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

Abstract

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 P_N2O 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.