Evaluation of bimetallic catalysts for the growth of carbon nanotube forests

We systematically study the growth of carbon nanotube forests by chemical vapor deposition using evaporated monometallic or bimetallic Ni, Co, or Fe films supported on alumina. Our results show two regimes of catalytic activity. When the total thickness of catalyst is larger than nominally 1nm, bimetallic catalysts tend to outperform the equivalent layers of a single metal, yielding taller forests of multi-walled carbon nanotubes (CNTs). In contrast, for layers thinner than ~1nm, bimetallic catalysts are notably less active than individually. However, the amount of small diameter and single-walled CNTs is significantly increased. This possible transition at ~1nm might be related to different catalyst composition after annealing, depending whether or not the films overlap during evaporation and alloy during catalyst formation. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Carbon nanotube forests growth using catalysts from atomic layer deposition

We have grown carbon nanotubes using Fe and Ni catalyst films deposited by atomic layer deposition. Both metals lead to catalytically active nanoparticles for growing vertically aligned nanotube forests or carbon fibres, depending on the growth conditions and whether the substrate is alumina or silica. The resulting nanotubes have narrow diameter and wall number distributions that are as narrow as those grown from sputtered catalysts. The state of the catalyst is studied by in-situ and ex-situ X-ray photoemission spectroscopy. We demonstrate multi-directional nanotube growth on a porous alumina foam coated with Fe prepared by atomic layer deposition. This deposition technique can be useful for nanotube applications in microelectronics, filter technology, and energy storage. © 2014 AIP Publishing LLC.

Facile route to fabricate large-scale silver microtubes

Large-scale uniform Ag microtubes with high length diameter ratios have been first successfully synthesized by a facile approach, using low-cost super fine glass fibers as templates. The samples were characterized by SEM and XRD. The investigations showed that calcining or adding of PEG-1000 and alcohol could greatly improve the mechanical strength of the sample. Especially the products exhibited favorable catalytic properties during the degradation of Rhodamine B by NaBH4. (c) 2007 Elsevier B.V. All rights reserved.

Pd/sulfated alumina - a new effective catalyst for the selective catalytic reduction of NO with CH4

Sulfated alumina (SA) is firstly found to be an effective support for Pd catalyst used in the SCR of NO with methane. The sulfation is important to increase support's acidity which is essential for the reduction of NO over Pd catalysts. On consideration of the lower cost and easier availability of SA, we believe that SA is more promising to act as the commercial support for Pd catalyst used in the SCR of NO with methane.

Investigation on the catalytic roles of silver species in the selective catalytic reduction of NOx with methane

Silver impregnated H-ZSM-5 zeolite catalysts with silver loading from 3 to 15 wt.% were investigated for the selective catalytic reduction (SCR) of NOx with CH4 in the excess of oxygen. X-ray diffraction (XRD) and UV-Vis measurements established the structure of silver catalysts. A relationship between the structure of silver catalysts and their catalytic functions for the SCR of NOx by CH4 was clarified. The NO conversion to N-2 showed a S-shape dependence on the increase of Ag loading. No linear dependence of catalytic activity on the amount of silver ions in the zeolite cation sites was observed. Contrastively, the activity was significantly enhanced by the nano-sized silver particles formed on the higher Ag loading samples (greater than or equal to7 wt.%). Temperature programmed desorption (TPD) and temperature programmed reduction (TPR) studies showed that nano-silver particles provided much stronger adsorption centers for active intermediates NO3-(s) on which adsorbed NO3-(s) could be effectively reduced by the activated methane. Silver ions in the zeolite cation sites might catalyze the reaction through activation of CH4 at lower temperatures. Activated CH4 reacted with NO3-(s) adsorbed on nano-silver particles to produce N-2 and CO2. (C) 2003 Elsevier B.V. All rights reserved.

Epoxidation of propylene over Ag-CuCl catalysts using air as the oxidant

Ag-CuCl catalysts were found to be active and selective for the epoxidation of propylene using air as the oxidant. Ag catalyst gives a propylene conversion of 31.6%, with a propylene oxide (PO) selectivity of 0.42% at a reaction temperature of 350 degreesC after 220 min of reaction. Addition of CuCl significantly improves the selectivity to PO, and suppresses the conversion of propylene. The Ag-CuCl (1/0.6) catalyst gives propylene conversion of about 3% and a PO selectivity of about 30% at a reaction temperature of 350 degreesC after 500 min of reaction. The activity of the Ag-CuCl catalyst increases with the reaction time and the selectivity to PO is very stable for this catalyst. It is found that AgCl and CuO phases formed during the catalyst preparation are beneficial to the epoxidation of propylene.

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo(2)N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo(2)N catalysts on three different surface sites: 4-fold vacancies, Mo(delta+) ( 0 < delta < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo(2)N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo(2)N phase shows lower electron density than pure Mo(2)N. On reduced passivated Mo(2)N catalysts the CO was found to adsorb mainly on Mo(4+) sites, although some slightly different surface Mo(delta+) d (0 < delta < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo(2)N/gAl(2)O(3) were similar to those on reduced MoO(3)/gamma-Al(2)O(3).

Promoting effects in hydrogenation and hydrodesulfurization reactions on the zirconia and titania supported catalysts

Tetralin hydrogenation (HYD) and thiophene hydrodesulfurization (HDS) were studied for the supported MoS2 and WS2 sulfides, either non-promoted or promoted with Co and Ni. The supports used were ZrO2, alumina-stabilized TiO2 and pure alumina. Preparation of catalysts included presulfidation of non-promoted system with subsequent addition of promoter and resulfidation. It has been found that the nature of promoter plays determining role for the catalytic performance. The most active in both HYD and HDS reactions are Ni-promoted Mo and W catalysts, supported on ZrO2. (C) 2003 Published by Elsevier B.V.

Monitoring catalytic degradation of dye molecules on silver-coated ZnO nanowire arrays by surface-enhanced Raman spectroscopy

Catalytic degradation of organic dye molecules has attracted extensive attention due to their high toxicity to water resources. In this paper, we propose a novel method for the fabrication of uniform silver-coated ZnO nanowire arrays. The degradation of typical dye molecule rhodamine 6G (R6G), as an example, is investigated in the presence of the as-prepared silver-coated ZnO nanowire arrays. The experimental results show that such composite nanostructures exhibit high catalytic activity, and the reaction follows pseudo-first-order kinetics. Furthermore, these nanowire arrays are desirable SERS substrates for monitoring the catalytic degradation of dye molecules. Compared with traditional UV-visible spectroscopy, SERS technology can reflect more truly the catalytic degradation process occurring on the surface of the catalysts.

One-step preparation and characterization of poly(propyleneimine) dendrimer-protected silver nanoclusters

Highly stable silver nanoclusters with narrow size distribution have been prepared by heating a third-generation poly(propyleneimine) dendrimer/AgNO3 aqueous solution without the additional step of introducing other reducing agents and protect agents. UV-vis absorption, transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS), and X-ray diffraction (XRD) have been used to characterize the resulting products. The as-obtained sample was in coexistence of Ag and Ag2O. It also suggested that increasing temperature resulted in both the decrease in number of small particles and the increase in size of large particles.

Catalytic properties of silica/silver nanocomposites

The catalytic properties of silver nanoparticles supported on silica and the relation between catalytic activity of silver particles and the support (silica) size are investigated in the present article. The silver nanoparticles with 4 nm diameters were synthesized and were attached to silica spheres with sizes of 40, 78, 105 nm, respectively. The reduction of Rhodamine 6G (R6G) by NaBH4 was designed by using the SiO2/Ag core-shell nanocomposites as catalysts. The experimental results demonstrated that the catalytic activity of silica/silver nanoparticles depends on not only the concentration of catalysts (silver) but also the support silica size. Silver particles supported on small SiO2 spheres (similar to 40 nm) show high catalytic activity. Moreover, by making a comparison between the UV-vis spectra of the catalyst before and after the catalytic reaction, we found that the position of surface plasma resonance (SPR) peak of Ag nanoparticles changes little. The above results suggested that the size and morphology of silver particles were probably kept unchanged after the reduction of R6G and also implied that the catalytic activity of silver particles was hardly lost during the catalytic reaction.

CHARACTERIZATION OF A POTASSIUM-PROMOTED COBALT MOLYBDENUM ALUMINA WATER-GAS SHIFT CATALYST

A series of potassium-promoted CoMo/Al2O3 has been investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). CoMoO4 was found in the CoMo/Al2O3 catalyst by XRD and is destroyed by the presence of potassium. The reducibility of molybdenum is enhanced by potassium in the CoMoK/Al2O3 catalyst and is easier to reduce to Mo(IV) during sulfidation. In the oxidic state catalyst cobalt is increased on the surface by the addition of potassium. After sulfidation this phenomena disappeared, the distribution of cobalt remains at a constant level and is unaffected by the potassium content. The addition of potassium leads to a monotonical decrease of the molybdenum dispersion with the impregnating amount of potassium in the oxidic state catalyst but is more complicated after sulfidation. Potassium is well dispersed on the surface in both the oxidic and sulfided state. The activity in the water-gas shift reaction was correlated with the potassium content of CoMoK/Al2O3.

On the selectively catalytic reduction of NOx with methane over Ag-ZSM-5 catalysts

The catalytic performance of silver-modified ZSM-5 catalysts in the selectively catalytic reduction (SCR) of NOx with methane was investigated. NO was selectively reduced by CH4 to N-2 in the presence of excess O-2, and the catalytic activity depended on both the activation of CH4 and the adsorption properties of NOx. Silver incorporated in ZSM-5 zeolite activated CH4 at low temperatures and lowered the "light-off" temperature for the CH4-SCR of NOx. Temperature-programmed (TP) spectroscopy studies depicted that surface nitrosyl species directly decomposed to N-2 in the absence of O-2. CH4 could not effectively reduce surface nitrosyl species, but might facilitate the direct decomposition of NO through the removal of surface oxygen. Surface nitrates were formed in NO and O-2 coexisting system and could be effectively reduced by CR4 to nitrogen. The priority of surface nitrates to O-2 in the reaction with CH4 clearly demonstrated that CH4 selectively and preferentially reduced the surface nitrate species to N-2 in the excess of oxygen. (C) 2002 Elsevier Science B.V. All rights reserved.

Templateless synthesis of catalysts with narrow mesoporous distribution

In this study, amorphous silica-alumina nanomaterials with narrow mesoporous distribution can be obtained by two novel sol-gel processes, without the use of any templates. The results of our experiments show that the preparation method has a great influence on the precursor sol structure as well as the specific surface area and mesopore volume of the final product, but has no effect on the pore size distribution.

The effect of chromium on sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation

Sulfur is a major poison to noble metal catalysts for deep aromatic hydrogenation in the petroleum refining industry. In order to study the sulfur resistance of Pd-based catalysts, a series of Pd, Cr, and PdCr catalysts supported on HY-Al2O3 were studied by NH3-TPD, pyridine-adsorption IR, TPR, IR spectra of adsorbed CO, and toluene hydrogenation in the presence of 3000 ppm sulfur as thiophene under the following conditions: 533-573 K, 4.2 MPa, and WHSV 4.0 h(-1). Cr has no influence on the acidity of the catalysts. TPR patterns and in situ IR spectra of adsorbed CO revealed a strong interaction between Cr and Pd, and the frequency shift of linear bonded CO on Pd indicates that the electron density of Pd decreases with the increase of the Cr/Pd atomic ratio. The catalytic performance of Pd, Cr, and PdCr catalysts shows that the sulfur resistance of Pd is strongly enhanced by Cr, and the activity reaches its maximum when the Cr/Pd atomic ratio equals 8. The active phase model "Pd particles decorated by Cr2O3" is postulated to explain the behavior of PdCr catalysts. (C) 2001 Academic Press.