Abstract
The transformation of Lewis acid sites to new Bronsted acid sites under hydrogen at 40 °C and atmospheric pressure on WO3/SiO2 and WO3/Al2O3 catalysts with various tungsten oxide loading (1, 5, 9, and 12 wt%) was investigated by the in situ DRIFTS of adsorbed NH3. The degree of Lewis acid transformation on WO3/SiO2 was higher than on WO3/Al2O3 because the weaker interaction between WO3 and SiO2 showed higher hydrogen accessibility as revealed by the H2-TPR and H2-TPD results. Moreover, Pt co-catalyst with supported tungsten oxide catalyst could enhance hydrogen uptake on WO3 catalyst, leading to higher Lewis acid transformation. However, the investigation of surface WO3 structures were observed by XRD, FT-Raman, UV–vis, and TEM to be the key point for allowing the Lewis acid transformation. After Lewis acid transformation, isolated tetrahedral WO3 were stable whereas octahedral polytungstate and crystalline WO3 decreased, converting to W4+ and W5+. The decreasing of crystalline WO3 after Lewis acid transformation was also found to be correlated well with the hydrogen uptake. Therefore, Lewis acid transformation on supported WO3 preferentially occurred on crystalline WO3 than on octahedral polytungstate WO3 and isolated tetrahedral WO3, respectively.
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