Effects of interfacial charge and the particle size of titanate nanotube-supported Pt nanoparticles on the hydrogenation of cinnamaldehyde

Abstract

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 Ptδ− 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 Ptδ− 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%.