CHARACTERIZATION OF ULTRA FINE SILICA SUPPORTED IRON CATALYSTS PREPARED BY THE AMMONIA METHOD

A. Loaiza-Gil1*, P. Rodríguez¹, W. Velasquez¹, D.Gómez¹, B. Fontal², M. Reyes², T. Suárez²

1. Laboratorio de Cinética y Catálisis, Departamento de Química, Facultad de Ciencias Universidad de Los Andes, Mérida 5101, Venezuela.
e-mail: loaizag@ciens.ula.ve
2. Laboratorio de Organometálicos, Departamento de Química, Facultad de Ciencias Universidad de Los Andes, Mérida, 5101 Venezuela.

Abstract
New ultra fine particle size and nonporous Fe/SiO2 catalysts ranging 20-160 nm were prepared with a modified high loading ammonia method. Scanning electron microscopy (SEM) associate to x-ray energy dispersion spectroscopy and nitrogen fisisorption studies of the fresh catalysts dried at 353 K shows a non porous structure constituted by superimposed sheet of rectangular crystals of Phyllosilicate type. The chemical and structural composition of such solids changes with the thermal pretreatment under hydrogen flow. FT-IR studies performed at programmed reduction temperature suggests that the fresh catalyst is constituted by an iron complex like Fe(H₂O)₆⁺³ sandwiched between bi-dimensional sheets of Si₂O₅ phyllosilicate. The amount of -OH groups bonded to the iron complex and those associates to the Si₂O₅ polymeric groups decrease progressively to disappears by slowly heating at 1 K/min from 353 to 973 K under hydrogen flow. At the final reduction temperature, a new catalyst structure constituted by silica support finely dispersed iron crystallites arise suggesting a catalyst morphology and properties governed by the temperature.

Keywords: Catalyst, Iron, ammonia method, Fischer Tropsch.

Resumen
Nuevos catalizadores no porosos de Fe/SiO₂ con tamaño de partículas en el orden de 20-160 nm fueron preparados con una modificación del método del amonio para altas cargas de metal. Los catalizadores frescos, secados a 353 K presentan una estructura no porosa de cristales en forma de laminas rectangulares sobrepuestas unas a otras similares a los filosilicatos. La composición química y estructural de estos sólidos cambia con el pre-tratamiento térmico en flujo de hidrógeno. Estudios de IR-FT realizados a temperatura de reducción programada sugieren que el catalizador fresco está formado por iones complejos de Fe del tipo Fe(H₂O)₆⁺³ atrapados entre laminas bidimensionales de filosilicatos Si₂O₅ .
La cantidad de grupos -OH enlazados a el complejo de hierro así como aquellos asociados a los grupos poliméricos Si₂O₅ disminuye progresivamente hasta desaparecer cuando se calienta lentamente el catalizador en presencia de hidrógeno desde 353 hasta 973 K. A la temperatura de reducción final, el catalizador presenta una nueva estructura constituida por pequeños cristalitos de hierro finamente dispersados sobre la superficie de la sílice sugiriendo que las propiedades finales del catalizador y su morfología están gobernadas por la temperatura.

1. Introduction
It is well known that transition metal (e.g. Fe, Co, Ni), unsupported or not, can catalyze the carbon monoxide hydrogenation reactions to forming either paraffins, olefins and alcohols. The activity and selectivity of such catalysts is strongly influenced by its chemistry and structural composition [1].
Non porous and small metal particle size catalysts seems to be a desired structural composition for the carbon monoxide hydrogenation processes because its avoid the presence of diffusion limitations of the reactants and are less influenced by the deactivation phenomena [2,3]. On the other hand, the iron based catalysts are shipper than cobalt catalysts and presents an acceptable selectivity for light olefins, gasoline and gas oil . A preparation method of iron based catalyst with such characteristic is mostly desirable for the synthesis of oxygenates or liquids hydrocarbons from the carbon monoxide hydrogenation.
The ammonia method [4] was used recently to prepare silica supported cobalt catalyst with metal particles of nanometric size and an acceptable metal dispersion [5]. The method consists of contacting silica aerosil 200 with a solution of Co(NO₃)₃.6H₂O to which ammonia solution was added.

Cobalt complexes of the solution like [Co(H₂O)_6-n (NH₃)_n]⁺², with n< 6 reacts with the silica surface [6] forming bi-dimensional compounds, probably of the Phyllosilicate type. The basic structural feature of such compounds [7] is a composite sheet in which a layer of octahedral coordinated cations is sandwiched between two identical layers of linked SiO₄ tetrahedra of Si₂O₅ composition. Additional hydroxyl ions together with the oxygen from the tetrahedral complete the octahedral coordination of the sandwiched cations. The space requirements of Fe⁺³ and Fe⁺² ions are similar to those of Ni⁺² , Co⁺² and Co⁺³. Such considerations, open the possibility to prepare silica supported iron catalysts with a similar method. The chemistry involved in the catalyst preparation is expected to be similar to that found for nickel and cobalt.
This work was aimed to study the feasibility of use the ammonia method to prepare nonporous silica supported iron catalyst with metal particle size in the nanometric range. The chemistry involved during the preparation and the structural changes induced by the thermal pretreatment under hydrogen flow was investigated by scanning electron microscopy (SEM) associate to x-ray energy dispersion spectroscopy (EDS), nitrogen physisorption, carbon monoxide chemisorption and Fourier transformed infrared (FT-IR) at programmed reduction temperature in the 353 to 973 K range and a heating rate of 1 K/min.

2. Materials and Methods

2.1 The catalyst
The silica supported iron catalyst were prepared substantially according to the method described by Barbier and coworkers [5] except for the inert atmosphere that was deemed not necessary because the salt precursor type. Since the sample calcinations were performed without previous centrifugation. The preparation of the catalysts was as follow: 1.6 g of Fe(NO₃)₃ . 9 H₂O (100% purity, J.T.Baker) is added to 16 mL of distilled water at room temperature and stirred by a magnetic rod. The iron hydroxide become precipitated by the addition of some drops of ammonia solution (25% NH₃, Riedel de-Haën). A large excess of ammonia solution ( 16-20 mL) is added to dissolve the former precipitated. After one hour stirring, 2 g. of silica aerosil 200 (specific surface area of 200 m²/g, Degussa) are added to the solution. The system is dried in an oven at 353 K for 48 hours after others 2 hours stirring. Except when otherwise indicated, the samples were reduced under hydrogen flow (10 mL/min) by slowly increasing the temperature at a heating rate of 1K/min. The final temperature was of 973 K and the reduction time was of 55 hours.

2.2 Apparatus and procedures

Micrographs of the reduced and unreduced Fe/SiO₂ catalysts were obtained in a scanning electron microscope Hitachi S-2500. The samples were dispersed by ultrasound in ethanol before its deposition in an aluminium disc pan.
Nitrogen physisorption measurements of the fresh samples were performed in a standard Sorptometer Micromeritics ASAP 2010. The samples, previously dried at 353 K were degassed during 4 hours at 625 K and 100 mmHg pressure and then exposed to a 5% nitrogen-helium mixture at 77 K. The carbon monoxide chemisorption of the reduced samples was carried out in the same sorptometer using the chemisorption mode. The experiments were performed at 308 K and a pressure range of 100-400 mmHg using the carbon monoxide isotherm of silica aerosil 200 as reference.
The infrared spectra of the samples obtained for each step of the reduction temperature in the 353-973 K range were obtained on KBr in a Perkin-Elmer 1725X FT-IR.

3. Results and Discussion

The structural feature of the Fe/SiO₂ catalysts obtained after precursors contact time of 2 hours dried in air at 353 K is shown in Figure 1. The micrograph reveals that the catalyst is constituted by superimposed bi-dimensional structures (sheets). The deformed rectangles presents in Figure 1 are similar to those observed in natural Phyllosilicate [7]. Figure 2 shows a single platy block of sheets with a well defined rectangular outline. Plastic deformations can be seen in both figures 1 and 2.

Fig. 1. Superimposed sheets of iron phyllosilicates synthesized with the ammonia method. Fresh catalysts dried at 353 K.

Fig.. 2. Single platy block (book) of iron Phyllosilicate with well-defined rectangular outline, obtained with the ammonia method. Fresh catalyst dried at 353 K.

Both micrographs shows around the two main crystals the presence of abundant smaller broken materials composed by superimposed sheets with no defined form probably resulting of the rupture of major size structures. The thermal pretreatment under hydrogen flow performed at 973 K caused a dramatic change in the catalyst morphology. Indeed, an agglomerate of mostly irregular particles of nanometric sizes are presents in the micrograph of Figure 3.

Fig. 3. Structural feature of the silica supported iron catalyst obtained from iron phyllosilicates at 923 K during the hydrogen pretreatment.

The nitrogen physisorption studies at 77 K performed both on the silica aerosil support and on the fresh silica support iron catalyst are shown in Figure 4. Both isotherms are quite similar and characteristic of non porous solids. The small hysteresis effect observed in Figure 4 can be attributable to a short time of pressure equilibration or the micro pores presence in the sample. The specific surface area of the silica supported iron catalyst decrease respect to that of support probably due to the presence of synterization effects and metal-support interactions. Such effect increases with the metal loading as can be seen in Table 1.

Fig. 4. Nitrogen physisorption isotherms at 77 K of the support silica aerosil (upside) and 10% Fe/SiO₂ fresh catalyst dried at 353 K.

 

Table 1. B.E.T. Surface area (m2/g)

Figure 5 shows the chemisorption isotherms after the reduction process under hydrogen flow at 973 K. The experiments were performed for both the 10% Fe/SiO₂ catalyst and the silica aerosil 200 support taken as reference. The silica aerosil 200 presents no adsorption of carbon monoxide (calculated at P = 0.0 mmHg) while the iron catalyst does it. The metal particle size as measured from the carbon monoxide chemisorbed (at zero pressure) is about 76 nm for the 10% Fe/SiO₂ catalyst and increase with the metal loading ( Table 2). This result can not be compared with the micrograph of Figure 3 because of the SEM spectroscopy resolution limitation. However, mostly of the agglomerate of particles shown in Figure 3 presents a size of around 100 nm.

Fig. 5. Carbon monoxide chemisorption on the silica aerosil support (low side) and 10% Fe/SiO₂ catalyst (upside) after a reduction process under hydrogen flow at 973 K.

 

Table 2. Influence of the metal loading on the metal particle size

Figure 6 shows the FT-IR spectra of the reduced iron catalysts in the 353 to 973 K temperature range. All of spectra shows the bands characteristic of silica. The shoulder of about 3468 and 3156 cm-1 can be attributable to O-H stretching vibration modes of hydrogen bonded to OH of polymeric association and hydrogen bonded to OH intermolecular or chelate compounds. The symmetric stretching Si-O vibration of silica can be observed at around 1100 cm^-1. At lower frequencies the bands at around 820 and 470 cm^-1 corresponds to asymmetric Si-O stretching and Si-O bending modes of silica in that order. At medium frequencies the bands at around 1640 and 406 cm^-1 corresponds to H-O-H bend of crystallization water and NO₃⁺ vibrations of the nitrate ion [8]. An increasing of the reduction temperature in the 353-898 K range does not affect the shoulder of about 3468 cm^-1 assigned to the O-H bonded to polymeric association like bi-dimensional Si₂O₅. At 973 K all of bands between 3500 and 3000 cm^-1 disappears while the band assigned to the bonded O-H intermolecular shoulder at 3156 cm^-1 disappears after 598 K. As the same way the intensity of the bands assigned to crystallization water and NO₃⁺ ions decrease with the temperature increase up to disappears at 973 K. At the last reduction temperature the FT-IR spectrum corresponds to that of the silica (SiO₂).

Fig. 6. FT-IR spectra of Fe/SiO₂ catalysts after programmed temperature reduction under hydrogen flow at 353-973 K temperature range and a heating rate of 1 K/min.

4. Conclusions

The ammonia method modified was used to prepared silica supported iron catalyst. The structural feature of the fresh catalyst consist in superimposed sheets of rectangular crystals . Some of those presents a well defined outline and plastic deformations characteristic of the natural Phyllosilicate. Such solids are nonporous. Its surface area decrease with the metal loading increasing. Iron crystallites of around 20 to 160 nm supported on silica (SiO₂) are resulting from the thermal pretreatment at high temperature under hydrogen flow. FT-IR studies at programmed temperature reduction indicates the progressive leaving of OH groups bonded to a polymeric structure like bi-dimensional Si₂O₅ and intermolecular OH groups probably belonging to the iron complex like Fe(H₂O)₆⁺³ suggesting a fresh catalyst structure constituted by Fe(H₂O)₆⁺³ complex sandwiched between sheets of Si₂O₅ phyllosilicates. Such structure disappears by a progressive leaving of the OH groups and a new structure constituted by small iron crystallites dispersed on the silica support arises by effect of the final reduction temperature. Similar structures were found on silica support cobalt catalysts synthesized with the same method [5,9].

Acknowledgements
This work was supported by Consejo Nacional de Investigaciones Cientificas (CONICIT-Venezuela) under Grant Nº S1-2000000809 and Consejo de desarrollo Científico, Humanìstico y tecnológico (CDCHT) de la Universidad de Los Andes, Project Nº C-942-99-08-AA.