or

om

i a

G.

stru

himi

a, U

safety. Thus, an idea is now spreading to associate the fuel cells

with reactors for hydrocarbons and alcohols reforming. TheCH3OH 1=2O2YCO2 2H2 DH- 191:9kJ mol

2exothermic partial

Solid State Ionics 176 (2005reactor should work at a relatively low temperature and be ableThe possibility of a wide use of hydrogen as fuel of proton exchange membrane fuel-cells forces to the development of selective catalytic

materials for the oxidative steam reforming of methanol (OSRM) to produce H2 essentially free from CO. Cu/ZnO/Al2O3 catalysts of OSRM

process have been obtained from hydrotalcite-like precursors with nominal formula Cu1xy Zny Alx(OH)2(CO3)x/2 (x =0.230.42, y =0.310.58) and prepared by homogeneous precipitation from metal chlorides solutions in the presence of urea. The catalysts were obtained after thermal

decomposition of the hydrotalcites at 450 -C, followed by in situ reduction with H2. X-ray powder diffraction (XRPD) patterns of the precursorsshowed the presence of the hydrotalcite phase with minor amounts of a Zn-rich paratacamite phase (Cu2xZnx(OH)3Cl) whose amount increaseswith increasing Cu content. XRPD patterns of thermally treated samples show only the lines of CuO and ZnO phases; Al2O3 and/or aluminates

may be present as amorphous phases. The BET surface areas of the samples are in the range 110220 m2 g1 and increase with increasing Alcontent. The catalytic activity in the OSRM process is appreciable from about 200250 -C and methanol conversions up to 9095% are obtainedat temperatures of 300400 -C. Hydrogen is the main product, and its yield reaches values up to 2.7 mol/mol of methanol. Carbon monoxidecontent is under the detection limit (500 ppm) of the detector.

D 2005 Elsevier B.V. All rights reserved.

PACS: -81.05Zx

Keywords: Hydrotalcites; Cu/ZnO/Al2O3 catalysts; H2 production; Methanol reforming

1. Introduction

Fuel cells with proton exchange membranes (PEMFCs) are

considered one of the most interesting alternatives to the

traditional internal combustion engine for the production of

energy for car-traction [1,2]. Although, on principle, the

PEMFCs can work with different fuels from the simplest

hydrocarbons to alcohols, the present technology is limited to

the employment of H2, but the feeding of the motor-vehicles

with hydrogen could cause problems for the fuel tank

construction, for the filling stations and, above all, for the

to feed the cell with the hydrogen produced during the reaction.

Among the fuels to be reformed, methanol is very attractive for

its low cost, the high H/C ratio and the absence of coke

formation [3]. Recently, an interesting methanol reforming

process, named Oxidative Steam Reforming of Methanol

(OSRM), has been proposed and is under study in various

academic and industrial laboratories [4,5]. OSRM combines

the two reactions:

CH3OH H2OYCO2 3H2 DH- 49:3kJ mol1 1

1AbstractCuZnAl hydrotalcites as precurs

hydrogen fr

U. Costantino a,*, F. Marmottini a, M. Sisan

M. Turco c,

a CEMIN, Centro di Eccellenza sui Materiali Innovativi Nanob Dipartimento di Ingegneria C

c Dipartimento di Ingegneria Chimic0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2005.09.051

* Corresponding author. Department of Chemistry, University of Perugia, Via

Elce di Sotto, 8, 06127 Perugia, Italy. Tel.: +39 075 585 5565.

E-mail address: ucost@unipg.it (U. Costantino).s of catalysts for the production of

methanol

, T. Montanari b, G. Ramis b, G. Busca b,

Bagnasco c

tturati, Dipartimento di Chimica, Universita` di Perugia, Italy

ca, Universita` di Genova, Italy

niversita` Federico II, Napoli, Italy

) 2917 2922

www.elsevier.com/locate/ssiendothermic steam reforming (1) andoxidation (2) and can be carried out in autothermic conditions

if the reactor is fed with mixtures of CH3OH, H2O and O2 in

proper concentration ratio. The reforming process must not

te Iproduce CO, since this by-product, even in ppm amounts,

damages the fuel cell operation [6]. Hence, the ideal OSRM

catalysts should possess, beside high chemical, thermal and

mechanical endurance to operate for long time with many

working cycles in the presence of water vapour and oxygen, a

high surface area and chemical properties (redox, acidbase) to

selectively activate the reagents for CO2 and H2 production.

Literature data indicate that good catalysts are obtained with

systems based on metallic copper well dispersed into zinc and/

or aluminium oxides [79]. A relatively simple way to obtain

such systems consists in preparing catalyst precursors consti-

tuted of Cu, Zn, Al hydroxycarbonates with hydrotalcite

structure [4,5,10]. The hydrotalcite-like compounds, also

known as anionic clays or lamellar double hydroxides, are

practically the unique example of lamellar solids having

lamellae with positive charges balanced with exchangeable

anions accommodated in the interlayer region. Their general

formula is [M(II)1xM(III)x(OH)2](Ax /n )ImH2O, whereM(II)=Mg, Zn, Ni, Cu, Mn; M(III)=Al, Cr, Fe; x =0.20.4;

An- is the intercalated anion of charge n; m =mol/mol of co-

intercalated water. The structure of the lamellae is of brucite

type and comes out from the linkage through corners of

M(OH)6 octahedra where M is the bivalent or trivalent metal.

More than one bivalent or trivalent metal can be introduced in

the brucite layer to achieve a large variety of composition

[11,12]. In any case, the brucite layer is constituted of metal

cations interdispersed at atomic level and ternary hydrotalcites

of the type CuxZn0.67xAl0.33(OH)2(CO3)0.17, calcined andreduced with a hydrogen flow can lead to catalysts containing

Cu highly dispersed on ZnO and Al2O3. In this line, previous

works [4,5] have shown that thermal decomposition and

reduction of ternary hydrotalcites produces catalysts that show

high catalytic performances in terms of methanol conversion

and high selectivity to produce H2 and CO2.

It was thus of interest to prepare CuZnAl hydrotalcite

precursors with different composition in order to obtain new

catalysts for the OSRM process. In this work a series of

hydrotalcite precursors with nominal formula Cu1xy ZnyAl-x(OH)2(CO3)x/2 (x =0.230.42, y =0.310.58) have beenprepared by homogeneous precipitation from metal chlorides

accomplished by urea hydrolysis [13]. The materials have been

characterised by chemical and thermal analyses, X-ray powder

diffraction (XRPD) and BET surface areas and the catalysts

thereby obtained have been used in OSRM process with the

aim to obtain information on the effect of Cu, ZnO and Al2O3molar ratio on the efficiency of the reaction.

2. Experimental

2.1. Synthetic procedures and chemical analyses

The hydrotalcite precursors were obtained with urea method

[13]. Aqueous solutions, obtained by mixing solutions 0.5 M of

AlCl3, ZnCl2 and CuCl2 in the proper volume ratio, were added

U. Costantino et al. / Solid Sta2918of solid urea until the molar ratio urea/Al(III) was 6 and then

refluxed for 3 days. The precipitates obtained were separated

from the mother solutions, washed with deionised water andthen suspended in a 0.05 M Na2CO3 solution (about 20 mL/g

of precipitate) for 1 day, in order to exchange chloride ions,

eventually present in the hydrotalcite materials, with carbonate

anions. After equilibration with the Na2CO3 solution the solids

were recovered, washed with deionised water and finally dried

at room temperature over P4O10. The catalysts were obtained

by heating the precursors in dry airflow at rate of 2 or 10 -Cmin1 up to 450 -C and maintaining it at this temperature for12 h. Then the samples were reduced in situ with 2% H2/He

mixtures. The reducing treatments were effected before

characterization measurements. The metal ions contents were

obtained by ion chromatography, after dissolution of the

samples in concentrated HCl. Carbonate and water contents

were evaluated from thermo gravimetrical analysis.

2.2. Instrumentation and catalytical characterisation

XRPD patterns of the samples were recorded with a

computer controlled Philips PW1710 diffractometer using

CuKa Ni-filtered radiation (40 kV, 30 mA). XRPD patternsat programmed temperatures have been taken in a HT A.

Paar diffraction camera. TG analyses were performed in air

by a Netzsch STA449C Thermal Analyser at a heating rate

of 5 -C min1.N2 adsorptiondesorption isotherms were obtained at 196

-C, on samples previously degassed at 100 -C, using acomputer controlled Micromeritics 2010 apparatus. The IR

spectra were recorded with a Nicolet Protege 460 Fourier

Transform instrument.

A Micromeritics 2900 apparatus equipped with a TCD

detector was employed for Temperature Programmed Reduc-

tion/Oxidation (TPR and TPO) measurements. TPR measure-

ments were carried out on calcined samples at a rate of 10 -Cmin1 using a 5% H2/Ar mixture. Catalytic activity measure-ments are carried out in a laboratory flow apparatus with a

fixed bed reactor. A gas-chromatograph (GC) HP 5890,

equipped with a Porapak-molecular sieve double-packed

column and a TCD detector, is employed for the analysis of

H2, CO, CO2, O2, CH4, CH3OH, H2O. From concentrations

and volume of effluent stream, total and partial conversions are

calculated. The tests were carried out at T=200400 -C,feeding with H2O/CH3OH/O2 mixtures at 1.1:1:0.12 molar

ratio and space velocity, GHSV=0.6106 or 1.2106 h1.

3. Results and discussion

3.1. Precursors preparation and characterisation

The CuZnAl precursors of the OSRM catalysts have

been prepared with the urea method. This method generally

allows the preparation of different hydrotalcite-like compounds

having a high crystallinity degree and a narrow particle size

distribution [13]. However, the presence of Cu2+ cations makes

the synthetic procedure more complex, at least for two reasons.2+

onics 176 (2005) 29172922First, Cu ions show the Jahn-Teller effect that favours the

formation of distorted octahedral structures [14], secondarily

Cu2+ ions can be depleted by ammonia originated from urea

hydrolysis. The preparation of CuZnAl precursors has

required modification of the original urea method. In particular,

the molar ratio urea/Al(III) has been decreased from 10 to 6

and addition of the CuCl2 solution has been carried out drop by

These data seem to indicate that the brucite sheet of well-

crystallised hydrotalcites is not able to accommodate more than

one copper atom every six different metal atoms. If one

considers that every metal centre in the brucite sheet is

Table 1

Metal content, phase analysis and surface area of Cu/Zn/Al hydrotalcite precursors (the surface area of the precursors, previously heated at 450 -C is also reported)

Sample Solid composition (% metals molar ratio) % Htlc phase % PTAC* phase Surface area (m2/g)

Cu Zn Al Precursor Oxides

C1u 9 48 43 100 0 216

C2u 11 52 37 100 0 216 254

C3u(NA) 18 33 49 85 15 134

C11u 24 43 33 65 35 48.7

C3u(bis) 45 31 23 56 44 67 125

C4u 60 20 19 40 60 116.1

PTAC* 75 25 0 100 10.8 9.4

PTAC*=paratacamite Cu2xZnx(OH)3Cl.

U. Costantino et al. / Solid State Ionics 176 (2005) 29172922 2919drop as the ZnAl hydrotalcite starts to be formed. These

modifications allowed to obtain materials having a Cu content

near, even if always lower, to that of the starting solutions.

Table 1 reports the composition, expressed as % molar ratio of

the metal ions content, the phase analysis and the BET surface

area of the samples prepared. The XRPD patterns of the

precursors are reported in Fig. 1. The patterns clearly indicate

the good crystalline order of the samples. It may be also seen

that most of them are biphasic and pure hydrotalcite phases

(PDF No. 14-0191) were obtained when the Cu-content was

lower than 15% (samples C1u and C2u). At higher Cu-content,

a new phase, identified as Zn-rich paratacamite (PDF N. 50-

1558) of formula Cu2xZnx(OH)3Cl, starts to be formed. Therelative amount of the two phases has been evaluated by

quantitative Rietveld procedure, using GSAS program [15].

Table 1 shows the increase of paratacamite phase amount with

the increasing Cu-content and, correspondingly, a decrease of

BET surface areas of related samples. It may be observed that

pure paratacamite sample, prepared with the urea method, has a

very low surface area when compared with that of pure

hydrotalcite phase (sample C1u).

10000

120005 10 15 20

0

2000

4000

6000

8000

Inte

nsity

(a.u.

)

Fig. 1. XRPD patterns of the indicated samples, conditioned at roosurrounded by six others metal centres, it may be deduced

that the brucite layer seems not to tolerate the presence of two

contiguous distorted Cu(OH)6 octahedra. It may be recalled

that hydrotalcite samples with a higher Cu content can be

prepared by the classical precipitation procedure, but the

samples obtained have a low degree of crystallinity and the

attempts to prepare more crystalline samples produce the

segregation of other Cu-rich phases [16].

3.2. Catalysts preparation and characterisation

As already mentioned, OSRM catalysts are obtained by

calcination and subsequent reduction of the precursors. It was

thus of interest to study the thermal behaviour of the

precursors. Fig. 2 shows the weight loss curves of the samples

prepared as a function of temperature. For sake of comparison,

the TG curve of the sample of paratacamite, prepared by the

urea method, is also reported. Thermal decomposition of

paratacamite occurs in two well differentiated steps. The first

one, attributed to the loss of condensation water, is sharp and

occurs at 300 -C. The second step, occurring between 450- and

C4u

PTAC25 30 35 40

C3u

C11u

C3ubis

C2u

C1u

2

m temperature over P4O10. See Table 1 for the abbreviations.

60

55

60

65

70

75

80

85

90

95

100

C4u

C3u

C3uNA

PTACC2u

% w

eigh

t los

s

rat

C1u C2u C3u C3uNA C4u Ptac

C1u

he

U. Costantino et al. / Solid State Ionics 176 (2005) 291729222920650 -C, is broad and can be attributed to the loss of chlorine,very likely as HCl. At 700 -C only a mixture of Zn and Cuoxides are present. The weight loss curve of the hydrotalcite

phase is represented by the curve of the C1u sample. It is

constituted by two broad weight losses, the first occurring

between room temperature and about 200 -C is mainlyattributed to the loss of co-intercalated water, the second loss,

between 200- and 450 -C, is attributed to the loss ofconstitutional water and carbon dioxide and leads to the

0 200 400Tempe

Fig. 2. Weight loss curves as a function of temperature of tformation of metal oxides and aluminates. The TG curves of

the other precursors show the typical features of the thermal

decomposition of hydrotalcite and paratacamite phases. The

loss below 150 -C can be attributed to the loss of water co-

Fig. 3. XRPD patterns at room temperature of the iintercalated with carbonates in the hydrotalcite phase, while the

loss over 450 -C to the decomposition of the paratacamitephase. The losses between 150 and 450 -C can be ascribed tothe loss of condensation water of hydrotalcite and of

paratacamite overlapping with the loss of carbonates. The total

weight loss of the precursors is very near to that calculated for

the thermal transformation of the precursors into CuO, ZnO

and Al2O3 metal oxides. It is, however, difficult to deduce the

formula of the mixed phase precursors since Cu and Zn are

0 800 1000 1200ure (C)indicated samples. Heating rate of 5 -C min1 in air flow.vicariant metals both in hydrotalcite and paratacamite phases.

The thermal decomposition of the precursor has been also

monitored by recording the XRPD patterns in a HT diffraction

camera. The structure of the hydrotalcite phase collapses at

ndicated samples, previously heated at 450 -C.

pre-treated under vacuum at 300 -C. Surface species after contact with the gas (a),of the catalyst after thermal treatment has been subtracted. Inset (c) (a) subtraction

te Ionics 176 (2005) 29172922 2921temperatures higher than 180 -C, while the characteristic peakof paratacamite is not present at temperatures higher than 260

-C, in agreement with the TG data. Over 320 -C, very weakreflections typical of the copper and zinc oxides start to appear.

Fig. 4. FT-IR adsorption spectra of CO adsorbed at 160 -C over C3u catalyst,after evacuation at 160 -C (b), after evacuation at 140 -C (c). The spectrumspectrum.

U. Costantino et al. / Solid StaTo have a better insight of the phases formed at temperatures

higher than 320 -C, the XRPD patterns, taken at roomtemperature, of samples previously heated for three hours in

an oven at 450 -C have been collected (see Fig. 3). The patternsshow diffraction peaks attributable to CuO and ZnO and only

broad reflections attributable to zinc (or copper) aluminates.

Reflections of Al2O3 are absent. From these data, it appears

that treatment at 450 -C is suitable for obtaining catalysts forOSRM. In fact, this temperature is sufficiently high for the

formation of metal oxides, but low enough to avoid sintering

phenomena that could cause loss of surface area. Subsequent

characterisation has been carried out on the C3u sample. TPR

signals showed that the reduction of Cu(II) species to Cu(0)

occurs in two steps in the 250400 -C temperature range, andgive evidence that the heating rate employed in the thermal

treatment influences the crystal size and the homogeneity of the

CuO phase. Correspondingly, TPO measurements gave evi-

dence that oxidation of Cu phase occurs in several steps in the

temperature range 200350 -C, suggesting that in the reactionconditions Cu(0), Cu(I) and Cu(II) can be present simulta-

neously. This was confirmed by IR measurements. Fig. 4

reports the IR spectra of carbon monoxide adsorbed at 160-C on the catalysts, pre-treated at 300 -C. The adsorption ofCO gives rise to a complex band characterised by a maximum

at 2128 cm1 with a shoulder at 2135 cm1 and two weakerbands at 2163 and 2187 cm1. According to previous works[17,18], the observed features can be assigned to CO adsorbed

on the Zn-aluminate support (2187 cm1), on Cu2+ sites (2163

Fig. 5. Methanol conversion (a) and hydrogen yield (b) as a function of

temperature in the OSRM process catalysed by the indicated catalysts.

GHSV=0.6106 h1.

cm1, decreasing in intensity by reduction), on Cu+ centres(2135 cm1, stable to out gassing at 110 -C) and on Cumetal deduced from the presence of a component at 2128

cm1, (see inset of Fig. 4), labile even at 110 -C.

3.3. Preliminary catalytic activity measurements

Preliminary catalytic activity tests indicated that the

catalysts, obtained from precursors listed in Table 1, have a

good activity and selectivity, giving almost complete CH3OH

This has allowed to obtain products with fairly high Cu

content, however the contemporary formation of CuZn

oxychloride (paratacamite phase) could not be avoided, it

seems in fact that the layer of brucite type does not tolerate

more than 15% of Cu(II) cations in octahedral coordination,

due to the Jahn Teller distorsion. However, the Cu/ZnO/Al2O3catalysts thereby obtained gave preliminary good results, in

terms of CH3OH conversion and H2 yield, together with low

CO production and this is a further stimulus to investigate the

hydrotalcite system as efficient precursor of very selective

OSRM catalysts.

U. Costantino et al. / Solid State Ionics 176 (2005) 291729222922limit=500 ppm) under conditions of interest for OSRM

process. The activity strongly depended on space velocity:

for GHSV=1.2106 h1, CH3OH conversion was low and O2conversion negligible at T