Exploring the Catalytic Activity of Zirconia, Zirconia Supported Metals and Metal Oxides for Oxidation of Phenol

doi:10.4236/mrc.2012.12004

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Modern Research in Catalysis, 2012, 1, 23-27

http://dx.doi.org/10.4236/mrc.2012.12004 Published Online July 2012 (http://www.SciRP.org/journal/mrc)

Exploring the Catalytic Activity of Zirconia, Zirconia

Supported Metals and Metal Oxides for

Oxidation of Phenol

Mohammad Sadiq1,2*, Mohammad Ilyas2

1Department of Chemistr y, University of Malakand, Chakdara, Dir (Lower), Khyber Pakhtunkhwa, Pakistan

2National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan

Email: *[email protected], *[email protected]

Received May 2, 2012; revised June 18, 2012; accepted July 10, 2012

ABSTRACT

Catalytic oxidation/degradation of phenol with molecular oxygen in aqueous medium by Zirconia, zirconia supported

metals and metal oxides at low temperature were studied in a three necked batch reactor. The catalysts prepared were

characterized by modern techniques like XRD, SEM and EDX. The activities of different catalysts for the oxidation of

phenol in aqueous medium were found to be in the order; Pt-Pd/ZrO2 > Bi-Pt/ZrO2 > Bi-Pd/ZrO2 > Pt/ZrO2 > Pd/ZrO2 >

Cu/ZrO2 > Mn/ZrO2 > Bi/ZrO2. The enhanced catalytic activity of Bi-Pt/ZrO2 is attributed to Bi which in contact with Pt

particles promotes its catalytic activity. In short, catalytic ox idation was found to be an effective tool, for the removal of

phenol, f rom indus trial wast e water.

Keywords: Oxidation; Phenol; Bismuth; Platinum; Zirconia

1. Introduction

Undesired phenol wastes are produced by many indus-

tries including the chemicals, plastics and resins, coke,

steel, and petroleum industries. Phenol is one of the

EPA’s Priority Pollutants. Under Section 313 of the

Emergency Planning and Community Right to Know Act

of 1986 (EPCRA), release of more than one pound of

phenol into the air, water and land must be reported, an-

nually, and entered into the Toxic Release Inventory

(TRI). Phenol has a high oxygen demand and can thus

readily deplete oxygen in the receiving water, with det-

rimental effects on those organisms that extract dissolved

oxygen from water, for their metabolism. It is also well

known that even low phenol level, in the parts p er billion

ranges, imparts disagreeable taste and odor to water.

Therefore, it is necessary to elimin ate maxi mum p oss ible

quantity of pheno l from the wastewater before it is being

discharged. Phenol may be treated by chemical oxidation,

bio-oxidation and adsorption. Chemical oxidation such as

with hydrogen peroxide or chlorine dioxide has a low

capital but a high operating cost. Bio-oxidation has a

high capital and a low operating cost. Adsorption has

both high capital and operating costs. The appropriate-

ness of any one of these methods depends on a combina-

tion of factors; the most important of which are the phe-

nol concentration and any other chemical pollutant that

may be present in the wastewater. Depending on these

variables, a sing le or a combination of treatmen ts may be

used. Currently, phenol removal is accomplished with

chemical oxidants, the most commonly used, being chlo-

rine dioxide, hydrogen peroxide and potassium perman-

ganate. Heterogeneous catalytic oxidation, of dissolved

organic compounds, is a potential mean for the remedia-

tion of contaminated ground and surface waters, indus-

trial effluents and other wastewater streams. The ability

to carry out the process at substan tially milder condition s

of temperature and pressure, in comparison to supercriti-

cal water oxidation and wet air oxidation, is achieved

through the use of an extremely active supported noble

metal catalyst. Catalytic Wet Air Oxidation (CWAO)

appears as one of the most promising processes but only

at elevated conditions of pressure and temperature in the

presence of metal oxide and supported metal oxide [1].

Although homo geneous copper catalysts are effective for

the wet oxidation of industrial effluents but the removal

of toxic catalyst has made the process debatable [2]. Re-

cently, Leitenburg et al. have reported that the activities

of mixed-metal oxides such as of ZrO2, MnO2, or CuO

for the oxidation of acetic acid can be enhanced by add-

ing ceria as a promoter [3]. Imamura et al. have studied

the catalytic activities of supported noble metal catalysts

for the wet oxidation of phenol and the other model pol-

*Corresponding a uthor.

M. SADIQ ET AL.

lutant compounds. Ruthenium, Platinum and Rhodium

supported on CeO2 were found to be more active than a

homogeneous copper catalyst [1]. Atwater et al. have

shown that several classes of aqueous organic contami-

nants, can be deeply oxidized, using dissolved oxygen

over supported noble metal catalysts (5% Ru-20% Pt/C),

at a temperature 393 - 433 K and pressure between 2.3

and 6 atmospheres [4]. Carlo et al. [5] hav e reported th at

lanthanum strontium manganites are very active catalysts

for the catalytic wet oxidation of phenol. Castro et al. [6]

have reported that polymer-supported metal complexes

have been used as catalysts for the catalytic wet hydro-

gen peroxide oxidation of Phenol with high yield; how-

ever the unacceptable point was leaching of the metal. In

the present work, we explored the effectiveness of zirco-

nia-supported noble metals (Pt and Pd) and bismuth

promoted zirconia supported noble metals for the oxida-

tion of phenol in aqueous solution.

2. Experimental

2.1. Materials

ZrOCl2·8H2O (Merck, 8917), NH4OH (BDH, 27140),

AgNO3 (Merck, 1512), PtCl4 (Acros, 19540), Palladium

(II) chloride (Scharlau, Pa 0 025), phenol (Acros, 41717),

alizarin (Acros, 400480250), Potassium Iodide (BDH,

102123B) and 2, 4-Dinitro phenyl hydrazine (BDH,

100099) were used as received. H2 (99.999%) was pre-

pared using hydrogen generator (GCD-300, BAIF). Ni-

trogen and Oxygen were supplied by BOC Pakistan Ltd.

and were further purified by passing through traps

(C.R.S.Inc.202268), to remove traces of water and oil.

Traces of oxygen from nitrogen gas were removed by

using specific ox ygen traps (C.R.S.Inc.202223).

2.2 Catalyst Preparation

Zirconia was prepared using an aqueous solution of zir-

conyl chloride [7-9] with a drop wise addition of NH4OH

for 4 hours (pH: 10 - 12) and continuou s stirring all along.

The precipitate was washed with triply distilled water for

24 hours, using a Soxhlet’s apparatus, until the chloride

ion test with AgNO3 was found to be negative. Precipi-

tate was dried at 110˚C for 24 hour s. After drying, it was

calcined with programmable heating at a rate of

0.5 Cmin



to achieve a temperature of 950˚C, which

was then maintained, for 4 hours. Nabertherm C-19 pro-

grammed control furnace was used for calcination.

2.3. Metals and Metals Oxides Supported on

Zirconia

Supported Catalysts {i. Pt(2wt%)/ZrO2 ii. Pd(2wt%)/ZrO2

iii. Pd(1wt%) and Pt(1wt%)/ZrO2 iv. Bi(0.5wt%)

Pt(2wt%)/ZrO2 v. Bi(0.5wt%)Pd(2wt%)/ZrO2} were

prepared, by incipient wetness technique. For this pur-

pose, calculated amount (wt%) of the precursor com-

pound (PdCl4 or PtCl4) wa s taken in a cr ucible and triply

distilled water was added to it, to make a paste. Then the

required amoun t of the suppor t (ZrO2) was mixed with it.

The paste was thoroughly mixed and dried in an oven at

110˚C for 24 hours and then ground. The catalyst was

sieved and 80 - 100 mesh portions were used for further

treatment. The ground catalyst was calcined again, at a

rate of 0.5˚C/min, to achieve a temperature of 950˚C,

which was then maintained, for 4 hours, followed by a

reduction in H2 flow (40 mL/min), at 280˚C, for 4 hours.

The supported multi-component catalysts, were prepared

by successive incipient wetness impregnation of the

support with bismuth and precious metals, followed by

drying and calcination. Bismuth was first added to zirco-

nia support by the incipient wetness impregnation pro-

cedure. After drying and calcination, Bi/zirconia was

then impregnated with the active metals such as Pd or Pt.

The final sample was then passed through the same dry-

ing and calcination procedures. The metal loading of the

catalyst was calculated from the weight of chemicals

used for impregnation.

2.4. Morphological Study

XRD analyses were performed using a JEOL (JDX-3532)

diffractometer with CuKa radiation (kα = 1.5406 Å) oper-

ated at 40 kV and 20 mA. SEM and EDX measurements

were performed using scanning electron microscope of

Joel 50 H super prob 733.

2.5. Oxidation of Phenol

Oxidation of phenol in aqueous medium was carried out,

in a magnetically stirred, Pyrex glass double walled flat

bottom three-necked batch reactor, equipped with a re-

flux co ndenser and a mercury thermometer. The reaction

temperature was maintained, by using water circulator

(WiseCircu, Fuzzy control system). A predetermined

quantity of the substrate solution (20 mL) was taken in

the reactor and a desired amount (0.2 g) of catalyst was

added to it. The reaction during heating period was neg-

ligible since no direct contact existed between oxygen

and the catalyst. O2 and N2 gases at atmospheric pressure

were allowed to pass through the reaction mixture at a

flow rate of 40 mL/min at a fixed temperature. When the

temperature and pressure reached the desired values, the

stirrer was set at 900 rpm and turned on. The reaction

mixture was analyzed by GC (Clarus 500, Perkin Elmer

equipped with (FID and TCD) and capillary column

(Elite-5, L 30 m, ID 0.25, DF 0.25), UV spectropho-

tometer (UV-160, Shamidzo, Japan) and COD was

measured by the potassium dichromate method [10].

M. SADIQ ET AL. 25

3. Results and Discussion

3.1. Characterization of Catalyst

X-ray diffraction pattern of the sample, reported in Fig-

ure 1, confirms the monoclinic structure of zirconia. The

major peaks indicating the monoclinic structur e of zirco-

nia appear at 2



= 28.18˚ and 31.38˚ while no peak,

characteristic for tetragonal phase (2



= 30.94˚), appears

suggesting that zirconia is purely present in monoclinic

phase. The reflections were observed for Pd at 2θ = 40.4˚

and 46.9˚ and for Pt at 2θ = 39.79˚ and 46.28˚ respec-

tively. For Bi2O3, the peaks appear at 2θ = 27.7˚, 30.5˚,

33˚, 42.4˚ and 47.2˚ while for MnO2 major peak appears

at 2θ = 26.1˚, 28.9˚. In all these catalysts zirconia main-

tains its monoclinic phase. SEM micrographs of fresh

samples, reported in Figure 2, show the homogeneity of

the crystal size of monoclinic zirconia. The micrographs

of Pt/ZrO2, Pd/ZrO2, Pt-Pd/ZrO2 and Bi2O3/ZrO2 reveal

that the active metals are well dispersed on the support.

Figure 3 shows the EDX analysis results for fresh and

used Bi(0.5wt%) Pt(2wt%)/ZrO2 and Bi(0.5wt%)

Pd(2wt%)/ZrO2 samples. The results show the presence

of carbon in the used samples which probably comes

from the total oxidation of the organic substrate. Many

researcher have reported the presence of chlorine and

carbon in the EDX analysis of freshly prepared samples

[7,8], suggesting that chlorine comes form the matrix of

zirconia while carbon from ethylene diamine. In our case,

we did not use ethylene diamine and no carbon was ob-

served, in the EDX analysis of fresh samples. Also, no

chlorine was found in our studies, because of careful

washing of the samples.

3.2. Catalytic oxidation of phenol

Oxidation of phenol was significantly higher over (2

wt%) Pt/ZrO2 catalyst. Combination of (1wt%) Pd and

(1wt%) Pt on ZrO2 gave an activity, comparatively

higher than that of the (2wt%) Pd/ZrO2 or (2wt%)

Pt/ZrO2 catalysts. Adding 0.5% Bismuth significantly

increased the activity of the (2wt%) Pt/ZrO2, which

shows promising activity for destructive oxidation of

organic pollutants in the effluents at 333 K and 101 kPa

in the liquid phase. Addition of 0.5% Bismuth however,

inhibited the activity of the ZrO2 supported (2wt%) Pd

catalyst.

3.3. Effects of Different Parameters

Alterations in different parameters of the reaction have

significant effect on the catalytic oxidation of phenol, in

aqueous medium. In comparison to homogeneous cata-

lytic oxidation of phenol, heterogeneous catalytic oxida-

tion appears to be less sen sitive to pH and more efficient

[9]. The conversion of the phen ol with time is reported in

Figure 1. XRD of different catalysts (m: peaks for mono-

clinic ZrO2).

Figure 2. SEM of different catalyst; (a) ZrO2; (b) Pt/ZrO2;

Figure 3. EDX of different cataly sts (fr esh and used).

M. SADIQ ET AL.

Figure 4 for Bi promoted zirconia supported platinum

catalyst. In the blank experiment, no conversion is ob-

tained after 3 hours while nearly total conversion is

achieved with Bi-Pt/ZrO2 in 3 h. Bismuth promoted zir-

conia-supported platinum catalyst shows a very good

specific activity for phenol conversion (Figure 4).

Leaching of the catalyst was checked and the reaction

was found purely heterogeneous. The activities of dif-

ferent catalysts were found in the order Pt-Pd/ZrO2 >

Bi-Pt/ZrO2 > Bi-Pd/ZrO 2 > Pt/ZrO2> Pd/ZrO2> Cu/ZrO2>

Mn/ZrO2 > Bi/ZrO2. Bi-Pt/ZrO2 is the most active cata-

lyst which suggests that Bi in contact with Pt particles

promotes metal activity as shown in Figure 5. However,

although very high conversions can be obtained (~91%),

total mineralization of phenol is never observed. It is

reported that organic intermediates are still present in the

solution [11]. Platinum and palladium loading influence

the catalytic activity. An in crease in Pt loading improves

the activity significantly and the conversion of phenol

increases linearly with increase in Pt loading. In contrast

to platinum, an increase in Pd loading improves the ac-

tivity not very significantly [12]. The influence of bismu-

th o n cata l ytic activities of Pt/ZrO2 catalysts is reported in

Figure 4. Time profile study. Reaction Conditions: Temp;

333 K, Cat; 0.2 g {Bi(0.5wt%)Pt(2wt%)/ZrO2}, substrate

solution; 20 mL (Conc; 10 g·dm–3) of phenol in water, pO2;

760 Torr and agitation; 900 rpm.

Figure 5. Comparison of different catalysts. Reaction Con-

ditions: Temp; 333 K, Cat; 0.2 g, substrate solution; 20 mL

(10 g·dm–3) of phenol in water, pO2; 760 Torr and agitation;

900 rpm.

Figure 6. Adding 0.5wt% Bi improves the activity of

Pt/ZrO2 catalyst, with a (1 and 2wt% Pt) loading. In con-

trast to supported Pt catalyst, the activity of supported Pd

catalyst with a (1wt%) Pd loading, decreases, by the ad-

dition of Bi to zirconia. The profound inhibiting effect

was observed with a Bi loading of 0.5wt%(2wt%)

Pd/ZrO2. Bi as an active metal supported on zirconia has

very poor activity while as a promoter shows remarkable

activity. High catalytic activity was obtained for reduced

catalysts as shown in Figure 7. Pt/ZrO2 was more reac-

tive than PtO/ZrO2. Similarly reduced Pd/ZrO2 were

found to be more reactive than the unreduced Pd sup-

ported on zirconia. This increase in catalytic activity with

reduction is attributed to the transformation of a fraction

of tetragonal zirconia into monoclinic phase. Further-

more, monoclinic zirconia is more active for oxidation

than tetragonal zirconia is [13,14]. Figure 8 reveals that

with increase in temperature, the conversion of phenol

increases, reaching a maximum conversion at 333 K. The

apparent activation energy is ~68.3 kJ mol. The value

of activation energy in the present case where the agita-

tion speed was kept 1200rpm shows that the reaction is

probably free of mass transfer limitation [15].

Figure 6. Effect of promoter Bi (0.5wt%) on catalytic activ-

ity of Pt(2wt%)/ZrO2 and Pt(1wt%)/ZrO2. Reaction Condi-

tions: Temp 333 K, Cat 0.2 g, substrate solution 20 ml (conc;

10 g·dm–3) of phenol in water, pO2 760 Torr and agitation

900 rpm.

Figure 7. Comparison of reduced (H2 flow at 280˚C for 4

hours) and unreduced cataly sts. Reaction Conditions: Temp

333 K, Cat 0.2 g, substrate solution 20 mL (conc; 10 g·dm–3)

of phenol in water, pO 2 760 Torr and agitation 900 rpm.

M. SADIQ ET AL.

Figure 8. Effect of temperature on the conversion of phenol.

Reaction condition: Temp 303 - 333K, Cat; 0.2 g Bi(0.5wt%)

Pt(2wt%)/ZrO2, substrate 20 mL (conc; 10 g·dm–3), pO2 760

Torr and agitation 900 rpm.

4. Conclusion

Both bismuth promoted Pt/ZrO2 and Pd/ZrO2 catalysts

are very promising for the destructive oxidation of the

organic pollutants in the industrial effluents. Addition of

Bi improves the activity of Pt/ZrO2 catalysts but inhibits

the activity of Pd/ZrO2 catalyst at a high loading of Pd.

Optimal conditions for better catalytic activity are: temp

333K, wt of catalyst 0.2g, agitation 900rpm, pO2 101kPa

and time 180min. Among the emerging alternative proc-

esses, the supported noble metals catalytic o xidation was

found to be the most effective for the treatment of several

pollutants like phenols, at milder temperatures and pres-

sures.

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