Controlled electrodeposition of metal nanocentres for catalysis

Abstract

In this research, electrodeposition was used with the aim of preparing surfaces with controlled dispersion and uniform small centre size to enhance the catalytic performance. Nanoparticle catalyst centres are designed to minimize the amount of expensive metal components loaded onto a cheaper substrate but with optimum catalytic activity. The electrodeposition of platinum and gold nanocentres have been investigated by varying the deposition parameters in hydrogen hexachloroplatinate(IV) acid and hydrogen tetrachloroaurate(III) hydrate respectively. Various substrates such as microdisc, microband and microwire electrodes were used for the study of controlled electrodeposition. Their smaller surface area provides a better tool for the study of nucleation and growth of nuclei. The effect of mass transport was studied with microdisc and rotating disc electrodes. The mechanism for the reduction of hydrogen hexachloroplatinate(IV) in hydrochloric acid was investigated. Voltammograms show two reduction waves and both waves lead to the deposition of platinum(0). At high overpotential, the reaction becomes diffusion controlled. No dissolution of platinum is observed on the reverse scan. In contrast, the reduction of hydrogen tetrachloroaurate(III) hydrate in acidic media produces a simple mass transfer controlled three-electron reduction wave with a stripping peak of gold observed on the backward scan. Several approaches, such as cyclic voltammetry, single potential step and doublepulse potential step deposition were used to electrodeposit platinum and gold nanocentres. Isolated and hemispherical nanocentres of platinum and gold were successfully electrodeposited onto carbon surfaces. In contrast, the electrodeposition of platinum onto a gold substrate quickly leads to complete layers when observed with a scanning electron microscope. The platinum and gold nanocentres were characterized by means of cyclic voltammetry in acid solution, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The electrodeposition of platinum from a lyotropic liquid crystalline mixture, specifically from a micellar solution, produced platinum deposits with higher specific catalyst area than those deposited in aqueous solution. The nanostructured platinum was not observable with the scanning electron microscope, even at high magnification; however the presence of nanostructured platinum was confirmed by cyclic voltammetry in sulphuric acid. On gold electrodes, the nanostructured platinum showed good catalytic response for hydrogen evolution and oxygen reduction reactions. This study has shown that a surface with small centres and high nuclei number density could be produced by electrodeposition at high overpotentials. A smooth surface with small centres deposited at low charge density will produce low roughness factor, and this makes it difficult to be confirmed with cyclic voltammetry in sulphuric acid. In addition, small centres of less than 50 nm cannot be observed using scanning electron microscopy. However, small centres are believed to be present on the surface as they lead to the centres observed at higher charge densities when deposited at very negative potentials

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ORCID for Guy Denuault: orcid.org/0000-0002-8630-9492

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