Chemistry of rhodium in zeolite Y

Exchange of [Rh(NH3)(5)(H2O)](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(NH3)(5)(H2O)](3+)-loaded NaY in argon up to 500 degrees 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 degrees C yields a mixture of the oxides RhO2 and Rh2O3 and the ions Rh3+ and Rh+. The oxides and Rh3+ ions are located in the supercages, while Rh+ is most likely in the small cages. After calcination to 500 degrees C, Rh2O3 is the only oxide present; some of the Rh3+ ions have migrated into sodalite cages and hexagonal prisms. Reaction of Rh2O3 with zeolite protons produces more Rh3+ ions; a maximum concentration is achieved with HY when heated to 500 degrees 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 approximate to 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.