Distinct activity and time-on-stream behavior of pure Pt and Rh metals and Pt-Rh alloys in direct N2O decomposition at high temperatures

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 1023 K 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 10 min 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 900 min 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.