Catalytic reduction of nitrous oxide by carbon monoxide in the presence of rhodium carbonyl and hydroxide. Evidence for an electron-transfer and an oxygen-transfer mechanism

The kinetics and mechanism of the reduction of N2O by CO to N2 and CO2 catalysed by the [Rh2(CO)4Cl2]–KOH–dmso (dimethyl sulphoxide) system were investigated. A first-order dependence of the rate on both PN2O and the concentration of [Rh2(CO)4Cl2] was established. The activation parameters ΔH‡ and ΔS‡ obtained from an Eyring plot are 59.0 kJ mol–1 and –149.4 J K–1 mol–1 respectively. During the course of catalysis [Rh2(CO)4Cl2] was mainly reduced to [Rh(CO)4]–. Two other rhodium species [Rh4(CO)11]2– and [Rh4(CO)11(N2O)]˙3– were also detected by ultraviolet–visible and EPR spectroscopies, respectively. The reaction of [Rh(CO)4]– with N2O yielding N2 and CO2 simultaneously is believed to be the process responsible for the product formation. Isotopic labelling studies suggest that a direct oxygen transfer from N2O to CO takes place in the observed catalysis. No reaction of [Rh(CO)4]– with N2O and no catalysis occurs if [Rh4(CO)11]2– is completely absent from the solution. A catalytic cycle including an electron transfer between [Rh(CO)4]– and N2O to give N2O˙–, the trapping of this radical by [Rh4(CO)11]2– to form [Rh4(CO)11(N2O)]˙3– and the evolution of CO2 and N2 from the latter radical species is proposed to account for the observed catalysis.