Electrochemistry of iridium single crystal surfaces: Part I. Structural effect on formic acid oxidation and poison formation on Ir (111), (100) and (110)

The geometrical arrangement of Ir sites favourable for the main oxidation reaction and the poison formation reaction in formic acid oxidation is determined on Ir (111), (100) and (110). The order of catalytic activity is (111) > (100) > (100) for the main oxidation and that for the poison formation is (111) < (110) < (100). This order is different from that on Pt single crystal planes, the order of which is (111) > (100) > (110) for the main oxidation and (111) < (100) < (110) for poison formation. Since the order is different on Pt and Ir, the chemical property determines the order. On both Ir and Pt, poison formation occurs independently of the existence of adsorbed hydrogen.