Precious metals are pivotal to a number of different technologies ranging from telecommunications to pollution control. Their scarcity has stimulated the search for effective solutions for their recovery from spent materials. However, commonly employed dissolution methods make use of harmful reagents such as aqua regia or cyanide solutions. In this study, instead, the team relies on the reactivity of radicals generated in situ when a suspension of TiO2 in acetonitrile, dichloromethane or a mixture thereof is irradiated with ultraviolet light (pictured). Electrons and holes on the surface of the semiconductor react with oxygen and solvent molecules resulting in the formation of superoxide radicals (O2•–) or alkyl radicals (R•) respectively, which further evolve and eventually react with the precious metal — also present in the suspension — enabling the oxidative dissolution process. The reaction pathway and the nature of the dissolved metal species depend on the complex-formation ability of each specific metal and the source of reactive radicals. Choosing the right solvent is therefore crucial to control the process. While gold dissolves in acetonitrile, platinum-group precious metals require a mixture of acetonitrile and dichloromethane. Combining electron paramagnetic resonance, infrared and X-ray adsorption spectroscopies together with density functional theory calculations, the authors interrogated the cascade reaction and resolved the chemical identity of the species evolving from the initial radicals. In particular, they established the relevance of CN• radicals and acetamide-like intermediates — the latter evolving into ammonium ions — that result in the complexation of gold as NH4Au(CN)2, whereas Cl• species proved central for the recovery of Pt, in the form of (NH4)2PtCl6, shedding light on the solvent dependence.
This work introduces an intriguing use of photocatalysis, and thanks to its simplicity and scalability in combination with the use of less hazardous reagents — at least when compared with conventional schemes — is expected to impact the recovery of precious metals on a practical level.
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