Cu Ag ModifiedCeriumOx

8/13/2019 Cu Ag ModifiedCeriumOx

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OU RNAL OF C ATALYSIS179,203221 (1998)ARTICLE NO.CA982213

Cu- and Ag-Modified Cerium Oxide Catalysts for Methane Oxidation

Lj. Kundakovic and M. Flytzani-Stephanopoulos1

D epartment of C hemical E ngineeri ng, Tufts Un iversity, M edfor d, M assachusetts 02155

R eceived Ja nua ry 21, 1998; revised June 24, 1998; accepte d J une 25, 1998

The catalytic activity of nanocrystalline doped ceria and Cu- and

Ag-modified ceria for the complete oxidation of methane was stud-

ied in this work. The catalyst structurewas studied byX-raydiffrac-

tion (XRD) and related to the availability of low-temperature oxy-

gen species. Selected samples were also analyzed by STEM/EDX,

HR TE M, and XPS . Temperature-programmed reduction (TP R ) by

H2and CH4, as well as oxygen chemisorption, measurements were

used to characterize the different oxygen species present on thecatalyst. La and Zr were used as dopants to modify the crystal size

and reduction properties of ceria. Enhanced activity for the com-

plete oxidation of methaneis discussed in terms of ceria reducibility,

crystal size, and formation of oxygen defects at the surface (extrin-

sic oxygen vacancies). Addition of transition metal oxides (CuO)

or transition metals (Ag) improves the low-temperature oxidation

activity of cerium oxide. The interaction of ceria with Ag and CuO

is a strong function of thecrystal size of ceria. In thepresence of the

transition metal or metal oxide, a small crystal size of ceria favors

the formation of highly reducible oxygen species and enhances the

methane oxidation activity. c 1998 Academic P ress

Key Words:cerium oxide; crystal size; reducibility; methane oxi-dation; silver; copper oxide.

INTRODUCTION

The low-temperature, complete oxidation of methane is

an a rea in cata lysis which, despite the large number of cata -

lytic systems studied, does not yet have an adequate so-

lution. The exhaust gas from natural gas-burning turbines

and vehicles contains unconverted methane which is emit-

ted into the a tmosphere (currently unregulated). Methane

is the most refra ctory of hydrocarbo ns, a nd its combustion

requires temperatures higher than typical exhaust temp-

erat ures (400C) (1). H owever, catalysts currently available

(noble metalsPd, Pt) are not adequate for low-tempera-

ture methane combustion, becauset hey require a fairly high

temperature for 100% conversion of methane (1).

The most active catalysts fo r methane oxidation are

Pd -based cata lysts and they ha ve been extensively studied

(26). However, the nature of active sites and sensitivity to

P d/P dO structure are no t w ell understoo d (7,8). Pd -ba sed

1 Corresponding author.

cata lysts ar e also stro ngly inhibited by the presence of

tion products (CO 2and H 2O), as reported by Ribeiro

(4). In addition, they give partial oxidation products

and H 2) under reducing conditions (9). Other metal o

studied include perovskite-like compounds (1012), m

exchanged zeolites (1316), and transition metal o

(17).

The methane oxidation rate on Pd-based catalyfirst order in methane, almost zero order in oxygen

is strongly inhibited by reaction products (water an

bon dioxide) (4,7,8). Kinetic data are consistent wit

La ngmuirH inshelwood reaction mechanism, whic

cludes dissociative a dsorption o f metha ne and oxyge

the other hand, methane oxidation on metal oxides

ovskites) is usually treated using the redox mech

(10,12). In both cases the abstraction o f the fi rst hyd

atom is considered to be the rate limiting step.

In this work we approach methane activation thr

the surface active oxygen species of mixed metal ox

We use cerium oxide as an active support for transmeta ls/meta l oxides, such a s silver a nd co pper.

Cerium oxide is an excellent catalyst for redox reac

The most importa nt a pplication is its use as a n a ddit

the three-way catalyst (TWC) for automotive exhau

treatment. U se of ceria as an active support for methan

CO oxidation reactions wa s recently reported (18,19).

Cu-modified, doped cerium oxide ma terials were fou

be excellent catalysts for the SO 2reduction by C O (2

In general, the high a ctivity of ceria in redox reaction

been attributed to the ceria reducibility and its high ox

storage capa city (OSC ) (2225), a nd format ion of desuch as oxygen vaca ncies (2628). Also, an a dva nta ge

TWC context, is the ability of ceria to disperse tran

metals and t o increase the thermal stability o f the alu

support (29,30).

The bulk and surfa ce properties of C eO2 can be mo

by d oping (30). D oping can improve t he sintering pr

ties of ceria, by stabilizing the ceria surface area and c

size. D oping w ith divalent and trivalent dopants lea

format ion of oxygen vacancies, and mo dificat ion of ox

mobility and ionic conductivity (28). The reduction pr

ties and oxygen storage capacity of ceria are also rep

203


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204 KU NDAKOVIC AND FLYTZA NI-STEP HA NOPO U LOS

to change by doping. C e-Z r-O solid solutions were exten-

sively studied recently because of their unusual reduction

behavior and high oxygen storage capacity (31). Zr and H f

doped CeO 2were found to b e good cata lysts for methane

combustion (32) and oxidation of isobutane (25). The re-

ducibility and higher oxygen storage capacity of Ce-Zr-O

solid solutions were attributed to the formation of defec-

tive fluorite structure by introduction of the smaller Zr4+

ion into the structure, which leads to higher oxygen mobil-ity (24). In addition Balducciet al. (23) reported promot ion

of the redox behavior of Ce-Zr-O solid solutions upon sin-

tering a t high t emperat ures (1000C ).

Addition of noble metals (Pt, Pd, R h) increases the re-

activity of the low-temperature o xygen species formed o n

ceria (29, 3339). As a result, the reducibility of ceria at

low t emperat ures is enha nced (40,41). R ecent reports ha ve

shown that the oxidation activity of ceria can be largely

enhanced not only by the platinum metals but also by tra n-

sition metals in general (4244). However, the interaction

of nanocrystalline ceria with metals and its ensuing higheractivity in oxidation rea ctions is not well understood.

In the present paper w e focus on the cata lytic activity of

nanocrystalline ceria a nd C u and A g-modified ceria f or the

complete oxidation of methane. La and Zr were used as

dopant s to modify the crystal size and sintering properties

of ceria. The dependence of the catalyst on ceria reducibil-

ity, crystal size, and forma tion of extrinsic oxygen vaca ncies

wa s examined. We report on the enha ncement of the reduc-

tion properties and a ctivity of ceria in redox reactions intro-

duced by doping and by add ition o f t ransition meta l/metal

oxides (Ag and Cu). The interaction of ceria with Ag and

CuO is a strong function of ceria crystal size. In the pres-ence of a t ransition metal, a small crystal size of ceria fa vors

the formation of highly reducible oxygen species.

EXPERIMENTAL

Catalyst P reparation

D oped fl uorite-type oxides were synthesized by copre-

cipitating nitrate salts by urea at about 100C (45). Ap-

proximately 25 mmol (NH 4)2Ce(NO 3)6(Aldrich, 99.99%)

and the desired amount of doping elements ZrO(NO 3)2(A ldrich, 99.99%), La (NO 3)3 5H 2O (A ldrich, 99.9%), a nd

24 g of urea (99%, A.C.S. grade, Aldrich) were dissolved

in 200 cm3 deionized water. The solution was heated to

100C a nd continuously mixed using a ma gnetic stirrer. Af -

ter coprecipitation, the resulting gels of Ce and Zr were

vigorously boiled for 8 h at 100C to remove excess urea

and age the gels. After aging, the precipitate was filtered,

washed twice in boiling deionized water, and dried in a

preheated vacuum oven (80100C) overnight. Only slight

vacuum was applied (5 mmHg). D ried samples w ere

crushed and calcined in air typically at 650C for 8 h. The

heating rate w as 2C /min. For comparison, C eO2wa

made from cerium-acetat e (Aldrich, 99.9%) decompo

at 750C.

Copper modified fluorite-type oxides were prepar

the ab ove urea coprecipitationgelation method. Ca t

containing Ag were prepared by the incipient w etne

pregnation method using the calcined supports pre

as above. The required amount of ammonia soluti

AgNO 3was added dropwise to the support powder uconstant stirring. The w et pow der w as dega sed in va

for 1 h so tha t the solution fully fi lled the pores of th

port. The samples were dried at 100C overnight an

cined typically a t 650C for 8 h (heating rate 2C /min

The catalyst composition throughout the pap

expressed as atomic percentage (metal/total me

100%).

Catalyst Characteri zation and Testing Pr ocedur es

For bulk composition analysis the catalysts wereyzed by inductively coupled plasma (IC P) Ato mic E m

Spectrometry (Perkin E lmer Plasma 40). The total B E

fa ce ar ea was routinely measured by single-point N2 ad

tion and desorption on a Micromeritics PulseChem

2705 instrument.

X-ray powder diffraction (XR D ) analysisof catalys

ples was performed on a R igaku 300 X-ray D iffracto

with R otating Anode G enerators and monochromat

tector. Copper Kradia tion wa s used with power sett

60 kV and 300 mA. For crysta l phase identifica tion, th

ical o peration pa rameters were: divergence slit o f 1

tering slit 1, receiving slit 0.3, and a scan rate 25with 0.02 data interval.

The cata lyst surface composition w as determined

ray photoelectron spectroscopy (XPS) on a Perkin-E

5100 system. For XPS analysis, the catalyst powde

pressed on a copper fo il an d placed in the vacuum cha

without a ny pretreatment. Mg X -ray source wa s used

the power setting at 300 W.

The catalyst microstructure analysis was perform

a Vacuum G enerato rs H B 603 scanning t ransmission

tron microscope (STEM) equipped with X-ray micro

of 0.14-nm optimum resolution for energy dispersive analysis (ED X). For STEM analysis, the catalyst

der was dispersed on a copper or nickel grid coated

a carbon film and elemental maps were obtained

128 128 da ta mat rix. Ag/Z rO 2 catalysts were also

lyzed by H RTE M/E D X. The a nalysis wa s performed

JEOL 2010 instrument. The powder was suspended i

propyl alcohol using an ultrasonic bath and deposit

the carbon-coated 200 mesh Cu grid.

All cata lysts w ere tested in a laboratory-scale pa

bed flow reactor, which consisted of 1-cm ID 50 cm

quartz tube with a porous quartz frit placed at the m


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CU - A G -M OD IFIED CER IU M OXID E CA TA LY S TS

An electric furnace (Lindberg) was used to heat the reac-

tor. Temperat ure was monitor ed by a K-type thermocouple

placed at the top of the catalyst bed and controlled by a

Wizard t emperature controller. The fl ow of reacting gases

was measured by mass flow meters. The typical feed gas

was 1% CH 4, 8% O 2, balance He, for methane oxidation

activity tests. All gases were certified calibration gas mix-

tures. The catalyst loading was 150 mg, unless otherwise

noted . The t ypical ca ta lyst pa cking density w as 1.8 g/cm3.The pressure drop through the reactor was


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FIG. 1. Effect of heat treatment of doped CeO 2(properties in Table 1) on methan e oxidat ion; light-off performance; 0.09 g s/cm3 (STP), 1

8% O 2, balance He.

(9.4 nm). La -doped CeO 2aft er t he 650C-calcination had

intermediate crystal size (8 nm). When heated to 800C

in air, Z r-doped cerias grow in crystal size close to the L a-

doped ones.

Ce-Zr-O solid solutions with Zr content


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FIG. 2. Variation of methane oxidation rate with partial pressure of methane, under constant oxygen pressure, Po = 0.05 bar; Ce(10%

calcined at 650C ( Ta ble 1).

FIG. 3. Variation of methane oxidation rate with partial pressure of oxygen under constant metha ne pressurePm = 0.01 bar; Ce(10%La)O 2 c

at 650C ( Ta ble 1).


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TABLE 2

Methane Oxidation K inetics over Ce(10% La)O2and 15% CuCe(4.5% La)O2C atalystsa

E q uation (4) E q ua tion (1)

A E a A b E a K Q

[mol/g/s/ba r1.23] [kJ /mol] [mol/g/s/bar0.23] [kJ /mol] [1/ba r] [kJ /mol]

Ce(10% La )O 2 8.9 109 113.0 5.7 108 113.0 1.03 24.3

15% CuCe(4.5% La)O 2 5.9 1010c 119.3c 7.84 107d 93.4d 3.46d 14.2d

a Ca lcined a t 650C ( Tab le 1).b For 15% CuCe(4.5% La)O 2in mol/g/s/ba r

0.18.c This work, BE T surface area = 32.3 m2/g.d From L iu and Flytzani-Stephanopoulos (1995) (18,19), B E T surface area = 30 m 2/g.

oxidation on noble metal catalysts is usually interpreted

ba sed on the La ngmuirHinshelwoo d mechanism which in-

cludes dissociative ad sorption of metha ne and oxygen (7,8),

with the abstraction of the first hydrogen atom being the

rate limiting step. On the other hand, methane oxidation

on metal oxides is usually treated using the redox mecha-

nism (1012). The experimental data (Figs. 2 and 3) were

best represented by

r = kmKm Pm P0.23

o

(1 + Km Pm), [1]

where kmis the surface reaction rate,

km = A exp(Ea/RT), [2]

a nd Km, the CH 4a dsorption eq uilibrium constant,

Km = Kexp(Q/RT). [3]

The La ngmuir adsorption-type dependence on Pm inEq.(1)

suggests t hat the rate-determining step involves a dsorbed

methane. The power order dependence on the partial pres-

sure of oxygen suggests a complex oxygen source for the

reaction. Due to the low value of the heat of adsorption,

Q (24.3 kJ/mol, Table 2), the experimental da ta may be

represented b y a simpler kinetic expression,

r = km Pm P0.23

o , [4]

km = A exp(Ea/RT), [5]

The values of the kinetic parameters for methane oxidationover Ce(10% La)O 2are listed in Tab le 2. The va lue of the

heat of adsorption is low, 24.3 kJ/mol, a nd the activation

ener gy is 113.0 kJ/mol.

E qua tion (4) is consistent with t he redox mechanism of

methane oxidation with a slow reduction and fa st oxidat ion

steps. Whether or not methane activation involvesadsorbed

methane species could not be determined by the methods

used in this study.

Since both Eqs. (1) and (4) give satisfactory represen-

tation of the experimental da ta, w e chose E q. (4) to com-

pare the various doped CeO 2catalysts, subjected to differ-

ent thermal treatment. E xperiments were conducted

Pm = 0.01 bar and Po = 0.05 bar, with conversion n

ceeding 10%. Figure 4 shows Arrhenius plots for La

Zr-doped CeO 2 and undoped CeO 2 for comparison

action rates are normalized by the total B E T surfac

in order to compare specific activities. Mea sured re

rates are in agreement with those reported for Ce

Zr)O 2catalysts (32). The values of the kinetic param

a re show n in Tab le 3.

For undoped C eO 2, heat ing to high temperat ure (7

prior to rea ction leads to a decrease in the preexpon

factor, while the activation energy remains approxim

the sa me (fr om 100.8 kJ /mol to 94.7 kJ /mol), ind icat in

of active sites after the 700C calcination in air, as the

ta l size of cer ia incr ea ses from 9.4 nm t o 13.5 nm (Tab

La-doped CeO 2has the highest preexponential facto

activa tion en ergy (113.0 kJ/mol) a mong t he ca ta lysts

ied. Calcination in air at 800

C d oes not significantly the kinetics of methane oxidation over C e(10% La )O

spite the loss of B E T surface a rea (from 91.7 to 41.9

Table 1). The surface area -normalized specifi c activity

ever, increases as the crystal size is increased from 7.2

12.5 nm). R esults indicate that the na ture of active s

changed after the high temperature treatment, which

TABLE 3

Kinetic Parameters for Methane Oxidation over Variou

Doped CeO2Materialsa

Surf

A E a compo

[mo l/m2/s/ba r1.23] [kJ/mo l] (at% d o

C eO 2-650C 9.6 106 100.8

C eO 2-700C 2.9 106 94.7

C e(10% Zr)O 2-650C 1.4 106 85.7 7

C e(10% Zr)O 2-800C 10.8 106 107.2 5

C e(10% La )O 2-650C 9.6 107 113.0 12

C e(10% La )O 2-800C 28.4 107 116.2 13

a For the kinetic expression shown by E q. [4]; see text.b

D etermined by XPS.


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FIG. 4. Arrhenius-type plots for methane oxidation over undoped and doped C eO 2catalysts; 0.04 g s/cm3 (STP), 1% CH 4, 5% O 2, balance

be attributed to La segregation to the surface (increase of

La concentration from 12.1 to 13.8% as determined by X PS

analysis). La enrichment of the surface of ceria was previ-

ously reported by Pijolat et al. (46). For Zr-doped CeO 2,

the observed activation energy is lower that the activation

energy for the undoped and La-doped CeO 2. This is prob-ably related to the reducibility of Zr-doped CeO 2 and its

small crystal size, 6.8 nm, as determined by XRD. After

the 800C calcination in air, the crystal size and kinetic

parameters of the Zr-doped ceria approach those of un-

doped ceria (calcined at 650C). The surface concentration

of zirconia decreases after the 800C calcination (Tab le 3).

However, loss of the specific activity cannot be solely at-

tributed to the surface composition change. It seems that

some other structural changes, which could not be ob-

served by the techniques used in this study occur at high

temperatures.

The methane oxidation activity of ceria supported CuOxcatalysts is shown in Fig. 5. Ceria-supported CuO x cata-

lysts have higher activity than the respective support, when

compared on a rate per gram or per surface area basis. In

this catalyst formulation copper is present in oxidized form

(18,19,44). Copper oxide itself is an active oxidation cata-

lyst. In a previous study we have reported on the effect of

copper oxide dispersion and oxida tion state on metha ne ox-

idation activity (53). In Cu-modified doped CeO 2 catalysts,

copper exists a s part ially oxidized clusters, strongly associ-

at ed w ith ceria (18,19). At high copper content (15 at%),

in addition to copper clusters, dispersed CuO part icles were

identifi ed by X R D and STE M/E D X. The activity of

(few nanometers) partially oxidized copper clusters is

tha n that of dispersed copper oxide particles (10 nm)

when copper is supported on inert zirconia. Comparis

kinetic dat a for La-doped CeO 2 and C u-modified L a-d

C eO 2 showed tha t a similar kinetic expression may b eto represent the kinetics of methane oxidation on bo th

lysts (Tab le 2). The rea ction ord er w ith respect to me

partial pressure is very close to 1, while the reactio

der with respect to oxygen partial pressure is close to

(0.23 for the La-doped and 0.18 for the Cu-modifie

terial). When Eq. [1] is used to represent the kinetic

the heat of adsorption of methane is low on both cat

(24.3 kJ/mol f or the La -doped material and 14.2 fo

Cu-modified catalyst (44)), while the activation ene

significantly lower on Cu-modified catalysts, indicatin

add ition of C u lowers the methane activation barrier.

parison of reaction rates for Ce(10% La)O2(Ta ble 2rates reported by L iu and Flytzani-Stephanopoulos (1

15% C uCe(4.5% L a)O 2, shows that addition of Cu t

doped CeO 2 increases the methane oxidation rates a

temperature (500C

tion rates approach those of La-doped CeO 2.

Although the structure of Ag catalysts is differen

havior similar to that of copper catalysts for methane

dat ion is observed. XR D ana lysis of fresh, 650C-cal

samples identified the presence of silver metal at lo

concentrations (


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FIG. 5. Arrhenius-type plots for methane oxidation of Ce(4.5% La)O 2-supported copper catalysts; all catalysts calcined at 650C (Ta

0.0060.02 g s/cm3 (STP), 1% CH 4, 5% O 2, balance He.

growth (e.g., from 8 nm to 12 nm for the C e(4.5% La )O 2support) as a result of impregnation and calcination, Ag-

modified catalysts have higher methane oxidation activity

than the Ce(4.5% La)O 2support, as shown in Fig. 7. The

FIG. 6. XR D profi le of Ag/Ce(4.5% La )O 2cata lyst; freshafter 650

C calcination.

crystal size of the ceria support appears to play an im

tant role. As the crystal size of ceria support decreases

12.1 nm fo r A g/C e(4.5% La )O 2 an d 9.4 nm fo r Ag /C e

Zr)O 2 to 6.8 nm for Ag/Ce(50% Z r)O2), the ac


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FIG. 7. Arrhenius-type plots for methane oxidation of C eO 2- and ZrO 2-supported Ag catalysts; all catalysts calcined at 650C (Tab le 1);

0.02 g s/cm3 (STP), 1% CH 4, 5% O 2, balance He.

increases. Also increasing Ag loading from 2 to 5% in-

creases the act ivity, a s is shown for Ag/C e(4.5% La )O 2. To

separa te the effect of the metal from that o f the support, we

also prepared silver supported on ZrO 2. At low metal load-

ing (2%), the activity of the ceria-supported Ag is higher

than that of zirconia-supported Ag. However, as shown inFig. 7, the 5% Ag supported on ZrO 2has activity similar

to that of Ag supported on doped CeO 2catalysts, although

Z rO 2 itself did not have a ny activity for methane oxidation

in the temperature range studied. Our recent data (54, 55)

indicate that t he structure of A g particles has a strong effect

on methane oxidation activity, with Ag supported on an in-

ert support, such as zirconia or a lumina. The turnover num-

ber for metha ne oxidat ion increases an ord er of ma gnitude

as the A g part icle size increases from 510 nm (determined

by HRTEM) (54,55).

Reduction Properties

D ifferent oxygen speciespresent in the doped CeO 2 cata-

lysts were characterized using temperature programmed re-

duction by H 2. The reducibility of ceria ha s been extensively

studied in the literature using temperature programmed

reduction techniques (TPR). In reduction of ceria by H 2,

a low-temperature peak (500C) is attributed to the re-

duction of surface capping oxygen species, while a high-

temperature reduction peak (800C) is attributed to bulk

reduction (29,40). More detailed studies of surface reduc-

tion of CeO 2were reported (5662).

Figure 8 shows plots of the weight change and the d

tive of the weight change of each sample with temper

Tab le 4lists the TP R r esults in the form of red uction e

x, of CeO x. Surface reduction of undoped C eO2 (Fi

starts a round 300C (Ta ble 4, x= 1.99). B ulk reducti

TABLE 4

R eduction E xtent of Modified-Ceria Catalysts in H2-TP

R eduction temperature (C )Ca talyst (calcination

t empera ture) 170 200 300 400 450 500 600

C eO 2(650C ) 2.00 2.00 1.99 1.98 1.97 1.96 1.93

Ce(10%Zr)O 2(500C) 1.96 1.95 1.93 1.91 1.89 1.86 1.76

Ce(10%Zr)O 2(650C) 1.96 1.95 1.92 1.90 1.88 1.85 1.73

Ce(10%Zr)O 2(800C) 1.97 1.96 1.95 1.94 1.92 1.88 1.77

Ce(50%Zr)O 2(500C) 1.97 1.95 1.91 1.86 1.81 1.72 1.53

Ce(50%Zr)O 2(800C) 1.99 1.99 1.98 1.95 1.92 1.87 1.68

Ce(4.5%La)O 2(650C) 2.00 2.00 1.99 1.98 1.98 1.96 1.90

Ce(10%La)O 2(650C) 1.97 1.96 1.96 1.94 1.93 1.91 1.84

Ce(10%La)O 2(800C) 1.99 1.99 1.98 1.97 1.97 1.95 1.89

5%CuCe(50%Zr)O 2 1.87 1.84 1.76 1.69 1.64 1.59 1.52

(650C )b

5%CuCe(4.5%La)O 2 1.96 1.93 1.91 1.88 1.86 1.84 1.78

(650C )b

aR eduction extent expressed as xin CeO x. TPR conditions: 5%

500 cm3/min (S TP), 10C /min in a TG A.b Reduction extent expressed as xin CeO x, assuming that all Cu

reduced to metal.c

Values exceeding the C e2O 3stoichiometry (see text).


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FIG. 8. H 2-TPR profiles of undoped and doped CeO 2; all after 650C calcina tion (Tab le 1); 5% H 2/H e, 500 cm

3/min (STP) , 10C /min: (a )

(b) Ce(4.5%La)O 2; (c) Ce(10%La)O 2; (d) C e(10%Z r)O 2.

C eO 2occurs at higher temperatures with maximum ra te a t

about 700C (Fig. 8a). D oping with La changes the bulk re-

duction pro perties of ceria a s shown in Fig. 8b f or C e(4.5%La)O 2(a t 600

C,x = 1.90 for Ce(4.5% La)O 2compared to

1.93 for undoped ceria). The maximum reduction rate oc-

curs around 650C. The amount of surface oxygen reduced

increases as the La level increases to 10 at%, and bulk re-

duction begins at lower temperature with maximum rate a t

600C (Tab le 4 and Fig. 8c). Similarly, the Z r-doped cata -

lyst has lower reduction temperaturetha n undoped CeO2 a s

shown in Fig. 8d. H igher reducibility o f C e-Z r-O mixed ox-

ides wa s recently reported by Fornasieroet al. (31). This wa s

attributed to the higher oxygen mobility (oxygen conduc-

tivity) of Ce-Zr-O mixed oxides, due to structural changes

introduced by doping. H ere we find tha t the fi nal redu

extent (at 850C) for the Ce(50%Zr)O 2-conta ining m

als exceeds the C e2O 3 stoichiom etr y (Ta ble 4). The refor this behavior are not clear.

The effect of calcination temperature on the redu

ity of Z r- and La -doped cat alysts is shown in Table 4

of surface area and crystal growth result in loss of su

oxygen for both types of materials. However, the bu

duction properties are only slightly influenced by c

growth. Bulk reduction of Ce(10% Zr)O 2occurs at

temperature than the Ce(10% La)O 2 catalyst even

high tempera ture a nnea ling (Tab le 4).

The higher reducibility of doped ceria materials

pared to undoped ceria was confirmed in CH4-TP


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FIG. 8Continued

C eO 2-based catalyst react with methane at temperatures

higher than 400C, which is the temperature range where

steady-state activity was observed (Fig. 1). CO 2 and H 2O

were the only reduction products below 650C. A t temper-

atures higher than 650C H 2and C O were detected, indi-cating the ability of partially reduced ceria to partially ox-

idize methane. U nder the rea ction conditions in this study,

no partial oxidation productswere observed. Figure 9shows

C O 2 evolution as a function of temperature for undoped

and La- and Zr-doped ceria. As expected, reduction tem-

peratures are higher (400C) than in H 2-TPR (200C )

when methane is used as the reductant. The onset for the

reduction in methane is the same for all catalysts studied

(400C ). B ulk an d surface reduction of ceria o ccur simul-

taneously, probably because of the oxygen diffusion from

the bulk to the surface at these high reduction temperatures.

Higher slopes in the carbon dioxide evolution profiles

cate higher reduction rate for both La-and Zr-doped

than for undoped ceria.

It is well known that the presence of noble metal

Rh) can improve the ceria reducibility at low tempera(26,29,40). Here we show that other transition meta

duce similar beha vior. Figure 10 show s H 2-TPR profi

Cu-Ce(La)O xand Cu-Ce(Zr)O xcata lysts. A low tem

ture peak which corresponds to CuO reduction is obs

at 160C for both supports. Literature data (63) show

the reduction peak of bulk CuO typically occurs bet

200 a nd 300C. The reduction tempera ture ob served h

significantly lower than the bulk CuO reduction tem

ture, indicating that the C uO reducibility is enhanced b

presence of ceria. The bulk reduction of doped CeO 2aff ected by the ad dition of copper. How ever, the redu


12/19


FIG. 9. C H 4-TPR profiles of undoped and doped CeO 2; a ll after 650C ca lcinat ion (Tab le 1); 5% CH 4/H e, 60 cm

3/min (STP ), 10C /min

extent shows that the surface oxygen reduction of Ce(4.5%

La)O 2 is increased by the a ddition o f copper (Table 4).

The same effect of the addition of Cu was observed on

Ce(50%Zr)O2 (Tab le 4). This is in a greeme nt w ith t he pre-

viously reported enha ncement of CeO 2 reducibility in phys-

ically mixed powders of na no-CuO and CeO 2(64).

Figure 11 shows CH 4-TPR profiles of copper-modifiedcatalysts in the form of CO 2 measured by mass spectro-

metry. These TP R tests were perf ormed in the pa cked-bed

reactor. R eaction of methane w ith these oxidized cata lysts

gives C O 2and H 2O as the main reduction products. Only

trace amounts of C O w ere observed at temperatures lower

than 650C. The onset of the reduction of a 5%C uCe(La)O 2catalyst in CH 4-TPR is about 400

C, with maximum reduc-

tion rate at about 450C. On the other hand, at a higher

copper content (>5 at%), reduction starts at 300C, and a

doublet is observed. Reduction of bulk CuO (prepared by

carbonate decomposition) is shown for comparison, indi-

cating that the doublet in the reduction profi le comes frombulk C uO reduction. A t low copper content (5 at%) the

amount of oxygen reduced exceeds the amount necessary

for complete reduction of CuO to metallic copper (53). As

in H 2-TPR , this is due to t he enhancement of ceria reduc-

tion at low temperatures.

Ag-modifi ed ceria showed different behavior. No distinct

silver oxide reduction peaks were observed in H 2-TPR,

which is in agreement with our XRD data which showed

the presence of metallic silver. The absence of a surface

reduction peak is attributed to the low surface area of the

A g-modifi ed samples. Although a ctive oxygen importa nt in

methane oxidation could not be identified by the H 2experiments of Ag-doped cerias, oxygen uptake ex

ments after reduction at 170C and 450C indicates

ability of oxygen for o xidat ion reactions on these cat

This is strongly influenced by the nature of the sup

Table 5 summarizes the oxygen uptake of variou

modifi ed doped CeO 2 cata lysts. Oxygen uptake of thesupports was also measured for comparison. This

pressed as reduction extent x in CeO x. Under the c

tions of the upta ke experiments, both ceria and Ag a

oxygen. Their separate contributions could not be c

lated exactly. The reduction extent of ceria in Ag mo

materials was calculated after the total oxygen uptak

corrected for o xygen adsorbed on Ag (a ssuming all A

oxidized to Ag2O, according to the preferred stoichio

at 170C (65)). A A g : O stoichiometry needs to be u

calculate the extent of ceria reduction. Thus, the redu

extent of ceria is a lower limit, since all Ag was ass

to be f ully exposed (100% dispersed). Ag wa s found metallic state aft er the oxygen uptake experiments, a

in the fresh cata lyst (determined by X R D ).

Neither the undoped nor the La- and Zr-doped

cata lysts had mea surable oxygen uptake after reduct

170C. A fter reduction at 450C, the oxygen uptake

doped and La-doped CeO 2wa s the same, while Z r-d

C eO 2 had the highest uptake. The oxygen uptake o

modified catalysts, however, clearly showed a syne

between the meta l and t he support. Indeed, the oxyge

ta ke aft er the 170C-reduction wa s much higher than w

correspond to Ag alone, even if all Ag was oxidized


13/19


FIG. 10. H 2-TPR profiles of Cu modified do ped CeO 2cata lysts; freshaft er 650C calcina tion (Tab le 1); 5% H 2/H e, 500 cm

3/min (S TP), 10

one-to-one stoichiometry. The excess oxygen must have

been adsorbed on surface oxygen vacancies formed on the

support. After reduction at 450C, how ever, the reduction

extent of the Ag-modified CeO 2was similar to the reduc-tion extent of the bare support at the same conditions.

The crystal size of ceria has a major influence on the

measured oxygen uptake. CeO 2 (24.2 nm) and Ce(4.5%

La)O 2 (12.1 nm) supported Ag ca talysts have lower oxygen

upta ke tha n the A g/C e(x% Zr)O 2(9.4 nm for 10% Zr and

6.8 nm for 50% Zr) catalysts after reduction at 170C. The

calcula ted dispersion (64.2% fo r 5% Ag/C eO 2and 85.4%

fo r 5% Ag/C e(4.5% La )O 2) is, however, high for this Ag

loading, especially if compared to the 5% A g/Z rO 2 cata-

lyst (9.5%, Tab le 5), so t hat part icipat ion o f t he support is

clearly indicated.

DISCUSSION

Kinetic expressions for the metha ne oxidation rep

in the literature fo r both noble metals and metal oxidfirst order in methane partial pressure and almost ze

der in oxygen partial pressure. Kinetic data for Pd-

catalysts are consistent with the reaction steps based o

LangmuirHinshelwood mechanism which includes d

ciative adsorption of methane and oxygen (4,7,8). Th

straction of t he fi rst hydrogen a tom from methane i

sidered to be the rate-limiting step. On the other

methane oxidation on metal oxides (perovskites) is u

treated using the Marsvan Krevelen redox mecha

(10,13). In the ca se of slow surface reduction by me

and fast reoxidation by oxygen, first order in me


14/19


FIG. 11. C H 4-TPR profiles of C u modified doped C eO 2 catalysts; freshafter 650C calcination (Table 1); 5% C H 4/H e, 60 cm

3/min

10C /min .

partial pressure, a nd zero order in oxygen partial pressure

are obta ined.

The kinetic data reported here for methane oxidation on

doped ceria are in agreement with those reported in the

TABLE 5

Oxygen Uptake of Ag Modified Doped CeO2C atalystsa

R eduction R eduction

tempera ture tempera ture D ispersionc

C atalyst (C ) xin CeO xd C a talyst (C ) xin CeO x

d [%] mol O 2/m

3.5% Ag /C eO 2e 170 2.00 24.4 0.06

C eO 2 170 2.00 5% A g/C eO 2 170 2.00 64.2 0.16

450 1.99 450 1.98 0.84

Ce(10% Zr)O 2 170 2.00 5% A g/C e(10% Z r)O 2 170 1.99 0.44

450 1.98 450 1.98 0.89

Ce(50%Zr)O 2 170 2.00 5% A g/C e(50% Z r)O 2 170 1.98 0.72450 1.95 450 1.95 1.14

Ce(4.5%La)O 2 170 2.00 5% A g/C e(4.5% La )O 2 170 2.00 85.4 0.21

2% Ag/C e(4.5% La )O 2 170 2.00 99.0 0.24

5% Ag /Z rO 2 170 9.53 0.02

3.5% Ag /Z rO 2 170 25.6 0.06

2% Ag /Z rO 2 170 40.8 0.10

a See Table 1 for the B E T surface area of cata lysts.b Measured amount of oxygen adsorbed divided by the amount of Ag in the sample.c C alculated, a ssuming 2Ag : 1O stoichiometry.d Reduction extent, x, calculated after the oxygen adsorbed on silver (with 2Ag :1O stochiometry and assuming 100% silver dispersio

subtracted from the actual amount of oxygen consumed.e C eO 2prepared by acetate decomposition at 750

C.

literature for similar systems. We demon strat ed using

TPR that d oped ceria ma terials can be reduced by me

so that the redox mechanism cannot be excluded.

surface can be reduced by metha ne at temperatures h


15/19


than 400C (Fig. 9). The same is true for copper conta ining

materials (Fig. 11). In both cases surface and bulk reduc-

tion cannot be separated, probably due to fast diffusion of

the lattice oxygen to the surface. Our results indicate that

the mechanism of methane oxidation over Ag-containing

materials might be different and that it probably includes

dissociative adsorption of oxygen and methane. XR D dat a

(Fig. 6) show the presence of metallic A g on Ag-modified

catalysts. We may, then, surmise that CuO provides activeoxygen for methane oxidation, while on A g-modified ca ta-

lysts, oxygen adsorbed on Ag must be active for methane

oxidation.

Methane interacts mainly with the surface oxygen anions

of ceria (coordinatively unsaturated oxygen (CU S) and lat-

tice oxygen) (66). Since the interaction of methane with

CUS oxygen is stronger than interaction with lattice oxy-

gen (67), we presume that C U S species are responsible for

methane activation on doped cerium oxide. Surface defects

of ceria are sites for oxygen a dsorption (67). D ecrease of

crystal size of ceria leads to the format ion of surface defectsand, therefore, a higher number of active sites. Thisis clearly

demonstrated in Fig. 1 for undo ped ceria. When heated to

750C and 800C, the crystal size of ceria grows to 20 nm

and 24 nm, respectively, and the low t emperat ure activity is

lost. When the ceria crysta l size is below 20 nm (650C and

700C treated ceria) t he specifi c activity per surface area is

a bo ut t he same (Fig. 4, Ta ble 3).

G enerally, doping of CeO 2 leads to improved methane

oxidation activity, as is clear from Fig. 4. Three different

factors are considered to be important for the catalytic ac-

tivity of doped ceria in redox reactions: crystal size, defect

formation, and reducibility of surface oxygen species (30).Those factors are not independent. Do ping of ceria leads to

defect format ion and in some cases to the reduction of t he

ceria crystal size. O n the o ther ha nd, nano crystalline mat e-

rials are known to be highly defective (68). The reduction

behavior of ceria depends on its surface area (surface oxy-

gen reduction) and also on oxygen ion conductivity (bulk

reduction). B oth the surface and bulk reduction of ceria can

be modified by doping.

The methane oxidation a ctivity of doped C eO 2cata lysts

can be related to the ability of surface oxygen to a ctivate

methane, leading to the removal of surface oxygen and tothe a bility o f ga s phase oxygen or bulk oxygen to fi ll a sur-

face oxygen vacancy. B oth La - and Z r-doped ceria show im-

proved oxidation activity. One reason is the smaller grain

size created by doping. Doping also changes the reduc-

tion properties of ceria. B oth L a and Z r improve the low-

tempera ture r educibility of ceria (Tab le 4).

The activity of Zr-doped materials can be clearly attri-

buted to the stabilization of smallcrystal size, and the conse-

quent creation of surface defects. Zr a s a tetra valent dopant

does not create extrinsic oxygen vacancies. After high tem-

perature treatment (800C) the specific activity (rate per

surface a rea) of Ce(10% Zr)O 2decreases and appro

that of pure ceria. La-doped CeO 2keeps its oxidatio

tivity due to stabilized crystal size, as well as to the

at ion extrinsic oxygen va cancies. A s Fig. 4 shows, the

La -doped ceria reta ined a much higher methane oxid

activity after the 800C-treatment than the 10% Zr-d

ceria. The specific activity of La-doped ceria increa

La surface concentration increases (Fig. 4). The pre

nential factors for metha ne oxidation over La -dopedare order of magnitude higher than those for undope

Zr-doped ceria.

The effect of ceria crystal size on methane oxid

activity is evident. Enhanced reactivity of nanocryst

ceria is in agreement with the results reported prev

for C O o xida tion. Tschope et al. (69) showed that no

chiometric nano crystalline cerium oxide ha s a 200C-

light-off temperature for CO oxidation than precip

stoichiometric CeO 2, presumably due to the rapid fo

tion of ad sorbed oxygen species (superoxide) on the fo

as a result of the presence of free electrons. It is kthat superoxide species (O 2) are formed on a pa

reduced CeO 2surface (66,70). However, the nonsto

metry of ceria has a much lower effect on the met

oxidation activity (44, 69) probably due to the higher

peraturesneeded for methane activation (>400C). A

temperatures electrophilic oxygen species, such as th

peroxide species, are quickly converted to lattice ox

(66,70).

In the presence of a transition metal both ceria an

metal (A g)/metal o xide (C uO) are active cata lyst co

nents. Addition of the transition metal clearly increase

reducibility of ceria a t low temperatures, as is demonshere by H 2- and C H 4-TPR for copper-containing cat

and by oxygen chemisorption for Ag-containing mate

On the other hand, ceria modifies the properties (di

sion, oxidat ion stat e, a nd reducibility (18,19,53,55)) o

supported metal/metal oxide. Strong interaction b et

C u+1 clusters and CeO 2, results in enhanced activi

CO oxidation (44). In recent work, we reported on th

ducibility an d activity in the complete oxida tion of me

of va rious copper species formed when copper oxide i

ported on ceria a nd zirconia (53). Co pper clusters, iden

at low copper load ing (5 at %) are less reducible tha n hdispersed copper oxide particles (present at higher c

loading, 15 at%), when copper is supported on zir

(53). Similar b ehavior wa s found f or copper support

ceria. H owever, copper species formed o n zirconia re

at low er temperature (both in H 2 and CH 4) rather tha

corresponding copper species on ceria due to the s

interaction between ceria and CuO x. When ceria is

as an active support, the properties of both ceria an

transition metal are modified so that their individua

tributions to t he overall methane oxidation a ctivity c

be separated. At low copper loading, the ra te of me


16/19


oxidation (expressed per gram or surface area of cata-

lyst) is higher on ceria-supported copper cata lysts than that

on zirconia-supported materials (53). H owever, a t higher

copper loading, zirconia-supported catalysts are more ac-

tive than the ceria-supported ones (53). This is attributed

to lower reducibility of copper oxide species formed on

zirconia.

Similar enhancement of the reducibility of ceria is re-

ported here for ceria-supported silver. The promotion ofceria reducibility is a strong function of crystal size a s the

results in Table 5indicate. Although the reasons for such be-

havior are not clear, this enhancement is probab ly the result

of a defective structure of na nocrysta lline ceria. Small crys-

ta ls of ceria intera ct with Ag more strongly, so that the reac-

tivity of ceria is enhanced. Addition of Ag to Ce-Al-O sup-

port was shown to enhance the format ion of O2 (38). Previ-

ous reports show that introduction of noble metals (Pt, Pd )

increasesthe reactivity of O2 species on ceria at low temper-

ature (38,39). Thisis the type of oxygen that more likely par-

ticipates in C O oxidat ion (55,71). The increased reducibilityof the doped CeO 2support leads to higher activity of the

superoxide species so that th e Ag-modifi ed Ce(50% Z r)O 2has the highest a ctivity for CO oxidation. The same is true

for methane oxidation, a lthough at the higher temperature

range of the latter, adsorbed oxygen species are quickly

converted to la ttice oxygen.

The availability of low temperature oxygen species on

ceria is not the only important factor for methane oxida-

tion over Ag-containing catalysts. Ag metal supported on

an inert support is an active catalyst as the data of Fig. 4

clearly show. When A g is supported on ceria, t he contribu-

tion of A g and ceria to t he overall cata lytic activity can notbe separa ted. A g promo tes the ceria red ucibility (Tab le 5).

However, ceria-supported Ag catalysts with 5% Ag load-

ing have lower a ctivity then Ag/Z rO 2, although high oxy-

gen uptake a t low temperature wa s measured (e.g., on 5%

A g/C e(50% Z r)O 2). These differences may be attributed

to the ef fect of A g part icle size. In other w ork (54), we ha ve

found that the methane oxidation activity is a strong func-

tion of Ag particle size, when Ag is supported on an inert

support (Z rO2), as wa s previously demonstrated fo r P d (8)

and Pt (72). The turnover rate increases as the Ag particle

size increases up to 10 nm (54). Further increase in the pa r-ticle size does not affect the activity. The difference in the

activity can be attributed to a different structure of small

(5 nm) and large (10 nm) Ag particles (54). As the Ag load-

ing increases, larger Ag particles are formed, which give

rise to XR D reflections of metallic Ag. These A g part icles

can be ea sily seen by H RTEM a s is illustrat ed in Fig. 12 for

5% Ag/Z rO 2.

To eliminate t he eff ect of ceria, Ag-supported o n low -

surface area ceria, CeO2, with large crystal size (20 nm)

was studied. The properties of this ceria and of the 3.4%

Ag/C eO 2 (ceria prepared by acetate decomposition) are

FIG. 12. H RTEM of 5% Ag/Z rO 2cata lyst.

listed in Tab le 1. The A g-conta ining undoped ceria

rials have low surface a rea , close to t hat of 3.5% Ag

catalysts. The CeO 2 material does not participate

reaction as it was shown earlier (Fig. 1). The mea

A g d ispersion in 3.4% Ag /C eO 2 is similar to that of A g/Z rO 2materials at a pproximately the same A g lo

H ow ever, the activity of Ag/Z rO 2is much higher tha

of A g/C eO 2, as shown in Fig. 13.

For this 3.4% Ag/C eO 2 catalyst, the silver particl

determined from oxygen chemisorption measuremen

suming that ceria does not contribute to the overall o

uptake, is much smaller (9.6 nm) than that measur

XR D (29.8 nm). This is attributed to nonuniform pa

size distribution. When the sample consists of a large

ber of small crystallites and a sma ll number of large cr

lites, the width of the diffraction peak is determined mby the sharp peak of the larger crystallites. Ag/Ce(L a,

materials could not be analyzed by HRTEM due t

strong interaction of ceria with the electron beam.

ever, STE M/E D X ana lysis of C e(La,Z r)O2-support

catalysts confirmed a nonuniform particle size dis

tion. Figure 14 shows elemental mappings of the 3.4

A g/C eO 2 cata lyst. Ag-rich and ceria-rich regions were

tified. In ceria-rich regions Ag appears to be uniforml

persed. H owever, low A g signal intensity indicates tha

a small fra ction of Ag is act ually dispersed on ceria. Th

of Ag forms large (400 nm) Ag particles. These lar


17/19


FIG. 13. Co mparison of activity of 3.4% Ag/CeO 2 (Tab le 1) a nd 3.5% Ag /Z rO 2cata lysts at the same Ag dispersion; 0.0060.02 g s/cm3

1% CH 4, 5% O 2, balance He.

particles are covered by ceria and are no t likely to partici-

pate in the reaction. Similar Ag distribution is observed for

Ag/C e(L a) O 2 and A g/C e(Z r)O 2cata lysts. STE M ana lysis

suggests tha t ceria-supported Ag materials contain only a

small amount of highly dispersed silver and that most of

the Ag is present as large (400 nm) particles. Since thelarge Ag pa rticles are covered by ceria, they are not the Ag

active phase. The active phase comprises highly dispersed

Ag clusters. Although the measured Ag dispersion is close

to t hat of A g/Z rO 2materials with similar Ag content, the

structure of supported silver on zirconia and ceria is dif-

ferent. Ceria as a support favors the formation of highly

dispersed Ag clusters, while intermediate size Ag particles

(10 nm) which have the highest activity for methane oxi-

dation (54) are not formed. The reasons for such behavior

are not clear, but the predominance of highly dispersed Ag

on ceria may explain their lower catalytic activity (Fig. 13).

CONCLUSIONS

Cu- and Ag-modified doped CeO 2 catalysts are active

cata lysts for the complete oxidation of methane. In the ab -

sence of the meta l, doped CeO 2is not inert. R ather, it pro-

vides sites for metha ne oxidation. The a ctivity of methane

oxidation on doped CeO 2 cata lysts depends on surface oxy-

gen availability. Loss of surface area and crystal size growth

of ceria lead to lower methane oxidation activity. The sinter-

ing properties of ceria can be mo dified b y doping. D opants,

such as La and Zr, decrease the ceria crystal size and

vent crystal growth at high temperatures, with concom

higher methane oxida tion activity. In ad dition, doping

ifies both the surface a nd b ulk reducibility of ceria.

La- a nd Z r-doped mat erials are more reducible tha

doped ceria. As a result, the methane oxidation activimproved. A lthough Z r-doped cata lysts have small c

size a nd a large number of surface d efects, doping of

with La which introduces extrinsic o xygen vacancie

ther enha nces the cat alytic a ctivity.

The addition of transition metals, such as Cu an

improves the low-temperature C H 4oxida tion activity

presence of the transition metal at low loading (


18/19


FIG. 14. STEM/E D X element al mappin g of 3.4% Ag/C eO 2cata lyst.

overall cata lytic activity of ceria-supported C uO a nd A g is

influenced by b oth th e ceria a nd the tra nsition meta l/meta l

oxide structure. A complex interaction betw een ceria and

the tra nsition metal/meta l oxide modifi es their properties

and activity for methane oxidation.

RE FE RE N CE S

1. Lampert, J., Kazi, J. K., and Farrauto, R. J., A p p l. Ca ta l. B: E

14, 211 (1997).

2. B aldwin, T. R., and B urch, R., A ppl. Catal. 66, 337 (1990).

3. B aldwin, T. R., and B urch, R., A ppl. Catal. 66, 359 (1990).

4. Ribeiro, F. H., Chow, M., and Dalla Betta, R. A., J. Catal. 1

(1994).

5. Farrauto, R . J., H obson, M. C., Kennelly, T., and Waterman,

A ppl. Catal. A : General81, 227 (1992).6. Farrauto, R . J., La mpert, J. K., Hobson, M. C ., and Waterm

A p p l. Ca ta l. B: En vir o n .6, 263 (1995).

7. Fujimoto, K., R ibeiro, F. H ., B ell, A. T., and I glesia, E ., in Sym

on Heterogeneous Hydrocarbon Oxidation, Division of Petr

Chemistry, 211 National Meeting, American Chemical Societ

Orleans, LA, March, 1996, p. 110.

8. Fujimoto, K., Ribeiro, F. H., Iglesia, E., and Avalos-Borja,

Symposium on C ata lytic Combustion, D ivision of Petroleum

istry, Inc., 213 Nat ional Meeting, American Chemical Socie

Fran cisco, CA , A pril, 1997, p. 190.

9. Oh, S. H., Mitchell, P. J. , and Siewert, R. M., J. Catal. 1

(1991).

10. Ara i, H., Yama da, T., Eguchi, K., and Seiya ma, T.,A ppl. Catal

(1986).11. McCa rty, J. G., a nd Wise, H., Catal. Today8, 231 (1990).

12. Klvana, D., Vaillancourt, J., Kirchnerova, J., and Cha ouki, J.

Catal. A : General109, 181 (1994).

13. Kucherov, A. V., Slinkin, A. A., G oryashenko, S. S., and Slove

K. I., J. Catal. 118, 459 (1989).

14. Kucherov, A . V., Kucherova, T. N., and Slinkin, A . A ., Kinet

33(3), 618 (1992).

15. Ishihara , T., Sumi, H., a nd Takita , Y.,Chem. L ett., 1499 (1994)

16. Li, Y., and A rmor, J. N.,A p p l. Catal. B: E n vir o n .3, 275 (1994)

17. Anderson, R. B., Stein, K. C., Feenan, J. J., and H ofer, L. J.,I n

Chem. 53, 809 (1961).

18. Liu, W., and Flytzan i-Stephano poulos, M., J. Catal. 153, 304 (1

19. Liu, W., and Flytzan i-Stephano poulos, M., J. Catal. 153, 317 (1

20. Liu, W., Sarofi m, A. F., and Flytza ni-Stephano poulos, M., Ap pB: En vir o n .4, 167 (1994).

21. Liu, W., Wadia , C., a nd Flytza ni-Stephano poulos, M., Catal. To

391 (1996).

22. Meriani, S.,M ater. Sci. E ng. A109, 121 (1989).

23. B alducci, G., Fornasiero, P., Di Mont e, R., Ka spar, J., Meriani,

G raziani, M., Catal. L ett.33, 193 (1995).

24. D e Leitenburg, C., Trovarelli, A., Za mar, F., Maschio, S., D olc

and L lorca, J., J. Chem. Soc., Chem. Comm un., 2181 (1995).

25. D e L eitenburg, C ., Trovarelli, A., Llorca, J., C avani, F., a nd B

A ppl. Catal. A : General139, 161 (1995).

26. Trovarelli, Q ., D olcetti, G . D e L eitenburg, C., Ka spar, J., Fin

and Santoni, A., J. Chem. Soc. Faraday Tr ans.88, 1311 (1992).

27. Tuller, H . L., a nd Now ick, A. S.,J. E lectochem. Soc.2, 255 (19

28. Inaba , H., and Tagaw a, H., Solid StateI onics83, 1 (1996).

29. H arrison, B., D iwell, A. F., and Ha llet, C., Platinum M etals Re

73 (1988).

30. Trova relli, A., Catal. Rev.-Sci. Eng.38(4), 439 (1996).

31. Fornasiero, P., D i Monte, R., R anga R ao, G ., Kaspar, L., Mer

Trovarelli, A., a nd G razia ni, M., J. Catal. 151, 168 (1995).

32. Z ama r, F., Trovarelli, A., D e Leitenburg, C., and D olcetti, G., J

Soc., Chem. Comm un.9, 965 (1995).

33. Za firis, G . S., and G orte, R. J., J. Catal. 139, 561 (1993).

34. Corda tos, H., and G orte, R. J.,J. Catal. 159, 112 (1996).

35. Co rdat os, H., B unluesin, T., Stubenra uch, J., Vohs, J. M., and

R. J., J. Phys. Chem. 100, 785 (1996).

36. Putna, E. S., Vohs, J. M., a nd G orte, R . J., J. Phys. Chem. 100

(1996).


19/19


37. B unluesin, T., G orte, R. J., and G raham, G . W., Appl. Catal. B: Envi-

r o n . 339, 1 (1997).

38. Tarasov, A . L ., P rzhevalskaya, L. K., Shvets, V. A., a nd K azanskii,

V. B., K inet. K atal. 29(5), 1181 (1988).

39. Li, C., C hen, Y., Li, W., and X in, Q., i n New A spects of Spillover in

Ca talysis (T. Inui et al., E ds.), p. 217. Elsevier Science, A msterdam,

1993.

40. Yao, H . C., and Ya o, Y. F. Y.,J. Catal. 86, 254 (1984).

41. Nunan, J. G., Silver, R. G., and Bradley, S. A., i n Cata lytic Control

of Air Pollution (R. G. Silver, J. E. Saeyer, and J. C. Summers, Eds.),Ch ap. 17. AC S Sy mposium Series, Vol. 495, Am. C hem. Soc., Wash-

ingto n, D C, 1992.

42. Aboukais, A ., B ennani, A., Aissi, C. F., G uelton, M., a nd Vedrine,

J. C., Chem. M ater.4, 977 (1992).

43. Soria, J., Conesa, J. C., Martinez-Arias, A., and C oronad o, J. M.,Solid

State Ion ics6365, 755 (1993).

44. Liu, W., D evelopment of Novel Metal O xide Composite C ata lysts

for C omplete O xidation R eactions, Sc.D. thesis, MI T, 1995.

45. Amenomiya, Y., Emesh, A., Oliver, K., and Pleizer, G., i n Proc. 9th

Int ern. Cong. C ata l. (M. P hillips and M. Ternan, E ds.), p. 634. Chem-

ical Inst. of Ca nada, O ttaw a, C anada , 1988.

46. Pijolat, M., P rin, M., Soustelle, M., Touret, O., a nd Nort ier, P., Solid

State Ion ics6365, 781 (1993).

47. P ijolat, M., P rin, M., Soustelle, M., Touret , O., and Nort ier, P., J. Chem.Soc. Faraday Tran s.91(21), 3941 (1995).

48. Pa lguev, S. F., A lyamosvkii, S. I., and Volchenkova, Z. S., R uss. J.

I norg. Chem.4, 1185 (1959).

49. Meriani, S.,M ater. Sci. E ng.71, 369 (1985).

50. Meriani, S., and Spinolo, G .,Powder D iffr action2(4), 255 (1987).

51. Yashima, M., Morimoto, K., I shizawa , N., and Yoshimura, M.,J. Am.

Cer. Soc.76(7), 1745 (1993).

52. Yashima, M., Arashi, H., Kakihana, M., and Yoshimura, M., J. A m.

Cer. Soc.77(4), 1067 (1994).

53. Kunda kovic, Lj., and Flytza ni-Stephano poulos, M., A p p l . C at al . A :

G eneral171(1), 13 (1998).

54. Kunda kovic, Lj., and Flytza ni-Stephano poulos, M., Ap p l. Ca

General, submitted.

55. Kund ako vic, Lj., Ph. D. t hesis, Tufts U niversity, 1998.

56. Johnson, F. L., an d Mooi, J.,J. Catal. 140, 612 (1987).

57. Laachir, A., Perrichon, V., Badri, A., Lamotte, J. , Catheri

Lavalley, J. C., El Fallah, J., Hilaire, L., Le Normand, F., Queme

Sauvion, G . N., and Touret, O., J. Chem. Soc. Faraday Trans.

1601 (1991).

58. Z otin, F. M. Z., J. Catal. 98, 99 (1993).

59. Perrichon, V., La achir, A., B ergeret, G., Frety, R., Tournaya n, Touret , O ., J. Chem. Soc. Faraday Trans.90, 773 (1994).

60. Bernal, S., Calvino, J. J., Cifredo, G. A., G atica, J. M., Perez

J. A., and Pintando, J. M., J. Chem. Soc. Faraday Trans.89

(1993).

61. B ernal, S., B otana , F. J., Calvino, J. J., Cauqui, M. A., C ifredo

Jobacho, J., Pintado, J. M., and Rodriguez-Izquierdo, J. M., J

Chem. 97, 4118 (1993).

62. B inet, C., B adri, A., and Lava lley, J.-C., J. Phys. Chem.98, 6392

63. Fierro, G., Jacono, M. L., Inversi, M., Porta, P., Lavecchia, R

Cioci, F., J. Catal. 148, 709 (1994).

64. Liu, W., and Flytzani-Stephanopoulos, M., Chem. Eng. J. 6

(1996).

65. Smeltzer, W. W., Tollefson, E. L ., and Ca mbron, A .,Can. J. Ch

(1956).66. Li, C., and Xin, Q.,J. Phys. Chem. 96, 7714 (1992).

67. Li, C., D omen, K., Maruya , K., and O nishi, T.,J. A m. Ch em. S

7683 (1989).

68. Z hang, Y., Andersson, S., and Muha mmed, M., A p p l. Ca ta l. B

r o n .6, 325 (1995).

69. Tschope, A., Liu, W., Flytzani-Stephanopoulos, and Ying, J. Y., J

157, 42 (1995).

70. Zha ng, X., and Klabunde, K. J., I norg. Chem.31, 1706 (1992).

71. Sass, A. S., Shvets, V. A., Saveleva, G. A., Popova, N. M

Kazanskii, V. B., K inet. Katal.27(4), 894 (1986).

72. Otto, K., L angmuir5, 1364 (1989).

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