Effect of the nature of the support on molybdenum catalytic behavior in diesel particulate combustion (original) (raw)
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Applied Catalysis A: General, 2014
A series of molybdenum oxide (2 to 15 wt% Mo) and mixed molybdenum-vanadium oxide (4 to 15 wt% Mo and 2 wt% V) on alumina catalysts have been synthesized by flame spray pyrolysis (FSP). The materials were structurally characterized by BET surface area, X-ray diffraction (XRD), Raman and diffuse reflectance UV-vis spectroscopy and evaluated as catalysts for the oxidative dehydrogenation (ODH) of propane. The results show that samples with high specific surface areas between 122 and 182 m 2 /g were obtained, resulting in apparent MoO x and VO x surface densities from 0.7 to 7.7 nm −2 and 1.5 to 1.9 nm −2 , respectively. Raman spectroscopy, UV-vis spectroscopy and XRD confirmed the high dispersion of molybdenum and vanadia species on ␥-Al 2 O 3 as the main crystalline phase. Only at the highest loading of 15 wt% Mo, with theoretically more than monolayer coverage, some crystalline molybdenum oxide was observed. For the mixed molybdenum-vanadium oxide catalysts the surface species were separate molybdenum oxide and vanadium oxide monomers at low loadings of molybdenum, but with increasing molybdenum loading interactions between surface molybdenum and vanadium oxide species were observed with Raman spectroscopy.
Applied Catalysis A: General, 2006
Mixed oxides, MoO 3-CeO 2 , were being used as catalysts for the cracking (TCC) of liquid hydrocarbon feedstocks. The dispersion and interactions of MoO 3 , CeO 2 and mixtures thereof impregnated into the silica-alumina surface were investigated using several techniques, which included X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). The loadings and the chemical states of metal oxides incorporated separately had significant effects on the catalytic activities of the resulting monocomponent catalysts. Addition of cerium to molybdenum had a favorable effect on the production of light olefins in the TCC of n-hexane up to a certain level of cerium loading. In fact, high loadings of molybdenum and/or cerium favored the formation of aromatics, instead. The catalytic performance of the bicomponent catalysts also depended significantly on the incorporation methods. It was found that the co-impregnation of MoO 3 and CeO 2 , which led to the highest production of light olefins, corresponded to the formation of (surface) cerium molybdate to the highest extent. On the other hand, the catalysts prepared by the two-step impregnation methods (sequential and reverse sequential impregnation) showed much lower catalytic performance due to low Mo-Ce interactions as suggested by an important segregation of the active phases, mostly MoO 3. The sequence of catalytic performance (to the desired products, i.e. light olefins) fully coincided with that of the dispersion of molybdate species on the support surface.
Applied Catalysis B-environmental, 1998
The catalytic combustion of diesel soot particles was studied on Co/MgO (12 wt% Co) and potassium-promoted Co/MgO ( 1.5 wt% K) that were calcined at different temperatures in the 300 to 700°C range. Catalyst samples were characterized by various techniques including nitrogen adsorption (BET), temperature programmed reduction (TPR), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS) and temperature programmed oxidation (TPO). As observed by TPO experiments, the catalyst activity depends strongly on the calcination temperature: calcination at 300 and 400°C produced samples that were much more active than those calcined at higher temperatures, on which an inactive Mg-Co mixed oxide is formed, as suggested by TPR, ESR and XRD results. FTIR shows carbonate species on the surface. Unpromoted samples seem to correlate their activity with the amount of reducible Co species present. Potassium not only increased the sample activity, probably due to the improvement in surface mobility, but also enhanced stability at high temperatures. Experiments with different soot to catalyst ratios showed no significant variation in combustion temperature. The K-promoted catalyst bums off soot at a temperature lower than the one needed for calcination, thus proving to be a promising catalyst. 0 1998 Elsevier Science B.V.
Catalytic combustion of diesel soot over metal oxide catalysts
Applied Catalysis B: Environmental, 1997
The activity of different metal oxides in the catalytic combustion of a diesel soot having a high amount of adsorbed hydrocarbons has been investigated and tested in a TPO apparatus. Two different steps have been observed. The first is related to the combustion of hydrocarbons adsorbed on soot and the second one to the combustion of the graphitic solid fraction. A high surface area Fe20 3 was found to be the most active catalyst for the oxidation of hydrocarbons, whereas VzQ was also able to promote the combustion of graphitic carbon. The oxidation of the hydrocarbon fraction has been correlated with the surface area and the strength of the metal-oxygen bond of the metal oxide. The combustion of graphitic carbon is favoured instead on metal oxides having a low melting point.
The Structure of Highly Dispersed SiO2-Supported Molybdenum Oxide Catalysts during Sulfidation
1994
The structure of sulfided Mo-catalysts and their oxidic precursors has been abundantly studied, but the genesis of the active phase has remained much less investigated. The sulfidation (in H2S/H2 atmosphere) of a series of MoO3/SiO2 catalysts has been examined by means of temperature-programmed sulfidation, X-ray absorption fine structure, and transmission electron microscopy.The oxidic, oligomeric clusters in a 5.6 wt 5% MoO3/Si02 catalyst are transformed into partly sulfided particles (MOOS,,) by 0 -S exchange at room temperature. A molybdenum sulfide species the structure of which resembles the MoS, structure is formed during sulfidation at 423 K. The MoS2 phase is formed at temperatures between 523 and 573 K, depending on the dispersion of the initial Moo3 phase. The transition of MoS3 into MoS2 can be monitored by the evolution of H2S from the catalyst with a simultaneous consumption of H2. The two-dimensional size of the MoS2 slabs can be derived from the EXAFS Mo-Mo coordination number by means of a theoretical model. TEM is required to elucidate the stacking height of the slabs.
Applied Catalysis A: General, 2001
A series of MoO 3 catalysts with Mo loadings, ranging from 2-12% w/w, supported on TiO 2 (anatase) and TiO 2 (rutile) were investigated by X-ray diffraction, temperature programmed reduction (TPR), 1 H magic angle spinning (MAS) NMR and oxygen chemisorption measurements. Dispersion of molybdena was determined by the oxygen chemisorption at 623 K and by a static method on the samples prereduced at the same temperature. At low Mo loadings, i.e. below 6% Mo, molybdenum oxide was found to be present in a highly dispersed state. Oxygen chemisorption results suggest that MoO 3 particles disperse better on TiO 2 (anatase) than when they are supported on TiO 2 (rutile). 1 H magic-angle-spinning NMR spectra of MoO 3 catalysts supported on anatase and rutile polymorphs reveal the presence of two types of OH groups on the surface: OH groups of acidic nature and the basic OH groups. Temperature programmed reduction profiles of MoO 3 /TiO 2 samples suggest that the reduction of MoO 3 to Mo proceeds in two stages and the reducibility of MoO 3 increases with Mo loading in the catalysts. The catalytic properties were evaluated for the vapor-phase ammoxidation of 3-picoline to nicontinonitrile and are related to oxygen chemisorption sites on the surface.
The potential of supported molten salts in the removal of soot from diesel exhaust gas
Applied Catalysis B-environmental, 1999
An exploratory study has been carried out to ®nd a satisfactory support for the promising catalytic phase based on eutectic mixtures of Cs 2 O, V 2 O 5 and MoO 3 for the oxidation of diesel soot. High-temperature, high-strength, low-porous ceramics and more conventional porous catalyst supports were investigated as candidate support, i.e.-alumina, g-alumina, cordierite, diatomaceous earth, silica, silicon carbide and silicon nitride. The molten salt was deposited at the external surface of the different supports. Synthetic soot was deposited in a loose contact manner on the supported catalysts. Oxidation rates of 15mg soot g À1 sootY initial s À1 at 650 K were observed. These rates are in the same order of magnitude as found for the best catalytic fuel additives, and, therefore, the molten salt catalyst is promising. Basic requirements for the support±molten salt interaction were formulated: the molten salt should wet the support, the molten salt should`anchor' to the support, and the support should not shield the molten salt from the soot. Low-porous, high-temperature, high-strength ceramics are promising candidate supports for the molten salt soot oxidation catalyst.