Michael A. Vuurman

Characterization of chromium oxide supported on Al2O3, ZrO2, TiO2, and SiO2 under dehydrated conditions

Abstract In the present investigation, Raman and IR spectroscopy were used to study the surface structures of chromium oxide supported on alumina, titania, zirconia, and silica, as a function of the loading under dehydrated conditions. It was found, that the dehydrated surface structures of chromium oxide differ strongly from those previously reported under ambient conditions, in which the surfaces are hydrated. Two species, each possessing one short terminal CrO bond, and one (or more) oligomer (s) are proposed to be present on the dehydrated alumina, titania, and zirconia surfaces. The relative concentrations of these different chromium oxide species is independent of the surface coverage. The chromium oxide species present on the dehydrated silica surface are completely different from those observed on the other three supports. The Raman and IR spectra indicate the presence of an isolated chromium oxide species possessing two short CrO bonds together with a small amount of surface species possessing a terminal CrO 3 unit, isolated or not. The gradual disappearance of the surface hydroxyl groups of all four supports upon addition of chromium oxide, as monitored by IR spectroscopy, suggests that the chromium oxide species interacts with the surface by removal of the surface hydroxyl groups.

Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Combined Raman and IR study of MOx–V2O5/Al2O3(MOx= MoO3, WO3, NiO, CoO) catalysts under dehydrated conditions

The influence of a second metal oxide (tungsten oxide, molybdenum oxide, nickel oxide, cobalt oxide) upon a V2O5/Al2O3 catalyst has been investigated by a combined Raman and IR study under dehydrated conditions. The presence of tungsten or molybdenum oxide was found to increase the concentration of polymerized surface vanadium oxide species, and this reflects the higher surface coverages of the metal oxides on the alumina support. This is probably caused by competition between vanadium and molybdenum oxide (or vanadium and tungsten oxide) species for reaction with the alumina hydroxy groups, since the IR spectra showed that on addition of these metal oxides the same type of alumina hydroxy groups are consumed. The presence of nickel or cobalt oxide on alumina increases the concentration of polymerized vanadium oxide species dramatically, which indicates that the presence of these oxides is also experienced by the surface vanadium oxide. However, the hydroxy groups are not affected as they are for the molybdenum and tungsten oxide systems.

Structural determination of surface rhenium oxide on various oxide supports (Al2O3, ZrO2, TiO2 and SiO2)

In the present investigation Raman and IR spectroscopy were used to determine the structure of Re2O7/Al2O3, Re2O7/ZrO2, Re2O7/TiO2 and Re2O7/SiO2 as a function of loading under ambient and in situ dehydrated conditions. Under ambient conditions, the surface rhenium oxide is hydrated and possesses the structure of the ReO4 ion in aqueous solution, independent of coverage or support type. Under dehydrated conditions, the in situ Raman and IR spectra show that only one surface rhenium oxide species is present on the silica support, whereas two surface rhenium oxide species are present on the alumina, zirconia and titania supports. The concentration ratio of the two species is a function of the coverage, and their structures are similar, possessing three terminal ReO bonds and one bridging ReO-support bond (C3v symmetry). Differences in properties between the surface rhenium oxide species were determined by TPR as a function of coverage and support type. The in situ Raman, IR and TPR measurements suggest that the bridging ReO-support bond strength decreases with increasing coverage, while the TPR data further indicate that the ReO-support bond strength decreases in the order Al2O3 ⪢ SiO2 ≈ ZrO2 ⪢ TiO2 for a given rhenium oxide loading.

Characterization of CrO3/Al2O3 catalysts under ambient conditions: Influence of coverage and calcination temperature

Abstract The CrO 3 /Al 2 O 3 catalyst system was investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and BET surface area measurements in order to determine the molecular structures and monolayer coverage of the surface chromium oxide phase under ambient conditions. Up to a surface coverage of 9% CrO 3 /Al 2 O 3 , the chromium oxide is stabilized by the alumina support in the +6 oxidation state after calcination at 120–1050°C. The molecular structures of the chromium(VI) oxide surface species are a function of the surface coverage and calcination temperature because under ambient conditions the surface structures depend on the net surface pH at point of zero charge of the hydrated oxide surface. Increasing the surface coverage results in a decrease of the net surface pH and formation of more polymerized chromium oxide species. High calcination temperatures (⩾950°C) cause a reduction of the BET surface area, as well as a phase transformation of γ-Al 2 O 3 into θ,δ-Al 2 O 3 , and also result in an increase in the surface density of the chromium oxide overlayer (more polymerized surface chromium oxide species). Monolayer coverage is reached at ca . 12% CrO 3 /Al 2 O 3 and crystalline Cr 2 O 3 particles are found on the alumina surface together with surface chromium oxide species at higher loadings. At high calcination temperatures (⩾800°C), the Cr 2 O 3 crystalline particles react with the alumina support to form Cr(III) in solid solution with α-Al 2 O 3 (corundum). The α-Al 2 O 3 lattice is slightly expanded due to the incorporation of chromia.

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.

Raman spectra of chromium oxide species in CrO3/Al2O3 catalysts

Abstract Raman spectra are reported of chromium(VI) oxide species in Cr/Al2O3 catalysts, and of aqueous solutions of chromium oxide used for the preparation of these catalysts. The Raman spectra reveal that after impregnation and drying at room temperature under vacuum, the chromium oxide is present as chromate submonolayer at 1 wt.% Cr/Al2O3, as dichromate between 5 and 15 wt.% Cr/Al2O3, as trichromate at 20 wt.% Cr/Al2O3 and as crystalline CrO3 and trichromate at 30 wt.% Cr/Al2O3. The supported chromium oxide species are in wet condition. The chromate in the 1 wt.% Cr/Al2O3 sample is stable upon calcination in air at 823 K. Preparation of the 1 wt.% Cr/Al2O3 catalyst from a Cr(NO3)3 solution gave the same result. It is shown that the molecular structure of the oxides is influenced by the chromium oxide concentration of the impregnation solutions, by the concentration in the catalysts and by the water content of the catalysts, which was varied by exposure to air and by using different laser powers.

Raman spectroscopy of V2O5, MoO3, Fe2O3, MoO3-V2O5, and Fe2O3-V2O5 supported on alumina catalysts: Influence of coverage and dehydration

Abstract In the present investigation the influence of addition of molybdenum oxide and iron oxide to a 5% V 2 O 5 /Al 2 O 3 sample has been studied by Raman spectroscopy under ambient as well as dehydrated conditions. The results show that under ambient conditions the presence of both vanadium oxide and molybdenum oxide on the hydrated alumina surface results in the conversion of hydrated metavanadate species into hydrated decavanadate species and of hydrated MoO 4 2− species into hydrated octahedrally coordinated clusters. This is due to the decrease in the net pH at point of zero charge relative to the individual metal oxide systems. For the mixed Fe 2 O 3 -V 2 O 5 /Al 2 O 3 sample it has been found that under ambient conditions the hydrated metavanadate species are converted into hydrated pyrovanadate and orthovanadate species, which is ascribed to the basic nature of the surface iron oxide species. Under dehydrated conditions the presence of both vanadium oxide and molybdenum oxide on the alumina surface results in increased concentrations of surface vanadate and molybdate species, which are found at much higher surface coverage in the corresponding individual metal oxide systems. A similar effect has been observed in the presence of iron oxide, which also promotes the formation of a moderately distorted vanadium oxide species.

A Cryogenic Cell for Liquid Noble Gases. Vibrational Spectroscopy and Photochemistry of Transition Metal Carbonyls in Liquid Xenon

The construction of a special cryogenic cell for spectroscopic and photochemical measurements in liquefied noble gases under pressure is described. The inner (sample) cell, withstanding a pressure of at least 700 psi, has no high-vacuum around it. It has two crossed IR and UV-visible optical pathways of 30 mm. The usefulness of these noble gases in vibrational spectroscopy is demonstrated for the following transition metal carbonyls, dissolved in liquid xenon (=LXe, pressure <150 psi, 170 < T < 240 K): [W(CO)6], [Mn2(CO)10l, and [Co2(CO)8]. The great advantage of LXe is its complete transparency over a wide spectral range. The limited solubility of many complexes in LXe in comparison with “normal” solvents is often compensated by the long optical pathway of the cell. Because of its complete inertness, reactive intermediates and products of photochemical reactions can be stabilized in LXe, even at moderate temperatures. The photochemical reaction is described of [W(CO)6] with a l,4-diaza-l,3-butadiene (=R-DAB; RN=CHCH=NR) ligand. During this reaction a photoproduct is identified as a stable complex which is so unstable in normal solvents that it can only be observed with rapid-scan FT-IR spectroscopy.

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.