Ultra-small metal nanoparticles supported on carbon nanotubes through surface chelation and hydrogen plasma reduction for methanol electro-oxidation

Abstract

This paper reports a new method to obtain ultra-small Pd and Pt nanoparticles (0.5–1 nm) supported on multi-walled carbon nanotubes (MWCNTs). Even at high loadings of both metals (22.3 and 31.5% (wt/wt) of Pd²⁺ and Pt²⁺, respectively), very narrow and unimodal particle size distributions are achieved. The complexing capabilities of polyethyleneimine covalently attached to the surface of the tubes are optimal for the retention of Pd²⁺ and Pt²⁺ from solution. We have addressed the reduction of the retained ions by two approaches: a classical treatment with NaBH₄ in aqueous solution, and a novel method using hydrogen cold plasma to preserve the structural features of the material. Cold plasma produced degrees of reduction similar or even larger than NaBH₄, supporting the advantage of hydrogen cold plasma as reducing agent as it is a simple, clean and fast (15 minutes) procedure. XPS analysis of the reduced materials show an increase in the electron density near the Fermi level. Pt/MWCNT materials have been tested as anode for methanol electrooxidation, showing a catalytic profile typical of that observed for platinum nanoparticles. The stability after 1000 cycles of the plasma-reduced materials is much larger than these reported for Pt/carbon materials, indicating the stability of the ultra-small nanoparticles.