Oxidation of carbon monoxide over Cu- and Ag-NaY catalysts with aqueous hydrogen peroxide

Oxidation of carbon monoxide over Cu- and Ag-NaY

catalysts with aqueous hydrogen peroxide

Zeinhom Mohamed El-Bahy *

Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, 11884 Cairo, Egypt

Received 21 October 2006; received in revised form 8 December 2006; accepted 5 January 2007

Available online 11 January 2007

Abstract

A novel oxidation reaction of CO with aqueous H2O2 over Cu-NaY (2–15 wt%) and Ag-NaY (5–15 wt%) catalysts has been

achieved at low temperatures (55–70 8C) using a flow mode system. The employed catalysts were prepared by the incipient wetness

impregnation of NaY zeolite (Si/Al = 5.6, surface area = 910 m2/g) with an aqueous solution of known concentrations of copper

acetate and silver nitrate. Solids were subjected to thermal treatment at 300–450 8C prior to catalytic measurements unless

subjected to subsequent reduction with hydrogen at 350 8C. The physicochemical characterization of the catalysts was probed using

X-ray diffraction (XRD), FT-IR and combined thermal analyses TGA–DrTGA. The XRD data indicated that, the Ag particles have

an ordered location in the sodalite cavity and the center of a single six-ring. The FT-IR data also proved the presence of a new peak at

1385 cm�1 that is assigned to Ag-coordinated with the framework.

A slow induced oxidation of CO (induction period, tind) took place at the initial stage of the CO oxidation reaction after which the

reaction obeyed first-order kinetics. The utilized metal ions are proposed to be reduced to lower oxidation states such as Cu+ and Ag0

during the first period of reaction, tind, where the reaction proceeded favorably on such sites. Such argument was evidenced by

carrying out the oxidation reaction over H2-reduced Cu10-NaYand Ag10-NaY catalysts. The reduction caused a decrease in the tind,

giving an evidence that the lower oxidation states Cu+ and Ag0 are the active sites in the studied oxidation reaction. The

enhancement in catalytic activity was interpreted in terms of the facile adsorption of CO on the low oxidation state species.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides; C. Infrared spectroscopy; C. X-ray diffraction; D. Catalytic properties

1. Introduction

The catalytic oxidation of CO with molecular oxygen has been extensively studied over numerous catalysts. This

simple reaction is being utilized in an increasing number of applications. For instance, the CO oxidation reaction is

often an integral component of pollution control devices designed to reduce industrial and automotive emissions [1].

Air purification devices [2–4] and CO gas sensors [5–7] commonly employ CO oxidation catalysts. Nevertheless, the

development of low temperature CO oxidation catalysts has received a considerable attention [8–10]. Haruta and co-

workers have prepared supported gold catalysts on Mn2O3, Fe2O3, Co3O4, etc., to determine their catalytic activities

toward the oxidation of H2 and/or CO [7,8].

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Materials Research Bulletin 42 (2007) 2170–2183

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E-mail address: [email protected].

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doi:10.1016/j.materresbull.2007.01.004

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http://dx.doi.org/10.1016/j.materresbull.2007.01.004

Many oxidation reactions in liquid phase using aqueous H2O2 as an oxidant have been of much attention in

presence of supported and non-supported catalysts [11–14]. H2O2 has been used in the oxidation of alcohols, ketons

[15–18] and amines [19–21] on titanium silicate (TS-1). The liquid phase ammoxidation of cyclohexanone over

Ti-zeolite catalyst in the presence of NH3 and H2O2 is an important process [22–24]. The conclusive oxidation

process of these reactions should lead to the production of CO gas. For instance, heterogeneous catalytic

epoxidation of olefins and parafins with oxygen usually gives poor selectivity because of the high reaction

temperature [14]. The low temperature oxidation of CO using H2O2 will be proficient in removing CO from gases in

hydrogen rich fuel produced from methanol in fuel cell. That will lead to avoid poisoning of Pt anode at low

temperatures (ca. 55–105 8C) and enhance the fuel cell performance. Under normal running conditions the product

hydrogen stream contains 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. Only we have reported the oxidation of CO

with H2O2 in aqueous medium in the presence of Au- and Pt-NaY catalysts [25] to reduce the CO concentration in

liquid phase conditions.

The aim of this work is to prepare and characterize Cu-NaY and Ag-NaY catalysts to make use of the high surface

area of the NaY zeolite and the activity of Cu and Ag species in the decomposition of H2O2 and adsorption of CO

which has a great role in the CO oxidation reaction. Thus, we wish to report a new attempt to explore the oxidation of

CO with aqueous H2O2 in presence of Cu- and Ag-NaY catalysts and make advantage of their activity towards H2O2

decomposition and the facile adsorption of CO on such sites.

2. Experimental

Cu-NaY and Ag-NaY catalysts were prepared by the incipient wetness impregnation of NaY zeolite (Si/Al = 5.6,

surface area = 910 m2/g) with an aqueous solution of known concentrations of copper acetate and silver nitrate,

respectively, to obtain different Cu (2, 5, 10 and 15%) and Ag (5, 10, and 15%) metal loadings. The metal ion

concentrations were ensured by the conventional complexometric titration using EDTA complexone. The mixture was

refluxed at 80 8C with a continuous stirring for 3 h, washed thoroughly with distilled water, then subsequent air-dried

overnight at 120 8C, according to the method described elsewhere [23]. The calcination of the prepared samples

was performed in air at 400 8C for 5 h till constant weight (unless otherwise stated). The samples were referred to as

Mx-NaY, where M and x referred to the metal and wt% of M loading.

The reduction of samples was performed in a closed circulation system by introducing hydrogen (99.99% purity,

120 Torr) during installation of liquid nitrogen trap to prevent the re-oxidation of the reduced species by water evolved.

X-ray diffractograms were obtained using a Philips Analytical (type PW 1840) diffractometer. The patterns were

run with nickel-filtered Cu Ka radiation (l = 1.5405 A) at 30 kV and 20 mA with scanning speed of 28 in 2u min�1.

The degree of relative crystallinity was calculated based on the five characteristic peak intensities at d-spacings of

13.79, 5.60, 3.73, 3.27 and 2.89 A. The diffraction lines of reference NaY zeolite were used as the base (100%) for

comparing the percentage of zeolitic NaY structure in the overall metal-loaded samples [26].

The Fourier transform infrared (FT-IR) spectra were recorded on a Bruker (Vector 22), single beam spectrometer

with a resolution of 2 cm�1. The samples were ground with KBr (1:100 ratio) as a tablet and mounted to the sample

holder in the cavity of the spectrometer.

Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DrTGA) were performed with a

Shimadzu-50 thermal analyzer. TGA and DrTGA curves were recorded with the following conditions: 9 mg; nitrogen

atmosphere of flow rate 30 ml min�1; platinum crucible; ramping rate of 10 K min�1 and a maximum temperature of

800 8C.

The oxidation of CO with aqueous H2O2 was carried out in a flow mode apparatus. In a typical reaction, 100 ml of

0.5 M H2O2 (BDH grade) was introduced into a double wall glass reactor. The gas mixture CO/N2 (1:1) was allowed to

pass through the reaction medium at a constant flow rate of 20 ml/min (unless otherwise stated) under constant stirring

conditions. The glass reactor was thermostated at the desired temperature by using temperature controller type (PEX-P

90). Then, 0.1 g of the catalyst was added to the reaction mixture supposing zero time. The kinetics of the reaction

were followed by introducing the gas mixture to a strong alkaline solution (NaOH) and measuring the change in its pH

value at interval times. The decrease in OH� concentration with time, indicated by the decrease of pH value, was taken

as equivalent to the amount of CO2 evolved from the reaction. The natural logarithm of [OH�] (ln C) was plotted

against time (t, min), applying the first-order rate equation: (kt = ln C0 � ln C) where, C0 and C are the initial OH�

Z.M. El-Bahy / Materials Research Bulletin 42 (2007) 2170–2183 2171

concentration and its concentration at time t, respectively. The specific rate constant was used to express the activity of

catalysts, km:

km ¼rate consant ðkÞcatalyst weight

:

3. Results and discussion

3.1. X-ray diffraction

Powder diffractograms of different Cu- and Ag-NaY loading samples, along with parent NaY zeolite were recorded

as shown in Figs. 1 and 2, respectively. The diffraction lines characteristic of NaY at d-spacings of 13.79, 5.60, 4.71,

4.32, 3.73, 3.27, 2.99, 2.89 and 2.84 A [PDF no. 38-0240], decreased with increasing metal loadings. Detailed analysis

of the X-ray diffraction of Cux-NaY and Agx-NaY showed evidence of CuO phase at d-spacings of 2.53, 2.33 and

1.87 A [PDF no. 48-1548]. Small lines at d-spacings of 2.51 and 2.30 A characteristic to Ag2O [PDF no. 75-1532]

were detected in the high Ag loading namely Ag10-NaY. This attenuation in the diffraction line intensities as

compared with those of NaY zeolite is caused by some loss of crystallinity evidently took place as shown in Table 1.

The crystallinity decreased especially in case of Ag10-NaY since it turned down to 58% and it was around 95% in case

of Cu10-NaY when compared with the parent NaY (taken at 100% crystallinity). This decrease can be a result of

the higher absorption coefficient of Cu and Ag species for the X-ray radiation [27] which increases with increasing

Cu-loading.

Z.M. El-Bahy / Materials Research Bulletin 42 (2007) 2170–21832172

Fig. 1. XRD spectra of (a) parent NaY, (b) Cu5-NaY, (c) Cu10-NaY and (d) Cu15-NaY catalysts calcined at 400 8C.

Fig. 3 shows the change in the diffraction patterns of Cu10-NaY with the variation of calcination temperature

in the range of 350–400 8C. Elevating the calcination temperature leads to progressive increase in the intensity

of CuO diffraction lines at d-spacings of 2.53, 2.33 and 1.87 A with a simultaneous decrease of the lines belonging

to NaY. This result undoubtedly shows a less effective interaction between surface CuO and NaY zeolite by

increasing temperature as due to increasing surface mobility of the latter phase and the agglomeration of CuO phase

on the NaY surface. The absence of a broad reflection in all samples between 2u value of 228 and 278, which would

indicate the presence of amorphous silica [28], demonstrates that the structure remains intact after the incorporation

of Cu and Ag inside the zeolite even with the decrease of crystallinity to a minimum value such as the case of Ag10-

NaY.

The unit cell parameters a0 for parent NaY and Cu- and Ag-Y samples has been determined by the help of

Rietveld method for optimizing the structural data and listed in Table 1. It is clear that the a0 value and the cell

volume has varied when compared with parent NaYas shown in Table 1. The cell volume of all samples is expected to

increase markedly when compared with the parent NaY suggesting the incorporation of Cu and Ag ions in the NaY

framework.


Fig. 2. XRD spectra of (a) parent NaY, (b) Ag5-NaY and (c) Ag10-NaY catalysts calcined at 400 8C.

Table 1

Unit cell parameters, cell volume and crystallinity of the prepared samples

Catalyst Cell constant, a0 (A) Cell volume (A)3 Crystallinity (%)

NaY 24.24 14,255 100

Cu5-NaY(400) 24.67 15,027 97.7

Cu10-NaY(350) 24.70 15,080 70.3

Cu10-NaY(400) 24.67 15,018 94.9

Cu10-NaY(450) 24.67 15,025 83.8

Cu15-NaY(400) 24.69 15,061 75.0

Ag5-NaY(400) 24.63 14,945 62.3

Ag10-NaY(400) 24.69 15,063 58.5

It has been observed that empirical derived relationship exists between the relative I331, I311, and I220

peak intensities and cation location in faujasite type zeolites [29]. Cations are randomly distributed within the

lattice if I331 > I220 > I311, but if I331 > I311 > I220, the cations assume positions at sites I�, II. Using these

empirical criteria, the diffraction patterns of the prepared samples, except for Ag10-NaY (Figs. 1–3), suggest

that the metal ions are probably present in random positions within the NaY cavities. Whereas in case of

Ag10-NaY, Fig. 2, the order of the peak intensities was I331 > I220 > I311 and consequently, the ions are located

at site I� (sodalite cavity) and site II (the center of a single six-ring, S6R) or displaced from this point into a

supercage [30].

3.2. Mid-infrared spectra

The framework IR spectra of parent NaY zeolite and Cu-NaY with different Cu-loading samples are shown in

Fig. 4. The Cu-loaded samples exhibited bands of varying intensity and width at about 1167, 1025, 793, 724, 578 and

457 cm�1 which are typical for NaY [31]. The bands at about 1025, 724 and 457 cm�1 are caused by internal


Fig. 3. XRD spectra of (a) parent NaY and Cu10-NaY calcined at (b) 350, (c) 400 and (d) 450 8C.

vibrations of (Si,Al)O4 tetrahedra of NaY, which tend to be insensitive to structure vibrations, whereas the bands at

about 1167, 793, 579 cm�1 are due to vibrations related to external linkages between (Si,Al)O4 tetrahedra, and are

sensitive to the framework structure [32]. The structure-sensitive bands did not show significant changes in the

spectrum of Cu5-NaY sample as compared with those of NaY. On the other hand, the 578 cm�1 absorption band

assigned to double ring polyhedra in the zeolite framework [33] decreased in intensity at Cu-loading of 15 wt%. This

indicates that a slight decomposition of the zeolite structure with increasing Cu concentration occurred. That is

confirmed also by the XRD data, since a decrease of the crystallinity was noticed up to 84% of the parent NaY. The

bands due to CuO, i.e. 610 and 500 cm�1 [34] were obscured by the absorption bands of NaY, though the XRD analysis

showed free CuO crystallites grew at the external surface of zeolite.

From the recorded spectra of NaY, Ag5-NaY and Ag10-NaY, Fig. 5, it can be observed that, the spectra have the

same NaY characteristic sensitive and insensitive peaks. It is obvious also that, a new peak at 1385 cm�1 is also

observed in Ag-NaY catalysts. This peak was not observed in either NaYor Cu-NaY spectra. The 1385 cm�1 peak can

be assigned to coordinated framework silver [35]. These data are well correlated with the XRD data where, the

imbedding of Ag in the lattice in the sites I� and II and forms a coordinated structure leads to an increase of the cell

volume and a great decrease of the crystallinity of the Ag-loaded NaY catalysts.

Fig. 6 shows the IR spectra of Cu10-NaY precalcined in air at increasing temperatures, i.e. 350, 400 and 450 8C.

The bands characteristic of these samples resembled those of NaY zeolite, indicating no prominent decomposition of

the zeolite framework occurred during the heat treatment in the studied temperature range.


Fig. 4. FT-IR spectra of: (a) parent NaY, (b) Cu5-NaY, (c) Cu10-NaY and (d) Cu15-NaY catalysts calcined at 400 8C.

3.3. Thermal analysis

The TGA and DrTGA data of Cu10-NaY showed that the TG curve consists of three successive temperature

regions, Fig. 7. These regions are centered at 100, 268 and 407 8C. The temperature regions were accompanied by

mass loss of 14.80, 12.01 and 1.14%, respectively. The total weight loss due to heating up to 800 8C attained

27.95%.

It is well known that NaY zeolite almost contains 20% as physisorbed water molecules. However, The first region

represents the loss of water of crystallization. The second region reflects the transformation of Cu-acetate to CuO

during heating up the sample. The XRD data of this sample showed that CuO is formed at a temperature close to

400 8C. So the second region is attributed to the decomposition of Cu-acetate and the formation of CuO at 268 8C,

Fig. 7. The third region may be due to surface dehydroxylation of OH groups attached to Al in zeolite lattice. However,


Fig. 5. FT-IR spectra of: (a) parent NaY, (b) Ag5-NaY and (c) Ag10-NaY catalysts calcined at 400 8C.

the dehydroxylation of pure Al2O3 took place at higher temperatures, e.g. 800–900 8C. It seems likely that the

presence of CuO species lowered the dehydroxylation temperature that extended over long range.

The TGA and DrTGA of Ag10-NaYare shown in Fig. 8. The figure displays four decomposition regions centered

at 47, 104, 166 and 566 8C. The first region is due to loosely held water (1.3%), the second is due to the water of

crystallization (8%) compared to�15% in case of Cu10-NaY. The loss of H2O of crystallization in Ag10-NaY is less

than that of Cu10-NaY which may be attributed to the higher dispersion of Ag+ in zeolite cage as proved before by the

XRD where, the order of the peak intensities I331 > I220 > I311, the large decrease of crystallinity and the large lattice

volume were identified. In addition, the presence of the peak at 1385 cm�1 in the FT-IR spectra of Ag-NaY catalysts

means the occlusion of Ag ions in the zeolite lattice. Although the decomposition of AgNO3 occurs at relatively

higher temperature, the third peak at 166 8C may be due to the decomposition of AgNO3 to the oxide form according

to TGA calculations of weight loss. A fourth peak in the range of 284–746 and centered at 566 8C is attributed to the

transformation of the phase Ag2O to Ag [36] and the removal of the OH groups from NaY lattice which is lower than

regular because of the presence of Ag species.


Fig. 6. FT-IR spectra of: (a) parent NaY and Cu10-NaY calcined at (b) 350 8C, (c) 400 8C and (d) 450 8C.

3.4. CO oxidation with aqueous H2O2 over Cu- and Ag-NaY catalysts

3.4.1. Effect of calcination temperature of Cu10-NaY catalyst

The catalytic performance of a given catalyst is generally governed by a number of variables in the experimental

conditions such as calcination temperature. The correlation between the catalytic activity (expressed as km) calculated

from the first-order plots and the calcination temperatures is shown in Fig. 9. It is clear that an induction period (tind)

was observed during the first stage of the reaction, after which the reaction obeyed the first-order kinetic equation. It

has been reported that the plot of H2O2 decomposition on bulk CuO did not display tind [37,38]. Therefore, tind might

be associated with CO chemisorption on charged Cu centers. It is noticed that the catalytic activity of Cu-NaY

catalysts increases gradually by raising the calcination temperature from 300 to 400 8C, and then decreases again with

increasing temperature. The maximum catalytic activity was observed for the catalyst calcined at 400 8C. That is in

agreement with the data obtained from thermal analysis and XRD. These results are in a harmony with the data

obtained for the CO oxidation with molecular oxygen over CuO–Al2O3, though the reaction was carried out at a higher

temperature such as 125–225 8C [39] and at 350 8C [40]. From preceding data, the oxidation of CO using H2O2 in

liquid phase occurs at lower temperatures, which may give an advantage for the use of H2O2 oxidation method over the

conventional molecular oxygen oxidation method. The thermal analysis presented the complete transformation of


Fig. 7. The TG and DrTGA profiles of Cu10-NaY.

Fig. 8. The TG and DrTGA profiles of Ag10-NaY.

copper acetate to CuO around 300 8C. In addition, the XRD showed agglomeration of the CuO phase on the surface of

the catalyst which leads to the decrease of the number of active sites.

3.4.2. Effect of catalyst reduction with hydrogen

To demonstrate the effect of catalyst reduction on the catalyst activity, Cu10-NaY and Ag10-NaY were reduced at

350 8C using 120 Torr of H2 prior to the reaction. Fig. 10 shows the first-order plots of the reaction performed using

reduced and unreduced Cu10- and Ag10-NaYat the studied reaction conditions. It is obvious that tind is affected by the

mode of catalyst treatment, where it becomes 15 min in the reduced Cu10-NaY instead of 25 min in the unreduced

sample and it decreased to 50 min in the reduced Ag10-NaY instead of 60 min in the unreduced one. It is obvious that

the km was very close for both reduced and unreduced catalysts, indicating that the nature and amount of active sites in

the steady state are the same. This data is different from the data of CO oxidation over reduced Pt-NaY since the

catalytic activity of the reduced form was 50 times more than that of the unreduced form [25]. These differences may

be due to the fact that Pt ions are easy to be reduced more than Cu or Ag. Another observation may be taken is that

Cu-NaY catalyst is more active than that of Ag-NaY catalyst which may be due to the imbedding of Ag ions in the

coordination of the lattice structure but CuO is present mainly on the surface of the NaY zeolite and hence can be

readily in contact with the reactant species.


Fig. 9. The correlation between the km values and the calcination temperatures.

Fig. 10. First-order plots of CO oxidation with (0.5 M) H2O2 at 60 8C over reduced and unreduced (a) Cu10-NaY and (b) Ag10-NaY.

It has been shown that no stable carbonyls can be formed when CO is adsorbed on Cu(II) sites at room temperature

[41,42]. Consequently, the catalyst is reduced with the action of CO to lower oxidation state in the first stage of

reaction that is known as the induction period, tind, i.e. the first-order kinetics was subsequent to the formation of active

sites of Cu+ and Ag0 from Cu2+ and Ag+, respectively, Scheme 1. However, such lower oxidation state species are

stabilized by back p-donation [43–45] and may be considered as the active sites for this CO oxidation due to the facile

chemisorption of CO on lower oxidation states [46].

3.4.3. Effect of reaction temperature

The first-order plots of CO oxidation over employed catalysts (not shown) gave an impression that, the reaction

temperature slightly affected the activity of the catalyst during the first stage of the reaction, i.e. the temperature did not

affect the tind in the first stage of reaction but it increased the reaction rate in the steady state. The activation energy for

the CO oxidation reaction over the utilized catalysts was calculated from Arrhenius plots used in the temperature range

of 55–70 8C, Fig. 11 (ln km versus 1/T). The km values increased respectively with increasing reaction temperatures,

i.e. 1.02, 1.79, 2.51 and 4.79 min�1 in case of Cu10-NaY and 0.249, 0.43, 0.67 and 1.21 min�1 in case of Ag10-NaY.

The evaluated activation energy from Arrhenius plots was 46 kJ mol�1 for Cu10-NaYand 48 kJ mol�1 for Ag10-NaY.

The activation energy of CO oxidation reaction over Cu10-NaY is slightly less than that of CO oxidation reaction

over Ag10-NaY catalyst indicating that the CO oxidation reaction pathway is the same in both catalysts.


Scheme 1. Proposed scheme for CO oxidation with H2O2 over Cu-NaY and Ag-NaY catalysts.

Fig. 11. Arrhenius plots of CO oxidation over: (upper) Cu10-NaY and (lower) Ag10-NaY catalyst.

3.4.4. Effect of flow rate of reactants

The effect of feed gas flow rate on CO oxidation with aqueous H2O2 over Cu10-NaY catalyst was studied at 60 8Cunder similar experimental conditions as described previously. The km values deduced from the first-order plots of CO

oxidation at different gas hourly space velocity (GHSV) ca. 12,000, 15,000 and 18,000 h�1 are given in Fig. 12. A

progressive increase of km values was observed as GHSV decreases, where they amounted 0.82, 1.23 and 1.66 min�1

at GHSV of 18,000, 15,000 and 12,000 h�1, respectively. Such increase of the km values with decreasing the GHSV

values is interpreted in terms of the increase of the contact time of reactants on the catalyst surface.


Fig. 12. The correlation between the GHSV (h�1) and the reaction rate constant (km).

Fig. 13. The correlation between the km values and (a) Cu- and (b) Ag-loadings in the oxidation of CO with (0.5 M) H2O2 at 60 8C.

3.4.5. Effect of metal loading

The Effect of Cu- and Ag-loading on the catalytic activity of CO oxidation was studied and the relation between

km values and metal loading is shown in Fig. 13. In case of Cu-NaY catalysts, a pronounced increase in the curve

when Cu-loading increased from 2 to 5%, then passing through a slight increase with further increase in Cu-loading,

i.e. 10–15 wt%. These results indicate that CuO is well dispersed in the interior surface of NaY zeolite up to 5 wt% as

indicated by XRD technique, though the population of Cu centers was not high enough. This reflects the extension of

tind in the curves of these samples. Aggregates of CuO particles have been formed by increasing Cu-loading beyond

5% on the zeolite surface thereby a slight increase in km values was observed. That is supported by XRD data which

demonstrates the increase of CuO lines at d-spacings of 2.53, 2.33 and 1.87 A. In case of Ag-NaY samples, the activity

did not change much with increasing the silver loading which may be due to the presence of Ag species inside the site

I� and II as mentioned and proved by XRD and FT-IR data.

4. Conclusions

The catalytic oxidation of CO over Cu- and Ag-NaY catalysts in aqueous H2O2 has been successfully done at low

temperatures in the range of 55–70 8C. The reaction temperature was much lower than that used for the oxidation of

CO by molecular oxygen. The reaction showed a period (tind) prior to the first-order kinetics. Reduction of Cu2+ and

Ag+ is necessary to enhance and complete the oxidation of CO to CO2 while CO is partially reducing the metal ions in

the beginning of reaction. The CO is then easily adsorbed on the reduced form and then being oxidized with the

adsorbed oxygen on the surface of the utilized catalyst. That was evidenced by the decrease of tind in the reduced

catalyst in both catalysts. Thus the activity of the catalyst is related to the transformation of higher oxidation states to

lower oxidation states. Coordinated Ag species are formed inside the supercage of NaY zeolite. On the other hand,

CuO forms aggregates on the surface of NaY zeolite which leads to the slight increase of the catalytic activity with

increasing Cu-loading.

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Oxidation of carbon monoxide over Cu- and Ag-NaY catalysts with aqueous hydrogen peroxide