Critical metals are significantly important in the preparation of high-tech materials associated with applications on, e.g., renewable energy, sustainable materials engineering and cleaner production. This importance together with supply risk to a substantial extent within the European Union (EU) has pushed their recovery from waste being highlighted. Electronic waste, usually from end-of-life electronic products, is a notable secondary resource for this purpose because of its distinctive features. A range of critical metals, including rare-earth metals, indium, cobalt and valuable metals, such as copper, silver and gold, are possibly recovered from electronic waste. On top of the current practices of electronic waste recycling, it requires innovations on technology and breakthroughs on process design in order to promote critical metal recovery or electronic waste treatment (in general) to be green and sustainable. Significant potentials are more and more noticed from hydrochemistry (metallurgy) technologies (processes) that contribute to this development because of its flexibility, relatively high recovery rate and extraction selectivity of critical metals, and possibilities of eliminating secondary waste. In this review, critical evaluation is carried out on the aspects of (1) understanding the features of different hydrochemistry processes for recycling of (critical) metals from electronic waste; (2) identifying the difficulties for a process to be implemented into industrial application which still originate from the high complexity of electronic waste and the secondary waste generation, e.g., wastewater; (3) defining circulability of metals to be recovered and recognizing their potentials to zero waste scheme. According to the evaluation, sustainable even zero waste processing is expected to be achieved for electronic waste treatment in the long term that it is preferred to reduce or prevent the generation of electronic waste and improve material efficiency from the whole life cycle of electronic products.
Heterogeneous Au-Pt nanostructures have been synthesized using a sacrificial template-based approach. Typically, monodispersed Au nanoparticles are prepared first, followed by Ag coating to form core-shell Au-Ag nanoparticles. Next, the galvanic replacement reaction between Ag shells and an aqueous H(2)PtCl(6) solution, whose chemical reaction can be described as 4Ag + PtCl(6)(2-)→ Pt + 4AgCl + 2Cl(-), is carried out at room temperature. Pure Ag shell is transformed into a shell made of Ag/Pt alloy by galvanic replacement. The AgCl formed simultaneously roughens the surface of alloy Ag-Pt shells, which can be manipulated to create a porous Pt surface for oxygen reduction reaction. Finally, Ag and AgCl are removed from core-shell Au-Ag/Pt nanoparticles using bis(p-sulfonatophenyl)phenylphosphane dihydrate dipotassium salt to produce heterogeneous Au-Pt nanostructures. The heterogeneous Au-Pt nanostructures have displayed superior catalytic activity towards oxygen reduction in direct methanol fuel cells because of the electronic coupling effect between the inner-placed Au core and the Pt shell.
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