Hydrogen activated WOx-supported catalysts for Lewis acid transformation to Bronsted acid observed by in situ DRIFTS of adsorbed ammonia: Effect of different supports on the Lewis acid transformation

摘要

The effect of hydrogen introduction on Lewis acid transformation to Bronsted acid was investigated over the WOx catalysts supported on different supports (HY-15, HY-500, MCM-22, SiO2, and Al2O3). The Lewis acid transformation on the catalysts was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed NH3 at 100 degrees C under atmospheric pressure for 130 min. According to the results of in situ DRIFTS, ammonia migration from Lewis acid sites to Bronsted acid sites occurred over the WOx catalysts supported on all different supports. It is proposed that the partial reduction of WOx occurred under the hydrogen atmosphere, resulting in the generation of oxygen vacancies sites. The generated oxygen vacancies of the supported WOx catalysts were found to play an important role for activating the hydrogen spillover, leading to Lewis acid transformation to Bronsted acid. It was suggested that the selected support is the crucial factor for Lewis acid transformation performances. The results from our experiments showed that the weak interaction between WOx and support led to the easy reduction of W species, which promoted the generation of oxygen vacancies sites. The large amount of oxygen vacancies leads to high Lewis acid transformation. Moreover, the influence of different supports was discussed and investigated by several analytical techniques such as H-2-TPD, H-2-TPR, O2TPD, UV-vis, XRD, XPS, and TEM-EDS techniques.