Towards stable catalysts by controlling collective properties of supported metal nanoparticles

Mass Spectrometry of Nanoparticles is Different

Secondary ion mass spectrometry, SIMS, is a method of choice for the characterization of nanoparticles, NPs. For NPs with large surface-to-volume ratios, heterogeneity is a concern. Assays should thus be on individual nano-objects rather than an ensemble of NPs; however, this may be difficult or impossible. This limitation can be side-stepped by probing a large number of dispersed NPs one-by-one and recording the emission from each NP separately. A large collection of NPs will likely contain subsets of like-NPs. The experimental approach is to disperse the NPs and hit an individual NP with a single massive cluster (e.g., C-60, Au-400). At impact energies of ~1 keV/atom, they generate notable secondary ion (SI) emission. Examination of small NPs (≤20 nm in diameter) shows that the SI emission is size-dependent and impacts are not all equivalent. Accurate identification of the type of impact is key for qualitative assays of core or outer shell composition. For quantitative assays, the concept of effective impacts is introduced. Selection of co-emitted ejecta combined with rejection (anticoincidence) of substrate ions allows refining chemical information within the projectile interaction volume. Last, to maximize the SI signal, small NPs (≤5 nm in diameter) can be examined in the transmission mode where the SI yields are enhanced ~10-fold over those in the (conventional) reflection direction. Future endeavors should focus on schemes acquiring SIs, electrons, and photons concurrently.

Monodisperse Aluminosilicate Spheres with Tunable Al/Si Ratio and Hierarchical Macro-Meso-Microporous Structure

While tremendous success has been seen in the development of ordered mesoporous silica by soft-templated methods, synthesis of hierarchical structures with controllable multiscale pore networks has remained a challenging topic. On the other hand, introduction of heteroatoms as an effective method of chemically functionalizing silica leads to difficulties in morphological control of the product, and multistep synthesis has been necessary for functionalized silica particles with hierarchical pore structure and uniform size. The present work demonstrates that the conflict between morphological control and heteroatom incorporation can be resolved in a CTAB-stabilized toluene-water-ethanol microemulsion system. For the first time, monodisperse macro-meso-microporous aluminosilicate spheres (MASS) are synthesized in one step at room temperature. Simultaneous tuning of Al/Si ratio (0-0.35) and the hierarchical pore structure is realized by Hofmeister anion effects of the Al source itself, [Al(OH)4](-), which change the geometry of CTAB micelles and giant vesicles. The Al is incorporated purely in a tetrahedrally coordinated status, and preliminary results from catalytic experiments show improved acidity of MASS as a catalyst support.

A facile strategy for enhancing FeCu bimetallic promotion for catalytic phenol oxidation

The mesoporous ZSM-5 zeolite obtained from alkaline treatment was found to be a superior support of bimetallic FeCu, minimizing the nanoparticle size, enhancing the bimetallic interaction, and promoting catalytic oxidation of phenol.

Silver modified carbon quantum dots for solvent-free selective oxidation of cyclohexane

The silver modified carbon quantum dots (Ag–CQDs) exhibit excellent catalytic ability for cyclohexane oxidation under visible light.

Pd nanoparticles partially embedded in the inner wall of nitrogen-doped carbon hollow spheres as nanoreactors for catalytic reduction of 4-nitrophenol

Pd@nitrogen-doped carbon nanoreactors (Pd@NC) were synthesized by partially embedding small Pd nanoparticles in the inner wall of the nitrogen-doped carbon shells. Such embedment is critical for improving catalytic activity and stability.

Mesoporous titanium dioxide coating on gold modified silica nanotubes: a tube-in-tube titanium nanostructure for visible-light photocatalysts

A novel tube-in-tube structured titanium dioxide (TiO₂) based visible-light photocatalyst with non-metal doping and plasmonic metal decoration was fabricated and characterized.

Fabrication of Cu–Ag bimetal nanotube-based copper silicates for enhancement of antibacterial activities

A novel Cu–Ag bimetal antibacterial system was developed, which showed higher antibacterial activities than single ones, and exhibited a synergistic effect.

Enhancement of the catalytic performance of a CNT supported Pt nanorod cluster catalyst by controlling their microstructure

Novel catalysts with Pt nanorod clusters distributed in both interior and exterior of CNTs were prepared and confirmed by TEM tomography. This structure benefits higher performance due to the CNTs' confinement effect.

Synthesis of dendrimer-templated Pt nanoparticles immobilized on mesoporous alumina for p-nitrophenol reduction

Dendrimer-templated mesoporous alumina-supported Pt nanocatalysts were prepared and used to catalyze reduction reaction after calcination at different temperatures in nitrogen.

Graphene-based catalysis for biomass conversion

This perspective summarizes the advances and challenges of graphene-based materials in the conversion of biomass into chemicals and biofuels.

An efficient route to prepare highly dispersed metallic copper nanoparticles on ordered mesoporous silica with outstanding activity for hydrogenation reactions

Highly dispersed copper nanoparticles synthesized on polyether-functionalized ordered mesoporous silica show outstanding activity for hydrogenation of cinnamaldehyde.

Triazole-stabilized gold and related noble metal nanoparticles for 4-nitrophenol reduction

The preparation of N-substituted triazole–polyethylene glycol-stabilized metal nanoparticles and their high catalytic activities for 4-nitrophenol reduction.

Direct imaging Au nanoparticle migration inside mesoporous silica channels

Supported metal nanoparticle (NP) catalysts have been widely used in many industry processes and catalytic reactions. Catalyst deactivation is mainly caused by the sintering of supported metal NPs. Hence, understanding the metal NPs' sintering behaviors has great significance in preventing catalyst deactivation. Here we report the metal particle migration inside/between mesochannels by scanning transmission electron microscopy and electron energy loss spectroscopy via an in situ TEM heating technique. A sintering process is proposed that particle migration predominates, driven by the difference of gravitational potential from the height of the uneven internal surface of the mesopores; when the distance of the gold nanoparticles with a size of about 3 and 5 nm becomes short after migration, the coalescence process is completed, which is driven by an "octopus-claw-like" expansion of a conduction electron cloud outside the Au NPs. The supports containing an abundance of micropores help to suppress particle migration and coalescence. Our findings provide the understanding toward the rational design of supported industrial catalysts and other nanocomposites with enhanced activity and stability for applications such as batteries, catalysis, drug delivery, gas sensors, and solar cells.

Oxidation of bioethanol using zeolite-encapsulated gold nanoparticles

With the ongoing developments in biomass conversion, the oxidation of bioethanol to acetaldehyde may become a favorable and green alternative to the preparation from ethylene. Here, a simple and effective method to encapsulate gold nanoparticles in zeolite silicalite-1 is reported and their high activity and selectivity for the catalytic gas-phase oxidation of ethanol are demonstrated. The zeolites are modified by a recrystallization process, which creates intraparticle voids and mesopores that facilitate the formation of small and disperse nanoparticles upon simple impregnation. The individual zeolite crystals comprise a broad range of mesopores and contain up to several hundred gold nanoparticles with a diameter of 2-3 nm that are distributed inside the zeolites rather than on the outer surface. The encapsulated nanoparticles have good stability and result in 50 % conversion of ethanol with 98 % selectivity toward acetaldehyde at 200 °C, which (under the given reaction conditions) corresponds to 606 mol acetaldehyde/mol Au hour(-1) .

A polyphenylene support for Pd catalysts with exceptional catalytic activity

We describe a solid polyphenylene support that serves as an excellent platform for metal-catalyzed reactions that are normally carried out under homogeneous conditions. The catalyst is synthesized by palladium-catalyzed Suzuki coupling which directly results in formation of palladium nanoparticles confined to a porous polyphenylene network. The composite solid is in turn highly active for further Suzuki coupling reactions, including non-activated substrates that are challenging even for molecular catalysts.

Nanoparticle growth in supported nickel catalysts during methanation reaction--larger is better

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO)4 ]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3-4 nm) were found to grow very large (20-200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO)4 ] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis.