Tungsten catalysts supported on activated carbon - I. Preparation and characterization after their heat treatments in inert atmosphere

Tungsten catalysts supported on activated carbon, and with different metal content, were prepared from both ammonium tungstate and tungsten hexacarbon yl precursors. The catalysts were subjected to heat treatments, in He flow, between 623 and 1223 K for 4 h and characterized to determine their surfac e area and pore texture, the chemical state and dispersion of the metallic phase, and the total surface acidity. For this purpose, different technique s were used such as adsorption of N-2 at 77 K and mercury porosimetry; X-ra y diffraction and X-ray photoelectron spectroscopy; temperature-programmed desorption of previously adsorbed NH3; and behavior in the decomposition re actions of isopropanol. Results show that in the case of the catalysts prep ared from ammonium tungstate an increase in the treatment temperature betwe en 623 and 1223 K leads to a drop in the O/W ratio to below 3, resulting in the appearance of nonstoichiometric oxides such as W25O73, W20O58, and W18 O49 At 1223 K, a mixture of W, tungsten carbides, and tungsten trioxide was detected. The catalysts prepared from tungsten hexacarbonyl presented a hi gher dispersion and no X-ray diffraction peaks. Total surface acidity, as m easured by ammonia desorption, decreased with increasing treatment temperat ure, and the study of the decomposition reactions of isopropanol showed tha t the catalysts were essentially of acid character, with a much higher sele ctivity (or rate) to propene than to acetone. Moreover, there was a linear relationship between the reaction rate to propene and the amount of NH3 des orbed from the catalysts, irrespective the method of preparation and treatm ent temperature. (C) 2000 Academic Press.