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Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO

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Science  21 Jan 2005:
Vol. 307, Issue 5708, pp. 403-407
DOI: 10.1126/science.1104168

Abstract

Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center–rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C–O vibration should arise via electron back-donation to the CO antibonding orbital.

The exceptional catalytic properties of small gold aggregates (1, 2) have motivated research (317) aimed at providing insights into the molecular origins of this unexpected reactivity of gold. Investigations on size-selected small gold clusters, Aun(2 ≤ n ≤ 20), soft-landed on a well-characterized metal oxide support [specifically, a MgO(001) surface with and without oxygen vacancies or F centers], revealed (4) that gold octamers bound to F centers of the magnesia surface are the smallest known gold heterogeneous catalysts that can oxidize CO into CO2 at temperatures as low as 140 K. The same cluster bound to a MgO surface without oxygen vacancies is catalytically inactive for CO combustion (4).

Quantum-mechanical ab initio simulations, in juxtaposition with laboratory experiments, led us to conclude (4, 5) that the key for low-temperature gold catalysis in CO oxidation is the binding of O2 and CO to the supported gold nanocluster, which activates the O–O bond to a peroxo-like (or superoxo-like) adsorbate state. This process is enabled by resonances between the cluster's electronic states and the 2π* antibonding states of O2, which are pulled below the Fermi level (EF). Charging of the metal cluster, caused by partial transfer of charge from the substrate F center into the deposited cluster, underlies the catalytic activity of the gold octamers (Au8), as well as that of other small gold clusters (Aun, 8 ≤ n ≤ 20) (4). These investigations predicted that (i) the F centers on the metal oxide support surface play the role of active sites (a concept that has been central to the development of heterogeneous catalysis); (ii) these sites serve to anchor the deposited clusters more strongly than sites on the undefective surface (thus inhibiting their migration and coalescence); and, most important, (iii) these active sites control the charge state of the gold clusters, thus promoting the activation of adsorbed reactant molecules (that is, formation of the aforementioned peroxo or superoxo species) (18).

We have studied the cluster-charging propensity of the F-center active sites, both experimentally and theoretically, by examining the vibrational properties of adsorbed CO. The internal CO stretch frequency ν(CO), measured in the presence of coadsorbed O2 for the octamer bound to the F center of the magnesia substrate [Au8/CO/O2/MgO(FC)], shifted to lower frequency by about 25 to 50 cm–1 compared to the ν(CO) frequency recorded for the gold octamer bound to the F-center–free MgO(001) surface (Au8/CO/O2/MgO). Systematic ab initio calculations (4, 5) reveal that this shift is caused by enhanced back-donation (from the gold nanocluster) into the antibonding 2π* orbital of the CO adsorbed on the cluster anchored to a surface F center. In addition, calculations addressing free Au8/O2/CO coadsorption complexes provide further evidence that the bonding characteristics and spectral shifts are related to each other and that they are correlated with, and sensitive to, the charge state of the cluster (18).

We reproduced experimentally the results of our earlier investigations pertaining to the enhanced catalytic activity of Au8 clusters deposited on F-center–defective MgO surfaces, and then went beyond those measurements by recording (under the same conditions as for the reactivity studies) the infrared (IR) spectra of the reactants as a function of the annealing temperature (Fig. 1). Size-selected Au8 cations were deposited at 90 K, with low kinetic energy, onto MgO(FC) thin films at low coverages (8 × 1012 clusters/cm2). Several experimental studies [such as the synthesis of monodispersed model catalysts by using soft-landing cluster deposition (19)] have shown that, in general, upon deposition, the clusters are neutralized, maintain their nuclearity, and stay well isolated at defect sites. These model catalysts were then saturated with isotopicaly labeled 18O and 13C162 O; the order of the exposure of the reactants (0.2 Langmuir) did not change the activity (unlike the case for other metals). Upon heating, 13C16O18O was produced at 140 and 280 K, as shown in Fig. 1A. No other isotopomer was detected, indicating that only the adsorbed O2 and CO participate in the reaction. We attribute the reaction at 140 K to an ensemble of Au8 clusters, with O2 bound to the top facet of the cluster; and the reaction at 280 K to an Au8 ensemble, with O2 bound to the perimeter of the cluster at the cluster-to-substrate interface (Fig. 2B). These assignments were made by us previously (4) on the basis of calculated activation barriers for the CO oxidation. Specifically for small gold clusters, only a single O2 molecule can be adsorbed, and thus the two ensembles are saturated with O2 via either direct adsorption or reverse spillover.

Fig. 1.

Mass spectrometric signals pertaining to the formation of CO2 on Au8 deposited on (A) F-center–rich and (B) F-center–free MgO(001) thin films. To unambiguously show that both CO and O2 are involved in the reaction, isotopically labeled 13CO and 18O2 were used. [(C) and (D)] Fourier-transform IR spectra measured for the same surfaces (defect-rich and defect-poor) and with the same CO and O2 exposures as in (A) and (B), respectively, at various annealing temperatures. The indicated temperatures cannot be compared directly with the ones in the temperature-programmed reaction spectrum but are lower limits. We have also observed an IR absorption band at 1300 cm–1, which we attribute to superoxo/peroxo-type O2 .Inorderto better disentangle the vibrational band of 13CO adsorbed on Au8 deposited on defect-poor films (B) from that of the CO weakly bound to the support material, the sample was annealed to 120 K. In this way, the 13CO frequency from MgO-adsorbed CO disappears, otherwise observed at 2127 cm–1.

Fig. 2.

Optimized configurations of (A) a bare Au8 cluster (yellow spheres) adsorbed on an F center of a MgO(001) surface (O atoms are in red and Mg atoms in green) and (B) a surface-supported gold octamer with O2 adsorbed at the interface between the Au8 cluster and the magnesia surface and a CO molecule adsorbed on the top triangular facet (the C atom is depicted in gray). There is a significant change in the geometry of Au8 compared to the one shown in (A). The inset between (A) and (B) shows a local-energy-minimum structure of the free Au8 cluster in the three-dimensional (3D) isomeric form with coadsorbed O2 and CO molecules. Although in the global ground state of the free Au8/O2/CO complex the gold octamer is characterized by a higher degree of 2D character (14), we chose to display here an isomer that closely resembles the structure of the surface-adsorbed complex: Compare the structure in the inset with that shown in (B). (C) Au8 on the magnesia surface [MgO(FC)] with three CO molecules adsorbed on the top facet of the cluster and an O2 molecule preadsorbed at the interface between the cluster and the magnesia surface. The molecule marked CO(2) lies parallel to the surface and is thus not IR-active in the experimental configuration used here. The C–O bond length d(CO(i)), the charge transferred to the CO(i) molecule ΔQ(i), and the calculated C–O vibrational frequency ν(i) (i =1, 2, and 3), as well as the corresponding values for the O2 molecule, are as follows: d(CO(i)) [Å] = 1.151, 1.158, 1.153; d(O2) = 1.422; δQ(i) [e] = 0.31, 035, 0.32; δQ(O2) [e] = 1.12; ν(i) [cm–1] = 1993, 1896, 1979. Isosurfaces of charge differences are as follows: (D) Au8 cluster adsorbed on defect-free MgO, δρ = ρtot – (ρAu8 + ρMgO), where ρtot = ρ[Au8/MgO]; (E) Au8 cluster anchored to a surface F center of MgO, δρ = ρtot – (ρAu8 + ρMgO(FC)), where ρtot = ρ[Au8/MgO(FC)]; (F) same as (E) but with O2 and CO molecules adsorbed on the gold cluster, δρ = ρtot – (ρAu8 + ρO2 + ρCO + ρMgO(FC)), where ρtot = ρ[Au8/O2/CO/MgO(FC)]. Pink isosurfaces represent δρ < 0 (depletion) and blue ones correspond to δρ > 0 (excess). All of the isosurfaces are plotted for the same (absolute) value of the density difference (δρ) in order to allow direct comparison between the different cases.

The corresponding IR study reveals absorption bands of the two reactants. In Fig. 1, C and D, we show only the spectra for adsorbed CO, with the decrease in intensity correlating with the formation of CO2 (Fig. 1C). The IR band corresponding to adsorbed O2 occurs around 1300 cm–1 for both the F-center–rich and perfect MgO surfaces. The IR frequencies (2049 and 2077 cm–1) are typical for on-top adsorbed 13CO on gold. In this context, we recall that the band at 2127 cm–1 originates from 13CO adsorbed directly on defect sites of the MgO(001) surface (20). On Au single crystals (2123), as well as on oxide-supported Au particles (9, 24, 25), sharp 13CO absorption bands occur at a single frequency around 2060 cm–1. We infer that the detection of two absorption features reveals the presence of (at least) two types of adsorbed CO molecules, which differ somewhat in how they bind to the Au8 cluster (Fig. 2C and discussion below).

Upon heating, the population of the three bands changes. At an annealing temperature of 220 K (subsequent to the 140 K combustion reaction), a small single absorption band at 2055 cm–1 was observed (Fig. 1C). Earlier studies (26) detected such a band for 13CO adsorbed to Au8/MgO(FC).

In contrast to the above, gold octamers adsorbed on an F-center–free MgO(001) surface were essentially inactive for the combustion reaction (Fig. 1B). In fact, even under quasi–steady-state conditions with pulsed molecular beams, no CO2 formation was observed. The absorption band of 13CO in this case (2102 cm–1) was shifted to a higher frequency (by 25 to 50 cm–1), as compared to the case of 13CO adsorbed on a gold octamer deposited on MgO(FC).

The above-noted red shift of the CO stretch when the molecule is adsorbed on Au8 supported on MgO(001)(FC) is a key for elucidating the change in the charge state of the gold octamer bound to F-center defects on the MgO surface. The absorption frequency of CO adsorbed on metal surfaces depends strongly on the population of the 2π* orbital, because occupation of this antibonding orbital weakens the C–O bond. Furthermore, results from extensive ab initio calculations, using the method developed in (27) [see also (18)], given in Table 1 for the isolated Au8/O2/13CO complex (which we present first in order to allow a clear distinction between charging and other support-related effects), reveal that the 13CO stretch vibration shifts to lower frequency in a manner that is correlated with increased charging of the complex (given by Qc in Table 1), as well as with the estimated increase in the population of the antibonding state [given by δQ(CO) in Table 1]. For a neutral free complex with zero spin [Qc = 0 and S = 0 in Table 1; see the corresponding atomic configuration shown in the inset of Fig. 2], a vibrational frequency of 2009 cm–1 was obtained for the adsorbed 13CO molecule. We attribute the calculated decrease in frequency (61 cm–1) in comparison to the value calculated for the free molecule (2070 cm–1) to a net excess charge [δQ(CO) = 0.28e, where e is the electron charge] on the adsorbed molecule. The excess charge on the molecule results from back-donation into the CO(2π*) antibonding state.

Table 1.

Results for free Au8/O2/13CO complexes as a function of the amount of excess electron charging Qc, shown for various values of the spin. For the neutral cluster (Qc = 0), we show the triplet (S = 1) and singlet (S = 0) states. The quantities that we display are: ν(13CO), the 13CO vibrational frequency; the calculated excess electronic charge on the adsorbed molecules δQ(CO) and δQ(O2), with the procedure used for achieving these estimates described in (18); and the bond distances d(CO) and d(O2). The calculated values for the isolated molecules are d(CO) = 1.14 Å and d(O2) = 1.24 Å, compared to the gas-phase experimental values of 1.13 and 1.21 Å, respectively. The calculated vibrational frequency of gas-phase 13CO is 2070 cm–1, which is 25 cm–1 smaller than the experimental value νexp(13CO) = 2095 cm–1.

Qc (e) S ν (cm-1) d(13CO) (Å) δQ(CO) (e) d(O2) (Å) δQ(O2) (e)
0 1 2005 1.149 0.29 1.336 0.71
0.25 0.875 1987 1.150 0.32 1.344 0.75
0.5 0.75 1968 1.154 0.35 1.350 0.77
1.0 0.5 1926 1.160 0.43 1.364 0.86
0 0 2009 1.148 0.28 1.378 0.88
0.25 0 1990 1.150 0.31 1.381 0.89
0.5 0 1975 1.153 0.34 1.385 0.92
1.0 0 1920 1.158 0.41 1.398 1.00

As expected, such donation of charge from occupied orbitals of the metal to unoccupied states of the adsorbed molecule is even more pronounced (0.88e) for the more electronegative O2 molecule. Upon charging the complex with up to 1.0e, the net excess charge on the 13CO molecule increases to 0.41e, and the CO stretch redshifts by 89 cm–1 to 1920 cm–1. The increased degree of charging of the metal cluster with back-donation, and the consequent decrease of ν(CO), increase the C–O bond length from 1.148 Å for Qc = 0 to 1.158 Å for Qc = 1.0e. Similar changes were seen starting from the triplet state of the neutral complex (Table 1).

We next focused on the adsorption of CO to gold complexes surface-supported on perfect or defective magnesia [with an oxygen molecule preadsorbed at the interface between the cluster periphery and the MgO surface (Fig. 2B)]; that is, Au8/O2/MgO or Au8/O2/MgO(FC). Three energy-optimized deposited cluster configurations pertinent to the experimental work are displayed in the top row of Fig. 2, A to C. The bare adsorbed Au8 cluster (Fig. 2A) exhibits only a slight distortion from the structure of the corresponding gas-phase neutral cluster (4, 5), consisting mainly of a displacement of the gold atom of the cluster closest to the surface oxygen vacancy. The cluster is anchored strongly to the defective MgO surface (with a binding energy of 3.44 eV) compared to a significantly weaker binding to the defect-free surface (1.22 eV). From examination of the charge-difference isosurfaces displayed in Fig. 2, we observe that bonding of the Au8 cluster to the MgO(FC) surface is accompanied by a significantly larger degree of charge transfer from the magnesia surface to the gold cluster (Fig. 2E) as compared to the case of adsorption on an F-center–free surface (Fig. 2D).

Optimal geometries with a single adsorbed CO molecule, Au8/O2/CO/MgO(FC), and at saturation coverage of the cluster [that is, with three adsorbed CO molecules, Au8/O2/(CO)3/MgO(FC)] are shown in Fig. 2, B and C, respectively. Comparison between the bare-cluster geometry in Fig. 2A with those shown in Fig. 2, B and C, reveals a significant change in the geometry of the gold cluster caused by the adsorption of the reactant molecules, mainly the peripherially adsorbed O2. This structural change, occurring during adsorption (or in the course of reaction), is a manifestation of the “structural fluxtionality” of clusters (5). The system shown in Fig. 2C possesses two IR-active 13CO molecules (CO(1) and CO(3)), as we had seen experimentally (18).

Because the bonding characteristics, and other properties of the system, with a single adsorbed CO molecule (Fig. 2B) are similar to the ones associated with the CO-saturated cluster (Fig. 2C), we focus next on the former (which is more convenient to analyze and illustrate). The energetics of the adsorption of the O2 and CO molecules to the defect-free and F-center clusters, corresponding in each case to two possible spin states (S = 0 or 1), together with the calculated amount of charge transferred to the deposited complex (gold cluster and adsorbed molecules), the bond length d(CO), and the calculated vibrational frequency are given in Table 2. Stronger binding of O2 to the cluster occurs in the presence of the F center (A and B in Table 2).

Table 2.

Binding energies: BE(O2), binding energy of O2 to Au8/MgO; BE(CO), binding energy of CO to Au8/O2/MgO. Excess electronic charge ΔQ(Au8/O2/CO) on the complex adsorbed on the (001) surface of magnesia is shown. The C–O bond length d(CO) and the C–O stretch vibrational frequency ν(13CO) are shown. Results are given for the gold octamer adsorbed on a MgO(001) surface with (F center), or without (F-center–free), a surface F center, and in each case we give results calculated for two spin states, S. The calculated amount of charge transferred to a bare gold octamer cluster adsorbed on the defect-free surface is ΔQ(Au8/MgO) = 0.82e, and it is significantly larger for the cluster adsorbed on a surface F center; that is, ΔQ(Au8/MgO(FC)) = 1.44e. The excess charge ΔQ is calculated as described in (18).

S BE(O2) (eV) BE(CO) (eV) ΔQ(Au8/O2/CO) (e) d(CO) (Å) ν (13CO) (cm-1)
F center
A 1 0.33 0.79 1.52 1.155 1937
B 0 0.47 0.65 1.58 1.156 1931
F-center—free
C 1 0.30 0.79 0.87 1.152 1965
D 0 0.15 0.91 1.01 1.150 1994

Adsorption is accompanied by a significant degree of charge transfer, which in the case of the defective surface is directed from the oxygen vacancy into the deposited gold cluster and adsorbed molecules. Comparison of the charge-difference isosurfaces for the bare cluster (Fig. 2E) and for the cluster with adsorbed O2 and CO molecules (Fig. 2F), bonded to MgO(FC), reveals a significant amount of charge excess on the adsorbed molecules. The consequent weakening of intramolecular bonds manifests itself in increased bond lengths and lower vibrational frequencies of the adsorbed molecules (Table 2) (28).

The correlation diagram (Fig. 3, A to C) depicts the local density of states (LDOS) for the two reactants, CO (Fig. 3A) and the Au8/O2/MgO(FC) complex (Fig. 3C), as well as for the product complex Au8/O2/CO/MgO(FC) with the CO adsorbed on the deposited cluster (Fig. 3B). The LDOS of Au8/O2/CO/MgO(FC) is reproduced in Fig. 4 along with images of the corresponding molecular orbitals, which aid visualization of the bonding mechanism discussed below. As expected, the 3σ and the nonbonding 4σ (oxygen lone-pair) molecular orbitals of the isolated CO are not involved in the bonding to the cluster complex, and the nonbonding 5σ (carbon lone-pair) orbital is stabilized by the interaction (by about 3eV) and thus it contributes to CO chemisorption (29). The 1π level is slightly pushed upward in energy (<1eV).

Fig. 3.

LDOS correlation diagram of (A) free CO, (C) Au8/O2/MgO(FC), and (B) after adsorption of a CO molecule, resulting in the complex Au8/O2/CO/MgO(FC). The results correspond to the spin singlet (S = 0) case. The electron populations of the various levels of the free and adsorbed CO molecule are given as 2e, 4e, etc., and isosurface images of the orbitals of the free CO molecules are also shown in (A). Black dashed lines indicate orbital shifts and redistribution caused by the adsorption of the CO molecule.

Fig. 4.

LDOS and orbitals of the Au8/O2/CO complex adsorbed on MgO(FC). Results are shown for the spin singlet S = 0 case. The LDOS projected on the CO molecule is shown in black and that projected on the gold cluster is colored orange. EF is indicated by a dashed line. The electronic population of the orbitals of the CO molecule (found to be mainly of 2π* character), with energies in the range that overlaps the gold cluster states, provides an estimate of the back-donated charge (1.27e). Included also are isosurface images of the wave functions of the Au8/O2/CO/MgO(FC) complex, which may be identified with the indicated orbitals of CO (compare to the orbital images of the free CO molecule in Fig. 3A).

The LDOS of the complex projected on the adsorbed CO also reveals contributions of the 2π* orbitals that are spread over the entire energy range of the cluster's electronically occupied spectrum (Fig. 3B, the black-shaded LDOS for energies between about –10eV and EF). The 2π* orbitals of the CO molecule are pushed below EF, which populates this state via back-donation [that is, hybridization and population of the initially unoccupied antibonding orbitals of the CO molecule through interaction with occupied surface orbitals (3032)]. All of these features are also present in the correlation diagram of the cluster complex bound to the defect-free MgO surface (not shown here), where back-donation, however, is less pronounced. We can estimate the amount of back-donated electronic charge by integrating over the squared amplitude of the 2π* orbital located below EF. For the Au8/O2/CO bound to the F center of the MgO support, 1.27e are back-donated into the 2π* orbital, whereas a smaller degree of back-donation (1.18e) occurs for the complex bound to the undefective surface.

The above-noted difference in the degree of back-donation is manifested in a variation of the stretch frequencies of the adsorbed CO molecules, and these correlate with the aforementioned variation in the degrees of substrate-induced charging of the gold octamer deposited on magnesia surfaces with or without F-center defects (33). Indeed, ν(CO) for the complex bound to an F-center–rich surface is measured to be redshifted by 25 to 53 cm–1 with respect to the frequency of a CO molecule bonded to the complex deposited on an F-center–free support (Fig. 1). This compares favorably with the corresponding calculated red shift; for example, from Table 2, a value of 34 cm–1 is estimated when comparing ν(CO) for the C(FC-free) and B(FC) states [both corresponding to the complexes exhibiting the largest binding energy of the oxygen molecule (Table 2)].

We conclude that partial electron transfer from the F centers to the adsorbed cluster complex correlates with frequency shifts of the intramolecular vibration of adsorbed CO. In addition, these sites serve to strongly anchor the deposited clusters, thereby inhibiting their coalescence into larger inert ones. Understanding of such issues pertaining to the interactions between deposited clusters and the support surfaces, and investigations of the dependencies of such interactions on the materials' identity, their size, and their chemical and physical properties, promise to enhance progress toward the design and use of specific nanocatalytic systems.

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