Gen Spin 647 the Sintering by Wanke in Catal Revs Sci Eng v 12 Iss 1 Pp 93 135 y 1975

7/29/2019 Gen Spin 647 the Sintering by Wanke in Catal Revs Sci Eng v 12 Iss 1 Pp 93 135 y 1975

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The Sinter ing o f Suppor ted Meta l Ca ta lys tsSIEGHARD E. W A N K EDepartment of Chemical EngineeringUniversity of Alberta,Edmonton. Alberta, CanadaPETER C. FLY NNSyncrude Canada, Ltd.Edmonton, Alberta, Canada

I. INTRODUCTION. . . . . . ... . . . . . . . . . . .. . . . . . . . . .11. MEASUREMENT O F DISPERSION. . . .. . . . . . . . . .. . . .. . . . .A. E lectron Microscopy. . .. ... . . . .. .. . . . . . . . .. . . .. . . . . . .B. X-Ray Diffraction.. . . . . . . . .. . .. . . . . .. . .... ..C. Selective Gas Adsorption . . . . . . . . . . . .... . . . . . . . ... . . .D. Comparisonof Techniques. . . . ... . ... . . . . .. ... ... . . ..

111. EXPERIMENTAL DATA AND EMPIRICAL CORRELAT IONS . . .A. Variable T ime-V ariable Temperature Sinteringof Supported Pt. .B. Constant Time-V ariable Temperature Sinteringof Supported Pt .C. Constant Temperature-V ariable Time Sinteringof Supported Pt .D. Thermal Treatmentof Supported Pt Resulting in Redispersion ..E. Sinteringof Supported Rh, Pd, and Ni Catalysts . . . . . . ... . . .F. Miscellaneous Sintering Results . . . . .. . . . .. . . . .. . . . .. .G. Empirical Correlation of Sintering Data . . . . . . .. . . . . . . . . . .H. General Comments on the Factors Affecting the RateofSintering............................................N. MECHANISTIC MODELS OF THE SINTERING PROCESS .. .. . .A. Support-Metal nteractions. . .. . . . .. . . . . .. . ..B. Crystallite Migration Model . . . . .. . . . . . . .. . . .. . . . . ...C. Atomic M igration Model. . . . . . .... . . . . .. . ... . . . . . . . . .D. Comparison of Crystallite and Atomic Migration Models .. . . ...

93

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Copyright 0 1975 I iy M.ircel Drkker, Inc. A ll Rights Hcserved. Neither this worh nor any partmay be reproduced or transmitted in any form or by any means. electronic or niechanical. i ncludingphotocopying. inicrofi lming. and recording. o r hy any i nformation storage and retrieval system.without permission in writing f rom the publisher.


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94 WANKE AND F L Y "V. CONCLUDINGREMARKS ............................... 128

APPENDIX A.DISPERSION .......................................... 129TIONENERGIES ....................................... 130REFERENCES ......................................... 132

CONVERSION OF REPORTED DATA TOAPPENDIX B. CALCULATION OF APPARENT ACTIVA -

I. INTRODUCTIONMetal catalysts are commonly employed in the form of metal dis-persed as small crystallites on high surface area supports. Theuseof these supported metal catalysts increases the util ization of themetal as a catalyst since a large fraction of the metal atoms are atthe surface of the small metal crystallites. Another important func-tion of the support is to physically separate the small metal crystal-lites and thereby hinder the agglomeration of the small metal crys-tallites into larger crystallites. This agglomeration would decreasethe number of surface metal atoms per unit mass of metal, andthereby decrease the utilization of the metal and the activity of the

catalyst.metal particles, growth of the metal crystall ites stil l occurs, es-pecially i f the catalyst isused at elevated temperatures. The pro-cess by which this occurs is generally referred to as catalyst sin-tering or aging. (Note this use of the term sintering isdifferent thanthe usual use of the term which refers to the agglomeration of par-ticles in physical contact.) In this paper the term sintering wil lrefer to any process which leads to achange in the metal particlesize distribution in supported metal catalysts. This change can in-clude increases aswell as decreases in the average metal crystallitesize. The situation where the term sintering will be used in the nar-rower sense of fusion of particles in contact wil l be clearly stated.The aims of this paper are: (1) to review the experimental re-sul ts of sintering of supported metal catalysts, (2 ) to present themethods used to correlate the sintering data, (3) to discuss themechanistic models proposed for the sintering process, and (4) topresent some suggestionsas to the typeof work which would increaseour knowledge of the sintering process and thereby lead to the devel-opment of catalysts with increased stability. In order to achieve theabove objectives it is necessary to describe the techniques used ininvestigating the sintering phenomenon. The experimental resul tspresented will be restricted to supported Group VII I metals; themajority of thedata being for platinum supported onalumina or silica.

Although the use of supported metal catalysts stabilizes the small


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SINTERING OF SUPPORTED METAL CATALY STS 9511 MEASUREMENT OF DISPERSION

Thermal treatment of supported metal catalysts resul ts in changesin metal surface area and average metal particle si ze. Hence it isrequired that the metal surface area and/or the metal particle sizebe obtained as a function of treatment conditions. In this paper themetal particle size will be expressed in terms of dispersion. Dis-persion is defined as the ratio of surface to total metal atoms.V arious methods and procedures have been reported in the li teraturefor obtaining the dispersion of supported metal catalysts; each tech-nique has advantages and limitations. I t is not the aim of thisworkto review the various methods in detail, as this has been done else-where [l-31. However, a brief description of the various methodsused to monitor changes in dispersion with thermal treatment, alongwith their l imitations, ar e required for subsequent discussion of ex-perimental sintering studies.A. Electron M icroscopy

For the characterization of supported metal catalysts, conven-tional bright-field transmission electron microscopy (TEM) is fre-quently employed. In TEM a thin specimen of the substance to beexamined is placed in an electron beam and the transmitted electronsare focused to form an image. The contrast in the image is due todiffraction contrast for large particl es and phase contrast for smallparticles [4]. While resolution is high, potentially down to singleatoms [5], the nature of the image generation for smal l particles(i.e., phase contrast) leads to major uncertainties as to the si ze anddetection of particles below -2.5 nm in size [4].Other problems associated with TEM investigations of supportedmetal catalysts include (1)the interference of the support and (2 ) rep-resentative sampling of the surface. The interference by the supportcan be eliminated by dissolving the support [6, 71. The dissolution ofthe support may result in an alteration of metal particle si ze distribu-tion since some of the metal parti cles may also be dissolved. Theproblem of obtaining a representative sample can be partly overcomeby measuring a large number of particles, but even measuring inexcess of 1000particles can result in a signif icant statistical err or

Electron microscopy, other than TEM, may also be used to char-[81.acterize supported metal catalysts. Rather than employing bright-field imaging, dark-field imaging may be employed. Thi s methodmay be used to obtain information on the crystal structure of thesupported metal parti cles [9]. If the metal particles are large(>lo0 nm), scanning electron microscopy (SEM) may be used to


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96 WANKE AND FLYobtain metal particle size distributions. For example, Huang and L i[lo]used SEM to study the growth of platinum particl es on variouscrystal faces of alumina. (For adescription of SEM, seeK imota andRuss [l l ].)A lthough the various electron microscopy techniques involve diffi -culties in interpretation of resultsand are limited in their applica-bility, they are very useful in the characterization of supportedmetal catalysts since they aretheonly direct methods of examin-ing the metal particles. I t should bekept in mind that the limitationsof TEM for examining highly dispersed supported metal catalyst(average metal particle si ze < 2 nm) makes this technique quali tativerather than quantitative.

B. X-Ray DiffractionM etal particle sizes can be obtained by two different methodsusing x-ray techniques: (1)by x-ray diffraction line broadening and(2) by small-angle x-ray scattering (SAXS) .The prelqence of small crystall ine particles in a sample beingexamined by x-ray diffraction causes abroadening in the diffractionlines. T his broadening can be related to the si ze of the particles.

A detailed description of x-ray diffraction line broadening is givenby K lug and A lexander [12], and Dorl ing [3] has reviewed the appli-cation of this method for the characterization of supported metalcatalysts. This technique is generally limited to detecting crystal-line particles with a si ze greater than 2 to 4 nm[13, 141, althoughA dams etal. [15], using a special spectrometer, detected muchsmal ler particles. The difficulty in detecting small particles l imitsthe use of this technique for characterization of well-dispersedcatalysts.to discontinuities in the electron density between the particles andthe surrounding medium (the particles need not be crystalline).SAXS has been used for several decades to determine particle si zein the range of 1to 50 nm [16], but its application to supported metalcatalysts is relatively recent. The main reason for not applyingSAXS to supported metal catalysts is that the low-angle scatteringby the porous supports obscures the scattering by the metal par-ticles. Somorjai [17] eliminated the support interference by com-pressing the catalyst samples at extremely high pressures to col-lapse the pores. Recently, pore maskants of electron density simi-lar to the support, such as CH& have been used to eliminate sup-port interference [8]. SAXS allows thedetermination of metal par-ticle size distribution, and it is potentially a very useful tool forcharacterization of supported metal catalysts.

Small particles in an x-ray beam will also scatter the x-rays due


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SINTERING OF SUPPORTED M ETAL CATAL Y STS 97C. Selective Gas A dsorption

By far the most common method for measuring metal dispersionsof supported metal catalysts is chemisorption. The method consistsof measuring the amount of gas chemisorbed by the metal in thecatalyst and converting this quantity to a metal dispersion by assum-ing an adsorption stoichiometry (adsorption stoichiometry is definedas the ratio of the number of adsorbate atoms or molecules adsorbedper surface metal atom).Several reviews of the use of chemisorption for determining metalsurface areas for supported metal catalysts exist [ l , 2 18- 20] .Therefore, the methods employed will not he discussed here, andonly a few general statements of caution when interpreting chemi-sorption resul ts wi ll be made. A s mentioned above, the conversionof chemisorption uptakes to metal dispersion requi res an assumptionof the adsorption stoichiometry, and in order for the adsorption tech-nique to be an absolute measure of the metal surface area, the cor-rect adsorption stoichiometry has to be used. This is not a seriousproblem, since as long as the adsorption stoichiometry is constant,i.e., it does not depend on metal particle size, catalyst pretreatmentconditions, etc., theobtained surface areas are correct on a relativebasis. There is evidence that adsorption stoichiometries may varyconsiderably with metal particle size [ 21- 241. This can lead toserious difficulties since the changes in adsorption uptake arenotdirectly proportional to the changes in dispersion i f the adsorptionstoichiometry isa function of the dispersion. For example, the ad-sorption stoichiometry for oxygen atoms on supported platinumcatalysts has been reported to be -0.5 for small P t crystals ($1.5 nm)and -1.0 for l arger P t crystals ( >2. 0 nm) [21, 231. Hence, i f oxygenchemisorption were used to monitor the change of dispersion of sup-ported platinum, the situation occurs where growth of metal parti cles(decrease in dispersion) is not reflected in a proportional decrease inthe amount of oxygen adsorbed.The majority of the sintering studies, results of which will bepresented in Section ID employed chemisorption for measuring themetal dispersion. The choice of adsorbates, adsorption and pretreat-ment conditions, and measuring techniques varied greatly among in-vestigators. In general, each investigator chose conditions which hebelieved would result in a measure of the metal dispersion. I t is be-yond the scope of this arti cl e to analyze the validity of the assump-tions, with regard to the adsorption stoichiometry, that the variousinvestigators used. In Section I 11 the adsorption conditions used bythe various workers investigating the sintering of supported metalcatalysts wi ll be presented. When interpreting these results, vari a-tion in adsorption stoichiometry should be kept in mind.


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98 WANKE AND FLY"D. Comparison of Techniques

The above sections have emphasized thedisadvantages and thelimitations of the various techniques used to measure metal disper-sions. Thepicture presented above is overly pessimistic sincenumerous studies (e.g., Refs. 8, 14, 21, 25) have compared metalparticle sizes determined by various techniques. The results areoften in excellent agreement. Poor agreement among the techniquesresults if the metal particle size distribution is broad or bimodal[8, 22, 26, 271. This isof concern in sintering experiments sincesintering frequently results in abroadening of metal particle si zedistributions.

111 EXPERIMENTAL DATA AND EMPIRICAL CORRELATIONSThere are few studies reported in the literature in which time andtemperature were systematically varied to determine the influenceof thermal treatment on the metal surface area of supported metalcatalysts. In many studies (e.g., Refs. 13, 28-31) thermal treatmentswere employed to change the metal crystall ite size in order to deter-

mine the effect of metal crystallite s i ze on adsorption uptakes and/orrates of reaction. In these studies, however, the rate of metal par-ticle growth as a function of treatment conditions was not of interest,and hence the conditions used are often not described in detail. Evenin studies where treatment conditions arepresented in detail, thedataare difficult to interpret because1. the problems associated with the measurement of metal dis-persion (Thi sproblem has been discussed in theprevious sec-tion.);2. the support material may undergo changes, such as col lapse ofthe pore structure resulting in the trapping of metal within thesupport (Thisproblem isparticularly severe i f silica is usedas the support.);3. the stateof the metal (elemental, oxide, or salt) during treat-ment, or at least for part of the treatment, is not known.In spiteof these limitations, general conclusions of the effectofvarious treatment conditions on metal sinteringwill be made. In

order to facilitate comparisons among thevarious studies, all thereported dataon metal surface area, particle size, etc. are con-verted to dispersion. The methods used for these conversions aredescribed in detail in Appendix A.


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SINTERING OF SUPPORTED M ETAL CATALY STS 99In the following section (II I-A to I I I-F) experimental results on thesintering of supported catalysts is presented. The analysi s of these

data arepresented in Section III-G (empirical correlation of the data)and III-H (general conclusions regarding rate of sintering).A. V ariable T ime-V ariable Temperature Sintering of Supported Pt

The main factors affecting the rate of sintering of a specific cata-lyst are the temperature, time, and atmosphere. Herrmann et al.[ 32]were among the f i rst to report adetailed study of the changesin Pt surfacearea for Pt/AhO, ca& lysts treated at various temper-atures and periods of time in a nitrogen atmosphere. T heir resultsfor the two extensively studied catalysts are summarized in Table1.Hydrogen chemisorption at 200C and 9 T orr was used to measurethe Pt area. The measured hydrogen uptakeswere such that adis-persion greater t han uni ty resulted for fresh catalysts i f an adsorp-tion stoichiometry of one hydrogen atom per surface Pt atom wasused. Thi s was probably due to the short reduction time ( 20sec at500C) and the long evacuation time (overnight) which could have re-sulted in surface contamination with oxygen. For this reason, thedispersions reported in Table 1arenormalized with respect to thedispersion of the fresh catalysts, i.e., D/D, is the ratio of thedis-persion of the sintered to the fresh catalyst.ducing atmospheres at 600and 700Cfor periods of up to 96hr onthe average metal particle size for a 5% Pt/AbO, catalyst. SAXSwas used to determine the average Pt crystal l ite size. T he resultsof this study are summarized in Table 2 (T he values reported inTable 2 were obtained from F igs. 5 and 6 in Ref. 17.)Recently, Bett et al. [ 33] investigated the sintering of P t/carboncatalysts in nitrogen and hydrogen at temperatures of 600, 700, and800C. They measured the Pt area by an electrochemical technique[34]. The results of this study are summarized in Table 3 (valuesobtained f rom Figs. 1and 5 in Ref. 33). The methods of catalystpreparation are described in Table 4.in Pt dispersion of a 0.4% Pt/AhO, catalyst caused by treatment inhydrogen at 900and 1000F for treatment times of up to 1000hr.They found that theCO adsorption uptake, U, as a function of treat-ment time, t, could be correlated by a function of the form (seeFig. 9, Ref. 35)

Somorjai [17] reports theeffect of treatment in oxidizing and re-

Hughes et al. [ 35] used CO chemisorption to measure the changes

U =atb (1)where a and b are constants at a fixed temperature. A ssuming that


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100 W A W AND F L Y "TABLE 1

Effect of Thermal Treatment in Nitrogen on Dispersionof Pt/Al,03 Catalysts [321Treatment conditions

_ _ _ _ _ _ _ _ _ _Hydrogen adsorptionuptake, cc (m)/g ofCatalyst description Temperature, "C Time, hr catalyst D/Do

0. 375%Ptiy-Al, 0,Support area =176m2gCommercial reformingcatalyst

-0. 7748P t / ~A I , 03 Fresh 0.68 1.00Support area =225 mz gPrepared by impregnation 564 44 0. 431 0. 63with H PtC1, solution 70.5 0.372 0. 55

167 0. 292 0. 43353 0. 116 0.17594 24 0. 326 0. 4848 0.215 0.3293 0. 135 0. 20625 4 0. 355 0. 528 0. 141 0.2118 0. 123 0. 1840 0. 080 0. 12

Fresh 0. 300 1.00564 44 0. 136 0. 4547.5 0. 057 0.19' "70. 5 0. 045 015167 0. 051 0. 17353 0. 030 0.10594 24 0. 117 0.3948 0. 077 0.2693 0. 043 0.14625 4 0. 088 0.298 0.035 0. 12

18 0.030 0.1040 0. 023 0. 08_ _ _ _ _ _"These two data points do not fit the general trend, i.e., 48 hr treatment at 594C

causesa smaller lossin dispersion than 47. 5hr treatment at 564"C, and 167 hr treatmentat 564 causes a smaller loss in dispersion than 70.5hr treatment at 564. Two other datapoints have been omitted because the original workers assumed them to be incorrect.CO adsorption occurs asoneCO molecule per Pt surface atom, aquestionable assumption [36], the results in Fig. 9 (of Ref. 35) canbe expressed as dispersion, D, as function of time. This yields

D :0.73t-O*l3 at 900F


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SINTERING OF SUPPORTED META L CATALY STS 101TABLE 2

Effect of Temperature, T ime, and Atmosphere on Dispersionof 5%Pt/r)--Al,O, [171 (area of support not given)Treatment conditions

Average PtparticleTemperature, T ime diameter,Atmosphere C hr nm Dispersion-Air

H, (or H,/CO)

~

600

700

600

700

1624481362448136244896136244896

12. 420. 023.625.422.027. 029.632.934.310.817. 018. 420. 421. 422.113. 618.620. 022.423. 424. 4

0. 0820. 0510. 0430.0400. 0460. 0380. 0340. 0310. 0300. 0940. 0600. 0550. 0500. 0480. 0460. 0750. 0550. 0510. 0450. 0440. 042

andD = 0.67t-0.4 at 1000F (3)

with t in hours. The results expressed by Eqs. (2) and (3) are validfor 2 t 6 1000hr.B. Constant T ime-V ariable TemDerature Si nterina of Sumorted Pt

A common method of obtaining catalysts with varying metal di s-persion is to treat a f resh catalyst sample at dif ferent temperaturesfor a fixed period of time. In T able 5 the resul ts of a number of such


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102 WANKE AND FLY"TABLE 3

Effect of Thermal Treatment on Dispersion of Pt/Carbon Catalysts[331Catalyst description' Treatment conditions

M ethod of Temperature, T ime,Composition preparation Atmosphere "C hr Dispersioh5%Pt/carbon Ih Fresh 0.31

N1 600 0.5 0. 301 0. 306 0. 2916 0.2730 0.2365 0.2490 0. 21700 1 0. 242 0. 216 0.2116 0.19800 03 0.231 0. 21

2 0. 1812%PUcarbon VII Fresh 0. 27

N* 600 3.5 0.268 0. 2616 0.2423 0.1946 0. 1996 0. 1920%Pt/carbon Ih N2 Fresh 0. 26600 05 0. 1615 0. 162 0. 166 0.1616 0.1465 0. 13

H i 600 0.5 0.162 0.156 0. 1616 0.13

aCarbon support was graphitized carbon (Vulcan XC-72;C abot Corp.) with a surfacearea of 80 ml/g.


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SINTERING OF SUPPORTED METAL CATALY STS 103TABLE 4

Methods of Catalyst PreparationMethoddesignation Description of preparation method

Iabdefgh

C

I 1I11IVV

ab

VIVI I

Impregnation with aqueous solutions containing the followingdissolved sal tsH2PtCl,Pt(NH3 )2(N02 zWNH, ),(OH),RhCl,H2PdC14H2PtCI, followed by treatment with H2SPt(NH3)z (NO,),Ion exchange with Pt(NH,); ;ref. 38Colloidal PtS deposited on Al(OH), in aqueous suspensionPt vacuum deposited onto y-A l,0, microcrystalsCommercial catalystsCyanamid Ketjen K atalysator, CK 306Engelhard,Lot 18-381Cogellingof alumina sol and H,PtCI , solution by addition ofaqueous ammoniaDeposition of colloidal Pt

Ni(N0, h

studies are summarized. The methods of catalyst preparation(column3) and the methods of measuring the Pt dispersion (column8)aredescribed in Tables 4 and 6, respectively.A more detailed description of the treatment methods presentedin Table 5 is necessary since this wi ll influence subsequent conclu-sions. Additional comments, generally relating to the stateof themetal during thermal treatment, are tabulated in Table 7.

C. Constant Temperature-V ariable T ime Sintering of Supported P tAnother method to vary the metal dispersion of supported cata-lysts is to treat catalysts at an elevated temperature for variousperiods of time. Results of such studies are summarized in Table8.

Gruber [42] carried out studies of this type with two Pt/q-A bO,catalysts. One catalyst, containing 0.7% Pt, was prepared by im-pregnation with Pt(NH,),(OH)2; theother one, containing 0.6% Pt,was prepared by impregnation with H,PtC!&. The areaof the support


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106 WANKE AND FLY"TABLE 6

Description of Methods Used for Measuring DispersionsMethoddesignation

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Description of method for measuring dispersionsHydrogen chemisorption using the following conditions:Flow system at 200Cwith addition of pulses containing5.7% H2 in N2Static system at 25C using extrapolated, zero-pressure uptakeas monolayer coverageFlow system at 0Cwith continuous addition of 0.13%H2 inargon streamStatic system at 250Cusing uptake at 100Torr as monolayercoverageFl owsystem at room temperature with addition of pure H2pulses to nitrogen carrier gasStatic system at 70C using the uptake at 1.0Torr as mono-layer coverageStatic system at -78C using the uptake at 250Torr as mono-layer coverageStatic system at 200Cusing uptake at9Torr as monolayercoverage (only 10sec reduction of catalyst at500Cand10Torr before adsorption measurement)Oxygen chemisorption in a flow system at room temperaturewith additionof pureO2pulse into helium carrier gasCarbon monoxide adsorption using the following conditions:Static system at room temperature with uptake after evacua-Flow system at room temperature with continuous addition ofHydrogen titration of adsorbed oxygen at 25C in a flowSmall-angle x-ray scatteringTransmission electron microscopyX-ray line broadening

tion with Toepler pump taken as monolayer coverage0.5%CO in helium.system

was 200 m2/g. The thermal treatment was carried out in hydrogenat 500C for 1to 82 days. The Pt dispersion was measured by COadsorption at 25C using pulse addition of CO to ahelium carrierstream. The results were correlated by Eq. (1).The results, ex-pressed asPtdispersion as a function of treatment time, are (from


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SINTERING OF SUPPORTED METAL CATALYSTS 107TABLE 7

Comments on Results Presented in Table 5Probable state metalRef. during thermal treatment Comments

8 Elemental Pt Samples were reduced in H2at 500C for 3hrbefore thermal treatment

38 Elemental Pt

37 Pt salt during early stages oftreatment and oxidizedPt during later stagesSamples were only mildly reduced in hydro-gen (2OO0C, length of time not given) be-fore thermal treatment in air. The resultsindicate that catalysts prepared by M ethod

l b were probably not reduced, and the lowdispersion resulted from the agglomerationof the saltSamples were reduced in H2at 500C for 1hrand subsequent treatment was in H2. t wasnot clear whether the treatments at 500,700, and 800C were carried out sequen-tially on the same sample, or whether adifferent sample was used foreach treat-ment temperature. A decrease in the silica

support surface area of 20%occurred dur-ing the 800C treatment39 Oxidized Pt40 Elemental Pt for catalyststreated inH2 xidizedPt for sample treated inair

Samples were oxidized prior to sintering in ai rSamples were reduced in H2at >400C be-fore thermal treatment. The length ofthermal treatment is only approximate

Fig. 4, Ref, 42): for the 0.6% Pt/q-A&O, CatalystD =0.38t-'"' (4 )

and for the 0.7% Pt/q-A&O, CatalystD =0.465t-0.073 (5)

with t in hours. The results in Eqs. (4) and (5) are for 24 C t C 2000hr.Huang and Li [lo] studied the sintering of large Pt crystals (>150nm) on various crystal faces of aluminum oxide (sapphire) at 900Cin air at 1/2 and 1atm. They used SEM to measure Pt particle sizechanges asa function of treatment time. Treatment times of up to


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SINTERING OF SUPPORTED METAL CATALY STS 1094 days were studied. The resul ts were well correlated by a power-law function of the form

- f =kt (6)where i- s the average Pt particle diameter at time t, and f,, is theaverage Pt particle diameter at t =0. The rate constant k varied bymore than a factor of 10, depending on which crystal face of thealumina was used as the support. The rate constant decreased byas much as a factor of 3 when the sintering atmosphere was chargedfrom ai r at 1 atm to ai r at 1/2 atm pressure.

D. T hermal Treatment of Supported Pt Resulting in RedispersionGenerally, treatment of supported metal catalysts to elevatedtemperature (>500C) resul ts in adecrease in metal surface area.Under certain treatment conditions it appears to be possible to causea redispersion of sintered metal catalysts. T his is very desirablefor the regeneration of deactivated catalysts. The patent l i teraturecontains various claims for the regeneration of deactivated reform-ing catalysts; for example, deactivated supported Pt catalyst was re-

generated by treatment at 370 to 550C in an inert gas stream con-taining 05 to 2% 0, [43]. The regenerated catalyst had ahigheractivity than the original fresh catalyst.redispersion has occurred after thermal treatment. The resul ts ofthese studies are summarized in Table 9. It isworth noting that theatmosphere for al l the cases resulting in an increase in dispersioncontained oxygen. For a more detailed description of the treatmentindicated in Table 9, the original references should be consulted.

T here are several studies in the li terature that indicate that metal

E. Sintering of Supported Rh, Pd, and Ni CatalystsM ost of the sintering studies reported i n the l i terature ar e forsupported platinum. Considerably less information is available onthe sintering of other supported noble metal catalysts. I n Table 10some resul ts of the effect of thermal treatment on the metal disper-sion of supported Rh, Pd, and Ni catalysts are presented. Sinteringstudies of catalysts in which the metal made up more than 40% ofthe total catalyst mass (e.g., Refs. 26, 53, 54) arenot included inTable 10, since these catalysts cannot be considered supported metalcatalysts.The Rh in the 5% Rh/q-A&O, sample [22, 481 was present as Rhmetal pri or to the treatment shown in Table 10since the samples


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SMTERING OF SUPPORTED METAL CATALY STS 113were reduced in hydrogen at 500C for several hours prior to thesintering in oxygen. The Rh in the 5% Rh/SiO, sample [49] waspresent as RhC1, prior to the ai r treatment shown in Table 10 sincethe dried impregnated catalyst was not reduced pr ior to the sintering.The state of the Rh and the N i in the 0.3% Rh/AbO, and 3% Ni/AbO,prior to the treatment in nitrogen is not known. The two Pd cata-lysts [50] were calcined at 500C before the treatment in hydrogen.Since the treatment temperatures were 2400"C, the oxidized Pd wasprobably converted rapidly to elemental Pd. The N i in the 10%Ni/AbO,-SiO, [51] and 6.7% Ni/SiO, [52] catalysts was present as thenitrate at thebeginning of the treatment. The treatment in hydrogenat 370"C, according to the authors [51], resulted in complete reduc-tion.

F. M iscellaneous Sintering ResultsNumerous miscellaneous observations of changes in catalyticactivity due to thermal treatment of supported metal catalysts canbe found in the l i terature. A rmstrong etal. [29] studied the effectof thermal treatment at 1000 to 1200C in steam on the ignition tem-perature of 1%0,-3% H2-95% He mixtures by supported Pt, I r, Pd-

Ru, Pd-Pt, Pd-I r, Ir-P t, Pt-Ru, and Pt-Rh. It is diffi cult to inter-pret the results in terms of loss of dispersion since signif icant alter-ations in the supports occurred at the elevated treatment tempera-tures. It is interesting to note that for some of the catalysts (Pt-Rhand Pt-Ru) steaming at 1000 to 1200C resulted in an increase inactivity.Conflicting results are reported on sintering in vacuum of re-duced supported Pt. Boudart et al, [55] found that treating a 1%Pt/carbon catalyst at 900C for 16 hr in vacuum did not resul t in a lossof dispersion for one sample, but decreased the dispersion by a fac-tor of -3 in another case. They attributed this difference in behaviorto a poor vacuum in the later case, and concluded that sintering invacuo at temperatures of up to 900C does not cause a loss of dis-persion. This is in agreement with the results of Spindler [56] whostates that treatment in high vacuum at 800C results in a metal dis-persion which is essential ly the same as that obtained by only reduc-tion in H, at 500C. These observations do not agree with the resultsof Renouprez et al. [8], which are reported in Table 5, since they ob-served a loss of dispersion for sintering in vacuo at temperaturesas low as 600C. There is general agreement that treatment in hy-drogen for prolonged periods (>15 hr) at temperatures >500"C re-sults in a loss of dispersion for supported Pt catalysts [25, 551 (alsosee data in Tables 5 and 8).


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114 WANKE AND FLY"The effect of thermal treatment of impregnated, unreduced cata-lysts is not well defined. Wilson and Hall [21] found that calcination

of adried unreduced 0.75% Pt/AbO,, prepared by impregnation withH , P Q solution, in pure nitrogen at 600C for 4 hr did not reduce thePt dispersion. Calcination of unreduced, supported Rh in air at538C for 4 hr caused the Rh dispersion to decrease by a factor of 2[49 .Unexpected growth ratesof Pd particles supported on charcoalhave been observed at temperatures below 50C. Pope et al. [57] ob-served a 30% loss of Pd surface due to reduction of a 10% Pd/char-coal catalyst at 25C for 2 hr. Brownlie et al. [58] employed a Pd/charcoal catalyst, prepared by vapor deposition of Pd, for the hydro-isomerization of 1-butene. They observed, by TEM, a change inaverage Pd crystall ite si ze from 14 nm for the fresh catalyst to130 nm for the catalyst used for reaction. The reaction temperaturewas 43"C, the feed consisted of a 1:l 1-butene:hydrogen mixture,and the total initial pressure was 100 T orr. L ittle particle growthwas observed for another Pd/charcoal sample which was preparedby impregnation with palladous chloride and exposed to the same re-action condition.V arious investigators report on regeneration techniques withoutspecifying the conditions. Spindler [56] states that complete redis-persion of Pt catalysts on clay supports canbeobtained by treatmentin air or other media. Blume et al. [59] states that the removal ofcoke from reforming catalysts can be accompanied by a redispersionof the Pt if proper conditions are chosen. No specific detai l of theregeneration conditions ispresented. Emelianova and Hassan [30]report that Pt redispersion occurs i f thermal treatment at 400 to650C was followed by rapid cooling to room temperature. If thecooling was carried out slowly, no redispersion was observed.Plank et al. [60] sintered commercial Pt/A&O, catalysts in K,hydrocarbons, and oxygen. They never observed a redispersion ofPt. They found that hydrocarbon and hydrogen atmospheres resultin a slow change in metal dispersion while in oxygen the loss of dis-persion was very much faster, e.g., treatment in H, at 870C for2 hr changed the average Pt crystal l ite si ze from 2.0 to 2.8 nm,while treatment in O2at 760C changed the Pt crystallite size from2.0 nm t o abimodal particle si ze distribution with 65% of the par-ti cles having an average diameter of 3.0 nm and 35% of theparticleshaving an average particle diameter of 42.5 nm. Similar bimodal Ptparticl e size distributions were observed for commercially agedcatalysts. The Pt crystal l ite sizes were determined by x-ray dif frac-tion line broadening.The use of supported noble metals in catalytic automobile mufflers


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SINTERING OF SUPPORTED METAL CATALY STS 115is a new use for these catalysts. The operating conditions encounteredin these catalytic converters can be very severe, e.g., intermittenttemperature in the excess of 1000C arenot uncommon. T hesesevere operating conditions, together with catalyst poisons in theexhaust, limit the lifeof the catalyst. V ery little information on theloss of metal dispersion with use is available in the l i terature. M ostof the reported life studies of exhaust catalysts deal with the catalyststability toward poisons, and the thermal deactivation has beenlargely ignored. The information available [61-641 is generallyqualitative in nature. Substantially more information on the sinter-ing behavior of these catalysts should become available in the nearfuture.

G. Empirical Correlation of Sintering DataIn the previous sections a large amount of experimental data onthe sintering of supported metals has been presented. M ost of theinvestigators discuss their data qualitatively. In this section themajority of the data presented in Tables 1-3,5,8, and 10 wil lbe cor-related by apower-law rate function of the form

-dD/dt = kDn ( 7)where the rate constant k wi ll be assumed to obey the A rrheniuslaw, i.e.,

Wherever possible, both thepower-law order, n, and the activationenergy, E, will be evaluated.sary to have D asa function of time at constant temperature. A tconstant temperature k isassumed to be constant, and Eq. ( 7) canbe integrated to yield

In order to obtain values of n from experimental data, it is neces-

k =+I n @ for n = 1 (9)

where Do is the initial metal dispersion (i.e., at t =0) and D is thedispersion at time t.


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116 WANKE AND FLY"The constant temperature-variable time sintering datacan thenbe fitted by Eqs. 9 and 10, and thevalue of n which results in rela-

tively constant values of k will be thepower-law order. For the twocases where the dispersion as a function of time was given byD=atb (1)

theorder isdetermined by the following method. If it is assumedthat n >5 and D,/D >1.4, then Eq. (10) canbe approximated by

By comparing Eqs. (11)and (1)it can be seen that1n =1- 3

The power-law orders obtained by the methods described aboveare presented in Table11. For many cases a rangeof n is presented,since the values of k were not constant for a specific value of n. In-creasing thevalue of n increases thevalue of k at large t in com-parison to the value at small t. The range of n shown in Table11issuch that at the low value of n thevalue of k at short sintering times(theexperimental times) is larger than the value of k at the longsintering times, while at thehigh value of n thevalue of k at shortsintering times is less than the value at long sintering times.The results in Table 11arearranged according to the atmos-phere in which the sintering was carried out. The general trend isthat the order for reducing atmospheres is larger than for oxidizingatmospheres. The orders in nitrogen atmospheres areapproximatelyequal to those for air. Another trend in thevalues of n is the effectof initial dispersion; higher initial dispersions generally result inhigher values of n. Mechanistic interpretations of the n values willbe presented inSection I V.variable temperature sinteringdata. Combining Eqs. (8) and (lo),one can obtain

Apparent activation energies can be obtained from variable time-

where A t, is the time required to change the dispersion from D, toD at a temperature T,, and A t, is the time required for the samechange in dispersion (i.e., D, toD) at temperature T,. (For ade-


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Documents

Gen Spin 647 the Sintering by Wanke in Catal Revs Sci Eng v 12 Iss 1 Pp 93 135 y 1975