Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts

Potential and limitations of molten alkali metal (Li, Na, and K) chlorides supported on Dy₂O₃/MgO were explored for the oxidative dehydrogenation of lower alkanes, such as ethane and propane. The catalysts have high activity and selectivity to olefins compared to conventional catalysts. Optimum performance is obtained with catalysts on which the alkali metal chloride phase is molten under reaction conditions. Lower chloride melting point correlates with higher selectivity. The high selectivity to ethene or propene is attributed to the high mobility of cations and anions, which facilitates desorption of alkene (limiting further oxidation) and the generation of spatially isolated hypochloride anions acting as the active sites for the primary C–H bond activation.     

Characterization of methanol-tolerant Pd–WO3 and Pd–SnO2 electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

Palladium (Pd) catalysts containing nanosized metal oxides, tungsten oxide (WO₃) and tin oxide (SnO₂), supported on carbon black (Pd–MO_x/C) were synthesized, and the effect of the metal oxide on the oxygen reduction reaction (ORR) in a direct methanol fuel cell (DMFC) was investigated. The SEM images showed that the Pd nanoparticles were highly dispersed on the carbon black, and the metal oxide particles were also distributed well. Pd/C and Pd–WO₃/C catalysts as cathode materials for the ORR in DMFCs showed activity similar to or better than that of Pt/C, whereas Pd–SnO₂/C showed no improvement in catalytic activity.     

A Comparison of Catalytic Oxidative Dehydrogenation of Ethane Over Mixed V-Mg Oxides and Over Those Prepared via a Mesoporous V-Mg-O Pathway

The catalytic oxidative dehydrogenation of ethane was investigated in a fixed-bed tubular microreactor at 500, 550 and 600 °C and a space velocity of 35 027ml g^-1h^-1. Two kinds of V-Mg oxides catalysts containing various V/Mg atomic ratios were employed. One group of catalysts was prepared by the solid reaction between fine powders of vanadium pentoxide and magnesium nitrate and the other ones were obtained from mesostructured V-Mg-Os. For the former catalysts, it was found that the selectivity to ethene increased and the conversion of ethane passed through a maximum with increasing V/Mg atomic ratio. For the catalysts obtained from the mesoporous materials, an optimum V/Mg atomic ratio was found, for which the conversion of ethane and the selectivity to ethene were maxima. Compared with the mixed-oxide catalysts, those obtained from the mesoporous materials exhibited much higher yields to ethene. Several new phases, such as pyro-Mg₂V₂O₇, ortho-Mg₃(VO₄)₂ and meta-MgV₂O₆, formed between magnesia and vanadia, were identified by XRD in the mixed V-Mg oxide catalysts; they may be responsible for the catalytic activity. In the catalysts prepared from mesoporous V-Mg-O, a V₂O₃ phase, which may contain highly dispersed magnesium, was identified and suggested to be responsible for the higher catalytic performance.     

Ordered mesoporous Fe (or Co)–N–graphitic carbons as excellent non-precious-metal electrocatalysts for oxygen reduction

Novel mesoporous Fe (or Co)–N_x–C non-precious-metal catalysts (NPMCs) have been fabricated by a simple nanocasting-pyrolysis method using 1,10-phenanthroline metal chelates as the precursors. Owing to the ordered hexagonal mesostructures, appropriate surface area, large-pore channels, and well-distributed metal–N_x moieties embedded within the graphitic carbon backbones, the prepared metal–N_x–C materials exhibit excellent catalytic activity for oxygen reduction reaction (ORR) in both alkaline and acidic media. The prepared Fe–N_x–C materials, when prepared with an optimized catalyst loading on the electrode, exhibit more positive ORR onset-potential and half-wave potential (E_1/2) than commercial Pt/C catalysts and the previously reported NPMCs in 0.1 M KOH electrolyte. They also have the comparable ORR onset-potential and current densities to Pt/C electrode in 0.1 M HClO₄ electrolyte. Moreover, ORR over mesoporous Fe–N_x–C was found to proceed by the direct four-electron mechanism with high selectivity in both electrolytes. The mesoporous Fe–N_x–C materials demonstrated higher ORR catalytic activity compared to the NPMCs made by alternative methods. Analysis of the catalytic behavior, structure and nature of surface species of N_x–C materials allows us to ascribe the origin of the excellent ORR catalytic activity of mesoporous Fe (or Co)–N_x–C in both electrolytes to Fe (or Co)–N_x moieties embedded within the graphitic carbon frameworks.     

Highly selective oxidative dehydrogenation of ethane to ethene over layered complex metal chloride oxide catalysts

Complex metal chloride oxides consisting of bismuth, alkali, alkaline earth, chlorine, and oxygen, were synthesized, characterized structurally, and tested as catalysts for the oxidative dehydrogenation of ethane to ethene with molecular oxygen. The catalysts were prepared by high-temperature solid-state reaction of appropriate mixtures of bismuth chloride oxide, bismuth oxide, alkali chloride, and alkaline earth chloride. We found that the catalysts containing strontium as the alkaline earth constituent and potassium as the alkali constituent were highly active and selective for the oxidative dehydrogenation of ethane. SrBi₃O₄Cl₃, which is a fundamental phase of the catalysts, was characterized by single and double chlorine sheets in its layer structure. The catalyst having the composition of KSr₂Bi₃O₄Cl₆ showed an extremely high ethene selectivity, more than 90%, even under high oxygen partial pressure conditions and at high molar ratios of oxygen to ethane, and gave 70% yield of ethene at 640°C under an optimized feed gas composition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of Methanol‐tolerant Oxygen Reduction Reaction at Pt–Bi/C Bimetallic Nanostructured Catalysts

The nanostructured platinum–bismuth catalysts supported on carbon (Pt₃Bi/C, PtBi/C and PtBi₃/C) were synthesised by reducing the aqueous metal ions using sodium borohydride (NaBH₄) in presence of a microemulsion. The amount of metal loading on carbon support was found to be 10 wt.‐%. The catalyst materials were characterised by X‐ray diffraction (XRD), X‐ray fluorescence (XRF), transmission electron microscope (TEM) and electroanalytical techniques. The Pt₃Bi/C, PtBi/C and PtBi₃/C catalysts showed higher methanol tolerance, catalytic activity for oxygen reduction reaction (ORR) than Pt/C of same metal loading. The electrochemical stability of these nano‐sized catalyst materials for methanol tolerance was investigated by repetitive cycling in the potential range of –250 to 150 mV_MSE. Bi presents an interesting system to have a control over the activity of the surface for MOR and ORR. All Pt–Bi/C catalysts exhibited higher mass activities for oxygen reduction (1–1.5 times) than Pt/C. It was found that PtBi/C catalyst exhibits better methanol‐tolerance than the other catalysts.     

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: Characterization and catalytic performance

This work demonstrates the high potential of a new class of catalytic materials based on nickel for the oxidative dehydrogenation of ethane to ethylene. The developed bulk Ni–Nb–O mixed oxides exhibit high activity in ethane ODH and very high selectivity (∼90% ethene selectivity) at low reaction temperature, resulting in an overall ethene yield of 46% at 400 °C. Varying the Nb/Ni atomic ratio led to an optimum catalytic performance for catalysts with Nb/Ni ratio in the range 0.11–0.18. Detailed characterization of the materials with several techniques (XRD, SEM, TPR, TPD-NH₃, TPD-O₂, Raman, XPS, electrical conductivity) showed that the key component for the excellent catalytic behavior is the Ni–Nb solid solution formed upon the introduction of niobium in NiO, evidenced by the contraction of the NiO lattice constant, since even small amounts of Nb effectively converted NiO from a total oxidation catalyst (80% selectivity to CO₂) to a very efficient ethane ODH material. An upper maximum dissolution of Nb⁵⁺ cations in the NiO lattice was attained for Nb/Ni ratios ⩽0.18, with higher Nb contents leading to inhomogeneity and segregation of the NiO and Nb₂O₅ phases. A correlation between the specific surface activity of the catalysts and the surface exposed nickel content led to the conclusion that nickel sites constitute the active centers for the alkane activation, with niobium affecting mainly the selectivity to the olefin. The incorporation of Nb in the NiO lattice by either substitution of nickel atoms and/or filling of the cationic vacancies in the defective nonstoichiometric NiO surface led to a reduction of the materials nonstoichiometry, as indicated by TPD-O₂ and electrical conductivity measurements, and, consequently, of the electrophilic oxygen species (O⁻), which are abundant on NiO and are responsible for the total oxidation of ethane to carbon dioxide.     

Decomposition of nitric oxide and its reduction by CO over superconducting and related cuprate catalysts

Catalytic activities of five non-conducting and three superconducting cuprates were measured for the decomposition of NO and the reduction of NO by CO. The concentration of the reactants and the space velocities approximate the conditions of automotive catalysts. In contrast to earlier results, obtained at 20 to 30 times higher partial pressures of NO and 20 to 100 times lower space velocities, none of the catalysts, including five perovskite-like cuprates, showed significant activity for the decomposition of NO at reaction temperatures up to 700 °C. All catalysts were fairly active for the reduction of NO. At temperatures above about 400 °C on the perovskite-like cuprates YBa₂Cu₃O_7-x and Ba₂CuO_3.5-x , the rates for NO reduction were higher than on CuO. All catalysts lost activity for NO reduction in the presence of oxygen (oxidizing conditions).     

Carbide and Nitride Supported Methanol Steam Reforming Catalysts: Parallel Synthesis and High Throughput Screening

A series of Mo₂C and Mo₂N supported catalysts have been synthesized using a parallel synthesis and high throughput screening approach. The high surface area Mo₂C and Mo₂N supports were prepared using temperature programmed reaction methods. Metals including Co, Cu, Fe, Ni, Pd, Pt, Ru, and Sn were impregnated onto these supports using a synthesis system. Methanol steam reforming (MSR) activities and selectivities for these materials were evaluated using a high throughput-screening reactor. The support type, metal type and concentration, and metal precursor type influenced the activity and selectivity patterns. Of more than 400 materials that were synthesized and evaluated, the Pt/Mo₂N, Pt–Ni/Mo₂N, Pt–Fe/Mo₂N, and Pd–Fe/Mo₂C catalysts possessed the highest activities. Some of these formulations were more active than a commercial Cu/Zn/Al₂O₃ catalyst, however, the CO₂ selectivities were typically lower. At similar conversions, materials that were highly active were not selective while the less active materials were very selective. Many of the highly active catalysts included noble metals while the highly selective catalysts included base metals.     

Influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbon over Cu-MFI zeolite

The influence of intracrystalline diffusion on the selective catalytic reduction of NO by hydrocarbons (propane, ethene) was investigated by using Cu-MFI catalysts of different zeolite crystal sizes. The influence of hydrocarbon as a reductant on the diffusion process was also investigated, employing kinetic and temperature-programmed desorption studies. In the NO–C₃H₈–O₂ reaction, the apparent reaction rate of NO conversion into N₂ did not depend on the zeolite crystal size, which indicates that the reaction is not controlled by intracrystalline diffusion. In the NO–C₂H₄–O₂ reaction, on the other hand, the apparent reaction rate evidently depended on the zeolite crystal size; the reaction rate over large crystal Cu-MFI (1.29 μm) was significantly less than that over a small one (0.09 μm), which indicates that the reaction is controlled by intracrystalline diffusion. The larger diffusion resistance in the NO–C₂H₄–O₂ reaction was attributed to the slower diffusion rate of ethene in zeolite channels than propane, which was due to much stronger interaction of ethene with Cu-MFI catalyst. Thus, the adsorption property of hydrocarbon on the Cu-MFI catalyst is revealed to play an important role in determining intracrystalline diffusivity and the diffusion influence on the selective reduction of NO over zeolite catalysts.     

Preparation of machinable cordierite/mica composite by low-temperature sintering

In order to fabricate machinable cordierite/mica composite at low temperatures, the mica-composition glass powder was mixed with the conventional magnesia, alumina and silica powders which are raw materials of cordierite, compacted and fired in a sealed platinum container. By the addition of the 40 mass% mica-composition glass powder, machinable cordierite/mica composite was obtained. The machinability was caused by the interlocking microstructure of mica developed in the composite. In the firing process, mica crystallized at about 730 °C, cordierite was suddenly formed at 1050–1100 °C and the densification progressed markedly at 1000–1100 °C. The formation and sintering of cordierite were strongly promoted by a small amount of gaseous fluorine and/or fluorides. It was considered that fluorine and fluorides such as AlF₃ evaporated from the mica-composition glass at >800 °C and gaseous HF was formed in the sealed platinum container by the reaction of fluorine with water evaporated from the glass.     

Effect of the composition of an oxide coating and the preparation method of block catalysts on their activity in the deep oxidation of methane

Deep methane oxidation catalysts containing 3d metal (Mn, Co), rare-earth (La) and alkali-earth (Ba, Sr) oxides in the porous matrices of secondary supports (Al₂O₃, ZrO₂, and their binary composition) formed on honeycomb blocks (cordierite, kaolin-aerosil) are studied by means of X-ray powder diffraction, the thermal desorption of nitrogen, and temperature-programmed reduction with hydrogen. It is shown that the activity and stability of the catalysts depend on the method for their preparation and the nature of the active components and secondary and block supports. After life cycle tests, the proposed catalysts with 80–100% conversion of methane into CO₂ at temperatures of 650–750°C can be recommended for use in systems for the catalytic purification of gases containing hydrocarbon admixtures (methane and C₂–C₄ homologues) and the combustion of hydrocarbon fuels in industrial and household catalytic heat generators.     

Outstanding electro-catalytic activity of Pt x –(RuO y –CeO2)1−x /C composites towards ethanol oxidation in acid media

Direct ethanol fuel cells are a key enabling technology for clean energy conversion; however, a major challenge is the determination of anode catalytic materials with high performance for complete ethanol oxidation. In this study, we synthesized binary Pt_0.50–(CeO₂)_0.50/C and Pt_0.50–(RuO _y )_0.50/C, as well as ternary Pt _x –(RuO _y –CeO₂)_1?x catalysts (x = 0.25, 0.50, or 0.75) and (y = 0, +2, or +3) by the sol–gel method and compared them in the ethanol oxidation reaction. Transmission electron microscopy images revealed the small particle size of the prepared catalysts (2.1–2.5 nm). Cyclic voltammetry, chronoamperometry, derivative voltammetry, and potentiostatic polarization were employed to analyze the ethanol oxidation reaction on binary Pt_0.50–(CeO₂)_0.50/C and Pt_0.50–(RuO _y )_0.50/C and ternary Pt _x –(RuO _y –CeO₂)_1?x catalysts, as well as Pt/C and Pt–Ru/C commercial catalysts, including some insights estimating a possible reaction mechanism. The results demonstrate, considering the activity outcomes approach, the highly superior performance of the Pt_0.25–(RuO _y –CeO₂)_0.75/C catalyst.     

Selective oxidation of ethane over hydrothermally synthesized Mo–V–Al–Ti oxide catalyst

Mo–V-based oxide catalysts are known highly active for various hydrocarbon selective oxidations. Particularly those which are monophasic giving XRD diffraction at d=4 Å are extremely active for alkane oxidations. We succeeded to synthesize this unique monophasic material by hydrothermal method and obtained Mo₆V₂Al₁O_x mixed oxide catalysts which showed activities for gas-phase ethane oxidation to ethene and acetic acid. The addition of titanium to the Mo₆V₂Al₁O_x oxide catalyst was found to result in a marked increase of the activity for the ethane selective oxidation, which was due to the morphological change of the catalyst particles and the increase of surface area by the addition of titanium. During the heat-treatment above 550°C under a nitrogen stream, the structural phase of the Mo₆V₂Al₁Ti_0.5O_x catalyst giving the XRD diffraction at d=4 Å transferred to (Mo_0.93V_0.07)₅O₁₄-like phase with drastic decreases of the oxidation activity and the surface area. On the basis of kinetic data and the fact that the lower reaction temperature and the existence of water vapor in the feed facilitated the formation of acetic acid, it is concluded that the breaking of C–H bond of ethane is the rate determining of the oxidation and acetic acid do not form through ethene.     

Support and promoter effects in the selective oxidation of ethane to acetic acid catalyzed by Mo-V-Nb oxides

Catalysts based on Mo-V-Nb oxides were examined in bulk and supported forms for the oxidation of ethane to ethene and acetic acid. Bulk Mo_0.61V_0.31Nb_0.08O_x powders showed rates and selectivities similar to those in previous reports. Precipitation in the presence of colloidal TiO₂ led to a 10-fold increase in ethene and acetic acid rates (per active oxide) without significant changes in selectivity relative to unsupported samples. Precipitation in the presence of colloidal ZrO₂ and Al₂O₃ suspensions, however, introduced unselective combustion sites without improving ethane oxidation rates. Mo₅O₁₄ structures, containing low-valent metal cation centers were detected in bulk Mo_0.61V_0.31Nb_0.08O_x and TiO₂-supported samples by Raman and UV–visible spectra and consistent with X-ray diffraction patterns, but not in Al₂O₃- or ZrO₂-containing catalysts. The introduction of trace amounts of Pd (0.0025–0.01 wt.%), as a physical mixture of separate 0.3 wt.% Pd/SiO₂, led to the near complete depletion of ethene intermediates and to a significant increase in acetic acid synthesis rate. Small PdO_x catalyze ethene oxidation to acetaldehyde, but require the rapid scavenging of these molecules by Mo-V-Nb oxides to prevent acetaldehyde combustion and loss of selectivity. Dispersed VO_x domains on TiO₂ were able to catalyze all steps required for ethane oxidation to acetic acid. CO_x selectivities, however, were much higher than on bulk and TiO₂-supported Mo_0.61V_0.31Nb_0.08O_x catalysts. Dispersed MoO_x domains were essentially inactive at these reaction conditions but their concurrent presence with VO_x increased acetic acid selectivity by titrating unselective sites and stabilizing more reducible VO_x species.     

Oxidative dehydrogenation of ethane over alkali‐metal doped sulfated zirconia catalysts

Alkali‐metal doped sulfated zirconia catalysts were tested for the oxidative dehydrogenation of ethane into ethene. The effects of metal precursor compounds and acidic anion promoters on the catalytic activity in this reaction were studied. It was found that sulfation of zirconia increases the selectivity of ethane towards ethene. Lithium‐, sodium‐, and potassium‐doped sulfated zirconia catalysts showed quite different activities in this reaction. Sulfated zirconia doped with lithium catalysts were found to be effective for the oxidative dehydrogenation of ethane, giving over 90% selectivity to ethene and 25% ethene yield at 650 °C. © 1999 Society of Chemical Industry     

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part II: Mechanistic aspects and kinetic modeling

In this work, transient and SSITKA experiments with isotopic ¹⁸O₂ were conducted to study the nature of oxygen species participating in the reaction of ethane oxidative dehydrogenation to ethylene and obtain insight in the mechanistic aspects of the ODH reaction over Ni-based catalysts. The study was performed on NiO, a typical total oxidation catalyst, and a bulk Ni–Nb–O mixed-oxide catalyst (Ni_0.85Nb_0.15) developed previously [E. Heracleous, A.A. Lemonidou, J. Catal., in press], a very efficient ethane ODH material (46% ethene yield at 400 °C). The results revealed that over both materials, the reaction proceeds via a Mars–van Krevelen-type mechanism, with participation of lattice oxygen anions. However, the ¹⁸O₂ exchange measurements showed a different distribution of isotopic oxygen species on the two materials. The prevalent formation of cross-labelled oxygen species on NiO indicates that dissociation of oxygen is the fast step of the exchange process, leading to large concentration of intermediate electrophilic oxygen species on the surface, active for the total oxidation of ethane. Larger amounts of doubly exchanged species were observed on the Ni–Nb–O catalyst, indicating that doping with Nb makes diffusion the fast step of the process and suppresses formation of the oxidizing species. Kinetic modeling of ethane ODH over the Ni_0.85Nb_0.15 catalyst by combined genetic algorithm and nonlinear regression techniques confirmed the above, since the superior model is based on a redox parallel-consecutive reaction network with the participation of two types of active sites: type I, responsible for the ethane ODH and ethene overoxidation reaction, and type II, active for the direct oxidation of ethane to CO₂. The kinetic model was able to successfully predict the catalytic performance of the Ni_0.85Nb_0.15 catalyst in considerably different experimental conditions than the kinetic experiments (high temperature and conversion levels).     

Ozone Initiated Oxidation of Hexadecane with Metal Loaded γ-Al2O3 Catalysts

The scope of metal loaded γ-Al₂O₃ materials as catalysts in the ozone initiated functionalisation of higher n-alkanes is investigated at moderate conditions (20 ± 1 °C and ～1 atm). In the ozone initiated oxidation of the higher hydrocarbon, n-hexadecane with 0.5% Pd or Ni or V loaded γ-Al₂O₃ catalysts lead to the keto-isomers as main products, and organic acids as minor products. This paper emphasises the effect of γ-Al₂O₃ catalysts on the conversion, selectivity and reaction products during ozonation of n-hexadecane.     

Hydration of ethene over W–P mixed metal oxide catalysts

W–P mixed metal oxide catalysts are active and selective for the gas-phase hydration of ethene to ethanol. The activity and selectivity of this catalytic reaction depend on the W/P atomic ratio. However, ethene conversion slightly decreases at higher W/(W + P) atomic ratio. The selectivity for ethanol increases with the W/P atomic ratio and reaches the highest value (92%) at W_0.81P_0.19O_x. The W_0.81P_0.19O_x catalyst is less active than the conventional H₃PO₄/SiO₂ catalyst, but the activity is maintained for a long time without the supply of any catalyst components. The reaction temperature does not affect substantially the rate of ethene hydration over the W_0.81P_0.19O_x catalyst. The H₂O/ethene molar ratio of 0.4 is the most appropriate for both reaction rate and selectivity. The active species of W–P mixed metal oxide are amorphous. But there is Keggin structure of W–P oxide species (PW₁₂O₄₀ ³⁻) in the presence of steam. And the species are the active sites for the hydration of ethene, confirmed by in situ Raman spectroscopy. This revised version was published online in July 2006 with corrections to the Cover Date.