Pre-oxidised surface

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

Pulsed-reactant procedures [192—196] allowed pulsing of the alcohol and/or the photon flux in systems involving alcohol vapour flowing over a powdered sample of the metal oxide (cf. Fig. 10). Surfaces of the metal oxide were preconditioned by heating to 623 K either in the N2 stream (referred to as mildly reduced surfaces) or in a stream of oxygen (referred to as pre-oxidised surfaces) and cooled prior to contact with a pulse of reactant(s). Such reactant pulses then passed through the packed GLC column for separation and subsequent quantitative analysis by the flame ionisation detector. Adsorption of reactant alcohol on the preconditioned... 

The IR spectra of CO adsorbed on different well-characterised chemical states and surfaces of Pd44 45 have helped in interpreting results for this system. Use of this technique has confirmed, for example, that CO adsorption on pre-oxidised supported Pd forms Pd° sites even at room temperature41 even in a stoichiometric CO + O2 mixture, although the bulk of the metal probably remains oxidised until significantly higher temperatures. This approach of course provides information only on molecularly adsorbed forms... 

Fig. 4a shows the TPD spectra after dosing NO at room temperature on a pre-reduced Pt-Rh/BaO/AbOs catalyst. As can be seen the NO is reduced on the surface as manifested in Na and N2O desorption peaks around 200°C. The only trace of NO is a small peak around 100°C. The integrated amount from these curves correspond to an adsorbed amount of 7.1-10 moles NO. The result of a similar experiment but with a pre-oxidised sample is shown in Fig. 4b. In this case there is no reduction of NO taking place. There is a small NO peak at around 90°C and a larger one at about 500°C. There is also an O2 peak around 500°C. It is likely that a chemisorbed oxygen layer on the noble metals prevents the dissociation of NO as observed by Lo6f et al. [6]. When NO2 rather than NO is dosed at room temperature, there is a much larger quantity adsorbed. Further, pre-reduced and pre-oxidised samples show similar TPD spectra indicating that the strong oxidising agent NO2 oxidises the sample at room temperature.

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. 

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 material had been delivered already in the pre-oxidised state. To achieve equal surface conditions for all materials, the initial oxide layer had to be ground off. This reduced the aluminium content to 4.0 wt.%. 

Surface colonisation by bacteria is a very rapid process in the case of pre-aged polyethylene but it also occurs even on polymer films containing transition metal ions that have not been deliberately pre-oxidised. Colonisation is followed rapidly by the bioerosion of he surface of the polymer (Fig. 3). 

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