Wet Oxidation of Phenol Catalyzed by Unpromoted andPlatinum-Promoted Manganese/Cerium Oxide

Safia Hamoudi and Faı1cal Larachi*

Department of Chemical Engineering and CERPIC, Laval University, Quebec, 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 thecatalytic 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 eliminatingcompletely phenol and total organic carbon (TOC), its poor selectivity to CO2 was demonstratedas 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. Evidencefor the reduction of the carbonaceous deposit over Pt-MnO2/CeO2 was confirmed via elementalanalysis and temperature-programmed oxidation (TPO). Surface analysis by X-ray photoelectronspectroscopy (XPS) indicated that the deposits built up more on cerium than on manganese andthat platinum increased the aromaticity of the deposit.

Introduction

Catalytic wet oxidation (CWO) is a subcritical aque-ous-phase abatement method that uses dissolved mo-lecular oxygen to destroy catalytically target organicpollutants contained in wastewater streams. Ideally,provided that temperature and pressure are sufficientlyhigh, any oxidizable CHO-containing pollutant moleculeis transformed into harmless CO2 and water. Incorpo-ration of solid catalysts not only allows one to ac-complish oxidative routes with lenient severity (90-150°C; 0.1-2 MPa O2)1 but also offers a versatile processwherein the catalyst, unlike a homogeneous one, maybe easily recoverable and eventually reusable. CWOinvolving solid catalysts has therefore attracted atten-tion as an alternate method for purifying wastewaters,and various solid catalysts have been tested on modelpollutant solutions.2-5

During the 1980s, prospective work and screeningtests on CWO solid catalysts identified MnO2/CeO2 asa potential deep oxidation catalyst for the removal ofseveral refractory organic compounds dissolved in waste-water.6,7 More specifically, past work on CWO ofaqueous phenolic solutions catalyzed by MnO2/CeO2demonstrated the remarkable activity of this catalystto achieve complete destruction of phenol and phenolintermediates [i.e., total organic carbon (TOC)] at lowtemperature within a few minutes.7-9 Notwithstand-ing, whether the removal of phenol and its dissolvedintermediates over MnO2/CeO2 translated into 100%selectivity to CO2 has not been thoroughly examined inthe literature. As a matter of fact, the carbon balanceover liquid, solid, and gas phases was scarcely reportedto confirm the TOC deep mineralization in CWO reac-tions.4 Furthermore, in their recent review Matatov-Meytal and Sheintuch5 recognized only two types of

deactivation of CWO solid catalysts: elution of activemetal from catalyst and poisoning due to trace contami-nants, such as halogen-containing compounds, formedduring CWO. Fouling deactivation due to surfacedeposition and strong adsorption of a polymeric carbon-aceous overlayer is another kind of deactivation thatwas poorly documented in the case of phenol CWOcatalyzed by MnO2/CeO2.

As shown in the present work, comparative surfacecharacterizations of fresh and used MnO2/CeO2 catalystrevealed that, after complete TOC abatement, an im-portant fraction of the initial carbon belonged to apolymeric product adsorbed on the catalyst (yield of CO2< 100%). On the other hand, CWO of phenol in thepresence of Pt/Al2O3 catalyst led to a lesser amount ofdeposit; however, the weak activity of this catalystrequired excessively longer reaction times and highertemperatures.10 Platinum very likely catalyzes the C-Cbond rupture of the organic molecule, while it simulta-neously prevents extensive polymerization of radicals.Promotion of MnO2/CeO2 with platinum in order toreduce the extent of deposits was therefore explored inthis work. Elimination or minimization of depositsalong with preservation of sufficient degradation ratesof phenol and TOC at mild conditions would be gainfulat several levels, e.g., accomplishment of (i) higher CO2yield, (ii) shorter residence times and smaller CWOreactor volumes, and (iii) eventually longer cycle life ofcatalyst due to a better activity.

This study was therefore intended to explore theimpact of Pt promotion of MnO2/CeO2 catalyst on phenolremoval and on the improvement of CO2 yield of theCWO reaction. The effect of platinum promotion wasdemonstrated through catalytic tests and various cata-lyst characterization techniques.

Experimental Section

Phenol (99+% purity) was purchased from BDH Co.and was used without further purification. Purified

* Corresponding author. Phone: (418)-656-3566. Fax: (418)-656-5993. E-mail: flarachi@gch.ulaval.ca.

† Phone: (54-1)-781-5021/29, ext. 360. E-mail: miryan@ferbat.uba.ar.

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

S0888-5885(98)00081-5 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 08/08/1998

acetic anhydride and pyridine used for phenol esterifi-cation were analytical-grade reagents. Naphthaleneused as an internal standard in GC analyses wassupplied by Fisher Scientific Co..

The MnO2/CeO2 catalyst (molar ratio ) 7/3) wassynthesized by coprecipitation of MnCl2 (Fisher Scien-tific Co.) and CeCl3 (Sigma Chemical Co.).6 Afterprecipitation, the mixture was filtered, washed, anddried overnight at 100 °C. Then it was calcined underflowing air at 350 °C for 3 h. Platinum was impreg-nated on MnO2/CeO2 according to the incipient wetnessmethod using an aqueous solution of H2PtCl6 (AldrichChemical Co.). The final platinum content was kept at1 wt %. After impregnation, the catalyst was calcinedin 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 withrespect to their adsorption isotherms and BET specificsurface area using N2 physical adsorption at 77 K on aMicromeritics Gemini 2360 sorption instrument. Frac-tal surface characterization of the two catalysts was alsoattempted by determining the surface fractal dimensionand the effect of CWO conditions on it. The surfacefractal dimension varies from 2 for very smooth surfacesup to 3 for very rough surfaces that tend to fill the space.The Brunauer-Emmett-Teller (BET) equation of ad-sorption isotherms, modified to account for the effect ofsurface roughness, was used,11,12 and the fractal dimen-sion for each sample was the fitting parameter.

The carbon content of the carbonaceous deposits onthe catalyst surface was quantified by CHN elementalanalysis (Carlo Erba, Model 1106). Burnoff profiles ofthese deposits were obtained by temperature-pro-grammed oxidation (TPO) using an Altamira AMI1instrument. In a typical TPO experiment, 60-100 mgof dehydrated samples were loaded in a U-shapedquartz microreactor. Dilute oxygen stream [5% (v/v) O2/He] at a constant flow rate of 30 cm3/min was used, andthe sample temperature was increased from roomtemperature to 610 °C at 8 °C/min heating rate.Analysis of the microreactor outlet gas was performedby thermal conductivity detection. Catalyst texture andmorphology were examined at different scales/magni-fications by scanning electron microscopy (SEM) on a515 Philips microscope. X-ray photoelectron spectros-copy (XPS) spectra for the fresh and used catalystsamples were recorded using a V.G. Scientific EscalabMark 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 channelwidths of 1.0 and 0.1 eV, respectively. Binding energy(BE) correction due to sample charging was done byreferencing Mn 2p3/2 core level in MnO2 (BE ) 642.2eV).

Phenol was oxidized in a 300 mL stainless steel high-pressure Parr agitated autoclave reactor (model 4842,Parr Instruments, Inc.) in the temperature range 80-130 °C, a catalyst loading of 5 g/L, and at 0.5 MPa O2pressure. This partial pressure corresponded to anoxygen/phenol stoichiometric ratio of 6-7 (assumingthat all phenol was transformed into carbon dioxide andwater) far in excess of the oxidation requirement. Theautoclave was charged with 100 mL of pure water. Itwas equipped with a reagent injection device connectedto a secondary oxygen inlet allowing addition of phenolafter the system had equilibrated to reaction conditions.

At preset reaction times, aliquots of the solution werewithdrawn and analyzed for (i) total organic carbon(TOC) using a combustion/nondispersive infrared gasanalyzer (Shimadzu 5050 TOC analyzer) and (ii) re-sidual phenol concentration whose derivatized ester wasanalyzed on a Hewlett-Packard GC (HP5890 series IIplus) equipped with a mass-selective detector (MSDmodel HP5972). For gas chromatography analyses, thesample volume injected was 1 µL and naphthalenedissolved in ethyl acetate was used as an internalstandard. A HP-5MS 30 m × 0.25 mm i.d. capillarycolumn was used in temperature-programmed mode andhelium carrier (ultrahigh purity) at a flow rate of 1 mL/min was the sweeping gas. The temperatures of theinjector and GC-MSD interface were 250 and 280 °C,respectively. The oven temperature was held at 50 °Cfor the first 2 min and then raised to 120 °C at a rate of5 °C/min.

The stability of the MnO2/CeO2 catalyst to leachingof active metal ingredients was verified by analyzingfiltered solution samples after complete TOC conversion,i.e., 30 min at 130 °C. The concentrations of dissolvedMn and Ce were measured by plasma emission spec-trometry using an Optima 3000 spectrometer fromPerkin-Elmer.

Analysis

Catalytic performances of fresh and spent (agedwithout regeneration) MnO2/CeO2 and 1% Pt-MnO2/CeO2 during the CWO of phenol at 130 °C are shown inFigure 1. For the fresh catalysts, complete removal ofphenol was achieved within 30 min (Figure 1a) whilevirtually no dissolved phenol byproducts remained insolution (TOC conversion > 98%). Although the twocatalysts exhibited comparable performances in elimi-nating phenol, more intermediates persisted in the caseof 1% Pt-MnO2/CeO2 as indicated by the slower die-out rate of the TOC profile (Figure 1b). It is worthmentioning that oxidation tests (not shown here) wererun between 80 and 130 °C with no catalysts to evaluatethe importance of the noncatalytic homogeneous wetoxidation. Less than 10% of phenol and TOC weredegraded in 30 min to conclude that the contribution ofthe noncatalytic wet oxidation was marginal.10

The stability and the efficacy of the two catalysts todestroy TOC was evaluated by running additionalconsecutive oxidation tests after catalyst recycling butwithout regeneration (dotted lines in Figure 1b). De-spite the fact that the catalyst degraded phenol andTOC within a reasonable time interval, a loss of catalystactivity was noticed (Figure 1b). This loss of activitywas more visible in the case of the Pt-promoted catalyst.No elution of manganese and cerium was detectable forthe CWO conditions of this work, precluding thereforea leaching type of deactivation.13 In fact, the concentra-tion of dissolved Mn was 10 ppm after complete conver-sion of TOC at 130 °C, which represents less than 0.5%of the total manganese present in the fresh catalyst. Inthe case of Ce, the concentration of dissolved ions wasbelow 0.2 ppm, confirming the negligible extent ofcatalyst leaching in the reaction medium.

Also, poisoning type of deactivation was unlikely tooccur since phenol is an S-, P-, and X-free molecule andthese are the heteroelements known to be poisonous tooxidation catalysts.5 Fouling deactivation due to surfacedeposition and strong adsorption of solid organic species

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

was the deactivation scenario that most likely occurred,which might explain the loss in activity.

The fate of carbon that was removed from the solutionwas tracked by monitoring the carbon content of bothcatalysts. The gaseous carbon (assumed to be CO2) wasdeduced by performing a carbon balance between theinitial TOC0, the running TOC, and the carbon belong-ing to the deposit. Typical time profiles of dissolvedcarbon (i.e., TOC), deposited carbon (CS), and CO2, aswell as CO2 yield, are shown in Figure 2 for MnO2/CeO2and 1%Pt-MnO2/CeO2. The CWO yield of CO2 isdefined on a carbon basis:

In the case of MnO2/CeO2, only 22% of carbon wasconverted into CO2 after completion of the CWO reac-tion. Under likewise conditions, the CO2 yield attainedwith the noble-metal-promoted catalyst was better(36%), even though the phenol and TOC degradationrates were slightly slower than with the unpromotedcatalyst (Figure 1a,b). In both cases, restoration ofcatalyst activity can be easily obtained by burning outthese carbon-containing deposits.

BET surface areas of MnO2/CeO2 and 1%Pt-MnO2/CeO2 catalysts are summarized in Table 1 for differenttemperatures, TOC conversions, and phenol initialconcentrations. There was a notable reduction insurface area, from 107 m2/g for the fresh MnO2/CeO2

catalyst to 95 m2/g after TOC was almost completelyremoved at 80 °C. As TOC conversion improved, asignificant increase in the fractal dimensions was notedfor the used catalysts with respect to those of the freshones. It is worth pinpointing the dramatic decrease inthe MnO2/CeO2 surface area from 107 to 3.5 m2/gobserved with a deliberately high phenol initial concen-tration (7.5 g/L). Hence, an increase in the fractaldimension and a decrease in the BET surface area canbe correlated to the amount of carbonaceous depositsthat built up on the catalyst surface.

From SEM photographs, the morphology of the twocatalysts consisted of agglomerates that appeared largerfor the Pt-promoted one. Higher magnification showedthat these agglomerates had rough surfaces composedof small irregularly shaped bulges (Figure 3a,b). Themicroscopic structure of the used catalysts was differ-

Figure 1. Phenol (a) and TOC (b) conversions at 130 °C and 0.5MPa, over MnO2/CeO2 and 1%Pt-MnO2/CeO2 catalysts (initialphenol concentration 1 g/L, catalyst loading 5 g/L). Solid lines,fresh catalysts; dotted lines, second consecutive oxidation runwithout catalyst regeneration.

YG ) 1 -[CS]

TOC0 - TOC

Figure 2. TOC, solid carbon, and carbon dioxide time profiles at130 °C and 0.5 MPa, over MnO2/CeO2 (a), 1%Pt-MnO2/CeO2catalysts (b), and carbon dioxide yield (c) (initial phenol concentra-tion 1 g/L, catalyst loading 5 g/L, fresh catalysts only). Lines arehere to show trends.

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

ently affected by the carbonaceous deposits dependingon whether the catalyst was promoted or not (Figure3c,d). The MnO2/CeO2 sample taken after 30 min ofCWO reaction indicated a uniform cover-up of the grainby the carbonaceous deposit and larger bulges. For theused 1%Pt-MnO2/CeO2 (after 10 min of reaction),despite the fact that a lot of bulges were embedded inthe carbonaceous deposit, some of them, having thesame average size as for the fresh sample, remainedvisible and thus active. In the Pt-promoted catalyst,more oxidation sites were left accessible to the reactantsto pursue the oxidation reaction.

Figure 4 shows the burnoff profiles of the two cata-lysts at their fresh state and after complete removal of

TOC at 130 °C. Oxygen uptakes in TPO profilesconfirmed that less carbonaceous deposits were formedover 1%Pt-MnO2/CeO2 than over MnO2/CeO2, whichagreed with the results in Figure 2. TPO profilesshowed two distinct combustion peaks: a sharp peakbetween 200 and 220 °C and a broad peak with amaximum between 250 and 280 °C. This may suggesteither that there were two kinds of carbon deposits, withone burning very quickly, or that the deposits locatedon manganese and cerium oxides burnt at differenttemperatures. These deposits were easily burnt below300 °C, presumably because their combustion wascatalyzed by the oxides. Oxygen uptake of the freshcatalyst samples occurred above 500 °C, far beyond the

Table 1. BET Surface Area, Fractal Dimension, and Surface C, Mn, Ce Atom % for Fresh and Used Catalysts

catalysttemperature (°C)(reaction time)

TOCconv (%)

fractaldimension

SBET(m2/g) C Mn Ce C/(Mn + Ce) Mn/Ce

MnO2/CeO2a 2.46 107 2.511 35.201 9.429 0.06 3.7

MnO2/CeO2 130 (30 min) 98.8 2.6 106.6 43.682 12.401 2.196 2.99 5.6130 (120 minb) 61.8 2.71 3.5

80 (15 min) 27.9 2.56 36.972 12.078 7.076 1.93 1.780 (45 min) 63.1 2.64 104 52.004 8.899 4.595 3.85 1.980 (90 min) 91.5 2.69 95 56.628 7.857 3.953 4.79 2.0

Pt-MnO2/CeO2a 2.46 88

Pt-MnO2/CeO2 130 (15 min) 91.2 2.71 83.280 (15 min) 22.5 2.5380 (45 min) 48.6 2.59 8880 (90 min) 78.8 2.7 83

a 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.

3564 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998

burnoff range of the deposits to assume that O2 uptakeof used catalysts came mainly from the combustion ofthe carbonaceous overlayer.

Occurrence of a carbonaceous overlayer on the cata-lyst surface was also confirmed by the XPS data asillustrated in Table 1 for MnO2/CeO2 catalyst. Atalmost complete TOC conversion, carbon coverage in-creased as the oxidation temperature was decreased.Surface carbon increased, and manganese and ceriumatomic fractions decreased as the TOC conversionincreased. C/(Mn + Ce) atomic ratio increased sharplyas the carbonaceous deposit built up on the catalystsurface. The Mn/Ce atomic ratio leveled off at temper-ature-dependent plateau values. The higher the tem-perature, the higher the Mn/Ce ratio. Hence, XPS datasuggest that the carbonaceous solid material was de-posited preferentially on cerium rather than on man-ganese. The detailed high-resolution C1s region spectrarevealed a number of overlapping features correspond-ing to different chemical natures of carbon: mainlyaromatic, aliphatic, and partially oxidized. The aroma-ticity of the deposit, as measured by the sp2 hybridizedcarbon and the plasmon loss feature,14 was sensitive tothe presence of Pt on the MnO2/CeO2. The decomposi-tion of the C1s region into individual line componentswas performed using reported binding energies for theC1s core level;15,16 see Figure 5. For MnO2/CeO2 cata-lyst, 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/CeO2catalyst, the C1s region was contributed by the sametypes of carbon but with a different distribution foraromatic and aliphatic carbons (Table 2). Aromaticity(sp2, 38%) of the deposit formed on the platinum-promoted catalyst was higher than for the unpromotedMnO2/CeO2 catalyst. This would explain why theformer catalyst, even though more selective to CO2formation, was (i) less efficient than the latter to oxidizephenol and its intermediates and (ii) more sensitive todeactivation as shown by the recycled nonregeneratedcatalysts (Figure 1b). Carbon species at BE < 284 eVwere previously assigned to carbidic carbon.16,17 In thisstudy, 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 lowand accounted for 7-8% of the overall C1s peak.

Concluding Remarks

Dissolved phenol and phenol intermediates were fullyoxidized in the presence of MnO2/CeO2 and 1%Pt-MnO2/CeO2 catalysts at mild conditions. Pt promotionof MnO2/CeO2 reduced the amount of carbonaceousdeposits and improved phenol deep oxidation (higherCO2 yield). However, platinum-promoted MnO2/CeO2was more sensitive to deactivation and exhibited sys-tematically slower degradation rates of phenol and TOC.Current work is focusing on the optimization of Ptloading and dispersion, as well as on the bimetalliccopromotion of MnO2/CeO2 in order to further maximizeCO2 yield.

Figure 4. TPO profiles of fresh and used MnO2/CeO2 and 1%Pt-MnO2/CeO2 catalysts. Oxygen uptake is expressed in micromolesper gram of catalyst.

Figure 5. XPS C1s high-resolution spectra and peak line-fittingof functional groups in the deposits of MnO2/CeO2 (a) and 1%Pt-MnO2/CeO2 (b) catalysts after 5 min of reaction at 130 °C and 0.5MPa.

Table 2. Position, Assignments, and Peak Areas of theC1s Peaksa

area (%)

peakno.

bindingenergy (eV) assignment

MnO2/CeO2

Pt-MnO2/CeO2

1 282.9 carbide 2.02 284.6 aromatics 18.6 38.23 285 aliphatics, â-carbons 50.8 27.54 286.1 C-OH, C-O-C 15.4 19.45 287.6 CdO 3.1 3.06 289.1 COOH, COOR 4.1 2.97 291.2 plasmon loss 8.0 7.1a 5 min at 130 °C, 0.5 MPa.

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

Acknowledgment

Financial support from the Natural Sciences andEngineering Research Council of Canada (NSERC) andthe Fonds pour la formation de chercheurs et d’aide ala recherche are gratefully acknowledged. Authors fromPINMATE acknowledge support from Universidad deBuenos Aires and CONICET. We also acknowledge Dr.A. Adnot for the XPS analyses and Prof. A. Sayari forlending us the Altamira instrument.

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Received for review February 10, 1998Revised manuscript received May 18, 1998

Accepted June 8, 1998

IE980081W

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