Interfacial Bonding Stabilizes Rhodium and Rhodium Oxide Nanoparticles on Layered Nb Oxide and Ta Oxide Supports

Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle–support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa₂Nb₃O₁₀, K₄Nb₆O₁₇, and RbTaO₃ were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na_0.66Mg_2.68(Si_3.98Al_0.02)­O_10.02F_1.96), and α-Zr­(HPO₄)₂·H₂O. High surface area SiO₂ and γ-Al₂O₃ supports were also used for comparison in the ITC experiments. A Born–Haber cycle analysis of ITC data revealed an exothermic interaction between Rh­(OH)₃ nanoparticles and the layered niobate and tantalate supports, with Δ<i>H</i> values in the range −32 kJ·mol^–1 Rh to −37 kJ·mol^–1 Rh. In contrast, the interaction enthalpy was positive with SiO₂ and γ-Al₂O₃ supports. The strong interfacial bonding in the former case led to “reverse” ripening of micrometer-size Rh­(OH)₃, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr­(HPO₄)₂·H₂O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh­(OH)₃ to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh­(OH)₃, Rh₂O₃, and Rh nanoparticles on niobate and tantalate supports