Heterogeneous catalysis of some redox reactions by platinum

Abstract

The heterogeneous catalysis by platinum of redox reactions in aqueous solution has been investigated. In Part I of the thesis the kinetics of the heterogeneous reactions of iodide with hexacyanoferrate(III), with aquo-iron(III) and with sulphato-iron(III) are examined with platinum as catalyst. The result for the Fe(CN)₆3-/I - reaction indicates that the catalytic rate is at least partly controlled by the rate of diffusion of Fe(CN)₆3- from the bulk solution up to the platinum surface. The state of the platinum surface is also shown to affect the observed reaction rate. It has been found that the heterogeneous reaction has a rate dependent upon [K+] and the results are consistent with an ion-pair, K+...Fe(CN) 3-/I-, being one species involved in exchanging electrons with the platinum surface. In the reaction of ferric ion with iodide the results indicate that Fe(OH)2+ is the species more suited to the heterogeneous process. In the FeSO₄+/I- reaction it has been observed that the. heterogeneous rate is due to FeSO₄ +. Both these latter heterogeneous reactions have an appreciable temperature co-efficient. The order of the heterogeneous component of reaction with respect to each reactant has always turned out to be one or less.

In Part II the rate of the reaction between tris (dipyridyl)-iron(III) and iodide has been measured spectrophotometrically by observing the optical density at 520 nm where absorption is virtually due to the formation of tris(dipyridyl)iron(II). The effects of several variations in reaction conditions are studied. The main reaction is shown to be first order in Fe(dipy)₃3+ and first to second order in I-. Finally a possible reaction mechanism is proposed. This reaction is unaffected in rate by the presence of platinum surface.

The effect of platinum on the reaction between cerium(IV) and thallium(I) in different acidic media such as HNO₃, H₂SO₄ and HClO₄ is also studied. It has been found that the reaction occurs almost exclusively via a heterogeneous catalysed path.

Finally in the reaction of peroxodisulphate with oxalate ion in the presence of silver(I) as a catalyst, it has been found that the rate of the reaction in the presence of silver(I) ion is uncatalysed further by the presence of platinum. Some implications of these results are discussed.