Low temperature decomposition of PCDD/PCDF, chlorobenzenesand PAHs by TiO2-based V2O5±WO3 catalysts

Roland Webera,*, Takeshi Sakuraia, Hanspaul Hagenmaierb

aResearch Institute, Ishikawajima-harima Heavy Industries Co., Ltd., 1, Shin-Nakahara-cho, Isogo-ku, Yokohama 235, JapanbInstitute of Organic Chemistry, University of TuÈbingen, D-72076 TuÈbingen, Germany

Received 2 October 1998; received in revised form 20 November 1998; accepted 27 November 1998

Abstract

The oxidation of representative congeners of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans

(PCDFs), polychlorinated chlorobenzenes (PCBzs), and of polyaromatic hydrocarbons (PAHs) was investigated on two

commercial V2O5±WO3/TiO2-based catalysts, optimized for the combined reduction of nitrogen oxides and decomposition of

dioxins.

The non-chlorinated polyaromatic compounds (including non-chlorinated dibenzodioxin and dibenzofuran) are destroyed at

temperatures as low as 1508C with an ef®ciency of more than 95%. PCDD and PCDF were also removed from the gas phase

with an ef®ciency of >98%. However, at 1508C they remained mainly unchanged (up to 75%) adsorbed on the catalyst.

A decrease in the oxidation rate with increasing chlorine substitution was found for the PCDD/PCDF. This could be

explained by an increasing `̀ redox potential'' with increasing chlorine substitution due to the electron withdrawing effect of

the chlorine.

For the more volatile monoaromatic PCBz, however, the effect of lowering the volatility with increasing chlorine

substitution (resulting in longer residence time on the catalyst) over-compensates the effect of the increasing `̀ redox

potential'' with higher degree of chlorination. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Vanadia±tungsta/titania; Decomposition (catalytic-); VOCs; PCDD; PCDF; Dioxins; PAHs; Chlorobenzenes;

Volatility

1. Introduction

The stringent limiting value for PCDD/PCDF emis-

sion of 0.1 ng I-TEQ/Nm3 for municipal and hazar-

dous waste incinerators has been in effect in several

European countries (e.g. Austria, Germany [1], Neth-

erlands) since the early 1990s and in Japan since

January 1997 for new municipal waste incinerators

(MWIs) [2,3].

At present primary measures such as design and

operation of the ®ring system to minimize the forma-

tion of `̀ products of incomplete combustion'' (com-

bustion technology) or boiler technology (i.e.

in¯uencing of the de novo synthesis in the cooling

of ¯ue gas) cannot guarantee compliance with this

limiting value [4]. Therefore, secondary measures are

needed to lower the emissions of PCDD/PCDF formed

Applied Catalysis B: Environmental 20 (1999) 249±256

*Corresponding author. Tel: +81-45-7592164; fax: +81-45-

7592149; e-mail: roland_weber@ihi.co.jp

0926-3373/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

P I I : S 0 9 2 6 - 3 3 7 3 ( 9 8 ) 0 0 1 1 5 - 5

to an extent that the limiting value imposed is not

exceeded. Two fundamentally different technologies

are applied in waste combustion facilities:

� Adsorption on a filtration material (fixed bed or

carbon spray in the flue gas) [5].

� Catalytic oxidation with direct destruction of the

PCDD/PCDF in the flue gas [6,7].

Both systems are considered proven technologies to

enable stack gas values below 0.1 ng I-TEQ/Nm3. The

contaminated ®ltration material needs additional treat-

ment to destroy the PCDD/PCDF, e.g. low tempera-

ture treatment under oxygen de®ciency [4,8]. The

catalyst is a true sink for the PCDD/PCDF due to

destruction of the compounds resulting in CO2, H2O

and HCl [9].

It has been shown that the TiO2-based V2O5±WO3

catalysts originally designed for the removal of

nitrogen oxides (NOx) by selective catalytic re-

duction (SCR) [10±14] are very effective in the

decomposition of PCDD/PCDF at the same tempera-

tures as are used for the DeNOx-reaction. With addi-

tion of ammonia, these catalysts can therefore be used

for the combined destruction of dioxins and NOx

[6,7,15,16].

In the last few years, the commercial SCR catalysts

have been optimized for the combined dioxin/NOx

destruction. This was achieved mainly by increasing

the oxidation potential of the catalysts by a higher

vanadium content. The impact of the vanadium con-

tent on the oxidation ef®ciency was also reported

recently for the decomposition of 1,2-dichlorobenzene

in a laboratory study [17].

In previous laboratory studies of the catalytic

destruction of chlorinated VOCs on metal oxide based

catalysts, assessment of the catalytic activity of var-

ious metal oxides was made usually on the basis of

only one or two compounds. In all these experiments

with chlorinated compounds, only highly volatile

organic compounds (VOCs) such as methylene chlor-

ide [18], dichloromethane [19], ethyl chloride [20],

dichloroethane [21], tetrachloroethene [9], tetrachlor-

omethane [18,19], chlorobenzenes [9,17,19,20,22] or

chloro¯uorocarbons [23] were used. The effective

temperature for the destruction of the chlorinated

VOCs found in these studies was above 3008C. Only

the TiO2-based V2O5±WO3 catalysts show effective

destruction of tetrachloroethene at a temperature of

2308C [15]. The only laboratory study reported

for the catalytic destruction of chlorinated semi-vola-

tile organic compounds in ¯ow experiments (hexa-

chlorobenzene, 2,4,8-trichlorodibenzofuran) focused

on the temperature region between 2608C and 5008C[9].

For the effective destruction of PCDD/PCDF, how-

ever, lower temperatures might be suf®cient. In pilot

plants, temperatures of 240±2608C have already been

tested [6,7,16] and shown to be effective. For eco-

nomical reasons, especially, if the catalyst is operated

downstream of wet scrubbers, it is desirable to operate

the catalysts at the lowest temperature which guaran-

tees the required emission limits. Therefore, pilot

plant tests have been carried out with catalyst tem-

peratures around 2008C [24]. However, laboratory

studies for the interesting compounds for waste com-

bustion, e.g. PCDD, PCDF, polychlorinated biphenyls

(PCB) or PAH which unequivocally prove the ef®-

ciency of the catalysts in these temperature regions are

missing. Such tests are expensive, time consuming and

dif®cult to carry out. Another problem is the high

toxicity of some compounds of these groups. There-

fore, the destruction behavior regarding PCDD/PCDF

have been investigated mainly in ®eld tests

[6,7,16,24]. The purpose of this investigation was to

use simpli®ed testing procedures for the laboratory

study of the catalytic destruction of chlorinated poly-

aromatic compounds on commercial TiO2-based

V2O5±WO3 catalysts, developed for the combined

destruction of dioxins and NOx, in the temperature

range 150±2508C.

2. Materials and methods

2.1. Catalysts

The catalysts used in this study are commercial

catalysts (V2O5±WO3 on TiO2 basis) especially devel-

oped for the simultaneous destruction of PCDD/PCDF

and nitrogen oxides. According to the producer the

BET surface of the two catalysts was between 70 and

90 m2/g.

The two catalysts were subjected to elemental

analysis according to the Japanese industrial standard

method (JISM) with a sequential plasma spectrometer

ICPS-7500 (Shimadzu, Kyoto, Japan). The results are

shown in Table 1. According to X-ray diffraction

250 R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256

analysis by a MXP-3 (Mac Science, Yokohama,

Japan), the structure of the TiO2 phase for both

catalysts is of the anatase type (Table 1).

2.2. Chemicals

All purchased from GL Science, Japan or Cam-

bridge Isotope Laboratories, USA:

Chlorobenzenes. 1,2-Dichlorobenzene (D2CBz),

1,2,3-trichlorobenzene (T3CBz), 1,2,4-T3CBz, 1,2,3,

4-tetrachlorobenzene (T4CBz), 1,2,4,5-T4CBz, and

hexachlorobenzene (H6CBz).

Polyaromatic compounds. Pyrene, biphenyl, diben-

zofuran, and dibenzodioxin.

PCDD/PCDF. 2,8-Dichlorodibenzofuran (D2CDF),

2,7-dibenzodioxin (D2CDD), 1,3,6,8/1,3,7,9-tetra-

chlorodibenzodioxin (T4CDD), 1,2,4,6,7,9-hexachlor-

odibenzodioxin (H6CDD), 1,2,4,6,8,9-hexachloro-

dibenzofuran (H6CDF), 1,2,3,4,6,7,9-heptachlorodi-

benzofuran (H7CDF), 1,2,3,4,6,7,9-heptachlorodiben-

zodioxin (H7CDD), octachlorodibenzodioxin

(O8CDD) (synthesized by condensation of the respec-

tive chlorophenols [25±27]), 2,4,8-trichlorodibenzo-

furan (T3CDF), octachlorodibenzofuran (O8CDF).

2.3. Experiments with compounds in flow-stream

The honeycomb catalysts are carefully crushed and

sieved. The particle size of the resulting ¯akes are

about 0.6 mm�1 mm�2±5 mm. Approximately 5 g

of the respective catalyst is placed in the quartz tube

(13 mm internal diameter) of the ¯ow reactor (Fig. 1).

The reactor is enclosed in a temperature controlled

furnace which provides a reactor temperature stability

of �28C. All ancillary tubing is maintained at a

temperature of 150±2008C with heating tape. The

testing substances (1,3,6,8-T4CDD, 1,3,7,9-T4CDD,

2,4,8-T3CDF, PCBz, biphenyl, dibenzodioxin, diben-

zofuran and pyrene) are applied to silica. For the

experiments, 500 mg of silica containing 1 mg of each

compound is placed in the preheating system. After

the reactor reaches stability at the respective tempera-

ture, the substances are evaporated resulting in an

average concentration of about 1 ppm in the gas phase.

Temperature program for evaporation: starts at 708C,

holds for 5 min, 108C/min to 1608C, and holds for

15 min resulting in a total time of 30 min for the

experiment. The composition of the gas mixture is

chosen analogous to MWIs (10% O2, 70% N2, and

20% H2O). The volumetric ¯ow through the catalyst

bed of 700 cm3/min corresponds to a space velocity

(SV) of 5000 hÿ1, and is comparable to that of SCR

catalyst in MWIs. The ¯ow rate is regulated using

mass ¯ow controllers (Shinagawa Seiki, Japan).

Table 1

Composition and structural data of the catalysts

Catalyst Detected

phase (XRD)

Catalyst composition

(wt%)

TiO2 V2O5 WO3

A Anatase 82.9 6.2 5.5

B Anatase 78.2 7.1 5.9

Fig. 1. Schematic of the reactor: (1) O2/N2 source, (2) flow controller, (3) water, (4) pump, (5) preheating system, (6) evaporation port for

compounds, (7) furnace, (8) pyrex reactor, (9) catalyst bed, (10) washing bottles for sampling (empty/toluene), (11) active carbon trap, and

(12) gas meter.

R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256 251

2.4. Stationary experiments for rate constant

calculation

A mixture of PCDD (P

500 ng) and PCDF (P

500 ng) is applied in toluene directly on catalyst B

(500 mg). The toluene is evaporated for 10 min at

508C. The furnace is heated to the respective tem-

perature and the catalyst is ®lled into the reactor. After

5 min, the catalyst is taken out of the reactor and

rapidly cooled to room temperature.

Measurements with a thermocouple show that the

catalyst reached the desired temperature after about

60 s. The cooling process lasts about 15 s.

2.5. Analysis and quantification

After the experiment the glass tubes after the cat-

alyst were rinsed with toluene. These rinses were

combined with the toluene in the washing bottle.

The catalysts were extracted separately by 12 h Soxh-

let extraction with toluene. This procedure ensures

complete removal of any adsorbed organic com-

pounds, as was proven in one case by a second 24 h

Soxhlet extraction with toluene and toluene/acetone

(95:5). The toluene in the washing bottle and the

extracts of the catalysts were analyzed separately.

In the ¯ow experiments the destruction removal

ef®ciency (DRE) corresponds to (input less output)/

input. The destruction ef®ciency corresponds in all

experiments to (input less (output plus unchanged

material adsorbed on catalyst))/input.

The clean-up procedures are described elsewhere

[28].

Analysis was carried out on a HP 6890 gas chro-

matograph coupled to a HP 5973 mass selective

detector. The chlorobenzenes, pyrene and the biphe-

nyls were quanti®ed by external calibration. The

quanti®cation for PCDD/PCDF and the dibenzodioxin

and dibenzofuran was carried out by an isotope dilu-

tion mass spectrometry with 13C-labeled standards.

The GC columns used were a CP-SIL 88 column

(50 m, 0.25 mm i.d., 0.2 mm ®lm thickness, CHROM-

PACK, Frankfurt/FRG) and a DB-5 fused silica col-

umn (30 m, 0.32 mm i.d., 0.25 mm ®lm thickness,

J&W Scienti®c, Folsom/USA).

3. Results

The destruction ef®ciency of the catalysts for the

decomposition of PCDD/PCDF, PAHs and chloroben-

zenes (PCBzs) were compared in ¯ow experiments

over the temperature range 150±3108C.

3.1. `̀ Polyaromatic'' compounds (PCDD, PCDF,

PAH)

From the amount of polyaromatic compounds

found downstream of the catalyst, it appeared that

both catalysts decompose these molecules in the

examined temperature range 150±3108C with an ef®-

ciency of more than 95%, the PCDD/PCDF (1,3,6,8-

T4CDD, 1,3,7,9-T4CDD, 2,4,8-T3CDF) even to an

extent of more than 98% (Fig. 2). However in the

experiments carried out at 1508C, 59±75% of the

PCDD/PCDF were found unchanged on the catalysts

(Table 2), while at 1908C less than 7% of the

unchanged PCDD/PCDF were found on the catalysts

after the experiments. Therefore, at temperatures

below about 2008C, polychlorinated aromatic com-

pounds remain adsorbed on the catalyst for several

minutes without being oxidized. At temperatures

below 2008C, the oxidizing potential of V2O5±WO3

is obviously not suf®cient to decompose polychlori-

nated aromatic compounds effectively.

For the non-chlorinated polyaromatic compounds

dibenzo-p-dioxin, dibenzofuran, biphenyl and pyrene

the destruction ef®ciency is even at 1508C higher than

90% (Fig. 2 and Table 2).

Table 2

Recovery of compounds adsorbed on the catalysts after the flow stream experiments at 1508C (space velocity: 5000 hÿ1; gas phase

composition: 10% oxygen, 70% nitrogen, and 20% water)

Compounds 1,2,4,5-T4CBz H6CBz Biphenyl Pyrene DF DD 2,4,8-T3CDF 1,3,6,8-/1,3,7,9-T4CDD

Catalyst A (% recovery) 0.3 11.3 0.5 0.7 0.8 0.3 68 59

Catalyst B (% Recovery) 0.5 12.4 0.8 1.1 0.5 0.4 74 63

252 R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256

Both catalysts exhibit quite comparable oxidation

behavior.

Chlorine substitution of the dibenzofuran and

dibenzodioxin molecules lower the rate of oxidation

decisively. The reason for this change in oxidation

behavior is most likely the electron withdrawal effect

of the chlorine substituents towards the aromatic ring

system. This seems to protect the polychlorinated

compounds from oxidation at low temperatures.

The in¯uence of the degree of halogenation of the

aromatic compounds on the rate of oxidation was also

observed in previous studies, when we investigated the

effect of chlorine and ¯uorine substitution in diben-

zodioxin and dibenzofuran with regard to the oxida-

tive degradation by sulfuric acid on silica [29,30]. In

these studies, the non-halogenated and low haloge-

nated dibenzodioxins and dibenzofurans are oxidized

to a considerable amount within several minutes at

about 708C. The tri- to octa-halogenated congeners are

nearly unaffected by this treatment.

3.1.1. Adsorption-affinity of the polyaromatic

compounds on the catalyst

As mentioned above, the polyaromatic compounds

are removed effectively from the gas phase by the

catalysts at 1508C and remain for several minutes on

the catalysts without desorption. The same com-

pounds evaporate however quantitatively from silica

at 1508C. The polyaromatic compounds are obviously

strongly adsorbed on the V2O5±WO3/TiO2 catalysts

by physisorption.

Our explanation for this physisorption quality of

these catalysts are the Lewis acid sites interacting with

the �-orbitals of the aromatic systems, and in the case

of dioxins and furans additionally with the free elec-

tron pairs of the oxygen atoms, both exhibiting partial

Fig. 2. Destruction removal efficiency (DRE) of biphenyl, dibenzofuran, dibenzo-p-dioxin, pyrene, 2,4,8-T3CDF and 1,3,6,8-/1,3,7,9-T4CDD

by catalysts A (^) and B (&) in the temperature range 150±3108C (space velocity: 5000 hÿ1; gas phase composition: 10% oxygen, 70%

nitrogen, and 20% water).

R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256 253

donor qualities. This is consistent with the fact that

V2O5±WO3/TiO2 catalysts show strong Lewis acid

properties [14] while the surface sites on silica are

only weakly Lewis acidic [31]. Capillary condensa-

tion phenomena seems unlikely to explain the adsorp-

tion quality hence the total amount of compounds in

one ¯ow experiment only corresponds to about 0.01±

0.1% of a monolayer on the catalyst surface. Also the

mean radius of the micropores for monolithic TiO2-

based honeycomb catalysts of 80±100 AÊ [32] seems

large compared to the size of the PCDD/PCDF mole-

cule with about 14 AÊ times 7 AÊ to explain the adsorp-

tion quality.

3.1.2. Estimation of destruction efficiency from

stationary experiments

When we recognized that at temperatures below

2008C, the PCDD/PCDF do not desorb from the

catalysts within minutes, it seemed feasible to inves-

tigate destruction rates for the catalytic oxidation of

these compounds by a simpli®ed procedure in a sta-

tionary system. In the ¯ow reactor experiments, it was

not possible to evaluate the exact residence time of the

molecules on the catalyst due to the `̀ time range'' of

evaporation of the molecules from silica.

For the stationary experiments, the compounds

were placed directly on catalyst B prior to heating.

The catalyst was heated for 5 min at the desired

temperature and the reaction was stopped by rapid

cooling.

Less than 1% of the compounds desorbed under

these conditions unchanged from the catalyst as was

predicted from the results in the ¯ow reactor.

The destruction ef®ciency in dependence of the

degree of chlorination and the temperature is shown

in Fig. 3. While at 1508C the non-chlorinated diben-

zodioxin and dibenzofuran are already oxidized after

5 min to an extent of about 95%, more than 85% of the

O8CDD and O8CDF remain unoxidized on the cata-

lyst. The destruction ef®ciency of the lower chlori-

nated compounds lay between these extreme values

(Fig. 3).

At 2008C after the 5 min residence time, the

destruction ef®ciency for O8CDD and O8CDF had

risen to about 80% and for the T3CDF/T4CDD to

about 95%. The destruction of the non-chlorinated

dibenzodioxin and dibenzofuran was practically com-

plete under these conditions (unchanged compounds

were below the detection limit) (Fig. 3)

3.2. Chlorobenzene±chlorine substitution and

competition of `̀ redox potential'' versus volatility

Above 3008C, more than 99% of the tested chlor-

obenzenes (PCBzs) are oxidized by these catalysts,

while at temperatures around 2008C the extent of

oxidation is about 50% (Fig. 4).

In contrast to the polyaromatic compounds the

degree of chlorination of PCBz does not have a

decisive in¯uence on the degree of their decomposi-

tion. With increasing chlorine substitution, the cata-

lytic decomposition even increases, which is more

emphasized at temperatures below 2008C.

When considering the results obtained with the

`̀ polyaromatic'' compounds, this result is somewhat

surprising at ®rst because the sensitivity towards

oxidation of the chlorobenzenes also decreases with

Fig. 3. Destruction of adsorbed PCDD (&) (dibenzodioxin, 2,7-

D2CDD, 1,3,6,8-T4CDD, 1,2,4,6,7,9-H6CDD, 1,2,3,4,6,7,9-

H7CDD, O8CDD) and PCDF (^) (dibenzofuran, 2,8-D2CDF,

2,4,8-T3CDF, 1,2,4,6,8,9-H6CDF, 1,2,3,4,6,7,9-H7CDD, O8CDF)

by catalyst B during 5 min heat treatment at 1508C and 2008C (gas

phase composition: 10% oxygen, 70% nitrogen, and 20% water).

254 R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256

the increasing degree of chlorination (e.g. ignition

temperature (Table 3) or oxidation in sulfuric acid).

On the other hand, with increasing degree of chlor-

ination, the volatility of the compounds is consider-

ably lowered as can be estimated from the boiling

points of the chlorobenzenes [33,34] and PCDD/

PCDF [35,36] (Table 3). This results in longer resi-

dence times of the molecules on the catalyst surface,

as is con®rmed by the fact that after the ¯ow experi-

ment at 1508C, 10±20% of H6CBz, only trace amounts

of T4CBz (0.3±1.0%) and no T3CBz and D2CBz were

detected on the catalyst (Table 2). With chloroben-

zenes, therefore, the effect of lowering the volatility

with increasing chlorine substitution compensates or

Fig. 4. Destruction removal efficiency (DRE) of 1,2-D2CBz, 1,2,3-T3CBz, 1,2,4,5-T4CBz and H6CBz by catalysts A (^) and B (&) in the

temperature range 150±3108C (space velocity: 5000 hÿ1; gas phase composition: 10% oxygen, 70% nitrogen, and 20% water).

Table 3

Boiling points of some polychlorinated dibenzo-p-dioxin, polychlorinated dibenzofurans [35,36] and boiling points and ignition temperature

of some chlorobenzenes [33,34]

Chlorobenzenes M1CBz 1,2-D2CBz 1,2,3-T3CBz 1,3,5-T3CBz 1,2,3,5-T4CBz P5CBz H6CBz

Boiling point (8C) 132 179 219 208 247 276 322

Ignition temperature (8C) 23 65 126 126 >170

PCDD DD 2,3-D2CDD 1,2,4-T3CDD 2,3,7,8-T4CDD 1,2,3,6,7,8-H6CDD H7CDD O8CDD

Boiling point (8C) 279 358 375 447 487 507 510

PCDF DF 2,3-D2CDF 2,3,8-T3CDF 2,3,7,8-T4CDF 1,2,3,6,7,8-H6CDF H7CDF O8CDF

Boiling point (8C) 287 375 408 438 488 507 537

R. Weber et al. / Applied Catalysis B: Environmental 20 (1999) 249±256 255

even overcompensates the effect of an increasing

`̀ redox potential''.

4. Conclusions

The test system described for the determination of

the temperature dependence of catalytic destruction

ef®ciencies (and destruction removal ef®ciencies) is

especially useful in comparing various catalysts with

regard to their destruction ef®ciencies for compounds

of low volatility.

� The catalytic decomposition strongly depends on

the volatility of the compounds and the oxidative

behavior, both related to the degree of chlorina-

tion.

� The catalysts tested show a destruction efficiency

for PCDD/PCDF of >98% above 2008C.

� At temperatures below 2008C, part of the chlori-

nated `̀ polyaromatic'' compounds remain unox-

idized on the catalysts for several minutes. In an

incinerator with continuous gas flow, this could

result in an accumulation of these compounds on

the catalysts. Saturation could result in the dis-

placement of adsorbed compounds and a

decreased rate of adsorption, resulting in an over-

all increased emission concentration.

� Non-chlorinated polyaromatic compounds are

effectively decomposed at temperatures as low

as 1508C, while for the effective destruction of

monoaromatic compounds (chlorobenzenes and

nitrobenzenes, etc.) operating temperatures above

2508C are necessary for >95% destruction.

The results of this laboratory study obtained with

PCDD/PCDF coincide with results obtained with

catalysts installed in MWIs with operating tempera-

tures above 2008C. Therefore, the test systems used

should allow further investigations with relevance to

applications in MWIs and other thermal processes.

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