Hydrogen Reduction of Adams' Catalyst in Ionic Liquids: Formation and Stabilization of Pt(0) Nanoparticles

The kinetic data on cyclohexene hydrogenation with PtO2 (Adams' catalyst) dispersed in 1-n-butyl-3methylimidazolium tetrafluoroborate (BMI.BF4) and hexafluorophosphate (BMI.PF6) ionic liquids at room temperature and 75 degrees C, with substrate/catalyst ratios of 1/1000 and 1/4000, suggest the formation of large agglomerates of bulk metal catalysts. The results of this study show that the kinetic curves, which show induction periods in some cases, only have a good fit with the double autocatalytic mechanism, indicating the formation of bulk metal and its probable participation in the catalytic activity of the system, in particular at high precursor concentrations. However, it was not possible to observe these large agglomerates in the ionic liquids with the naked eye. In fact, in situ TEM analysis of several samples, including those obtained at the end of the induction period and at the end of the reaction, showed that only platinum nanoparticles with a mean diameter of around 3.0 nm were formed. Moreover, similar nanoparticle sizes were indicated by XRD analysis of isolated metal nanoparticles, suggesting that no bulk metal was formed. These results show that although the kinetic curves are consistent with a double autocatalytic mechanism, they do not seem to represent truly formation of large agglomerates of bulk metal. The direct application of such a mechanism is not appropriate for the formation of Pt(0) nanoparticles via salt metal reduction in imidazolium ionic liquids. Therefore, the nanoclusters of zerovalent platinum in imidazolium ionic liquids resulting from the reduction of the Adams' catalyst are mainly responsible for the high catalytic activity in the hydrogenation of olefins.