Metal-support effects and catalytic properties of platinum supported on zinc aluminate

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Applied Catalysis A: General, 90 (1992) 25-34 Elsevier Science Publishers B.V., Amsterdam APCAT A2336

Metal-support effects and catalytic properties of platinum supported on zinc aluminate

G. Aguilar-Rios Znstituto Mexican0 de1 Petrbleo, A.P.14-80S, 07730, M&ico D.F. (Mexico)

M.A. Valenzuela Znstituto Mexican0 de1 Pethleo, A.P.14-805,07730, Mkcico D.F. and Universidad Authoma Metropolitana-Z,A.P.53-534,09340, M&co D.F. (Mexico)

H. Armendariz, P. Salas and J.M. Dominguez Znstituto Mexican0 del Petrbleo, A.P.l4-805, 07730, Mkxico D.F. (Mexico)

D.R. Acosta Znstituto de Fkica, UNAM., A.P.20-364,01000,

M&co D.F. (Mexico)

and I. Schifter Znstituto Mexican0 de1 Petrbleo, A.P.14-805,07730,

Mgxico D.F. (Mexico)

(Received 7 April 1992, revisedmanuscriptreceived 14 July 1992)

Abstract Pt/ZnAlz04 catalystswith platinum contents ranging from 0.1 to 1.17 wt.-% were characterizedby temperature-programmedreduction, hydrogen chemisorption, high resolution transmission electron microscopy, electron energy loss spectroscopy and tested for isobutane dehydrogenation,using helium or hydrogen as reaction media. For low metal contents, the resultssuggestthat platinum diffuses into the oxygen vacancies in the spine1lattice while at higher loadings, metal-metal interactions are dominant, leading to particle formation. The catalytic behaviour points to the necessity of strong Pt-H interactionsto preserveactivity. Keywords: characterization (TPR, HRTEM, EELS, XRD), platinum, zinc aluminatespinel, dehydrogenation,support effects. Correspondence to: Dr. Gabriel Aguilar-Rfos, Institute Mexican0 de1 Petrdleo, Gerencia de Catilisis y Mat&ales, Eje L&ear0Cardenas No. 152, C.P. 07730, MBxico, D.F., Mexico. Tel. ( +52915) 3689226, fax. (+52-915) 5679270.

0926-3373/92/$05.00

0 1992 Elsevier Science Publishers B.V. All rights reserved.

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INTRODUCTION

In the search for improved properties of dehydrogenation catalysts, great interest has been focused on spine1 type structures like magnesium and zinc aluminates [l-5]. The main characteristics of those solids are hydrophobicity, high mechanical resistance and very low surface acidity, which are well suited to perform in the severe conditions imposed by dehydrogenation reactions of light alkanes [ 31. Those properties make them interesting as carriers for noble metals to substitute the more traditional Cr203/A1203 catalysts [ 2,4]. Rennard and Free1 [5] have shown that platinum supported on magnesium aluminate is more resistant to sir&ring due to regeneration by oxidative procedures than the Pt/Al,O, and Pt/SiO, systems. There is a vast literature concerning the platinum on alumina system [6-G], but for the metal supported on zinc aluminate no complete picture of both surface and bulk properties related to catalytic activity is available. It is the aim of this work to contribute with a multi-technique characterization of the Pt/ZnAlzOl system, including high resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), temperature-programmed reduction (TPR), hydrogen chemisorption (HC) and catalytic performance. Some of the fundamental aspects like metal-support interaction and selectivity of the Pt/ZnAIPO1 catalysts for isobutane dehydrogenation leading to C, alkenes were tackled in this work. EXPERIMENTAL

Catalysts preparation Zinc aluminate was prepared by coprecipitation from the nitrates (Baker, 99.0 wt.-% ) with ammonium carbonate. The precipitate was carefully washed with demineralized water and then calcined at 800’ C in air for 6 h. The support (So-100 mesh) was impregnated by an incipient wetness technique using aqueous solutions of H,PtCl, (Alfa Products) to obtain platinum contents between 0.1 and 1.17 wt.-%. Catalysts were dried at 80°C in vacuum. Calcination was done at 500’ C for 4 h after heating at a rate of 2’ C /min in dry air. In some cases a commercial chromia on alumina catalyst was used as a reference during catalyst testing [ 161. Characterization techniques Chemical composition was determined by atomic absorption spectroscopy in a Perkin Elmer 2380 apparatus. The standards employed for quantitative analysis were: AlC& in water (1.0 g 2 0.002 g Al, Merck) and ZnClz in 0.06% HCl(l.00 g + 0.002 g Zn, Merck). Reported measurement represent an average

G. Aguikar-Rios et al.fAppl. Catal. A 90 (1992) 25-34

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value of three determinations. X-ray diffraction patterns served to identify the crystalline phases present and were obtained with a D-500 Siemens diffractometer coupled to a copper anode tube. The Cu K& radiation was selected with a nickel filter. Textural properties were calculated from nitrogen physisorption data (Micromeritics ASAP-2000 ) . TPR experiments were performed in a conventional system [ 171, using 2 l/h of a mixture consisting of 4 mol-% of hydrogen in argon and temperature ranges from - 80 to 800” C at a heating rate of 10” C/min. The chemisorption measurements were carried out at 25 ’ C on a volumetric installation; firstly the overall hydrogen uptake was determined, the samples were then evacuated and redosed to measure the reversible hydrogen uptake. The irreversible uptake used to calculate platinum dispersion was obtained by subtracting the irreversible from the total uptake in the extrapolated isotherms to zero hydrogen pressure. An H/Pt ratio of 1 for a completely dispersed sample was assumed for calculations [ 181. Catalysts were pretreated at 500’ C for 18 h in vacuum before performing either TPR or hydrogen chemisorption. HRTEM observations were carried out in a Jeol-400 kV electron microscope equipped with a high resolution pole piece (Cs = 1 mm). Also, analytical TEM observations were performed in a lOO-CX electron microscope fitted with a goniometer stage and EDS system (Tracer 5500). EELS experiments were carried out in a 607-Gatan spectrometer, coupled to a Jeol-200 electron microscope. The powder samples were prepared by direct deposition onto a holey copper grid; this avoided interference effects from hydrates or from oxygen in the plastic support materials, which are commonly used in TEM work. A detailed description of the techniques employed has been published elsewhere [ 191. Isobutane dehydrogenation was carried out using a conventional continuous flow microreactor system, operated at 550°C and 590 mm Hg total pressure. Measurement of isobutane conversion and the product distribution were made using on-line gas chromatographic (GC ) analysis. The GC (Varian-3700) was fitted with a flame ionization detector and a 3 m silica gel column. In all the experiments 0.05 g catalyst and a total flow-rate of 2.4 l/h were used. Feed stream was an equimolar mixture of isobutane with either helium or hydrogen. Before the reaction test, catalysts were pretreated with hydrogen (1.8 l/h) at 550’ C for one hour. The conversion of isobutane is defined as the percentage of isobutane converted to all products. The selectivity to isobutene (S) is defined as the amount of isobutane converted to isobutene divided by the amount of reactive converted to all products, and reported as a mole percentage. Reaction rate ( - rA) was calculated assuming differential behaviour of the reactor. RESULTS AND DISCUSSION

BET surface area of about 20 m2/g and pore volume of 0.12 cm3/g were obtained for the support. The normal spine1 structure of zinc aluminate was

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et al./Appl. Catal. A 90 (1992) 25-34

corroborated by X-ray diffraction (XRD); the obtained patterns were similar to that reported in the JCPDS-PDF card # 5-669 [ 201. An excess of aluminium with respect to the stoichiometric needed for zinc aluminate spine1 was used to avoid undesirable Pt-Zn interactions [ 211, therefore chemical analysis reported an Al/Zn atomic ratio equal to 2.12. Aluminium free phases were not detected by XRD. TPR results (Fig. 1) suggest the presence of two platinum species with reduction peaks located at around 300 and 500” C, respectively. In samples with platinum contents lower than 0.6 wt.-%, the H,/Pt mole ratio calculated from the TPR total hydrogen consumption is near two, so platinum is present in an oxidation state of 4 + . For higher platinum loadings, the mole ratio H,/Pt decreases; thus platinum apparently is either in a lower oxidation state or as aggregates of lower dispersion. This behaviour is evident in Fig. 2 in which the total amount of hydrogen consumed (open symbols) are compared with the theoretically needed in terms of the reaction: PtO, + 2Hz+Pt+ 2H20 (filled symbols). The low-temperature reduction peak at about 10” C assigned by some authors [8] to bulk PtOz phase in high concentrations on the y-alumina support, is not detected in our case. To explain the presence of two platinum species in oxidation state 4 + , one must take into account a previous work made on platinum-y-alumina catalyst [ 91, which concludes that two platinum surface oxides as well as two oxychlorinated compounds are present with metal in 4 + oxidation state. PtOz species are reduced at lower temperatures than oxychlorinated compounds. In our case, it is possible to postulate the formation of the latter compounds because chloroplatinic acid was used for preparation of the catalysts. Table 1 shows that the total amount of chemisorbed hydrogen is almost independent of metal content up to 0.6 wt.-% Pt, for higher platinum loadings chemisorption is very sensitive to platinum content. However, the mole ratio I

2

Id0

300

TEMPERATURE

700

5&J (“C)

-

Fig. 1. Temperature-programmed reduction profiles. Code from Table 1

G. Aguilar-Rim

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et al./Appl. Catal. A 90 (1992) 25-34

0.2

1.2

0.6 w t % Pt -

Fig. 2. Total hydrogen consumption; Pt0,+2H,+Pt”+2H,0.

(0 ) experimental from TPR studies, (0)

calculated for

TABLE 1 Hydrogen chemisorption results of Pt/ZnAlzOl catalysta

Sample

Platinum (wt.-%)

Hydrogen adsorbed @mol Hz/g cat. )

1 2 3 4 5 6 7

0.00 0.10 0.16 0.47 0.59 0.81 1.17

0.00 1.95 2.01 1.93 2.00 2.20 3.60

Dispersion (%)

H/Pt atomic ratio

75.9 49.0 16.0 13.2 13.0 12.0

0.76 0.49 0.16 0.13 0.13 0.12

H/Pt decreases continuously as platinum increases, which suggests particle formation. Conventional transmission electron microscopy in the 100 kV microscope was used to obtain the structural characteristics by selected area electron diffraction. Zinc aluminate was fully identified in all cases, but platinum particles were observed only in samples with content above 0.5 wt.-% Pt. In samples with low platinum contents, the metal was not detected even by using long acquisition times in EDS analysis. In fact, the La1 and Ma1 lines of platinum at 9.49 keV and 2.051 keV, respectively, do not show up upon point analysis (200 A diameter beam at 100 kV); as the limit of detection by this technique is around lo-l7 g, we conclude that platinum is in a very dispersed state.

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HRTEM images are shown in Fig. 3, sections la and lb correspond to 1.17 wt.-% Pt catalyst; sections 2a and 2b are images from the 0.47 wt.-% Pt catalyst, section 2c is an enlargement of the particle indicated by the arrow in section 2b and finally, sections 3a and 3b were taken from the 0.16 wt.-% Pt. In all the cases, aggregates of zinc aluminate crystallites with many grain boundaries were observed. Even under different tilt orientations, particles were only observed in samples with platinum contents above 0.5 wt.-%. These showed a match with the crystallographic planes of the support material. Faceting was not clearly observed because of the low contrast conditions between particles and support. In all the cases lattice distortions of the support material were not observed. The fact that platinum was not detected with EDS or HRTEM analysis in samples with low content may suggest a specific interaction with the support material. EELS was used in order to provide information about the metal-support

Fig. 3. HRTEM images of the Pt/mO, catalysts la and lb are from 1.17 wt.-% pt; 2a and 2b from 0.47 wt.-% Pt; 2c is the particle indicated by the arrow in 2b; 3a and 3b are from 0.16 wt.-% Pt.

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interactions, searching for specific modifications of the Al-K and the O-K absorption edges of the support. This might indicate modifications in the coordination of the probed atoms that could take place in the solid due to possible metal intrusion in the spine1 lattice. In this case, the EELS spectra of zinc aluminate with and without metal should be compared in terms of the Al-K (E= 87 eV) and the O-K (E = 531 eV) absorption edges, respectively. The latter results are condensed in Fig. 4, Figs. 4a and 4b show the O-K absorption edges for 0.16 and 0.47 wt.-% Pt catalysts; on the other hand, Figs. 4c and 4d are the corresponding edges of the 1.17 wt.-% Pt catalyst and the pure ZnAlzOI support, respectively. As observed, either set is different from each other: for high platinum content catalysts (i.e. 1.17 wt.-% Pt), the O-K edges are similar to the pure ZnAlpOl phase; which means that supported metal phase might not disturb the oxygen coordination of the spine1 lattice. However, the low platinum content catalysts show distinct EELS absorption edge profiles indicating a modification of the chemical environment of the lattice oxygen with respect to the previous case. This change might be caused by the intrusion of atomic platinum metal in the spine1 oxygen vacancies. In fact, spine1 type structures have about a third of the lattice oxygen sites unoccupied (02- ionic radius equal to 1.4 A); then if the platinum metal atoms (platinum ionic radius equal

F’ig.4. EELS absorption edges for lattice oxygen in Pt/ZnAllOl catalyrh (a) 0.16 wt.-% Pt ; (b) 0.47 wt.-% pt; (c) 1.17 wt.-% pt; (d) pure ZnA1201.

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et al./Appl. Catal. A 90 (1992) 25-34

6.0

(Ih)

TOF S-’

3,0

I I

/

0.5 wt %

1.0 Pt-

Fig. 5. Turnover frequency (TOF) numbers and selectivity (% S) for isobutane dehydrogenation obtained for Pt/ZnAlp04 catalysts (0,. ) hydrogen and ( V, v ) helium. TABLE 2 Catalytic activity of Pt/ZnA&O, catalysts in isobutane dehydrogenation with hydrogen or helium as reaction media AU the values in the table were taken at 1 h reaction time Sample

Helium

Hydrogen -r*XlO”

Sb

(mol/s g)

(mol-%)

1

2.4

69.3

1.1

3 4

18.9 21.7 22.1

91.7 97.5 97.1

30.0 22.8

25.6

97.6

41.5 29.9

29.2

96.0

29.1

5 6 7

k,xlOk (min-‘)

TOFd (5-l)

-r*x1os”

s*

kdXIOk

TOP

(mol/s g)

(mol-%)

(min-‘)

(s-l)

-

2.3 6.3 5.2

58.2

5.1 5.5

86.3 80.0

4.5

75.4

4.7 5.6 5.5 4.8 4.1

63.7 86.1

1.9 69.6 70.2

1.5

49.6

1.3 1.3

49.7 45.2

0.6

1.0

a r&action rate, mol/s g. bS Selectivity to isobutene, mol-96. ‘kd First order deactivation constant, min-‘. dTOF Turn over frequency number, 8-l.

to 1.36 A) diffuse into the spine1 lattice, they should settle preferentially in oxygen vacancy sites. This is a lattice change at the sub-unit-cell level which might be caused by intrusion of the platinum metal atoms in the spine1 osygen vacancies. The metal in this case does not cluster on the surface but it might be partially spread out almost into atomic dispersion. These results are supported by the EELS Al-K absorption edge profiles (not shown) which present

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a different shape for low platinum contents with respect to that in the pure ZnAlz04 spinel. Results on catalytic conversion of isobutane are condensed in Fig. 5 and Table 2. When hydrogen is used instead of helium as diluent, turn over frequency (TOF ) numbers are higher and reach a maximum for a platinum concentration around 0.5 wt.-%. In contrast, in helium the activity decreases continuously as metal concentration increases. Concerning selectivity, the undesirable reaction observed was cracking of isobutane to propene and methane. When the hydrogen-isobutane mixture is used, selectivity attains the highest values (97% ) remaining constant for platinum loadings higher than 0.5 wt.-%. When helium-isobutane is used, selectivity shows a well defined maximum at about 0.5-0.6 wt.-% Pt. Deactivation is more important in helium atmosphere than in hydrogen; in fact, deactivation constants (&) obtained by means of a first order parallel deactivation model [22] are always greater when catalysts are tested in helium, see Table 2. The higher activity and selectivity values found when hydrogen is the reaction medium suggest that the interaction Pt-H plays an important role. When helium instead of hydrogen is used, hydrogen evolving from the reaction is not sufficient to transform all the platinum present to a Pt-H species, remaining platinum would promote cracking of the isobutane molecule. CONCLUSIONS

TPR data suggest the presence of two well defined platinum species on the surface; up to 0.6 wt.-% Pt, hydrogen consumption corresponds to that required for the reaction: PtO, + 2Hz + Pt + 2H,O; for larger platinum loadings the HJPt mole ratio is less than two. This means either that not all of the platinum present is reduced or that it is in a lower average oxidation state. The platinum on zinc aluminate might interact strongly with the support as evidenced by HRTEM and EELS studies; that is, it may diffuse into the zinc aluminate spine1 structure. This fact corroborates a distinct metal-support interaction of platinum in zinc aluminate with respect to y-alumina; this interaction explains why zinc aluminate prevents metal sintering. Catalytic experiments on isobutane dehydrogenation point out that hydrogen as reaction medium is very important to achieve high activity and selectivity.

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