The effect of reduction conditions on the chloride content of IrTiO2 catalysts and their activity for benzene hydrogenation

Abstract Reduction of K 3 IrCl 6 , on TiO 2 occurs in two well-defined stages, the first of which occurs between 350 and 470 K and consumes an amount of H 2 independent of Ir content. No HCl is released during this stage. It is suggested that IrCl 6 3− ions interact with TiOH groups to form a monolayer of IrCl x (OTi) 6− x species which on reduction afford highly dispersed Ir particles (1–2.5 nm), and that HCl formed by elimination and during reduction chlorinates other TiOH groups. K 3 IrCl 6 in excess of that required to form a monolayer exists as less reducible microcrystals leading to larger Ir particles on reduction. Complete removal of Cl − under H 2 cannot be achieved. The effect of precalcination, and of oxidation after reduction, on the Cl − content has been examined. Residual Cl − affects the O 2 titration of chemisorbed H, giving anomalously low results for the number of surface Ir atoms. Resulting turnover frequencies (TOF) are therefore unreliable, but Ir TiO 2 reduced at low temperature shows specific activity for benzene hydrogenation comparable to that for Ir SiO 2 . Low TOF's for Ir TiO 2 after high-temperature reduction are attributed to crowding of free sites by TiO 2− x , species. The role of Cl − in TiO 2 -supported catalysts needs a complete reappraisal.