B ENVIRONMENTAL

ELSEVIER Applied Catalysis B: Environmental 13 ( 1997) 175-184

Combustion of non-halogenated volatile organic compounds over group VIII metal catalysts

Panagiotis Papaefthimiou, Theophilos Ioannides, Xenophon E. Verykios*

Department of Chemical Engineering and Institute of Chemical Engineering and High Temperature Processes, University of Patras, 26500 Patras, Greece

Received 26 May 1996; received in revised form 26 October 1996; accepted 26 October 1996

Abstract

Catalytic combustion of volatile organic compounds (VOCs), present in low concentrations (10-1000 ppm) in industrial effluent streams, is a promising air abatement technology. The oxidation of benzene, butanol and ethyl acetate over group VII metal catalysts supported on alumina carriers has been investigated. Pt, Pd and Co were found to be the most active among group VIII metals, while ethyl acetate was found to be the most-difficult-to-oxidize compound. Benzene and ethyl acetate oxidations over Pt/A1203 were found to be structure sensitive reactions with the turnover frequency (TOF) increasing with increasing mean metal particle size. The presence of chloride on the catalyst surface, originating from chloride-containing metal precursor compounds was found to exert an inhibiting effect on the activity of Pt. Apparent activation energies of the reactions over Pt and Pd catalysts were found to be in the 70-120 kJ/mol range while the reaction order with respect to the VOC was positive in all cases. During oxidation of benzene-butanol mixtures, benzene oxidation was completely suppressed as long as butanol was present in the reaction mixture. 0 1997 Elsevier Science B.V.

Keywords: Benzene; Butanol; Ethyl acetate; Catalytic oxidation; Palladium; Platinum; Volatile organic compound

1. Introduction

Volatile organic compounds (VOCs) are recognized as major contributors to air pollution, either directly through their toxic or malodorous nature, or indirectly as ozone precursors [ 11 and smog precursors. Mobile sources, such as cars and trucks, are responsible for a large fraction of VOC emissions. The USA Environ- mental Protection Agency (EPA) estimates that - 40% of the VOC emissions in the USA result from transportation activities, primarily from light cars and

*Corresponding author. Fax: (+30 61) 991527

trucks. The remaining 60% result from stationary sources and are about equally divided between solvent emissions, industrial manufacturing and fuel combus- tion [1,2].

Environmental legislation in the USA and the EU has established stringent regulations to control VOC emissions. The USA Clean Air Act of 1990 calls for a 90% reduction in emissions of 189 toxic chemicals, 70% of them VOCs, over the next nine years [3]. Catalytic oxidation is one of the most important processes for VOC destruction, since VOC oxidation over a catalyst takes place at temperatures much lower than those required for thermal destruction [4-71.

0926-860x/97/$17.00 c 1997 Elsevier Science B.V. All rights reserved. PII SO926-3373(96)00103-S

176 P Papaefthimiou et al/Applied Catalysis B: Environmental 13 (1997) 175-184

The commercial catalysts which are used for the combustion of non-halogenated VOC can be classified into the following categories [1,8]: ?? supported noble metals (mainly Pt and Pd); ?? metal oxides (supported or not); and . mixtures of noble metals and metal oxides.

The majority of commercial catalysts belong to the first category and consist of Pt or Pd or both, supported on alumina carriers (y-AlzOs). The catalysts are either in pellet or ceramic monolith form. In general, metal catalysts possess greater activity than the oxide cata- lysts, contributing to the reduction of operating costs despite their higher manufacturing cost.

The activity of metal catalysts in the oxidation of volatile organic compounds depends considerably on the nature of the organic compound. In practice, the gas streams which must be treated consist of a mixture of organic compounds and, in the majority of situa- tions, the behavior of the catalyst cannot be predicted a priori.

An extensive review of the literature prior to 1993 has been published by Catalytica [ 11. Referring to the combustion of benzene, n-butanol and ethyl acetate (EAc), Sawyer et al. [93 studied the mechanism of ethyl acetate oxidation over a Pt/A120s catalyst. They found that acetic acid and ethanol are the primary products of the reaction which are subsequently oxi- dized to CO* and H20. Conversion of ethyl acetate greater than 90% was obtained at temperatures higher than 300°C with a space velocity of 48 000 hh’. Tichenor and Palazzolo [lo] obtained destruction of ethyl acetate greater than 90% at temperatures higher than 350°C using a noble metal catalyst and space velocity of 50 000 hh’. Hermia and Vigneron [ 1 l] studied the combustion of butanol over a monolithic Pt/AlzOs catalyst. Conversion of butanol greater than 90% was obtained in the 180-200°C range for a space velocity of N 15 000 hh’. The same researchers attained 95% conversion of benzene at 236°C and 99% at 300°C using space velocity of 30000 hh’. The oxidation of benzene has been studied more extensively than the oxidation of butanol and ethyl acetate. In general, destruction efficiency of benzene greater than 90% is attained in the 180-250°C temperature range [ 12-141 using catalysts which contain Pt.

It is obvious from the above that butanol and benzene are oxidized more easily than ethyl acetate

over Pt catalysts. Therefore, the activity of a catalyst under ethyl acetate oxidation should determine the catalytic behaviour under oxidation of mixtures which include ethyl acetate (or generally acetates).

The main scope of the present work was to deter- mine the most active group VIII metal catalysts for benzene, n-butanol and ethyl acetate oxidation. Ben- zene was chosen as a representative VOC of aromatic nature while butanol and ethyl acetate, as representa- tive model compounds for alcohols and esters, respec- tively. The present communication concerns identification of the temperature range required for destruction efficiencies greater than 95%, determina- tion of kinetic parameters, examination of the effects of metal particle size on catalyst activity and the investigation of catalyst behavior during the oxidation of binary mixtures.

2. Experimental

2.1. Catalyst preparation

Supported group VIII metal catalysts were prepared by the method of incipient wetness impregnation of y- A1203 (surface area = 100 m2/g) and two different a- A120s carriers of low (< 1 m2/g) and high (160 m*/g) surface area, using the corresponding chloride salts for Pt and Ru and nitrate salts for Pd, Co, Rh and Ni, as precursor compounds. All precursor compounds and carriers were obtained from Alfa Products. Metal loading was in most cases 0.3 wt%, with the exception of Co and Ni catalysts where higher loadings were used (3-17%). Chloride-free Pt catalysts were pre- pared by impregnation of the support with appropriate amounts of a H,Pt(OH)e solution acidified with HNOs (pH of the solution was in the range l-2). Impregnated supports were dried at 110°C overnight. The dried material was then ground and placed in a quartz tube for reduction. The material was heated to 250°C with a heating rate of lO”C/min under nitrogen flow (50 cm3/ min). The flow was then switched to pure Hz (50 cm3/ min) and the temperature increased slowly to 300°C where it was maintained for 2 h. The flow was then switched to N2 and the catalyst was slowly cooled to room temperature and stored in air-tight vials until further use. In the case of Co and Ni catalysts, reduction was performed at 500°C for 5 h.

P. Papaefthimiou et al. /Applied Catalysis B: Environmental 13 (1997) 175-184 177

Low dispersion Pt and Pd catalysts supported on ~-A1~0s were prepared by calcination of highly dispersed catalysts under air flow (50 cm3/min) at S50-800°C for 2 h.

2.2. Catalyst characterization

The dispersion and mean crystallite size of the supported metal catalysts were determined by equili- brium hydrogen chemisorption at room temperature in a constant-volume high-vacuum apparatus (Micro- meritics, Accusorb 2100E). Pressure detection was done by means of a specially designed autoranging digital readout capacitance manometer with dual transducers. Adsorption isotherms of Hz were deter- mined at 25°C (at 60°C for Pd catalysts and 100°C for Ru catalysts), at equilibrium pressures in the 240 Torr range. Prior to adsorption measurements, the catalyst samples (- 1 g) were evacuated at 200°C for 1 h, exposed to static Hz at 300°C for 2 h (Co and Ni catalysts at 450°C for 4 h) and placed under dynamic vacuum at 300°C for 3 h. Uptakes of H2 at monolayer coverage of the metal particles were obtained by extrapolation of the linear portions of the adsorption isotherms to zero pressure. Uptakes by unmetallized supports were found to be negligible, in all cases. The gases used were of ultra-high purity and they were further purified by passing them through a Deoxo purifier and a molecular sieve trap.

2.3. Experimental apparatus and procedures

Mass flow controllers (MKS Instruments) were used for accurate and stable control of gas flow rates. Product and reactant analysis was carried out by a gas chromatograph (Shimadzu 14A) equipped with ther- mal conductivity and flame ionization detectors. A Carboxen 1000 column was used for CO2 separation and a 10% CW-20M on Supelcoport column, for VOC analysis. The detection limit of the analytical appa- ratus was 10 ppm for CO2 and generally less than 1 ppm for the organic species.

The reactor system consists of a vertical tubular furnace of 30 cm length controlled by a PID controller. The reactor itself is a quartz tube of 6 mm o.d., at the middle of which the catalyst bed is contained. A thin thermocouple (K-type) is positioned into the catalyst bed (inside a quartz thermowell) for accurate mea-

surement of the catalyst temperature. All gas lines of the apparatus are heated continuously at 80°C in order to minimize VOC adsorption on tube walls.

Benzene in air (3000 ppm), n-butanol in air (600 ppm) and ethyl acetate in air (2000 ppm) mix- tures were prepared in the laboratory. Desired VOC concentrations were obtained by appropriate dilution of these mixtures with air.

The procedure for each experiment was as follows: the catalyst in the 30-60 mg mass range, sieved in the 63-125 pm range (in order to avoid mass- or heat- transfer effects) was placed in the reactor, supported by quartz wool, and was heated to 300°C at lO”C/min under He flow. The flow was then switched to air and the temperature was raised to 450°C for 30 min.

Subsequently, the catalyst was cooled under He flow to 3OO”C, then the flow was switched to Hz for 1 h. (Co and Ni catalysts were reduced at 500°C for 2 h). The sample was then cooled at the desired reaction temperature under He flow. The catalytic tests were performed in the 100-400”C temperature range under atmospheric pressure using a feed con- sisting of 50-3000 ppm VOC in air.

The space velocity employed in tests under differ- ential conditions varied depending on the VOC mole- cule and the catalyst activity. In the case of benzene and EAc oxidation, space velocity was in the 30 OOO- 200000 hh’ range. In the case of butanol oxidation, space velocity was in the 30000-960000 hh’ range. VOC destruction efficiency as a function of reaction temperature was measured at a space velocity of 30000 h-‘.

3. Results and discussion

3.1. Catalyst screening

The metal dispersion was estimated from the ratio of the hydrogen atoms adsorbed per total metal atoms in the catalyst, H/M, which was determined from equilibrium static chemisorption of hydrogen. Typical hydrogen-adsorption isotherms for the group VIII metal catalysts studied are shown in Fig. 1. The corresponding H/M ratio is presented in Table 1 for all catalysts which were studied. Generally, highly dispersed noble metal catalysts were obtained using y- A1203 as the support. As expected, calcination at high

178 P Papaefthimiou et al./Applied Catalysis B: Environmental 13 (1997) 175-184

I Hydrogen chemisorption

10 -2; I I 10 20 30 40

PH2, (torr) Fig. 1. Isotherms of hydrogen adsorption on metal catalysts supported on y-A1203.

Table 1 Dispersion of group VIII metal catalysts, as determined by equilibrium hydrogen adsorption

Catalyst H/h4 Catalyst

0.3% Pt

~41203

y-Also3 (air, 600°C) a y-AlZ03 (air, 700°C) y-A1203 (air, 750°C) ~A1203 cx-A1203 (air, 750°C) wA1203 b

0.3% Pd ~-Al203 y-Al203 (air, 550°C) ~-A1203 (air, 650°C) y-Al203 (air, 750°C) ~-Al203 (air, 800°C)

1.00 0.70 0.14 0.09 0.11 0.02 1 .oo

1.00 0.63 0.38 0.24 0.04

3% Nil?-AlZ03 17% Nil?-Al,03

3% Co/y-A1203 5% Co/y-Alz03 10% Co/y-A1203

0.3% Ru y-Al203 a-A1203

0.3% Rh

~-Al203 wAlZ03

0.20 0.05

0.02 0.04 0.04

0.20 0.01

1.00 0.05

a Calcination conditions of highly dispersed catalysts. b Sample with high surface area (- 160 m’/g).

temperatures (550-800°C) causes a considerable decrease of dispersion. As it is shown in Table 1, Pd catalysts seem to be more resistant to calcination than Pt catalysts. At a calcination temperature of 75O”C, Pd dispersion decreases from 1 originally to

10'" R pd Co Ru Rh Ni

Fig. 2. Turnover frequency (TOF, s-l) of benzene, butanol and ethyl acetate oxidation over Group VIII metal catalysts supported on alumina.

0.24, compared to the reduction of Pt dispersion to 0.09 under the same conditions. The WMratios for Co and Ni catalysts underestimate the true dispersion of the metal. This is due to the fact that these metals are reduced with difficulty and reduction is not complete due to spine1 formation.

The intrinsic activity (expressed as turnover fre- quency, TOF (s-l)) of group VIII metal catalysts for total oxidation of benzene, butanol and ethyl acetate was determined under differential conditions (conver- sion to CO2 < 10%) at 18O”C, and comparative results are presented in Fig. 2. The catalysts employed in the initial screening were: 0.3% Pt/y-Alz03 (H/Pt = l), 0.3% Pd/y-A1203 (H/Pd = 0.3), 5% Co/y-AlzOX (H/Co = 0.04), 0.3% Ru/y-Alz03 (H/Ru = 0.2), 0.3% Rh/y-AlzOX (HYRh = 1) and 3% Nily-Alz03 (H/Ni = 0.2). The most active metal for benzene and butanol oxidation was found to be Pt, followed by Pd, which is approximately an order of magnitude less active than Pt in the case of butanol oxidation. All other metals (Co, Ru, Rh, Ni) are one to two orders of magnitude less active than Pt, on the basis of turnover frequency. The activity with respect to the VOC molecule was observed to follow the sequence: buta- no1 > benzene > ethyl acetate. In the case of ethyl acetate, which is the most difficult to oxidize mole- cule, Co was found to be the most active metal, although differences among the activity of the various metals were not so significant, as in the case of

F! Papaefihimiou et al./Applied Catalysis B: Environmental 13 (1997) 175-184 179

benzene and butanol oxidation. It must be noted, however, that the TOF of Co catalysts is overestimated by as much as two to three times due to underestima- tion of its dispersion, as determined by hydrogen adsorption. A notable observation is the very low activity of Pt in ethyl acetate oxidation, in contrast to its activity in benzene and butanol oxidation.

Under benzene oxidation conditions, the only pro- duct in all cases was found to be COa. Homogeneous oxidation of 250 ppm benzene in air with 30 cm3/min gas flow becomes appreciable at N 300°C while almost complete conversion of benzene to CO2 is achieved at 400°C. Oxidation of n-butanol, on the other hand, was observed to proceed via formation of butanal. The reaction scheme appears to be:

butanol + butanal -+ CO2 (1)

It was found that a stream of 150 ppm butanol in air with 30 cm3/min gas flow is quite efficiently des- tructed homogeneously and the conversion as a func- tion of temperature is presented in Fig. 3. Conversion of butanol to butanal starts at temperatures as low as 180°C and is maximized at - 260°C. The total oxida- tion product (COZ) appears at 240°C and at - 340°C complete conversion of butanol to CO2 is achieved homogeneously.

Small amounts of two partial oxidation products were also observed in the case of ethyl acetate oxida- tion at low and intermediate conversions. In most probability, ethyl acetate decomposes to ethanol and

100

r? 80 P ” 60

5 .- 11 P

40

C

(2 20

Homogeneous reaction

200 250 300 Temperature, (“C)

Fig. 3. Conversion of reactants and products under homogeneous gas-phase oxidation of n-butanol.

acetic acid [9]. Homogeneous oxidation of 250 ppm ethyl acetate in air with 30 cm3/min gas flow becomes appreciable at - 280°C while almost complete con- version of EAc to CO2 is obtained at x 380°C.

Employment of a group VIII metal catalyst shifts the conversion curve to significantly lower tempera- tures. The conversion of the three VOCs to CO2 as a function of reaction temperature is presented in Fig. 4a, 4b and 4c for 0.3% Pt/y-AlzO3, 0.3% Pd/y- A1203, and 5% Co/y-AlzOa catalysts, respectively. The reactor feed consisted of 250 ppm VOC in air (150 ppm for butanol) and the space velocity was 30000 h-l.

The only CO, product was CO2 in all cases. The behavior of the carbon balance depended on the VOC molecule, the temperature and the catalyst used (high or low surface area). In the case of catalysts supported on low-surface-area a-Alz03, the carbon balance was satisfied in all cases. In the case of catalysts supported on high-surface-area carriers, a significant gap in the carbon balance at low reaction temperatures was observed, especially under butanol oxidation condi- tions, where no butanol was observed in the reactor effluent stream, although conversion to CO2 was low. It was found by microbalance experiments that this is due to VOC adsorption on the alumina support. This transient period may be as high as lo-15 h due to the low concentration of the VOC molecule in the feed. Increase of the reaction temperature leads to desor- ption or reaction of adsorbed species, and the tran- sient period shortens drastically. Throughout the present paper, the conversion of VOC to CO2 and rtot the total conversion of VOC is used for establish- ing catalytic activity. It is obvious that the use of total VOC conversion may lead to erroneous conclusions regarding catalyst activity, if measurements are made during the period of transient adsorption of the VOC on the support.

In the case of Pt catalysts (Fig. 4a), the conversion of butanol and benzene to CO2 becomes appreciable in the 120-140°C range, while conversions greater than 95% are obtained at z 200 and 220°C respectively. These results are in agreement with literature results referring to butanol [ 1 l] and benzene oxidation [ 12- 14] over Pt catalysts. Under ethyl acetate oxidation conditions, the conversion to CO;? becomes appreci- able at 220°C and greater than 95% at 320°C also in accordance with the results of Sawyer et al. [9].

180 F! Papaefthimiou et al. /Applied Catalysis B: Environmental 13 (1997) 17%I84

60

0 100

2 - 80

64

’ 60 s

60

40

20

0 100 150 200 250 300 350 400

Temperature. (“C)

Fig. 4. VOC conversion as a function of temperature over (a) 0.3% Wy-AltO?, (b) 0.3% Pd/y-A1203 and (c) 5% Co/y-Al203 catalysts (SV = 30000 h-’ and VOC concentration = 150-250 ppm).

In the case of Pd catalysts (Fig. 4b), which are less active than Pt catalysts for butanol and benzene oxi- dation, conversions greater than 95% were obtained at 220 and 260°C respectively. Under ethyl acetate oxidation conditions, Pd appears to be more active than Pt, and almost complete conversion to CO2 is attained at 280°C.

In the case of Co catalysts (Fig. 4c), a loading of 5 wt% Co on y-AlzOa was used. Differences among

the activity of the catalysts with various Co loadings (3-10 wt%) were not significant. Thus, employing the 5% Co/y-A1203 catalyst, destruction efficiencies greater than 95% were obtained at 280°C for benzene and at 220°C for butanol and ethyl acetate. It can be seen that Co catalysts compete favorably with Pt and Pd catalysts in the case of ethyl acetate oxidation, which is the most difficult molecule to oxidize.

3.2. Dispersion effects on VOC oxidation

The effect of metal dispersion on VOC oxidation reactions was investigated over Pt and Pd catalysts, so that the catalysts could be optimized with respect to this parameter. Samples of variable metal dispersion were synthesized by calcination of highly dispersed Pt and Pd/y-AlzOa catalysts at high temperatures (550- 800°C). Low dispersion samples were also prepared employing low-surface-area ~-A1203 carriers. All Pt and Pd samples which were used and their correspond- ing dispersions, as determined by equilibrium hydro- gen adsorption, are listed in Table 1.

The effect of metal particle size, d (nm), or, alter- natively, of dispersion, on the intrinsic activity of the catalyst is presented in Fig. 5a and 5b for Pt and Pd catalysts, respectively. The intrinsic activity, TOF (s-l), was determined under differential conditions at 180°C using a constant feed composition of 250 ppm VOC in air. The relationship between parti- cle size of the metal, d (nm), and the metal dispersion, 0 < D < 1, which was used is: d = 1.1/D [15].

In the case of Pt catalysts (Fig. 5a), the TOF of benzene and ethyl acetate oxidation was found to increase significantly with increasing Pt particle size, levelling off at a crystallite size larger than 10 nm. Thus, a Pt\alumina catalyst, with dispersion lower than 10% (d > 10 nm) was found to be N 40 times more active than the highly dispersed Pt catalyst for benzene oxidation and N 25 times more active for ethyl acetate oxidation. On the other hand, the TOF of butanol oxidation is essentially independent of Pt particle size. It can be seen that Pt catalysts of similar dispersion supported on y-Alz03 and ~A1203 carriers have comparable activities, which implies that the observed effects are not due to modification of the state and morphology of Pt, induced by calcination of the Ptly- A1203 catalysts [ 161, but most probably are due to the structure-sensitive character of the reaction. In the

P. Papaefthimiou et al. /Applied Catalysis B: Environmental I3 (1997) 175-184 181

Dispersion 1.0 0.5 0.2 9.1 0.05 0.02 1[ :

Fig. 5. Effect of (a) Pt and (b) Pd particle size (d, nm) on TOF of benzene, butanol and ethyl acetate oxidation, (filled symbols: a- .41203 carriers, open symbols: ~-Al~0~ carriers, T=lSO”C and feed: 250 ppm VOC in air).

case of Pd catalysts (Fig. 5b), the intrinsic activity for benzene oxidation is independent of dispersion, while the activity for butanol oxidation increases slightly with decreasing dispersion and the activity for ethyl acetate oxidation decreases slightly with decreasing dispersion. In general, the TOF of all three oxidation reactions is only weakly dependent on Pd particle size in contrast to what was observed for Pt catalysts.

In the case of benzene and ethyl acetate oxidation over Pt/A120s catalysts, the optimal dispersion should be of the order of 0.10-O. 15. In all other cases, the optimal catalyst should be of as high dispersion as possible. Comparison of Fig. 5a and 5b shows that Pd is approximately an order of magnitude less active than Pt for butanol oxidation, has comparable activity

to highly dispersed Pt for benzene oxidation, but N 40 times lower activity compared to poorly dispersed Pt. On the other hand, Pd is generally more active than Pt for ethyl acetate oxidation, as it has already been shown in the previous section.

3.3. Effect oj’chloride on the activity of Pt catalysts

It is well known that the presence of chloride ions on the catalyst surface can significantly affect the activity of Pt catalysts in oxidation reactions. The presence of chloride was found to have a poisoning effect on Pt activity for heptane oxidation, especially for highly dispersed samples [16].

The Pt catalysts described earlier were prepared from H,PtCl,, a Cl-containing precursor compound. Based on the aforementioned findings, it was deemed necessary to examine the effect of chloride ions on Pt activity for VOC oxidation. Toward this end, two Pt catalysts were prepared: the first was a highly dis- persed Ptly-Al203 catalyst (I-UPt = 1) and the second was a low dispersion Pt/o-AlzOs catalyst (H/ Pt = 0. I l), employing a Cl-free precursor compound, namely H,Pt(OH),, all other parameters of prepara- tion being the same.

The Cl-free catalysts were tested in the reactions of benzene and ethyl acetate (EAc) oxidation, where significant dispersion effects had been already observed. The intrinsic activity of Cl-free and Cl- containing Pt catalysts for benzene and ethyl acetate oxidation is presented in Fig. 6 as a function of dispersion (or crystallite diameter, 2) of the Pt metal. It can be seen that in both reactions, the samples prepared from the Cl-containing precursor compound exhibit significantly lower activity than the Cl-free samples. In the case of highly dispersed catalysts, the Cl-free sample is 15 times more active than the Cl- containing sample in benzene oxidation and 40 times more active in ethyl acetate oxidation. Concerning low dispersion catalysts, the Cl-free sample is 10 times more active than the Cl-containing sample in ethyl acetate oxidation while both catalysts have compar- able activity in benzene oxidation.

It appears that chloride ions, which remain on the surface even after reduction, exert an inhibiting effect on the activity of Ptly-Al203 catalysts. This inhibiting effect appears to be more severe in the highly dis- persed samples, which is in accordance with literature

182 F! Papaefthimiou et al. /Applied Catalysis B: Environmental 13 (1997) 175-184

Dispersion

10 +’ ..I: 1

d, $I)

1 100

Fig. 6. Effect of Pt-precursor compound on intrinsic activity (TOF) of Pt, supported on y-Al203 (open symbols) and a-AlTO (filled symbols), for benzene and ethyl acetate oxidation (dashed line: Cl- free precursor compound and solid line: Cl-containing precursor compound).

findings [16]. It has also been suggested that the poisoning effect of chloride is due to stabilization of [Pt4+] surface species, which are much less active than Pt crystallites in oxidation reactions. According to Barbier et al. [ 171 and Reyes et al. [ 181 the existence of chloride in reduced Pt/A1203 catalysts (Cl/Pt < 1) causes the stabilization of Pt”+ species and the formation of platinum oxide at low temperatures (130-230°C) which is totally decomposed above 630°C yielding reduced metallic Pt and inducing metal sintering.

3.4. Kinetic parameters of VOC oxidation

The simplest kinetic model of a catalytic reaction is a power law rate equation which expresses the rate as a product of a ‘rate constant’ and the reactant concen- trations raised to a power, e.g. for the catalytic oxida- tion of VOCs, the rate could be expressed as:

For the conditions encountered in most VOC con- trol applications, the concentration of oxygen will always be much larger than the concentration of the organic reactant (for a feed of 250 ppm VOC in air, co,lCvoc = 840). Thus, because the oxygen partial pressure will be essentially constant during the reac-

tion, the observed rate will depend only on the con- centration of VOC.

Although, this simple type of rate law does not provide any insight into what is occurring at the catalyst surface, it is useful for quick comparison of the kinetic behavior of different catalysts. Based on Eq. (2), the apparent activation energies of benzene, n-butanol and ethyl acetate oxidation as well as reac- tion orders with respect to VOC and oxygen concen- tration were determined over Pt, Pd and Co catalysts. The reaction order with respect to oxygen concentra- tion was determined so as to probe the inhibiting effect of the large excess of oxygen on the observed rate. Measurements were performed at differential condi- tions (conversions in the region of 10% or less) by adjusting space velocity. Determination of the reaction orders with respect to the VOC was performed at constant temperature and oxygen concentration (using air as oxidant) and 40-600 ppm butanol, 40- 2000 ppm ethyl acetate and 50-3000 ppm benzene. The corresponding experiments with respect to oxy- gen concentration were performed at constant tem- perature, constant VOC concentration of 250 ppm VOC (100-150 ppm in the case of butanol) and oxy- gen concentrations in the 2.5-21% range. It should be noted that different temperatures have been employed for each VOC, due to differences in reactivity. The temperatures chosen were generally below 200°C for benzene and butanol oxidation and somewhat higher than 200°C for ethyl acetate oxidation in order to ensure that data were obtained in the surface kinetic region. The results are presented in Table 2.

The apparent activation energies of the reactions were determined using a feed of 250 ppm VOC in air and measuring the rate of CO2 production at different temperatures. The highest activation energy of 121 kJ/ mol over Pt and Pd and 151 kJ/mol over Co was observed in the case of ethyl acetate oxidation, which is also the most-difficult-to-oxidize molecule. The activation energies measured over Pt and Pd were found to be comparable and did not show any specific trend with varying dispersion. No trend was also observed over Co catalysts by varying metal loading (3-10 wt%).

The reaction order with respect to oxygen was observed to be close to zero or negative. On the other hand, the reaction order with respect to the VOC was always positive with values in the O-l range, with the

P Papaejihimiou et al/Applied Catalysis B: Environmental 13 (1997) 175-184 183

Table 2 Kinetic parameters of VOC oxidation reactions

Metal voc

Pt

I’d

Co

Benzene Butanol Ethyl acetate Benzene Butanol Ethyl acetate Benzene Butanol Ethyl acetate

Temperature range a (“C)

120-200 120-160 240-300 120-200 140-200 1 SO-260 140-200 14&200 16G220

E act (!cJ/mol)

96+6 ’ lOOZt8 121fll 92flO 71*7

121*10 100*5 12511.5 lSlf12

Tb

(“C)

180 140 240 180 160 220 180 180 200

0.22~tO.06 0.15f0.04 0.84?cO.O7 -0.61f0.10 0.4 1 ~tO.06 -0.12&0.07 05310.03 -0.15*0.03 0.94f0.09 -0.73~tO.12 0.19f0.04 0.02~tO.09 0.06f0.04 0.10*0.09 0.25f0.15 0.1210.11

-0.31f0.05 0.631tO.14

” Temperature range of determination of activation energy. ’ Temperature of reaction order measurement. ’ 95% confidence limit.

notable exception of EAc oxidation over Co. The highest order with respect to VOC was found in the case of butanol oxidation over Pt and Pd catalysts, where the largest inhibition by oxygen also exists. Oxygen inhibition has also been observed by VGlter et al. [16] during n-heptane oxidation over Ptly-AlzOs. These results imply that the reaction follows a Lang- muir-Hinshelwood type of mechanism.

J.5. Oxidation of benzene-butanol mixtures

In practice, a VOC-contaminated exhaust stream is expected to consist of a rather complicated mixture of VOCs. Therefore, it is of interest to be able to predict the behavior of the catalyst under these conditions based on data concerning the oxidation of each com- ponent separately, or of simple mixtures. In this work, the oxidation of a benzene-butanol mixture was stu- died over Pt/y-Al203 and Pd/y-AlzOs catalysts. The reactor feed consisted of 250 ppm benzene plus 150 ppm butanol in air at a space velocity of 30 000 hh’. The conversion of each compound in the mixture as a function of temperature is presented in Fig. 7a and 7b for Pt and Pd catalysts, respectively. The conversion of each compound, when oxidized alone, is also shown for comparison.

It can be seen that for both catalysts, benzene oxidation is almost completely suppressed, as long as there is unreacted butanol in the reaction mixture. When complete conversion of butanol is achieved, benzene oxidizes rapidly and the temperature at which almost complete conversion of benzene is obtained is

80

60

s 100r : .- k? p 80- C 0

0 60-

40 -

20 -

(b)

150ppm Butanol

250ppm Benzene

Butanol (alone): o (in mixture): 0

Benzene (alone): 0 (in mixture): n

250 Temperature, (“C)

3c

Fig. 7. VOC conversion as a function of temperature during oxidation of a benzene-butanol mixture and of each component alone over (a) 0.3% Pt/y-AlaOs and (b) 0.3% Pd/y-AlaOs catalysts.

3

184 P: Papaefthimiou et al./Applied Catalysis B: Environmental 13 (1997) 175-184

the same as the one corresponding to oxidation of benzene alone (220°C over Pt and 260°C over Pd). In the case of Pt catalysts (Fig. 7a), butanol oxidation is also inhibited by benzene, and its conversion curve is shifted to higher temperatures by lo-20°C. Thus, almost complete conversion of butanol is obtained in the mixture at 220°C. Similar behavior has been observed by Barresi et al. [19] during oxidation of a benzene-ethanol mixture over a Pt-based commercial catalyst. On the other hand, a positive effect of ben- zene on butanol conversion is observed over Pd cata- lysts (Fig. 7b), shifting the conversion curve to lower temperatures by N 20°C. Thus, destruction efficiency of butanol greater than 95% is obtained at 200°C.

These results reveal the complexity of the processes that take place during oxidation of mixtures, implying that predication of the catalyst behavior is not possible in most cases and requires experimental verification.

4. Conclusions

The following conclusions can be drawn from the results of the present study:

Pt and Pd are the most active group VIII metals for the oxidation of benzene and butanol, while Pd and Co are the most active metals for the oxidation of ethyl acetate. The oxidation activity of the various metals is highest for butanol and lowest for ethyl acetate; Pt particle size, or, alternatively, Pt dispersion influences the intrinsic activity of the catalyst for benzene and ethyl acetate oxidation. This is not true in the case of Pd catalysts; using a chloride-containing precursor compound for the preparation of Pt/y-A1203 catalysts exerts an inhibiting effect on the activity of Pt; apparent activation energies of the reactions over Pt and Pd catalysts were found to be in the 70- 120 kJ/mol range, while the reaction order with respect to the VOC was always positive with values ranging O-l ; and

. mixture effects during oxidation of benzene-buta- no1 mixtures are significant. Benzene oxidation is completely suppressed, as long as there is butanol, not oxidized completely.

Acknowledgements

Financial support by the Commission of the Eur- opean Union (Contract No. EVW-CT93-0253) is gratefully acknowledged.

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