Tuning the coordination environment of single-atom catalyst M-N-C towards selective hydrogenation of functionalized nitroarenes

Fine-tuning of the coordination environment of single-atom catalysts(SACs)is effective to optimize their catalytic performances,yet it remains challenging due to the vulnerability of SACs.Herein,we report a new approach to engineering the coordination environment of M-N-C(M=Fe,Co,and Ni)SACs by using glutamic acid as the N/C source and pyrolysis atmosphere as a regulator.Compared with that in N2,NH3 was able to promote the doping of N at 7<700℃yet etch the N-species at higher temperatures,by which the M-N coordination number(CN)and the electronic structure were delicately tuned.It was found that the electron density of Ni single atoms increased with the decrease of Ni-N CN.As a consequence,the capability of Ni-N-C to dissociate H2 was greatly enhanced and a higher catalytic activity in chemoselective hydrogenation of functionalized nitroarenes was achieved.Moreover,this modulation method could be applied to other transition metals including Fe and Co.In particular,the as-synthesized Co-N-C SAC afforded a turnover frequency of 152.3 h~1 with 99%selectivity to 3-vinylaniline in the hydrogenation of 3-nitrostyrene,which was the highest ever reported thus far and was at least one order of magnitude more active than state-of-the-art noble-metal-free M-N-C catalysts,demonstrating the great potential of engineering the coordination environment of SACs.     

One-step synthesis of carbon nanotubes–copper composites for fabricating catalyst supports of methanol electrooxidation

One-step synthesis of carbon nanotubes–copper composites was established by catalytic chemical vapor deposition (CCVD) of acetylene over Co–Cu–Al mixed metal oxides derived from layered double hydroxides (LDHs). Power X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Raman spectra, thermogravimetric and differential thermal analysis (TG-DTA) and N₂ adsorption–desorption measurements revealed that multi-walled carbon nanotubes were synthesized during cobalt-catalyzed CCVD, and copper nanoparticles were simultaneously in situ formed in CNTs matrix. Electrodes modified with platinum particles supported on as-fabricated CNTs–Cu composites showed much higher electrocatalytic activity for the oxidation of methanol than that modified with Pt particles supported on the commercial CNTs. The present study greatly enlarges the practical application of hybrid CNTs-based nanocomposites.     

Combination of supported bimetallic rhodium–molybdenum catalyst and cerium oxide for hydrogenation of amide

Hydrogenation of cyclohexanecarboxamide to aminomethylcyclohexane was conducted with silica-supported bimetallic catalysts composed of noble metal and group 6–7 elements. The combination of rhodium and molybdenum with molar ratio of 1:1 showed the highest activity. The effect of addition of various metal oxides was investigated on the catalysis of Rh–MoO_x/SiO₂, and the addition of CeO₂ much increased the activity and selectivity. Higher hydrogen pressure and higher reaction temperature in the tested range of 2–8 MPa and 393–433 K, respectively, were favorable in view of both activity and selectivity. The highest yield of aminomethylcyclohexane obtained over Rh–MoO_x/SiO₂ + CeO₂ was 63%. The effect of CeO₂ addition was highest when CeO₂ was not calcined, and CeO₂ calcined at >773 K showed a smaller effect. The use of CeO₂ as a support rather decreased the activity in comparison with Rh–MoO_x/SiO₂. The weakly-basic nature of CeO₂ additive can affect the surface structure of Rh–MoO_x/SiO₂, i.e. reducing the ratio of Mo–OH/Mo–O⁻ sites.     

Effects of various poisoning compounds on the activity and stereospecificity of heterogeneous Ziegler–Natta catalyst

Abstract

A TiCl₄/ethylbenzoate/MgCl₂ Ziegler–Natta catalyst was pretreated with chemically different poisoning compounds to investigate their effects on the catalyst activity and stereospecificity for propylene polymerization. The poisoning power on the activity was in the order of methanol > acetone > ethyl acetate. A kinetic analysis using the stopped-flow method revealed that addition of the poisoning materials decreased the activity through the reduction of the number of active sites, whereas the catalyst isospecificity was hardly affected by these materials.     

Effects of various poisoning compounds on the activity and stereospecificity of heterogeneous Ziegler–Natta catalyst

A TiCl₄/ethylbenzoate/MgCl₂ Ziegler–Natta catalyst was pretreated with chemically different poisoning compounds to investigate their effects on the catalyst activity and stereospecificity for propylene polymerization. The poisoning power on the activity was in the order of methanol > acetone > ethyl acetate. A kinetic analysis using the stopped-flow method revealed that addition of the poisoning materials decreased the activity through the reduction of the number of active sites, whereas the catalyst isospecificity was hardly affected by these materials.     

Growth of graphene sheets under an oxyacetylene flame without a catalyst北大核心CSCD

High quality graphene sheets that had a low I-D/I-G and a high 2D intensity in Raman spectra were prepared by a catalyst-free acetylene flame method. The sheets were grown vertically on the surface of carbon particles to form a petal-like morphology. A high temperature, high pressure and short residence time of the flame intensified the decomposition and cyclization reaction's of acetylene, leading to the formation of graphene sheets in the gas phase. The turbulent flame and the gases released during the formation of the graphene sheets from carbon nuclei could be responsible for their petal-like morphology instead of an onion-like structure.     

Nano-morphology of a polymer electrolyte fuel cell catalyst layer—imaging, reconstruction and analysis

The oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) of polymer electrolyte fuel cells (PEFC) is one of the major causes of performance loss during operation. In addition, the CCL is the most expensive component due to the use of a Pt catalyst. Apart from the ORR itself, the species transport to and from the reactive sites determines the performance of the PEFC. The effective transport properties of the species in the CCL depend on its nanostructure. Therefore a three-dimensional reconstruction of the CCL is required. A series of two-dimensional images was obtained from focused ion beam — scanning electron microscope (FIB-SEM) imaging and a segmentation method for the two-dimensional images has been developed. The pore size distribution (PSD) was calculated for the three-dimensional geometry. The influence of the alignment and the anisotropic pixel size on the PSD has been investigated. Pores were found in the range between 5 nm and 205 nm. Evaluation of the Knudsen number showed that gas transport in the CCL is governed by the transition flow regime. The liquid water transport can be described within continuum hydrodynamics by including suitable slip flow boundary conditions.      

Ni/γ-Al2O3 catalyst from kaolinite for the dry reforming of methane

A catalyst consisting of a 5% Ni over a γ-Al₂O₃ novel support was evaluated under high-temperature reaction conditions. The γ-Al₂O₃-rich phase was obtained by selective dissolution of siliceous components of heat-treated kaolinite. The support was appropriate to prepare a catalyst with high surface area (170 m²/g) that showed to be active and stable for dry reforming of methane at 650 °C and a CH₄/CO₂=0.5 molar ratio. Compared to the conventional Ni/α-Al₂O₃ catalyst, this new material showed an improved sulfur resistance when it was reduced in hydrogen steam at the reaction temperature. In industrial conditions, CH₄/CO₂=1, deactivation by carbon deposition was not increased. The content and type of deposited carbon were analyzed by TPH and TPO techniques. As regeneration methods of the coked catalysts, hydrogen and oxygen carbon treatments were employed. After the treatment with hydrogen the catalyst reaches the initial activity, but with oxygen the activity only is partially regenerated.     

Suzuki–Miyaura reaction and solventfree oxidation of benzyl alcohol by Pd/nitrogen-doped CNTs catalyst

Suzuki–Miyaura C–C coupling reactions were investigated with Pd/nitrogen-doped carbon nanotubes (Pd/N-CNTs) as a catalyst. Also, the same catalyst was examined for the solventfree oxidation of benzyl alcohol to benzaldehyde. Nitrogen-doped carbon nanotubes (N-CNTs) were synthesized from 1-ferrocenylmethyl(2-methylimidazole) and benzophenone via a chemical vapour deposition technique. Acetonitrile was used as a solvent and source of both carbon and nitrogen constituents of N-CNTs. Pd nanoparticles (Pd NPs) were successfully dispersed on N-CNTs via a metal organic chemical vapour deposition method. SEM, TEM, XRD, elemental analysis and ICP-OES measurements were used to characterize the nanomaterials. From the TEM analysis, it was observed that Pd NPs were spherical and with particle sizes ranging from 3 to 8 nm. For Suzuki C–C coupling reactions, phenylboronic acid, aryl halide, Pd/N-CNTs catalyst and a base (NaOAc, K₂PO₄, K₂CO₃, NaOH, Et₃N and Na₂CO₃) were used. The optimized experiments indicate that K₂CO₃, as the base, and ethanol/water (1:1 v/v, 10 mL) mixture, as a solvent, are the best reaction conditions. The solventfree oxidation reactions of benzyl alcohol were also done with Pd/N-CNTs catalyst and benzyl alcohol as a substrate. In both sets of reactions, C–C coupling and oxidation, the increase in pyrrolic nitrogen species was found to be responsible for higher catalytic activities of Pd/N-CNT catalysts, and this was attributed to the ease of Pd NP dispersion on N-CNTs, relative to pristine CNTs. Also, the higher catalytic activity of Pd/N-CNTs could be ascribed not only to the smaller Pd NP size or surface area, but to also the surface properties and the nature of the support when compared with the undoped counterpart, Pd/CNTs.     

Towards a ‘catalyst activity map’ regarding the nucleation and growth of single walled carbon nanotubes

Quantification using scanning electron microscopy (SEM) of single walled carbon nanotubes (SWNTs) grown per unit area using a Co-Fe (50:50) catalyst system, prepared by the incorporation of the appropriate metal salts into a Spin-On Glass substrate, at 900°C. The effects of substrate, as well as catalyst precursor concentration, were investigated. SWNT growth density is maximised with a catalyst precursor concentration of ≥2.5 mM, associated with the formation of catalyst nanoparticles of a critical size for SWNT nucleation. Samples were subjected to secondary growth, using a range of H₂:CH₄ ratios to determine the optimum precursor composition. It was found that nucleation and growth stages are optimal under different conditions. Optimum conditions for nucleation resulted in >10× increase in SWNT density. Optimisation is dependent on temperature and the partial pressure of reagent gas species.     

Synthesis,characterization and dielectric properties of polynorbornadiene–clay nanocomposites by ROMP using intercalated Ruthenium catalyst

Polynorbornadiene clay nanocomposites were prepared for the first time by the ring opening metathesis polymerization (ROMP) using modified montmorillonite and polynorbornadiene the latter of which is used commonly in electric–electronic industry. The Na–MMT clay was modified by a quaternary ammonium salt containing Ruthenium complex as a suitable catalyst and intercalant as well. The norbornadiene monomers were polymerized within the modified montmorillonite layers by in-situ polymerization method in different clay loading degrees. Intercalation ability of the Ru catalyst and partially exfoliated nanocomposite structure were proved by powder X-ray Diffraction (XRD) Spectroscopy and Transmission Electron Microscopy (TEM) methods. The nanocomposite materials with high thermal degradation temperature and low dielectric constant compared to the pure polynorbornadiene were obtained. The dielectric constants decreased with the increase of the clay content.     

Crystallization of γ-Fe2O3 particles with the aid of barium oxide as a catalyst and the effects on magnetic properties

-Fe₂O₃ particles with BaO additives (up to 20 mol%) have been crystallized by solid state reaction of the stoichiometric compositions containing 20 mol% B₂O₃ as a sintering aid. This markedly effects the crystallization and magnetic properties of -Fe₂O₃. The microstructure of the samples shows growth of crystallites of considerably smaller sizes and with fairly sharp size distribution after the additions. Crystallites as small as 5 m size (normally 25 m) were obtained using 15 to 20 mol% BaO additives in the reaction performed at 1230 °C/20h. This leads to a variation in coercivity over a wide range from 35 to 3500 Oe. Measurements of X-ray diffractometry, magnetization, microstructure and magnetic resonance have been carried out to characterize the magnetic applications of the material. The results are all consistent and elucidate promotion by thermal-treatment of the incorporation of Ba²⁺ into the -Fe₂O₃ particle cores.     

Preparation of the BaTiO3–SiO2 hybrid particles for the catalyst of methane steam reforming process

BaTiO₃ nano-coated SiO₂ (BaTiO₃–SiO₂) hybrid particles were prepared by liquid phase deposition and sol–gel process. The obtained BaTiO₃–SiO₂ hybrid particles have relatively high surface area (20 m² g⁻¹) at 600 °C annealing temperature. Ni component was impregnated to the obtained BaTiO₃–SiO₂ hybrid particles, and the obtained catalyst was used for the methane steam reforming process to consider the effect of the surface area on the catalytic activity. The catalytic activity of the Ni/BaTiO₃–SiO₂ catalyst was approximately three times as large as that of the reported Ni/BaTiO₃ catalyst, even in the lower process temperature. However, the limitation temperature for methane steam reforming process of this hybrid material was 600 °C, because of the diffusion of the Ba component.     

Enhanced photocatalytic degradation of tetrabromobisphenol A by tourmaline–TiO<Subscript>2</Subscript> composite catalyst

The TiO₂ supported on tourmaline composite catalyst was successfully prepared by a sol–gel method and was characterized by scanning electron microscopy, X-ray diffraction and nitrogen adsorption–desorption analysis. The results showed that the addition of tourmaline increased crystallite sizes of TiO₂. Furthermore, the photocatalytic activity of the tourmaline–TiO₂ composites obtained was evaluated by the degradation of tetrabromobisphenol A (TBBPA). Compared with bare TiO₂, the tourmaline–TiO₂ composites exhibited the excellent photocatalytic activity for the degradation of TBBPA and this degradation enhanced was correlated with the tourmaline dosages and the optimum photocatalyst is found to be T(20%)-TiO₂ catalyst. It implied that the higher dosage of tourmaline facilitated migration of charge carriers and higher photon-to-electron conversion efficiency. The photocatalytic reaction followed pseudo-first-order kinetics and the Langmuir–Hinshelwood model. The photodegradation efficiency of TBBPA enhanced with the increase in pH value. Additionally, metal ions influenced the photocatalytic degradation of TBBPA by T(20%)-TiO₂ catalyst, Cu²⁺ could accelerate TBBPA degradation, whereas inhibited by Mn²⁺ and Fe³⁺. Importantly, five intermediate products of TBBPA at m/z 463, 355, 293, 255 and 137 were observed by HPLC–MS/MS. The possible photodegradation pathway of TBBPA by tourmaline–TiO₂ composite catalyst was proposed. Generally, the incorporation of mineral with spontaneous permanent poles to TiO₂ with photocatalytic activity offers a promising technology to the practical application of TiO₂ in contamination control.     

The interaction of oxygen with high loaded Ru/γ-Al2O3 catalyst

The interaction of oxygen with the Ru/γ-Al₂O₃ catalyst comprising metal particle with sizes of 1-16 nm, was examined over a temperature range 20-400 °C. The catalyst loaded with 10.8 wt.% Ru was prepared by incipient wetness from RuCl₃ precursor. The structure of the Cl-containing catalyst and the catalyst after elimination Cl⁻ ions was characterized using H₂ and O₂ chemisorption, O₂ uptake, BET, XRD and TEM. The Cl⁻ ions in the catalyst decreased the H₂ and O₂ chemisorption capacity of Ru and caused large discrepancies between the mean particle size calculated from gas chemisorption and from TEM. Exposure to O₂ at 100-200 °C caused oxidation of small Ru particles, while larger particles were covered with very thin Ru_xO_y skin (undetected by XRD and TEM). The O/Ru ratio increased up to 200 °C implying high affinity of the small Ru particles to oxygen. Oxidation at 250 °C led to the formation of poorly crystalline RuO₂ particles with a mean size of 4 nm, and coverage of large Ru particles with 1.6 nm thick oxide layer. At 300 and 400 °C crystallization of the RuO₂ phase, as well as significant agglomeration of oxide particles was observed. However, even at 400 °C, metallic Ru was detected by XRD, TEM and SAED suggesting that large metal particles were not fully oxidized under the used conditions. Also, the O₂ uptake at 400 °C was lower than expected for oxidation of Ru metal to RuO₂. For the catalyst after elimination Cl ions the O₂ uptake (O/Ru ratio = 1.50) was higher, than for sample with large amount of Cl ions (O/Ru ratio = 1.34), indicating that the presence of Cl inhibits ruthenium oxidation in the Ru/Al₂O₃ catalyst.     

Magnesia supported Au and Ag catalysts for the preparation of few-layer graphene–metal nanocomposites: relationship between catalyst structure and the properties of graphene composites

Magnesia supported Au, Ag, and Au–Ag nanostructured catalysts were prepared, characterized, and used to synthesize few-layer graphene–metal nanoparticle (Gr–MeNP) composites. The catalysts have a mezoporous structure and a mixture of MgO and MgO·H₂O as support. The gold nanoparticles (AuNPs) are uniformly dispersed on the surface of the Au/MgO catalysts, and have a uniform round shape with a medium size of ~8 nm. On the other hand, the silver nanoparticles (AgNPs) present on the Ag/MgO catalyst have an irregular shape, larger diameters, and less uniform dispersion. The Au–Ag/MgO catalyst contains large Au–Ag bimetallic particles of ~20–30 nm surrounded by small (5 nm) AuNPs. Following the RF-CCVD process and the dissolution of the magnesia support, relative large, few-layer, wrinkled graphene sheets decorated with metal nanoparticles (MeNPs) are observed. Graphene–gold (Gr–Au) and graphene–silver (Gr–Ag) composites had 4–7 graphitic layers with a relatively large area and similar crystallinity for samples prepared in similar experimental conditions. Graphene–gold–silver composites (Gr–Au–Ag) presented graphitic rectangles with round, bent edges, higher crystallinity, and a higher number of layers (8–14). The MeNPs are encased in the graphitic layers of all the different samples. Their size, shape, and distribution depend on the nature of the catalyst. The AuNPs were uniformly distributed, had a size of about 15 nm, and a round shape similar to those from Au/MgO catalyst. In Gr–Ag, the AgNPs have a round shape, very different from that of the Ag/MgO catalyst, large size distribution and are not uniformly distributed on the surface. Agglomerations of AgNPs together with large areas of pristine few-layer graphene were observed. In Gr–Au–Ag composites, almost exclusively large bimetallic particles of about 25–30 nm, situated at the edge of graphene rectangles have been found.     

Conversion of plastic waste into CNTs using Ni/Mo/MgO catalyst—An optimization approach by mixture experiment

A combustion technique is used to study the synthesis of carbon nano tubes from waste plastic as a precursor and Ni/Mo/MgO as a catalyst. The catalytic activity of three components Ni, Mo, MgO is measured in terms of amount of carbon product obtained. Different proportions of metal ions are optimized using mixture experiment in Design expert software. D-optimal design technique is adopted due to nonsimplex region and presence of constraints in the mixture experiment. The activity of the components is observed to be interdependent and the component Ni is found to be more effective. The catalyst containing Ni_0.8Mo_0.1MgO_0.1 yields more carbon product. The structure of catalyst and CNTs are studied by using SEM, XRD, and Raman spectroscopy. SEM analysis shows the formation of longer CNTs with average diameter of 40–50 nm.