Broad reactivity trends for oxygen-isotope exchange from the near-surface regions of some metal (hydr)oxide solids

Abstract

The flux of 18O from suspensions of isotopically enriched Cr(III) and Rh(III) hydroxide solids at varying temperature and pH was measured in a series of experiments. Most of these solids are metal hydroxide nanospheres that have a large surface area and a narrow distribution in particle sizes and contain inert metals (Cr(III) and Rh(III)). Using rate data for dissolved multimeric complexes as a guide, the solids were enriched in 18O under conditions that were intended to affect mostly bound water molecules ( η-OH 2) at the surface, but this point could not be verified. Nevertheless, the fluxes of 18O back into solution from the isotopically enriched surfaces indicate that increased pH, which partly deprotonates the surface, is surprisingly unimportant to the rate and does not measurably affect 18O fluxes. Although these data are sparse, Rh(III) solids react at rates that are lower than for Cr(III) solids, and the rates of exchange for crystalline and amorphous solids are relatively close. The results indicate that rates of ligand exchange at these surface sites are controlled dominantly by the local metal–oxygen bond strengths and that long-range forces are relatively unimportant. These experiments also indicate a strategy for measuring rates of ligand exchange from solid surfaces.