Total oxidation of volatile organic compounds by vanadium promoted palladium-titania catalysts:...

Total oxidation of volatile organic compounds by vanadium

promoted palladium-titania catalysts: Comparison of

aromatic and polyaromatic compounds

Tomas Garcia a, Benjamin Solsona a, Diego Cazorla-Amoros b,Angel Linares-Solano b, Stuart H. Taylor a,*

aCardiff University, School of Chemistry, Main Building, Cardiff Cf10 3AT, UKbDepartamento de Quımica Inorganica, Universidad de Alicante. Apartado 99, E-03080 Alicante, Spain

Received 2 May 2005; received in revised form 8 June 2005; accepted 11 June 2005

Available online 26 August 2005

Abstract

Vanadium oxide, palladium oxide and mixed Pd/V-supported on titania catalysts have been prepared and tested in the total oxidation

of volatile organic compounds (VOCs). A comparative study with two different aromatic VOCs (benzene and naphthalene) has been

carried out. For benzene, the mixed Pd/V-catalysts presented the highest catalytic activity. However, whilst studies with benzene led to

the formation of CO2 only, the total conversion of naphthalene to CO2 was not achieved throughout the full temperature range for

naphthalene conversion. A naphthalene conversion to CO2 of 99% was obtained over Pd/TiO2, V/TiO2 and Pd/V/TiO2 catalysts at 275, 325

and 300 8C, respectively. Therefore, the requirements for an effective benzene total oxidation catalyst cannot be readily extrapolated to

larger polycyclic aromatic compounds, as in the naphthalene oxidation the most active catalyst from an environmental point of view is Pd

supported on TiO2.

# 2005 Elsevier B.V. All rights reserved.

Keywords: VOCs; Benzene; Naphthalene; Catalytic oxidation; Palladium; Vanadium

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 62 (2006) 66–76

1. Introduction

In recent years, the design of catalytic systems for

reducing emissions of environmentally unacceptable com-

pounds is an important development for the protection of the

environment. Problems that have been addressed catalyti-

cally are clean up technologies for mobile emission control,

NOx removal from stationary sources, liquid and solid waste

treatment, greenhouse gas abatement or conversion, sulphur

compound removal and volatile organic compound (VOC)

combustion [1]. Anthropogenic VOC emissions are released

in a variety of processes, such as exhaust gases from the

internal combustion engine, industry and power generation

plants. VOCs include a wide range of different compounds,

* Corresponding author. Tel.: +44 29 2087 4062; fax: +44 29 2087 4030.

E-mail address: [email protected] (S.H. Taylor).

0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2005.06.016

such as oxygenates, aliphatic, aromatic and halogenated

hydrocarbons. Assessing the available technology, catalytic

combustion is one of the most promising, for the removal of

VOCs from polluted air streams [2].

In the literature, several catalysts have been used for the

total oxidation of aliphatic compounds. These catalysts have

previously been reviewed [3,4] and it can be concluded that

noble metal supported catalysts are preferred due to their

higher activity. One particular advantage of supported metal

catalysts is that the metal is dispersed over a greater surface

area of the support and shows different activity from the

unsupported metals due to interactions of the metal with the

support. The support also reduces thermal degradation. The

modification of the catalyst properties by addition of a

promoter in order to improve the catalytic performance of

the supported noble metal catalysts has been widely studied.

Recently, a new class of vanadium promoted palladium

T. Garcia et al. / Applied Catalysis B: Environmental 62 (2006) 66–76 67

titania catalysts for complete aliphatic hydrocarbon oxida-

tion were studied [5,6]. A strong synergistic effect between

palladium and vanadium in the complete oxidation of

aliphatic compounds was established. Comparison of the

0.5%Pd/1.5%V/TiO2 catalyst with other palladium cata-

lysts, tested under similar conditions, showed that it was

the most active. These studies clearly demonstrated that

the Pd/V/TiO2 catalyst possesses high complete oxidation

activity.

Various catalysts have also been investigated for the

total oxidation of benzene [7–9]. As for aliphatic

compounds, vanadium promoted catalysts on different

supports were found to be highly active in the catalytic

combustion of benzene [10–13]. However, the catalytic

combustion of polycyclic aromatic hydrocarbons (PAH),

such as naphthalene has rarely been studied. The limited

number of studies are almost exclusively focussed on the

study of naphthalene total combustion on different noble

metal and metal oxide catalysts, such as Pt, Pd, Ru, Co, Mo

and W supported on g-Al2O3 [14], and Pt supported on g-

Al2O3 [15]. Naphthalene catalytic oxidation in gas mixtures

simulating emissions from combustion of biofuels has also

been studied. Pt commercial catalysts [16], Pd modified

zeolites [17] and catalysts based on Mn/Cu oxides, mixed

with Pt or Pd, supported on a-Al2O3 alone or doped with La

[18] exhibited high activities for the oxidation of the

model pollutants. Finally, the catalytic combustion of

PAH mixtures in an incinerator exhaust gas has been

investigated [19,20]. In these investigations, V/TiO2 [19] and

V-W/TiO2 [20] have been shown to be a highly efficient way

of removing gas phase PAH at very low temperatures.

Therefore, the vanadium promoted palladium titania catalyst,

which we have recently reported, could be a very promising

catalyst to abate atmospheric pollution from a variety of

emission sources. To the best of our knowledge, this is the first

study not only showing the performance of V promoted

Pd/TiO2 catalysts in catalytic combustion of PAH, but also

comparing the catalytic activity of aromatic and polyaromatic

compounds over the same catalysts. At this point, only the

catalytic combustion of benzene and aliphatic compounds

have been compared previously [13,18].

Table 1

Chemical composition, BET surface area, TPR hydrogen consumption and CO c

Catalysts Pd (%) V (%) SBET

0.5PdT 0.5 0.0 50c

0.5VT 0.0 0.5 48

1.0VT 0.0 1.0 48

1.5VT 0.0 1.5 47

3.0VT 0.0 3.0 42

0.5Pd0.5VT 0.5 0.5 47c

0.5Pd1.0VT 0.5 1.0 47c

0.5Pd1.5VT 0.5 1.5 45c

0.5Pd3.0VT 0.5 3.0 39c

a Data obtained by TPR.b Fraction of vanadium monolayer (%), data obtained by CO chemisorption.c Data obtained from [6].

2. Experimental

2.1. Catalyst preparation

The catalysts were prepared by dissolving a known

amount of PdCl2 (Aldrich 99%) in deionised water. The

solution was heated to 80 8C and stirred continuously. An

appropriate quantity of ammonium metavanadate (Aldrich

>99%) and oxalic acid (Aldrich >99%) were added to the

fully dissolved PdCl2 solution. Titanium oxide (Degussa

P25. SABET = 50 m2 g�1) was added to the heated solution

and stirred at 80 8C to form a paste. The resulting paste was

dried at 110 8C for 16 h. The catalyst contained V in the

range 0.5–3.0 wt.%. A similar method was used to prepare

TiO2 based materials without vanadium and without

palladium. Final catalysts were prepared by calcination in

static air at 550 8C for 6 h. The range of catalysts prepared is

detailed in Table 1.

2.2. XPS measurements

The oxidation state of V and Pd in the calcined and

used catalysts was determined by the XPS technique. X-ray

photoelectron spectroscopy (XPS) measurements were

performed with a VG-Microtech Multilab, using a Mg

Ka (1253.6 eV) source. Spectra were obtained with a

constant pass energy of 50 eV. The pressure in the analysis

chamber of the spectrometer was 5 � 10�10 mbar during

the measurements. The BE values were obtained using a

Peak-fit Program.

2.3. Catalyst activity

Benzene total oxidation experiments have been carried

out in a 10 mm stainless steel reactor (BTR-Jr.) coupled to a

mass spectrometer (Thermocube, Balzers). The reaction

feed consisted of 100 vppm benzene and 10% O2 in He. A

total flow rate of 100 ml min�1 was used and catalysts were

packed to a constant volume to give a gas hourly space

velocity of 45,000 h�1 for all studies. Catalytic activity was

measured over the range 100–450 8C in incremental steps,

hemisorption uptake

H2 consumption (mmol H2/gcat)a CO uptake (mmol CO/gcat)

b

�1 8.7

61 –

116 –

224 –

390 –

31c 5.3

45c 3.6

313c 3.3

563c 2.4

T. Garcia et al. / Applied Catalysis B: Environmental 62 (2006) 66–7668

Table 3

XPS results for TiO2-supported catalysts

Catalyst Ti 2p O 1s V 2p3/2 Pd 3d

0.5PdT 458.0 463.8 529.6 – 335.2 340.4

0.5PdTu 457.9 463.7 529.5 – 335.4 340.6

1.5VT 458.7 464.3 529.8 516.9 – –

3.0VT 458.7 464.3 530.0 516.9 – –

0.5Pd0.5VT 458.6 464.3 530.0 516.8 – –

0.5Pd1.0VT 458.7 464.2 530.0 516.6 337.2 342.0

0.5Pd1.5VT 458.6 464.3 530.0 516.7 337.1 342.0

0.5Pd1.5VTu 458.7 464.4 530.0 516.7 337.2 342.0

0.5Pd3.0VT 458.6 464.3 529.9 516.7 337.2 341.9

and temperatures were measured by a thermocouple placed

in the catalyst bed. To ensure the measurement of steady

state data the same collection procedure outlined above was

adopted. For both reactors conversion data were calculated

by the difference between inlet and outlet concentrations and

all carbon balances were in the range 100 � 10%.

A different catalytic reactor was required for determina-

tion of naphthalene oxidation, due to the difficulties

associated with the potential deposition of naphthalene in

a reactor which was not fully trace heated. Catalyst activity

for naphthalene oxidation was determined using a fixed bed

laboratory micro reactor. Catalysts were tested in powdered

form using a 1/400 o.d. stainless steel reactor tube. The

reaction feed consisted of 100 vppm naphthalene in air.

A total flow rate of 50 ml min�1 was used and catalysts were

packed to a constant volume to give a gas hourly space

velocity of 45,000 h�1 for all studies. Analysis was

performed by an on-line gas chromatograph with thermal

conductivity and flame ionisation detectors. Catalytic acti-

vity was measured over the temperature range 100–325 8Cin incremental steps, and temperatures were measured by a

thermocouple placed in the catalyst bed. Data were obtained

at each temperature after a stabilization time. Three analyses

were made at each temperature to ensure that steady state

data were collected. The reaction temperature was increased

and the same procedure followed to determine each data

point.

3. Results and discussion

3.1. Catalyst characterisation

The Pd/V/TiO2 catalysts used in the present study have

been extensively characterised previously [5,6]. Character-

ization techniques applied were BET surface area, CO

chemisorption, H2 temperature programme reduction (H2-

TPR), laser Raman spectroscopy, X-ray diffraction (XRD)

and electron paramagnetic resonance spectroscopy (EPR).

Table 1 shows a summary of important characterisation data

[6]. From the characterization data, the following were

evident: (i) there was a reduction of the catalyst surface area

as Pd and/or V loading increase; (ii) a dramatic decrease

of the CO uptake, as the V loading increased was observed;

Table 2

Chemical surface ratios calculated by XPS

V/Ti Pd/Ti

0.5PdT 0.031

0.5PdTu 0.024

1.5VT 0.15

3.0VT 0.16

0.5Pd0.5VT 0.021 0.0093

0.5Pd1.0VT 0.082 0.0097

0.5Pd1.5VT 0.15 0.010

0.5Pd1.5VTu 0.15 0.011

0.5Pd3.0VT 0.23 0.013

(iii) the addition of palladium increases the amount of

polyvanadate species and V2O5 crystallites on the surface of

Pd/V/TiO2 catalysts, but no major modification of the nature

of the vanadium species was detected; (iv) the reducibility of

vanadium sites was dramatically enhanced for Pd/V/TiO2

catalysts when compared to V/TiO2; (v) only diffraction

peaks corresponding to TiO2 anatase, and in minor quantity

TiO2 rutile, were detected in all the catalysts; (vi) the

addition of Pd increased the overall abundance of

paramagnetic V4+ ions (i.e., affecting the oxidation state

of the supported vanadium ions) but it did not structurally

perturb the nature of the supported vanadium oxide species

themselves.

X-ray photoelectron spectroscopy was used to complete

the previous characterisation results and data obtained are

shown in Tables 2 and 3. Table 2 compares the surface

chemical composition of the catalysts. The fact that similar

values for the V/Ti ratio were found for 1.5%V/TiO2 and

3.0%V/TiO2, in spite of the two-fold higher nominal amount

of vanadium in the latter catalyst, is in agreement with the

previous results reported for V2O5/TiO2 catalysts, which

were explained by a model consisting of ‘towers’ of

crystalline vanadia covering the monolayer of VOx [21]. The

V/Ti ratio in the case of Pd/V/TiO2 increased with the

vanadium concentration, but it must be noted that the

increase of the V/Ti synthesis ratio did not scale directly

with the measure V/Ti surface ratio. This trend can be

explained by the presence of V2O5 crystallites over the VOx

monolayer. However, the V/Ti surface ratio were higher for

0.5%Pd/3.0%V/TiO2 than for 3.0% V/TiO2 catalysts. These

data confirm Raman spectra results [6], showing that the

addition of palladium increases the amount of polyvanadate

species on the surface of Pd/V/TiO2 catalysts. No change

was observed in the vanadium surface concentration of

0.5%Pd/1.5%V/TiO2 after these catalysts were used in the

benzene catalytic combustion for 24 h.

The XPS results also allow the determination of the

relative concentrations of Ti and Pd on the catalyst surface,

see Table 2. These results showed that the Pd/Ti ratio for

the Pd/TiO2 catalyst was higher than for the Pd/V/TiO2

catalysts, independently of the vanadium loading. This result

agrees with the CO uptake [6], indicating that the co-

impregnation of Pd and V over a TiO2 support most likely

resulted in less dispersed Pd particles. Moreover, it was also


Fig. 1. XPS spectra of V 2p 3/2 in the case of: (A) 1.5%V/TiO2 and 0.5%Pd/

1.5%V/TiO2; (B) 3.0%V/TiO2 and 0.5%Pd/3.0%V/TiO2.

reported in Table 2 that whilst the Pd/Ti ratio was constant at

vanadium loading lower than the monolayer for Pd/V/TiO2

catalysts, a slight increase in the Pd/Ti ratio at vanadium

loading higher than 1.5% was observed. Conversely, CO

uptake values for these catalysts decreased with the increase

of vanadium loading. Therefore, a straightforward explana-

tion for these results is not immediately apparent. Again, the

presence of crystalline vanadia covering the monolayer of

VOx could explain the XPS results, as less Ti and V can be

detected on the catalyst surface. Unfortunately, the lack of

agreement between XPS results and CO chemisorption does

not allow us to reach any definitive conclusion about the Pd

particle dispersion. Table 2 also shows that whilst the Pd/Ti

ratio remained almost constant for fresh and used 0.5%Pd/

1.5%V/TiO2, this ratio decreased after reaction for 0.5%Pd/

TiO2. Therefore, the addition of vanadium seems to modify

the sintering in the palladium particles during use.

The O 1s binding energies are also shown in Table 3.

These values are due to the overlapping contribution of

oxygen from vanadium and titania in the case of V/TiO2;

palladium and titania in the case of Pd/TiO2 and palladium,

titania and vanadia in the case of the Pd/V/TiO2 catalysts.

The maximum contribution to the O 1s in all these catalysts

was TiO2 since its concentration was higher than those of

other constituents. In Table 3, it is observed that the binding

energy of the most intense O 1s peak for V/TiO2 and Pd/V/

TiO2 catalysts was almost constant, and it was independent

of the vanadium loading. However, a shift to lower binding

energies can be noted in the case of the un-promoted Pd/

TiO2 catalyst. Table 3 also reports the binding energies of Ti

2p photoelectron peaks. It was observed that the binding

energies of Ti 2p3/2 and Ti 2p1/2 lines for vanadium

promoted and un-promoted catalysts were 458.6 � 0.1 and

464.3 � 0.1 eV, and 457.9 � 0.1 and 463.8 � 0.1 eV,

respectively. As mentioned for O 1s binding energy values,

a shift to lower binding energies has also been found. This is

in agreement with laser Raman spectroscopy data for these

samples [6], as they showed a Raman frequency shift for the

TiO2 bands when vanadium was present. This peak shift is

due to the different transfer of charge from TiO2 particles to

Pd or V/Pd species. The auger parameters of Ti 2p3/2 and Ti

2p1/2 lines in the case of V- and V/Pd-catalysts showed very

small and insignificant variations for both V/TiO2 and Pd/V/

TiO2 catalysts. In this case, as charge effects did not affect

the parameter; the Ti seems to be in a similar chemical state

for all these samples. Thus, it can be assumed that Pd

particles are not interacting directly with the TiO2 support on

vanadium promoted Pd/TiO2 catalysts, and during the

calcination process, the palladium particles migrate to the

catalyst surface. It is worthy of comment that a possible

ensemble effect between the Pd and V particles during this

migration process could be ruled out, as a change in the

nature of the vanadium species should be expected.

However, EPR and Raman spectra have shown that although

the addition of vanadium can influence the relative

concentrations it did not structurally perturb the nature of

the supported vanadium oxide species themselves as no new

species were identified.

Finally, it can be tentatively proposed that in the case of

vanadium promoted Pd/TiO2 catalysts, Pd particles with a

larger size than in the case of Pd/TiO2 catalysts are supported

over the VOx species. With increasing vanadium loading there

was a decrease of the CO uptake, which could be due to a

partial coverage of the Pd particles with V species and/or a

change in the Pd particle size. Unfortunately, it has not been

possible to find other similar systems in the literature prepared

by co-impregnation to compare the above suggestions.

Complementary characterisation studies using HRTEM are

currently in progress to provide a more detailed explanation.

Table 3 shows the XPS binding energies for some

representative catalysts. The binding energies of V 2p3/2

photoelectrons for both V- and V/Pd-catalysts are 516.9 and

516.7 � 0.1 eV, respectively. It is well known that the BE

of the photoelectrons for V4+ and V5+ are 515.9 � 0.2

and 517 � 0.2, respectively [22,23]. Therefore, it can be

concluded that surface vanadium was mainly present as a

mixture of V4+ and V5+. XPS spectra were further processed

in order to estimate the V4+/V5+ surface ratios for the

different catalysts. The effect of palladium addition in the

case of 0.5%Pd/1.5%V/TiO2 and 0.5%Pd/3.0%V/TiO2

catalysts is shown in Fig. 1. It can be observed that the


addition of palladium produced an increase in the V4+/V5+

ratio and this increase was more marked in the case of the

0.5%Pd/3.0%V/TiO2 catalyst. Also, the V4+/V5+ ratio for

the V/TiO2 catalysts remained approximately constant for

the two different vanadium loading. However, no change

was observed in the V4+/V5+ ratio for Pd/V/TiO2 catalysts

with vanadium loading lower than the monolayer (not

shown). Although it is true that the nature of the vanadium

species changes the binding energy and the presence of

palladium modifies the proportion of the V species, the shift

of the band to lower energies agree with our previously

published EPR spectroscopy studies, which also demon-

strated an increase of V4+ content when palladium was

present in the catalyst [6]. Therefore, it can be concluded

that the addition of palladium to the vanadium catalysts

modified the redox properties of the vanadium sites, as

demonstrated by TPR analysis [6]. No significant differ-

ences were found between the vanadium binding energies

and the V4+/V5+ ratio of the fresh and used Pd/V/TiO2

catalysts.

According to the binding energy, the oxidation state of

palladium can also be estimated. For palladium the Pd 3d

binding energy of metallic palladium is 335 and 339 eV,

whereas for PdO it is 336 and 341 eV. As for our monometallic

Pd-catalysts, the binding energy were 335.2 and 340.4 eVand

for the mixed V/Pd-catalysts the binding energies were

337.1 � 0.1 and 342.0 � 0.1 eV, it can be concluded that

the average oxidation state of mixed V/Pd-catalysts was

Fig. 2. XPS spectra of Pd 3d in the case of: (A) 0.5%Pd/TiO2 and 0.5%Pd/

1.5%V/TiO2; (B) 0.5%Pd/TiO2 and 0.5%Pd/3.0%V/TiO2.

higher than that of Pd-catalysts. XPS spectra were

processed in order to estimate the Pd0/Pd2+ ratio in these

catalysts. The effect of vanadium addition in the case of

0.5%Pd/1.5%V/TiO2 and 0.5%Pd/3.0%V/TiO2 catalysts is

shown in Fig. 2. Only one palladium species, most likely a

metallic state, was found in the case of Pd/TiO2 catalysts.

However, different behaviour in the case of Pd/V/TiO2

catalysts was observed. Thus, whilst only Pd2+ seem to be

detected at vanadium loading lower than the monolayer

(data not shown for 0.5%Pd/0.5%V/TiO2 and 0.5%Pd/

1.0%V/TiO2 catalysts), a second peak most likely due to the

existence of metallic palladium particles was also detected

at vanadium loading higher than the monolayer. This

indicates that there were strong electronic interactions

between vanadium and palladium particles in the vanadium

promoted catalysts and that the addition of vanadium helps

the existence of palladium particles as Pd-O on the catalyst

surface. The Pd 3d binding energy values in the case of V/

Pd-catalysts at different vanadium loading are also

compared in Table 3 (in the case of 0.5%Pd/3.0%V/TiO2

the highest peak corresponding to Pd2+ and not the shoulder

was considered). It can be seen that these values remained

approximately constant for the different loadings. It has

been previously reported that the strength of the Pd–O bond

depends on the crystallite size [24]. Thus, it can be

tentatively proposed that the increase of the vanadium

loading for Pd/V/TiO2 catalysts does not change the size of

the Pd particles and, therefore, the decrease of CO uptake at

different vanadium loading [6] is likely to be due to the Pd

sites being covered by vanadium particles or partially

covered by the vanadium layer.

3.2. Catalyst activity

3.2.1. Benzene oxidation

Fig. 3 shows the catalytic activity for V/TiO2 and

0.5%Pd/V/TiO2 catalysts with different vanadium loadings

Fig. 3. Benzene conversion over V promoted and un-promoted Pd/TiO2

catalysts with varying vanadium loading as a function of reaction tempera-

ture. Pd promoted catalysts: (line) (�) 0.5%Pd/TiO2; (&) 0.5%Pd/0.5%V/

TiO2; (~) 0.5%Pd/1.0%V/TiO2; (^) 0.5%Pd/1.5%V/TiO2; (*) 0.5%Pd/

3.0%V/TiO2. Pd un-promoted catalysts: (dotted line) (&) 0.5%V/TiO2; (~)

1.0%V/TiO2; (^) 1.5%V/TiO2; (*) 3.0%V/TiO2.


for benzene oxidation as a function of the temperature. CO2

was the only product formed. The catalysts without

palladium showed the lowest activity. Benzene conversion

did not reach 100% at 450 8C with any of the V/TiO2

catalysts. The activity of the V/TiO2 catalysts was strongly

dependent on the vanadium loading. Therefore, there was a

direct relationship between the number of vanadium sites

and the catalytic activity. The general order of activity for

benzene total oxidation was:

3:0%V=TiO2 > 1:5%V=TiO2 > 1:0%V=TiO2 >

0:5%V=TiO2

Recently, Lichtenberger and Amidiris [9] have studied

the benzene reaction mechanism on V/TiO2. A two step

mechanism has been suggested where the first step is an

adsorption of the aromatic ring on the catalyst via

nucleophilic attack and the second step is an electrophilic

substitution of the adsorbed species. Partial oxidation

products were formed in both the presence and the absence

of gas-phase oxygen, indicating that the surface oxygen was

involved in the oxidation process. The nucleophilic attack

seems to be the slow and kinetically significant step in the

reaction mechanism. On the other hand, Ferreira et al. [25]

studied the benzene catalytic combustion over V/Al2O3

catalysts. The performance of this catalytic system cannot

directly be extrapolated, as Al2O3 is a support without redox

properties, but some similarities can be obtained on the role

of the vanadium sites. These authors reported that the

catalytic activity of V/Al2O3 catalysts could be correlated

with the vanadium content and that a higher ratio of V4+ at

higher vanadium loading was responsible for the higher

activity of these catalysts. Similar conclusions can be

suggested in this work as the catalyst with higher vanadium

loading shows higher activity. However, in this catalytic

system the characterization results show that the catalytic

behaviour of V/TiO2 catalysts could be linked to the increase

in the overall amount of V4+, but not to the increase of the

V4+/V5+ ratio with the vanadium loading.

In Fig. 3, it can also be observed that the activity of the

Pd//TiO2 catalyst was greater than all of the V/TiO2

catalysts, irrespective of the vanadium loading. Therefore,

benzene was oxidized at lower temperatures on palladium

noble metal catalyst supported on TiO2 than on vanadium

supported-TiO2. This type of catalytic behaviour was

previously observed when comparing the catalytic oxidation

of benzene oxidation over MnO2 and Pt/TiO2 catalysts [26].

Regarding noble metal catalysts, a different oxidation

mechanism has been proposed [27]. In this mechanism, an

irreversible adsorption of the aromatic compounds and non-

equilibrium adsorption of the oxygen over different sites was

assumed for the benzene catalytic oxidation over Pt/Al2O3

catalysts. Unfortunately, this behaviour cannot directly be

extrapolated to our system, as previously demonstrated by

Papaefthimiou et al. [7]. These authors, assuming a very

simple mechanism, have reported that there are significant

differences between the kinetic parameters of platinum

catalysts supported on Al2O3 or TiO2 in the benzene

oxidation, implying specific aspects of the oxidation

mechanism of these catalysts over the different supports.

Moreover, differences in the kinetic parameters in the case

of palladium or platinum catalysts supported over Al2O3

have also been reported [8].

It can also be seen in Fig. 3 that the activity of the Pd/V/

TiO2 catalysts was greater than all of the Pd/TiO2 catalysts,

irrespective of the vanadium loading. Therefore, it was

clearly observed that the presence of vanadium in the Pd/

TiO2 catalyst greatly increased the catalytic combustion of

benzene at all temperatures. The Pd/V/TiO2 catalysts all

showed complete benzene conversion to CO2 below 350 8C.

From Fig. 3, it was also apparent that the catalytic activity in

the case of Pd/V/TiO2 catalysts increased with the vanadium

loading. The general order of catalyst activity for benzene

oxidation was:

0:5%Pd=3:0%V=TiO2 > 0:5%Pd=1:5%V=TiO2 >

0:5%Pd=1:0%V=TiO2 > 0:5%Pd=0:5%V=TiO2

These data show that there is a synergistic effect between

TiO2 supported palladium and vanadium in the catalytic

combustion of benzene. Also, it can be seen that the addition

of vanadium to Pd/TiO2 significantly decreased the light off

temperature of benzene oxidation to temperatures as low as

125 8C. Other authors [10,11] have previously reported this

synergistic effect in the catalytic combustion of benzene in

the case of Pd/V2O5/Al2O3 catalysts. Vassileva et al. [11]

reported a promoting effect of Pd addition to a 30%V2O5/

Al2O3 catalyst. The results were attributed to a modification

of oxidation state of vanadium atoms by the palladium. On

the other hand, Ferreira et al. [10] claimed that a redox

mechanism was probably taking place, but it was not the

main reason for the better catalytic behaviour observed on

the Pd/V2O5/Al2O3 catalyst. These authors observed that the

turnover frequency of Pd/V2O5/Al2O3 catalysts increased as

the dispersion decreased suggesting that the benzene

oxidation on Pd/V2O5/Al2O3 catalysts was a structure

sensitive reaction. They also suggested that in spite of the

higher amount of V4+ species on the Pd/V2O5/Al2O3

catalysts containing 10 and 20 wt.% of vanadium, the effect

of palladium particle size plays a much more important role

in benzene combustion. However, as described above, the

catalytic mode of action of these systems cannot be directly

extrapolated to our system due to significant differences

between the chemistry of the supports used.

In previous work [6], we have reported that the propane

combustion on Pd/V/TiO2 catalysts was not solely a

palladium size dependent reaction, and the modification

of the redox properties of the catalyst was also an important

factor in determining catalyst activity. The comparison

between the catalytic activity of propane [6] and benzene

over Pd/V/TiO2 catalysts at different loading shows a

similar trend. Therefore, the strong synergistic effect for the


Fig. 4. (A) Naphthalene conversion over V/TiO2 catalysts with varying

vanadium loading as a function of reaction temperature. (B) CO2 yield over

V/TiO2 catalysts with varying vanadium loading as a function of reaction

temperature. (&) 0.5%V/TiO2; (~) 1.0%V/TiO2; (^) 1.5%V/TiO2; (*)

3.0%V/TiO2.

benzene catalytic combustion could again be linked to the

enhancement of the redox properties and the different Pd

particles size. In this paper, these assumptions were further

verified by XPS analysis. XPS data suggested that the

addition of vanadium to the palladium catalyst increases the

palladium particle size and helps the existence of palladium

particles as Pd-O in the catalyst surface. Pd or PdO

supported catalysts have been extensively used in the VOCs

total oxidation [7,28], nevertheless, their behaviour is not

understood well, and for example, the nature of the true

active phase (metal or metal oxide) still remains a matter of

discussion [29,30]. Recently, it has been reported that the

relationship between the oxidation state of palladium and its

catalytic activity indicated that the catalytic activity depends

on the oxidation state of palladium, and that partially

oxidized palladium is more effective for propane combus-

tion [31]. Therefore, the presence of oxidized palladium in

the case of Pd/V/TiO2 catalysts could favour the benzene

total oxidation. On the other hand, XPS results have also

shown that the addition of palladium to the vanadium

catalysts increased the V4+/V5+ ratio. This fact agrees with

previously published results [6] and it could also explain the

improvement in the catalytic behaviour of Pd/V/TiO2

catalysts. Finally, it is suggested that the increase in the

catalytic activity in the case of Pd/V/TiO2 catalysts with

increasing vanadium loading could be due to a higher

number of vanadium sites and to the enhancement of the

redox properties of the catalysts. Therefore, the mixed

catalysts could improve the benzene adsorption at the active

sites and his latter oxidation to CO2. In this case, the

experimental evidence seems to indicate that the palladium

particle size is remaining constant at the different vanadium

loading, and therefore, it was not a key factor affecting the

change of the benzene catalytic activity over Pd/V/TiO2

catalysts at different vanadium loading.

3.2.2. Naphthalene oxidation

The conversion and the yield to CO2 for the catalytic

oxidation of naphthalene over V/TiO2 catalysts are

presented in Fig. 4A and B, respectively. The comparison

of these two figures shows that although the vanadium

catalysts were very active in the naphthalene catalytic

oxidation, as all the naphthalene feed to the reactor

disappeared, complete oxidation to CO2 was not achieved

over the temperature range studied. As it has previously been

reported [7] the carbon balance depends on the VOCs

molecule, the temperature and the catalyst used. For benzene

the carbon balance was always close to 100%. Whilst for

the case of naphthalene a significant carbon deficit was

observed. At each temperature, the deficit decreased until

steady state was reached and this was due to both the

naphthalene adsorption and the by-product formation. Once

the adsorption equilibrium was reached, the deficit only

depended on the by-product formation and it was constant

during data acquisition. The total time to reach the

adsorption equilibrium decreased with the reaction tem-

perature ranging from 30 min to 1 h [32]. All catalytic

activity results were obtained once the naphthalene

adsorption equilibrium was reached.

The deficit in the carbon balance due to the formation of

by-products is dependent on the temperature and on the

nature of the catalysts. Weber et al. [20] have reported that

V2O5-WO3/TiO2 catalysts were active in decomposing

polychlorinated dibenzo-p-dioxins (PCDDS), polychlori-

nated dibenzofurans (PCBZs), and PAHs (such as biphenyl

and pyrene). They showed that destruction of reactants

may generate by-products as secondary pollutants, and the

disappearance of reactants may not represent the full extent

of oxidation to CO2. This fact has also been observed by

Zhang et al. [14]. These authors showed that during

naphthalene decomposition over Pt catalysts, little carbon

dioxide was produced at 200 8C, whilst more than 50% of the

naphthalene feed was consumed, suggesting that naphthalene

was partially converted to other by-products. Recently, the

same group have reported the reaction intermediates and

the possible pathways involved in the naphthalene oxidation

over 1%Pt and 5%Co/g-Al2O3 catalysts [33]. Naphthalene

derivatives, polymerized polycyclic aromatic hydrocarbons

and polymerized-oxygenated polycyclic aromatic com-

pounds, were found in the reaction products, not only in

the gas phase but also on the catalyst surface.

In the present work, we have studied the nature of the by-

products formed. A cartridge containing a Tennax adsorbent


Fig. 5. (A) Naphthalene conversion on Pd/V/TiO2 catalysts as a function of

reaction temperature: (&) 0.5%Pd/TiO2; (*) 3%V/TiO2; (~) 0.5%Pd/

3.0%V/TiO2. (B) Yield to CO2 (line) or yield to CO (dotted line) on Pd/V/

TiO2 as a function of reaction temperature: (&) 0.5%Pd/TiO2; (*) 3%V/

TiO2; (~) 0.5%Pd/3.0%V/TiO2.

was placed in line after the catalyst bed in order to trap any

by-products desorbed to the gas phase. To analyse for any

by-products the cartridge was placed into a GC/MS and the

trapped products desorbed onto the chromatography

column. Studies were carried out at 225 and 250 8C using

the 0.5Pd/1.5V/TiO2 catalysts. At 225 8C naphthalene was

detected along with the products ethyl benzene, dimethyl

benzene, trimethyl benzene, p-xylene, benzaldehyde,

benzylalcohol and acetophenone. The most predominant

by-products were benzaldehyde and dimethyl benzene.

Increasing the reaction temperature to 250 8C resulted in the

detection of ethyl benzene, dimethyl benzene, trimethyl

benzene, p-xylene, benzaldehyde, benzylalcohol, acetophe-

none, benzoic acid and phthalic anhydride. The most

predominant by-products were phthalic anhydride, benzoic

acid and benzaldehyde. Naphthalene was also still detected

at 250 8C. Interestingly, at 250 8C, trace quantities of

Chlorobenzene were also detected, indicating that residual

chloride from the catalyst preparation was reactive. It is

important to note that increasing the reaction temperature, so

that the CO2 yield was 100%, did not result in the formation

of any by-products.

In the present study, it was clearly observed that whilst

the activity of the V/TiO2 catalysts was dependent on the

amount of vanadium present, the yield to CO2 was

practically independent. The most active catalyst in terms

of naphthalene conversion was the one with the highest

vanadium loading, 3.0%V/TiO2. Naphthalene conversions

of about 10% were reached in the same temperature range,

170–180 8C, over all the V/TiO2 catalysts. On the other

hand, conversions of 90% were obtained around 240 8C over

3.0%V/TiO2 and 1.5%V/TiO2 catalysts, around 250 8C over

1.0%V/TiO2 catalyst and around 270 8C over the 0.5%V/

TiO2 catalyst. Therefore, it can be concluded that whilst the

number of vanadium sites in the case of V/TiO2 catalysts

seem to affect the naphthalene conversion, there is no effect

over the naphthalene conversion to CO2. During combustion

processes, it has been reported that the adsorbed oxygen can

be activated by catalysts to electrophilic species [34] before

attacking the naphthalene adsorbed on the surface of the

catalyst. It was proposed that one of the aromatic rings of

naphthalene was more easily ruptured once the other was

attached to the catalyst surface [35]. Therefore, both

benzene and naphthalene seem to have a similar reaction

mechanism over V/TiO2 catalysts, where the first step is an

adsorption of the aromatic ring on the catalyst via a

nucleophilic attack and the second step is an electrophilic

substitution of the adsorbed species. In these reaction

mechanisms, it seems that naphthalene could be more easily

bound to the catalyst surface than the benzene molecule, as

the temperature to activate the naphthalene molecule was

lower. However, once the naphthalene was attached to the

catalyst surface, its total oxidation seems to be more

difficult, due to the possible formation of very stable

intermediate products, which cannot be oxidised at low

temperatures.

The catalytic activity of 0.5%Pd/TiO2, 3.0%V/TiO2 and

0.5%Pd/3.0%V/TiO2 catalysts are compared in Fig. 5. The

naphthalene conversion and the yield to COx over these

catalysts as a function of the reaction temperature are reported

in Fig. 5A and B, respectively. In Fig. 5A, it is observed that

the naphthalene conversion is higher over 3.0%V/TiO2 than

over 0.5%Pd/TiO2 catalyst in the whole temperature range.

However, if these two catalysts are compared in terms of the

total conversion to CO2, although 3.0%V/TiO2 shows activity

at very low temperatures, it was evident that 0.5%Pd/TiO2

catalyst is remarkably more active at temperature higher than

250 8C. According to some studies [14,17], weak physisorp-

tion bonding between naphthalene and the catalyst surface

allows the second ring to interact with the catalyst surface

leading to a facile oxidation of the second ring once the first

ring of naphthalene was destroyed. Thus, higher naphthalene

conversion to CO2 over the catalysts with weakly physisorbed

naphthalene could be observed at higher reaction tempera-

tures. It is likely that the strength of the interaction between

naphthalene and the catalyst surface is stronger over V-sites


Fig. 6. (A) Naphthalene conversion over Pd promoted V/TiO2 catalysts with

varying vanadium loading as a function of reaction temperature. Pd promoted

catalysts: (&) 0.5%Pd/0.5%V/TiO2; (~) 0.5%Pd/1.0%V/TiO2; (^) 0.5%Pd/

1.5%V/TiO2; (*) 0.5%Pd/3.0%V/TiO2. (B) CO2 yield over Pd promoted

V/TiO2 catalysts with varying vanadium loading as a function of reaction

temperature. Pd promoted catalysts: (&) 0.5%Pd/0.5%V/TiO2; (~) 0.5%Pd/

1.0%V/TiO2; (^) 0.5%Pd/1.5%V/TiO2; (*) 0.5%Pd/3.0%V/TiO2.

than on Pd-sites. Strong interactions between naphthalene and

Mo and W sites have been reported [14], and it is interesting

to note that these elements have similar redox properties and

are mildly acidic like vanadium. However, in addition to

the adsorption characteristics of the catalysts the specific

reactivity of the active sites must be considered. It is widely

known that for a large number of reactions that vanadium sites

tend to attack C–H bonds leading to products of dehydro-

genation or partial oxidation rather than to the total oxidation

to CO2 [36], most likely due to the presence of selective

nucleophilic oxygen sites. The preference of vanadium for the

partial oxidation products is demonstrated in this work when

low reaction temperatures are sufficient to decompose

naphthalene in the case of V/TiO2 catalysts. However, higher

temperatures are required to achieve total oxidation. On the

other hand, it is well known that Pd supported catalysts are one

of the most active materials in VOC catalytic combustion [5],

so it can be expected that non-selective electrophilic oxygen

species are dominant on the catalyst surface.

In Fig. 5, it can be observed that the 0.5%Pd/3.0%V/TiO2

catalyst also showed higher naphthalene conversion than the

0.5%Pd/TiO2 catalyst. However, as it was mentioned above

with 3.0%V/TiO2 catalysts, the complete naphthalene

oxidation to CO2 was achieved at lower temperatures over

the 0.5%Pd/TiO2 catalyst; demonstrating total naphthalene

oxidation to CO2 at a temperature 25 8C lower than the

0.5%Pd/3.0%V/TiO2 catalyst, see Fig. 5B. Moreover, it was

observed in Fig. 5A that 3.0%V/TiO2 and 0.5%Pd/3.0%V/

TiO2 catalysts have similar activity at all the temperatures

studied, in terms of naphthalene conversion, but different

conversion to CO2 at temperatures higher than 250 8C.

Surprisingly, at temperatures higher than 250 8C, the total

conversion to CO2 in the case of 0.5%Pd/3.0%V/TiO2

changes from a CO2 yield similar to 3.0%V/TiO2 to a CO2

total conversion close to that of 0.5%Pd/TiO2. Therefore,

whilst aromatic adsorption could happen on the nucleophilic

vanadium sites of the Pd/V/TiO2 catalysts, its total oxidation

to CO2 seems to be mainly on the electrophilic palladium

sites. It can also be concluded that there was no synergistic

effect between vanadium and palladium in the naphthalene

catalytic oxidation.

The naphthalene conversion and the yield to CO2 over Pd

promoted V/TiO2 catalysts with varying vanadium loading

as a function of reaction temperature are shown in Fig. 6A

and B, respectively. It was observed that the naphthalene

conversion over 0.5%Pd/3.0%V/TiO2 catalyst conversion

depends on the vanadium loading. The higher the vanadium

loading, the greater the naphthalene conversion at all

temperatures. Nevertheless, as for the V/TiO2 catalysts with

varying vanadium loading, the total naphthalene conversion

to CO2 on the vanadium promoted Pd/TiO2 catalyst was

essentially independent of vanadium loading (Fig. 6B). An

increase in the vanadium loading produced only a slight

increase in the yield to CO2. However, independently of

the vanadium loading, all the Pd/V/TiO2 catalysts showed

complete oxidation of naphthalene to CO2 at 300 8C. This is

in agreement with the preference of vanadium sites for

the nucleophilic attack, and palladium sites for the total

combustion of the adsorbed molecules to CO2.

Finally, the catalyst activity of 0.5%Pd/TiO2, 3.0%V/

TiO2 and 0.5%Pd/3.0%V/TiO2 were studied at 300 8C for an

extended period to assess catalyst stability. No change in

activity was observed for any of the samples after 100 h time

on line and these data indicated that the catalysts did not

undergo significant deactivation.

Fig. 7 shows a comparative study of 0.5%Pd/TiO2,

3.0%V/TiO2 and 0.5%Pd/3.0%V/TiO2 catalysts for the

oxidation of propane, benzene and naphthalene. T10, T50 and

T90 (reaction temperatures for levels of conversion of 10, 50

or 90%) are plotted for each substrate. Whilst experiments

with propane and benzene led only to the formation of

CO2, complete naphthalene oxidation could not be achieved.

Consequently data are presented for the naphthalene

conversion and the yield to CO2. Mixed V/Pd-catalysts

presented the highest catalytic activity for all substrates.

However, in the naphthalene oxidation the highest yields to

CO2 were obtained by the Pd-catalyst without modification

by vanadium. Hence, although vanadium promoted Pd/TiO2

catalysts seemed to be very promising environmental

catalysts for the VOC removal, in the case of aliphatic [6]

and one-ring aromatic compounds, this behaviour cannot be

extrapolated to a Polyaromatic Compound like naphthalene.


Fig. 7. (A) Reaction temperature for a propane, benzene and naphthalene

conversion of 10%. The last set of bars indicates the reaction temperature for

a yield to CO2 of 10% in the naphthalene oxidation. (B) Same but for a level

of conversion (or yield to CO2) of 50%. (C) Same but for a level of

conversion (or yield to CO2) of 90%. Symbols: (black bars) 3%V/TiO2,

(grey bars) 0.5%Pd/TiO2, (white bars) 0.5%Pd/3.0%V/TiO2.

Therefore, it can be concluded that the nature of the VOC

substrate is a very important consideration in the design of

catalysts for environmental protection.

4. Conclusions

For the two different substrates and three groups of

catalysts (V, Pd and V/Pd) studied in the work, it has been

observed that benzene was more difficult to oxidise than

naphthalene. In the case of benzene, a synergistic effect was

observed whenvanadium was added to Pd/TiO2 catalysts. The

high catalytic activity for Pd/V/TiO2 catalysts for benzene

was related to the presence of two catalytic sites: readily

reducible vanadium sites, since the presence of palladium

increases the reducibility of vanadium species, and palladium

sites in a higher oxidation state and with a larger particle size.

However, in the case of naphthalene, it was observed that there

is only an additional effect between palladium and vanadium

active sites, and Pd/TiO2 was the most active catalyst from an

environmental point of view as the total conversion to CO2

was reached at the lowest temperature.

Acknowledgements

T. Garcia would like to thank the Ministry of Education

and Science, MEC, (Spain) for the FPU fellowship. The

authors would also like to thank Rob Jenkins for his

assistance with GC/MS analysis.

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