ethanol steam reforming over nilazr and niculazr mixed metal oxide catalysts

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Catalysis Today xxx (2013) xxx– xxx

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Catalysis Today

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thanol steam reforming over NiLaZr and NiCuLaZr mixed metalxide catalysts

uan Bussia,∗, Mauricio Mussoa, Santiago Veigaa, Nicolás Bespalkoa, Ricardo Facciob,nne-Cécile Rogerc

Laboratorio de Fisicoquímica de Superficies, DETEMA, Facultad de Química, UdelaR, Gral. Flores 2124, Montevideo, 11800 UruguayLaboratorio de Cristalografía y Química del Estado Sólido, DETEMA, Facultad de Química, UdelaR, Gral. Flores 2124, Montevideo, UruguayLaboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, LMSPC-ECPM, UMR CNRS 7515, 25, rue Becquerel 67087 Strasbourg Cedex 2, France

a r t i c l e i n f o

rticle history:eceived 13 December 2012eceived in revised form 25 March 2013ccepted 3 April 2013vailable online xxx

eywords:opper-nickel catalysts

a b s t r a c t

NiLaZr and NiCuLaZr mixed metal oxide catalysts were prepared by co-precipitation with oxalic acid inalcoholic medium, followed by calcination. XRD analysis of the unreduced catalysts showed the formationof crystalline phases corresponding to the pyrochlore structure La2Zr2O7 and NiO following calcinationat 850 ◦C, 900 ◦C and 950 ◦C. TEM microscopy of the amorphous solids obtained by calcination at 700 ◦Cshowed the formation of nanoparticles 20–30 nm in size. TPR analysis showed a shift in the Ni andCu reduction temperature towards lower values with increasing calcination temperature. All catalystswere active in runs of ethanol steam reforming, leading to the formation of gaseous mixtures containing

ydrogenio-ethanolteam reforminganthanum–zirconium mixed oxide

hydrogen, carbon monoxide, carbon dioxide and methane. Deactivation due to carbon formation wasobserved at the lowest reaction temperature tested (500 ◦C). XRD, FTIR and thermogravimetric analysisrevealed differences in the textural properties of catalysts before and after reforming, which varied indegree according to the catalyst calcination temperature. The NiCuLaZr catalysts showed a lower activitythan their NiLaZr analogues, a fact that was ascribed to the formation of a nickel–copper solution, witha lower catalytic activity in the cleavage of C C bonds.

. Introduction

Hydrogen combustion appears as a preferred means of energyeneration leading to zero-carbon and -nitrogen oxide emissions.n addition, hydrogen-based fuel cells have been reported to be0–90% more efficient than conventional internal combustionngines [1].

The development of renewable, non-fossil fuels ensures thevailability of adequate amounts of bio-ethanol as a suitable feed-tock for large-scale hydrogen production. Easy-storage conditionsnd a relatively low toxicity enable the safe delivery of ethanolhrough the current gasoline distribution network. Ethanol can beonverted into hydrogen by means of steam reforming [2], partialxidation [3], auto-thermal reforming [4] and dry reforming [5].

Ethanol steam reforming has a higher hydrogen yield on anthanol feed basis although the endothermic nature of the reaction

Please cite this article in press as: J. Bussi, et al., Ethanol steam reforming(2013), http://dx.doi.org/10.1016/j.cattod.2013.04.013

equires an external heat supply to keep the process at constantemperature. The stoichiometric amount of H2 that can be obtained

∗ Corresponding author. Tel.: +598 29248352; fax: +598 29248352.E-mail addresses: [email protected], [email protected] (J. Bussi).

920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2013.04.013

© 2013 Elsevier B.V. All rights reserved.

by total conversion of ethanol in steam reforming can be repre-sented by the following equation:

C2H5OH + 3H2O → 6H2 + 3CO2 (1)

High H2 yields have been associated with reaction conditionsentailing the use of high temperatures (above 450–500 ◦C) atatmospheric pressure in the presence of excess water [2,6]. Thecomplexity of the set of reactions involved in the overall mech-anism results in the formation of minor light hydrocarbons (C2species like acetaldehyde and ethylene in addition to other, C3–4species) in variable amounts. Critically, a number of side reactionsresult in the formation of carbonaceous build-up on catalyticallyactive surfaces.

Metal oxide-supported nickel catalysts have proven highly effi-cient in activating C C and C H bonds [7–14]. The chemical,physical and surface properties of supports can greatly affect thebehaviour of catalytic species, such as their thermal stability,reducibility, and sintering and coke resistance. Ni-based catalysts

over NiLaZr and NiCuLaZr mixed metal oxide catalysts, Catal. Today

prepared by co-precipitation with other metals contain oxidesstructures bearing highly stable Ni species capable of reductionat high temperatures. Tests conducted at 600 ◦C and atmosphericpressure showed high H2 yields and significant coke resistance [14].

dx.doi.org/10.1016/j.cattod.2013.04.013


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A NiLaZr mixed oxide mineral has been used to provide highly dispersed NiO phase with good reducibility between00 ◦C and 650 ◦C [15]. H2 yields using these catalysts inthanol steam-reforming have been reported to be close to ther-odynamically expected values. The basic character of these

atalysts favours the dehydrogenation route involving acetalde-yde as a first reaction intermediate of ethanol decomposition16,17].

Other transition metals like Cu, Co, and Rh, reported to have aood activity and selectivity in ethanol steam reforming, may beonveniently used to increase catalytic performance [18–22]. Inarticular, the addition of Cu to Ni supported on several commer-ial oxides (Al2O3 and SiO2) or other synthetised materials has beeneported to increase H2 selectivity and reduce the rate of carbonormation [18–20].

Here, the physicochemical properties and catalytic performanceere studied of NiLaZr catalysts containing 5% of Ni and NiCu-

aZr catalysts containing 4% and 1% of Ni and Cu, respectively. Theffect of the final calcination temperature used for catalyst prepa-ation on the catalytic performance of these catalysts in ethanolteam reforming was assessed over the temperature range of00–650 ◦C.

. Experimental

.1. Materials

Nickel nitrate Ni(NO3)2·6H2O, copper nitrate (Cu(NO3)2·2.5H2O,anthanum nitrate (La(NO3)3·5H2O, 70% (w/w) zirconium propox-de Zr(OCH2CH2CH3)4 in 1-propanol, and oxalic acid (HO2CCO2H)

ere purchased from Sigma–Aldrich. Ethanol was purchased fromiopack. All chemicals were reagent-grade and were used aseceived.

.2. Catalyst preparation

NiLaZr and NiCuLaZr catalysts were prepared by co-recipitation following the addition of oxalic acid to an ethanolicolution of Cu(NO3)2, Ni(NO3)2, La(NO3)3 and Zr(OCH2CH2CH3)4.he precipitates were washed with ethanol, dried at 80 ◦C, andalcined for 2 h at different temperatures: 700 ◦C, 850 ◦C, 900 ◦Cnd 950 ◦C. The resulting NiLaZr catalysts contained 5% of Ni, andhe NiCuLaZr catalysts contained 4% of Ni and 1% of Cu, on a weightasis. Where required, the final calcination temperature (◦C) is

ndicated in the catalyst denotation.

.3. Catalyst characterisation

The fresh catalysts were characterised on an X-ray diffractome-er (Rigaku Ultima IV) fitted with a CuK� (� = 1.5418 A) sourcend coupled to a data acquisition system. BET surface area wasetermined based on nitrogen adsorption–desorption isothermsbtained at −196 ◦C (Micromeritics Tristar 3000). The reducibilityf metals in the fresh catalysts was assayed by thermal pro-rammed reduction using 0.05 g of catalyst and a 50 mL min−1 flowf H2/Ar at a molar ratio of 3.85%, with temperature increasing to00 ◦C at a rate of 15 ◦C min−1. Before and after use in reform-

ng tests, catalysts were subject to thermogravimetric analysisShimadzu TGA-50) under 50 mL min−1 air flow, with tempera-ure increasing from 20 ◦C to 850 ◦C at a rate of 5 ◦C min−1. FTIRpectra were obtained for both fresh and used catalysts (Bomem


artmann & Braun MB-Series). The fresh and used catalysts werenalysed by TEM microscopy (Jeol JEM-1010). The carbonaceousesidue was analysed for its C and H contents (Carlo Erba EA1108HNS-0).

PRESSy xxx (2013) xxx– xxx

2.4. Catalytic tests

Steam reforming tests were performed at atmospheric pressurein a fixed bed continuous flow quartz reactor (12 mm i.d.) heatedby a temperature-controlled electrical furnace. The liquid reactionmixture (ethanol:water at a 1:9 molar ratio) was fed (Cole Parmer74900 Series) into a heated chamber upstream of the reactor whereit vaporised and was mixed with a flow of Ar carrier gas stabilisedby means of mass flow controllers (Aalborg AFC Series). The cata-lyst (0.10 g) was first subject to in situ reduction under a 3 mL min−1

pure hydrogen flow by heating from room temperature to 650 ◦C, ata rate of 10 ◦C min−1 and keeping the final temperature for 1 h. Fol-lowing reduction, the H2 flow was stopped and the reactor purgedwith argon at a flow rate of 20 mL min−1. Tests were run using 0.10 gcatalyst, a 20 mL min−1 argon flow rate, a 5.58 × 10−5 mol min−1

ethanol feed, and a 1/9/14.3 ethanol/water/argon molar ratio(GHSV 41,000 h−1). The reaction temperature was kept constantthroughout the run. The gaseous mixture containing argon andreaction products was passed by a cold trap (−70 ◦C) for conden-sation of non-reacted ethanol and reaction intermediates, mainlyacetaldehyde and acetone, in addition to excess water, replacingthe trap every 4 h of reaction time. Reaction products were ana-lysed on a gas-chromatograph (Shimadzu GC-14B) equipped withFID and TCD detectors in series. Non-condensable products, mainlyH2, CO, CO2, CH4, and C2 (ethane and ethylene) and C3 (propaneand propylene) hydrocarbons, were analysed with an on-line col-umn (Supelco CarboxenTM-1000). The liquid and gaseous phases ofproducts retained in the cold traps were analysed at the end of therun with a Porapak Q column. Ethanol conversion (Xethanol), productselectivity (Si) and product yields (Yi) were determined as follows:

Xethanol = F INethanol − FOUT

ethanol

F INethanol

× 100

Si = Fi

˙Fi× 100

Yi = Fi

F INethanol

where Fi is the molar flow of product i (i = H2, CO, CO2, CH4, acetalde-hyde, acetone, C2 and C3). Carbon build-up on the catalyst surfaceconsistently accounted to less than 0.1% of the total carbon feedand was therefore not considered in the above calculations.

3. Results and discussion

3.1. Catalyst characterisation

3.1.1. XRD analysisX-ray patterns of the NiLaZr and NiCuLaZr catalysts obtained

by calcination at 700 ◦C, 850 ◦C, 900 ◦C and 950 ◦C are shown inFig. 1. The catalyst NiLaZr700 showed a highly amorphous struc-ture formed by a homogeneous mixture of the three metal oxides,whereas the catalysts calcined at higher temperatures showed dis-tinct phases corresponding to NiO (signals at 2� = 37.20◦, 43.46◦ and63.12◦) and the compound La2Zr2O7, with a cubic structure typi-cal of pyrochlore compounds (signals at 2� = 28.74◦, 33.31◦, 47.83◦

and 56.76◦). These results are similar to those observed in previousstudies of the same system with 17 wt% Ni [15] and confirm the


spontaneous conversion of the amorphous oxide structure into abiphasic system, as follows:

(NiLaZr mixed oxides)amorphous → NiO + La2Zr2O7 (2)


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Table 1Cell parameters of Ni and NiCu solid solutions.

Catalyst NiLaZr NiCuLaZr Ni NiCu

Lattice parameter (A) 3.5391 3.5513 3.5239 3.5396

Table 2BET surface area of the fresh catalysts.

Catalyst Calcination temp. (◦C) SBET (m2 g−1)

NiLaZr 700 25.4850 8.8950 5.7

NiCuLaZr 700 25.6

Fig. 1. XRD patterns of NiLaZr and NiCuLaZr catalysts.

X-ray patterns of the NiCuLaZr catalysts obtained by calci-ation at temperatures above 850 ◦C show the formation of theame phase-separated system composed of NiO and La2Zr2O7. AuO phase observed for CuLaZr catalysts prepared by the sameo-precipitation procedure [16] was not detected in the studiedatalysts, presumably as a result of the small amount of Cu used forhe preparation of these catalysts.

Fig. 2 shows the results of XRD analysis for the catalystsiLaZr950 and NiCuLaZr950 following H2 reduction at 650 ◦C for

h, with both reduced catalysts exhibiting the same cubic structuref the pyrochlore phase observed for the unreduced catalysts. How-ver, the signals corresponding to Ni do not coincide with thosef the unreduced catalysts and the diffraction peak at 2� = 44.44◦

an be assigned to the (1 1 1) plane of a metallic Ni phase. Theiagram magnification in the proximity to this 2� value is closelyimilar to that of pure metallic Ni (2� = 44.48◦). In the case of theiCuLaZr catalyst, the signal corresponding to the metallic phaseas slightly shifted to a lower 2� value (2� = 44.32◦), presumablyue to the formation of a NiCu solution in the reduced form ofhis multi-metal oxide material. This shift was consistent withhat observed between the signals of pure metallic Ni and a NiCuolution (4:1 mass ratio) prepared by the same co-precipitationrocedure. Table 1 shows the crystal cell parameters of these metal-


ic phases estimated by full-pattern profile fitting using the Rietveldethod. The difference between the calculated cell parameters for

he metallic phase of the two catalysts (+0.0122 A) closely coin-ided with that calculated for pure metallic Ni and a NiCu solution

Fig. 2. XRD patterns of reduced NiLaZr950 and NiCuLaZr950 catalysts.

850 15.7950 7.3

(+0.0157 A). Rao et al. reported that the reduction of Cu/Ni mixedoxides to a CuNi alloy observed on ceria supports is reflected ina gradual shift of the (1 1 1) reflection [23]. The formation of theNiCu alloy in the reduced NiCuLaZr catalysts could be favoured bythe low tendency of both transition metals to interact with La andZr oxides, as suggested by the biphasic system obtained by calcina-tion at temperatures above 850 ◦C. Fig. 2 also shows a broad signalcentred at 2� = 43.58◦ overlapped with a structural signal of theLa2Zr2O7 phase which may be ascribed to either metallic Cu or aNiCu solution with a higher Cu content.

3.1.2. BET areaAll catalysts showed a low specific surface area (Table 2). The

highest values obtained for catalysts prepared by calcination at700 ◦C (25 m2 g−1) are consistent with the occurrence of nano-sizedcrystallites of diameters ranging from 20 to 30 nm, which were alsoobserved by TEM, as shown below. Catalyst surface area was foundto decrease with increasing calcination temperature, in line withthe promotion of sintering. The presence of Cu in the NiCuLaZrcatalysts appeared to delay this effect. Although low, the specificsurface areas of these catalysts are considerably greater than thoseobtained for the same trimetallic system prepared by a pseudosol–gel technique [24].

3.1.3. TPR analysisSeveral H2 consumption peaks were observed for the fresh

NiLaZr and NiCuLaZr catalysts (Fig. 3a and b). NiLaZr700 showeda peak centred at 434 ◦C and another at 671 ◦C, ascribed to thereduction of Ni species in the NiO outside and within the amor-phous structure of the NiLaZr mixed oxide, respectively. A thirdbroad peak at 840 ◦C was ascribed to the loss of oxygen taking placeduring the formation of the pyrochlore phase, associated with thegeneration of oxygen vacancies within the crystal lattice by partialsubstitution of Zr4+ and/or La3+ by Ni2+ and/or Cu2+ [25]. The aboveresults are consistent with TGA results shown below.

The same low temperature peak was found for the NiLaZr850followed by a broad band that progressively decays with increas-ing temperature, suggesting a greater exposure of Ni species in thebiphasic system. This trend was confirmed by the TPR profile of theNiLaZr950 catalyst where the H2 consumption was greatly reducedat temperatures above 700 ◦C. It has been suggested that oxygenvacancies like those formed within the pyrochlore structure canpromote the reduction of adjoining Ni species in the NiO phase[26].

The NiCuLaZr catalysts showed similar behaviour to that of the


NiLaZr catalysts. However, the three H2 consumption peaks werefainter for NiCuLaZr700 than for NiLaZr700, presumably on accountof a higher degree of incorporation of Ni in the mixed oxide struc-ture. Also, the H2 consumption peaks of those catalysts calcined at




4 J. Bussi et al. / Catalysis Today xxx (2013) xxx– xxx

8aaimtoio

3

3

yl

TE

E

Fig. 3. TPR profiles of catalysts. (a) NiLaZr; (b) NiCuLaZr.

50 ◦C and 950 ◦C were sharper and shifted towards lower temper-ture values (355 ◦C and 364 ◦C, respectively). These results suggest

synergistic interaction between CuO and NiO resulting in an eas-er reduction of Ni species in the NiO phase [18]. This behaviour

ay be explained in terms of the capacity of group 11 elements ofhe periodic table to decrease the reduction temperature at whichther metals are reduced [27]. The small peak centred at 195 ◦Cn Fig. 3b may be ascribed to the reduction of a Cu-rich fractionccurring on the crystallite surface.

.2. Reforming tests


.2.1. NiLaZr catalystsIn addition to H2, CO2, CO and light hydrocarbons (CH4, eth-

lene, propylene), other common intermediates or by-productsike acetaldehyde and acetone were detected in variable amounts

able 3thanol steam reforming using NiLaZr catalysts.

Catalyst Reaction temp. (◦C) Xethanol (%) Selectivity (%)

H2 CH4 CO

NiLaZr700 500 100 65.3 2.0 7.8

NiLaZr700 550 100 68.6 1.7 6.9

NiLaZr700 600 100 68.4 1.6 7.1

NiLaZr700 650 100 69.6 1.4 7.1

NiLaZr850 650 90 66.9 2.1 8.2

NiLaZr950 650 100 65.9 2.0 10.4

thanol conversion (Xethanol), selectivities and yields are average values corresponding to

Fig. 4. Ethanol steam reforming using NiLaZr700 catalysts.

in steam reforming tests conducted between 500 ◦C and 650 ◦C.Table 3 shows the results obtained for the NiLaZr catalysts over4 h of operation. Ethanol conversion (Xethanol) over the catalystNiLaZr700 was practically 100% at all tested reaction tempera-tures. Increasing H2 selectivities were obtained with increasingtemperature, the H2 yield at 650 ◦C (YH2 = 5.02) representing84% of its maximum value (YH2 = 6) according to Eq. (1). Theselectivities to acetaldehyde and acetone decreased with increas-ing temperature and these compounds were undetectable at650 ◦C. These results are in line with the reaction mechanisminvolving acetaldehyde as the main intermediate derived frominitial ethanol dehydrogenation [15,17,18]. At low temperatures,the promotion of C C bond cleavage imparted by Ni is ineffi-cient and the decarbonylation of acetaldehyde competes with thealdol condensation and further formation of acetone, favouredby the basic properties of these materials. At higher tempera-tures, the cracking properties of Ni are predominant and lead tothe production of H2 and carbon oxides by reforming of C2 andC3 oxygenated intermediates [28]. Overall, an apparent agree-ment was found between the amount of carbon contained in theethanol feed and that constituting the above reaction products,which shows that no significant amounts of other unidentifiedproducts were formed under the tested conditions. The amountof carbon build-up on the used catalysts amounted consistently toless than 0.1% of the total amount of carbon contained in the feed.

Although ethanol conversion at 650 ◦C was also high forNiLaZr850 and NiLaZr950, small amounts of acetaldehyde and ace-tone among the reaction products suggest a smaller number ofactive sites and/or changes in textural properties reflected in adecrease in the rate of reaction for the conversion of such inter-mediary species to H2 and the associated final products. This is in


line with a lower specific surface area found for these catalysts thanthat found for NiLaZr700 and NiCuLaZr700.

The evolution of gaseous products was monitored over longeroperation times. Fig. 4 shows that H2 yields were nearly constant

H2 yield CO2 yield

CO2 C2 C3 CH3CHO CH3COCH3

19.9 0.3 – 0.7 4.1 2.42 0.7421.3 0.6 – 0.4 0.7 3.61 1.1220.7 1.4 – 0.3 0.5 4.07 1.2321.3 0.6 – – – 5.02 1.5321.1 0.5 – 0.7 0.5 3.10 0.9718.6 2.7 – – 0.4 3.52 0.99

the 4 h of operation.





Table 4Ethanol steam reforming using NiCuLaZr catalysts.

Catalyst Reaction temp. (◦C) Xethanol (%) Selectivity (%) H2 yield CO2 yield

H2 CH4 CO CO2 C2 C3 CH3CHO CH3COCH3

NiCuLaZr700 500 80.5 51.1 1.9 2.2 19.5 14.6 1.6 1.4 7.7 2.31 0.88NiCuLaZr700 550 98.5 57.3 3.2 4.7 19.4 11.3 0.3 0.5 3.3 2.59 0.90NiCuLaZr700 600 99.8 61.5 3.9 8.1 19.0 6.3 – 0.1 1.1 3.09 0.96NiCuLaZr700 650 100 68.6 2.5 10.5 17.8 0.6 – – – 4.56 1.18NiCuLaZr850 650 100 66.5 2.3 8.5 20.7 2.0 – – 0.1 3.67 1.14

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NiCuLaZr950 650 100 58.0 4.5 9.0

thanol conversion (Xethanol), selectivities and yields are average values correspond

ver the first 8 hours’ operation using NiLaZr700 at 650 ◦C, butecreased slightly (from 2.5 to 2.0) in runs conducted at 500 ◦C. Aonstant decrease in the amount of CH4 with increasing operationime at 500 ◦C was ascribed to changes in the surface propertiesf the catalyst. Also constant H2 yields observed at 650 ◦C for bothiLaZr850 and NiLaZr950 (not shown here) suggest a low deacti-ation rate of all these catalysts.

.2.2. NiCuLaZr catalystsNiCuLaZr700 had a lower activity than that of NiLaZr700

Table 4). At 500 ◦C, ethanol conversion was incompleteXethanol = 80.5% at 500 ◦C). H2 selectivity (51.1%) was lowerhan the value observed for the NiLaZr700 catalyst (65.3%) andhe H2 yield (2.31) amounted to 39% of its maximum value.emarkably significant selectivities to acetaldehyde and acetone1.4% and 7.7% respectively) were found at the lowest reactionemperature. These results were ascribed to a lesser capacity ofhe Ni-Cu catalyst to activate C C and C H bonds, as requiredor the production of H2 and carbon oxides as final products.his may result from changes in the electronic properties of Niue to the proximity of Cu atoms, leading to the formation of

NiCu solution, as discussed in Section 3.1.1. Copper-bearingatalysts have been reported to exhibit poor catalytic activity inhe steam reforming of ethanol, mainly in promoting the cleavagef C C bonds, thus leading to relatively high concentrations ofntermediates like acetaldehyde and ethylene [18,29,30]. Vizcaínot al. reported decreasing ethanol conversion and H2 selectivityith increasing Cu load for a series of supported Ni-Cu catalysts

18]. The high amounts of ethylene formed at low temperaturesay reflect changes in the acid-base properties of catalytic

ites due to carbon deposition and/or the cracking of oligomeric


ntermediates formed in high amounts under low-temperatureonditions.

Fig. 5 shows a progressive decrease in H2 yield (from 2.5 to 1.8)or the NiCuLaZr700 catalyst which was slightly higher than the

Fig. 5. Ethanol steam reforming using NiCuLaZr700 catalysts.

8 8.5 – – 1.2 1.83 0.59

the 4 h of operation.

observed for NiLaZr70 over the first 8 h of operation at 500 ◦C. Sucha decrease, found for both NiLaZr700 and NiCuLaZr700 at 500 ◦C,suggests some degree of catalyst deactivation at this temperature.Changes in the surface state of catalysts, such as sintering and/orto carbon formation may have led to deactivation. The formationof carbon has been reported to derive from reactions of condensa-tion/dehydration of different oxygenated intermediates, includingthose formed during these runs, has been reported [19,28,31]. Theoligomerisation of intermediates — mainly acetaldehyde, aceticacid and acetone — may have been favoured by the reduced crack-ing activity of the Ni and Ni-Cu catalysts at low temperatures. Thehigh amount of ethylene — known to be a coke promoter — foundfor this catalyst at low temperature is consistent with the formationof carbon build-up. The formation of carbon by CH4 decomposi-tion in steam reforming over Ni crystals has been associated witha greater ability of the catalyst to dissociate C H bonds [32]. Thisis typical of CH4 reforming catalysts, among which Cu is usuallynot included [33]. Probably, the presence of Cu forming a NiCusolution in NiCuLaZr700 resulted in the depletion of large Ni ensem-bles necessary for carbon deposition [18]. Thus the formation ofcarbon by CH4 decomposition would be inhibited by partial substi-tution of Ni by Cu and it could not explain the higher deactivationhere observed for the NiCuLaZr700 catalyst. The Boudouard reac-tion: C + CO2 → 2CO must also be taken into account. This reactionis endothermic (�H298

◦ = 173.3 kJ mol−1) and is favoured with theincrease of temperature. The partial substitution of Ni by Cu mayhave led to a reduction in the rate of this reaction preventing therapid removal of carbon from the catalyst surface, in particular inruns conducted at low temperatures. Sintering of metallic Ni andCu could not be ruled out as a cause of catalyst deactivation, affect-ing the long-term behaviour of these catalysts. As described below,sintering can result from structural changes in the amorphous cat-alysts NiLaZr700 and NiCuLaZr700, leading to the formation of thebiphasic system.

3.3. Post-reaction characterisation

3.3.1. FTIR analysisFTIR spectra of fresh and used NiLaZr700 (Fig. 6) show clear

bands at 849, 1069, 2346 and 1390 cm−1, similar to those reportedfor surface carbonates of La(OH)3 [34]. Lanthanum oxy-carbonate,La2O2CO3, is a stable intermediate in the thermal decomposition oflanthanum carbonate to La2O3 and CO2 and has been reported tooccur in other reforming catalysts containing lanthanum [35,36]. Inthe catalyst NiLaZr700, La2O3 was not integrated in the crystallinestructure of the La2Zr2O7 pyrochlore and may have retained, at leastpartially, its reported reactivity with CO2 to form oxy-carbonategroups. In the fresh catalyst, CO2 can derive from the thermaldecomposition of metal oxalates during the thermal treatment of


the catalyst precursor prior to calcination. Oxy-carbonates in theused catalyst could have been formed by CO2 resulting from theethanol reforming reaction. According to literature, oxy-carbonatesfavour the gasification of carbon by the Boudouard reaction:




6 J. Bussi et al. / Catalysis Today xxx (2013) xxx– xxx

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Table 5Full width at half maximum (FWHM) for the reflection Ni(1 1 1) as a function of thereaction time and the corresponding average crystalline size Dp.

Time (h) 2� (◦) FWHM (◦) Dp (nm)

Fig. 6. FTIR spectra of NiLaZr catalysts.

+ CO2 → 2CO, thus preventing catalyst deactivation [17,36]. Theands corresponding to surface carbonates were greatly attenuated

n the spectrum of NiLaZr850. The higher thermodynamic stabilityf La integrated in the pyrochlore phase of this catalyst appearso have reduced its reactivity with CO2 and may have disfavouredhe formation of carbonates and oxy-carbonates within the testedemperature range.

.3.2. XRD analysisNiLaZr700 was subject to XRD analysis at several times of

eforming operation at 650 ◦C. As can be seen in Fig. 7, at the firsttages of the chemical reaction the catalyst consisted of a predom-nantly amorphous phase, with well defined signals correspondingo crystalline Ni. As the reaction proceeded, the La2Zr2O7 phasehowed more definite diffraction peaks, indicating an incipientrystalline growth, that became more pronounced with increasingeaction time. In all cases, the crystalline Ni phase was detectedith the corresponding diffraction peaks always located in the

ame 2� position and showing similar scattering, on account of a


imilar integrated area. In the case of 0 h, the sample had been sub-ect to a reduction process in presence of H2, with a broad Ni (1 1 1)eflection indicating the presence of incipient crystallites. Table 5hows the crystallite size calculated according to the Scherrer

ig. 7. XRD patterns of NiLaZr700 after use in ethanol steam reforming at 650 ◦C atifferent operation times.

0 44.56 0.80 11.134 44.52 0.58 15.57

12 44.50 0.47 18.97

equation [37]. It can be seen that crystallite size increases as long asthe reaction proceeds. Clusters started with crystallites of 11.10 nmin size (0 h), and then grew up to a size of 18.97 nm over the lapse of12 h. These structural changes can affect the surface properties ofthe catalyst (e.g. specific surface area, pore size distribution, num-ber of nickel sites) and its catalytic behaviour. Nevertheless, thecatalyst efficiency in producing H2 was not affected over the first8 h of operation as shown in Fig. 4.

3.3.3. Thermogravimetric analysisFig. 8 shows the thermograms in air of reduced NiLaZr700 before

and after use in runs of reforming at 650 ◦C for 4 h. The weight gainobserved for the fresh catalyst over a wide temperature intervalwas close to the theoretical maximum amount of oxygen requiredfor total re-oxidation of constituent Ni to NiO. A weight loss above780 ◦C was ascribed to oxygen desorption following the formationof oxygen vacancies in the pyrochlore compound, consistently withabove TPR results. Such oxygen vacancies have been reported to beactive in steam reforming reactions through reversible exchangeof oxygen between the catalyst and different reactants like carbon,CO2 and H2O, leading to an improvement in the rate of removalof deposited carbon [26]. The used catalyst exhibited a weight lossbelow 300 ◦C, ascribed to the combustion of carbon deposited onthe transition metal [38]. A weight gain starting from 300 ◦C wasfound to occur at a higher rate than that found for the reduced freshcatalyst, a fact which may be ascribed to changes in the surface stateof metallic Ni after its use in the reforming test. As shown in X-raydiffractograms of the used catalyst (Fig. 7), changes in crystallinitytaking place gradually under the reforming conditions may haveled to a higher exposition and a easier reducibility of metallic Ni.The reduction in the rate of weight gain above 430 ◦C was ascribedto the loss of carbon residues formed mainly on the metal oxidestructure.


The catalyst NiLaZr850 also showed Ni re-oxidation, but theweight-temperature profiles did not differ considerably for thefresh and used catalysts in the low-temperature range below 430 ◦C(Fig. 9). This suggests a higher structural stability of this catalyst,

Fig. 8. TG of NiLaZr700 catalysts before and after use in ethanol steam reforming at650 ◦C, 4 h.





Fig. 9. TG of NiLaZr850 catalysts before and after use in ethanol steam reforming at6

Table 6Elemental analysis of carbon deposits on used NiLaZr and NiCuLaZr catalysts. Reac-tion time: 12 h.

Catalyst Reaction temperature (◦C) C (mg g−1) H/C

NiLaZr700 500 3.39 0.028NiLaZr700 650 0.40


50 ◦C, 4 h.

Fig. 10. TEM images of NiLaZr catalysts. (a) and (b) Fresh and unreduced NiLaZr

NiCuLaZr700 500 1.16 0.134NiCuLaZr700 650 0.19

in particular, for the segregated Ni fraction, during its use in thereforming test. Above 430 ◦C, the weight gain profile of the usedcatalyst was lower than that of the fresh catalyst, which may bedue to the amount of CO2 released by the combustion of carbonresidues formed during the run, as also noted for NiLaZr700.

The TGA profiles of the NiCuLaZr catalysts showed a similarbehaviour and results are therefore not included in this paper.

Carbon formation was also revealed by TEM microscopy andCHNS elemental analysis. Fig. 10a and b shows that the morphologyof the unreduced fresh NiLaZr700 was characterised by nanoparti-cle aggregates of uniform size (20–30 nm). The same aggregates


were observed for the catalysts used at 500 ◦C for 12 h, findingamorphous carbon deposited on the catalyst surface (Fig. 10c andd). According to these results, no significant sintering of the catalysttook place in the reforming at 500 ◦C. Table 6 shows that the great-

; (c) NiLaZr after use at 500 ◦C, 12 h; (d) NiCuLaZr after use at 500 ◦C, 12 h.


ING Model

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399 (2003) 167–170.

ARTICLEATTOD-8433; No. of Pages 8

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st amount of elemental carbon was found in the catalysts used at00 ◦C, with clear deactivation after 12 hours’ reaction. Table 5 alsohows a higher H/C atomic ratio for those residues deposited onhe NiCuLaZr catalysts, suggesting that polymers formed by con-ensation of intermediary species were dehydrogenated to a lesserxtent, probably due to the deleterious effect of Cu on surface reac-ions of this kind.

. Conclusions

NiLaZr and NiCuLaZr catalysts prepared by co-precipitation ofetals with oxalic acid showed significant differences in crystalline

tructure according to calcination temperature. The highly amor-hous state observed for the catalysts calcined at 700 ◦C shifted atalcinations temperatures of 850 ◦C and above to a biphasic systemomposed of the transition metal oxide(s) and the La2Zr2O7 com-ound. Both catalysts showed significant catalytic activity in steameforming of ethanol over the temperatures range of 500–650 ◦C.ydrogen was produced as the main reaction product, in addition

o CO, CO2 and CH4. The partial substitution of Ni by Cu reduced theatalyst’s ability to activate C C and C H bonds, thus favouringondensation reactions of acetaldehyde and other intermediates,eading to the formation of low-volatility polymers. Catalyst deacti-ation due to carbon formation was observed at low temperatures.ngoing research seeks to further characterise the physicochemicalroperties of these materials and describe their long term catalyticehaviour in steam reforming of ethanol and other liquid biomasserivatives.

cknowledgements

Research project supported by the Sectorial Commission forcientific Research (CSIC–UdelaR). National Program for the Devel-pment of Basic Sciences (PEDECIBA–PNUD).

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ethanol steam reforming over nilazr and niculazr mixed metal oxide catalysts

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