Study of the Preparation and Electrochemical Properties of Cobalt(II) Methyl Pyridyl Porphyrin Adsorbed on the SiO 2 =TiO 2 =Sb 2 O 5 Matrix Prepared by Sol-Gel Method Ana M. Castellani, Jose ´ E. Gonc ¸alves, and Yoshitaka Gushikem* Instituto de Quı ´mica, Unicamp, C P 6154, 13083-970 Campinas, SP, Brazil; e-mail: gushikem@iqm.unicamp.br Received: September 18, 2000 Final version: December 29, 2000 Abstract This work describes the use of the mixed oxide SiO 2 =TiO 2 =Sb 2 O 5 (STSb) prepared by the sol-gel processing method. The material had an ion exchanger capacity of 2.0 mmol g 1 and was used to absorb a Co(II) methyl-pyridyl-porphyrin complex (CoTmPyP) by an ion exchange reaction. The resulting material, STSb=CoTmPyP, showed a specific surface area of 554m 2 g 1 and 84 mmol g 1 of adsorbed cobalt complex. A carbon paste electrode made with STSb=CoTmPyP showed that, in a cyclic voltammetry experiment, it can electro- catalytically reduce dissolved oxygen in water solution (pH between 3 and 6.8) under a cathodic potential of 170mV. The electrode showed a linear response for dissolved O 2 concentration in water between 1 and 10.1 ppm. Keywords: Silica-titania-antimonia, Sol-gel, Cobalt porphyrin, Oxygen electroreduction, Cyclic voltammetry, Amperometry 1. Introduction The sol-gel process can be described as a very convenient method to prepare binary oxides such as SiO 2 =M x O y , with high purity and homogeneity [1–6]. Among these, SiO 2 =TiO 2 having high concentration of titanium(IV) homogeneously dispersed in the matrix can be obtained by this process [7]. The metal oxide incorporated in the matrix is coordinatively unsaturated and can furthermore react with oxoacids, resulting in an immobilized solid acid system [8]. As an example, the reactive  TiOH groups on the SiO 2 =TiO 2 surface can react with phosphoric acid, resulting in the solid acid (  TiO) 2 P(O)OH where the hydrogen phosphate group is adsorbed and bound to the surface by a Ti–O–P bond [9]. Antimony(V) ions in acid solution can also be adsorbed on the SiO 2 =TiO 2 surface in a similar way, forming a new solid acid phase [7, 9–11]. The resulting amorphous titanium anti- monate [10, 12], represented as (  TiO) n Sb(OH) 5n , was used as adsorbent mainly because of its high ion exchange capacity. Contrary to bulk phase Sb 2 O 5 , normally obtained as fine powder with low mechanical and thermal resistance, the silica matrix confers to the (  TiO) n Sb(OH) 57n desirable characteristics of mechanical and thermal resistance [12–15]. Solid acids attached to porous SiO 2 =M x O y have been successfully used in electrochemistry studies as versatile mate- rials to prepare modified electrodes [11, 16]. As they are good ion exchangers, metal complexed methyl-pyridyl-porphyrin, a positively charged water soluble porphyrin complex, has been immobilized by an ion exchange reaction and used as an elec- trocatalyst for dissolved oxygen reduction [11]. The main advantage in using porous material substrates is that the surface immobilization density yield of the electroactive species is normally very high, significantly improving the performance of electrodes made with the material. An additional advantage is that the catalysts are not leached out during the reactions and can be used many times without loss of their activities. In this work the immobilization of the cobalt(II) complex (CoTmPyP) of 5,10,15,20-tetrakis(1-methyl-4-pyridyl)- 21H,23H-porphyrin on SiO 2 =TiO 2 =Sb 2 O 5 , as well as its char- acterization and further application in dissolved dioxygen reduction, is described. 2. Experimental 2.1. Preparation [10] SiO 2 =TiO 2 binary oxide was prepared in three steps, according to the following procedures: 1) 12.1 mL of 0.85 mol L 1 HNO 3 aqueous solution was added to 250 mL of 50 % (v=v) tetraethoxysilane=ethanol solution, and the mixture was stirred at 353 K for 2.5 h. 2) About 34 mL of titanium(IV) butoxyde and 490 mL of ethanol were then added and the mixture was stirred for 2 h at room temperature. 3) 66 mL of 0.6 mol L 1 HNO 3 solution was slowly added and allowed to rest for gelation. The formed xerogel was dried at 383 K for 24 h, ground and sieved in order to obtain particles size between 75 and 250 mm. About 15 g of the xerogel SiO 2 =TiO 2 was immersed in 500 mL of a 1.2610 2 mol L 1 acidic Sb(V) solution (pH 2; prepared from SbCl 5 ) and the mixture was heated at 333 K for 8 h. The solid was filtered, washed with 1.0 mol L 1 aqueous HNO 3 solution in order to avoid Sb(V) hydrolysis and then washed with deionized water. 2.2. Chemical Analyses The titanium and antimony contents in SiO 2 =TiO 2 and SiO 2 =TiO 2 =Sb 2 O 5 were analyzed by the X-ray fluorescence analysis technique on a TRACOR Northern spectrometer equipped with a beryllium window. For the sake of brevity, SiO 2 =TiO 2 and SiO 2 =TiO 2 =Sb 2 O 5 will hereafter be designated as ST and STSb, respectively. 2.3. Ion Exchange Capacity The ion exchange capacity of STSb was determined by weighing about 100 mg of the material and immersing this into 50.0 mL of 1.0 mol L 1 KCl solution and then shaken for 8 h at 298 K. The supernatant solution (25.0 mL) was titrated with standard 0.05 mol L 1 KOH solution. 1165 Electroanalysis 2001, 13, No. 14 # WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 1040-0397/01/1410–1165 $17.50þ.50=0