CO-Driven Au Recovery and Nanoparticle Synthesis By in-Situ CO2 Reduction on Gas-Diffusion Electrodes

Abstract

Effective synthesis methods for gold nanoparticles (NPs) have been extensively researched due to their wide-ranging applications in biomedicine, bioanalytical processes, and electrochemistry. Chemical reduction is the most common technique for producing precious metals nanoparticles, with the synthesis method greatly influencing the morphology of the resulting product. In most instances, the formation of polydisperse nanoparticles (NPs) is obtained (Reverberi, Kuznetsov et al. 2016). CO is a known reducing agent for and , offering fast reduction rates (Pretzer, Nguyen et al. 2010) and easy removal, resulting in fewer purification steps.We recently reported a synthesis method called Gas-Diffusion Electrocrystallization (GDEx), which can be applied to the platinum group metals (PGMs). The precipitation of the water-soluble PGM precursors is accomplished by the in situ production of H2 and CO, issued from the electrochemical reduction of CO2 and water electrolysis, respectively. This is typically accomplished using gas diffusion electrodes (Martinez Mora, Pozo et al. 2023). H2 acts as a reducing agent and CO as a size-controlling agent. In this work, we propose the synthesis of Au0 NPs driven by the GDEx process fed by CO2 in aqueous media.Different current densities (i.e., 10 and 40 mA cm-2) and two gas-flow regimes were tested –flow-by (FB) and flow-through (FT). Under FB, excess gas was fed into the gas chamber of the electrochemical set-up (i.e., 200 mL min-1), while under FT, the gas chamber overpressure was increased to force the gas through the GDE in convective regime, (i.e., the outlet of the gas chamber was closed and the flow rate was set to 5 mL min-1).In the first instance, the CO-driven Au(III) reduction process was modeled, including the CO(g)-CO(l) mass transport and equilibrium. It was discovered that the CO mass transport is the bottleneck in the reduction rate.In the GDEx electrochemical set-up, lower current densities and FT conditions achieved smaller particle size (i.e., 46 nm at -10 mA cm-2 and FT, table 1) and higher current efficiencies for CO production (~19 %, where the competing reaction was H2 evolution, table 1). Conversely, higher current densities led to faster reduction kinetics, yet lower current efficiencies (see Fig. 1).The GDEx process enables Au recovery and nanoparticle synthesis from Au(III)-containing synthetic streams, and can be applied to industrial leachates from end-of-life products. Conditions favoring CO mass transport and availability in the electrolyte improved the reduction kinetics. Furthermore, GDEx allows controlled CO production, mitigating the risk associated with using pure CO.Table 1. Summary of CO yield, current efficiencies and NP size for the different operating conditions j / mA cm-2 FB/FT CO generation / mmol min-1 CECO / % CEAu / % NP Size / nm Crystallite size / nm -10 FB 0.0023 7.3 4.2 70 ± 32 29 ± 7 FT 0.0081 18.6 6.6 46 ± 17 29 ± 6 -40 FB 0.0046 3.3 2.0 91 ± 36 31 ± 6 FT 0.0081 5.5 2.1 72 ± 20 27 ± 6 This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 958302 (PEACOC project). Martinez Mora, O., G. Pozo, L. F. Leon-Fernandez, J. Fransaer and X. Dominguez-Benetton (2023). "Synthesis of platinum group metal nanoparticles assisted by CO2 reduction and H2 cogeneration at gas-diffusion electrodes." RSC Sustainability.Pretzer, L., Q. Nguyen and M. Wong (2010). "Controlled Growth of Sub-10 nm Gold Nanoparticles Using Carbon Monoxide Reductant." The Journal of Physical Chemistry C 114.Reverberi, A. P., N. T. Kuznetsov, V. P. Meshalkin, M. Salerno and B. Fabiano (2016). "Systematical analysis of chemical methods in metal nanoparticles synthesis." Theoretical Foundations of Chemical Engineering 50: 59-66. Figure 1