Comparative study of Nickel-based perovskite-like mixed oxide catalysts for direct decomposition of NO

The mixed oxides LaNiO3, La0.1Sr0.9NiO3, La2NiO4 and LaSrNiO4 were prepared and used as catalysts for the direct decomposition of NO. The catalysts were characterized by means of XRD, XPS, O-2-TPD, NO-TPD and chemical analysis. By comparing the physico-chemical properties and catalytic activity for NO decomposition, a conclusion could be drawn as follows. The direct decomposition of NO over perovskite and related mixed oxide catalysts follows a redox mechanism. The lower valent metal ions Ni2+ and disordered oxygen vacancies seem to be the active sites in the redox process. The oxygen vacancy plays an important role favorable for the adsorption and activation of NO molecules on one hand and on the other hand for increasing the mobility of lattice oxygen which is beneficial to the reproduction of active sites. The presence of oxygen vacancies is one of the indispensable factors to give the mixed oxides a steady activity for NO decomposition.

Relationship between the structure and catalytic activity of La2-xSrxNiO4-lambda(0<=x<=1) mixed oxides for NH3 oxidation

A series of samples having the composition of La2-xSrxNiO4(0 less than or equal to x less than or equal to 1) were prepared and used as catalysts for NH3 oxidation. It was found that the La and oxygen vacancies exist in the La2-xSrxNiO4-lambda(0 less than or equal to x less than or equal to 1). The unit cell volume decreases with the increase of x. For bath c and a parameters there appeared a turning point at x = 0.5. Doping with a lower valence cation Sr2+ in the case of La2NiO4 resulted in an increase of Ni3+, consequently the formation of oxygen vacancies, the increase of reducing ability and the increase of catalytic activity. In the oxygen TPD of La2-xSrxNiO4(0 less than or equal to x less than or equal to 1) appeared three peaks, the alpha' peak at about 400K was attributed to the surplus oxygen desorption, the a peak at 700K which approaches to a maxium at x = 0.6 was attributed to the oxygen adsorbed at oxygen vacancies. The beta peak at about 1000K which depends closely on the x and favors the catalytic activity was attributed to the reduction of Ni3+. The catalytic activity of La-2-x SrxNiO4 mixed oxides in the NH3 oxidation in general could be attributed to the extent of the redox reaction: 2Ni(2+) + O-2 + V-0(..) reversible arrow 2Ni(3+) + 20(-) where V-0(..) representes the oxygen vacancies and O- the oxygen species adsorbed at the vacancies.

SOLID DEFECT STRUCTURE AND CATALYTIC ACTIVITY OF PEROVSKITE-TYPE CATALYSTS LA1-XSRXNIO3-LAMBDA AND LA1-1.333XTHXNIO3-LAMBDA

Two series of La1-xSrxNiO3-lambda and La1-1.333xThxNiO3-lambda catalysts have been prepared, and the relationships between the solid defect structure and catalytic activity for NH3 oxidation were measured. The results showed that in the range of x < 0.3, the samples possessed single perovskite-type structure, and as the content of Sr2+ decreased and that of Th4+ increased the catalytic activity increased which was paralleled with the Ni3+ concentration within the catalysts. The active oxygen species (O- or O2(2-)) were present not only on the surface but also in the bulk of the samples. The synergistic effect of transition metal ions with higher oxidation states and randomly distributed oxygen vacancies was the key factor determining catalytic activity of perovskite-type oxides. A redox mechanism for NH3 oxidation over ABO3 is proposed.

NO reduction and CO oxidation over Cu/Ce/Mg/Al mixed oxide catalyst in FCC operation

Simultaneous NO reduction and CO oxidation in the presence of O-2,H2O and SO2 over Cu/Mg/AUO (Cu-cat), Ce/Mg/Al/O (Ce-cat) and Cu/Ce/Mg/Al/O (CuCe-cat) were studied. At low temperatures (<340 degreesC), the presence of O-2 or H2O enhanced the activity of CuCe-cat for NO and CO conversions, but significantly suppressed the activity of Cu-cat and Ce-cat, At high temperature (720 degreesC), the presence of O-2 or H2O had no adverse effect on the NO and CO conversions over these catalysts. The addition of SO2 to NO + CO + O-2 + H2O system had no effect on the, reaction of CO + O-2 over Cu-cat, but deactivated this catalyst for NO + CO and CO + H2O reactions; over Ce-cat, all of these reactions of NO + CO, CO + O-2 and CO + H2O were suppressed significantly; over CuCe-cat, NO + CO and CO + O-2 reactions were not affected while the reaction of CO + H2O was slightly inhibited. (C) 2002 Elsevier Science B.V. All rights reserved.

One-step heterogeneously catalytic oxidation of o-cresol by oxygen to salicylaldehyde

Salicylaldehyde (selectivity = 57.3% at a conversion = 73.8%) was prepared for the first time by the oxidation of o-cresol in a single step using impregnated CuCo/C catalysts.

Epoxidation of styrene on Si/Ti/SiO2 catalysts prepared by chemical grafting

Titania-silica (Ti/SiO2) and silica-titania-silica (Si/Ti/SiO2) catalysts were:prepared by chemical grafting using TiCl4 and tetraethyl orthosilicate (TEOS) as precursors and SiO2 as support. The prepared catalysts were characterized by UV Raman and visible Raman spectroscopies, XRD and the epoxidation of styrene; Ti/SiO2: catalyst grafted with only titanium species is not very active for epoxidation using H2O2 (30%), but is active and-selective when one uses tert-butyl hydroperoxide (TBHP). The catalyst grafted at high temperatures shows better epoxide selectivity. Si/Ti/SiO2 catalyst, the titanium-silica grafted further with TEOS, is active and selective for the epoxidation of styrene using either dilute H2O2 or TBHP, possibly due to the fact that the grafting of Ti/SiO2 with TEOS modifies the coordination structure of titanium and makes the titanium sites of Si-O-Ti-O-Si species less hydrophilic. A characteristic band at 1085cm(-1) due to Ti-O-Si species is detected for the grafted catalysts by UV resonance Raman spectroscopy. Reaction between TiCl4 and SiO2 at high temperatures favors the formation of Ti-O-Si species. Better activity and selectivity to epoxide,is found for the catalysts with more Ti-O-Si species. It is assumed that the active sites are the highly isolated Ti-O-Si species. For Si/Ti/SiO2 catalyst, the gas phase O-2 can participate in the catalytic oxidation of styrene when H2O2 is present ana:ii causes the formation of benzaldehyde. (C) 2000 Elsevier Science B.V. All rights reserved.

Wet air oxidation of nitrobenzene enhanced by phenol

Simultaneous nitrobenzene and phenol wet air oxidation was investigated in a stainless autoclave at temperature range of 180-220 &DEG; C and 1.0 MPa oxygen partial pressure. Compared with the single oxidation of nitrobenzene under the same conditions, the presence of phenol in the reaction media greatly improved the removal efficiency of nitrobenzene. The effect of temperature on the reaction was studied. Phenol was considered as a type of initiator in the nitrobenzene oxidation. © 2004 Elsevier Ltd. All rights reserved.

Catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone over Co3O4 nanocrystals with molecular oxygen

Co3O4 nanocrystals with average particle sizes of 30 and 50 run were synthesized using cobalt nitrate as precursor, and were characterized by X-ray diffraction (XRD), nitrogen adsorption, transmission electron microscopy (TEM), and Fourier transform infrared (FT-IR) spectroscopy. Catalytic oxidation of cyclohexane with molecular oxygen was studied over Co3O4 nanocrystals. These catalysts showed obviously higher activities as compared to Co3O4 prepared by the conventional methods, Co3O4/Al2O3, or homogeneous cobalt catalyst under comparable reaction conditions. The 89.1% selectivity to cyclohexanol and cyclohexanone at 7.6% conversion of cyclohexane was realized over 50 nm sized Co3O4 nanocrystals at 393 K for 6 h. (c) 2005 Elsevier B.V. All rights reserved.

Preparation and characterization of multi-walled carbon nanotubes supported PtRu catalysts for proton exchange membrane fuel cells

A series of PtRu nanocomposites supported on H2O2-oxidized multi-walled carbon nanotubes (MWCNTs) were synthesized via two chemical reduction methods - one used aqueous formaldehyde (HCHO method) and the other used ethylene glycol (EG method) as the reducing agents. The effects of the solvents (water and ethylene glycol) and the surface composition of the MWCNTs on the deposition and the dispersion of the metal particles were investigated using N-2 adsorption. TEM. ICP-AES. FTIR and TPD. The wetting heats of the MWCNTs in corresponding solvents were also measured. The characterizations suggest that combination of the surface chemistry of the MWCNTs with the solvents decides the deposition and the dispersion of the metal nanoparticles. These nanocomposites were evaluated as proton exchange membrane fuel cell anode catalyts for oxidation of 50 ppm CO contaminated hydrogen and compared with a commercial PtRu/C catalyst. The data reveal superior performances for the nanocomposites prepared by the EG method to those by the HCHO method and even to that for tile Commercial analogue. Structure performance relationship of the nanocomposites was also studied. (C) 2005 Elsevier Ltd. All rights reserved.

Butene catalytic cracking to propene and ethene over potassium modified ZSM-5 catalysts

Catalytic cracking of butene over potassium modified ZSM-5 catalysts was carried out in a fixed-bed microreactor. By increasing the K loading on the ZSM-5, butene conversion and ethene selectivity decreased almost linearly, while propene selectivity increased first, then passed through a maximum (about 50% selectivity) with the addition of ca. 0.7-1.0% K, and then decreased slowly with further increasing of the K loading. The reaction conditions were 620 degrees C, WHSV 3.5 h(-1), 0.1 MPa 1-butene partial pressure and 1 h of time on stream. Both by potassium modification of the ZSM-5 zeolite and by N(2) addition in the butene feed could enhance the selectivity towards propene effectively, but the catalyst stability did not show any improvement. On the other hand, addition of water to the butene feed could not only increase the butene conversion, but also improve the stability of the 0.7%K/ZSM-5 catalyst due to the effective removal of the coke formed, as demonstrated by the TPO spectra. XRD results indicated that the ZSM-5 structure of the 0.07% K/ZSM-5 catalyst was not destroyed even under this serious condition of adding water at 620 degrees C.

Structure and chemical composition of supported Pt-Sn electrocatalysts for ethanol oxidation

Carbon supported PtSn alloy and PtSnOx particles with nominal Pt:Sn ratios of 3:1 were prepared by a modified polyol method. High resolution transmission electron microscopy (HRTEM) and X-ray microchemical analysis were used to characterize the composition, size, distribution, and morphology of PtSn particles. The particles are predominantly single nanocrystals with diameters in the order of 2.0-3.0 nm. According to the XRD results, the lattice constant of Pt in the PtSn alloy is dilated due to Sn atoms penetrating into the Pt crystalline lattice. While for PtSnOx nanoparticles, the lattice constant of Pt only changed a little. HRTEM micrograph of PtSnOx clearly shows that the change of the spacing of Pt (111) plane is neglectable, meanwhile, SnO2 nanoparticles, characterized with the nominal 0.264 nm spacing of SnO2 (10 1) plane, were found in the vicinity of Pt particles. In contrast, the HRTEM micrograph of PtSn alloy shows that the spacing of Pt (111) plane extends to 0.234 nm from the original 0.226 nm. High resolution energy dispersive X-ray spectroscopy (HR-EDS) analyses show that all investigated particles in the two PtSn catalysts represent uniform Pt/Sn compositions very close to the nominal one. Cyclic voltammograms (CV) in sulfuric acid show that the hydrogen ad/desorption was inhibited on the surface of PtSn alloy compared to that on the surface of the PtSnOx catalyst. PtSnOx catalyst showed higher catalytic activity for ethanol electro-oxidation than PtSn alloy from the results of chronoamperometry (CA) analysis and the performance of direct ethanol fuel cells (DEFCs). It is deduced that the unchanged lattice parameter of Pt in the PtSnOx catalyst is favorable to ethanol adsorption and meanwhile, tin oxide in the vicinity of Pt nanoparticles could offer oxygen species conveniently to remove the CO-like species of ethanolic residues to free Pt active sites. (C) 2005 Elsevier Ltd. All rights reserved.

Selective oxidations on recoverable catalysts assembled in emulsions

Catalysts assembled in emulsions are found to be potentially recoverable and efficient for a number of catalytic reactions. The catalysts composed of polyoxometalate anions and quaternary ammonium cations have been designed and synthesized according to the catalytic reactions and by optimizing the structures of cations and anions. The catalysts act essentially as surfactants, which are uniformly distributed in the interface of the emulsion droplets, and accordingly behave like homogeneous catalysts. The catalysts show remarkable selectivity and activity in the oxidation of sulfur-containing molecules to sulfones in diesel and the selective oxidation of alcohols to ketones, using H2O2 as oxidant. For an example, the catalyst demonstrated over 96% efficiency of H2O2 and similar to 100% selectivity to sulfones for the selective oxidation of sulfur-containing molecules in real diesel. Moreover, the catalysts can be separated and recycled by a simple demulsification and re-emulsification.

Enhancement of the catalytic performance of supported-metal catalysts by pretreatment of the support

We report an interesting finding that the catalytic performance of supported Ag/SiO2 catalysts toward selective catalytic oxidation of CO in hydrogen at low temperatures can be greatly enhanced by pretreatment of the SiO2 support before catalyst preparation. Calcination of SiO2 at appropriate temperatures preferentially removes the H-bonded SiOH, which results in the highly dispersive Ag/SiO2 catalyst and thus improves the catalytic performance. (c) 2005 Elsevier Inc. All rights reserved.

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H-2) are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 similar to 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H-2 treatment at low temperatures (100 similar to 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H-2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (> 300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 similar to 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (> 300 degrees C) is still higher than that directly pretreated with H,. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.