Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2006"
TL;DR: Facile autoreduction of iron oxide encapsulated within carbon nanotubes has been observed at a temperature 200 degrees C lower than those on the outer surface, opening a new route to tune the state of confined nanoparticles of d-band metals by the confinement of CNTs.
Abstract: Facile autoreduction of iron oxide encapsulated within carbon nanotubes has been observed at a temperature 200 °C lower than those on the outer surface. This opens a new route to tune the state of confined nanoparticles of d-band metals by the confinement of CNTs.
239 citations
TL;DR: The confinement of mesopores played a key role in improving the thermal stabilities of silver nanoparticles (stable up to 773 K without any observable coarsening), which is essential to the further investigations on their chemical (e.g., catalytic) properties.
Abstract: A novel in situ autoreduction route has been developed, by which monodispersed silver nanoparticles with tunable sizes could be easily fabricated on silica-based materials, especially inside the channels of mesoporous silica (MPS). 13C CP/MAS NMR spectroscopy was employed to monitor the whole assembly process. It was demonstrated that the amino groups of APTS (aminopropyltriethoxyl silane)-modified MPS can be used to anchor formaldehyde to form novel reducing species (NHCH2OH), on which Ag(NH3)2NO3 could be in situ reduced. Monodispersed silver nanoparticles were thus obtained. In situ XRD and in situ TEM experiments were used to investigate and compare the thermal stabilities of silver nanoparticles on the external surface of silica gels (unconfined) and those located inside the channels of SBA-15 (confined). It was observed that unconfined silver nanoparticles tended to agglomerate at low temperatures (i.e., lower than 773 K). The aggregation of silver nanoparticles became more serious at 773 K. However...
232 citations
TL;DR: IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment.
Abstract: Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag+ and Zn2+ with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag]+ the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn2+ forms a deprotonated dimer complex with Phe, [Zn+Phe2−H]+, in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly su...
222 citations
TL;DR: Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers.
Abstract: Localized electronic defects on the surface of a 4 monolayer (ML) thin MgO(001) film deposited on Ag(001) have been investigated by low-temperature scanning tunneling microscopy and spectroscopy. Depending on the location of the defect, we observe for the first time different defect energy levels in the band gap of MgO. The charge state of defects can be manipulated by interactions with the scanning tunneling microscope tip. Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers.
143 citations
TL;DR: In this article, density functional theory was used with B3LYP gradient-corrected exchange correlation functional to study the mechanism for the reaction of H2 + O2 → H2O over a Pt catalyst via direct oxygen reduction.
Abstract: Density functional theory (DFT) was used with the B3LYP gradient-corrected exchange–correlation functional to study the mechanism for the reaction of H2 + O2 → H2O over a Pt catalyst via direct oxygen reduction.
Within these studies we first examined the binding characteristics and energetics for each likely intermediate chemisorbed on the Pt(111) surface, modeled by a 35 atom cluster: O, H, O2, H2, OH, OOH, H2O.
Then, the pathways for the dissociation processes of the various intermediates on the Pt35 cluster were calculated. For convenience in comparing different reaction steps, these energetics were used to calculate heats of formation (ΔHf), which were combined with the dissociation barriers. Two main reaction pathways were found for the formation of H2O from H2 and O2:
•(OO-Dissociation: Here O2 adsorbs on the surface, dissociates, and finally reacts with H sequentially to firstly form OH and then water. The rate-determining step (RDS) for this mechanism is the Oad + Had → OHad with a barrier of 31.66 kcal mol–1 and not the dissociation of O2ad (barrier of 15.02 kcal mol–1).
•OOH-Formation: Here O2 reacts firstly with Had to form OOHad, which then dissociates to form OHad and Oad (the RDS, with a barrier of 17.13 kcal mol–1), which finally reacts with another Had to form water.
Thus, under gas-phase conditions, the OOH-Formation mechanism is found to be the most favorable.
119 citations
TL;DR: Investigation of soot particulate collected from a Euro III heavy duty diesel engine run under black smoke conditions reveals that technical carbon black is not a suitable model for the chemistry of the diesel engine soot.
Abstract: Soot particulate collected from a Euro III heavy duty diesel engine run under black smoke conditions was investigated using thermogravimetry, transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. The characterization results are compared with those of commercial carbon black. The onset temperature toward oxidation of the diesel engine soot in 5% O2 is 150 °C lower than that for carbon black. The burn out temperature for the diesel engine soot is 60 °C lower than that of the carbon black. The soot primary particles exhibit a core−shell structure. The shell of the soot particles consists of homogeneously stacked basic structure units. The commercial carbon lamp black is more graphitized than the diesel engine soot, whereas the diesel engine soot contains more carbon in aromatic nature than the carbon black and is highly surface-functionalized. Our findings reveal that technical carbon black is not a suitable model for the chemistry of the diesel engine soot.
119 citations
TL;DR: In the complex alkane/P123/TEOS/H2O emulsion system, an emulsion engineering method to modulate pore length and morphological architecture of mesoporous materials has been built and a series of chemically significant mesostructures with tunable pore Length and Morphological architecture have been successfully constructed.
Abstract: In the complex alkane/P123/TEOS/H2O emulsion system, an emulsion engineering method to modulate pore length and morphological architecture of mesoporous materials has been built. With fine tuning of the synthetic parameters (e.g., the composition of the synthetic mixtures, temperature, stirring, etc.), a series of chemically significant mesostructures (i.e., short-pore SBA-15 materials) with tunable pore length and morphological architecture have been successfully constructed. The effects of alkane solubilizates on pore length and particle morphology are discussed. The resulting short-pore materials would have potential applications in the fields of adsorption/separation of biomolecules and inclusion chemistry of guest species, etc.
115 citations
TL;DR: The dynamics of heterogeneous electron transfer from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum to verify the validity of applying the rate equation model with an optical Bloch equation model.
Abstract: The dynamics of heterogeneous electron transfer (ET) from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum. Data from ultraviolet photoelectron spectroscopy and from linear absorption spectroscopy showed that the donor state of the chromophore was located around 900 meV above the lower edge of the conduction band. With the wide band limit fulfilled the rate of the heterogeneous ET reaction was only controlled by the strength of the electronic coupling and not reduced by Franck−Condon factors. Two different time constants for the electron transfer, i.e., 13 and 28 fs, were measured with carboxylic acid and phosphonic acid as the respective anchor groups. The difference in the ET time constants was explained with the different extension of the donor orbital onto the respective anchor group to reach the empty electronic states of the semiconductor. The time constants were ...
111 citations
TL;DR: N nanoscale silica particles with ordered large mesopores (approximately 13 nm) have been successfully constructed and have an unusual ultrafast enzyme adsorption speed and the amount of enzyme that can be immobilized is larger than that of conventional mesoporous silica, which has potential applications in the fast separation of biomolecules.
110 citations
TL;DR: This Minireview describes some of the main theoretical methodologies that are used to study catalytic properties and reactions on surfaces and studies the seemingly simple reaction of water formation out of hydrogen and oxygen on Pt and Pt/Ni alloy catalysts.
Abstract: In the past, the modelingof catalytic processes was limited by the size and complexity of the systems involved. However, the enormous progress in both computer power and theoretical methods has made computational modelinga valuable tool in increasingour knowledg e of catalytic reactions on the atomic scale. While complex reactions can be studied by dividingthe overall reaction into a series of steps calculable by quantum mechanics, the combination with methods appropriate for larger time and length scales enables the gap between these regimes to be bridged. This provides a more realistic modeling of the experimental system and allows important environmental effects such as solvation to be taken into account. In this Minireview we describe some of the main theoretical methodologies that are used to study catalytic properties and reactions on surfaces. Using these methods, we study the seemingly simple reaction of water formation out of hydrogen and oxygen on Pt and Pt/Ni alloy catalysts. To provide a more realistic description we also discuss the interestingeffects determined by hydratingthe system or using alloy nanoparticles rather than extended surfaces.
110 citations
TL;DR: In this paper, the authors scrutinize the accuracy of the pseudopotential approximation in density-functional theory calculations of surfaces by systematically comparing to results obtained within a full-potential setup.
Abstract: We scrutinize the accuracy of the pseudopotential approximation in density-functional theory calculations of surfaces by systematically comparing to results obtained within a full-potential setup. As model system we choose the CO oxidation at a RuO2110 surface and focus in particular on the adsorbate binding energies and reaction barriers as target quantities for the comparison. Rather surprisingly, the major reason for discrepancy does not result from the neglected semicore state relaxation in the frozen-core approximation, but from an inadequate description of the local part of the Ru pseudopotential, responsible for the scattering of f-like waves. Tiny, seemingly irrelevant, imprecisions appearing in these properties can have a noticeable influence on the surface energetics. At least for the present example, we obtain excellent agreement between both approaches, if the pseudopotential describes these scattering properties accurately.
TL;DR: In this article, the first comprehensive analysis of glyoxal (CHOCHO) by passive multi-axis differential optical absorption spectroscopy (MAX-DOAS) was reported, which was conducted at the Massachusetts Institute of Technology (MIT), Cambridge, USA and on board the research vessel Ron Brown in the Gulf of Maine as part of the International Consortium for Atmospheric Re- search on Transport and Transformation (ICARTT) 2004 campaign.
Abstract: The direct detection of glyoxal (CHOCHO), the smallest -dicarbonyl, in the open atmosphere by active dif- ferential optical absorption spectroscopy (DOAS) has re- cently been demonstrated (Volkamer et al., 2005a) and trig- gered the very recent successful detection of CHOCHO from space (Kurosu et al., 2005; Wittrock et al., 2006; Beirle et al., 2006). Here we report the first comprehensive analysis of CHOCHO by passive multi axis differential optical absorp- tion spectroscopy (MAX-DOAS). CHOCHO and NO 2 slant column measurements were conducted at the Massachusetts Institute of Technology (MIT), Cambridge, USA, and on board the research vessel Ron Brown in the Gulf of Maine as part of the International Consortium for Atmospheric Re- search on Transport and Transformation (ICARTT) 2004 campaign. For a day with nearly clear sky conditions, radia- tive transfer modeling was employed to derive diurnal CHO- CHO mixing ratios in the planetary boundary layer (PBL) for both sites. CHOCHO mixing ratios at MIT varied from 40 to 140 ppt, with peak values observed around noon. Mixing ra- tios over the Gulf of Maine were found to be up to 2.5 times larger than at MIT. The CHOCHO-to-NO2 ratio at MIT was <0.03, and enhancements of this ratio by up to two orders of magnitude were found over the Gulf of Maine. This paper focuses on the methodological aspects involved with MAX- DOAS measurements of CHOCHO.
TL;DR: In this article, a general and facile approach for the encapsulation of enzymes of various sizes in ordered mesoporous silica is reported, where the enzymes are entrapped in macroporous cages connected by uniform mesoporus channels.
Abstract: There is currently great interest in enzyme immobilization to enhance enzyme stability and reusability, and to aid in separation from the reaction mixture, [1–17] but immobilized enzymes on commonly used inorganic and organic solid supports show low activities. This is a result of the leaching of the enzymes from the solid supports and the limited conformational transitions available to the enzymes for chemical interaction on the supports. [1–4] Enzymes encapsulated by a sol–gel/polymer [3–10] show good activity, but the wide pore-size distribution in sol–gel/polymers cannot be well controlled, and this adversely influences the diffusion of reactants and products during biocatalysis to the detriment of their practical application. [3,4,16] Recently, a number of successful examples of good enzyme activity resulting from enzyme immobilization in uniform mesopores of ordered mesostructured materials have been reported. [14–17] However, enzyme immobilization in mesopores is limited by the pore size of the mesostructured materials, so that bulky enzymes or enzyme aggregates larger than the mesopores cannot be immobilized. A general and facile approach for the encapsulation of enzymes of various sizes in ordered mesoporous silica is reported here, where the enzymes are entrapped in macroporous cages connected by uniform mesoporous channels. These encapsulated enzymes show good activity, long-term stability, and excellent recycling characteristics. The concept of “fish-in-net” encapsulation of enzymes in ordered mesoporous silica under mild conditions is illustrated in Figure 1. Tetraethylorthosilicate (TEOS) was first assembled from a triblock ethylene oxide (EO)/propylene oxide (PO) copolymer surfactant ( EO20PO70EO20, P123) in ethanol. After evaporation of the ethanol and addition of glycerol, preformed precursors with ordered mesostructured silica particles were obtained in the glycerol solution, which is a non-denaturing solvent for enzymes. The preformed precursors were mixed with the enzyme solution under stirring at 4°C. During the interaction between the enzymes and the preformed precursors, active enzymes (acting as the “fish”) were gradually entrapped in the “net” formed by the polymerization and condensation of the ordered mesostructured silica particles. After removal of the glycerol and water by evacuation, the xerogels with encapsulated enzymes were washed with ethanol and water several times to remove polymer surfactants in the mesopores. In contrast to the “ship-in-a-bottle” technique, [18] the enzymes in this work were used as templates for the formation of the macroporous cages. The encapsulated enzymes in the mesoporous silica are typical nanoreactors, which combine the advantages of native enzymes with those of mesoporous channels. When water is introduced into the cages, the chemical environment of the enzymes in the cages is similar to that of native enzymes in aqueous solution. This is beneficial for protein rotation and conformational transitions, and provides for high biocatalytic activity. [3,16] Moreover, the or
TL;DR: In this paper, the influence of oxygen on the structure and morphology of a Pd/Fe3O4 model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM).
Abstract: We have studied the influence of oxygen on the structure and morphology of a Pd/Fe3O4 model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe3O4 thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10−6 mbar). These surface oxides are initially generated at the Pd/Fe3O4 interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe3O4 support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.
TL;DR: The gamma-turn motif observed here is intriguing since the condensed phase structure is known to be a beta-turn, and it emphasizes the subtle balance between intra- and intermolecular forces, which is responsible for the relative stability of the different secondary structure motifs.
Abstract: Infrared/UV hole-burning spectroscopy is performed on individual conformers of the protected dipeptide Z-Aib-Pro-NHMe. The extended IR range probed in this study allows one to elucidate both the H-bonding motif (5−7 μm) as well as the backbone structure (7−10 μm). Comparison with DFT calculations shows that the backbone is locally constrained to an α-conformation by Aib and to a γ-turn by Pro. The γ-turn motif observed here is intriguing since the condensed phase structure is known to be a β-turn. This is the first actual observation of such a discrepancy, and it emphasizes the subtle balance between intra- and intermolecular forces, which is responsible for the relative stability of the different secondary structure motifs.
TL;DR: The decomposition of ethene on the Pd(111) surface was studied at effective pressures in the 10(-8) to 10(-7) mbar range and at sample temperatures between 300 and 700 K, using an effusive capillary array beam doser for directional adsorption, LEED, AES, temperature programmed reaction, and TDS.
Abstract: The decomposition of ethene on the Pd(111) surface was studied at effective pressures in the 10(-8) to 10(-7) mbar range and at sample temperatures between 300 and 700 K, using an effusive capillary array beam doser for directional adsorption, LEED, AES, temperature programmed reaction, and TDS. In the temperature range of 350-440 K increasingly stronger dehydrogenation of the ethene molecule is observed. Whereas at 350 K an ethylidyne adlayer is still present after adsorption, already at temperatures around 440 K complete coverage of the surface by carbon is attained, while the bulk still retains the properties of pure Pd. Beyond 440 K a steady-state surface C coverage is established, which decreases with temperature and is determined by detailed balancing between the ethene gas-phase adsorption rate and the migration rate of carbon into the Pd bulk. This process gives rise to the formation of a "partially carbon-covered Pd(x)C(y) surface". Above 540 K the surface-bulk diffusion of adsorbed carbon becomes fast, and in the UHV experiment the ethene adsorption rate becomes limited by the ethene gas-phase supply. The carbon bulk migration rate and the steady-state carbon surface coverage were determined as a function of the sample temperature and the ethene flux. An activation energy of 107 kJ mol(-1) for the process of C diffusion from surface adsorption sites into the subsurface region was derived in the temperature range of 400-650 K by modeling the C surface coverage as a function of temperature on the basis of steady-state reaction kinetics, assuming a first-order process for C surface-subsurface diffusion and a second-order process for C(ads) formation by dissociative C2H4 adsorption.
TL;DR: In this article, a density functional theory was employed to calculate the structure and the vibrational frequencies of the O 2 species on the surface of the V 2 O 3 (0, 0, 0, 0) surface.
TL;DR: In this article, it was shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature.
TL;DR: In this paper, a detailed investigation of the preparation routine for the multi-metal oxide Mo 1 V 0.30 Te 0.23 Nb 0.125 O x used as catalyst for the selective oxidation of propane to acrylic acid is presented.
Abstract: This work presents a detailed investigation of the preparation routine for the multi-metal oxide Mo 1 V 0.30 Te 0.23 Nb 0.125 O x used as catalyst for the selective oxidation of propane to acrylic acid. In situ Raman spectroscopy on the initial aqueous polyoxometalate solution prepared from ammonium heptamolybdate, ammonium metavanadate and hexaoxotelluric acid reveals the coexistence of Anderson-type anions [TeM 6 O 24 ] n − , M = Mo, V; n ≥ 6 and protonated decavanadate species [H x V 10 O 28 ] (6− x )− . Raman analysis showed that the monomeric motif of the Anderson-type tellurate is preserved after addition of the Nb precursor and the subsequent spray-drying process. Calcination of the X-ray amorphous spray-dried material in air at 548 K seems to be the essential step, leading to a re-arrangement of the tellurate building blocks, generating nanocrystalline precursors of the phases finally established during treatment in helium at 873 K.
TL;DR: In this paper, an enhanced electron localization was used to interpret recent scanning tunneling microscopy results of ultrathin epitaxial metallic films, where the electron motion is only confined in the z direction normal to the film, which is similar to the bulk band from which they are derived.
Abstract: Two-dimensional quantum well states in ultrathin metal films generally exhibit a dispersion relation of s-p-derived states that can be described through an effective mass of the corresponding bulk band. By contrast, the effective masses in Pb quantum well states on Si111, measured through angle-resolved photoemission, are up to an order of magnitude larger than those from the bulk states or predicted by slab calculations, while similar anomalies are not observed in the related In/Si111 system. We interpret these data in terms of an enhanced electron localization, and use them to interpret recent scanning tunneling microscopy results. DOI: 10.1103/PhysRevB.73.161308 PACS numbers: 73.21.Fg, 68.55.Jk, 79.60.Dp The physical properties of ultrathin epitaxial metallic films exhibit marked differences compared to those of the bulk, through the effect of electron confinement in the film, and the influence of film-substrate and film-vacuum interfaces. Electron confinement leads to the formation of discrete quantum well states QWS, 1 which may play a decisive role in many film properties, such as growth morphology, 2 “magic” or “critical” thicknesses, 3 oscillations in the magnitude of the superconducting transition temperature, 4 and oscillations in the direction and magnitude of the Hall effect. 5,6 Moreover, the study of confinement in metal films has led to important observations of fundamental properties of metals, such as a detailed study of electron-phonon coupling. 7 In extended metal films, electron motion is only confined in the z direction normal to the film. Hence, quantum well states are expected to exhibit a dispersion relation Ekx,ky for the component of electron wave vector k along the x and y directions parallel to the film, that is similar to the bulk band from which they are derived. This is in fact found in studies of quantum well states in s-p metals such as Ag, 8 Al, 9 and Mg. 10 Effective masses m * are used to describe the disper
TL;DR: The obtained results demonstrate that intensities of IR C-H stretching bands can be also used as an index of chemical activation of adsorbed paraffins via their polarization by the low-coordinated cations.
Abstract: The present study reports an unusual diffuse reflection Fourier transform (DRIFT) spectrum of ethane adsorbed by gallium oxide. One of the stretching C-H bands in this spectrum with a maximum at 2753 cm(-1) is more than by 100 cm(-1) shifted toward lower frequencies in comparison with gaseous ethane. In addition, the relative intensity of this band is unusually high. This indicates a very strong polarizability of the corresponding chemical bonds resulting from perturbation of ethane by the low coordinated Ga(3+) cations. The assignment of this band to the very strongly perturbed initially fully symmetric nu(1) C-H stretching vibration is confirmed by a DFT modeling of ethane adsorption by the simplest Ga(2)O(3) cluster. The obtained results also indicate heterolytic dissociative adsorption and dehydrogenation of ethane by Ga(3+) Lewis sites at elevated temperature. This is evidenced by the appearance of new IR bands from zinc alkyl fragments and acidic protons followed by decomposition of resulting zinc ethyl species. In parallel, the most intense IR band at lower frequency from the most strongly polarized C-H chemical bond decreased in intensity. The obtained results indicate that these vibrations are involved in subsequent heterolytic dissociative adsorption. The obtained results demonstrate that, similar to the shifts of C-H stretching vibrations to the low-frequency, intensities of IR C-H stretching bands can be also used as an index of chemical activation of adsorbed paraffins via their polarization by the low-coordinated cations.
TL;DR: The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorptive measurements, providing a reliable calibration for both Zn O, Zn, and oxygen coverage.
Abstract: The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2×2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1×1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400−650 K) arising from upper Zn layers and a second peak (800−1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a wel...
TL;DR: In this paper, the effects of the oxidation state, the ligand, and the solvent on structures and energetics of cationic iron complexes are investigated by means of electrospray-ionization mass spectrometry.
Abstract: The effects of the oxidation state, the ligand, and the solvent on structures and energetics of cationic iron complexes are investigated by means of electrospray-ionization mass spectrometry Insights into the potential-energy surfaces of FeX+, FeX2+, and XFe(OCH3)+ ions (X = F, Cl, Br, I) with a variable number of coordinated methanol molecules are obtained by means of collision experiments and complementary thermochemical considerations It is shown that upon change of the halide ligand, the weakly solvated ions respond differentially to the increasing need for stabilization of the partial charge on the metal center For example, whereas F2Fe(CH3OH)n+ ions tend to lose mainly HF for n = 1–3, the loss of HBr is not observed at all for Br2Fe(CH3OH)n+ Instead, the iron-bromide cations undergo reductive loss of atomic bromine Cl2Fe(CH3OH)n+ bears an intermediate position in that both reduction as well as elimination of HCl can occur (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2006)
TL;DR: Clear evidence is found for structure sensitivity in CO oxidation over clean planar Ir(210) versus that over clean facetedIr(210): the CO2 desorption rate is sensitive to the surface morphological differences, however, no evidence has been found for size effects inCO oxidation over faceted Ir( 210) for average facet size.
Abstract: We report results on the catalytic oxidation of carbon monoxide (CO) over clean Ir surfaces that are prepared reversibly from the same crystal in situ with different surface morphologies, from planar to nanometer-scale facets of specific crystal orientations and various sizes. Our temperature-programmed desorption (TPD) data show that both planar Ir(210) and faceted Ir(210) are very active for CO oxidation to form CO2. Preadsorbed oxygen promotes the oxidation of CO, whereas high coverages of preadsorbed CO poison the reaction by blocking the surface sites for oxygen adsorption. At low coverages of preadsorbed oxygen (≤0.3 ML of O), the temperature Ti for the onset of CO2 desorption decreases with increasing CO coverage. At high coverages of preadsorbed oxygen (>0.5 ML of O), Ti is <330 K and is independent of CO coverage. Moreover, we find clear evidence for structure sensitivity in CO oxidation over clean planar Ir(210) versus that over clean faceted Ir(210): the CO2 desorption rate is sensitive to the...
TL;DR: In this paper, single phase samples of PdGa and Pd3Ga7 have been synthesized by inductive melting and a special chemical etching procedure was applied which significantly increased the catalytic activity of both binary intermetallic compounds.
Abstract: Single phase samples of PdGa and Pd3Ga7 have been synthesized by inductive melting. Afterwards a special chemical etching procedure was applied which significantly increased the catalytic activity of both binary intermetallic compounds. The increase in activity can be attributed to the partial dissolving of surface gallium oxides. Chemically etched samples are characterized by an activity/palladium mass ratio comparable to the commercial catalysts (Pd dispersed on metal oxides) and simultaneously show higher selectivity and increased long-term stability. According to XPS investigations the chemical etching influences the chemical composition of the surface. BET measurements show an increase in surface area, while a significant decrease in crystallite size or the presence of new compounds were not detected by X-ray powder diffraction. The concept of applying intermetallic compounds as stable, well characterized and highly selective catalysts should also be transferable to other catalytic reactions and appropriate intermetallic compounds.
TL;DR: The relative intensities of IR C-H stretching bands are much better criterion of perturbation of light paraffins upon adsorption than the frequencies of these bands, which are traditionally used for this purpose.
Abstract: Polarization of ethane and propane resulting from adsorption of these hydrocarbons by protons and different cations in mordenite, ZSM-5, and Y zeolites was studied by diffuse reflection Fourier transform IR spectroscopy (DRIFTS). Perturbation of adsorbed molecules by protons and sodium cations is weak, while positions of absorption bands for both these zeolites are very close to each other. In contrast, distributions of C-H IR stretching bands in intensities are somewhat different. This effect is pronounced much stronger for adsorption of light paraffins by bivalent alkaline earth and zinc cationic forms of these zeolites. Distribution of relative intensities of absorption bands strongly depends in this case both on the nature of cations and on the zeolites, while the most strongly perturbed vibrations are the initially fully symmetric C-H stretching vibrations. The corresponding low-frequency shifts and relative intensities of IR bands are increasing for different cations and zeolites in the following sequences: Na < Ca < Mg < Zn and Y < Mor approximately ZSM-5, while the difference in distribution of relative intensities of C-H stretching bands is pronounced much stronger than for the low-frequency shifts of these bands. Therefore, the relative intensities of IR C-H stretching bands are much better criterion of perturbation of light paraffins upon adsorption than the frequencies of these bands, which are traditionally used for this purpose. In addition, distribution of C-H IR stretching bands in intensity also provides unique information on anisotropy of polarizability of different C-H bonds created by their vibrations. For the acid and acid-base catalysis, where the main source of chemical activation arises from polarization of adsorbed molecules, such information is most important, while the anisotropy of polarizability provides a unique information on selective activation of different chemical bonds resulting from their stretching. The obtained results also demonstrate the possibility to use for testifying of the strength of Lewis acid sites instead of adsorption of the model molecular probes adsorption of the paraffins themselves.
TL;DR: In this article, the properties of the hydrated NaCl nanocrystal were studied by calculating its kinetic energy distribution and vibrational spectrum, which show a good agreement with experiment, and the characteristics of NaCl dissolution dynamics, such as ion sequence, dissolved species, dissolution force and dynamical role of water molecules during the dissolution were further investigated based on the statistical analysis.
Abstract: The interfacial structure and dynamical processes of a NaCl nanocrystal in liquid water have been studied in detail through classical molecular dynamics simulations. Before dissolution, the radial distribution of water molecules around the nanocrystal exhibits a multi-shell structure at the solid–liquid interface. The thermodynamic properties of the hydrated NaCl nanocrystal were studied by calculating its kinetic energy distribution and vibrational spectrum, which show a good agreement with experiment. The characteristics of NaCl dissolution dynamics, such as ion sequence, dissolved species, dissolution force, and dynamical role of water molecules during the dissolution were further investigated based on the statistical analysis. An ion sequence of Cl−, Na+, Cl−... and intermitted neutral pairs taking place from corner then ledge sites is preferred in NaCl dissolution processes.
TL;DR: By numerical simulations of the Oregonator model, it is shown that this instability and, thus, the Winfree turbulence may also be observed in the Belousov-Zhabotinsky reaction.
Abstract: Excitable media support self-organized scroll waves which can be unstable and give rise to three-dimensional wave chaos. Winfree turbulence of scroll waves results from the negative-tension instability of scroll waves; it plays an important role in the cardiac tissue where it may lead to ventricular fibrillation. By numerical simulations of the Oregonator model, we show that this instability and, thus, the Winfree turbulence may also be observed in the Belousov-Zhabotinsky reaction. The region in the parameter space, where the instability takes place, is determined, and a relationship between the negative-tension instability and the meandering behavior of spiral waves is found. The application of global periodic forcing to control such turbulence in the Oregonator model is discussed.
TL;DR: In this article, Co was found to wet the niobia film and be partially oxidized at 300 K in contrast to Co deposited on thin alumina films, where three-dimensional metal particles are stable up to 600 K.
Abstract: Cobalt was deposited by physical vapor deposition onto thin well-ordered niobia films in order to model niobia supported Co catalysts. Adsorption of CO on the Co/niobia surfaces was studied by temperature programmed desorption and infrared reflection absorption spectroscopy. Structural characterization was performed by photoelectron spectroscopy and scanning tunneling microscopy. Cobalt was found to wet the niobia film and be partially oxidized at 300 K in contrast to Co deposited on thin alumina films, where three-dimensional metal particles are stable up to 600 K. The combined results clearly indicate a strong interaction of Co with the niobia surface including Co migration into the film, which may have implications for the effects of niobia observed in real catalysts.