Increasing Mo Loading Research Articles

CVD Synthesis of Alumina-supported Molybdenum Carbide Catalyst

Abstract The alumina-supported Mo carbide catalyst was synthesized using a chemical vapor deposition (CVD) technique. The CVD carbide exhibited an extremely high activity for the reduction of CO2, compared to the alumina-impregnated carbide. The XPS and XRD analyses showed the formation of Mo carbide on the alumina. The increase in the BET surface area of the CVD carbide with increasing Mo loading suggested that very fine particles of Mo carbide were formed on the alumina.

Dehydrogenation and aromatization of methane in the absence of oxygen on Mo/HZSM-5 catalysts before and after NH4OH extraction

Mo/HZSM-5 catalysts show good catalytic reactivity in the absence of oxygen for the dehydrogenation and aromatization of methane at 973 K. The active Mo species were investigated by combining catalytic studies on Mo/HZSM-5 catalysts before and after NH4OH extraction with XRD, BET, NH3-TPD and TPR analysis. The XRD patterns show that Mo species are well dispersed on the zeolite surface. The specific surface areas decrease with increasing Mo loading but they can be restored to a large extent by NH4OH extraction. NH3-TPD results suggest that the Mo species prefer to deposit on the strong acid sites of HZSM-5 zeolite. TPR profiles show that there is a kind of Mo species which is easily reduced. No TPR peaks could be obviously observed if the Mo/ HZSM-5 catalysts were extracted by NH4OH solution. The results of NH4OH extraction experiment and other relevant characterization studies suggest that there are several kinds of Mo species deposited on the surface. By referring to the Mo species on Al2O3 supported MoO3 samples, we propose that the dissolvable Mo species in NH4OH solution are MoO3 crystallites and their aggregates in octahedral coordination, while the unsoluble Mo species mainly are Al2(MoO4)3 and MoO 4 2− in tetrahedrally coordinated form. The catalytic performance of Mo/HZSM-5 catalysts before and after NH4OH extraction illustrates that Mo species in small MoO3 crystallites with octahedral coordination form are active for methane activation in the absence of oxygen on Mo/HZSM-5 catalysts, while Mo species in tetrahedrally coordinated form is less active for the reaction.

Silica-Alumina-Supported Acidic Molybdenum Catalysts - TPR and XRD Characterization

A series of silica-aluminas with controlled composition has been synthesized and used as supports for Mo oxide catalysts. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) were used to study the role of support composition upon the stability of various oxidic species, including products of solid state reaction between Mo oxide and silica-alumina. In general, the reducibility of Mo increases with increasing Mo loading, regardless of support composition. For a fixed Mo loading, however, extent of reduction increases with SiO 2 content and shows a maximum at a composition SiO 2 : Al 2O 3 = 75 : 25 (wt%). Except for silica-rich catalysts, the average oxidation state of Mo does not reach below +4 at the conditions used for TPR (maximum temperature: 550°C). Most of the support compositions yield XRD-amorphous Mo oxide. Only Mo-rich and silica-rich catalysts (MoO 3 ≥ 12 wt%; SiO 2 ≥ 50 wt%) contain crystalline phases of Mo: orthorhombic MoO 3 on 100% SiO 2, and MoO 3 and Al 2(MoO 4) 3 On silica-aluminas.

The structure and function of supported molybdenum nitride hydrodenitrogenation catalysts

A series of γ-Al 2O 3 supported Mo nitrides were prepared by the temperature programmed reaction of supported molybdates with NH 3. Several loadings, heating rates, and space velocities were employed in an attempt to vary the properties of the nitride. The structure and composition of the supported nitrides depended on the structure of the precursor oxide and the conditions employed in nitriding the oxide. The ease with which the oxide was nitrided improved with increasing Mo loading. Nitride domains in the low-loaded materials (4 and 8 wt% Mo) were highly dispersed and X-ray amorphous while the higher-loaded catalysts (16 wt% Mo) consisted of γ-Mo 2N crystallites. The response of the high-loaded materials to the various nitriding conditions was similar to that observed for unsupported γ-Mo 2N. The supported Mo nitrides were active for the hydrodenitrogenation of pyridine. In fact, their activities were superior to that of a commercial sulfided NiMo hydrotreating catalyst and comparable to those of the unsupported Mo nitrides. The catalytic properties of the supported Mo nitrides depended on the size and composition of the Mo nitride domains. We believe that the most active sites were located at the perimeters of two-dimensional, raft-like domains. Regions near the perimeter also appeared to be nitrogen deficient. Lower activity sites were associated with the γ-Mo 2N crystallite surfaces.

Morphology and activity of MoS 2 on various supports: Genesis of the active phase

The morphology and hydrodesulfurization (of thiophene) activity of unpromoted MoS 2 supported on alumina, silica, titania, and zirconia have been examined. In the oxidic catalyst, no surface molybdenum phases could be identified by transmission electron microscopy. For all supports except titania, an additional bulk molybdenum oxide phase appeared above a particular Mo loading. In the case of the alumina support, bulk Al 2(MoO 4) 3 also appeared. In the sulfided catalysts, the bulk species retained their original morphology and were surrounded by a skin of MoS 2 several layers thick. The core of the bulk MoO 3 was reduced to Mo metal. On the surface of the SiO 2 and Al 2O 3, the MoS 2 was present as flakes up to five layers thick, sitting vertically on the support. For TiO 2 and ZrO 2 the MoS 2 was present as “islands,” which expanded with increased Mo loading, eventually encapsulating the support particles. The reactivity behavior also fell into two groups, corresponding to the two forms of MoS 2 morphology. The specific activity of MoS 2 supported on TiO 2 and ZrO 2 was higher than that for the case of Al 2O 3 and SiO 2 supports.

X-ray absorption spectroscopy studies of calcined MoAl 2O 3 and CoMoAl 2O 3 hydrodesulfurization catalysts

X-ray absorption spectroscopy has been used to obtain information on the Co and Mo structures present in calcined CoMoAl 2O 3 and MoAl 2O 3 catalysts. The results indicate that the elements are present in disordered structures. From a detailed analysis of the X-ray absorption fine structure (EXAFS), information on the oxygen coordination around Co and Mo is obtained. An apparent decrease in the oxygen coordination number around Mo with increasing Mo loading is explained by a change in the Debye-Waller factor, presumably due to a decrease in the support interactions. The presence of Co in the catalysts is found to have very little effect on the local structure around the Mo atoms. The Co EXAFS results indicate that Co is predominantly octahedrally coordinated.

Physicochemical characterization of MoO 3-NaY zeolite catalysts

A series of Mo-impregnated NaY zeolite catalysts, with a Mo loading in the range 1–15 wt%, MoO 3, have been studied by X-ray diffraction, transmission electron microscopy, ir and diffuse reflectance spectroscopy, temperature-programmed reduction, specific surface area, water content, and extraction measurements, in order to characterize them structurally. The crystallinity, specific surface area, and water content of the Mo-NaY were found to decrease almost linearly with increasing Mo loading. These effects were accompanied by an increasing perturbation of the zeolite framework. The ir spectra and, particularly, the diffuse reflectance spectra suggest that Mo is predominantly present as a tetrahedrally coordinated MoO 4 2− species, presumably within the zeolite cavities. The Mo extraction and temperature-programmed reduction results revealed the existence of a strong interaction between Mo species and zeolite, which apparently decreases with increasing Mo loading.

Hydrodesulphurization activity and coking propensity of carbon and alumina supported catalysts

Co—Mo, Fe and Mo catalysts were prepared on alumina and carbon supports. The reactivity of sulphided catalysts for thiophene hydrodesulphurization and the propensity for coke deposition of nonsulphided catalysts during propylene cracking and anthracene carbonization were measured. The hydrodesulphurization activity of Co—Mo and Fe catalysts increased in the order γ- Al 2 O 3 < C-black composite < active carbon. For a given metal, considerable differences in activity were found both between the carbons and alumina and between the carbons themselves. Possible reasons for the different activities include the influence of the supports on metal dispersion and the presence of impurities in the carbons. For alumina supported catalysts there is a correspondence between hydrodesulphurization activity and coking propensity. On carbon supported catalysts, the rates of carbon deposition appear insensitive to the nature of the metal component(s) whereas hydrodesulphurization activity is not. Thus it is possible to enhance the latter without attendant increases in the susceptibility to coke-forming reactions. Increasing Mo loading from 0–12 wt% was found to substantially increase the rate of carbon deposition on alumina, whereas the effect on a carbon support was comparatively small. This behaviour appears to relate to the inherent support acidity and its modification on metal addition.