DETERMINATION OF THE SURFACE COVERAGE OF MO TIO2 CATALYSTS BY ISS ANDCO2 CHEMISORPTION/

A series of Mo/TiO2 catalysts was prepared by equilibrium adsorption f rom aqueous solutions of (NH4)(6)Mo7O24 by varying the pH of the solut ion from 9.8 to 4. The Mo loading increases from 0.8 to 3.6 wt.% as th e pH of preparation decreases. The surface coverage of the calcined ca talysts was monitored by CO2 chemisorption and ion scattering spectros copy (ISS) measurements. Carbon dioxide uptake was determined in a sta ndard BET system from the adsorption isotherms by extrapolating to zer o pressure. Coverage measurements from ISS were obtained by monitoring the changes of the ISS Ti signal with Mo addition. Additional informa tion about the dispersion of the Mo phase of these catalysts was obtai ned from x-ray photoelectron spectroscopy (XPS) measurements. The XPS results evidenced the presence of a highly dispersed Mo phase. Carbon dioxide chemisorption results for Mo/TiO2 catalysts indicated that the Mo coverage increases linearly from 0.16 to 0.93 with increasing Mo l oading from 0.6 to 3.6 wt.%. These results were consistent with those determined by ISS measurements and are in good agreement with the pred icted values for monolayer dispersion. These findings are in contrast with those obtained for alumina-supported catalysts, where the CO2 che misorption method has been shown to overestimate the surface coverage. The origin of the apparent suitability of the CO2 chemisorption metho d for monitoring the surface coverage of TiO2-based systems is discuss ed. (C) 1998 John Wiley & Sons, Ltd.