Hydrogen reduction of metal ions: An electrochemical model

Rate laws for the hydrogen reduction of metal ions are typically of the form: r = -d[M(II)]/dt = k[M(II)]^xP_H2^y where k is an apparent rate constant, [M(II)] and P_H2 represent dissolved metal concentration and hydrogen pressure respectively, while x = 0 - 1 and y = 0.5 - 2. Half-order dependences have been reported for some M(II)/H₂ systems; however, no satisfactory explanations have been offered. In an effort to bridge this gap in our understanding, the published precipitation rate data (for M(II) = Cu(II), Ni(II), and Co(II)) are reviewed (with emphasis on the reported reaction orders) and compared with the corresponding but limited electrochemical polarization data. Based on the available literature on the electrochemical kinetics of the M/M(II) and H₂/H⁺ systems it is demonstrated theoretically that, under certain conditions (e.g., in the presence of electron-conducting solids), the rate of the M(II)/H₂ reaction can be expressed as: -d[M²⁺]/dt = k[M²⁺] ^1/2P_H2^1/2 It is concluded that electrochemical processes probably play an important role in the hydrogen reduction of metal ions, especially for systems that involve heterogeneous catalysis.