Du Soung Kim

Surface structures of supported tungsten oxide catalysts under dehydrated conditions

The molecular structures of the WO3/support (Al2O3, TiO2, Nb2O5, ZrO2, SiO2, and MgO) catalysts under in situ dehydrated conditions have been investigated by Raman spectroscopy. The series of catalysts was synthesized by the aqueous incipient wetness method. The WO3/support catalysts, with the exception of the WO3SiO2 and WO3MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short WO bond (mono-oxo tungsten oxide species) at high surface coverages. The WO3SiO2 catalysts exhibit strong Raman features of crystalline WO3 particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO3MgO catalysts possess non-stoichiometric compounds, Mgx(WO4)y and Cax(WO4)y, at low tungsten oxide contents and crystalline MgWO4 and CaWO4 at high tungsten oxide contents. This result is attributed to the high aqueous solubility of MgO as well as the CaO impurity and the strong acid-base interaction between WO2−4 with Mg(OH)2 and Ca(OH)2. The current findings for supported tungsten oxide catalysts parallel the previous findings for supported molybdenum oxide catalysts and reflect the similar surface structural chemistry of these two oxides.

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 NH4+ cations which coordinate to the surface molybdenum oxide species for compensation of net charge: Mo7O246− species in NH4+-rich concentrations (high pH region) favor formation of (NH4)6Mo7O24 · 4H2O. Upon calcination, the NH4+ 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.

Physicochemical properties of MoO3TiO2 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, Mo7O246 ions, are present, while in the basic solutions it is the monomeric MoO42− 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 MoO3 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 MoO3. 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.

Surface structure and reactivity of CrO3/SiO2 catalysts

The molecular structure of the two-dimensional chromium oxide overlayer on the silica support at different chromium oxide contents has been investigated by in situ Raman spectroscopy. Under dehydrated conditions, the surface chromium oxide species on silica consists of a highly distorted, tetrahedral monochromate species regardless of chromium oxide content. The catalysts above 2% CrO3/SiO2 also contain crystalline α-Cr2O3 particles in addition to the surface monochromate species. The methanol oxidation studies reveal that the catalytic activity per Cr atom, the turnover number (TON), decreases as the chromium oxide content increases. The somewhat higher TON of the initial rate as compared to the steady state rate for the methanol oxidation reaction reflects the higher activity of the fully oxidized chromium oxide species relative to the partially reduced chromium oxide species for the methanol oxidation reaction. The major reaction product is HCHO, while HCOOCH3, CO, and C02 are next in abundance. The selectivity for HCHO increases as the chromium oxide content increases and the precalcination temperature is increased. The opposite trend is observed for the selectivity of HCOOCH3. These selectivity changes are due to dehydroxylization of the silica surface by precalcining at elevated temperatures and increasing the surface chromium oxide species. The current results suggest that HCHO and HCOOCH3 are produced on two different catalytic sites: HCHO is formed on the Cr site, whereas HCOOCH3 is produced via hemiacetal intermediates, which are formed by interaction between HCHO adsorbed on the Cr site and CH3O adsorbed on the silica site. The CO and CO2 combustion products are produced on both the Cr and silica site.

Preparation and characterization of WO3/SiO2 catalysts

Two sets of WO3/SiO2 catalysts were prepared from (NH46H2W12O40 (aqueous method) and W(η3-C3H5)4 (non-aqueous method). The molecular structures and dispersions of the surface tungsten oxide species for the WO3/SiO2 catalysts under ambient and in situ dehydrated conditions were investigated by Raman spectroscopy. The samples prepared from (NH4)6H2W12O40 (aqueous method) exhibit very strong Raman features due to the presence of crystalline WO3 and the samples prepared from W(η3-C3H5)4 (non-aqueous method) do not possess crystalline WO3. These results suggest that the preparation method exerts an influence on the dispersion of the surface tungsten oxide species on SiO2. The surface tungsten oxide species under ambient conditions possess polytungstate clusters, W12O4212−, on the silica support. Upon dehydration at elevated temperatures, the hydrated polytungstate clusters decompose and interact with the silica support via the formation of isolated, octahedrally coordinated tungsten oxide species.

Molecular design of supported metal oxide catalysts: An initial step to theoretical models

Abstract Molecular design of supported metal oxide catalysts is now possible from molecular level information obtained from Raman spectroscopy and the methanol oxidation reaction. The important factors that influence the molecular design of the supported metal oxide catalysts are the specific oxide support and the specific surface metal oxide. The structure or modification of the oxide support, however, has no effects on the surface metal oxide structure and reactivity. The surface coverage of the specific surface metal oxide, however, influences the reactivity during the methanol oxidation reaction. The synthesis method is not critical since it does not influence the surface metal oxide structure or reactivity. Calcination temperature is not important as long as moderate temperatures (350–500°C) are used. The current fundamental information available about the physical and chemical characteristics of the supported metal oxide catalysts provides a foundation for theoretical models to be developed with respect to their solid—solid and solid—gas interactions.

Relationship between structure and point of zero surface charge for molybdenum and tungsten oxides supported on alumina

Laser Raman spectroscopy was used to characterize alumina-supported molybdenum and tungsten oxides at loadings ranging from 0.5 to 15 wt% Mo and 0.5 to 30 wt% W. The structure of calcined Mo6+/Al2O3 and W6+/A12O3 was governed by the point of zero surface charge of each system, with the point of zero surface charge being dependent on metal loading. The structures formed by the molybdenum and tungsten overlayers at the sample point of zero surface charge were found to be analogous to the structures formed by molybdenum and tungsten oxyanions in aqueous solution at a solution pH equal to the sample point of zero surface charge.

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 Mo7O246-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.

Molecular Design of Supported Metal Oxide Catalysts

Abstract This study demonstrates that molecular design of supported metal oxide catalysts is possible from molecular level information obtained from combined Raman spectroscopy and the methanol oxidation reaction. The important factors that influence the molecular design of the supported metal oxide catalysts are the specific oxide support (factor of ∼10 3 ) and the specific surface metal oxide (factor of ∼10 1 ). The synthesis method is not critical since it does not influence the surface metal oxide structure or reactivity. Calcination temperature is not important as long as moderate temperatures (350–500°C) are used.