Extrinsic field effect

Extraction process, 26 381 Extractive chemisorption, 37 80 Extra-framework titanium, 41 293 Extrinsic field effect, 27 26-48 catalysts for, 27 49 theory, 27 50, 51 Extrusion, 28 83... 

The catalyzed ortho-para hydrogen conversion rate may be measured in either a flow, or a static, reactor. The former is the more convenient, the latter is generally used for obtaining absolute rates. Both methods have been used to study the extrinsic field effect, but most of the data have been obtained by the flow method. 

Fig. 1. Flow reactor assembly for measuring extrinsic field effects.

The static reactor method used for absolute rate determinations, and almost always for ortho -para deuterium studies, is generally that based on the micro-Pirani gauge analysis chamber as adapted by Ashmead et al. (3). The time necessary for a single determination of the extrinsic field effect by this method is unfortunately likely to be measured in hours or days rather than in seconds as for the flow reactor. To date the only application of this method to the extrinsic field effect appears to be that of Eley et al. (4). Van Cauwelaert and Hall (5) have described a recirculating adaptation of the static reactor that would seem to be useful for studying the field effect. 

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. 

In some respects the rare earths are almost like oxides of isotopes of the same metal. In other respects they differ in ways that are so subtle as almost to defy understanding. Data will be presented for extrinsic field effects at relatively high and low fields over most of the rare earths in normal (referred to here as self-supported ) form and also in lanthana-supported form. One example of a rare earth in alumina-supported form will be given. 

K followed by rapid cooling to room temperature or by cooling in vacuum or pure helium. Details of preparative procedures, the kinds of activity generated and the possible reasons thereof have been described by the writer (27). It is also to be noted that the catalyst surface prepared in this way is slowly poisoned by molecular hydrogen at room temperature, more rapidly as the temperature is raised (27). These considerations sharply limit the maximum temperature at which the extrinsic field effect may be studied on this catalyst. 

Europium monoxide, EuO, is another ferromagnetic solid that may be used for studying the extrinsic field effect although the low Curie temperature restricts the usefulness of the results. Data are available (27) on two samples, one from the insulative part of the phase diagram with Tc = 69.3 1.0 K and surface 1.4 m2 g 1. (All samples of EuO are very sensitive to moist air and must be handled accordingly.) This sample is referred to below as EuO (ins). Another sample of EuO, from the conductive part of the diagram, had Tc = 129 3 K and surface 0.8 m2 g-1. This is referred to as EuO (con). 

The writer (30) has also attempted to measure the extrinsic field effect over pretreated nickel wire, and over nickel supported on silica gel, at 77... 

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. 

It is obvious that no theory of the extrinsic field effect can be developed without a good understanding of conversion theory for the nondissocia-tive process in the absence of an applied magnetic field. The status of k0 calculations for various systems has been reviewed many times and will be summarized very briefly here. The original development by Wigner... 

Understanding the several extrinsic field effects has proved to be difficult because of our failure, in more than a few cases, to identify the... 

This review will be concluded with some speculative remarks on possible sources of the several extrinsic field effects. There is one kind of behavior that appears to have a simple explanation. The ferromagnetic catalysts all show zero field effect below Tc The reason for this must be that in all such substances the Weiss molecular field is of the order of 10s to 107 Oe. An extrinsic field of a few kOe could produce little additional effect. 

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