Determination of the surface coverage of Mo/TiO2 catalysts by ISS and CO2 chemisorption

Abstract

A series of Mo/TiO2 catalysts was prepared by equilibrium adsorption from aqueous solutions of (NH4)6Mo7O24 by varying the pH of the solution from 9.8 to 4. The Mo loading increases from 0.8 to 3.6 wt.% as the pH of preparation decreases. The surface coverage of the calcined catalysts was monitored by CO2 chemisorption and ion scattering spectroscopy (ISS) measurements. Carbon dioxide uptake was determined in a standard BET system from the adsorption isotherms by extrapolating to zero pressure. Coverage measurements from ISS were obtained by monitoring the changes of the ISS Ti signal with Mo addition. Additional information about the dispersion of the Mo phase of these catalysts was obtained 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 loading from 0.6 to 3.6 wt.%. These results were consistent with those determined by ISS measurements and are in good agreement with the predicted values for monolayer dispersion. These findings are in contrast with those obtained for alumina-supported catalysts, where the CO2 chemisorption method has been shown to overestimate the surface coverage. The origin of the apparent suitability of the CO2 chemisorption method for monitoring the surface coverage of TiO2-based systems is discussed. © 1998 John Wiley & Sons, Ltd.