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

Journal Article · Wed Sep 01 04:00:00 EDT 1993 · Journal of Catalysis; (United States)

OSTI ID:5738640

Koehler, K; Schlaepfer, C W; Zelewsky, A von ^[1]; Nickl, J; Engweiler, J; Baiker, A ^[2]

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1. Univ. of Fribourg, Perolles (Switzerland)
2. Swiss Federal Institute of Technology, Zuerich (Switzerland)

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(NO[sub 3])[sub 3] [center dot] 9H[sub 2]O. After impregnation, mononuclear, dinuclear ([sigma]-signal), and polynuclear ([beta]-signal) Cr(III) surface complexes as well as Cr(V) species ([gamma]-signal) assigned to magnetically interacting Cr(III) surface ions and the narrow [gamma]-signal with g[sub perpendicular] = 1.975 and g[sub parallel] = 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 (v = 9.5 and 35 GHz), and are interpreted in terms of dipolar and exchange interactions and exchange narrowing due to different Cr[sub 2]O[sub 3] cluster sizes. The Cr(V) complexes are antiferromagnetically exchanged coupled to other surface species as deduced from the temperature dependence of the [gamma]-signal. For higher Cr contents (>5 wt% Cr[sub 2]O[sub 3]/TiO[sub 2]), the simultaneous existence of antiferromagnetic [alpha]-Cr[sub 2]O[sub 3] and ferromagnetic CrO[sub 2] 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 by Cr(VI) by calcination. The relative intensities of the EPR signals were determined. 30 refs., 12 figs., 3 tabs.

• OSTI ID: 5738640
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 143:1; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ANTIFERROELECTRIC MATERIALS
CALCINATION
CATALYST SUPPORTS
CATALYSTS
CHALCOGENIDES
CHEMICAL PREPARATION
CHEMICAL REACTIONS
CHROMIUM COMPOUNDS
CHROMIUM OXIDES
DECOMPOSITION
DIELECTRIC MATERIALS
ELECTRON SPIN RESONANCE
FERROMAGNETIC MATERIALS
MAGNETIC MATERIALS
MAGNETIC RESONANCE
MATERIALS
OXIDES
OXYGEN COMPOUNDS
PYROLYSIS
REDUCTION
RESONANCE
SYNTHESIS
TEMPERATURE DEPENDENCE
THERMOCHEMICAL PROCESSES
TITANIUM COMPOUNDS
TITANIUM OXIDES
TRANSITION ELEMENT COMPOUNDS