Al2O3 Catalysts.

A. Crucq (Editor), Catalysis and Automotive Pollution Control ZI 0 1991 Elsevier Science Publishers B.V., Amsterdam

58 1

CHARACTERIZATION OF BIMETALLIC SURFACES BY 180/160 ISOTOPIC EXCHANGE. APPLICATION TO THE STUDY OF THE SINTERING OF PtRh/A1203 CATALYSTS. S. Kacimi (ab) and D. Duprez (a)

(a)Laboratoire de Catalyse en Chimie Organique, 40 Avenue du Recteur Pineau 86022 Poitiers France (b)Present adress :University of Sidi-Bel-Abbes, Algeria. ABSTRACT A series of PtRh/A1203 catalysts designated PtRhx, where x is the atomic percentage %Rh/Rh+Pt was prepared by successive impregnation of a delta-alumina, pretreated in H2 at 850"C, with aqueous solutions of H2PtC16 and RhC13. They were calcined at 450°C (fresh catalysts) and then sintered (2% vol.O2/Ar for 2h) at 700, 800 and 900°C. They were characterized by hydrogen chemisorption HC and oxygen titration and by oxygen isotopic equilibration (1602 + 1802 3 2 160180). The surface compositions were evaluated from the total metal surface area deduced from and from the rhodium surface area deduced from re (rate of equilibration at 300°C). re being very sensitive to the presence of chlorine, the catalysts were dechlorinated in a stream of H2 + H2O at 400°C. This treatment induces a decrease of the metal area of Pt while there is no change for Rh. No surface enrichment is found in these catalysts : a linear variation of re with x is obtained from pure Pt (0.16 x 1019 at.0 min-1m-2) to pure Rh (3.06 x 1019 at.0 min-1,-2). On the contrary a significant change in the surface composition is observed for the sintered catalysts which are strongly enriched in Rh at high Rh content and in Pt at low Rh content. The inversion of composition occurs for x = 25 Rh at.-% at 700°C and for x = 40 Rh at.-% at 900°C. Two models are proposed to explain these results; they take into account the structural and morphological changes of Rh and Pt in oxidizing atmosphere as well as the degree of interaction between the two composents of the bimetallics. INTRODUCTION

Surface composition of bimetallic catalysts in an essential parameter for their catalytic performances. This is particularly true for the PtRh system used in exhaust gas catalysis since each of the components possesses a specificity with respect to the oxidation reactions (platinum) or the reduction reactions (rhodium) (ref.1). The surface composition of bimetallics can be measured by physical methods (XPS, ISS ...) (ref.2). Complementary methods (X-Ray Diffraction, Temperature-Programmed Reduction) can also be used

582

for obtaining information on the degree of alloying in the bimetallics. Physical methods are well adapted to the study of bulk catalysts (powder, film...) or of highly loaded supported catalysts whereas physico-chemical methods are more effective for highly dispersed supported catalysts with a low metal loading. Nevertheless physico-chemical methods require the stoichiometries of chemisorption of the probe molecules on each metal to be quite different. Recently we showed that the precious metals used in threeway catalysts could promote the isotopic exchange between gaseous oxygen and the surface oxygen of the support and this reaction was used for the study of the oxygen mobility on the catalyst surface (ref.3). Moreover we showed that the same metals could catalyze the isotopic equilibration reaction : 1602 + 1802 2 160180 (1) As rhodium and platinum presented very different intrinsic rates of equilibration, it was decided to use this reaction to measure the surface composition of PtRh/AlzOs catalysts. The method was applied to a series of catalysts calcined at 450°C and then sintered at 700°C and 900°C.

*

PRINCIPE AND EXPERIMENTAL

The measurement of the surface composition is based on the two complementary experiments : (i) the determination of the total surface of the metals by appropriate chemisorption or titrations at room temperature, after reduction of the catalysts at 450°C.Hydrogen chemisorption (Hc) and hydrogen titration of chemisorbed hydrogen (OT) were used to determine the dispersion D, and the total surface area Am, of platinum and rhodium. We have : Am (m2 metal g-1) = a + b (2) where a and b represents the surface areas of platinum and of rhodium respectively (ii) the determination of the rate re of oxygen equilibration. For monometallics, re was shown to be proportional to the metal surface area. Assuming that there is no synergy effect between Pt and Rh, re will be equal to the sum of the contribution of each component in the bimetallics, i.e. : re = aa + pb (3) where a and p are the intrinsic rates of equilibration of platinum and of rhodium respectively. a and p being determined separately with monometallic catalysts, the measurement of Am and re give a and b by resolving the system (2) + (3). Eq.3 can be expressed in terms of intrinsic rate of equilibration (per m2 of metal area) : r*e = re/a+b = ~1 (a / a +b ) + p ( b / a +b ) or r*e = 01 (1 - XS)+ p XS (4) where xs represents the fraction of the surface occupied by rhodium atoms which is close to the atomic percentage of rhodium at the surface. Moreover,

583

if a linear plotting of r*e with x, atomic percentage of rhodium in the catalyst, is found, i.e. : r*e = m(l - x) + n x (5) then m = a,n = p and x = xs. This means that there is no surface enrichment in the bimetallics. For eq.4 to be valid, there must be no synergy effect between platinum and rhodium on the isotopic equilibration reaction, i.e. each surface atom behaves in the same manner as in the monometallics. A series of PtRWAlzO3 catalysts (E 1% metal) was prepared by coimpregnation of an alumina (pretreated at 900°C before use) with aqueous solutions of chloroplatinic acid and rhodium chloride. After drying at 120"C, the catalyst samples were calcined in air flow at 450°C. They are referred to as PtRhx where x is the atomic percentage of rhodium. Aliquot portions of these catalysts were sintered at 700°C and at 900°C in oxygen (oxygen pulses injected every 30s in an argon flow for 2h, corresponding to a mean value 0 2 of 2%).The sintered catalysts are designated as PtRhx700 or PtRhx900. Hydrogen chemisorptions and oxygen titrations were carried out in a pulse chromatographic apparatus using ultrapure argon (less than 1 vpm impurities) as a carrier gas (ref.4). H c values were corrected from the amounts of weakly adsorbed hydrogen (10 min. desorption). Temperatureprogrammed reduction (TPR) were carried out in the same apparatus. Isotopic equilibration measurements were made in a recycle reactor described elsewhere (ref.3). The catalyst samples (0.02 to 0.5g) were first treated at 450°C in natural oxygen in order to eliminate any carbonaceous impurities, then reduced in a flow of H2 at the same temperature (lh) and subsequently outgassed (450"C, lh, 10-4 mbar).They were cooled down to 300°C for isotopic equilibration measurements. (50 mbar, 1602 50% + l * 0 2 50%). The rates were deduced from the initial slopes dP34/dt of the formation of 160180.

RESULTS

Fresh catalysts (calcined at 450 "c) The characteristics of the chlorinated samules are given in Table 1 which shows that all catalysts are relatively well-dispersed and that their chlorine content is randomly distributed as a function of the atomic percentage of rhodium (x in PtRhx). The values of OT are found in accordance with those of HC as far as pure rhodium or rhodium-rich catalysts are concerned : a stoichiometry of OT : HC = 2 : 1 is then obtained in agreement with previous results on rhodium catalysts (ref.6 and 7). On the contrary, there is no agreement between the HC and OT values on pure platinum or platinum-rich catalysts, the stoichiometry H : Pt being close to 2. This is most likely due to the very high dispersion of platinum in these

584

catalysts (ref.8). That is why the dispersion characteristics (D and Am) given in table 1 were calculated from the values of OT. TABLE 1

Characteristics of the chlorinated catalyst samples (calcined and reduced at 450°C).

Pt Rh 12 Pt Rh 19 Pt Rh 28 Pt Rh45 Pt Rh 100

0.50 0.00

Rh wt % 0.00 0.036 0.061 0.1 1 0.22 0.5 1

wt % 0.49

pmole g-1 38 I 32

0.7 1 0.76 0.59

52 46

%

95

69

m2g-1 1.07 1.52 1.84 2.00

The TPR profiles of the chlorinated samples are shown in Fig.1. Rhodium reduces more easily than platinum in these catalysts : the difference between the two temperatures TM for the maxima of reduction reaches 100°C. However both metals seem to reduce together in the bimetallics : in every case a single peak of rduction is observed with a maximum increasing regularly from pure rhodium to pure platinum (Table 2). Moreover, the initial oxidation state of the metals is close to +4 for platinum and to +3 for rhodium and a regular change of the H/M values is observed from pure platinum to pure rhodium in the bimetallics. pmoi H min-lg”

Fig.1 TPR profiles of the chlorinated PtRhlA1203 catalysts.

1 : WA1203 ;2:PtRh45 ; 3 :Pt Rh28 ;4 : PtRh 19; 5 : Pt Rh 12; 6 : PdA1203

585

TABLE 2

Temperatures TM of the maxima of reduction and amounts of hydrogen taken by the chlorinated PtRh/A1203 during TPR from 25 to 450°C.

H2

Dechlorinating the catalvsts (to less than 0.15 wt-% C1) in a stream of

+ 10%H 2 0 results in a significant change in the metal accessible fraction

(Fig.2). Nevertheless the dechlorination treatment affects essentially platinumcontaining catalysts and has virtually no effect on rhodium. Temperature programmed reduction of dechlorinated catalysts shows that the metals are almost completely reduced after the dechlorination treatment : hydrogen uptakes at 25°C are observed on Pt and PtRh catalysts whereas Rh/A1203 shows a single peak for about 100°C (Fig.3). These hydrogen uptakes can be

Fig.2 :EJrfect of the dechlorination in H2 + 10% H 2 0 on the metal sulface area of the PtRhlA1203 catalysts. Cl = chlorinated samples ;DCl = dechlorinated samples.

586

ascribed to hydrogen titrations of chemisorbed oxygen on the metals : it is well-known that these titrations occur readily at room temperature on Pt catalysts whereas on Rh, they require higher temperatures for them to be attained. mol H min-lg-’

-o l

Fig.3 :TPR profiles of PtRh catalysts a f e r dechlorination a - RhlA1203 ;b - PtRh 19 ;c - PtIAl203. 1 6 0 / 1 8 0 isotopic equilibration experiments were carried out on the chlorinated and on the dechlorinated samples. The changes of r*e (rate of equilibration) with the atomic percentage of rhodium in the bimetallics are shown in Fig.4. The presence of chlorine in the catalyst provokes a very important decrease of r*e (by a factor of 10). Moreover no correlation can be found between r*e and the composition of the bimetallics (Fig.4a). As shown in Table 1, the chlorine content of the catalysts amounts to 0.62 f 0.15 wt-% but it seems that these small changes in C1 content can induce large variations of the rates of equilibration. On the contrary, a linear plotting of r*e with the atomic percentage of rhodium is obtained on dechlorinated samples (Fig.4b). This means that the surface composition of the dechlorinated, calcined bimetallics is very close to the bulk composition. This shows also that, there is no synergy effect, between Pt and Rh in this samples. A linear plotting r*e vs x would have been obtained only in the case of a hypothetical compensation between a surface enrichment in platinum and a synergy effect of the rhodium on the platinum. This would increase the activity of this latter metal. This unlikely hypothesis, can be rejected. We can thus consider that the equations 3 and 4 are valid for PtRWA1203 bimetallics with : 01 = 0.16 1019 at. 0 min-l(m2 Pt)-1 and p = 3.06 1019 at. 0 min-l(m2 Rh)-1 at 300°C.

587

r:

1 0 ' ~atoms

o

min-1 m2m-,m,

a

I

0

.

.

.

50

.

50

100

x

100

Rh at.-%

Fig.4 :Variations of the rates of 160t180 equilibration with the atomic percentage of rhodium in the PtRhlAl203 bimetallics. (a - chlorinated samples ;b - dechlorinated samples). Sintered catalysts The chlorinated catalysts were sintered in an oxidizing dry atmosphere at different temperatures. Their relative metal surface areas are given in Table 3. TABLE 3

Effect of the sintering (pulses of 0 2 in Ar for 2h at T"C corresponding to a 2% 0 2 average composition) on the relative metal surface area. The reference is the metal surface area of the catalyst calcined at 450°C denoted Am (450). Catalysts

Am(450)

Pt Rh 0 PtRh 12 PtRh 19 Pt Rh 28 Pt Rh 45 Pt Rh 100

1.07 1.08 1.29 1.52 1.84 2.00

m2/g

Am(T)/Am(450), 96 T = 700°C T = 800°C T = 900°C 15.5 16.6 15.5 15.8 16.5 15.8

7.3 7.3 7.7

8.2 10.2

7.4

1.6 1.8

2.4 2.7

2.8

3.7

588

There is a continuous decrease of the metal surface area with the sintering temperature. At 700"C, the rate of sintering does not depend on the catalyst composition while at higher temperature, the effect of this composition is noticeable. At 8OO"C, it seems that the bimetallics resist better to sintering whereas at 900"C, there is an acceleration of the sintering rate for platinum and platinum-rich catalysts. These results will be discussed in the light of the different models of sintering for platinum and for rhodium. In the first column of Table 4, are given the values of the rates of equilibration on catalysts sintered at 700°C. These values show a non-coherent change of re with the rhodium content of the catalysts. Moreover the value of a (intrinsic rate of isotopic equilibration of Rh) calculated from the data obtained with PtRh 100 (2.66 x 1018 at. 0 min-1 g-1 for a rhodium surface area of 0.31 m2 g-1) is much smaller than the value obtained with the standard non-sintered catalyst (0.86 x 1019 instead of 3.06 x 1019 at. 0 min-1 m-2). It seems thus that the sintered catalysts still contain chlorine which can poison the equilibration reaction. Therefore this series of sintered catalysts was dechlorinated in a stream of H2 + H20. The results obtained after this treatment are reported in the second column of Table 4.

llUiLE4 Rates of 1 6 0 / 1 * 0 isotopic equilibration at 300°C on catalysts sintered at 700"C, before (a) and after (b) a further treatment of dechlorination (H2+H20 at 400°C). dechlorinated Pt Rh 0 Pt Rh 12 Pt Rh 19 Pt Rh 28 Pt Rh 45 Pt Rh 100

(4 sintered at 700°C 0.20

(b) sintered and dechlorinated 0.21

2.66

6.95

The total surface areas of metals (Pt+Rh) were not controlled after dechlorination. Nevertheless the values of a and p calculated from the metal areas measured with the non dechlorinated samples are 0.13 x 1019 and 2.3 x 1019 at. 0 min-1 per m2 of Pt and of Rh, respectively. These values, coherent with those measured on the fresh catalysts, were used for the determination of the surface composition of the samples sintered at 700°C. Similar measurements were carried out on the catalysts sintered at 900°C. The intrinsic rate of isotopic equilibration found for Rh (2.2 x 1019 at. 0 min-1 m-2) is coherent with the value obtained on the fresh catalyst.

589

Moreover no change was noted after a further treatment of dechlorination, which shows that the sintering at 900°C eliminates practically all the chlorine contained in the catalyst

X,

~h surface %

I

50

0

100

0

.

.

.

50

.

1 0

Fig.5 :Sulface composition of the PtRhlAl203 bimetallics sintered at 700"C (left) and 900 "c (right). The surface compositions deduced from the 160/1*0 isotopic equilibration rates are shown in Fig.5 for the catalysts sintered at 700 and 900°C. In every case, two kinds of phenomena are observed : (i) the metal surfaces are enriched in at least one of the metals (ii) an inversion of the surface composition can occur for a rhodium content x which depends on the sintering temperature. For x < 25 at.-% (catalysts sintered at 700°C) or x < 40 at.-% (catalysts sintered at 900"C), the bimetallics are strongly enriched in platinum while the opposite tendency (enrichment in rhodium) can be observed for x>25 or 40 at.-% . DISCUSSION

Chemical state of metals during isotopic equilibration During the experiments of isotopic e uilibration, the reduced catalysts are placed in contact with a dose of 1602 + 1 0 2 at 300°C. It has already been shown that, at this temperature, a surface oxidation occurs for platinum (ref.10-12) while rhodium is most probably oxidised in the bulk (ref.6,8,1315). The question arises whether or not the method itself could change the surface composition of the bimetallics. The results obtained on the fresh catalysts seem to show that, this change of surface composition, if it occurs, is

1

590

rather small inasmuch as no apparent surface enrichment is found in these samples. Moreover it can be argued that the characterization method by 160/*80 isotopic equilibration gives the surface composition of the bimetallics in oxidizing atmosphere, which is of the utmost interest in exhaust gas catalysis. After sintering at 700-9OO0C, there is a significant change of the surface composition with a rhodium enrichment for Rh/Rh+Pt > 25 at.-% (700°C) or 40 at.-% (900°C). Similar surface enrichments in Pt-Rh alloys or in supported Pt-Rh catalysts have been found by Williamson et al. (ref.16) and by Schmidt and coworkers (ref.17-18). These works report that apparently the rhodium surface enrichment occurs, even at low Rh content, on unsupported PtRh alloys. The inversion of surface composition found in the present study would be specific of aluminasupported catalysts.

Role of the support in the surj5ace segregation of PtRh bimetallics It is generally assumed that in an alloy treated in oxidizing atmosphere the element forming the most stable oxide segregates to the surface (ref.19). This is the case for Rh in PtRh alloys which forms Rh2O3 stable up to about 900°C while Pt02 decomposes beyond 550°C. The presence of platinum can accelerate the decomposition of Rh2O3 which starts then at 800°C (ref.20). However this does not change the general tendency for Rh to segregate to the surface of PtRh in 0 2 or air. The inversion of surface composition for low Rh contents can be related to the ionic mobility of Rh3+ ions in the alumina matrix : rhodium diffuse in alumina forming a "diffuse oxide phase" (DOP) no longer accessible to gases and, in particular, non-reducible in H2 (ref.8 and 14). Table 5 shows the change of the DOP content in Rh/A1203 with the sintering temperature. The DOP corresponds to the fraction of rhodium non reducible at 450°C, the reducible fraction being determined by oxygen uptakes at 500°C assuming a reoxidation of Rho into Rh2O3. TABLE 5

Degree of reduction of Rh (%R) in Rh/A1203 as a function of the sintering temperature Tox ("C). Tox ("C)

%R

450 (fresh) 100

700 18

800 9

900 4

It appears that the proportion of diffuse oxide phase depends largely on the rhodium content in the catalyst : the degree of reduction of a 2% Rh/A1203 catalyst was, after a treatment in oxidizing atmospher, 63% at 700"C, 26% at 800°C and 22% at 900°C (ref.8). Thus the proportion of

59 1

rhodium lost by diffusion in alumina increases significantly when the rhodium content is decreased. Two models, differing by the degree of interaction between Pt and Rh in the metal particles, can be proposed to explain the results of the present study : - In the model "with interaction", Pt and Rh are present together in the metal particles. Taking into account the TPR profiles shown in Fig.1, this is a realistic model, at least for the catalysts calcined at 450°C. Upon sintering in 0 2 at high temperature (700-9OO0C), rhodium oxidizes and segregates to the surface of the particles. Simultaneously part of the rhodium ions diffuse into alumina. For low Rh content, the fraction of this element remaining at the support surface is close to 0, the external particles becoming extremely rich in platinum because there is practically no longer Rh in the metal particles. For higher Rh contents, the fraction of rhodium remaining in the particles increases. As this element is segregated in the outer layer of the particles, the surface becomes enriched in rhodium. - In the model "without interaction", Pt and Rh are located in separate particles. The apparent enrichment in platinum at low Rh-content would be caused by the same phenomenon as in the model "with interaction", i.e. a significant diffusion of Rh3+ ions in the alumina matrix. The apparent enrichment in rhodium in Rh-rich bimetallics would be due to a good dispersion of the Rh2O3 phase remaining accessible at the alumina surface while Pt particles would suffer a severe sintering. For this model to be valid for catalysts calcined at 450°C, the particle sizes of Pt and Rh must be identical since the surface composition was found very close to the bulk composition. This is not the case in dechlorinated catalysts for which platinum was shown to sinter during the H2/H20 treatment. Thus it seems the most likely that in the fresh catalysts, Pt and Rh would form alloy particles. However in the course of sintering at high temperature, we could have a phase separation and with these catalysts, the two models (with or without interaction) could coexist. Further studies, including also the role of steam, are presently in progress to examine this eventuality. ACKNOWLEDGEMENTS

We thank the Groupement de Recherche sur les Catalyseurs de postcombustion automobile" (Centre National de la Recherche Scientifique, Institut FranGais du Ptrole et Agence FranGaise pour la Maitrise de 1'Energie) for its financial support. MM. Prigent, Mabilon and Gravelle are thanked for their constant interest and helpful discussions concerning this work.

592 REFERENCES 1. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

A. Crucq and A. Frennet (Eds.) "Proceeding of the 1st International Symposium on Catalysis and Automotive Pollution Control, Brussels, 1986" Stud. Surf. Sci. Catal. Vo1.30, Elsevier Publ., Amsterdam (1987). F. Delannay and B. Delmon in "Characterizationof heterogeneouscatalysts" Base1 (1984). (F. Delannay Ed.) p.1, M. Dekker Publ., New York and H. Abderrahim and D. Duprez, p.359 of ref.1. D. Duprez, J. Chim. Phys. 80 (1983) 487. H. Abderrahim and D. Duprez in "Proc. 9th Int. Congr. Catalysis, Calgary, 1988" (M.J. Philipps and M.Ternan, Eds) Vo1.3 p.1246. The Chem. Inst. Canada, Ottawa, 1988. T. Paryjczak, W.K. Jozwiak and T. Goralski, J. Chromatogr. 166 (1978) 65 and 75. D. Duprez and A. Miloudi, J. Catal. 86 (1984) 441. D. Duprez, G. Delahay, H. Abderrahim and J. Grimblot J. Chim. Phys.83 (1986) 465. M. Kobayashi, Y. Inoue, N. Takahashi, R.L. Burwell Jr, J.B. Butt and J.B. Cohen, J. Catal., 64 (1980) 74. H.C. Yao, M. Sieg and H.K. Plummer Jr, J. Catal., 59 (1979) 365. H. Lieske, G. Lietz, H. Spindler and J. Vlter, J. Catal., 81 (1983) 8. J. Barbier, D. Bahloul and R. Szymanski, Bull. Soc. Chim. F., 1(1988) 478. S.E. Wanke and N.A. Dougharty, J. Catal., 24 (1972) 367. H.C. Yao, S. Japar and M. Shelef. J. Catal. 50 (1977) 407. C. Wong and R.W. Mc Cabe, J. Catal., 119 (1989) 47. W.B. Williamson, H.S. Gandhi, P. Wynblatt, T.J. Truex and R.C. Ku Aiche, Symp. Ser. Nx201,76 (1980) 212. M. Chen, T. Wang and L.D. Schmidt, J. Catal., 60 (1979) 356. T. Wang and L.D. Schmidt, J. Catal., 71 (1981) 411. F.L. Williams and M. Boudart, J. Catal., 33 (1973) 438. A.J.S. Chowdhury, A.K. Cheetham and J.A. Cairns, J. Catal., 95 (1985) 353.