Molecular layer-by-layer re-stacking of MoS2–In2Se3 by electrostatic means: assembly of a new layered photocatalyst

2D-layered transition metal chalcogenides are useful semiconductors for a wide range of opto-electronic applications. Their similarity as layered structures offers exciting possibility to modify their electronic properties by creating new heterojunction assemblies from layer-by-layer restacking of individual monolayer sheets, however, the lack of specific interaction between these layers could induce phase segregation. Here, we employed a chemical method using n-BuLi to exfoliate MoS2 and In2Se3 into their monolayer-containing colloids in solution. The bulky Se atoms can be selectively leached from In2Se3 during Li treatment which gives positively charged surface monolayers in neutral pH whereas the strong polarization of Mo–S with moderate S leaching gives a negatively charged surface. Specific interlayer electrostatic attraction during their selective assembly gives a controllable atomic AB-type of layer stacking as supported by EXAFS, STEM with super-EDX mapping, TAS/TRPL and DFT calculations. Using this simple but inexpensive bottom-up solution method, a new photocatalyst assembled from layers for photo water splitting can be tailor-made with high activity

Ultraselective nanocatalysts in fine chemical and pharmaceutical synthesis

Surface catalysed reactions play an important role in chemical productions. Developments of catalyst requiring high activity whilst improving on product selectivity can potentially have a profound effect in the chemical industry. Traditional catalyst modifications were focused on tuning the size, shape and foreign metal doping to form well defined metal nanoparticles of unique functionalities. Here, we show new approach to engineering of metal nanocatalysts via a subsurface approach can modify the chemisorption strength of adsorbates on the surface. Carbon modified nanoparticles were synthesised using glucose to stabilise Pd nanoparticles at a molecular level. Upon heat treatment, the carbonised glucose encapsulated the Pd nanoparticles with carbon atoms take residence in the octahedral holes (15 at.%). These materials were tested in liquid phase stereoselective hydrogenations of 3-hexyn-1-ol and 4-octyne. The former has importance in the fragrance industry towards the production of leaf fragrance alcohol. It was shown for the first time that the geometrically and electronically modified Pd with interstitial carbon atoms reduced the adsorption energy of alkenes, ultimately leading to higher reaction selectivity. Boron modified Pd nanoparticles was synthesised using BH3.THF in the liquid phase. The material possess high B interstitial saturation (20 at.%), which can be synthesised for the first time below 100°C. These materials were tested in the liquid phase selective hydrogenation of various alkynes and 2-chloronitrobenzene, of which the latter has importance in the pesticides industry. Kinetic modelling on the hydrogenation of 4-octyne suggests these subsurface occupied B does play a pivotal role on increasing the reaction selectivity, as removal of these species lead to decreased selectivity. Au nanoparticles were synthesised and characterised using H13COOH NMR. The new liquid NMR characterisation method is successfully applied to examine the chemisorption strength of metal nanoparticles. An attempt to synthesise PVP capped B modified Pd nanoparticles with the above NMR characterisation was investigated. It is believed the examples of subsurface atom modifications as shown here may offer future catalyst developments in this area.</p

Nitrogen-enriched carbonaceous materials with hierarchical micro-mesopore structures for efficient CO2 capture

Carboxyl-rich porous carbons (CPCs) derived from glucose are prepared by a hydrothermal method in the presence of acrylic acid and non-ionic surfactant Brij 72 as structure-directing agents. Tetraethylenepentamine molecules are then grafted onto the surfaces of CPCs via acylation-amidation route, yielding nitrogen-enriched porous carbons (NPCs). Material characterizations reveal that the CPCs display high surface areas with micro and mesoporous structures. Despite the surface functionalization of CPCs via acylation-amidation route the porous structures of NPCs are maintained. As a result, the NPCs exhibit excellent CO2 capture capacity (3.2 mmol g(-1)) with high selectivity of CO2 over N-2 (>46) at 298 K and 1 bar. In addition, good reversibility of CO2 adsorption can be achieved at different temperatures. (c) 2012 Published by Elsevier B.V.EPSRC; China Scholarships Counci

Morphology-Dependent Catalytic Activity of Ru/CeO₂ in Dry Reforming of Methane

Three morphology-controlled CeO2, namely nanorods (NRs), nanocubes (NCs), and nanopolyhedra (NPs), with different mainly exposed crystal facets of (110), (100), and (111), respectively, have been used as supports to prepare Ru (3 wt.%) nanoparticle-loaded catalysts. The catalysts were characterized by H2-temperature programmed reduction (H2-TPR), CO&#8315; temperature programmed desorption (CO-TPD), N2 adsorption&#8315;desorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (XDS). The characterization results showed that CeO2-NRs, CeO2-NCs, and CeO2-NPs mainly expose (110), (100) and (111) facets, respectively. Moreover, CeO2-NRs and CeO2-NCs present higher oxygen vacancy concentration than CeO2-NPs. In the CO2 reforming of methane reaction, Ru/CeO2-NR and Ru/CeO2-NC catalysts showed better catalytic performance than Ru/CeO2-NPs, indicating that the catalysts with high oxygen vacancy concentration are beneficial for promoting catalytic activity

Dramatic Effects of Gallium Promotion on Methanol Steam Reforming Cu–ZnO Catalyst for Hydrogen Production: Formation of 5 Å Copper Clusters from Cu–ZnGaO_<i>x</i>

A new class of copper, zinc, and gallium mixed oxides (CuZnGaO_<i>x</i>) with different chemical compositions obtained by a coprecipitation technique is identified as a highly active catalyst for the low-temperature, direct steam reforming of methanol to supply hydrogen gas to portable fuel cell devices. Their catalytic activity and selectivity are found to be critically dependent on the copper surface area, catalyst structure, and metal–support interaction, etc. As a result, temperature-programmed reduction has been used to investigate the copper ion reducibility and resulting copper speciation; N₂O chemisorption and advanced microscopies to determine specific copper surface area, dispersion, and particle size; XRD to investigate the catalyst structure; EPR spectroscopy to probe the environment of Cu²⁺ species; and AC impedance spectroscopy to probe the mobility of trapped ions in solids. It is proposed that Ga incorporation into Cu–Zn oxide leads to the formation of a nonstoichiometric cubic spinel phase containing interstitial Cu⁺ ions, which can produce in situ a high population of extremely small 5 Å copper clusters at high dispersion on a defective ZnGa₂O₄ surface for effective catalysis

Shape Effect of Pd-Promoted Ga₂O₃ Nanocatalysts for Methanol Synthesis by CO₂ Hydrogenation

In this paper, we present a new approach to investigate metal–support interaction in catalysis. First, we have carried out a controlled growth of two semiconductive Ga₂O₃ nanocrystals in distinctive shapes, namely, plate and rod with the majority of their surfaces covered with polar and nonpolar facets, respectively. We have then placed the same contents of Pd on these nanocrystals and carried out a systematic testing and characterization for methanol synthesis from CO₂ hydrogenation under industrial applicable conditions. It is found that a low indexed (002) polar Ga₂O₃ surface is highly unstable, which gives oxygen defects and mobile electrons in the conduction band more readily than those nonpolar (111) and (110) surfaces. A significantly strong metal–support interaction between the (002) polar Ga₂O₃ surface and Pd was determined, and it gave rise to higher metal dispersion and facilitated electron transfer between them, leading to the formation of PdGa_<i>x</i>. This renders such composite nanocatalysts active for methanol production

Atomic-Scale Determination of Cation and Magnetic Order in the Triple Perovskite Sr3Fe2ReO9

Pseudo-cubic (pc) perovskite oxides (ABO3) that can have different magnetic cations with different types and degrees of order at B sites have attracted considerable interest as a result of their tunable magnetic properties. Nanoscale inhomogeneity in cation order on the B sites can lead to different magnetic ground states and electronic band structures in local sample regions. Here, we determine cation order on the atomic scale in a nanosized Sr3Fe2ReO9 phase that has a 1:2 B-site-ordered triple perovskite structure using aberration-corrected analytical transmission electron microscopy (TEM), revealing that the Fe and Re cations form tripled-layered repeats with –[Fe–Fe–Re]n– sequences along [111]pc and an ordering vector of 1/3[111]*. To the best of our knowledge, this 1:2 B-site-ordered triple perovskite Sr3Fe2ReO9 phase has not been reported before. Based on a relaxed theoretical model that is consistent with the experimental images, density functional theory calculations are performed to determine the magnetic ground states and exchange parameters of the newly discovered Sr3Fe2ReO9 phase, in which nearest-neighbour Fe and Re cations are coupled antiferromagnetically. This combination of aberration-corrected analytical TEM and ab initio calculations provides physical insight into cation order and magnetic coupling in perovskite oxides at the atomic level.11Nsciescopu