AlPO4—SiO2 catalysts by TEM and XRD

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Surfacer.6 (1982) 227-239 Ebevier Scientific Publishing Company, Amsterdam-Printed s . *

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6ATALYSTS. II. DEiTERMINATION OF OF Rh/AlP04--SiU~ CATALYSTS BY ‘NM AND XRD

iiX&UiA

DTtiPERSION

J.M. CAMPELO,

227

. 1.

AiP04~~i6~R~;,”

METAL

fn The Netherlands

A. GARCIA,

D. LUNA and J.M. MARINAS

of drgan& Chernisfay. Cordoba Uniuersify of Sciences. Corcfoba-5 (Spainl (ReceIved 7 December 1981; accepted in Tmalfoxm 6 Jury 1982) Department

ABSTRACT The appfIcationsOPX-ray diffxaction(XRD) line bxoacfenillgand txanamtilon elmtxon microscopy (TEM) In ddenninlng metal crystallite size and size distribution In Rh/AlPO,SIO, catalystshavinqwide rangesof rhadIuum Ioading were fnvdatigated. The averagecrystatIite diameter. &, at&dmetal eurface ~h’a were obtained from X-ray measurements,and croschecked &Wi the crystallitesfze distributionobsecued by ett&rbn microscopy. The resuIts otatahed in both experimentalmethodswem In agreement.Tile rhodium aucfaceaxea was

Iineartydependent on the mebl loadirqqup to 4 WC%Rh, ThL & a result of B change In the number of mete1particleswithout xemerkablechange in par1kle dimensions (dy from

TBM). Qn increasing the metal loading up to C--5 wt% Rh, a deviatton from linearity is &secved and the metal surface area increaxs mare gradually.

INTRODUCTION

Supported metal catalysts IJZ widely used in petrochemical, chemfcal, and automotive industries. Thus, Ihcdium supported cataJy&s find application in a number of impartant prowls: hydrbgenation of unsaturated compounds [l--4], st&n reftirming of hy&ocarbons [5--S], fuel cell eIectrocataI$sis [Q] ,

Fischemopsch ayritheais[:!O--141 and hydrogenolyais reactions [I& 16]_ -The active metal surfa@ a+:~eaper unit we&ht of met@ affecti the activity and airmetiqies the selecfivitirj cf the catalysts. The specific-metal surface area depends ‘on&e size of thb.n+re&iparticles, Meawrement of the size of me&WC rhodium barticles is the&fo:.ti important. From a kn&&cIge’of this parameter the metal’di&xsicm; defined ti the ratio-of-the nutihtir of surface metal atoms to the total number of metal atims, and, hence, kinetic parameters, can be c&&.$& [q q ; ..;i; .: ! : . ’ . . . . :: -. : ’ -:-TFe Str$dtllre! of the metal stifice, is strongIy modified bjt ‘changesin the size ef thr. ti+l Ory&lIi&; It ig thus impqrtant tii &ablish how catalyst - prop&ti&,ikh &i r&tal~diap&&ii tid‘metal lo~&ling~‘mi&ttiff&t @A metal :part~l~.~~~e~~\t.;i.i z,,; :: .::.:(;:i’_.,!. __-. ._:, : .’ .. :*.I . ..-, : -:.-z+ Variotii’metlii~I8 for

e&matir,& thtt’ifzetif aUppo+d metal pa&&s have been sun+ailired &XdiscUs&d hi &4$literat& [19,201 .‘A &mm& &Mice

220

consists of measuring the mean diameter by hydrogen chemisorption (surface voIume mean) and X-ray diffraction Iine broadening (weight mean) and detarmining the size distribution by transn&sion electron microscopy. rn the case of rhodium, if’has become cXearthat the data for hydrogen, oxygen and carbon monoxide chemisoqtions axe not amenable to interpretations in terms of simple and t:nique surface stoichiometries [l&21-26J. Rhodium appears to be perhaps c,-en =-ore complex than platinum. especially in CO chemisorption. Recently, we have shown the ~fu!ness of the AiPOa and AIPOI-AI~O, and AIPOe--SiOl systems, as support, for dispezsed metals such as Ni [Z6-281, Pd [29] and Pt [30]. In the present paper, we report on a study of the preparation of AIPOaSi& supported rhodium catalysts, This support exhibits a smaller metalsupport interaction [27J. The present study wa undertaken to determine and compare the accuracy of two technic;uesfor the measurement of rhodium metal particles and to determine the eff&s of rhodium Loading on the metal particte size. Because of the uncertain stoichiometry between metal z.tomsand adsorbed mrlleculesIn chemisorption methods, we used X-ray diffmction line broadening (XRD) and transmigsionelectron microscopy ITEM) to determine cry&& Iite size. particle size distribution and morphology. Based on experimental resuIts obtained for AtPOd-SD, catalysts with various Rh contents, the agreement between the two methods used for deterninIng metal dispersion are dixussed,

A se&s of supported rhodium catalysts containing up to 6 w&5% Rh were prqared by impregnation of AIPOa-SiOl (E, ZO0-2w meah, 326 ml g-l) prerioualy caMted for 3 h af 923 K, Impregnation was carried out using an aqueous satution of rhodium trichloride hydrate (Merck) to incipient wetness. The ca&Mystswere.subsequentIy dried at 383 K for 24 h, reduced af 473 K in a dream of hydrogen (99.99976,Ha0 < 3 ppm) at 200 mI minqL, and then cc&d to room temperature under the same hydrogen stream, The deWIs of the preparation of the support ixirvebeen described previousIy [31]* Previous resuI+~1321indicate that, for cataIytic purposes, reduction temperatures in the lange 423-473 K are suffiiient to reduce the rhodium chloride. Hfgher temperatures are unnecessary and may @t in loss of metal surface area by sinte&.g. A reduction as short as lO.min was suificient [32-341. . The rhodium content of the catalysts was determind by atomic absorption sp=trophotometric anaIyt$s.Jt is leporttwi as a ye&M perc&age. The rhodium ccccentration in the cat&y&s was vati@ in the rang0 of 0.26-6 ~6%.

229

A sample of A1poc--SiOt (E) was soaked with thesame volumeof hydrachloric acid solution as thatusedfortherhodiumtrichlorideimpregnation. It was subsequently dried and “reduced” in a hydrogen stream in the same conditions as the imp; egnatcd systems. This was used as “treated” support, ET.

X-my diffiactZon measuremenfrs X-ray diftiaction Iine bxoadening determinations were carried out using a Philips diffractometer witi Fe-filtered Co-K, radiation (A = 1.76026 A). To provide adequate count accumulation for recording the proWe of the rhodium reflections, the instrument was operated with a scanning speed of 7.6Oh-‘. The average crystallite diameter, dv, was calculated from X-ray diffraction patterns using the width of the (111) rhodium peak at half the maximum peak height, using the Scherrer [36] relation: dv =

KX p cos28

(1)

where A is the radiation wavelength and 28 is the Bragg angle. The value of the Scherrer constant, K, depends on the definitions of crystalMe size and profile width, the shape of the crystal!ites and the reflection being examined. Assuming that the crystallites were present as spheres, the K value is 1.1. InsWunental broadening correction was made using the Warren equation (361: f12= (Ba -

b=)

(2)

where S is the experimentd width and & isthe instrumental factor, which appeated to be 0.2Ousing a quartz reference siunpte(190 nm < d < 1000 nm). The analysis of the line broadening for the (111) peak yielded a volume mean diameter, dv, for comparison with TRM results. This technique is limited to particle sizes >3 nm [lQJ,which is confirmed in our case, although &soIution of the support was used in the present study. indeed, for the samples 0.26 and 0.6wt% Rh, no metal peak could be detected, since it could not be distinguished from the background, Specific surface areas, determined from the rhodiuln crystallite sizes (c&, nm), were caiculated using the relation

a,=

6. US

PS

(3)

where p ja the density of rhodium (g ml-‘) and S isthr: surface area per gram of rhodium (m* g”). @ecimen preparation for TEBf inuet@gufion: Since observation of Rh crystaltitesin cataXystsis very difficuIt at low Rh loadings (0.26-l wt%), the Rh crystaliites have been concentrated by dissohing the support with 20% HF/

aatone solution. Wi#A this procedure?,#he detection’bf Rh tiettli is a&y possible when the metal toadIng is over 0.6 wt% This treatment, identical to that used in preparatton of extmctive replicas, does not affect‘ Rh particte size.

TEA2measswremerr ts Election micrograpbs were taken with a Philips HM-300 electron t&roscope operated at 100 KV, tith a lesofution of 0.3 NIL An erttracticereplica prop+ dure c37,38] was us~cifor specimen preparation. AU catalyst samples were examined at a tignification of 48,000X. An average of 100~1200 particles from at least eight pfctures were coUnted for each sample using photographs enlarged to a magnifkatidn of 670,~OOX. Accuracy to within 1-0.3nm was obtained for the micrographs analyzed. Expressing the particle diameter as di ami the number of particles in each diameter increment z1.3 ni, the number mean, surface mean and volume mean diameters (d,, & aciddy, respectively) were calculated from the folkwing equations: (4)

Surface areas from dw were d&ermined RESULT3

XRD meawrements

according to eq. (3).

231

Fig. 1. X-ray diffractbn profiles of supyort (E), “treated” suppurt (ET) and 6 wt% Rh/ auppotted pyatzm (5% Rh).

AlPO,*tO,

timpariscn

of average diameters for duppotted syyaterm

232

Pig. 2.

Etection micragrapb

of 0.25% Rh/AIP04-SIO,.

TEA? siudtes U&g TEM we were able to measure crystallite size and size dtitribution; in addition, we could observe morphological characBristics such rrsshape and den&y of Rh crystals and could even determine whether the onetal pa&i&s are randomly distributed or whether they axe concentrated in ch~sters. After dissoIution of the support, these latter assumptions aze not accurate. Particle size distribution arzd an average particle size can be determined, since the TEM data allows us to assume a spherical geometry of the metal particle. Representative TEM micrographs for all the catalysts are shown in Figs. 2-S The samples were generally very easy to analyze with the electron

233

q~q

0

°-I E~ t~ O

0

.i

i,

234 B, ~ ' "

.¢2 {s~3 I O.q

c~

e .o i=t

¢t

o !

p.

0

it~ o

Fig. 7. Electron micrographof 4% Rh/AlPO,-SiO:.

0.15%

0.51

Rh

Rh

1%

ib,

h

---+-i7. Fig. 9. Size

diptribufion

J

af Rh arystilites

in 0 26-l

Sh

wt% Rh/AU?O,--SiO,

I

catdysfa

TEM micrographs.

2%

3% Rh

Rh

I&8. SfRh

. .:

--

.:.

.

from

1

L

3

4

c

w8%Ah

Pig. 11. Metal surface area as a function af the rhodium Ioading.

microscope due to the extra&iv@ replica procedure used in specimen preparation.

The particle size distributions were obtained by tabulating the number of Farticles in a.specific range. Histograms, that were prepared by expressing the number of the particks in each size range, are presented in Fig. 9 and 10. The histograms reveal that cry&Ute size distribution is very narrow for catalysts with 0.26-2 wt% Rh. An increase in the metal loading resuks in a RrogzGssively broader crystallite size distribution and a sIight shift to larger crystailite diameters. From crystalhte size distributions, average.cry+llite diameters w&e calculated for the catalysts according to. eqs. (4), (5) and (6). These are compared in T&b16 I with crystalli%e diameters from XRD (t&,) for the studied catalysfs. -The reprducibijity of -the measurement of the average crystallite size by TEM is within akut +lO%.

less than 3.0 nm. However, this behaviour has been p=vioualy

described by Bartholomew and Mustard [40] in Ni/SiOz, Ni/Al,O, and Ni/TiOz catalysts, *here the values of d, from XRD (for those cases where it was possible to obtain data) were typically 30-40% smaller than corresptinding d, values from TEM in relatively well dispersed samples, iuld in better agreement for the moderately or poorly dispersed samples, These authors (401 indicated that dy from TEM data calculated according to eq. (6) involves di to the third and fourth powers, thus gi-ring the greatest weight to the larger particlesCONCLUSIONS

This paper has been restricted to a consideration of the application of highresolution electron microscopy and X-ray diffraction to the study of Rh/ A1P04--Si0~ catalysts, and to our knowledge, this work is the fit quantitative comparison of average rhodium crystallite size determined by TEM and XRD, although similar studies have been reported for supported Pt 1411, Pd 1421 and Ni (431. The agreement betwwn both techniques is satisfactory, despite a difference of 26%, and comparable to the results in the literature. TEM and XRD are therefore adequate techniques for estimating Rh c@tallite size in R!I/AIPOCSiO:, catalysts over a wide range of Rh loading. These conclusions are supported by two obsenrations: (I) reproducibiIity of each-&f these techniques in measuring dB to within aboutl 10%;

(2) the support displays a low degree of tirystalli#ty and does not obscure refletion from Rh crystals by XRD, although this technique is not sensitive to metal loading of 0.26 &I 2 wt% Rh. The crystallite size, &,-of Rh supported Catalysts increases smoothly with the increase of the meti laading kom’ O.OS~to 5 wt%;’ The results obtained by the& t&hniquCs_ Wili be’used to intc-.rpretthe kinetic data of cyclohexene hydrogenation ori’the Rti/AIPOp-SiOz catalysts. that will-b& reported in a frituru ptiper. . ’ . _-

239 11 12 13 14 15 16 17

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