Metal Oxide Thin Films Grafted on Silica Gel Surfaces: Recent Advances on the Analytical Application of these Materials

Journal of the Brazilian Chemical Society

Gushikem, Yoshitaka¹  Rosatto, Simone S.¹  Universidade Estadual de Campinas, Campinas, Brazil¹ 

关键词: modified silica;    monolayer attached metal oxides;    preconcentration;    modified electrodes;    electrochemical sensors;    biosensors    1. Introduction The surface of silica gel contains many different types of silanol groups;    ºSiOH;    which can react with reagents such as metal alkoxides or halides. Preparation methods using these reagents in order to obtain active metal oxides;    dispersed as monolayer films on the surface of SiO2;    have been an area of growing interest in recent years. Experimental procedures;    aiming to produce monolayers of the coating metal oxides with thermal stability;    have been designed1-13. Different from metal oxides supported on solid matrices prepared by impregnation or co-precipitation processes;    grafting of the reagent on the solid surface gives rise to oxides whose acidic properties are associated with high thermal and chemical stability;    which are not characteristic of the respective oxides in the bulk phase14. From the view point of basic research;    this procedure is particularly advantageous because formation of a well defined surface species allows a more precise study of the acid-base character of the active sites15;    16. Although SiO2 has been considered as a relatively inert material;    surface silanol groups or strained siloxane groups;    as stated above;    can react according to the following equations:;   

DOI  :  10.1590/S0103-50532001000600002

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The surface of silica gel contains many different types of silanol groups, ºSiOH, which can react with reagents such as metal alkoxides or halides. Preparation methods using these reagents in order to obtain active metal oxides, dispersed as monolayer films on the surface of SiO2, have been an area of growing interest in recent years. Experimental procedures, aiming to produce monolayers of the coating metal oxides with thermal stability, have been designed1-13. Different from metal oxides supported on solid matrices prepared by impregnation or co-precipitation processes, grafting of the reagent on the solid surface gives rise to oxides whose acidic properties are associated with high thermal and chemical stability, which are not characteristic of the respective oxides in the bulk phase14. From the view point of basic research, this procedure is particularly advantageous because formation of a well defined surface species allows a more precise study of the acid-base character of the active sites15,16. Although SiO2 has been considered as a relatively inert material, surface silanol groups or strained siloxane groups, as stated above, can react according to the following equations: where MXn is an active metal compound and ºSiOH stands for the silanol group. Careful hydrolysis of attached metals leads to formation of a bidimensional oxide structure in which the metals are bound to the surface by a ºSiO-M bond. In this work some features of monolayer attached oxides on a silica gel surface are discussed. Many of these materials have been designed to satisfy the demands of activity and selectivity in a catalytic system by a thorough control of surface morphology. For this purpose many studies have been carried out, aiming a better understanding of the surface structure of the oxide coated species4,6,17-22. Thus, most of the metal oxide coated silica materials, SiO2/MxOy, have been prepared for using in catalytic reactions3-5,7,10,11,23-26. Highly dispersed metal oxides on a porous silica surface are characterized to present coordinatively unsaturated metal oxide (Lewis acid sites, LAS) in addition to the Brfnsted acid sites (BAS), mainly due to the MOH groups. These LAS or BAS can adsorb many molecular species which can be immobilized by covalent bonding or electrostatic interaction. The preparation procedures, properties and applications of these species immobilized on the SiO2/MxOy surface, apart from applications in catalytic reactions, are described in this work. We aim to give an overview on new applications of these materials in various fields such as selective adsorbents, enzyme immobilization supports, new packing materials for HPLC columns, carbon paste modified electrodes, electrochemical sensors and biosensors.   2. Preparation and Characteristics of Oxide Monolayers Supported active oxides are prepared as mentioned above by a careful hydrolysis of ºSiOMXn-1 according to the following reaction equations:andIt is supposed that the metal oxides form a complete and coherent layer. However in no instance, in the various examples cited in the literature, is the surface of the porous solid entirely coated. For instance, the density of niobium atoms for Nb2O5 coated on a SiO2 surface27, i.e. SiO2/Nb2O5, is 1.7 atoms nm-2 while complete coverage28 requires 2.2 atoms nm-2. Since in most cases the porous solid specific surface areas decrease after the coating treatment, it is not difficult to conceive that the finest pores remain untouched or blocked. In order to assure complete surface coverage, the treatment of the solid surface, i.e., the reactions described by equations1-4, can be repeated. However, treatment of the solid by successive coating of the porous surface may result in agglomeration of the particles.   3. Thermal Stability The mobility of the oxides on the surface under thermal treatment depends on the particular metal, i.e. of the Si-O-M energy bond. An indication of how mobile the metal particles are on the surface is given by XPS measurements. The results are presented in Table 1.   For SiO2/Sb2O5 and SiO2/Nb2O5, heat treatments of the samples down to 1373K, in the first case, and 1273K, in the second case, practically do not change the atomic ratio while for SiO2/TiO2, the atomic ratio decreases from 0.082 down to 0.045 between 673K and 1273K. Mobility of titanium oxide on the surface is therefore considerable if compared to that observed for antimony and niobium oxides. Diffusion of titanium into the matrix or agglomeration into larger particles may be occurring in this case. Bulk phase binding energies of the metals in the oxides are (in eV): Sb2O5, 3d3/2= 540.529,30; Nb2O5, 3d3/2 =210.2 and 3d5/2= 207.431; TiO2 (rutile or anatase), 2p3/2= 459.032. Comparing these results with those of the metals in the coating oxides, only Sb(V), Nb(V) or Ti(IV) are detected. In every

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