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Electrochimica Acta 47 (2002) 3595 /3609 www.elsevier.com/locate/electacta

Catalytic oxidation of CO by platinum group metals: from ultrahigh vacuum to elevated pressures A.K. Santra, D.W. Goodman * Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, USA Received 10 October 2001; received in revised form 15 February 2002

Abstract CO oxidation over platinum group metals has been investigated for some eight decades by many researchers and is considered to be the best understood catalytic reaction. Nevertheless, there has been a renewed interest in CO oxidation recently because of its technological importance in pollution control and fuel cells. Removal of COx from automobile exhaust is accomplished by catalytic converters using supported Pt, Pd and Rh catalysts. Catalysts are used in fuel cells to remove traces of COx from the H2 feed gas to the few ppm level necessary for their efficient operation. Efforts have been made in our laboratory to understand the adsorption of CO and the kinetics of CO-oxidation on both single crystals and supported metal catalysts over a wide temperature (100 /1000 K) and pressure (1/10 7 /10 Torr) range. By comparing the results of single crystals, model supported catalysts, and supported technical catalysts the relationship between particle size and catalytic activity can be better understood. Also discussed is CO oxidation on model supported Au catalysts, a promising new candidate for low temperature CO oxidation. # 2002 Published by Elsevier Science Ltd. Keywords: CO oxidation; Catalysts; Automobile exhaust

1. Introduction Catalytic oxidation of CO over platinum group metals (Pt, Ir, Rh and Pd) has been the subject of many experimental and theoretical investigations [1 /46] since the classic work of Langmuir [47] in 1922 and has been extensively reviewed [10,48/50]. Recently, CO oxidation has attracted renewed attention due to its technological importance in the area of pollution control [51] and fuel cells [52,53]. Currently the removal of CO from automobile exhaust is accomplished by the oxidation of CO in catalytic converters using supported Pt, Pd and Rh catalysts. It is well established that for optimum operation of low temperature fuel cells it is essential to have a continuous supply of CO-free hydrogen. Although, proton exchange membrane fuel cells can tolerate a few ppm level of CO in the hydrogen stream, alkaline fuel cells require CO-free hydrogen. The

* Corresponding author. Tel.: /1-979-845-6822; fax: /1-979-8450214 E-mail address: [email protected] (D.W. Goodman).

conventional hydrogen production technologies such as steam reforming, partial oxidation and autothermal reforming of hydrocarbons produce large amounts of CO as a by-product [52,53]. Therefore, it is extremely important to have a CO oxidation catalyst with very high efficiency and one that can preferentially oxidize CO for the production of CO-free hydrogen stream. Numerous adsorption and kinetic studies on single crystals and supported metal catalysts have been reported in the last two decades from our laboratory as a function of O2 and CO partial pressure from ultrahigh vacuum (UHV) to elevated pressures (10 Torr) over a wide temperature range of 100 /1000 K [1 /6,10 /12,54 / 59]. Although the CO oxidation reaction is the best understood among the industrially important catalytic reactions, there are many complex aspects to be resolved with respect to a unified reaction mechanism. Historically, gold is chemically inert compared with the other Pt group metals, however, recently it has been shown that Au, deposited as finely dispersed, small particles (B/5 nm diameter) on reducible metal oxides like TiO2, is an excellent catalyst for CO oxidation at relatively low temperatures [54,55,60 /63]. Furthermore,

0013-4686/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 3 3 0 - 4

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A.K. Santra, D.W. Goodman / Electrochimica Acta 47 (2002) 3595 /3609

this catalytic activity has been shown to be a critical function of the Au cluster size. In this article, we will compare CO adsorption and oxidation on single crystals with model supported catalysts of Pt, Ir, Rh, Pd and Au. Primary emphasis will be given to the results obtained in our laboratory over wide temperature (100 /1000 K) and pressure ranges (1 /107 /10 Torr). The goal of this work is an understanding of the effects of temperature, pressure and particle size on CO oxidation that spans the material and pressure ‘gaps’ between ‘real world’ catalysis and ‘surface science’.

2. Experimental The UHV systems used for this work were equipped [64] with Auger electron spectroscopy (AES), a quadrupole mass spectrometer for temperature programmed desorption (TPD), low energy electron diffraction (LEED), scanning tunneling microscopy/spectroscopy (STM/STS) and an Ar ion sputter gun, contiguous to a high-pressure reaction chamber. The single crystal samples were mounted on a retractable manipulator, allowing the sample to be moved between the two chambers in situ. Pt(100), Ir(110) and Pd(110) crystals of 0.92 cm in diameter and 0.11 cm in thickness were used for the experiments. The Ir(111) sample was elliptical in shape and 0.75 /0.55 /0.03 cm3 in size. The samples were heated resistively by two high purity (0.051 cm) tungsten leads spot-welded to the back of the crystal; sample temperature was measured by a 0.08 cm chromel-alumel thermocouple spot-welded to the sample edge. AES was used to check the cleanliness of the samples. In addition, the Pt(100) sample was cleaned of carbon by O2 adsorption/desorption and of Si and Ca impurities by high temperature oxidation (1123 K, 1 /10 7 Torr O2) and Ar sputtering. The Pd crystal was cleaned in the reactor with 8 Torr of CO and 8 Torr of O2 and heating to 600 K for 1 /2 min. One to three cycles of this treatment produced a clean surface. Large carbon impurities were cleaned from the Ir samples by oxidation in 1 /105 Torr O2 at 1000 K for 5 /10 min, followed by a 3 min anneal to 1600 K. Small traces of carbon were removed by reaction in 8 Torr O2 and 4 Torr CO for 2 min at 600/625 K, followed by a brief flash to 1600 K. The Pt sample was cleaned by sputtering at 1100 K in 5/105 Torr of Ar for 30 min (1 kV beam energy) mainly to remove Si and Ca impurities. This treatment was followed by heating in 0.1 Torr of O2 at 1100 K for 30 min to remove traces of carbon and then annealing at 1300 K. Repeated cycles of this procedure produced a clean Pt surface, which could not be oxidized under UHV conditions at high temperature. Rh crystals were cleaned by oxidation (2 /

107 Torr O2 at 1300 K) followed by annealing at 1500 K. The Ru(0001) crystal was cleaned by oxidation (2 / 107 Torr O2 at 1450 K for 3 min) followed by annealing at 1500 K. The IR cell with CaF2 window is connected to the UHV chamber through a double differentially pumped sliding seal. The configuration of the elevated pressure cell is similar to that described by Campbell et al. [65]. This arrangement allows IR experiments in the pressure range of 10 8 /103 Torr and also provides convenient access to the sample without opening the UHV chamber. The pressure in the IR cell was determined using an ionization gauge and a capacitance manometer at their respective working pressure ranges. TiO2(110) single crystals (Commercial Crystal Laboratories) were used in these studies mainly due to their suitability for atomically-resolved STM and STS experiments. The n-type semiconductor form, sufficiently conductive for STM and electron spectroscopic measurements, was prepared by cycles of Ar sputtering and annealing to 700/1000 K. Deposition of the metal was typically accomplished by resistive evaporation of high-purity metal wire wrapped around a W or Ta filament in vacuum. Such dosers provide an excellent means of obtaining a clean and stable metal flux after thorough outgassing. By controlling the filament current, doser to substrate distance and the substrate temperature, fine control can be exercised over clustersize and density. Gas chromatography with flame ionization detection (FID) was used to analyze the reaction products in which CO and CO2 were catalytically converted to methane before analysis. Rates of reaction are expressed as turnover frequencies (TOF), defined as the number of CO2 molecules produced per active metal site per second. For Pd, Rh or Ir, the entire crystal (front, back and edge) were included in determining the total number of sites; for Pt, only the front face was included, as the back and edge of the crystal were not subjected to the sputter cleaning procedure. Research grade CO (99.99%) and O2 (99.995%) were supplied by Matheson. The CO was further purified by slowly passing it through a molecular sieve trap at 77 K. No metal carbonyls (e.g. Ni(CO)4 were detected in any experiment in post-reaction AES analysis. The experimental procedure has been described in detail elsewhere [64]. Briefly, after cleaning, the sample was moved to the reactor and charged with reactants. Most experiments were performed with 16 Torr of CO and 8 Torr O2. The sample is heated to the desired temperature for a specified time, the products are then allowed to mix for 15 min and then an aliquot of the product mixture was analyzed by GC. The reactor was then evacuated, and the sample returned to the UHV chamber for post-reaction analysis.


Fig. 1. IR spectra of CO on Pd(111) at Pco /10.0 Torr as a function of temperature. The spectra were collected from high temperature to low temperature [85a,85b].

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0.86 ML. The species has been identified as the a-top type and three different ordered structures at uCO / 0.33, 0.5 and 0.80 ML have been identified by LEED. A regular increase in the C /O frequency in the IRAS data along with an increase in the CO desorption temperature with respect to the increase in coverage (uCO) have been observed and understood to be due to lateral CO /CO repulsion. On the reconstructed (5 /1) as well as on the (1 /1) Ir(100) surface, formation of a c(2 /2) overlayer structure has been reported [76]. In this article, we would like to concentrate on the adsorptive behavior of CO over a wide pressure range from UHV to 10 Torr and a temperature range of 100/ 1000 K using Pd as an example. We will also discuss the effect of particle size on the catalytic activity. CO adsorption on Pd(100) and Pd(111), studied under UHV conditions using an array of surface science techniques, exhibits structure sensitivity depending on the face of the Pd crystal. For example, on Pd(111), at a CO coverage (uCO)B/1/3 ML, CO adsorbs at 3-fold hollow sites, with a structure corresponding to (3/ 3)R 308 as revealed by LEED and a carbon /oxygen stretching frequency of 1850 cm 1 [77,78]. However at a coverage (uCO) of 0.5 ML, a c (2 /2) LEED pattern with a C /O stretching frequency of 1918 cm 1 has been observed and is assigned to a CO adsorbed onto bridgebound sites. At very high coverages, a-top and bridgebound CO co-exist yielding a (2 /2) LEED pattern with C /O frequencies near 1951 and 2097 cm 1, respectively. In contrast, only bridge-bonded CO has been

3. CO adsorption Carbon monoxide adsorption under UHV conditions especially on transition metal surfaces has been studied extensively [10,48/50,66 /68]. Three different types of adsorption sites have been observed, in general, namely a-top, bridge-bound and 3-fold hollow. The interaction of a CO molecule with a transition metal can be understood by a simple donor /acceptor model first described by Blyholder [69]. Both linear and bridge bonded CO has been observed on Rh(100) for the entire coverage and temperature range of 90 /300 K [70]. A transition from a well ordered c (2 /2) structure at uCO /0.5 ML to a p (42 / 42)R 458 coincidence structure at uCO /0.7 ML has been observed by LEED and infrared reflection absorption spectroscopy (IRAS). On Pt single crystal surfaces, however, all three types of CO have been observed depending on the CO coverage (uCO), adsorption temperature, pressure and crystal face [20,35,45,67,71/ 74]. Three different desorption features of CO have been observed on the Ir(110) surface [75], whereas the corresponding IR spectra showed a continuous increase in the C /O stretching frequency from 2001 cm 1 at the lowest coverage to 2086 cm 1 at a coverage (uCO) near

Fig. 2. IR spectra of CO on Pd(111) at Pco /1.0 /10 6 Torr as a function of temperature. The spectra were collected from high temperature to low temperature [85a,85b].

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Fig. 3. CO /Pd(111) equilibrium pressure /temperature phase diagram showing the different CO adsorption phases: a-top/3-fold hollow, bridging, 3-fold hollow and non-adsorbed [85a,85b].

observed on Pd(100) even up to uCO /0.5 ML with a c (22 /2)R 458 LEED pattern [79 /84]. In this study, for uCO /0.5 ML, a uniaxial compression takes place resulting in the formation of incommensurate overlayers. As a consequence of this compression of the CO overlayer, a sharp decrease in the heat of adsorption has been observed at uCO /0.5 ML due mainly to lateral repulsive interactions.

3.1. Effects of temperature and pressure A series of IR spectra of adsorbed CO on Pd(111) obtained at a CO pressure of 10 Torr is shown in Fig. 1 [85a,85b]. At high temperatures (i.e. low uCO) only 3fold hollow sites are occupied. With decreasing temperature (i.e. increasing uCO) the population at the bridging sites increases. At temperatures near 200 K, a sudden change in the spectrum is apparent, corresponding to a transition from bridging sites to a mixture of atop and 3-fold hollow sites. The bridging0/a-top/3-fold hollow transition takes place within a very narrow temperature range while the transition from the 3-fold hollow to bridging sites is much less defined. The narrow spectral line-width, due to the 3-fold hollow sites, indicates the presence of a very well ordered CO adsorbate layer, and has been confirmed by a very sharp (2 /2) LEED pattern. In addition to the 10.0 Torr data shown in Fig. 1, isobaric adsorption data were collected at every decade from 107 to 10 Torr. In each of these isobaric series, adsorption site progression was identical; the only apparent difference was the temperature at which the adsorption site transformation took place. As a repre-

sentative case, 1/106 Torr data are shown in Fig. 2 [85a,85b]. Transition from 3-fold hollow to bridging sites can clearly be seen as the C /O stretching frequency changes from 1855 to 1900 cm 1. As the population of CO increases the frequency shifts toward higher values and the peak becomes sharper. This blue-shift has been attributed to repulsive lateral interactions between CO molecules. The frequencies of the a-top and 3-fold hollow sites are 2110 and 1895 cm 1. The highest frequency observed for the bridge-bound CO was 1962 cm 1. An equilibrium phase diagram for CO on Pd(111) is shown in Fig. 3, based on the adsorption data collected at the temperature range 90/1000 K and a CO pressure range of 107 /10 Torr [85a,85b]. It is important to mention that this phase diagram is valid only at equilibrium conditions. This is the case for the bridging 0/a-top/3-fold hollow phase transition. Nonequilibrium CO adsorption can also occur at low adsorption temperatures (crosshatched region of Fig. 3). In this region appropriate adsorption conditions have to be used to ensure a fully equilibrated adsorbate layer. This equilibrium phase diagram illustrates that low pressure/low temperature adsorption information can be extrapolated into the high pressure/high temperature regime provided that certain initial adsorption conditions are used. In Fig. 4 two series of IR spectra of CO adsorption at Pd(100) surface are shown at CO pressures of 1/106 and 1 Torr [85a,85b], respectively. In agreement with previous studies [83,84] only bridge-bound CO was detected near the C /O stretching frequency of 1895 cm 1 at the lowest coverage. The vibration frequency that corresponds to a coverage near 0.5 ML was blueshifted to 1950 cm 1, which is further blue-shifted to 1995 /1998 cm1 as the coverage is increased to 0.8 ML. 3.2. Particle size effects As we have seen in the previous section CO adsorption behavior changes dramatically with respect to the structure of the crystal surface indicating particle size effects. In Fig. 5, IRAS data for CO adsorbed on Pd/ Al2O3/Ta(110) model catalysts for uPd /5.0 and 1.0 ML as a function of temperature is presented [86]. These results were independent of the direction of temperature variation provided the samples were annealed in CO to 600 K. On the uPd /5.0 ML catalyst (Fig. 5a), a peak at 1894 cm1 was observed at 500 K. With a decrease in temperature, this peak shifted towards higher frequency, became broader, and at 300 K split into two peaks at 1984 and 1942 cm.1 In addition, a peak at 2076 cm 1 appeared at 400 K. These three features continued to grow, gradually shifting to higher frequency until, at 150 K, a new peak appeared at 1890 cm1. These peaks have striking similarities to those observed on single


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Fig. 4. IR spectra of CO on Pd(100) as a function of sample temperature at CO pressures of (a) 1.0 Torr and (b) 1/10 6 Torr [85a,85b].

crystal Pd(100) and Pd(111) surfaces as discussed in the previous section. The broad peak corresponding to 1894

cm 1 at 500 K can be attributed to a combination of 3fold hollow and bridge-bound CO species, shifting

Fig. 5. Temperature dependent IR spectra of CO adsorbed on uPd /5.0 and 1.0 ML Pd/Al2O3/Ta(110) catalysts at 1/10 5 Torr [86].

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Fig. 6. Arrhenius plot of the CO/O2 specific rates of reaction (TOF) for (a) single crystal (Pd, Ir and Pt) and their supported catalysts and (b) for Rh single crystals [10,11].

towards exclusively bridge-bound species with decreasing temperature. At 300 K, the features at 1984 and 1942 cm 1 are assigned to contributions from bridging CO from Pd(100) to Pd(111) facets, respectively. The feature at 2076 cm 1 and 400 K is associated with an a-top species on 111 facets. Near saturation coverage, the peak at 1998 cm 1 corresponds to a bridging species on 100 facets while the features at 2108, 1960 and 1890 cm 1 correspond to a-top, bridging and 3-fold hollow sites, respectively, on 111 facets. The results obtained for a uPd /1.0 ML catalyst (Fig. 5b) differ significantly from the uPd /5.0 ML sample. The broad peak near the region of bridge-bound species never splits cleanly into individual components as is the case for larger particles. Moreover, no 3-fold hollow feature is evident at saturation coverage and the ratio of a-top/bridge intensity is higher compare to the larger particles indicating that the smaller particles contain a higher proportion of edge/defects sites. The broader features observed at saturation coverage for the smaller particles are due to higher surface curvature on the small particles and a correspondingly less compressed CO overlayer assuming roughly hemispherical shape of the particles. That the types of transitions (bridging 0/a-top/ 3-fold hollow) occur on the Pd(111) single crystal and, to a more limited extent, on the uPd /5.0 ML catalyst do not occur on the uPd /1.0 ML particles is consistent with the reduced CO density on the smaller particles. The effect of particle size with respect to CO oxidation will be discussed subsequently.

4. CO oxidation 4.1. Pt, Ir, Rh and Pd single crystals and model supported catalysts 4.1.1. Steady-state reaction kinetics The CO2 formation rate as a function of inverse temperature (1/T ) for Pd, Pt and Ir single crystals is shown [10] in Fig. 6a and compared with the data obtained on several supported metal catalysts [38]. Data obtained on Rh(100) and Rh(111) single crystals are shown [11] in Fig. 6b. The Pd, Pt and Ir single crystal data are for a (1:2) O2: CO mixture at a total pressure of 24 Torr, whereas, the Rh single crystal data are for a (1:1) O2: CO mixture at a total pressure of 16 Torr. Within this pressure range the reaction rate is zero-order with respect to total pressure. The TOF for the singlecrystal catalysts traverse four orders of magnitude over a temperature range of 450/600 K. Kinetic measurements over such a wide temperature range with supported catalysts are not possible due to heat and mass transfer limitations encountered at high temperatures. Thus a direct comparison between the two types of catalysts is limited to a relatively small temperature range. Nevertheless, it is very clear from Fig. 6a and b that there is excellent agreement between the single crystal and model supported systems with respect to the specific reaction rates and apparent activation energies [28,38]. Fig. 7a /c [10,11] show reaction rate dependence on CO partial pressure for Pd(110), Ir(111) and Rh(111) /


Fig. 7. CO partial pressure dependence at constant oxygen pressure and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and Rh(100) [10,11].

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Rh(100), respectively. In these experiments the CO pressure was allowed to vary keeping the oxygen pressure fixed. In the case of Pd and Ir, up to a CO: O2 ratio of approximately 1:12 the reaction was firstorder with respect to CO partial pressure; whereas below this ratio the reaction became negative-first-order with respect to CO. For Rh(111) and Rh(100) such a roleover with respect to the partial pressure of CO has been observed with a maximum activity occurring approximately at a O2: CO ratio of 30:1. However, at a lower partial pressure of O2 (8 Torr), for Pd(110), Rh(100) and Rh(111), the reaction order has been observed to be negative-first-order with respect to CO partial pressure (Fig. 3a and c, respectively). It is noteworthy (Fig. 7c) that the rate of CO2 formation on Rh(100) is higher compared with that on Rh(111) at any given CO partial pressure indicating that the reaction may be structure sensitive. For Pd, Ir and Rh the reaction order with respect to the partial pressure of O2 (Fig. 8a/c) is generally positive-first-order [10,11]. Above an O2:CO ratio of 12:1, the same ratio at which the CO order changes from negative to positive, the reaction rate begins to decrease, becoming negative-order in O2 partial pressures. For example, at extremely high O2:CO ratios, the order of reaction for O2 is /0.79/0.2 on Ir(111). For Pd, changing the CO partial pressure at a constant temperature only shifts the curve, i.e. the maximum rate and the ratio at which the rate turns over, remain unchanged. Whereas, for Ir the O2:CO ratio at which the rate varies from first-order oxygen dependency changes somewhat (from 12:1 to 16:1) as does the order of the reaction. The reaction rate on both Rh surfaces (Fig. 8c) increases linearly at low oxygen partial pressures. This first-order dependence is altered at high oxygen pressures where the rates roll over and become negativeorder with respect to the partial pressure of oxygen. Note that this rollover occurs at different oxygen partial pressures on the two single crystal surfaces indicating the possibility of structure sensitivity. On Pt(100), the order of the reaction with respect to CO partial pressure changes with temperature (Fig. 9) [10]. In the range where the activation energy is changing (425 /490 K), the reaction order varies from 0.0 to 0.6. Above 500 K the order becomes more negative and rapidly approaches negative-first-order. The reaction never becomes positive-first-order with respect to CO partial pressure even at an O2: CO ratio of 200:1, as observed for Pd, Ir and Rh. The Pt(100) surface shows (Fig. 10) [10] only positivefirst-order behavior with respect to the partial pressure of oxygen. The range of O2:CO ratios studied at a given temperature was limited to TOF’s where there was differential conversion (B/5%) of CO. No decrease in the order of reaction is observed for O2:CO ratios of 1:5 to 150:1, and temperatures from 475 to 650 K, indicat-

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Fig. 8. O2 partial pressure dependence at constant CO pressure and temperature: (a) on Pd(110), (b) on Ir(111) and (c) on Rh(111) and Rh(100) [10,11].

ing that under these experimental conditions no strongly bound, deactivating oxygen species is present. In order to form a similar species on Pt, much higher O2 pressure and/or temperatures would be necessary, conditions not accessible in our experiments. 4.1.2. Structure sensitivity and particle size effects Although it appears from the data in Fig. 1 that the CO-oxidation reaction is structure insensitive, it should be noted that the single crystal rates are compared with the least dispersed supported catalyst. Very recently, under UHV conditions structure sensitivity has also been observed by Niemantsverdriet and co-workers [90]

on Rh single crystal surfaces wherein the CO2 desorption temperature differs by /50K on Rh(100) and Rh(111) surfaces. The Rh(100) surface shows significantly higher reaction rates compared with that on the Rh(111) surface. The other difference between the two systems is that on Rh(100) all CO is oxidized to CO2, whereas on Rh(111) a fraction of the CO desorbs. The reason for such high activity and selectivity on Rh(100) towards CO2 formation is assumed to be due to the surface reaction step, COads/Oads 0/CO2(g), being intrinsically faster on Rh(100) than on Rh(111). In contrast to the high-pressure data (Fig. 7c and Fig. 8c), the reaction exhibits first order kinetics with respect


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Fig. 9. CO partial pressure dependence as a function of temperature at constant oxygen pressure on Pt(100). (a) The Arrhenius plot for CO oxidation on Pt(100) is shown to illustrate the temperature regimes in which (b) the CO pressure dependence data were obtained [10].

Fig. 10. O2 partial pressure dependence at constant CO pressure and temperature on Pt(100). In all cases the reaction order is 1.09/0.1 [10].

to both the partial pressure of CO and O2 under UHV conditions, however, the structure sensitivity is consistent with the high-pressure data (Fig. 7c and Fig. 8). Dramatic structure sensitivity has been observed on Ir/SiO2 catalyst [38] (Fig. 11). It is clear from Fig. 11 that larger particles are more active for CO oxidation and the data more comparable to the single crystal data. The data for the larger particles were calculated from

Fig. 11. Effect of particle size on the CO2 formation rate on Ir/SiO2 and Ir single crystal catalysts [38].

the dispersion values obtained by Cant and co-workers [38]. It is noteworthy that the single crystal data in Fig. 11 corresponds to the supported data extrapolated to a particle size of 40 nm. This is close to the ‘effective’ particle size of the single crystals used in this study,

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Fig. 12. CO oxidation with O2 over model Pd/SiO2/Mo(100) and a conventional 5% Pd/SiO2 catalyst. Reaction conditions were PTorr / 0.5 Torr and CO /O2 /0.2 [56,86].

Fig. 13. Dependence of the rate of CO oxidation on metals on the oxygen bond energy EMO (793 K, PO2 /PCO /10 7 Torr) [94].

taking into account step densities and edge effects. The decreasing activity of the reaction with Ir particle size can be understood as due to a preferential poisoning of the active sites by carbon formed by CO dissociation, a competing process to CO oxidation under such highpressure conditions. This reaction would be expected to occur more rapidly on defect sites and step edges, present at much higher concentration on the smaller particles. CO oxidation has also been investigated over Pd/ SiO2/Mo(100) model catalyst [56,86]. The reaction conditions for the catalysts were 10 Torr CO, 5.0 Torr O2, and reaction temperatures in the range 540 /625 K. The conversions were maintained at less than 50% and were measured by monitoring the pressure decrease in a static reactor of known volume (750 cm3). The average cluster sizes shown in Fig. 12 were determined by CO TPD, O2 TPD and ex situ STM/AFM and all were in good agreement. The specific reaction rates were somewhat higher for the model catalysts than the highsurface-area catalysts, but the activation energies are remarkably similar. There was no noticeable dependence of the CO2 formation rate on the Pd cluster size, indicating that CO oxidation over Pd/SiO2 is structure insensitive. Structure sensitivity has also been observed for Pt. Yates and co-worker [91 /93] using IRAS and TPD have

shown that on Pt(335) surface dissociation of CO and O2 occurs preferentially at step sites. It also has been observed that CO adsorbed at 111 terraces is more active compared with CO at 100 step sites whereas chemisorbed oxygen atoms at step sites is more active compared with CO adsorbed at terraces. Using isotopically labeled 13C18O molecules, it has been shown that the oxidation rate at step sites is twice the oxidation rate at terrace sites. However, recent STM studies of CO oxidation on Pt(111) have shown that the reaction occurs exclusively at the boundaries between (2 / 2)Oads and c(4 /2)COads domains when co-adsorbed under UHV condition [17,87 /89]. Therefore, to increase the reactivity of the surface it is necessary to increase the interface between the Oads and COads domains, i.e. to increase the coverage of Oads. 4.1.3. CO oxidation versus metal /oxygen bond energy The oxidation rate of CO under steady-state conditions on various Pt group metals are compared in Fig. 13 [94]. The temperature of the reaction was chosen specifically as 793 K in order to avoid surface site blocking due to CO adsorption; the pressure of CO and O2 was 1 /107 Torr. From the data of Fig. 13 it is clear that the most active metals (Pd, Pt, Ir and Rh) for CO oxidation have M /O bond energies within the range 320 /390 kJ mol 1. In the case of metals having M /O bond energies less than 320 kJ mol 1, e.g. Ag or Au, the rate-determining step for CO oxidation is the adsorption and dissociation of oxygen. On the other hand, for metals having M/O bond energies larger than 390 kJ mol 1, the rate-determining step is the reaction between COads and Oads. In other words those metals on the right hand side of Fig. 13 have a higher tendency to form stable oxides. In fact, on Rh(111) and (100) single crystals, high O2:CO ratios result in a decrease in the overall rate and a change from positive-order in oxygen to negative-order. This change was directly correlated with the formation of an oxide-like species, as determined by post-reaction AES and TPD [10,11]. Pd(110), Ir(111) and Ir(110) exhibit partial pressure dependence and high oxygen pressure behavior similar to Rh. For Rh, the formation of a near surface oxide (probably Rh2O3 [10,11] which is much less active) is responsible for the deactivation. The similar behavior of Pd and Ir suggests a similar deactivation mechanism. In contrast, on Ru the oxide was found to be substantially more active than the clean surface, and the reaction order in oxygen pressure increases by approximately 3-fold [9,13,80,95]. 4.1.4. Reaction mechanism The Langmuir /Hinshelwood reaction between COads and Oads is well established as the dominant reaction mechanism for conditions where CO is the primary surface species [48,49]. This mechanism has been con-


Fig. 14. Reactive temperature programmed desorption (RTPD) of CO2 as a function of emission angle after predosing 1.5 /1015 cm 2 molecules of O2 and 2.7/1015 cm2 molecules of CO successively at Ts /100 K [99].

firmed by numerous UHV studies of the co-adsorption of reactants, transient kinetic studies, and steady-state kinetics [1 /46]. The reaction steps can be written as CO(g) 0 COads O2 (g) 0 2Oads COads Oads 0 CO2 (g)

(1) (2) (3)

where the recombinative desorption of adsorbed Oads atoms (reverse of reaction 2) and the dissociative chemisorption of CO2 (reverse of reaction 3) are neglected. It is assumed that CO is the dominant surface species [96]. Considering the reaction steps (1/3) and using the above assumptions, an approximate rate expression, originally proposed by Langmuir [47] for Pt can be derived as: d[CO2 ]=dtk exp (Edes;CO =F )PO2 =PCO

(4)

where the reaction rate is independent of total pressure, first-order in O2 pressure and negative-first-order with respect to the CO pressure. The rate of the reaction is then governed by the desorption of CO or the lifetime of CO (tCO) on the surface [48], depending on the reaction temperature, whereas the pressure dependence simply reflects the competition for adsorption sites between oxygen and CO. It has been found [10] that the kinetics on Pd, Ir and on Pt at high temperatures, are consistent with this model in that the pressure dependencies predicted by equation 4 are observed. In addition, a correlation of the activation energies between supported and single crystal data, and among different singlecrystal planes [28,38] reflect the fact that the binding energy of CO does not vary greatly among these metal catalyst surfaces.

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4.1.5. Angle-resolved temperature programmed desorption In search of the possible reaction mechanism of CO oxidation on both polycrystalline and Pt(111) surfaces, angle resolved reactive temperature programmed desorption (RTPD) has been performed using time of flight (TOF) mass spectrometry [32,97 /99]. The results show dramatic angular and velocity dependence of CO2 desorption from the reaction of CO and oxygen coadsorbed on a Pt(111) surface at 100 K. The velocity integrated desorption spectra [99] (Fig. 14) show four different peaks (a, b3, b2 and b1) for CO2 formation at 145, 210, 250 and 330 K, respectively. These feature have been attributed to four different reaction mechanisms operating at various temperatures depending upon the relative binding of oxygen and the geometric arrangement and coverage of the adsorbed species. Although the precise origin of b3 and b2 processes are not clear at present, the a-CO2 formation temperature coincides with that of the molecular O2 desorption. The b1-CO2 formation is most likely the mechanism proposed in the previous section due to the reaction at the overlapping regions of COads and Oads island boundaries. Very interesting oscillatory behavior of the CO oxidation reaction has been observed on Pt single crystal under UHV conditions [15,16,100 /102], however, this behavior is beyond the scope of the present article. 4.2. CO oxidation on Au In the bulk form, Au is known to be chemically inert compared with the other Pt group metals. However recently it has been shown that Au clusters, deposited as finely dispersed, small particles (B/5 nm diameter) on reducible metal oxides like TiO2, Fe2O3 and Co3O4, dramatically enhance the rate of a number of industrially important reactions. Reactions catalyzed by Au particles on TiO2 supports are CO oxidation, hydrogenation and partial oxidation of hydrocarbons and the selective oxidation of higher alkenes [54,55,60 /63]. It has also been observed that catalytic activity of these catalysts is a function of cluster size. 4.2.1. Characterization of the Au clusters by STM and STS The constant current STM micrographs in Fig. 15 show [103] changes in the clusters with respect to the amount of Au deposited. At a relatively low coverage of Au (uAu /0.1 ML) hemispherical 3D clusters are observed with diameters of 2 /3 nm and heights of 1/ 1.5 nm. Interestingly the clusters mainly grow along the step edges. Well-dispersed quasi-2D clusters, having a height of 0.3 /0.6 nm and a diameter of 0.5 /1.5 nm, can be seen on the terraces. With increasing Au coverage (uAu), the clusters steadily grow larger while the increase

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Fig. 15. A set of 50 /50 nm2 STM images (2.0 V, 1.0 nA) of TiO2(110) /(1/1) with different Au coverage (uAu): (A) 0.10 ML, (B) 0.25 ML, (C) 0.50 ML, (D) 1.0 ML, (E) 2.0 ML and (F) 4.0 MLE. With increasing coverage, Au clusters grow and gradually cover the surface [103].

in cluster density is minimal. However, even at uAu /4.0 ML, some portions of the TiO2 substrate are still visible. In Fig. 16A, the constant current STM micrograph of Au (uAu /0.25 ML) deposited onto single crystal TiO2(110) /(1 /1) [55,103] is shown. The metal deposition was performed at 300 K, followed by annealing to 850 K for 2 min to stabilize the clusters. In Fig. 16A only the Ti cations are visible; whereas the O2 anion are not seen. The inter-atomic distance between the 001 rows is /0.65 nm, which can be observed along the terraces corresponding to the length of the unit cell along the [110] direction of the unreconstructed TiO2(110) /(1 /

1). Three-dimensional Au clusters, imaged as bright protrusions, have average diameters of /2.6 nm and heights of /0.7 nm (corresponding to 2/3 atoms thick) and are preferentially nucleated at the step edges. Quasitwo-dimensional clusters are characterized by heights of 1 /2 atomic layers. Previous studies have shown that the Au clusters upon annealing form large microcrystals with well-defined hexagonal shapes. Fig. 16B shows STS taken over various clusters on the surface, where the tunneling current (I) as a function of bias voltage (V) across the STM tip is measured. The I/ V curves correlate with the Au cluster size on the TiO2


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Fig. 17. CO oxidation TOFs as a function of the Au cluster size supported on TiO2. (A) The Au/TiO2 catalysts were prepared by a precipitation method, and the average cluster size was measured by TEM, 300 K. (B) The Au/TiO2 catalysts were prepared by vapordeposition of Au on planner TiO2 films on Mo(100). The CO /O2 mixture was 1:5 at a total pressure of 40 Torr, 350 K [55].

Fig. 16. (a) A CCT /STM image of a Au (uAu /0.25 ML) deposited onto TiO2(110) /(1/1) prepared just prior to a CO:O2 reaction. The sample had been annealed to 850 K for 2 min; (b) STS data acquired for Au clusters of varying sizes on the TiO2(110) /(1/1). An STS of TiO2 substrate, having a wider band-gap than the Au cluster, is also shown as a point of reference [55].

surface. The length of the observed plateau at the zero tunneling current is a measure of band-gap (along the bias voltage axis) of electrons tunneling between the valence and conduction band of the cluster and tip. The electronic character of these clusters vary between that of a metal and a non-metal depending on their size. With an increase in size the clusters gradually exhibit metallic character with an enhanced density of states at the Fermi level. Note that clusters of 2.5 /0.7 nm2 size have a larger band gap than that for a cluster 5.0 /2.5 nm2 in size. Smaller clusters have a non-metallic character resulting in significant band-gap and a reduced density of states near the Fermi level. A similar metal to non-metal transition with respect to cluster size has also been observed for Fe clusters deposited on

Fig. 18. The specific activity for CO conversion as a function of reaction time at 300 K on a model Au/TiO2/Mo(100) catalyst. The Au coverage (uAu) was 0.25 ML, corresponding to an average cluster size of /2.4 nm [103].

GaAs(110) [104]. Very small clusters then are nonmetallic and exhibit electronic and chemical properties unlike those of the corresponding bulk metal. 4.2.2. Particle size effects Interestingly, a marked correlation between the cluster size and catalytic activity has been observed for CO oxidation over Au/TiO2 system [54,55,60 /63]. Studies have been carried out on Au/TiO2/Mo(100) as well as on Au/TiO2(110) /(1 /1) for comparison. Fig. 17a and b show plots of CO oxidation activity (TOF) at 350 K as a function of Au cluster size supported on TiO2(110) / (1 /1) and TiO2 /Mo(100) substrates, respectively.

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These results show similarities in the structure sensitivity of CO oxidation with a maximum activity evident at /3 nm Au cluster size on both TiO2 supports. For each catalyst, the activity and the selectivity of the supported Au clusters are markedly size-dependent. Although the TiO2 supported Au catalysts exhibit a high activity for the low-temperature CO oxidation, rapid deactivation was observed as a function of reaction time. Fig. 18 shows a plot of TOF versus time for CO oxidation at 300 K on a Au(uAu /0.25 ML)/TiO2/Mo(100) model catalyst. The model catalyst, which exhibited a high initial activity, deactivated after a CO/O2 (1:5) reaction of /120 min at 40 Torr. This deactivation is due to agglomeration of the Au clusters with reaction time, as detailed by post-reaction STM measurements. The STM data clearly demonstrate [54,55,60 /63] that under reaction conditions the Au clusters ripen via an Ostwald mechanism; i.e. large clusters grow at the expanse of small ones leading to a bimodal size distribution. This ripening mechanism depends upon the strength of the cluster/support interaction as well as gas pressure. The TOF of CO oxidation, which maximizes with respect to cluster size, correlates with a metal to non-metal transition at a particle size of /3 nm, as revealed by a detailed STM /STS investigation. This behavior has been discussed in the previous section and there is no unified theory to understand the catalytic activity of the small gold particles at the moment. However, the fact that CO oxidation activity increases with decreasing cluster size as long as the particles are metallic suggests that an overall catalytic activity that depends on both electronic as well as geometric factors. Electronic factor means the size-induced changes in the electronic levels of the small clusters namely electronegativity, charge state and metallicity leading to changes in the cluster /CO and /O2 interaction strength thereby the overall reaction itself. On the other hand, geometric factor leads to changes in the shape with respect to the changes in the particles size leading to change in the number of steps, size of the terraces and facetes.

5. Conclusions An approach that combines both surface science and traditional catalytic methodologies has been shown to be extremely advantageous in bridging the material and pressure ‘gaps’ between ‘real world catalysis’ and ‘surface science’. Although, it appears that the CO oxidation reaction follows a simple Langmuir-Hinshelwood mechanism, many complicating factors influence the overall activity. The example of Pd shows that the knowledge acquired from single crystal data can be used to understand the results for model supported catalysts, including structure sensitivity, reaction order determination, nature of the bonding of adsorbed molecules, the

optimum reaction temperature, deactivation, etc. Finally it has been shown that ultra-small gold particles, unlike bulk gold metal, are promising catalysts for low temperature CO oxidation.

Acknowledgements The support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and the Robert A. Welch Foundation is gratefully acknowledged.

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