ELECTRON PARAMAGNETIC RESONANCE ABSORPTION OF CHROMIA—ALUMINA CATALYSTS¹

Abstract

The electron paramagnetic resonance ahsorption of ehroniia supported on alumina has been investigated a t X-band (9.5 khfcps.) and K-band (23.9 kMcps.). Spectra of chroinia-alumina reduced in hydrogen a t 500" show two distinct phases: a dispersed (6) phase whic,h predominates a t low concentrations of chromium and a bulk ( B ) phase which is prevalent at, the higher concentrations. 1 he temperature dependence of the intensity of these phases indicates the &phase consists of rather isolated Cr+3 ions not coupled electronically and the p-phtHe consists of clusters of Cr+3 ions with strong exchange roupling of 3d electrons. Intensity calibrations of e.p.r. spectra have been made and these indicate all the chromium present in the rcduced catalysh to be in the trivalent state.A relatively sharp resonance absorption appears upon oxidation of reduced chromia-alumina with oxygen. A spin-Hamiltonian for each of the observed types of resonances is proposed and discussed.Absolute amounts of 6-and @-phase chromium also arc given.

Quantitative EPR spectroscopy: Comparison between primary standards and application to MgO-MnO and α-Al2O3-Cr2O3 solid solutions

“…This signal is due to isolated Cri ions, and has been analyzed with the fine-structure terms D = -0.1930 cm-' and EID = 0, determined on ruby single crystal [29]. In concentrated ACX solid solutions (Cr-content >_ 11.5%) the spectrum consisted of a single exchange-narrowed line of Lorentzian shape at g = 1.98, assigned to clustered Cri ions [5,30]. Samples with intermediate Cr-content yielded both signals (Fig.…”

Microstructural Evolution in α-Al2O3 Compacts During Laser Irradiation

“…8) shows the high degree of inhomogeneity of distribution of the chromium in our samples. Note that, in most works on the EPR investigation of the -Al 2 O 3 + n-Cr 2 O 3 samples with even a small value of n, a similar situation can be observed (Carman & Kroenke, 1968;O'Reilly & Maciver, 1962). This indicates that the preparation of -Al 2-х Cr х O 3 samples with a homogeneous distribution of chromium impurities by solidstate synthesis is an intricate problem.…”

“…The EPR spectrum of Cr 3+ (singularities I-III) in Al 2 O 3 polycrystalline samples is well known (Carman & Kroenke, 1968;de Biasi & Rodrigues, 1985;O'Reilly & Maciver, 1962;Stone & Vickerman, 1971). In order to identify signal IV, we compared the EPR spectrum for polycrystalline samples with an angular dependence of EPR curves of Cr 3+ i n r u b y c r y s t a l s ( F i g .…”

“…A large number of studies by different methods were performed on dilute both lowchromium and high-chromium α-Al 2 O 3 solid solutions (Carman & Kroenke 1968;de Biasi & Rodrigues 1985;Galoisy & Calas 1993;Manenkov & Prokhorov, 1955;O'Reilly & Maciver, 1962;Statz et al, 1961;Stone & Vickerman 1971;). Gradual increases in the a and c parameters of the hexagonal unit cell of the corundum structure with increasing chromium concentration were noted (de Biasi & Rodrigues 1985).…”

“…www.intechopen.com Analysis of the EPR spectrum of Cr 3+ (singularities I-III) in Al 2 O 3 polycrystalline samples we have made in part 3.3 (see also (Carman & Kroenke, 1968;de Biasi & Rodrigues 1985;O'Reilly & Maciver, 1962;Stone & Vickerman, 1971). Figures 9 and 18 a shows that singularity I corresponds to the transition -3/2 ↔ -1/2 of ions Cr 3+ in Al 2 O 3 (θ = 90°, where θ is the angle between crystal axes c and the direction of the magnetic field).…”