Supported Ionic-Liquid “Semi-Heterogeneous Catalyst”: An Interfacial Chemical Study

The Langmuir–Blodgett (LB) technique has been employed for the construction of a model of interfacial supported ionic liquids (ILs) catalyst. In this model a hybrid LB film consists of three components: ionic liquids (imidazolium chloride with different length of substituent alkyl chain), catalyst carrier (saponite and laponite), and catalyst species (platinum nanocrystals). It was found imidazolium ILs with carbon number more than six in the alkyl substituents can form stable Langmuir monolayer over diluted clay dispersions. By recording surface pressure versus time (π– t ) kinetic curves, we found that the adsorption of imidazolium ILs or platinum nanocrystals by clay elementary layers reaches equilibrium within 200 s, and the molar Gibbs free energy change −d G (kJ mol –1 ) for the adsorption lies in the range 0.34–5.6 kJ mol –1 . Surface pressure versus area (π– A ) isotherms revealed that platinum nanocrystals are adsorbed by the IL–clay Langmuir films at the air–water interface, and there are two phase transition points in the π– A isotherms during the compression of IL–clay–Pt. We utilized two methods, i.e., interfacial chemical methods (π– t, π– A ) and spectroscopic measurements (UV–vis absorption, atomic absorption spectroscopy (AAS)) to quantify the amount of imidazolium IL and Pt that are presented in the LB films. Results showed that the amount of adsorbed 1-hexadecyl-3-methylimidazolium chloride (C 16 MIM) in the hybrid film C 16 MIM–saponite–Pt at the first deposition layer is around 0.65 ng mm –2 . Laponite gives a higher adsorption capacity for loading IL and Pt nanocrystals. Attenuated total reflectance (ATR)–Fourier transform infrared spectra (FTIR) confirmed the presence of IL in the LB film which shares ordered vibrational conformation, and the degree order of C 16 MIM molecules becomes worse after incorporation of Pt nanocrystals. Transmission electron microscopy (TEM) verified that Pt nanoparticles with size of ∼5.0 nm tend to be adsorbed on the edges of clay layers, and the surface coverage of Pt particles can be tuned by the compression pressure. The Pt-containing LB films were deposited on a glass carbon electrode (GCE), and the electrocatalytic performance toward methanol electro-oxidation and synergistic effects between the three components have been studied by means of cyclic voltammetry (CV) and chronoamperometry (CA). The results indicated that the film IL–clay–Pt exhibits a remarkable enhancement in catalytic activity and stability compared to clay–Pt in terms of the onset potential for methanol oxidation, the specific mass current density based on the amount of Pt catalyst, and the slower decay of steady-state current in the CA profile.