STRUCTURE OF CR-V 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 O-2, 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). Upon adsorption of small H2O doses on the CrOx/SiO2 sample, the Cr-4c(v) (A) species is transformed into Cr-5c(v) (A) and Cr-6c(v) (A). In the presence of excess water, the chromyl species on SiO2 becomes unstable, undergoing disproportionation to Cr-III and Cr-VI On adsorption of NH3, Cr-5c(v) (B) is formed from Cr-4c(v) (A) at RT. With (NH3)-N-15, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us to assign the Cr-5c(v) (B) species on SiO2 to a slightly distorted chromyl complex with slightly nonplanar equatorial ligands.