Transient studies on the effect of oxygen on the high-temperature NO reduction by NH3 over Pt-Rh gauze

The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt-Rh (95-5 wt.%) alloy gauze at 1023-1073 K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O2-pretreated gauze accelerate the reaction of NH3 with NO with respect to the as-received gauze. N2 was the main reaction product and traces of N2O were comparatively formed (N2O/N2 ∼ 10−3). Pulsing of an equimolar O2-15NH3-NO mixture over the Pt-Rh gauze mainly produces 15NO, while the formation of N2 is largely suppressed. The selectivity to 15NO and N2O in the ternary O2-15NH3-NO system diminished upon decreasing the O2/(15NH3 + NO) ratio, in favor of N2. This ratio was qualitatively varied by changing the time delay between O2 and 15NH3-NO in sequential pulse experiments. Our results indicate that the NH3 oxidation by O2 to NO is much faster than the NO reduction by NH3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N2 and N2O in ammonia burners within nitric acid manufacture.