www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 69 (2007) 226–234

Effect of dopants on the performance of CuO–CeO2

catalysts in methanol steam reforming

Joan Papavasiliou a,b, George Avgouropoulos a, Theophilos Ioannides a,*a Foundation for Research and Technology-Hellas, Institute of Chemical Engineering and High Temperature Chemical

Processes (FORTH/ICE-HT), P.O. Box 1414, GR-26504 Patras, Greeceb Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece

Received 13 February 2006; received in revised form 30 June 2006; accepted 10 July 2006

Available online 17 August 2006

Abstract

Steam reforming of methanol was carried out over a series of doped CuO–CeO2 catalysts prepared via the urea–nitrate combustion method.

XRD analysis showed that at least part of the dopant cations enter the ceria lattice. The addition of various metal oxide dopants in the catalyst

composition affected in a different way the catalytic performance towards H2 production. Small amounts of oxides of Sm and Zn improved the

performance of CuO–CeO2, while further addition of these oxides caused a decrease in catalyst activity. XPS analysis of Zn- and Sm-doped

catalysts showed that increase of dopant loading leads to surface segregation of the dopant and decrease of copper oxide dispersion. The addition of

oxides of La, Zr, Mg, Gd, Y or Ca lowered or had no effect on catalytic activity, but led to less CO in the reaction products. Noble-metal modified

catalysts had slightly higher activity, but the CO selectivity was also significantly higher.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Copper oxide; Cerium oxide; Hydrogen production; Methanol; Steam reforming; Combustion method

1. Introduction

Solid polymer fuel cells (SPFCs), which consume hydrogen

and oxygen to produce electricity, appear to be a clean and

efficient energy solution for both mobile and stationary

applications. The use of hydrogen for mobile applications is

hindered by problems of storage, safety and refueling.

Alternatively, H2 can be produced onboard from various liquid

fuels, such as methanol. Methanol is a strong candidate fuel,

since it is readily available and can be catalytically converted

into a H2-rich gas at moderate temperatures (200–300 8C). It

has a high H/C ratio and no C–C bonds, hence minimizing the

risk for coke formation. Production of hydrogen from methanol

is also attractive because of the relatively low selectivity to

byproducts, such as carbon monoxide and methane compared

to alkane or higher alcohol reforming processes [1]. Moreover,

methanol can be produced from renewable sources and, as a

consequence, may be considered as a sustainable energy carrier

[2]. Ultra-small methanol processors in combination with fuel

* Corresponding author. Tel.: +30 2610 965264; fax: +30 2610 965223.

E-mail address: theo@iceht.forth.gr (T. Ioannides).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.07.007

cells have been proposed as Li-battery substitutes in demanding

applications [3].

Copper-based catalysts, especially with the composition

Cu–ZnO–(Al2O3), have been widely used for generating

hydrogen from methanol via the steam reforming (SRM)

process [4–13]. The activity and CO selectivity are greatly

dependent on catalyst morphology (high copper dispersion is

desirable) and the redox properties of the catalyst [2,4,14]. For

the same process we have recently examined copper–

manganese spinel oxide catalysts, prepared via the combustion

method [15]. These catalysts showed high catalytic activity and

selectivity towards hydrogen production. On the other hand,

cerium oxide has attracted much attention in environmental

catalysis due to its high-oxygen storage capacity and the ability

of fast transfer of bulk oxygen to its surface. Additionally, ceria

can affect the valence state of various metal oxides. Prereduced,

coprecipitated Cu/CeO2 catalysts were found to be more active

in the SRM process than the state-of-the-art Cu/Zn/Al2O3

catalysts [16]. Recently, we reported on the synthesis of CuO–

CeO2 catalysts via the urea–nitrate combustion method and

their catalytic performance for the SRM reaction [14]. These

catalysts were produced in a single step in ready-to-use form.

The resulting material characteristics and their catalytic

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234 227

properties were strongly influenced by the urea/nitrate molar

ratio, with urea-rich samples having more favorable catalytic

properties.

Doped ceria has been the subject of considerable interest,

because addition of dopants can increase the concentration of

oxygen vacancies [17] or improve the thermal stability of the

parent oxide [18]. The dopant cations with ionic radius and

electronegativity close to those of cerium cation are thought to

be the most appropriate modifiers of structural and chemical

properties of ceria [19]. The similarity of the ionic radii is also

the criterion to predict the presence or not of significant solid

solubility. The latter is also affected by the preparation method

and the calcination temperature [19].

In the present study, we report on the effect of type and

content of eight different dopant cations on the catalytic

properties of CuO–CeO2 catalysts in the reaction of methanol

steam reforming. The doped catalysts were prepared by the

urea–nitrate combustion method.

2. Experimental

2.1. Catalyst preparation

Doped copper–cerium oxide catalysts were prepared via the

urea–nitrate combustion method [20]. Nitrate salts of copper

[Cu(NO3)2�3H2O], cerium [Ce(NO3)3�6H2O] and the dopant

(Zn2+, Zr4+, La3+, Sm3+, Mg2+, Y3+, Gd3+, Ca2+) were mixed

with urea (in 75% excess) in an alumina crucible in the

following atomic ratios: Cu/(Cu + Ce + M) = 0.15, M/

(M + Ce) = 0–0.25 (where M is the dopant cation added in

each case). The following encoding of catalysts will be used

throughout the paper: CuCe1�xMx, where x is the atomic ratio

M/(M + Ce). Two additional samples containing 0.11% Pd or

0.11% Rh were prepared with the same method. These values

were optimized in the case of non-modified CuO–CeO2

catalysts in a previous study [14]. The initial urea-to-nitrates

molar ratio was adjusted to excess value according to the

principle of propellant chemistry [21]. The mixed solutions

were preheated on a hot plate at �80 8C, so that excess water

was removed. The resulting viscous gel was placed in an open

muffle furnace maintained at 400–500 8C, where it was

autoignited yielding finally a voluminous solid product within

few minutes. The color and the existence or not of a flame was

dependent on the type of the dopant and the M/(M + Ce) ratio.

Fresh catalyst powders were further heated at 550 8C for 1 h

right after synthesis. All the produced powders were sieved to

obtain the desired fraction (90 < dp < 180 mm), so that,

significant pressure drop, internal concentration and tempera-

ture gradients over the catalyst bed could be negligible. A

commercial CuO–ZnO–Al2O3 catalyst was also examined for

comparison purposes.

2.2. Catalyst characterization

The specific surface area (SBET), the pore volume (Vp) and

the pore size distribution of the samples were determined from

the adsorption and desorption isotherms of nitrogen at�196 8C

using a Quantachrome Autosorb-1 instrument. The specific

surface area (SBET) of the samples was calculated following the

BET (Brunauer–Emmett–Teller) procedure with six relative

pressures of nitrogen in the range of 0.05–0.3. Prior to the

measurements, the samples were outgassed at 200 8C for 2 h

under vacuum.

The crystalline structure of the catalysts was analyzed by

means of X-ray powder diffractometer (Philips PW1830/40)

employing Cu Ka radiation (l = 0.15418 nm). The X-ray was

operated at 40 kV and 30 mA. The mean particle diameter of

CeO2 was calculated from the X-ray line broadening of the

(1 1 1) diffraction peak according to Scherrer’s equation [22].

Temperature-programmed reduction (TPR) experiments

were performed under a flow of a 3% H2/He mixture

(50 cm3 min�1) over 60 mg of catalyst using a heating rate

of 10 8C min�1. Prior to TPR, the catalysts were treated under

air flow (20 cm3 min�1) at 400 8C for 30 min. A mass

spectrometer (Omnistar/Pfeiffer Vacuum) was used for on-line

monitoring of TPR effluent gas.

The surface composition of selected doped copper–cerium

oxide catalysts, in terms of atomic ratios, was calculated on the

basis of XPS peak area intensities using a Shirley type

background and empirical cross section factors [20]. X-ray

photoelectron spectra of the samples were recorded on a

commercial ultrahigh vacuum system, equipped with a

hemispherical electron energy analyzer (SPECS LH-10) and

a twin anode X-ray gun. The base pressure was

5 � 10�10 mbar. Unmonochromatized Al Ka line at

1486.6 eV and an analyzer pass energy of 97 eV were used

in all XPS measurements. The binding energies were calculated

by reference to the energy of C 1s peak of contaminant carbon

at 284.6 eV and the highest binding energy peak for Ce 3d at

916.5 eV [20].

2.3. Catalytic activity

Activity and selectivity measurements for the SRM reaction

were carried out at atmospheric pressure in a conventional

fixed-bed reactor system, which has been described previously

[20,23]. The samples were pretreated in a flowing 20% O2/He

mixture at 400 8C for 30 min. The catalyst weight was 0.3 g and

the total flow rate of the reaction mixture was 70 cm3 min�1 (W/

F = 0.257 g s cm�3). The feed gas contained 5% MeOH and

7.5% H2O (H2O/MeOH = 1.5) in helium. Product and reactant

analysis was carried out by a gas chromatograph (Shimadzu

GC-14B) equipped with TCD and FID. The selectivity of CO

was defined as the molar ratio of CO to that of the sum of CO

and CO2 produced.

3. Results and discussion

The XRD patterns of doped catalysts, as well as of ceria and

non-modified CuO–CeO2, are shown in Fig. 1. The absence of

copper oxide reflections is in agreement with previous studies

that ascribed the missing copper oxide phase in XRD to: (a)

solid solution formation due to incorporation of some amount

of copper oxide in the subsurface region of the CeO2 lattice and/

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234228

Fig. 1. XRD patterns of doped CuO–CeO2 catalysts: (a) CeO2, (b) CuO–CeO2,

(c) CuCe0.95La0.05, (d) CuCe0.90La0.10, (e) CuCe0.95Sm0.05, (f) CuCe0.90Sm0.10,

(g) CuCe0.90Zr0.10, (h) CuCe0.75Zr0.25, (i) CuCe0.95Zn0.05, (j) CuCe0.80Zn0.20,

(k) CuCe0.95Mg0.05, (l) CuCe0.90Mg0.10, (m) CuCe0.95Y0.05, (n) CuCe0.90Ca0.10,

(o) CuCe0.95Gd0.05, (p) CuCe0.998Pd0.002, and (q) CuCe0.998Rh0.002.

Table 1

Lattice constant of doped CuO–CeO2 catalysts

Cation Ionic radius (A) M/(M + Ce) Lattice constant, a (nm)a

Ce4+ 0.970 0.5412

Cu2+ 0.650 0.5412

La3+ 1.160 0.05 0.5449

0.10 0.5461

Sm3+ 1.079 0.05 0.5415

0.10 0.5436

Mg2+ 0.890 0.05 0.5396

0.10 0.5393

Zr4+ 0.840 0.10 0.5362

0.25 0.5313

Zn2+ 0.900 0.05 0.5396

0.20 0.5386

Ca2+ 1.120 0.10 0.5381

Y3+ 1.019 0.05 0.5407

Gd3+ 1.053 0.05 0.5396

a Calculated from the (3 1 1) crystallographic plane.

Table 2

Surface composition of selected doped copper–cerium oxide catalysts

Catalyst Cu=ðCuþ CeþMÞ M=ðCeþMÞ

Nominal XPS Nominal XPS

CuCe0.95Sm0.05 0.15 0.380 0.05 Trace

CuCe0.90Sm0.10 0.15 0.256 0.10 0.426

CuCe0.95Zn0.05 0.15 0.490 0.05 0.496

CuCe0.20Zn0.20 0.15 0.233 0.20 0.610

CuO–CeO2a 0.15 0.55 – –

a Ref. [20].

or (b) fine dispersion of copper oxide clusters (XRD-invisible

due to small size, i.e. less than 3 nm) on the surface of ceria

[20]. The distinct fluorite-type oxide structure of CeO2 (face-

centered cubic lattice, fcc) was the only observable structure in

all the samples. The tetravalent cations, such as Ce4+, have an

eight-coordination of oxygen ions. The unit cell contains four

cations occupying the opposite four corners. These tetravalent

cations can be replaced by trivalent (e.g. La3+, Sm3+, Y3+, Gd3+)

or divalent cations (e.g. Mg2+, Zn2+, Ca2+, Cu2+). Oxygen

vacancies are created by this substitution and are desirable in

redox-type reactions [24].

The ionic radii of the cations used as dopants [25] and the

lattice constant ‘‘a’’, calculated with the use of Materials Studio

4.0 software are shown in Table 1. The lattice constant of the

cubic cell of ceria was found to be equal to 0.5412 nm, in

agreement with previous studies [16]. Taking into account the

XRD patterns of the doped catalysts, it can be said that with the

incorporation of a cation with higher ionic radius than that of

Ce4+, such as La3+ or Sm3+, in the lattice, the lattice constant

becomes larger and the XRD reflections of the doped ceria are

shifted to lower degrees. On the other hand, the lattice constant

is getting smaller and the reflections are shifted to higher

degrees with the incorporation of a cation with smaller ionic

radius than that of Ce4+, e.g. Zn2+, Mg2+. In previous studies,

great solubility and complex lattice parameter dependences

were reported among Y2O3, Gd2O3 and CeO2 [19]. In the

present work, it was also found that despite the larger ionic radii

of the trivalent cations Y3+ and Gd3+, the lattice constant of the

doped ceria was smaller compared to that of the unpromoted

one (Table 1). Although the ionic radius of Ca2+ (0.112 nm) is

similar to that of La3+ (0.116 nm), the Ca-doped sample neither

has a greater lattice constant nor it shifts the XRD peaks to

smaller degrees. A possible explanation is that the low number

of electrons of Ca2+ might have provoked some kind of matrix

effect that affects the real lattice constant so that the calculated

one is plasmatic. The introduction of smaller isovalent cations,

like Zr4+, into the fcc cell of ceria resulted in a defective fluorite

structure with increased oxygen mobility [26].

The addition of noble metals (Pd, Rh) did not cause any

observable shift in the XRD reflections of the doped sample.

The noble metals should be present in the form of small

crystallites on the surface of the catalysts taking into account

that their content is very low (M/(M + Ce) = 0.002).

The surface composition of Zn- and Sm-doped catalysts, in

terms of atomic ratios, was calculated by XPS and is presented

in Table 2. Comparing these values with the nominal

compositions of the samples and with the corresponding

values of the undoped sample [20], it can be observed that the

calculated values are significantly higher (up to 10 times) than

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234 229

Fig. 2. H2-TPR profiles of doped CuO–CeO2 catalysts: (a) CuO–CeO2, (b)

CuCe0.95La0.05, (c) CuCe0.90La0.10, (d) CuCe0.95Sm0.05, (e) CuCe0.90Sm0.10, (f)

CuCe0.90Zr0.10, (g) CuCe0.75Zr0.25, (h) CuCe0.95Zn0.05, (i) CuCe0.80Zn0.20, (j)

CuCe0.95Mg0.05, (k) CuCe0.90Mg0.10, (l) CuCe0.95Y0.05, (m) CuCe0.90Ca0.10, (n)

CuCe0.95Gd0.05, (o) CuCe0.998Pd0.002, and (p) CuCe0.998Rh0.002.

the nominal ones, implying an enrichment of the surface with

copper and dopant cation species. Only in the case of

CuCe0.95Sm0.05 catalyst, samarium oxide was undetectable

on the surface, indicating full incorporation into the ceria

lattice, in agreement with XRD measurements. Further increase

of samarium oxide loading resulted in a high surface coverage

of samarium of 0.4 (40%). The copper and dopant oxide species

can be considered as highly dispersed on the ceria surface since,

on one hand, they are XRD-invisible and, on the other hand,

they present a high surface coverage (Table 2). XPS

measurements cannot exclude the possibility of simultaneous

solid solution formation (this is strongly evidenced in the case

of CuCe0.95Sm0.05 catalyst), since the XRD reflections of doped

ceria were clearly shifted. The surface coverage of Sm3+ and

Zn2+ dopant cations increases with increase of their nominal

content, however, which indicates that most of the additional

dopant is not incorporated in the ceria lattice, but resides on the

surface. It can be also observed that the surface coverage of

copper ions decreases upon increase of the dopant content. This

most probably indicates a smaller dispersion of copper oxide

and may be attributed to perturbation of the interaction between

copper and cerium ions by the dopants. Another possibility is

that the dopant oxides cover part of the copper oxide species.

Both alternative explanations point towards a smaller

concentration of exposed copper oxide species at high content

of Sm3+ and Zn2+ dopants.

The BET specific surface areas of the various doped CuO–

CeO2 catalysts are listed in Table 3, together with the crystallite

sizes of CeO2 and the total pore volume, Vp. Addition of Zn,

Sm, Gd, Rh and Pd, led to a significant increase in the specific

surface area and to a decrease in the CeO2 crystallite size. On

the other hand, the addition of Zr, La, Mg, Yand Ca provoked a

decrease in the specific surface area. This result was not always

accompanied with a prospective increase in the CeO2 crystallite

size. In all cases, addition of larger amounts of dopant caused a

decrease in the surface area.

Table 3

Characteristics of doped CuO–CeO2 catalysts

Catalyst SBET

(m2 g�1)

d(1 1 1)a

(nm)

Vp

(cm3 g�1)

H2 consumption

(mmol g�1)

CuO–CeO2 43.2 12.0 0.18 1.75

CuCe0.95Zn0.05 63.5 8.4 0.20 2.00

CuCe0.20Zn0.20 47.1 10.2 0.16 1.59

CuCe0.90Zr0.10 24.5 12.6 0.11 1.79

CuCe0.75Zr0.25 19.8 9.5 0.07 1.88

CuCe0.95La0.05 35.4 10.8 0.16 1.31

CuCe0.90La0.10 31.5 11.2 0.17 1.23

CuCe0.95Sm0.05 58.1 8.2 0.21 1.75

CuCe0.90Sm0.10 48.7 9.0 0.18 1.70

CuCe0.95Mg0.05 29.3 13.2 0.16 1.67

CuCe0.90Mg0.10 26.1 17.0 0.15 1.87

CuCe0.95Y0.05 40.7 12.4 0.18 1.58

CuCe0.95Gd0.05 62.7 8.0 0.19 1.58

CuCe0.95Ca0.10 41.2 6.6 0.21 2.48

CuCe0.998Pd0.002 58.5 9.0 0.17 1.72

CuCe0.998Rh0.002 55.3 9.9 0.20 2.34

a Measured from line broadening of CeO2 (1 1 1) peak.

The H2-TPR profiles of doped CuO–CeO2 catalysts are shown

in Fig. 2. The x-axis is up to 350 8C so that differences among the

various catalysts are more discernible (no other peaks were found

above 350 8C). The H2-TPR profile of the non-modified CuO–

CeO2 catalyst is also included in the same figure for comparison

purposes. The CuO–CeO2 catalyst exhibited two overlapping

reduction peaks, i.e. a low temperature peak at �150 8C, called

peak a, and a pair of peaks at �200 and 230 8C, called peak b1

and peak b2, respectively. These results are in agreement with

previous studies [20]. It has been pointed out that peak a may be

ascribed to the reduction of copper ions in close contact with

ceria. On the other hand, peaks b1 and b2 may be attributed to

surface and bulk copper oxide species which are less associated

with ceria. As mentioned in previous studies, there is a

proportional relation between the reduction temperature and

the size of the copper oxide species [27]. The latter is affected by

the surface area of the support, which is dependent on the

preparation method. Copper–cerium oxide catalysts prepared by

different methods showed a varying intensity of b2-peak (high

temperature region) [28], correlating with the degree of sintering.

As it can be seen in Fig. 2, doped CuO–CeO2 catalysts maintain

the same characteristics in their profiles. For example, Sm, Zn

and Y dopants did not shift the position of neither of the peaks,

but the a-peak area was increased (as the reduction started at

lower temperatures around 100 8C) and the b2-peak area was

decreased. However, the TPR profiles of Zr- and Mg-doped

catalysts had an additional peak in the high temperature region,

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234230

Fig. 3. Methanol conversion and CO selectivity values of doped CuO–CeO2

catalysts at a reaction temperature of 240 8C. Operating conditions: W/

F = 0.257 g s cm�3, 5% MeOH, H2O/MeOH = 1.5.

called peak b3. The addition of lanthanum oxide in the CuO–

CeO2 catalyst shifted slightly all peaks to higher temperatures.

The same is true for the Gd-doped catalyst, while the TPR

peaks of Ca-doped samples were significantly shifted to higher

temperatures. In the case of Pd and Rh containing catalysts, the

addition of the noble metal shifted peaks a and b1 to lower

temperatures and eliminated peak b2. A new shoulder-peak in

the temperature range of 75–130 8C may be attributed to the

reduction of the noble metal [26]. Addition of further amount

of the dopant leads to a shift of peaks to higher temperatures

(for all dopants with the exception of Zn) with contemporary

decrease of a-peak area. Table 3 presents the amounts of H2

consumed during TPR up to the temperature of 500 8C. The

theoretical H2 consumption corresponding to complete

reduction of Cu2+ to Cu0 is 0.94 mmol g�1. In the case of

the reference CuO–CeO2 catalyst, the consumed amount of H2

is 1.75 mmol g�1. Bera et al. [29] have also observed this and

have attributed the higher-than-stoichiometry H2 consumption

to simultaneous reduction of two Ce4+ cations together with a

Cu2+ cation according to the reaction: Cu2+ + 2Ce4+ + 2O2� +

2H2! Cu0 + 2Ce3+ + 2H2O. Pintar et al. [30] have proposed

on the basis of TPR and TPO experiments that the higher-than-

stoichiometry H2 consumption is due to H2 incorporation into

the catalyst. The amounts of consumed H2 during TPR were

also higher than the ones corresponding to the Cu2+! Cu0

reduction in the case of doped catalysts. In comparison with the

undoped CuO–CeO2 catalyst it can be observed that the H2

consumption is: (i) lower for La-, Y- and Gd-doped samples,

(ii) similar for Zr-, Sm-, Mg-doped samples, and for the Pd-

containing sample and (iii) higher for Zn- and Ca-doped

samples, as well as for the Rh-containing sample. There does

not appear any correlation to exist between the ease of

reducibility and the amount of consumed H2. For example,

both La and Ca dopants shift peaks to higher temperatures, but

the amount of consumed H2 is lower than CuO–CeO2 for La,

but higher for Ca.

All the catalysts were tested for the production of hydrogen

via steam reforming of methanol without any prereductive step.

The main products of the SRM reaction were H2, CO2 and CO.

Other byproducts such as formaldehyde, formic acid, methyl

formate or dimethyl ether, which are often reported [16,31,32]

to be produced in methanol-involving reactions, were detected

in small amounts, only at low reaction temperatures and/or low

methanol conversions. In the following presentation of

activity–selectivity results, the unmodified copper–cerium

mixed oxide catalyst is used as reference.

Fig. 3 illustrates the methanol conversion and CO selectivity

values of all the samples at a reaction temperature of 240 8C,

while Fig. 4 presents conversion and selectivity curves as a

function of reaction temperature. First of all, it can be observed

that the commercial reforming catalyst has considerably higher

activity than all other catalysts tested. For the dopants, which

were examined at two different dopant levels, it was found that

increase of the dopant level always leads to decrease of

catalytic activity. Focusing, therefore, on the catalysts with a

low level of dopant, these can be classified in three categories

with regard to the dopant effect on catalyst activity.

3.1. Dopants with positive effect

A positive effect is found only in the case of Zn2+ and Sm3+

dopants, as well as for the Pd-containing catalyst. It can be

observed that the CuCe0.95Zn0.05 catalyst reaches �99%

methanol conversion at 280 8C with �1.2% CO selectivity

(Fig. 4b), while the same conversion is achieved over unmodified

CuO–CeO2 at 295 8C with �3.7% CO selectivity. In a similar

manner, the CuCe0.95Sm0.05 catalyst had �99% methanol

conversion at 280 8C with �1% CO selectivity (Fig. 4f). It

should be recalled here that Zn- and Sm-doped catalysts had

specific surface areas of 63.5 and 58.1 m2 g�1, respectively, i.e.

higher than that of CuO–CeO2 (43.2 m2 g�1). In addition, the

observed trend in activity upon addition of small amounts of Zn

or Sm correlates well with the H2-TPR profiles of these samples,

in which: (i) reduction starts at lower temperatures (�100 8C),

which corresponds to a higher fraction of easily reduced and

highly dispersed CuO clusters and (ii) there is a significant

decrease of the b2-peak (which represents larger copper oxide

particles, less responsible for catalytic activity). Increase of

activity is also found for the Pd-containing sample (Fig. 4e),

which achieves almost complete methanol conversion at 280 8C.

The Pd-containing catalyst had larger specific surface area than

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234 231

Fig. 4. Activity and selectivity for SRM over doped CuO–CeO2 catalysts. Operating conditions: W/F = 0.257 g s cm�3, 5% MeOH, H2O/MeOH = 1.5: (a)

CuCe0.95Mg0.05 (~), CuCe0.90Mg0.10 (&), (b) CuCe0.95Zn0.05 (~), CuCe0.80Zn0.20 (^), commercial ($), (c) CuCe0.90Zr0.10 (&), CuCe0.75Zr0.25 (*), (d)

CuCe0.95La0.05 (~), CuCe0.90La0.10 (&), (e) CuCe0.998Pd0.002 (&), CuCe0.998Rh0.002 (*), (f) CuCe0.95Sm0.05 (~), CuCe0.90Sm0.10 (&), and (g) CuCe0.95Y0.05 (~),

CuCe0.90Ca0.10 (^), CuCe0.95Gd0.05 (&), commercial ($). Activity–selectivity curves of unmodified CuO–CeO2 (open circle) catalyst as well as the CO selectivity

equilibrium curve (dotted line) are also presented.

CuO–CeO2 and showed enhanced reducibility with TPR peaks

shifted to lower temperatures.

3.2. Dopants with a negligible effect

Mg, Zr, La and Gd dopants belong in this category. The

addition of these dopants at a molar ratio of 0.05 (0.10 for Zr)

almost did not affect the catalytic activity, but lowered the CO

selectivity, especially at high temperatures (Fig. 4a, c, d and g).

A direct relationship between the physicochemical character-

istics of these catalysts (as obtained from BET, XRD and TPR

measurements) and their catalytic performance is not obvious.

For example, Mg-, Zr- and La-doped catalysts had lower

surface area than CuO–CeO2, while the Gd-doped sample had a

larger one. The TPR a-peak was smaller in intensity in these

doped catalysts and, in the case of Zr- and Mg-doped catalysts,

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234232

Fig. 4. (Continued ).

a third high-temperature b3-peak appeared implying a higher

fraction of less active copper oxide species. Gd- and La-doped

catalysts, on the other hand, had TPR peaks shifted to 10–20 8Chigher temperatures. The Rh-containing catalyst had also

similar activity to CuO–CeO2, but a much higher CO selectivity

close to its equilibrium value.

3.3. Dopants with a negative effect

Ca- and Y-doped catalysts had lower activity than the

unmodified CuO–CeO2 catalyst (Fig. 4g). The decrease in the

activity correlates well with the TPR results in the case of the

Ca-doped sample, since there is a shift of all peaks by 50 8C to

higher temperatures (the largest shift among all catalysts).

Comparison of TPR profiles of the Y-doped and the undoped

catalyst, on the other hand, shows that the Y-doped catalyst has

a smaller area of the a-peak.

The results of this work indicate, first of all, that the

combustion method employed for the synthesis of doped CuO–

CeO2 catalysts leads to incorporation of at least part of the

dopant cations into the lattice of CeO2. In most of the cases, the

presence of the dopant did not enhance the reducibility of

J. Papavasiliou et al. / Applied Catalysis B: Environmental 69 (2007) 226–234 233

Fig. 5. Activity and selectivity for SRM over CuCe0.95Zn0.05 (triangles),

CuCe0.95Sm0.05 (circles) and CuO–CeO2 (rectangles) catalysts for different

W/F ratios: 0.103 g s cm�3 (solid symbols), 0.257 g s cm�3 (open symbols) and

0.686 g s cm�3 (open-crossed symbols). Operating conditions: 5% MeOH,

H2O/MeOH = 1.5.

copper oxide species, the two exceptions being Sm- amd Zn-

doped catalysts (increase of a-peak in TPR), which also showed

increased catalytic activity. The performance of Zn- and Sm-

doped catalysts in methanol steam reforming was further

examined by variation of the W/F ratio and the results are given

in Fig. 5. A common feature of both doped catalysts at all W/F

ratios is that their CO selectivity is noticeably lower than the

one of undoped CuO–CeO2 at all temperatures. The improved

performance of the Zn-doped catalyst was observed for all

W/F ratios employed, while the Sm-doped catalyst showed

higher activity than the undoped catalyst only at the

intermediate W/F ratio.

The addition of a larger amount of dopant leads always to

decrease of catalytic activity. One explanation for this could be

the segregation of the dopant oxide on the catalyst surface (as

found by XPS). The dopant oxide may partially cover copper

oxide crystallites or disturb the interaction between copper and

ceria leading to decrease of copper oxide dispersion. As far as

CO selectivity is concerned, the most pronounced effect is

found in the case of Pd and Rh-containing catalysts, for which

CO selectivity is remarkably (three times or more) higher and

quite close to WGS equilibrium. To further investigate this

behavior, the performance of CuO–CeO2 and Pd–CuO–CeO2

catalysts in methanol decomposition was examined. It was

found that, in the absence of steam, methanol conversion over

CuO–CeO2 is almost zero at 280 8C and becomes 7% and 20%

at 300 and 320 8C, respectively. Taking into account that

methanol conversion is �90% at 280 8C in the presence of

steam, it can be inferred that methanol decomposition is not an

important pathway on CuO–CeO2 under steam reforming

conditions. The Pd–CuO–CeO2 catalyst was more active than

CuO–CeO2 in methanol decomposition achieving methanol

conversion of 5%, 13% and 35% at 280, 300 and 320 8C,

respectively. This means that methanol decomposition is not an

important pathway at least at temperatures lower than 300 8C.

As the CO selectivity is close to equilibrium over the Pd and Rh

containing CuO–CeO2 catalysts even at temperatures at

which methanol decomposition does not take place (i.e. at

T < 280 8C), it can be concluded that this is due to the high

activity of these metals (when supported on CeO2) for the WGS

and the reverse WGS reaction [33,34]. On the other hand, CO

selectivity decreases with the addition of dopant cations and

this is desirable for the use of CuO–CeO2 catalysts in fuel cell

applications.

4. Conclusions

Doping of CuO–CeO2 catalysts with small amounts of

oxides of Sm and Zn improves their catalytic performance in

methanol steam reforming, while doping with oxides of La, Zr,

Mg, Gd, Y or Ca lowers or has negligible effect on catalytic

activity. All doped catalysts produce less CO than CuO–CeO2.

Addition of larger amounts of dopant leads always to a decrease

of catalytic activity. Pd and Rh-containing catalysts have

similar (Rh) or higher (Pd) activity compared to CuO–CeO2,

but their CO selectivity is significantly higher and close to WGS

equilibrium. At least part of dopant cations gets incorporated

into the CeO2 lattice leading to solid solution formation, while

increase of dopant loading leads to its surface segregation and

decrease of copper oxide dispersion. As the activity of CuO–

CeO2 catalysts is, in most cases, not improved by doping, it

appears that the effect of surface segregation outweighs the

effect of the modification of the ceria lattice by dopant

incorporation.

Acknowledgement

The work was carried out in the frame of EPAN E-25 project

with funding from the General Secretariat for Research and

Technology (Ministry of Development, Greece).

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