Catalyst Preparation Conditions Research Articles

TiO2 photocatalytic oxidation: I. Photocatalysts for liquid-phase and gas-phase processes and the photocatalytic degradation of chemical warfare agent simulants in a liquid phase

The results of studies on the effect of the preparation procedure on the properties of TiO2-based photocatalysts and the kinetics and mechanism of the photocatalytic oxidation of organic water pollutants are surveyed. The effects of calcination temperature, surface modification with platinum, and acid-base treatment of the surface of titanium dioxide on its activity in model gas-phase and liquid-phase reactions are considered. Optimal catalyst preparation conditions were found in order to achieve maximum activity, and conceivable reasons for the effects of the above factors on the activity were revealed. The intermediate products and mechanisms of the photocatalytic and dark reactions of solutes that simulated chemical warfare agents in water are considered. All of the test simulants can undergo complete oxidation to form inorganic products in an aqueous TiO2 suspension under irradiation with UV light. It was found that, in addition to oxidation, the dark steps of hydrolysis also play an important role in the degradation of these substances. The low-frequency ultrasonic treatment (20 kHz) of a photocatalyst suspension in the course of the photocatalytic oxidation of dimethyl methylphosphonate can accelerate the reaction because of the facilitated transport of reactants to the surface of photocatalyst particles.

Activity of CoMo/γ-Al 2O 3 as a catalyst in hydrodesulfurization: effects of Co/Mo ratio and drying condition

A series of γ-alumina-supported cobalt–molybdenum hydrodesulfurization catalysts have been prepared with different ratios of cobalt to molybdenum (0.0–1.0) and under different drying temperatures (323, 373 and 423 K). The active components ions were loaded on the alumina via an incipient wetness impregnation method. The catalysts were converted to active sulfide form using dimethyldisulfide. Hydrodesulfurization (HDS) of dibenzothiophene (DBT) was tested at 550–683 K and 10 atm. Samples of the catalysts were characterized using SEM and XPS techniques. Changes in grain sizes and binding energy due to the cobalt–molybdenum interactions have been observed. The catalyst activities based on dibenzothiophene conversion rate constants and product (biphenyl BP and cyclohexylbenzene CHB) distributions, depended strongly on the Co/Mo ratios. CHB/BP ratio as high as 0.2 was obtained for Co/Mo of 0.2–0.4, while for higher Co/Mo ratios, no CHB was observed. The ratio of about 0.4 exhibited the highest rate constant. The drying temperature was found to improve the activities, with the lowest temperature showing the best performance in terms of dibenzothiophene degree of conversion. Such results suggest that cobalt and molybdenum are associated in a fixed proportion and that their distribution on the support surface, which in turn improves catalyst performance, could be modified by the catalyst preparation conditions.

Catalytic CVD synthesis of double and triple-walled carbon nanotubes by the control of the catalyst preparation

We report the influence of catalyst preparation conditions for the synthesis of carbon nanotubes (CNTs) by catalytic chemical vapour deposition (CCVD). Catalysts were prepared by the combustion route using either urea or citric acid as the fuel. We found that the milder combustion conditions obtained in the case of citric acid can either limit the formation of carbon nanofibres (defined as carbon structures not composed of perfectly co-axial walls or only partially tubular) or increase the selectivity of the CCVD synthesis towards CNTs with fewer walls, depending on the catalyst composition. It is thus for example possible in the same CCVD conditions to prepare (with a catalyst of identical chemical composition) either a sample containing more than 90% double- and triple-walled CNTs, or a sample containing almost 80% double-walled CNTs.

Polymerization rate modeling of ethylene polymerization with supported chromium oxide catalysts

AbstractSupported chromium oxide catalysts have long been used for the polymerization of ethylene to high‐density polyethylene. Unlike Ziegler–Natta catalysts, chromium oxide catalyst systems do not require cocatalysts to obtain high polymerization activity. One of the most distinctive characteristics of the chromium oxide catalyst is the presence of an induction period during the initial period of polymerization. The duration of induction period is dependent on many factors such as catalyst preparation, activation procedures, and polymerization conditions. After the induction period, the polymerization rate increases steadily with reaction time until it reaches a stationary value. Although chromium oxide catalysts have been used for years in industrial processes, there is still a dearth of literature concerning the kinetic modeling of chromium oxide catalyzed polymerization processes. In this article, a kinetic model is developed for the chromium oxide catalyzed ethylene polymerization in gas and slurry phases. In this model, it is proposed that the reaction byproduct generated by the reduction of hexavalent chromium species to Cr2+ by ethylene poisons the active sites. With the desorption of the poison species, the deactivated sites are transformed to active sites to polymerize ethylene. The validity of the proposed model is illustrated by comparing the model calculations with the experimental data obtained from gas phase and liquid slurry polymerization experiments. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2923–2927, 2004

Study of the Influence of the Preparation Method on the Formation of the Phase Composition and Structure of V–Ti–O and W–Ti–O Catalysts for Selective Catalytic Reduction of NO by NH3

The influence of the preparation method on the structure and phase composition of V–Ti–O and W–Ti–O catalysts for selective catalytic reduction of NO by NH3 was studied. The preparation conditions were found to insignificantly affect the structure of the resulting V–Ti–O catalysts, whereas in the case of W–Ti–O catalysts, such an influence was distinctly observed. The introduction of tungsten ions into the lattice of titanium dioxide leads to the formation of both local defects (solid solutions of the substitution type) and extended, so-called Wadsley's defects in the framework of TiO2. The concentrations of the defects of both types depend on the catalyst preparation conditions.

Nanostructured PEFC Electrode Catalysts Prepared via In-situ Colloidal Impregnation

ABSTRACTNanostructured Platinum-based electrode catalysts were prepared via in-situ colloidal impregnation for polymer electrolyte fuel cells. Crystallite size, grain size, and distribution of Pt nanoparticles on carbon support materials were characterized by XRD, TEM, high-resolution FESEM, and STEM. Effective surface area and kinetically-controlled current density of Pt electrode catalysts were analyzed by cyclic and hydrodynamic voltammetry using rotating disk electrodes. PEFCs with these electrode catalysts were also prepared and their I-V characteristics were examined at 80°C.We have succeeded to develop Pt electrode catalysts with a diameter of a few nm, supported on carbon nanofibers with different structures (including herringbone-type fibers, platelet-type fibers, and highly-conductive vapor-grown fibers), carbon nanotubes, as well as carbon black. The dependencies of nanostructure and electrochemical properties on crystallographic structure of carbon support materials and preparation conditions of electrode catalysts are analyzed and discussed. Nanostructural design of PEFC electrode catalyst layers using carbon nanofibers as catalyst supports and electrode fillers is also discussed.

Low temperature CO oxidation in hydrogen rich streams on Pt-SnO 2/Al 2O 3 catalyst using Taguchi method

Taguchi method of experimental design was used to optimize the catalyst preparation conditions of the sol–gel method to design a Pt-SnO 2/Al 2O 3 catalyst for the low temperature oxidation of carbon monoxide in hydrogen rich stream. HNO 3, H 2O, aluminum nitrate concentrations and the stirring rate in sol–gel process were optimized to obtain the maximum CO conversion using Taguchi method. It was found and experimentally verified that lower levels of HNO 3 and H 2O concentrations and higher levels of aluminum nitrate concentration and stirring rate maximized CO conversion without inversely affecting selectivity. The effects of reaction conditions such as temperature, time-on-stream, O 2 concentration, CO concentration and space velocity on CO conversion and selectivity were also investigated in a microreactor flow system using the optimum catalyst obtained. A 100% CO conversion was obtained with reasonable selectivity at a temperature of 100 °C, CO concentration of 0.5–1.1%, O 2/CO ratio of about 2 and space velocity of 24,000 cm 3/(g h).

Nitric Oxide Reduction by Carbon Monoxide over Supported Hexaruthenium Cluster Catalysts. 1. The Active Site Structure That Depends on Supporting Metal Oxide and Catalytic Reaction Conditions.

Ruthenium site structures supported on metal oxide surfaces were designed by reacting organometallic Ru cluster [Ru6C(CO)16](2-) or [Ru6(CO)18](2-) with various metal oxides, TiO2, Al2O3, MgO, and SiO2. The surface Ru site structure, formed under various catalyst preparation and reaction conditions, was investigated by the Ru K-edge extended X-ray absorption fine structure (EXAFS). Samples of [Ru6C(CO)16](2-)/TiO2(anatase) and [Ru6C(CO)16](2-)/TiO2(rutile) were found to retain the original Ru6C framework when heated in the presence of NO (2.0 kPa) or NO (2.0 kPa) + CO (2.0 kPa) at 423 K, i.e., catalytic reaction conditions for NO decomposition. At 523 K, the Ru-Ru bonds of the Ru6C framework were cleaved by the attack of NO. In contrast, the Ru site became spontaneously dispersed over TiO2 (anatase). When being supported over TiO2 (mesoporous), MgO, or Al2O3, the Ru6C framework split into fragments in gaseous NO or NO + CO even at 423 K. The Ru6 framework of [Ru6(CO)18](2-) was found to break easily into smaller ensembles in the presence of NO and/or CO at 423 K on support. Taking into consideration the realistic environments in which these catalysts will be used, we also examined the effect of water and oxygen. When water was introduced to the sample [Ru6C(CO)16](2-)/TiO2(anatase) at 423 K, it did not have any effects on the stabilized Ru6C framework structure. In the presence of oxygen gas, however, the Ru hexanuclear structure decomposed into isolated Ru cations bound to surface oxygen atoms of TiO2 (anatase).

Appropriate conditions or maximizing catalytic reduction efficiency of nitrate into nitrogen gas in groundwater

This study focused on the appropriate catalyst preparation and operating conditions for maximizing catalytic reduction efficiency of nitrate into nitrogen gas from groundwater. Batch experiments were conducted with prepared Pd and/or Cu catalysts with hydrogen gas supplied under specific operating conditions. It has been found that Pd–Cu combined catalysts prepared at a mass ratio of 4:1 can maximize the nitrate reduction into nitrogen gas. With an increase in the quantity of the catalysts, both nitrite intermediates and ammonia can be kept at a low level. It has also been found that the catalytic activity is mainly affected by the mass ratio of hydrogen gas to nitrate nitrogen, and hydrogen gas gauge pressure. Appropriate operating values of H 2/NO 3-N ratio, hydrogen gas gauge pressure, pH, and initial nitrate concentration have been determined to be 44.6 g H 2/g N, 0.15 atm, 5.2 (-), 100 mg L −1 for maximizing the catalytic reduction of nitrate from groundwater.

Activation of propane and butanes over niobium- and tantalum-based oxide catalysts

Niobium and tantalum are important elements for the activation of alkanes in the viewpoints of acidic property and the formation of unique mixed metal oxides. And the difference of the ability of alkane activation between niobium- and tantalum-based oxide catalysts is studied. Although hydrated niobium and tantalum oxides show strong acid property, only hydrated tantalum oxide is activated to a solid superacid by the treatment with sulfuric acid, and isomerizes n-butane to isobutane at room temperature. The sulfuric acid treated tantalum oxide activates P–Mo–V heteropolyacid compounds for the selective oxidation of isobutane to methacrolein (MAL) and methacrylic acid (MAA). The difference of ability of alkanes activation between niobium and tantalum is studied by using surface science technique. Mo–V–Nb–Te mixed metal oxide catalysts are active for the ammoxidation of propane to acrylonitrile (AN). However, Mo–V–Ta–Te mixed metal oxide is less active. The effect of catalyst preparation condition is studied. Mo–V–Nb–Te mixed metal oxide catalysts are also active for the oxidation of propane to acrylic acid (AA).

ZSM-5 acid zeolite supported metallocene catalysts for ethylene polymerization

Zeolite H-ZSM-5 was used as support for the impregnation of Cp 2ZrCl 2 catalyst in the synthesis of polyethylene. The catalyst preparation conditions were varied and the statistic program was employed with the purpose of evaluating the best catalyst preparation method. In order to utilize zeolite as support material it was necessary to perform a thermal treatment, which is composed by a pre-drying at 120 °C and afterwards a calcination at 300 °C under heating rate of 2 °C/min before treatment with methylaluminoxane (MAO) and bis(cyclopentadienyl) zirconium dichloride. It was observed that high amounts of zirconium fixed on the support surface did not lead to catalysts with higher activities, but in general the molecular weights of the obtained polyethylenes were very high. Furthermore, the supported catalyst was submitted to a longer MAO pre-contact before starting the polymerization by feeding the monomer, causing a sharp increase in the catalyst activity.

A study of catalyst formulations for isomerization of C 7 hydrocarbons

Zeolite and alumina based catalysts are screened for activity and selectivity in the isomerization of n-heptane. Pt, Ni, Co, Fe, Zn, La and combinations thereof are introduced to the catalysts via inorganic (chloride, nitrate) or organometallic (carbonyl, acetylacetonate) precursors. Pt/UY and Pt/VUY catalysts show the greatest activity and selectivity to isomers, with up to 94% selectivity to isomers at 72% conversion. Their isomerization activity and selectivity increase with Pt loading up to 1.5 and 1.0%, respectively. For Pt/faujasite catalysts that show good selectivity to isomers over cracked products, isomerization of n-C 7 to multibranched isomers occurs in consecutive steps. The ratio of monobranched to dibranched isomers is a unique function of conversion regardless of catalyst formulation or processing conditions. However, the selectivity to isomers over cracked products at a given conversion is a strong function of catalyst preparation and processing conditions. Less selective catalysts yield monobranched isomers, multibranched isomers, and cracked products in parallel. Pt/faujasite catalysts strongly resist inhibition by naphthenic and aromatic cyclic compounds, and are therefore suitable for the isomerization of a realistic naphtha cut. Methylcyclohexane (MCH) is isomerised without ring opening to an equilibrium mixture of C 7 naphthenes while toluene is hydrogenated and partially isomerised. Conversion of n-C 7 and mixed feeds yields a product stream containing branched isomers having much higher research octane rating.

Effect of fluorine addition on the formation of active species and hydrotreating activity of NiWS/Al 2O 3 catalysts

Fluorine-added Ni/Al 2O 3, W/Al 2O 3 and NiW/Al 2O 3 catalysts were prepared and their surface properties investigated using temperature-programmed reduction, diffuse reflectance spectroscopy and temperature-programmed sulfidation. The hydrodesulfurization (HDS) of thiophene was used as a model reaction to examine the activity of fluorinated NiW/Al 2O 3. Fluorination led to an increase in the formation of polymeric tungstates in W/Al 2O 3 and the surface nickel spinel species in the case of Ni/Al 2O 3. Fluorination promoted the sulfidation of the NiW/Al 2O 3 catalyst and the interaction of nickel with tungsten to produce the Ni–W–O species, a precursor of active catalytic sites. The activity of NiW/Al 2O 3 was increased by fluorination up to a fluorine content of 4 wt.%, and then decreased at higher fluorine contents. Two opposite effects of fluorination determine the characteristic changes in the activity of NiW/Al 2O 3: the enhanced sulfidation of the catalyst and incorporation of nickel with tungsten, both of which promote the activity, and a decrease in the dispersion of active species, which lowers the activity. The activity results obtained in this study are different from those reported previously [Appl. Catal. A 144 (1996) 343], suggesting that the extent of catalyst modification by fluorination is strongly dependent on the conditions of catalyst preparation.

Highly selective formation of aldehydes in the hydrogenation of the corresponding acid chlorides with silica-supported palladium catalysts prepared by a complexing agent-assisted sol–gel method

Synthesis of aldehydes from acid chlorides was carried out using four types of supported Pd/SiO 2 catalysts, which were prepared by a complexing agent-assisted sol–gel method and two types of impregnations with organic solvents or water, and a commercial Pd/BaSO 4 catalyst. The performance of the catalysts was much affected by the catalyst preparation and activation conditions. The sol–gel catalysts activated under milder conditions exhibited the highest performance for the formation of aldehydes. The high performance of the sol–gel catalysts was due to the organic residues derived from acetylacetone and ethylene glycol used in the sol–gel catalyst preparation process. The organic residues play a role similar to that of a poison added into the reaction solution for the traditional Rosenmund reduction, while being kept in the silica gel matrices.

Atomic Scale Observations of the Chemistry at the Metal–Oxide Interface in Heterogeneous Catalysts

Here we present a preliminary analysis of the atomic scale interface chemistry in a model heterogeneous catalyst system, Pt/SiO2. We show that the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) exhibits a sensitivity to surface oxidation and changes in the interfacial chemistry that is higher than that of conventional analytical methods such as energy-dispersive X-ray spectroscopy (EDS), chemisorption, and X-ray absorption near edge structure analysis (XANES). In particular, the presence of a few monolayers of platinum oxide can be clearly seen, and changes in the chemistry of the SiO2 support within ∼1 nm of the metal–oxide interface can be characterized as a function of the catalyst preparation conditions. These results demonstrate that this combination of novel techniques can provide unparalleled information that is potentially the key to understanding the activity and selectivity of heterogeneous catalyst systems.

Palladium-based supported liquid phase catalysts: influence of preparation variables on the activity and enhancement of the activity on recycling in the Heck reaction

Supported liquid phase catalysts (SLPC) were prepared using a porous silica, ethylene glycol, palladium acetate, and triphenylphosphine trisulfonate sodium salt (TPPTS). The SLPC samples prepared were used for Heck reaction of iodobenzene and butyl acrylate with triethylamine base in toluene and the influence of catalyst preparation conditions was examined. The rate of reaction depends on the concentration of palladium–TPPTS complexes in the dispersed phase of ethylene glycol but not on the quantity of the dispersed liquid used. The SLPC sample can be easily separated by simple filtration and it is recyclable; interestingly, the rate of reaction is promoted on the repeated runs, due to the formation and accumulation of Et 3NHI adduct in the dispersed phase.

Substrate–support interactions in metal-catalyzed carbon nanofiber growth

Catalyst–support interactions are critical in CVD processes for nanotube synthesis. In this article, the relative contributions of the catalyst electronic structure and support chemical composition are evaluated with Cu, Fe and Ni as catalysts and Al 2O 3, CaO, SiO 2 and TiO 2 as support media. The impact of the interaction is judged qualitatively based on nanotube growth and structure. Results are interpreted in terms of electron charge donation to the metal nanoparticle enabled by either strong-metal support interaction (SMSI) or by interaction of the catalyst nanoparticle with exposed Lewis base sites on the support material. The role of the physical structure of the support medium is explored by comparison of nanotubes grown upon powdered and fumed phases of the support oxides. Carbon nanotubes catalyzed by metal nanoparticles generated in-situ or preformed illustrate the advantage of presynthesized particles for size uniformity with attendant greatly lessened dependence upon catalyst preparation conditions. Catalyst retention and dispersion under rapid heating conditions is evaluated for the same support-catalyst systems listed above as a preliminary test for flame synthesis. Results show that SMSI interaction is critical to using the supported catalyst method in a flame.

Surface active structure of ultra-fine Cu/ZrO 2 catalysts used for the CO 2+H 2 to methanol reaction

Some copper/zirconia catalysts were prepared by way of deposition–precipitation (DP); for comparison, others were prepared by impregnation (IM) and co-precipitation (CP). Catalytic reactions were carried out in a fixed-bed minireactor with 0.25 ml catalyst. The results have proved that the zirconia aerogel prepared by supercritical fluid drying has higher specific surface area and larger pore volume; the catalyst composition and the catalyst preparation methods and conditions exert influence on the surface structure of catalysts. The DP CuO/ZrO 2 catalysts, especially that with CuO/ZrO 2 ratio of about 30/70 (wt.%) and calcined at 350°C have finer ultra-fine particles than the others, leading to higher catalytic activity and favoring the methanol synthesis.

Stopped-Flow Techniques in Ziegler Catalysis

The stopped-flow method, by which a quasi-living polymerization process can be realized within an extremely short period (ca. 0.2 s), has been proven to be one of the most powerful techniques for studies on the nature of the active sites and elucidation of the olefin polymerization mechanism based on the information from the polymers obtained in the initial stage of polymerization. It has been demonstrated that a better and deeper understanding of many controversial problems in Ziegler catalysis has been achieved, such as arguments concerning the non-uniformity of the active sites, the intrinsic kinetic parameters, the effects of catalyst preparation conditions, the role of cocatalysts, hydrogen and electron donor, etc. The successive modifications to the basic stopped-flow system have led to extensive applications in investigating the catalyst pretreatment effects induced by various reagents, as well as developing a series of novel block copolymers.

Supported Gold Catalysts Prepared from a Gold Phosphine Precursor and As-Precipitated Metal-Hydroxide Precursors: Effect of Preparation Conditions on the Catalytic Performance

Supported gold catalysts highly active for low-temperature CO oxidation were prepared by using AuPPh3NO3 complex as a Au-metal precursor and wet as-precipitated iron hydroxide and titanium hydroxide as support precursors. Effects of catalyst preparation conditions on the performance for low-temperature CO oxidation were studied. Metal-hydroxide precipitation conditions and heating rates of temperature-programmed calcination altered significantly the performance of the supported gold catalysts. To elucidate key factors responsible for the effects the catalysts were characterized by EXAFS, TEM, XRD, and N2 adsorption. Changes in the activities of both Au/Fe oxide and Au/Ti oxide catalysts originated mainly from changes of the Au particle size distribution. It was also found that states of the mesoporous support precursors upon attaching the Au precursor onto them and during the temperature-programmed calcination appeared to be of great importance in obtaining dispersed Au nanoparticles on oxide surfaces.