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 hexacarbonyl precursors. The catalysts were subjected to heat treatments, in He flow, between 623 and 1223 K for 4 h and characterized to determine theirsurface area and pore texture, the chemical state and dispersion of the metallic phase, and the total surface acidity. For this purpose, different techniques were used such as adsorption of N2 at 77 K and mercury porosimetry; X-ray diffraction and X-ray photoelectron spectroscopy; temperature-programmed desorption of previously adsorbed NH3; and behavior in the decomposition reactions of isopropanol. Results show that in the case of the catalysts prepared from ammonium tungstate an increase in the treatment temperature between 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 W18O49. At 1223 K, a mixture of W, tungsten carbides, and tungsten trioxide was detected. The catalysts prepared from tungsten hexacarbonyl presented a higher dispersion and no X-ray diffraction peaks. Total surface acidity, as measured by ammonia desorption, decreased with increasing treatment temperature, and the study of the decomposition reactions of isopropanol showed that the catalysts were essentially of acid character, with a much higher selectivity (or rate) to propene than to acetone. Moreover, there was a linear relationship between the reaction rate to propene and the amount of NH3 desorbed from the catalysts, irrespective the method of preparation and treatment temperature.