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
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.
C
opyright © 2012 SciRes. MRC

M. SADIQ ET AL.
24
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;
(c) Pd/ZrO2; (d) Pt-Pd/ZrO2; (e) Bi2O3; (f) Bi2O3/ZrO2.
Figure 3. EDX of different cataly sts (fr esh and used).

M. SADIQ ET AL.
26
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.
27
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.
REFERENCES
[1] S. Imamura, “Catalytic and Noncatalytic Wet Oxidation,”
Industrial & Engineering Chemistry Research, Vol. 38,
No. 5, 1999, pp. 1743-1753.
doi:10.1021/ie980576l
[2] R. Wang, Y. Qi, Z. Shen and Z. Wu, Huadong Huagong
Xueyuan Xue, Vol. 4, 1982, p. 411.
[3] C. Leitenburg, D. Goi, A. Primavera, A. Trovarelli and G.
Dolcetti, “Wet Oxidation of Acetic Acid Catalyzed by
Doped Ceria,” Applied Catalysis B: Environmental, Vol.
11, No. 1, 1996, pp. L29-L35.
[4] J. E. Atwater, J. R. Akse, J. A. Mckinnis and J. O.
Thompson, “Aqueous Phase Heterogeneous Catalytic
Oxidation of Trichloroethylene,” Applied Catalysis B:
Environmental, Vol. 11, No. 1, 1996, pp. L11-L18.
doi:10.1016/S0926-3373(96)00063-X
[5] R. Carlo, C. Federico, B. Silvia, P. Ombretta and B.
Guido, “Catalytic wet oxidation of phenol over lanthanum
strontium manganite,” Applied Catalysis B: Environ-
mental, Vol. 84, No. 3-4, 2008, p. 678.
[6] I. U. Castro, D. C. Sherrington, A. Fortuny, A. Fabregat,
Stüber, F. Font and J. C. Bengoa, “Synthesis of Poly-
mer-Supported Copper Complexes and Their Evaluation
in Catalytic Phenol Oxidation,” Catalysis Today, Vol.
157, No. 1-4, 2010, pp. 66-70.
doi:10.1016/j.cattod.2010.02.006
[7] L. D. Souza, J. S. Subaie and R. Richards, “A Colloid
‘Digesting’ Route to Novel, Thermally Stable High Sur-
face Area ZrO2 and Pd/ZrO2 Catalytic Materials,” Journal
of Colloid and Interface Science, Vol. 292, No. 2, 2005,
pp. 476-485
[8] L. D. Souza, A. Suchopar, K. Zhu, D. Balyozova, M.
Devadas and R. M. Richards, “Preparation of Thermally
Stable High Surface Area Mesoporous Tetragonal ZrO2
and Pt/ZrO2: An Active Hydrogenation Catalyst,” Mi-
croporous and Mesoporous Materials, Vol. 88, No. 1-3,
2006, pp. 22-30.
[9] J. M. Tatibouet, E. Guelou and J. Fournier, “Catalytic
Oxidation of Phenol by Hydrogen Peroxide over a Pil-
lared Clay Containing Iron. Active Species and pH Ef-
fect,” Topics in Catalysis, Vol. 33, No. 1-4, 2005, pp.
225-232.
[10] A. E. Greenberg, L. S. Clesceri and A. D. Eaton, “Stan-
dard Methods—For the Examination of Water and
Wastewater 18a Ed.,” Part 5000, 6, APHA-AWWA-
WPCF, 1992.
[11] Q. Zhang and K. T. Chuang, “Alumina-Supported Noble
Metal Catalysts for Destructive Oxidation of Organic
Pollutants in Effluent from a Softwood Kraft Pulp Mill,”
Industrial & Engineering Chemistry Research, Vol. 37,
No. 8, 1998, pp. 3343-3349.
doi:10.1021/ie980111v
[12] C. Resini, F. Catania, S. Berardinelli, O. Paladino and G.
Busca, “Catalytic Wet Oxidation of Phenol over Lantha-
num Strontium Manganite,” Applied Catalysis B: Envi-
ronmental, Vol. 84, No. 3-4, 2008, pp. 678-683.
doi:10.1016/j.apcatb.2008.06.005
[13] M. Ilyas and M. Sadiq, “Kinetics of Heterogeneous Sol-
vent-Free Liquid Phase Oxidation of Alcohol Using ZrO2
Catalyst with Molecular Oxygen,” Chinese Journal of
Chemistry, Vol. 26, No. 5, 2008, pp. 941-946.
doi:10.1002/cjoc.200890172
[14] M. Ilyas and M. Sadiq, “Pt/ZrO2: An Efficient Catalyst
for Aerobic Oxidation of Toluene in Aqueous Solution,”
Chinese Journal of Chemistry, Vol. 28, No. 11, 2010, p.
2216.
[15] D. V. Bavykin, A. A. Lapkin, S. T. Kolaczkowski and P.
K. Plucinski, “Selective Oxidation of Alcohols in a Con-
tinuous Multifunctional Reactor: Ruthenium Oxide Cata-
lysed Ox idatio n of Benzyl Alc ohol, ” Applied Catalysis A:
General, Vol. 288, No. 1-2, 2005, pp. 175-184.
doi:10.1016/j.apcata.2005.04.042