Revealing the Active Species for Aerobic Alcohol Oxidation by Using Uniform Supported Palladium Catalysts

The synthesis of single-atom catalysts for heterogeneous catalysis

Heterogeneous catalysis is an important class of reactions in industrial production, especially in green chemical synthesis, and environmental and organic catalysis. Single-atom catalysts (SACs) have emerged as promising candidates for heterogeneous catalysis, due to their outstanding catalytic activity, high selectivity, and maximum atomic utilization efficiency. The high specific surface energy of SACs, however, results in the migration and aggregation of isolated atoms under typical reaction conditions. The controllable preparation of highly efficient and stable SACs has been a serious challenge for applications. Herein, we summarize the recent progress in the precise synthesis of SACs and their different heterogeneous catalyses, especially involving the oxidation and reduction reactions of small organic molecules. At the end of this review, we also introduce the challenges confronted by single-atom materials in heterogeneous catalysis. This review aims to promote the generation of novel high-efficiency SACs by providing an in-depth and comprehensive understanding of the current development in this research field.

A Liquid-Liquid-Solid System to Manipulate the Cascade Reaction for Highly Selective Electrosynthesis of Aldehyde

Electrocatalytic synthesis of aldehydes from alcohols exhibits unique superiorities as a promising technology, in which cascade reactions are involved. However, the cascade reactions are severely limited by the low selectivity resulting from the peroxidation of aldehydes in a traditional liquid-solid system. Herein, we report a novel liquid-liquid-solid system to regulate the selectivity of benzyl alcohol electrooxidation. The selectivity of benzaldehyde increases 200-fold from 0.4 % to 80.4 % compared with the liquid-solid system at a high current density of 136 mA cm^-2 , which is the highest one up to date. In the tri-phase system, the benzaldehyde peroxidation is suppressed efficiently, with the conversion of benzaldehyde being decreased from 87.6 % to 3.8 %. The as-produced benzaldehyde can be in situ extracted to toluene phase and separated from the electrolyte to get purified benzaldehyde. This strategy provides an efficient way to efficiently enhance the selectivity of electrocatalytic cascade reactions.

Tailoring Bond Microenvironments and Reaction Pathways of Single-Atom Catalysts for Efficient Water Electrolysis

Single-Atom Sites (SASs) are commonly stabilized and influenced by neighboring atoms in the host; disclosing the structure-reactivity relationships of SASs in water electrolysis are the grand challenges originating from the enormous support materials with complex structures. Through a multidisciplinary view of the design principles, synthesis strategies, characterization techniques, and theoretical analysis of structure-performance correlations, this timely review is dedicated to summarizing the most recent progress in tailoring bond microenvironments on different supports and discussing the reaction pathways and performance advantages of different SAS structures for water electrolysis . The essences and mechanisms of how SAS structures influence their electrocatalysis and the critical needs for their future developments are discussed. Finally, the challenges and perspectives are also provided to stimulate their practically widespread utilization in water-splitting electrolyzers.

A Novel Pd-P Nano-Alloy Supported on Functionalized Silica for Catalytic Aerobic Oxidation of Benzyl Alcohol

Catalytic aerobic oxidation of benzyl alcohol (BnOH) to benzaldehyde (PhCHO) over supported noble metal catalysts has grabbed the attention of researchers due to the critical role of PhCHO in numerous industrial syntheses. In the present study, a novel catalyst, Pd-P alloy supported on aminopropyl-functionalized mesoporous silica (NH2-SiO2), was prepared through in situ reduction and characterized by BET-BJH analysis, SEM, TEM, XRD, FTIR, TG-DTA, and XPS. Chemical properties and catalytic performance of Pd-P/NH2-SiO2 were compared with those of Pd° nanoparticles (NPs) deposited on the same support. Over Pd-P/NH2-SiO2, the BnOH conversion to PhCHO was much higher than over Pd°/NH2-SiO2, and significantly influenced by the nature of solvent, reaching 57% in toluene at 111 °C, with 63% selectivity. Using pure oxygen as an oxidant in the same conditions, the BnOH conversion increased up to 78%, with 66% selectivity. The role of phosphorous in improving the activity may consist of the strong interaction with Pd that favours metal dispersion and lowers Pd electron density.

Single-Atom Pt Stabilized on One-Dimensional Nanostructure Support via Carbon Nitride/SnO2 Heterojunction Trapping

Catalysis with single-atom catalysts (SACs) exhibits outstanding reactivity and selectivity. However, fabrication of supports for the single atoms with structural versatility remains a challenge to be overcome, for further steps toward catalytic activity augmentation. Here, we demonstrate an effective synthetic approach for a Pt SAC stabilized on a controllable one-dimensional (1D) metal oxide nano-heterostructure support, by trapping the single atoms at heterojunctions of a carbon nitride/SnO₂ heterostructure. With the ultrahigh specific surface area (54.29 m² g^-1) of the nanostructure, we obtained maximized catalytic active sites, as well as further catalytic enhancement achieved with the heterojunction between carbon nitride and SnO₂. X-ray absorption fine structure analysis and HAADF-STEM analysis reveal a homogeneous atomic dispersion of Pt species between carbon nitride and SnO₂ nanograins. This Pt SAC system with the 1D nano-heterostructure support exhibits high sensitivity and selectivity toward detection of formaldehyde gas among state-of-the-art gas sensors. Further ex situ TEM analysis confirms excellent thermal stability and sinter resistance of the heterojunction-immobilized Pt single atoms.

Design and synthesis of a highly efficient heterogeneous MnCo2O4 oxide catalyst for alcohol oxidation: DFT insight into the synergistic effect between oxygen deficiencies and bimetal species

We report the design and synthesis of an oxygen vacancy-abundant spinel-structured MnCo₂O₄ oxide as a highly efficient catalyst for alcohol oxidation.

Supported cluster catalysts synthesized to be small, simple, selective, and stable

Molecular metal complexes on supports have drawn wide attention as catalysts offering new properties and opportunities for precise synthesis to make uniform catalytic species that can be understood in depth. Here we highlight advances in research with catalysts that are a step more complex than those incorporating single, isolated metal atoms on supports. These more complex catalysts consist of supported noble metal clusters and supported metal oxide clusters, and our emphasis is placed on some of the simplest and best-defined of these catalysts, made by precise synthesis, usually with organometallic precursors. Characterization of these catalysts by spectroscopic, microscopic, and theoretical methods is leading to rapid progress in fundamental understanding of catalyst structure and function, and to expansion of this class of materials. The simplest supported metal clusters incorporate two metal atoms each-they are pair-site catalysts. These and clusters containing several metal atoms have reactivities determined by the metal nuclearity, the ligands on the metal, and the supports, which themselves are ligands. Metal oxide clusters are also included in the discussion presented here, with Zr6O8 clusters that are nodes in metal-organic frameworks being among those that are understood the best. The surface and catalytic chemistries of these metal oxide clusters are distinct from those of bulk zirconia. A challenge in using any supported cluster catalysts is associated with their possible sintering, and recent research shows how metal nanoparticles can be encapsulated in sheaths with well-defined porous structures-zeolites-that make them highly resistant to sintering.