Mechanistic Investigations on the Belousov−Zhabotinsky Reaction with Oxalic Acid Substrate. 2. Measuring and Modeling the Oxalic Acid−Bromine Chain Reaction and Simulating the Complete Oscillatory System

Abstract

The aim of the present paper is to study radical reactions important in the mechanism of the Belousov-Zhabotinsky (BZ) reaction with its simplest organic substrate, oxalic acid, and to model the oscillatory system applying the newly determined rate constants. We considered five radical species in this BZ system: carboxyl radical, bromine atom, dibromine radical ion, and bromine monoxide and dioxide radicals. To study separately reactions of only three radicals, • CO 2 H, • Br, and • Br 2 -, semibatch experiments were performed. The semibatch reactor contained oxalic acid, elemental bromine, and bromide ions in a solution of 1 M H 2 SO 4 at 20 °C, and a continuous inflow of Ce 4+ generated carboxyl radicals. The carboxyl radicals initiate a chain reaction: first they react with elemental bromine and produce bromine atoms (CR1); then the bromine atoms react with oxalic acid, producing carboxyl radicals again (CR2). Consumption of elemental bromine in the chain reaction was followed with a bright Pt electrode. By measuring the stoichiometry of the chain reaction, it was possible to determine or estimate several rate constants. It was found that CR1 is a fast reaction with an estimated k value of more than 10 9 M -1 s -1 . The rate constant of CR2 is 7 × 10 5 M -1 s -1 , and the k value for the Ce 4+ -• CO 2 H reaction is 1.5 × 10 9 . These values were obtained by comparing experiments with model calculations. Such simulations also suggested that a reaction of • Br 2with oxalic acid, analogous to CR2, plays a negligible role or no role here. Simulations of the oscillatory system applied rate constants, which were known from the literature, or determined here or in the first part of our work, and some unknown rate constants were estimated on the basis of analogous radical reactions. To obtain an optimal fit between experiments and simulations, only one rate constant was used as a variable parameter. This was the reaction of carboxyl radical with acidic bromate with an optimal k value of 1.0 × 10 7 M -1 s -1 . Agreement between experimental and simulated oscillations was satisfactory at low bromine removal rates (that rate was controlled by a nitrogen gas flow), but a disagreement was found at higher flow rates. Possible reasons for this disagreement are discussed in the conclusion.

Periodic CO and CO2 Evolution in the Oscillatory Briggs-Rauscher Reaction

“…Such a chain reaction was already found in a BZ system in which carboxyl radicals and bromine atoms were the chain carriers. 21 Present mechanisms of the BR reaction exclude the possibility of such chain reactions as none of the two inorganic radical species ( • OOH or • IO 2 ) is able to react with organic molecules according to the schemes. Nevertheless, such reactions probably can occur.…”

Mapping Sequential Oscillations in the Bromate–Oxalic Acid–Acetone–Ce(IV) Reaction

“…Instead of a change in the mechanism of reduction, the system might be going through a change in the oxidation mechanism. The mechanism proposed by Pelle et al 26 considers a series of reactions between Ce 3+ and radicals from bromine. Some of the reactions are as followsThese reactions may be an alternative to P2 on Ce 3+ oxidation, even at high Br – , since they rely on Br • instead of HBrO 2 .…”

Advances in Treatment of Brominated Hydrocarbons by Heterogeneous Catalytic Ozonation and Bromate Minimization

“…These reactions lead to the inhibition of Br− to HOBr/OBr−, thus resulting in lower BrO3− formation. The Ce3+ surface ions underwent oxidation by Br• and BrO• to form Ce4+ [20] according to the following pathway:Ce3++BrO•+H+ → Ce4++HOBr Ce3++Br• → Ce4++Br− Ce 3+ also reacts with H2O2 to form Ce4…”