Ultra-Small Ceria Nanocrystals at Carbon Surface Synthesized by Ultrasound Sonication: A Study of Highly Active Platinum-Cerium Bifunctional Catalysts for Methanol Oxidation and Oxygen Reduction
Authors: Nguyen Huy Quí Vinh , Nerut Jaak , Kasuk Heili , Thomberg Thomas , Romann Tavo , Aruväli Jaan , Lust Enn 
Published: September 2023

ABSTRACT

Cerium (IV) oxide (called ceria) has gained a lot of interest from researchers due to its high oxygen storage capacity (1) and high tolerance toward CO-like intermediates of methanol oxidation reaction (MOR) (2,3). In our previous study (4), the ceria crystals were synthesized by the modified solvothermal synthesis with urea (5) and grain size of ceria was around 30 µm, even if the crystallite size was only 11 nm. However, even this huge grain-size CeOx particles contributed insignificantly to the catalytic synergetic effect of ceria at the PtCe catalysts toward MOR.
The parameters influencing the deposition of Pt nanoparticles (NPs) on the ceria-carbon support by the ethylene glycol (EG) reduction method were investigated in previous study (4). It was found that the influence of energy supply methods on the reaction system needs to be simplified to control the nucleation and growth of Pt NPs better.
Although the role of catalytically active components (Pt and CeOx) in catalyst is important, the structure of carbon support affects the catalytic behaviour of the catalyst significantly. The chromium carbide-derived carbon, noted as C(Cr-CDC), synthesized in our previous study (6) is active materials toward oxygen reduction reaction in 0.1 mol dm─3 KOH due to its high number of the active sites at the surface, large specific surface area, and unique micro-mesoporous structure. Therefore, it is a good support material for this study.
The purpose of this study was to synthesize a ceria with small grain size, improve the activity of Pt NPs synthesized by tuning the parameters of EG reduction method, and compare the activity of the novel PtCe/C(Cr-CDC) materials and PtCe/C (commercial Ketjenblack carbon) materials.
In detail, the ceria-carbon materials were synthesized firstly by ultrasound sonication method in the mixture of EG and sodium hydroxide solutions, called sonochemical method. Later, the Pt NPs were deposited onto the ceria-carbon materials by EG reduction method using the refluxing system. All the materials were characterized with thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) methods. All the PtCe/C materials were measured in the 3-electrode system in 0.5 mol dm─3 H2SO4 and in a mixture of 0.5 mol dm─3 H2SO4 + 1 mol dm─3 CH3OH to investigate the electrochemical behaviour and MOR activity, respectively.
The results of XRD and SEM-EDX revealed that it was possible to synthesize the ultra-small ceria NPs by the sonochemical method. The crystallite size is smaller than 2 nm. EDX confirmed the presence of cerium in measuring areas, however, the grain size of ceria in ceria-carbon was too small to be revealed with SEM. The TGA confirmed that all cerium was deposited onto carbon with approximately 11 wt. % of the total ceria-carbon weight. Besides, XRD and TGA also confirmed the success of Pt deposition method. The electrochemical activity at PtCe/C materials revealed high electrochemically active surface area value (55-80 mPt
2 gPt
−1).

ACKNOWLEDGEMENT

This work was supported by the EU through the European Regional Development Fund TK141 “Advanced materials and high-technology devices for energy recuperation systems” (2014-2020.4.01.15-0011) and Personal Research Grant PRG676. The SEM-EDX measurements were conducted using the NAMUR+ core facility funded by the Estonian Research Council (TT 13).

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