Evolution of the structure and chemical state of Pd nanoparticles during the in situ catalytic reduction of NO with H2

Nano-gold diggers: Au-assisted SiO(2)-decomposition and desorption in supported nanocatalysts

An investigation of the thermal stability of size-selected Au nanoparticles (NPs) synthesized via inverse micelle encapsulation and deposited on SiO2(4 nm)/Si(100) is presented. The size and mobility of individual Au NPs after annealing at elevated temperatures in ultrahigh vacuum (UHV) was monitored via atomic force microscopy (AFM). An enhanced thermal stability against coarsening and lack of NP mobility was observed up to 1343 K. In addition, a drastic decrease in the average NP height was detected with increasing annealing temperature, which was not accompanied by the sublimation of Au atoms/clusters in UHV. The apparent decrease in the Au NP height observed is assigned to their ability to dig vertical channels in the underlying SiO2 support. More specifically, a progressive reduction in the thickness of the SiO2 support underneath and in the immediate vicinity of the NPs was evidenced, leading to NPs partially sinking into the SiO2 substrate. The complete removal of silicon oxide in small patches was observed to take place around the Au NPs after annealing at 1343 K in UHV. These results reveal a Au-assisted oxygen desorption from the support via reverse oxygen spillover to the NPs.

Electrochemical post-treatment of infinite coordination polymers: an effective route to preparation of Pd nanoparticles supported onto carbon nanotubes with enhanced electrocatalytic activity toward ethanol oxidation

This study describes an effective method to prepare highly dispersed palladium nanoparticles supported onto single-walled carbon nanotubes (SWNTs) with high electrocatalytic activity toward the oxidation of ethanol. This method is essentially based on electrochemical post-treatment of Pd-based infinite coordination polymer (ICP). The Pd-based ICP is synthesized through the coordination reaction between Zn(2+) and metallo-Schiff base (MSB) to form Zn-MSB-Zn (ZMZ) ICP that precipitates from ethyl ether. The as-formed Zn-MSB-Zn ICP is then subjected to an ion-exchange reaction with Pd(2+) to obtain the Zn-MSB-Pd (ZMP) ICP. To prepare Pd/SWNT nanocomposite, the ZMP ICP is mixed into the SWNT dispersion in N-dimethylformamide (DMF) to form a homogeneous dispersion that is then drop-coated onto a glassy carbon (GC) electrode. Electrochemical post-treatment of ZMP ICP to form Pd/SWNT nanocomposite is thus performed by polarizing the coated electrode at -0.2 V for 600 s in 0.5 M H2SO4. The results obtained with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveal that the resulting Pd nanoparticles are highly dispersed onto SWNTs and the particles size are small and narrowly distributed (2.12 ± 0.32 nm). X-ray photoelectron spectroscopy (XPS) analysis shows that, after the electrochemical post-treatment, no detectable ZMP ICP precursors are left on the surface of SWNTs. The electrocatalytic activity of the as-formed Pd/SWNT nanocomposite toward ethanol oxidation is investigated by cyclic voltammetry and chronoamperometry. The results show that the Pd/SWNT nanocomposite prepared here shows a more negative potential and higher mass catalytic activity, as well as higher stability for the oxidation of ethanol than the commercial Pd/C catalyst. This work demonstrates a novel approach to the formation of ultrasmall and highly dispersed Pd/SWNT nanocomposite with enhanced electrocatalytic activity toward ethanol oxidation.

One-pot, water-based and high-yield synthesis of tetrahedral palladium nanocrystal decorated graphene

This paper reports a facile, water-based and one-pot synthesis of tetrahedral Pd nanocrystals (Pd-TNPs) with high yield and good size monodispersity supported on reduced graphene oxide (RGO) nanosheets via a co-chemical reduction method. The key synthetic strategy employed a positively charged polyallylamine-Pd(II) complex (PAH-Pd(II)) with un-coordinated amine groups as a linker molecule to immobilize Pd(II) species on the negatively charged graphene oxide (GO) surface through electrostatic interaction. As characterized by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) techniques, well-defined Pd-TNPs with an average size of 9 nm were uniformly distributed on the RGO surface. The as-prepared Pd-TNPs/RGO nanohybrid with excellent colloidal stability in aqueous solution exhibits superior catalytic activity towards the degradation of methylene blue (MB) compared to both unsupported Pd-TNPs and Pd black. Thus, the resultant Pd-TNPs/RGO nanohybrid, as a promising heterogeneous catalyst, might have wide potential applications in water-based catalysis systems for the future.

Metal nanoparticle catalysts beginning to shape-up

The field of heterogeneous catalysis has received a remarkable amount of interest from scientific and industrial perspectives because of its enormous impact on the world's economy: more than 90% of chemical manufacturing processes use catalysts. Catalysts are also essential in converting hazardous waste into less harmful products (car exhaust) and in generating power (fuel cells). Yet in all applications, it remains a challenge to design long lasting, highly active, selective, and environmentally friendly catalytic materials and processes, ideally based on Earth-abundant elements. In addition, the field needs more satisfactory experimental and theoretical approaches to minimize trial and error experiments in catalyst development. Nanocatalysis is one area that is developing rapidly. Researchers have reported striking novel catalytic properties, including greatly enhanced reactivities and selectivities, for nanocatalysts compared to their bulk counterparts. Fully harnessing the power of nanocatalysts requires detailed understanding of the origin of their enhanced performance at the atomic level, which in turn requires fundamental knowledge of the geometric and electronic structures of these complex systems. Numerous studies report on the properties that affect the catalytic performance of metal naoparticles (NPs) such as their size, interaction with their support, and their oxidation state. Much less research elucidates the role played by the NP shape. Complicating the analysis is that the preceding parameters are not independent, since NP size and support will affect which NP shapes are most stable. In addition, we must consider the dynamic nature of NP catalysts and their response to the environment, since the working state of a NP catalyst might not be the state in which the catalyst was prepared, but rather a structural and/or chemical isomer that responded to the particular reaction conditions. In order to address the complexity of real-world catalysts, researchers must undertake a synergistic approach, taking advantage of a variety of in situ and operando experimental methods. With the continuous shrinking of the scale of material systems, researchers require more sensitive experimental probes and computational approaches that work across a wide range of temperatures and chemical environments. This Account provides examples of recent advances in the preparation and characterization of NP catalysts with well-defined shapes. It discusses how to resolve the shape of nanometer-sized catalysts via a combination of microscopy and spectroscopic approaches, and how to follow their evolution in the course of a chemical reaction. Finally, it highlights that, for structure-sensitive reactions, controlled synthesis can tune catalytic properties such as the reaction rates, onset reaction temperature, activity, and selectivity.

In situ coarsening study of inverse micelle-prepared Pt nanoparticles supported on γ-Al2O3: pretreatment and environmental effects

The thermal stability of inverse micelle prepared Pt nanoparticles (NPs) supported on nanocrystalline γ-Al(2)O(3) was monitored in situ under different chemical environments (H(2), O(2), H(2)O) via extended X-ray absorption fine-structure spectroscopy (EXAFS) and ex situ via scanning transmission electron microscopy (STEM). Drastic differences in the stability of identically synthesized NP samples were observed upon exposure to two different pre-treatments. In particular, exposure to O(2) at 400 °C before high temperature annealing in H(2) (800 °C) was found to result in the stabilization of the inverse micelle prepared Pt NPs, reaching a maximum overall size after moderate coarsening of ∼1 nm. Interestingly, when an analogous sample was pre-treated in H(2) at ∼400 °C, a final size of ∼5 nm was reached at 800 °C. The beneficial role of oxygen in the stabilization of small Pt NPs was also observed in situ during annealing treatments in O(2) at 450 °C for several hours. In particular, while NPs of 0.5 ± 0.1 nm initial average size did not display any significant sintering (0.6 ± 0.2 nm final size), an analogous thermal treatment in hydrogen leads to NP coarsening (1.2 ± 0.3 nm). The same sample pre-dosed and annealed in an atmosphere containing water only displayed moderate sintering (0.8 ± 0.3 nm). Our data suggest that PtO(x) species, possibly modifying the NP/support interface, play a role in the stabilization of small Pt NPs. Our study reveals the enhanced thermal stability of inverse micelle prepared Pt NPs and the importance of the sample pre-treatment and annealing environment in the minimization of undesired sintering processes affecting the catalytic performance of nanosized particles.