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 IrO2 nanotubes (IrO2NT) 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 COad and methanol oxidation. Lattice oxygen of IrO2NT 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 IrO2NT can be preserved via pore generation in the oxygen removal process. The tubular wall was transformed from a thin dense plate of IrO2(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[1(1)over-bar0] parallel to the IrO2NT 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-IrO2NT catalyst reduced at 500 degrees C exhibits a significantly higher activity than Pt-IrO2NT 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.