Wet Oxidation of Phenol Catalyzed by Unpromoted and Platinum

Ind. Eng. Chem. Res. 1998, 37, 3561-3566

3561

Wet Oxidation of Phenol Catalyzed by Unpromoted and Platinum-Promoted Manganese/Cerium Oxide Safia Hamoudi and Faı1c¸ al Larachi* Department of Chemical Engineering and CERPIC, Laval University, Que´ bec, Canada G1K 7P4

Graciela Cerrella and Myrian Cassanello† PINMATE, Department Industrias FCEyN, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina

Manganese/cerium composite oxide (MnO2/CeO2) proved to be a potent solid catalyst for the catalytic wet oxidation (CWO) of aqueous phenol under mild treatment conditions (80-130 °C, 0.5 MPa O2 pressure). Despite the fact this catalyst exhibited an important activity in eliminating completely phenol and total organic carbon (TOC), its poor selectivity to CO2 was demonstrated as a result of deposition on the catalyst surface of carbonaceous deposits with high carbon content. Promotion of MnO2/CeO2 with platinum improved the CO2 yield of phenol oxidation. Evidence for the reduction of the carbonaceous deposit over Pt-MnO2/CeO2 was confirmed via elemental analysis and temperature-programmed oxidation (TPO). Surface analysis by X-ray photoelectron spectroscopy (XPS) indicated that the deposits built up more on cerium than on manganese and that platinum increased the aromaticity of the deposit. Introduction Catalytic wet oxidation (CWO) is a subcritical aqueous-phase abatement method that uses dissolved molecular oxygen to destroy catalytically target organic pollutants contained in wastewater streams. Ideally, provided that temperature and pressure are sufficiently high, any oxidizable CHO-containing pollutant molecule is transformed into harmless CO2 and water. Incorporation of solid catalysts not only allows one to accomplish oxidative routes with lenient severity (90-150 °C; 0.1-2 MPa O2)1 but also offers a versatile process wherein the catalyst, unlike a homogeneous one, may be easily recoverable and eventually reusable. CWO involving solid catalysts has therefore attracted attention as an alternate method for purifying wastewaters, and various solid catalysts have been tested on model pollutant solutions.2-5 During the 1980s, prospective work and screening tests on CWO solid catalysts identified MnO2/CeO2 as a potential deep oxidation catalyst for the removal of several refractory organic compounds dissolved in wastewater.6,7 More specifically, past work on CWO of aqueous phenolic solutions catalyzed by MnO2/CeO2 demonstrated the remarkable activity of this catalyst to achieve complete destruction of phenol and phenol intermediates [i.e., total organic carbon (TOC)] at low temperature within a few minutes.7-9 Notwithstanding, whether the removal of phenol and its dissolved intermediates over MnO2/CeO2 translated into 100% selectivity to CO2 has not been thoroughly examined in the literature. As a matter of fact, the carbon balance over liquid, solid, and gas phases was scarcely reported to confirm the TOC deep mineralization in CWO reactions.4 Furthermore, in their recent review MatatovMeytal and Sheintuch5 recognized only two types of * Corresponding author. Phone: (418)-656-3566. Fax: (418)656-5993. E-mail: [email protected]. † Phone: (54-1)-781-5021/29, ext. 360. E-mail: miryan@ ferbat.uba.ar.

deactivation of CWO solid catalysts: elution of active metal from catalyst and poisoning due to trace contaminants, such as halogen-containing compounds, formed during CWO. Fouling deactivation due to surface deposition and strong adsorption of a polymeric carbonaceous overlayer is another kind of deactivation that was poorly documented in the case of phenol CWO catalyzed by MnO2/CeO2. As shown in the present work, comparative surface characterizations of fresh and used MnO2/CeO2 catalyst revealed that, after complete TOC abatement, an important fraction of the initial carbon belonged to a polymeric product adsorbed on the catalyst (yield of CO2 < 100%). On the other hand, CWO of phenol in the presence of Pt/Al2O3 catalyst led to a lesser amount of deposit; however, the weak activity of this catalyst required excessively longer reaction times and higher temperatures.10 Platinum very likely catalyzes the C-C bond rupture of the organic molecule, while it simultaneously prevents extensive polymerization of radicals. Promotion of MnO2/CeO2 with platinum in order to reduce the extent of deposits was therefore explored in this work. Elimination or minimization of deposits along with preservation of sufficient degradation rates of phenol and TOC at mild conditions would be gainful at several levels, e.g., accomplishment of (i) higher CO2 yield, (ii) shorter residence times and smaller CWO reactor volumes, and (iii) eventually longer cycle life of catalyst due to a better activity. This study was therefore intended to explore the impact of Pt promotion of MnO2/CeO2 catalyst on phenol removal and on the improvement of CO2 yield of the CWO reaction. The effect of platinum promotion was demonstrated through catalytic tests and various catalyst characterization techniques. Experimental Section Phenol (99+% purity) was purchased from BDH Co. and was used without further purification. Purified

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3562 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998

acetic anhydride and pyridine used for phenol esterification were analytical-grade reagents. Naphthalene used as an internal standard in GC analyses was supplied by Fisher Scientific Co.. The MnO2/CeO2 catalyst (molar ratio ) 7/3) was synthesized by coprecipitation of MnCl2 (Fisher Scientific Co.) and CeCl3 (Sigma Chemical Co.).6 After precipitation, the mixture was filtered, washed, and dried overnight at 100 °C. Then it was calcined under flowing air at 350 °C for 3 h. Platinum was impregnated on MnO2/CeO2 according to the incipient wetness method using an aqueous solution of H2PtCl6 (Aldrich Chemical Co.). The final platinum content was kept at 1 wt %. After impregnation, the catalyst was calcined in an air flow at 350 °C, cooled at room temperature, and then submitted to flowing hydrogen (30 mL/min) at 250 °C for 2 h to reduce platinum to its metallic state. Fresh and used catalysts were characterized with respect to their adsorption isotherms and BET specific surface area using N2 physical adsorption at 77 K on a Micromeritics Gemini 2360 sorption instrument. Fractal surface characterization of the two catalysts was also attempted by determining the surface fractal dimension and the effect of CWO conditions on it. The surface fractal dimension varies from 2 for very smooth surfaces up to 3 for very rough surfaces that tend to fill the space. The Brunauer-Emmett-Teller (BET) equation of adsorption isotherms, modified to account for the effect of surface roughness, was used,11,12 and the fractal dimension for each sample was the fitting parameter. The carbon content of the carbonaceous deposits on the catalyst surface was quantified by CHN elemental analysis (Carlo Erba, Model 1106). Burnoff profiles of these deposits were obtained by temperature-programmed oxidation (TPO) using an Altamira AMI1 instrument. In a typical TPO experiment, 60-100 mg of dehydrated samples were loaded in a U-shaped quartz microreactor. Dilute oxygen stream [5% (v/v) O2/ He] at a constant flow rate of 30 cm3/min was used, and the sample temperature was increased from room temperature to 610 °C at 8 °C/min heating rate. Analysis of the microreactor outlet gas was performed by thermal conductivity detection. Catalyst texture and morphology were examined at different scales/magnifications by scanning electron microscopy (SEM) on a 515 Philips microscope. X-ray photoelectron spectroscopy (XPS) spectra for the fresh and used catalyst samples were recorded using a V.G. Scientific Escalab Mark II system. A nonmonochromatized Mg KR (hν ) 1253.6 eV) was used as X-ray source for all samples. Survey and detailed spectra were acquired at channel widths of 1.0 and 0.1 eV, respectively. Binding energy (BE) correction due to sample charging was done by referencing Mn 2p3/2 core level in MnO2 (BE ) 642.2 eV). Phenol was oxidized in a 300 mL stainless steel highpressure Parr agitated autoclave reactor (model 4842, Parr Instruments, Inc.) in the temperature range 80130 °C, a catalyst loading of 5 g/L, and at 0.5 MPa O2 pressure. This partial pressure corresponded to an oxygen/phenol stoichiometric ratio of 6-7 (assuming that all phenol was transformed into carbon dioxide and water) far in excess of the oxidation requirement. The autoclave was charged with 100 mL of pure water. It was equipped with a reagent injection device connected to a secondary oxygen inlet allowing addition of phenol after the system had equilibrated to reaction conditions.

At preset reaction times, aliquots of the solution were withdrawn and analyzed for (i) total organic carbon (TOC) using a combustion/nondispersive infrared gas analyzer (Shimadzu 5050 TOC analyzer) and (ii) residual phenol concentration whose derivatized ester was analyzed on a Hewlett-Packard GC (HP5890 series II plus) equipped with a mass-selective detector (MSD model HP5972). For gas chromatography analyses, the sample volume injected was 1 µL and naphthalene dissolved in ethyl acetate was used as an internal standard. A HP-5MS 30 m × 0.25 mm i.d. capillary column was used in temperature-programmed mode and helium carrier (ultrahigh purity) at a flow rate of 1 mL/ min was the sweeping gas. The temperatures of the injector and GC-MSD interface were 250 and 280 °C, respectively. The oven temperature was held at 50 °C for the first 2 min and then raised to 120 °C at a rate of 5 °C/min. The stability of the MnO2/CeO2 catalyst to leaching of active metal ingredients was verified by analyzing filtered solution samples after complete TOC conversion, i.e., 30 min at 130 °C. The concentrations of dissolved Mn and Ce were measured by plasma emission spectrometry using an Optima 3000 spectrometer from Perkin-Elmer. Analysis Catalytic performances of fresh and spent (aged without regeneration) MnO2/CeO2 and 1% Pt-MnO2/ CeO2 during the CWO of phenol at 130 °C are shown in Figure 1. For the fresh catalysts, complete removal of phenol was achieved within 30 min (Figure 1a) while virtually no dissolved phenol byproducts remained in solution (TOC conversion > 98%). Although the two catalysts exhibited comparable performances in eliminating phenol, more intermediates persisted in the case of 1% Pt-MnO2/CeO2 as indicated by the slower dieout rate of the TOC profile (Figure 1b). It is worth mentioning that oxidation tests (not shown here) were run between 80 and 130 °C with no catalysts to evaluate the importance of the noncatalytic homogeneous wet oxidation. Less than 10% of phenol and TOC were degraded in 30 min to conclude that the contribution of the noncatalytic wet oxidation was marginal.10 The stability and the efficacy of the two catalysts to destroy TOC was evaluated by running additional consecutive oxidation tests after catalyst recycling but without regeneration (dotted lines in Figure 1b). Despite the fact that the catalyst degraded phenol and TOC within a reasonable time interval, a loss of catalyst activity was noticed (Figure 1b). This loss of activity was more visible in the case of the Pt-promoted catalyst. No elution of manganese and cerium was detectable for the CWO conditions of this work, precluding therefore a leaching type of deactivation.13 In fact, the concentration of dissolved Mn was 10 ppm after complete conversion of TOC at 130 °C, which represents less than 0.5% of the total manganese present in the fresh catalyst. In the case of Ce, the concentration of dissolved ions was below 0.2 ppm, confirming the negligible extent of catalyst leaching in the reaction medium. Also, poisoning type of deactivation was unlikely to occur since phenol is an S-, P-, and X-free molecule and these are the heteroelements known to be poisonous to oxidation catalysts.5 Fouling deactivation due to surface deposition and strong adsorption of solid organic species

Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3563

Figure 1. Phenol (a) and TOC (b) conversions at 130 °C and 0.5 MPa, over MnO2/CeO2 and 1%Pt-MnO2/CeO2 catalysts (initial phenol concentration 1 g/L, catalyst loading 5 g/L). Solid lines, fresh catalysts; dotted lines, second consecutive oxidation run without catalyst regeneration.

was the deactivation scenario that most likely occurred, which might explain the loss in activity. The fate of carbon that was removed from the solution was tracked by monitoring the carbon content of both catalysts. The gaseous carbon (assumed to be CO2) was deduced by performing a carbon balance between the initial TOC0, the running TOC, and the carbon belonging to the deposit. Typical time profiles of dissolved carbon (i.e., TOC), deposited carbon (CS), and CO2, as well as CO2 yield, are shown in Figure 2 for MnO2/CeO2 and 1%Pt-MnO2/CeO2. The CWO yield of CO2 is defined on a carbon basis:

YG ) 1 -

[CS] TOC0 - TOC

In the case of MnO2/CeO2, only 22% of carbon was converted into CO2 after completion of the CWO reaction. Under likewise conditions, the CO2 yield attained with the noble-metal-promoted catalyst was better (36%), even though the phenol and TOC degradation rates were slightly slower than with the unpromoted catalyst (Figure 1a,b). In both cases, restoration of catalyst activity can be easily obtained by burning out these carbon-containing deposits. BET surface areas of MnO2/CeO2 and 1%Pt-MnO2/ CeO2 catalysts are summarized in Table 1 for different temperatures, TOC conversions, and phenol initial concentrations. There was a notable reduction in surface area, from 107 m2/g for the fresh MnO2/CeO2

Figure 2. TOC, solid carbon, and carbon dioxide time profiles at 130 °C and 0.5 MPa, over MnO2/CeO2 (a), 1%Pt-MnO2/CeO2 catalysts (b), and carbon dioxide yield (c) (initial phenol concentration 1 g/L, catalyst loading 5 g/L, fresh catalysts only). Lines are here to show trends.

catalyst to 95 m2/g after TOC was almost completely removed at 80 °C. As TOC conversion improved, a significant increase in the fractal dimensions was noted for the used catalysts with respect to those of the fresh ones. It is worth pinpointing the dramatic decrease in the MnO2/CeO2 surface area from 107 to 3.5 m2/g observed with a deliberately high phenol initial concentration (7.5 g/L). Hence, an increase in the fractal dimension and a decrease in the BET surface area can be correlated to the amount of carbonaceous deposits that built up on the catalyst surface. From SEM photographs, the morphology of the two catalysts consisted of agglomerates that appeared larger for the Pt-promoted one. Higher magnification showed that these agglomerates had rough surfaces composed of small irregularly shaped bulges (Figure 3a,b). The microscopic structure of the used catalysts was differ-

3564 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 Table 1. BET Surface Area, Fractal Dimension, and Surface C, Mn, Ce Atom % for Fresh and Used Catalysts catalyst MnO2/CeO2a MnO2/CeO2

Pt-MnO2/CeO2a Pt-MnO2/CeO2

a

temperature (°C) (reaction time)

TOC conv (%)

fractal dimension

SBET (m2/g)

130 (30 min) 130 (120 minb) 80 (15 min) 80 (45 min) 80 (90 min)

98.8 61.8 27.9 63.1 91.5

2.46 2.6 2.71 2.56 2.64 2.69

107 106.6 3.5

130 (15 min) 80 (15 min) 80 (45 min) 80 (90 min)

91.2 22.5 48.6 78.8

2.46 2.71 2.53 2.59 2.7

88 83.2

104 95

C

Mn

Ce

C/(Mn + Ce)

Mn/Ce

2.511 43.682

35.201 12.401

9.429 2.196

0.06 2.99

3.7 5.6

36.972 52.004 56.628

12.078 8.899 7.857

7.076 4.595 3.953

1.93 3.85 4.79

1.7 1.9 2.0

88 83

Fresh. b Phenol initial concentration ) 7.5 g/L.

Figure 3. SEM photographs showing fresh MnO2/CeO2 (a), fresh 1%Pt-MnO2/CeO2 (b), used MnO2/CeO2 (c), and used 1%Pt-MnO2/ CeO2 (d): evidence for organic deposits.

ently affected by the carbonaceous deposits depending on whether the catalyst was promoted or not (Figure 3c,d). The MnO2/CeO2 sample taken after 30 min of CWO reaction indicated a uniform cover-up of the grain by the carbonaceous deposit and larger bulges. For the used 1%Pt-MnO2/CeO2 (after 10 min of reaction), despite the fact that a lot of bulges were embedded in the carbonaceous deposit, some of them, having the same average size as for the fresh sample, remained visible and thus active. In the Pt-promoted catalyst, more oxidation sites were left accessible to the reactants to pursue the oxidation reaction. Figure 4 shows the burnoff profiles of the two catalysts at their fresh state and after complete removal of

TOC at 130 °C. Oxygen uptakes in TPO profiles confirmed that less carbonaceous deposits were formed over 1%Pt-MnO2/CeO2 than over MnO2/CeO2, which agreed with the results in Figure 2. TPO profiles showed two distinct combustion peaks: a sharp peak between 200 and 220 °C and a broad peak with a maximum between 250 and 280 °C. This may suggest either that there were two kinds of carbon deposits, with one burning very quickly, or that the deposits located on manganese and cerium oxides burnt at different temperatures. These deposits were easily burnt below 300 °C, presumably because their combustion was catalyzed by the oxides. Oxygen uptake of the fresh catalyst samples occurred above 500 °C, far beyond the

Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3565

Figure 4. TPO profiles of fresh and used MnO2/CeO2 and 1%PtMnO2/CeO2 catalysts. Oxygen uptake is expressed in micromoles per gram of catalyst.

burnoff range of the deposits to assume that O2 uptake of used catalysts came mainly from the combustion of the carbonaceous overlayer. Occurrence of a carbonaceous overlayer on the catalyst surface was also confirmed by the XPS data as illustrated in Table 1 for MnO2/CeO2 catalyst. At almost complete TOC conversion, carbon coverage increased as the oxidation temperature was decreased. Surface carbon increased, and manganese and cerium atomic fractions decreased as the TOC conversion increased. C/(Mn + Ce) atomic ratio increased sharply as the carbonaceous deposit built up on the catalyst surface. The Mn/Ce atomic ratio leveled off at temperature-dependent plateau values. The higher the temperature, the higher the Mn/Ce ratio. Hence, XPS data suggest that the carbonaceous solid material was deposited preferentially on cerium rather than on manganese. The detailed high-resolution C1s region spectra revealed a number of overlapping features corresponding to different chemical natures of carbon: mainly aromatic, aliphatic, and partially oxidized. The aromaticity of the deposit, as measured by the sp2 hybridized carbon and the plasmon loss feature,14 was sensitive to the presence of Pt on the MnO2/CeO2. The decomposition of the C1s region into individual line components was performed using reported binding energies for the C1s core level;15,16 see Figure 5. For MnO2/CeO2 catalyst, the C1s region consisted of three main components (aromatic C, 19%; aliphatic C, 51%; alcohol-ether C, 15%) as shown in Table 2. For 1%Pt-MnO2/CeO2 catalyst, the C1s region was contributed by the same types of carbon but with a different distribution for aromatic and aliphatic carbons (Table 2). Aromaticity (sp2, 38%) of the deposit formed on the platinumpromoted catalyst was higher than for the unpromoted MnO2/CeO2 catalyst. This would explain why the former catalyst, even though more selective to CO2 formation, was (i) less efficient than the latter to oxidize phenol and its intermediates and (ii) more sensitive to deactivation as shown by the recycled nonregenerated catalysts (Figure 1b). Carbon species at BE < 284 eV were previously assigned to carbidic carbon.16,17 In this study, the carbidic carbon shoulder appeared at a BE ) 282.9 eV and contributed for ca. 2% of the overall peak (Table 2). Also, the plasmon loss contribution was low and accounted for 7-8% of the overall C1s peak.

Figure 5. XPS C1s high-resolution spectra and peak line-fitting of functional groups in the deposits of MnO2/CeO2 (a) and 1%PtMnO2/CeO2 (b) catalysts after 5 min of reaction at 130 °C and 0.5 MPa. Table 2. Position, Assignments, and Peak Areas of the C1s Peaksa area (%) peak no.

binding energy (eV)

1 2 3 4 5 6 7

282.9 284.6 285 286.1 287.6 289.1 291.2

a

assignment

MnO2/ CeO2

Pt-MnO2/ CeO2

carbide aromatics aliphatics, β-carbons C-OH, C-O-C CdO COOH, COOR plasmon loss

18.6 50.8 15.4 3.1 4.1 8.0

2.0 38.2 27.5 19.4 3.0 2.9 7.1

5 min at 130 °C, 0.5 MPa.

Concluding Remarks Dissolved phenol and phenol intermediates were fully oxidized in the presence of MnO2/CeO2 and 1%PtMnO2/CeO2 catalysts at mild conditions. Pt promotion of MnO2/CeO2 reduced the amount of carbonaceous deposits and improved phenol deep oxidation (higher CO2 yield). However, platinum-promoted MnO2/CeO2 was more sensitive to deactivation and exhibited systematically slower degradation rates of phenol and TOC. Current work is focusing on the optimization of Pt loading and dispersion, as well as on the bimetallic copromotion of MnO2/CeO2 in order to further maximize CO2 yield.

3566 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998

Acknowledgment Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Fonds pour la formation de chercheurs et d’aide a` la recherche are gratefully acknowledged. Authors from PINMATE acknowledge support from Universidad de Buenos Aires and CONICET. We also acknowledge Dr. A. Adnot for the XPS analyses and Prof. A. Sayari for lending us the Altamira instrument. Literature Cited (1) Levec, J. Wet Oxidation Processes for Treating Industrial Wastewaters. Chem. Biochem. Eng. Q. 1997, 11, 47. (2) Sadana, A.; Katzer, J. R. Involvement of Free Radicals in the Aqueous-Phase Catalytic Oxidation of Phenol over Copper Oxide. J. Catal. 1974, 35, 140. (3) Ohta, H.; Goto, S.; Teshima, H. Liquid-Phase Oxidation of Phenol in a Rotating Catalytic Basket Reactor. Ind. Eng. Chem. Fundam. 1980, 19, 180. (4) Pintar, A.; Levec, J. Catalytic Oxidation of Organics in Aqueous Solutions. I. Kinetics of Phenol Oxidation. J. Catal. 1992, 135, 345. (5) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic Abatement of Water Pollutants. Ind. Eng. Chem. Res. 1998, 37, 309. (6) 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. (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) Imamura, S.; Dol, A.; Ishida, S. Wet Oxidation of Ammonia Catalyzed by Cerium-Based Composite Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 75.

(9) Duprez, D.; Delanoe, F.; Barbier, Jr.; Isnard, P.; Blanchard, G. Catalytic Oxidation of Organic Compounds in Aqueous Media. Catal. Today. 1996, 29, 317. (10) Hamoudi, S.; Larachi, F.; Sayari, A. Wet Oxidation of Phenolic Solutions over Heterogeneous Catalysts: Degradation Profile and Catalyst Behavior. J. Catal. 1998 (in press). (11) Fripiat, J. J.; Gatineau, L.; van Damme, H. Multilayer Physical Adsorption on Fractal Surfaces. Langmuir 1986, 2, 562. (12) Pfeifer, P.; Obert, M.; Cole, M. W. Fractal BET and FHH Theories of Adsorption: A Comparative Study. Proc. R. Soc. London A 1989, 423, 169. (13) Ding, Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 3257. (14) Kelemen, S. R.; Rose, K. D.; Kwiatek, P. J. Carbon Aromaticity Based on XPS Π to Π* Signal Intensity. Appl. Surf. Sci. 1993, 64, 167. (15) Desimoni, E.; Casella, G. I.; Morone, A. Salvi, A. M. XPS Determination of Oxygen-Containing Functional Groups on CarbonFibre Surfaces and the Cleaning of these Surfaces. Surf. Interface. Anal. 1990, 15, 627. (16) Paål, Z.; Schlo¨gl, R.; Ertl, G. The Surface State and Catalytic Properties of Pt Black after O2-H2 Cycles. Catal. Lett. 1992, 12, 331. (17) Levis, R. J.; DeLouise, L. A.; White, E. J.; Winograd, N. Defect Induced Surface Chemistry: a Comparison of the Adsorption and Thermal Decomposition of C2H4 on Rh {111} and Rh {331}. Surf. Sci. 1990, 230, 35.

Received for review February 10, 1998 Revised manuscript received May 18, 1998 Accepted June 8, 1998 IE980081W