A remarkable anion effect on palladium nanoparticle formation and stabilization in hydroxyl-functionalized ionic liquids

Abstract

Pd nanoparticles (NPs) with a small size and narrow size distribution were prepared from the decomposition of Pd(OAc)₂ in a series of hydroxyl-functionalized ionic liquids (ILs) comprising the 1-(2′-hydroxylethyl)-3-methylimidazolium cation and various anions, viz. [C₂OHmim][OTf] (2.4 ± 0.5 nm), [C₂OHmim][TFA] (2.3 ± 0.4 nm), [C₂OHmim][BF₄] (3.3 ± 0.6 nm), [C₂OHmim][PF₆] (3.1 ± 0.7 nm) and [C₂OHmim][Tf₂N] (4.0 ± 0.6 nm). Compared with Pd NPs isolated from the non-functionalized IL, [C₄mim][Tf₂N] (6.2 ± 1.1 nm), it would appear that the hydroxyl group accelerates the formation of the NPs, and also helps to protect the NPs from oxidation once formed. Based on the amount of Pd(OAc)₂ that remains after NP synthesis (under the given conditions) the ease of formation of the Pd NPs in the [C₂OHmim]⁺-based ILs follows the trend [Tf₂N]⁻, [PF₆]⁻ > [BF₄]⁻ > [OTf]⁻ > [TFA]⁻. Also, the ability of the [C₂OHmim]⁺-based ILs to prevent the Pd NPs from undergoing oxidation follows the trend [Tf₂N]⁻ > [PF₆]⁻ > [TFA]⁻ > [OTf]⁻ > [BF₄]⁻. DFT calculations were employed to rationalize the interactions between Pd NPs and the [C₂OHmim]⁺ cation and the various anions.