Pt−Ir−IrO₂NT Thin-Wall Electrocatalysts Derived from IrO₂ Nanotubes and Their Catalytic Activities in Methanol Oxidation

The structural features and catalytic activities of PtIr electrocatalysts derived from vertical IrO 2 nanotubes (IrO 2 NT) of 1100 nm in height and 80−100 nm in diameter have been studied using scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, and cyclic voltammetry toward CO ad and methanol oxidation. Lattice oxygen of IrO 2 NT is removed under high-vacuum thermal annealing to facilitate nucleation of 3−5 nm Ir grains and subsequent synthesis of PtIr catalyst on the tube walls. Interestingly, the apparent dimensions and orientation of IrO 2 NT can be preserved via pore generation in the oxygen removal process. The tubular wall was transformed from a thin dense plate of IrO 2 (110) single crystal into a porous plate consisting of connected Ir grains that exhibit lattice fringes of the Ir{110} spacing with preferential orientation of Ir[11̄0] parallel to the IrO 2 NT growth direction. The amount of Ir being reduced, the Ir grain size, and the deposited Pt size strongly influence the surface area and the catalytic activity. The Pt−Ir−IrO 2 NT catalyst reduced at 500 °C exhibits a significantly higher activity than Pt−IrO 2 NT and Pt−IrNT in methanol oxidation and also a higher current density than that of a Johnson−Matthey PtRu catalyst in the high potential region.