On the selection of the catalyst among the commercial platinum-based ones for total oxidation of...

Applied Catalysis B: Environmental 27 (2000) 243–256

On the selection of the catalyst among the commercial platinum-basedones for total oxidation of some chlorinated hydrocarbons

José Corella∗, José M. Toledo, Ana M. PadillaDepartment of Chemical Engineering, University ‘Complutense’ of Madrid, 28040 Madrid, Spain

Received 28 November 1999; received in revised form 14 March 2000; accepted 26 March 2000

Abstract

Catalytic total oxidation of some selected chlorinated hydrocarbons is studied with several Pt-based catalysts. Chlorinatedhydrocarbons used have been ethyl chloride, trichloroethylene and dichloromethane, alone or mixed with some hydrocarbons,such as toluene. Eighteen different catalysts have been tested from eight different manufacturers (Degussa AG, Süd-ChemieAG, Kataleuna GmbH, Chimet, Johnson Matthey, Prototech Co.,. . . ) and from three research institutions (Universities ofLeiden (NL) and Wroclaw (PL) and Spanish CSIC-ICP). Catalysts both in the form of spheres (particulates) and of monolithsare used. Selection of the best catalyst(s) is made based on their activity, selectivity and life. Apparent energies of activationfor these reactions on these catalysts, using an empirical first-order reaction rate, are given. There are interesting or noticeabledifferences in activity and selectivity among the tested Pt-based catalysts. In overall they are not so active as the chromia andvanadia-based catalysts but they have an high life, reason why they can be recommended for this application. © 2000 ElsevierScience B.V. All rights reserved.

Keywords:Chlorinated hydrocarbons; Total oxidation; Platinum-based catalysts; Methylene chloride; Waste incineration; Catalytic gascleaning

1. Introduction

Heterogeneous catalytic total (complete or deep)oxidation of volatile organic compounds (VOCs) is aquite well known process [1] and nowadays it doesnot present important (major) difficulties. Neverthe-less, chlorinated organic compounds (Cl-VOCs) aremore difficult to convert/destroy than VOCs and in themarket there are not many catalysts for total oxidationof such Cl-VOCs.

Even more, very often they present some problemslike their deactivation, high cost or relative low ac-

∗ Corresponding author. Tel.:+34-91-394-41-64;fax: +34-91-394-41-64.E-mail address:[email protected] (J. Corella)

tivity. Selection of the best catalyst for eliminatingCl-VOCs is still doubtful. This paper is focussed thuson the selection of the best catalysts for total oxidationof Cl-VOCs.

Most of the existing commercial catalysts (made byUS and European manufacturers) as well as some newones being developed by several research institutionsworld-wide will be here tested.

Cl-VOCs whose catalytic total destruction would beof interest can be divided in two broad groups: chlo-rinated compounds of low molecular weight (from C1to C10) and polychlorinated dibenzodioxins and furans(PCDD/Fs). The content of these two types or lumps ofCl-VOCs in a flue gas at industrial scale or in a incin-eration plant is usually very different. In the first case,typical concentrations are 200–2000 ppm (Table 1)

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244 J. Corella et al. / Applied Catalysis B: Environmental 27 (2000) 243–256

Table 1Some previous work on total oxidation of Cl-VOCs with Pt-based catalysts

Reference Authors Catalyst Halogenated compound Content (ppm) GHSV (h−1)

[2] Yang and Reedy (1977) Pt–Ru Vinyl chloride and mixtures 2400–4800[3] Hiraoka et al. (1991) Pt/SiO2–TiO2 PCDDs 1000–5000[4] Klinghoffer and Rossin (1992) 1% Pt/Al2O3 CAN 250–1000[5] Yu et al. (1992) 4% PdO/g-Al2O3 TCE 200 4000–24000[6] Mendyka et al. (1992) 0.05–0.15% Pt TCE 2000–4000 10000–30000[7] Noordally et al. (1993) Precious metals on

cordierite monolithTCE, DCM, 1-2 DCE 500 12000

[8] Rossin and Farris (1993) 2% Pt/a-Al2O3 Chloroform 100–5000[9] Müller et al. (1993) Pt, Pd/Al2O3 1-2 DCE, methyl chloride 1000 15000[10] Agarwal et al. (1993) 2.15% Pt/a-Al2O3 CNCl 5300[11] Windawi and Wyatt (1993) Pt/ceramic honeycomb MC and C1 and C2 halocarbons 10–1500 26000[12] Freidel et al. (1993) Pt, Pd/ceramic honeycomb TCE, CCl4 500–3500 5000–15000[13] Sharma et al. (1995) Pt, Pd/ceramic MC and C1 and C2 halocarbons 65–350 3000–15000

while in the second case, contents are usually below1 ng/m3. Even more, first type of Cl-VOCs are usuallyfound in atmospheres or flue gas without dust or partic-ulates, while PCDD/Fs usually need to be destroyed influe gas with dust or particulates, as it is the case of mu-nicipal solid waste incineration plants. These two factshave generated two different types of catalysts (and oftechnologies) for Cl-VOCs abatement: spheres, extru-dates or pellets, and monoliths. As an starting pointfor the catalytic abatement of Cl-VOCs from waste in-cinerators, which include PCDD/Fs, only some ‘lowmolecular weight Cl-VOCs’ will be used in this pa-per. Such ‘low molecular weight Cl-VOCs’ selectedfor this study are: ethylene chloride (CH3–CH2Cl,EC), dichloromethane or methylene chloride (CH2Cl2,DCM), trichloroethylene (TCE), and chlorobenzene(C6H5Cl, CB).

The catalysts used in the said objective can be as-sembled into four main groups: (i) supported noblemetals, (ii) chromium-based, (iii) vanadia-based, and(iv) others, including single/mixed metal oxides likeperovskites. This paper is devoted only to supportednoble metals of which the most world wide used isplatinum. Results in this paper with Pt-based catalystswill be compared then with results got with the othertypes of total oxidation catalysts.

Concentrating thus on the use of Pt-based cata-lysts for total oxidation of Cl-VOCs, a lot of work,Table 1, has been already or previously made. Themost used or targeted Cl-VOCs have been DCM andTCE [2–17] which will be thus the Cl-VOCs most usedin this paper. This work will be concentrated thus in

the selection at lab scale of the best catalyst(s) amongthe commercial/existing/available ones for total oxi-dation of Cl-VOCs.

2. Experimental

2.1. Installation used

The facility at laboratory scale is shown in Fig. 1. Itcomprises: (a) feeding system, (b) catalytic reactors,and (c) gas sampling and analysis. Feeding systemincludes some mass flow meters for air and for gaseousCl-VOCs, like EC, and syringe pumps (from 0.1 to5 ml) volume for liquid Cl-VOCs, like DCM and TCE.Air flow is bubbled in a water tank to get a flow with1 vol.% H2O.

Several glass-made reactors have been used, from16 to 30 mm internal diameter. These sizes are big-ger than the common catalytic laboratory-scale reac-tors. Some gradients of temperature could happen inthem but these sizes had to be used to work withcommercial Pt-based catalysts, full size, without be-ing ground. Most commercial Pt-based catalysts arespheres/beads of 2 to 4 mm and, to avoid wall effectsor flow by-passing, reactors with diameters of at least6 times the particle diameter have to be used.

Since these glass reactors could not support tem-peratures higher than 500–540◦C, all tests ended atabout 450–470◦C. Temperature was measured just be-fore and after the catalytic bed. Differences of temper-ature between inlet and exit were always below 5◦C.

J. Corella et al. / Applied Catalysis B: Environmental 27 (2000) 243–256 245

Fig. 1. Facility of UCM for testing total oxidation catalysts for targeted Cl-VOCs.

A stainless-steel reactor of square cross (15 cm×15 cm) section was also manufactured and used forthese catalysts but the stainless-steel was attacked bythe reaction atmosphere which includes HCl and per-haps some Cl2. The overall conversion was then dueboth to catalyst and to reactor wall. For such reason, re-sults with this metallic reactor were discarded and arenot presented here. The 15 cm×15 cm monoliths werecut then to 20 mm×20 mm smaller ones and testedin smaller and glass-made reactors (without catalyticactivity).

Most of the monoliths used had a length of 20 cm.Monoliths were sealed (using glass wool or carborun-dum) with the reactor wall in order to prevent gasby-passing.

Main experimental conditions have been the follow-ing ones:

GHSV 10,000 h−1 (normal conditions)Halocarbon contentin the flue gas

1000 ppm for DCM

200 ppm for TCETemperature 200–450◦C

2.2. Sampling and analysis

Sampling was made at two locations in the facil-ity: at the inlet and at the exit of the catalytic reac-tor. Fig. 1 shows the sites of such sampling points.The first sampling point provided the baseline to knowthe difference with the sample from the catalytic bedexit. The sampling is on line with a gas chromato-graph (GC) and a mass spectrometer. The GC is a5890 Series II Hewlett Packard, equipped with anelectronegative conductivity detector (ECD), a capi-lar column (HP-5 Crosslinked 5% Ph Me Silicone,50 m×0.32 cm×0.17mm (thickness)), and a HP 3396Series II integrator.

2.3. Catalysts used

Excepting eight experimental or new catalystsmade by Universities of Leiden (NL) and of Wro-claw (Poland) and by the Spanish CSIC, all catalystsused/tested were commercial ones. The catalysts usedare shown in Table 2. Secrecy agreements with man-ufacturers don’t allow to publish here more details

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Table 3Textural analysis of some commercial Pt-based catalysts by N2

adsorption

EF 257 H/D HAC-Exp02 PRO*CLEAN*500

BET (m2/g) 106 104 112Total pore

volume (cm3/g)0.57 0.54 0.60

Average porediameter (Å)

215 205 212

of the said commercial catalysts. Only a few moreproperties of some catalysts are shown in Table 3.

Some catalysts as the EF 257 H/D from Degussa AGor the PRO*CLEAN*500 from Süd-Chemie AG (with0.15 wt.% Pt each) have the noble metal deposited in athin and outer shell of the particle while some others,as the ones from Univ. of Leiden (NL) with 0.5 to2.0 wt.% Pt, have the Pt homogeneously distributedalong the particle radius.

The commercial catalysts were used ‘as received’,as supplied by manufacturers. They were notpre-treated before the tests.

2.4. Experimental error

Before starting the comparison among the catalysts,some reproducibility tests were made in order to knowthe error or scatter in the data [18]. With some se-lected catalysts up to five tests were made with thesame experimental conditions using in each test a dif-ferent sample of catalyst. Results from such five dif-ferent tests were the same, indicating accuracy. Possi-ble differences in activity among the catalysts can bewell detected thus in these catalytic tests.

3. Results

3.1. Effect of the Cl-VOC concentration at inlet (C0)

Several tests were made using the same catalystand space-velocity but varyingC0. The light-off orconversion (X)–T curves for total oxidation of EC,DCM and TCE with several concentrations of Cl-VOCat inlet are shown in Figs. 2–5.

For some catalysts and Cl-VOCs, case of Figs. 2and 3, theX–T curves for differentC0 values are the

Fig. 2. Effect of the inlet concentration of EC on its oxidationover tow different catalysts.

same. This result would indicate that the reaction hasan apparent first order respect to the main reactant(EC and DCM in such figures). In fact, an apparent

Fig. 3. Effect of the inlet concentration over EF-257 H/D in the totaloxidation of DCM (SV (n.c.): 10,000 h−1; C0: DCM=Toluene;10,000 ppm stream; 21% O2).


Fig. 4. Effect of the inlet concentration of DCM on its totaloxidation with mixtures of DCM and toluene. Catalyst: LEY#1Leiden (In brackets the number of the order of test. SV (n.c.):10,000 h−1; C0: DCM=Toluene; 21% O2).

first-order reaction, if the reactor has piston plug flowand if it is isothermal, leads to the following integratedor macrokinetic equation

SV[−ln(1 − X)] = k0 exp

(−Eapp

RT

)(1)

A different kinetic behaviour or model appear withsome other catalysts and/or reactants. It is here shownwith results got with the experimental catalysts LEY#1

Fig. 5. Conversion of TCE over the Pt/SM-1 catalyst from Univ.Wroclaw (SV (n.c.): 10,000 h−1; C0: DCM=Toluene; 21% O2).

Fig. 6. Activities of some noble metal catalysts (particles) in totaloxidation of ethylchloride (SV (n.c.): 10,000 h−1; C0: 1000 ppmEC; 21% O2).

from Univ. of Leiden and Pt/SM-1 from Techn. Univ.of Wroclaw. Five tests were made using DCM with theLeyden cat. varyingC0 from 70 to 1000 ppm, Fig. 4,and eight tests with the Pt/SM-1 one, feeding TCEfrom 30 to 300 ppms, Fig. 5. This trend (conversionincreases whenC0 decreases) would correspond tokinetic equations of the type

(−rCl-VOC) = k1CCl-VOC · · ·1 + k2CCl-VOC + k3CHCl + · · · (2)

in which there is some rate or activity inhibition dueto adsorption of reactants and/or of products like HCl.

Two different microkinetic models (first order andEq. (2)) are needed at least to fit the kinetics of thesereactions, thus.

3.2. Activity of the catalysts

3.2.1. Tests with ECThe activities of six different catalysts for to-

tal oxidation of EC are shown in Fig. 6 for fourparticle-shaped catalysts and in Fig. 7 for two mono-liths.

Some conclusions can already be drawn from thesetwo figures: (i) for total oxidation of EC the differ-ences in activity (among the four catalysts indicatedin Fig. 6) are small and (ii) among the two monolithstested from Johnson Matthey, the ‘high temperature’one (EP 1279) is much more active than the ‘low


Fig. 7. Activities of some Pt-based catalysts (monoliths) in totaloxidation of ethylchloride (SV (450◦C): 5000 h−1; C0: 500 ppmEC; 21% O2).

temperature’ one (EP 1271). In fact, such EP 1279 cat-alyst has proved to be very active for total oxidationof EC (near 100% conversion is obtained at 290◦C).

3.2.2. Tests with TCESeven Pt-based catalysts have been tested for total

oxidation of TCE. The activities of five particle-shapedcatalysts is given in Fig. 8 as theirX–T curves. The

Fig. 8. Activities of some Pt-based catalysts (particles) in to-tal oxidation of TCE (SV (n.c.): 10,000 h−1; C0: 200 ppm TCE;10,000 ppm stream; 21% O2).

Fig. 9. Activities of some Pt-based catalysts (monoliths) in totaloxidation of TCE (SV (n.c.): 1000 h−1; C0: 20 ppm TCE; 1000 ppmstream 21% O2. (*) See Table 3 for SV).

activity of two monoliths is presented in Fig. 9. Ex-perimental conditions of such tests are given in thecorresponding figure captions.

From Fig. 8, it is deduced how the two most ac-tive catalysts are the ground EP 1273 from JohnsonMatthey and the 7714k from Kataleuna GmbH. Re-sults in Fig. 9 indicate that the ‘experimental’ (notcommercialised yet) monolith called HAC-Exp01from Degussa AG is very active.

3.2.3. Tests with DCMThe Cl-VOC most studied in this work has been

DCM. In total, twelve different Pt-based catalysts,both particles and monoliths, from seven manufactur-ers have been tested with DCM.

Activities at different temperatures are shown inFig. 10 for six Pt-based spherical catalysts and inFig. 11 for three different monoliths. Experimentalconditions in these tests are given in the figure captionsand in Table 2. From these results the authors considerthat the most active catalysts are LEY#1, from Univ.Leiden; 714k, from Kataleuna GmbH; EP-1280 andEP-1272, from Johnson Matthey; HC-Exp01, fromDegussa AG.

When temperatures to get 50 and 90% conversions,T50%, T90%, of DCM are plotted against the Pt-contentin the catalyst (according to the manufacturers’ cat-alogues), there is not a relationship because, as it is


Fig. 10. Activities of some Pt-based catalysts (particles) in totaloxidation of DCM (SV (n.c.): 10,000 h−1; C0: 1000 ppm DCM;10,000 ppm steam; 21% O2).

well-known, Pt-content is not enough to characterisethe activity of the catalyst. For instance, three catalystswith 0.15 wt.% Pt have very different values ofT50%andT90%. Location of the Pt atoms (in the outer partor shell of the catalyst, or homogeneously distributed)and Pt-dispersion affect DCM conversion, and theseparameters are not given in the manufacturers’ cata-logues.

Fig. 11. Activities of some Platinum catalysts (monoliths) in totaloxidation of DCM (SV (n.c.): 10,000 h−1; C0: 1000 ppm DCM;1000 ppm steam 21% O2. (*) See Table 4 for SV).

Fig. 12. Activities of different Pd-based catalysts in the catalytictotal oxidation of dichloromethane over paladium catalysts (parti-cles).

The experimental catalyst from Univ. of Leiden(LEY#1) with a 2.0 wt.% Pt-content has the lowestvalues forT50% andT90%. Such a 2.0 wt.% Pt is a good(active) catalyst thus but it is expensive too.

Activities of three Pd-based commercial catalystsare shown in Fig. 12. Comparing results in Fig. 12 withthose of Figs. 10 and 11, it is deduced that commercialPd-based catalysts are not so active as most of thePt-based ones.

3.2.3.1. Effect of the H2O and O2-contents in the fluegas. Several tests were made with a very active cat-alyst, LEY#1, with two different O2-contents (21 and10.7 vol.% O2) in the flue gas and with three differentH2O-contents too (0, 1 and 2 vol.% H2O). Results areshown in Fig. 13.

Results with 1 and 2 vol.% H2O are, under theseconditions, the same than the ones without H2O in theflue gas. It is probably due to two facts: (i) in thesetests there was also some toluene in the flue gas (in thesame amount that DCM) which provided H-atoms toget a high H:Cl ratio, five in these tests and (ii) theC0content was quite low (70 ppm). Another behaviourcould happen with higher Cl-VOC contents. This isthe reason why this variable (H2O-content in the fluegas) will be studied more in detail with the catalystsselected (from this paper).


Fig. 13. Effect of the atmosphere used over the LEY#1 cat-alysts in total oxidation of DCM (SV (n.c.): 10,000 h−1; C0:DCM=Toluene=70 ppm).

3.2.4. Ranking of the catalysts according to theiractivity

A detailed kinetic study was absolutely out thescope of this work and it was not made, thus. As itwas above said, results got with the broad spectrum ofcatalysts here used need at least two different microki-netic models to fit them. Nevertheless, several differ-ent institutions (Universities of Leyden, Strasbourg,Wroclaw, Twente, Budapest and Madrid) workingin join EU-financed projects have to compare theirresults. Besides, results presented in this paper willbe further compared with results got with chromia[17] and vanadia–titania-based [19] catalysts whichprobably generate different reaction mechanisms andkinetic equations. Since comparing results from sodifferent catalysts and institutions is quite difficult, itwas decided to select and use the simplest, althoughempirical, microkinetic model: single and first orderreaction. All X–T data were fitted to Eq. (1), thus.Fitting was always acceptable. Using a given (con-stant) value for the space-velocity (SV) for each test,‘apparent’ pre-exponential factors (k0) and energies ofactivation (Eapp) were calculated from Eq. (1). Thesetwo parameters (k0 andEapp) are apparent ones, thus:they are calculated under an empirical and/or forcedapproach.

Eapp so obtained ranged between 8 and 13 kcal/mol(32–53 kJ/mol). These values are too low for three rea-

sons at least: (1) kinetic approach used is not goodenough, of course. Effect of temperature on kineticparameters in denominator of Eq. (2) makes to mod-ify temperature dependence on kinetic constant in nu-merator when such denominator is omitted as it isthe case of a first-order kinetic equation. (2) Thereis some internal diffusion control because of the rel-atively big size of the commercial catalyst particlesused. (3) True gas residence time in the catalytic beddecreases onT increasing in all tests here presented.The space-velocity or the space-time calculated under‘normal conditions’ is the same along the experimentor test. Nevertheless, in theX–T curve each point hasa different gas residence time or SV value when it iscalculated at its corresponding bed temperature. Whenthis fact is taken into account in Eq. (1), somewhathigherEapp values are obtained.

On the other hand, it is well known the so-calledcompensation effectbetweenEappandk0. Both param-eters are affected by the catalyst activity. For a givenEapp value, k0 increases with catalyst activity.k0 insuch case can be used as an index of the apparent ac-tivity of the catalyst.

To compare the activity of the catalysts used in thiswork, an averaged value forEapp of 44 kJ/mol wasselected for all catalysts. For such a constant value ofEapp(=44 kJ/mol) allX–T curves were fitted again toEq. (1) andk0 were re-calculated. Suchk0 values areshown in Table 4.

Table 4Activity index (pre-exponential factors of the Arrhenius law, con-sidering a first-order kinetic equation andEapp=44 kJ/mol) fortotal oxidation of Cl-VOCs on different Pt-based catalysts

Catalyst Cl-VOC k0×10−3 (s−1)

EP 1279 HTHC EC 65D-4768 EC 25EF-257 H/D EC 22EP 1271 LTHC EC 21

LEY#1 DCM 114HAC-Exp01 DCM 73EF-257 H/D DCM 307714k DCM 32HAC-Exp02 DCM 31D-4768 DCM 23

EP 1273 LTHC TCE 62HAC-Exp01 TCE 437714k TCE 31LEY#4 TCE 30


Table 5Activity index (pre-exponential factor of the Arrhenius law, con-sidering first-order kinetic equation andEapp=44 kJ/mol) for totaloxidation of Cl-VOCs on different chromia-based catalysts (Datafrom [17])

Catalyst Cl-VOC k0×10−3 (s−1)

PRO*HHC*Base2 DCM 31ARI Econ Abator DCM 24PRO*CLEAN*110 DCM 13R-3-20 DCM 7.3

PRO*HHC*Base2 TCE 31ARI Econ Abator TCE 21R-3-20 TCE 18PRO*CLEAN*110 TCE 23

The ‘activity index’ (k0) so calculated indicateshow there are ‘quite interesting’ differences in activityamong the catalysts tested in this work. For instance,the ‘experimental’ catalysts LEY#1 and HAC-Exp01have activities (for total oxidation of DCM) 2–4 timeshigher than most of the commercial catalysts used inthis work.

The same methodology or approach was applied toresults got with commercial chromia-based catalysts[17]. The ‘activity index’ for such catalysts is shownin Table 5. When results got with Pt-based (Table 4)and chromia-based catalysts (Table 5) are compared,both groups/types of catalysts have an activity for to-tal oxidation of pure Cl-VOCs of the same order ofmagnitude.

3.3. Selectivity of the catalysts by-products

3.3.1. Tests with ECBy-products detected in total oxidation of EC are

shown in Fig. 14 for two commercial catalysts. Us-ing G-68B catalyst, three co-products were detected,D-4768 produces two co-products, and EF 257 H/D,PRO*PEL*VOC*1414 and EP 1279 produces severalcompounds in very small amounts. The most selective(towards CO2 and HCl) catalysts, among the ones heretested, were the EF 257 H/D from Degussa AG and thePRO*PEL*VOC*1414 from Prototech. EP 1279 cata-lyst proved to be very active but not enough selective.

3.3.2. Tests with TCETotal oxidation of TCE with the seven catalysts

indicated in Table 2 produces (besides CO2, HCl,

Fig. 14. By-product formed in total oxidation of ethylchloride overtwo catalysts from Chimet and Prototech (SV (n.c.): 10,000 h−1;C0: 1000 ppm EC; 21% O2).

and H2O of course) three main co-products: CHCl3,CCl4 and C2Cl4. The amounts of these co-productsat the exit of the catalytic bed are shown in Fig. 15.The most abundant co-product is C2Cl4. EP 1273,HAC-Exp02 and 7714k catalysts generate importantamounts of C2Cl4. HAC-Exp01 catalyst produces thelowest amount of C2Cl4. This catalyst would be themost selective among all the catalysts, although it alsoproduces trace amounts of CHCl3 and CCl4. Except-ing the LEY#4 catalyst which had the lowest selectiv-ity, the remaining catalysts had a similar selectivity;no big differences (in amounts of co-products) weredetected between them.

3.3.3. Tests with DCMThree main by-products have been identified in the

catalytic oxidation of DCM: CHCl3 and CCl4.CHCl3 and CCl4 formed with seven catalysts

are shown in Figs. 16 and 17. LEY#1, LEY#4


Fig. 15. Amounts of CHCl3, CCl4 and C2Cl4 formed in totaloxidation of TCE over some noble metal (particles and monoliths)catalysts (SV (n.c.): 10,000 h−1; C0: 200 ppm TCE; 10,000 ppmsteam; 21% O2. (*) See Table 3).

and HAC-Exp01 are the catalysts which form theless amount of CHCl3. LEY#4 and HAC-Exp01are the catalysts which form the less amount ofCCl4

The most selective catalysts for deep oxidationof DCM are, thus, HAC-Exp01, from Degussa AG;7714k, from Kataleuna GmbH; LEY#1, and LEY#4from Leiden University.

Fig. 16. CHCl3 formed in total oxidation of DCM over some noblemetal catalysts (particles and monoliths) (SV (n.c.): 10,000 h−1;C0: 1000 ppm; 10,000 ppm steam; 21% O2. (*) See Table 4).

3.4. Catalyst deactivation

The selection of a catalyst depends, of course, notonly in its activity and selectivity but also on its life.Two good catalysts, having proved their high activ-ity and selectivity were selected for this checking:EF 257 H/D and HAC-Exp01 both from DegussaAG. Conversion with time-on-stream for the EF 257H/D catalysts at 400◦C under 1000 ppm of DCM isshown in Fig. 18. This test lasted 120 h. According tothis figure, under these experimental conditions therewould be a very small deactivation (from 87 to 84% in120 h).

DCM conversion at different times-on-stream, un-der the same conditions of the HAC-Exp01 catalystis shown in Fig. 19. No noticeable deactivation wasfound with this catalyst.

Condensates got in these tests were analysedby inductively coupled plasma-mass spectrometry(IEC-MS) and neither Pt not Pd were found in con-


Fig. 17. CCl4 formed in total oxidation of DCM over some noblemetal catalysts (particles and monoliths) (SV (n.c.): 10,000 h−1;C0: 1000 ppm; 10,000 ppm steam; 21% O2. (*) See Table 4).

Fig. 18. Long-time test for the EF 257 H/D catalyst from DegussaAG (SV: 10,000 h−1; C0: 1000 ppm DCM; T: 400◦C; 1 vol.%steam).

densates which is a positive fact (remember thatchromia-based catalyst lost Cr by chlorine attack,[17]).

Fig. 19. Long-time test for the HAC-Exp01 catalyst from DegussaAG (SV: 10,000 h−1; C0: 1000 ppm DCM; T: 400◦C; 1 vol.%steam).

4. Conclusions

There are interesting or noticeable differences inthe apparent activity among the tested commercialPt-based catalysts for total oxidation of Cl-VOCs.These differences are still something more importantwhen these commercial catalysts are compared withnew or experimental catalysts not commercialisedyet. It would mean that catalyst manufacturers canstill improve their Pt-based catalysts, concerning thisapplication.

Due to the high price and intrinsic activity of Pt(and of Pd), it is recommended to use it depositedonly on the outer shell of the catalyst particle (asit already happens in most of the commercial cat-alysts for this application). But due to the fact thatthey are designed for their use in fixed beds, theyhave a relative big size (2–5 mm diameter) whichin turn makes to have a low apparent activity if itis based on weight of the overall catalyst. To find asolution to this problem (higher catalyst activity bykg of catalyst) would bring a high benefit. Aboutthis point, Pt-based monoliths with a thin wall couldbe a good solution. In fact, some monoliths heretested (such as HAC-exp01 from Degussa AG andEP 1279 from Johnson Matthey) have proved to beamong the most actives ones (by unit of weight ofcatalyst).


Catalyst selectivities are quite different among thecatalysts here tested. They have to be taken into ac-count. The ranking of the catalysts concerning theirselectivity has been indicated in the text.

Perhaps, the most important aspect of these catalystsis their life or deactivation. Of course, it depends ontemperature and H:Cl ratio but, in general, for thePt-based catalysts here tested, their deactivation hasnot been a problem. Its life seems (with the limitationsof this research about long-term tests) quite high. NoPt has been detected in condensates when they wereanalysed for it.

Comparing results in this paper with Pt-based cata-lysts with the ones obtained with chromia-based [17]and with V–W–Ti monoliths [19], the Pt-based oneshad the longest life or the smallest deactivation (forrelative low H:Cl ratios). Chromia and vanadia-basedcatalysts do not resist the attack by the nascent Cl2(formed by the reaction HCl with O2) and Cr andV are quickly lost from the catalyst surface (17, 19).Pt-based catalysts are thus the best ones, among thetypes studied and for these authors of course, concern-ing deactivation or life.

Comparison of these three types or groups of cat-alysts for total oxidation of Cl-VOCs can be sum-marised in the following way

Catalyst Pt-based Cr-based V2O5-based

Activity Moderate Somewhathigher thanPt-based

High

Life Acceptable Low Low

Deactivation detected in some Pt-based catalysts isattributed to the support (usually Al2O3) and not bythe platinum. The key aspect would be the support,thus. Many Al2O3-types are not good enough becauseit does not resist the attack of the nascent chlorine.Catalyst manufacturers should focus attention to thesupport of these catalysts, and perhaps TiO2 or ZrO2might be better supports than Al2O3 which, of course,needs to be verified.

5. Nomenclature

CSIC Spanish Council for ResearchEC ethylene chloride

MC, DCM methylene chloride or dichloromethaneDCE dichloroethaneTCE trichloroethyleneC0 concentration of Cl-VOC at the reactor

inlet (ppm)Eapp apparent energy of activation (kJ or

kcal/mol)k0 pre-exponential factor of the Arrhenius

law for an apparent first-order reaction(s−1 in Tables 4 and 5 or h−1 in Eq. (1))

k1 kinetic constant, Eq. (2) (mol h−1 g percatalyst ppm−1)

k2, k3 adsorption parameters, Eq. (2) (ppm−1)n kinetic order of the reactionSV gas hourly space velocity (normal

conditions) (h−1)T temperature at the exit of the catalytic

bed (◦C)rCl-VOC rate of reaction of the Cl-VOC

(mol h−1 g per catalyst)X conversion of the halocarbon

Acknowledgements

This work has been carried out under theEC, DGXII, contracts no EV5V-CT94-0530 andINCO-DEMO-2094-1996. The authors thank to theEuropean Commission its financial support. The au-thors also thank very much the comments from R.Louw, Ben Nieuwenhuys, Manon F. Jeurissen andRuud W. van den Brink from University of Leiden(NL), and to Dr. Reisinger from Degussa AG, toDr. Trawczynski from The Techn. Univ. of Wroclaw(PL), to Kataleuna GmbH, Süd-Chemie AG, Chimet(Viciomaggio, Arrezo, Italy), Johnson-Matthey andPrototech Co. for the supply of samples of catalystsfor their testing at UCM, and to Mercedes Gutierrezfor carrying out some experiments.

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