Nickel catalysts magnetization measurements

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... 

In this respect, magnetic measurements give valuable information. They can show whether poisons react not only with surface metal atoms but also with subsurface atoms as for the case of H2S chemisorption over Nt/Si02 catalysts (refs. 11. 12) the loss of magnetization caused by H2S chemisorption at saturation at room temperature is twice that produced by H2 chemisorption at atmospheric pressure and room temperature in a separate experiment. Assuming that hydrogen chemisorption is restricted to surface metal atoms, then, it can be deduced that H2S is able to attack the nickel particle in depth (corrosive chemisorption). 

Table I summarizes the characteristics of nickel catalysts prepared onto these supports. For brevity these catalysts will be referred to by a notation in the form aA-fi. For example, 7AAP-573 represents a 7 wt % Ni catalyst supported on A O 2A1P0 reduced at 573 K for 1 h. Incidentally, this sample did not reduce under these conditions and was excluded from further kinetic studies. Notations for the other catalysts are shown in the first column of Table I. All samples were reduced at the specified temperature for 1 h unless noted otherwise. The percent reduction was determined by measuring oxygen uptake at 673 K in a commercial thermogravimetric system (Cahn 113). The average particle size was determined by either X-ray diffraction line broadening or magnetic measurements (see below).

Curve 1 for nickel shows a maximum at about 4 nm, and curve 2 may also be at a maximum at around 10 nm. Recent data of Richardson and Koveal (300) (not shown in Fig. 18) also exhibit a maximum turnover rate at about 10 nm. They studied Ni/Si02 catalyst in the size range d of about 2.5 to 25 nm, and the Ni particle sizes were measured by magnetic methods. Also reported are results on H2 chemisorption (H/M). As d and Ms were known from the magnetic measurements, it was shown that H/Ms... 

The purpose of this paper is to review the present status of a method whereby the electron density in a functioning catalyst may readily be measured under in situ conditions. The method is a magnetic one, and it depends for success on the fact that active supported nickel catalysts often contain particles of nickel of less than 100 A. diameter. 

However, there also appears to be a fault in the argument by which Professor Selwood has tried to establish the cleanliness of his catalysts. Even if we accept that the magnetic behavior of the materials reported on is essentially that of bulk nickel and that the admission of O2 or H20 results in a measurable effect, this does not establish that at the start of such an experiment the surfaces are clean. It shows only that if there were impurities present, they could not be detected by this technique. To establish cleanliness, however, it would be necessary to start with a demonstrably clean surface, to contaminate this to a known extent and to follow the magnetization. It is of interest here that in Professor Selwood s paper there are data showing that, under some circumstances, adsorption does not affect the magnetization initially. Thus, it may be advisable to qualify Professor Selwood s claim to cleanliness with the proviso as judged by magnetic measurements, and to subject these to further tests. 

With plahnum and palladium catalysts, supported on siUca, alumina and active carbon, both H2, O2 and CO probe molecules are available for dispersion measurements. For rhodium, the various values are taken from the work of Ferretti et al. [102]. For ruthenium and iridium, O2 cannot be used as a probe molecule for dispersion measurements, because there is formahon of bulk oxides. With nickel, only H2 gives reUable results, O2 and CO cannot be used as probe molecules for dispersion measurements, because there is formation of bulk oxides with O2 and metal-carbonyls with CO, but the dispersion of the sample can be additionally measured by magnetic measurements. 

Breeder etal. (Ill) have carried out S.P. and magnetization experiments to distinguish between ionic and covalent bonding for the adsorption of H2 and O2 on Ni. Nickel contains 9.4 electrons, 0.6 hole, and 0.6 unpaired electron spin per atom in the d band, the latter being responsible for the magnetic properties of the metal. The S.P. measurements were made on an evaporated Ni film and the magnetization studies on a nickel-silica catalyst, the properties of which were regarded as strictly comparable with the metal film. 

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