Intercalation compounds of α-zirconium hydrogen phosphate with Rh 3+ ions and Rh 3+-diamine complexes. Part II. Their behaviour towards CO, CO 2 and H 2 and their use in the CO catalytic oxidation

Abstract

The reactivity of Rh 3+ ions and Rh 3+-diamine α-Zr(HPO 4) 2·H 2O complexes intercalated in α-zirconium hydrogen phosphate towards small molecules (CO, O 2, H 2) was studied. The compounds only containing Rh 3+ ions, of composition ZrH x Rh y (PO 4) 2·4H 2O ( x = 2 – 3 y; 0 < y ≤ 0.66) react with CO at atmospheric pressure and temperatures ranging from 80 to 100°C, and undergo selective reduction of Rh 3+ to Rh 1+. The resulting materials containing Rh 1+ are reoxidized to Rh 3+ by molecular dioxygen under the same pressure and temperature conditions. The simultaneous action of a CO O 2 mixture determines the catalytic oxidation of the CO to CO 2 and the system acts as a stable catalyst of this reaction. At higher temperatures, the reduction of Rh 3+ is no longer selective and in these conditions Rh 0 is formed, which escapes from the support and causes its deactivation. Similar behaviour is found in systems containing Rh 3+-diamine complexes, which react with CO at temperatures higher than 120°C and undergo an irreversible reduction of Rh 3+ to Rh 0. The reaction with H 2 (70 < T < 100°C) also causes a non selective reduction of the Rh 3+ to Rh 1+ and Rh 0. The progress over time of the catalytic activity of some compounds with different contents of Rh 3+ in converting CO to CO 2 has shown not only that these materials maintain a constant catalytic activity, indicating the stability of the systems to the loss of metal during working cycles, but also that Rh 3+ supported in these matrixes is more active and selective in this type of reaction than Rh 3+ in solution.