Aqueous H2O2 as an oxidant for CO over Pt- and Au-NaY catalysts

Article abstract of DOI:10.1016/j.molcata.2006.08.003

The oxidation of carbon monoxide with aqueous hydrogen peroxide was carried out over Pt- and Au-NaY catalysts, that were prepared by the impregnation of NaY zeolite (Si/Al = 5.6, S_BET = 910 m²/g) with aqueous solutions of tetramine platinum(II) nitrate and hydrogen tetrachloroaurate trihydrate, respectively. The Pt- and Au-NaY catalysts exhibited high activity at relatively low temperatures as low as 303 K in the CO-H₂O₂ reaction. Pt-NaY was compared favorably with Au-NaY at similar reaction conditions, where the specific reaction rate constant, K_m, over Pt-NaY was five times of magnitude higher than that of Au-NaY at 323 K. These data were confirmed by the values of activation energy evaluated from Arrhenius plots. The Pt-NaY was reduced in flowing H₂ at 623 K for 2 h. The catalytic activity of reduced Pt-NaY was about 44 times more active than that of analogous unreduced catalyst at 303 K. The catalytic activity of Au-NaY was greatly affected by thermal treatment. A higher K_m value was obtained at 353 K over Au-NaY, exceeding this temperature led to a decrease in activity due to sintering of Au particles. It is proposed that a dual Au(0)/Au(I) site is necessary to catalyze the reaction, where, Au(0) accelerates the H₂O₂ decomposition and the CO adsorption capability was enhanced on Au(I) site.

Full text of DOI:10.1016/j.molcata.2006.08.003

Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
Aqueous H2O2 as an oxidant for CO over Pt– and Au–NaY catalysts
Tarek M. Salama, Zeinhom M. El-Bahy, Farouk I. Zidan∗
Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City 11884, Cairo, Egypt
Received 27 May 2005; accepted 2 August 2006
Available online 13 October 2006
Abstract
The oxidation of carbon monoxide with aqueous hydrogen peroxide was carried out over Pt– and Au–NaY catalysts, that were prepared by the
impregnation of NaY zeolite (Si/Al = 5.6, SBET = 910 m2/g) with aqueous solutions of tetramine platinum(II) nitrate and hydrogen tetrachloroaurate
trihydrate, respectively. The Pt– and Au–NaY catalysts exhibited high activity at relatively low temperatures as low as 303 K in the CO–H2O2
reaction. Pt–NaY was compared favorably with Au–NaY at similar reaction conditions, where the specific reaction rate constant, Km, over Pt–NaY
was five times of magnitude higher than that of Au–NaY at 323 K. These data were confirmed by the values of activation energy evaluated from
Arrhenius plots. The Pt–NaY was reduced in flowing H2 at 623 K for 2 h. The catalytic activity of reduced Pt–NaY was about 44 times more active
than that of analogous unreduced catalyst at 303 K. The catalytic activity of Au–NaY was greatly affected by thermal treatment. A higher Km value
was obtained at 353 K over Au–NaY, exceeding this temperature led to a decrease in activity due to sintering of Au particles. It is proposed that
a dual Au(0)/Au(I) site is necessary to catalyze the reaction, where, Au(0) accelerates the H2O2 decomposition and the CO adsorption capability
was enhanced on Au(I) site.
© 2006 Elsevier B.V. All rights reserved.
Keywords: CO oxidation; Gold; Platinum; NaY; Hydrogen peroxide
1. Introduction
catalytic oxidation of carbon monoxide on transition metal sur-
faces has gained increasing importance in recent years in chemi-
cal industry and automobile emission control as well as a model
reaction for surface science investigations. The reaction takes
place between dissociatively adsorbed oxygen and molecularly
adsorbed carbon monoxide by which CO2 immediately desorbs
after formation.
Moreover, the hydrogen used as a fuel in polymer elec-
trolytemembranefuelcell(PEMFC), asusedinelectricvehicles,
operates at relatively low temperatures between 328 and 378 K
[4]. Hydrogen rich fuel produced from methanol, by the steam
reforming and water gas shift reaction processes, should be
essentially free of CO to avoid poisoning of the Pt anode at these
low temperatures. CO, if present, strongly adsorbs onto anode
Pt catalyst surface in the fuel cell and substantially reduces the
overall fuel cell performance [5]. Under normal running condi-
tions the product hydrogen stream contains 25 vol.% CO2, a few
vol.% H2O, and about 0.5–1 vol.% CO. Thus, in order to obtain
optimum performance, the total concentration of CO in the gas
stream should be reduced if possible to below 100 ppm.
The main target of the present work is to explore whether
H2O2 could serve as a potential oxygen donor for the oxidation
of CO in solution by comparing the catalytic properties of
Oxidation processes play a significant role in chemical indus-
try, being the basis for production of important compounds [1].
Benefits deriving from availability, low cost and absence of
wastes select molecular oxygen as the oxidant of choice. How-
ever, the sever reaction conditions often required or the radical
nature of the reaction involved, are common obstacles to a wide
use of the molecular oxygen in synthetic chemistry. As an alter-
native, monooxygen donors are available as milder oxidants:
hydrogen peroxide, peracids, organic hydroperoxides, inorganic
and metallorganic peroxides, sodium hypochlorite and nitrous
oxide. In catalytic oxidation, these are generally characterized
by good activity at moderate temperatures and, often by good
selectivity as well. Among many oxidants, hydrogen peroxide
(H2O2) is a quite unique. It has high active oxygen content (47%)
and producing water as the only by-product [2].
Carbon monoxide is usually one of the discarded by-products
in bulk chemical processes as an environmental issue [3]. The
∗
Corresponding author. Tel.: +202 2629357/8; fax: +202 2629356.
E-mail address: zeinelbahy2020@yahoo.com (Z.M. El-Bahy).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.08.003

T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
129
Au–NaY with Pt–NaY. However, supported gold catalysts are,
in contrast to Pt-group metal catalysts, intrinsically more active
for CO oxidation than for H2 oxidation [6], and the catalytic
activity of Au is enhanced by moisture [7,8] and almost
insensitive to CO2 [9]. Furthermore, it has been emphasized
that the conversion of CO oxidation was dramatically increased
by an order of six times of magnitude with the introduction
of water vapor compared with the system free of steam [10].
Although the catalytic oxidation of carbon monoxide by
molecular oxygen gas has been extensively studied in the last
decades [11–14], to the best of our knowledge the oxidation of
CO with aqueous H2O2 has not yet been reported in literature.
recorded with the following conditions: 9 mg; nitrogen atmo-
sphere of flow rate 30 ml min−1; platinum crucible; ramp rate
10◦ min−1 and a maximum temperature of 1073 K.
2.4. Catalytic reactions
The oxidation of CO with aqueous H2O2 was carried out in
a continuous flow mode system. In a typical reaction, 100 ml
of 0.5 M H2O2 aqueous solution was introduced into a dou-
ble jacket glass reactor. The pH of the solution was ≈6.2.
Using gas flowmeters, the gas mixture consisting of CO and
N2 (CO/N2 = 0.1) was allowed to pass through the catalyst bed
with space velocity (SV) of 12 000 ml g−1 h−1. The temperature
of the glass reactor was adjusted at the desired temperature by
using a temperature controller type (PEX-P 90). 0.1 g of cata-
lyst stock (unless otherwise stated) was added to the reaction
medium supposing zero time. To pursue the progress of the
reaction with time, the resulting gas mixture was introduced
in a strong alkaline solution with an online pH meter. Since
the decrease of OH− (indicated by the decrease of pH value)
is equivalent to the amount of CO2 evolved from the reac-
tion. The natural logarithm of [OH−] (ln C) was plotted against
(time, min) as an application of the first order rate equation
(Kt = ln C0 − ln C). The specific rate constant Km was calculated
as (Km = rate consant (K)/catalyst weight) and used to express
the activity of the prepared catalysts.
2. Experimental
2.1. Materials and regents
Tetraamine platinum(II) nitrate [Pt(NH3)4](NO3)2 from
strem Chemical, USA. Gold(III) was in the form of hydro-
gen tetrachloroaurate trihydrate (HAuCl4·3H2O) from Aldrich,
A.C.S. Hydrogen peroxide was provided as an aqueous solu-
tion (30%, w/w) BDH grade. NaY zeolite (Si/Al = 5.6), surface
area = 910 m2/g, HZS-320 NAA, lot no. 3001, Japan.
2.2. Preparation of catalysts
5 and 10 wt.% Pt and 10 wt.% Au were deposited by incip-
ient wetness impregnation onto NaY zeolite with solutions
of [Pt(NH3)4](NO3)2 and HAuCl4·3H2O, respectively. The Pt
and Au supported on NaY precursors slurry were refluxed at
333–353 K with continuous stirring for 3 h. The preparations
were thoroughly washed with hot water and therefore Au sample
is considered to be almost free of chloride ions. The prepared Pt
and Au supported precursors were air-dried overnight at 383 K
unless otherwise stated and heated in air at 333–673 K for 3 h.
The prepared samples were referred to as Mx–NaY(t), where
M, x and t stand for the name of the precious metal, its weight
percentage and the treatment temperature, respectively. The
Pt–NaY sample was reduced in a closed glass manifold circula-
tion system by introducing hydrogen (99.99% purity, 120 Torr)
while installing liquid nitrogen trap to prevent evolved water
from re-oxidation of the samples.
3. Results and discussion
3.1. X-ray diffraction
In Fig. 1, the x-ray diffraction patterns of Pt10–NaY(673)
and NaY are compared. The Pt free sample displayed the
expected pattern of NaY zeolite [16]. The crystallographic
changes induced by Pt10–NaY(673) exhibited virtually the
unshifted Pt lines in the normal face-centered cubic (fcc) pat-
tern of metallic Pt at 2␪ = 39.8◦ (1 1 1) and 46.4◦ (2 0 0) [15]. The
2.3. Catalyst characterization
2.3.1. X-ray diffraction
X-ray diffractograms were obtained using a Philips Ana-
lytical (type PW 1840) diffractometer. The patterns were run
˚
withnickel-filteredCopperradiation(λ = 1.5405 A)at30 kVand
20 mA with scanning speed of 2◦ in 2θ min−1. Calculations for
average crystallite size were made using Pt(1 1 1) linewidths in
Scherrer equation [15], with Gaussian lineshape approximation.
2.3.2. Thermal analysis
Thermogravimetric analyses were performed with
Shimadzu-50 thermal analyze. TG and DTG curves were
a
Fig. 1. XRD spectra of NaY and Pt10–NaY catalyst heated at 673 K.

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T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
Fig. 3. The TG and DTG profiles of Pt10–NaY.
3.2. Thermal behavior of uncalcined samples
Figs. 3 and 4 show the representative TG and DTG curves of
the Pt10– and Au10–NaY solid samples along with the weight
percentage loss at different steps. The DTG profiles exhibited
four endothermic peaks of maximal located at 330, 391, 554
and 695 K for Pt10–NaY (Fig. 3), whilst at 348, 393, 469 and
699 K for the Au10–NaY (Fig. 4). The first, second and fourth
peaks in the profiles of both samples are found at nearly iden-
tical temperatures. The first and second maximal correspond to
elimination of loosely bound and strongly held water molecules
in NaY zeolite, respectively, where they resemble those in the
curve of unloaded NaY (not shown).
The third maximum of weight loss in the curve of Pt10–NaY
at 554 K corresponds to the maximum rate of decomposition of
[Pt(NH3)4]2+ complex to metallic Pt (Fig. 3). The formation of
Pt(0) particles in the calcined sample at 673 K has been con-
firmed by the XRD analysis, 2θ = 39.8◦ and 46.4◦. However, the
formation of some metallic Au in the early stage shortly after
heating of Au10–NaY is likely occurred since the decomposi-
tion of some Au precursors exposed to the surface of zeolite was
reportedly taking place at low temperatures as 338–353 K [19].
This conclusion has been confirmed by the XRD analysis of this
sample. Where a diffraction line was observable at 2θ = 38.2◦
and 44.4◦ characteristic to Au(0). The loss of intrazeolite chlo-
ride associated with Au sample in the ␤-cage represents the third
temperature region of weight loss in the profile of Au10–NaY.
In fact, the investigated samples exhibited a slow loss process
Fig. 2. XRD spectra of NaY and Au(10)–NaY catalyst treated at 353 and 423 K.
formation of Pt crystallites was likely occurred due to decom-
position of Pt precursor complex, [Pt(NH3)4](NO3)2, during the
calcination at 673 K. The unshifted position of all Pt peaks
indicated that Pt forms a separate phase at the external sur-
face of NaY zeolite. Average Pt crystallite size was calculated
from the Pt10–NaY(673) pattern using line broadening of the
Pt(1 1 1) line, Scherrer equation, and a correction for instru-
mental line broadening. The 18.6 nm average size of Pt crys-
tallites thus calculated was much smaller than those reported for
Pt(1.2–5.0)/WO3/SiO2 catalysts, e.g. 50 nm, which decreased
with WO3 loading [17]. This indicates that, the distribution of
Pt particles in NaY zeolite is better than that on the SiO2 sub-
strate. Nevertheless, other oxidized forms of Pt moieties could
also present in NaY, though we did not reduce this sample. A
careful inspection of the pattern did not demonstrate evidence of
either PtO or PtO2, which may not show appreciable crystallinity
at this calcination stage. Thus, we cannot exclude completely the
presence of these phases since powder diffraction X-ray tech-
nique will not detect amorphous solids and those having particle
˚
size less than 50 A.
Fig. 2 shows the XRD diffractograms of pure NaY and
Au10–NaY(353) and Au10–NaY(423). The XRD pattern of
Au10–NaY(353) was characterized by the presence of new
diffraction lines at 2θ = 38.2◦ (1 1 1) and 44.4◦ (2 0 0) due to
metallic Au particles [6]. The average crystallites size of Au
were estimated (using Scherrer equation) to be 55.7 and 46.4 nm
from the peak-width of Au(1 1 1) for the sample heated at 353
and 423 K, respectively. As far as the impregnation method was
used it was difficult to deposit Au on zeolite with dispersion as
high as that for Pt metal, though gold has low sublimation energy
and very low Tammann temperature [18]. It was evidenced in a
previous study by EXAFS/XANES technique that the Au par-
ticles existed in the supercage of NaY when AuCl3/NaY was
thermally treated at 328–353 K [16]. The decrease of Au parti-
cle size with temperature shows, as unexpected, that Au particles
diffuse inside or nearby the surface (␣-cages) with increasing the
temperature.
Fig. 4. The TG and DTG profiles of Au10–NaY.

T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
131
Fig. 6. First order plots of CO oxidation with aqueous (0.5 M) H2O2 over unre-
duced Pt10–NaY catalyst at different reaction temperatures.
increases with increasing the Pt-content. It is also noticed that
the XRD pattern of these catalysts showed the characteristic
lines of Pt metal (ca. 2θ = 39.8◦ and 46.4◦). Therefore it seems
interesting to explore the effect of reduction of Pt10–NaY cata-
lyst on its catalytic activity towards CO oxidatin with aqueous
H2O2 at different temperatures. One hundred and 20 mg of unre-
duced and reduced catalysts, respectively, were used for the CO
oxidation reaction at relatively low temperatures in the range
of 303–333 K. Taking in consideration that a small amount of
reducedcatalyst(20 mg)wasusedduetoitshighactivitytowards
the H2O2 decomposition which in turn difficult to follow up the
reaction. Figs. 6 and 7 illustrate the first order plots (ln C versus
time) of CO oxidation with aqueous H2O2 (0.5 M) over unre-
duced and reduced catalysts, respectively, and at the studied
temperatures. The Km values as a function of temperature were
calculated and represented in Table 1. A similar trend of activ-
ity, increasing then decreasing with temperature, is obvious. Two
temperature regions were observed in case of both catalysts with
a maximum value of Km at 313 K for the reduced catalyst while
it was at 323 K for the unreduced one. The Km values listed in
Table 1 demonstrate that, a dramatic increase of the catalytic
Fig. 5. (a) First order plots of CO oxidation with (0.5 M) H2O2 at 313 K over
Pt–NaY of different Pt loadings and (b) the correlation between the Km values
and the Pt loadings.
by heating in a wide range (573–973 K) followed by mass loss
of 2.1 and 3.8 wt.% for Pt and Au samples, respectively. This
process might indicate complete removal of OH groups from
NaY lattice.
3.3. Oxidation of CO over Pt-NaY catalysts
3.3.1. Influence of Pt content
Fig. 5a shows the first order plots (ln C versus time) of
CO oxidation with H2O2 at 333 K over the Pt2–, Pt5– and
Pt10–NaY(673) catalysts. It is worthy to mention that, water
gas shift reaction did not contribute in the production of CO2
in the studied temperature. It is obvious that first order kinet-
ics is obeyed only after a certain period of about 20–40 min.
During such period the rate of CO oxidation is less than H2O2
decomposition, therefore Oad resulting from the decomposition
of H2O2 is favorably adsorbed more than CO on the catalyst
surface leading to slow rate of CO oxidation at the first stage
of reaction. After that period the reaction assumes a significant
rate and obeys first order kinetics. Similar results were observed
in case of Au–NaY and also over Cu–NaY catalysts [20] for
the same reaction. The specific first order rate constant Km was
evaluated from the linear portion of the curves.
The correlation of the catalytic activity (Km value) with Pt
content is shown in Fig. 5b. It is clear that the catalytic activity
of Pt–NaY catalysts increases systematically as the Pt content
increases. This might suggest that the Pt particles have well dis-
persed in Y-zeolite, which gave good activity in the CO oxidation
reaction with aqueous H2O2.
3.3.2. Influence of reaction temperature
From the kinetic study of CO oxidation over Pt–NaY cata-
lysts, itwasconcludedthatthecatalyticactivityofthesecatalysts
Fig. 7. First order plots of CO oxidation with aqueous (0.5 M) H2O2 over
reduced Pt10–NaY catalyst at different reaction temperatures.

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T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
Table 1
sites is mutually changed to an oxidic form, Pt(O)x [10], leading
to a drastic monotonic decrease in the reaction rate. From the
data, it is clear that the CO oxidation in aqueous H2O2 solution
reduces the activation energy for the reaction drastically espe-
cially in case of the reduced catalyst. However, similar results
were reported where the activation energy of CO oxidation was
estimated to be ≈74 and 37 kJ mol−1 in the absence and presence
of water vapor, respectively [10].
Reaction rate constants of CO oxidation using (0.5 M) H2O2 on the surface of
reduced and unreduced Pt10–NaY catalysts at different reaction temperatures
Temperature (K)
303
308
313
318
323
328
333
Km (min−1
Unreduced Pt/NaY
Reduced Pt/NaY
)
0.09
4.01 5.1
–
0.98
6.6
1.54
–
3.01
4.9
0.65
–
0.44
2.7
It is of great interest to note that the activation energy of
CO oxidation in aqueous H2O2 solution has low Eac value for
reduced catalyst compared with that of the non-reduced one gave
further evidence that highly dispersed metallic Pt is served as
the active site for this reaction.
activity of the reduced catalyst compared with the unreduced
one was observed especially at low temperature (303 K). For
instance, at that temperature, the value of the specific rate con-
stant (Km) was 4.45 min−1 for reduced Pt10–NaY while it was
weighing up 0.09 min−1 at the same temperature over unreduced
Pt10–NaY. Based on the above experimental observations, it is
evident that Pt metal in NaY functions as the active site for
CO oxidation with aqueous H2O2 as a large area of Pt surface
becomes accessible for CO adsorption. However, the XRD data
have shown that the particle size of Pt metal was obtained as
small as 18.6 nm. Also, the linear relationship between Pt con-
tent and the activity of such catalysts gives further evidence that
the active sites are Pt species which are dispersed on the catalyst
surface.
3.4. Oxidation of CO over Au–NaY catalysts
Gold is well known for its inertness among group Ib metals
[18], so, the Au atoms may be considered as less active than other
precious metals for adsorption and catalysis. Recently, Haruta
et al. [23,24] have found that gold is very active for the low
temperature oxidation of CO, if dispersed on metal oxides such
as TiO2. Therefore, it seems interesting to test the activity of
Au supported on NaY zeolite towards the oxidation of CO with
H2O2 in aqueous media.
The Arrhenius plots for reduced and unreduced catalysts,
Fig. 8, illustrated a reverse trend at the high temperature region
(303–323 K). Similar results were invoked by several authors for
gaseous CO oxidation with O2 over Pt–Al2O3 catalysts [21,22].
The calculated activation energies were ≈20 and ≈55 kJ mol−1
for reduced and unreduced catalysts, respectively. From these
results, it is obvious that the reaction mechanism changed com-
pletely with temperature. The inversion of the Arrhenius trend
at relatively high temperatures is discussed on the basis of the
higher decomposition of H2O2 on Pt surface, resulting in the
coverage of Pt with Oad. The subsequent assess of CO to Pt sites
appreciably decreased, indicating a competitive O–CO adsorp-
tion on Pt surface. In addition, the rate of H2O2 decomposition
increased with temperature in which the nature of the active
The effect of thermal treatment of Au10–NaY catalyst on
CO oxidation was studied by heating the samples in air for 3 h
Fig. 9. (a) First order plots of CO oxidation with aqueous H2O2 (0.5 M) over
Au10–NaY treated at different temperatures and (b) relationship between the
Km values and the treatment temperatures.
Fig. 8. Arrhenius plots of CO oxidation over reduced and unreduced Pt10–NaY
catalysts.

T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
133
Fig. 10. (a) First order plots of CO oxidation with aqueous H2O2 over Au10–NaY at different temperatures and (b) Arrhenius plot of CO oxidation over Au10–NaY
catalyst.
at temperatures ranging between 338 and 503 K. Fig. 9a illus-
trates the first order plots of CO oxidation at 333 K with 0.5 M
H2O2 in presence of Au10–NaY catalysts treated at different
temperatures. 0.1 g of the prepared samples was used. The Km
values were 0.47, 0.95, 0.8, 0.41, 0.32 and 0.20 min−1 at 338,
353, 383, 423, 473 and 503 K, respectively. It is clear that the
activity of Au10–NaY catalyst heated at 353 K was found to
have superior activity over other Au treated catalysts at higher
temperatures (Fig. 9b).
The high activity of Au10-NaY catalyst heated at 353 K
should be attributed to the reduction of Au(III) to Au(I) and
Au(0) in the presence of H2O [14,16]. Au(I) is considered as
the active site for CO adsorption over Au–NaY catalysts as
confirmed in our previous work [25]. It was shown by XRD anal-
Scheme 1. Proposed scheme for CO oxidation with (0.5 M) H2O2 over Au–NaY
catalyst.
ysis that the sample heated at 353 K possesses gold crystallites
and since, this sample showed the highest catalytic performance
among all the Au–NaY catalysts. It is suggested that, a dual
oxidation reaction over Au10–NaY catalyst is higher than that
site Au(I)–Au(0) is crucial for this reaction. The present results
over reduced Pt10–NaY catalyst indicating that the oxidation of
indicate that three steps are possible for CO oxidation by aque-
CO with aqueous H2O2 proceeds more readily over the reduced
ous H2O2 over gold catalysts: (i) the oxidation of Au(0) by Oad
Pt catalyst than over Au–NaY.
derived from H2O2 leading to the formation of Au–O–Au link-
ages [8]; (ii) the hydrolysis of Au–O–Au to Au(I)–OH−; (iii)
CO replaces the OH− forming Au(I)–CO which reacts with Oad
forming CO2. The elimination of Au(I)–OH− is expected due to
a very low activation energy of Au–O bonds [16,26] (Scheme 1).
According to the proposed mechanism, one must consider that
the delay of the desorption of CO2 (tind) is due to sequential
formation of Au(I).
The influence of reaction temperature on the activity of
Au10–NaY(353) catalyst was studied at three different tem-
peratures namely 323, 328 and 333 K at similar reaction con-
ditions (Fig. 10a). The activities, Km, values were 0.52, 0.70,
0.95 min−1 at 323, 328 and 333 K, respectively. The Km values
increase with increasing the reaction temperature. The activa-
tion energy of CO oxidation calculated from the Arrhenius plot
was 27 kJ mol−1. The value of activation energy obtained in this
study is less that the values observed by Iwasawa and co-workers
(50–87 kJ mol−1) [27,28] which indicates that the aqueous CO
oxidation by H2O2 is much easier than other methods since it
has lower activation energy. The activation energy of the CO
4. Conclusions
The results in this study present the CO oxidation in aqueous
H2O2 solution over precious metals namely Au and Pt catalysts
supported on NaY zeolite. The reaction proceeded successfully
at ambient temperature on the prepared catalysts. Pt–NaY cata-
lyst showed much better catalytic activity than Au–NaY catalyst
in the CO oxidation under the studied conditions. Four possible
reasons may be considered as good evidences for suggesting that
dispersed metallic Pt is served as the active site for CO oxidation:
(i) The catalytic activity of Pt–NaY catalysts increases as the
Pt content increases.
(ii) The XRD study of Pt10–NaY catalyst confirms the pres-
ence of Pt-metal on the surface of NaY zeolite.
(iii) ThecatalyticactivityofthereducedPt10–NaYintheoxida-
tion of CO is approximately 44 times that of the unreduced
catalyst at 303 K, and under the same conditions.

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T.M. Salama et al. / Journal of Molecular Catalysis A: Chemical 264 (2007) 128–134
(iv) Activation energy of CO oxidation over the reduced cat-
alyst (ca. 20 kJ mol−1) is much lower than that over the
unreduced catalyst (ca. 55 kJ mol−1).
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est catalytic activity was obtained for the catalyst treated at
353 K. A dual Au(I)/Au(0) sitewassuggested asthe activecenter
for CO oxidation over Au–NaY catalysts.
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