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Collecting information for a presentation [academic high-school] about catalysts, I stumbled upon the terms desorption and adsorption as important parts of the Haber process.

However, I could neither find a good explanation for these processes (why and how do they occur?) nor the difference between ad- / desorption and the process of the reactants "diffusing through the phase boundary to the active site", as it's stated on German Wikipedia.

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All chemical catalysis can be grouped into 1 of 2 categories: homogeneous or heterogeneous catalysis. Homogeneous catalysts are ones that are in the same phase (i.e. solid, liquid, gas BUT USUALLY IN SOLUTION) as the reactants they are helping to react. Heterogeneous catalysts are in a different phase than the reactants they are working on. Industrial processes (like the Haber-Bosch process for making ammonia) frequently use heterogeneous catalysts due to their comparatively inexpensive nature, and there ease of handling on a large scale. Doing it this way can cause some added problems, namely the reactants need to bind to the catalyst's surface (adsorption) for the catalyst to work. In the H-B process this is gaseous N2 and H2 binding to a mainly iron catalyst. To help this process along the iron is made with a high surface area with holes and such (think like the texture of a sponge except on a much smaller scale). After the reaction happens the products then need to release from the catalyst (desorption) so they can be separated from the catalyst and be collected. The active site simply refers to the place on the catalyst where the reaction happens. Adsorption is helped by the high temperature and massive pressures in the H-B process, as the gasses are flowed through a chamber at high pressure containing the solid heterogeneous catalyst. Not all of the molecules stick and not all that do stick react, so the underacted gas is recycled and passed over the catalyst again. Long story long...the very question you asked won the Gerhard Ertl the 2007 Nobel Prize in Chemistry.

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The first few slides seem to give a nice simple explanation. There is a Van der Waal force interaction between the solid and a gas. In your case, nitrogen, hydrogen and the catalyst. http://www.ece.eng.wayne.edu/~jchoi/Adsorption%20of%20gases%20on%20solids.pdf

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    $\begingroup$ This is intriguingly close to a link-only answer; it would be better if you could include more of the process in your answer since we do not know how long the link will be reachable. $\endgroup$
    – Jan
    Nov 26, 2015 at 13:13

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