Steady-state kinetics of the catalytic reduction of nitrogen dioxide by carbon monoxide on platinum

The steady-state kinetics of the reduction of nitrogen dioxide (NO₂) with carbon monoxide (CO) on a polycrystalline platinum foil have been investigated using reactant pressures between 1 × 10^-7 and 5 × 10^-6 Torr. At temperatures less than 400 K, the primary products formed are carbon dioxide (CO₂) and nitric oxide (NO). The CO₂ formation rate at temperatures less than 400 K is at least an order of magnitude greater for this reaction than for those of either CO + O₂ or CO + NO under similar reaction conditions. The reaction rate is first-order in CO pressure when P_CO P_NO2 < 0.25 and first-order in NO₂ pressure when P_CO P_NO2 > 1. The activation energy when P_CO P_NO2 = 0.10 is 7.2 kcal mole^-1 which is consistent with the activation energy for CO + O₂ under similar conditions. However, when P_CO P_NO2 = 1.5, the activation energy is only 4.3 kcal mole^-1. This value is 29 kcal mole^-1 lower than the activation energy for CO + O₂ at low temperatures when the reaction is first-order in the oxidant. Consistent reaction mechanisms for both regimes are proposed based upon recent reports of the chemisorption of NO₂ on Pt surfaces.