LOCAL STRUCTURE IN BIFUNCTIONAL NiO-MoO3 ZEOLITE CATALYST BY K-Ni EXAFS AND XANES SPECTROSCOPY

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LOCAL STRUCTURE IN BIFUNCTIONAL NiO-MoO3 ZEOLITE CATALYST BY K-Ni EXAFS

AND XANES SPECTROSCOPY

A. Clozza, J. Garcia, A. Bianconi, A. Corma

To cite this version:

A. Clozza, J. Garcia, A. Bianconi, A. Corma. LOCAL STRUCTURE IN BIFUNCTIONAL NiO-

MoO3 ZEOLITE CATALYST BY K-Ni EXAFS AND XANES SPECTROSCOPY. Journal de

Physique Colloques, 1986, 47 (C8), pp.C8-313-C8-316. �10.1051/jphyscol:1986861�. �jpa-00226183�

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Colloque C8, suppl6ment au n o 12, Tome 47, decembre 1986

LOCAL STRUCTURE I N BIFUNCTIONAL NiO-Moo3 ZEOLITE CATALYST BY K-Ni EXAFS AND XANES SPECTROSCOPY

A. CLOZZA, J. GARCIA('), A. BIANCONI and A. CORMA*

D i p a r t i m e n t o d i F i s i c a ,

~ n i v e r s i t s

" L a S a p i e n z a " , P i a z z a l e A l d o Moro, 2 , I - 0 0 1 8 5 Roma, I t a l y

" ~ n s t i t u t o d e C a t a l i s i s y P e t r o l e o q u i m i c a , C.S.I.C. S e r r a n o , 1 1 2 Dup., E-28000 M a d r i d , S p a i n

ABSTRACT

The local structure around the Ni atom has been studied by EXAFS and XANES spectroscopy in NiO-Moo3 Zeolite supported catalyst. Coordination, geometry and degree of dispersion of the NiO component has been determined for different concentrations of the active component.

1 INTRODUCTION

A process which has been very useful in dealing with low quality and highly contamined feeds is hidroc;acking. The hydrocracking catalyst of commercial interest are of t h e bifunctional type with a hidrogenation-dehidrogenation cor?po_?ent on an acidic support and the most usual components are nickel, cobalt, molybdenum and tungsten( t L I .

The final product distribution obtained with a given hydrocracking catalyst does not only depend on the individual characteristics of the two functionals, but also on the interaction between the support and the hydrogenating components. This interaction is strongly affected by the degree of dispersion of the active components on the support and is directly correlated with the preparation metod.

In this work we have studied by EXAFS and XANES spectroscopy the local structure around Ni atom in bifunctional NiO-Moo3 Y-zeolite catalysts prepared with different.procedure and concentration of the active components.

2 EXPERIMENTAL

Seven samples of metal containing zeolite are prepared for this study. An ultrastable HY zeolite (HYUS) is obtained by repeated ion exchange of the sodium form of SK-40 (SiIAI =2.4) with amonium acetate solution, with intermediate deep bed calcinations at 823 K, until the Na+ of the zeolite is less than 2% of the original. The nickel and molybdenum are incorporated into the HYUS zeolite by vacuum impregnation from an acqueous solution of Ni(NO& andlor ammonium heptamolybdate, forming NiO or

-

Following this procedure a series of catalyst are prepared containing: 8 wt % Moo3 and 4 % NiO with the molybdenum impregnated before, after or simultaneously to the nickel ; 6 wt % Ni018 WW/O Moo3 and 9% Ni0/12°/o Moog respectively, with the NiO first incorporated; and 4 ^O/O, 6"/0 and 8% NiO containing sample.

Absorption spectra of the Ni K-edge has been carried out at the PULS Frascaty radiation Facility using a channel cut Si(l1 I) single crystal monocromator. Our samples consist of tablets of powdered catalyst in a boric acid matrix.

("permanent address : Departement of Therrnology, University of Zaragoza.. Spain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986861

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3 RESULTS and DlSCUSSlON

In fig.1 are shown the EXAFS spectra and the Fourier transforms of the catalyst containing only NiO.

The EXAFS signal for the NiO 4% containing sample shows a single oscillation corresponding to a first shell of scattering atoms. Increasing the NiO concentration, the spectra became more structurated aproximating the shape to that of crystalline NiO (lower curve of fig. 1). The Fourier transforms of these spectra exhibit the precedent evolution giving only one peak for the lowest concentration to two peaks when the NiO concentration grows up. The conventional data analysis of these spectra, taking as reference the spectrum crystalline NiO and assuming phase transferability, shows that Ni is coordinated by six oxygen atoms. For 6 and 8% containing samples appears a second shell contribution corresponding to the Ni-Ni distance in crystalline NiO, but with smaller coordination than that of the reference sample. This fact gives evidence of the formation of small clusters of NiO dispersed in the zeolite. Structural parameters, given in table 1, show that interatomic distances are from 2 to 5% smaller than that of NiO reference sample, such has been observed in other dispersed catalysd3).

Table 1. Interatomic distances and approximate number of Ni atoms in the aggregates of NiO.

Sample r lSt shell r 2" shell NiNO atoms/cluster Ni0-4%

NiO-6Yo Ni0-8%

Ni0-4% Moo3-8%

Ni0-4% MOO 8x5.

Moo 8%~i8:4%

N~o-&L~ Moo3-8Yo Ni0-9% Moo3-1 2%

N i i

We have investigated the coordination geometry by measuring the XANES ~ ~ e c t r a . ( ~ * ~ ) The spectrum of Ni0-4% sample coincides with that of ~ iin water solution (upper curve in fig.2), this means that ~ + same local structure is present in these two systems. Theoretical XANES spectra, using the multiple scattering approach, for octahedral clusters with a transition metal surrounded by six oxygens, show that ~ iin water solution is octahedrically coordinated by six oxygen atoms.(5) In the spectra of ~ + crystalline NiO a new feature (labeled A) grow up. The structure A came from the effect of the presence of a second Ni shell. Evolution in the different spectra in relation to these features confirm the previous conclusions obtained from the EXAFS analysis.

The influence in the local structure at Ni site due to the different method of incorporation of the Moo3 component in the zeolite have been investigated by comparison of the EXAFS and XANES spectraof Ni0-4% Moo3-8%. The presence of Moo3 in the supported catalyst, does not produce significant changes in the XANES spectra of the Ni0-4% zeolite, this fact implies that octahedral geometry is conserved, and NiO is higly dispersed on the zeolite matrix. We observe an assymmetry of the radial distribution function, in the Fourier transform of the EXAFS spectra, in the case of simultaneous incorporation and/or when the molybdenum is incorporated before. This results can be interpreted assuming that the presence of Moo3 in the zeolite affects the position in which the NiO is fixed. Probably the different Ni-0 bond distances are due to an interaction between Ni and Mo atoms through oxygen bonds.

We have also investigated the catalyst gOhNiO-12%Mo03 and 6%Ni0-8%Mo03 with NiO first incorporated. The comparison with the Fourier transform of the NiO crystalline sample shows a similar structure starting from the second peak to higher distances. In fact, the filtered spectra of the reference and of the last sample, subtracting the contribution of the first shell, gives practically the same signal.

This result implies that, in this sample, NiO is poorly dispersed and forms small clusters of crystalline NiO.

The differences observed in the second coordination shell are not well correlated with the concentration ratio of Ni and Mo in the catalyst. This suggest that the dispersion is strongly dependent on firie details in the preparation method.

In table 1 are reported the obtained distances for the first and second shells, the coordination ratio of the second shell related to the crystalline NiO and an estimation of the number of atoms in each aggregate for the different catalysts.(6)

Fig.1

EXAFS spectra (panel a) and their Fourier transforms (panel b) at the Ni K-edge of NiO zeolite supported catalyst with different concentrations of NiO. The upper and lower spectra correspond to CI2Ni acqueous solution and crystalline NiO respectively.

-20 0 20 40 60

E-E, (eV)

Fig.2

XANES spectra at the Ni K-edge of NiO zeolite catalyst.

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4 CONCLUSIONS

We can point out that, in NiO-Moog zeolite catalyst, the Ni atom is octahedrically surrounded by six oxygen atoms in all the samples. The interaction with the support is probably given by bonds with the hydroxyl groups of the zeolite. Generally NiO tends to form aggregates, only the 4OA containing sample gives a totally dispersed structure. Dimension of this aggregates depends principally on the NiO concentration and also on littles details in the preparation method. Effects of the presence of Moo3 not - seem significative for this tendence.

The absence of significative changes in the spectra of Moog containing samples, when compared with samples containing only NiO, show that the interaction between Ni and Mo atoms are through the support and not due to direct interaction.

References

1. Ahuja S.P., Derrien M.L. and Le Page J.F.; Ind. Eng. Chem. P.R.D.

9,3,

2. Konings A.J.A., Brentjens W.L.J., Koningsberger D.C. and De Beer V.H.J.; J. Catal. 67,145, (1 981)

3. Lagarde P., Dexpert H.; Adv. Phys. 33,567, (1984)

4. Bianconi A., Garcia J., Marcelli A,, Benfatto M., Natoli C.R. and Davoli I.; J. de Phys. 46, 89, (1 985)

5. Garcia J., Bianconi A., Benfatto M. and Natoli C.R.; in these proceedings 6. Greegor R.B. and Lytle F.W.; J. Catal. 63,476, (1 980)