Vacuum pre-treatment

To understand these results, we must look at the band structure of the MgO surface (Fig. 5). In the band gap a number of lattice defects and surface states can be observed. The various transitions are shown by arrows. Vacuum pre-treatment at 300 °C will cause oxygen chemisorbed as O2 or 0 79) to be removed from the surface or to migrate into the lattice, thereby annihilating an oxygen vacancy. Physically and chemically adsorbed water will be removed, thus occupying another O2 vacancy in the surface ... 

The preparation methods of 105 °C diying and vacuum pre-treatment yield samples with different residual moisture contents (see Table 1). The pressure/volume curves of dry (Bl Gl) and wet (B2 G2) samples are compared in Fig. 1 and 2. 

Fig. 1. Total average curves with uncertainty interval of cylindrical samples dried by various methods (Bl -105 °C drying, B2 - vacuum pre-treatment)...

Although the activities have been determined under differing reaction temperatures, they have been extrapolated to 300°C and 350°C as a means of comparison. The data in the tables illustrates a number of features of the reaction. Comparison has been made between the conditions of pretreatment, such as vacuum versus oxygen atmospheres. The temperature of the vacuum pre-treatment was carefully controlled, as previous studies had shown that when the sample was pre-heated too high in vacuum, a... 

Traces of acid adhering to glassware are sufficient to induce explosive decomposition of the alcohol dining distillation, and must be neutralised by pre-treatment with ammonia gas. Low pressures and temperatures are essential during distillation. Explosions during distillation using water pump vacuum and bath temperatures above 115°C were frequent. 

Micro-calorimetric adsorption measurements require a proper in situ vacuum activation at a higher temperature than the adsorption process. A first pre-treatment under oxygen is performed in the calorimetric cell in order to eliminate the impurities present on the sample (essentially carbonates, nitrates, carbonaceous residues and water present from the preparation, calcination and exposure to atmosphere) and to avoid the partial reduction of the surface of an oxide that is easily reduced under vacuum. 

Extremely valuable information on the chemical state of the elements can be obtained by combining the XPS analysis with a pre-treatment operation on the sample under reactive gas. The XPS analysis is conducted under vacuum, which hinders genuine in situ studies being carried out but does have the advantage of preventing, for example, the reoxidation of reduced catalysts. The treated sample must be introduced under inert gas either from a glove box preparation or from a treatment cell connected to the equipment as illustrated in Figure 5.13. 

Similar to XPS in terms of the analysis and interpretation of spectra, the cost of LEISS analysis is relatively high with regard to the preparation of the sample. This is because constraints on vacuum quality and the cleanliness of the sample surface are stricter. A pre-treatment operation lasting several hours is often required. 

In situ adsorption cells enable adsorption under gas flow (dynamic mode) or under pressure (static mode). The catalysts are generally studied afler pre-treatment, under specific gases or under secondary vacuum, at high temperature. Adsorption may be carried out between 450 C and liquid nitrogen temperature depending on the material used. 

The novel feature of our approach is to prepare freestanding, thin membranes by vacuum deposition on to a suitable temporary substrate that can easily and cleanly be removed. Important requirements for the substrate material are that it be very smooth and free of contaminants, pinholes and surface defects. The materials should also have decent thermal stability and be inexpensive (relative to the membrane), reusable and/or recyclable. Based on these requirements, two initial approaches were envisioned (1) deposition of the membrane onto a polymeric substrate, which can be subsequently chemically dissolved or (2) deposition onto a rigid substrate with and without pre-treatment with a thin release coating that may be water soluble. 

Metals other than A1 have been successfully introduced in numerous zeolite frameworks. Aluminum substitution by other metals, such as Fe, Ga, Zn, Co or Cu in the zeolite framework results in modified acidity, and subsequently modified catalytic activity, for certain reactions such as selective catalytic reduction of NOx by hydrocarbons. For example, a calorimetric and IR spectroscopic study of the adsorption of N2O and CO at 303 K on Cu(II)-exchanged ZSM-5 zeolites with different copper loadings has been performed by Rakic et al. [92]. The active sites for both N2O and CO are Cu (I) ions, which are present on the surface as a result of the pre-treatment in vacuum at 673 K. The amounts of chemisorbed species adsorbed by the investigated systems and the values of the differential heats of adsorption of both nitrous oxide (between 80 and 30 kJ mof ) and carbon monoxide (between 140 and 40 kJ mol ) demonstrate the dependence of the adsorption properties on the copper content. 

Pre-treatment procedure Ml. After inserting the fresh catalyst into the vacuum system, it was heated up to 573 K at 10 K/min The catalyst... 

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