Investigating Particle Size Effects in Catalysis by Applying a Size-Controlled and Surfactant-Free Synthesis of Colloidal Nanoparticles in Alkaline Ethylene Glycol: Case Study of the Oxygen Reduction Reaction on Pt

Abstract

Colloidal platinum nanoparticles are obtained via a surfactant-free polyol process in alkaline ethylene glycol. In this popular synthesis, ethylene glycol functions as solvent and reducing agent. The preparation procedure is known for its reproducibility to obtain 1-2 nm nanoparticles, but at the same time the controlled preparation of larger nanoparticles is challenging. A reliable size control without the use of surfactants is a fundamental yet missing achievement for systematic investigations. In this work it is demonstrated how the molar ratio between NaOH and the platinum precursor determines the final particle size and how this knowledge can be used to prepare and study in a systematic way supported catalysts with defined size and Pt to carbon ratio. Using small-angle X-ray scattering, transmission electron microscopy, infrared spectroscopy, X-ray absorption spectroscopy, pair distribution function and electrochemical analysis it is shown that changing the NaOH/Pt molar ratio from 25 to 3, the Pt nanoparticle size is tuned from 1 to 5 nm. This size range is of interest for various catalytic applications, such as the oxygen reduction reaction (ORR). Supporting the nanoparticles onto a high surface area carbon, we demonstrate how the particle size effect can be studied keeping the Pt to carbon ratio constant, an important aspect that in previous studies could not be accomplished.

Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants

“…Organic compounds such as alcohols, polyalcohols (polyols), and phenols are attractive for the synthesis of colloidal metal nanocrystals as they can play multiple roles in a reaction, acting as a solvent, and/or stabilizer, in addition to a reductant . Among various alcohols, methanol and ethanol are the most commonly used because of their abundance and relatively lower prices than other alcohols such as n‐propanol, n‐butanol, and 1‐hexanol . In general, primary alcohols always work as reductants due to their superior reactivity relative to secondary and tertiary alcohols.…”

Controlled Synthesis of Surfactant‐Free Water‐Dispersible Colloidal Platinum Nanoparticles by the Co4Cat Process

“…In the polyol process, the NaOH/Pt molar ratio affects the obtained NP size . To probe a similar effect in alkaline methanol, 2.5 and 65 m m H 2 PtCl 6 solutions in 50 m m NaOH were prepared.…”

“…However, once the nucleation proceeds, no strong influence of the NaOH/Pt ratio on the NP size is observed. This is in clear contrast to the polyol process, in which the NaOH/Pt molar ratio allows an accurate control of the size of the NPs . The NaOH/Pt ratio, however, affects the colloidal stability and the formation of agglomerates, as is detailed below in parallel to a discussion on colloidal stability.…”

“…Evidently, the NaOH/Pt ratio strongly affects the NP colloidal stability (Figure ). In the polyol process the NaOH/Pt molar ratio needs to be reduced to a value of approximately 3 to obtain a stable colloidal dispersion of NPs with a size of approximately 5 nm . Reducing the NaOH/Pt molar ratio to 5 in the Co4Cat process (entry 10) already results in a very poor colloidal stability; that is, the Pt material sediments owing to the formation of larger agglomerates.…”

“…The agglomerates have a size of approximately 50 nm but seem to consist of individual NPs with a size of approximately 5 nm (Figure S11 in the Supporting Information). This indicates that the NaOH/Pt ratio influences, to some extent, the size of the individual NPs but not as markedly as for the EG synthesis . Most importantly, the individual NPs are not stabilized against agglomeration.…”