Gas-phase reduction of oxides of nitrogen with CO catalyzed by atomic transition-metal cations.

important reactions occurring in automotive catalytic converters where both reactants are undesirable pollutants. Base-metal oxides, mixed metal-oxide compounds such as perovskites, supported metal catalysts, metal zeolites, and alloys have all been investigated as heterogeneous catalysts for this reaction, which does not occur directly in the gas phase at room temperature to any measurable extent. For example, copper in the + 2 oxidation state has been found to be active both as a base-metal-oxide catalyst and in a metalsupported or perovskite form. We report here the first example of homogeneous catalysis by the reaction described in Equation (1), which occurs in the gas phase with atomic transition-metal cations serving as catalysts. The catalysis occurs in two steps in which NO is first reduced to N2O. An analogous three-step catalytic reduction of NO2, in which NO2 is first reduced to NO, was also discovered. The overall catalytic scheme that was established in the study reported here consists of the three catalytic cycles shown in Figure 1. These three cycles were characterized with laboratory measurements of reactions of each of the three nitrogen oxides NO2, NO, and N2O with up to 29 different transition-metal cations M [Eqs. (2)–(4)]: