Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

Thermally induced hydrogel formation in aqueous solutions of poly(N-isopropylacrylamide) and fluorocarbon-modified poly(oxyethylene)s

A number of perfluorooctyl-modified poly(oxyethylene)s (F_QPEOs) of molar masses of approximately 2000 and 4000 were synthesised using a two-step procedure: (a) preparation of PEO precursors bearing tertiary amino groups (NPEOs) and (b) their quaternisation with perfluorooctyl iodide to obtain one-ended (F_QE_n), telechelic (F_QE_nF_Q) or PEO modified with a C₈F₁₇-group in the mid of the polyether chain (E_nF_QE_n). F_Q is abbreviation of the C₈F₁₇-group attached to a PEO chain (E_n) through a quaternary ammonium group. The aqueous solution properties of F_QPEOs were studied by viscosity measurements at 25 °C. The cationic charge affects the association ability of the F_Q group and the one-ended F_QE_n behaves like PEO modified with a C₆F₁₃-group through urethane linkage. Temperature induced interactions in aqueous mixtures of F_QPEOs with poly(N-isopropylacrylamide) (PNIPAM) were studied by turbidity and rheological measurements. In the semi-dilute regime, the aqueous polymer mixtures exhibit a large viscosity enhancement upon heating as soon as the temperature reaches 35 °C. In the range from 35 to 40 °C the mixtures of PNIPAM with F_QE_n and E₅₀F_QE₅₀ form gels with viscoelastic response. The solution of PNIPAM mixed with the telechelic F_QE₁₀₀F_Q reveals shear-thickening at 35 and 40 °C.     

Development of vanadium-based mixed oxide catalysts for selective oxidation of H2S to sulfur

Various vanadium-based binary and multi-metallic oxides were prepared and their catalytic activities for the selective oxidation of H₂S to elemental sulfur were tested. Because the deactivation of vanadium-based catalysts originated from a relatively slow rate of reoxidation of the reduced vanadium oxide [PhD thesis, Pohang University of Science and Technology, 2000], the focus was given to increase the redox ability, especially in the reoxidation step. Stable and improved activity was observed in BiVO_x, TiVO_x, and ZrV₂O₇ at 250°C, but TiVO_x was the only catalyst that could maintain its activity below 250°C. Much higher activity was observed when VO_x/TiO₂ became multi-metallic by the incorporation of Fe, Cr, and Mo. TPR–TPO, microbalance, and XPS techniques were used to explain the redox properties of VO_x/SiO₂, VO_x/TiO₂, and V-Fe-Cr-Mo-O_x/TiO₂ catalysts in the reoxidation step.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

Photocatalytic performance of nanometer-sized FexOy/TiO2 particle synthesized by hydrothermal method

The nanometer particles of two Fe_xO_y/TiO₂’s with high photocatalytic activities were obtained through hydrothermal treatment and impregnation method. The XRD result did not show the peaks assigned to the Fe components (for example Fe₂O₃, Fe₃O₄, FeO₃, and Fe metal) on the external surface of the anatase structure in the Fe_xO_y/TiO₂ attained through hydrothermal treatment. This meant that Fe components were well incorporated into the TiO₂ anatase structure. In addition, it exhibited uniform anatase structure with particle size of below 50 nm. The FeO₃ component on the external surface of the TiO₂ anatase structure was identified in the Fe-loaded TiO₂ prepared through the impregnation method. In particular, the FT-IR spectroscopy revealed that the Fe_xO_y/TiO₂ particle attained through hydrothermal treatment had higher hydrophilic property compared to the other catalysts. Together with the Fe component, they absorbed wavelength of above 370 nm. The band slightly shifted to the right without tail broadness, which was the UV absorption of Fe oxide in the Fe_xO_y/TiO₂ particle attained through hydrothermal method. This meant that Fe components were well inserted into the framework of the TiO₂ anatase structure. Despite the red shift in UV-Vis absorption, however, CHCl₃ decomposition on the Fe_xO_y/TiO₂ catalyst was not largely enhanced compared to pure TiO₂.     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

Oxidation of carbon black over various Pt/MOx/SiC catalysts

Catalytic activities of various Pt/MO_x/SiC systems for carbon oxidation under simulated diesel exhaust gas were investigated in temperature-programmed reactions. When Pt/MO_x (MO_x=TiO₂, ZrO₂, Al₂O₃) was loaded onto silicon carbide (SiC), the oxidation activities became higher than those of Pt/MO_x alone or other Pt/MO_x/SiC systems (MO_x=Ta₂O₅, WO₃, Nb₂O₅, SnO₂, SiO₂, CeO₂, MoO₃, V₂O₅). Among them, Pt/TiO₂/SiC exhibited the highest activity. We discuss the activity of MO_x=TiO₂, ZrO₂, and Al₂O₃ in connection with NO oxidation activity, adsorption of sulfate onto the support, Pt dispersion, and specific surface area of the catalyst. Furthermore, we investigated the catalytic performance of Pt/TiO₂/SiC in more detail under isothermal conditions and in a staged arrangement.     

Electrical resistivity and thermopower of (La1−xSrx)MnO3 and (La1−xSrx)CoO3 at elevated temperatures

Electrical resistivity and Seebeck (S) measurements were performed on (La_1−xSr_x)MnO₃ (0.02x0.50) and (La_1−xSr_x)CoO₃ (0x0.15) in air up to 1073 K. (La_1−xSr_x)MnO₃ (x0.35) showed a metal-to-semiconductor transition; the transition temperature almost linearly increased from 250 to 390 K with increasing Sr content. The semiconductor phase above the transition temperature showed negative values of S. (La_1−xSr_x)CoO₃ (0x0.10) showed a semiconductor-to-metal transition at 500 K. Dominant carriers were holes for the samples of x0.02 above room temperature. LaCoO₃ showed large negative values of S below ca. 400 K, indicative of the electron conduction in the semiconductor phase.     

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La_0.5Sr_xCe_yFeO_z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO₃ and SrFeO_3−x and oxides -Fe₂O₃ and CeO₂ depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH₄+O₂ (“lean-NO_x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La³⁺ cation at A-site with divalent Sr⁺² and tetravalent Ce⁺⁴ cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La_0.5Sr_xCe_yFeO_z materials to reduce NO by CO or by CH₄ under “lean-NO_x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s⁻¹) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al₂O₃ catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La_0.5Sr_xCe_yFeO_z solids and their catalytic activity. O₂ TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O₂ TPD studies was observed for the La_0.5Sr_0.2Ce_0.3FeO_z and La_0.5Sr_0.5FeO_z solids.     

Total oxidation catalysts based on manganese or copper oxides and platinum or palladium I: Characterisation

Temperature-programmed reduction (TPR), oxidation (TPO), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterise catalysts based on manganese oxides, copper oxides or one of them mixed with platinum or palladium-supported on γ-alumina. The catalysts were characterised before and after they had been exposed either to high temperature in the presence of steam or to sulphur dioxide. Raman spectroscopy, XRD, XPS and TPR performed on the fresh samples of MnO_x, mixed MnO_x–Pt and MnO_x–Pd revealed the presence of a mixture of manganese oxides, particularly Mn₂O₃. In the fresh mixed MnO_x–Pd and CuO_x–Pd samples, Pd catalysed the reduction of both MnO_x and CuO_x, whereas Pt only catalysed the reduction of MnO_x. After hydrothermal treatment at 900°C of the MnO_x, mixed MnO_x–Pt and MnO_x–Pd samples, there was a formation of new manganese oxide phase, Mn₃O₄ detected by Raman spectroscopy. TPR revealed increasing interaction between the metal oxides and the noble metals in the hydrothermally treated mixed MnO_x–Pd and CuO_x–Pd samples, and also the appearance of interaction in the treated mixed CuO_x–Pt sample. The sulphur adsorbed in all the MnO_x samples formed sulphate, which was more difficult to reduce than the oxides. Also, the reduction temperature of sulphates was lowered when noble metals are present.     

The Nature of Soft Mode and Ferroelectric Phase Transition in SbOxS1-xI and SbSexS1-xI Mixed Crystals

The ferroelectric phase transition temperature has different dependence upon the mixture composition x in mixed SbO_xS_1-xI and SbSe_xS_1-xI crystals. From dependence of potential energy V(z) upon normal coordinates of the B_1u soft mode at a Sb site of SbO_xS_1-xI and SbSe_xS_1-xI crystals we proved that main reason for the growth of T_C in SbO_xS_1-xI crystals is that atomic formfactor _VI decrease with increasing x and decrease of T_C in SbSe_xS_1-xI crystals is caused by the increase of _VI with increasing x.     

Impact of redox conditions on thermal deactivation of NOx traps for diesel

Performance of NO_x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO_x trap, Pt–Ba/Al₂O₃. These were done at 950 °C for 3 h, in air and in 1% H₂/N₂, respectively. Lean aging had a severe impact on NO_x trap performance, including HC and CO oxidation, and NH₃ and N₂O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO_x trap and two model NO_x traps, Pt–Ba/Al₂O₃ and Pt–Ba–Ce/Al₂O₃. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO_x, HC, CO, NH₃ and N₂O activities for the ceria-containing NO_x traps, but had no impact on Pt–Ba/Al₂O₃. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO_x traps for light-duty diesel vehicles.     

On the promotional effect of Pd on the propene-assisted decomposition of NO on chlorinated Ce0.68Zr0.32O2

The selective catalytic reduction (SCR) of NO_x assisted by propene is investigated on Pd/Ce_0.68Zr_0.32O₂ catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO₂ reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C₃H₆. These catalysts display very stable deNO_x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N₂ selectivity obtained on Pd/CZ suggests that the lean deNO_x mechanism occurring on these catalysts is different from that occurring on Pd⁰/SiO₂. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO₂ and NO decomposition to N₂, whereas PdO_x activates C₃H₆ via ad-NO₂ species, intermediately producing R-NO_x compounds that further decompose to NO and C_xH_yO_z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C₃H₆-assisted NO decomposition mechanism stresses the key role of NO₂, R-NO_x and C_xH_yO_z as intermediates of the SCR of NO_x by hydrocarbons.     

Effect of carrier type on coupled transport kinetics of thiocyanate ions through liquid membranes

Coupled transport of thiocyanate ions through a liquid membrane containing quaternary ammonium chloride salts in chloroform was examined. The influences of the carrier type in the membrane phase on the coupled transport of thiocyanate ions were investigated. The kinetics of the coupled transport were analyzed in the formalism of two consecutive irreversible first-order reactions and the kinetic parameters (k_1d, k_2m, k_2a, R_m^max, t_max, J_d^max, J_amax) were also calculated. For the coupled transport of thiocyanate ions, quaternary ammonium chloride salts having different carbon atom numbers (C_n) used as carriers were found to increase thiocyanate ions transport efficiency with increasing carbon atom numbers. Tetradecyl trimethylammonium chloride (TDTMACl), hexadecyl trimethylammonium chloride (HDTMACl), and tetraoctyl ammonium chloride (TOACl) were found to be the most effective carrier types for transport of thiocyanate ions when C_n is greater than 16. The activation energies for maximum membrane entrance and exit fluxes were calculated as 32.45 kJ/mol and 34.75 kJ/mol, respectively. The values of the activation energy indicate that the process is controlled by species diffusion.     

CO oxidation over promoted Pt catalysts

A study of CO oxidation by O₂ over Pt catalysts, promoted by MnO_x and CoO_x, is described. The activities of Pt/SiO₂, Pt/MnO_x/SiO₂ and Pt/CoO_x/SiO₂ are compared with commercial Pt/Al₂O₃, Pt/Rh/Al₂O₃ and Pt/CeO_x/Al₂O₃ catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO_x/SiO₂ > Pt/MnO_x/SiO₂, Pt/CeO_x/Al₂O₃ > Pt/Al₂O₃, Pt/Rh/Al₂O₃, Pt/SiO₂. Over Pt/CoO_x/SiO₂ CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO_x/SiO₂ in the CO/O₂ reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO_x/SiO₂, Pt/MnO_x/SiO₂ or Pt/CeO_x/Al₂O₃ catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO_x/SiO₂ catalyst in CO oxidation.     

Alumina supported cobalt–palladium catalysts for the reduction of NO by methane in stationary sources

In order to improve a “Three Function Catalysts Model”, the present paper deals with alumina based catalysts containing cobalt and palladium for the NO reduction by methane.

The deNO_x temperature window was estimated by adsorption and subsequent desorption of NO in lean conditions. Two NO_x desorption peaks were detected for both catalysts. For Pd(0.63)Co(0.58)/Al₂O₃, the two desorption peaks appeared at 205 and 423 °C, whereas for Pd(0.14)Co(0.57)/Al₂O₃, the maxima desorption temperature peaks were at 205 and 487 °C. In addition, NO oxidation was also studied to evaluate the catalyst first function. It was found that, the oxidation begins on Co–Pd/Al₂O₃ around 250 °C. On Pd(0.63)Co(0.58)/Al₂O₃, 8% of deNO_x were found in the range of the second NO_x desorption peak temperature (410 °C). During TPSR, C_xH_yO_z species such as formaldehyde were detected. These oxygenate species are the reactive intermediate for deNO_x by methane.     

Catalytic dehydrogenation of ethylbenzene with carbon dioxide: promotional effect of antimony in supported vanadium–antimony oxide catalyst

Alumina-supported vanadium oxide, VO_x/Al₂O₃, and binary vanadium–antimony oxides, VSbO_x/Al₂O₃, have been tested in the ethylbenzene dehydrogenation with carbon dioxide and characterized by S_BET, X-ray diffraction, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction and CO₂ pulse methods. VSbO_x/Al₂O₃ exhibited enhanced catalytic activity and especially on-stream stability compared to VO_x/Al₂O₃ catalyst. Incorporation of antimony into VO_x/Al₂O₃ increased dispersion of active VO_x species, enhanced redox properties of the systems and formed a new mixed vanadium–antimony oxide phase in the most catalytically efficient V_0.43Sb_0.57O_x/Al₂O₃ system.     

Analysis of the thermodynamic feasibility of NOx decomposition catalysis to meet next generation vehicle NOx emissions standards

Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO_x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N₂ and O₂) conditions, the NO decomposition (NO↔(1/2)N₂+(1/2)O₂) and NO oxidation (NO+(1/2)O₂↔NO₂) equilibria impose lower bounds on the NO_x concentrations achievable by thermodynamic equilibration or NO_x decomposition, and these equilibrium NO_x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO_x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO_x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO_x values can fall below NO_x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO_x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO_x standards will require selective NO_x catalysts that vigorously promote NO_x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO_x catalyst system to reduce NO_x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.