Abstract
Cobalt complexes of three water-soluble porphyrins have been examined as catalysts of H/sub 2/O reduction to H/sub 2/. They have been shown to catalyze H/sub 2/ production via controlled-potential electrolysis (-0.95 V vs. SCE at Hg-pool electrode; 0.1 M trifluoroacetic acid) at rates approximately 10-fold greater than background and with nearly 100% current efficiency. Reversible cyclic voltammograms were observed in dry Me/sub 2/SO, with Co(III)/Co(II) and Co(II)/Co(I) potentials near their expected values. Addition of water (0.5-2%) increased the cathodic peak and decreased the anodic peak at the Co(II)/Co(I) couple, consistent with electrocatalytic H/sub 2/O reduction. In aqueous buffers, catalytic currents were observed for CoTMPyP, which increased with decreasing pH, but at a rate less than expected, due to porphyrin adsorption. Adsorption was more pronounced for CoTMAP, which showed no catalytic current except at very low pH. The rate of Co/sup I/TMAP reaction with H/sub 2/O, however, is extremely fast as shown by spectrophotometry in dry acetonitrile; a lower limit for the Co(I)-H/sub 2/O reaction rate constant was estimated to be approximately 10/sup 4/ M/sup -1/s/sup -1/. Co(I) reactivity and cobalt hydride stability are discussed in comparison with the reactivity and stability of related compounds.
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