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 Cr^v species in a square-pyramidal configuration, Cr^v_5c(A), on all supports with the exception of SiO₂ where Cr^v is in a tetrahedral configuration, Cr^v_4c(A). H₂O or NH₃, both at room temperature (RT), yielded the Cr^v_6v(A) species from Cr^v_5c(A), that is, the complex changes its coordination from five to six. After H₂O adsorption and evacuation at RT, Cr^v_5c(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^v_5c(B), the perpendicular component is split into three lines with ¹⁴NH₃(A_14N= 4.0 G) and two lines with ¹⁵NH₃(A_15N= 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial O^2– ligand replaced by a nitrogen-containing NH^x–3_x species, possibly the NH^–₂ anion. With H₂O or NH₃ at RT, the Cr^v_5c(B) species is transformed into the corresponding hexacoordinated species, Cr^v_6c(B).

Upon adsorption of small H₂O doses on the CrO_x/SiO₂ sample, the Cr^v_4c(A) species is transformed into Cr^v_5c(A) and Cr^v_6c(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^v_5c(B) is formed from Cr^v_4c(A) at RT. With ¹⁵NH₃, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us to assign the Cr^v_5c(B) species on SiO₂ to a slightly distorted chromyl complex with slightly non-planar equatorial ligands.