DISPERSION AND COVERAGE OF CR/AL2O3 CATALYSTS

A series of chromia-alumina catalysts was prepared by equilibrium adsorption from ammonium chromate solutions at various pH's. The uptake of Cr increased from 0.4 to 2.5 wt % Cr with decreasing pH from ca. 10 to 2.7. The results were interpreted on the basis of an electrostatic model of adsorption from solution and a reaction between the Cr anions and OH of the alumina support. X-ray photoelectron spectroscopy (XPS) Cr 2p(3/2)/Al2p intensity ratios for dried and calcined catalysis indicated that the Cr phase is highly dispersed or present as a surface compound. The surface coverage of the calcined Cr/Al2O3 catalysts was monitored by CO2 chemisorption and ion scattering spectroscopy (ISS). CO2 chemisorption was found to overestimate surface coverage of the Cr species. This was attributed to selective adsorption of Cr on alumina sites which also bind CO2. The ISS results indicated that the Cr phase covers only a small fraction of the alumina surface (up to 26%). The surface coverages measured by ISS were generally consistent with those predicted from theoretical considerations.