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The HzDz equilibration at 78°K has been studied in a flow system over three varieties of MgO powders with or without addition of impurities. The only impurity that affected the catalytic activity consisted of protons, the surface content of which could be regulated by pretreatment of the samples in vacuo at temperatures between 500 and 900°C. Thus, samples were prepared with catalytic activity differing by more than five orders of magnitude. The activity was correlated with the spin density of an electron-deficient paramagnetic center which was shown to be a surface center. The exchange-narrowed symmetrical epr line, corresponding to this new V-like center called VI, was assigned to a triangular array of 0-ions situated on a (111) plane of MgO. The catalytic site then consists of an OH-ion adjacent to the VI center which adsorbs the deuterium molecule to form a quasi-sorbed Rideal-Eley transition state bringing about exchange. The proposed ion-molecule type reaction explains its low activation energy (2 kcal mol-1) and the possibility of carrying out the reaction at 78°K without the participation of transition metal ions. The apparent necessity of metastable (1 11) planes on MgO for obtaining catalytic sites and VI centers was suggested by the irreversible loss of activity and spin density upon heating MgO samples in the presence of HzO vapor at temperatures at which recrystallization sets in with a change of crystal habit. Thus the catalytic site is associated with a proton impurity, an electronic defect, and a specified crystallographic plane.authors to catalyze the Hz-D2 exchange at 78"K, without the need for any irradiation before or during the reaction. Much interesting work on MgO powders preevacuated at various temperatures and preirradiated with uv light was published subsequently by Lunsford6 and by Harkins, et aZ.2 who studied the Hz-D2 exchange at 195°K and room temperature (which will be abbreviated hereafter: 298"K), respectively. This later work, which also involved epr spectroscopy, led the authors to propose V-type centers as catalytic sites and in particular' to assign to 0ions the role of sites responsible for the H2-D2 reaction at 78°K on MgO preevacuated at 300°C.

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