Magnetic effects on the ortho-parahydrogen conversion over α-Cr 2O 3, CoO, and MnO

Abstract

Catalyzed ortho-parahydrogen conversion rates were measured on two antiferromagnetic oxides, α-Cr 2 O 3 and CoO over a range of temperature and both with and without an extrinsic magnetic field. A few measurements were made on MnO. The conversion mechanism was, in all cases, nondissociative. Results for the three catalysts, while qualitatively similar, differed in detail. A correlation between the zero field conversion rate and the extrinsic field rate suggests a relation to the paramagnetic-antiferromagnetic phase transition in catalysts of high surface site density. A positive extrinsic field effect was observed only at temperatures above the Neel point ( T N ). Negative effects were found, in some cases, both above and below T N . The latter results were characterized by a maximum rate decrease just below T N and by a rate saturation effect that was a function of applied field strength. The several effects observed suggest that, on high site density surfaces, two or more mechanisms are operative in an extrinsic field. These, and other related results, show that the current state of conversion theory over magnetic surfaces is still incomplete.