The Palladium Alumina System: Influence Of The Preparation Procedures On The Structure Of The Metallic Phase

463

G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III © 1983 Elsevier Science Publisbers B.Y., Amsterdam - Printed in The Netherlands

THE PALLADIUM ALUMINA SYSTEM : INFLUENCE OF THE PREPARATION PROCEDURES ON THE STRUCTURE OF THE METALLIC PHASE S. VASUDEVAN, J. COSYNS, Institut

Fran~ais

E. LESAGE, E. FREUND and H. DEXPERT

du Petrole, B.P. 311, 92506 Rueil-Malmaison Cedex (France)

ABSTRACT Two preparation procedures for alumina supported palladium catalysts are compared using conventional and scanning transmission electron microscopy. Different structural relationships have been found between the metallic phase and the carrier with respect to the chosen impregnation method. When the alumina surface structure is damaged a maximum dispersion of only 20% is non destructive route from

ohtained. A

an organometallic precursor leads to a monomodal

higher dispersion.

INTRODUCTION Though the concept of demanding reactions in the field of supported metallic catalysts has been proposed by Boudart (ref. 1) as early as 1966, and has been correlated to such parametemas crystallite size, electronic properties varied by alloying or metal-support interactions, or even preparation procedures, fundamental studies aimed at fully characterizing the metallic particle structure and/or its relationships with the underlying carrier have been generally limited to model systems: evaporated films (ref. 2, 3), or monocrystalline samples (ref. 4) with flat or stepped surfaces. Modern

transmission electron microscopes allow lattice imaging down to

0.15 nm or even below so that lattice images of small (3-10 nm) metallic particles are rather easily obtained (see for example ref. 5). However, the information contained in these images must be interpreted with care. A major advance has been brought about by the advent of high resolution dedicated scanning transmission electron microscopes, which provide simultaneously high resolution images, microanalytical and microstructural facilities down to a few atoms (see for example ref. 6). Applied to the characterization of supported metallic catalysts, electron microscopy may give not only a detailed picture of crystallite size, composition, microstructure, crystallographic relationships between the metallic crystallites and the carrier (ref. 7), but also may help in understanding

464

the influence of each step of a given preparation procedure, or in comparing different preparation procedures. This will be exemplified in the present paper for the case of a palladium catalyst supported on a Y alumina c~rrier. The t metallic phases obtained from two different procedures will be compared : A) a classical impregnation method, which leads to a poor or moderate metal dispersion

; B) preparation by an organometallic route leading to high metallic dis-

persion. EXPERIMENTAL I. Preparation of catalysts

The carrier used is a Y alumina manufactured by Rhone Poulenc with a specit fic surface area of 69 m2/g. Procedure A (classical impregnation) : This procedure consists of dry impregnation of the support with a slight excess of palladium nitrate aqueous solution so as to fix 0.3 % wt palladium. Palladium fixes on the periphery of each pellet of the carrier (penetration depth < I rom). The catalyst is oven-dried (373 K) overnight, calcined at medium (673 K) or high (1173 K) temperature and reduced by hydrogen under atmospheric pressure at 373 or 773 K. Procedure B (organometallic route) : The detailed

procedure is reported in

another paper (J.P. Boitiaux et aI, this conference) wherein a five-fold excess (with respect to pore volume) of a palladium acetylacetonate benzenic solution is left in contact with the carrier for at least 48 hours. The palladium content of the catalyst is controlled by varying the palladium concentration in the benzenic solution. The impregnated alumina is oven-dried overnight at 393 K, then air-calcined at 573 K and reduced under hydrogen at 573 K. Metal dispersion may be varied by subsequent treatment with hydrogen or argon at temperatures varying between 373 and 1073 K. 2. Characterization of catalysts a) CO chemisorption The dynamic method is used, with a catharometric detection. The catalyst sample (1-2 g) is reduced at 523 K for 2 hours, treated with argon at 573 K for 2 hours, cooled to room temperature before

chemisorption measurements. Disper-

sion is computed assuming a 1 : I stoichiometry for CO chemisorption on palladium. b) Electron microscopy A Jeol 120 CX equipped with high resolution pole pieces is used to image lattice fringes of the metal and the carrier. X-ray emission microanalysis (Kevex detector) and microdiffraction is carried out on a V.G. Microscope

HB5 STEM

equipped with analytical pole pieces allowing a point to point resolution of 0.45 nm. The microdiffraction facility has been modified as described in reference (8). Sensitivity and spatial resolution for X-ray emission microanalysis

465

is discussed in reference {9}. c} Temperature programmed reduction Reduction by a 2 or 5 % volume H2-Ar mixture is followed with a catharometer. Temperature is increased linearly up to 873 K. (maximum temperature 1000 K). Raw data are processed using a H.P. 2645 A computer. RESULTS I. Samples obtained from procedure A The samples studied are listed in Table I together with the pretreatment conditions and the dispersion values obtained from CO chemisorption. TABLE I Procedure A Sample

Tcalcination(K)

AI A2 A3 A4 A5 A6

673 673 673 673 1173 1173

Remark

freduction(K)

-

373 573 373

-

Dispersion (%)

-

-

water

after

373

26 16 26

wash

-

i~regnation

-

14

The series of samples calcined at 673 K is first considered. The standard procedure {reduction at 373 K} yields a catalyst of 20% dispersion

(sample A2).

Electron micrographs reveal two types of crystallites : - a few large crystals up to 20 nmin diameter. Sucherystals are present (probably as palladium oxide) in the precursor Al ; - small crystals with an average diameter of 5 nm. However palladium is also present in a highly dispersed phase - palladium metal clusters or palladium ions as revealed by microanalysis. Temperature programmed reduction of sample AI shows that reduction takes place in two steps

a peak at room temperature and another at high temperature of

613 K can be observed. The origin of this second peak is still unclear. Sample A3 has been reduced at a temperature high enough to ensure a complete reduction. A new.population of small crystallites (3-5 nm) is observed in accorddance with the above hypothesis (reduction and/or sintering of the highly dispersed palladium phase). High temperature calcination (1173 K, sample AS) leads to a poorly dispersed catalyst even when reduced at low temperatures {373 K, sample A6}. Only large palladium crystals are visible, however X-ray microanalysis still reveals a highly dispersed (may be unreduced) palladium phase (fig. I). Lastly if the precursor AI is washed before calcination, no large {palladium

466 oxide) particles are observed and a monomodal dispersion is obtained even after low temperature reduction.

Fig. I. X-ray emission evidence for a highly dispersed palladium phase. Left part (I) : zone imaged before analysis and the corresponding spectrum; Right part (2) : after analysis a palladium signal is seen coming from the dark zone formed under the electron beam. Some characteristic high resolution images obtained with tilted illumination, on sample A2 are shown in figures 2 and 3. The following conclusions may be derived from these images - the (Ill) lattice fringes

are generally obtained in the palladium crystals

images. Thus the most frequent zone axes must be [110), [123) or [211). This can be checked directly for instance on figure 2 where the (III) and (002) lattice fringes are imaged ; - the same zone axes are also obtained for the alumina carrier, as it is visible on figure 3, the [lID] zone axes being the most frequent. A much more systematic study is possible, using microdiffraction with STEM. Typical results are presented in figure 4 for sample A3. The microdiffraction

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Fig. 2. CTEM high resolution image of a palladium crystallite oriented along the [lID] axis.

Fig. 3. CTEM high resolution image of a metal-carrier association. Alumina zone axis [\12] .

Fig. 4. Palladium typical parallel microdiffraction pattern. The diffracting area is 4 nm2.

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interpretation yields the following results: - particles in the size range 3 to 7 nm are mono crystalline. Some larger particles exist and are polycrystalline , - the above quoted zone axes are confirmed in most cases, the layer of alumina on which the palladium is deposited is poorly crystallized or even amorphous. 2. Samples obtained from procedure B High dispersions are obtained with this procedure, for reduction temperatures not exceeding 573 K.

~ll

samples are partly reduced at room temperature. Several

high temperature peaks are seen for calcination temperatures below}73 K, confirming that palladium is partly present in the unreduced state for all the samples considered. For the higher dispersion, no

pallad~um

phase can be imaged in the CTEM

though palladium is detected by X-ray emission microanalysis and shown to be rather uniformely located on the surface of the alumina carrier. As small crystallites (characteristic dimension less than 3 nm). cannot be easily (or at all) investigated, we studie4 a sample treated under argon at 973 K. Typical microdiffraction patterns are given in figure 5. Concerning the metallic phase, the situation is roughly similar to what is observed for an equivalent sample (same overall dispersion) obtained from procedure A. All observed particles are monocrystalline and have [110), [123) or [211) ~s

zone axes. However the alumina

beneath the metal is crystalline and partial epitaxy is generally obtained (i.e.

5. [123) palladium-alumina epitaxy for the two arrowed crystallites.

Fig~

469 along one lattice plane: (III)). This conclusion is valid in the particles size range of 3-7 nm, and for all three zone axes. Furthermore, the recording system used for microdiffraction acquisition is rapid enough (down to a few tenth of a second) to follow the fluctuations of the crystallographic orientations of a . . -10 g~ven part~cle under the electron beam (beam current = 10 A in I to 10 nm2). DISCUSSION Two important points for each procedure studied can be observed and will be discussed : - the chemistry involved in the formation of the metallic phase - the crystallographic relationship between palladium particles and the carrier; In the precursors issued from procedure A, palladium is present in three phases: bulk oxide (10-50 nm)" divided

oxide (3-7 nm) and highly dispersed

oxide or palladium ions. Bulk oxides may be eliminated by carefully washing the precursor after impregnation as shown by the examination of sample A4. The hiehly divided palladium phase cannot be eliminated by air calcination, even at 1173 K. Thus, a monomodal palladium dispersion cannot be easily obtained from procedure

A. On the contrary, starting from procedure B, no bulk oxide phase is formed, most of palladium becomes reducible at low temperature for calcination temperatures above 573 K. Thus, very high monomodal dispersions may be obtained. The microstructural study by microdiffraction and high resolution conventionnal transmission microscopy demonstrates that epitaxy is a normal phenomenon in the palladium/alumina system, provided that the alumina surface remains undamaged, condition which is respected with procedure B but not with procedure A (amorphisation of the structure during impregnation and/or calcination) As a consequence of epitaxy, a rather strong interaction is produced in the case of small crystallites, which explains the very high resistance to sintering of catalysts obtained from procedure B. The preliminary results suggest that it is worthwhile : 1/ to examine if the phenomenon observed in the palladium/alumina system is general and if it

c~n

be extended to other metal/carrier systems ;

2/ to study in more detail the formation of a metallic phase from the precursor. Work is

in progress in this direc don.

ACKNOWLEDGE~ffiNTS

One of the authors (S. V.) wishes to thank Engineers India Ltd (India) for the necessary study leave and the French Governement for the scholarship to do this work.

470 REFERENCES M. Boudart, A. Aldag, J.E. Benson, N.A. Dougharty and E.G. Harking, J. of cae .; 6(1966)92. 2 M. Gillet and A. Renou, Surf. Sci., 90(1979)91. 3 M. Gillet and A. Renou, Thin Solid Films, 41(1977)15. 4 G.A. Somorjai and coli., J. de Catal., 67(1981)371, Appl. Surf. Sci., 2(1979) 352 and Surf. Sci., 92(1980)489. 5 D.J. Smith and L.D. Marks, Phil. Mag., 44(1981)735. 6 H. Dexpert, E. Freund and J.P. Lynch, Proceedings of Quantitative microanalysis with high spatial resolution, Manchester, March 1981, The Metals Society, London, 1981, p , 101. 7 A. Howie i~ J.M. Thomas and R.M. Lambert (Ed.), Characterization of Catalyst, J. Wiley and sons, 1980, p. 89. 8 J.P. Lynch, E. Lesage, H. Dexpert and E. Freund, lnst. Phys. Conf., 61(2) (1981)67. 9 H. Dexpert, J.P. Lynch and E. Freund, lnst. Phys. Conf., 61(4)(1981)171.

471 DISCUSSION J.W.E. COENEN In several laboratories (Eindhoven, Leiden) evidence was obtained that in well-dispersed supported noble metal catalysts, metal ions were present, which presumably could act as a glue layer between metal crystallite and support and thereby enhance stability against sintering. The same idea was put forward by me for Ni/Si02 already 10 years ago. E. FREUND: According to our TPR results concerning the palladium on nitrate system, the proportion of unreduced palladium is small if not'zero. As the metal dispersion is not very high (maximum 25%), the presence in sufficient number of unreduced palladium cations cannot be ruled out. However, for another system: platinum on Yc chlorinated alumina (reforming catalysts) we have carried out a det~iled EXAFS and XANES study, (to be published very shortly), which clearly shows the absence of any significant amount of unreduced platinum. In this case, the metal dispersion is 100% (maximum crystallite size 0.9 nm). R. VAN NORD STRAND : With your Pd nitrate preparation, the TPR curve was explained as a reduction of Pf at low temperature, followed by reduction of the nitrate at a considerably higher temperature. How is this possible, that the nitrate ion remains while its cation is reduced? E. FREUND / I agree that the nitrate anions located near the palladium cations are likely to be reduced at low temperature. However, because low pH solution is used for the impregnation, nitrate anions will be fixed on the alumina carrier away from palladium. These nitrate anions will not be so easily reducible. H. CHARCOSSET: 1. A short comment on TPR. The diagrams are often complicated by superimposition of H2 consumption due to reduction and of H2 desorption at the same time. We have observed this phenomenon over (Pt,Ru)/y-AI203' 2. With your techniques could you easily observe a reduction of the support at the interface Pd/AI203' if for instance you would reduce your catalysts at higher temperatures ? E. FREUND: 1. We do not observe H2 desorption (except hydride decomposition below 373K) in the temperature range considered in our study. 2. No,except if a well defined Pd-Al intermetallic compound is formed. The precision for parameter determination from an electron microdiffraction pattern (or more generally an electron diffraction pattern) is rather poor (5%), so that a Pd-Al solid solution would not be detected. H. CHARCOSSET : What experimental evidence have you to say that the high temperature TPR small peak is due to reduction of SO~or/and Fe 2 + and Fe 3 + in the support ? E. FREUND This attribution was arrived at by considering different alumina carriers having different impurity contents (especially titanium, iron and sulphate) .