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Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties

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Science  11 Sep 1998:
Vol. 281, Issue 5383, pp. 1647-1650
DOI: 10.1126/science.281.5383.1647

Abstract

Gold clusters ranging in diameter from 1 to 6 nanometers have been prepared on single crystalline surfaces of titania in ultrahigh vacuum to investigate the unusual size dependence of the low-temperature catalytic oxidation of carbon monoxide. Scanning tunneling microscopy/spectroscopy (STM/STS) and elevated pressure reaction kinetics measurements show that the structure sensitivity of this reaction on gold clusters supported on titania is related to a quantum size effect with respect to the thickness of the gold islands; islands with two layers of gold are most effective for catalyzing the oxidation of carbon monoxide. These results suggest that supported clusters, in general, may have unusual catalytic properties as one dimension of the cluster becomes smaller than three atomic spacings.

An atomic-level understanding of structure-activity relations in surface-catalyzed reactions is one of the most important goals of surface science studies related to heterogeneous catalysis. Planar model catalysts (1–3) consisting of metal clusters supported on thin (2.0 to 10 nm) oxide films simulate the critical features of most practical high surface area metal catalysts, yet are tractable for a wide range of surface analytical probes. The oxide films (SiO2, Al2O3, TiO2, MgO) used are thin enough to be suitably conductive for use with various electron spectroscopies including STM and STS.

We have used such model catalysts to study the unusual and as yet unexplained catalytic properties of nanosize Au clusters. STM, STS, and elevated pressure reaction kinetics measurements demonstrate that the structure sensitivity of the CO oxidation reaction on Au clusters supported on TiO2 is related to a quantum size effect with respect to the thickness of the Au clusters; clusters with a two-layer thickness of Au, which exhibit a band gap uncharacteristic of bulk metals, are shown to be particularly suited for catalyzing the oxidation of CO.

Gold has long been known as being catalytically far less active than other transition metals. Quite recently, however, it was found that when dispersed as ultrafine particles and supported on metal oxides such as TiO2, Au exhibits an extraordinary high activity for low-temperature catalytic combustion, partial oxidation of hydrocarbons, hydrogenation of unsaturated hydrocarbons, and reduction of nitrogen oxides (4). For example, Au clusters can promote the reaction between CO and O2 to form CO2 at temperatures as low as 40 K (5). The catalytic properties of Au depend on the support, the preparation method, and particularly the size of the Au clusters. The structure sensitivity of the low-temperature oxidation of CO on supported Au clusters manifests as a marked increase in the reaction rate per surface Au site per second, or turnover frequency, as the diameter of the Au clusters is decreased below ∼3.5 nm (6, 7) (Fig. 1). A further decrease in cluster diameter below ∼3 nm leads to a decrease in the activity of the Au. Despite the extensive recent efforts addressing this extraordinary catalytic behavior, no atomic-level understanding currently exists.

Figure 1

CO oxidation turnover frequencies (TOFs) at 300 K as a function of the average size of the Au clusters supported on a high surface area TiO2 support (7). The Au/TiO2 catalysts were prepared by deposition-precipitation method, and the average cluster diameters were measured by TEM. The solid line serves merely to guide the eye.

Constant current topographic (CCT) images were obtained by applying a positive bias voltage of 2.0 V to the sample with a tunneling current of 2.0 nA. Figure 2A shows a CCT STM image of 0.25 monolayers (ML) Au on TiO2(110)-(1×1) after deposition of Au at 300 K and annealing at 850 K for 2 min (8, 9). The TiO2(110) surface consists of flat (1×1) terraces separated by monoatomic steps. Recently, it was shown that Ti cations instead of O anions are generally imaged in STM (9). Individual atom rows separated by ∼0.65 nm can also be resolved on the terraces corresponding to the length of the unit cell along the [11̅0] direction of the unreconstructed TiO2(110)-(1×1) surface. The Au clusters are imaged as bright protrusions with a relatively narrow size distribution. The clusters, with an average size of ∼2.6 nm in diameter and ∼0.7 nm in height (two to three atomic layers), preferentially nucleate on the step edges of the TiO2(110)-(1×1) substrate. By varying the initial Au coverage, clusters from 1 to 50 nm can be synthesized with relatively narrow cluster size distributions (9, 10).

Figure 2

(A) A CCT STM image of Au/TiO2(110)-(1×1) as prepared before a CO:O2reaction. The Au coverage is 0.25 ML, and the sample was annealed at 850 K for 2 min. The size of the images is 50 nm by 50 nm. (B) STS data acquired for Au clusters of varying sizes on the TiO2(110)-(1×1) surface. For reference, the STS of the TiO2(110)-(1×1) substrate is also shown.

STS spectra were recorded during the CCT imaging by stopping the scan at a certain point of interest, interrupting the STM feedback loop, and measuring the tunneling current (I) as a function of the bias voltage (V). TheseI-V curves can then be correlated with the corresponding geometric features on the surface. Figure 2B shows STS data acquired for the bare TiO2(110)-(1×1) surface and for overlying Au clusters of varying sizes.

To address the basic issues relating to the structure sensitivity of CO oxidation over supported Au catalysts, we investigated the reaction of CO and O2 on Au clusters of varying size supported on TiO2(110)-(1×1) at reaction conditions similar to those used in actual technological applications and in (6).

The reaction studies of this surface indeed show a marked size effect of the catalytic activity of the supported Au clusters for the CO oxidation reaction, with Au clusters in the range of 3.5 nm exhibiting the maximum reactivity (Fig. 3A).

Figure 3

(A) The activity for CO oxidation at 350 K as a function of the Au cluster size supported on TiO2(110)-(1×1) assuming total dispersion of the Au. The CO:O2 mixture was 1:5 at a total pressure of 40 Torr. Activity is expressed as (product molecules) × (total Au atoms)−1 s−1. (B) Cluster band gap measured by STS as a function of the Au cluster size supported on TiO2(110)-(1×1). The band gaps were obtained while the corresponding topographic scan was acquired on various Au coverages ranging from 0.2 to 4.0 ML. (•) Two-dimensional (2D) clusters; (□) 3D clusters, two atom layers in height; (▴) 3D clusters with three atom layers or greater in height. (C) Relative population of the Au clusters (two atom layers in height) that exhibited a band gap of 0.2 to 0.6 V as measured by STS from Au/TiO2(110).

The I-V characteristics obtained from several Au clusters supported on TiO2(110)-(1×1) for various Au coverages from 0.2 to 4.0 ML are shown in Fig. 3B in terms of their band gaps as a function of the Au cluster size. A metal-to-nonmetal transition occurs as the cluster size is decreased below 3.5 by 1.0 nm2 (3.5 nm in diameter and 1.0 nm in height, ∼300 atoms per cluster). This result is similar to that of Pd/TiO2(110) for which the metal-to-nonmetal transition occurs at a cluster size of 3.0 nm by 1.1 nm (∼300 atoms per cluster) (9).

The relative population of the Au clusters with a band gap of 0.2 to 0.6 V measured by STS from Au/TiO2(110) is shown in Fig. 3C. This band gap is associated primarily with those Au clusters whose thickness corresponds to that of two Au atoms. Clusters that are only one atom thick have band gaps significantly larger, whereas clusters with thicknesses of three atoms or greater exhibit metallic properties. The population of two-atom-thick clusters is clearly peaked at diameters ranging from 2.5 to 3.0 nm, which coincides with the maximum of the specific activity of the model and high-area Au/TiO2catalysts (Figs. 1 and 3A).

A series of CCT STM images of the TiO2(110)-(1×1) surface of Fig. 2 show the effect of separate 120-min exposures of CO, O2, and CO:O2 (2:1), respectively, at a total pressure of 10 Torr in the sealed reactor chamber and at 300 K. The CO exposure has no effect on the morphology of the Au/TiO2(110)-(1×1) surface, whereas significant changes occur after exposure to O2 or CO:O2 (Fig. 4, A and B). In the latter exposures, the Au cluster density was greatly reduced as a result of sintering to form much larger clusters with an average diameter and height of ∼3.6 and ∼1.4 nm, respectively. In addition to the significant agglomeration of the Au clusters, extremely small, presumably TiO2clusters (∼1.5 nm in diameter), were formed. X-ray photoemission spectra (XPS) before and after the CO exposure show no changes in the chemical composition of the Au/TiO2(110)-(1×1) surface; however, the TiO2(110) surface oxidized after the CO:O2 (and O2 alone, not shown) exposure (Fig. 5). A small shoulder at the low binding energy side of the XPS Ti 2p transition, owing to the presence of Ti3+ species, was completely absent after the 120-min CO:O2 exposure at 300 K. Because all of the structural and surface chemical changes upon exposure to O2and CO:O2 were identical and because there were no detectable changes after exposure to CO, we conclude that the Au/TiO2(110) surface exhibits an exceptionally high reactivity toward O2 at 300 K that promotes the sintering of the Au nanocrystallites. The possible effect of thermal sintering can be excluded because of the anneal to 850 K before the adsorption experiments. Although O2 adsorption on atomically flat metal single crystals of Au is a highly activated process with an extremely low sticking probability at 300 K (11), Au nanoclusters can activate O2 and produce atomically adsorbed O atoms on Au clusters (12).

Figure 4

A series of CCT STM images of Au/TiO2(110)-(1×1) as prepared in Fig. 2 (A) after 120 min of O2 exposure at 10 Torr, and (B) after 120 min of CO:O2 (2:1) exposure at 10 Torr. The Au coverage was 0.25 ML, and the sample was annealed at 850 K for 2 min before the exposures. All of the exposures are given at 300 K. The size of the images is 50 nm by 50 nm.

Figure 5

Core-level spectra (Ti 2p) of Au/TiO2(110)-(1×1) measured at grazing emission, ϕ = 45° off the crystal normal, (A) before CO:O2exposure and (B) after 120 min of CO:O2 exposure at 10 Torr and 300 K. The Au coverage was 0.25 ML and the sample was annealed at 850 K for 2 min before the CO:O2 exposure.

In the reaction kinetics studies (discussed above), the Au clusters exhibited a very high activity toward CO oxidation; however, the surface was effectively deactivated after reaction for 120 min. This deactivation is believed to be caused by O2-induced agglomeration of the Au clusters as seen in Fig. 4B. The oxidation of the slightly oxygen-deficient TiO2 surface after the 120-min CO:O2 exposure (Fig. 5) is likely lowering the activity of Au/TiO2 even further, because the fully oxidized stoichiometric TiO2 surface can no longer adsorb O2 at 300 K (13). Oxidation of the TiO2 surface during CO oxidation also provides direct evidence that the deactivation is not likely caused by encapsulation of Au clusters by reduced Ti suboxides as, for example, in the case of Pt/TiO2(110) (14).

In order to better understand the role of O2 in CO oxidation and in various O2 pretreatments that are commonly applied to Au/TiO2 catalysts to improve their activity (15), the clean TiO2(110) surface (Fig. 6A) was exposed to O2 at 2 × 10−8 Torr. After O2 treatment, small islands randomly nucleated on TiO2(110) and finally covered the entire surface (Fig. 6B). Low-energy electron diffraction (LEED) showed a (1×1) pattern indicating that the islands are growing pseudomorphically. XPS measurements of this rough TiO2surface after the O2 treatment indicate that the surface is not significantly changed in chemical composition and thus is still slightly O-deficient. Recently, it was suggested that partially reduced Tin+ (n ≤ 3) ions can be formed in a vacuum-annealed and Ar+-bombarded TiO2(110) surface by annealing at 800 K and reoxidizing to TiO2(110)-(1×1) terraces in an O2 ambient of 7.5 × 10−8 Torr (7). A similar kind of oxidation of the reduced Tin+ ions may occur during the O2 treatment used here.

Figure 6

Two CCT STM images of TiO2(110)-(1×1). (A) Clean surface, before O2 exposure, and (B) after a 12-L exposure of O2 at 650 K (1 L = 10−6Torr-s). The size of the images is 30 nm by 30 nm. Two CCT STM images of Au/TiO2(110)-(1×1). (C) Before CO:O2 exposure, and (D) after 120 min of CO:O2 (2:1) exposure at 10 Torr and 300 K. The Au coverage was 0.25 ML and the sample was exposed to 2 × 10−8 Torr of O2 at 650 K for 10 min (∼12 L) before the exposures. Image sizes are 50 nm by 50 nm.

The influence of the O2-exposed, rough TiO2 surface on the sintering of the Au clusters during CO oxidation at 300 K is shown in Fig. 6, C and D. If we compare a CCT STM image of 0.25 ML Au on TiO2(110)-(1×1) after deposition of Au at 300 K, annealing at 850 K for 2 min, and a subsequent O2 exposure of 2 × 10−8 Torr at 650 K for 10 min (Fig. 6C) with one for which no O2 treatment was made (Fig. 2A), the only difference is the general disorder of the TiO2 surface. The cluster density and size distribution of the Au clusters are identical for both surfaces. Upon exposure of the rough Au/TiO2(110) surface to CO:O2 for 120 min at a total pressure of 10 Torr at 300 K, the cluster density and size distribution of the Au clusters remain unchanged (Fig. 6D). The Au/TiO2(110) surface was oxidized after the high pressure CO:O2 exposure (Fig. 4) and the cluster density of the TiO2 clusters increased. The O2-exposed, rough TiO2 surface then prevents sintering of the Au clusters. Furthermore, a similar kind of atomically rough TiO2 phase may be formed during the high-temperature reduction, calcination, and low-temperature reduction (HTR/C/LTR) procedure used on high–surface area Au/TiO2 catalysts (15). After this treatment Au/TiO2 catalysts exhibit a higher degree of resistance toward sintering of the Au clusters during CO oxidation at low temperatures (15).

These results indicate that the pronounced structure sensitivity of CO oxidation on Au/TiO2 originates from quantum size effects associated with the supported Au clusters. The observed tailoring of the properties of small metal clusters by altering the cluster size and its support could prove to be universal for a variety of metals and will likely be quite useful in the design of nanostructured materials for catalytic applications.

  • * Visiting Scientist, Department of Physics, Tampere University of Technology, Tampere, FIN 33101, Finland.

  • To whom correspondence should be addressed: E-mail: goodman{at}chemvx.tamu.edu

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