Solvation of Palladium Clusters in an Ionic Liquid: A QM/MM Molecular Dynamics Study

The mechanism of solvation and stabilization of palladium nanoparticles in the 1,3-dimethylimidazolium tetrafluoroborate ionic liquid (IL) has been studied using a combination of density functional theory and molecular dynamics (MD) simulations-with hybrid quantum mechanics/molecular mechanical (QM/MM) potentials. It is shown that the IL induces a strong polarization in Pd-6 and Pd-19 clusters, which were taken as computationally tractable models of palladium nanoparticles. The clusters have large induced dipole moments and, as a result, interact strongly with the IL. MD simulations demonstrate an accumulation of the IL layer of high density and a negative charge around the Pd-6 and Pd-19 clusters as a result of interactions with the anions of the IL. A single palladium atom does not show any noticeable preference for the positive or negative ions and interacts only very weakly with the IL, which can, to some extent, protect the palladium atom from the energetically favorable process of aggregation into Pd clusters only sterically.

Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study

Ning Yan

Analysis of infrared spectra of palladium nanoparticles (NPs) immersed in the tri-tert-butyl-R-phosphonium-based ionic liquids (ILs) demonstrates that both cations and anions of the ILs interact with the NPs. According to quantum-chemical simulations of these interactions, the binding energy of anions to the Pd-6 cluster, taken as a minimal-size model of the NPs, increases from similar to 6 to similar to 27 kcal mol(-1) in the order PF6 approximate to BF4 < Tf2N < OTf < Br < TFA. In contrast, the binding energy for all types of the (Bu3PR)-P-t cations slightly varies at about similar to 22 kcal mol(-1) only moderately depending on the choice of the R moiety (n-pentyl, 2-hydroxyethyl, 2-methoxyethyl, 2-ethoxy-2-oxoethyl). As a result, the energies of interaction between a Pd-6 cluster and various ion pairs, formed by the abovementioned counter-ions, follow the order found for the anions and vary from similar to 24 to similar to 47 kcal mol(-1). These values are smaller than the energy of addition of a Pd atom to a Pd-n cluster (similar to 58 kcal mol(-1)), which suggests kinetic stabilization of the NPs in phosphonium-based ILs rather than thermodynamic stabilization. The results are qualitatively similar to the trends found earlier for interactions between palladium clusters and components of imidazolium-based ILs, in spite of much larger contributions of the London dispersion forces to the binding of the (Bu3PR)-P-t cations to the cluster (up to 80%) relative to the case of 1-R-3-methylimidazolium cations (up to 40%).

Royal Soc Chemistry2014

Rationalization of Solvation and Stabilization of Palladium Nanoparticles in Imidazolium-Based Ionic Liquids by DFT and Vibrational Spectroscopy

Paul Joseph Dyson, Ning Yan, Xiao Yuan

A combined DFT/vibrational spectroscopy approach is used to determine the interactions of the 1,3-dimethylimidazolium ([Mmim]+) and 1-ethyl-3-methylimidazolium ([Emim]+) cations and [BF4]- anions with each other in the liquid state and with Pd nanoparticles (NPs) immersed in ionic liquids (ILs) composed of these ions. Formation of aggregates of the counter-ions in the liquid does not have a strong influence on the interaction energy of the IL components with the palladium clusters used to model NPs, which is smaller than the energy of addition of a Pd atom to the cluster. Stronger PdPd interactions in comparison to the interactions of the palladium cluster with the IL suggest kinetic stabilisation of Pd-NPs in 1,3-dialkylimidazolium tetrafluoroborates rather than thermodynamic stabilisation. Moreover, the palladium clusters interact more strongly with the anions than with the cations, and this suggests an important role of the anions in formation and stabilisation of Pd-NP in ILs. At the same time, binding between an isolated Pd atom and [Mmim]+ cation is stronger than Pd[BF4]- binding. IR and Raman spectral simulations reveal that surface-enhanced Raman spectroscopy is much less sensitive to interactions of Pd-NPs with the [BF4]- anion compared to interactions with [Mmim]+ cations. In contrast, IR spectroscopy is better suited to study the anionmetal interactions, whereas IR spectral manifestations of the cationmetal interactions are rather modest. The splitting of the strong ?as(BF4-) IR band into three components appears to be a convenient spectroscopic marker for Pd[BF4]- interactions, which is confirmed by actual spectra of Pd-NPs stabilised by [Emim][BF4]. The IR spectra and their assignment with quantum chemical computations suggest that both the anions and cations of [Emim][BF4] interact with the Pd-NP surface in the IL. The cation ring orientation close to the surface normal appears to be the dominant interaction. The anion is bound to the surface through either two or three fluorine atoms.

2012

Rationalization of Solvation and Stabilization of Palladium Nanoparticles in Imidazolium-Based Ionic Liquids by DFT and Vibrational Spectroscopy

Paul Joseph Dyson, Ning Yan, Xiao Yuan

2012