Formation of Cationic Gold Clusters on the MgO Surface from Au (CH3

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J. Phys. Chem. C 2007, 111, 5154-5161

Formation of Cationic Gold Clusters on the MgO Surface from Au(CH3)2(acac) Organometallic Precursors: A Theoretical Analysis Sabrina Sicolo, Cristiana Di Valentin, and Gianfranco Pacchioni* Dipartimento di Scienza dei Materiali, UniVersita` di Milano-Bicocca, Via Cozzi, 53 - 20125 Milano, Italy ReceiVed: NoVember 9, 2006; In Final Form: January 12, 2007

The formation of gold atoms and clusters from the reduction by hydrogen of mononuclear [Au(CH3)2(acac)] organometallic complexes deposited on the MgO surface has been studied by first principle density functional theory calculations. The comparison of measured and computed infrared data suggests the occurrence of a dissociative adsorption of the complex on the surface of the oxide, although a given fraction of undissociated molecules can cohexist. The reduction can follow various mechanisms, leading to the formation of neutral or positively charged mononuclear gold intermediates. We show that some of these intermediates can easily diffuse on the surface at room temperature and lead to nucleation of stable partially oxidized gold clusters.

1. Introduction Metal clusters on oxide supports are widely used in heterogeneous catalysis, and one of the metals that has attracted more interest in the past decade is definitely gold,1 after the surprising discovery that, once prepared in the form of small particles of few nanometers, even this chemically unreactive metal becomes an active catalyst, for example in CO oxidation.2-6 An enormous activity has then been directed to the elucidation of the origin of this chemical activity, with particular attention to the role of the dimensions of the metal particle, of the support where it is deposited, and of the charged state of the cluster.7-20 This latter point in particular has stimulated several accurate studies with physical-chemical methods to prepare and characterize gold clusters under controlled conditions. The conclusions are rather contradictory. Some authors have presented evidence for an enhanced activity of negatively charged gold,13,15,21 where charging is supposed to occur through the interaction with specific sites of the oxide surface like point defects (in particular oxygen vacancies). According to other studies, the active catalyst is indeed cationic gold,22-27 although in this case it is possible that the positively charged state of the gold catalyst is produced during the catalytic reaction and is not related to the charged state of the as prepared metal particle.28,29 Clearly, the nature of the gold catalyst can differ substantially depending on the preparation of the particles,5,6 on the form and nature of the oxide substrate,6 and on the size of the particles.3,4,30,31 For instance, according to some investigations, flat, two-dimensional gold clusters are more active than thicker three-dimensional aggregates.1,32 In some cases it has been possible to deposit well-defined gold clusters all having the same nuclearity, and it has been found that the catalytic activity can change substantially by simply adding or removing one atom to the cluster.13 The preparation methods of gold model catalysts on oxide surfaces or thin films includes therefore various techniques ranging from soft landing of mass selected clusters13,33 to self-assembly of gold atoms deposited on the support,15 to more classical methods based on wet chemistry.22-27 In the latter case, gold nanoparticles have been prepared by reduction of gold salts like HAuCl4 or complexes like Au(PPh3)* Corresponding author. E-mail: [email protected].

(NO3). This procedure, however, suffers from the fact that contaminants such as chlorine or phosphorus are left on the surface.22 Recently, a new approach to the chemical preparation of supported gold nanoclusters has been proposed by Guzman and Gates.22-27 This consists in the deposition at the surface of polycrystalline MgO of an organometallic precursor, [Au(CH3)2(acac)], where (acac) stands for C5H7O2. This is followed by reduction of the complex either in He or in H2 atmosphere. The first report showed the formation of nanoclusters of Au, possibly Au6, on the MgO surface.22 The technique has been refined and further analyzed in a series of papers where the catalytic activity of the supported gold clusters has also been examined.26 X-ray absorption near edge spectroscopy, XANES, has shown the presence of both zerovalent and cationic gold in these particles. This is true in particular when the reduction is done in He, whereas the use of H2 seems to lead to essentially complete reduction.22,23 These results open new questions for theory, and in this paper, we will try to address some of them. First of all, we will consider the nature of the interaction of the [Au(CH3)2(acac)] organometallic precursor with the MgO surface, looking in particular at the competition between molecular and dissociative adsorption of the precursor. Then, we will analyze the interaction with molecular hydrogen and the mechanisms which lead to partial and then to complete reduction of the organometallic fragments. In this phase, some positively charged mononuclear gold complexes can form on the surface. We have investigated the diffusion probability of these fragments on the MgO surface at operating temperatures, as well as the possibility to build small cationic clusters from their combination. In the end, this shows the possible formation of partially oxidized gold clusters on the surface of MgO as the consequence of an incomplete reduction of the mononuclear precursors. To address these points, we have performed first principle density functional theory (DFT) calculations using embedded clusters to represent the MgO surface. The identification of the surface species has been done by computing the vibrational properties of some intermediates. The paper is organized as follows. In section 2, we provide the essential ingredients of the calculations. In section 3.1, we discuss the adsorption of the [Au(CH3)2(acac)] complex and, in particular, its dissociation into charged fragments. The next

10.1021/jp067403r CCC: $37.00 © 2007 American Chemical Society Published on Web 03/15/2007

Analysis of Gold Clusters on the MgO Surface

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Figure 1. Computed harmonic frequencies (scaled) in cm-1 and intensities in km/mol of the most important modes of the Au(CH3)2(acac) free complex. Yellow, O atoms; green, C atoms; small blue, H atoms; gray, Au atom.

section, 3.2, deals with the mechanisms of reduction of the [Au(CH3)2]+ fragment. Section 3.3 is dedicated to the diffusion and aggregation of [Au(CH3)2]+, [Au(CH3)]+, Au(CH3), Au+, and Au fragments, whereas in section 3.4, we comment on the formation of dinuclear organometallic complexes in positive oxidation state, [Au2(CH3)2]+/MgO. In section 3.5, we discuss the mechanism of reduction of [Au2(CH3)2]+/MgO complexes, and in section 3.6, we present the properties of bare cationic gold nanoclusters deposited on the MgO surface. The conclusions are summarized in the last section. 2. Computational Method To represent the MgO surface, we used an embedded nanocluster containing up to several thousands of atoms.34 The entire system is partitioned into two main regions. Region I includes the quantum-mechanical (QM) cluster, surrounded by interface ions and a large set of about 400 polarizable ions modeled within a classical shell model (SM). Formal charges +2 and -2 have been used for Mg and O ions, respectively. Interface atoms between the QM cluster and classical shellmodel ions are needed in order to prevent the oxygen electron density from an artificial spreading out of the QM region; semilocal effective pseudopotentials (ECP)35 have been associated to Mg2+ cations at interface. All constituents of region I are constraint-free. Region II contains about 3000 point charges in order to correctly reproduce the long-range Madelung potential and the short-range interactions with region I. The scheme has been implemented within the GUESS code36 interfaced with Gaussian 9837 for the calculation of the QM cluster. The GUESS code allows one to calculate forces acting on region I, both QM and SM centers, and simultaneously optimize their positions. The method has been successfully used in a number of previous studies,34,36,38,39 and the reader is referred to the existing literature for further details. The total energy and the electronic structure of the QM cluster are calculated in the framework of DFT solving the KohnSham equations which include the matrix elements of the electrostatic potential due to all classical ions in regions I and II, computed on the basis functions of the cluster. The gradient corrected Becke’s three parameters hybrid exchange functional40 with the correlation functional of Lee, Yang and Parr41 (B3LYP)

has been used. We employed the lanl2 scalar relativistic effective core potential42 (ECP) for Au, which explicitly consider 5s2 5p6 5d10 6s1 electrons as valence electrons (19-electron ECP), combined with the lanl2dz basis set.42 C, O, and Mg atoms are treated at an all electron level. C and O atoms of the complex have been modeled with 6-311G* and 6-311+G* GTO basis sets, respectively, and H atoms, with the 6-31G43 basis set. The surface O and Mg atoms directly interacting with the adsorbed complex have been treated with 6-311+G* and 6-31+G* GTO basis sets,44,45 respectively, whereas the 6-31G basis has been used on the other centers of the QM cluster. Different stoichiometric QM clusters have been used to simulate portions of the ECP MgO surface and in particular O10Mg10MgECP 18 , O12Mg12Mg18 , ECP ECP and O15Mg15Mg19 modeling a terrace and O12Mg12Mg16 for the edge site. Adsorption energies, when reported, are not corrected by the basis set superposition error (BSSE). Harmonic vibrational frequencies (ω0) have been determined and compared, when possible, with experimental data. In order to facilitate a comparison of the calculated with the measured spectra, a scaling factor 0.967 has been applied to all computed values. This has been derived by rescaling the C-H frequency of the Au(CH3)2(acac) free complex. With the present method, scaling and basis set, the vibrational frequencies of the gas-phase Au(CH3)2(acac) complex are well reproduced, see Figure 1 and Table 1. The GUESS code (SM embedding) does not allow the calculation of the intensities of the vibrational modes, and these have been estimated only based on the molecular calculations with the Gaussian 98 code. 3. Results and Discussion 3.1. Adsorption of Au(CH3)2(acac) on MgO. The first question to address is the adsorption mode of the organometallic complex on the MgO(100) surface. To this end, we have considered both molecularly and dissociatively adsorbed complexes, Figure 2. In the first case (molecular adsorption, Figure 2a), the complex is bound by -0.3 eV (negative values indicate exothermic processes) mainly by dispersion, electrostatic, and polarization forces; in the dissociative adsorption, we used a O15Mg15MgECP 19 cluster large enough to accommodate the two

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TABLE 1: Frequencies, in cm-1, of the Free [Au(CH3)2(acac)] Complex and of [Au(CH3)2]+ and [acac]Fragments Adsorbed on the MgO(100) Surfacea free complex

[acac]-/MgO

theory

exp.27

theory

exp.27

1402 1437 1461 1504 1570 2957, 2960 3060, 3062 3088

1386 1421 1465 1519 1559 2967, 2990

1438 1451 1489 1515 1621 2967

1427 1472 1519 1613 2965

3088

3123

3080

a

[Au(CH3)2]+/MgO theory

exp.27

2955, 2957 3057 3061

3088

See Figures 1 and 4 for the associated normal modes.

ionic fragments, [Au(CH3)2]+ and [acac]-, Figure 2b, and to reduce as much as possible their steric repulsion. This second structure is a local minimum on the potential energy surface but is unbound with respect to MgO and Au(CH3)2(acac) by +0.1 eV. The cost to dissociate Au(CH3)2(acac) in gas-phase into [Au(CH3)2]+ and [acac]- ionic fragments is 7.5 eV; thus, the simultaneous adsorption of the two fragments on the large MgO cluster, Figure 2b, leads to an overall binding of 7.4 eV. This is about 1.4 eV higher than the adsorption of the separated fragments (4.4 eV for [Au(CH3)2]+ and 1.6 eV for [acac]-, respectively). This shows the existence of an attractive interaction between the two charged fragments which overcompensates their steric repulsion. Thus, the calculations seem to indicate a preference for the molecularly adsorbed complex, although the energy difference is relatively small (0.4 eV). This energy difference could be further reduced by the inclusion of basis set superposition errors (not considered here). In principle, it is possible that the formation of the dissociated species is more favorable on the low-coordinated sites. In order to evaluate this second point, we have considered the adsorption of isolated [Au-

Figure 2. Optimal structures of molecularly adsorbed (a), and dissociatively adsorbed (b) Au(CH3)2(acac) complex on the MgO(100) surface. Only the surface layer of the O15Mg15MgECP embedded 19 cluster used in the calculations is shown for semplicity. Large blue, Mg atoms; yellow, O atoms; green, C atoms; small blue, H atoms; gray, Au atom.

(CH3)2]+ and [acac]- fragments on four-coordinated ions on the edge sites of MgO, and we have compared the corresponding adsorption energies with those of the (100) terraces, Figure 3ad. The [Au(CH3)2]+ fragment is bound to two surface oxide anions by -4.75 eV on the terrace site (Figure 3a) and by -4.12 eV on the edge (Figure 3c), whereas the [acac]- unit is bound to two Mg cations by -1.59 eV on the terrace (Figure 3b) and -1.58 eV on the edge site (Figure 3d). Thus, the results do not show a preference for the formation of the surface complexes on the edge sites. This is clearly due to the tendency of the two fragments to bind in a similar way to the surface on a terrace and on an edge; in both cases, the molecular unit binds to two nearest neighbor oxide anions or magnesium cations on the (100) surface, separated by 2.98 Å. The binding to two cations or to two anions along the step edge, separated by 4.21 Å, clearly reduces the electrostatic interaction with the surface. On this basis, we can conclude that the low-coordinated sites do not seem to have a special role in the thermodynamic of the process, but it is not excluded that on the line defects the barrier for dissociation is lower, thus affecting the kinetic of the process (not investigated here). Based on these results, in the following, we have considered only adsorption of Au complexes on the terrace sites. We have seen that the calculations do not indicate a clear preference for molecular or dissociative adsorption. Experimentally, the situation is also not well defined since the vibrational signatures of the adsorbed complex are similar to those of the molecular precursor. In fact, molecular adsorption was proposed in ref 22, whereas in ref 27, it has been concluded that the process is dissociative. We have compared the vibrational frequencies of the undissociated molecular complex in gas phase (see section 2 and Figure 1) with those of the dissociated fragments adsorbed on MgO, see Figures 3a,b and 4 and Table 1. We do not observe any shift in the frequencies for the methyl group on the free complex and on the adsorbed [Au(CH3)2]+ fragment. This is also what has been observed experimentally, Table 1. Some change in the spectrum is found for the other part of the complex, [acac]-. This holds in particular for the intense CsO stretching which in the free molecule is found at 1570 cm-1 while on [acac]-/MgO surface complex is at 1621 cm-1, Figure 4 and Table 1. The other modes of the [acac]- group do not shift significantly, being the changes of the order of 10-20 cm-1 at most. This observation is quite consistent with the experimental data which show that the Cs O stretching, found at 1559 cm-1 in the organometallic complex,27 is shifted by about 50 cm-1 to the blue in the adsorbed species (1613 cm-1).27 This is the same shift computed in our models, 51 cm-1, Table 1. On this basis, we believe that the IR data provide sufficient evidence for the presence of dissociated fragments on the MgO surface. However, given the similar stability of molecular and dissociative adsorption, one cannot exclude that the two modes coexists and that the formation of one species or the other depends on the experimental conditions (temperature, surface morphology, concentration of adsorbed species, etc.). On the basis of the conclusion that dissociative adsorption occurs, we have considered possible reduction mechanisms of the [Au(CH3)2]+ fragment by exposure to molecular hydrogen. 3.2. Mechanisms of Reduction of [Au(CH3)2]+/MgO. We should mention that only thermodynamic aspects have been considered, and no attempt has been made to evaluate the corresponding energy barriers. According to mechanism A, Figure 5, two CH3 fragments combine to form C2H6, and then



Figure 3. Optimal structures of [Au(CH3)2]+ and [acac]- fragments adsorbed on a terrace site (a and b) and on an edge site, (c and d), of the MgO ECP surface. For atoms labeling and details see Figures 1 and 2. Clusters used: (a and b) O12Mg12MgECP 18 ; (c and d) O12Mg12Mg16 .

Figure 4. Vibrational modes and frequencies in cm-1 of [Au(CH3)2]+ and [acac]- fragments adsorbed on the MgO(100) surface (only the two surface anions or cations of the surface are shown). Large blue, Mg atoms; yellow, O atoms; green, C atoms; small blue, H atoms; gray, Au atom. For the clusters used see Figure 3, panels a and b.

a H atom reduces Au+ to Au, forming a proton which remains attached to the surface. The first step, desorption of C2H6, is almost thermoneutral and implies an energy gain of -0.03 eV only. The second step, Au+ reduction by hydrogen, is exothermic by -1.32 eV. This leads to a neutral Au atom bound to an oxide anion with a AusO distance of 2.30 Å and a proton adsorbed on a nearby oxygen anion, Figure 5, forming a surface OH group. Notice that this reaction consumes 0.5 mol of hydrogen per mole of Au formed. The second possibility (mechanism B) implies that hydrogen first reacts with a methyl group forming a methane molecule which leaves the complex, Figure 6. This reaction is exothermic by -0.3 eV. Then, the reaction can follow two routes. In the first one, B1, the [AuCH3]+ complex is reduced leading to an adsorbed neutral AuCH3 species and an adsorbed proton, Figure 6, with a gain of -2.50 eV; this is then followed by the removal of a methyl group with formation of one methane molecule with a cost of +0.62 eV. In an alternative route, B2, a CH4 molecule is formed by reaction of [AuCH3]+ with hydrogen leaving on the surface a Au+ cation (energy gain of -0.56 eV); this is attacked by another H atom with formation of neutral Au0 and

an adsorbed proton, Figure 6, ∆E ) -1.31 eV. Mechanism B consumes 1.5 mol of hydrogen per mole of Au formed. This is what has been experimentally observed together with the formation of methane molecules.24 Thus, there is no doubt that this second mechanism is that occurring under real conditions. The theoretical results are consistent with this conclusion. In fact, mechanism A, Figure 5, leads to an overall stabilization of -1.35 eV, whereas in mechanism B, Figure 6, the total energy gain is -2.17 eV. Thus, from a purely thermodynamic point of view, the reduction of [Au(CH3)2]+ via formation of two methane molecules is clearly preferred. When we consider the two possible paths, B1 and B2, Figure 6, we notice that in B1 (left side of Figure 6) there is one step associated with a large energy gain, -2.50 eV, followed by a step which implies an energy cost of +0.62 eV. This means that the intermediate complex Au(CH3)/MgO is very stable and its conversion into atomic gold and methane implies that it overcomes a significant energy barrier (at least 0.6 eV). In mechanism B2 (right side of Figure 6) there is one step associated with a release of -0.56 eV, followed by a second step exothermic by -1.31 eV. Although this does not necessarily imply lower energy barriers,


Figure 5. Mechanism A for the reduction of [Au(CH3)2]+ by 0.5 mol of H2. See also Figure 2 for definitions. Clusters used: O12Mg12MgECP 18 .

it is likely that B2 will be kinetically more favorable than B1. The final products of the reaction are two gas-phase methane molecules, a Au atom adsorbed on an oxide anion, and a hydroxyl group on the MgO surface. 3.3. Diffusion and Aggregation of [Au(CH3)2]+, [Au(CH3)]+, Au(CH3), Au and Au+ Fragments. We consider now the possible diffusion and aggregation on the MgO surface of the various molecular fragments or of Au atoms (neutral and charged) generated in the reduction process to form gold complexes of higher nuclearity. All of the species of interest, [Au(CH3)2]+, [Au(CH3)]+, Au(CH3), Au, and Au+ adsorb preferentially on the oxide anions of the MgO surface but with different characteristics depending on their structure. The [Au(CH3)2]+ fragment, where Au is formally AuIII, binds in a bridge position over two O ions of the surface, Figure 3a, forming a square planar complex; in the diffusion process, the lowest path corresponds to the Au atom being on top of a single oxide anion. For the other fragments, [Au(CH3)]+, Au(CH3), Au, and Au+ the preferred adsorption site is on top of O, and the diffusion process implies going through the 4-fold hollow sites on the MgO terraces.46 The estimate of the energy barrier for diffusion has been done by placing the Au atom of the adsorbed complex at the center of the 4-fold hollow site (on top for the case of [Au(CH3)2]+) and optimizing the geometry under this constraint. The resulting structures do not correspond to real transition states, and the computed barriers are upper bounds to the actual values. Still, this provides an estimate of the mobility of the adsorbed species. From the values of Table 2, it is apparent that there are three highly mobile species, Au, Au+, and [Au(CH3)]+, and two fragments that can diffuse only for temperatures well above RT. For the [Au(CH3)2]+ fragment, the high barrier can be explained by the fact that in the transition state the complex is threecoordinated, whereas AuIII prefers square-planar structures; in

Sicolo et al.

Figure 6. Mechanism B for the reduction of [Au(CH3)2]+ by 1.5 mol of H2. See also Figure 2 for definitions. Clusters used: O12Mg12MgECP 18 .

TABLE 2: Adsorption Energies, Eb, Estimated Diffusion Barriers, Ed, and Residence Time, τ, for Gold Molecular Fragments and Gold Atoms Adsorbed on the MgO Surface

a

species

Eb (eV)

Ed (eV)

τ (s)

[Au(CH3)2]+ Au(CH3) [Au(CH3)]+ Au Au+

4.75 1.38 5.31 0.83 5.51

0.87 0.68 0.12 0.24a 0.22

102 101 10-11 10-9 10-9

From ref 46.

a similar way, Au(CH3) is linearly coordinated to a surface oxygen, typical of AuI, whereas in the transition state, the complex is forced to assume a coordination number of three. Using the calculated diffusion barriers, Ed, and an Arrhenius expression from transition state theory,47 the residence times τ are calculated at RT. [Au(CH3)2]+ and Au(CH3) units, with residence times of the order of 10-100 s are quite immobile on the surface, whereas [Au(CH3)]+, Au, and Au+ have residence times of the order of nanoseconds or less. Thus, both neutral and charged gold species can diffuse on the surface. This can lead in principle to aggregation and formation of dimeric gold fragments and larger aggregates in the positive oxidation state. In the next paragraph, we describe the structural properties of these surface complexes and their possible reduction by reaction with molecular hydrogen. 3.4. Properties of [Au2(CH3)2]+/MgO Complexes. We have seen above that Au, Au+, and [Au(CH3)]+ units are highly mobile on the MgO surface. They can combine with the rather “immobile” [Au(CH3)2]+ or Au(CH3) fragments, respectively, to form, among others, a dimeric [Au2(CH3)2]+ charged species which can be a precursor of a partially oxidized gold nanocluster. On the flat MgO terrace, there are initially two possible isomers



Figure 7. Optimized geometries of three isomers of [Au2(CH3)2]+ adsorbed on a MgO(100) surface. Distances in Å. See also Figure 2 ECP for definitions. Clusters used: O12Mg12MgECP 18 and O10Mg10Mg18 .

for the [Au2(CH3)2]+ complex, one with one methyl per Au atom, Figure 7a, and one where both methyl groups are bound to the same Au atom, Figure 7b. The calculations show that the first isomer, Figure 7a, is more stable by -0.56 eV. There are various conformers of this structure depending on the orientation of the methyl groups; the structure shown in Figure 7a is asymmetric, with a methyl group normal to the surface and the second one almost parallel to it. The actual structure of this complex has been investigated using a larger MgO cluster. We found that the symmetric structure with the methyl groups almost perpendicular to the surface, Figure 7c, is preferred, although by only 0.3 eV. The preference for the symmetric structure is probably due to the possibility to better delocalize the positive charge. In this structure, the two Au atoms are separated by 2.95 Å, a distance which is slightly longer than the Au-Au distance in bulk Au, 2.88 Å. This suggests the existence of some metal-metal bonding also in the dinuclear charged complex. To estimate the stability of the dinuclear complex, we have considered the following dimerization processes:

Au(CH3)2+/MgO + Au/MgO f Au2(CH3)2+/MgO ∆E ) -0.98 eV (1) Au(CH3)+/MgO + Au(CH3)/MgO f Au2(CH3)2+/MgO ∆E ) -0.17 eV (2) Au/MgO + Au(CH3)+/MgO f Au2(CH3)+/MgO ∆E ) -1.43 eV (3) Au+/MgO + Au(CH3)/MgO f Au2(CH3)+/MgO ∆E ) -0.46 eV (4) Clearly, reaction 1 is energetically very favorable and leads to the formation of a dinuclear gold complex which is much more stable than the constituting fragments. Thus, a neutral Au0 atom (mobile) can bind to a Au(CH3)2+ unit (immobile) and form a dimeric species. The resulting Au2(CH3)2+ complex, however,

Figure 8. Mechanism C for the reduction of [Au2(CH3)2]+ by 1.5 mol of H2. See also Figure 2 for definitions. Clusters used: O10Mg10MgECP 18 .

is only 0.17 eV more stable than two isolated Au(CH3)+ and Au(CH3) fragments, reverse of reaction 2. This means that, if the barrier for the dissociation is not too high, it is possible that at RT there is enough thermal energy for the dimer Au2(CH3)2+ to convert into monomeric Au(CH3)+ and Au(CH3) species. Also the combination of a neutral Au atom and of the Au(CH3)+ fragment, reaction 3, is very exothermic and leads to a stable Au2(CH3)+/MgO complex also toward fragmentation into Au+ and Au(CH3), reverse of reaction 4. These results show that several fragments can form by aggregation or fragmentation in the course of the reaction and that their interaction can follow rather complex mechanisms where dynamic aspects are important. 3.5. Mechanism of Reduction [Au2(CH3)2]+/MgO Complexes. Here we consider the mechanism of reduction of the dimeric gold species [Au2(CH3)2]+/MgO, Figure 8 (mechanism C), assuming that these form according to reactions 1 or 2 and that the species can exist on the surface long enough to interact with the hydrogen reducing agent. We also assume that the reduction process proceeds via release of methane, as shown above for the monomeric unit and by the experimental results. We start from the most stable isomer of the [Au2(CH3)2]+/MgO complex, Figure 7c. The first step is the elimination of a methyl group by interaction with hydrogen with formation of CH4 and [Au2(CH3)]+, Figure 8. This step is exothermic by -0.81 eV. At this point, two mechanisms are possible. In the first one (C1, left side of Figure 8), there is a reduction of the dinuclear complex with formation of Au2(CH3) and a surface OH group. The process is accompanied by an energy gain of -0.96 eV. The following step implies the reaction with another H atom to form a second methane molecule leaving on the surface a neutral


Sicolo et al. The total energies of the adsorbed Au2+, Au+, and Au units have been computed on the same cluster model. It turns out that reaction 5 is exothermic by -0.87 eV, showing the formation of a net Au-Au bond despite the positive charge. Therefore, we conclude that the collision of a diffusing neutral Au atom to a Au+ cation can lead to a stable surface complex. Since both Au atoms and Au+ cations diffuse on the surface at RT, we consider now the possible formation of a cationic gold trimer. Au3+ has a singlet ground state and a triangular equilateral shape in the gas phase with Au-Au distances of 2.71 Å, as for the dimer. On the surface, the preferred geometry is with the cluster plane normal to the surface, two Au atoms nearly on top of the oxide anions and the third Au atom pointing away from the surface, Figure 9b. The basal Au-Au distance expands to 2.81 Å, whereas the other two distances remain similar as in gas-phase. This cluster cation is bound by 5.16 eV to the MgO(100) surface. The reaction

Au2+/MgO + Au/MgO f Au3+/MgO Figure 9. Supported cluster cations on the MgO(100) surface. Distances in Å. See also Figure 2 for definitions. Clusters used: O10Mg10MgECP 18 .

Au2 species. This step is exothermic by -0.98 eV. The global reaction consumes 1.5 mol of H2 per mole of Au2 formed. In the second variant (C2, right side of Figure 8), the methyl group dissociates from the [Au2(CH3)]+ complex with formation of a methane molecule and an adsorbed Au2+ unit. This step is endothermic by +0.14 eV and suggests therefore a higher barrier than for mechanism C1. This endothermic step is followed by reduction of Au2+ to Au2 with formation of an adsorbed proton, a process which leads to a large energy gain, -2.09 eV. The results suggest that the reduction process may involve, besides monomeric species (see § 3.2), also dimeric (and possibly higher nuclearity) gold complexes formed by diffusion at room temperature of various reaction intermediates. 3.6. Structure and Properties of Small Au Clusters in Positive Oxidation State. In this section, we discuss the electronic and geometric structure of small Aun+ cluster cations with n ) 2-4 under the assumption that these units can form in the process of aggregation-reduction of charged precursors (see section 3.5). Although the structure of neutral and anionic gold clusters has been investigated in several theoretical papers, less work has been dedicated to cationic gold clusters. A combined study of ion mobility measurements and DFT calculations has shown that gold cluster cations containing up to 7 atoms are planar.48 Here the interest is in the stability and binding ability of the MgO supported gold cluster cations containing up to four atoms. Au2+ has a doublet ground state and in the gas phase is characterized by a Au-Au distance of 2.71 Å; when deposited on the MgO terraces, the dimer lies flat on the surface with the two Au atoms almost on top of two five-coordinated oxide anions, Figure 9a. The Au2+ dimer is bound by 5.33 eV with respect to desorption from the MgO surface. An important question related to the nucleation mechanism is if the combination of a neutral Au atom diffusing on the MgO surface with an adsorbed Au+ cation is energetically favorable. To this end, we have considered the energy change associated to the following process:

Au+/MgO + Au/MgO f Au2+/MgO

∆E ) -0.87 eV (5)

∆E ) -2.05 eV (6)

occurs with a large energy gain of more than 2 eV, further showing the easy nucleation of a cationic gold cluster. Once formed, Au3+ is stable also toward dissociation into Au2 and Au+ supported on MgO

Au3+/MgO f Au+/MgO + Au2/MgO

∆E ) +1.17 eV (7)

The last case considered is that of Au4+. Two isomers have been considered in the gas phase: a rhombic one, more stable, and a distorted tetrahedral shape higher in energy by 0.3 eV. This is in agreement with previous studies.48 Both structures have a doublet ground state. On the surface, we found only one stable structure, the rhombus, Figure 9c. Attempts to optimize the cluster shape starting from an adsorbed tetrahedron lead to the same adsorbed rhombus. In the optimal structure, the rhombus is binding with only two Au atoms to the MgO surface and the cluster plane perpendicular to the surface, Figure 9c. The basal Au-Au distance is elongated to 3.01 Å. Also in this case, the binding of Au4+ to MgO is very strong, 5.19 eV. The addition of a neutral Au atom to an adsorbed Au3+ cluster, reaction (8) is only moderately exothermic, by -0.43 eV

Au3+/MgO + Au/MgO f Au4+/MgO

∆E ) -0.43 eV (8)

The reason for the weaker bonding is that Au3+ is very stable and Au4+ has an open shell structure. Notice that Au4+ is stable also with respect to fragmentation into supported dimers

Au4+/MgO f Au2+/MgO + Au2/MgO

∆E ) +0.74 eV (9)

4. Conclusions Recent experimental work by Guzman and Gates22-27 has shown how to prepare ultra-fine gold clusters deposited on a MgO support by reduction of an organometallic precursor, [Au(CH3)2(acac)]. The first report showed the formation of Au6 or of clusters of similar size.22 Further studies have shown the presence of both zerovalent and cationic gold in these particles.23 The presence of cationic gold contrasts with other reports that indicate the formation of negatively charged gold particles by aggregation or diffusion of gold neutral atoms or clusters on specific defects of an oxide, like in particular the oxygen vacancies.13,15 In other studies, it has been suggested that cationic

Analysis of Gold Clusters on the MgO Surface gold forms in the course of the catalytic reactions and that this is the active species.28,29 Clearly, the charged nature of the gold particle strongly depends on the preparation method and on the working conditions. However, we should notice that only if the reduction process is incomplete the gold cluster will present a positive oxidation state. Once a Au0n cluster is formed on the surface, its interaction with the oxide anions leads to a small charge transfer to the cluster not from the cluster.17 In other words, neutral gold clusters deposited on the MgO surface become slightly electron rich by effect of the interaction with the substrate (this effect is much stronger when the interaction involves a defect like an F center).15 In this study, we have investigated the formation of gold clusters from the agglomeration of mononuclear gold fragments formed during the reduction process. To this end, we have performed first principle DFT calculations using cluster models of the MgO surface. Once adsorbed on the surface, the [Au(CH3)2(acac)] complex tends to stay intact as no clear energetic benefit is associated with the adsorption of the two charged fragments, [Au(CH3)2]+ and (acac)-. On the other hand, the analysis of the computed and measured IR frequencies suggests that most likely what forms on the surface are [Au(CH3)2]+ and (acac)- ionic fragments. The reduction process has then been considered starting from the adsorbed [Au(CH3)2]+ mononuclear complex. Various reaction mechanisms in the presence of hydrogen have been considered. On the basis of the energy data (barriers have not been computed), we come to the conclusion that the preferred mechanism involves the release of two CH4 molecules, followed by reduction of Au+ to Au0 and formation of an adsorbed proton, with a total consumption of 1.5 mol of H2, as suggested experimentally.24 Depending on the route followed in the reduction process, various mononuclear gold intermediates can form on the surface: [Au(CH3)]+, Au(CH3), Au+, Au0, in addition to the [Au(CH3)2]+ precursor. Some of these species, in particular [Au(CH3)]+, Au+, and Au0 have a high mobility on the surface at RT, suggesting that they can lead to aggregation even before the reduction process is completed. For instance, the calculations show the possible formation of a [Au2(CH3)2]+ complex from the aggregation of [Au(CH3)2]+ and Au0 or of Au(CH3) and [Au(CH3)]+. Even this dinuclear gold complex, [Au2(CH3)2]+, can be reduced leading directly to Au2+ and Au20 species. In general, this raises the possible existence on the surface of partially oxidized gold particles. The stability of low nuclearity gold cluster cations on the MgO surface has then been analyzed. We found that Au2+, Au3+, and Au4+ species are stable toward fragmentation into smaller supported units. These results show that gold cluster cations or gold particles with partial positive oxidation state can in principle be formed under reaction conditions in the reduction process of the Au(CH3)2(acac) organometallic precursor. Acknowledgment. This work is supported by the EU Project STREP GSOMEN and by the Italian Ministry of University and Research (Cofin 2005). References and Notes (1) Chen, M.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (2) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (3) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (4) Hayashi, T.; Tanaka, H.; Haruta, M. J. Catal. 1998, 178, 566.

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Formation of Cationic Gold Clusters on the MgO Surface from Au (CH3

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