Pre-oxidised

Reduction of substituted nitrobenzenes under alkaline conditions, usually with aqueous sodium acetate as electrolyte and a nickel cathode, is the classical method due to Elbs [45] for the formation of azo- and azoxy-compounds. Protons are used in the electrochemical reaction so that the catholyte becomes alkaline and under these conditions, phenylhydroxylamine reacts rapidly with nitrosobenzene to form azoxybenzene. Finely divided copper has long been known to catalyse the reduction of nitrobenzene to aniline in alkaline solution at the expense of azoxybenzene production [46]. Modem work confirms that whereas reduction of nitrobenzene at polycrystalline copper in alkaline solution gives mainly azoxybenzene, if the electrode is pre-oxidised in alkaline solution and then reduced just prior to the addition of nitrobenzene, high yields of aniline are obtained with good current efficiency... 

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. 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

Fig. 3a, b GC-MS chromatograms showing the different degradation product patterns after ageing of pre-oxidised LDPE-PO in a biotic mineral medium inoculated with a mixed culture of compost microorganisms and b sterile mineral medium... 

Fig. 4 Changes in carbonyl index for pre-oxidised LDPE-PO degraded at 50 °C in air, in sterile mineral medium and in biotic mineral medium with compost microorganisms...

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... 

Figure 2. Conversion of CO (circles) and HC (squares) over pre-oxidised Pd/Ce/A Os using net oxidising (fdled symbols) and net reducing (open symbols) CO/C3H6/O2/N2 gas mixtures.

Pd/Co/Al203 (when pre-oxidised) does not seem to exhibit an improved low temperature activity compared with Pd/Al203, suggesting that pre-oxidised cobalt oxide does not promote the activity of Pd. Thus, pre-oxidised cobalt oxide promotes the activity of Pt, but not of Pd, for oxidation of CO and propene. However for pre-reduced Pd/Co/Al203, there is an obvious promoting effect on the low-temperature activity (see below). 

In Figure 5, however, it is seen that the pre-treatment atmosphere has a significant effect on the low temperature activity of Pd/Co/AbOa. The effect of catalyst pre-treatment is most pronounced for the cobalt oxide catalyst promoted with Pd or Pt (see Table 2). In lean reactant gas, pre-reduced Pd/Co/AbOs has light-off temperatures at 169°C and 177°C for CO and HC, respectively, whereas the light-off temperatures over the same catalyst, but pre-oxidised, are 246°C for both CO and HC. A clear effect of pre-reduction is also seen for Pt/Ce/Ab03, whereas no obvious effect of the pre-treatment atmosphere on the oxidation activities for CO or HC is seen for Pd/Ce/AbOs. 

An even more pronounced difference in light-off performance between pre-oxidation and pre-reduction is seen for C0/AI2O3 (see below). The reason that pre-reduced Pd/Co/AbOs and Pt/Co/Ab03 start to react at lower temperatures than the pre-oxidised samples (see Figure 5) is most likely due to that the initial reaction on cobalt sites starts at lower temperatures for the pre-reduced samples than for the pre-oxidised catalysts. The pre-reduction may also induce alloy formation with lower adsorption energy of CO compared with the noble metal [18]. It is also possible that 0-vacancies form on the cobalt oxide which promote the dissociative adsorption of O2. Reaction with CO will then proceed on the noble metal or at the interface between Pt (or Pd) and cobalt oxide [9-10]. 

N 0 plus "C 0 over pre-oxidised materials The trace amounts of N2 and produced over pre-oxidised Ce02 by RT contact with equimolar N O + CO were similar to those observed from RT contact with only. Under T-ramp the first... 

Over pre-oxidised Rh203 the most notable difference observed under the equimolar 0... 

C °0 mixture was that uptake of N 0(s) commenced at 280°C which was ca. 220°C lower than onset of a much smaller decrease over the same material under N 0 only. Furthermore that onset at 280°C was accompanied by a parallel decrease in C 0(g) and by onset of increases in CO2 products, thereby indicating that the pre-oxidised Rh203 sample promoted reaction(s) between N 0(g) and C 0(g) at temperatures 280 -> 600°C. Since no N2 0 or N2 0 was detected, reaction with 1 1 stoichiometry appeared to be favoured so that C 0 0 could be expected as the primary product fi-om 0 + -> /2 N2... 

Comparison between the composition profiles versus ramp temperature observed over pre-oxidised 0.5% RhOx/Ce02 when in contact with N 0 + C 0 (cf. fig. 2a) rather than with N 0 only (cf fig la) reveals the absence of detectable 0 - N 0 in the former. Furthermore, the profiles in fig. 2c confirm the lack of any evidence for the oxygen isotope exchange process over the 0.5% Rh/Ce02 sample when in the HTR condition in contact with the equimolar mixture, thereby doubly emphasising the inhibitory role of C 0 co-reactant upon the O.i.x process observed over the same material equivalently pre-treated but in contact only with Blockage or removal by CO of the sites or species active for o.i.x. on... 

Fig. 2. Changes in gas phase composition following RT introduction of 5 mbarr each of N 0 and and T-ramp to 700°C over 0.5% RhOx/Ce02 when pre-oxidised (2a) LTR (2b) and HTR (2c). (Vertical scale MS peak height in mutually consistent a.u., except for small upward displacements of CO2 profiles to avoid overlap) Bottom scale °C.

TPD measurements were performed for the NOx-storage catalyst with Pt-Rh/Ba0/Al203 and for samples without the storage medium (BaO) or noble metals, respectively. TPD experiments were performed by exposing pre-reduced and pre-oxidised catalysts, respectively, to either NO or NO2 at room temperature. 

Figure 4. TPD after exposing the Pt-Rh/Ba0/Al203 sample to NO at room temperature. A) pre-reduced catalyst, B) pre-oxidised catalyst.

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.

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... 

Extent of adsorption of alcohols on to pre-oxidised rutile layers and rate constants for subsequent photoreaction with 1802... 

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