Chemisorption on chromia

In the past, there have been two main types of interpretations of chemisorption and catalysis on chromia. One is more or less of the type given in this chapter (for example, 39, 56, 63a, 66). The other is based upon theories of semiconductors (for example, 68-70). Chromia is a semiconductor at high temperatures (20). No complete theory of chemisorption on chromia is possible at present one can only use approximate treatments. However, in our opinion, the first type of approximation (which is related to coordination chemistry and cry.stal field theory) is much more useful than the second type for reactions in reducing atmospheres at lower temperatures, say below 300°. Morin (70a) has given an analysis of transition metal oxides which indicates that the 3d band in a-Ci 203 is so narrow as to correspond to 3d charge carriers localized on the cations. 

C -C, on platinum-silica, 30 352 chain termination bed residence time, 39 255-256 probability, CO pressure effects, 39 258 chemical mechanisms for reactions with deuterium, on chromia, 20 73-84 chemisorption, carbon atom complexes, 32 167-167, 175-176 coupling, 27 235-238 double, 27 238, 239 cracking, 39 283 cyclic, catalysis of, 20 309-311 cyclization, 28 295 degree of strain, 25 135 dehydrogenation of, 19 88, 89 deuteration of, 25 140, 141 dimerization, 20 304... 

In Sections III-V, we speculated about the nature of active sites on chromia and the relations of such speculations to chemisorption and heterogeneous catalytic reactions. In particular, we suggested that many types of active sites would involve coordinatively unsaturated surface (cus) ions of Cr3+ and 0 - and that the following types of chemisorption might occur at such sites simple coordinative adsorption at Cr3+(cus), adsorption of generalized acids at 02-(cus), heterolytic dissociative adsorption at pair sites of Cr +(cus) and 02-(cus), and reductive adsorption. In addition, we considered the possibility of ligand displacement adsorption which does not depend upon (cus) ions. 

Since (cus) sites should develop and increase in number on chromia as the temperature of activation is increased, it appeared of interest to compare chemisorption with catalytic activity as a function of activation temperature. 

Garcia de la Banda (67) has measured the chemisorption of hydrogen and oxygen on chromia at temperatures of about 100-150°. The amounts adsorbed increase with temperature. He considers that active sites are formed endothermically and increase in number with temperature. There are temperature ranges and adsorption conditions in which one could observe such behavior on our catalysts, but we believe that such phenomena are better explained by the treatment which we have given. 

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