Luis A. Cárdenas ¹, Alfonso Rodríguez ¹, Fulgencio Rueda ¹, Rodrigo Casanova ^1*,

 Juan Mendialdua ¹, Alfonso Loaiza-Gil ²

1: Laboratorio de Física de Superficies, Dpto. de Física, Facultad de Ciencias, Universidad de Los Andes, Mérida, Venezuela.

2: Laboratorio de Cinética y Catálisis, Dpto. de Química, Facultad de Ciencias, Universidad de Los Andes, Mérida, Venezuela.

* E-mail: rodrigoc@ula.ve

Publicado On-Line el 19-May-2006

Disponible en: www.polimeros.labb.usb.ve/RLMM/home.html

Abstract

The aim of the present work is to study the different oxygen species adsorbed on an iron phyllosilicate catalyst using the Kelvin probe technique. The results of the oxygen and hydrogen adsorption studies on samples with different iron content, Fe(10%)SiO₂, Fe(20%)SiO₂ and Fe(30%)SiO₂ , are presented in the temperature range between 323 K and 723 K. In the Fe(10%)SiO₂ sample the oxygen species adsorbed is O^-₂ for temperatures lower than 473 K, while the O^-2 species is found in the temperature range between 573K and 723K. In the Fe(20%)SiO₂ sample, the O^- oxygen species is adsorbed in this sample at 473K predominantly; while the O^-2  is found on the surface in the temperature range between 623 K and 698 K. The oxygen species adsorbed on  the Fe(30%) SiO₂  sample is  O^-2.

Keywords: Kelvin probe technique, Oxygen and hydrogen adsorption, Iron phyllosilicate catalyst.

Resumen

El objeto del presente trabajo es el estudio de las diferentes especies de oxígeno adsorbidas sobre los catalizadores de filosilicatos de hierro utilizando la técnica de Kelvin.  Se exponen aquí los resultados de los estudios de adsorción de hidrógeno y oxígeno sobre muestras con diferentes concentraciones de hierro, Fe(10%)SiO₂, Fe(20%)SiO₂ y  Fe(30%)SiO₂, en el rango de temperatura de  323 K hasta 723K. En la muestra de Fe(10%)SiO₂  el oxígeno se adsorbe en la forma O^-₂  para temperaturas  menores a  473 K,  mientras que en el rango de 573 K a 723 K la especie que se encuentra es el O^-2. En la muestra Fe(20%)SiO₂, la especie O^- del oxígeno se adsorbe a temperaturas menores de 473 K, mientras que la especie  O^-2   se encuentra en el rango de 623 K a 698 K. Finalmente, la especie O^-2  predomina en todo el rango de temperatura para la muestra de  Fe(30%)SiO₂.

Palabras claves: Técnica de Kelvin, Adsorción de hidrógeno y oxígeno, Catalizadores de filosilicatos de hierro.

Revisado: 21-Feb-2006; Aceptado: 10-May-2006

2. Introduction

 It is well known that Iron, cobalt, nickel and ruthenium belonging to the VIII group of periodic system are active catalysts for the Fischer-Tropsch synthesis [1]. These elements can be taken, in the presence of hydrogen or carbon monoxide under reaction conditions, to lower oxidation states metallic or carbide states. Similarly metallic oxidation states in catalytic can be obtained by means of a reduction process at high temperature [2]. In these conditions, the catalytic active elements are capable of physisorbing or chemisorbing carbone monoxide and/or hydrogen. It has been found that Fischer-Tropsch synthesis is selective to the formation of linear olefins when iron catalysts are used [3]. For this reason we have used three samples of iron catalyst supported on silica with different iron contents, in this work.

We have studied the oxygen and hydrogen adsorption on the surface of catalysts by means of the Kelvin technique [4] to complement the surface characterization performed in our laboratory by means of AES and XPS spectroscopy [5-7], where the surface chemical composition iron oxidation

states were determined. Whit the Kelvin technique we can obtain the oxygen specie adsorbed on the catalysts.

2. Experimental Methods

The method used for the preparation of the iron based catalysts, for this work, supported on silica aerosil is a modification of the ammonia method [8], employed to prepare nickel catalysts supported on silica with small size particles. The authors report particle size of the order of nanometers in catalysts with an acceptable metal dispersion. This method was also employed to prepare   silica supported cobalt catalysts by the same researchers. It is important to mention that this is the first time this method is used to prepare iron based catalysts with some modifications.

The method consists of contacting silica aerosil with a solution of ferric nitrate [Fe(NO₃)₃.9H₂O] to which ammonia solution was added. The role of ammonia is two folded.  On one hand, the silica strongly attacks the silica (catalysts support) according to the reaction:

            Si-OH + NH₃® SiONH₄,

and on the other side, allows the formation of amino-ferric ions of the form:

            Fe(H₂O)_9-n[(NH₃)_n]³⁺

with n < 9. These ions are exchanged with the NH₄ ions bonded to silica to in order to form bi-dimensional compounds, probably of the phyllosilicate type.

The determination of the oxygen species adsorbed on the samples studied in this work was done by measuring the surface potential changes by means of the vibrating capacitor technique [9], in this technique the reference electrode used is graphite because of the inertness of this material to different gases up to a temperature of 723 K.

Finally, the fluctuations of the capacitor potential are measured when the sample (deposited on one of the electrodes) is in the presence of a given gas. Each sample was previously stabilized in the presence of Ar and Oxygen (0.02 Atm) by means of heating and cooling cycles in the temperature range between 323 K and 723 K till the surface potential shows reversibility.

3.   Results and discussion

The results of the surface potential measurements are shown in table 1. Figure 1 deals with the kinetics of the surface potential changes (V vs t) at 623 K the three samples under study. The oxygen adsorption on the catalysts surfaces can be described by the following reaction:

                                (1)

where ½ and ¼ are non-dimensional stoichiometric coefficients and N^- is the adsorbed oxygen species (O^-, O⁼ and O^-₂).

The surface potential V time plots follow the Slovich model [10], as it is evidenced in figure 1, once the experimental data are fitted using the equation:

                            (2)

where K is the Boltzman constant, e the electron charge,  b the number of electrons transfered, t the time and t_o the gas arrival time to the sample, V the surface potential.

The figure 1 corresponding to the adsorption kinetic and the figure 2 the surface potential changes with oxygen pressure.

Figure 1. Surface Potential V as a function of time at 623 K for the three samples under study in the presence of oxygen.

Figure 2. The surface potential V is represented as function of oxygen pressure at 623 K for the samples Fe(10%)SiO₂ and Fe(20%)SiO₂

 The surface potential versus oxygen pressure plots are fitted using the equation:

               (3)

where PO₂ is the oxygen pressure. We found reasonable agreement for both fittings. 

The results presented in table 1, were obtained from the experimental data of figures 1 and 2, using equations (2) and (3).

 Table 1. Experimental results for the samples studied

Catalyst Fe(10%)SiO₂  shows a tendency to adsorb oxygen in atomic form in which case oxygen transfer two electrons to the substrate for the temperature greater than 573 K which is indicated  by an average surface potential of 126 mV. It is observed that for adsorption of the molecular species O^-₂ a bigger change in surface potential is reached (160mV.).

The oxygen species O^-2 predominate in the catalyst Fe(20%)SiO₂ in the temperature range between 623 K to 698 K and the change in the surface potential  is 25 mV, which is indicative of low oxygen adoption. This   specie predominate in the Fe(30%)SiO₂ sample and the surface potential (40 mV) indicating  low adsorption as well.           

4.  Conclusions

It is concluded that the kinetics of the potential changes occurring upon the uptake of oxygen or hydrogen reach the equilibrium conditions in an inverse proportion to the Fe concentration in the catalysts. It is also observed that the kinetics is compatible with the existence of O^-2 predominantly in the temperature region (473-723) K for each sample.

5.  Acknowledgements

The authors like to thank CDCHT-ULA for funding project C-1130-02-05 AA. We also like to thank Mr. J. Sarmiento from ULA for this technical assistance and cooperation.

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