Chemistry of rhodium in zeolite Y

Abstract

Exchange of [Rh(NH 3) 5(H 2O)] 3+ ions from aqueous solution into NaY starts at the surface of the zeolite grains; ion penetration into subsurface cavities is slow. A marked rhodium concentration profile from the surface to the interior of the granuli vanishes only after an exchange time of three days, as evidenced by X-ray photoelectron spectroscopy (XPS). Heating the [Rh(NH 3) 5(H 2O)] 3+-loaded NaY in argon up to 500°C leads to 100% autoreduction of the rhodium and formation of rather large rhodium particles. Heating the same precursor in 1 bar of O 2 up to 380°C yields a mixture of the oxides RhO 2 and Rh 2O 3 and the ions Rh 3+ and Rh +. The oxides and Rh 3+ ions are located in the supercages, while Rh + is most likely in the small cages. After calcination to 500°C, Rh 2O 3 is the only oxide present; some of the Rh 3+ ions have migrated into sodalite cages and hexagonal prisms. Reaction of Rh 2O 3 with zeolite protons produces more Rh 3+ ions; a maximum concentration is achieved with HY when heated to 500°C. Reduction of the calcined samples in flowing H 2 produces small rhodium particles located primarily inside the zeolite supercages. The extent of this reduction depends on the proton concentration and the temperature because the equilibrium between Rh 0, protons, and Rh + prevents 100% formation of Rh 0 when the H + concentration is appreciable; in HY a Rh 0 Rh + ratio ≈1 is found. As a consequence of strong proton anchoring, the rhodium particle size in HY remains <1 nm after H 2 reduction. Formation of rhodium-proton adducts lowers the propensity of rhodium to adsorb H 2 at low temperature.