Comparative Study of Lewis Acid Transformation on Non-reducible and Reducible Oxides Under Hydrogen Atmosphere by In Situ DRIFTS of Adsorbed NH 3

摘要

The Lewis acid transformation to Bronsted acid was investigated over the Pt/γ-Al~(2)O~(3)hybrid catalysts in the presence of hydrogen atmosphere by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed NH~(3). The changes of FTIR spectra were monitored during the introduction of hydrogen at 40?°C and atmospheric pressure for 130?min. The degrees of Lewis acid transformation were varied by addition of non-reducible (SiO~(2)and Al~(2)O~(3)) and reducible (ZrO~(2), TiO~(2)and CeO~(2)) oxides to the Pt/γ-Al~(2)O~(3)catalysts as the hybrid catalysts. According to the in situ DRIFTS, the hydrogen temperature programmed reduction (H~(2)-TPR), and the hydrogen temperature programmed desorption (H~(2)-TPD) results, the introduction of hydrogen resulted in a decrease in the amount of ammonia adsorbed on Lewis acid sites, and an increase in the amount of ammonium ions on Bronsted acid sites with time on stream. It is proposed that ammonia migration from Lewis acid sites to Bronsted acid sites occurred during the introduction of hydrogen in the presence of Pt particles when compared to the observation of only observed catalysts (without Pt particles). The addition of reducible oxides led to the high rate of Lewis acid transformation, which was higher than those of the non-reducible oxides. Weaker Lewis acid sites and higher amount of hydrogen spillover over the observed catalysts enhanced the rate of Lewis acid transformation in this study. However, the amount of Lewis acid sites at the initial stage did not play an important role in these transformations.