Electron microscopy characterization of Ni/Hfl-zeolite ... · Electron microscopy characterization...

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REVISTA MEXICANA DE F ´ ISICA 50 SUPLEMENTO 1, 69–71 JUNIO 2004 Electron microscopy characterization of Ni/Hβ -zeolite catalyst prepared by deposition-precipitation methods I. Puente-Lee Laboratorio de Microscopia Electr´ onica, Depto. De Ingenier´ ıa Qu´ ımica, Facultad de Qu´ ımica, Universidad Nacional Aut´ onoma de M´ exico, 04510, D.F., M´ exico, e-mail: [email protected] R. Nares and J. Ram´ ırez UNICAT, Departamento de Ingenier´ ıa Qu´ ımica, Facultad de Qu´ ımica, Universidad Nacional Aut´ onoma de M´ exico, Cd. Universitaria, 04510, D.F., M´ exico, e-mail: [email protected] P.S. Schabes-Retchkiman Instituto de F´ ısica, Universidad Nacional Aut´ onoma de M´ exico, Apartado Postal 20-364, 01000, D.F. M´ exico, e-mail: schabes@f´ ısica.unam.mx Recibido el 27 de marzo de 2003; aceptado el 18 de julio de 2003 In the present work we present the characterization by electron microscopy (TEM and SEM) and microanalysis of Ni/Hβ catalysts prepared by the deposition-precipitation method (DP). During such preparation nickel hydrosilicates are formed mainly, but not exclusively, in the external surface of the Hβ zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during calcination and reduction of the catalysts, and TEM shows clearly that an homogeneous distribution of Ni metal particles, of approximately 3-5nm in size is obtained. The catalysts obtained by the DP method showed better catalytic activity in hydrogenation of naphthalene than the catalysts prepared by other methods. Keywords: Electron microscopy; Ni-Zeolite catalysts. En este trabajo se presenta la caracterizaci´ on por microscop´ ıa electr´ onica (TEM y SEM) y microan´ alisis de catalizadores Ni/Hβgreparados por el m´ etodo de deposici´ on-precipitaci´ on. En la preparaci´ on mencionada se forman hidrosilicatos de n´ ıquel principalmente pero no de manera excluyente en la superficie externa de la zeolita Hβ La interacci´ on fuerte entre metal y soporte inducida por el m´ etodo DP de preparaci´ on, previene la sinterizaci´ on de las part´ ıculas de Ni met´ alico en los procesos de calcinado y reducci´ on y TEM muestra claramente que se obtiene una distribuci´ on homog´ enea de part´ ıculas en el soporte con tama˜ nos t´ ıpicos de 3-5 nm. Los catalizadores obtenidos por este etodo muestran una actividad cat´ alitica mayor en reacciones de hidrogenaci ´ on de naftaleno que aquellos preparados por otros m´ etodos. Descriptores: Microscop´ ıa electr ´ onica; catalizadores Ni-Zeol´ ıticos. PACS: 61.43.Gr; 68.37.-d 1. Introduction The preparation of supported catalysts with high dispersion and high metal loading is a field of great interest in cataly- sis. For instance, Ni catalysts are widely used in hydrogena- tion reactions. Among the most used preparation methods, ion exchange can provide high metal dispersions but lim- ited metal loadings whereas impregnation can achieve high metal loading but a limited dispersion. In the case of Ni/SiO 2 catalysts, high loadings and high metal dispersions can be achieved with the deposition-precipitation (DP) method de- veloped by Geus [1-3]. This method is based on the pre- cipitation of a nickel(II) phase onto silica thanks to the slow and homogeneous basification of the solution containing the metal precursor and the support by urea hydrolysis at 90 C. The nature of the Ni(II) phase depends on the DP time and the silica surface area [4]. On silica of high surface area (400 m 2 .g -1 ), 1:1 nickel phyllosilicate is the main Ni(II) phase. It may be noted that the source of Si for the formation of 1:1 nickel phyllosilicate is the support itself, which is grad- ually consumed during DP. The Ni loading increases with the DP time and 20 wt % of Ni are deposited after 4 h of DP. After reduction in hydrogen, highly dispersed Ni particles, whose average size also depends on the DP time and on the silica surface, are obtained (from 2.7 to 7.9 nm) [5]. In addi- tion, the metal particles do not easily sinter because of their strong interaction with the support [4,5]. Hence, it appears interesting to attempt to apply this method to the preparation of Ni catalysts supported on zeolites with high Si/Al ratios. Among zeolites, the beta zeolite (Hβ) presents interest- ing properties as a support because of its three-dimensional 12-ring pore system with an intersecting channel system 6 , and its rather large pores (7.6 × 6.4 ˚ A and 5.5 × 5.5 ˚ A). Hence, in this study, we will show the characterization by TEM and SEM of Ni/Hβ-zeolite catalysts obtained by the DP method with high Ni loading. Electron Microscopy will allow us to find answers to the following questions: (i) to determine how the structural properties of the zeolite are affected by the deposition of nickel;

Transcript of Electron microscopy characterization of Ni/Hfl-zeolite ... · Electron microscopy characterization...

REVISTA MEXICANA DE FISICA 50 SUPLEMENTO 1, 69–71 JUNIO 2004

Electron microscopy characterization of Ni/Hβββ-zeolite catalyst prepared bydeposition-precipitation methods

I. Puente-LeeLaboratorio de Microscopia Electronica, Depto. De Ingenierıa Quımica, Facultad de Quımica,

Universidad Nacional Autonoma de Mexico, 04510, D.F., Mexico,e-mail: [email protected]

R. Nares and J. RamırezUNICAT, Departamento de Ingenierıa Quımica, Facultad de Quımica, Universidad Nacional Autonoma de Mexico,

Cd. Universitaria, 04510, D.F., Mexico,e-mail: [email protected]

P.S. Schabes-RetchkimanInstituto de Fısica, Universidad Nacional Autonoma de Mexico,

Apartado Postal 20-364, 01000, D.F. Mexico,e-mail: schabes@fısica.unam.mx

Recibido el 27 de marzo de 2003; aceptado el 18 de julio de 2003

In the present work we present the characterization by electron microscopy (TEM and SEM) and microanalysis of Ni/Hβ catalysts preparedby the deposition-precipitation method (DP). During such preparation nickel hydrosilicates are formed mainly, but not exclusively, in theexternal surface of the Hβ zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metalparticles from sintering during calcination and reduction of the catalysts, and TEM shows clearly that an homogeneous distribution of Nimetal particles, of approximately 3-5nm in size is obtained. The catalysts obtained by the DP method showed better catalytic activity inhydrogenation of naphthalene than the catalysts prepared by other methods.

Keywords: Electron microscopy; Ni-Zeolite catalysts.

En este trabajo se presenta la caracterizacion por microscopıa electronica (TEM y SEM) y microanalisis de catalizadores Ni/Hβgreparadospor el metodo de deposicion-precipitacion. En la preparacion mencionada se forman hidrosilicatos de nıquel principalmente pero no demanera excluyente en la superficie externa de la zeolita Hβ La interaccion fuerte entre metal y soporte inducida por el metodo DP depreparacion, previene la sinterizacion de las partıculas de Ni metalico en los procesos de calcinado y reduccion y TEM muestra claramenteque se obtiene una distribucion homogenea de partıculas en el soporte con tamanos tıpicos de 3-5 nm. Los catalizadores obtenidos por estemetodo muestran una actividad catalitica mayor en reacciones de hidrogenacion de naftaleno que aquellos preparados por otros metodos.

Descriptores: Microscopıa electronica; catalizadores Ni-Zeolıticos.

PACS: 61.43.Gr; 68.37.-d

1. Introduction

The preparation of supported catalysts with high dispersionand high metal loading is a field of great interest in cataly-sis. For instance, Ni catalysts are widely used in hydrogena-tion reactions. Among the most used preparation methods,ion exchange can provide high metal dispersions but lim-ited metal loadings whereas impregnation can achieve highmetal loading but a limited dispersion. In the case of Ni/SiO2

catalysts, high loadings and high metal dispersions can beachieved with the deposition-precipitation (DP) method de-veloped by Geus [1-3]. This method is based on the pre-cipitation of a nickel(II) phase onto silica thanks to the slowand homogeneous basification of the solution containing themetal precursor and the support by urea hydrolysis at 90◦C.The nature of the Ni(II) phase depends on the DP time andthe silica surface area [4]. On silica of high surface area(≈400 m2.g−1), 1:1 nickel phyllosilicate is the main Ni(II)phase. It may be noted that the source of Si for the formationof 1:1 nickel phyllosilicate is the support itself, which is grad-

ually consumed during DP. The Ni loading increases with theDP time and≈20 wt % of Ni are deposited after 4 h of DP.After reduction in hydrogen, highly dispersed Ni particles,whose average size also depends on the DP time and on thesilica surface, are obtained (from 2.7 to 7.9 nm) [5]. In addi-tion, the metal particles do not easily sinter because of theirstrong interaction with the support [4,5]. Hence, it appearsinteresting to attempt to apply this method to the preparationof Ni catalysts supported on zeolites with high Si/Al ratios.

Among zeolites, the beta zeolite (Hβ) presents interest-ing properties as a support because of its three-dimensional12-ring pore system with an intersecting channel system6,and its rather large pores (7.6× 6.4A and 5.5× 5.5A).

Hence, in this study, we will show the characterizationby TEM and SEM of Ni/Hβ-zeolite catalysts obtained by theDP method with high Ni loading. Electron Microscopy willallow us to find answers to the following questions:

(i) to determine how the structural properties of the zeoliteare affected by the deposition of nickel;

70 I. PUENTE-LEE, R. NARES, J. RAMIREZ, AND P.S. SCHABES-RETCHKIMAN

(ii) to determine where and which Ni phase is formed inthe Hβ zeolite, especially after reduction.

2. Experimental methods

2.1. Sample preparation

The Hβ-zeolite sample was provided by Zeolyst Interna-tional CP811E-75 (lot 1822-74; SiO2/Al2O3=75, surfacearea =580 m2.g−1). The Ni/Hβ zeolite samples were pre-pared by the DP method decribed elsewhere [1-4], the sam-ples were subjected to different deposition times from 1to 4 hours.

2.2. Sample characterization

For SEM observation, the samples in the form of powderswere stuck on carbon tabs and then inserted to the microscopeat Low Vacuum Operation mode, no coating of the specimenswas needed, which could obscure some structural character-istics of the samples to be studied. The Ni content, of theNi/Hβ samples was determined with a JEOL JSM-5900LVmicroscope equipped with an analytical EDX accessory.

For the TEM study of the reduced catalysts, the sampleswere reduced in a quartz gas flow reactor at atmospheric pres-sure during 5 h at 450◦C under a flow of H2. For TEM ob-servation the samples in the form of a powder were ground,floated in water free alcohol, dispersed with an ultrasoundfor a couple of hours. Then a drop was collected from the topsurface mounted on carbon coated copper grids and allowedto dry. The Ni zeolite catalysts were observed in a JEOLJEM 2010 high-resolution electron microscope operating at200 kV. The average of metal particle sizes was establishedfrom the measurement of at least 300 particles.

3. Results

3.1. SEM

A SEM micrograph of the Ni free zeolite is shown in Fig. 1together with an EDS spectrum of the sample, the character-istic peaks correspond to the zeolite Al, Si, O. Figure 2 showsthe SEM micrograph and microanalysis of typical Ni/Hβsamples, the spectrum shows clearly the zeolite peaks plusthe Ni peaks. The micrograph is a characteristic image of agranulated surface on the zeolite powder particles. As the DPtime increases, the surface looks more and more granulated.These observations are well in line with the dissolution ofpart of the silica surface in the Hβ zeolite and the formationof Ni phases on the external zeolite surface.

3.2. TEM

Figures 3,4, and 5 display the micrographs of samples Hβ,Ni/Hβ1 and Ni/Hβ4 after drying and after reduction. InFig. 3, an image of Hβ, lamellar structures are observed. In

FIGURE 1. a) Typical SEM image of the beta zeolite (Hβ) powder;b) Corresponding X-ray Microanalysis.

FIGURE 2. a) Typical SEM image of the Hβ/Ni powder; b) Corre-sponding X-ray Microanalysis.

FIGURE 3. TEM bright-field micrograph of the typical beta zeolite(Hβ).

FIGURE 4. TEM bright-field of the typical Ni/Hβ 1 hour DP, afterreduction; the shape of the Ni particles is clearly shown, an MTPparticle is observed.

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ELECTRON MICROSCOPY CHARACTERIZATION OF NI/Hβββ-ZEOLITE. . . 71

FIGURE 5. TEM bright-field micrograph of the typical Ni/Hβ 4hour DP, after reduction for 5 hours.

the micrographs of the reduced Ni loaded samples, Figs. 4and 5, the lamellar structures, after reduction, are replacedby a distribution of small particles. The samples reducedat 450◦C for 5 h, Fig. 5 show an homogeneous distribu-tion of Ni metallic particles. In some areas of the reducedNi/Hβ4 sample, lamellar structures are still observed, in-dicating an incomplete reduction of the Ni in the silicatestructures. These observations are consistent with those inNi/SiO2 systems [7].

The average particle size in the reduced Ni/Hβ samplesincreases with the DP time from 2.9 nm to 4.4 nmA. Onlyin Ni/Hβ1, i.e. with 1 hour treatment, particles smaller than1nm were observed.

4. Discussion

The granulated surface which appeared on the zeolite parti-cles (SEM micrographs in Figs. 1 and 2 ) as the DP time in-creases, is also consistent with the formation of a new lamel-lar Ni(II) phase. This lamellar phase may be nickel phyllosil-icate or nickel hydroxide or a mixture of them since both arelamellar compounds. TPR and IR spectroscopy (not shownhere) provided us valuable information. At the beginning ofthe DP, nickel hydroxide is the main species present on theHβ zeolite. As the DP time increases, more and more 1:1 Niphyllosilicate is formed and its crystallinity increases. Hence,

the Ni/Hβ samples contain mainly nickel hydroxide for DPtimes lower than 2 h. A growing proportion of 1:1 nickelphyllosilicate exists for 3 and 4 h of DP time.

After reduction of the Ni/Hβ samples, the TEM micro-graphs show that the average size of the Ni metallic particles(Figs. 3-5) is small (between 2.9 and 4.4 nm) for Ni loadingsfrom 5.5 to 22 wt %. These result are consistent with thoseobtained on Ni/SiO2 samples prepared by DP and reducedat similar conditions [8]. The strong interaction between theNi(II) phase and the Hβ zeolite notably inhibits the sinteringof the nickel particles during reduction treatments, and leadto small metallic particles, as in the case of Ni/SiO2 samplesprepared by deposition-precipitation [9]. It may be noted thatmost of the metallic particles are larger than the pore size ofthe Hβ zeolite, confirming that the Ni(II) phase and most ofthe Ni metallic particles after reduction are deposited on theexternal surface of the Hβ zeolite.

5. Conclusions

The following conclusions can be drawn. The deposition-precipitation of nickel on Hβ zeolite occurs in the same wayas for the DP of nickel on silica of low surface area. The Niloading increases with the DP time (1 to 4 h) in a highly re-producible fashion, from 5.5 to 22 wt %. For short DP times(≤2 h), nickel hydroxide is the main Ni(II) phase on Hβ zeo-lite whereas for longer DP times (3 and 4 h), the Ni(II) phaseis a mixture of nickel hydroxide and 1:1 nickel phyllosilicate.The characterization study of the Hβ support indicates that itis mostly the external surface that is involved for supportingthe Ni(II) phase. It also confirms the consumption of the zeo-lite framework for the formation of 1:1 nickel phyllosilicate.Nickel hydroxide and 1:1 nickel phyllosilicate which are bothlamellar structures, lead to an increase in the mesoporous sur-face area of the samples.

Hence, the method of deposition-precipitation of nickelapplied to the Hβ zeolite leads to high Ni loading and high Nidispersion (average metallic particle size from 2.9 to 4.4 nm).However, the metallic nickel particles are mainly located onthe external surface of the zeolite.

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2. J.A. van Dillen, J.W. Geus, L.A. Hermans, and J. van derMeijden, Proc. 6Intern. Congr. Catal., London, 1976;Bond,G.C.; Wells, P.B.; Tompkins, F.C. Eds.; Elsevier: Amsterdam,(1977); pp 677.

3. M.J.F.M. Verhaak, A.J. van Dillen, and J. W. Geus,Appl. Catal.105(1993) 251.

4. P. Burattin, M. Che, and C. Louis,J. Phys. Chem. B101 (1997)7060.

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6. J.B. Higginset al., Zeolites8 (1988) 446.

7. D.W. Breck, Zeolite Molecular Sieve: Structure, Chemistry,and Use(John Wiley and Son: New York, 1974) p. 415.

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