Pre-oxidised platinum sponge

Fig. 8. A [ N]-NH3 pulse was adsorbed on the pre-oxidised platinum sponge kept under He flow (40 cm /min) at 373 K followed after 170 s by a temperature programmed desorption experiment (a) TPD spectrum (b) PEP image, the colour intensity represents the concentration of (c) normalised concentration as function of time...

The role of NO as an reaction intermediate is further investigated in an experiment, in which a [ N]-NHs pulse was adsorbed on pre-oxidised platinum sponge kept imder He flow at 323 K, followed by the removal of the adsorbed species with nitric oxide (Fig. 12). Figures 12(b) and 12(c) show... 

Ammonia oxidation is conducted on a pre-oxidised platinum sponge catalyst. Figure 20 shows the conversion and selectivity at 373 K. The same selectivity characteristics as on the reduced platinum sponge catalyst are observed (Fig. 14). Thus, a high oxygen surface coverage does not favour initial nitrous oxide formation. The main difference with the reduced platinum sponge is the faster deactivation of the pre-oxidised catalyst below 413 K. 

Fig. 20. Ammonia oxidation reaction performed at 373 K on a pre-oxidised platinum sponge catalyst (a) concentration of N2, N2O and H2O versus time for (b) conversion of NH3 and O2 versus time (GHSV = 5600 hr , NH3/O2 = 2/1.5, flow = 46.5cm /min.).

A pre-oxidised catalyst deactivates much faster than reduced platinum sponge. Ammonia adsorption and dissociation are accelerated by the presence of oxygen. Thus, the NHx species cover much faster the platinum surface. The concentration profiles for nitrogen and nitrous oxide do not change, which indicates that the reaction mechanism is not changed for the pre-oxidised catalyst. 

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