Titania rhodium activities

Although the decomposition of ozone to dioxygen is a thermodynamically favoured process,126 it is thermally stable up to 523 K and catalysts are needed to decompose it at ambient temperature in ventilation systems, in the presence of water vapour and at high space velocity. A limited number of catalysts have been evaluated and active components are mainly metals such as platinum, palladium and rhodium, and metal oxides including those of manganese, cobalt, copper, iron, nickel and silver. Supports that have been used include 7-alumina, silica, zirconia, titania and activated carbon.125,170... 

The purpose of the present work is to study the precursor (metallic chlorides or carbonyl compounds), particle size and support (silica, magnesia and titania) effects in the selective hydrogenation of carvone employing rhodium as active metal. 

Rh dispersion appears to be a key factor in the activity of zeolite-supported systems. In studies of the effect of Si/Al ratio in Rh/NaX (faujasite-type zeolite), prepared by cation exchange of NaX with [Rh(NH3)5Cl](OH)2, maximum carbonylation activity (ca. 8 mol/(gRh-h) at 200°C) correlates with maximum Rh dispersion in the prepared catalyst (48). Similarly, cation exchange of NaX with [Rh(NH3)5Cl]Cl2 has been reported to give carbonylation rates of ca. 1 mol/(gRh h) at 150°C (49), and NaY exchanged with rhodium salts (50) gives carbonylation activity of 0.4 mol/(gRh-h) at 170°C. These rates are claimed to be higher than those for Rh impregnated alumina, silica-alumina, silica, or titania (49,50). Optimum Rh dispersion corresponded to approximately two Rh atoms per vmit cell (50). 

The problem of reactive versus spectator species in the SCR-HC mechanism was recently investigated for noble metals (Pt or Rh) supported on Ti02 [60], Catalysts based on platinum and rhodium supported on titania show comparable activity, but quite a different nature of chemisorbed NO c species after interaction with a flow of NO + O2 in helium. With Pt-Ti02, the IR spectrum is characterized by the intense bands due to nitrate, while with Rh-Ti02 the spectmm is... 

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