X-ray diffraction and X-ray absorption and Raman spectroscopies were used to determine the structure of dispersed and crystalline structures in MoOx/ZrO2 catalysts useful in the oxidative dehydrogenation of alkanes. The MoOx surface density on ZrO2 was varied over a wide range (0.35−50 Mo/nm2) by changing the Mo content (1−44 wt % MoO3) and the treatment temperature (393−973 K). Raman spectra showed that MoOx/ZrO2 samples with low surface density (<5 Mo/nm2) treated at temperatures below 873 K initially contain isolated tetrahedral MoOx species; these species oligomerize to form two-dimensional structures with bridging MoOMo bonds as the surface density increased to values typical for a polymolybdate monolayer (∼5 Mo/nm2). An increase in surface density led to a shift in the ν(MoO) Raman band to higher frequencies and to changes in the near-edge X-ray absorption spectra. Both of these are consistent with the growth of these polymolybdate domains with increasing Mo surface density, as also suggested by the concurrent decrease in the UV−visible absorption energy. Thermal treatment at 973 K led to the dissociation of MoOMo bonds and to the formation of tetragonal−pyramidal OMoO4 species. For MoOx/ZrO2 samples with Mo surface densities greater than 5 Mo/nm2, MoO3 and Zr(MoO4)2 were detected by Raman and for larger crystallites also by X-ray diffraction. Treatment of these samples in air at 723 K led to the predominant formation of MoO3, while higher temperatures led to a solid-state reaction between MoO3 and ZrO2 to form Zr(MoO4)2. This structural evolution was confirmed by the evolution of pre-edge and near edge features in the X-ray absorption spectra of these high surface density samples. Zr(MoO4)2 contains Mo6+ cations in a distorted tetrahedral coordination with one oxygen bonded only to molybdenum and the other three shared by Zr and Mo atoms. The Raman bands observed for Zr(MoO4)2 at 750, 945, and 1003 cm-1 were assigned to νsym(OMoO), νasym(OMoO), and ν(MoO) vibrational modes, respectively, based on the analysis of the Raman bands observed after 18O2 exchange with lattice oxygen atoms. Bridging O atoms in MoOMo species exchanged with gas phase 18O2 more readily than terminal MoO species.
A new method based on absolutely localized molecular orbitals (ALMOs) is proposed to measure the degree of intermolecular electron density delocalization (charge transfer) in molecular complexes. ALMO charge transfer analysis (CTA) enables separation of the forward and backward charge transfer components for each pair of molecules in the system. The key feature of ALMO CTA is that all charge transfer terms have corresponding well defined energetic effects that measure the contribution of the given term to the overall energetic stabilization of the system. To simplify analysis of charge transfer effects, the concept of chemically significant complementary occupied-virtual orbital pairs (COVPs) is introduced. COVPs provide a simple description of intermolecular electron transfer effects in terms of just a few localized orbitals. ALMO CTA is applied to understand fundamental aspects of donor-acceptor interactions in borane adducts, synergic bonding in classical and nonclassical metal carbonyls, and multiple intermolecular hydrogen bonds in a complex of isocyanuric acid and melamine. These examples show that the ALMO CTA results are generally consistent with the existing conceptual description of intermolecular bonding. The results also show that charge transfer and the energy lowering due to charge transfer are not proportional to each other, and some interesting differences emerge which are discussed. Additionally, according to ALMO CTA, the amount of electron density transferred between molecules is significantly smaller than charge transfer estimated from various population analysis methods.
Abstract The CO hydrogenation activity and selectivity of the fcc phases of Mo 2 C and Mo 2 N are found to be identical, whereas the activity of the Mo 2 C hcp phase is only half that of these catalysts but its olefin selectivity is higher.
While supported particles of metals and oxides exhibit changes in specific activity with particle size and shape, the identification of what is meant by an active site on such catalysts is very difficult because of the diversity of possible active site. As a result, it is usually difficult to identify the exact cause for changes in catalyst activity and selectivity when changes are made in catalyst composition and structure. By contrast, single-site catalysts are exceptionally good models systems for exploring the consequences of site composition and structure on the mechanism and kinetics of catalyzed reactions because such catalysts can be prepared in such a way that almost all sites are identical, or nearly identical. Example of single sites include metal cations present in either framework or extra-framework positions in zeolites, metal oxo species dispersed onto oxide supports, and supported metal atoms and small clusters. This talk will illustrate the characterization of such structures by means of XANES and EXAFS, aided by simulations of such data based on theoretical models. Methods for studying the progress of elementary processes involved in catalyzed reactions will be illustrated for the oxidation of methanol to formaldehyde on supported vanadate species and the oxidative carbonylation of methanol to dimethyl carbonate on Cu-exchanged zeolites. These studies will also show that the hypotheses of reaction mechanism and the influence of site and support composition can be understood from first principles through the application theoretical analysis.
Read moreRaman and UV−vis diffuse reflectance spectroscopy were used to characterize the structure of vanadia dispersed on high surface area zirconium oxide. Two-dimensional vanadia species with tetrahedral coordination appear on the surface of the ZrO2 and expand in size with increasing V loading. Crystalline V2O5 appears when the vanadia loading exceeds an apparent surface density of 7.0 V atoms/nm2, and ZrV2O7 is formed as a consequence of zirconia migration into the V2O5 crystallites. A model for the structure of two-dimensional vanadia overlayer is proposed based on the experimental data and information taken from the literature. Vanadia is found to absorb the light scattered by the support, and this gives rise to a reduction in the intensity of the Raman bands for zirconia as the surface loading of vanadia increases. Partial reduction of the dispersed vanadia increases the absorbance of the vanadia and alters the profile of absorbance versus frequency in such a manner as to increase the intensity of the Raman bands for zirconia relative to those for vanadia. Examination of Raman spectra taken after repeated reduction−oxidation cycles suggests that reduction occurs via removal of oxygen from the vanadia monolayer without agglomeration or reorganization of the surface species.
Read moreViews Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Geoffrey M. Wise, Morton M. Denn, Alexis T. Bell, Jimmy W. Mays, Kunlun Hong, Hermis Iatrou; Surface mobility and slip of polybutadiene melts in shear flow. Journal of Rheology 1 May 2000; 44 (3): 549–567. https://doi.org/10.1122/1.551100 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentThe Society of RheologyJournal of Rheology Search Advanced Search |Citation Search
Read moreLow temperatures and low pressures suffice for the sulfonation of methane with a suitable free-radical initiator and promoter [Eq. (1)]. Since the RhCl3 promoter can be recycled, and the initiator complex is stable and easy to handle, development of this reaction into an industrial process is promising. MSA=methanesulfonic acid.
Read moreDensity functional theory has been used to investigate the mechanism and kinetics of the liquid-phase, oxidative carbonylation of toluene to p-toluic acid (C7H8 + CO + 1/2O2 → p-C7H6COOH + H2O) catalyzed by Rh(III) cations. In toluene solution containing trifluoroacetic acid and dissolved CO, Rh(III) is coordinated to three trifluoroacetate (TFA) anions and two CO molecules as Rh(CO)2(TFA)3. The oxidative carbonylation of toluene is initiated by the addition of toluene across one of the Rh−O bonds of Rh(CO)2(TFA)3 to form (C7H7)Rh(CO)2(TFAH)(TFA)2. The latter species undergoes isomerization and CO migration to produce (C7H7CO)Rh(CO)(TFAH)(TFA)2, which then coordinates another molecule of CO. The mixed anhydride of toluic and tirfluoroacetic acid, C7H7C(O)O(O)CCF3 and Rh(CO)3(TFA), are produced by reductive elimination from (C7H7CO)Rh(CO)2(TFAH)(TFA)2. Para-toluic acid is then formed by hydrolysis of C7H7C(O)O(O)CCF3. The proposed reaction mechanism explains many of the observations reported in our previous experimental work (Zakzeski, J. J.; Bell, A. T. J. Mol. Catal. A 2007, 276, 8) and, in particular, the effect of temperature on the ratio of p- to m-toluic acid, the effects of H2O and the partial pressure of CO on the loss of catalyst activity, and the effect of Rh concentration on the formation of a catalytically inactive Rh dimer species.
Read moreAcidic protons in zeolites are known to be mobile at elevated temperatures. In this study, density functional theory was used to identify the reaction pathways for proton migration in a model that represents the zeolite ZSM-5. In the absence of water, the acidic proton "hops" or migrates between two of the four O atoms surrounding an aluminum center with an activation barrier of 28 kcal/mol. During proton transfer, the O atoms stretch closer together in order to stabilize the transition state. This is revealed by a 13.4° decrease in the O−Al−O bond angle. Adsorbed water bridges the proton donor and acceptor sites, reducing the barrier height by 24 kcal/mol. Hence proton migration depends heavily on the local geometry and conditions of the zeolite. We show that experimentally undetectable amounts of water can greatly influence the measured rates and apparent activation barriers. We broaden the scope of our study to consider hydrogen exchange with other gas-phase species of the form RO−H (RO = CH3O, CH3CH2O) and R−H (R = H, CH3, C2H5, C3H7, C6H5). It is evident that to a first approximation the activation barrier increases with an increase in the polarizability of the species RO−H. For the chemical series R−H, the activation energy increases with the deprotonation energy of the interacting species R−H. We also calculate the overall reaction rate constants for proton hopping and hydrogen exchange.
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