M.A. Fortin, M. Bouchard, A. Turgeon
Keywords: plasma electrochemistry, dielectric barrier discharge, gold nanoparticles, platinum group nanoparticles, continuous-flow process
Summary:Abstract: Platinum group metals (PGM: Pd, Pt, Ir, Rh, etc) are commonly used in catalysis. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime of the catalytic devices. Due to the ever-increasing demand for the production of PGM nanoparticles used in catalytic applications, new synthesis routes must be found to facilitate their efficient production, if possible using a continuous-flow process, while maintaining a high purity level. Here we report on the development of a plasma reactor based on a dielectric barrier discharge, used to synthesize PGM and Au NPs (Figure 1).[2-4] In this atmospheric plasma reactor, an argon plasma (alternatively a hydrogen plasma) is generated at the surface of an aqueous solution containing Au, or PGM salts (Pd, Pt, Ir, Rh..) and surfactant molecules, which are redirected in the reactor in a closed loop for several minutes. This method yields a continuous production of stable noble metal NP suspensions directly in water. In only 45 minutes, a 50 mL solution containing 1 mM of AuCl4- can be reduced into NPs with a reduction yield of 99.3 ± 0.7 % (Figure 1). Reduction yields in the order of 50% were reached after 10 minutes of synthesis only. The size of particles can be tuned by adding various ratios of surfactants (in Figure 2: dextran, a polysaccharide). The reactor can be scaled-up to accommodate larger flows and volumes, and many geometrical configurations. Different acidic solutions of PGM precursors (1 mM of Pd, Pt, Ir, Rh chlorides) were treated with and without surfactants (10 min. of treatment), and the process was monitored using in situ UV-spectroscopy (Figure 3). Each one of the precursor solutions treated by plasma lead to a very efficient decrease of the ionic species, and increased presence of absorption bands related to PGM NPs (confirmed by transmission electron microscopy TEM and ICP-MS). Reduction rates, overall mass production efficiency, as well as energy consumption of the process, will be presented. Overall, plasma electrochemistry could enable the efficient, continuous-flow production of Au and PGM NPs for various types of applications (catalysis, biomedical, etc), and under in situ monitoring using UV-vis spectroscopy. References:  J. A. Kurzman et al, Dalton Trans., 2013, 42, 14653.  M.-A. Fortin et al, Radioactive and/magnetic metal nanoparticles and process and apparatus for synthesizing same; Patent CA2859694 A1 (2012)/ US 2014/0227176 A1.  M.-A. Fortin et al, Dielectric barrier discharge plasma method and apparatus for synthesizing metal nanoparticles; PCT2015/CA051326.  M.-A. Fortin, M. Bouchard et al, MRS Fall meeting (Boston), Dec 4th, 2015.