Equilibrium Adsorption Method Research Articles

Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

Supported molybdenum oxide catalysts were prepared by an equilibrium adsorption method. The molecular structures of the molybdenum oxide overlayers on different oxide supports (Al 2O 3, TiO 2, ZrO 2, SiO 2, and MgO), under in situ conditions, were investigated by Raman spectroscopy. The molybdenum oxide species on TiO 2, ZrO 2, and Al 2O 3 possess a highly distorted, octahedrally coordinated surface molybdenum oxide species with one short MoO bond regardless of the molybdenum oxide content. The MoO 3/SiO 2, catalysts primarily contain crystalline MoO 3 because of the lower density and reactivity of the silica surface OH groups. The MoO 3/MgO catalysts possess MgMoO 4 and CaMoO 4 compounds due to the high aqueous solubility of MgO and CaO (an impurity in the MgO support) and the strong acid-base interaction between molybdenum oxide and MgO/CaO. The methanol oxidation reaction studies revealed that the MoO 3/TiO 2 (anatase and rutile) and MoO 3/ZrO 2 catalysts are the most active catalysts and that their activities (TOFs) are at least 1-2 orders of magnitude higher than those of the MoO 3/Al 2O 3, MoO 3SiO 2, and MoO 3/MgO catalysts. It was also found that the catalytic activities correlate with the reducibility of the surface molybdenum oxide species on the oxide supports as well as their surface morphology (e.g., molybdenum oxide dispersion and molybdate compound formation). These studies demonstrate that the specific oxide support controls the reactivity of the supported molybdenum oxide phases.

Structure of Crv species on the surface of various oxides : reactivity with NH3 and H2O, as investigated by EPR spectroscopy

Samples containing chromium (both 53Cr-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO2, γ-Al2O3, SnO2, TiO2(anatase) and SiO2 as supports. Heating in O2, generally at 773 K, yielded mononuclear Crv species in a square-pyramidal configuration, Crv5c(A), on all supports with the exception of SiO2 where Crv is in a tetrahedral configuration, Crv4c(A). H2O or NH3, both at room temperature (RT), yielded the Crv6v(A) species from Crv5c(A), that is, the complex changes its coordination from five to six. After H2O adsorption and evacuation at RT, Crv5c(A) is reversibly restored; however, NH3 adsorption and evacuation at increasing temperature gives a new Crv species at 473 K. In the EPR signal of this species, designated Crv5c(B), the perpendicular component is split into three lines with 14NH3(A14N= 4.0 G) and two lines with 15NH3(A15N= 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial O2– ligand replaced by a nitrogen-containing NHx–3x species, possibly the NH–2 anion. With H2O or NH3 at RT, the Crv5c(B) species is transformed into the corresponding hexacoordinated species, Crv6c(B).Upon adsorption of small H2O doses on the CrOx/SiO2 sample, the Crv4c(A) species is transformed into Crv5c(A) and Crv6c(A). In the presence of excess water, the chromyl species on SiO2 becomes unstable, undergoing disproportionation to CrIII and CrVI On adsorption of NH3, Crv5c(B) is formed from Crv4c(A) at RT. With 15NH3, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us to assign the Crv5c(B) species on SiO2 to a slightly distorted chromyl complex with slightly non-planar equatorial ligands.

Preparation of highly dispersed niobium oxide on silica by equilibrium adsorption method

Two series of Nb 2O 5/SiO 2 catalysts are prepared by an equilibrium adsorption method (type A) and a conventional evaporation to dryness method (type E). The loading amounts of type A catalysts were low ( ~ 0.2 wt% as Nb 2O 5) and those of type E catalysts were varied from 0.1 to 10 wt%. UV/VIS diffuse reflectance spectra of the catalyst samples show that micro particles of Nb 2O 5) are present on the catalysts with high-loading and monomeric or oligomeric niobate species are present on the catalysts with low-loading. By analyzing luminescence spectra of the low-loading samples, we conclude that the type A catalyst contains monomeric NbO 4 tetrahedra and the type E catalyst oligomeric NbO 4 tetrahedra. Propene photo-oxidation on type A catalysts yields propene oxide selectively, formed on monomeric NbO 4, whereas the photo-oxidation on the type E catalyst with low-loading yields propanal selectively. Propanal is the product in decomposition of propene oxide on oligomeric NbO 4 tetrahedra.

Nature of the active site for propene hydrogenation on CrO x/ZrO 2 catalysts

The catalytic activity of CrO x /ZrO 2 for propene hydrogenation has been investigated at 195 K in a flow apparatus. Samples containing 0.05 to 0.8% chromium by weight (0.1 to 1.9 Cr atoms nm −2) were prepared by the equilibrium adsorption method. After heating in O 2 at 773 K (average oxidation number of Cr, n̄=5.5), samples were reduced to a controlled extent with CO at 393 to 623 K. The activity at 195 K of partially reduced samples ( n̄=4.4) is low, but it increases by nearly two orders of magnitude when n̄ is further decreased by 0.2 units. Thereafter the activity changes only slightly with increasing reduction (down to n̄=2.5). The concentration of the Cr v species observed by ESR after heating the samples with O 2 was monitored after each reduction treatment, and it was found to decrease by 17% in the same reduction region in which the catalytic activity increases sharply. The average turnover frequencies per total chromium ( N Cr/molecules s −1) atom −1 of CrO x /ZrO 2 catalysts reduced to the same extent ( n̄=2.5) at 623 K differ by a factor 2.6, in spite of the large variation (factor of 20) of Cr content. The Cr III(NO) 2 complex formed on the surface of reduced CrO x /ZrO 2 by adsorption of NO at room temperature was reacted with CO at room temperature. By IR spectroscopy, it was found that a fraction of the Cr III(NO) 2 species leads to mixed ligand complexes, Cr III(NO) 2CO. Relying on IR, ESR and catalytic results, it is inferred that active sites for the low temperature hydrogenation of propene on reduced CrO x /ZrO 2 catalysts are surface mononuclear Cr III, species with three coordinative vacancies.

Surface structures of supported molybdenum oxide catalysts under ambient conditions

Two sets of supported molybdenum oxide catalysts, wet (dried at room temperature) and calcined (calcined at 773 K) samples, were prepared by an equilibrium adsorption method at different pH values of the impregnating solution. The adsorbed amounts of molybdenum oxide species onto the oxide support are strongly dependent on the pH of the impregnating solution and increase with decreasing pH. The Raman spectroscopic studies reveal that the surface molybdenum oxide species under ambient conditions, wet and calcined, are hydrated and essentially in an aqueous medium. Furthermore, the surface structures of molybdenum oxide species on the oxide support were found to depend on the net surface pH at point of zero charge (PZC) under ambient conditions. The net surface pH at PZC under ambient conditions is determined by the specific oxide support and surface molybdenum oxide coverage. The surface molybdenum oxide structures in the wet, uncalcined, samples are not only dependent on the net surface pH at PZC but also on the number of NH 4 + cations which coordinate to the surface molybdenum oxide species for compensation of net charge: Mo 7O 24 6− species in NH 4 +-rich concentrations (high pH region) favor formation of (NH 4) 6Mo 7O 24 · 4H 2O. Upon calcination, the NH 4 + ions are removed and the surface molybdenum oxide species become rehydrated upon exposure to air by adsorbing moisture. Consequently, the structures of surface molybdenum oxide species in the calcined samples which have been exposed to ambient are also dependent on the net surface pH at PZC.

The dispersion of Cr V and Mo V on the surface of various oxides, as investigated by ESR spectroscopy

Samples containing chromium ( 53Cr-enriched or not) or molybdenum ( 95Mo-enriched or not) were prepared by the equilibrium adsorption or impregnation methods at two different loadings (∼ 0.05 and 0.5%-i% by weight) using as supports ZrO 2, SiO 2, TiO 2, Y 2O 3, Al 2O 3, Ga 2O 3 or La 2O 3. After thermal activation (heating with O 2 at 773 K for the Cr samples followed by reduction with H 2 for the Mo samples). ESR signals from surface Cr V or Mo V species are observed. The spectra are satisfactorily resolved on ZrO 2, SiO 2, TiO 2, Y 2O 3, whereas they are broad (and always structureless in the case of chromium) on Al 2O 3, Ga 2O 3 or La 2O 3, whether 53Cr-enriched ( 95Mo-enriched) was used or not for the preparation of samples. Resolved spectra are assigned to mononuclear Cr V or Mo V surface species. Notably, the ESR spectrum of 95Mo V on Al 2O 3, in contrast to that of 53Cr V on the same matrix, shows partially resolved hyperfine structure which allows for the assignment of the spectrum to mononuclear Mo V species. The finding that the ESR signals of Cr V or Mo V on the surface of Al 2O 3, Ga 2O 3 or La 2O 3 are much broader than on the other matrices is attributed to unresolved interaction with neighbouring 27Al. 69.71Ga or 139La (superhyperfine interaction, shf). The shf interaction is small on Y 2O 3 in view of the low nuclear spin and magnetic moment of the 89Y nucleus and in fact resolved spectra are obtained on this matrix. Computer-calculated spectra assuming mononuclear Cr V or Mo V species interacting with two equivalent 27Al nuclei (slightly anisotropic shf interaction 6 to 8.5 gauss) are in substantial agreement with experimental spectra.

Supported molybdena catalysts: Characterization of oxidized, reduced, and sulfided MoO3 prepared by an adsorption method

Highly dispersed molybdena-titania catalyst can be prepared by an equilibrium adsorption method. In this method, molybdate anions adsorb onto the positively charged titania surfaces via electrostatic attraction by controlling the pH of the impregnating solution and they increase as an inverse function of the pH. 95Mo-NMR and UV spectroscopic studies of impregnating solution show that the polymeric species like Mo7O24 6-ions are adsorbed on titania in the acidic impregnating solution. XRD, Raman, and XPS data of the calcined samples show that mono-layer coverage of molybdenum oxide over-layer possesses a highly distorted MoO6 group with a molecular geometry resembling the distorted square pyramid. The catalytic oxidation of methanol over the surface molybdate species on titania possesses higher turnover numbers and higher selectivities of partial oxidation products than the catalysts supported on alumina, silica, zirconia, or magnesia. Changes of the surface properties either after reduction and sulfiding treatment over monolayer catalyst on titania have also been investigated. The NO chemisorption and XPS studies show that two types of active sites appeared after reduction treatment: one site is active for hydrogenation of 1,3-butadiene and the other site is active for metathesis of propene. A higher degree coordinative unsaturations of MO is required for hydrogenation than metathesis. After sulfiding treatments of the catalyst, hydrogenation of 1,3-butadiene also requires triply coordinative unsaturation, and hydrogenolysis of thiophene requires the ensemble of doubly or triply coordinative unsaturations.

Ion scattering and electron spectroscopic study of catalysts prepared by adsorption of molybdate on alumina

A series of Mo/Al 2 O 3 catalysts was prepared by using an equilibrium adsorption method. The dispersion of the molybdenum phase and coverage of the alumina surface by Mo species as a function of Mo loading were monitored by ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS, ESCA). By modeling changes in the ISS Mo/Al intensity ratio as a function of molybdenum coverage, It is shown that, for Mo/Al 2 O 3 catalysts prepared by equilibrium adsorption, coverage of the alumina surface should be complete at a loading of 47×10 13 Mo atoms/cm 2 for calcined catalysts

ChemInform Abstract: Spectroscopic Studies of Molybdate Species Deposited on a Nb2O5 Support

Abstract(XRD, Raman, FTIR, diffuse reflectance, ESCA, FABMS‐SIMS) show that the nature and structure of surface molybdate species mainly depend on the Mo loading and to a lesser extent on the preparation method (impregnation at pH 0.5 and 5.5, equilibrium adsorption methods).

Hydrodesulfurization activity of WO 3/γ-alumina prepared by the equilibrium adsorption method

The electrophoretic properties of WO 3/γ-Al 2O 3 catalysts prepared by the adsorption equilibrium method at pH values between 1 and 10 were studied. This study indicates the formation of a tungsten monolayer structure if pH values lower than six are used, and a multilayer structure if pH values higher than six are employed. Activity for the hydrodesulfurization of gas-oil shows that multilayer structure leads to higher activity than monolayer structure.

Supported molybdenum oxide catalyst. Structure and chemistry of oxidized, reduced, and sulfided surfaces.

Highly dispersed molybdena-titania catalyst can be prepared by an equilibrium adsorption method. In this method, molybdate anions adsorb onto positively charged titania surfaces via electrostatic attraction by controlling pH of the impregnating solution. The amounts of absorbed molybdate anion are strongly dependent on pH of the impregnating solution and they increase as an inverse function of pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide over-layer possesses a highly distorted MoO6 group with a molecular geometry resembling a distorted square pyramid. The Mo species are fixed by an oxygen bridge with Ti, as Mo-O-Ti, or with neighboring Mo, as Mo-O-Mo.The catalytic oxidation of methanol over the surface molybdate species on titania possess higher turnover numbers and higher selectivities of partial oxidation products than do the catalysts exceeding monolayer coverage which possess the crystalline MoO3 phase. Changes of the surface properties either, after reduction and sulfiding treatment over monolayer catalyst, have also been investigated. The NO chemisorption and XPS studies show that two types of active sites appeared after reduction treatment: one site is active for hydrogenation of 1, 3-butadiene and the other site is active for metathesis of propene. A higher degree of coordinative unsaturations of Mo is required for hydrogenation than for metathesis. After subjecting sulfiding treatments to the catalyst, hydrogenolysis of thiophene requires the ensemble of doubly or triply coordinative unsaturations.

Physicochemical properties of MoO 3TiO 2 prepared by an equilibrium adsorption method

The adsorption phenomena of molybdena species onto titania surfaces and the surface properties of the catalysts have been studied by using an equilibrium adsorption method. 95Mo NMR and UV spectroscopic studies show that the aqueous molybdena species vary as a function of the pH of the impregnating solution. For acidic pH values, polymeric species, Mo 7O 24 6 ions, are present, while in the basic solutions it is the monomeric MoO 4 2− ions that are present. The adsorbed amounts of molybdate anion are strongly dependent on the pH of the impregnating solution and increase as an inverse function of the pH. XRD, Raman, and XPS data of the calcined samples show that monolayer coverage of molybdenum oxide is established at pH 3.98 (6.6 wt%). The Raman studies reveal that the molybdenum oxide monolayer is composed of distorted octahedra. At more acidic pH regions, pH < 3.98, crystalline MoO 3 is formed above monolayer coverage. The results of catalytic oxidation of methanol show that the catalysts up to monolayer coverage of surface molybdate species possess higher turnover numbers than the catalysts possessing more than monolayer coverage (presence of crystalline MoO 3. The primary methanol oxidation product is dimethoxymethane at low conversions; methyl formate is next in abundance. The selectivity for dimethyl ether, which occurred as a side reaction on the acidic sites of catalysts, increases as the Mo loading increases.

Preparation and characterization of equilibrium adsorption-prepared molybdena-alumina catalysts

The preparation of molybdena-alumina catalysts was studied using an equilibrium adsorption method at 50 °C in which the molybdenum anions from solution were adsorbed on the alumina surface at pH values between 5.70 and 7.70 near to its isoelectric point. In these experimental conditions a two-step adsorption isotherm was found, indicating that the loading of the carrier was very dependent on the solute concentration. By combining different characterization techniques, namely, temperature-programmed reduction and kinetics of reduction, reflectance spectroscopy, chemisorption of NO and O 2 probe-molecules, infrared spectroscopy of NO, and extraction by ammonia, it is shown that at low solute concentrations the Mo is highly dispersed, mostly as tetrahedrally coordinated (by oxygen ions) Mo species. However, poorer dispersion was found at intermediate and high solute concentrations. In this latter case, the combined use of chemisorption and extraction data indicated that on calcining at 500 °C the surface of the carrier becomes populated with both tetrahedrally (monolayer) and octahedrally (small clusters) coordinated Mo-species.

Fischer-tropsch synthesis on charcoal-supported molybdenum: The effect of preparation conditions and potassium promotion on activity and selectivity

High dispersion and high activity charcoal-supported molybdenum catalysts were prepared using incipient wetness and equilibrium adsorption methods. Activities and selectivities of these catalysts were tested after reducing the catalysts at 773 K in flowing hydrogen. It was found that all the catalysts, particularly those prepared by the equilibrium adsorption method, showed high activities in Fischer-Tropsch synthesis of C 1 to C 3 hydrocarbons. Potassium promotion increased the selectivity towards C 2 and C 3 hydrocarbons while reducing moderately the activity. Without potassium promotion, C 2+ selectivities of all the catalysts with metal loading of three weight percent or higher were about 50 ± 5% under differential reactor conditions. Promotion by potassium increased the C 2+ selectivity to as much as 70%. It was found that there was a critical potassium loading at which point the activity decrease levelled off. The method of potassium addition to the catalyst also plays a major role in determining the final activity.

Preparation of Co-Mo-γAl 2O 3 and Ni-Mo-γAl 2O 3 catalysts by ph regulation of molybdenum solution. characterization of supported species and hydrogenation activities

A new preparation method of the oxidic precursors of Co-Mo-γAl 2O 3, and Ni-Mo-γ Al 2O 3 hydrotreating catalysts has been investigated. The oxo-molybdenum species were added first by an equilibrium adsorption method in a fluidised bed with a pH control of the solution by a regulation process. Different pH were chosen assuming we have different oxo-molybdenum species in solution checked by Laser Raman Spectroscopy (LRS). After subsequent drying and calcination, cobalt or nickel were added by a pore filling method. The oxide precursors have been characterized by X-ray photoelectron Spectroscopy (XPS) and LRS. Then the sulphided catalysts were tested in Hydrogenation of toluene and cyclohexene and the spent sulphided samples have been characterized by XPS. The hydrogenation activities were found to be always more important for Ni-Mo-Al 2O 3 than for Co-Mo-Al 2O 3, catalysts and dependent of the preparation conditions (pH or metal loading). Maximum activities were found for samples impregnated at neutral pH with medium metal loading of the carrier.