Influence of the Support on the Catalytic Properties of NickelCeria in Carbon Monoxide and Benzene...

8/12/2019 Influence of the Support on the Catalytic Properties of NickelCeria in Carbon Monoxide and Benzene Hydrogentaion

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Applied Catalysis, 46 1989) 269-279Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands269

Influence of the Support on the Catalyticproperties of Nickel Ceria in Carbon monoxide and enzeneHydrogenationJ. BARRAULT* andA ALOUCHELaboratoire de Catalyse en Chimie Organique, UA RS 350, 40 Avenue du Recteur Pineau,86022 Poitiers Cedex FranceV. PAUL BONCOURLaboratoire de Chimie Metallurgique des Terres Rares, ER RS 209, 1 Place Aristide Briand,92190 Meudon FranceL HILAIRELaboratoire de Catalyse et Chimie des Surfaces, UA RS 423, 4 Rue Blaise Pascal, 67070Strasbourg Cedex FranceandA. PERCHERON GUEGANLaboratoire de Chimie Metallurgique des Terres Rares, ER RS 209,1 Place Aristide Briand,92190 Meudon FranceReceived 20 June 1988, revised manuscript received 22 September 1988)

ABSTRACTIn this work the catalytic properties ofnickel supported on various supports AI : l , Si02 , Ce02)in syngas conversion are compared. The influence of th e temperature of reduction pretreatmentwas studied. The characterization of the catalysts was performed by temperature programmed

reduction, isothermal reduction, CO and H2chemisorption, X-ray diffraction, X ray absorptionspectroscopy, magnetization and X ray photoelectron spectroscopy. The modification of the catalytic properties of Ni/Ce02 catalysts with reduction pretreatment is correlated to the transformation of the Ce02 support and to strong interactions between these species and metal particles.

INTRODUCTION

Previous work on the catalytic behaviour of intermetallic compounds of rareearths and transitionmetals in the syngas reaction showed intermetallic trans-formation into nickel particles and rare earth oxides [1 4] Their interestingcatalytic properties led us to prepare catalysts of nickel supported on rare earthoxides by an impregnation method in order that we might compare them withNi/Al 3 and Ni/Si0 catalysts.

0166-9834/89/ 03.50 1989 Elsevier Science Publishers B.V.


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270TABLE 1Catalytic properties of nickel in the Co H 2 reaction as a function of pretreatment conditions fordifferent supports

Ni 7.5 )/Ce0 2 Ni 11.4 )/Al2 3 Ni 8 )/Si02Calcination 0 C) 500 500Reduction 0 C) 250 250 400 400 400 500 500Reaction 0 C) 250 250 250 400 400 250 250Activity X 103 ) 6 5 0 6 5 54 25 ole CO h-1g- 1Ni)Selectivity ) 39 37 10 12 41 35CH 4C2 C 4 46 52 20 18 44 41C5 4 13 13CO2 11 11 70 70 2 11 Olefinsin C C4 37 55 90 90 40 38

The present paper deals with the study of the catalytic properties of Ni/Ce02 in carbon monoxide and benzene hydrogenation with special emphasison the role played by the temperature of the reduction pretreatment.Characterization of the catalysts by the following physicochemical andphysical measurements is reported: temperature-programmed reductionTPR ), isothermal reduction, CO and H2 chemisorption, X-ray diffractionXRD ), magnetization, X-ray absorption spectroscopy XAS) and X-rayphotoelectron spectroscopy XPS).A correlation between the change of catalytic properties and the physicochemical and physical transformation of the NijCe02 catalyst as a function ofthe pretreatment shows the strong influence of the Ce02 support.EXPERIMENTAL mplesThe catalysts were preparedby impregnation of an aqueous solution of nickel

nitrate 98 , Fluka) on cerium dioxide 99.9 EGA-Chemie with 20 m2g- 1BET area), y A 203 hone Poulenc, GFSC with 210 m2g- 1 BET area) andSi02 hone Poulenc, PBS 300 with 260 m2g- 1 BET area). After completedrying on a sandbath the powders were heated in an oven at 393 K for 12 h.Before use for the catalytic reactions the solids obtainedwere, in some cases,calcined at 500 0 C see Table 1) then reducedwith hydrogen in situ at differenttemperatures see Table 1Catalytic reactions were performed in a dynamic fixed bed reactor describedin ref. 1. The experimental conditions were as follows:


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271 O, H 2) reaction: CO H2=1 at 1 atm and T=25 C or 400CCsH s, H2) reaction: CsHs H2= 1 20 at 1 atm and T=5 Ch r cteriz tion methodsTPR and adsorption experiments were carried out using a pulse chromatographic method described previously [2]. Before each TPR experiment, thesamples were outgassed for 2 h at the temperature of reduction, then cooleddown to room temperature. The heating rate was 4C min 1 Hydrogen andcarbon monoxide adsorption were determined on samples reduced for 2 h un

der hydrogen at a given temperature.For the following characterizations, the samples were treated as for the catalytic reaction, handled in an argon filled glove box and then sealed eitherunder argon or under vacuum in an adapted container.

The Debye Sherrer methodwas used for X ray powder diffractionMagnetization measurements were performed on a vibrating magnetometerin the temperature range 77 620 K and magnetic fields from 0 to 10 kGs.X-ray absorption datawere collectedat Lure using the synchroton radiationprovided either by the DCI ring for the Ni K edge or by the ACO ring for theCeMrv_v edge. EXAFS spectra at the Ni K edge were collected with a silicon311) double crystal monochromator on an EXAFS device. CeMrv-v edgespectra were collected by photoelectron yield using a double crystal monochromator equippedwith beryl crystals [5]. This method allowed us to analyzea thickness of 100at the surface of the samples.

For the XPS experiments, the samples were dried at 120C and the oxidation and reduction treatments carried out in situwithout exposure to air beforeanalysis. These experiments were carried out in a Vacuum Generators ESCA spectrometer using Al radiation u= 1486.6 eV).RESULTS t lytic properties rbon monoxide hydrogen tionThe catalytic properties of Ni7.5/Ce02 submitted to different pretreatmentsofcalcination and reduction are compared in Table 1with the results for Ni l1 A /

203 and Nis Si02which were not calcined butwere reduced at 500C. Thesecatalytic values were obtained after catalyst stabilization and the product distributions are compared for similar carbon monoxide conversions < 10 ).

First we see that the calcination treatment neither changed the activity northe selectivity of the Ni Ce02 catalyst while the temperature of reduction hada drastic effect: When reduced at 250C, Ni Ce02 was less active than Ni20 3 and Ni Si02but exhibited the same products distribution. However, if


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272reduced at 4OOC, Ni/CeOI was not active if the catalytic reaction was per-formed at 250C and it was necessary to increase this temperature to 400Cto attain the same level of actlvlty as in the previous case. The selectivity wasalso very different, with 70~ of carbon dioxide being produced instead of 10%and 90% of olefins in the C C afraction.

Benzene hydrogenationThe activity of the catalvsts 1s given in Table 2. When reduced at 250CNi/CeO, exhibited slightlv greater activity than the other two catalysts. But,as for the previous reactron. reduction at 400 C induced deactivation of theNi/CeO, catalyst.Characterizat ion

The study of the catalvtir properties of Ni- J CeO, showed that the temper-ature of reduction plays a verv Important role whereas calcination treatmenthas no effect in this range of concentration. So we selected for characterizationthe two samples with the most different properties i.e. Ni7.s/Ce02 reduced at250 and 400 C respectivelc However, the samples with A1203 and SiO, sup-ports and CeO, alone are compared with the catalysts to show the differencesin reducibility (TPR) and chemisorption behaviour.TPRThe results for the three catalysts (nickel supported on CeO,, A1203, SiOz)and for CeO, alone are presented in Fig. l It appears that nickel is reduced ata lower temperature when It is deposited on CeO, than on the other supportsand the time of reduction ISshorter : hydrogen uptake became very importantfrom 300 C and decreased from 400 C. The reduction stopped after about 40min in this range of temperature. In contrast, on SiO, and A1203 supports thereduction began at 350 and 450 C respectively with a broad maximum at 450

and 550C respectively. However, for CeO,>alone the hydrogen uptake beganTABLE 2Catalytic activity of nickel in benzene hvdrogenation as a function of the temperature of reductionpretreatment for different supports

Ni(7.Fii 4 CO Ni(11.4%)/ Ni(3.7%)/SiO,AI,O,)

Temperature ofreduction ( C )Activity X 10)(mole C,H,h-g-Ni)

250 400 500 5005.5 I 0;3 4.8 4.6


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T C) Time mn)Fig. 1. Temperature programmed reduction of nickel on different supports precalcined at 500 a C,(0) Ni(i.Swt.- )/CeO, (m) Ni(8wt: )/SiO, (A) Ni(ll.4wt.- )/Al,O,andofCeO, (*).

TABLE 3Reducibility of nickel supported by different oxides at constant temperature

Ni(7.5 )/CeO, Ni(ll.4 )/AlL0.1 Ni(8 )/SiOT CeO,Temperatureof reduction 250 400 500 500 500 800pretreatment ( C )Percentage ofreduction 126 132 92 97 0 3

only from 500 c C with one maximum around 620 o C and a second, broader onearound 800 c C.Isothermal reduct ionThe reduction rate of the catalysts during treatment for 10 h under 1 atm ofhydrogen at a constant temperature is given in Table 3.For Ni,,,/CeOz the reduction rate was about the same, independent of thetemperature of reduction and was higher than loo%, whereas, on the othersupports nickel was not completely reduced during the same time at 500C.We deduced therefore, that for Ni,_,J Ce02 it is necessary to take into accountthe reduction of the CeO, support from 250 C. From the percentage of reduc-tion, with the hypothesis that all the nickel is reduced, we obtained the follow-ing stoichiometry for the support after reduction: CeO, at 250C and

CeO,.,,.,.,, at 400.We note that, as in the TPR experiment, CeO, alone did not take up hydro-gen before 500C. Therefore, CeO, is more easily reduced in the presence ofnickel.


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Ni 71-Ce02-v_-c-- _a--- co

,/-,/,

Fig. 2. Effect of reduction temperature on hydrogen and carbon monoxide adsorption on Ni/CeO,.The reduction time is 2 h.

TABLE 4Hydrogen and carbon monoxide chemisorption of catalysts and dispersion of nickel (0% ) as afunction of the temperature of reduction T,)

Ni(7.5%)/CeO, Ni(11.4%)/Al,O:, Ni(8%)/SiO,Ta 2) 250 400 500 500pmole CO g _ Icat 17 37 119 80D (s) 1.3 2.9 6.1 6,umole H,g- Icat 10 0.5 117 42D ( I 0.8 0.04 6 6.1

H ydr ogen and car bon monoxi de chem i sor pti onThe variation of carbon monoxide and hydrogen chemisorption on Ni7,5/

CeO, as a function of the temperature of reduction is reported in Fig. 2. As thetemperature increased, the carbon monoxide chemisorption increased, whereashydrogen chemisorption vanished at 350C. This contrasting behaviour didnot allow us to deduce the number of active sites for the CeO, support as forthe SiO, and A1203 supports. Indeed, the dispersions calculated for the threecatalysts are summarized in Table 4: good agreement was obtained for thenumber of moles chemisorbed and the calculated dispersion of the metal, what-ever the gas, for the SiO, and Al,O, supports, whereas for the CeOz supportthere was no correlation between the two sets of values.

XRDFor both compounds, i.e. Ni,,,/CeO, reduced at 250 and 400C narrow dif-

fraction lines were obtained and all of them were indexed as CeOz and nickelmetal.

M agneti zat ionField cooled magnetization curves were measured between 77 and 620 K. For

both samples (i.e. NiJCeO, reduced at 250 C and 400C ), below 500 K pro-


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-0.090 200 400 600

E eV)

Fig. 3. EXAFS spectra at the Ni-K edge of Ni/CeOa reduced at 250 and 400C and the nickelreference.

TABLE 6Influence of the oxidation and reduction treatments on the surface composition, elemental bindingenergies of components and cerium valencyTreatment Ni(Ni+Ce) E Ni2p3,Yat(%)+5% (eV)

Cevalency

Main peak Small peaksDriedat 120C 63 855.6 3.34Oxidation (O,, 500 C) 58 854 855.6 3.27Reduction (H,, 25OC) 68 852.7 854 3.27Reduction (H2, 400C) 50 852.5 3.16

are exactly superimposed. This means that large nickel particles were presentin both cases.The spectrum at the Ce Mrv_v edge of the sample reduced at 250 C wasidentical to that of the other sample and to the CeO, spectra [B].

X SThe influence of the oxidation and reduction treatments on the surface com-position of the samples and the electronic state of the constituents are sum-marized in Table 6. Binding energies of Ni2p3,2 nd Ce3d5,2 eaks were calculatedwith a precision of t 0.2 eV to determine the nickel and cerium electronic state.The relative spectral weight of Ce3+-Ce4+ states (valency) was deduced bya method described elsewhere [9]. The surface compositions were estimated


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A 655.6INi 2p3* 873.5I a

854

zi 652.5 ,t c

k_d

E eV)Fig. 4. Evolution of the Ni 2p photoemission spectra of Ni/CeO, as a function of pretreatment.(a)Driedat 120C; (b) oxidizedat 500C; (c) reduced at 25OC (d) reduced at 400C.

from the area of the photoelectron peaks. The evolution of the photoelectronpeaks of nickel is shown in Fig. 4. Drying of the Ni/CeO, sample resulted in apeak at 855.6 eV (Fig. 4a) which was ascribed to nickel nitrate. Indeed thewhole spectrum of cerium was characteristic of CeO, [9].After oxidation at 500C (Fig. 4b), the nickel was almost completely oxi-dized (854 eV) with only a small amount of nitrate (855.6 eV) present. Thecerium remained as CeO, with a slight decrease of the valency. After reduc-tion at 250C (Fig. 4c), the nickel was almost completely reduced (852.7 eV)to the metal, with a small contribution from nitrate and oxide. Cerium re-mained in the same state as after oxidation. After reduction at 400 C (Fig. 4d)the nickel was completely reduced to the metal (852.5 eV) and the valencyof cerium decreased to 3.16 showing the presence of Ce3+ species. However,these species were present only in small amounts, since we could not resolveany transition around 885 eV which was characteristic of Ce3+.The change of the percentage of nickel at the surface depending on the var-ious treatments was not determined with accuracy, however it is noteworthythat the mean value, around 55 wt.-%, is quite high compared to the bulk valueof 7.5 wt.-% (17.3 at.-% ) showing an important nickel enrichment of the sur-face. Moreover a relative decrease of about 25% was observed between thesamples reduced at 250 SC and 400 3C respectively.DISCUSSION

We have shown that nickel supported on CeO, exhibits different behaviourfrom that which is observed with Al,O, and SO, supports.


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Firstly, concerning the reducibility, nickel on CeO, was reduced at a lowertemperature and the hydrogen uptake showed partial reduction of the CeO,support leading to a nonstoichiometric oxide in addition to the reduction ofnickel. Indeed, unlike Al,O, and SiOr, CeO, is a reducible oxide. The TPRexperiments on CeO, alone show two hydrogen uptake peaks at 600 and 800 C,which agreed with the results of Yao and Yu Yao [lo] who associat.ed themwith the reduction of surface capping oxygen and bulk oxygen anions respec-tively. Moreover, these authors observed a lowering of the reduction temper-ature of CeO, in the presence of precious metals as for nickel. Inversely, thereduction of NiO was easier in the presence of CeO,.

Secondly, we showed that the catalytic properties of Ni/CeO, dependedmainly on the temperature of reduction pretreatment. After a 250 C treatmentNi/CeO, exhibited the same catalytic behaviour as Ni/Al,O, and Ni/SiO, re-duced at 500 s C : the selectivities were the same, and the difference in activitiescould be ascribed to the difference of the specific area of the supports andconsequently to the lower dispersion of the metal (Table 4). Indeed the char-acterizations showed that, for the three catalysts, nickel was almost completelyreduced in each case.

Deactivation of the Ni/CeO, catalyst reduced at 400 C was observed for thetwo catalytic reactions, although the specific area of nickel deduced from mag-netic measurements was unchanged and the bulk characterizations (XAS,XRD) did not allow us to detect a difference between the two samples pre-treated at 250 and 400 C respectively; only Ni and CeO, were identified. How-ever the chemisorption and XPS experiments, sensitive to the surface, showedimportant differences: An increase of carbon monoxide chemisorption was ob-served as the temperature increased whereas hydrogen chemisorption de-creased. The nickel concentration at the surface, determined by XPS, showeda decrease of about 25% between the sample reduced at 250 and that reducedat 400 C. In addition, a decrease of the oxidation state of cerium by reductionat 400 0 C was shown at the surface.

A similar effect of the reduction temperatures on catalytic properties hasbeen observed for other transition metals supported on reducible oxide cata-lysts such as TiO, or ZrO, [ 11, 121, and is known as strong metal-supportinteraction (SMSI ) [ 111. In fact, some characteristics of Ni/CeO, resemblethose of Ni/TiO+ reduction of the TiO, support led to the formation of Ti,07with the presence of Ti+ species [ 13,141 and the hydrogen and carbon mon-oxide chemisorption changed as the temperature increased.

In order to explain this behaviour, some authors suggest a migration of thereduced species of the support on the transit.ion metal [ 15, 161 leading to thecreation of new active sites. Thus the chemisorption properties of the transi-tion metal would be modified by the interaction wit.h the reduced species of thesupport.

This mechanism seems to be in agreement with our observations. When the


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reduction temperature increased we showed, by XPS, an increase in the re-duction of cerium at the surface creation of Ce+ species) and a decrease inthe nickel concentration which should correspond to the presence of the oxidespecies CeO, _ r on the surface of the nickel particles.Moreover, the lowering of the hydrogen chemisorption could be explainedby a decrease in the accessible nickel surface. As for the increase in the carbonmonoxide chemisorption, it could be due to carbon monoxide adsorption onCeO,_,, followed by a large desorption of carbon dioxide which was observedin the products of the reaction of carbon monoxide hydrogenation at 400Cwith the 400 C reduced Ni/CeO,.

In conclusion, we have shown the influence of the temperature of the reduc-tion pretreatment on the catalytic properties of nickel supported on ceria. Anincrease in this temperature induced deactivation of the catalysts in carbonmonoxide and benzene hydrogenation. This deactivation is ascribed to a mod-ification of the active sites with a strong interaction occurring bet.ween thenickel and the reduced species present at the surface of the support, since nochange of the specific area was observed.

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Influence of the Support on the Catalytic Properties of NickelCeria in Carbon Monoxide and Benzene...

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