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Abstract: The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt–Rh (95–5wt.%) alloy gauze at 1023–1073K 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. [Copyright &y& Elsevier]

The effect of the iron distribution in steam-activated FeZSM-5 and Fe-silicalite on their catalytic performance for direct N2O decomposition and for the oxidative dehydrogenation of propane (ODHP) with N2O and O2 was investigated by means of steady-state and transient pulse experiments. Steam-activated FeZSM-5, containing a large fraction of iron as oligonuclear FexOy clusters, is more active and selective in ODHP and direct N2O decomposition than steam-activated Fe-silicalite, where isolated iron species predominate. This suggests that the same type of iron centers is involved in both conversions. The higher activity of steam-activated FeZSM-5 is associated to the higher mobility of atomic oxygen species attached to oligonuclear iron species, which lead to a faster oxygen desorption in direct N2O decomposition as well as to a faster transfer to propane in the ODHP reaction. Steam-activated FeMFI zeolites were found to be highly effective catalysts for ODHP with N2O, with initial propene yields up to 24%. Relatively high yields of propionaldehyde are also obtained, which mainly originates from the selective oxidation of propene. The unique ODHP performance of iron zeolites requires the use of N2O as monooxygen donor. Such species cannot be created by O2. Only a fraction of oxygen species deposited over iron sites of FeMFI upon N2O activation was found to be active towards propane oxidation under transient vacuum conditions. This is explained by a fast transformation of short-living highly reactive atomic oxygen species into less reactive ones. [Copyright &y& Elsevier]

Abstract: The effect of reaction conditions and rhodium content in H2-pretreated Pt–Rh materials on their initial activity and time-on-stream behavior in direct N2O decomposition at 1023K was investigated by means of continuous-flow catalytic tests at ambient pressure and X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Reduced pure Pt (Pt100) gauze shows a slightly higher initial activity (after 10min on stream) in the reaction than reduced pure Rh (Rh100) wire. The initial N2O conversion over various Pt–Rh alloys (95–5, 90–10, and 80–20) was considerably lower than that of the pure metals. The following order of initial activity was obtained: Pt100 >Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20. However, Pt100 gauze rapidly deactivates with time-on-stream. This deactivation is tentatively attributed to the penetration of oxygen (generated upon N2O decomposition) into the bulk of the metal. In contrast to Pt100 gauze, Rh100 wire activates with time-on-stream leading to a significantly enhanced and stable N2O conversion. The activity trend after 900min on stream is changed to: Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20 >Pt100. The different catalytic performance of Rh is related to the gradual transformation of metallic Rh to Rh2O3 under reaction conditions as concluded from surface composition obtained by XPS analyses. Our results indicate that the Pt–Rh alloy gauzes are inactive for direct N2O decomposition in the presence of excess oxygen, as typically found in ammonia burners. [Copyright &y& Elsevier]