Low-Temperature Exsolution of Rh from Mixed ZnFeRh Oxides toward Stable and Selective Catalysts in Liquid-Phase Hydroformylation

The exsolution of metal nanoparticles offers a promising strategy to enhance catalyst stability and fine-tune metal-support interactions. Expanding the use of exsolved nanoparticles in heterogeneous catalysis requires the development of low-temperature (T < 400 °C) exsolution processes. In this study, we report the synthesis of phase-pure ZnFe_2-xRh_xO₄ metal oxide precursors with a spinel-type crystal structure. The isomorphic substitution of Fe³⁺ in the host lattice by Rh³⁺ was confirmed by X-ray diffraction and Raman spectroscopy combined with DFT calculations. The hydrothermal synthesis method of the oxide precursors was specifically chosen so that very small oxide particles of 10-20 nm were obtained, which enabled the exsolution of Rh nanoparticles with a particle size of about 1 to 2 nm at temperatures below 200 °C in a hydrogen-containing atmosphere. Compared to a Rh catalyst prepared by conventional wet impregnation of ZnFe₂O₄, the catalysts obtained by low-temperature exsolution show superior properties in terms of selectivity toward aldehydes in the hydroformylation of 1-hexene in the liquid phase. In addition, there is no Rh loss due to leaching, which is the main challenge for heterogeneous Rh catalysts used in liquid phase reactions. The exceptionally strong metal-support interaction imparts unique nanostructures and electronic properties to the exsolved metal nanoparticles, as revealed by electron energy loss spectroscopy (EELS) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The specific adsorption sites on the exsolved Rh particles lead to stronger metal-hydride and weaker metal-carbonyl bonds on the surface, steering the reaction pathway toward hydroformylation rather than olefin isomerization.