Kinetics of CO Oxidation over Pt Modified CuO Nanocatalysts

Kinetics of CO Oxidation over Pt-Modi

fi

ed CuO Nanocatalysts

Luu C. Loc

1,+

, Nguyen Tri

2

, Hoang T. Cuong

1

and Ha C. Anh

2 1_{Institute of Chemical Technology, Vietnam Academy of Science and Technology,}

01 Mac Dinh Chi Str., Ho Chi Minh City, 70100 Vietnam

2_{Ho Chi Minh City University of Technology, Vietnam National University - Ho Chi Minh City,}

268 Ly Thuong Kiet Str., Ho Chi Minh City, 70100 Vietnam

Three Pt-CuO nanocatalysts PtCu/Al, PtCu/CeAl and PtCu/Ce have been successfully prepared. The characterization of the catalysts was examined by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray energy dispersive analysis (EDS), temperature-programmed reduction (TPR), nitrogen physisorption measurements, and IR-CO adsorption. The kinetics of CO oxidation using these catalysts was studied in a gradientlessflow-circulating system at 398­498 K. The obtained kinetic equation confirmed that the reaction proceeds in medium surface coverage with the participation of CO molecules and oxygen atoms. [doi:10.2320/matertrans.MA201545]

(Received January 30, 2015; Accepted May 29, 2015; Published August 25, 2015)

Keywords: carbon monoxide oxidation, kinetics, platinum-modified copper oxide nanocatalysts

1. Introduction

The advantage of low-temperature oxidation is to reduce fuel consumption for conversion of large volume of polluted air. Metal oxides and multioxide owning high activity and thermal stability are considered as alternative catalyst for the existing expensive noble metals. In fact, promising results were obtained by adding a small amount of noble metals to metal oxide catalysts. Particularly, the highest activity

in oxidation of CuO/CeO2 catalysts modified with Pt is

rationalized by the strong link between the Pt with CuO/

CeO2.1)The synergic effects between metal oxides and noble

metals results in the increase of reducibility, which may enhance the oxygen transfer from the metallic oxides to the noble metals.2)

Research on kinetics of oxidation of single CO on noble

metal and oxide catalysts have intensively studied.3­11)

Among various forms of suggested kinetic equations for the oxidation of CO, power-law kinetic expressions were

repeatedly proposed. Indeed,first-order of oxygen and

zero-order of CO concentrations for CO oxidation on bulk copper

oxide were reported by Garner et al.3) In contract, over a

silica-supported copper oxide catalyst,first-order of CO and

zero-order of oxygen concentrations were observed and

Eley-Rideal mechanism was proposed.4)_{In addition, a power-law}

rate equation was found to satisfactorilyfit the experimental

data of carbon monoxide oxidation with CO at partial pressure ranging from 0.0015 to 0.0125 atm over CuO

supported on nanosized CeO2.5)Kinetics of CO oxidation in

the CO-PROX process (H2-rich gases) has been investigated

in afixed-bed reactor by Caputoet al.6)and a power-law rate

equation was found. On the basis of Langmuir-Hinshelwood mechanism, the following expression for the reaction rate of

CO oxidation on CuO/£-Al2O3 was proposed by Vannice

et al.:7)

r¼k KCOPCO

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

KO2PO2 p

ð1þKCOPCOþ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKO2PO2 p

Þ2 ð1Þ

wherePCO,PO2 are the partial pressures of CO and O2;KCO

and KO2-adsorption coefficients for CO and O species,

respectively. However, order of O2pressure was found to be

near zero and the equation became power-law kinetic expressions:7)

r¼kðKCOPCOÞ0:7 ð2Þ

In our previous publications,8,9) oxide catalysts 10 mass%

CuO/£-Al2O3 (Cu/Al), 10 mass% CuO/20 mass% CeO2+

£-Al2O3 (Cu/CeAl) have been reported to be the most

active and stable catalysts in the complete oxidation of CO. Furthermore, high active catalysts at low temperature reaction

was obtained when Pt of 0.1 mass% was introduced to CuO

catalysts.10) _{The kinetics of deep oxidation of CO}8) _and

p-xylene and its mixtures with CO9) _{over Cu/Al and Cu/}

CeAl have been investigated at the temperature range of 473­ 543 K. The following rate equations were achieved for deep

oxidation of sole CO:8)

rCO¼ kCOPCOP 0:5

O2

ð1þk1PO0:25þk3PCO2Þ

ð3Þ

and p-xylene:9)

rxyl¼

kxylPxylPO0:25

ðP0:5

O2 þk3PCO2þk4Pxylþk5PH2OÞ

ð4Þ

It has been revealed that a complicated mutual effect associated with the formation of new intermediates takes

place in the simultaneous oxidation of CO and p-xylene to

change the reaction kinetics. The following rate equations

were obtained for deep oxidation of CO andp-xylene in their

mixture on the Cu/CeAl catalyst:9)

r_i ¼ k

iPiP_O0:₂5

ð1þk₁P_O0:₂5þk₂PCOþk₃PCO2þk4Pxylþk5PH2OÞ

ki PCOPxyl

ð1þk₂PCOÞð1þk₆PCOPxylÞ ð5Þ

where ri* ­ reaction rate of CO or p-xylene oxidation in

their mixture; ki*,ki** ­ constants; Pi ­ partial pressure of i

components;i-CO orp-xylene.

The aim of this study is to establish the kinetics of CO

oxidation using Pt-modified CuO catalysts.

+_{Corresponding author, E-mail: lcloc@ict.vast.vn}

Special Issue on Nanostructured Functional Materials and Their Applications

2. Experimental Procedure

Two catalysts: 0.1 mass% Pt+10 mass% CuO/£-Al2O3

(PtCu/Al) and 0.1 mass% Pt+10 mass% CuO/(20 mass%

CeO2+69.9 mass% £-Al2O3) (PtCu/CeAl) have been

pre-pared by sequential impregnations as described in our

previous works.10,12) Catalyst 0.1 mass% Pt+7.5 mass%

CuO/CeO2 (PtCu/Ce) was prepared by the urea nitrates

combustion method described by H. Matralis et al.11)

with molar ratios of urea/nitrate=4.17. Ce(NO3)3·6H2O,

Cu(NO3)2·3H2O, H2PtCl6·6H2O complex, urea (CO(NH2)2)

and £-Al2O3 were purchased from Merck. All of the

precursors were used without further purification. The IR

spectra were recorded from apparatus Nicolet­Spectrometer

460 in the range of 4000­400 cm¹1 _{with a resolution of}

4 cm¹1_{. The catalyst samples were pretreated in a pure}

oxygen flow of velocity 5 L/h for 1 h at 873 K for metal

oxide catalyst and at 573 K for Pt containing sample. The

kinetics of CO oxidation was studied in a gradientless fl

ow-circulating system at 398­498 K. Ranges of initial partial

pressures of CO, O2 and CO2 were 2.5­20, 35­140 and 0­

25(hPa), respectively. The follow gas have been used: O2

(99.999%); N2(99.999%); Air (21 mol%O2+79 mol%N2);

mixture CO (6 mol%)+N2 (94 mol%); and mixture CO2

(6 mol%)+N2(94 mol%).

3. Results and Discussions

3.1 Physico-chemical characteristics and activity of the obtained catalysts

Physico-chemical characteristics of catalysts were

previ-ously reported.12) _{Briefly, from XRD pattern of catalysts}

(Fig. 1), alumina exists in an amorphous state, cerium oxide

exists in crystalline state of cubic fluorite structured CeO2

(2ª=28.6°, 33°, 47.4°, 56.3°, 59°, 69.4°, 76.7° and 79°).13)

The significantly weak intensity CeO2 peaks in PtCu/CeAl

catalyst indicated that the interaction of CeO2 with Al2O3

resulted in cerium oxide to be crystallized in small

agglomerate. The XRD patterns of PtCu/CeAl and PtCu/

Ce showed very weak CuO reflections. This can be explained

by the existing of copper oxide phase in a highly divided or amorphous state on the surface of ceria or the formation of

solid solution.11) _{At the same time on PtCu/Al catalyst,}

copper oxide exits in a state of good crystalline. In all samples the characteristic peaks of Pt with low intensity were observed. From Table 1 and TEM image (Fig. 2), platinum

exists infine dispersed state with particle size of 1­3 nm. The

EDS image of PtCu/Ce catalyst (Fig. 3(a)) show that Cu and

Cr are fairly evenly distributed on the surface of CeO2. On

the surface of PtCu/CeAl catalyst (Fig. 3(b)), the different

regions of metal particles distribution can be observed; Pt and

Cu are concentrated more on CeO2 than on £-Al2O3. TPR

diagram on all CuO-based catalysts modified by Pt, showed

only the peak of CuO reduction while the characteristic peak of Pt did not appear, probably due to its very low

concentration.12)_{Thus, in comparison with non-Pt modified}

catalysts8)_{the addition of Pt did not change the character in}

XRD pattern of the sample Cu/Al. Instead, it enhanced the

reducibility of catalysts by decreasing reduction temperature

and increasing reduction extent KRed (Table 1), further

enhancing the activity of catalysts. When 0.1 mass% of Pt

was introduced to the Cu/Al catalyst, the temperature for

50%-conversion of CO reduced from 498 K to 438 K, and the

temperature for 100%-conversion of CO reduced from 573 K

to 548 K. Similarly, the PtCu/CeAl catalyst was capable of

converting 50% CO at 362 K and 100% CO even at 383 K

(15 K lower than that for Cu/CeAl catalyst).

It has been shown in Table 1, compare to PtCu/Al

catalyst, the CeO2-contained catalysts (PtCu/CeAl and

PtCu/Ce) offered much higher activity in CO oxidation,

the temperature for 50%-conversion of CO was as low as

358­462 K (80 K lower than that of the catalysts without

CeO2). The results might come from the fact that in catalysts

containing CeO2the copper oxide exists in a highly divided

or amorphous state.

3.2 Kinetics of CO oxidation over the obtained catalysts

The Arrhenius plot of CO oxidation rate (rCO), logrCO

versus 1/Tis nonlinear, showing that the reaction rate obeyed

a fractional rational equation rather than a power law one (Fig. 4). The dependence of reaction rate upon CO partial pressure for all the catalysts was nearly linear (Fig. 5).

[image:2.595.314.540.66.300.2]

Fig. 1 XRD patterns of catalysts: (1) PtCu/Al; (2) PtCu/CeAl; (3) PtCu/ Ce (Pt-Pt; Cu-CuO; Al-Al2O3; CuAl-CuAl2O4; Ce-CeO2).

Table 1 The values of surface specific area (SBET), crystal size of CeO2 at 2ª=28.6° (dCeO2) and CuO at 2ª=35° (dCuO), particle size of Pt determined from TEM imagine (dPt), maximum reduction temperature (Tmax), reduction extent (KRed) and temperatures for 50%conversion of CO (T50) of the catalysts.

Catalysts SBET, m2_/g

dCeO2, nm

dCuO, nm

dPt, nm

Tmax, K

Kred,

%

T50, K

PtCu/Al 95.9 ® 18.8 1­3 547, 673 36.7 438

PtCu/Ce 14.8 11.8 n.d 1­3 457, 487,

818, 960 32.2 358

PtCu/CeAl 80.1 7.1 n.d ¯1 528 45.8 362

[image:2.595.304.550.401.480.2]

Therefore, it is conclusive that CO pressure appears in the

numerator of kinetic equation infirst power. The convex form

of dependence of reaction rate versus O2 partial pressure in

Fig. 6 indicates that oxygen concentration appeared in both the numerator and denominator of kinetic equation. The

concave shape of the conversion curves, rCO versus CO

conversion (XCO), revealed that the reaction was inhibited by

at least one of the products.14)The dependence of 1/rCO vs

PCO2 is linear (Fig. 7), meaning that PCO2 appeared in the

denominator of kinetic equation in power of unit. Thus, the reaction rate in general form should be described by the following equation:

rCO¼ kCOP

n1

COPOn22

ð1þk1POm12 þk2P m2

COþk3Pm3CO2Þ2¡

ð6Þ

Where: kCO, k1, k2, k3 - constants of kinetic equation; 2¡

-surface coverage;PCO,PO2,PCO2- partial pressures of CO, O2

and CO2, respectively. The optimal coincidence between

experimental and calculated results has been observed

when n1=m2=m3=1; n2=m1=0.5; ¡=0.5, k2=0

and reaction rate is described in form of eq. (3).

The values of the kinetic constants of eq. (3) were given in Table 2. The error of the calculation of the reaction rates

via eq. (3) was 19­22%. Results in Table 2 showed that the

remarkable higher value of kCO was obtained on CeO2

-contained catalysts referring to high catalytic activity.

In comparison with non-Pt modified catalysts,8) _the

addition of Pt does not change the form of kinetic equation and expression (3) is the common equation for CO oxidation

(a)

(b)

(c)

Fig. 2 TEM images of the catalysts: (a) PtCu/Al; (b) PtCu/CeAl; (c) PtCu/Ce.

Fig. 4 The Arrhenius plot of CO oxidation rate, logrCOversus 1/T, over the catalysts: (1) PtCu/Al; (2) PtCu/CeAl and (3) PtCu/Ce atXCO=0.4;

PCO=3 hPa;PO2¼104hPa;PCO2¼2hPa. (a)

(b)

[image:3.595.69.267.67.527.2] [image:3.595.314.542.70.394.2] [image:3.595.317.536.454.587.2]

on CuO-based catalysts under tested conditions. However, it

reduced activation energy of the reaction (reflected in the

decrease of the value of ECO), subsequently increased the

activity of Pt containing catalysts and lowered the

temper-ature region of reaction. Moreover, on the Pt-modified

catalysts, value oxygen adsorption constants (k2) is much

lower than that of non-modified catalyst,8) _{indicating the}

relatively strong adsorption of CO. It is likely that the presence of Pt make oxidation degree of copper becomes lower. Indeed, IR spectra of CO-adsorption showed

charac-teristic bands Cu+­CO on PtCu/Al sample slightly shifted to

the shorter wavenumber region as seen on the Cu/Al sample

(2123 cm¹1 _{compared to 2125 cm}¹1_{) (Fig. 8). It has been}

demonstrated that shifts in CO band frequencies have often

been related to the change in exposed Cu surface planes.15)

4. Conclusion

Kinetic studies of CO oxidation over three Pt-modified

CuO nanocatalysts were performed. The experimental results provided that characteristics of the carriers and Pt additive affected the properties, activity, and adsorption capacity of catalysts. However, form of the kinetic equation was kept intact. On these catalysts the reaction proceeds in the average coverage with participation of CO molecules and oxygen

Fig. 5 Rate of CO oxidation (rCO) versus the partial pressure of CO (PCO) over the catalysts: (1) PtCu/Al; (2) PtCu/CeAl and (3) PtCu/Ce at

T=448 K;XCO=0.4;PO2¼104hPa;PCO2¼2hPa.

Fig. 6 Rate of CO oxidation (rCO) versus the partial pressure of O2(PO2) for catalysts: (1) PtCu/Al; (2) PtCu/Ce; and (3) PtCu/CeAl atT=448 K;

XCO=0.4;PCO=3 Pa;PCO2¼2hPa.

[image:4.595.47.546.85.149.2]

Fig. 7 The dependence of reversed values of reaction rate (1/rCO) on partial pressure of CO2(PCO2) for catalysts: (1) PtCu/Al; (2) PtCu/Ce; and (3) PtCu/CeAl at T=448 K; XCO=0.4; PCO=3 hPa; PO2¼ 104hPa.

Table 2 The values of the kinetic constants in the eq. (3).

Catalysts PtCu/Al PtCu/Ce PtCu/CeAl

kCO, mol·g¹1·h¹1·hPa¹1.5 4.2©exp(¹1421/RT) 1.1©103©exp(¹1496/RT) 9.7©102©exp(¹1139/RT)

k1, hPa¹0.5 10¹7©exp(13846/RT) 5©10¹3©exp(7028/RT) 6.9©10¹8©exp(16130/RT)

k3, hPa¹1 2©10¹16©exp(30302/RT) 0.2©10¹3©exp(8756/RT) 1.7©exp(1323/RT)

Variance,% 22 19 21

R=1.987 cal.mol¹1_.K¹1_{; k}

i=k0i©exp(¹Ei/RT); Ei(cal.mol¹1)

(a)

(b)

[image:4.595.305.547.95.440.2] [image:4.595.62.279.181.335.2] [image:4.595.63.279.404.543.2] [image:4.595.61.277.612.739.2]

atoms. Furthermore, CeO2 depressed the formation of

massive CuO leading to the increase in catalyst reduction

and reaction rate. Addition of 0.1 mass% Pt decreased the

activation energy of the reaction and increased CO adsorption, leading increased the activity of CuO-based catalysts.

Acknowledgments

This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grand No. 104.03-2012.60.

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