The nature of active centres in hydrocarbon reactions on Pt catalysts

315

Cemlysis Today 10 (1991) 315-322 Eleevier Science Fubliihers B.V., Amsterdam

TEE NATURE OF ACTIVE CENTRES IN HYDROCARBON REACTIONS ON PT CATALYSTS R Burch and V.Pitchon Catalysis Research Group, Chemistry Department, University of Reading, Whiteknights. Reading, RG6 ZAD, U.K. SUMMARY The reaction of neohexane on various supported Pt catalysts has been studied after different pretreatments. Very large changes in product distribution are ohserved, although the mode of adsorption of the neohexane is essentially unchanged. The Smgle TurnOver technique, using puhcs of I-butene, has been used in an attempt to determine the relative proportions of di&rent sites on the Pt cataIysts after the various pretreatments. It is found that the changes in neohexane &ectivity cannot be accounted for by changes in the numbers of sites Alternative ~~~atio~ are considered INTRODUCTION The nature of the active centres required for hydrocarbon skeletal rearrangement reactions is not known We have presented evidence previously that for titania-supported Pt catalysts only a single metal atom may be required for the activation of n-hcxane and metbylcyclopentane [l,Z]. In an attempt to determine the types of active centres required we have now investigated neohexane as a model reactant. Neohexane is an excellent probe molecule because it can adsorb in three distinctly different modes, and the products obtained can be used to ascertain which type of adsorption has occur&. In principle this provides a sensitive means of monitoring changes in the surface proper&

of small metal

particles. The Sin8le TurnOver technique (STO) developed by Au8ustine and Wagner [3] providea a further means of characterising the surface sites on a metal particje. Augustine defined sites (3&f) capable of adsorbing three ligands, and identified as comer or kink atoms; 2M sites on which 2 species can adsorb, identified as edge or step atoms; and 1M sites on which only 1 hgand can adsorb, identified as face or terrace atoms. In the present work we have combined the two techniques in an attempt to identify and quantity the sites required for neohexane reactions on Pt.

EXP-AL

Catahw 0rc;garation Three catalysts were prepared and used in addition to EUROPT-1 (6.3% ptlsiO,J and EUROPT-3 (0.3% Pt/y-A&O& The catalysts (2 wt.% Pt) were prepared by wet impregnation of titania (Degussa PZS) or y-Al&

(Akzo, CK300) using aqueous sohitions of Pt(NH&Cls or Pt(NO&(OQ

~pHwasad;j~~to8andthewaterwasthen~by~~evaporation.~eca~~

0920-5861/91/$03.60

0 1991Elsavier Sciance Publishers B.V. All righta raaarvad.

316 dried in an oven at 393 K overnight, and then calcined in flowing oxygen by heating at 10 K til 673 K and holding at this temperature reactor or

stored

in

for 2 h. The catalyst was then transferred

a desiccator. Before catalytic measurements,

pretreatmenta,

as summarised in Table 1.

Chemisorution

measurements

Adsorption

of hydrogen was performed

apparatus. Prior to each measurement,

to

into the catalytic

the samples were submitted to various

at room temperature

in a conventional

volumetric

the sample was reduced by heating in flowing hydrogen at 10

K min-’ to the required reduction temperature.

The catalyst was then evacuated at 10d torr for 2 h.

The double isotherm method [4] was used in which two adsorption isotherms are measured, with weakly adsorbed hydrogen being removed by evacuation at room temperature

prior to the second

isotherm measurement. The amount of strongly adsorbed hydrogen was determined by subtraction. The stoichiometry for hydrogen chemisorption

Catalvst

on platinum was assumed to be 1:l.

testin The

skeletal rearrangement

of neohexane was studied using a conventional

differential flow

microreactor operating at atmospheric pressure and 563 K. The sample (200-300 mg) was reduced in hydrogen in &

at the required

temperature

and then cooled to the reaction temperature.

neohexane (50 torr, Fhrka puriss grade) was introduced into the reactor via a thermostatted In all experiments,

the conversions

The

saturator.

were kept below 15%, (generally much lower). Under these

conditions deactivation was minimal and the activity was essentially stable after about 15 minutes on stream. The effluent was analysed on line using a Perkin Elmer S44Klgas chromatograph

equipped with

a capillary column (Chrompack Plot fused silica 50 m x 0.32 mm i.d. column coated with AlsOs/KCI) and a Nelson data handling system. For convenience Cl; ethane, c2, propane, C3; 2-methylpropane, methylpentane,

3MP; 2,3dimethylbutane,

the following abbreviations

lc4, 2-methylbutane,

2,3DMP; neopentane,

are used methane,

iC5; 2-methylpentane,

2MP; 3-

neoC5.

The rate of the reaction is expressed in mmole of reactant converted per gram of Pt per hour. The selectivity Si of each product is defined as: Si = (iCi/ciCi) x 100% where Ci is the number of moles of neohexane converted into a product containing “i” carbon atoms. The Single Turnover

technique (STO) using I-butene

(Aldrich 99%) can be summarised as

follows [3]. First, the surface of a known amount of catalyst is saturated by a pulse of hydrogen, and the excess is removed in an inert carrier gas stream. A pulse of 1-butene is then introduced.

The

products resulting from a stoichiometric surface reaction are analysed on line by gas chromatography. A second pulse of Hz is introduced to remove the partially hydrogenated

species still adsorbed on the

surface. The amount of butane formed from the first pulse of hydrogen with a rem sweep-off time corresponds

to the total number of 3M sites; the 3M, sites are found from the amount of butane

formed after the reversibly adsorbed hydrogen has been removed, the number of 3Mn sites being the

317

TARLH 1. C&&sorption

data and pretreatment

CATALYST

CODE

EIJROPT-1 2PtKi 2PVH

A

0.69 0.63

Wi

:

0.58

2PUTi EUROPT-3 EUROPT-3 EUROPT-3 2Pt/AWc 2pt/AI(cI) 2Pt/Al 2PtfAl

D E I? G I-I I J K

_ o&i 0.62 0.90 0.86

conditions for Pt catalysts.

PRHIRHATMENT H#73/1 hb Oa/673m, H.d57Jllh Oa/673&, H,/173/lh Oa/673m, stored 5 days; Ha/S73/lh Od673/2h, stored 5 days; Hfl3/lh 0a/673/2h; H@73/lh Oa/673& Ha/773/lb Od673/2h; stored 3 weeks; Hd573tlh Oa/673/2h; Ha/57311h 0#673nh; H$773/lh 0a/673f2& H$SWlh Oa/67312h; H&‘73/lh

*afterref.

[5]. bindicates the treatment gas, the temperature, arid the time of treatment. %dicates catalyst prepared using a chloride-containing precursor.

difference between these two values. The amount of cis- and trans-butene the number of sites on which the half-hydrogenated determined

represent the 2Mo sites, and

state, or metal alkyl, are formed, 2Ms, is

by the amount of butane generated from a second pulse of Ha. The number of 1M sites

is determined

from a knowledge of the total quantity of II2 adsorbed and the amount of each site

present, as calculated from the amount of 1-butene which has reacted. (In the terminology used by Augustine et al, the pretIx numbex indicates the degree of unsaturation subscripts isomer&ion

are: I, irreversibly

adsorbed

hydrogen;

R, reversibly

sites; S, sites where adsorbed species are h~gena~

of the metal site, and the

adsorbed

hydrogen;

C, alkene

by a second pulse of hydrogen.)

RFsuLzs Neohexane reaction Neohexane can adsorb via ag, ay, or ay’intermedlates,

leading, respectively, to (Cl + neoCS),

or to (Cl + iCS) + 2-MP, or to 2,3DMEl+ 2MP + (Cl + iCS) + (C2 + iC4). The selectivity patterns and rates of reaction for EIJROPT-1,

EUROPT-3

and our catalysts are reported in Table 2 In only

one case is neeC5, the product of a$-scission, observed in a significant amount. Moreover, 3MP, from ay-isomerisation

ls never detected.

EUROPT-1

is a moderately active catalyst and all the products produced by a single step

originate from an ay’-intermediate.

A fresh Pt/I’iO, catalyst reduced at 573 K is more active. and has

a higher hydrogenolysis

selectivity

~me~tion.

of a fresh catalyst at 773 K leads to drastic changes in activities and

sekctivitles.

Reduction

when compared

with EUROpT-1,

but is leas effective

at

The rate of the reaction is decreased by a factor about 43, and the main reaction is now

318

CATALYST

CODEI Cl

C2

C3

iC4

nC4

neoC5

KS

8C5

2MP A”

2,3DhiB

isomerisation, represented by the formation of 2,3DMB and, to a lesser extent, 2h@. When a PtA’iOz catalyst is stored for 5 days in a deaiceator, reduction at 573 K leads to a remarkable change: the catalyst now shows the same selectivity pattern as a HTR catalyst i.e. high ~me~tion

se&iv&y due to the ~~ation

of 2,3DMB. The activity is deerea& by a Factor of about

4.5, ague

the &emisorption of bin

is hardly ~~

The louger the catalyst is stored the

lower the activity becomes. For exampieS a catalyst stored 1 moath in a desiccator has an activity of ordyI0 mmolg”tht after reduction at 573 K. For the catalyst stored for 5 days, reduction at 773 K totally eliminates the Maiytic act&&yfor this reaction. WROPT-3 reduced at 573 K shows a high hydrogenolysis sehztivity~ and similar behaviour is observed for the 2% Pt/A&O~catalysts. An increase in the reduction temperature to 773 K agaiu leads to marked changes of the properties of the catalyst: there is a shift towards isomerisation, with the selectivity increasing from 10-20 to 54-80%. In contrast to the Pt/TiOz cata@t, there is a large increase iu the activity when the alumina-supported catalyst is reduced at 773 K. (Nate that the activity reported io Table 2 for 2% P&&OS reduced at 773 K has been recorded at 523 K, the conversion being too high at 563 K to give a reliable result.) The effect of storage has been studied for EUROPT-3. After 3 wee& this catalyst shows the same features as P@I102 when compared to a fresh catalyst, ie., a shift towards isomerisation and a decrease in activity, both phenomena being less pronoumxxi in the case of EUROPT-3. when there is no chlorine in the precursor, the catalyst reduced at 573 K gives the saxme features as report& above The se&x&it7 is mainly dominated by the ~a~en~~n

reaction but the

rate is Iower when compared to a Waiyst prepared from a ~h~o~e~n~g

prwr.

reduction at 773 K, the ~rne~ti~~o~o~

After

ratio is hardly changed but the activity is imxeased,

319

TABLE 3. Conuxnration of different typea of surface sites on Pt cataly&.

CATALY8T

TJK

H!Pt

3M

ZM,

ZM,

1M

EUROPT-1 EURoPT-1 EBBOPTEUROPT-1 EUROFT-1 EuRoPT*lb

573 573 573 573 573 573 573 573 573 773 573 773

0.65 0.65

O.lOl(18) O.l12(2lj 0.103(19) 0*096(1-r) 0*107(20) 0.125(20) 0.166(x) 0.185(32) 0.171(20) 0215(35) 0.230(26) 0.508(59)

O.t326(5)

0.003(1)

0.549(76)

0.65 0.65 0.65

a63

0.58 0.84 0.62 0.90 0.86

0.025(5)

0*029(4) ~013(2~ o_olq3) O-028(4) 0.073(12) 0.033(6) O-0560 0.104(17) 0.07q8) 0.157( 18)

O.Olo@) o-v9 0.005(1)

0.014(2) a~2) 0.028(4) 0.018(3) 0.032(6) 0.032(4) 0.066(11) 0.071(S) 0.019(Z)

O&547(77) 0554(78) 0.543(76) 0.470(72) 0.373(59) 0.3290 0.581(69) 0.235(38) 0.523(58) 0.176(B)

BThe~~n~atio~

are given in mole of sites per mole of Pt; the figure in parentheais indica~ the percentageof each type of site. badapted from ref. 6. 1Butene reaction The results for the ST0 experiments with I-butene are presented in Table 3. The calculations are made after 5 ST0 runs, the results being reproducible after the third run. The first five rows of the Table ihustrate the reproducibility of the method, and the agreement with the results of Augustine et aL[6] for EUROPT-1. On this catalyst, the main product is butane(l)

(butane formed after the

introductior~ of the first puke of l-butene), representing about 80% of the overall selectivity. The activity, represented by the sum of the sites (3M+2Mo+2Ms), is low and is mainly due to the presence of 3M sites. When a freshly calcined PQTiO, catalyst is reduced at 573 K, the main product formed is butane(l):

the amount of isomers formed is not negligible when compared to EUROPT-1, while

butane(Z) - formed from the second pulse of reactant - remains insignificant. The number of inactive sites decreases at the expense of both 3M and 2MC sites but are still in a majority. If the catalyst is stored for 5 days, the surface composition is oniy shghtIy affectfx&the percentage of the 3M sites being increased by only 6% at the expense of Z& sites. For the titania-supported catalyst reduced at 773 K, hydrogen chemisorption is completely suppressed, so it is not possrbie to calculate the number of different sites. However, an analysis of the product distribution is still possible. The remarkable point about this analysis is that only butane(2), formed by a two-step process, is observed. ‘lIms, a catalyst which cannot adsorb hydrogen, and doea not therefore hydrogenate l-butene,

is capable of adsorbing 1-butene which can subsequently be

hydrogenated by a pulse of gaseous hydrogen. Both ~~a-sup~~~

catalysts (i.e. with or without c~o~e~n~~g

precursor) show

320 similar trends when reduced at 573K the site distribution is similar to that of PVTiOa with a domhmnce of the 1M type. The ST0 activity is mainly due to the presence of 3M sites, the chknine-containing cataiyst showing a higher overall activity. Reduction at 773 K leads to an obvious change in both catalysts. The catalyst become more active, with the chlorine-free catalyst showing the highest activity of all the catalysts teated. The selectivities are also greatly affect& a sharp decrease of the percentage of 1M sites is observed, while the number of sites which perform a reaction is augmented with the appearance of a noticeable amount of isomerisation sites.

Changes of pretreatment conditions produce drastic modi&ations in the reaction of neohexane over platinum. Whereas other reactant molecules, such as hexane or methylqclopentane, do not show major changes in selectivity [l,Z], neohexane is a good probe molecule to explore subtle changes in a catalyst. While hexane or methylcyclopentane are known to react mainly via a cyclic intermediate, particularly for this range of Pt particle sixes [7], neohexane cannot form Cs-cyclic intermediates and must react via a bond shift mechanism, possibly involving a metallacyclobutane intermediate [S,9], If we consider our results for Pt/TlO, going from LTR to HTR, we can see that the rate of h~o~no~~

is decreased more than the rate of isome&ation {ratio 8:l). Moreover, when the

PVTiOa catalyst is stored before use, and although the hydrogen adsorption capacity is hardly at&ted, the rate of hydrogenolysii is decreased by a factor of 9. On the other hand the isomerisation activity is only diminished by 10%. On Pt/AlaOs, the rate of isomerisation is increased more than the rate of hydrogenolyais. This means that on Pt/AlaO, isomerisation sites are created by HTR, a phenomenon which could not be observed directly on Pt/TiOa because of a superimposed SMSI effect_ Although eventually, TiOx species would cover all types of sites on the surface, defect sites (corners, edgea etc.) are likely to be bIocked p~fe~nti~ the decrease in the h~~~~~

because of their coordinative aeon;

activity suggests that these sites are very active for this reaction. On

the other hand, certain sites remain uncovered and are able to perform the isomerisation reaction It seems likely that these sites are situated on a flat surface. This would explain the temperature effects we observed. When a catalyst is reduced at higher temperature, the resulting surf&e is likely to be smoother: surface defects wig be eliminated. This idea agrees with the work of Davis and Somorjai [lo] on isobutane isomerisation, where they have shown that the selectivity for isomerisation was high on Pt(100) and particularly on the vicinai (13,1,1) surface as compared to Pt(ll1) and Pt(lO,S,7), whereas hydroc&&g

was performed on the Pt(lO,8,7) surface which has a high density of kink sites. This idea

is supported by the work published recently by Maire et al. [ll]. They induced reconstrncdon of a stepped platinum surface Pt(s)[6(111)x(lOO)]by adding a very small amount of sulphur, previous work of LanxiUotto et al. [12] having shown a preferential adsorption of the sulphur at the (100) step edge of the stepped surface. Such a reconstruction creates new sites specific for the bond shift isomerisation mechanism and suppresses sites responsible for hydrogenoiysis. The effect of storage leads in a way to similar results: the presence of ambient water is weil

321 known

in the

case of TIO+upgorted

catalysts to increase the mobility of reduced species on the

surface, so under these comlitions, the catalyst goes into a partial SMSI state more easily. In this case, only the hydrogenolysis sites are strongly affecmd, whereas the rate of isomerisation remains the same. Augustine et al [13] have used the ST0 technique to demmtrate,

by analogy with single

crystal studies [14], that in the conversion of cyclohexane there was a direct correlation between the formation of benzene and the number of 3M sites, while hexane formation was a function of the terrace atom density. The ~p~~~~~

of our results, and their agreement with those of Ache

et al. for EUROPT-1 [6], demonstrate the validity of the experimental method. ‘Ipro main points emerge horn the experimental data: first, the sites able to perform a reaction are not all the same, and for the catalysts studied there are always a large number of sites which do not participate directly in the formation of a particular product (up to 75% in the case of EIJROPT-1). Second, a change of pretreatment conditions leads to a change of site distribution, so the method provides a convenient way to obtain evidence of structural changes. It is clear from our experiments that the sites identified by the ST0 technique do not correlate with our expectation for the neohexane reaction. Thus, we have interpreted our neohexane results in terms of variations in the relative numbers of defect and non-defect sites, the former being responsible for hydrogenolysis of the ay’ adsorbed intermediate, and the latter responsible for isomerisation of the same type of intermediate. Inspection of the information in Table 3 shows that even though there are large changes in the numbers of the diiIerent types of surface sites, none of these changes correlate exactly with the variations in neohexane activity or selectivity. Thus, although there is a consistent increase in the number of 3M sites as the reduction temperature is increased, and a parallel decrease in the nmber of 1M sites, which correspond to changes in the neohexane selectivity, this correlation does not apply in any q~ti~~

mamrer. We are forced to the conclusion that the changes in

selectivity of the neohexane cannot readily be attributed to variations in the mu&em of sites identiiied by the ST0 technique. The adsorption of neohexane is almost exclusively through an ay/ intermediate and the difference in selectivity arises because this intermediate can either isometise or hydrogenolyse. A stoichiometric hydrogenolysis reaction consumes hydrogen whereas isomerisation does not. We know from our own unpublished work that the selectivity of hydnxarbon reactions on these catalysts is sensitive to hydrogen pressure. Therefore, it is possrble that the selectivity in the neohexane reaction arisea because of variations in the ~~~ neolwme.

of hydrogen at wjiw

sitm ac-fjacentfo the ~~

What we are suggesting is that the variations in sele&ivity which we observe as a function

of reduction temperature, time of storage, etc., may regect subtle changea in surface morphology, but mainly influence the effective hydrogen coverage of the surface. For example, if hydrogen is activated or adsorbed only at a small number of surface sites when a hydrocarbon is present, then vety small changes in surface morphology could have a large infhrence on the amount of hydrogen present. A neohexane molecule adsorbed at a Pt site adjacent to hydrogen atoms may have a high probabiity of hydrogenolysis. The bond shift isomerisation m&anism requims a transfer of a CII, group from one

322 carbon atom to another in neobexane Jf, at the point at wbicb tbe bond shift occurs, hydrogen is available on an adjacent site, this may insert into one of tbe C-C bonds (eitber tbe CC! bond b&g broken or tbe C-C bond being made as the CHs group shifts), &us interrupt&tg tbe bond shift isomerisation. Addition of further bydrogen atoms will lead to bydrwgenolysisproducts being deaorbed from the catalyst surface. Following the ideas proposed by King and coworkers concerning tbe hydrogenation of ethylene [U] and the bydrogenolysis of etbane [la], it is possible ‘that hydrogen activation and adsorption is Eavoured at defect sites. Blocking of these sitea would then inhibit bydrogenolysis as we seem to observe in OUTesperiments. ACKNOWLEDGEMENTS We are grateful to the EEC for pi

financial support for this maearch under contract SCI

OOB!K(EDB).We are pleased to acknowled~ the marry stimulating discumions a3nceming this work with Professor J.K.A. Clarke and Profmr

J.J. Rooney.

J.B.F. Anderson, R Burch and J.A. Cairns, J. CataL, 167 (19S7) 364. J.B.F. Anderson, R Burcb and Xk Cairns, J. CataL, 107 (1987) 351. RL Augustine and RW. Wagner, J. CataL, 80 (1983) 3!%. M. Boudart and HIS. Wang, J. Catal., 39 (1975) 44. personal communication from G,C.Bond on EUROPT-1. RL Augustme, D.R Baum, KG. Higb and LS. O’Leary, personal communication, preprint (19903. F. Gault, F. Garin and G. Maire in Y&wtb and Proper&r of Metal clusters” p-451; Ed. J. Bourdon, Ehevier (19EtO) J.J. Rooney and J.KA_ Clarke, AMY.CataL, 25 (1976) 125. F. Ganlt, Adv. CataL, 30 (1981) 1. SM. Davis, F_ Zaera and GASomorjai, J. Amer. Cbem. Sot., 104 (1982) 7453. G. Maire, G. Undauer, F. Garin, P. L&x&, M. cheval and M Vayer, J. Chem. Sot. Faraday Trams, 86 (1990) 2791. A.M. Lanaillotto and S.L Bemtwek,J. Chem. Phw, S4 (19%) 3553. R.L Augustine, KP. Keby and Y.M. Lay, Appl. CataL, 19 (19@) 87, RK. Hem, W.D. Gillepsie, EE Petersen, and G.ASomorjai, J. CataL, 67 (1981) 371. M. Sprock, M. Pruski,B.C. Gerstein and T.S. King, Catal. J.&t., 5 (1990) 3%. M.W. &vale and TS. King, J. CataL, 119 (1989) 441.