Promoters poor crystallinity

Fe solutions have been aerated at pH 7.2 in the presence of various minerals and rocks including quartz and basalt (Posey Dowty et al., 1986). The major product in all cases except that of quartz (goethite) was lepidocrocite. These authors also noted that lowering the dielectric constant of the solvent (by replacing water with a mixture of water and dioxane) promoted goethite over lepidocrocite. Poorly crystalline lepidocrocite was also the sole product when Fe was oxidized at pH 7 and RT in the presence of bacteria (Bacillus suhtilis Escherichia coli) (Chatellier et al. 2001) (see also chap. 17). 

The synthesis of Al-substituted lepidocrocite needs special precautions because A1 may promote goethite in a Fe system. However, poorly crystalline lepidocrocites with up to 12 mol% A1 in the stmcture have been produced by oxidation of mixed FeCl2/Al(N03)3 solutions with C02-free air at pH 8 in a NH3/NH4Cl-buffer (0.2 M NH3 0.2 M NH4CI 1 19) with the pH being kept constant by adding M NH3 dropwise (Schwertmann and Wolska, 1990 Wolska et al. 1994). Note that A1 can only be incorporated into the lepidocrocite structure, if the pH of the system is close to 8. 

Less frequently, diffraction patterns such as those seen in Fig. 2.30b were obtained, typically from areas rich in promoter oxides. They arise from the spinel structure of magnetite/hercynite particles, with little long-range order. Such particles can only diffract X-rays very poorly and may well remain undetected in a phase analysis by XRD. The existence of a disordered phase has also been mentioned in Ref. 2. The poor crystallinity of the promoter oxides is in full agreement with their genesis as a product of exsolution from the oxide precurser, which has been described in the preceding sections. The dispersion of this poorly ordered phase was found to be much higher in catalysts 2, 3, and 5 than in catalyst 1, where this phase formed seams within the activated material. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

The increase in crystallinity after soaking in phenol solution was estimated using these equations and is shown in Table 2. The crystaUinity changed from 38 to 52 % for sPS and from 4.7 % to 6.5 % for sPPMS after soaking in phenol solution. In our previous paper concerning the absorption of aliphatic alcohols in sPS and sPPMS,[34] there was no trace of further crystallization. Phenol is basically a poor solvent for these pol)nners, but it was established that the crystallization can be promoted by soaking in the phenol solution. 

---