Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters

The identity and reversibility of the elementary steps required for catalytic combustion of dimethyl ether (DME) on Pt clusters were determined by combining isotopic and kinetic analyses with density functional theory estimates of reaction energies and activation barriers to probe the lowest energy paths. Reaction rates are limited by C-H bond activation in DME molecules adsorbed on surfaces of Pt clusters containing chemisorbed oxygen atoms at near-saturation coverages. Reaction energies and activation barriers for C-H bond activation in DME to form methoxymethyl and hydroxyl surface intermediates show that this step is more favorable than the activation of C-O bonds to form two methoxides, consistent with measured rates and kinetic isotope effects. This kinetic preference is driven by the greater stability of the CH3OCH2* and OH* intermediates relative to chemisorbed methoxides. Experimental activation barriers on Pt clusters agree with density functional theory (DFT)-derived barriers on oxygen-covered Pt(111). Measured DME turnover rates increased with increasing DME pressure, but decreased as the O2 pressure increased, because vacancies (*) on Pt surfaces nearly saturated with chemisorbed oxygen are required for DME chemisorption. DFT calculations show that although these surface vacancies are required, higher oxygen coverages lead to lower C-H activation barriers, because the basicity of oxygen adatoms increases with coverage and they become more effective in hydrogen abstraction from DME. Water inhibits reaction rates via quasi-equilibrated adsorption on vacancy sites, consistent with DFT results indicating that water binds more strongly than DME on vacancies. These conclusions are consistent with the measured kinetic response of combustion rates to DME, O2, and H2O, with H/D kinetic isotope effects, and with the absence of isotopic scrambling in reactants containing isotopic mixtures of 18O2-16O2 or 12CH3O12CH3-13CH3O13CH3. Turnover rates increased with Pt cluster size, because small clusters, with more coordinatively unsaturated surface atoms, bind oxygen atoms more strongly than larger clusters and exhibit lower steady-state vacancy concentrations and a consequently smaller number of adsorbed DME intermediates involved in kinetically relevant steps. These effects of cluster size and metal-oxygen bond energies on reactivity are ubiquitous in oxidation reactions requiring vacancies on surfaces nearly saturated with intermediates derived from O2.     

Reactivity of chemisorbed oxygen atoms and their catalytic consequences during CH4-O2 catalysis on supported Pt clusters

Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O(2) pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption-desorption equilibrium was maintained. The virtual O(2) pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C-H and O═O activation steps. O* coverages and virtual pressures depend on O(2) pressure when O(2) activation is equilibrated and on O(2)/CH(4) ratios when this step becomes irreversible as a result of fast scavenging of O* by CH(4)-derived intermediates. In three of these kinetic regimes, C-H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen-oxygen (O*-O*), to oxygen-oxygen vacancy (O*-*), and to vacancy-vacancy (*-*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*-O* site pairs activate C-H bonds in CH(4) via homolytic hydrogen abstraction steps that form CH(3) groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH(4) pressure but are independent of O(2) pressure. The observed normal CH(4)/CD(4) kinetic isotope effects are consistent with the kinetic-relevance of C-H bond activation; identical (16)O(2)-(18)O(2) isotopic exchange rates in the presence or absence of CH(4) show that O(2) activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C-H bond activation barriers are large, because of the weak stabilization of the CH(3) fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*-* site pairs activate C-H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH(3) interactions with the vacancies, which lead to much higher turnover rates than on O*-O* pairs. In this regime, O(2) activation becomes irreversible, because fast C-H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O(2)/CH(4) ratios instead of the O(2) pressures. CH(4)/CD(4) kinetic isotope effects are much larger for turnovers mediated by O*-* than by O*-O* site pairs, because C-H (and C-D) activation steps are required to form the * sites involved in C-H bond activation. Turnover rates for CH(4)-O(2) reactions mediated by O*-* pairs decrease with increasing Pt dispersion, as in the case of O*-O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O(2)/CH(4) ratios and O* coverages become smaller, O(2) activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O(2) pressures and independent of CH(4) pressure and no CH(4)/CD(4) kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O(2) activation step is essentially barrierless. In the absence of O(2), alternate weaker oxidants, such as H(2)O or CO(2), lead to a final kinetic regime in which C-H bond dissociation on *-* pairs at bare cluster surfaces limit CH(4) conversion rates. Rates become first-order in CH(4) and independent of coreactant and normal CH(4)/CD(4) kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C-H bond activation transition states more effectively via stronger binding to CH(3) and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH(4) oxidation reactions using O(2), H(2)O, or CO(2) as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps.     

CH3O decomposition on PdZn(111), Pd(111), and Cu(111). A theoretical study

Methanol steam re-forming, catalyzed by Pd/ZnO, is a potential hydrogen source for fuel cells, in particular in pollution-free vehicles. To contribute to the understanding of pertinent reaction mechanisms, density functional slab model studies on two competing decomposition pathways of adsorbed methoxide (CH(3)O) have been carried out, namely, dehydrogenation to formaldehyde and C-O bond breaking to methyl. For the (111) surfaces of Pd, Cu, and 1:1 Pd-Zn alloy, adsorption complexes of various reactants, intermediates, transition states, and products relevant for the decomposition processes were computationally characterized. On the surface of Pd-Zn alloy, H and all studied C-bound species were found to prefer sites with a majority of Pd atoms, whereas O-bound congeners tend to be located on sites with a majority of Zn atoms. Compared to Pd(111), the adsorption energy of O-bound species was calculated to be larger on PdZn(111), whereas C-bound moieties were less strongly adsorbed. C-H scission of CH(3)O on various substrates under study was demonstrated to proceed easier than C-O bond breaking. The energy barrier for the dehydrogenation of CH(3)O on PdZn(111) (113 kJ mol(-)(1)) and Cu(111) (112 kJ mol(-)(1)) is about 4 times as high as that on Pd(111), due to the fact that CH(3)O interacts more weakly with Pd than with PdZn and Cu surfaces. Calculated results showed that the decomposition of methoxide to formaldehyde is thermodynamically favored on Pd(111), but it is an endothermic process on PdZn(111) and Cu(111) surfaces.     

Structures and properties of zirconia-supported ruthenium oxide catalysts for the selective oxidation of methanol to methyl formate

The effects of RuO(x) structure on the selective oxidation of methanol to methyl formate (MF) at low temperatures were examined on ZrO(2)-supported RuO(x) catalysts with a range of Ru surface densities (0.2-3.8 Ru/nm(2)). Their structure was characterized using complementary methods (X-ray diffraction, Raman and X-ray photoelectron spectra, and reduction dynamics). The structure and reactivity of RuO(x) species change markedly with Ru surface density. RuO(x) existed preferentially as RuO(4)(2-) species below 0.4 Ru/nm(2), probably as isolated Zr(RuO(4))(2) interacting with ZrO(2) surfaces. At higher surface densities, highly dispersed RuO(2) domains coexisted with RuO(4)(2-) and ultimately formed small clusters and became the prevalent form of RuO(x) above 1.9 Ru/nm(2). CH(3)OH oxidation rates per Ru atom and per exposed Ru atom (turnover rates) decreased with increasing Ru surface density. This behavior reflects a decrease in intrinsic reactivity as RuO(x) evolved from RuO(4)(2-) to RuO(2), a conclusion confirmed by transient anaerobic reactions of CH(3)OH and by an excellent correlation between reaction rates and the number of RuO(4)(2-) species in RuO(x)/ZrO(2) catalysts. The high intrinsic reactivity of RuO(4)(2-) structures reflects their higher reducibility, which favors the reduction process required for the kinetically relevant C-H bond activation step in redox cycles using lattice oxygen atoms involved in CH(3)OH oxidation catalysis. These more reactive RuO(4)(2-) species and the more exposed ZrO(2) surfaces on samples with low Ru surface density led to high MF selectivities (e.g. approximately 96% at 0.2 Ru/nm(2)). These findings provide guidance for the design of more effective catalysts for the oxidation of alkanes, alkenes, and alcohols by the synthesis of denser Zr(RuO(4))(2) monolayers on ZrO(2) and other high surface area supports.     

Structure, stability, and mobility of small Pd clusters on the stoichiometric and defective TiO2 (110) surfaces

We report on the structure and adsorption properties of Pd(n) (n = 1-4) clusters supported on the rutile TiO(2) (110) surfaces with the possible presence of a surface oxygen vacancy or a subsurface Ti-interstitial atom. As predicted by the density functional theory, small Pd clusters prefer to bind to the stoichiometric titania surface or at sites near subsurface Ti-interstitial atoms. The adsorption of Pd clusters changes the electronic structure of the underlying surface. For the surface with an oxygen vacancy, the charge localization and ferromagnetic spin states are found to be largely attenuated owing to the adsorption of Pd clusters. The potential energy surfaces of the Pd monomer on different types of surfaces are also reported. The process of sintering is then simulated via the Metropolis Monte Carlo method. The presence of oxygen vacancy likely leads to the dissociation of Pd clusters. On the stoichiometric surface or surface with Ti-interstitial atom, the Pd monomers tend to sinter into larger clusters, whereas the Pd dimer, trimer, and tetramer appear to be relatively stable below 600 K. This result agrees with the standard sintering model of transition metal clusters and experimental observations.     

Chemisorption of CO and mechanism of CO oxidation on supported platinum nanoclusters

Kinetic, isotopic, and infrared studies on well-defined dispersed Pt clusters are combined here with first-principle theoretical methods on model cluster surfaces to probe the mechanism and structural requirements for CO oxidation catalysis at conditions typical of its industrial practice. CO oxidation turnover rates and the dynamics and thermodynamics of adsorption-desorption processes on cluster surfaces saturated with chemisorbed CO were measured on 1-20 nm Pt clusters under conditions of strict kinetic control. Turnover rates are proportional to O(2) pressure and inversely proportional to CO pressure, consistent with kinetically relevant irreversible O(2) activation steps on vacant sites present within saturated CO monolayers. These conclusions are consistent with the lack of isotopic scrambling in C(16)O-(18)O(2)-(16)O(2) reactions, and with infrared bands for chemisorbed CO that did not change within a CO pressure range that strongly influenced CO oxidation turnover rates. Density functional theory estimates of rate and equilibrium constants show that the kinetically relevant O(2) activation steps involve direct O(2)* (or O(2)) reactions with CO* to form reactive O*-O-C*=O intermediates that decompose to form CO(2) and chemisorbed O*, instead of unassisted activation steps involving molecular adsorption and subsequent dissociation of O(2). These CO-assisted O(2) dissociation pathways avoid the higher barriers imposed by the spin-forbidden transitions required for unassisted O(2) dissociation on surfaces saturated with chemisorbed CO. Measured rate parameters for CO oxidation were independent of Pt cluster size; these parameters depend on the ratio of rate constants for O(2) reactions with CO* and CO adsorption equilibrium constants, which reflect the respective activation barriers and reaction enthalpies for these two steps. Infrared spectra during isotopic displacement and thermal desorption with (12)CO-(13)CO mixtures showed that the binding, dynamics, and thermodynamics of CO chemisorbed at saturation coverages do not depend on Pt cluster size in a range that strongly affects the coordination of Pt atoms exposed at cluster surfaces. These data and their theoretical and mechanistic interpretations indicate that the remarkable structure insensitivity observed for CO oxidation reactions reflects average CO binding properties that are essentially independent of cluster size. Theoretical estimates of rate and equilibrium constants for surface reactions and CO adsorption show that both parameters increase as the coordination of exposed Pt atoms decreases in Pt(201) cluster surfaces; such compensation dampens but does not eliminate coordination and cluster size effects on measured rate constants. The structural features and intrinsic non-uniformity of cluster surfaces weaken when CO forms saturated monolayers on such surfaces, apparently because surfaces and adsorbates restructure to balance CO surface binding and CO-CO interaction energies.     

Effect of steps on the decomposition of CH3O at PdZn alloy surfaces

The decomposition of methoxide (CH(3)O) on a PdZn alloy is considered to be the rate-limiting step of steam re-forming of methanol over a Pd/ZnO catalyst. Our previous density functional (DF) studies (Langmuir 2004, 20, 8068; Phys. Chem. Chem. Phys. 2004, 6, 4499) revealed only a very low propensity of defect-free flat (111) and (100) PdZn surfaces to promote C-H or C-O bond breaking of CH(3)O. Thus, we applied the same DF periodic slab-model approach to investigate these two routes of CH(3)O decomposition on PdZn(221) surfaces that expose Pd, (221)(Pd), and Zn, (221)(Zn), steps. C-H bond cleavage of CH(3)O is greatly facilitated on (221)(Pd): the calculated activation energy is dramatically reduced, to approximately 50 kJ mol(-1) from approximately 90 kJ mol(-1) on flat PdZn surfaces, increasing the rate constant by a factor of 10(8). The lower barrier is mainly due to a weaker interaction of the reactant CH(3)O and an enhanced interaction of the product CH(2)O with the substrate. The activation energy for C-O bond scission did not decrease on the (221)(Pd) step. On the (221)(Zn) step, the calculated reaction barriers of both decomposition routes are even higher than on flat surfaces, because of the stronger adsorption of CH(3)O. Steps (and other defects) appear to be crucial for methanol steam re-forming on Pd/ZnO catalyst; the stepped surface PdZn(221)(Pd) is a realistic model for studying the reactivity of this catalyst.     

C-H bond activation over metal oxides: a new insight into the dissociation kinetics from density functional theory

The C-H activation on metal oxides is a fundamental process in chemistry. In this paper, we report a density functional theory study on the process of the C-H activation of CH(4) on Pd(111), Pt(111), Ru(0001), Tc(0001), Cu(111), PdO(001), PdO(110), and PdO(100). A linear relationship between the C-H activation barrier and the chemisorption in the dissociation final state on the metal surfaces is obtained, which is consistent with the work in the literature. However, the relationship is poor on the metal oxide surfaces. Instead, a strong linear correlation between the barrier and the lattice O-H bond strength is found on the oxides. The new linear relationship is analyzed and the physical origin is identified.     

Ligand effects on hydrogen atom transfer from hydrocarbons to three-coordinate iron imides

A new β-diketiminate ligand with 2,4,6-tri(phenyl)phenyl N-substituents provides protective bulk around the metal without exposing any weak C-H bonds. This ligand improves the stability of reactive iron(III) imido complexes with Fe═NAd and Fe═NMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates Fe═N-C bending, a longer Fe═N bond, and greater access to the metal as potential reasons for the increase in C-H bond activation rates.     

Reactivity of site-isolated metal clusters: propylidyne on gamma-Al2O3-supported Ir4.

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma-Al2O3 (Ir4/gamma-Al2O3) with propene and with H2. Infrared, 13C NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir4/gamma-Al2O3 at 298 K gave stable, cluster-bound mu3-propylidyne. Propene adsorbed onto Ir4/gamma-Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). H2 reacted with Ir4/gamma-Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir4 was stable in helium or H2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir4 did not undergo isotopic exchange in the presence of D2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H2 (or D2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of H2, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.     

Mechanisms of initial propane activation on molybdenum oxides: a density functional theory study

We report the first detailed density functional theory study on the mechanisms of initial propane activation on molybdenum oxides. We consider 6 possible mechanisms of the C-H bond activation on metal oxides, leading to 17 transition states. We predict that hydrogen abstraction by terminal Mo=O is the most feasible reaction pathway. The calculated activation enthalpy and entropy are 32.3 kcal/mol and -28.6 cal/(mol/K), respectively, in reasonably good agreement with the corresponding experimental values (28.0 kcal/mol and -29.1 cal/(mol/K)). We find that activating the methylene C-H bond is 4.7 kcal/mol more favorable than activating the methyl C-H bond. This regioselectivity is correlated with the difference in strength between a methylene C-H bond and a methyl C-H bond. Our calculations suggest that a combined effect from both the methylene and the methyl C-H bond cleavages leads to the experimentally observed overall kinetic isotopic effects from propane to propylene on the MoO(x)/ZrO(2) catalysts.     

SCF MO pseudopotential studies of chemisorption. The Pd(111) oxygen system

Following previous work on Pt(111)/hydrogen system, SCF MO pseudopotential calculations have been done for the case of atomic oxygen chemisorption on Pd(111), using clusters of up to four metal atoms. No site preference is found for oxygen chemisorption, and the chemisorption bond appears to involve only nearest-neighbor metal atoms. The use of a double-zeta 4d function for the metal results in significant changes in the bonding pattern with respect to single-zeta.     

Oxidation state and symmetry of magnesia-supported Pd13O(x) nanocatalysts influence activation barriers of CO oxidation

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.     

甲烷在清洁 Pd(111) 及氧改性的 Pd(111) 表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.     

Total oxidation of methanol on Cu(110): a density functional theory study

The partial and total oxidation of methanol on clean and oxygen-precovered Cu(110) has been studied by periodic density functional theory calculations within the generalized gradient approximation. Reaction paths including the geometry and the energetics of several reaction intermediates and the activation barriers between them have been determined, thus creating a complete scheme for methanol oxidation on copper. The calculations demonstrate that the specific structure of oxygen on copper plays an important role in both the partial and the total oxidation of methanol. For lower oxygen concentrations on the surface, the partial oxidation of methanol to formaldehyde is promoted by the presence of oxygen on the surface through the removal of hydrogen in the form of water, which prevents the recombinative desorption of methanol. At larger oxygen concentrations, the presence of isolated oxygen atoms reduces the C-H bond breaking barrier of adsorbed methoxy considerably, thus accelerating the formation of formaldehyde. Furthermore, oxygen also promotes the formation of dioxymethylene from formaldehyde, which then easily decays to formate. Formate is the most stable reaction intermediate in the total oxidation. Thus the formate decomposition represents the rate-limiting step in the total oxidation of methanol on copper.     

Catalytic coupling of haloolefins with anilides

A strategy in which C-H activation reactions promoted by Pd(II) have been combined with beta-heteroatom elimination to create a catalytic cycle achieving the arylation of haloacrylates is reported. The catalytic cycle can be subdivided into four parts: (1) C-H activation; (2) the functionalization step, migratory insertion of the olefin into a metal-carbon bond; (3) beta-heteroatom elimination; and (4) exchange of metal halide (if X = halogen) for a less coordinating anion. In this catalytic cycle, the oxidation state of the metal does not change, and an oxidant is not required. The method is more functional group tolerant compared with the existing alkene-arene coupling methods based on electrophilic C-H activation.     

Infrared spectra of CH3-MH, CH3-M, and CH3-MH- prepared via methane activation by laser-ablated Au, Ag, and Cu atoms

Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to generation of CH(3)-MH, CH(3)-M, and CH(3)-MH(-), which are identified in the product infrared spectra on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal that C-H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or electron addition also follows. Each type of product has similar photochemical properties regardless of the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the insertion complexes. The methyl metal species have the shortest C-M bonds, consistent with their highest calculated effective bond order, and the CH(3)-MH complexes have higher electron affinities than the metal atoms.     

Selective catalytic oxidation of ethanol to acetic acid on dispersed Mo-V-Nb mixed oxides

The direct oxidation of ethanol to acetic acid is catalyzed by multicomponent metal oxides (Mo-V-NbO(x)) prepared by precipitation in the presence of colloidal TiO(2) (Mo(0.61)V(0.31)Nb(0.08)O(x)/TiO(2)). Acetic acid synthesis rates and selectivities (~95 % even at 100 % ethanol conversion) were much higher than in previous reports. The presence of TiO(2) during synthesis led to more highly active surface areas without detectable changes in the reactivity or selectivity of exposed active oxide surfaces. Ethanol oxidation proceeds via acetaldehyde intermediates that are converted to acetic acid. Water increases acetic acid selectivity by inhibiting acetaldehyde synthesis more strongly than its oxidation to acetic acid, thus minimizing prevalent acetaldehyde concentrations and its intervening conversion to CO(x). Kinetic and isotopic effects indicate that C-H bond activation in chemisorbed ethoxide species limits acetaldehyde synthesis rates and overall rates of ethanol conversion to acetic acid. The VO(x) component in Mo-V-Nb is responsible for the high reactivity of these materials. Mo and Nb oxide components increase the accessibility and reducibility of VO(x) domains, while concurrently decreasing the number of unselective V-O-Ti linkages in VO(x) domains dispersed on TiO(2).     

Mechanism of methanol conversion on ZrO<Subscript>2</Subscript> and 5% Cu/ZrO<Subscript>2</Subscript> according to in situ IR spectroscopic data

It is demonstrated by in situ IR spectroscopy that, in methanol conversion on ZrO₂ and 5% Cu/ZrO₂ catalysts, methoxy groups are present on the catalyst surface, which result from O-H or C-O bond breaking in the methanol molecule. Two types of formate complexes, localized on ZrO₂ and CuO, are also observed. The formate complexes form via the oxidative conversion of the methoxy groups. There are two types of linear methoxy groups. First-type linear methoxy groups condense with the formate complex located on CuO to yield methyl formate and then CO and H₂. Second-type methoxy groups appear as intermediate products in the formation of dimethyl ether. The main hydrogen formation reactions are the recombination of hydrogen atoms (which result from the interconversion of surface complexes) on copper clusters and the decomposition of methyl formate. The source of CO₂ in the gas phase is the formate complex, and the source of CO is methyl formate. The effect of water vapor and oxygen the surface reactions and product formation is discussed.     

CO adsorption on Cu-Pd alloy surfaces: ligand versus ensemble effects

The CO adsorption on ordered Cu-Pd alloy surfaces and surface alloys has been studied using density functional theory (DFT) within the framework of the generalized gradient approximation (GGA). On the surface alloys, the CO adsorption energy at the top sites decreases with increasing concentration of the more reactive metal Pd. This surprising ligand effect is caused by the effective compressive strain induced by the larger size of the Pd atoms. On the other hand, at the most favorable adsorption sites the CO binding becomes stronger with increasing Pd concentration which is caused by an ensemble effect related to the availability of higher coordinated adsorption sites. At the surfaces of the bulk alloys, the trends in the adsorption energy as a function of the Pd concentration are less clear because of the strong Pd-Cu interaction and the absence of effective strain effects.