Uniform PtIr catalysts supported on carbon nanotubes prepared with assistance from phosphomolybdic acid, and their enhanced performance in the oxidation of methanol

Abstract

Multiwalled carbon nanotubes (MWCNTs) are functionalized with phosphomolybdic acid (PMo) by ultrasonication. With the assistance of PMo, PtIr nanoparticles are homogeneously dispersed on the surface of MWCNTs. The size of PtIr nanoparticles can be controlled from 1.6 to 3.2 nm by tuning the composition of Pt catalysts. The composition of PtIr nanoparticles can be optimized at 4 : 1 Pt/Ir ratio, and as-prepared PMo/Pt₄Ir₁/MWCNT catalysts possess unique properties including a small size of PtIr nanoparticles, a high electroactive surface area and a large current density of methanol oxidation. For comparison, some Pt catalysts, Pt₄Ir₁/MWCNT and PMo/Pt/MWCNT, are also prepared in a controlled experiment. In the absence of PMo, PtIr nanoparticles are unevenly dispersed and aggregated on the surface of MWCNTs. Their average diameter is 3.1 nm, much larger than that of PtIr nanoparticles which are prepared in the presence of PMo (1.6 nm). This accounts for the difference in the catalytic activity of methanol and CO oxidation between Pt₄Ir₁/MWCNT and PMo/Pt₄Ir₁/MWCNT. On the other hand, the introduction of Ir makes PMo/Pt₄Ir₁/MWCNT catalysts superior to PMo/Pt/MWCNT catalysts in terms of a better CO-tolerance and a higher catalytic efficiency of methanol oxidation.