| Summary: | 碩士 === 國立彰化師範大學 === 化學系 === 100 === Small Pt metal particles(3-5 nm) were deposited on the external surfaces of alkali metal titanante nanotube bunbles (Pt/M2Ti3O7, M=Na+, K+, Rb+, Cs+) by photoreduction method using Pt(NH3)4(NO3)2 in 50% v/v aqueous methanol. The peak area of the Pt signals in the XPS spectra for Pt particles in Pt/MTNTs prepared by this method are 3-6 times higher than those prepared by ion exchange method followed by H2 reduction. Furthermore, 4f7/2 signals in all Pt/MTNTs were at around 70.2 eV, whose value was lower than that of a metallic Pt0 (at 71.0 eV), suggesting that these Pt metal particles were in negative oxidation state, Ptδ-. Adsorption of pyridine coupling with DRIFTS showed that MTNTs contained only Lewis acid sites, but upon being promoted with small amount of Pt metal (0.3 wt %), MTNTs generated some Brönsted acid sites with a corresponding decreased number of Lewis acid sites. Futher increase in Pt loading, however, did not significantly affect the number and the strength of these Brönsted acid sites. It is assumed that due to the loss of its electron density to the Pt particles, some hydroxyl groups in MTNTs and near the Pt-MTNTs interface became acidic and was observed in DRIFTS. Thermal desorption of adsorbed pyridine with DRIFTS indicated that the strength of these Brönsted acid sites followed the order Pt/CsTNTs > Pt/RbTNTs > Pt/KTNTs > Pt/NaTNTs, and Pt/RbTNTs contained the highest number of acid sites among the four Pt/MTNTs. The acidity of these Pt/MTNTs catalyst was tested against an acid-catalyzed reaction, formation of cyclic acetal from cyclic hexanone and ethylene glycol. The results seems consistent with the spectroscopic characterization : MTNTs showed no catalytic activity at all, but Pt-promoted MTNTs were active for the reaction; the activity followed the order of Pt/RbTNTs > Pt/KTNTs > Pt/CsTNTs > Pt/NaTNTs. More acidic catalysts, 3% Pt/RbTNTs and 3% Pt/KTNTs, could reach the equilibrium conversion of around 80% at 353 K. The lower activity observed for the most acidic 3% Pt/CsTNTs was due to the molecular diffusion imposed by the bulky cyclic acetal product in a more stereo-restricted pore channel in CsTNTs.
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