Reduction of Aqueous Ag+ Steered by Electrochemical Plasma: Connecting the Bulk pH Variation with the Reaction Pathways for Hydrated Electrons

Reduction of silver cations followed by nanoparticle (Ag-NPs) synthesis is a model process to understand the reduction mechanism induced by a discharge over an aqueous surface, termed electrochemical plasma. This work aims at studying the silver reduction reaction steered by electrochemical plasma in the presence of other chemically active plasma-related interfaces, namely the plasma-gas and the liquid-gas interfaces. As no other plasma-induced species are able to reduce silver cations, the reduction of silver cations is employed as strategy to selectively detect the presence of hydrated electrons (e-h). The results demonstrate that the global rise of pH (increase in the content of OH-), observed for discharge in the helium gas phase, occurs in connection with the silver reduction, which is interpreted as a vivid experimental evidence of the second-order recombination reaction of the e h − 2 e aq − + 2 H 2 O → H 2 + 2 OH −. On the other hand, the global decrease of pH (increase in the content of H+) observed for discharge in mixed oxygen and nitrogen gas phase, is an event primarily driven by the Birkeland-Eyde process, and it is concomitant but spatially distinct from the electron injection. The acidification interferes in the NP formation, as NPs promptly dissolve in presence of HNO3. Only in complete suppression of the acidification, an experimental evidence of the reaction pathways for hydrated electron could be captured: e-h is competitively consumed through a scavenger-like reaction (reduction of silver cations in this work) and through the second-order recombination reactions of the e-h. The kinetic model proposed in this work further corroborates this interpretation.

银阳离子还原与银纳米颗粒（silver nanoparticles, Ag-NPs）合成的联合过程，是解析水相表面放电诱导还原机制的经典模型体系，该类放电过程被称为电化学等离子体（electrochemical plasma）。本研究旨在探究在其他与等离子体相关的化学活性界面——即等离子体-气体界面与液-气界面——存在时，电化学等离子体驱动的银阳离子还原反应。由于其他等离子体诱导产生的物种均无法还原银阳离子，因此本研究采用银阳离子还原作为检测策略，以选择性识别水合电子（hydrated electrons, eₐq⁻）的存在。研究结果表明，在氦气氛围下放电时观测到的整体pH升高（氢氧根离子OH⁻浓度上升）现象，与银阳离子还原过程相关联，这一直观实验证据可佐证水合电子的二级复合反应：2eₐq⁻ + 2H₂O → H₂ + 2OH⁻。另一方面，在氧气与氮气混合氛围下放电时观测到的整体pH降低（氢离子H⁺浓度上升）现象，主要由伯克兰-艾德反应（Birkeland-Eyde process）驱动，该过程与电子注入同时发生，但在空间上相互独立。酸化过程会干扰银纳米颗粒的形成，因为纳米颗粒在硝酸（HNO₃）环境中会迅速溶解。唯有完全抑制酸化过程时，才能获取水合电子反应路径的实验证据：水合电子会通过两类反应被竞争性消耗——一类是本研究中的清除剂类反应（即银阳离子还原），另一类是水合电子自身的二级复合反应。本研究提出的动力学模型进一步验证了上述解释。