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The homogeneous catalytic activation of molecular hydrogen by cupric salts in aqueous solution Peters, Ernest

Abstract

Hydrogen, which is relatively inert at ordinary temperatures, was found to be, activated homogeneously in aqueous solution try dissolved cupric salts, as shown by their catalytic effect on the reactions between H₂ and reducible substrates such as Cr₂O₇⁼, IO₃⁻, and Ce⁺⁺⁺⁺. From kinetic studies of the Cu(ll)-catalyzed hydrogenation of Cr₂O₇⁼, it was shown that the catalytic activity of Cu⁺⁺ is greatly influenced by complex-forming reagents. The catalytic activities of cupric complexes were found to decrease in the order: butyrate, propionate > acetate > sulphate > chloride > H₂O (i.e. the uncomplexed Cu⁺⁺ ion) > glycine, ethylene-diamine . In all systems that were studied, the reaction was.found to be second order kinetically, the rate being proportional to the concentrations of Cu(ll) and H₂. The occurrence of an H⁺ ion as a product of the initial step of the reaction was postulated to account for the perchloric acid dependence of the rate. The promoting effect of various negative ions, which follows the order of their basicities, was thus explained by assigning to them the role of stabilizing the H⁺ ion. The following mechanism was postulated to account for the observed kinetics: Cu(ll) + H₂ [symbol omitted] CuH⁺ + H⁺ CuH⁺ + substrates [symbol omitted] Cu(ll) + products CuH⁺, the activated intermediate suggested by this mechanism, also seems on energetic grounds to be the most plausible of the possible intermediates having reasonable classical structures. It is suggested that the catalytic activity of Cu⁺⁺ is related to its electron affinity. An attempt is made to extend this interpretation to other homogeneous and heterogeneous hydrogenation catalysts.

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