Catalysts magnetic field effect

Mechanistic details of the microwave-induced oligomerization of methane on a microporous Mn02 catalyst were studied by Suib et al. [67], with emphasis on fundamental aspects such as reactor configuration, additives (chain propagators, dielectrics), temperature measurements, magnetic field effect, and reaction conditions. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

IR spectra, 27 283, 284 magnetic measurements, 27 280 oxidized state, 27 289 Raman spectra, 27 284 reduced state, 27 291 reflectance spectroscopy, 27 279 X-ray diffraction, 27 272, 273 support interactions, 27 290 Cobalt monoxide, field effect, 27 44, 45 Cobalt(nickel)-molybdenum-sulfide catalysts, 42 417... 

Electronics of Supported Catalysts Georg-Maria Schwab The Effect of a Magnetic Field on the Catalyzed Nondissocitive Parahydrogen Conversion Rate P. W. Selwood... 

Extrinsic field effects have been studied on a large number of solid catalysts of quite diverse magnetic properties. Presentation of results on these substances will be started with four oxides that are normally considered to be diamagnetic. 

Certain correlations are possible but it must be kept in mind that comparisons of activity in heterogeneous catalysis are likely to be treacherous. Minor differences in surface pretreatment may make major differences in properties. Nevertheless, it is clear that there are recognizable patterns of extrinsic field effects. The several kinds are shown in Fig. 22 wherein AkH is plotted against H (log scale). The curves drawn are meant to be representative, but except for relatively minor details, all of the samples studied to date fall into one of the six patterns shown. This covers over 30 different catalyst preparations, a variety of temperature-sensitive magnetic phases, and various pretreatments. Figure 22 shows one example of catalysts found to have the indicated kind of field effect. Pretreatment conditions are stated in Table IV which includes all samples on which measurements have been made. Pretreatments are abbreviated as follows (H2773q298) means that the sample had been heated in hydrogen for an hour or more at 773 K and then cooled rapidly to 298 K. One or two temperatures at which each A k pattern has been observed are also shown in Table IV. 

Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

Although relatively little used in catalysis, Mossbauer spectroscopy has given important information on the state of iron and cobalt in Fischer-Tropsch and hydrodesulphurization catalysts. Mossbauer spectroscopy provides the oxidation state, the magnetic field and the lattice symmetry of a number of elements such as iron, tin, iridium, and cobalt, and can be applied in situ. We will first describe the theory behind the Mossbauer effect and explain how a nuclear technique gives information on the state of atoms. 

Fig. 1 shows the Moessbaucr spectra of fresh and used (in an industrial apparatus) partially deactivated catalysts. The parameters of the spectra (isomer shift 5, quadrupole splitting A, effective magnetic field Hcff, and relative weight of components C) are presented in Table 2. 

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