]

Au44(SR)28

Ligand exchange

MALDI-TOF-

method

MS

Brust method

#

ESI-MS and

460-480

380

MALDI-TOFMS 43

44

2012

2011

[100]

Au54(C≡CPh)26

[101]

Au55(SR)31

Ligand exchange

MALDI-TOF-

-

method

MS

Etching of large

ESI-MS

760 835

MALDI-TOF-

500

MS

970

NaBH4 reduction of

MALDI-TOF-

-

HAuCl4 via one-pot

MS

clusters 45

46

2014

2013

Au64(S-c-C6H11)32[102] [103]

Au67(SR)35

Brust method#

THF method 47

2009

[104]

Au68(SR)34

NaBH4 reduction of

MALDI-TOF-

HAuCl4 via one-pot

MS

-

THF method 48

2015

[105]

Au99(SR)42

Brust method#

MALDI-TOF-

730,

Etching of large

MS

600, 490,

clusters 49

2007

Au102(p-MBA)44

[43]

400

Modified Brust method

#

ESI-MS

269

MALDI-TOFMS Single crystal XRD

50

2013

Au103(SR)45, Au104(SR)45,

Brust method

#

Brust method

#

Au104(SR)46,Au105(SR)46[106] 51

52

2014

2015

[107]

Au130(SR)50

Au133(SPh-tBu)52

[54]

MALDI-TOF-

-

MS

Brust method

#

ESI-MS

360,

MALDI-TOF-

508,

MS

708

ESI-MS

-

Etching of large

MALDI-TOF-

clusters

MS

99

Page 99 of 104

Single crystal XRD 2011

53

[108]

Au144(SR)60

Brust method

#

ESI-MS

517

MALDI-TOF-

700

MS #

-

Brust method: two phase (water-toluene) reduction of AuCl4 by sodium borohydride in the presence of an

alkanethiol [109]. Usually, tetraoctylammonium bromide is usually employed as the phase-transfer reagent.

Table 2. Selected examples of the NC-catalyzed selective hydrogenation of unsaturated aldehydes and ketones. Entr

Precatalyst

Substrate

a

y 1

Sub to Au

Au25(SR)18

benzalacetone

0.03–0

P(H2)/

T/

bar

C

t/h

Conb/%

S(UA)c/

Ref.

%

1

60

4

N.M.

1

60

4

1

60

d

76

[28]

N.M.

67

[28]

4

N.M.

65

[28]

.06 2

Au38(SR)24

benzalacetone

0.03– 0.06

3

Au144(SR)60

benzalacetone

0.03–0 .05

4

Au25(SC2H4Ph)18

benzalacetone

30

1

0

3

22

100

[186]

5

Au25(SC12H25)18

benzalacetone

30

1

0

3

20

100

[186]

6

Au25(SC2H4Ph)18/Fe2

benzalacetone

30

1

0

3

43

100

[186]

benzalacetone

30

1

0

3

40

100

[186]

benzalacetone

30

1

0

3

19

100

[186]

HMFe

800

38

12

2

>90

~80

[208]

O3 7

Au25(SC2H4Ph)18/TiO 2

8

Au25(SC2H4Ph)18/SiO 2

9

Au NC/Al2O3

0 10

Pt NCs/UiO-66-NH2

cinnamaldehy

1240

40

80

12

98.7

91.7

[209]

1064

40

60

72

>95

>99

[188]

50

30

25

6

99

86

[210]

de 11

Au NCs

cinnamaldehy de

12

Cu60@PAMAM−OH(

cinnamaldehy

G6)

de

a

Sub to Au: the molar ratio of the substrate relative to Au. bCon: conversion of the substrate. cS(UA): Selectivity

for unsaturated alcohols. dN.M.: not mentioned. eHMF: 2-hydroxymethyl-5-furfural.

100

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Table 3. Selected examples of NCs that catalyze the selective hydrogenation of 4-nitrobenzaldehyde. Conb/

S(1)c/

%

%

4

7.8

n.d.

100

[218]

80

4

10.2

n.d.

100

[218]

20

80

4

16.0

n.d.

100

[218]

13

20

80

4

20.6

n.d.

100

[218]

50

30

25

12

88

n.d.

100

[210]

T/

Entry

Precatalyst

Sub to Aua

P(H2)/bar

1

Au15(GSH)13

33

20

80

2

Au18(GSH)14

28

20

3

Au25(GSH)18

20

Au38(GSH)24

4 5

Cu60@PAMAM− OH(G6)

C

t/h

S(2) d

/%

Ref.

6

Au99(SPh)42

10

20

80

12

8.7

n.d.

100

[187]

7

Au99(SPh)42/SiO2

10

20

80

12

27.9

n.d.

100

[187]

8

Au99(SPh)42/TiO2

10

20

80

12

69.8

n.d.

100

[187]

9

Au99(SPh)42/CeO2

10

20

80

12

93.1

n.d.

100

[187]

10

Au25(SPh)18/CeO2

10

20

80

10

15

0

100

[219]

11

Au36(SPh)24/CeO2

10

20

80

10

14

0

100

[219]

10

20

80

10

90

0

100

[219]

10

20

80

10

91

0

100

[219]

12

13

Au11(PPh3Py)7Br3/ CeO2 Au11(PPh3Py)7Cl3/ CeO2

a

Sub to Au: molar ratio of substrate relative to Au. bCon: conversion of the substrate. cS(1): selectivity for

4-aminobenzaldehyde. dS(2): selectivity for 4-nitrobenzyl alcohol.

Table 4. Selected examples of NC-catalyzed epoxidation reactions with styrene. Entr

Precatalyst

Oxidant

1

Au55/BN

2

Po2/

Sub to

T/

t/h

Conb/

S(1)c/

S(2)d

%

%

/%

Ref.

bar

Au

C

O2

1.5

3750

100

15

19

14

82

[252]

Au55/SiO2

O2

1.5

3500

100

15

26

12

82

[252]

3

Au25/HAP

TBHP

-

300

80

100

92

N.M.

[255]

4

Au25(SCH2CH2Ph)18

O2

1

~6

100

24

27

24

70

[28]

5

Au25(SC6H13)18

O2

1

~6

100

24

25

26

69

[28]

6

Au38(SCH2CH2Ph)24

O2

1

5

100

24

14

24

72

[28]

7

Au38(SC12H25)24

O2

1

~6

100

24

15

25

71

[28]

8

Au144(SCH2CH2Ph)60

O2

1

~5

100

24

12

20

80

[28]

9

Au144(SC12H25)60

O2

1

~5

100

24

11

16

84

[28]

TBHP

-

150

80

12

86

-

~100

[256]

TBHP

-

150

80

12

86

-

~100

[256]

y

10 11

Au25(SCH2CH2Ph)18/ SiO2 Au38(SCH2CH2Ph)24/

a

101

Page 101 of 104

SiO2 12

Au144(SCH2CH2Ph)60 /SiO2

TBHP

-

150

80

12

90

-

~93

[256]

13

Au25/SiO2

TBHP

-

150

80

12

90

-

~92

[256]

14

Au38/SiO2

TBHP

-

150

80

12

95

-

~95

[256]

15

Au144/SiO2

TBHP

-

150

80

12

100

-

~100

[256]

TBHP

-

150

80

12

90

-

~65

[256]

TBHP

-

150

80

12

78

-

~75

[256]

TBHP

-

150

80

12

78

-

~78

[256]

16

17

18

Au25(SCH2CH2Ph)18/ HAP Au38(SCH2CH2Ph)24/ HAP Au144(SCH2CH2Ph)60 /HAP

19

Au25/HAP

TBHP

-

150

80

12

100

-

~62

[256]

20

Au38/HAP

TBHP

-

150

80

12

100

-

~65

[256]

21

Au144/HAP

TBHP

-

150

80

12

100

-

~71

[256]

22

Au25-i@SiO2

TBHP

-

66

46

48

[257]

23

Au25-bi@SiO2

TBHP

-

300

75

24

43

16

75

[257]

24

Au25@SiO2

e

TBHP

-

300

75

24

77

22

70

[257]

25

Au25@SiO2f

TBHP

-

300

75

24

67

53

47

[257]

PhI(OAc)2

-

20

70

10

59

44

54

[205]

PhI(OAc)2

-

20

70

10

91

10

90

[205]

O2

1

2400

80

24

~5

Trace

100

[258]

O2

1

2400

80

24

~5

Trace

100

[258]

TBHP

-

2400

80

24

99

44

32

[258]

TBHP

-

2400

80

24

98

14

36

[258]

26

27

28

29

30

31

Au25(SCH2CH2Ph)18/ TiO2 Pt1Au24(SCH2CH2Ph )18/TiO2 Au25(SCH2CH2Ph)18/ CeO2 NRs Au25(SCH2CH2Ph)18/ CeO2 NPs Au25(SCH2CH2Ph)18/ CeO2 NRs Au25(SCH2CH2Ph)18/ CeO2 NPs

32

Ptn(L-Cys)m

O2

1

13

80

12

21

20

77

[259]

33

Ptn(L-Cys)m

H2O2/O2

1

13

80

12

28

10

83

[259]

34

Ptn(L-Cys)m

H2O2

-

13

80

12

65

8

90

[259]

35

Ptn(L-Cys)m/TiO2

O2

1

13

80

12

23

0

100

[259]

36

Ptn(L-Cys)m/TiO2

H2O2/O2

1

13

80

12

30

0

100

[259]

37

Ptn(L-Cys)m/TiO2

H2O2

-

13

80

12

92

10

82

[259]

38

Ptn/TiO2

O2

1

13

80

12

8

26

70

[259]

39

Ptn/TiO2

H2O2/O2

1

13

80

12

14

20

72

[259]

40

Ptn/TiO2

H2O2

-

13

80

12

40

10

85

[259]

TBHP

-

-

80

24

30

~0

~100

[175]

TBHP

-

-

80

24

~100

~0

~100

[175]

41 42

Au25(SCH2CH2Ph)18/ SBA-15-SH Au25/SBA-15-SH

102

Page 102 of 104

43 44

Au25/SBA-15-SH Au144(SCH2CH2Ph)60 /SBA-15-SH

O2

1

-

80

24

28

11

75

[175]

TBHP

-

500

80

24

32

~0

~100

[175]

45

Au144/SBA-15-SH

TBHP

-

500

80

24

~100

~0

~100

[175]

46

Au144/SBA-15-SH

O2

1

500

80

24

14

22

71

[175]

47

Au25(MHA)18/HAP

TBHP

-

500

80

16

71

30

-

[178]

Au25/HAP(1)

g

TBHP

-

500

80

166

71

17

-

[178]

49

Au25/HAP(2)

h

TBHP

-

500

80

16

89

20

-

[178]

50

Au25/CNT

TBHP

-

250

65

24

73

20

66

[260]

51

Ag44/CNT

TBHP

-

250

65

24

44

6

93

[260]

52

Ag46Au24/CNT

TBHP

-

250

65

24

68

1

96

[260]

53

Ag32Au12/CNT

TBHP

-

250

65

24

70

58

38

[260]

48

a

Sub to Au: molar ratio of substrate relative to Au. bCon: conversion of the substrate. cS(1): selectivity for styrene

oxide. dS(2): selectivity for benzaldehyde. eAu25-i@SiO2 after calcination. fAu25-bi@SiO2 after calcination. Thermal treatment of Au25(MHA)18/HAP at 300C for 2 h. hTBHP solution treatment of Au25(MHA)18/HAP for

g

36 h at 50C.

Table 5. Selected examples of NC-catalyzed aerobic oxidation of benzyl alcohol using O2 as the oxidant. Entr y 1 2

S(1)c/

S(2)d/

%

%

%

6

0

-

-

[266]

30

6

0

-

-

[266]

Po2/

T/

Aua

bar

C

Au25(SCH2CH2Ph)18/CNT

50

1

30

Pd1Au24(SCH2CH2Ph)18/H

50

1

AP

t/h

Ref.

3

Au25/CNT

50

1

30

6

22

37

29

[266]

4

Pd1Au24/CNT

50

1

30

6

74

24

53

[266]

5

Ru NC/SiO2

100

1

80

6

100

90.4

-

[267]

Au25(SCH2CH2Ph)18/HPC

230

1

30

6

0

-

-

[268]

1

30

6

84

74

21

[268]

Air

60

n.d.

n.d.

n.d.

n.d.

[269]

6 7 8

S Au25/HPCS

230

Au25(SPh-pNH2)17/SBA-1

500

5

9

Au25/SBA-15

500

Air

60

0.75

50

73

n.d.

[269]

10

Au25(MHA)18/HAP

11 12 a

Conb/

Sub to

Precatalyst

Au25/HAP(1)

90

5

30

16

0

-

-

[178]

e

90

5

30

16

46.7

65.2

8.8

[178]

f

90

5

30

16

23.6

92.1

5.9

[178]

Au25/HAP(2)

b

c

Sub to Au: molar ratio of the substrate relative to Au. Con: conversion of the substrate. S(1): selectivity for

103

Page 103 of 104

benzaldehyde. dS(2): selectivity for benzoic acid. eThermal treatment of Au25(MHA)18/HAP at 300C for 2 h. f

TBHP solution treatment of Au25(MHA)18/HAP for 36 h at 50C.

104

Page 104 of 104