Al2O3

Sang-Kyung Kim, and Son-Ki Ihm* ... Platinum was impregnated on γ-Al2O3 by using two different precursors: one is anionic (i.e., H2PtCl6) and the oth...
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Ind. Eng. Chem. Res. 2002, 41, 1967-1972

1967

Effects of Ce Addition and Pt Precursor on the Activity of Pt/Al2O3 Catalysts for Wet Oxidation of Phenol Sang-Kyung Kim and Son-Ki Ihm* National Research Laboratory for Environmental Catalysis, Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Korea

The effects of Ce addition on the activity of Pt/Al2O3 catalysts were investigated for the wet oxidation of phenol. Platinum was impregnated on γ-Al2O3 by using two different precursors: one is anionic (i.e., H2PtCl6) and the other is cationic (i.e., Pt(NH3)4Cl2). The Pt catalysts from the former showed much higher activity than that from the latter, because the former resulted in a better metal dispersion than the latter. Cerium addition lowered the catalytic activity of Pt/Al2O3 catalysts from H2PtCl6, while it improved the activity of Pt/Al2O3 catalysts from Pt(NH3)4Cl2. The effect of Ce addition on the wet oxidation activity of Pt-Ce/Al2O3 could be explained by the differences in Pt dispersion and the Pt-Ce interactions. Introduction Wet air oxidation (WAO) is recognized as an important technique for wastewater treatment, particularly for highly toxic organic wastewaters.1,2 Although there are many advantages of WAO, the high material costs of WAO plants and high operating costs covering severe reaction conditions remain the limiting points of using WAO. The use of effective catalysts can solve this problem by making the reaction conditions milder and also by decomposing even refractory pollutants. Many catalytic researches have been focused mainly on the wet oxidation performance and partially on the reaction kinetics and mechanisms. A better understanding of reactions and catalysis taking place during WAO is important, as it leads to the reliable design of oxidation reactors and also to cost reduction by optimization of the operating conditions. Metals, metal oxides, and metal salts were used as catalysts for wet oxidation, like other oxidation reactions. Homogeneous catalysts, particularly copper salt, showed good activities,3 but the dissolved catalysts are generally so toxic that they should be separated from the solution. Mixtures of metal oxide catalysts frequently exhibit greater activity than the single oxide. Co/Bi (5/1) catalyst4 and Mn/Ce catalysts5 showed good activity for the wet oxidation of acetic acid. Levec and Pintar6 reported that CuO/ZnO/γ-Al2O3 catalyst (42/47/ 10 wt %) and CuO/ZnO/CoO catalyst (9.3/6.9/1.4 wt %) supported on steam-treated porous cement were effective for phenol oxidation. Supported noble metal catalysts are generally more active than metal oxides,7 and many catalysts used in commercial wet oxidation processes are based on noble metals, especially platinum. Gallezot et al.8 reported that platinum catalysts supported on active charcoal could be used under very moderate conditions to oxidize carboxylic acid. Hamoudi et al.9 reported that Pt promotion of MnO2/CeO2 reduced the amount of carbonaceous deposits and improved phenol deep oxidation (higher CO2 yield). Taguchi and Okuhara10 studied NH3 oxidation over titania-supported * To whom correspondence should be addressed. Phone: +82-42-869-3915. Fax: +82-42-869-5955. E-mail: skihm@ mail.kaist.ac.kr.

platinum and palladium catalysts, and Pt/TiO2 showed the superior activity of N2 formation among other titania-supported noble metal catalysts. There are several researches on the promoting effect of Ce for wet oxidation reactions. The promoting effect of Ce was reported for a Co/Bi catalyst, which was explained in terms of the variation in the electronic states of Co and Mn.4 The promoting effect of Ce on mixed-metal oxides, CeO2/ZrO2/CuO and CeO2/ZrO2/ MnOx, for acetic acid oxidation was attributed to the enhancement of the redox properties of the active centers in the catalysts.11 Zhang and Chuang12 reported that Ce promoted the activity of an alumina-supported Pt catalyst but inhibited the activity of an aluminasupported Pd catalyst for the wet oxidation of pulp mill wastes. In this study, the wet oxidation of phenol was investigated over alumina-supported Pt catalysts. The Ce addition effect on metal dispersion, the reducibility of catalysts, and the catalytic activity of the wet oxidation of phenol was observed for two different kinds of Pt precursors. Discussions were made on the factors affecting the activity of alumina-supported Pt catalysts by Ce addition. Experimental Section Preparation of Catalysts. γ-Al2O3 from Strem was used as support. Pt(NH3)4Cl2, H2PtCl6, and Ce(NO3)3 were obtained from Aldrich and used as received for the precursors of Pt and Ce, respectively. Pt and Ce were supported on γ-Al2O3 catalysts by successive impregnation. Ce was loaded first on Al2O3 support by an incipient wetness impregnation procedure. After being dried at 110 °C for 12 h and calcined at 500 °C for 5 h, Ce/Al2O3 was then impregnated with platinum. The final sample was dried and calcined with the same procedure and then reduced at 400 °C for 2 h with H2 for the wet oxidation reactions. The Ce loading was chosen to be 1, 3, and 5 wt % respectively, and the Pt loading was fixed at 1 wt %. For simplicity, each catalyst will be designated according to the kind of loaded metal, its loading, and the kind of Pt precursor (Table 1); for example, Pt-Ce(1-3) denotes the Al2O3-supported catalyst containing 1 wt % Pt using H2PtCl6 as a Pt

10.1021/ie010590p CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002

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Table 1. Metal Loading, BET Surface Area, and Pt Dispersion of Catalysts

catalyst Al2O3 Ce(1) Pt+(1) Pt+Ce(1-1) Pt+Ce(1-3) Pt+Ce(1-5) Pt-(1) Pt-Ce(1-1) Pt-Ce(1-3) Pt-Ce(1-5)

Pt Ce loading loading (wt %) (wt %)

1 1 1 1 1 1 1 1

0 1 3 5 0 1 3 5

Pt precursor

Pt(NH3)4Cl2 Pt(NH3)4Cl2 Pt(NH3)4Cl2 Pt(NH3)4Cl2 H2PtCl6 H2PtCl6 H2PtCl6 H2PtCl6

surface Pt area dispersion 2 (m /g) (%) 208 208 207 204 204 197 205 207 200 195

66 76 63 65 91 78 76 68

precursor and 3 wt % Ce, and Pt+Ce(1-3) denotes the Al2O3-supported catalyst containing 1 wt % Pt using Pt(NH3)4Cl2 as Pt precursor and 3 wt % Ce. Characterizations of Catalysts. The surface areas of the prepared catalysts were measured by a nitrogen adsorption method at liquid nitrogen temperature using a Micromeritics ASAP 2000. The chemisorption of carbon monoxide was carried out at 35 °C in the pressure range of 200-400 mmHg. The sample was oxidized in situ at 350 °C under O2 flow to eliminate impurities on the surface of sample and was then evacuated. For CO chemisorption, the sample was reduced under H2 flow at 350 °C for 2 h, evacuated at 350 °C, and cooled to 35 °C. After the first isotherm was obtained, the sample was evacuated, and then the second isotherm was obtained. The two isotherms were extrapolated to zero pressure, and the difference of two isotherms at zero pressure was taken as the amount of chemisorbed CO. Powder X-ray diffraction patterns of the prepared catalysts were obtained with a Rigaku D/MAX-III diffractometer using monochromic Cu KR (λ ) 0.1506 nm) radiation. The data were scanned from 5° to 70° (2θ) in steps of 0.01°, and the scanning rate was 6°/min. Temperature programmed reduction (TPR) of the prepared catalysts were carried out using a 5% H2/95% Ar gas mixture. The samples were pretreated with O2 at 400 °C for 2 h. The hydrogen consumption was recorded with a TCD cell while the sample was heated linearly from 50-600 °C at 10 °C/min. Temperature-programmed oxidation (TPO) of the used catalysts was carried out by using a 5% O2/95% He gas mixture. The samples were pretreated with He at 100 °C for 2 h to remove physically adsorbed organic molecules. During the TPO experiments, the variations of CO2, CO, and O2 content in the effluent gas were detected with a quadrupole mass spectrometer (QMS200M3, Pfeiffer Vacuum). FT-IR spectra of the used catalysts were obtained from NEXUX of Nicolet equipped with MCT detector at a resolution of 4 cm-1 and 32 scans per spectrum. The used catalysts were dried at 100 °C for 5 h to remove physically adsorbed organic molecules and then pelletized with KBr to prepare the IR samples. Reaction Procedure. Phenol (+99% purity) was purchased from Aldrich and was used without further purification. A stirred reactor (1 L; Autoclave Co.) with suspended catalysts was used batchwise for the wet oxidation of phenol. It was equipped with a Teflon liner, magnetically driven impeller (Hastelloy), sampling line (Hastelloy), control units of temperature and agitation speed, and a liquid-injection vessel (SS316; 100-mL capacity) mounted on the top of the autoclave. To

conduct a reaction experiment, 225 mL of distilled water, 0.75 g of catalyst, which corresponds to 3-g of catalyst/1-L of reactant solution, and air of 2.02 MPa at ambient temperature were charged into the autoclave. It was stirred with a 1000 rpm blade rotating speed and heated to the desired temperature, between 155 and 200 °C. Catalyst particle size used was chosen in the range of 0.038-0.075 mm because it was confirmed that there were no internal diffusion limitations for three different particle sizes in the range of 0.0380.180 mm. In the meantime, 25 mL of 10 000 ppm phenol solution and air of 2.02 MPa was charged in the injection vessel and preheated to about 100 °C. When the temperature of reaction vessel was reached to a desired point, preheated reactant solution was injected with 5.05 MPa air to obtain 250 mL of a 1000 ppm phenol solution. The used oxygen partial pressure (1.06 MPa) corresponded to an oxygen/phenol mole ratio of about 81, far exceeding the oxidation stoichiometry (which is 7 if it is assumed that all phenol was transformed into carbon dioxide and water). As soon as the phenol was injected, a sample was withdrawn, and it was referred to as the sample at time zero. The concentrations of phenol were determined using a HPLC (Young-Lin 930) system, equipped with Rheodyne injection valve and UV detector. Analysis was carried out using a µ-Bondapak C18 column and a mobile phase of a mixture of water and acetonitrile (4:6) with a flow rate of 0.8 mL/min. The initial reaction rate was calculated from the slope of each plot of C/C0 versus time at the start of reaction. Results and Discussion Catalyst Characterization. Table 1 summarizes results of BET surface area and CO chemisorption. The BET surface areas of the prepared catalysts were hardly changed by metal impregnation. However, there was a little decrease in BET surface area for the 5 wt % Ce loaded catalysts (Pt-Ce(1-5) and Pt+Ce(1-5)), which may be due to the clogging of micropores of support. It has been proposed that the use of base metal oxide additives such as CeO2 or MoO3 increase the dispersion of a noble metal on γ-Al2O3.13-15 It was also proposed that base metal additives inhibit the agglomeration of dispersed Pt, which has been attributed to a Pt-CeO2 interaction.16-18 This interaction helps to maintain Pt in an oxidized state which is more difficult to sinter than metallic Pt.16,17,19 On the other hand, Tiernan and Finlayson20 reported that the addition of Ce, particularly at higher levels (Pt/Ce e 1:8), resulted in the decrease in Pt dispersion, which agreed with Summers and Ausen21 who found that, as the level of Ce increased between 0 and 13 wt %, the apparent dispersion of fresh samples decreased. It was found in this study that the effect of Ce addition on Pt dispersion varied depending upon the nature of Pt precursor. It can be seen that Pt-(1) catalysts showed higher Pt dispersion than Pt+(1) catalysts (Table 1). This must be due to different ionic state of Pt species in precursor solution (i.e., cationic state for Pt(NH3)4Cl2 and anionic state for H2PtCl6). The Ce addition in Pt-(1) resulted in a decrease of Pt dispersion. Considering the low loading of Pt and Ce, the decrease of Pt dispersion with increasing Ce content is unlikely to be attributed to the loss of support surface area. It seemed more likely that the loading of Ce on Al2O3 changed the surface properties of support, which

Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1969

Figure 2. TPR profiles of Pt+Ce catalysts (precursor: Pt(NH3)4Cl2): (a) Ce(1), (b) Pt+(1), (c) Pt+Ce(1-1), (d) Pt+Ce(1-3), and (e) Pt+Ce(1-5).

Figure 1. X-ray powder diffraction patterns of prepared catalysts: (a) Pt+(1), (b) Pt+Ce(1-1), (c) Pt+Ce(1-3), (d) Pt+Ce(15), (e) Pt-(1), (f) Pt-Ce(1-1), (g) Pt-Ce(1-3), (h) Pt-Ce(1-5); (b) PtO2, (1) CeO2, and (9) Al2O3.

affected the Pt dispersion. In case of Pt+(1), a 1 wt % addition of Ce increased the Pt dispersion and a 3 or 5 wt % addition of Ce slightly decreased the Pt dispersion. The change of Pt dispersion might be attributed to the interaction of Pt and Ce or Ce and Al2O3. Figure 1 shows the XRD patterns for the prepared catalysts using Pt(NH3)4Cl2 (traces a-d) and H2PtCl6 (traces e-h) as Pt precursors, and they showed the similar XRD patterns. Pt appears to be well-dispersed on the γ-Al2O3 surface because no diffraction lines associated with Pt or PtO2 were detected. The formation of CeO2 crystallite phase was evident only for the highest (5 wt %) Ce loading, suggesting that Ce is largely dispersed as XRD-undetectable crystallites at a lower Ce loading (under 3 wt %) but agglomerated on Al2O3 surface at a higher Ce loading (5 wt %). The reducibility of the prepared catalysts was determined by TPR (Figure 2 and Figure 3). There was a broad peak at about 450 °C for Ce(1) (Figures 2 (trace a) and 3 (trace a)) which may be assigned to the removal of surface oxygen from CeOx surfaces. A peak at about 250 and 370 °C for the Pt+Ce catalysts (Figure 2 (traces b-e)) appeared to be associated with the reduction of dispersed PtOx. The peak at 370 °C appearing only for

Figure 3. TPR profiles of Pt-Ce catalysts (precursor: H2PtCl6): (a) Ce(1), (b) Pt-(1), (c) Pt-Ce(1-1), (d) Pt-Ce(1-3), and (e) PtCe(1-5).

the Pt+Ce samples could be from the reduction of bulk PtOx because there is no peak at 370 °C for the Pt-Ce samples. The TPR profile for Pt+Ce(1-1) catalyst was not a direct superimposition of those for Ce(1) and Pt+(1), indicating the existence of a bimetallic Pt and Ce interaction on the surface. Tiernan and Finlayson20 also reported the possible surface interaction between Pt and Ce on γ-Al2O3 support, particularly when Pt and Ce were coimpregnated on γ-Al2O3. Especially for higher Ce loading more than 8 wt %, the characteristic TPR peak due to the surface interaction between Pt and Ce was reported to appear even with Pt-Ce samples. Furthermore, Shyu and Otto22 reported that Pt interacted preferentially with Ce rather than Al2O3. Peaks at about 160 °C for Pt+Ce catalysts (Figure 2 (traces c-e)) were attributed to the combined reduction of Pt and Ce surface species with the main contribution from the Ce component because the peak intensity increased with Ce loading. The presence of Pt resulted in more facile surface Ce reduction, in agreement with a proposal that the presence of noble metals can increase the oxygen storage capacity of CeO2 because of noble metal-

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Figure 5. Initial reaction rate of wet oxidation of phenol at 5.05 MPa air over a Pt-Ce catalyst (b) or Pt+Ce catalyst (O).

Figure 4. Reduction of phenol concentration during wet oxidation at 170 °C and 5.05 MPa air over (a) Pt+Ce catalyst or (b) Pt-Ce catalyst: (b) blank, (9) Pt(1), (2) Pt-Ce(1-1), (1) Pt-Ce(1-3), and ([) Pt-Ce(1-5).

CeO2 interactions.23 In contrast to Pt+Ce catalysts, however, Pt-Ce catalysts with a lower loading of Ce (Figure 3 (traces b-c)) did not show the new peak induced by the surface interaction between Pt and Ce. Effect of Pt Precursors and Ce Addition on the Activity of Phenol Wet Oxidation. Figure 4 shows reduction of phenol concentration during wet oxidation of phenol over Pt+Ce or Pt-Ce catalysts at 170 °C and 5.05 MPa of air. Compared to the uncatalyzed (blank) run, there was a significant increase in reduction of phenol concentration over the prepared Pt+Ce or PtCe catalysts, as expected. The Pt-(1) catalyst showed a higher wet oxidation activity than the Pt+(1) catalyst. This kind of Pt precursor effect could be evidenced by the difference in Pt metal dispersion (i.e., 91% of Pt dispersion for the H2PtCl6 precursor versus 66% of Pt dispersion for the Pt(NH3)4Cl2 precursor). It is interesting to note that the addition of Ce increased the wet oxidation activity of the Pt+(1) catalysts using Pt(NH3)4Cl2 as a precursor (Figure 4a), while it inhibited the activity of Pt-(1) catalysts using H2PtCl6 as a precursor (Figure 4b). The promoting effect of Ce for Pt+Ce catalysts with a cationic Pt precursor was also reported for the wet oxidation of pulp mill waste.12 The reduc-

ibility of the Pt/Al2O3 catalyst was enhanced by additional loadings of Ce for Pt+Ce catalysts, as evidenced by TPR profiles (Figure 2). This must lead to the improvement in the redox properties of the catalysts and enhanced the wet oxidation activity. On the other hand, the inhibiting effect of Ce addition for the Pt-Ce catalysts using an anionic Pt precursor could not be explained in terms of redox properties only. The platinum dispersion of the Pt-(1) catalyst decreased with Ce addition, probably leading to a decrease in activity, while the Pt dispersion of the Pt+(1) catalyst did not change significantly, except for Pt+Ce(1-1), although Ce addition promoted the wet oxidation activity of Pt+(1) catalyst. Figure 5 shows that the initial reaction rates varied with Ce loadings. The initial reaction rate decreased with Ce addition in the case of Pt-(1) catalysts showing a minimum of reaction rate, while the initial reaction rate increased with Ce addition in the case of Pt+(1) catalysts showing the maximum rate. Figure 6 shows the similar effect of Ce addition on the conversion. As the reaction proceeded and the reaction temperature increased, the effect of Ce loadings seemed to disappear, as expected. The effects of Ce addition on phenol conversion were found to be more pronounced at lower temperatures, especially in the early stages of reaction. There should be an optimum loading of cerium. Ce seemed to be largely dispersed into XRD-undetectable crystallites at a lower Ce loading (under 3 wt %) so that a Ce-Al2O3 interaction may exist. However, at a higher Ce loading (5 wt %), Ce could cover the Al2O3 surface and agglomerate to the XRD-detectable crystallites; hence, the Ce-Al2O3 interaction might be weaker than that of lower Ce loaded sample. Even if the nature of nonphenolic byproducts depends on the catalyst, reaction temperature, and reaction time, they are largely low molecular weight carboxylic acids, hydroquinone, catechol, and so forth, according to the

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Figure 7. TPO/MS profiles for O2 consumption and CO2 evolution during the thermal incineration of fresh catalyst and used catalysts at 170 °C and 5.05 MPa air for 3 h.

Figure 6. Effects of reaction temperature and Ce loading on the conversion of phenol after 60 min of wet oxidation at 5.05 MPa air over (a) Pt+Ce catalyst or (b) Pt-Ce catalyst.

reaction pathway proposed by Devlin and Harris.24 The yield of nonphenolic byproducts estimated from the total organic carbon (TOC) was less than 10% after 180 min reaction at 170 °C, which is similar to the reports by Hamoudi et al.25 Carbonaceous Deposits During Reaction. TPO/ MS data of Pt+Ce catalysts illustrate the time evolution of CO2 formation and O2 consumption during the temperature-programmed burning off of the used catalysts (Figure 7). It can be seen that CO2 was detected in the used catalysts, indicating that there were carbonaceous deposits. The TPO/MS profile of Pt-Ce catalysts showed the same pattern as that of Pt+Ce catalysts. Pintar and Levec26 characterized the adsorbed polymer deposits on the used CuO/ZnO/Al2O3 catalyst for wet oxidation of phenol by means of 13C CPMAS NMR and assigned the spectra to a mixture of a copolymer of phenol and glyoxal and a polymer of glyoxal. Hamoudi and coworkers25,27 reported the existence of carbonaceous deposits on the used Pt/Al2O3 catalyst and Mn/Ce catalyst for the wet oxidation of phenol. The nature of carbonaceous deposit was proposed as aromatic-/graphitic-, aliphatic-, and oxygen-bearing carbon by XPS and S-SIMS, of which the composition depended on the reaction time, temperature, and the type of catalysts. FT-IR spectra of used Pt-Ce catalysts were obtained to investigate the chemical nature of carbon deposits

Figure 8. FT-IR spectra of used catalysts at 170 °C and 5.05 MPa air for 3 h: (a) Pt-(1) fresh catalyst, (b) Pt-(1), (c) Pt-Ce(1-1), (d) Pt-Ce(1-3), and (e) Pt-Ce(1-5).

(Figure 8). New peaks appeared for the spectra of used catalysts, although there were no such peaks for fresh catalysts. The spectra were similar in both Pt-Ce and Pt+Ce catalysts. The characteristic peaks were assigned as follows: a shoulder peak around 3100-3000 cm-1 as CH stretch from aromatics; peaks around 1625-1430 cm-1 as aromatic CdC stretches from phenyl; a peak around 1250-1025 cm-1 as CH in-plane bending from phenyl; and a shoulder peak around 3350-3250 cm-1 as OH stretch from alcohols or phenol.28 Therefore, the chemical species of the deposited carbon on the used catalysts seemed to be mainly aromatics, especially some phenolics. Acknowledgment This work was supported by National Research Laboratory Project and the Brain Korea 21 Project. Literature Cited (1) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2.

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(2) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic Abatement of Water Pollutants. Ind. Eng. Chem. Res. 1998, 37, 309. (3) Imamura, S.; Hirano, A.; Kawabata, N. Wet Oxidation of Acetic Acid-Catalyzed by Co-Bi Complex Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 570. (4) Imamura, S.; Doi, A.; Ishida, S. Wet oxidation of Ammonia Catalyzed by Cerium-Based Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 75. (5) Imamura, S.; Nakamura, M.; Kawabata, N.; Yoshida, J.; Ishida, S. Wet Oxidation of Poly(ethylene glycol) Catalyzed by Manganese-Cerium Composite Oxide. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 34. (6) Levec, J.; Pintar, A. Catalytic Oxidation of Aqueous Solutions of Organics. An Effective Method for Removal of Toxic Pollutants from Wastewaters. Catal. Today 1995, 24, 51. (7) Imamura, S.; Fukuda, I.; Ishida, S. Wet Oxidation Catalyzed by Ruthenium Supported on Cerium(IV) Oxides. Ind. Eng. Chem. Res. 1988, 27, 718. (8) Gallezot, P.; Laurain, N.; Isnard, P. Catalytic Wet-Air Oxidation of Carboxylic Acids on Carbon-Supported Platinum Catalysts. Appl. Catal. B. 1996, 9, L11. (9) Hamoudi, S.; Larachi, F.; Cerrella, G.; Cassanello, M. Wet Oxidation of Phenol Catalyzed by Unpromoted and PlatinumPromoted Manganese/Cerium Oxide. Ind. Eng. Chem. Res. 1998, 37, 3561. (10) Taguchi, J.; Okuhara, T. Selective Oxidative Decomposition of Ammonia in Neutral Water to Nitrogen over Titania-Supported Platinum or Palladium Catalyst. Appl. Catal. A. 2000, 194, 89. (11) Leitenburg, C.; Goi, D.; Primavera, A.; Trovarelli, A.; Dolcetti, G. Wet Oxidation of Acetic Acid Catalyzed by Doped Ceria. Appl. Catal. B. 1996, 11, L29. (12) Zhang, Q.; Chuang, K. T. Alumina-Supported Noble Metal Catalysts for Destructive Oxidation of Organic Pollutants in Effluent from a Softwood Kraft Pulp Mill. Ind. Eng. Chem. Res. 1998, 37, 3343. (13) Yao, H. C.; Gandhi, H. S.; Shelef, M. Interactions of Base and Noble Metals with Insulator Supports. Stud. Surf. Sci. Catal. 1982, 11, 159. (14) Yermakov, Y. I. Supported Catalysis Obtained by Interaction of Organometallic Compounds of Transition Elements with Oxide Supports. Catal. Rev.sSci. Eng. 1976, 13, 77. (15) Yermakov, Y. I.; Kuznetsov, B. N. Supported Metallic Catalysts Prepared by Decomposition of Surface Organometallic Complexes. J. Mol. Catal. 1980, 9, 13.

(16) Silver, R. G.; Summers, J. C.; Williamson, W. B. Design and Performance Evaluation of Automotive Emission Control Catalysts. Stud. Surf. Sci. Catal. 1991, 71, 167. (17) Diwell, A. F.; Rajaram, R. R.; Shaw, H. A.; Truex, T. J. The Role of Ceria in Three-Way Catalysts. Stud. Surf. Sci. Catal. 1991, 71, 139. (18) Gandhi, H. S.; Shelef, M. The Role of Research n the Development of New Generation Automotive Catalysts. Stud. Surf. Sci. Catal. 1987, 30, 199. (19) Murrel, L. L.; Tauster, S. J.; Anderson, D. R. Laser Raman Characterization of Surface Phase Precious Metal Oxides Formed on CeO2. Stud. Surf. Sci. Catal. 1991, 71, 275. (20) Tiernan, M. J.; Finlayson, O. E. Effect of Ceria on the Combustion Activity and Surface Properties of Pt/Al2O3 Catalysts. Appl. Catal. B. 1998, 19, 23. (21) Summers, J. C.; Ausen, S. A. Interaction of Cerium Oxide with Noble Metals. J. Catal. 1979, 58, 131. (22) Shyu, J. Z.; Otto, K. Characterization of Pt/γ-Alumina Catalysts Containing Ceria. J. Catal. 1989, 115, 16. (23) Yao, H. C.; Yu-Yao, Y. F. Ceria in Automotive Exhaust Catalysts. J. Catal. 1984, 86, 254. (24) Devlin, H. R.; Harris, I. J. Mechanism of the Oxidation of Aqueous Phenol with Dissolved Oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387. (25) Hamoudi, S.; Larachi, F.; Sayari, A. Wet Oxidation of Phenolic Solutions over Heterogeneous Catalysts: Degradation Profile and Catalyst Behavior. J. Catal. 1998, 177, 247. (26) Pintar, A.; Levec, J. Catalytic Oxidation of Organics in Aqueous Solutions I. Kinetics of Phenol Oxidaion. J. Catal. 1992, 135, 345. (27) Hamoudi, S.; Larachi, F.; Adnot, A.; Sayari, A. Characterization of Spent MnO2/CeO2 Wet Oxidation Catalyst by TPO/ MS, XPS, and S-SIMS. J. Catal. 1999, 185, 333. (28) Lambert, J. B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G. Introduction to Organic Spectroscopy; Macmillan: New York, 1987.

Received for review July 10, 2001 Revised manuscript received November 12, 2001 Accepted February 1, 2002 IE010590P