Structural Construction of Au-Pd Nanocomposite for Alkali-Free Oxidation of Benzyl Alcohol

A bimetallic Au-Pd system is an alternative candidate to catalyze primary alcohol oxidation and is of crucial importance for the sustainable chemical industry. However, understanding the bimetallic system in terms of the nanostructure is still challenging. Herein, we adopt the in situ colloid immobilization to obtain a series of bimetallic Au_xPd_y/CNT samples supported by carbon nanotubes (CNTs). Elaborate characterizations confirmed the bimetallic structure of AuPd alloy particles with randomly dispersed Pd²⁺ on the surface, forming the AuPd@PdO structure on CNTs. Unlike the monometallic samples, bimetallic ones, particularly Au₁Pd₁/CNT, efficiently transformed benzyl alcohol in an alkali-free mild condition. The DFT simulation confirmed the electron-rich gold atoms as a steric and electronic regulator to confine the electron-deficient Pd atoms in alloy particles. The interacted metal sites in the alloy system activated the alcohol with optimized adsorption configuration. Surface Pd²⁺ transported active oxygen to capture the abstracted H on alcohol. The collaboration between metal sites facilitated the transformation of benzyl alcohol to benzaldehyde with the selectivity of 91.8% by a fast TOF of 1274 h^-1 at only 80 °C.

Chemical kinetics and promoted Co-immobilization for efficient catalytic carbonylation of ethylene oxide into methyl 3-hydroxypropionate

The carbonylative transformation of ethylene oxide (EO) into methyl 3-hydroxypropionate (3-HPM) is a key process for the production of 1,3-propanediol (1,3-PDO), which is currently viewed as one of the most promising monomers and intermediates in polyester and pharmaceuticals industry. In this work, a homogeneous reaction system using commercial Co₂(CO)₈ was first studied for the carbonylation of EO to 3-HPM. The catalytic behavior was related to the electronic environment of N on aromatic rings of ligands, where N with rich electron density induced a stronger coordination with Co center and higher EO transformation. A reaction order of 2.1 with respect to EO and 0.3 with respect to CO was unraveled based on the kinetics study. The 3-HPM yield reached 91.2% at only 40°C by Co₂(CO)₈ coordinated with 3-hydroxypyridine. However, Co-containing colloid was formed during the reaction, causing the tough separation and impossible recycling of samples. Concerning the sustainable utilization, Co particles immobilized on pre-treated carbon nanotubes (Co/CNT-C) were designed via an in situ reduced colloid method. It is remarkable that unlike conventional Co/CNT, Co/CNT-C was highly selective toward the transformation of EO to 3-HPM with a specific rate of 52.2 mmol·gCo-1·h-1, displaying a similar atomic efficiency to that of coordinated Co₂(CO)₈. After reaction, the supported Co/CNT-C catalyst could be easily separated from the liquid reaction mixture, leading to a convenient cyclic utilization.

1D Colloidal chains: recent progress from formation to emergent properties and applications

Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.

Boosting the catalytic behavior and stability of a gold catalyst with structure regulated by ceria

In this work, a series of colloidal gold nanoparticles with controllable sizes were anchored on carbon nanotubes (CNT) for the aerobic oxidation of benzyl alcohol. The intrinsic influence of Au particles on the catalytic behavior was unraveled based on different nanoscale-gold systems. The Au/CNT-A sample with smaller Au sizes deserved a faster reaction rate, mainly resulting from the higher dispersion degree (23.5%) of Au with the available exposed sites contributed by small gold particles. However, monometallic Au/CNT samples lacked long-term stability. CeO2 was herein decorated to regulate the chemical and surface structure of the Au/CNT. An appropriate CeO2 content tuned the sizes and chemical states of Au by electron delivery with better metal dispersion. Small CeO2 crystals that were preferentially neighboring the Au particles facilitated the generation of Au-CeO2 interfaces, and benefited the continuous supplementation of oxygen species. The collaborative functions between the size effect and surface chemistry accounted for the higher benzaldehyde yield and sustainably stepped-up reaction rates by Au-Ce5/CNT with 5 wt% CeO2.

Excellent Catalytic Performances of a Au/C-CuO Binary System in the Selective Oxidation of Benzylamines to Imines under Atmospheric Oxygen

A green method of the oxidation of benzylamines to imines was developed using a novel binary system of Au/C-CuO. This system was evaluated under atmospheric oxygen, and the corresponding imines were obtained in up to 100% yields by loading 0.006 mol % of Au/C and 1.25 mol % of CuO under mild conditions. This system was also successfully applied to the syntheses of N-containing functional molecules, as well as that of imines on the scale of several grams. Furthermore, the turnover number of the system (more than 8000 times on a gold basis) as well as its ability to be reused more than 10 times for benzylamine oxidation demonstrates the excellent durability and recyclability of the developed system.

Nano-Sized NiO Immobilized on Au/CNT for Benzyl Alcohol Oxidation: Influences of Hybrid Structure and Interface

Tiny gold nanoparticles were successfully anchored on carbon nanotubes (CNT) with NiO decoration by a two-step synthesis. Characterizations suggested that Ni species in an oxidative state preferred to be highly dispersed on CNT. During the synthesis, in situ reduction by NaBH₄ and thermal treatment in oxidation atmosphere were consequently carried out, causing the formation of Au-Ni-O_x interfaces and bimetal hybrid structure depending on the Ni/Au atomic ratios. With an appropriate Ni/Au atomic ratio of 8:1, Ni atoms migrated into the sub-layers of Au particles and induced the lattice contraction of Au particles, whilst a higher Ni/Au atomic ratio led to the accumulation of NiO fractions surrounding Au particles. Both contributed to the well-defined Au-Ni-O_x interface and accelerated reaction rates. Nickel species acted as structure promoters with essential Au-Ni-O_x hybrid structure as well as the active oxygen supplier, accounting for the enhanced activity for benzyl alcohol oxidation. However, the over-layer of unsaturated gold sites easily occured under a high Ni/Au ratio, resulting in a lower reaction rate. With an Au/Ni atomic ratio of 8:1, the specific rate of AuNi₈/CNT reached 185 μmol/g/s at only 50 °C in O₂ at ordinary pressure.

Atomic-Scale Observation of Bimetallic Au-CuOx Nanoparticles and Their Interfaces for Activation of CO Molecules

Supported gold nanoparticles with sizes below 5 nm display attractive catalytic activities for heterogeneous reactions, particularly those promoted by secondary metal (e.g., Cu) because of the well-defined synergy between metal compositions. However, the specific atomic structure at interfaces is less interpreted systematically. In this work, various bimetallic Au-CuO_x catalysts with specific surface structures were synthesized and explored by aberration-corrected scanning transmission electron microscopy (AC-STEM), temperature-programmed experiments and in situ DRIFT experiments. Results suggest that the atomic structure and interfaces between gold and CuO_x are determined by the nucleation behaviors of the nanoparticles and result in subsequently the distinctive ability for CO activation. Bimetallic CuO*/Au sample formatted by gold particles surrounded with CuO_x nanoclusters have rough surface with prominently exposed low-coordinated Au step defects. Whereas the bimetallic Au@CuO sample formatted by copper precursor in the presence of gold nanoparticles have core-shell structure with relatively smooth surface. The former structure of CuO*/Au displays much accelerated properties for CO adsorption and activation with 90% CO converted to CO₂ at 90 °C and nice stability with time on stream. The results clearly determine from atomic scale the significance of exposed gold step sites and intrinsic formation of defected surface by different nucleation. The above properties are directly responsible for the induced variation in chemical composition and the catalytic activity.

Defective graphene@diamond hybrid nanocarbon material as an effective and stable metal-free catalyst for acetylene hydrochlorination

The nanodiamond–graphene hybrid material (ND@G) exhibits superior catalytic activity comparable to Au/C catalysts due to abundant surface defects.

Regulation of Ni-CNT Interaction on Mn-Promoted Nickel Nanocatalysts Supported on Oxygenated CNTs for CO2 Selective Hydrogenation

Mn-promoted Ni nanoparticles (NPs) supported on oxygen-functionalized carbon nanotubes (CNTs) were synthesized for CO₂ hydrogenation to methane. This novel metal-carbon catalytic system was characterized by both experimental and computational studies. An anomalous metal-support interaction mode (i.e., a higher temperature would lead to a weakened Ni-CNT interaction) was observed. Deep investigation confirmed that surface oxygen groups (SOGs) on CNTs played a key role in tuning the Ni-CNT interaction. We proposed that high calcination temperature would firstly lead to the decomposition of SOGs (>400 °C), then causing a loss of anchoring sites and the anchoring effect of SOGs on Ni NPs, thus cutting off the connection between interfacial Ni atoms and CNT body, resulting in the migration and coalescence of fine flat Ni NPs into larger sphere ones at 550 °C (geometric effect). Density functional theory calculation study clarified that this kind of anchoring effect stemmed from the formation of covalent bonding between the interfacial Ni atom and C or O elements of SOGs, causing the electrons to be transferred from Ni atoms to CNT support because of the intrinsic electronegativity of -COOH (electronic effect). Besides, Mn promotion notably boosts the activity compared with unpromoted catalysts, which was irrelevant to the size effect, but enhanced CO₂ adsorption and conversion according to the result of CO₂-temperature programmed desorption and transient response experiment. The optimized NiMn350 catalyst endowed with Mn promotion and robust Ni-CNT interaction showed both high activity and sintering resistance for more than 140 h. Our findings paved the way to reasonably design the metal-carbon catalyst with both high activity and stability.

Probing the enhanced catalytic activity of carbon nanotube supported Ni-LaOx hybrids for the CO2 reduction reaction

Oxygenated functionalized carbon nanotube (oCNT) supported LaO_x-promoted Ni nanoparticles (10Ni-xLa/oCNT) were prepared by the co-impregnation method and tested for synthetic natural gas from the CO₂ reduction reaction. Several advanced characterization methods, including atomic resolution scanning transmission electron microscopy (STEM), temperature programmed experiments (TPSR, CO₂-TPD, and H₂-TPR) and X-ray photoelectron spectroscopy (XPS), were applied to explore, for the first time, the origin of structure modulation of LaO_x species on oCNT supported Ni-LaO_x hybrids and the structure-activity relationship over the CO₂ reduction reaction. The Z-contrast STEM-HAADF results revealed that the LaO_x species are mostly in the size of the sub-nano scale and highly dispersed on the surface of Ni nanoparticles and oCNT, and consequently no diffraction peak of LaO_x was observed from XRD results. TEM analysis showed that the Ni nanoparticle sizes were similar among all samples either after reduction or after reaction due to the relatively strong interaction between Ni and oxygenated groups on CNT supports, regardless of the influence of the La mass loading. It was suggested that the catalytic performance trend was due to the structural variation rather than the size effect. The LaO_x modulation catalyst with 2 wt% of La metal loading not only presented low CO₂ activation temperature at only 163 °C, but also resulted in extremely high CH₄ selectivity (100%) compared with the initial supported Ni catalyst (52.7% of CH₄ selectivity at 300 °C).