Mechanism of Nitrous Oxide Formation by Metal-Catalyzed Reduction of Nitric Oxide in Aqueous Solution

Kinetics data were collected for the palladium-catalyzed reduction of nitric oxide (NO) to nitrous oxide (N2O) with cuprous chloride reductant in 2 M hydrochloric acid (2NO + 2CuCl + 2HCl → N2O + 2CuCl2 + H2O). The rate-determining step was first order in the palladium concentration and NO partial pressure. The cuprous chloride dependence was first order below 0.1 M; at higher concentrations saturation kinetics were observed. The rate of reaction was independent of H+ and Cl- concentrations. Kinetics results were consistent with the initial, reversible attack (k 1/k -1) of free NO on the bound nitrosyl of [PdCl3NO]2- yielding [PdCl3(N2O2)]2-, which is then reduced by Cu(I) (k 2) to generate products and recycle the palladium. A k 1 value of (6.0 ± 0.4) × 10-6 (PNO)-1 s-1 at 20 °C was calculated, with a k -1/k 2 ratio of 0.116 ± 0.004 Μ. Rate measurements show that NO reduction by Cu(I) is the rate-limiting step in the Wacker-style catalysis of the CO + 2NO → CO2 + N2O reaction. The current mechanism resembles the nitric oxide reductase activities of cytochrome c oxidases, which proceed by Cu(I) reduction of a heme bound nitrosyl, and cytochrome P450nor.