Oxygen Vacancy-Rich Ni-CeO2 Heterojunction Catalyst for Hydrogenating Halogenated Nitroarenes with High Activity and Selectivity

Halogenated arylamines are important intermediates for the synthesis of dyes, pesticides, herbicides, and other chemicals. One important way to prepare halogenated arylamines is catalytic hydrogenation of halogenated nitroarenes. Ni-based catalysts have been used in the hydrogenation of halogenated nitroarenes but suffer from low activity and dehalogenation side reaction. In this paper, Ni-CeO₂/SiO₂ heterojunction catalyst with a "raisin-bun" structure was prepared by reverse microemulsion. A built-in electric field and more oxygen vacancies were formed due to electron transfer from Ni to CeO₂ as a result of their work function difference. The built-in electric field leads to the heterolytic cleavage of H₂, thereby improving the hydrogenation activity. Oxygen vacancies preferentially adsorb and activate nitro groups, inhibiting the dehalogenation side reaction. Through the cooperation of built-in electric field and oxygen vacancy, synchronous enhancement of the activity and selectivity is obtained successfully. This finding provides a new view for the design of non-noble metal-based catalysts with high activity and selectivity.

Pd-Nanoparticles@Layered Double Hydroxide/ Reduced Graphene Oxide (Pd NPs@LDH/rGO) Nanocomposite Catalyst for Highly Efficient Green Reduction of Aromatic Nitro Compounds

A facile chemical method is developed to fabricate well-dispersed and an approx. 5 nm sized Pd-nanoparticles (Pd-NPs) deposited ZnAl-LDH/rGO nanocomposite (Pd NPs@LDH/rGO) as a highly efficient and stable catalyst for...

Surface electronic states mediate concerted electron and proton transfer at metal nanoscale interfaces for catalytic hydride reduction of -NO2 to -NH2

Concerted electron and proton transfer is a key step for the reversible conversion of molecular hydrogen in both heterogeneous nanocatalysis and metalloenzyme catalysis. However, its activation mechanism involving electron and proton transfer kinetics remains elusive. With the most widely used catalytic hydride reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction, we evaluate the catalytic activity of noble metal nanoparticles (NPs) trapped in porous silica in aqueous NaBH4 solution. By virtue of a novel combination of catalyst design, reaction kinetics, isotope labeling, and multiple spectroscopic techniques, the real catalytic site for the conversion of -NO2 to -NH2 is identified to be the water-hydroxyl transition metal complex, which could further react with NaBH4 to form a new triangular configuration metal complex of H3B-water-hydroxyl with dynamic features. It yields an ensemble of surface electronic states (SESs) though space overlapping of p orbitals of one B and several O atoms (including the O atoms of 4-NP), which could act as an alternative channel for concerted electron and proton transfer. This work highlights the critical role of the conceptual SESs model in heterogeneous catalysis to tune the chemical reactivity and also sheds light on the intricate working of the [FeFe]-hydrogenases.

Effect of Polyvinyl Alcohol Ligands on Supported Gold Nano-Catalysts: Morphological and Kinetics Studies

The effect of polyvinyl alcohol (PVA) stabilizers and gold nanoparticles supported on active carbon (AuNPs/AC) was investigated in this article. Polymers with different molecular weights and hydrolysis degrees have been synthesized and used, like the stabilizing agent of Au nano-catalysts obtained by the sol-immobilization method. The reduction of 4-nitrophenol with NaBH4 has been used as a model reaction to investigate the catalytic activity of synthesized Au/AC catalysts. In addition, we report several characterization techniques such as ultraviolet-visible spectroscopy (UV-Vis), dynamic light scattering (DLS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) in order to correlate the properties of the polymer with the metal nanoparticle size and the catalytic activity. A volcano plot was observed linking the catalytic performance with hydrolysis degree and the maximum of the curve was identified at a value of 60%. The Au:PVA-60 weight ratio was changed in order to explain how the amount of the polymer can influence catalytic properties. The effect of nitroaromatic ring substituents on the catalytic mechanism was examined by the Hammett theory. Moreover, the reusability of the catalyst was investigated, with little to no decrease in activity observed over five catalytic cycles. Morphological and kinetic studies reported in this paper reveal the effect of the PVA polymeric stabilizer properties on the size and catalytic activity of supported gold nanoparticles.

Simultaneous catalytic reduction of p-nitrophenol and hydrogen production on MIL-101(Fe)-based composites

MIL-101(Fe)-based composite materials and their application for the generation of H₂ by the catalytic reduction of nitro organics are reported in this study.

Sustainable Protocol for the Reduction of Nitroarenes by Heterogeneous Au@SBA-15 with NaBH4 under Flow Conditions

A gold-incorporated SBA-15 catalyst was prepared by a solvent-free ball-milling approach. The catalyst showed high reactivity and selectivity in the reduction of a variety of nitroarenes to anilines operating in absolute EtOH with NaBH₄ as reducing agent. The catalyst was reused in batch conditions over five consecutive runs without any losses of activity or selectivity. Considering the high chemical stability and reusability of the catalytic system, a continuous-flow protocol was also investigated and defined to minimize the generation of waste and optimize the continuous reuse of the catalyst. Benefits of flow conditions were proven by turnover numbers that increased from 47.5 to 1902 and also by the minimization of both leaching (9.5 vs. 1 ppm) and E-factor values (8 vs. 23 in batch).

Synthetic strategies and application of gold-based nanocatalysts for nitroaromatics reduction

With the increasing requirement of efficient organic transformations on the basic concept of Green Sustainable Chemistry, the development of highly efficient catalytic reaction system is greatly desired. In this case, gold (Au)-based nanocatalysts are promising candidates for catalytic reaction, especially for the reduction of nitroaromatics. They have attracted wide attention and well developed in the application of nitroaromatics reduction because of the unique properties compared with that of other conventional metal-based catalysts. With this respect, this review proposes recent trends in the application of Au nanocatalysts for efficient reduction process of nitroaromatics. Some typical approaches are compared and discussed to guide the synthesis of highly efficient Au nanocatalysts. The mechanism on the use of H₂ and NaBH₄ solution as the source of hydrogen is compared, and that proposed under light irradiation is discussed. The high and unique catalytic activity of some carriers, such as oxides and carbons-based materials, based on different sizes, structures, and shapes of supported Au nanocatalysts for nitroaromatics reduction are described. The catalytic performance of Au combining with other metal nanoparticles by alloy or doping, like multi-metal nanoparticles system, is further discussed. Finally, a short discussion is introduced to compare the catalysis with other metallic nanocatalysts.

Monolithic carbon foam-supported Au nanoparticles with excellent catalytic performance in a fixed-bed system

A MCF/Au monolith can be innovatively utilized to construct a fixed-bed system, which exhibits exceptional reduction performance for 4-nitrophenol.