Abstract
The preparation of molybdena-alumina catalysts was studied using an equilibrium adsorption method at 50 °C in which the molybdenum anions from solution were adsorbed on the alumina surface at pH values between 5.70 and 7.70 near to its isoelectric point. In these experimental conditions a two-step adsorption isotherm was found, indicating that the loading of the carrier was very dependent on the solute concentration. By combining different characterization techniques, namely, temperature-programmed reduction and kinetics of reduction, reflectance spectroscopy, chemisorption of NO and O 2 probe-molecules, infrared spectroscopy of NO, and extraction by ammonia, it is shown that at low solute concentrations the Mo is highly dispersed, mostly as tetrahedrally coordinated (by oxygen ions) Mo species. However, poorer dispersion was found at intermediate and high solute concentrations. In this latter case, the combined use of chemisorption and extraction data indicated that on calcining at 500 °C the surface of the carrier becomes populated with both tetrahedrally (monolayer) and octahedrally (small clusters) coordinated Mo-species.
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