Catalysts and in situ experiments

ES S studies of catalysts are described in Chapter 7. These experiments present a particular set of problems that need to be addressed. Foremost is the fact that INS is not an intrinsically surface sensitive technique. This is overcome by using large samples to maximise the number of surface sites and hydrogenous adsorbates to give the highest possible contrast. Supported metal catalysts, zeolites and oxides often have low densities, while metal powders have high densities but low surface areas. Both situations require a large volume cell to place sufficient sample in the beam. 

Catalysts with a heavy-metal content are an exception to the rule that chemical samples do not usually become radioactive. This radioactivity must be allowed to decay to safe levels before it can be handled without very special apparatus or precautions. To avoid delays whilst waiting for this decay, several cells are run in parallel while one is being measured, another is being treated and a third is being left to decay. This method 

A further option is to carry out all the operations on the sample in the cell while it is in the cryostat, at the correct temperature, in the beam. This has considerable advantages only one cell is needed and there are no concerns about sample relocation. The experiment becomes much more flexible and readily accommodates changes in the original plan, e.g. increasing the amount of adsorbate. The disadvantages are that the experimental arrangements are more complex the transfer lines transporting the adsorbate to the sample must be kept warm to prevent 

The experience of the authors after several iterations is that the best system is to use steel cells with the minimum wall thickness permitted for the required temperature and pressure conditions. An off-the-shelf sealing system, such as Conflat flanges with copper gaskets or metal tubes with Swagelok fittings work well. There is also the need to allow the adsorbate access to the sample and all-metal bellows type valves are the best choice. These survive temperature cycling well. A variety of cells that have been used at ISIS are shown in Fig. 3.33. Fig. 3.34 shows a centrestick equipped for gas handling. 

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