Transient studies on the effect of oxygen on the high-temperature NO reduction by NH 3 over Pt–Rh gauze

Abstract

The effect of oxygen on the activity and selectivity of the NO reduction by NH 3 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 NH 3 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 O 2-pretreated gauze accelerate the reaction of NH 3 with NO with respect to the as-received gauze. N 2 was the main reaction product and traces of N 2O were comparatively formed (N 2O/N 2 ∼ 10 −3). Pulsing of an equimolar O 2– 15NH 3–NO mixture over the Pt–Rh gauze mainly produces 15NO, while the formation of N 2 is largely suppressed. The selectivity to 15NO and N 2O in the ternary O 2– 15NH 3–NO system diminished upon decreasing the O 2/( 15NH 3 + NO) ratio, in favor of N 2. This ratio was qualitatively varied by changing the time delay between O 2 and 15NH 3–NO in sequential pulse experiments. Our results indicate that the NH 3 oxidation by O 2 to NO is much faster than the NO reduction by NH 3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N 2 and N 2O in ammonia burners within nitric acid manufacture.