"Structure of CrV Species on the Surface of Various Oxides: Reactivity with NH3 and H2O, as investigated by EPR Spectroscopy"

Samples containing chromium (both Cr-53-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO2, gamma-Al2O3, SnO2, TiO2 (anatase) and SiO2 as supports. Heating in O2, generally at 773 K, yielded mononuclear Cr(v) species in a square-pyramidal configuration, Cr-5c(v) (A), on all supports with the exception of SiO2 where Cr(v) is in a tetrahedral configuration, Cr-4c(v) (A). H2O or NH3, both at room temperature (RT), yielded the Cr-6v(v) (A) species from Cr-5c(v) (A), that is, the complex changes its coordination from five to six. After H2O adsorption and evacuation at RT, Cr-5c(v) (A) is reversibly restored; however, NH3 adsorption and evacuation at increasing temperature gives a new Cr-v species at 473 K. In the EPR signal of this species, designated Cr-5c(v) (B), the perpendicular component is split into three lines with (NH3)-N-14 (A(14N) = 4.0 G) and two lines with (NH3)-N-15 (A(15N) = 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial 0(2-) ligand replaced by a nitrogen-containing NHxx-3 species, possibly the NH2- anion. With H2O or NH3 at RT, the Cr-5c(v) (B) species is transformed into the corresponding hexacoordinated species, Cr-6c(v) (B).