NO decomposition on mordenite-supported Pd and Cu catalysts

In this work palladium and copper catalysts supported on mordenite were prepared by ionic exchange, characterized by X-ray diffraction (XRD), porosimetry (BET surface areas), temperature programmed reduction (TPR), H₂ and CO chemisorption, induced coupled plasma optical emission spectroscopy (ICP-OES) and tested in the direct decomposition of NO. The results showed that the monometallic catalysts maintained their structure and surface area. This did not happen with the bimetallic catalysts that had their surface area decreased, probably due to blockage of the zeolite pores. All the catalysts were active in the NO decomposition reactions and more selective to N₂ and O₂ than catalysts based on alumina reported in a previous work.     

Nitrate catalytic reduction in water using niobia supported palladium–copper catalysts

Niobia and alumina supported palladium catalyst promoted by copper were investigated in the reaction of nitrate catalytic reduction in water and characterized by temperature programmed reduction, physisorption, H₂ chemisorption and X-ray diffraction. Niobia supported Pd–Cu catalysts were as active and selective as an alumina supported catalyst. All catalysts had similar turnover frequencies independent of the support. The control of pH and the interaction between Pd and Cu were critical to improving the selectivity and activity of Pd–Cu/Nb₂O₅ catalysts.     

Enhanced activity of in and Ga-supported sol-gel alumina catalysts for NO reduction by hydrocarbons in lean conditions

Nitrogen monoxide was reduced efficiently by hydrocarbons in the presence of oxygen over sol-gel alumina supported indium, gallium, cobalt and tin catalysts. The support alumina prepared by a sol-gel method had high surface area and accordingly active alumina sites for the reaction. Particularly indium/alumina showed a high activity to reduce NO preferably by propene, propane and ethene but also by alcohols in the absence and the presence of water vapor. The activities of alumina supported cobalt, silver and tin catalysts were increased when calcinating the catalysts at 800°C instead of 600°C. In the case of gallium/alumina, NO₂ has higher reactivity than NO to nitrogen when propene was used as a reductant, proving the significance of the oxidation step of NO to NO₂. The step of NO oxidation was promoted by preparing a physical mixture of 5 wt% Mn₃O₄ with indium/alumina or gallium/alumina. The NO conversion to nitrogen was increased from 58 to 84% with the manganese oxide promotion over indium/alumina in the presence of water. The reaction mechanistic differences between the alumina supported catalysts and Cu/ZSM-5 were also discussed.     

Highly dispersed MgO-supported model Pd-Mo catalysts prepared from bimetallic clusters: Chemisorption and selective catalytic reduction of NO

Model supported palladium and palladium-molybdenum catalysts prepared from organometallic precursors and previously characterized by a variety of chemical and physical methods were examined by IR spectroscopy of NO chemisorption and tested for their activities as catalysts in the competitive NO + CO + O₂ reaction. The IR results reveal distinctive behavior of the catalyst made from a bimetallic precursor, and the activity results show that this catalyst is more selective for NO reduction than the other catalysts, but its stability is vulnerable to the reaction conditions. The high selectivity is attributed to Pd-Mo interactions, which are inferred to be stronger in the catalyst prepared from a bimetallic precursor than in catalysts prepared from monometallic precursors.     

Effect of support modification by carbon coverage in the dehydrogenation activity of Cu/Al2O3 catalyst

Gamma alumina and carbon covered gamma alumina supported Cu catalysts were prepared and characterized by adsorptive decomposition of N₂O, NH₃ chemisorption and temperature-programmed reduction. The decrease in interaction of copper species with alumina of carbon covered alumina has influence on the conversion and selectivity of the reaction of cyclohexanol to cyclohexanone.     

Influence of Pd Precursor and Method of Preparation on Hydrodechlorination Activity of Alumina Supported Palladium Catalysts

A series of alumina supported Pd catalysts were prepared by the novel deposition-precipitation method adopting the chloride precursor (DP-Cl) of Pd and varying the metal content from 0.25 to 1.0 wt%. The catalytic properties of prepared catalysts were studied by various characterization techniques such as N₂ adsorption, CO chemisorption, TPR, XRD, XPS, and TEM techniques. The activity and stability of the catalysts were evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene operating at atmospheric pressure. At 1 wt% of Pd the catalyst showed higher chlorobenzene conversion with good stability when tested for a period of 25 h, whereas the other catalysts exhibited a loss in activity with time. In order to elucidate the exceptional activity and stability of this catalyst, a few more catalysts with 1 wt% Pd were prepared by impregnation technique and also using a non-chloride precursor, palladium nitrate. The 1 wt% DP-Cl catalyst again was found to be the best among the others. The activity and stability of the DP-Cl catalyst was also found to be superior to two low-dispersed catalysts, each with 10 wt% Pd, prepared by conventional impregnation method using the chloride and nitrate as the precursors. The characterization results reveal that the high activity and stability of the DP-Cl catalyst is related to the formation of electron deficient Pd species and its stabilization in the octahedral vacancies of alumina.     

Hydrogenation of carbon dioxide over alumina supported Fe-K catalysts

The hydrogenation of CO₂ has been studied over Fe/alumina and Fe-K/alumina catalysts. The addition of potassium increases the chemisorption ability of CO₂ but decreases that of H₂. The catalytic activity test at high pressure (20 atm) reveals that remarkably high activity and selectivity toward light olefins and C₂₊ hydrocarbons can be achieved with Fe-K/alumina catalysts containing high concentration of K (K/Fe molar ratio = 0.5, 1.0). In the reaction at atmospheric pressure, the highly K-promoted catalysts give much higher CO formation rate than the unpromoted catalyst. It is deduced that the remarkable catalytic properties in the presence of K are attributable to the increase in the ability of CO₂ chemisorption and the enhanced activity for CO formation, which is the preceding step of C₂₊ hydrocarbon formation.     

负载型Ni催化剂上对硝基酚加氢合成对氨基酚的研究

研究了以硅藻土、锐钛型TiO2、Al2O3和高岭土为载体制备的负载型Ni催化剂催化对硝基酚加氢制备对氨基酚的活性。采用XRD与TPR技术表征了催化剂的结构与还原性能。负载型催化剂具有单一的对氨基酚选择性。NiO与载体硅藻土和锐钛型TiO2有弱的相互作用,制备的催化剂还原后有较高的催化活性。Al2O3和高岭土与NiO有较强的相互作用,抑制了还原后金属Ni的催化加氢活性。     

Effect of the fluorine-addition order on the hydrodesulfurization activity of fluorinated NiW/Al2O3 catalysts

Fluorinated NiW/Al₂O₃ catalysts with different orders of fluorine addition have been prepared, tested for hydrodesulfurization (HDS) of thiophene, and characterized using nitric oxide chemisorption and temperature-programmed sulfidation. The catalyst surface area has been affected by fluorine addition but not by the order of fluorination. The fluorine addition-order does not affect the amount of fluorine retained in the catalysts after the calcination and the reaction steps, either. On the other hand, the order of fluorine addition changes the dispersion of the nickel and the tungsten species, incorporation of nickel with the tungsten edge sites, and consequently the HDS activity of the catalysts. The catalyst fluorinated in the last step, i.e., after addition of both tungsten and nickel, shows the highest activity in thiophene HDS, which is supported by other experimental results indicating the most nitric oxide chemisorption and the largest incorporation of nickel with the tungsten species. Accordingly, enhancement of the catalyst activity by fluorination is due to the repartition of the metal species rather than to partial solubilization of alumina in the fluorine-addition step.     

Selective Catalytic Reduction of NO x Over Nano-Sized Gold Catalysts Supported on Alumina and Titania and Over Bimetallic Gold–Silver Catalysts Supported on Alumina

The reduction of NO with octane under lean conditions was examined over gold supported on alumina and titania and over alumina supported bimetallic gold–silver catalysts. The silver loading was either 1.2 or 1.9 wt% whereas 0.3, 1 or 5 wt% gold was used. The catalysts were characterized by means of EDXS, N₂-adsortion, UV–Vis and TEM to correlate recorded results with different preparation methods. UV–Vis measurements indicated that gold was present in the form of fine Au particles, single Au ions and small (Au)_n ^δ+ clusters on the catalysts and silver was mainly present in the form of single Ag ions. The highest NO to N₂ reduction activity was recorded over the 0.3Au–Al₂O₃ catalyst. The Au–TiO₂ catalysts did not result in significant NO to N₂ reduction.     

The effect of preparation variables on the dispersion of supported platinum catalysts

The effects of several preparation variables on the dispersion of supported platinum catalysts were examined in this study. Supported catalysts were prepared using two different platinum salts, four high surface area support materials, and two aqueous deposition techniques. The catalysts were characterised by hydrogen chemisorption, oxygen chemisorption, and the hydrogen titration reaction. In addition, X-ray diffraction measurements were conducted on all catalysts. Results from this study show that one of the platinum salts, chloroplatinic acid, always gave an equally or more highly dispersed catalyst than the other salt, tetraammine platinum (II) nitrate. The incipient wetness preparation technique produced equal or better dispersions than did the excess water method, with the possible exception of the silica-alumina support. The highest dispersions were attained with alumina, and the lowest with carbon. In some samples, a lack of good agreement existed between X-ray diffraction and chemisorption measurements of crystallite size which indicated broad or bimodal particle size distributions. However, these X-ray data were qualitatively helpful in understanding the unusual chemisorption behaviour of Pt/C catalysts, one of which had a dispersion higher than any previous reported Pt/C catalyst prepared by aqueous impregnation techniques. Experiments were also conducted which showed that either a 1 h or a 12 h treatment in hydrogen at 723 K was sufficient to reduce the platinum salt, and 1 h or 12 h evacuations at 698 K gave similar results. Finally, it was found that approximately one-half of the hydrogen monolayer on supported Pt could be removed by evacuating for 1 h at 300 K.     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

Dehydrogenation of Ethylbenzene in the Presence of CO2 over V Catalysts Supported on Nano-sized Alumina

Carbon-covered alumina (CCA) were synthesized from mesoporous alumina and a series of carbon sources (including sucrose, furfuryl alcohol, and benzene). They had structural properties of alumina and surface characteristics of carbon. When they were used as supports for molybdenum carbide, nitride, and phosphide catalysts, significantly higher activities were obtained in hydrazine decomposition as compared to those supported on the conventional alumina. The difference in the interactions of catalytic active sites with the CCA and with the alumina supports was preliminarily deemed to be the main cause of the better performance of CCA supported catalysts. Carbon contents on alumina and carbon sources were found to be important for CCA to be a good support. Carbon deposited on alumina in a near monolayer form showed the best activities. In contrast with sucrose and furfuryl alcohol, benzene as the carbon source readily yielded CCA supports with a hydrophobic surface, which resulted in relatively low dispersions of metal and, in turn, decreased activity of the supported catalysts.     

Activation of Ni/A12O3 catalysts through modification of the Al2O3 support

The effect of alumina pretreatment on the performance of alumina supported nickel catalysts was demonstrated in gas phase hydrogenation of toluene to methylcyclohexane. The state of the alumina was changed from pure to pure phase through various heat treatments in air. The catalysts were prepared from vapor phase by saturating the accessible binding sites on the pretreated alumina with the nickel precursor. The highest number of active sites for hydrogenation was observed for catalysts prepared on alumina having an incomplete phase transition and a / alumina phase ratio between 0.5 and 10. Results from temperature programmed desorption (TPD) studies revealed that a maximum in weakly chemisorbed hydrogen as well as in total amount of desorbed hydrogen was found for the same catalysts. By hydrogen chemisorption studies the total hydrogen uptake was found to correlate with the observed hydrogenation maximum. It is suggested that both the chemical and physical properties of the alumina influence the activity. An optimal metal-support interaction and structural defects on the alumina due to the phase transition can explain the observed maximum in the number of active sites and in hydrogen uptake.     

Porous carbon as support for iron and ruthenium catalysts

Iron and ruthenium catalysts have been supported on a porous carbon prepared by pyrolysis and activation of the copolymer Saran. For comparison, a graphitized carbon black (V3G) has also been used as support for both metals. The catalysts have been characterized by chemisorption of H₂ and CO₂ at 298 K (373 K in some cases) and by X-ray line broadening. The hydrogen chemisorption on iron catalysts was very low and increased with adsorption temperature, whereas the CO chemisorption results indicate the formation of subcarbonyl species. However, H₂ and CO uptakes led to similar dispersion values for the ruthenium catalysts. The X-ray results were in good agreement with the chemisorption results except in the case of highly dispersed Fe catalysts. The results obtained in the hydrogenation of CO indicate that in the case of Fe catalysts the highest selectivity toward hydrocarbons was given by the catalyst supported on V3G, with large metal particle size which, at the same time, exhibited a lower decrease in activity with reaction time than the other Fe catalysts with smaller average particle size. The olefin/paraffin ratio is very large for the catalyst prepared from Fe(CO)₅.The Ru catalysts are essentially of the methanation type.     

Support effect on the low-temperature hydrogenation of benzene over PtCo bimetallic and the corresponding monometallic catalysts

PtCo bimetallic and Co, Pt monometallic catalysts supported on γ-Al₂O₃, SiO₂, TiO₂ and activated carbon (AC) were prepared and evaluated for the hydrogenation of benzene at relatively low temperatures (343 K) and atmospheric pressure. Results from flow reactor studies showed that supports strongly affected the catalytic properties of different bimetallic catalysts. AC supported PtCo bimetallic catalysts exhibited significantly better performance than the other bimetallic catalysts, and all the bimetallic catalysts possessed higher activity than the corresponding monometallic catalysts. Results from CO chemisorption and H₂-temperature-programmed reduction (H₂-TPR) studies suggested that different catalysts possessed different properties in chemisorption capacity and reduction behavior, and AC supported PtCo catalysts possessed significantly higher CO chemisorption capacity compared to the other catalysts. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) analysis provided additional information regarding the formation of Pt–Co bimetallic bonds and metallic particle size distribution in the PtCo bimetallic catalysts on different supports.     

Stability of supported platinum catalysts in aqueous phase under hydrogen atmosphere

Two supported platinum catalysts (1.0 wt.% Pt/alumina (D=51%) and 1.0 wt.% Pt/silica (D=41%)) were treated at room temperature under nitrogen or hydrogen in aqueous media of various pH (pH=0.7, HCl; pH=6, water; pH=10, NH₄OH). Then, the catalysts were filtered, dried and activated following various procedures. The dispersions of the different samples were determined from transmission electron microscopy (TEM) and/or chemisorption measurements.

On preoxidized samples, a sintering of platinum is observed under hydrogen bubbling whatever the pH of the solution and it is higher on the silica support. An in situ reduction allows to prevent the metal accessibility loss on alumina and to restrict that observed on silica. Otherwise, the activation mode (direct reduction or calcination–reduction) can modify the platinum accessibility of the Pt/alumina catalyst treated in HCl medium.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

New insight in the solid state characteristics, in the possible intermediates and on the reactivity of Pd–Cu and Pd–Sn catalysts, used in denitratation of drinking water

Bimetallic palladium-based supported catalysts were tested in the liquid phase hydrogenation of nitrates. They were characterised by XPS, CO chemisorption, TPD–TPR and DRIFT. The effect of the preparation method, the support, the precursors, the relative amount of active metals and their role in the formation of intermediates and products are tentatively discussed. The catalytic activity and the formation of intermediate nitrite depend on the Pd–Cu ratio. Catalysts presenting a Pd/Cu atomic ratio >1 display the highest activity and the lowest intermediate nitrite than those presenting a Pd/Cu atomic ratio <1. Sol–gel method gives catalysts with a high activity and a low nitrite formation. The Pd–Cu-based catalyst supported on zirconia is more active and selective in N₂ compared to the corresponding Pd–Sn catalyst. An enrichment of the surface by Pd is responsible for a low intermediate nitrite formation and high selectivity in N₂. The reduction of NO is activated on Pd–Cu catalysts, contrary to Pd–Sn catalysts. Sn promotes the formation of ammonia.     

Adsorption and decomposition of NO on carbon and carbon-supported catalysts

The interactions of NO with carbon and carbon-supported catalysts have been investigated by means Fourier transform infrared spectroscopy. Nitric oxide direct decomposition over carbon-supported catalysts (Cu, Pt) was studied in a temperature ranging from 473 to 623 K. NO conversion increased with increasing reaction temperature in the whole temperature range. The carbon-supported Pt catalyst has a very high activity for the decomposition of NO in the absence of oxygen. As a result of NO chemisorption isocyanate (-NCO) species on the surface of carbon containing Cu were observed. When the reaction temperature was increased, the -NCO band at 2229 cm⁻¹ became more intense.