Reducing dyes without the noble metals

Environmental contaminants such as nitroarenes and organic dyes can be reduced catalytically to more benign substances without the need for costly and rare noble metal catalysts. Instead, the process exploits sulfur vacancies formed by the activation of two-dimensional molybdenum sulfide nanosheets using wet-chemical sulfur vacancy engineering, according to research published in the journal Applied Materials Today. [García-Dalí, S. et al. Appl. Mater. Today (2020); DOI: 10.1016/j.apmt.2020.100678]

Nitrophenols and their derivatives are toxic compounds that can persist in the environment. As such, where they are present in the waste streams from the manufacture of pesticides, herbicides, and dyes they muse be degraded to less harmful compounds before discharge into waterways.The compounds can be reduced to biodegradable and far less toxic anilines. Common catalysts for such processes usually contain noble metals. The new work makes such a reduction far more cost effective and precludes the need for sourcing rare elements, such as platinum, palladium and gold, by turning to transition metal dichalcogenide. In this case, molybdenum disulfide.

Molybdenum disulfide and its chemical cousins molybdenum diselenide and tungsten disulfide have been the focus of much research in recent years for their potential to usurp noble metal catalysts in many different areas. They have been widely studied in the bulk, layered form, but this latest work takes them down a dimension to two-dimensional monolayer materials that have even greater potential in terms of the massively boosted surface to volume ratio of such materials coupled with the possibility of far high densities of catalytic sites being made available.

The team has explains that the introduction of sulfur vacancies into their 2D material prevents it from reverting from the active metastable state to a thermodynamically stable state. The vacancies also effectively convert the non-catalytic 2D material into a potent catalyst for the target reactions. The team has found that by fine-tuning the mild preparation they can far exceed the catalytic activity of metallic molybdenum disulfide and other related compounds.

The researchers have demonstrated proof of principle with the successful catalytic reduction of ni troarenes (4-nitrophenol and 4-nitroaniline) and organic dyes (methyl orange and methylene blue). They add that the catalysts can be immobilized on melamine foam which makes them far easier to handle than the free materials and activity, they add, is sustained under such conditions for several consecutive catalytic cycles.