266

CATALYTIC AND ELECTRONIC PROPERTIES OF PLATINUM PARTI-CLES ENCAGED IN ZEOLITE. PARTICLES SIZE AND ENVIRONMENTEFFECTS ON N-BUTANE CONVERSION

TRAN HANH TRI, J. HASSARDIER, P. GALLEZOT and B. UIELIKInstitut de Recherches sur la Catalyse - C.N.R.S. - 2, avenueA. Einstein - 69626 Villeurbanne Cedex - France.

ABSTRACT: The catalytic activity in n-butane conversion of plati-num particles encaged in Y-type zeolites has been studied as afunction of particle size, support acidity, exchange of Ce 3+ ionsand presence of Mo atoms. A correlation has been found between thehydrogenolysis rates and the electrophilic character (e.c.) of themetal evaluated from X-ray absorption edge spectroscopy. It is assu-med that the e.c. of Pt in PtCeY and PtNaHY zeolites modifies theadsorption equilibrium between H2 and n-C4H10 on the surface. Theplatinum is not electrophilic in PUloY zeolite. The high activityof this catalyst seems to be due to a synergetic effect of Pt andHo both participating in the reaction mechanism.

1. INTRODUCTIONThe catalytic properties of metals can possibly be tailored to

obtain a given activity or selectivity by altering their electronicstructure. This happens when the number of atoms involved in a par-ticle decreases from a few tens to a few unities or when additionalperturbations are induced by the particle environment including thesupport. Platinum zeolite catalysts are among the best suited mate-rials to investigate the importance of these effects 1). Thus homo-geneous states of platinum dispersion can be obtained with particlesizes ranging down to nearly atomic scale 2,3,4,5) and the acidic

and redox properties of the support can be easily tuned by changingthe charge compensating cations. Indeed it has been shown that theplatinum particles encaged in Y zeolite exhibit an electron defi-cient or electrophilic character with respect to larger particles4,

6,7,8,9,10)and that their catalytic behaviour can be different fromthat of other supported platinum catalysts 4,6,9,11). In this work,

the catalytic properties of PtY zeolites in n-butane conversionhave been studied as a function of particle size, support acidity,

Properties of Platinum Particles Encaged in Zeolite 267exchange of Ce 3+ ions and presence of Mo atoms in an attempt toestimate the importance of these modifications.

2. EXPERIMENTALThe PtNaHY zeolites were obtained as already described 5,6). The

PtCeY zeolites were prepared in the same way from CeY zeolites. ThePt zeolites were treated to obtain either 2 nm Pt particles occlu-ded in zeolite bulk or 1 nm Pt particles fitting in the zeolitesupercages 5,6) (treatment A and B, table 1). The zeolite composi-tions and nomenclature are given in Table 1. Some of the reducedzeolites were further modified by ion exchanging Na+ or Ce 3+ ionsas described in table 2. The PtMoY zeolite has been prepared byadsorption at 360 K of a known amount of MO(CO)6 vapor on the 10.9%PtNaHY sample. The adsorbed carbonyl was then decomposed at 580 Kunder 300 Torr of hydrogen pressure. The treatment and nomenlatureof the modified samples are given in table 2. The particle sizeswere measured by electron microscopy 12). The Pt dispersion was alsodetermined from the H2 adsorption isotherms at 300 K. The crystalstructure of platinum in the 13%Pt,7%CeY zeolite was determined aspreviously described 13) using the Radial Electron Distribution(RED) method. The electronic properties of platinum have been inves-tigated by X-ray absorption edge spectroscopy using the synchrotonradiation available at the LURE facility (Orsay, France). Reactionsof n-butane have been carried out in a conventional flow reactor atlow conversion. The zeolites (0.02-0.04 g batch) were reactivatedin flowing H2 at 600 K for 3 h. Reactions were performed at atmos-pheric pressure (gas flow rates 0.5-3 ml s-l) and within 560-640 K.

3. RESULTSTables 1 and 2 give the particles sizes and the dispersion used

to determine the turnover frequencies. The RED corresponding to the13%Pt,7%CeY zeolite under hydrogen is given in figure 1.

The X-ray absorption data at PtLI I I edge were analyzed as alrea-dy described 8). The areas of the white lines relative to that of areference Pt foil are given in table 3. The spectra were taken af-ter outgassing at 650 K and H2 adsorption at 300 K. The absorptionspectrum of the 10.9%PtNaHY zeolite was also taken after adsorptionof NH3 and H2S at 300 K.

The n-butane (nC 4) gives methane (Cl), ethane (C2), propane (C3)and isobutane (iC 4) with the same amount of Cl and C3. The turnoverfrequencies Nl, N2, Ni (in h- l) of the reactions, nC 4 - C1 + C3 ;nC 4 - 2C2 ; nC4- iC 4, the total hydrogenolysis rate (NH=Nl+N2),the total reaction rate (NT=NH+Ni), the isomerization selectivity

268

TABLE

Particleb) Alternativesize (nm) Nomenclature(dispersion)2 (60) 2 nm PtNaHY

(100) 10.9% PtNaHY

(l00) 4.4% PtNaHY

0.8 (100) 5% Pt, 1% CeY

0.8 (100) 5% Pt., 4% CeY

O.S (100) 5% Pt, 12% CeY

1 (l00) 13% Pt, 7% CeY

I Pt7.6Na30.9H9.9Y A

II Pt7.6Na30.9H9.9Y B

III Pt3.2Na29.5H20.1Y B

IV pt3.3CeO.6NalS.4H29.2Y B

V Pt3.4Ce3.SNa19H15.SY B

VI Pt3.7Ce12.6Na12.4Y B

VII Pt9.9Ce7.6Nall.7H2Y B

Composition, treatment and nomenclature of the zeolites

~Ple Unit cell composition Treatment a)

a) Treatment A : heating at 820 K under O2, reduction at 820 Kunder H2" Treatment B : heating at 650 K under 02' reduction at600 K under H2. b) Determined by E.M. (± 0.3 nm). Dispersion inpercentage of exposed atoms.

TABLE 2

Treatment and nomenclature of the modified zeolites

Starting a) Particle Alternative~ample Treatment size (nm)sample (dispersion) Nomenclature

lIb II Ion exchange in 1 (100) 10.9% PtNaHY+ Ce~+Ce(N03)3 solution

II II Adsorption and 2 (60) 10% Pt, 5% fuYc decompositionof Mo(CO)6

IIIb III Treated with O.lNb) 1 (100) 4.4% PtNaY b)NaOH solution

Vb V Ion exchange in 0.8 (l00) "5 % Pt, CeY - ce3+NaN0 3 solution

VIII Impregnation of 2 (60) 5 % Pt/Si02aerosil with Ptsalt

a) See table 1. b) I.R. spectra after treatment show the totaldisappearance of v OH bands.

269

TABLE 3

Electrophilic character of platinum particles from X-ray absorptionedge spectroscopy

ISample Particle

size (nm)Relative whiteline area

Pt foilPtNaY b)

(II)10.9%PtNaHYPtHY b)

(lIb) 10.9%PtNaHY+Ce3+

(III) 5%Pt, 12%CeY10.9%PtNaHY+NH310.9%ptNaHY+H2S(IIc)10%Pt,5%MoY

19 1.21 10 25 1.3

33 48 1.61 1.90.8 2.01 0.91 1.62

a) Number of protons per unit cell before and after reduction.b) Results taken from ref. 8 and 10.

TABLE 4

Reaction rates (h- 1) in n-butane conversion at 590 K (activationenergies (kcal.mole-1) in parentheses)

Sample PH /P C N1 (E 1) N2 (E 2) N. NT NH Ni/NT2 n 4 1

I 20 5(30} 3 (36) 5 13 8 0.38

II I 20 13 (26) 6(30) 2 21 19 0.0980 16 7 2 25 23 0.08

III 20 8 (28) 4 (34) 1 13 12 0.08IV 20 29(26) 16(30} 4 49 45 0.08

V I 20 26 (26) 13 (31) 4 43 39 0.0980 44 21 4 69 65 0.06

VI 20 25 (26) 11 (32) 4 40 36 0.10

Illb I 20 6 2 2 10 8 0.2080 1 1 1 3 2 0.33

VIII 20 4 (36) 2 (36) 5 11 6 0.45II a} 20 80 (34) 56(34} 5 141 136 0.03c

a) In PtMoY zeolite the turnover frequencies are calculated withthe total number of atoms (Pt + Mo) and with a 60 % dispersion

270 T.M. Tri, J. Massardier, P. Gallezot, B. Imelik

Ni/NT and the apparent activation energies are given in table 4.Figures Za and Zb give the logarithm of NH as a function of thelogarithm of nC4 and HZ pressures respectively.

471r 2p lrlte2/nm)

0.133I

40000

20000

oo 0.2 0.4 0.6

rlnm)

Fig. 1-(sampletances,

logN

Radial electron distribution of the 13%Pt,7%CeY zeoliteVII). The peak at 0.165 nm corresponds to (Si,Al)-O dis-other peaks correspond to the pt-pt distances.

logNlie (+1,11

2,0

1,0II

.

~III 'VIII

7co.e r

1,S

1,0

Yt-O.OIlI

O,SL :'::- -::':: __

1,5 2.0 2,5 log PH,

Fig. Zb. log NH at 590 K(N in h- l) vs. log PH2 (P intorr). Reaction order in pa-rentheses (P C = 40 torr) •

n 4

1,0ol-------'-----'------'-----'--__

Fig. 2a. log NH at 590 Kvs. log PnC4 (P in torr)orders in parentheses.

4. DISCUSSION4.1. State of platinum in the PtNaHY and PtCeY zeolites

Treatment B (table 1) leads to the formation of platinum parti-cles fitting in the zeolite supercage as previously shown 5,6). Thepresence of Ce 3+ ions exchanged before reduction (samples IV, V, VI)favors the formation of smaller particles (size distribution cente-red at 0.8 nm in PtCeY instead of 1 nm in PtNaHY, table 1). The par-ticle sizes do not change during subsequent ion exchanges (table 2,samples lIb' IIIb, Vb)'

The RED of the 13%Pt,7%CeY zeolite (figure 1) shows that all the

Properties of Platinum Particles Encaged in Zeolite 271

pt-pt interatomic vectors corresponding to the successive coordina-tion shells of the f.c.c. structure are present and regular distan-ces are observed. Therefore the 1 nm Pt particles covered with H2have the normal f.c.c. structure as in the PtNaY zeolite 13). Thereis no direct bonding interaction between the metal and the Ce 3+

ions which otherwise would have perturbed the atom packing. However,under reaction conditions, the structure might be modified by theadsorbed hydrocarbons. The structure has been studied after thereaction of nC4 at 600 K. The distribution function obtained wasidentical with that shown in figure 1. However, during the coolingunder the reagents flow, the equilibrium of the adsorbed speciesis liable to change toward a predominant hydrogen adsorption sothat the regular structure appears again.

The magnitude of the PtLI I I white line is related to the numberof unoccupied electronic states in the 5d and 6s bands thereforefrom the relative white line area given in table 3, the platinum inthe different zeolite samples can be compared in terms of electrondeficiency or electrophilic character (e.c.). It turns out that theplatinum encaged in PtNaHY and PtCeY zeolites exhibits a stronge.c •. In the PtNaHY series, the platinum e.c. is closely related tothe protonic acidity of the zeolite support (table 3). This is dueto an electron transfer from the metal to the electron acceptorsites of the zeolite 7). In the PtCeY samples, the e.c. can be in-terpreted either by a size effect or by the presence of Ce 3+ ions.The first interpretation can be ruled out because the e.c. is thesame in sample lIb obtained by loading Ce3+ ions in the 10.9%PtNaHYzeolite containing I nm particles and in the 5%Pt,12%CeY zeolitecontaining 0.8 nm particles. The e.c. of platinum is therefore en-hanced by the presence of ce3+ ions. Recently, Foger and Anderson 9)reported a similar effect in PtLaY zeolites. The effect of multiva-lent cations on the electronic structure of platinum is probablyindirect because under the reaction conditions, the zeolite is de-hydrated and the cations are in SI or SI' sites (hexagonal prismsand sodalite cages) of the lattice 14). The high electrostaticfield associated with the Ce 3+ cation can modify the charge distri-bution in the tetrahedra of the cage wall and thus can perturbe theelectronic structure of a Pt particle possibly located in the nextsupercage. Edge spectroscopy appears to be a useful tool to monitorthe charge transfer from the metal to the support and also betweenmetal and adsorbates. Thus, the electron donor NH3 molecule adsor-bed on the 1 nm particles in 10.9%PtNaHY produces a large decreaseof the platinum e.c., whereas electron acceptor sulfur atom produ-ces a large increase of the platinum e.c. (table 3).

272 T.M. Tri, J. Massardier, P. Gallezot, B. Imelik

4.2. Conversion of n-butane on the PtNaHY and PtCeY zeolitesThe question may arise whether the reaction proceeds essentially

on the metal or via a bifunctional mechanism. It has been checkedthat the NaHY and CeY zeolites are inactive under present condi-tions. The activities of the Pt zeolites are proportional to thetotal surface area of the metal provided the Pt particles conside-red have the same e.c. This has been checked in the PtCeY series upto 15wt% Pt loading, such a proportionality is unlikely to occur ina typical bifunctional mechanism. Moreover the rates of nC4 hydro-genolysis are comparable to those measured on Pt films by Andersonand Avery 15) who also reported as in the present case a low isome-rization selectivity whereas high isomerization selectivities areusually observed with bifunctional catalysts 9,16).

From figure 2a, the samples can be classified in two groups :catalysts of group 1 exhibit a positive order with respect to thenC4 pressure whereas catalysts of group 2 are showing a negativeorder (although a positive order must exist at very low pressure) .Furthermore, the catalysts of group 2 are more active, and theirapparent activation energies and isomerization selectivities aresmaller than those of group 1 (Table 4). In this study, only twosamples belong to group 1, the 5%Pt/Si02 (VIII) and the 4,4%PtNaHY

++ Na (IIIb). Both have a neutral support since the zeolite acidityhas been neutralized by NaOH and the platinum is no longer electro-philic. On the other hand, the catalysts of group 2 comprise thePtNaHY and PtCeY zeolites where the 1 nm Pt particles are electro-philic (table 3), the larger the e.c., the more marked the charac-teristics of group 2. Thus larger e.c. and higher hydrogenolysisactivities are observed on PtCeY zeolites (samples IV, V, VI, lIb)compared to PtNaHY zeolites (samples II, III). Moreover, the repla-cement of part of the Ce 3+ ions by Na+ ions in the reduced5%Pt,4%CeY zeolite (V -Vb) lowers the hydrogenolysis activity tothe level of the PtNaHY zeolites (II, III, figure 2a). At thereverse, the replacement of part of the Na+ ions by Ce 3+ ions inthe reduced IO.9%PtNaHY, enhances the platinum e.c. from 1.3 to 1.9(II - lIb' table 3) and simultaneously enhances the rate of reac-tion at low pressures and changes the reaction order with respectto nC4 toward higher negative values (II - lIb' figure 2a). All thecatalysts in the PtCeY series (IV, V, VI) have the same catalyticproperties although the amount of Ce 3+ ions varies from 1 to 12 wt%.This indicates that Ce 3+ ions do not have a role per se in thereaction mechanism. The hydrogenolysis rates on sample I, involving2 nm particles in a NaHY matrix, are intermediate between those ofgroup 1 and 2 but the reaction order with respect to nC4 pressure

Properties of Platinum Particles Encaged in Zeolite 273is more like that of group 2. Indeed, the 2 nm Pt particles areelectrophilic since a positive shift of XPS peaks has been reportedin contrast with the 2 nm Pt particles in Pt/Si02 7) Therefore thesupport produces an appreciable effect even in the case of 2 nmparticles.

The preceding discussion has shown that the kinetics of hydroge-nolysis reaction on electrophilic platinum in zeolite has charac-teristic fingerprints. The following interpretation can be tentati-vely proposed. In the case where the Pt particles do not exhibitany e.c. like in catalyst IIIb and VIII, at low nC4 pressure (5-10Torr) and high H2 pressure (~760 Torr), the metal surface is cove-red with adsorbed H2 and the sticking probability of nC4 is weak.The reaction rate is slow and a positive order with respect to nC 4pressure shows up. On the other hand, if the Pt particles exhibitane.c., their affinity for nC 4 should be much higher because the car-bon atoms of the molecules are negatively charged (-0.372 and-0.200 electron on the primary and secondary carbon respectively 17~Therefore a higher sticking probability of nC 4 is expected and theequilibrium of adsorbed species on the metal surface should be dis-placed toward a higher concentration of hydrocarbon intermediatesat the expense of hydrogen adsorption. Accordingly, higher reactionrates are expected at low nC4 pressures. Indeed, figure 2a showsthat the rates of hydrogenolysis reactions on catalysts of group 2have already run past their maximum at the lowest nC 4 pressure in-vestigated. Any increase of nC4 pressure results in a loss of acti-vity so that a negative reaction order occurs. One is led to con-clude that because of the strong metal to carbon affinity, the con-centration of hydrocarbon intermediates on the surface is too largewith respect to that of adsorbed hydrogen so that a deactivationsimilar to a self poisoning occurs. This is not an irreversible poi-soning due for instance to coke formation because once the nC4 pres-sure is decreased, the rate increases along the curves given infigure 2a.

As far as the reaction order with respect to hydrogen is concer-ned (figure 2b) the influence of the platinum e.c. is less markedbut still obvious. At a constant nC 4 pressure (40 Torr), the reac-tion order is positive (n ~+ 1) for all the zeolites up to 60-100Torr of H2 pressure. At higher pressures, the order is slightlynegative (-0.1) for the electrophilic Pt particles (III, V) but ahigher negative value (-0.3) is observed on non electrophilic Ptparticles (IIIb). This different behaviour is consistent with thepreceding interpretation. The sticking probability of nC4 underincreasingly higher H2 pressures remains high on electrophilic

274 T.M. Tri, J. Massardier, P. Gallezot, B. Imelik

platinum and the rate is almost constant whereas both the nC4 ad-sorption and the hydrogenolysis rates decrease on non electrophilicplatinum because of the overwhelming hydrogen adsorption.4.3. Conversion of n-butane on the Pt-MoY zeolite

The PtMoY zeolite (sample IIc) has distinctive features withrespect to the PtNaHY and PtCeY zeolites. The decomposition at580 K of Mo(CO) 6 adsorbed in the 10.9%PtNaHY zeolite containing1 nm particles leads to 2 nm particles (table 2) occluded in zeoli-te bulk. Previous studies 5,12) have shown that the 1 nm particlesare very stable and cannot sinter under the present conditions,therefore the increase of particle size from 1 to 2 nm is probablydue to the addition of Mo atoms onto the Pt particles. One is ledto conclude that Pt-Mo associations are formed. The Pt atoms bondedto the Mo atoms or ions do not exhibit any e.c. as shown by absorp-tion edge spectroscopy (table 3). This very important point hasbeen confirmed independantly by XPS and IR spectroscopy. The elec-tronic state of platinum in the Pt-Mo association is normal unlikethat claimed by Yermakov 18) in Pt-Mo/Si02 catalysts or that pre-sent in PtCeY zeolites.

The catalytic behaviour of PtMoY is also completely differentfrom that of the PtNaHY and PtCeY zeolites (figure 2a, 2b, table 4)Very high hydrogenolysis rates with a maximum as a function of nC4pressure, positive reaction order with respect to the H2 pressureover the range investigated cannot be explained with the argumentsdeveloped in section 4.2. One can suggest that each component inthe Pt-Mo association has a specific role in the reaction mechanismresulting in a strong synergetic effect.

CONCLUSIONSThe platinum particles in Y-type zeolite exhibit an electrophilic

character (e.c.) which depends upon the particle size and to a lar-ger extent upon the particle environment. Thus protonic acidity andmultivalent cations enhance the e.c. of platinum because of an elec-tron transfer from the metal to the electron acceptor sites of thesupport. The e.c. modifies drastically the kinetics of nC 4 conver-sion. The most striking features such as the high hydrogenolysisrate at low nC4 pressures and the negative reaction order can beBxplained by the strong affinity of the electrophilic platinum forthe negatively charged carbon atoms of the nC4 molecule. Even atlow pressures, the nC4 competing with H2 can stick to the surfacebut as pressure increases, the equilibrium is soon displaced towarda saturation of the surface by adsorbed hydrocarbon intermediates.On the other hand, the kinetics of nC4 conversion on the PtMoY

Properties of Platinum Particles Encaged in Zeolite 275zeolite cannot be explained in the same way because the electronicstructure of the platinum in PtMoY is not perturbed. The most pro-bable interpretation is that the 2 nm particles formed by MO(CO)6decomposition on 1 nm Pt particles involves Pt-Mo associations.Unlike the Ce 3+ ions in PtCeY zeolites which acted indirectly tomodify the electronic structure and the activity of platinum, themolybdenum component has a role per se in the reaction mechanismand a synergetic effect occurs.

REFERENCES1. P. Gallezot, Catal. Rev.-Sci. Eng., 20, 121 (1979).2. J.A. Rabo, V. Schomaker and P.E. Pickert, in Proc. of the 3rd

Int. Congr. on Catalysis, Amsterdam, 1964, Vol. 2, p. 1264,North Holland, Amsterdam (1965).

3. T. Kubo, H. Arai, H. Tominaga and K. Kunugi, Bull. Chern. Soc.Jpn, 45, 607 (1972).

4. R.A. Dalla Betta and M. Boudart, in Proc. of the 5th Int. Congr.on Catalysis, Palm Beach, 1972, vol. 2, p. 1329, North Holland,Amsterdam (1973).

5. P. Gallezot, A. Alarcon-Diaz, J.A. Dalmon, A.J. Renouprez andB. Imelik, J. Catal., 39, 334 (1975).

6. P. Gallezot, J. Datka,~. Massardier, M. Primet and B. Imelik,in Proc. of the 6th Int. Congr. on Catalysis, London, 1976,Vol. 2, p. 696, Chemical Society, London (1977).

7. J.C. Vedrine, M. Dufaux, C. Naccache and B. Imelik, J. Chern.Soc., Faraday Trans 1, 74, 440 (1978).

8. P. Gallezot, R. Weber, ~A. Dalla-Betta and M. Boudart,Z. Naturforsch., 34A, 40 (1979).

9. K.F. Foger and J.~Anderson, J. Catal., 54, 318 (1978).10. R. Weber, P. Gallezot and M. Boudart, Pro~ of the 32th meeting

of the Societe de Chimie Physique, Lyon, 1979, in press.11. C. Naccache, N. Kaufherr, M. Dufaux, J. Bandiera and B. Imelik,

"Molecular Sieves II", p. 538, American Chemical Society,Washington D.C. (1977).

12. P. Gallezot, 1. Mutin, G. Dalmai-Imelik and B. Imelik,J. Microsc. Spectrosc. Electron 1, 1 (1976).

13. P. Gallezot, A. Bienenstock and M. Boudart, Nouv. J. Chim., ~'263 (1978).

14. J.V. Smith "Zeolite Chemistry and Catalysis (J.A. Rabo ed.)",p. 3, American Chemical Society, Washington (1976).

15. J.R. Anderson and N.R. Avery, J. Catal., 5, 446 (1976).16. P.E. Pickert, J.A. Rabo, E. Dempsey and V~ Schomaker, Proc. of

the 3rd Int. Congr. on Catalysis, Amsterdam, 1964, vol. 1,p. 714, North Holland, Amsterdam (1965).

17. R. Hoffman, J. Chern. Phys., 39, 1397 (1963).18. Y.I. Yermakov, B.N. KuznetsoV-and Y.A. Ryndin, J. Catal., ~'

73, (1976).

DISCUSSION

D. Delafosse (Univ. P. et M. Curie, Paris)We have studied the interaction of Ce 3+ on Ni 2+ in zeolites

and have found that indeed Ce 3+ influences, firstly the locationof Ni2 + during thermal pretreatment; secondly the redox pro-perties of the zeolite matrix increasing the number of TCNE-

276 T.M. Tri. J. Massardier, P. Gallezot, B. Imelik

radical anions; thirdly the reducibility of Ni 2+, and finally,stabilizes small Ni particles of 7 X in diameter. have thustwo questions.

1) Have you some information on the distribution of PtO andCe 3+ within your zeolite samples?

2) I am not clear about the valence state of Mo. Can youclarify this point? And give some information on the Mo and Ptdistribution in zeolite.

P. Gallezot1) Previous studies have shown that the Pt particles occupy

the supercages under present treatment conditions (Ref. 5, 6,12) and the Ce 3+ ions occupy SI and SI' sites in the zeoliteframework (P. Gallezot, J. Chim. Phys., 68, 816 (1971)). Theinteraction between ptO and Ce 3 + is therefore indirect as sug-gested in our paper.

2) We have studied the PtMoY zeolite with a high resolutionelectron microscope coupled with X-ray microanalysis. It appearsthat the fraction of molybdenum is not constant in the zeolitecrystal whereas the platinum distribution is homogeneous. Thestudy of PtMoY by ESCA shows that part of the molybdenum isoxidized.

G. Leclersg (Univ. Poitiers)1) Some years ago, we studied the hydrogenolysis of saturated

hydrocarbons and we have been surprised to find that, when aPt/A1203 catalyst was treated either with sodium hydroxide orwith ammonium carbonate not only the overall activity for thehydrogenolysis of butane decreased, but also the ratio of therates of isomerization (NI) to hydrogenolysis (NH) increasednotably from a very low value of 0.025 to 0.3, exactly as youreported in your paper for your sample III. We thought, atthat time, that one possible explanation for these observationscould be a poisoning of the catalyst by some electron donorspecies and that isomerization of butane would be less sensitiveto that poison than hydrogenolysis. Do you have another ex-planation?

2) Our kinetic study on Pt/A1203 with 60% dispersion ofplatinum showed that the two kinds of hydrogenolysis of butane,the one giving methane and propane and the one leading toethane production, obey two different kinetic laws; They havenot only different energies of activation, but also differentorders in hydrogen and in hydrocarbon. Did you find similar

Properties of Platinum Particles Encaged in Zeolite 277

results on your catalysts?3) In your discussion you classify platinum catalysts in two

categories; group I with positive order in hydrocarbon, lowactivity and rather high ratio of rates of isomerization tohydrogenolysis (NIjNH), group II with negative order in hydro-carbon, high activity and low ratio NIjNH" But I think thatthe classification is certainly more complex, since we havefound PtjA1203 catalysts with positive orders in butane, butalso with high activity and low selectivity for isomerizationof butane.

P. Gallezot1) Your results on PtjA1203 are very well understood with the

interpretation given in this paper for PtY zeolite. The actionof electron donor species in both cases reduces the electro-phi1ic character of platinum which results in a decrease of thehydrogenolysis activity.

2) There are indeed higher activation energies for the C2bond rupture than for the C1 bond rupture and the selectivity forC2 bond rupture is larger at low nC4 pressure.

3) The classification of the reaction in two groups was usedmerely to describe the present results. The PtMoY catalyst isin agreement with what you describe namely positive reactionorder in butane, high activity in hydrogenolysis, low selectivityfor isomerization.

W.O. Haag (Mobil Res., Princeton)The increased butane conversion with increase in zeolite

acidity would be consistent with a bifunctional mechanism. Theauthors argue against this mechanism partly from the observationthat butane is unreactive on the metal-free zeolites. However,this argument can not be valid, because in bifunctional paraffinconversion it is the metal-generated olefin that is converted onthe acid (zeolite) sites. Specifically, Dr. Weisz in our labora-tory has shown that addition of Pt to H-mordenite increases thebutane conversion, mostly to cracked products, from zero to highvalues under conditions similar to those used in the paper.I would like the speaker's comments on these observation.

P. GallezotWe do not argue against bifunctional mechanism as a whole, we

do not prove by one observation but by many others that thereaction takes place on metal under present conditions charac-terized by high metal loading (5-15 wt %) compared to the con-

278 T.l\l. Tri, J. Massardier, P. Gallezot, B. Imelik

ditions where bifunctional mechanism is usually claimed (0.5wt %). The turnover frequencies reported are in agreement withinvestigations carried out on unsupported metal (Ref. 15) orwith neopentane hydrogenolysis data (Ref. 9) which cannot reactunder the bifunctional scheme, or with platinum on other support(see Dr. G. LECLERCQ comment). We also report a proportionalityof the activity with the total metal surface up to 15 wt % of Ptwhereas bifunctional scheme is usually associated with a plateauat much lower loading. Finally, the fact that the hydrogenolysisrate is increased when Pt is added to mordenite does not provethat it is a bifunctional mechanism; although this may be trueyou should still prove it.

P.B. Weisz (Mobil Res., Princeton)The X-Ray absorption edge measurements are intriguing; it

would be good to explore their relevance to the catalytic be-havior further. The butane conversion (and many others) on suchcatalysts involves an interaction of chemically different sites,via intermediates like olefins at very low vapor pressure. Wehave seen subtle effects also such as the inhibition (reduction)of metal hydrogenolysis by the existence of acidic sites some-where else (1) on the catalyst, thereby effecting apparent re-action order, etc.. I would urge thorough extension of measure-ments to include catalytic behavior due to physically mixed-yetchemically independent components of Pt-Na-Y and Ce-Y, forexample, and others.

P. Ga 11 ezotThe X-ray absorption edge measurements are in agreement with

XPS measurements carried out in this laboratory (Ref. 7) andelsewhere (Ref. 9) on similar materials which also pointed to aplatinum electron deficiency. As far as the possibility of abifunctional mechanism is concerned see the answer to Dr. HAAG.