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Direct Spectroscopic Observation of the Role of Humidity in Surface Diffusion through an Ionic Adsorbent Powder. The Behavior of Adsorbed Pyridine on Nanocrystalline MgO
Abstract
The influence of water vapor on the surface diffusion behavior of pyridine adsorbed on powdered MgO surfaces has been studied by Fourier transform IR (FTIR) absorption spectroscopy. It has been found that the introduction of water vapor significantly increases the pyridine surface diffusion coefficient. FTIR spectra showed that water vapor converted Lewis acid Mg2+ sites to Brønsted Mg−OH sites. These measurements also detected the change in surface bonding of pyridine to the two types of sites. The activation energy for escape of chemisorbed pyridine into a mobile precursor state is lower for pyridine bound to Mg−OH sites than for binding to Mg2+ sites, causing the hydroxylated MgO to exhibit a higher diffusivity than that found for dry MgO containing no surface hydroxyl groups. This effect was confirmed by DFT calculations of the binding energy of pyridine to MgO(100) sites and to defect sites on MgO(100), where hydroxylation decreases the binding energy by 30 kJ mol−1 on each type of site.
... In the most common neutralization technologies use is made of mixtures that consist of nanoparticles of such metal oxides as MgO, CaO, ZnO, SrO, TiO 2 , Al 2 O 3 , CuO, Fe 2 O 3 with grain diameters ranging from a few nanometers to several dozen micrometers . The nanocrystalline MgO with grains of a 4 µm diameter and a specific surface exceeding 500 m 2 /g is characterized by a particularly large surface and activity [14][15][16][17][18]30,34]. Reducing the size of nanocrystallites causes a significant increase in the share of surface atoms, including those occupying edge and corner places, particularly as their network defects comprise the active sites of the oxide neutralizer [34]. ...
Undertaking effective and prompt action during chemical events is an important measure in preventing the spread of contamination. Studies have shown that despite very high surface activity of nanocrystalline metal oxides, their use does not reduce the fire hazard for hydrocarbon spills and alcohols more effectively than commonly used sorbents during fire rescue operations. Other limitations of specimens containing nanocrystalline oxides have also been proven, such as the maximum absorption similar to commonly used specimens, as well as high costs of conducting the acid neutralization process without increasing the level of safety of operations. Results obtained allowed the determination of proposed principles of effective and economically appropriate application of nanocrystalline metal oxides in rescue units.
We report an efficient and easily available transition metal-free catalyst, barium oxide, for D/H exchange reaction between pyridine and D2. BaO enables the selective deuteration at α-position of pyridine, which...
Hydrocarbon diffusion and binding within porous molecular networks are critical to catalysis, separations, and purification technologies. Fundamental insight into n-butane uptake and mobility within a new class of materials for separations, metal-organic frameworks (MOFs), has been gained through in situ infrared spectroscopy. These UHV-based measurements revealed that adsorption of n-butane within UiO-66 proceeds through the formation of hydrogen bonds between the alkane molecules and hydroxyl groups located at the inorganic node of UiO-66. Modeling the gas transport of n-butane with Fick’s 2nd law yielded diffusion coefficients at several temperatures. The Arrhenius parameter for the activation energy for diffusion was found to be 21.0 ± 1.2 kJ/mol. These studies have further shown that the rate-determining step for diffusion is the dissociation of n-butane from a binding site located within the tetrahedral pores of UiO-66.
Short chain aniline oligomers are of interest for applications in organic electronics and as corrosion inhibitors for steel, requiring an improved understanding of their interactions with metal oxide films. Here we investigate the interactions of N,N’-Diphenyl-1,4-benzoquinonediimine (B2Q1, oxidized form of an aniline dimer) with iron (III) oxides. B2Q1 transforms into its semiquinone form when interacting with α-Fe2O3. The resulting charge transfer between B2Q1 and α-Fe2O3 is demonstrated with mid-IR, visible and Raman spectroscopy. Atomic force microscopy shows the first layer of B2Q1 to be oriented face-on. Thermal analysis also confirms this orientation for sub-monolayer coverage, whereas molecules start standing up on their edges upon multilayer formation. Thermal analysis shows that the first monolayer of B2Q1 is chemisorbed on the α-Fe2O3 surface, and the following multilayers are strongly interacting with each other. The behavior of the oxidized aniline dimer B2Q1 is in stark contrast to its reduced counterpart (DPPD), which also undergoes charge transfer to iron oxide (in opposite direction). B2Q1 interacts less strong with the surface, causing it to be more mobile. The mobility of B2Q1 provides a possible clue towards understanding the self-healing behavior of polyaniline corrosion-inhibiting films on steel.
The study of the surface behavior of high area solids can be carried out using extensions of the fundamental measurement methods developed for studies of low area single crystal surfaces. Among the most widely used methods in single crystal studies is temperature-programmed desorption (TPD), often called thermal desorption spectroscopy (TDS).
The study of supported catalysts using transmission IR spectroscopy is a well-established technique employed widely throughout the field. Over 50 cell designs have been devised and reported in the literature (Basu et al. Rev Sci Instrum 59:1321, 1988 [1]). The cell described in Fig. 47.1 has special features of importance for some types of applications, especially where a simple design for use
at cryogenic temperatures is desired.
Metal oxides are mainly used in powder
or particulate form for their application as functional materials
. Insights into functional as well as unwanted properties of different defect types ranging from oxygen vacancies to grain boundaries and pores require an integrated characterization approach and are extremely difficult to establish. After a brief introduction into the complexity of particle systems and related heterogeneities, we discuss examples where for defects and other distinct structural features of MgO or TiO2 particle systems firm structure-property relationships
have been established. Moreover, we want to point out that processing of particle matters. Related microstructural changes can induce the formation of solid-solid interfaces upon transformation of nanoparticle powders into mesoporous nanoparticle networks. This change in aggregation level and microstructure can substantially modify the electronic, optical and spectroscopic properties of metal oxide nanoparticle ensembles and, for this reason, plays a critical role for the generation of structural and—at the same time—functional defects inside particle ensembles.
An important and so far neglected class of structural elements affecting the overall properties of metal oxide nanopowders are interfaces between individual nanocrystals. In this work, we show experimentally that these defects inside a powder of compressed MgO nanocubes are subject to photoexcitation in the UV light range. In particular, we identify a so far unobserved photoluminescence emission process at 2.5 eV. First-principles calculations of the optical properties of nanocrystal interfaces provide plausible candidates for both light absorbing and emitting sites, which involve different types of interface features. It was found that edge dislocations that arise from interfaces between nanocube edges and terraces induce a significant electrostatic perturbation of the interfacial electronic states. This leads to exciton generation and luminescence at even lower energies than those related to corners and edges of MgO nanocubes.
The photooxidation of methanol, n-hexane, and carbon monoxide using TiO2 nanotubes (TNTs) has been investigated by a new operando IR technique. Following the photocatalytic reaction by time-resolved IR spectroscopy coupled with mass spectrometry (MS) allowed a surface study of the photocatalyst with on-line analysis of the products. Identification of the species adsorbed on the photocatalyst surface and those detected in the gas phase led to further clarification of the photooxidation mechanisms. The photocatalyst was characterized by IR, Raman, UV–visible, XRD, N2 sorption, SEM, and TEM techniques. The activity and selectivity of the photocatalyst were determined by quantitative studies using gas-phase IR spectroscopy and MS. For comparison, photooxidation reactions using TiO2 P25 as a reference were performed under the same conditions. The effects of different parameters such as temperature, VOC concentration, and UV irradiation intensity on the reactivity and selectivity of the photocatalytic reaction were investigated. The effect of temperature was observed by TPD measurements (from room temperature to 200 °C). The TNT material showed a higher reactivity and CO2 selectivity than TiO2 P25.
Gold (Au) nanoparticles supported on reducible oxides such as TiO2 demonstrate exceptional catalytic activity for a wide range of gas phase oxidation reactions such as CO oxidation, olefin epoxidation, and water gas shift catalysis. Scientists have recently shifted their hypotheses on the origin of the reactivity of these materials from the unique electronic properties and under-coordinated Au sites on nanometer-sized particles to bifunctional sites at the Au-support interface. In this Account, we summarize our recent experimental and theoretical results to provide insights into the active sites and pathways that control oxidation over Au/TiO2 catalysts. We provide transmission IR spectroscopic data that show the direct involvement of the Au-Ti(4+) dual perimeter sites, and density functional theory results that connect the electronic properties at these sites to their reactivity and to plausible reaction mechanisms. We also show the importance of interfacial Au-Ti(4+) sites in adsorbing and activating O2 as a result of charge transfer from the Au into antibonding states on O2 causing di-σ interactions with interfacial Au-Ti(4+) sites. This results in apparent activation energies for O2 activation of 0.16-0.60 eV thus allowing these materials to operate over a wide range of temperatures (110-420 K) and offering the ability also to control H-H, C-H, and C-O bond scission. At low temperatures (100-130 K), adsorbed O2 directly reacts with co-adsorbed CO or H2. In addition, we observe the specific consumption of CO adsorbed on TiO2. The more strongly held CO/Au species do not react at ∼120 K due to high diffusion barriers that prevent them from reaching active interfacial sites. At higher temperatures, O2 directly dissociates to form active oxygen adatoms (O*) on Au and TiO2. These readily react with bound hydrocarbon intermediates via base-catalyzed nucleophilic attack on unsaturated C═O and C═C bonds or via activation of weakly acidic C-H or O-H bonds. We demonstrate that when the active Au-Ti(4+) sites are pre-occupied by O*, the low temperature CO oxidation rate is reduced by a factor 22. We observe similar site blocking for H2 oxidation by O2, where the reaction at 210 K is quenched by ice formation. At higher temperatures (400-420 K), the O* generated at the perimeter sites is able to diffuse onto the Au particles, which then activate weakly acidic C-H bonds and assist in C-O bond scission. These sites allow for active conversion of adsorbed acetate intermediates on TiO2 (CH3COO/TiO2) to a gold ketenylidene species (Au2═C═C═O). The consecutive C-H bond scission steps appear to proceed by the reaction with basic O* or OH* on the Au sites and C-O bond activation occurs at the Au-Ti(4+) dual perimeter sites. There is a bound-intermediate transfer from the TiO2 support to the Au sites during the course of reaction as the reactant (CH3COO/TiO2) and the product (Au2═C═C═O) are bound to different sites. We demonstrate that IR spectroscopy is a powerful tool to follow surface catalytic reactions and provide kinetic information, while theory provides atomic scale insights into the mechanisms and the active sites that control catalytic oxidation.
Necklace-like MgO/TiO2 heterojunction structures were synthesized by a simple and economical precursor calcination method for the first time. The obtained unique MgO/TiO2 heterojunction structures exhibit an excellent removal performance towards toxic As(v) ions combined with good photocatalytic properties towards organic dyes.
In this paper, one-dimensional (1-D) dypingite (hydrated magnesium carbonate hydroxide, Mg5(CO3)4(OH)2·5H2O) nanowires with several hundreds micrometers in length were synthesized at 100 °C by a simple solution method without using any template or catalyst for the first time. A plausible diffusion growth model for the growth of the dypingite nanowires was proposed. The influences of the magnesium ion concentrations and the magnesium sources on the structures of the products were also studied. The as-synthesized nanowires were an excellent precursor for the fabrication of 1-D nanoporous magnesium oxide (MgO) nanowires via a simple thermal transformation process. Room temperature photoluminescence studies suggest that the nanoporous MgO nanowires show a strong ultraviolet emission and a weak blue emission. The adsorption properties of the porous MgO nanowires were investigated for As(V) removal. A high As(V) adsorption capacity of 384.6 mg g−1 was realized, which was resulted from the reaction between the As(V) species and MgO in aqueous solution.
The chemisorption and surface diffusion/desorption of pyridine from TiO2 powder (P25) have been measured as a function of temperature using transmission IR spectroscopy. Two classes of diffusion have been measured with activation energies of 36 kJ/mol (fast) and 90 kJ/mol (slow). By comparing density functional theory (DFT) calculations of bonding energies on TiO2 rutile (110) and anatase (101), the dominant crystal planes expected in P25 TiO2, it is found that fast diffusion would be expected on the rutile phase and that much slower diffusion would be expected on the anatase phase. In addition, the presence of oxygen defect sites will produce more strongly bound pyridine than either of the model crystal planes selected for investigation. To both types of TiO2 surfaces, pyridine bonding occurs to coordinatively unsaturated Ti cation sites through the N lone pair of the pyridine molecule. More than 85% of the molecules exhibit slow surface diffusion, and this is attributed to diffusion on the dominant anatase crystallites as well as to diffusion on defective sites. Diffusing molecules exhibit a ν19b ring-breathing mode, with the rapidly diffusing molecules exhibiting a mode frequency of 1438 cm−1 and the slowly diffusing species exhibiting a mode frequency of 1445 cm−1. Electron stimulated desorption ion angular distribution (ESDIAD) studies of the C−D bond directions for pyridine-d5 on the rutile TiO2(110)-1 × 1 surface show that the ring plane is rotated by 37 ± 1° with respect to the [001] azimuth on the crystal surface, in good agreement with the 39° rotational angle calculated by DFT. Several ring-breathing modes of pyridine are slightly broadened on rutile sites compared to anatase sites, and this may be due to more freedom for librational vibrations of the more weakly bound pyridine molecules.
The adsorption of O2 from air is an essential step in the use of TiO2 as a photo-oxidation catalyst. This work explores the properties of a weakly bound O2 species which is probably the precursor to “active” O2 needed for the photo-oxidation of molecules by TiO2. Adsorbed molecular oxygen was observed via FTIR for the first time on nanosized TiO2 particles. The observed O−O stretching mode frequency at 1550 cm−1 is identical to the Raman displacement of O2 gas, showing that the interaction between the IR active O2 molecules and TiO2 is relatively weak. It is noteworthy that weak adsorption of O2 on TiO2 is accompanied by the development of a weak IR absorption band due to the production of a small dipole in the molecule. In addition, the constant νO2 observed for increasing coverage indicates that the O2 molecules exhibit only small interactions with each other in the adsorbed layer. The adsorption enthalpy of O2 on TiO2 is measured to be between −16 to −21 kJ mol−1. The O2− species formed by O2 adsorbing on TiO2 surface defect sites (likely to be the active O2 in photochemistry), with a calculated vibrational frequency of 1097 cm−1, is not observed in this work. The influence of coadsorbed CO, CO2, and H2O with the adsorbed O2 species was also investigated. It is found that when the adsorbed surface species, such as CO, CO2, and excess H2O, screen the TiO2 surface electric field and TiO2 surface dispersion forces, the IR active O2 species are no longer observed.
Composite BaO/MgO nanoparticles have been prepared by chemical vapor synthesis and subsequent annealing in controlled gas atmospheres. High resolution transmission electron microscopy and X-ray diffraction reveal that a part of the obtained nanoparticles can be characterized as support particles with hemispherical BaO phases. The structural and energetic properties of BaO units dissolved inside the MgO host and adsorbed on MgO(100) were investigated by density functional theory (DFT) calculations. Moreover, ab initio thermodynamics was used to explore the shape of BaO and MgO particles in a water environment as a function of temperature. The calculations suggest that the spherical shapes of the segregates result from the growth process and become thermodynamically stabilized by surface hydroxylation.
The prevailing view of CO oxidation on gold-titanium oxide (Au/TiO(2)) catalysts is that the reaction occurs on metal sites at the Au/TiO(2) interface. We observed dual catalytic sites at the perimeter of 3-nanometer Au particles supported on TiO(2) during CO oxidation. Infrared-kinetic measurements indicate that O-O bond scission is activated by the formation of a CO-O(2) complex at dual Ti-Au sites at the Au/TiO(2) interface. Density functional theory calculations, which provide the activation barriers for the formation and bond scission of the CO-O(2) complex, confirm this model as well as the measured apparent activation energy of 0.16 electron volt. The observation of sequential delivery and reaction of CO first from TiO(2) sites and then from Au sites indicates that catalytic activity occurs at the perimeter of Au nanoparticles.
Duales Reaktionszentrum: Die katalytische Reaktion H2+O2 an einem Au/TiO2-Katalysator wurde durch Transmissions-IR-Spektroskopie und DFT-Rechnungen untersucht. Für die O2-unterstützte H2-Dissoziation wurde gefunden, dass sie über eine Ti-OOH-Zwischenstufe am Au/TiO2-Perimeter verläuft. Die berechneten Energiebarrieren (0.13–0.25 eV) für die Abfolge von niedernergetischen Reaktionsschritten stimmen mit dem experimentellen Ea-Wert von 0.22 eV überein.
Stop or Go on Oxide Surfaces
Direct studies of surface diffusion with instruments such as the scanning tunneling microscope (STM) have often focused on species on metal surfaces, but surface diffusion can play an important role for reactions on metal oxide surfaces. Li et al. (p. 882 ) used STM and density functional theory calculations to study how catechol (a benzene ring bearing two −OH groups) diffuses on the surface of the rutile phase of titanium dioxide. Both mobile and immobile species were observed on the time scale of minutes while making repeated STM scans. Hydrogen atom transfers between surface OH groups and the molecule changed the interaction energy between the molecule and the surface, and hence the barrier for diffusion.
Traditionally, magnesium oxide has been considered a typical basic catalyst, catalyst carrier and/or adsorbent. In this study MgO was prepared using Mg-ethoxide dissolved in ethanol and hydrolyzed with various aqueous inorganic acids and bases. We have found that it is possible to induce Brnsted and Lewis acid sites depending on the method of preparation and, more specifically, on the type of inorganic acids used in the hydrolysis stage. In the FTIR spectra of adsorbed pyridine on MgO obtained using aqueous HCl (pH = 3), the bands corresponding to Lewis (1603, 1496 and 1444 cm–1) and Brnsted (1550 cm–1) acid sites are observed. The fact that upon vacuum and high temperature treatment, i.e., at 773 K, the above mentioned bands are clearly distinguishable indicated that the acid sites are very strong. On the other hand, the MgO materials obtained using aqueous acetic acid as a hydrolysis catalyst (pH = 5), and that without any catalyst (pH = 7) showed only Lewis acid sites which decreased markedly upon higher temperature treatment. When aqueous NH4OH (pH = 9) was used as the hydrolysis catalyst, the MgO obtained showed only Lewis acid sites. In all cases the number of Lewis acid sites was greater than that of Brnsted sites.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
Generalized gradient approximations (GGA’s) seek to improve upon the accuracy of the local-spin-density (LSD) approximation in electronic-structure calculations. Perdew and Wang have developed a GGA based on real-space cutoff of the spurious long-range components of the second-order gradient expansion for the exchange-correlation hole. We have found that this density functional performs well in numerical tests for a variety of systems: (1) Total energies of 30 atoms are highly accurate. (2) Ionization energies and electron affinities are improved in a statistical sense, although significant interconfigurational and interterm errors remain. (3) Accurate atomization energies are found for seven hydrocarbon molecules, with a rms error per bond of 0.1 eV, compared with 0.7 eV for the LSD approximation and 2.4 eV for the Hartree-Fock approximation. (4) For atoms and molecules, there is a cancellation of error between density functionals for exchange and correlation, which is most striking whenever the Hartree-Fock result is furthest from experiment. (5) The surprising LSD underestimation of the lattice constants of Li and Na by 3–4 % is corrected, and the magnetic ground state of solid Fe is restored. (6) The work function, surface energy (neglecting the long-range contribution), and curvature energy of a metallic surface are all slightly reduced in comparison with LSD. Taking account of the positive long-range contribution, we find surface and curvature energies in good agreement with experimental or exact values. Finally, a way is found to visualize and understand the nonlocality of exchange and correlation, its origins, and its physical effects.
Time-resolved FTIR techniques are used to investigate the sorption and sorption kinetics of pyridine upon uptake into zeolite structures. The strength of acidic Brønsted sites and Lewis centres in hydrogen mordenite and hydrogen ZSM-5 are compared. From the measurements, diffusion coefficients are derived. The zeolite samples used possess both types of sites. At a sorption temperature of 395-475 K, in H-MOR the Lewis sites are saturated at partial pressures of pyridine as low as 11.5 Pa, whereas the Brønsted sites show an increasing population with increasing partial pressures up to p(Py) >1000 Pa. The opposite is true for H-ZSM-5. For HMOR the diffusion coefficients, which are significantly dependent on the coverage at 475 K from 10 -12to 10 -11cm 2. s -1whereas those for H-ZSM-5 are greater by one order of magnitude.
A new approach to the construction of first-principles pseudopotentials is described. The method allows transferability to be improved systematically while holding the cutoff radius fixed, even for large cutoff radii. Novel features are that the pseudopotential itself becomes charge-state dependent, the usual norm-conservation constraint does not apply, and a generalized eigenproblem is introduced. The potentials have a separable form well suited for plane-wave solid-state calculations, and show promise for application to first-row and transition-metal systems.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
The sorption and reaction of but-1-ene over a ferrierite sample was studied by in situ IR spectroscopy to relate the selectivity to isobutene and the sorbed species. The spectra highlighted the presence of various unsaturated species including polyenes, which were readily formed upon contacting H-FER with but-1-ene while a poor selectivity to isobutene was observed. Aromatic compounds were concluded to be subsequently formed via dehydrogenation/cyclization of the polyenic deposits while the isobutene selectivity increased via suppression of by-products. These data support earlier assumptions that the initial high yield of by-products was produced by the reaction of butenes with reactive deposits, which were gradually converted into more inert aromatic deposits. Upon purging the catalyst with an inert, branched C8 hydrocarbons and aromatic molecules such as xylenes desorbed. The rates of desorption showed an initially linear variation with the square root of time and were fitted to a monodimensional Fickian model. Because the slowly diffusing species were rather abundant during the course of the reaction, it is proposed that the skeletal isomerization was effectively limited to the acid sites close the pore mouth of H-FER crystallites.
In situ FTIR spectroscopy was used to monitor the sorption and desorption kinetics of pyridine into and out of microporous materials such as mordenite, ZSM-5 and silicalite. A previously described experimental technique and method of evaluation, which had been successfully used to study sorption and sorption kinetics of aromatics in H-ZSM-5, was now employed for the investigation of the behavior of pyridine. A modification of the original flow-through IR cell, however, was necessary to provide better resistance to and handling at higher sorption temperatures. Sorption and desorption of pyridine was measured through the intensity changes of the IR bands typical of sorbed pyridine. At sufficiently high temperatures (525–575 K), the uptake of pyridine even into strongly acidic hydrogen mordenite (H-MOR) and H-ZSM-5 could be monitored and formally described by a solution of Fick's second law using uniform diffusion coefficients for the whole sorption processes, i.e. D=1×10−12 and D=6×10−11 cm2 s−1 for H-MOR (at 525 K) and H-ZSM-5 (at 575 K), respectively. The process was, however, not reversible, i.e. no `random walk' of the sorbate molecules occurred. Therefore, it was tentatively assumed that the slow transport of pyridine into the micropores is the rate-determining step, which may indeed be described as a diffusion, but followed by a rapid reaction with the acidic Brönsted and/or Lewis sites. The reaction step is, at the temperatures applied, almost irreversible. By contrast, with less strongly acidic sorbents such as Li-ZSM-5 and Na-ZSM-5, pyridine sorption was found to be reversible, the diffusivities determined at 575 K amounted to 8×10−12 and 2×10−11 cm2 s−1, respectively. In the case of pyridine/silicalite, reversibility was observed as well. Pyridine in silicalite showed a diffusion behavior similar to that of benzene in H-ZSM-5, exhibiting a Darken-corrected diffusivity of about 5×10−10 cm2 s−1 at 395 K. In fact, the isosteric heats of adsorption of pyridine in Li-ZSM-5 and Na-ZSM-5 were determined to 195–155 kJ mol−1 and 120 kJ mol−1 and were thus significantly higher than those previously measured for benzene in H-ZSM-5 (65±5 kJ mol−1).
The sorption and reaction of but-1-ene over a ferrierite sample was studied by in situ IR spectroscopy to relate the selectivity to isobutene and the sorbed species. The spectra highlighted the presence of various unsaturated species including polyenes, which were readily formed upon contacting H-FER with but-1-ene while a poor selectivity to isobutene was observed. Aromatic compounds were concluded to be subsequently formed via dehydrogenation/cyclization of the polyenic deposits while the isobutene selectivity increased via suppression of by-products. These data support earlier assumptions that the initial high yield of by-products was produced by the reaction of butenes with reactive deposits, which were gradually converted into more inert aromatic deposits. Upon purging the catalyst with an inert, branched C8 hydrocarbons and aromatic molecules such as xylenes desorbed. The rates of desorption showed an initially linear variation with the square root of time and were fitted to a monodimensional Fickian model. Because the slowly diffusing species were rather abundant during the course of the reaction, it is proposed that the skeletal isomerization was effectively limited to the acid sites close the pore mouth of H-FER crystallites.
First principles molecular dynamics (MD) simulations, of water adsorption on the MgO[100] surface, was performed to determine the molecular structure and chemical nature of the adsorbed water at varying coverage. Dissociative adsorption was stabilized by hydrogen bond donation from neighboring water molecules. The dissociation barrier had a strong dependence on coverage. Spontaneous dissociation was observed in picosecond MD simulations above 1/2 monolayer (ML) coverage. Ordered structures with c[3×2] symmetry were examined at 2/3 and 1 ML coverage. The ordered c[3×2] phase consisted of a mixture of dissociated and molecular adsorbed molecules.
Using the ab initio total energy method based on the gradient-corrected local density approximation we have modeled the experimentally observed ( 3×2) water monolayer on the MgO (100) surface. The lateral interactions between the adsorbed water molecules, the formation of hydrogen bonds, and the resulting strong dimerization of the adsorbate promote the dissociation of two out of six water molecules in the surface unit cell. Although, on the theoretical grounds, water dissociation on a defective MgO surface has been already reported, this is the first theoretical evidence of water dissociation on the perfect MgO (100) surface.
The Car–Parrinello method has been applied to study the adsorption of water on solid magnesium oxide with surface defects. A step consisting of an (100) and an (010) surface on an (011) base plane allows us to model the experimentally observed microfaceting. In and on this step dissociation of water into a hydroxyl group and a H‐atom took place following a complicated pathway only accessible by the simulation of thermal motion. Under comparable conditions physisorption only was observed on a regular (001) plane. This solves an experimental controversy and it is in agreement with the observation, that disordered surfaces are more active in initiating the dissociation of the water molecules. Our work allows us to identify an important active center. We can also account for the experimentally observed broadening and shifting to the red of the stretching mode of hydrogen bonded hydroxyl groups, and we provide a detailed explanation of the origin of this effect. This allows us to verify earlier theories of hydrogen bonding such as that of the adiabatic separation of the proton dynamics. © 1995 American Institute of Physics.
Pyridine‐d5 has been prepared by exchange between pyridine vapor and heavy water in the presence of a palladium catalyst. Infrared and Raman spectra are reported for pyridine and pyridine‐d5 over the spectral range 300–4000 cm−1. Interpretation of the spectra with the help of the product rule and by analogy with the fundamental frequencies of benzene and benzene‐d6 leads to complete sets of frequencies for the 27 vibrational degrees of freedom of pyridine and of pyridine‐d5. The frequency assignment for pyridine does not differ greatly from previous ones, but is believed to be more reliable because of the additional spectroscopic data.
An analysis of the vibrations of the pyridine molecule was made on the basis of infra‐red, Raman, and ultraviolet absorption spectra. The fundamental frequencies were assigned and the thermodynamic properties of pyridine were calculated on the basis of such an assignment.
Sorption and diffusion of O2, N2, CH4, CO2, cyclo-C3H6, and cis-C4H8 in commercial 5A zeolite has been studied chromatographically. Equilibrium constants and heats of sorption are in satisfactory agreement with previously reported gravimetrically measured values. Dispersion of the chromatographic response for O2, N2, CH4, and CO2 is controlled mainly by axial dispersion with some contribution from macropore diffusion but for cyclo-C3H6 and cis-C4H8 intracrystalline diffusional resistance is dominant. Intracrystalline diffusivities for these species are in reasonable agreement with the extrapolation of lower temperature gravimetric values.
CO adsorption at low temperatures has been used to probe Al3+ Lewis acid sites created upon dehydroxylation of Al2O3, using transmission infrared spectroscopy. Al2O3 was dehydroxylated in the temperature range from 475 to 1200 K. There is an approximate linear correlation between the decreasing integrated absorbance of the Al-OH IR features and the increasing integrated absorbance of the adsorbed CO features throughout the full coverage range of hydroxyl groups. Two CO adsorption sites have been identified: the first (nu-CO = 2195 cm-1) appears immediately upon mild dehydroxylation, while a second site (nu-CO = 2213 cm-1) appears after an 800 K dehydroxylation treatment. CO adsorption on the Lewis acid sites formed by dehydroxylation of Al2O3 involves a binding energy of about 21 kJ/mol.
A method for the preparation of high surface area solids for transmission infrared spectroscopic surface chemistry studies is described. This method consists of mechanically pressing the solid into tiny holes of a support grid, resulting in many tiny self-supported pellets surrounded by the metal grid framework. This allows for maximum efficiency heat transport in the temperature range of 100-1500 K in the sample. Studies of adsorption and desorption phenomena from the pressed powder indicate that diffusion of gases into and out of the pressed sample is rapid. There are no apparent differences in the adsorption and desorption behavior of samples produced by this method and by a spray deposition method. Furthermore, contaminant-free samples can be prepared by this method since no solvents are used.
The molecular transport of pyridine through nanosized MgO particles has been investigated by Fourier transform IR spectroscopy. Two regimes of chemisorbed pyridine diffusion are observed. A fast diffusion process is associated with pyridine bound to nondefective smooth MgO(100) facets which exhibit an activation energy barrier of 35.7 kJ mol−1 for escape into a mobile precursor. The slow pyridine diffusion process occurs from defect sites, where the activation energy of escape to the mobile precursor is measured to be 64.6 kJ mol−1. The escape barriers controlling pyridine diffusion through MgO powders are similar to the calculated adsorption energy on MgO(100) and to the calculated adsorption energies on various defect sites on MgO(100) having lower Mg2+ coordination numbers than those on MgO(100) facets.
Impact of the parent material used to yield magnesia on its surface properties was the prime objective of the present investigation. Accordingly, four test magnesias were obtained by calcination at 600 °C of four different parents: magnesium oxalate (to yield MgO−OX), carbonate (MgO−CA), hydroxide (MgO−HY), and perchlorate (MgO−PC). The surface accessibility was determined by N2 sorptiometry at liquid nitrogen temperature, whereas the surface acid and base properties were assessed qualitatively and quantitatively by means of thermogravimetry, differential scanning calorimetry, and in-situ infrared spectroscopy of adsorption/desorption of pyridine and CO2 at RT-400 °C. The reactivity of the surface acid and base sites thus revealed was gauged versus the decomposition of methylbutynol at 100 °C. The results obtained have shown the surface accessibility, basicity, and base site reactivity to assume the following descending order: MgO−OX > MgO−HY > MgO−CA > MgO−PC. Moreover, the results emphasize the importance of implementing the above-indicated combination of studies, if the nature, amount, and strength of surface acid and base sites are to be exhustively assessed
A vibrational force field for pyridine has been determined by making use of ab initio calculated force constants as the starting point. These constants were adjusted to fit the observed frequencies for pyridine, pyridine-d5, and partially deuterated pyridines. The calculated frequencies allowed an assignment of the uncertain frequencies for the partially labeled compounds, and a constrained adjustment of the ab initio F matrix to give a "best fit" to the observed frequencies provided a fairly accurate force field. The infrared band intensities for pyridine and pyridine-d5 were determined in solution and were converted to dipole moment derivatives. The relationship between these quantities and the charge distribution in pyridine is discussed.
A modified autoclave hypercritical drying procedure has been used to prepare a hydrated form of MgO from Mg(OCH3)2 in a methanol-toluene solvent mixture. This material was prepared with 1000 m2/g surface area and 35-angstrom crystallite size. Heat treatment of this precursor at 500-degrees-C under vacuum yielded the dehydrated MgO with 500 m2/g surface area and 45-angstrom crystallite size. The samples were further characterized by Fourier transform infrared/photoacoustic spectroscopy (FT-IR/PAS), X-ray diffraction, scanning electron microscopy, and chemical analyses. The hydrated precursor contained some residual -OCH3 groups and was much less crystalline in appearance than conventionally prepared MgO-Mg(OH)2 samples. The dehydrated material was free of -OCH3 groups and was made up of much smaller crystallites than conventionally prepared MgO.
C-H and C-D stretching vibrations of h(5)- and d(5)-pyridine adsorbed on various metal oxides have been investigated by infrared spectroscopy. Experimental spectra were further compared with density functional (DFT) calculations of the interaction of pyridine with cluster models. Both vCH(D) frequencies and intensities are sensitive to the adsorption mode and allow one to distinguish between H-bonded, protonated and coordinated species. In the case of coordinated species, an increase in v(CH) frequencies is observed and it could be correlated with the v(8a). and v(19b) frequency shifts which are commonly used to assess acid strength of metal oxides. It is found that the study of v(CH) modes is more reliable than that of the v(8a) and v(19b) modes to distinguish H-bonded from weakly coordinated species. An example of such an application is given for the cases of CeO(2), MgO and CaO. (c) 2006 Elsevier B.V. All rights reserved.
Inverse Gas Chromatography (IGC) is used to study the influence of relative humidity and chemical treatment with sulfochromic acid on the surface properties of glass beads. The humidity of the carrier gas to the chromatograph is controlled by means of a specially designed humidity generator to allow investigation of the surface of powders attributed to the progressive coverage by water molecules. The solid under test is glass beads with a very a low specific surface and the properties studied are the specific surface, thermodynamic parameters and the proportions of silanol and siloxane groups on the surface.
Published breakthrough time, adsorption rate, and capacity data for components of organic vapor mixtures adsorbed from flows through fixed activated carbon beds have been analyzed. Capacities (as stoichiometric centers of constant pattern breakthrough curves) yielded stoichiometric times τ, which are useful for determining elution orders of mixture components. Where authors did not report calculated adsorption rate coefficients kv of the Wheeler (or, more general, Reaction Kinetic) breakthrough curve equation, we calculated them from breakthrough times and τ. Ninety-five kv (in mixture)/kv (single vapor) ratios at similar vapor concentrations were calculated and averaged for elution order categories. For 43 first-eluting vapors the average ratio (1.07) was statistically no different (standard deviation 0.21) than unity, so that we recommend using the single-vapor kv for such. Forty-seven second-eluting vapor ratios averaged 0.85 (standard deviation 0.24), also not significantly different from unity; however, other evidence and considerations lead us to recommend using kv (in mixture)=0.85kv (single vapor). Five third- and fourth-eluting vapors gave an average of 0.56 (standard deviation 0.16) for a recommended kv (in mixture)=0.56kv (single vapor) for such.
Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH3, pyridine, CH3CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH3; (ii) much more strongly adsorbed than CO and CH3CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν8a mode of νCCN vibrations at 1630–1600 cm−1, where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH+) is identifiable by (i) an ν8a-absorption at ≥1630 cm−1; (ii) an ν19b-absorption at 1550–1530 cm−1; as well as (iii) νN+H and δN+H absorptions occurring, respectively, near 2450 and 1580 cm−1, and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH−and O2−) involved in formation of acid–base site pairs.
Experimental and theoretical evidence for the existence of mobile, transitory precursor states and for the importance of lateral interactions between chemisorbed species is briefly surveyed. Generalised rate expressions for adsorption and desorption are derived which specifically take into account the existence of intrinsic and extrinsic precursor states, and which can be readily adapted to account for dissociative or non-dissociative adsorption, with and without interactions, and which also account for the possibility of direct transfer to or from the chemisorbed state, without trapping in the intrinsic precursor state. These expressions, derived using kinetic schemes, are shown to be identical with rate expressions obtained from a successive site statistical model. Thermodynamic conditions are used to restrict the number of parameters in these expressions.
Assignment modifications for the pyridine fundamentals are proposed, based on first time Raman vapor measurements, and a more complete set of infrared and Raman spectra for the gas and liquid. Raman polarization measurements are newly given for some 70 lines between 1600 and 3000 cm−1. The Kakiuti et al. (Kakiuti et al., J. Mol. Spectrosc. 61 (1976) 164) assignment for the nonplanar modes in CS2 solution is shown to be the only previous correct assignment. Planar assignment modifications, though generally small, are warranted because of the benchmark nature of pyridine. A previous set of vibrational frequencies, derived from a scaled 4-21G force field calculation (Pongor et al., J. Am. Chem. Soc. 106 (1984) 2765; Pongor et al., J. Mol. Spectrosc. 114 (1985) 445), is in excellent agreement with the observed fundamentals, and in fact, tips the balance in favor of assigning ν22 to 1053 cm−1. Ideal-gas entropies derived from spectroscopic constants are also seen to be in excellent agreement with those from calorimetry (Chirico et al., J. Chem. Thermodyn. 28 (1996) 797; Chirico and Steele, J. Chem. Thermodyn. 28 (1996) 819).
We present a detailed description and comparison of algorithms for performing ab-initio quantum-mechanical calculations using pseudopotentials and a plane-wave basis set. We will discuss: (a) partial occupancies within the framework of the linear tetrahedron method and the finite temperature density-functional theory, (b) iterative methods for the diagonalization of the Kohn-Sham Hamiltonian and a discussion of an efficient iterative method based on the ideas of Pulay's residual minimization, which is close to an order N-atoms(2) scaling even for relatively large systems, (c) efficient Broyden-like and Pulay-like mixing methods for the charge density including a new special 'preconditioning' optimized for a plane-wave basis set, (d) conjugate gradient methods for minimizing the electronic free energy with respect to all degrees of freedom simultaneously. We have implemented these algorithms within a powerful package called VAMP (Vienna ab-initio molecular-dynamics package), The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semi-conducting surfaces, phonons in simple metals, transition metals and semiconductors) and turned out to be very reliable.
The basic principles of a Fourier Transform IR (FTIR) method are introduced for measurements of diffusivities (D0) in single-component diffusion, co- and counter-diffusion (in the case of binary mixtures) and related data (activation energy of diffusion, EA, heat of adsorption, Q) of systems with microporous sorbents. As microporous sorbents H-ZSM-5, H-mordenite, H,Na-ZSM-5, silicalite, Li-ZSM-5 and Na-ZSM-5 are employed. Adsorbates studied are benzene, ethylbenzene, p-xylene and pyridine. It is shown how the reliability of the FTIR method was checked and confirmed via comparison of the FTIR results derived for the system benzene/H-ZSM-5 with the large body of literature data reported for the same system but obtained with various independent techniques. Similarly, the FTIR results of the other systems are discussed in view of literature data, if available. In comparison to single-component diffusion, it is generally found via the FTIR method that the diffusivities in co- and counter-diffusion are lower by about 50%. It is observed that, in H-ZSM-5, D0(p-xylene)>D0(benzene), a result which is not a priori expected. The investigation of pyridine diffusion in H-mordenite and H-ZSM-5 was hampered by the strong interaction of the base with the Brønsted acid sites of the zeolite and the upper limitation of the temperature of the experiment (575 K); it could be studied, however, in the absence of strong acid sites, e.g. in the systems pyridine/silicalite or pyridine/Li-ZSM-5 and pyridine/Na-ZSM-5. To cite this article: H.G. Karge, C. R. Chimie 8(2005).
We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal–amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nosé dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.
We study the adsorption of water on MgO surfaces with the Car-Parrinello method. Our simulation shows that an H2O molecule in the proximity of a perfect (001) surface is physisorbed. The binding energy is rather small and the molecule desorbs at modest temperatures. We also simulated a stepped surface. On this surface the dissociation of water proceeds very rapidly. Our simulation is the first of this kind and agrees with experimental evidence.
A new method to study the diffusion properties of molecules into porous materials using transmission IR spectroscopy is employed. A measurement of the diffusion of the 2-chloroethylethyl sulfide (2-CEES) molecule into two types of gamma-Al2O3 powder is performed, showing that the diffusion rate into subnanometer crystallite particle size gamma-Al2O3 powders (subnano-Al2O3) is higher than that into the larger crystallite particle size powder. It is shown that a surface diffusion mechanism can be used to model the diffusion process giving good agreement with the experimental results, where Dsubnano-Al2O3 is approximately 5 times larger than Dmultinano-Al2O3 at 170 K for the 2-CEES molecule.
The adsorption of triethylenediamine (TEDA) at 300 K is observed to occur via hydrogen bonding to isolated Al-OH groups on the surface of partially dehydroxylated high area gamma-Al(2)O(3) powder. This form of bonding results in +0.3 to +0.4% blue shifts in the CH(2) scissor modes at 1455 cm(-1) and a -0.4% red shift in the CN skeletal mode at 1060 cm(-1), compared to the gas-phase frequencies. Other modes are red shifted less than 0.1%. The isolated OH modes are red shifted by -200 to -1000 cm(-1) due to the strong hydrogen bonding association of Al-OH groups with an N atom in TEDA. Thermal desorption of adsorbed TEDA from the surface occurs in the range 300-700 K. Mass spectral and infrared studies indicate that the decomposition of TEDA occurs on Al(2)O(3) above 725 K, and that C-H bonds are broken, forming adsorbed species with N-H bonds which are stable to 1000 K or above. In contrast to adsorption at 300 K, adsorption of TEDA at 85 K results in the formation of a condensed ice of TEDA, which covers the outer surface of the porous Al(2)O(3) and which does not interact with Al-OH groups inside the porous powder due to immobility.