Susceptibility isothermal

Dispersion of supported oxides can be studied by determining the effect of concentration on susceptibility at a single temperature. A susceptibility isotherm for chromium in chromium oxide on alumina at -188 C is shown as the top portion of Figure 2 ( 5). This isotherm shows a sharp decrease in susceptibility as the chromium concentration is increased to 9%. At higher concentrations, a smaller decrease in susceptibility with increasing concentration is observed. The susceptibilities for supported 2 3 does not attain the value for bulk Cr 0 which is about 30 X 10 at 188°C. 

From this discussion it will be clear that supported, or dispersed, transition group oxide systems may be studied by the measurement of magnetic susceptibility. If the susceptibility is plotted against concentration for a supported oxide there are obtained curves characterized in most cases by a great increase in susceptibility at low concentration. The effect is especially pronounced at low temperatures. It is convenient to have a name for such curves. They are called susceptibility isotherms. In many cases the susceptibility rises sharply at a critical point, conveniently referred to as point 1. ... 

From the susceptibility isotherms it may be expected that three kinds of information may be obtained first, the oxidation state of the paramagnetic ion second, evidence of intercation covalence and third, the effectiveness of dispersion of the paramagnetic ions. There will now be presented specific applications of the method. 

Magnetic susceptibility measurements were made on the several samples by the Gouy method. Susceptibility isotherms for two temperatures are shown in Fig. 9. These curves gives the susceptibility per gram of chromium in the several catalysts, as a function of chromium concentration. The expected rise of susceptibility with decreasing chromium concentration, and the position of point I are clearly shown, especially at low temperatures. In calculating the susceptibility of chromium in each sample it has been assumed that the susceptibilities of aluminum ions and oxide ions remain constant. This procedure is... 

An attempt will now be made to explain the shape of the susceptibility isotherm for the chromia-alumina system in terms of the previous discussion. Provided that the Curie-Weiss law holds for this system it will be possible to determine whether the great increase of susceptibility at low concentration is due to change of oxidation state, the relaxation of... 

Experiments have also been made on alumina samples of different areas. For instance, an impregnated chromia catalyst on a sample of boehmite, the specific area of which as 170 square meters per gram gave a susceptibility isotherm almost identical in form with that on high-area 7-alumina. Point I occurred at the same concentration, namely 6 per cent chromium by weight. This surprising result is understandable... 

Chromia-alumina catalysts are often prepared by procedures other than the method of impregnation. A precipitated chromia was prepared as follows y-alumina was suspended in 25 per cent ammonium hydroxide solution. The mixture was stirred rapidly while chromic nitrate solution was added from a buret. The resulting mixture was then dried, ignited, and reduced in the same manner as for impregnated samples. A total of four samples was prepared. The susceptibility isotherm for this series is of the same general form as for the impregnation series except that point I is virtually absent. But the most striking... 

The general form of the susceptibility isotherm is not unlike that obtained for supported chromia. The magnetic moment is very close... 

Fig. 14. Susceptibility isotherms for high-ignition manganese oxide supported on...

The situation with respect to the low-ignition manganese on alumina is very different. Figure 17 shows the susceptibility isotherms for this series. Fig. 18 shows the magnetic moment and the chemical oxidation state, and Fig. 19 shows the Weiss constant. The susceptibility isotherm differs from all those previously presented in the sharpness of point 1. 

The expected result was obtained, namely, all the manganese remained in the -1-4 state even at the lowest manganese concentration investigated, 1.3 per cent. The susceptibility isotherm was a typical example of the chromia-alumina type in which practically the whole change is due to a change of ionic environment, and not to a change of oxidation state. The magnetic moment corresponded satisfactorily with the theoretical value for Mn+ . 

Susceptibility isotherms for the nickel alumina samples at three temperatures are shown in Fig. 21. It will be noted that the large increase of susceptibility at low concentrations is present, but that point I is difficult or impossible to determine. 

Fio. 21. Susceptibility isotherms for nickel oxide supported on alumina. 

Below 6 per cent nickel the first observation is that the Weiss constant is zero. The form of the susceptibility isotherm is thus in the case of nickel in no way related to the exchange interaction between adjacent nickel ions. This is not to say that the nickel ions are at infinite magnetic dilution. For nickel in massive nickel oxide the exchange integral, the paramagnetic neighborhood (z), and the number of unpaired electrons are smaller than they are for chromium ions in massive chromia. The quantity. A, is understandably smaller for the case of nickel, and it... 

Susceptibility isotherms for supported iron oxide are given in Fig. 27. Mimetic moments and chemical oxidation states are given in Fig. 28. 

Susceptibility isotherms for the supported copper oxide are shown in Fig. 29. Copper shows the dispersion effect, the great increase of sus-... 

Susceptibility isotherms at —180° for initial (oxide), reduced, and reoxidized samples are compared in Fig. 30. It is clear that reduction in all cases causes the susceptibility to drop substantially to zero. This result poses a difficulty. The isotherm for the oxidized form suggests that below 10 per cent copper there is an approach to infinite magnetic dilution, because the susceptibility is not rising very rapidly. 

Fig. 30. Susceptibility isotherms for original, reduced, and reoxidized copper supported on alumina, all at —180 .

Preliminary data on a few other systems have been obtained in the writer s laboratory and as they show the general scope of the susceptibility isotherm method they will be described briefly. 

Supported vanadium oxide prepared by impregnation of y-alumina with ammonium vanadate solution, followed by ignition and reduction at 850 in hydrogen, gives the peculiar susceptibility isotherms shown in... 

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