Magnesium oxide activation energies

Calculations of the interaction energy in very fine pores are based on one or other of the standard expressions for the pair-wise interaction between atoms, already dealt with in Chapter 1. Anderson and Horlock, for example, used the Kirkwood-Miiller formulation in their calculations for argon adsorbed in slit-shaped pores of active magnesium oxide. They found that maximum enhancement of potential occurred in a pore of width 4-4 A, where its numerical value was 3-2kcalmol , as compared with 1-12, 1-0 and 1-07 kcal mol for positions over a cation, an anion and the centre of a lattice ceil, respectively, on a freely exposed (100) surface of magnesium oxide. 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

To use adsorption as a unitary operation in industrial, pollution abatement, or energy production applications, in most cases, a reactor where a dynamic adsorption process will take place is packed with a concrete adsorbent. The adsorbents generally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Magnesium oxide has been investigated by Robertson and by Henney and Jones. Surface diffusion was identified as the dominant mechanism of transport in both of these studies. The activation energies appeared excessive for this interpretation. In the reevaluation of these data it was shown that interpretation as volume diffusion produced = 0.02 exp (—70,000/i 7, which is in agreement with the tracer and mass spectrometer results compiled by Harrop in which the mean activation energy for Mg and O migration is 70,500 cal/mole. 

The impurity diffusion coefficient of Fe impurities in a magnesium oxide, MgO, single crystal is given in Table 7.7(e). Estimate the activation energy for diffusion. 

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