Synthesis and acidic properties of the SiO2/SnO2 mixed oxides obtained by the sol–gel process. Evaluation of immobilized copper hexacyanoferrate as an electrochemical probe

SiO2/SnO2 mixed oxides were prepared by the sol–gel processing method using SnI4 as the tin oxide precursor reagent. Solids with Sn compositions (in wt.%) of 4.1, 12.9 and 18.4 and presenting specific surface areas (determined by the BET method) of 492, 658 and 712 m2 g−1, respectively, were obtained. Transmission microscopic images showed nanosized SnO2 particles with average dimensions of (5.3±0.5) nm for samples having 12.9 wt.% Sn and (7.0±0.7) nm for samples having 18.4 wt.% Sn. For the sample presenting 4 wt.% of Sn the crystallites were poorly defined, barely being observed. The amorphous SnO2 particles started to crystallize at different temperatures, i.e., 1273, 1173 and 1073 K for samples with 4.1, 12.9 and 18.4 wt.% of Sn, respectively. The X-ray diffraction patterns showed that only cassiterite crystallites were present in every case and, even at a temperature of 1473 K, the SiO2 remained as an amorphous matrix. The Lewis and Brønsted acid sites were thermally stable up to a temperature of 523 K for all the compositions, as probed using pyridine molecules. The infrared spectra showed that Si–O–Sn bonds are formed at the interface between SiO2 and SnO2 particles. These bonds are the ones mainly responsible for the low mobility of the oxide particles, avoiding crystallization of SnO2. hexacyanoferrate was prepared in situ on the SiO2/SnO2 surface and cyclic voltammetry tests were carried out by sweeping the potential between 0.2 and 1.0 V. The midpoint potential corresponding to the redox process: SnOH2+/{KCu[Fe(CN)6}−⇄SnOH2+/{Cu[Fe(CN)6}−+K++e− was observed at about 0.7 V. The electrochemical impedance spectroscopic data showed a charge transfer resistance of 17.8 Ω. This low value favors the oxidation–reduction process in the pores of the material.