Synthesis of BiRh Nanoplates with Superior Catalytic Performance in the Semihydrogenation of Acetylene

Highly uniform and well-crystallized nanoparticles of the intermetallic compound BiRh were obtained by low-temperature synthesis at 240 °C using the microwave-assisted polyol process. In this time- and energy-efficient reaction the polyol acts as solvent, reducing agent, and surfactant, while the microwave radiation leads to fast and homogeneous nucleation and crystal growth. Electron microscopy studies confirmed the presence of pseudohexagonal nanoplates with a primary particle diameter of 60 nm and high crystallinity. As indicated by high-resolution transmission electron microscopy, the plate normal is generally not parallel to [001] but coincides with [421]. Powder X-ray diffraction and energy dispersive X-ray spectroscopy revealed the single-phase nature and the equimolar composition. The specific surface area (0.54 m² g^–1) and the particle size distribution were measured by fractional sedimentation. According to the analysis of the chemical bonding by means of quantum chemical calculations, 0.62 electrons are transferred from Bi to Rh. Covalent homoatomic Rh–Rh as well as heteroatomic three-center Rh–Bi–Rh bonds define a three-dimensional bonding network. Unsupported BiRh nanoparticles exhibit an extraordinary high selectivity of 88 to 93% in the semihydrogenation of acetylene, which makes them an interesting model compound as well as a promising candidate for the application as an industrial catalyst

Synthesis of a Cu-Filled Rh₁₇S₁₅ Framework: Microwave Polyol Process Versus High-Temperature Route

Metal-rich, mixed copper–rhodium sulfide Cu_3−δRh₃₄S₃₀ that represents a new Cu-filled variant of the Rh₁₇S₁₅ structure has been synthesized and structurally characterized. Copper content in the [CuRh₈] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu₃Rh₃₄S₃₀ obtained by a rapid, microwave-assisted reaction of CuCl, Rh₂(CH₃CO₂)₄ and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu_3−δRh₃₄S₃₀ (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu_3−δRh₃₄S₃₀ is metallic just like Rh₁₇S₁₅, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh₁₇S₁₅ host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh–Rh interactions in favor of Cu–Rh interactions as revealed by quantum-chemical calculations