Reduction of precious metal ions in aqueous solutions by contact-electro-catalysis.

Precious metals are core assets for the development of modern technologies in various fields. Their scarcity poses the question of their cost, life cycle and reuse. Recently, an emerging catalysis employing contact-electrification (CE) at water-solid interfaces to drive redox reaction, called contact-electro-catalysis (CEC), has been used to develop metal free mechano-catalytic methods to efficiently degrade refractory organic compounds, produce hydrogen peroxide, or leach metals from spent Li-Ion batteries. Here, we show ultrasonic CEC can successfully drive the reduction of Ag(ac), Rh3+, [PtCl4]2-, Ag+, Hg2+, Pd2+, [AuCl4]-, and Ir3+, in both anaerobic and aerobic conditions. The effect of oxygen on the reaction is studied by electron paramagnetic resonance (EPR) spectroscopy and ab-initio simulation. Combining measurements of charge transfers during water-solid CE, EPR spectroscopy and gold extraction experiments help show the link between CE and CEC. What's more, this method based on water-solid CE is capable of extracting gold from synthetic solutions with concentrations ranging from as low as 0.196 ppm up to 196 ppm, reaching in 3 h extraction capacities ranging from 0.756 to 722.5 mg g-1 in 3 h. Finally, we showed CEC is employed to design a metal-free, selective, and recyclable catalytic gold extraction methods from e-waste aqueous leachates.

Mechanism for Generating H2 O2 at Water-Solid Interface by Contact-Electrification.

The recent intensification of the study of contact-electrification at water-solid interfaces and its role in physicochemical processes lead to the realization that electron transfers during water-solid contact-electrification can drive chemical reactions. This mechanism, named contact-electro-catalysis (CEC), allows chemically inert fluorinated polymers to act like single electrode electrochemical systems. This study shows hydrogen peroxide (H2 O2 ) is generated from air and deionized water, by ultrasound driven CEC, using fluorinated ethylene propylene (FEP) as the catalyst. For a mass ratio of catalyst to solution of 1:10000, at 20 °C, the kinetic rate of H2 O2 evolution reaches 58.87 mmol L-1  gcat -1  h-1 . Electron paramagnetic resonance (EPR) shows electrons are emitted in the solution by the charged FEP, during ultrasonication. EPR and isotope labelling experiments show H2 O2 is formed from hydroxyl radicals (HO• ) or two superoxide radicals (O2 •- ) generated by CEC. Finally, it is traditionally believed such radicals migrate in the solution by Brownian diffusion prior to reactions. However, ab-initio molecular dynamic calculations reveal the radicals can react by exchanging protons and electrons through the hydrogen bonds network of water, i.e., owing to the Grotthuss mechanism. This mechanism can be relevant to other systems, artificial or natural, generating H2 O2 from air and water.

Degradation of methyl orange by dielectric films based on contact-electro-catalysis.

Contact-electro-catalysis (CEC) has been recently proposed for the effective degradation of methyl orange, but the reactivity of catalysts in the CEC process needs further investigation. Here, we have used dielectric films, such as fluorinated ethylene propylene (FEP), modified by inductively coupled plasma (ICP) etching with argon, to replace the previously employed micro-powder due to their potential scalability, facile recycling process, and possible lower generation of secondary pollution. It has been found that ICP creates cone-like micro/nano structures on the surface, and thus changes the contact angle and specific surface area. The value of the contact angle varies non-linearly with etching time and attains a maximum after 60 seconds of etching. Concurrently, an increased electron transfer is observed, as well as an enhanced degradation efficiency, thus suggesting a special role of the surface structure. Finally, KPFM measurements show a lower electron affinity at the summit of the nanocones. This observation suggests that the structures are endowed with higher charge transfer ability. In addition, this film-based CEC has been observed in several polymer materials, such as PET, PTFE, and PVC. We view this work as a stepping stone to develop CEC into scalable applications, based on film technologies.