Israel E. Wachs

Raman and IR studies of surface metal oxide species on oxide supports: Supported metal oxide catalysts

Abstract Raman and infrared spectroscopy provide complementary information about the nature of the surface metal oxide species present in supported metal oxide catalysts. This paper reviews the type of fundamental information that is typically obtained in Raman and IR characterization studies of supported metal oxide catalysts. The molecular structures of the surface metal oxide species are reflected in the terminal M=O and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports is determined by directly monitoring the specific surface hydroxyls of the support that are being titrated. The surface coverage of the surface metal oxide species on the oxide supports can be quantitatively obtained since at monolayer coverage all the reactive surface hydroxyls are titrated and additional metal oxide results in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites present in supported metal oxide catalysts are determined by adsorbing basic probe molecules like pyridine. Information about the behavior of the surface metal oxide species during catalytic reactions are provided by in situ characterization studies. Such fundamental information is critical for the development of molecular structure-reactivity relationships for supported metal oxide catalysts. This paper will be limited to supported metal oxide catalysts containing group V-VII transition metal oxides (e.g., V, Nb, Cr, Mo, W and Re) on several different oxide supports (alumina, titania, zirconia, niobia and silica).

Titania–silica as catalysts: molecular structural characteristics and physico-chemical properties

Abstract Recent results on characterization and applications of titania–silica materials as photocatalysts, acid catalysts and oxidation catalysts are reviewed. The similarities and differences in structural characteristics and physico-chemical properties between titania–silica mixed and supported oxides are emphasized. The generation of new catalytic active sites either on the silica surface or in the silica matrix is discussed with respect to the formation of Ti–O–Si bonds and the local structure. The insights obtained from these studies allow a fundamental understanding of the relationships between the structural characteristics and the physico-chemical/reactivity properties of titania–silica catalysts.

Structure and reactivity of surface vanadium oxide species on oxide supports

Abstract Supported vanadium oxide catalysts, containing surface vanadia species on oxide supports, are extensively employed as catalysts for many hydrocarbon oxidation reactions. This paper discusses the current fundamental information available about the structure and reactivity of surface vanadia species on oxide supports: monolayer surface coverage, stability of the surface vanadia monolayer, oxidation state of the surface vanadia species, molecular structures of the surface vanadia species (as a function of environment and catalyst composition), acidity of the surface vanadia species and reactivity of the surface vanadia species. Comparison of the molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during hydrocarbon oxidation reactions: (1) the role of terminal VO, bridging VOV and bridging VO-support bonds, (2) the number of surface vanadia sites required, (3) the influence of metal oxide additives, (4) the influence of surface acidic and basic sites, (5) the influence of preparation methods and (6) the influence of the specific oxide support phase. The unique physical and chemical characteristics of supported vanadia catalysts, compared to other supported metal oxide catalysts, for hydrocarbon oxidation reactions are also discussed.

The selective oxidation of CH3OH to H2CO on a copper(110) catalyst

The oxidation of methanol to formaldehyde was studied on a Cu(110) single crystal by flash decomposition spectroscopy. The Cu(110) surface was preoxidized with oxygen-18; deuterated methanol, CH3OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Cu(110) surface, but the ability of the copper surface to chemisorb methanol was greatly enhanced by surface oxygen. This enhancement was absent subsequent to reduction of the surface by methanol itself. CH3OD was selectively oxidized upon adsorption at 180 °K to adsorbed CH3O and D218O. The methoxide was the most abundant surface intermediate and decomposed at about 365 °K to formaldehyde and hydrogen with a first-order rate constant equal to 5.2 ± 1.6 × 1012 exp(−22.1 ± 0.1 kcal/mole/RT) s−1. To a lesser extent some methanol was oxidized to HC16O18O which subsequently decomposed to C16O18O and hydrogen with a first-order rate constant equal to 8 ± 2 × 1013 exp(−30.9 ± 0.2 kcal/mole/RT) s−1. A pronounced maximum in oxidation activity with surface oxygen coverage was observed.

The oxidation of methanol on a silver (110) catalyst

The oxidation of methanol was studied on a Ag(110) single-crystal by temperature programmed reaction spectroscopy. The Ag(110) surface was preoxidized with oxygen-18, and deuterated methanol, CH3OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Ag(110) surface, and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen. CH3OD was selectively oxidized upon adsorption at 180 K to adsorbed CH3O and D218O, and at high coverages the D218O was displaced from the Ag(110) surface. The methoxide species was the most abundant surface intermediate and decomposed via reaction channels at 250, 300 and 340 K to H2CO and hydrogen. Adsorbed H2CO also reacted with adsorbed CH3O to form H2COOCH3which subsequently yielded HCOOCH3 and hydrogen. The first-order rate constant for the dehydrogenation of D2COOCH3 to DCOOCH3 and deuterium was found to be (2.4 ± 2.0) × 1011 exp(−14.0 ± 0.5 kcalmole · RT)sec−1. This reaction is analogous to alkoxide transfer from metal alkoxides to aldehydes in the liquid phase. Excess surface oxygen atoms on the silver substrate resulted in the further oxidation of adsorbed H2CO to carbon dioxide and water. The oxidation of methanol on Ag(110) is compared to the previous study on Cu(110).

Acidic properties of supported niobium oxide catalysts: An infrared spectroscopy investigation

Abstract Chemisorption of pyridine was applied as a method for studying the acidic properties of niobium pentoxide supported on silica, magnesia, alumina, titania, and zirconia. The infrared spectra of adsorbed pyridine were used to evaluate the concentration and the relative strength of Bronsted and Lewis acid sites. Lewis acidity was found in all the supported niobium oxide systems, while Bronsted acid sites were only detected for niobia supported on the al umina and silica supports. The origin and characteristics of the surface acid sites present in supported niobium oxide catalysts are discussed in the present study.

The interaction of vanadium pentoxide with titania (anatase): Part I. Effect on o-xylene oxidation to phthalic anhydride

Abstract Two types of vanadia are present in active V205/TiO2(anatase) catalysts:a surface vanadia species coordinated to the TiO2 support and crystallites of V205. The surface vanadia is the active site in V20/TiO2(anatase)catalysts for the oxidation of o-xylene to phthalic anhydride. The TiO2(anatase)support must be covered by a complete monolayer of the surface vanadia species since exposed titania sites lead to complete combustion of the partial oxidation products. The surface vanadia species possess a higher activity and selectivity than crystalline V205 for this oxidation reaction. Moderate amounts of crystalline V905 do not significantly affect the catalytic performance of V205/TiO2(anatase) because of the low effective surface area and poor catalytic activity of this phase. The unique properties of the surface vanadia species are related to the vanadiatitania interaction.

Oxidation of sulfur dioxide to sulfur trioxide over supported vanadia catalysts

The objectives of this research are to establish the fundamental kinetics and mechanism of sulfur dioxide oxidation over supported vanadia catalysts and use these insights to facilitate the design of SCR DeNOx catalysts with minimal sulfur dioxide oxidation activity. A series of supported vanadia catalysts were prepared on various metal-oxide supports: ceria, zirconia, titania, alumina and silica. Raman spectroscopy was used to determine the coordination of surface species. At low vanadia loadings, vanadia preferentially exists on oxide support surfaces as isolated tetrahedrally coordinated (M‐O)3V a5 aO species. At higher vanadia loadings, the isolated (M‐O)3V a5 aO species polymerize on the oxide support surface breaking two V‐O‐M bonds and forming two V‐O‐V bridging bonds. The turnover frequency for sulfur dioxide oxidation was very low, 10 ˇ4 to 10 ˇ6 s ˇ1 at 4008C, and was independent of vanadia coverage suggesting that only one vanadia site is required for the oxidation reaction. As the support was varied, sulfur dioxide oxidation activity of the supported vanadia catalysts varied by one order of magnitude (Ce>Zr, Ti>Al>Si). The basicity of the bridging V‐O‐M oxygen appears to be responsible for influencing the adsorption and subsequent oxidation of the acidic sulfur dioxide molecule. Over the range of conditions studied, the rate of sulfur dioxide oxidation is zero-order in oxygen, first-order in sulfur dioxide and inhibited by sulfur trioxide. The turnover frequency for sulfur dioxide oxidation over WO3/TiO2 was an order of magnitude lower than that found for V2O5/TiO2, and no redox synergism between the surface vanadia and tungsten oxide species was evident for a ternary V2O5/ WO3/TiO2 catalyst. This suggests that WO3 promoted catalysts may be suitable for low-temperature SCR where minimal sulfur dioxide oxidation activity is required. # 1998 Elsevier Science B.V. All rights reserved.

Methanol: A “Smart” Chemical Probe Molecule

A novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts (MgO, CaO, SrO, BaO, Y2O3, La2O3, CeO2, TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, Cr2O3, MoO3, WO3, Mn2O3, Fe2O3, Co3O4, Rh2O3, NiO, PdO, PtO, CuO, Ag2O, Au2O3, ZnO, Al2O3, Ga2O3, In2O3, SiO2, GeO2, SnO2, P2O5, Sb2O3, Bi2O3, SeO2 and TeO2). The number of surface active sites for methanol dissociative adsorption corresponds to ∼3 μmol/m2 on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites on metal oxides since redox sites yield H2CO, acidic sites yield CH3OCH3 and basic sites yield CO2. The distribution of the different types of surface active sites was found to vary widely for the different metal oxide catalysts. In addition, the commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover frequency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10−3 to 104 s−1). An inverse relationship (for metal oxide catalysts displaying high (>85%) selectivity to either redox or acidic products) was found between the methanol oxidation TOF values and the decomposition temperatures of the surface M–OCH3 intermediates reflecting that the decomposition of the surface M–OCH3 species is the rate-determining step during methanol oxidation over the metal oxide catalysts.

The interaction of V2O5 with Ti02(anatase): Catalyst evolution with calcination temperature and O-xylene oxidation

The interaction of V2O5 with the surface of TiO2(anatase) was studied over the temperature range 110–750 °C. The V2O5TiO2(anatase) system was characterized with laser Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, X-ray diffraction, thermal gravimetric analysis, BET, and catalytic performance for o-xylene oxidation to phthalic anhydride. The state of V2O5TiO2(anatase) possessing high loadings of vanadia is strongly dependent on calcination temperature. In the presence of vanadia the TiO2(anatase) support exhibits a simultaneous loss in surface area and structural transformation to rutile at elevated calcination temperatures. The morphology of the supported vanadia phase also depends on calcination temperature. At low calcination temperatures, 110–200 °C, the vanadia exists as vanadyl oxalate, the starting vanadia salt. At intermediate calcination temperatures, 350–575 °C, vanadia is present as a complete monolayer of surface vanadia species coordinated to the titania support and V2O5 crystallites. At calcination temperature of 575 °C and above, the supported vanadia phase reacts with the TiO2(anatase) support to yield VxTi1 −xO2(rutile). These structural changes have a pronounced effect on the catalytic performance of V2O5TiO2(anatase) catalysts for the oxidation of o-xylene. The optimum catalytic performance is observed for prolonged calcination at intermediate temperatures, 350–575 °C, where a complete monolayer of surface vanadia exists on the TiO2(anatase) support. The complete monolayer of surface vanadia and crystalline vanadia phases remain intact during the o-xylene oxidation reaction and become partially reduced by the reaction environment.