Preoxidation increasing

An application of an electrochemical quartz crystal microbalance (EQCM) in the study of the A11/HCIO4 system shows that even at a potential about 0.5 V more negative than the onset of AuO formation (the so-called preoxide region), the resonant frequency of the Au-covered quartz crystal decreases as that of the surface mass increases. A comparison of a voltammogram with the potential dependence of the micro-balance frequency for an Au electrode is shown in Figs. 6a and 6b. 

This increase of the mass was ascribed earlier to the adsorption of perchlorate ions, °° a conclusion that found no confirmation in work published later. It turns out that other weakly adsorbing anions of different masses (NOJ, CFySOf) give the same values of frequency decrease as was observed for C104. Ultimately, the increase of the electrode mass in the preoxide region was explained in terms of the three-dimensional hydration of AuOH, which is present in small amounts at the gold surface. The mass increase was consistent with the surface hydration for a cluster of about 32 water molecules per one AuOH site. ... 

A single N(ls) peak at 402 eV is observed on preoxidized nickel [with clean nickel at 80 K the N(ls) peak is at 399.5 eVj. The intensity of the 402-eV peak is both temperature and pressure dependent, increasing in intensity with decreasing temperature and increasing pressure (46). 

The depolymerization reaction was modified by Ouchi, Imuta, and Yamashita (3), who substituted p-toluenesulfonic acid (PTSA) for BF3 as the catalyst and increased the reaction temperature to 180° to 185°C. A very high degree of depolymerization was achieved under these conditions, with pyridine-soluble product yields over 90%. Darlage and Bailey (4) investigated the effects of reaction temperature, various solvents, and coal preoxidation on depolymerization product yields using a number of acid catalysts. They found that meta-substituted phenols were more effective aromatic substrates for the depolymerization reaction than phenol. The preoxidation of coal, particularly of some sulfur-rich bituminous coals, with dilute aqueous nitric acid considerably increased the yield of depolymerization products (5),... 

The porous structure of chars from a high volatile bituminous coal from mine Pumarabule in Spain, initial and preoxidized, then steam activated, was characterized by carbon dioxide and benzene adsorption measurements, as well as by immersion calorimetry molecular probes with increasing critical dimensions were used. The influence of preoxidation of the coal on the values of parameters describing the pore size distribution, with particular attention to micropores, evaluated according to each of the applied methods, is discussed. 

Figure 2. Porosity formation in steam activated chars from initial and preoxidized high volatile bituminous coal with increasing bum-off.

Preoxidation of the coal causes a strong development of submicropores, what favours the creation of microporosity in the obtained char (Figure 1). In consequence, increased volumes of micropores are also developed in the resulting activated chars (Figure 2). 

Obviously, some of the dichloromethane accessible micropores are closed for the benzene molecules (Figure 5a). Adsorption of carbon dioxide, expressed as V co2 (Figure 5b), is not very much alfected by the degree of activation of the chars. This could mean, that the main structural elements of the chars on which carbon dioxide can be adsorbed, are both present and accessible in the chars before their activation. Preoxidation causes an increase of carbon dioxide adsorption. 

Chars from the initial coal, activated to low bum-off (5 % and 10%), indicate a narrow micropore size distribution in the region of widths between 0.33 and 0.41 nm (accessible for dichloromethane, inaccessible for benzene). Preoxidation of the coal shifts this distribution towards slightly wider micropores - between 0.41 and 0.54 nm (accessible for benzene, inaccessible for cyclohexane), accompanied by an increase of the volume of these pores. 

In case of the char activated to the highest studied bum-oflf (50%), the molecular sieve properties in this range of micropores disappear, but the pronounced effect of micropore volume increase due to preoxidation is clearly visible. Figure 7... 

These experiments demonstrated that the passivating qualities of steam pretreatment may be partially associated with the generation of a silica-rich oxide. However, brief attempts to increase the surface concentration to approach a continuous silica layer excluding all the alloying metals (by variations in preoxidation conditions) met with failure, exhibiting bulk inhomogeneity and oxide spallation problems. Further, many ESC plants are constructed with radiant coil alloys low in silicon content, and so preoxidation can only be considered a partial palliative. Hence, attention turned to the prospect of coating the internal surfaces of radiant coils with a thin, continuous layer of silica. 

It is seen that coke formation on the sample of foil is dependent on the material of the tubular reactor as is the product gas composition. With the steel reactor, preoxidation leads to a substantial increase in the coke formation on the steel foil (Figures 6 and 7). The effect of preoxidation is even more pronounced with Ni as the foil material (Figure 8). On the other hand, preoxidation does not lead to a high rate of coke formation on the steel foil if a quartz liner is used in the tubular reactor (Figure 9). 

N2 to 02 resulted in the formation of nodules of intermixed Ti02 and AI2O3 interspersed with thin areas of protective A1203 which would cover the entire surface in the absence of N2.The area density of these nodules increased as the concentration of N2 increased until the surface was completely covered with the mixed oxides when the gas contained 90 % N2. One effect of N2 appears to involve the nucleation and initial growth of the scale since preoxidation in a nitrogen-free gas develops an alumina scale which remains protective during subsequent exposures in air [60] even under cyclic conditions [68]. 

In order to determine the value of Cq, Imre (40) has used preoxidized Ag-Cd alloy samples in which the diffusion distance Ax is relatively large, for example, 30 p, and has followed further advancement of internal oxidation by measuring the weight increase per unit area AmjA of the sample during time At. With the help of Fick s first law, the value of Cq is found to be... 

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