Water Chemisorption and Reconstruction of the MgO Surface

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with Delta E = -90.2kJ/mol. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.Comment: RevTeX, 4 pages, 1 Encapsulated Postscript Figur

Effect of mill type on the size reduction and phase transformation of gamma alumina

The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; γ-Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d90 of 37 µm to 2.9 µm compared to planetary ball milling (30.2 µm) and single ball milling (10.5 µm). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired α-Alumina resulting in a notable decrease in surface area from 136.6 m2/g to 82.5 m2/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure

CO hydrogenation catalyzed by alumina-supported osmium: Particle size effects

Alumina-supported catalysts were prepared by conventional aqueous impregnation with [H2OsCl6] and by reaction of organoosmium clusters {[Os3(CO)12], [H4Os4(CO)12], and [Os6(CO)18]} with the support. The catalysts were tested for CO hydrogenation at 250-325 [deg]C and 10 atm, the products being Schulz-Flory distributions of hydrocarbons with small yields of dimethyl ether. The fresh and used catalysts were characterized by infrared spectroscopy and high-resolution transmission electron microscopy. The catalyst prepared from [H2OsCl6] had larger particles of Os (~70 A). The cluster-derived catalysts initially consisted of molecular clusters on the support; the used catalysts contained small Os aggregates (typically 10-20 A in diameter). The catalytic activity for hydrocarbon formation increased with increasing Os aggregate size, but the activity for dimethyl ether formation was almost independent of aggregate size. The hydrocarbon synthesis was evidently catalyzed by the Os aggregates, and the ether synthesis was perhaps catalyzed by mononuclear Os Complexes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25555/1/0000097.pd

Structure and Morphology of Vanadia-Promoted Rh/SiO₂: A Transmission Electron Microscopy Study

Vanadia-promoted Rh/SiO2-catalysts have been prepared by impregnation followed by calcination at 573, 773, 973, and 1173 K. The structure and morphology of these materials in the oxidized state, and after low (523 K) and high (773 K) temperature reduction, were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), microdiffraction, X-ray diffraction (XRD), and CO chemisorption. During calcination at temperatures ≥ 973 K, a RhVO4 phase is formed, which consists of well-crystallized rod-like particles after calcination at 1173 K. After reduction in H2, the catalysts consist of highly dispersed Rh° particles as judged from the electron micrographs. This high dispersion is presumably stabilized by interaction of the zerovalent Rh° with the promoter oxide V2O3. The well-crystallized sample (calcination at 1173 K) cannot be reduced at 523 K, but at 773 K the rhodium, originally present as RhVO4, is quantitatively reduced to small metal particles in contact with V2O3. In contrast to the high dispersion derived from TEM, CO chemisorption gave unexpectedly low CO/Rh ratios, which were even zero for the catalyst calcined at 1173 K. The CO/Rh ratios decreased with increasing calcination temperature (RhVO4 formation) at constant reduction temperature and with increasing reduction temperature at a given calcination temperature. It is suggested that the surface of the highly dispersed Rh° particles is decorated and blocked by VOx species. This effect, though more pronounced at higher reduction temperature, already occurs after reduction at 523 K. Highly dispersed Rh° particles are produced in the materials studied, particularly when a RhVO4 precursor phase was present, these particles being in intimate contact with a V2O3 promoter oxide. In extreme cases (calcination at 1173 K), the encapsulation of Rh° particles seems to be complete, so that no metal surface is exposed

Sb2o3/sb2o4 in Reducing/oxidizing Environments - An In-situ Raman-spectroscopy Study

An in situ Raman characterization of alpha-Sb2O4 and Sb2O3 is reported. The aim of the study was to understand the spectroscopic behavior of these oxides in oxidizing and reducing atmospheres. No detectable variations in the Raman spectra of Sb2O4 can be attributed to either the loss of lattice oxygen (under reducing conditions) or an incorporation of O-18 (during reoxidation). All the observed spectral changes must be due to changes of the Raman scattering tenser with temperature, suggesting a higher symmetry at high temperatures. H2O present in the gas phase has no detectable influence on the exchange properties of Sb2O4. Sb2O3 used in this study is a mixture of molecular (alpha-senarmontite) and polymeric (beta-valentinite) polymorphs. Under reducing conditions the suboxide Sb2O3-x seems to be formed, and reoxidation in O-18(2) leads to a spectrum different from those of (Sb2O3)-O-16 and (Sb2O4)-O-16. Reexchange in 16(2)(O) Or H2O-saturated O-16(2) does not alter the Raman spectra. This may be explained by a stable incorporation of O-18 into the lattice, thus rendering the reexchange impossible, and by an incomplete oxidation to Sb2O4-x under the conditions applied

In-situ Raman-spectroscopy Characterization of O-18 Exchange in Physical Mixtures of Antimony Oxides and Molybdenum Oxide

The interaction of O-18(2) With physical mixtures of MoO3 and Sb2O4 and of MoO3 and Sb2O3 has been investigated by in situ Raman spectroscopy. Signal intensity ratios of the two O-18-related high-frequency Raman bands of MoO3 at 788 and 945 cm(-1) relative to the corresponding O-16-related bands at 820 and 995 cm(-1) lead to the assignment of these bands to the two different terminal Mo=O groups along the a and b axes. At elevated temperatures, the scattering cross section of MoO3 is strongly diminished relative to antimony oxides. MoO3 in the mixture with Sb2O4 does not show enhanced O-18 exchange as compared to pure MoO3, suggesting that efficient spillover of oxygen does not occur in this mixture, probably because of the very slow kinetics of that process. Some spillover during the reexchange with O-16(2) may be indicated by a slightly increased signal intensity of the O-18-related bands. As in the pure compounds, the reexchange with O-16 atoms occurs very slowly and is not influenced by the presence of H2O. Significant reduction of MoO3 in contact with Sb2O3, after evacuation at 648 K as compared to pure MoO3 possibly indicates an intimate cross talk between the two phases, by which MoO3 is reduced via a ''reverse spillover''. Reoxidation of the evacuated sample shows an incorporation of heavy oxygen into the MoO3 lattice. The comparable signal intensities of the O-18-related band and of the line attributed to a suboxide reveal that only oxygen defects are refilled during the O-18 treatment in MoO3. A further oxygen exchange does not seem to take place. In contrast to the reexchange in dry O-16(2), the presence of H2O in the gas phase leads to a distinct decrease of the O-18-related signals. A comparison of the experiments carried out after evaucation at 393 and at 648 K, only shows signals due to O-18 incorporation into MoO3 if MoO3 in the sample had a higher degree of reduction, thus revealing the essential role of lattice defects in the oxygen exchange. The results suggest that in the Sb2O4/MoO3 mixture, surface mobile (spillover) species should be involved to explain the catalytic synergy between the two phases

Oxygen-exchange Properties of Moo3 An In-situ Raman-spectroscopy Study

In situ Raman spectroscopy was used to investigate the oxygen-exchange properties of MoO3 and the role of bulk defects during redox processes. A change of the absorption coefficients between MoO3 and molybdenum suboxides as evidenced by a change in color from pale yellow to gray blue may contribute to a general decrease in Raman intensities. An alteration of the Raman scattering tenser of MoO3-x, is indicated by additional Raman bands and by changes in signal intensity ratios compared to stoichiometric MoO3. This different scattering tenser may also contribute to the lower Raman efficiency of Mo suboxides. Furthermore, besides temperature-induced broadening and signal shifts, Raman spectra of MoO3 seem to be sensitive to the crystallite size due to sintering and crystal growth. Calcination in O-18(2) after evacuation at 393 K (sufficient to remove physically adsorbed H2O and CO2 from the surface) does not result in any detectable spectral changes due to O-18 exchange. On the other hand, after evacuation at 648 K leading to a higher degree of reduction, calcination in O-18(2) gives rise to bands due to an O-18 incorporation into MoO3. Therefore, the intensity ratio of the O-18-related bands relative to those of the O-18-free stoichiometric oxide may correlate with the initial bulk defect concentration in oxygen-deficient MoO3. The long time required for the O-16 reexchange of MoO3 when fully reoxidized in O-18(2), constitutes a proof of the essential role played by defects in the anion vacancy conductor MoO3 during the exchange process. H2O present in the gas phase has no detectable influence on the O-16 reexchange in MoO3