Nowadays, nanoparticles (NPs) with sizes between 2 to 50 nm become more and more popular, because they can be applied in various fields such as materials science, chemistry, catalysis, medicine or biology. Until now, noble metal NPs were used as I generation catalysts, as they exhibit good electrocatalytic activity towards ethanol oxidation reaction (EOR) [1]. The best EOR performance, however, was observed for ternary catalysts consisting of PtRh nanoalloy NPs supported by SnO₂ NPs, designed and synthesized in Adzic’s group, [2-5]. Indeed, the break of the C=C bond in the ethanol molecule occurs at the interface between PtRh and SnO₂ particles. Therefore their physical contact is imperative for the effectiveness of the catalyst.

The aim of reducing the expensive noble metal content in the catalyst has lead to the design the II generation of catalysts having a core/shell structure, where only the shell consists of Pt and the core comprises a cheaper metal.

Nanoparticles with fancy shapes can be considered as the III generation catalysts. In particular, hollow metallic nanoparticles become attractive as fuel cells catalysts, since they make use of the large surface area they offer [6-8]. These three-dimensional (3D) structures, such as nanoframes, nanoboxes or nanocages can be fabricated either by etching away a certain volume of the solid nanoparticles (NPs) or by galvanic replacement reactions (GRR), transforming solid metal NPs into multimetallic hollow NPs [8]. During the GRR, the template material is oxidized and dissolved, while metal ions are reduced and the resultant atoms are deposited on the surface of the template. The driving force of the GRR is the difference in redox potential between the two metals.

The present study is focused on several types of catalytic nanoparticles (NPs) studied using TEM techniques combined with energy-dispersive X-ray spectroscopy (EDX). Tomographic reconstruction was applied to confirm the 3D structure of the NPs.

In the ternary PtRh/SnO₂ nanocatalysts, the role of Rh is to cleave the C-C bond of ethanol, while SnO₂ provides OH species to oxidize intermediates and to free Pt and Rh sites for further ethanol oxidation [2]. Because rhodium is not available in Poland it was aimed to be replaced by Re. Pt/Re/SnO₂ NPs were synthesized individually by colloidal and polyol methods and interconnected in a controlled way. For that purpose zeta potentials of the respective NPs were measured as a function of the solution pH. Subsequently NPs with opposite zeta potentials were mixed to obtain binary and ternary combinations.

Three-dimensional (3D) catalysts having a rhombic dodecahedron shape, composed of a Pt frame surrounding a Ni core, which was subsequently removed by etching or GRR with tin were investigated. The morphology of these 3D PtNi particles strongly depends on the synthesis parameters allowing fabricating dodecahedrons, core-shell or even dendritic structures. A number of GRR between Ni and Sn was performed in order to track the reaction over time. The impact of the concentration of tin ions on the obtained nanoframes was investigated and the Pt:Ni:Sn atomic ratio at different reaction stages was determined.

References

1. Antolini, E. Catalysts for direct ethanol fuel cells. J. Power Sources 170 (2007) 1.
2. Kowal et all, Nature Materials 9 (2009) 325.
3. Adzic, A. Kowal, (Brookhaven National Laboratory), Patent Application Publication, Pub. No. US2009/0068505 A1 (2009).
4. Li et all, Electrochimica Acta 55 (2010) 4331.
5. Kowal et all, Electrochem. Comm. 11 (2009) 724.
6. Chen et all, Science 343 (2014) 1339.
7. Ruditskiy and Y. Xia, ACS Nano 11 (2017) 23.
8. Hajfathalian et al., ACS Nano 10 (2016) 6354.

Acknowledgements

The authors thank the Institute of Engineering Materials and Biomaterials of the Silesian University of Technology and the Department of Chemistry, University of Warsaw for the use of the Titan FEI TEM and the Talos F200 FEI TEM instruments, respectively. Financial support from the Polish National Science Centre (NCN), grant UMO-2014/13/B/ST5/04497 is acknowledged. Partial financial support by Pik-Instruments is greatly acknowledged.