A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au—TiO2 catalysts

6.ENVlRCMENlAL Applied Catalysis

B: Environmental

8 (1996) 417-443

A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO, catalysts Mark A. Bollinger, M. Albert Vannice

*

Department of Chemical Engineering, Penn. State University, Unic;ersity Park, PA 16802, USA Received 5 September

1995; revised 14 November

1995; accepted

14 November

1995

Abstract Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions. Impregnated Au-TiO, is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated Au-TiO, and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO, overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of Au-TiO, catalysts and argues for the formation of active sites at the Au-TiO, interface produced by the mobility of TiO, species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated Au-TiO, reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO,; however, a band for adsorbed CO is observed on the pure TiO,. Kinetic studies with a 1.0 wt.-% impregnated Au-TiO, sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on 0, between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and 0, adsorbing on TiO,, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and carboxylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO, (near 2340 cm-‘) which may be competing with CO for adsorption sites. Keywords: Carbon monoxide

* Tel. (+ l-814) 8634803,

oxidation;

Kinetics of CO oxidation;

fax. (+ l-814) 8657846,

Gold-titania;

e-mail [email protected].

0926-3373/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0926-3373(95)00077-l

DRIFTS of CO on Au-TiO,

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Catalysis B: Environmental 8 (1996) 417-443

1. Introduction Gold has attracted little attention during the development of heterogeneous catalysts over the past 50 years because it is typically less catalytically active than other platinum group metals [l-5]. However, it has recently been found that well dispersed gold on certain metal oxides has high activity for CO oxidation at temperatures as low as 200 K [6,7]. The ability of such catalysts to oxidize CO at room temperature could be utilized in a number of practical applications including CO detection devices, the reduction of CO in industrial and automotive emissions, the purification of air in homes and offices, and the recombination of CO and 0, in orbiting, closed-cycle CO, lasers used for weather monitoring or other remote sensing applications [8-l I]. The utilization of these catalysts to remove CO from ultra high purity 0, and O,-containing gas mixtures is also a possibility. In our research and that of Haruta and coworkers, gold on titania has been found to be one of the most active catalysts for low temperature CO oxidation [7,12]. The Au-TiO, system is particularly interesting because neither bulk Au nor titania alone is an active CO oxidation catalysts at 300 K and, in addition, the preparation and pretreatment of these catalysts can profoundly affect their catalytic behavior and their deactivation rates [7,12,13]. The emphasis in this study was to examine these catalysts utilizing kinetic measurements and infrared spectroscopy to gain additional insight about the synergism that exists with Au-TiO, systems and to learn more about the surface chemistry associated with this reaction over TiO,-supported Au.

2. Experimental 2.1. Catalyst preparation The impregnated gold catalysts, noted by (I) after the catalyst name, were prepared by an incipient wetness method using AuCl, (Johnson-Matthey, 99%) and TiO,. Details of this preparation method, together with adsorption behavior, have been reported earlier [14].Following the general procedure for the deposition-precipitation method described by Haruta et al. [13], a catalyst, designated (DP), was prepared using AuCl, and TiO,. TiO, was mechanically stirred into distilled, deionized H,O and ammonium hydroxide was added to raise the pH. An aqueous AuCl, solution was added dropwise over a 40 min period and stirred for 30 additional min. This mixture was filtered, washed, and dried in air at room temperature for two h then evacuated overnight. Catalysts prepared from ultra high purity Au powder (Johnson-Matthey, 99.9994%), designated (UHP), were prepared by first dissolving Au powder in high purity aqua regia which was made from ‘ultrapure’ HCl (Johnson-Matthey, 99.999%) and ‘ultrapure’ HNO, (Johnson-Matthey, 99.999%). The solution was

M.A. Bollinger, M.A. Vannice /Applied Catalysis B: Encironmental

8 (1996) 417-443

419

then evaporated slowly to near dryness by heating to form a red AuCl, powder. The standard incipient wetness impregnation procedure was then followed. Chloride-free catalysts, designated (EtHx), were prepared by dissolving Au 2-ethylhexanoate, Au(C,H i502>s, (OM Group) in pentane (Sigma-Aldrich, 99 + %) and impregnating the TiO, using an incipient wetness method. Since the pentane evaporated rapidly, excess pentane was added to one sample, 2.2% Au-TiO, (EtHx), to form a slurry and more easily mix the Au solution with the TiO,. Coprecipitated catalysts, designated (P), were prepared using the following procedure. HAuCl, was added to a solution that contained TiCl, dissolved in 2 M HCl to form a 2 wt.-% precursor solution. Ammonium hydroxide was added dropwise to form a white precipitate. The precipitate was washed 3 times with deionized water and dissolved in HNO,. This solution was added dropwise to a Na,CO, solution to form a Ti-Au coprecipitate. The fresh sample had a BET surface area of 140 m2/g. After a HTR/C/LTR or calcination pretreatment, the surface area decreased to 30 and 70 m2/g, respectively. A TiO,-covered Au powder sample (TiO,-Au) was prepared from a precursor solution of Ti’” n-nonylate, Ti(C,H,,O),, (Alfa Products) dissolved in pentane. This solution was added to UHP Au powder in the appropriate amount to form 10 theoretical monolayers of TiO, on the Au surface and stirred for 40 min while the pentane evaporated. It was then calcined in dry air (MG Ind.. Med. Grade) at 673 K for 2 h. 2.2. Catalyst pretreatment The standard pretreatment used for these samples is designated by HTR/C/LTR: High temperature reduction (773 K in flowing H, (MG Industries, 99.999%) for 1 h), followed by calcination (673 K in a flowing mixture of 20% 0, in He for 1 h), followed by low temperature reduction (473 K in flowing H, for 2 h). For the samples examined by DRIFTS, Ar was used in place of He and the HTR was done with 20% H2 in Ar rather than pure H 2. Catalysts given portions of this pretreatment will be designated by variants of HTR/C/LTR, e.g., HTR/C to represent high temperature reduction followed by calcination. Another pretreatment used was calcination (C) at 673 K in a 20% 0,/80% He mixture for 4 h. The UHP Au powder was calcined in a 20% 0,/80% He mixture at 573 K for 1 h, then reduced in H, at 573 K for 1 h. Details are given elsewhere [ 151. 2.3. Kinetic studies The reactor system used for these CO oxidation studies was previously described, but essentially it involved a microreactor connected to a gas chromatograph utilizing Carbosieve G columns for product stream analysis [ 121. The

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Catalysis B: Environmental 8 (1996) 417-443

typical sample size was 50 mg; however, larger samples were used for the catalysts with low activity. The pretreatments were done in situ followed by cooling in He to the reaction temperature. For kinetic activity measurements, oxygen was introduced slowly to eliminate any large surge and allowed to stabilize for five min, then CO was added to the stream. Partial pressure dependence runs were carried out by varying either CO (Matheson, 99.99%) or 0, (MG Industries, 99.99%) with the other gas held constant at 5% in the feed gas (typically 38 Torr) with the balance composed of He (MG Industries, 99.999%). Arrhenius runs were conducted with 5% CO and 5% 0, in the feed gas. All catalysts exhibited some deactivation with time on stream, but the rate of deactivation varied with pretreatment, temperature, and catalyst preparation method. Typically, to check for any significant deactivation during these measurements, a sequence with increasing pressure followed by decreasing pressure was used during the partial pressure runs and increasing temperature followed by decreasing temperature was employed in the temperature dependence runs. A total flow rate of 35 cm3 (STP)/min was used in all the experiments and after 20 min under reaction conditions a sample was taken for GC analysis. A typical weight hourly space velocity (WHSV) was 17 l/g/h for a sample size of 50 mg. 2.4. Di#use rejlectance Fourier transform infrared spectroscopy

(DRIFTS)

The infrared studies were conducted with a recently upgraded Sirius 100 FTIR system (Mattson Instr., Inc.) using a DRIFTS cell (HVC-DRP, Harrick Sci. Corp.). The system is described in more detail elsewhere [16]. The sample compartment was filled with either 1.0% Au-TiO,(I) or pure TiO,. The interferograms consisted of either 100 or 1000 signal-averaged scans obtained using a post-amplifier gain of 4, an iris setting of 50 and 4 cm- ’ resolution. Each interferogram was Fourier transformed to its equivalent frequency component spectrum. The ratio of these two spectra gave the transmittance spectrum from which the absorbance spectrum was obtained. These results were also used to obtain diffuse reflectance Kubelka-Monk spectra, but because of the similarity that routinely occurred and the need to present spectra with negative absorbances after certain pretreatments, only absorbance spectra are shown here. To resolve the intensity due to adsorbed species, spectra with no gas-phase CO and CO, bands were obtained by subtracting either a CO or a CO, background spectrum with WinFIRST software. 2.5. Catalyst characterization The low BET surface areas reported for pure Au and TiO,-Au were determined using a stainless-steel UHV adsorption system with Ar as the adsorbate at liquid N, temperature. Other BET surface areas were determined

M.A. Bollinger, M.A. Vannice / Applied Catalysis B: Environmental

8 (1996) 417-443

421

using N, physisorption and a Quantasorb adsorption system. The Au crystallite sizes were obtained from X-ray diffraction line broadening measurements using a Rigaku Geigerflex diffractometer and the Scherrer equation with Warren’s correction for instrumental line broadening [ 151.

3. Results 3.1. DRIFTS experiments

with Au-TiO,

3. I. 1. Catalyst pretreatment A fresh sample of 1.0% Au-TiO,(I) was loaded into the DRIFTS cell and purged in 10 cm3 (STP)/ min Ar overnight at room temperature; the spectrum of the catalyst at this stage before pretreatment, referenced to pure Ar, is shown in Fig. la. A very prominent, broad band at 3400 cm-’ is seen along with peaks at 1685 and 1610 cm-’ and a smaller band at 1365 cm-‘. The bands at 3400 and 1610 cm-’ can be attributed to stretching and bending modes, respectively, of adsorbed molecular water based on previous assignments [17-211. At 773 K under H, (HTR step), no peaks are detected as shown in Fig. Id. The subsequent calcination step at 673 K generated a band around the 3670-3700 cm-’ range, shown in Fig. lc, probably due to hydroxyl groups on anatase and i-utile surfaces as previous assignments to hydroxyl groups have been made in this region [17,19-221. The freshly pretreated sample at room temperature in flowing Ar can be seen in Fig. lb. The difference spectrum, i.e., the after-pretreatment spectrum minus before-pretreatment spectrum, is shown in Fig. 2. 3.1.2. Spectra with only CO present 5% CO was added to the Ar feed, corresponding to a CO partial pressure of 38 Tot-r, and the sample was exposed to CO for 30 min at 300 K. The following spectra have been referenced to an appropriate background spectrum and bands due to gas-phase CO (and CO, when it is present) have been removed by subtracting the contribution of the gas-phase CO and CO, species. Fig. 3 shows spectra with gas-phase CO subtracted after various exposure and purge times. All spectra were referenced to that of the sample just before adding CO. Five min after the introduction of CO a sharp band is seen at 2105 cm-‘, which has been assigned to CO adsorbed on Au [23-251. Bands are seen in the 1700-1000 cm-’ region; however, they are not well resolved and involve considerable overlap. Peaks near 1670 and 1250 cm-’ have been previously assigned to bidentate carbonate species on anatase [20] and to carbonate-like species in Au-TiO, samples [23]. Noncoordinated carbonate species with a band at 1430 cm-’ have been reported on Au-TiO, [26] and may be responsible for part of this broad band; however, their contribution is certainly not the dominant one. The most intense peak at 1580 cm-’ could be due to a

422

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Catalysis B: Environmental 8 (1996) 417-443

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Fig. 1. DRIFT spectra of 1.0% Au-TiO,(I) during HTR/C/LTR pretreatment: (a) initial sample at 303 K prior to pretreatment, (b) in H, at 773 K after HTR, (c) in 20% 0, at 673 K after HTR/C and (d) in Ar at 303 K after HTR/C/LTR pretreatment. The spectra are referenced to a pure Ar background. Marks on the y-axis represent intervals of 0.5 absorbance units.

carboxylate species or a bidentate carbonate species bound to a single Ti atom [27]. The carboxylate assignment also requires a peak at around 1320 cm- ’ [26] or in the 1420-1350 cm-’ range due to the symmetric stretching mode [27]. The carboxylate assignment may be more preferable because the bidentate carbonate should show an asymmetric stretch around 1270- 1250 cm- ’ and the intensity in this region is small [27]. Monodentate carbonate species may also be present, as they have peak positions typically around 1530-1470 and 1370- 1300 cm-’ [27]. The contributions from formate and bicarbonate species are assumed to be small because no peaks due to the modes involving C-H in formate (2880, 2885, 2950 and 2970 cm- ’ on anatase) [28] or those from O-H stretching in bicarbonate (36 15 cm- ’ on r-utile) [22] can be seen. The major contributions are tentatively assigned to bidentate carbonate (bound to either one or two Ti

MA. Bollinger, M.A. Vannice /Applied

Catalysis B: Enuironmental

8 (1996) 417-443

423

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cations) and carboxylate ions, with monodentate carbonate and free carbonate species possibly being present also. After 20 min under CO, a band below the main CO band at 2105 cm-’ can be seen at 2071 cm-’ along with a reduction in intensity of the main band. The band frequency at 2071 cm-’ is too high to be a bridge-bonded CO molecule on Au, as such a peak has been assigned at 1900-2000 cm-’ [29], but a doublet peak at 2075 and 2094 cm- ’ has been assigned to linearly adsorbed CO on t-utile [30]. Five min after stopping CO, the main adsorbed CO peak at 2105 cm-’ is reduced as CO desorbs from the surface, and an accompanying shift to higher wavenumber (2113 cm- ‘> is seen at the lower surface CO concentration. This type of shift to higher wavenumbers as the surface concentration of CO on Au is lowered has been reported by others [23-251. The peak at 2071 cm-’ has been replaced by a broader band centered around 2038 cm- ’; the origin of this latter band is not clear at this time. One h after CO was stopped, all IR-active

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M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Enoironmental8 (1996) 417-443

I-3500

3ti

(d)

2500

zoo0

Wavenumber (cm-‘) Fig. 3. DRIFT spectra of surface species on 1.0% Au-TiO,(I) at 300 K after addition of 38 Torr CO: (a) under CO for 5 min, (b) under CO for 20 min, (c) 5 min after purging CO and (d) 60 min after purging CO. The spectra are referenced to the sample prior to the addition of CO. Marks on the y-axis represent intervals of 0.2 absorbance units.

adsorbed CO was removed, implying weak CO chemisorption TiO,, in agreement with a previous study of Au-TiO, [26].

on Au as well as

3.2. Spectra under reaction conditions Spectra were obtained after the addition of both CO and 0, to the feed gas to examine the behavior of surface species under reaction conditions. Immediately following the previous run with only CO, a second run was performed using the following procedure: start 5% CO, after 20 min start 5% O,, stop 0, after 70 min exposure and stop CO 30 min after stopping 0,. The spectra shown in Fig. 4 are again referenced to the spectrum of the sample just prior to the initial introduction of CO. Five min after CO addition (spectrum a), a sharp peak at 2106 cm- ’ appears with a weaker, broader counterpart at 2062 cm- ‘. No new

MA.

Bollinger,

MA.

Vannice /Applied

Catalysis

B: Environmental

8 (1996) 417-443

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(38 Torr CO, 38 Torr 0, (CO remains) and addition of CO (i.e., represent intervals of

carbonate-type bands were present at this time. Five min after 0, addition in the presence of CO (spectrum b), a sharp band occurs at 2342 cm- ’ with a shoulder near 2320 cm-’ and another small spike appears at 2378 cm- ‘. These peaks are associated with CO, adsorbed on Au and TiO, as adsorbed CO, peaks have been previously reported at 2350 cm-’ on both r-utile [22] and anatase [20]. One study reports the peak position can shift from 2345 to 2353 cm-’ as the surface Hz0 concentration on anatase decreases [21]. The CO band at 2062 cm-’ is no longer present and the 2106 cm- ’ band has decreased and shifted to 2 117 cm- ’ because of the decreased CO coverage on the Au surface. Bands in the carbonate/carboxylate region now increase, specifically bands near 1670, 1550, 1426, 1350 and 1250 cm-‘, similar to the case after the initial CO addition. After removing 0, a single adsorbed CO, peak at 2342 cm-’ remained

426

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Catalysis B: Environmental 8 (1996) 417-443

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on 1.0% Au-TiO,(I) under reaction conditions at 300 K: (a) under 38 and 71 Torr 0,. (c) under CO and 130 Torr 0, and (dl under CO and as that used in Fig. 3; CO and CO, gas-phase spectra have been intervals of 0.2 absorbance units.

(spectrum c) while the main CO band shifted back to 2110 cm- ’ and intensified and two very small bands at 2175 and 2055 cm-’ could be detected. The peak at 2175 cm-’ is near values of 2182-2208 cm-’ reported for CO on rutile [ 171, anatase [21] and Au-TiO, [26]. Thirty-five min after CO was stopped, the adsorbed CO, is almost completely removed (spectrum d), thus indicating weak CO, chemisorption on this catalyst in agreement with previous studies which showed weak CO, chemisorption on rutile and/or anatase [17,21,3 11. It has not yet been determined if the CO, is adsorbed primarily on Au or on TiO,. The partial pressure of oxygen was varied to examine its effect on CO and CO, adsorption. Fig. 5 shows the spectra obtained from the following sequence: start 5% CO with 5% 0, already flowing, increase 0, partial pressure from 38 to 71 Torr, increase 0, partial pressure from 71 to 130 Torr, and finally decrease 0, partial pressure from 130 back to 38 Torr. The spectra are

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Environmental 8 (1996) 417-443

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essentially independent of the 0, partial pressure, which is consistent with partial pressure measurements showing a near zero-order dependence on 0, at this temperature (300 K). Both CO and CO, adsorption, occurring here at ca. 2120 and ca. 2345 cm-‘, show little change with 0, partial pressure, a result consistent with the assumption of noncompetitive adsorption of O,, i.e., it adsorbs on different sites than those adsorbing CO and CO,. Finally, one run was conducted to examine the transitory behavior of adsorbed CO in the presence of 0, as gas-phase CO was removed. The procedure involved stopping the CO feed after 20 min under reaction conditions (spectrum 6a was taken before the end of this period) and the spectra are shown in Fig. 6. Upon CO addition (spectrum a), the intensity of bands near 1670 and 1580 cm-’ increases. These bands have been assigned to bidentate carbonate and carboxylate (or possibly bicarbonate) groups, respectively. Adding 0, increased

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the peaks at 1671 and 1242 cm-‘, decreased that at 1583 cm-’ and introduced the adsorbed CO, band at 2341 cm- ‘. Removing gas-phase CO caused the adsorbed CO and CO, peaks to decrease simultaneously. Although the 1671 cm-’ peak also decreases, its absolute intensity is large both before and after reaction, thus this peak does not appear to be a ‘reactive’ intermediate but rather a fairly stable spectator species whose surface concentration has a small dependence on the CO, (and possible CO) partial pressure in the catalyst bed. 3.3. DRIFTS experiments

with TiO,

3.3.1. Spectra with only CO present CO was added to the Ar in the same manner as that for Au-TiO, (P,, = 38 Tort-). Fig. 7 shows the spectra recorded after 5 min and 50 min under CO; both spectra show the appearance of a peak at 2182 cm- ’ which can be assigned to

M.A. Bollinger, M.A. Vannice / Applied Catalysis B: Environmental 8 (1996) 417-443 Table 1 Au-TiO,

catalyst

429

designations

Catalyst

Preparation

1.O% Au-TiO,(I) 0.3% Au-TiO,(I) I .4% Au-TiO, 2.2% Au-TiO, 1.7% Au-TiO, 1.6%Au-TiO, 0.1% Au-TiO, 2.4% Au-TiO,

impregnation impregnation pentane slurry pentane slurry impregnation impregnation deposition-precipitation coprecipitation

(EtHx) (EtHx) (UHP) (UHP) (DP) (PI

method

Precursor AuCl, AuCl, Au 2-ethyl hexanoate Au 2-ethyl hexanoate Au powder (99.9994%) Au powder (99.9994%) AuCl, /Au(OH), AuCI, /Au(OH),

CO on TiO, [17,21,26]. Peaks at 1580 and 1360 cm-’ appear which are consistent with assignments to carboxylate or bicarbonate groups. Free carbonate may also be present near 1440 cm-‘. After leaving this sample under flowing Ar overnight, the CO adsorption band near 2182 cm-’ was absent, but the growth of a large band at 1691 cm- ‘, which was also present in the spectrum of the TiO, before pretreatment and a band at 1173 cm-’ occurred similar to what was observed in the spectrum of the Au-TiO, sample after remaining overnight in argon. 3.4. Kinetic results for Au-TiO, Table 1 lists the five different methods used to prepare TiO,-supported catalysts, and the designation in parentheses indicates the method of preparation. Most of this study of Au-TiO, catalysts has involved impregnated catalysts; however, to gain a better understanding of variables possibly influencing the activity of these samples, catalysts were prepared to examine specific factors, i.e., Cl-free catalysts to examine the effect of surface Cl, high-purity Au catalysts to examine the possible effect of impurities in the AuCl,, coprecipitated and deposition-precipitated catalysts to examine the effect of Au crystallite size and pretreatment conditions, a low wt.-% Au catalyst to examine the effect of Au loading and a TiO,-covered Au powder (TiO,-Au) to probe for any electronic effect associated with the Au particles. As stated earlier, deactivation was routinely observed but was very dependent on different factors, and a set of representative decay rates is shown in Fig. 8. The activities listed in Tables 2 and 3 were ‘initial’ values typically obtained during the first 30 min on stream. However, Arrhenius runs and partial pressure dependency measurements were not conducted until after deactivation rates were quite low, i.e., after 2-3 h on stream. Over the time period required for these runs, quite reproducible data could be obtained with little influence from deactivation [15], and an example is presented in Fig. 9. Tables 2 and 3 list the ‘initial’ activity of typical samples at 313 K under 38 Torr CO and 38 Torr 0,

430

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Catalysis B: Environmental 8 (1996) 417-443

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Table 2 Catalytic

properties

of Au-TiO,

Catalyst

catalysts

after an HTR/C/LTR

‘Initial’ activity

pretreatment

Au crystallite

1.O% Au-TiO,(I) 0.3% Au-TiO,(I) 1.4% Au-TiO, (EtHx) 1.7% Au-TiO, (UHP) 0.1% Au-TiO, (DP)

2.3 0.3 0.6 0.2 0.35

230 100 40 10 350

2.4% Au-TiO, (P) TiO, -Au powder

3.0 0.013

130 0.013

33 _ 22 32 4.5 10 /.Lm

Reaction conditions: PC0 = 38 Torr, PO* = 38 Torr, T = 313 K. a From XRD line broadening.

a Dispersion

3.6 5.5 3.8 _ 27 0.01

Activation energy (kcal/mol)

TOF

7.1, 6.8 5.9 6.5 4.4 6.3

1.26 0.14 0.052 _

7.4 5.6

0.95 0.023

(s-‘1

M.A. Bollinger, M.A. Vannice/Applied Table 3 Catalytic

properties

of Au-TiO,

catalysts

Catalysis B: Encironmental8 (1996) 417-443

after a calcination

Catalyst

‘Initial’ activity

1.O% Au-TiO,(I) 0.1% Au-TiO, (DP)

none measured 1.1

431

(C) pretreatment Au crystallite size (nm)

( < 2) 1100

2.4% Au-TiO, (P) TiO, -Au powder

0.63 0.002

26 0.002

Reaction conditions:

Pco = 38 Torr, Paz = 38 Torr, T = 313 K.

Activation energy (kcal/mol)

33 _ 4.5 10 pm

7.3 9.0

along with Au crystallite sizes estimated from XRD (when available) and the apparent activation energies after either a HTR/C/LTR or a calcination only pretreatment. The tables show that all samples have measurable activity at 313

3.2

Fig. 9. Arrhenius plot for CO oxidation over 1.O% Au-TiO, (I) after HTR/C/LTR = 38 Torr). The descending-temperature sequence (filled symbols) was recorded perature sequence (open symbols).

3.8 pretreatments ( PC0 = PO2 prior to the ascending-tem-

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Table 4 Titania on Au surfaces markedly

enhances

Catalysis B: Environmental

8 (1996) 417-443

activity

Catalyst

‘Initial’ activity

Au powder (0.025 m*/g) (99.9994%) TiO, (50 m*g) TiO, -Au (0.070 m* /g) ( 10 monolayers 1.0% Au-TiO,(I) (42 m*/G)

0 (up to 573 K) 0 (0.004 at 573 K) 0.013 2.3

Reaction conditions:

TiO, on Au powder)

0 0 (8. lo-“) 0.19 0.056

Pco = 38 Torr, PO, = 38 Torr, T = 313 K.

K and have activation energies in the range of 4-7 kcal/mol, which suggests a similar mechanism for these samples. The coprecipitated catalyst has an average Au crystallite size of 4.5 nm, similar to that reported for deposition-precipitated Au-TiO, catalysts measured by TEM [26]. In contrast, samples prepared by impregnation show average Au crystallite sizes between 20 and 30 nm. The TiO,-Au powder has a calculated crystallite size of 10000 nm based on its BET surface area of 0.025 m2/g, a site density of 1.2 . 1019 Au atoms per m*, and the relationship d = 1.2/D where d is the particle diameter (nm) and D is the dispersion (fraction exposed). TiO,-Au showed a remarkably high activity at 313 K compared with that of pure Au powder, as shown in Table 4. Au powder had no measurable activity at temperatures up to 573 K; thus, the activity enhancement of titania on the gold surface is clearly demonstrated. The addition of TiO, to the Au powder increased the BET surface area by a factor of less than 3, which is far too small to account for the magnitude of the activity enhancement. The activity of 1.0% Au-TiO,(I) and pure TiO, are shown in the table for comparison. On a surface area basis, the TiO,-Au has a higher specific activity than 1.0% Au-TiO,(I) by a factor of 3 and both were well above the activity of pure TiO, alone, which was inactive below 363 K. A summary of results which illustrates some of the effects of pretreatment on impregnated Au-TiO, catalysts, including the activity of a 2.3% Au-TiO,(I) sample previously reported [12], is given in Table 5. For 2.3% Au-TiO,(I), HTR provided an activity 40 times larger than the LTR pretreatment and about one-quarter the activity of a HTR/C/LTR sample (I) [12]. In the current study, it was found that a four h calcination at 673 K did not activate the 1.0% Au-TiO,(I) catalyst ( < 0.02 pmol/g cat/s at 313 K> and an HTR/C pretreatment provided an activity similar to that after a HTR/C/LTR step. Fig. 10 shows the effect of the addition of a high concentration of water vapor on a sample of 1.0% Au-TiO,(I) which was first given an HTR/C/LTR pretreatment, then held under reaction conditions at 3 13 K until a reasonably steady activity was measured. Water vapor was added to the feed at 4.5 Torr

M.A. Bollinger, M.A. Vannice / Applied Catalysis B: Environmental 8 (19961417-443 Table 5 Pretreatment

significantly

Catalyst

1.O% 2.3% 2.3% 1.O% 1.O%

Au-TiO,(I) Au-TiO,(I) Au-TiO,(I) Au-TiO,(I) Au-TiO,(I)

effects CO oxidation Pretreatment

Reaction conditions: a From Ref. [12].

PC0 = 38Torr,

catalysts

‘Initial’ activity

C (673 K) LTR (473 K) HTR (773 K) HTR/C HTR/C/LTR

’ a

over Au-TiO,

433

[S]

[F]W

0 0.007 0.28 2.0 2.3

0 0.02 0.68 4.9 5.6

Poq=38Torr,T=313K.

(0.6 mol-%) and it severely deactivated the catalyst. Purging with the dry reactant gas mixture did not restore the activity, and heating to 393 K for 30 min while purging restored the activity to about half that prior to adding the water

0

4 Add H,O Vapor

Heat to 12O’C for 30 min.

Stop H,O Vapor

a0 \

O0

500

1CKM.l 1500 Time (min.)

Fig. 10. Deactivation of 1.O% Au-TiO,(I) (after HTR/C/LTR) feed: PC0 = 37 Torr, Po, = 37 Torr, balance He, T = 313 K.

following

2ooo

2500

addition of 4.5 Torr water vapor to

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M.A. Bollinger, M.A. Vannice/Applied

Table 6 Partial pressure (HTR/C/LTR)

dependencies pretreatment

Catalysis B: Environmental

(with 95% confidence

intervals)

8 (1996) 417-443

for CO oxidation

over

1.0% Au-TiO,(I),

r= kP;oPoJ

T (K)

x

Y

273 293 313

0.41 + 0.05 0.43 + 0.05 0.56 + 0.09

0.02 f 0.26 0.03 f 0.04 0.13+0.04

vapor. An LTR step did not restore the activity at this point, but a C/LTR step essentially reproduced both the high initial activity and the deactivation curve shown for the freshly pretreated sample. Table 6 lists the partial pressure dependencies along with their 95% confidence limits for 1.0% Au-TiO,(I) at 273, 293 and 313 K after an HTR/C/LTR pretreatment. Partial pressure dependencies were also determined for 0.3% Au-TiO,(I) and 1.4% Au-TiO, (EtHx). The 0.3% Au-TiO,(I) sample showed partial pressure dependencies on CO and 0, very close to that for 1.0% Au-TiO,(I) at 3 13 K but showed a slightly lower CO dependence (0.14) and a slightly higher 0, dependence (0.34) at 273 K. The 1.4% Au-TiO, (EtHx) sample showed near half-order dependencies on both CO and 0,.

4. Discussion Disagreement exists in the literature regarding CO oxidation over supported gold. This reaction has been reported to proceed on a Au wire [32] and a Au sponge [33] near room temperatures; however, Haruta found no activity over Au powder up to 543 K [34]. Consistent with the last study, the low surface area UHP Au powder used in our study was found to be inactive for CO oxidation at temperatures as high as 573 K under 38 Torr CO and 38 Torr 0,. The differences among these studies are presumably due primarily to impurities in the Au, which have not been previously reported [35]. The inactivity of unsupported Au is expected due to the limited adsorption on clean bulk Au surfaces, as suggested by several studies [32,35-381. Daglish and Eley suggested that only a small number of sites exist on Au for 0, adsorption, such as dislocation sites or impurity atoms, while CO can be adsorbed on ‘normal’ sites [32]. Trapnell found that CO is adsorbed on evaporated Au films while 0, is not at temperatures up to 273 K [36]. MacDonald and Hayes obtained a surface coverage of oxygen of less than 2% at 273 K on 99.999% Au powder [37], while a clean Au(1 lo)-(1 x 2) surface did not dissociatively adsorb 0, between 300 and 500 K under oxygen pressures up to 1400 Torr [38]. Carbon monoxide

M.A. Bollinger, M.A. Vannice /Applied Catalysis B: Environmental 8 (1996) 417-443

435

adsorption is very weak ( < 8 kcal/mol) and no CO adsorption was observed on a Au(ll0) surface down to 125 K [35]. In contrast to this clean UHP Au powder, the same powder showed significant activity at 313 K after TiO, had been deposited on its surface and it was given an HTR/C/LTR or a calcination (C) pretreatment. This activity cannot be explained by either an increase in surface area (from 0.025 to 0.07 m2/g> or the activity of TiO,, because the latter is inactive at these temperatures and the 10 pm Au crystallites are far too large to expect any electronic effect to occur, as argued previously by Vannice and Sudhakar for TiO,-covered Pt powder [39]. Consequently, some kind of synergism exists between TiO, and Au, and a previous proposal of special active sites at the metal-support interface is applicable here [12,39]. The high specific activities of both TiO,-Au (with large Au and small TiO, crystallites) and Au-TiO, (with small Au and large TiO, crystallites) strongly suggest an interfacial effect is involved. Per g the 1.0% Au-TiO, sample has a larger activity than TiO,-Au at 313 K by a factor of about 180, while on a total surface area basis the TiO,-Au catalyst is 3 times as active. This is not surprising since the Au dispersion in the 1.0% Au-TiO, sample is 480 times higher than that in TiO,-Au sample. It would be preferable to compare specific activities based on the exposed Au surface areas but these values are not known accurately because well defined chemisorption methods for Au do not exist [14]. Regardless, the estimated turnover frequencies based on the average crystallite sizes in Table 2 provide some measure of this. A number of supported Au catalysts have now been reported to be very active for CO oxidation, and they are summarized in Table 7. Au-TiO, catalysts are in this group; [7,12,13,26] however, a word of caution is appropriate here because these Au catalysts can exhibit significant deactivation and some of these rates were reported without specifying time on stream. This present study has confirmed the high activity of Au-TiO, catalysts prepared via impregnation [ 121, coprecipitation [ 131 and deposition-precipitation techniques [26]. It has also shown that preparation methods and the pretreatment can have a profound influence on the activity of these catalysts. For example, as shown in Table 5, a Au-TiO,(I) catalyst is inactive after only a calcination step and an LTR pretreatment induces only a low activity. An HTR step alone produces a large increase in activity, but the most active samples require both an HTR and a calcination step. In contrast, Au-TiO, catalysts prepared by deposition-precipitation and coprecipitation methods can be activated by just a calcination step, as reported by Haruta and coworkers. An HTR/C/LTR pretreatment enhances the activity of the (P) catalyst and decreases that of the (DP) sample (see Tables 2 and 3). The Cl-free 1.4% Au-TiO, (EtHx) sample was active after an HTR/C/LTR pretreatment, with an activity about one-sixth that of the 1.0% Au-TiO, after the same pretreatment, which demonstrates that active impregnated catalysts can be prepared without Cl present. The possibility has been stated that Cl may have

436

M.A. Bollinger, M.A. Vannice/Applied

Table 7 CO oxidation

Catalysis B: Environmental 8 (1996) 417-443

kinetics reported on Au and supported

Catalyst

P (Torr)

PC0

3.3% Au-TiO, (DP) 1.2% Au-Co,O, (DP) 0.66% Au- cu-Fe,O, (DP)

7.6 1.6 7.6

3.3% Au-TiO, (DP) 1.2% Au-Co,O, (DP) 0.66% Au- cr-Fe,O, (DP)

38 38 38

20% Au-MnO, (P) a 29% Au-MnO, (P) a 1% Au-Z@ (P) 1% Au-Z@ (P) Au powder 1.8% Au-SO, (DP) 0.1% Au-TiO, (DP)

1.6 1.6 1.9 1.9 7.6 38 7.6

T(K)

Au

E,(kcal/ mol)

P02

‘Initial’ activity

pmol/ s/g cat

pmol/

Partial pressure Ref. dependency CO

0,

s/g Au

156 156 156

313 313 313

8.2 3.9 8.4

51 12 1.2

1500 1000 1100

0.05 0.05 0

0.24 0.27 0.05

[261 [261 [261

38 38 38

313 313 313

8.2 3.9 8.4

39 8.9 6.7

1200 740 1000

0.05 0.05 0

0.24 0.27 0.05

1261 [261 [261

3.8 3.8 1.9 76 156 37 156

328 308 373 373 546 313 263

3.6 -

-

-

1.3 0.9 0.2 b 1.3 b 0.5 0.069 0.48

6.5 3.1 20 130 0.5 3.9 480

[11,411 [401 1421 [421 [341 [121 I71

a Wt. loading assuming MnO,. b Initial activity.

a negative impact on activity, since it could block active sites, thus accounting for the low activity of LTR pretreated samples [12]. Unfortunately, these experiments do not unambiguously determine the effect of surface Cl on catalyst activity because catalytic activity is lower than the best Au-TiO,(I) catalysts, but they imply that Cl is not a major concern after an appropriate pretreatment. The question of the role of metallic impurities has been raised by previous investigators [35]. The 1.7% Au-TiO, (UHP) sample was active after an HTR/C/LTR pretreatment and had a similar low E, value, thus, arguing against impurities in the Au accounting for the high activity of the catalysts prepared with the AuCl, precursor; however, this result is admittedly somewhat ambiguous because the overall activity was decidedly lower. The lower activity compared to the other Au-TiO, catalysts is attributed to the poorer Au dispersion and the preparation procedure because the migration of TiO, species on gold cannot be controlled (or measured) yet. The turnover frequencies in Table 2 are based on crystallite sizes determined by XRD line broadening and are similar to those reported by Haruta et al. [26] although the 1.0% Au-TiO, sample has a noticeably higher value. The variations in activity among these catalysts are attributed to the inability to control the migration of TiO, species on the surface of the Au crystallites; consequently, the interfacial regions can vary markedly. The lack of an adsorbate to measure Au surface areas compounds this difficulty. Although the deactivation process was not of major interest to us in this

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Environmental 8 (1996) 417-443

431

study, various rates of decay have been reported by different groups, and the roles of H,O and CO, in this process were of some interest from a kinetic point of view. Hoflund and coworkers have shown that Au-MnO, catalysts have very slow, long-term decay rates [ 11,40,41], whereas Wokaum and coworkers have reported much more rapid deactivation with Au-ZrO, catalysts [42]. However, the latter authors found that deactivation was greatly reduced and quite minimal with a stoichiometric CO/O, ratio of 2 whereas O&h feeds markedly increased deactivation [42]. As essentially all of the runs were conducted under a nonoptimum O,-rich environment, deactivation rates were undoubtedly enhanced with our catalysts. However, as stated previously, after several h on stream decay rates were slow enough to allow kinetic runs to be conducted with little or no effect of deactivation. The addition of 4.5 Torr water vapor at 313 K deactivated the 1.0% Au-TiO,(I) catalyst, as shown in Fig. 10. Au-Co,O, catalysts have been found to be insensitive to water vapor concentrations between 6.5 * 10e4 Torr and 4.6 Torr, and this behavior was ascribed to the absence of adsorption of H,O on Au surfaces [43]. The deactivation of this 1.0% Au-TiO, catalysts by water vapor may be due to H,O adsorption on TiO,, since water is known to adsorb both associatively and dissociatively (as OH-) on reduced Ti cations and to oxidize the surface [44], thus, blocking the titania inter-facial sites. The activity of 1.0% Au-TiO,(I) was not restored by removing the water vapor from the feed stream thus indicating irreversible adsorption. A portion of the catalyst activity was restored after heating at 393 K for 30 min, and complete reactivation was obtained after a C/LTR treatment. This is consistent with the DRIFTS results which suggest that partial, but not complete, water removal could occur after heating at 393 K for 30 min. This pretreatment should also decompose carboxylate species which have been associated with the initial activity decrease in Au-TiO, catalysts [45]. This strong deactivation dependence on water allows the possibility that some of the observed rate decay may have been due to trace amounts of water in the feed stream. Some kinetic aspects of low temperature CO oxidation on Au-TiO, have been previously reported [ 12,261. In the present study, partial pressure dependencies in a power rate law were found to be near 0.4 for CO and near zero for 0, for 1.0% Au-TiO,(I) at 273 K, and the activation energy was 7.1 kcal/mol. For a 3.3% Au-TiO, (DP) catalyst at 273 K, Haruta et al. have reported dependencies of 0.05 for CO and 0.24 for 0, with an activation energy of 8.2 kcal/mol [26]. The activation energies agree satisfactorily but the CO and 0, dependencies do not; however, the pressure dependencies were measured under different conditions, i.e., PO, = 38 Torr vs. 156 Ton: during CO measurements and P = 38 Torr vs. 7.6 Torr during 0, measurements. Comparison of 1.O% AETiO,(I) with the previously reported 2.3% Au-TiO,(I) catalyst at 313 K shows a somewhat higher CO order and a somewhat lower 0, order [12]. This is attributed to changes in the 2.3% Au-TiO, catalyst caused by more extensive

438

M.A. Bollinger, MA. Vannice/Applied

Catalysis B: Enuironmental

8 (1996) 417-443

deactivation before the data were collected; i.e., CO was flowed continuously during the experiments with 2.3% Au-TiO,(I) whereas with 1.0% Au-TiO,(I) only He was flowed between runs. The quality of the data in this latter study is also better due to improved reactor and GC performance. To better understand the kinetics of this reaction, it is important to examine the effect of CO,. The results of this study provide evidence that CO and CO, compete for adsorption on the same sites on the Au surface. The DRIFTS results demonstrate the presence of adsorbed CO and CO, on 1.0% Au-TiO,(I) under reaction conditions. The adsorption band near ca. 2110 cm- ’ has been assigned to CO adsorbed on Au [23,25], while the most intense band attributed to CO,, 2342 cm-‘, is near that reported for CO, on TiO, [20,22], and the shoulder at 2320 cm-’ and the band at 2378 cm-’ could indicate adsorption on different sites such as on Au. Under reaction conditions, the amount of chemisorbed CO is decreased compared to the amount when only CO is flowing (see Figs. 4 and 6). Although this may possibly be due to a lo-15 K temperature increase in the sample because of the large exothermic heat of reaction, the most likely explanation is competitive adsorption between CO and CO, which decreases the surface coverage of CO. The evidence supporting this is the following: (1) an inverse linear relation was found between adsorbed CO and adsorbed CO, integrated peak areas [15], i.e., an increase in CO, is accompanied by a decrease in CO on the surface; (2) the CO peak shift to higher wavenumbers in the presence of CO, indicates a dilution effect on Au [24,25]; (3) adsorption of 0, on Au-TiO, is extremely low [12] and there is a zero-order dependence on 0, of both the rate and CO coverage, as shown in Fig. 5; and (4) addition of CO, to the feed in kinetic experiments indicated a reversible inhibitory effect [15]. The DRIFT spectra of TiO, and 1.0% Au-TiO,(I) show some similarities in the 1100-1700 cm-i region, which is typically associated with carbonate or carboxylate-type surface species on transition metal oxides [27] and with -OH bending and stretching modes. Before pretreatment both samples have bands near 1550 and 1360 cm- ’ which can be assigned to carboxylate ions or monodentate carbonate species [27] (see Fig. 1). A distinct band at 1690 cm-’ is seen on both samples but is easily removed upon heating to 423 K in Ar; the origin of this band is not clear but may be due to the C-O stretch of a carboxylic acid species [16]. Both samples contain adsorbed H,O and hydroxyl groups which are removed along with carbonate-carboxylate species by a pretreatment at 773 K in H,, as shown in Fig. 2; however, some strongly bound hydroxyl groups remain on the TiO, surface after the subsequent calcination step at 673 K, as shown in Fig. lc. As Figs. 3 and 7 indicate, exposing these samples to 5% CO in Ar increases the intensity of bands assigned to bidentate carbonate (1620- 1670 and 1250 cm- ‘> and carboxylate or monodentate carbonate (1580, 1530 (shoulder) and ca. 1380 cm-‘) [27]. Similar behavior exists under reaction conditions (Fig. 41, but an additional band intensity increase occurs at 1426 cm- ‘, which has been assigned to a free carbonate species [27].

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Environmental

8 (1996) 417-443

439

Adsorption of CO on pure TiO, is indicated by the presence of a band at 2182 cm- ‘, while on Au-TiO, only a band at 2105 cm- ’ associated with gold is observed. With Au-TiO,, Haruta et al. reported only a very small amount of adsorption on TiO,, indicated by a peak at 2183 cm-r assigned to CO adsorbed on Ti4+ cations, and most of the CO adsorption occurred on Au, as indicated by a peak at 2118 cm-’ [26]. The reason for the absence of a band in this catalyst for CO adsorbed on titania is not clear at this time, but one possibility is that Au and/or Cl could interact with the titania sites normally capable of adsorbing CO. These DRIFTS results argue against reactive CO species existing on the titania surface. Although some surface carbonate species may form during CO adsorption, the absolute amount is small because little or no irreversible CO adsorption could be detected within the uncertainty of the isotherm measurements [ 121. These DRIFTS experiments provide some insight into possible reaction intermediates. Bidentate carbonate species have been proposed as a reaction intermediate over Au-TiO, catalysts [26] and Haruta et al. have proposed a model in which a bidentate carbonate species is a reaction intermediate reacting with CO adsorbed on Au near the TiO, interface to form CO, [26,45]. Under only CO, 1.0% Au-TiO,(I) sometimes showed an increase in bidentate carbonate and carboxylate species (Fig. 61, and under reaction conditions the peak associated with the bidentate carbonate increased as shown in Figs. 4 and 6. These results do not contradict those of Haruta et al. although the small 1670 cm-’ peak in Fig. 4 might be expected to disappear in the presence of CO; however, they can also be interpreted to mean that the intensity of the 1670 cm-’ band is primarily dependent on CO, pressure and this is just a spectator species. The partial pressure dependency results represented in Table 6 were used to test the validity of various sequences of elementary steps. Similar to a previous mechanism and one with adsorption as the rate study [ 121, both a Rideal-Eley determining step (rds) can be eliminated, since they require a first-order dependence on one of the reactants. Four Langmuir-Hinshelwood (L-H) models were considered: (a) dissociative 0, adsorption and competitive adsorption between CO and 0, (1 type of site) with the reaction between CO and 0 atoms as the rds, (b) dissociative 0, adsorption and noncompetitive adsorption between CO and 0, (2 types of sites) with the reaction between CO and 0 atoms as the rds, (c) nondissociative 0, adsorption and competitive adsorption between CO and 0, with the reaction between adsorbed CO and 0, molecules as the rds and (d) nondissociative 0, adsorption and noncompetitive adsorption between CO and 0, with the reaction between adsorbed CO and 0, as the rds. The rate equations derived from these four L-H models were fitted to the data using a nonlinear regression technique contained in a SAS computer package employing the Gauss-Newton method and a convergence criterion requiring a change in the residual to decrease below lo-‘. Each set of partial pressure data

440

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Enuironmenral8

Table 8 Parameters

from the NDNC reaction model for CO oxidation

T (K)

k”

over Au-TiO, a Kc,

213 293 313

1.11*0.5 2.4 + 0.3 11*4

13+8 14+3 6.9k2.6

(1996) 417-443

(HTR/C/LTR) b KG 150*370 38+564 67&-M

- ASfd (cal/mol/K)

- AH: (kcal/mol) co

0,

co

02

2.1

3.2

18

14

a Uncertainty represented by 95% confidence limits. b K,, and Koq are reported in units of atm-‘, units for k are pmol/s

g cat.

was normalized to standard reaction conditions (Pco = PO, = 38 Torr) at that temperature to correct for any deactivation which may have occurred. The equilibrium adsorption constants for CO and O,, Kc0 and Ko2, were obtained from the curve fitting along with the rate constant for the rds, and the equilibrium constants were evaluated for physical consistency. The parameters obtained from these L-H models were evaluated and the values for the entropy and enthalpy of adsorption were consistent for all models, i.e., they were all negative and the adsorption entropy losses adsorption were less than the gas-phase entropies; [ 151 however, the nondissociative, competitive adsorption sequence (Model c) gave a noticeably poorer statistical fit. The model most consistent with both the kinetic behavior and the DRIFTS results is that chosen previously; [ 121 i.e., nondissociative, noncompetitive (NDNC) adsorption of 0, and CO with the reaction involving two types of sites at the Au-titania interface. This two-site model is consistent with the physically meaningful thermodynamic parameters obtained, with the capability to give zero-order dependencies on both 0, and CO [26], with the DRIFTS results showing no effect of 0, pressure on CO coverage, with the fact that both Au and TiO, alone are inactive, and with the report that nondissociative 0, species have been detected by ESR on anatase and rutile [46]. Since CO adsorption only on Au was observed, it is now proposed that 0, is activated on TiO, at the TiO,-Au interface, which is opposite to our earlier supposition [ 121.With the exception of their proposed carbonate intermediate, the model of Haruta et al. is very similar [26]. The rate equation for this sequence is r =

KdL1pCOpOZ

(1 +&o&,)(1 + Ko/oJ at low conversions when CO, can be ignored. The values for the three p&&eters at each temperature are listed in Table 8 and the capability to fit the data is shown in Fig. 11. The thermodynamic parameters clearly indicate weak adsorption, as do those from all the models [15], which is consistent with chemisorption and DRIFTS results.

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Environmental 8 (1996) 417-443

441

6J

(4 5-

43 P P

-

% 5

3:

0 ..:

_

Y

2-

1-

0

3.0 -

25

50

75

LM)

125

150

175

200

(b)

2.8 2.6 2.4 2.2 2.0 P

1.8-

Q 2 1.6zi ‘; 1.4.? .$

1.2l.O0.8 0.6 -

0

20

40

60

80

100

120

140

160

180

200

pm me

Fig. 11. Fitting of NDNC (nondissociative, noncompetitive) model to partial pressure data collected ( X 1, 293 K (+ ) and 313 K (A ): (a) activity vs. CO pressure. and (b) activity vs. 0, pressure.

at: 273 K

442

M.A. Bollinger, M.A. Vannice/Applied

Catalysis B: Environmental 8 (1996) 417-443

5. Summary TiO,-supported Au catalysts can have extremely high initial activities for CO oxidation at 300 K and below; however, 50% to 70% losses in rate are observed over times on stream of 3-15 h. Impregnated Au-TiO, is most active after a HTR/C/LTR pretreatment, active after HTR and HTR/C pretreatments, far less active after a LTR pretreatment, and inactive after only calcination (C). Coprecipitated and deposition-precipitated catalysts could be activated by either a calcination step or an HTR/C/LTR pretreatment. After an HTR/C/LTR pretreatment, an impregnated catalyst with an average Au crystallite size of 25 nm has activity comparable to a coprecipitated catalyst with an average Au crystallite size of 4.5 nm. The ease of migration of TiO, species onto Au particles and the inability to control the TiO, coverage are the most likely reasons for the lack of correlation between Au crystallite size and activity. Deposition of TiO, overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the high activity of Au-TiO, catalysts and it strongly supports the proposal that special Au-TiO, inter-facial sites are the most likely explanation for the high activity in these systems. A LangmuirHinshelwood model invoking CO adsorbed on Au which reacts with oxygen activated at the Au-TiO, interface is proposed which is consistent with these results and the assumption that CO and 0, adsorption occurs on two different types of sites. Variations in 0, pressure showed no effect on adsorbed CO or on reaction rate which supports this model. Only CO adsorbed on Au was detected by DRIFTS and reversible CO, adsorption was detected during reaction which appeared to inhibit the reaction by competing with CO for adsorption sites. The kinetic studies showed a near half-order rate dependence on CO and a rate dependence on 0, varying from zero to one-half between 273 and 313 K, and an activation energy typically near 7 kcal/mol was obtained. Water vapor at 4.5 Torr deactivated 1.0% Au-TiO,(I), but the activity was partially restored upon heating to 303 K and was fully restored after a C/LTR treatment.

Acknowledgements This study was supported by the US Department of Energy BES under Grant No. DE-FG02-84ER13276. We would like to thank Paul Fanning, who provided assistance with DRIFTS work and Dr. Raj Selvaraj, who prepared the coprecipitated Au-TiO, sample used in this study.

References [l] M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 115 (1989) 301. [2] G.C. Bond and P.A. Sermon, Gold Bull., 6 (1973) 102.

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