Pt nanoparticles of diameter 0.5-1.5 nm were dispersed homogeneously on the inner and outer surfaces of alkali metal titanate nanotubes (M 2 Ti 3 O 7 , M ) Li + , Na + , and K + ) by ion exchange with Pt(NH 3 ) 2 Cl 2 followed by calcination in ambient air at 673 K and reduction in H 2 at 448 K. Both XPS and DRIFTS revealed that these titanate nanotubes donated their electrons to the Pt particles, yielding a negative Pt oxidation state, and that the degree of electron donation increased with the size of the alkali metal cation. The high electron density of the Pt particles promoted the activation of both CdC and CdO functional groups in cinnamaldehyde. Therefore, the order of the catalytic activity in the hydrogenation of cinnamaldehyde was Pt/KTNTs > Pt/ NaTNTs > Pt/LiTNTs. Since the Pt particles were very small, Pt/MTNTs selectively hydrogenated the CdC bond, yielding a hydrocinnamaldehyde with selectivities of 80-87% (at cinnamaldehyde conversions of 27-50%). The alkali metal ions of the titanate nanotubes were able to modify the mode of adsorption of cinnamaldehyde. Therefore, when a KOH promoter was added to the reaction mixture, cinnamyl alcohol became the main product and the cinnamyl alcohol selectivities were 77-83% (at cinnamaldehyde conversions of 49-74%).

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The oxidation state and size of Pt nanoparticles attached to alkali metal titanate nanotubes (MTNTs=M2Ti3O7, M = Li(+), Na(+), K(+), Cs(+)) via ion exchange (indicated by the added label '-IE') and wet impregnation (indicated by the added label '-IMP') methods varied systematically with the cation of the MTNTs. X-ray photoelectron spectroscopy revealed that the binding energy of Pt was reduced to a low value when the support was changed from LiTNTs to CsTNTs, yielding a Pt(δ-) oxidation state. Thus, a space charge layer (SCL) was constructed at the interface between the Pt particle and MTNT support; the former carried the negative charge, and the alkali cation and proton in the hydroxyl group of the latter carried the positive charge. Due to a higher M/Ti atomic ratio in MTNTs, a higher electron density accumulated on Pt particles in Pt/MTNTs-IMP than on those in Pt/MTNTs-IE. Sub-ambient temperature temperature-programmed reduction and transmission electron microscopy revealed that because of the difference in reducibility of PtOx/MTNTs, the mean Pt particle size followed the order Pt/CsTNTs> Pt/KTNTs > Pt/NaTNTs > Pt/LiTNTs and Pt/MTNTs-IMP > Pt/MTNTs-IE. DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) showed that owing to its interaction with SCL, cinnamaldehyde adsorbed on Pt mainly through the C=C bond at the Pt-MTNT interfaces, and the small Pt particles in Pt/LiTNTs adsorbed three times more cinnamaldehyde than those in Pt/CsTNTs. Due to the competition between the adsorption of cinnamaldehyde and C=C activation, Pt/KTNT-IMP is the most active Pt/MTNT catalysts, achieving a conversion of 100% in the hydrogenation of cinnamaldehyde at 2 atm and 313 K. The carbonyl stretching of adsorbed cinnamaldehyde was almost unperturbed by adsorption (at 1705 cm(-1)), suggesting that Pt(δ-) and the π electrons in the carbonyl group repel each other, so the CH=O group points upward and away from the Pt surface, preventing it from being hydrogenated and causing Pt/MTNTs to exhibit high 3-phenyl propionaldehyde selectivities of 75-80%.

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