Pre-oxidised platinum

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.).

However, above 413 K and also on the pre-oxidised catalyst, the high activity and selectivity towards nitrogen sustains. The presence of oxygen at the platinum surface apparently does not cause a permanent deactivation of the catalyst. Above 413 K, the catalyst is reduced by ammonia. 

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. 

The above FIA systems are based on monitoring the anodic decomposition of hydrogen peroxide at a platinum electrode set at 600-700 mV vs a Ag/AgCl electrode. However, at such high potentials other electroactive species, notably ascorbic acid, uric acid and hypoxanthine, will also be oxidised unless appropriate sample pre-treatment is taken. 

Platinum catalysts were prepared by an ion-exchange method [16,17]. Oxidised sites on the surface of an activated carbon support (CECA SOS) were created by pre-treatment with sodium hypochlorite (3%) the associated protons were subsequently exchanged with Pt(NH3)4 " ions, in an aqueous ammonia solution, and reduction was carried out on the dry catalyst under a flow of hydrogen at 300°C. A surface redox reaction was subsequently employed to deposit the bismuth whereby the catalyst was suspended in a glucose solution, under an inert nitrogen atmosphere, and the required volume of a solution of BiONOs, dissolved in hydrochloric acid (IM), was added [18]. 

A simple comparison between the reference spectra of Pt foil, hulk Pt02 and Pt-Zn/AI2O3 spectrum (Figure 1) allows the conclusion that after the pre-treatment, the platinum is fully oxidised. It is less than likely that it could be reduced during the course of the reaction since this one occurs in a very oxidising media (6% O2). This result is very important in the sense that NO does not dissociate on platinum oxide and therefore implying that one of the intermediate is likely to be an adsorbed hydrocarbon species reacting with NO. 

---