Sonochemical deposition of platinum nanoparticles on polymer beads and their transfer on the pore surface of a silica matrix

Abstract

This study reported the sonochemical deposition of platinum on the surface of polystyrene beads (PSBs) and the transfer of obtained Pt nanoparticles into a porous silica matrix using the PSB as a sacrificial template. Platinum nanoparticle deposition was ensured by the sonochemical reduction of Pt(IV) at room temperature in latex solutions containing polystyrene beads in the presence of formic acid under Ar or under Ar/CO atmosphere without any additives. After ultrasonic treatments for few hours, well dispersed Pt nanoparticles within the range of 3–5 nm deposited on PSB were obtained in both studied conditions. Samples were then mixed with TEOS, dried, and heated at 450 °C to ensure the PSB removal from the silica matrix. TEM and SEM results clearly show that final silica pore size is within the same order of magnitude than initial PSB. Finally, platinum decorated silica matrix with chosen pore sizes was successfully prepared.

Introduction

Noble metal nanoparticles have been widely studied, especially for their unique size-dependent properties [1]. Several metals have drawn the attention like gold, palladium, or platinum, which can be found in many applications such as catalytic converters, water–gas-shift reaction, or large scale chemical synthesis [2], [3], [4]. In catalysis, noble metal nanoparticles (NPs) can be used alone or deposited on different supports in order to facilitate their use, to be stabilized toward the aggregation or to enhance their catalytic activity [1], [5], [6]. Various supports can be considered from silicon or metal oxides to carbon black as well as polymeric materials. For latter thermosensitive supports, the conventional impregnation/hydrogen reduction method is not applicable. To date, several alternative synthetic routes have been considered for such kind of supports: electrodeposition [7], preformed metal particle sorption on polystyrene beads (PSBs) [8], and chemical reduction including electroless deposition [9], [10], [11]. Other techniques can also be considered for the synthesis of nanomaterials. Sonochemistry for instance was successfully applied for the preparation of noble metal particles with or without surfactants or additives [8], [12], [13], [14]. In fact, when submitted into a liquid, power ultrasound induces the nucleation, growth, and violent collapse of cavitation bubbles. This phenomenon known as acoustic cavitation produces a non-equilibrium plasma inside the cavitation bubbles which is responsible for in situ radical formation [15]. In pure water, homolytic split of H₂O molecules leads to the formation of H₂ within the bubble and H₂O₂ at the bubble–liquid interface due to the mutual recombination of H atoms and OH° radicals, respectively [16]. Local heating within the solution surrounding the bubble during its collapse can accelerate the chemical processes with non-volatile species [17]. Recently, it was reported that platinum(IV) could be reduced under ultrasonic irradiation in formic acid media and also in pure water under Ar/CO atmosphere [18]. In homogenous solutions, the sonochemical reactions could be achieved within few hours at room temperature. The strength of this process relies in the fact that the platinum reduction could be envisaged in a large array of experimental conditions and in presence of various supports, especially thermal sensitive materials.

In the current paper, the platinum deposition on polystyrene beads was investigated under two different experimental conditions namely within formic acid media and also in presence of Ar/CO gas mixture in latex solutions containing PSB. Obtained materials were then characterized and used as a template for the preparation of controlled porous silica with platinum decorated pores.