When Gold Is Not Noble

When Gold Is Not Noble

(Parte 1 de 2)

When Gold Is Not Noble: Nanoscale Gold Catalysts

A. Sanchez,† S. Abbet,† U. Heiz,† W.-D. Schneider,† H. Ha1kkinen,‡ R. N. Barnett,‡ and Uzi Landman*,‡

UniVersite de Lausanne, Institut de Physique de la Matiere Condensee CH-1015 Lausanne, Switzerland, and Georgia Institute of Technology, School of Physics, Atlanta, Georgia 30332-0430

ReceiVed: October 8, 1999

While inert as bulk material,nanoscalegold particlesdispersedon oxide supports exhibit a remarkablecatalytic activity. Temperature-programmed reaction studies of the catalyzed combustion of CO on size-selected small monodispersed Aun (n e 20) gold clusters supported on magnesia, and first-principle simulations, reveal the microscopic origins of the observed unusual catalytic activity, with Au8 found to be the smallest catalytically active size. Partial electron transfer from the surface to the gold cluster and oxygen-vacancy F-center defects are shown to play an essential role in the activation of nanosize gold clusters as catalysts for the combustion reaction.

Introduction

Investigations of reactions heterogeneously catalyzed by metals and alloys are a major area of research endeavors.1 Here we focus on a class of catalystswhich led recentlyto remarkable and rather surprising findings. In contrast to the inert nature of gold as bulk material,2 nanosize particles of gold supported on various oxides,3-5 as well as two-monolayer-thick gold islands of up to 4 nm diameter on titania,6 were found to exhibit an enhanced catalytic activity, in particular for the low-temperature oxidation of CO.3,5,6 While the novel heterogeneous catalysis by nanosize gold aggregates supported on oxides is of great significance in current and future environmental, sensor, and chemical technologies,3 the processes underlying the catalytic activity of gold in reduced dimensions and the reaction mechanisms are not yet understood. In this study we address these issues through investigations of the low-temperature combustion of CO on size-selected gold clusters, Aun (n e 20), supported on defect-poor and defect-rich MgO (100) films, demonstrating the size dependence of the activity of nanoscale gold clusters, with Au8 found to be the smallest size to catalyze the reaction, and revealing that charging of the cluster via

(partial) electron transfer from the oxide support, as well as the presence of oxygen- vacancy (F-center) defects in the substrate, play an essential role in the activation of nanosize gold model catalysts.

Catalyst Preparation and Structure

Monodispersed cluster ions, selected from a distribution of cluster sizes obtained via supersonic expansion of a cold (40 K) laser-generated metal plasma, were deposited with low kinetic energy (<0.2 eV/atom) on well-defined oxide surfaces;7 under these conditions the clusters soft-land (that is, without fragmentation and significant structural distortion) on the substrate.7-9 Deposition of less than 1% of a monolayer (1 ML ) 2.2 1015 clusters/cm2) at a substrate temperature of 90 K assures isolated supported clusters and prevents agglomeration.10-12 The magnesium oxide supports were prepared in situ by epitaxially growing thin films on a Mo (100) surface.13 Different annealing temperatures result in defect-poor (10 K) and defect-rich (840 K) MgO (100) thin films, and the defect densities (which we attribute mainly to oxygen-vacancy F- centers (FC)) were estimated by titration with NO (see upper spectra in Figure 1d,e).14

Equilibriumstructuresof Au8 clustersadsorbedon MgO (100) were investigated with ab initio simulations.15,16 for both a defect-free surface and for one containing an FC, MgO (FC). The energy-optimal structure of a gold octamer adsorbed on MgO (FC) shown in Figure 2a,b was obtained through energy minimization via unconstrained structural relaxations of both the cluster and substrate ions, starting from the equilibrium geometry of a free Au8 cluster deposited on the surface (thus modeling the soft-landing process) in the vicinity of the FC defect. The doubly-occupied FC orbital of the bare surface, whose energy lies in the band-gap of the MgO (100) spectrum split by 2.45 eV from the O (2p)-derived band, is shown in Figure 2c exhibiting an expanded electron distribution above the surface plane localized about the oxygen vacancy.

The binding energy of the cluster in the optimal adsorption geometry17 (Figure 2a,b) to the surface is rather high (5.56 eV), and the adsorption process is accompanied by charge transfer of 0.5 e into the cluster as shown in Figure 2d,e. Similar (partial) charging of the cluster occurs also for adsorption of the gold octamer on a defect-free MgO (100) surface, with only a minor deformation of the cluster from its gas-phase structure, but the binding energy to the surface is reduced by 2.2 eV from the MgO (FC)/Au8 case. We remark here that the charging of the adsorbed gold clusters is expected to be most significant for their size-selective catalytic activity in light of (i) the enhancedbindingof Au8 clustersto the MgO (FC)surfacewhich anchors and localizes them near the defect sites, thus reducing their propensity for agglomeration, and (i) early investigations of molecular oxygen adsorption on (free) gas-phase gold clusters, where only even numbered anionic clusters (Au2n-,2 e n e 10) were found to react with gaseous O2 molecules with the reactivity correlating with the size-dependent pattern of the electron affinities of the gold clusters.18 This implies that the oxide surface is indeed essential for the (electronic) catalytic

* Author to whom correspondence should be addressed. † Universite de Lausanne. ‡ Georgia Institute of Technology.

10.1021/jp9935992 C: $18.0 © 1999 American Chemical Society Published on Web 1/09/1999

activity of the supported gold clusters, beyond serving merely as a substrate for dispersing the metal particles.

Catalyzed Oxidation Reactions and Mechanisms

Subsequentto verificationthat oxidationof CO does not occur on bare MgO substrate films (with and without defects), see lower spectra in Figure 1d,e, the catalyzed combustion of CO by nanosize gold clusters Aun (1 e n e 20) adsorbed on the two different MgO films was studied by temperature-pro- grammed reaction (TPR) experiments. Isotopically labeled 18O2 and 13CO were used to disentangle the CO2 production on the cluster from an eventual catalytic oxidation of CO involving oxygen atoms from the MgO substrate. Indeed, the 13CO molecule is exclusively oxidized by 18O2 since only the 13C16O18O isotopomeris detected.Furthermore,the experiments revealed that the gold clusters up to the heptamer are inert for the oxidationof CO (see Figure1c) and that the low-temperature

(T < 250 K) combustion of CO is most effective for Au8. Consequently, we set out to elucidate experimentally and theoretically the oxidation of CO by gold octamers supported on magnesia.

The catalytic activity (measured via the amount of desorbed carbon dioxide)on Au8 (0.4% ML) clustersdepositedon defectpoor MgO (100)filmsobservedduringone heatingcycle(Figure

1b), reveals a very small combustion of CO at temperatures between 150 and 250 K. Surprisingly, CO combustion on Au8 dramatically increases when depositing the octamer (0.4% ML) on defect-rich MgO-films (see Figure 1a). This particular behavior pertains even after several reaction cycles up to temperatures of 350 K, indicating that the model catalyst is thermally stable. This is further supported by infrared studies, with the vibrational frequencies of 13CO adsorbed on Au8 (at 90 K before and after annealing to 350 K, and with no oxygen present) showing no change in frequency and integrated absorption intensity (note here that for small metal clusters the vibrational frequency and linewidth of adsorbed CO are rather sensitive to the size and structure of the cluster). In the absence of oxygen, CO desorbs from the supported Au8 cluster between 150 and 180 K. The catalytically formed 13C16O18O desorbs alreadyat 140 K with the main desorptionpeak at 240 K (Figure 1a). Evaluating the area of the TPR signal in Figure 1a yields the production of about one CO2 molecule per Au8 cluster.1,12

Each cluster size (Aun with 1 e n e 20) supported on defectrich MgO (100) films reveals a distinctly different reactivity, with Au8 being the smallest cluster size active for the combustion of CO (see Figure 1c). For all active cluster sizes we observed in addition to the relatively small desorption at 140 K a higher temperature channel with a transition from a narrow desorptionmaximumat 240 K for Au8 (see Figure1a) to a broad

To explorethe energeticsand reactionmechanismsunderlying the above observations we performed extensive ab initio simulations,15 summarized in Figure 3 where we display atomic configurations corresponding to various reaction paths of the oxidation of CO catalyzed by Au8 clusters adsorbed on MgO

(FC). Both O2 and CO adsorb on the Au8 cluster supported on the MgO (100) surface (with or without FC defects). However, the observedsinglesharplypeakedstretchfrequencyof adsorbed cluster and a rather minor influence of adsorptionon the internal structure and bonding of the molecule. Indeed, we found that in the optimal adsorption configuration CO binds on-top of an

Au atom of the upper triangularfacet of the adsorbedAu8 cluster (see Figure 3d-g) with d(CO) ) 1.14 Å (compared to 1.13 Å in the free molecule) and d(AuC) ) 1.92 Å.

On the other hand, an O2 molecule may readily adsorb at several sites, both on the upper Au8 facet and at the interface between the adsorbed gold cluster and the underlying magnesia surface, and in all cases the adsorbed molecule is found to be activated to a peroxo O2* molecular state20,21 with a weakened highly stretched intramolecular bond length (1.41-1.46 Å) compared to that in the free molecule (1.24 Å). On the Au8 triangular top-facet the most stable molecular adsorption configurations are (i) an edge-top (ET) configuration, with an adsorption energy Ea ) 1.2 eV (see oxygen atoms marked O1 and O2 in Figure 3a,b), and (i) an edge (E) configuration with Ea ) 0.8 eV (see Figure 3d,e). Upon coadsorption of CO and

O2 the latter is found in the E adsorption configuration (Figure 3d,e). Another molecular oxygen adsorption site which we consider is the Au8 cluster periphery at the interface with the MgO surface. Here the molecule adsorbs with the two oxygens lying above an Au-Au bridge of the Au8 interfacial layer (see Figure 3f,g), and the absorption energies are Ea ) 1.05 eV and 1.85 eV to MgO(FC)/Au8 and MgO/Au8, respectively, with

Figure 1. CO combustion on Au8 (0.4% ML) supported on defectrich (a) and defect-poor (b) MgO (100) films shown by the production of 13C16O18O obtained by TPR experiments.MgO (100)/Au8 is exposed at 90 K first to an average of 20 molecules of 18O2 and subsequently to 20 molecules of 13C16O per deposited Au atom; these exposures correspondto saturationas the CO2 productiondid not enhanceat higher exposures. (c) Size-dependent overall reactivity of gold clusters, Aun, supported on defect-rich MgO(100) films, expressed as the number of

CO2 molecules per cluster. Note that clusters up to the heptamer are inert (reactivity < 0.2). (d,e) Results of the titration of NO from defect- rich (d) and defect-poor (e) films (without gold clusters). Note that no

CO2 is formed on both films (lower spectra in each panel) under the same experimental conditions as used for the deposited-cluster experi- ments.

9574 J. Phys. Chem. A, Vol. 103, No. 48, 1999 Letters

preadsorbed CO. The activation of the adsorbed oxygen molecule involves occupation of the antibonding ð|* molecular orbital (i.e., the molecular oxygen ðg* orbital oriented parallel to the adsorbing surface) which hybridizes with the orbitals of neighboring gold atoms of the partially negatively charged adsorbed gold cluster (Figure 3a). Reaction Mechanisms. Starting from the optimal ET con- figurationof O2* adsorbed on the MgO(FC)/Au8 catalyst, a gas-

Figure 2. (a,b) Two views of the energy-optimal structure of Au8 adsorbed on the MgO (100) surface containing an oxygen-vacancy F-center

(FC). In (a) the C2v plane of the Au8 cluster is normal to the page, and in (b) it is parallel to the page. The configuration in (a) is tilted forward by 20° about the horizontal axis to expose the (100) plane of the MgO surface. The FC is in the middle of the top layer of the MgO surface under the

Au atom marked 5. The structure of the adsorbed Au8 cluster may be described as a deformed piece of an hexagonal close-packed crystal with stacked five (bottom)- and three (top)-atom layers. Red, blue, and yellow (numbered) spheres depict O, Mg, and Au atoms, respectively. Here, and in the following (including Figure 3) only a portion of the MgO surface used in the calculations is shown. (c) Isovalue surface of the 2e- FC wavefunction. The selected surface encloses about 50% of the orbital charge, and the sign of the wavefunction is color coded: (+) in green, and (-) in purple. Red and blue spheres depict O and Mg ions. (d) Total valence electron distribution F(z) across the slab (in the direction of surface

F(z;Au8) and F(z;substrate) are the isolated subsystem densities calculated with atoms in the positions of the fully optimized system. The dotted line denotes the midpoint in the z-direction, zj, between the surface plane and the Au atom (marked 5 in a and b) adsorbed 1.57 Å above the color center.

Integrating äF(z) from the bottom of the substrate to zj yields transfer of äe 0.5 e from the magnesia surface to the gold octamer; this result is rather insensitive to physically reasonable variations in the upper integration limit (for example when taken at zj( 0.4 Å we obtain äe ) 0.45e).

Letters J. Phys. Chem. A, Vol. 103, No. 48, 1999 9575

phase CO molecule brought to the vicinity of the peroxo molecule (Figure 3b) reacts spontaneously (without an energy barrier) forming a carbon dioxide molecule (the molecule marked O1CO3 in Figure 3c) weakly bound to the catalyst( 0.2 eV); the total exothermicity of the reaction MgO (FC)/Au8 + O2(g) + CO(g) f MgO (FC)/Au8O (CO2(a)) is 4.8 eV and it

Figure 3. (a) Top view of the isovalue surface of the highest occupied antibonding molecular-orbital (HOMO) ð|* of the oxygen molecule adsorbed in the peraxo ET configuration on the top-facet of the Au8 cluster supported on the MgO (FC) surface. The wave function is color coded with green

(+) and purple (-). The selected isovalue surface encloses 50% of the orbital charge. The locations of the underlying atoms of the Au8 cluster are depicted by the yellow spheres numbered as in Figure 2a. The locations of the oxygen atoms of the peroxo oxygen molecule are given by the red spheres marked O1 and O2. The occupied state next to the HOMO is an antibonding ð⊥* orbital with the p-like lobes perpendicular to the plane (not shown). (b,c) The (direct) ER-type reaction mechanism: (b) snap-shot of the CO molecule (the carbon is colored in light-blue and the oxygen, numbered O3, in red) approaching the adsorbed peroxo molecule (red spheres marked O1 and O2). The Au8 cluster and underlying portion of the MgO (FC) surface as in Figure 2b; the reaction products consist of a weakly adsorbed carbon dioxide molecule (marked CO1O3), and an oxygen atom (marked O2). (d,e) Two views of the initial configuration for the LHt reaction. The peroxo oxygen molecule (red spheres numbered O1 and

O2) is adsorbed on top of the Au8 cluster in an E configuration, and the CO molecule is coadsorbed on top of an Au atom (numbered 1). The reaction product is the same as that shown in (c). (f-h) The LHp reaction mechanism. In (f) and (g) we show two views of the initial configuration with the CO molecule adsorbed on top of the Au8 cluster and the peroxo oxygen molecule (red spheres numbered O1 and O2) coadsorbed on the Au5-Au4 bridge at the interfacial periphery of the gold octamer. The reaction products consist of a carbon dioxide molecule (marked O1CO3 in (h)) weakly bound to the gold cluster and an adsorbed oxygen (red sphere numbered O2).

9576 J. Phys. Chem. A, Vol. 103, No. 48, 1999 Letters is 5 eV when the CO2 product is desorbed,2 here (g) and (a) denote gas-phase and adsorbed molecules, respectively. This

Eley-Rideal (ER)-type reaction mechanism corresponds to a low-temperature generation of (weakly adsorbed) CO2 through direct reaction of gaseous CO with a preadsorbed peroxo O2* molecule. This reaction channel can occur even at 90 K (that is, during the initial dosing stage in the experiment) with the weakly adsorbed CO2 product desorbing upon slight heating, correlating with the observed CO2 desorption peak at 140 K. Another reaction pathway which may contribute to the low- temperature oxidation of CO is of the Langmuir-Hinshelwood (LH) type, where the two reactants are initially coadsorbed on the top-facet of the Au8 cluster, with the distance between the carbon atom and one of the peroxo oxygens in this local minimum state equal to 3.1 Å (d(CO1) in Figure 3d,e); in the following we refer to the LH top-facet reaction mechanism as

LHt. Through mapping of the potential energy surface along the C-O1 reaction coordinate (via total relaxation of the system with the variable C-O1 distance as a constraint) we determined a rather low energy barrier ¢Eb (LHt) ) 0.1 eV (occurring at db(CO1) 2.0 Å) for the LHT oxidation channel with the same end product, i.e., a weakly adsorbed carbon dioxide molecule, as shown in Figure 3c. Such ER and LHt low-temperature oxidationmechanismswere foundby us (withsimilarenergetics) for reactions on the gold cluster deposited on either a defectfree MgO (100) surface or one containing an FC, and both are expected to be relatively insensitive to the Aun cluster size, correlating with the experimental results.

As aforementioned,the higher-temperaturechannelis strongly enhancedfor Au8 supportedon a defect-richMgO (100) support

(compare the CO2 signals at 240 K in Figure 1a,b. This trend correlateswith our simulationsof a LH-periphery(LHp)reaction mechanism, starting with the CO adsorbed on the top-facet of the Au8 cluster and the peroxo O2* molecule bonded to the periphery of the interfacial layer of the cluster (see Figure 3f,g), where the distance between the C atom and the oxygen marked

O1 is d(CO1) ) 4.49 Å). Indeed,mappingof the potentialenergy surface along the C-O1 reaction coordinate revealed for MgO (FC)/Au8 a rather broad reaction barrier ¢Eb(LHp) 0.5 eV (at db (CO1) 2.0 Å), while for the defect-free substrate a significantly higher barrier23 was found, ¢Eb(LHp) 0.8 eV; the reaction product is shown in Figure 3h for the MgO (FC)/

Au8 catalyst.Additionally,the availabilityof alternativeclusterperiphery adsorption sites for the oxygen molecule, resulting in a distribution of distances between the adsorbed CO and the

O2* with a consequent distribution of reaction barriers, may underlie the observed relatively larger width of the higher temperature desorption peak (see Figure 1a).

Acknowledgment. Research at Georgia Tech is supported by the U.S. DOE, the AFOSR and the Academy of Finland, and at the Universityof Lausanneby the Swiss NationalScience Foundation. Simulations were performed on an IBM SP2 computer at the GIT Center for Computational Materials Science.

(Parte 1 de 2)

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