Flash filament method

Abb. 19. Bestimmung des Endvakuums nach der Flash-filament-Methode bei Ktihlung der Fallen mit C08 bzw. fliissigem Stickstoff (nach Hagstrum). 

Abb. 43. Sekund relektronenausbeute von He+-Ionen 200 eV) an Molybdan in Abh ngigkeit von der Gas bedeckung (obere Kurve). Die Gasbedeckung ist nach der Flash-filament-Methode gemessen (untere Kurve... 

Apker, L. Flash filament method. Ind. Engng. Chem. 40, 846 (1948). 

Various techniques are used to obtain information on the active centers of catalysts, such as selective poisoning, measurement of the catalyst acidity and its strength, field electron and ion microscopy, infrared spectroscopy, fiash-filament desorption, differential isotopic method, etc. A temperature-programmed desorption method, which will be described and discussed in the present article, is in principle similar to the fiash-filament desorption method, reviewed recently by Ehrlich (1). It differs, however, from it in several respects. Modifications have been necessary in order to make the construction and operation of the apparatus easier and to adapt it to studies of materials other than metals, for example the conventional oxide catalysts. The conditions employed are much more similar to those ordinarily used in catalytic reactions than is the case with the fiash-filament method. An additional important feature of the modified technique is that it permits in some cases simultaneous study of a chemisorption process and the surface reaction which accompanies it. At the same time the modifications made have sacrificed some of the simplicity of the flash-filament method. For example, an obvious complication may arise from the porous structure of the conventional catalytic materials, in contrast to the relatively smooth surfaces of metal filaments. The potential presence of this and other complications requires extension of the relatively simple theoretical treatment of flash-filament desorption to more complicated cases. 

Temperature-programmed desorption (TPD) (Cvetanovic and Amenomiya 1972) yields not only the heat of adsorption but also information on groups of different active sites, which are useful in characterizing reactions. While the flash filament method in which an adsorbate is desorbed from a rapidly heated filament in an ultra-high vacuum environment has been used for a long time, the usefulness of the TPD method lies in the fact that the adsorbed gas is desorbed in a programmed... 

As noted in Section 2.1.2, measurement of the kinetics of chemisorption on clean metal surfaces generally requires ultra high vacuum techniques, in order to accomplish the experiment in a reasonable period of time. The variant of the classical adsorption method known as the flash-filament technique has been developed by several groups of workers and recently summarised by Ehrlich . 

The flash filament experiment as first described by Becker and Hartman (14) has since been used extensively in studies of the adsorption of gases onto refractory metals, particularly in association with other techniques. The basic method is to allow gas introduced at a known input rate to adsorb for a measured time onto a previously cleaned wire or ribbon. The gas is then desorbed by heating the sample, and the resulting pressure bursts monitored. The pressure versus time curve is referred to as a desorption spectrum, as illustrated in Fig. 4 and 5. Sticking probabilities can then be obtained from the relative adsorption times and desorption quantities. Methods of analysis of these desorption spectra (15, 16) and of the variation in thermal resolution by different heating schedules such as linear or reciprocal increase in temperature with time, have been discussed extensively by a number of authors... 

Low energy (usually less than 100 eV) low intensity (10" -10 A/cm ) electron beams, causing the dissociation and desorption of adsorbed phases have been used increasingly, in recent years, as a surface studies tool (59). This approach has an advantage over flash-filament techniques in that the method does not appreciably alter the surface film being studied, provided that the surface is probed with a sufficiently low current density of electrons. Invariably, electron probe current densities used in these experiments can be a factor of a hundred lower than those used in LEED experiments, in which the processes involved are very similar. The method is applicable for the study of bulk single crystals or ribbons as well as for polycrystalline samples, unlike FEM and LEED measurements which are restricted in their application. 

A proper judgment of the validity of these findings, as well as any extension of such work, must rest upon a detailed appreciation of the experiments involved. It is the aim of this article to review the experimental methods upon which these advances have been based—the flash filament technique, flash desorption, field emission and field ion microscopy, and the use of ultrahigh vacuum procedures. 

Two procedures have been described, (I) flash filament pyrolysis (Section 6.3.1) and (ii) heating the polymer in a micro-reactor and collecting the total volatiles produced, (Section 6.3.2). In yet another more recent approach, the pyrolyser probe method discussed below, the polymer is placed in a quartz tube which is then inserted into a platinum coil heater element. The coil heater is switched on to release the pyrolysis products which are then directly swept into the gas chromatograph. 

The equipment needed in flash evaporation is comparatively modest, and this method is also suitable for the preparation of amorphous alloy films whose constituents have different vapour pressures. It consists of a single heated filament usually made of molybdenum. Powder of the alloy is fed continuously onto the filament, the temperature of which is sufficiently high for evaporation. No shifts in composition of the alloy occur since all the material is evaporated to completion. Devices for monitoring the vapour flux and the source temperature are not needed. The method is restricted to materials that can be obtained in powdered form. 

Two of the most widely used methods for preparation of alpha sources on a metal backing for counting or spectroscopyare (1) direct evaporation of an aqueous or organic solution and (2) electrodeposition. Other methods, such as volatilization of the sample in a vacuum, adsorption from an aqueous solution, are not so widely used. However, the method of vacuum "flashing" from a tungsten filament produces very satisfactory sources and is in routine use in some laboratories. (See for example Procedure 4 in Sect. VUI.)... 

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