Atomically Monodisperse Gold Nanoclusters Catalysts with Precise Core-Shell Structure

The emphasis of this review is atomically monodisperse Aun nanoclusters catalysts (n = number of metal atom in cluster) that are ideally composed of an exact number of metal atoms. Aun which range in size from a dozen to a few hundred atoms are particularly promising for nanocatalysis due to their unique core-shell structure and non-metallic electronic properties. Aun nanoclusters catalysts have been demonstrated to exhibit excellent catalytic activity in hydrogenation and oxidation processes. Such unique properties of Aun significantly promote molecule activation by enhancing adsorption energy of reactant molecules on catalyst surface. The structural determination of Aun nanoclusters allows for a precise correlation of particle structure with catalytic properties and also permits the identification of catalytically active sites on the gold particle at an atomic level. By learning these fundamental principles, one would ultimately be able to design new types of highly active and highly selective gold nanocluster catalysts for a variety of catalytic processes.


Introduction
Gold was initially considered to be catalytically inactive for a long time [1,2].This changed when gold was seen in the context of the nanometric scale, which has indeed shown it to have excellent catalytic activity as a homogeneous or a heterogeneous catalyst [3][4][5][6][7][8][9][10][11][12].The comprehensive reviews and books about gold nanoparticles as catalysts have appeared, which cover many important aspects related to preparation of gold catalysts and their catalytic properties [13][14][15][16][17][18][19].However, almost all of the current studies only give rise to an ensemble average of the catalytic performance due to the structural polydispersity and heterogeneity of conventional nanoparticles catalysts.Although significant efforts have been invested in preparing well defined nanoparticles, fundamental nanocatalysis research still lags significantly behind.Due to the size dispersity of conventional nanoparticles, it is not possible to achieve an in-depth understanding of the origin of the sizedependence of nanogold catalysts; moreover, it is impossible to identify the catalytically active species in nanoparticle catalysis.
Therefore, it is of paramount importance to attain atomically precise gold nanoparticles and use such nanoparticles as well defined catalysts.By solving their atomic structure of the nanoparticles, one will be able to precisely correlate the catalytic properties with the exact atomic structure of the nanoparticles and to learn what controls the surface activation, surface active site structure and catalytic mechanism.Atomically monodisperse gold nanoclusters (referred to as Au n , n = number of metal atom in particle, n ranging from a dozen to hundreds) are ideally composed of an exact number of gold atoms and are unique and vastly different from their larger counterparts-gold nanocrystals (typically 3-100 nm).Small Au n nanoclusters (n < 100) behave like molecules and exhibit strong quantum confinement effects; relatively larger ones (100 < n <200) exhibit intermediate properties between molecular behavior and metallic properties [20].Overall, the non-metallic behavior of Au n in both size regimes is particularly important for nanocatalysis.More importantly, on the basis of their atom packing structures and unique electronic properties, one could indeed study a precise correlation of structural properties with catalytic properties, identify the catalytically active sites on the gold particle and unravel the nature of gold catalysis [21][22][23][24][25][26][27].This has long been an important task in nanocatalysis [28][29][30][31][32][33][34][35][36].
The emphasis of the review will be on the preparation of monodisperse gold nanocluster catalysts and precise structure-catalytic activity relationships, the investigation of which is currently being actively pursued.

Atomic Structure of Gold Nanoclusters
The atom packing structures of Au n nanoclusters are critical for understanding the catalytic properties of nanoclusters and theoretical modeling of mechanistic steps.The synthesis of gold nanoclusters in solution phase has been developed, including the electrophoretic separation [37,38], the kinetic control approach [39,40], and the thiol etching method [41,42].Interestingly, these structures of Au n nanoclusters do not resemble the face-centered cubic (fcc) structure of their larger counterparts: gold nanocrystals or bulk gold.Firstly, the structural model emerged from the density functional calculation provides a glimpse into a prevailing structural concept with an atomically Au/ligand interface and compact gold core [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].
The early theoretical studies confirmed that Au 16 and Au 20 clusters have tetrahedral structures.Au 20 cluster contains a prolate Au 8 core and four level-3 extended staple motifs -RS-Au-RS-Au-RS-Au-RS-.This highly stable cluster may represent a structural evolution of thiolate protected gold clusters from the homoleptic core-free structure to the core-stacked structure (Figure 1) [58].The geometry of Au 16 is derived from the T d -symmetric Au 20 by removing the four vertex atoms and allowing for an outward relaxation of the 4 face-centered atoms [59].Au 34 cluster has been predicted for a C 3 structure constructured from a more symmetric C 3v geometry via a twist (see Figure 2) [60].Au 39 cluster has an approximate D 3 point group symmetry, with the gold atoms forming a hexagonal antiprismatic cage, filled by one bulk-coordinated gold atom (Figure 3) [61].The core of Au 38 (SR) 24 is a face-fused biicosahedral Au 23 (13 + 13 − 3 = 23) (Figure 7(A)) [65].The fusion of the two icosahedra occurs along a common C 3 axis.Note that the way the structure is anatomized does not necessarily mean the real growth mechanism of the cluster.The Au 23 rod is structurally strengthened by three monomeric -SR-Au-RS-staples (Figure 7(C)).Then, the top icosa-hedron is further capped by three -SR-Au-SR-Au-RS-dimeric staples, which are arranged in a rotative fashion, resembling a tri-blade "fan" or "propeller" (Figure 7(D) and (E)).A similar arrangement of the other three staples is found on the bottom icosahedron, but the bottom "propeller" rotates by approximately 60° relative to the top one, forming a staggered dual-propeller configuration.In fact, the entire cluster is chiral due to the rotative arrangement of the dimeric staples.A larger Au n (SR) m nanocluster is Au 102 (SR) 44 [62,69].The gold particles were coated with p-MBA and crystallized from a solution containing 40% methanol, 300 mM sodium chloride, and 100 mM sodium acetate, at pH 2.5.Interestingly, this cluster has a truncated Au 49 Marks decahedral kernel (Figure 8B and C), which is based on a fivefold symmetric 19-atom kernel (Figure 8(A)).The Au 49 kernel is further capped by two 15-atom caps at the top/bottom, respectively (Figure 8(D) and (E)).The resultant Au 79 kernel is capped by five -SR-Au-SR-monomeric staples on the top and another five at the bottom, and nine monomers and two dimers (-SR-Au-SR-Au-SR-) at the waist.The arrangement of 13 gold atoms at the waist (from nine monomers and two dimers, 9 + 2 × 2 = 13) destroys the fivefold symmetry of the entire Au 102 cluster, so Au 102 cluster has two chiral isomers.

Catalytic Performance of Atomically Precise Gold Nanocluters
Such nanoclusters provide a new opportunity for unraveling catalysis at an atomic level.The catalytic activity has been explained by various complementary mechanisms, such as charging effects, geometric fluxionality, particle-size-dependent metal-insulator transition and electronic quantum size effects [70][71][72][73][74][75][76].The theoretical investigations of the catalytic activity of these small gold nanoclusters (up to a few tens of atoms) were studied a decade ago [29,75].Density functional calculations showed that the gold particle sizes fall into a region where quantum size effects are expected to dominate the reactivity of gold.Hakkinen et al. showed, by considering a series of structurally well-defined gold clusters with diameter between 1.2 and 2.4 nm, that electronic quantum size effects, particularly the magnitude of HOMO-LUMO energy gap, have a decisive role in activated-form of the nanocatalysts [70].The experiments have given strong indications of the catalytic activity of supersmall well-defined gold clusters.Recently, Turner et al. [77] reported the selective oxidation of styrene with O 2 by nanocatalysts derived from solution phase protected Au 55 clusters and found the activity of Au 55 nanocatalyst was super to that of Au nanocrystals (>3 nm).A sharp size threshold in catalytic activity was found such that that, when fed with O 2 alone, the catalytic activity is quenched for Au particles with diameters greater than or equal to 2 nm (Figure 10(d)).Since the crystal structure of Au 55 cluster has been utterly unknown so far, it is not easy to correlate structural properties with catalytic properties.They proposed that electron transfer from the anionic gold core into LUMO(π*) of O 2 forms superoxoor peroxo-like species, which may play an essential role in the oxidation of alcohol (Figure 11).This work provides a principle for the synthesis of aerobic oxidation catalysts based on the electronic structures of Au clusters and more electronic charge should be deposited into the high-lying orbitals of Au clusters by doping with electropositive elements or by interaction with nucleophilic sites of stabilizing molecules.[23,26].The catalytic activity of Au n nanocluster catalysts exhibits a strong dependence on size (n); the smaller Au n (SR) m nanoclusters give rise to a much higher catalytic activity.Among the three sizes, Au 25 nanocluster catalyst shows the highest conversion of styrene, followed by Au 38 and Au 144 .The effect of thiolate ligands was investigated and found that the ligands do not affect the catalytic activity and selectivity.Therefore, the catalysis of Aun(SR)m nanoclusters are mainly determined by the gold core rather than by ligands shell.A mechanism has been proposed for selective oxidation of styrene catalyzed by Au 25 (SR) 18 nanoclusters (Figure 12) [26].The three oxidant systems were investigated:  Tsukuda et al. [22] immobilized Au 25 (SR) 18 nanoclusters on a hydroxyapatite support for the selective oxidation of styrene in toluene solvent.They achieved a 100% conversion of styrene and 92% selectivity to the epoxide product.These results demonstrate that atomically monodisperse Au n nanocluster catalysts exhibit excellent catalytic activity in the selective oxidation processes.
Au n nanolcusters catalysts have also made significant advances in selective hydrogenation processes.Herein, Au 25 nanocluster is chosen as a model for a discussion of selective hydrogenation.The crystal structures of [Au 25 (SR) 18 ] q (q = −1, 0) show a core-shell type structure: a Au 13 icosahedral core and an exterior Au 12 shell.The charge distribution on the Au 13 core and the Au 12 shell is quite different: the Au 13 core possesses eight (when q = −1) or seven (q = 0) delocalized valence electrons originated from Au(6s).These electrons are primarily distributed within the Au 13 core, whereas the Au 12 shell bears positive charges due to bonding with thiolates and electron transfer from gold to sulfur.Selective hydrogenation of α,β-unsaturated ketones/aldehydes, conventional supported gold nanoparticle catalysts have been demonstrated to be capable of selective hydrogenation of α,β-unsaturated ketones to produce predominant α,β-unsaturated alcohols but with side products of saturated ketones from C=C hydrogenation as well as saturated alcohols from further hydrogenation.Although conventional gold nanoparticles can achieve high conversion and selectivity of the unsaturated alcohol in the hydrogenation of α,β-unsaturated ketones, a ~100% selectivity for the unsaturated alcohol has not been achieved [21].Using Au 25 (SR) 18 nanoclusters as hydrogenation catalysts, selective hydrogenation of the C=O bond in α,β-unsaturated ketones (or aldehydes) with 100% selectivity for α,β-unsaturated alcohols can be obtained.The extraordinary selectivity and activity of Au 25 catalysts correlate with the electronic structure of the Au 25 nanocluster and its nonclosed Au 12 exterior shell.The volcano-like eight uncapped Au 3 faces of the icosahedron from the exposure of Au 13 core should favor adsorption of the C=O group by interaction of the active site with the O atom of the C=O group (see Figure 13).Subsequently, the weakly nucleophilic hydrogen attacks the activated C=O group, and then form the unsaturated alcohol product.The surface Au atoms with low-coordination character, coordination number N = 3, should provide a favorable environment for the adsorption and dissociation of H 2 , and H 2 dissociation should occur on the gold atoms of the exterior shell (Figure 13) [21].The electron-rich Au 13 core has no ability to active C=C bond in α,βunsaturated ketone at mild temperatures, therefore there are no side products from the hydrogenation of C=C in α,β-unsaturated ketone.

Conclusions
These Au n catalyst examples demonstrate the huge power of atomically precise Au n nanocatalysts for achieving super selective oxidation and hydrogenation performance and atomically precise structure-property relationships.Au n nanoclusters possess a unique core-shell structurean electronrich core with delocalized valence electrons and an electron-deficient shell.Such nanoclusters will not only provide further insight into the nature of gold nanocatalysis at an atomic level, but also promote the exploration of new chemical processes with Au n as well-defined, highly efficient catalysts.Au n nanocluster catalysts will ultimately bring gold nanocatalysis to an exciting new level.

Figure 1 .
Figure 1.Structures of Au 20 cluster and structural decomposition of the Au 8 core and four level-3 staple motifs [58].

Figure 9 .
Figure 9. Atomic structure for the most favorable energy profile for O 2 dissociation on Au 38 cluster [75].

Figure 11 .
Figure 11.Mechanism for the activation of molecular oxygen by Au cluster [78].
(a) TBHP (tert-butyl hydroperoxide) as the oxidant; (b) TBHP as an initiator and O 2 as the main oxidant; (c) O 2 as the oxidant [26].The three different oxidant systems can undergo different reaction pathways to activate the oxidants and generate a common peroxyformate intermediate Au 25 -O 2(ad) (species D).In the case of TBHP as the oxidant, interaction of anionic Au 25 (species A) with TBHP forms a hydroperoxy species B, and then species B loses one H 2 O molecule and rearranges to form the Au 25 -O 2(ad) species D. In the case of TBHP as an initiator and O 2 as a main oxidant, initiation of TBHP forms species BuO*/*OH and hence activates O 2 to form the superoxolike O 2 *.The O 2 * is proposed to adsorb via a side-on fashion to the gold surface with two partial Au-O bonds to produce a low-barrier transition state species C, and then the peroxo-like species C transforms to the Au 25 -O 2(ad) species D. In the case of sole O 2 as oxidant, O 2 may directly attack the Au 13 core to form the Au 25 -O 2(ad) species D. The presence of partial positive charges on the surface gold atoms of the Au 12 shell should greatly facilitate activation of the nucleophilic C=C group of styrene (species E) since the positive Au atoms at the shell are electrophilic.Then the activated C=C bond reacts with the O 2(ad) species through side-by-side interaction on the Au 25 surface sites, leading to species F. Subsequently, the catalytic selectivity is triggered by the dissociation and rearrangement in three competing pathways that lead to the three products.The formation of benzaldehyde is from the breaking of the C-C bond (species G); the epoxide is created by the transfer of oxygen to the olefinic bond to form a metalloepoxy intermediate (species H); and acetophenone is produced by the breaking of the C-O bond (species I).Finally, the oxidized [Au 25 (SR) 18 ] 0 catalyst can be reduced to the anionic [Au 25 (SR) 18 ]  by gaining an electron when the C=C bond leaves the Au 25 cluster, hence, one catalytic cycle is completed [26].