Chromium magnetic susceptibility

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

Observed magnetic susceptibilities of homo- and heterometallic alkoxides of latter 3d metals (Cr, Mn, Fe, Co, Ni, or Cu) are in consonance with their structures, which were derived on the basis of electronic spectra (7, 19). For example, chromium(III) alkoxides as well as their substituted derivatives, Cr(OEt)(acac)2 and Cr(OEt)2(acac), all depict a magnetic susceptibility of 3.8/xB, indicative of an octahedral geometry for chromium with three unpaired electrons. 

Oxides. E.s.r. spectra and the temperature dependence of the magnetic susceptibility of chromium oxide powders show differences from the properties found for the bulk material. These differences are proportional to both the increase in surface area and to the concentration of lattice defects. ... 

Solid chromium adopts a body-centered cubic structure and crystallizes in the space group Irnim with a = 288.46pm its density is 7.19gcm (at 293K). The metal melts at 2130 ( 20) K and boils at 2945 K the corresponding enthalpies are A//fusion = 15.3kJmoU and AT/vap = 348.78 kj mol Values of various thermodynamic functions are listed in Table 2. Some other values are thermal conductivity 93.7 W m (at 300 K), electrical resistivity 12.7 X 10 m (at 273 K), magnetic susceptibility 3.5 x... 

Their H NMR spectra are characterized by resonances due to the PFe in the positions found for the symmetrical PFe-O-FeP dimer, and a resonance at 38 ppm that is due to the pyrrole-H of the chromium porphyrin unit. The temperature dependence of the magnetic susceptibility of the mixed-metal dimer leads to its formulation as antiferromagnetically coupled Fe S = 5/2) and Cr S = 3/2), yielding S =... 

In the chromia-alumina catalyst system we are primarily concerned with the phenomena of paramagnetism and antiferromagnetism, both of which are due to the magnetic moments of the chromium ions. The magnetic susceptibility (y) of a paramagnetic material such as chromia-alumina is related to the absolute temperature (T) according to the Curie-Weiss law... 

Historically, the first major advance in our understanding of the physical-chemical structure of a chromia-alumina catalyst resulted from a series of magnetic susceptibility studies carried out by Selwood and Eischens (5). They prepared catalysts by both coprecipitation and impregnation techniques, and measured the magnetic susceptibility as a function of catalyst composition. From the results they were able to draw important conclusions concerning the valence state of the chromium, and the manner in which this chromium was combined with the diamagnetic alumina. The same technique has since been applied by other workers with considerable success. 

It would be expected that chromia-alumina catalysts prepared by coprecipitation techniques should differ from impregnated catalysts, and this difference has been demonstrated by Eischens and Selwood (5) who measured the magnetic susceptibility of a chromia-alumina catalyst (35 wt % Cr) prepared by coprecipitation with ammonium hydroxide from a solution of aluminum nitrate and chromium nitrate. The susceptibility of the reduced catalyst indicated a much greater dispersion of the chromium than was characteristic of the impregnated catalysts. This was attributed to the presence of a three-dimensional dispersion of the chromium in the coprecipitated catalysts, as compared to a two-dimensional dispersion in the case of the impregnated catalysts. 

One of the first applications of electron spin resonance (ESR) spectroscopy to catalysis was in a study of the chromia-alumina system, and during the last five years or so a number of publications have appeared dealing with this subject. The ESR spectra of supported chromia catalysts have been interpreted in terms of various chromium ion configurations or phases, each of which wiU be discussed below. It will be seen that these data substantiate many of the conclusions drawn from the magnetic susceptibility data described above, and, in addition, they provide a deeper insight into the molecular structure of chromia-alumina catalysts than can be obtained from static susceptibility measurements alone. This body of research serves as a very good illustration of the potential usefulness of ESR spectroscopy to the catalytic chemist, particularly when one considers that all of the data to be discussed below were obtained on poorly crystallized, high surface area powders, typical of practical catalysts. 

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