Highly Efficient Photoinitiation in the Cerium(III)-Catalyzed Aqueous Autoxidation of Sulfur(IV). An Example of Comprehensive Evaluation of Photoinduced Chain Reacions

Abstract

The photoinitiated and cerium(III)-catalyzed aqueous reaction between sulfite ion and oxygen has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Cerium(III) is identified as the photoactive absorbing species, and the production of cerium(IV) initiates a radical chain reaction. To interpret all the experimental findings, a simple scheme is proposed, in which the additional chain carriers are sulfite ion radical (SO3(-.)), sulfate ion radical (SO4(-.), and peroxomonosulfate ion radical (SO5(-.). The overall rate of oxidation is proportional to the square root of the light intensity per unit volume, which is readily interpreted by the second-order termination reaction of the proposed scheme. It is also shown that the reaction proceeds for an extended period of time in the dark following illumination, and a quantitative analysis is presented for this phase as well. The postulated model predicts that cerium(III) should have a cocatalytic or synergistic effect on the autoxidation of sulfite ion in the presence of other catalysts. This prediction was confirmed in the iron(III)-sulfite ion-oxygen system. The experimental method and the mathematical treatment used might be applicable to a wide range of photoinduced chain reactions.

Kinetics of the autoxidation of sulfur(iv) co-catalyzed by peroxodisulfate and silver(i) ions

“…It is known from previous studies that the appearance of the break point most likely refers to the total depletion of dissolved oxygen from the system. 21,38,40 This was demonstrated in this work by showing that dissolved oxygen is the limiting reagent in the first phase under the conditions used in this study ( Fig. S2 in ESI †).…”

“…This is different from what was observed in the case of cerium(III) initiated photocatalysis, where the reaction rate was independent of the pH. 38 There are more similarities with the case of iodide ion initiated photocatalysis, where decreasing acidity accelerated the reaction. 40 The findings displayed in Fig.…”

“…However, each of these steps has significance in this model and independent evidence for their existence in earlier experimental studies. The high rate of autoxidation and the zeroth order dependence on the oxygen concentration make it clear that a radical chain mechanism similar to the cases of photocatalysis by cerium(III) 38 and iodide ion 40 must be operative in this system.…”

“…This may seem a rather interesting observation in an autoxidation process, where oxygen acts as a reactant, but it is common in similar systems. 38,40 The entire process is not zeroth-order, though, primarily because sulfur(IV) could not be used in high enough excess to be considered constant during a single kinetic run. The rate of reaction (2) as a function of sulfur(IV) concentration is plotted in Fig.…”

Aqueous photochemical reactions of chloride, bromide, and iodide ions in a diode-array spectrophotometer. Autoinhibition in the photolysis of iodide ions

“…Continuous and interrupted illumination methods 19,25 have both been used in this study. In the latter method, periods of illumination are regularly interrupted by dark periods of various lengths by closing the shutter of the spectrophotometer.…”