INTRODUCTION
Gold nanoparticles supported on titania show a high activity in many catalytic processes, including the oxidation of CO and the destruction of SO2. This is a remarkable phenomenon because surfaces of bulk metallic gold are not good catalysts. In recent years, many experimental and theoretical studies have been focused on understanding the high catalytic activity of gold nanoparticles supported on titania. Quantum effects related to the small size of the particles could be responsible for the enhancement in catalytic activity with respect to bulk gold, but it is becoming increasingly clear that interactions between the gold nanoparticles and the titania support plays a very important role. In this contribution, a review is presented of studies dealing with the behavior of a well-defined system such as Au/TiO2(110). The edge and corner sites of a gold nanoparticle (i.e., sites having three to four metal atom neighbors) can bond well adsorbates such as CO, O2, and SO2. They can even perform the catalytic oxidation of CO, but for more demanding reactions, the chemical activity of the isolated Au nanoparticles is not enough. A comparison of the DeSOx activity for the Au/ TiO2(110) and Au/MgO(100) surfaces illustrates the important role played by gold-titania interactions. The titania support is not a simple spectator.
CATALYTIC ACTIVITY OF BULK GOLD AND GOLD/TITANIA
Bulk metallic gold typically exhibits a very low chemical and catalytic activity.[1-4] Among the transition metals, gold is, by far, the least reactive and is often referred to as the ”coinage metal.” Fig. 1 shows a valence photoemis-sion spectrum for metallic gold.[5] The zero of binding energy represents the Fermi level of the system. States with Au 6s,p character appear from 0 to 2 eV, while the Au 5d states extend from 2 to 8 eV.[5] The low reactivity of metallic Au is a consequence of combining a deep-lying valence 5d band and very diffuse valence 6s,p orbitals.[3A6]
In the last 10 years, gold has become the subject of considerable attention because of its unusual catalytic properties when dispersed on some oxide supports.[7-29] The Au/ TiO2 system is particularly interesting.
Gold particles supported on titania are active catalysts for the low-temperature oxidation of CO, as shown in Fig. 2. This phenomenon was originally discovered by Haruta and coworkers in the early 1990s,[8] and has been corroborated by many subsequent studies.[7,10,19,25,27,29] The exact catalytic activity of the Au/TiO2 system depends on the method of preparation and the dispersion of the metal on the support,[,,,] but in general, Au particles with sizes between 2 and 4 nm display a catalytic activity for CO oxidation much larger than that of bulk metallic gold. New preparation methods aim for the synthesis of very small Au particles (< 2 nm) with an extremely high catalytic activity.[25] The Au/TiO2 systems lose catalytic activity over time as a consequence of the sintering of the Au particles.[8,25] The smaller the initial size of the particles, the more dramatic the negative effects of sintering.
Au particles supported on titania are also efficient catalysts for the complete oxidation of methane, the selective or partial oxidation of propene, the hydrogenation of CO and olefins, the reduction of NO with hydrocarbons, and the decomposition of so2.[8,9,11,24,28-31] Depending on the conditions, Au/TiO2 is a useful catalysts for the destruction of the three major contaminants produced during the combustion of fossil-derived fuels: CO, NO, and SO2.[8,29,31] Several models have been proposed for explaining the activation of supported gold:[8,10,17-19,23,29,31] ranging from special electronic properties resulting from the limited size of the active gold particles (usually less than 10 nm),[8,10,19,31] to the effects of metal-support interactions (i.e., charge transfer between the oxide and gold).[15,18,23,31] In principle, the active sites for the catalytic reactions could be located only on the supported Au particles or on the perimeter of the gold-oxide inter-face.[8,10,12,16,31] The Au/Ti02(110) surface appears as an ideal and well-defined system to examine some of these hypothesis in a controlled manner.[10,12,14-18,22,23,28]
GOLD NANOPARTICLES ON TiO2(110)
Fig. 3 shows a schematic model for the (110) face of titania in its rutile phase. The surface exposes pentacoor-dinated Ti cations, while O atoms are present at in-plane and bridging positions. O vacancies are usually created by the removal of O atoms from bridging positions. Several studies indicate that gold grows on Ti02(110) epitaxially, forming two- or three-dimensional particles (Volmer-Weber growth mode).[10,12,16] The formation of two-dimensional (2-D) clusters has been detected only at low gold coverages [<0.2 monolayers (ML)] and moderate temperatures (<350 K).[10,16] Although, thermodynami-cally, gold would prefer to form three-dimensional (3-D) islands from the onset of growth, kinetic limitations constrain the growth initially to two-dimensional (2-D) islands.[16] The critical gold coverage for a 2-D^3-D transition decreases with temperature, and increases with the defect density of the TiO2 surface.[10,16] At elevated temperatures 750 K), 3-D particles are usually present on the oxide surface, without encapsulation of the Au islands by Ti suboxides.[10,12,16]
Fig. 1 Valence photoemission spectrum for metallic gold.
Fig. 2 Turnover frequencies for CO oxidation at 300 K on Au/ TiO2 as a function of the mean particle diameter of Au.
Fig. 3 Schematic model for the Ti02(110) surface. Dark spheres represent Ti atoms, while white spheres represent O atoms.
Fig. 4 shows an image of scanning tunneling microscopy (STM) for 0.25 ML of Au on Ti02(110) after deposition of Au at 300 K and annealing at 850 K for 2 min.[10] The Ti02(110) surface consists of flat (1 x 1) terraces separated by monoatomic steps. The Au clusters are imaged as bright protrusions with a relatively narrow size distribution.1-10-1 The clusters preferentially nucleate on the step edges of the Ti02(110) substrate and have an average size of ~ 2.6 nm in diameter and ~ 0.7 nm in height (two to three atomic layers). Studies of scanning tunneling spectroscopy (STS) indicate that the clusters have a small band gap (0.2-0.6 V) and electronic properties different from those of bulk metallic Au.[10] This is important, because such difference could be responsible for the variation in chemical activity when going from the nanoparticles to bulk gold.[10]
Fig. 4 STM image for 0.25 ML of Au on Ti02(110).
The nature of the interactions between Au and Ti02(110) has been examined in several theoretical studies. In agreement with experimental observations, density-functional (DF) calculations show weak bonding interactions between Au atoms and stoichiomet-ric Ti02(l 10). Au—Au bonds are stronger than Au—Ti02 bonds, which explains the formation of mostly 3-D particles in Fig. 4. The results of STM and DF calculations show that Au adatoms prefer to interact with the O-vacancy sites present on the oxide support.[18,23,30] Theoretical studies[18,23] and photoemission measure-ments[15,23] indicate that the Au atoms bonded to these sites receive electron density from the oxide substrate. Fig. 5 displays Ti 2p photoemission spectra collected before and after dosing Au to a TiO2(110) surface.[23] The surface without gold (top) was initially annealed at 750 K for 2 min to induce the formation of O vacancies. In this way, one obtains a distribution of vacancies from the surface to the bulk of the sample. The Ti 2p spectrum for this system is well fitted[23] by a set of two doublets with p3/2 components at 458.03 eV (Ti4+) and 455.96 eV (Ti3+). Under these conditions, the near surface region contains O vacancies with a density of ~7%. When Au is deposited on this surface at 300 K, the features between 454 and 456 eV gain relative intensity with respect to the main feature at ~ 458 eV and the resulting Ti 2p spectrum needs three doublets for a good fit (center of Fig 5). The p3/2 components of these doublets appear at 458.06, 456.93, and 455.41 eV. The first peak is attributable to Ti4+ cations, the last one corresponds to Ti3+ species, and the middle one can be assigned to Ti3 + ions weakly oxidized (Ti8 +) by interaction with Au.[23] Final annealing of the Au/TiO2(110) surface at 750 K produces a clear increase in the signal covering the 454456 eV region because of a rise in the intensity of the Ti3+ and Tid+ peaks. This phenomenon is not a consequence of the oxidation of Au, or the evolution of O2 into gas phase. It originates in the migration of O vacancies or Ti3+ interstitial sites from the bulk to the surface of the oxide.[23] One has a complex situation, in which the admetal modifies the rate of exchange of defects between the bulk and surface of the oxide, and at the same time, the presence of O vacancies in the surface electronically perturbs the gold, probably making it more chemically active.
Fig. 5 Ti 2p photoemission spectra taken before and after dosing 0.5 ML of gold to a TiO2(110) surface. In the first step, the clean oxide was annealed at 750 K, and then the top spectrum was recorded. This was followed by the dosing of Au at 300 K, middle, and final heating to 750 K, bottom.
OXIDATION OF CARBON MONOXIDE ON Au/TiO2(110)
High surface area Au/TiO2 catalysts are very efficient for the oxidation of CO.[8,25] The top panel in Fig. 6 shows how the CO oxidation activity of a Au/TiO2(110) surface changes as a function of particle diameter.[10] There is a marked size effect on the catalytic activity, with Au clusters in the range of 3.5 nm exhibiting the maximum reactivity. For this size, most of the particles have a band gap of 0.2-0.6 V according to STS (Fig. 6B and C). Particles with a larger band gap (> 1 V) display a lower reactivity, and particles with metallic character (band gap ~ 0 V) are the least active. Thus there is a correlation between the electronic and chemical properties of the supported Au nanoparticles. Studies of STM indicate that exposure to CO has no effect on the morphology of the Au/TiO2(110) surface.[10] On the other hand, significant morphological changes occur after exposure to O2 or CO:O2 mixtures. In these cases, the Au cluster density is considerably reduced as a result of sintering. The Au/ TiO2(110) surfaces exhibit an exceptionally high reactivity toward O2 at 300 K that promotes the sintering of the Au nanocrystallites.[10,14] This sintering eventually leads to a decrease in the CO oxidation activity of the Au/ TiO2(110) systems.[10]
Fig. 6 A) The activity for CO oxidation at 350 K as a function of the Au cluster size supported on TiO2(110). B) Cluster band gap measured by STS as a function of the Au cluster size supported on TiO2(110). C) Relative population of the Au clusters that exhibited a band gap of 0.2-0.6 V as measured by STS.
A transfer of electrons from the titania support to atoms in the Au nanoparticles could help to explain the high catalytic activity of Au/TiO2[10,15,18] DF calculations have been used to study the CO oxidation process on an isolated (i.e., nonsupported) Au10 cluster.[19] The results are summarized in Fig. 7. Two different reaction paths were considered: one where O2 dissociates and one where adsorbed O2 directly reacts with adsorbed CO. Both reactions were found to be extremely facile on the Au10 particle, with reaction barriers of less that 0.4 eV indicating that the CO oxidation reaction should be possible well below room temperature.[19] This is contrary to the behavior found for a Au(111) surface. The small Au10 cluster offers special geometrical configurations (corner and edge sites) that cannot be found on the extended sur-face.[19] The size and shape of the Au particle are important parameters. Also, the electronic structure of the Au atoms in the cluster is different from that of the Au atoms at the surface of a large crystal. The d states of Au10 are 0.75 eV higher in energy than the d states of the surface atoms of an Au(111) surface.[19] These DF results show that an isolated Au nanoparticle could catalyze the oxidation of CO.
No theoretical study has been published examining the role of the oxide support during the oxidation of CO on Au/TiO2(110). Similar studies have been carried out in the case of the Au/MgO(100) system.[26,27] It appears that the smallest gold cluster that catalyzes the CO oxidation reaction is Au8.[26] A key aspect for the reactivity of this particle is its structural fluxionality that allows the adsorption and activation of O2.[26] DF calculations indicate that the active sites for CO oxidation on Au/MgO(100) involve low-coordinated Au atoms and Mg cations.[27] The oxide stabilizes a peroxolike intermediate, CO • O2, and then the oxidation reaction proceeds in the metal-oxide in-terface.[27] Such a reaction pathway is consistent with studies for Au/TiO2 high-surface area catalysts,[31] which show that a strong contact between the Au nanoparticles and oxide support is indispensable for high catalytic activity because the periphery sites probably carry out the oxidation of CO.
Fig. 7 Calculated reaction profile for CO oxidation on a Au10 particle. All energies are given with respect to CO and O2 in gas phase. Black color, direct path; gray, indirect path. Thicker lines represent stable states, while thinner lines correspond to transition states. Light gray spheres represent Au atoms, dark spheres represent O atoms, and gray spheres represent C atoms.
DECOMPOSITION OF SULFUR DIOXIDE ON Au/TiO2(110)
Surfaces of metallic gold interact very weakly with SO2 and the molecule desorbs intact at temperatures below 200 K.[23] Titania is the most common catalyst used in the chemical industry and oil refineries for the removal of SO2 through the Claus reaction: SO2+ 2H2S! 2H2O + 3Ssolid.[1,2,32] The main product of the adsorption of SO2 on stoichiometric TiO2(110) are SO3 and SO4 spe-cies.[23,33] A substantial concentration of O vacancies on the oxide surface is necessary to induce the decomposition of SO2 at high temperatures (>400 K). In contrast, Au/ TiO2(110) surfaces fully dissociate SO2 at room temperature.1-23-1 Fig. 8 shows S 2p photoemission spectra for the adsorption of SO2 on TiO2(110) and Au/TiO2(110) at 300 K. The Au/TiO2(110) surfaces were prepared following a procedure similar to that used to obtain the surface in Fig. 4.[10] The S 2p spectra indicate that upon adsorption of SO2 on Au/TiO2(110), SO4 and atomic S (produced by the full dissociation of SO2) coexist on the surface. For the systems in Fig. 8, the larger the Au coverage on titania (0.05-0.5 ML range), the bigger the amount of atomic S deposited. This trend points to a direct involvement of gold in the dissociation of SO2. A large shift in the corresponding Au 4f core level spectra also supports this idea.[23]
Fig. 8 S 2p photoemission spectra for the adsorption of SO2 on TiO2(110) and Au/TiO2(110) at 300 K.
Fig. 9 Relative amounts of atomic S formed by the full dissociation of SO2 on Au/MgO(100) and Au/TiO2(110) surfaces at 300 K. Each surface was exposed to the same amount of SO2. The S coverage is assumed to be proportional to the area under the corresponding S 2p XPS features.
Fig. 9 compares S 2p areas measured for atomic S after dosing the same amount of SO2 to Au/TiO2(110) and Au/ MgO(100) surfaces at 300 K.[28] Neither TiO2(110) nor MgO(100) is able to dissociate SO2 on their own. On both oxide supports, the largest activity for the full dissociation of SO2 is found in systems containing Au coverages smaller than 1 ML when the average diameter of the nanoparticles is below 5 nm.[10,28] Clearly, the Au/ TiO2(110) systems are much more chemically active than the Au/MgO(100) systems. Catalytic tests also show that Au/TiO2 is substantially more active than Au/MgO for the Claus reaction or the reduction of SO2 with CO.[23,28] These data indicate that titania either play a direct active role in the dissociation of SO2 or modifies the chemical properties of the supported Au nanoparticles.
DF calculations have been used to examine the adsorption of SO2 on Au(100) and a series of clusters: Au6, Au8, and Au14.[28] Very weak bonding interactions were observed on the extended Au surface with adsorption energies smaller than 0.15 eV. On the other hand, the corner atoms in the Au clusters were able to interact reasonably well with SO2, giving adsorption energies of 0.430.65 eV. However, none of the isolated clusters was able to dissociate the SO2 molecule. The DF calculations indicate that such a process is very endothermic on Au6, Au8, and Au14.[28] One of the most favorable cases found in shown in Fig. 10.[28] The adsorption of SO2 on the Au14 particle is an exothermic process but, upon heating, the molecule should desorb instead of dissociating. The chemistry observed experimentally for SO2 on Au/MgO(100) seems to reflect mainly the intrinsic reactivity of Au nanoparticles with the oxide support playing only a minor role.[28]
Fig. 10 Bottom: Diagrams showing an isolated Au14 cluster and the cluster with the SO2 molecule bonded. The molecule is bonded via the oxygen atoms (z2-O,O). Each oxygen atom is bonded to a corner site of the Au14 cluster. Top: Change in energy for the dissociation of a SO2 molecule on the Au14 cluster. The zero of energy corresponds to an initial state with SO2 and the Au14 separated. Then, the SO2 molecule is adsorbed in the configuration shown at the bottom of the figure. This is followed by cleavage of a S-O bond, and at the end, the molecule is fully dissociated into one S and two O atoms.
Variations in the strength of metal-support interactions are probably the key to the large difference in chemical activity seen in Fig. 9 for Au/MgO and Au/TiO2. During the preparation of the active Au/TiO2(110) surfaces, the system is annealed to temperatures as high as 700-750 K to induce the formation of O vacancies and migration of bulk defects to the surface of the oxide.[23] Au particles like to interact with O vacancies,[15,18,23,30] and on these adsorption sites an oxide!gold charge transfer has been predicted from theoretical studies[15,18,23] and X-ray photoelectron spectroscopy (XPS) measurements.[10,15,23]
The active Au/TiO2(110) surfaces combine electronically perturbed Au atoms and an oxide substrate with a significant amount of defects. Both factors probably contribute to the high activity of these surfaces for the dissociation of SO2.[23] DF calculations for SO2/Au/
TiO2(110) systems show several bonding conformations in which Au- and O-vacancy sites work in a cooperative way to dissociate the SO2 molecule.[23] Thus the active sites should be at the Au-TiO2 interface. In the case of MgO (100), the formation of O vacancies is a highly endo-thermic and difficult process.[34] In the Au/MgO(100) surfaces, a negligible number of O vacancies is expected and this probably leads to a low activity for the dissociation of SO2.
CONCLUSION
Gold nanoparticles supported on titania show a high activity in many catalytic processes not seen for bulk metallic gold or titania. Quantum effects related to the small size of the particles could be responsible for the enhancement in catalytic activity with respect to bulk gold. The edge and corner sites of a gold nanoparticle have distinctive electronic properties and can bond well adsorbates such as CO, O2, and SO2. They can even perform the catalytic oxidation of CO, but for more demanding reactions, it is becoming increasingly clear that interactions between the gold nanoparticles and the titania support can play a very important role in determining chemical activity.
