In situ Investigations of the Formation Mechanism of Metastable γ-BiPd Nanoparticles in Polyol Reductions.

Synthesizing intermetallic phases containing noble metals often poses a challenge as the melting points of noble metals often exceed the boiling point of bismuth (1560 °C). Reactions in the solid state generally circumvent this issue but are extremely time consuming. A convenient method to overcome these obstacles is the co-reduction of metal salts in polyols, which can be performed within hours at moderate temperatures and even allows access to metastable phases. However, little attention has been paid to the formation mechanisms of intermetallic particles in polyol reductions. Identifying crucial reaction parameters and finding patterns are key factors to enable targeted syntheses and product design. Here, we chose metastable γ-BiPd as an example to investigate the formation mechanism from mixtures of metal salts in ethylene glycol and to determine critical factors for phase formation. The reaction was also monitored by in situ X-ray diffraction using synchrotron radiation. Products, intermediates and solutions were characterized by (in situ) X-ray diffraction, electron microscopy, and UV-Vis spectroscopy. In the first step of the reaction, elemental palladium precipitates. Increasing temperature induces the reduction of bismuth cations and the subsequent rapid incorporation of bismuth into the palladium cores, yielding the γ-BiPd phase.

In situ investigation of the formation mechanism of α-Bi2Rh nanoparticles in polyol reductions.

The synthesis of intermetallic phases formed from elements with very different melting points is often time and energy consuming, and in extreme cases the evaporation of a reactant may even prevent formation completely. An alternative, facile synthesis approach is the reduction of metal salts in the polyol process, which requires only moderate temperatures and short reaction times. In addition, the starting materials for this procedure are readily available and do not require any special treatment to remove or prevent passivation layers, for example. Although the formation of intermetallic particles via the polyol process is an established method, little attention has been paid to the mechanism behind it. However, it is precisely a deeper understanding of the underlying mechanisms that would enable better and more targeted synthesis planning and product design. Taking the well-known formation of Bi2Rh particles from Bi(NO3)3 and various rhodium salts in ethylene glycol as an example, we studied the chemical process in detail. We investigated the effects of anion type and pH on the polyol reaction. The reaction was also probed by in situ X-ray diffraction using synchrotron radiation. Products, intermediates and solutions were characterized by X-ray and electron diffraction, electron microscopy and optical spectroscopy. In the first step, co-reduction of the metal cations leads to BiRh. Only with increasing reaction temperature, the remaining bismuth cations in the solution are reduced and incorporated into the BiRh particles, leading to a gradual transition from BiRh to α-Bi2Rh.

Formation of Bi2Ir nanoparticles in a microwave-assisted polyol process revealing the suboxide Bi4Ir2O.

Intermetallic phases are usually obtained by crystallization from the melt. However, phases containing elements with widely different melting and boiling points, as well as nanoparticles, which provide a high specific surface area, are hardly accessible via such a high-temperature process. The polyol process is one option to circumvent these obstacles by using a solution-based approach at moderate temperatures. In this study, the formation of Bi2Ir nanoparticles in a microwave-assisted polyol process was investigated. Solutions were analyzed using UV-Vis spectroscopy and the reaction was tracked with synchrotron-based in situ powder X-ray diffraction (PXRD). The products were characterized by PXRD and high-resolution transmission electron microscopy. Starting from Bi(NO3)3 and Ir(OAc)3, the new suboxide Bi4Ir2O forms as an intermediate phase at about 160 °C. Its structure was determined by a combination of PXRD and quantum-chemical calculations. Bi4Ir2O decomposes in vacuum at about 250 °C and is reduced to Bi2Ir by hydrogen at 150 °C. At about 240 °C, the polyol process leads to the immediate reduction of the two metal-containing precursors and crystallization of Bi2Ir nanoparticles.

Metal-Dependent and Selective Crystallization of CAU-10 and MIL-53 Frameworks through Linker Nitration.

The reaction of the V-shaped linker molecule 5-hydroxyisophthalic acid (H2 L0 ), with Al or Ga nitrate under almost identical reaction conditions leads to the nitration of the linker and subsequent formation of metal-organic frameworks (MOFs) with CAU-10 or MIL-53 type structure of composition [Al(OH)(L)], denoted as Al-CAU-10-L0, 2, 4, 6 or [Ga(OH)(L)], denoted as Ga-MIL-53-L2 . The Al-MOF contains the original linker L0 as well as three different nitration products (L2 , L4 and L4/6 ), whereas the Ga-MOF mainly incorporates the linker L2 . The compositions were deduced by 1 H NMR spectroscopy and confirmed by Rietveld refinement. In situ and ex situ studies were carried out to follow the nitration and crystallization, as well as the composition of the MOFs. The crystal structures were refined against powder X-ray diffraction (PXRD) data. As anticipated, the use of the V-shaped linker results in the formation of the CAU-10 type structure in the Al-MOF. Unexpectedly, the Ga-MOF crystallizes in a MIL-53 type structure, which is usually observed with linear or slightly bent linker molecules. To study the structure directing effect of the in situ nitrated linker, pure 2-nitrobenzene-1,3-dicarboxylic acid (m-H2 BDC-NO2 ) was employed which exclusively led to the formation of [Ga(OH)(C8 H3 NO6 )] (Ga-MIL-53-m-BDC-NO2 ), which is isoreticular to Ga-MIL-53-L2 . Density Functional Theory (DFT) calculations confirmed the higher stability of Ga-MIL-53-L2 compared to Ga-CAU-10-L2 and grand canonical Monte Carlo simulations (GCMC) are in agreement with the observed water adsorption isotherms of Ga-MIL-53-L2 .