Chromia chemisorption

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... 

Matsunaga (15) applied the magnetic techniques of Eischens and Selwood and the chemisorption and chemical techniques of Voltz and Weller to a series of chromia-alumina catalysts. He found that in the limit of low chromia contents, where Eischens and Selwood deduced a two-dimensional distribution of chromium ions, treatment with oxygen at 450°C. resulted in an average valence number of six for all of the chromium ions in the sample. 

Later studies by Garner and his co-workers showed that the fraction of carbon monoxide or hydrogen reversibly chemisorbed at room temperature varied from oxide to oxide. Zinc oxide was shown to be a case where the adsorption of carbon monoxide at room temperature was completely reversible. The heat of adsorption, determined both calorimetrically (5) and isosterically 6), was in the range 12-20 kcal./mole. For several other oxides, however, notably chromia, Mn20s and Mn20s Cr20s, the heat of adsorption of carbon monoxide was higher and the chemisorption was... 

The surface of chromia appears to be an ideal case for study at the present. By activation at increasing temperatures, one can vary the number of active sites from none to some maximum number. At a low density of sites, one can hope that the sites are well separated and noninteracting. One can compare chemisorption of various gases with specific catalytic activities at various levels of site densities and hope to gain information about site heterogeneity. This chapter reports a first approach to this objective. 

Fig. 7. Chemisorption of oxygen and of carbon monoxide on amorphous chromias as a function of the temperature of activation squares, carbon monoxide solid circles, carbon monoxide after helium flush at 25 open circles, oxygen x, carbon monoxide on microcrystalline rt Cr203, axis at the right triangles, oxygen on microcrystalline a-Cr203, axis at the right.

We attempted to measure an adsorption isotherm for the adsorption of ethylene at 25° on a chromia activated at 337°. At 2, 8, 18, 28, 36, and 48 torr, the weight rapidly reached a steady state at 60 torr, the weight very slowly increased at 90 torr it increased rather rapidly. In 12 minutes at 90 torr, 0.5 rnolecules/100 of extra ethylene adsorbed. Adsorption from about 25 to 50 torr is linear with pressure. If one extrapolates the linear region to zero pressure as we did with carbon monoxide, one computes a chemisorption of 0.6 molecules/100 A. Most of the ethylene adsorbed at 90 torr is not removable by helium flushing at either 25 or 100°. 

---