Aqueous slurries of thorium oxide and thorium oxide containing uranium were investigated for their catalytic activity for the reaction of hydrogen and oxygen to form water. Pure thorium oxide, thorium-uranium oxide mixed crystals prepared by calcining coprecipitated oxalates, and thorium oxide with uranium oxide sorbed on the surface were used after calcining at 65O°, 800 °, and 1000°. The reaction rates were found to "be first order with respect to hydrogen pressure and zero order with respect to oxygen pressure in all cases at temperatures from 230 to 300 and total gas pressures from 100 to 2000 p. s. i. For the pure thorium oxide an average activation energy of k-1 kcal./mole and an average frequency factor of 4=6 x 10" moles/p. s. i. H2~hr.-g of ThO2 were found. Addition of uranium lowered both factors, the maximum effect giving a Ea of ~14 kcal. with an A of ~10-2 . Actual rates for all catalysts were within one order of magnitude when compared on a unit surface area basis. This compensation effect was explained on the basis of a two-site process, one site being related to the uranium concentration on the catalyst surface and the other characteristic of pure thorium oxide. A few tests on uranium trioxide slurries gave initial fast rates followed by slow ones, the change being accompanied by reduction of the surface uranium under the experimental conditions. The apparent activation energy for both surface conditions was 26 kcal./mole based on first order rate constants with frequency factors of 2.2 x 10 and 2.5 x 1° moles/p.s.i. Hp-hr.-g. for the initial and final rates, respectively.
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