Y. L. SANDLER
54
Vol. 58
T H E ADSORPTION AND THE MAGNETIC ORTHO-PARA CONVERSION OF HYDROGEN ON DIAMAGNETIC SOLIDS. I. SOME EXPERIMENTS I N SURFACE YARAMAGNETISRill BY Y. L. SANDLER~ Division of Chemistry, National Research Laboratories, Ottawa, Canada Received M a y ‘7, 1055
The rates of the magnetic ortho-para conversion of hydrogen adsorbed on some diamagnetic solids are determined and the true half lives are found by means of adsorption measurements. True half lives of 1 to 6 minutes are found with BaSOl and 2 samples of Ti02 at 90 ’IC. The appearance of a strong blue color in a sample of Ti02 due to loss of oxygen on heating in uacuo and disappearance of the color on contact with oxygen at room temperature is accompanied by changes in surface paramagnetism, as determined by the parahydrogen conversion. A method for estimating the number of extra-electrons due to surface defects is indicated ; for this, the found true conversion rate is compared with the true conversion rate in presence of measured amounts of molecular oxygen adsorbed on diamagnetic surfaces. The true half life on a solid D20 surface is found to be about 10 hours; this is shown to be’the expected order of magnitude if compared with conversion rates of hydrogen, dissolved in diamagnetic liquids.
Charcoal a t liquid air temperatures is a good catalyst for the ortho-para conversion of hydrogen.a A short review of the work done on this type of reaction up to 1934 is found in Farkas’ book.4 The “low temperature mechanism” in this case involves a nuclear spin inversion in the hydrogen molecule due to inhomogenous magnetic fields in the surface of the catalyst; the conversion is of the same type as found with paramagnetic The magnetic fields have been assumed to be due to “unsaturated valences” in the surface of the charcoal. The true half-lives of the conversion,* as measured in a static system, were between a few seconds and about 40 minutes for different charcoals. Conversions of a similar nature were also observed on other diamagnetic solid^.^ However, in most cases no quantitative conclusions can be drawn as adsorption data were not always given, and in some cases a chemical mechanism, analogous to the H2 D Preaction, may have been involved. I n other cases again in which no conversion was found,S the magnetic centers may have been poisoned by chemisorbed hydrogen. In these cases apparently no precaution was taken to cool the catalyst to a sufficiently low temperature before contacting with hydrogen (for example, by using helium as a heat exchanger). One might also suspect that some sort of a residual conversion may exist with solids, analogous to the conversion of hydrogen, dissolved in diamagnetic liquids. The experiments described below were performed
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(1) Presented a t the 122nd National Meeting of the American Chemical Society, Atlantic City, N. J . , September 1952. (2) Laboratory for Insulation Research, Massachusetts Institute of Technology, Cambridge 39, Mass. (3) I
