Chromia supported on titania: I. An EPR study of the chemical and structural changes occurring during catalyst genesis

The chemical and structural changes occurring during the preparation of chromia on titania catalysts have been investigated using electron paramagnetic resonance (EPR). The various chromium species after impregnation, calcination in excess oxygen, and temperature-programmed reduction were characterized. Catalysts with chromia contents ranging from 0.1 to 32 wt% were prepared by impregnation of titania with aqueous solutions of Cr(NO₃)₃ · 9H₂O. After impregnation, mononulear, dinuclear (δ-signal), and polynuclear (β-signal) Cr(III) surface complexes as well as Cr(V) species (γ-signal) were observed. The EPR spectra of the calcined catalyst consist of a broad line at g = 1.98 (β-signal) assigned to magnetically interacting Cr(III) surface ions and the narrow γ-signal with g_⊥ = 1.975 and g_∥ = 1.955-1.964 corresponding to Cr(V) surface complexes. The intensities and linewidths of the signals are strongly dependent on chromium content, preparation conditions, recording temperature (77-423 K). and the microwave frequency (ν = 9.5 and 35 GHz), and are interpreted in terms of dipolar and exchange interactions and exchange narrowing due to different Cr₂O₃ cluster sizes. The Cr(V) complexes are antiferromagnetically exchanged coupled to other surface species as deduced from the temperature dependence of the γ-signal. For higher Cr contents (>5 wt% Cr₂O₃/TiO₂), the simultaneous existence of antiferromagnetic α-Cr₂O₃ and ferromagnetic CrO₂ crystallites on the surface is proposed. At temperatures >773 K, Cr(III) ions are partially incorporated into the rutile and anatase lattice. A considerable amount of chromium is oxidized to Cr(VI) by calcination. The relative intensities of the EPR signals were determined.