Structure of Crv species on the surface of various oxides : reactivity with NH3 and H2O, as investigated by EPR spectroscopy

Abstract

Samples containing chromium (both 53Cr-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO2, γ-Al2O3, SnO2, TiO2(anatase) and SiO2 as supports. Heating in O2, generally at 773 K, yielded mononuclear Crv species in a square-pyramidal configuration, Crv5c(A), on all supports with the exception of SiO2 where Crv is in a tetrahedral configuration, Crv4c(A). H2O or NH3, both at room temperature (RT), yielded the Crv6v(A) species from Crv5c(A), that is, the complex changes its coordination from five to six. After H2O adsorption and evacuation at RT, Crv5c(A) is reversibly restored; however, NH3 adsorption and evacuation at increasing temperature gives a new Crv species at 473 K. In the EPR signal of this species, designated Crv5c(B), the perpendicular component is split into three lines with 14NH3(A14N= 4.0 G) and two lines with 15NH3(A15N= 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial O2– ligand replaced by a nitrogen-containing NHx–3x species, possibly the NH–2 anion. With H2O or NH3 at RT, the Crv5c(B) species is transformed into the corresponding hexacoordinated species, Crv6c(B).Upon adsorption of small H2O doses on the CrOx/SiO2 sample, the Crv4c(A) species is transformed into Crv5c(A) and Crv6c(A). In the presence of excess water, the chromyl species on SiO2 becomes unstable, undergoing disproportionation to CrIII and CrVI On adsorption of NH3, Crv5c(B) is formed from Crv4c(A) at RT. With 15NH3, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us to assign the Crv5c(B) species on SiO2 to a slightly distorted chromyl complex with slightly non-planar equatorial ligands.