The stringent environmental protection measures make it necessary to eliminate nitro groups from pollutant nitro compounds. This can be achieved via catalytic reaction to reduce 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH4. The conversion of nitrophenols to aminophenols has been extensively explored for potential industrial applications, including the production of paracetamol and aniline. Such catalytic reactions are reportedly dependent on NaBH4 concentration, the surface area of the metal nanoparticles and the initial concentration of the 4-NP. Several metal nanostructures such as silver, platinum and copper have been used in the catalytic reduction of 4-NP. Compared to monometallic nanoparticles, trimetallic and bimetallic nanoparticles exhibit higher performance towards 4-NP catalytic reduction.
Nevertheless, despite several studies on reducing 4-NP to 4-AP, mechanistic studies of such reactions are still limited. Platinum is always considered the best catalyst for various chemical reactions. However, its practical application is limited by its high cost and scarcity. To this end, platinum has been alloyed with other metals not only to reduce costs but also to improve the physicochemical properties of the resulting alloys. Among the metals, silver has drawn considerable research interest as a potential alloying candidate. It improves the physicochemical and photocatalytic properties desired for many catalytic and electrocatalytic reactions. Specifically, silver-platinum bimetallic nanoparticles (Ag-Pt BMNPs) have exhibited potential practical applications such as methanol oxidation. To date, numerous studies on Ag-Pt BMNPs have mainly focused on their preparation and characterization, with only a few concentrating on their catalytic reduction of 4-NP.
With the increasing environmental concerns and urgent need to devise effective strategies for eliminating nitro group from toxic aromatic nitro compounds, Shalaka Varshney (PhD candidate) and Dr. Tomer Zidki from Ariel University in Israel, together with Dr. Ronen Bar-Ziv from Nuclear Research Center Negev, studied the catalytic reduction of 4-NP on the surfaces of the metallic and alloy nanoparticles. Their main aim was to explore the activity and performance of silver-based alloy nanoparticles in the catalytic reduction of 4-NP to 4-AP and their potential industrial applications. The original article can be found in the research journal, ChemCatChem.
In this approach, the authors synthesized nine different unsupported compositions of Ag-Pt and Au-Pt in aqueous suspensions and studied their catalytic activities in the presence of NaBH4. Upon alloying, tunable surface properties were provided to obtain optimal catalyst-reactant interaction, considered beneficial in enhancing the catalytic reduction of 4-NP. The catalytic performance of the Ag-based alloy nanoparticles was discussed and compared with those previously reported. Also, the authors conducted a mechanistic study on the obtained results.
The research team found a remarkably higher catalytic performance for silver-based alloy nanoparticles (Ag-Pt) to reduce 4-NP than the previously reported catalysts. Silver-based catalysts with relatively low platinum content (10% Pt) exhibited the highest catalytic activity with an apparent rate constant () and high activity parameter value () of and , respectively. Even though hydrides were identified as the active species for effective reduction of the 4-NP, the composition of the alloy also exhibited considerable effects on both the reactivity and competition with hydrogen evolution reaction. Additionally, the importance of tuning the surface properties upon alloying in enhancing the catalytic process was demonstrated.
In summary, the authors synthesized gold, silver and platinum metallic nanoparticles as well as silver-platinum and gold-platinum bimetallic alloy nanoparticles without any external support or stabilizer and studied their catalytic activities towards the reduction of 4-NP using NaBH4. For the first time, remarkable apparent rate constant and high activity parameter values were reported. The excellent catalytic activity of the silver-based catalysts, especially at low amounts of platinum, fulfilled the cost and performance requirements for reducing 4-nitrophenol to 4-aminophenol. In a statement to Advances in Engineering, Dr. Tomer Zidki explained that the study presented new methods for the design of efficient and low-cost catalysts for various applications like the neutralization of toxic organic pollutants.
Varshney, S., Bar‐Ziv, R., & Zidki, T. (2020). On the Remarkable Performance of Silver‐based Alloy Nanoparticles in 4‐Nitrophenol Catalytic Reduction. Chemcatchem, 12(18), 4680-4688.