HYDROGEN ACTIVATION BY MAGNESIA CATALYSTS

Hydrogen activation by magnesia catalysts was investigated by studying H2D2 exchange. The number of active sites, surface composition, rate, and mechanism of the reaction were investigated as a function of the activation temperature. Behavior of a commercial magnesia and a catalyst synthesized from Mg(OH)2 were similar, although the latter catalyst had a much larger surface area. The upper limit for the number of active sites was determined in the classical manner by using selective poisoning by CO. A novel technique involving monitoring the stoichiometric reaction between magnesia and D2(g) enabled the very unusual determination of the lower limit for the number of active sites. Thus, the true number of active sites is bracketed within the upper and lower limits. Maximal activity occurred after activation at 700°C. The number of active sites is about 1012/cm2, which is 103-fold higher than formerly reported on the basis of EPR data. The turnover frequency at 273 K and a partial pressure of 20 Tort of an equimolar mixture of H2D2 is 4 s-1, roughly 103-fold less than previously reported. The site density and activity are now consistent with expected values, rather than the anomalous values previously reported. The hydroxyl coverage of the surface was determined in a novel manner using thermogravimetric analysis over the temperature range of 300 to 1400 K. The catalysts are of low activity when the surface is either of very high or very low hydroxyl content. A mechanism in which the active site includes an ensemble consisting of a Mg2+ center and neighboring surface OH and O2- is proposed. © 1992.