Showing papers in "Physical Chemistry Chemical Physics in 2003"

Solvent-solute interactions in ionic liquids

Abstract: A range of ionic liquids has been investigated using the Kamlet–Taft parameters α, β, and π*. It was found that π* is high for all of the ionic liquids studied and varies with both anion and cation, α is generally moderate and depends mainly on the cation, β is also moderate and depends mainly on the anion. Comparison is made with other polarity measurements in ionic liquids.

Chemical capacitance of nanostructured semiconductors: its origin and significance for nanocomposite solar cells

Abstract: The capacitance measured in dye-sensitised nanocrystalline TiO2 solar cells (DSSC) is interpreted in terms of a chemical capacitance, which is found to be a crucial feature for describing the dynamic operation of solar cells based on nanoscaled materials.

Nanocrystallinity effects in lithium battery materials

Abstract: Nanostructured materials offer the possibility to make use of small transport lengths and small separation distances almost like in fluids, but unlike fluids, the higher structural stability of the solid state can be taken advantage of. Recent findings in the field of Li-batteries highlight the potential for room temperature applications. This paper addresses advantages and disadvantages of nanostructured matter with respect to stability, storage capacity, voltage and charging/discharging rates. In this context we discuss a novel interfacial storage mechanism for lithium which, in the mesoscopic case, forms a bridge between batteries and capacitors.

The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism

Abstract: The heterogeneous reaction of NO2 with water on the surface of laboratory systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO2 and in water vapor, and the formation of a complex between NO2 and water at the air–water interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO2 and the formation of the products HONO, NO and N2O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO3, N2O4 and NO2+. We propose a new reaction mechanism in which we hypothesize that the symmetric form of the NO2 dimer, N2O4, is taken up on the surface and isomerizes to the asymmetric form, ONONO2. The latter autoionizes to NO+NO3−, and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO3. Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our experimental data and those of previous studies, as well as the chemistry of such intermediates as NO+ and NO2+ that is known to occur in solution. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chemistry is that in the atmospheric boundary layer where human exposure occurs and many measurements of HONO and related atmospheric constituents such as ozone are made, a major substrate for this heterogeneous chemistry is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE = Surfaces, Urban and Remote: Films As a Chemical Environment) has received relatively little attention compared to reactions in the gas and liquid phases, but in fact may be quite important in the chemistry of the boundary layer in urban areas.

Oxidation of glycerol using supported Pt, Pd and Au catalysts

Abstract: The oxidation of aqueous solutions of glycerol is described and discussed for Pd, Pt and Au nanoparticles supported on graphite and activated carbon. The oxidation in a batch reactor at 60 °C and 1 bar pressure using air as oxidant was initially investigated. Under these conditions, supported Pd and Pt catalysts give some selectivity to glyceric acid, but the main reaction products are considered to be non-desired C1 by-products, e.g. CO2, HCHO and HCOOH. In addition, under these conditions, supported Au catalysts were totally inactive. Using an autoclave with pure oxygen at 3 bar pressure gave a significant improvement in reactivity and, for Pt and Au catalysts, the formation of C1 by-products was eliminated when NaOH was added. In particular, it was noted that, in the absence of NaOH, the Au/C catalyst was inactive. For 1 wt.% Au/graphite or activated carbon, 100% selectivity to glyceric acid at high conversion was readily achieved. The role of the base is discussed and it is proposed that the base aids the initial dehydrogenation via H-abstraction of one of the primary OH groups of glycerol and, in this way, the rate limiting step in the oxidation process is overcome.

The hydrophobic effect

Abstract: The thermodynamics of the hydrophobic effect, as measured primarily through the temperature dependence of solubility, is reviewed, and then a class of models that incorporate the basic mechanism of hydrophobicity is described. These models predict a quantitative relation between the free energy of hydrophobic hydration and the strength of the solvent-mediated attraction between pairs of solute molecules. It is remarked that the free energy of attraction being just of the order of the thermal energy kT may be important for the effective operation of the hydrophobic effect in proteins. Deviations from pairwise additivity of hydrophobic forces are also briefly discussed.

On the origin of binding energy shifts of core levels of supported gold nanoparticles and dependence of pretreatment and material synthesis

Abstract: X-ray photoelectron spectroscopy (XPS) investigations of supported nanoparticles smaller than 10 nm show a significant shift of the electron binding energy of core levels compared with the bulk values. In this work, such shifts were examined at differently supported and prepared gold nanoparticles for the 4f electron level. Special attention was paid to the influence of reducing pretreatment in hydrogen and, moreover, the influence of different oxide supports. Surprisingly, in most cases, lower binding energies than the Au 4f7/2of 84.0 eV were observed depending on the oxidic support as well as the pretreatment conditions. The origin of these differences of the core level values are discussed in terms of different models like electron transfer from the support to the particles, size and geometric effects. It seems that especially geometric factors like the particle shape play an important role.

Direct synthesis of hydrogen peroxide from H2 and O2 using Pd and Au catalysts

Abstract: The direct synthesis of hydrogen peroxide from H2 and O2 using a range of supported metal catalysts is described and discussed. A detailed study of the factors influencing the formation and decomposition of hydrogen peroxide is presented for a Pd/sulfonated carbon catalyst in a methanol/water solvent. The use of low temperatures (1–2 °C) and short reaction (residence) time are identified as the key factors that favour high selectivity to hydrogen peroxide. Decomposition of hydrogen peroxide, mainly via further hydrogenation, prevents the formation of high concentrations of hydrogen peroxide. Combustion of hydrogen to water is a competing reaction that becomes significant at higher temperatures, but this can be partially inhibited by the addition of HBr. A second set of supported Pd and Au catalysts are evaluated for the direct synthesis of hydrogen peroxide using supercritical CO2 as a solvent. The use of supercritical CO2 is shown to be beneficial when compared with hydrogen peroxide formation at a temperature just below the critical temperature for CO2. However, at the critical temperature of CO2 (31.1 °C), the decomposition of hydrogen peroxide is rapid and only low rates of hydrogen peroxide formation are observed. At low temperature (2 °C) supported Au catalysts are shown to be very selective for the synthesis of hydrogen peroxide. The rate of hydrogen peroxide synthesis is enhanced markedly when Pd is present with Au and a detailed scanning transmission electron microscopy study shows that the 2–9 nm metal nanoparticles present in this supported catalyst are a Au∶Pd alloy.

Determining the active site in a catalytic process: Operando spectroscopy is more than a buzzword

Abstract: Determining the active site in a catalyst material and elucidating the related reaction mechanism remain intellectual challenges and are of paramount importance for the rational development of new or better catalysts. Identifying active sites should in principle be possible by studying the catalyst at its working place under real reaction conditions. This is the field of operando spectroscopy, a research area, which is under intense development and of high interest to the catalysis community. This feature article presents an attempt of a roadmap for future operando spectroscopy studies. Two case studies will illustrate the potential and limitations of the operando approach. A first study deals with operando vibrational spectroscopy to obtain mechanistic insight in the destruction of chlorinated hydrocarbons with steam over lanthanide oxide-based catalysts. The second case study elaborates on the use of operando electronic spectroscopy to elucidate the active alkane dehydrogenation site in supported chromium oxide catalysts.

Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates

Abstract: α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4 prepared by precipitation and solid-state reactions showed intense absorption bands in the visible light region due to band gap transitions Comparison of a diffuse reflectance spectrum of α-NaVO3 with that of α-AgVO3 with a diopside-type structure revealed that a band gap (25 eV) of α-AgVO3 was 06 eV smaller than that (31 eV) of α-NaVO3 The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of α-AgVO3 was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals Among α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4, only Ag3VO4 showed a photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation Holes photogenerated in Ag3VO4 can migrate to the reaction sites on the surface more easily than those of other silver vanadates, because the content of silver forming a valence band is large It resulted in that holes photogenerated in Ag3VO4 are able to oxidize H2O to form O2

Multiparametric probing of intermolecular interactions with fluorescent dye exhibiting excited state intramolecular proton transfer

Abstract: Excited-state intramolecular proton transfer (ESIPT) in 3-hydroxyflavone dyes allows us to record, in addition to common spectroscopic parameters, the positions of absorption (νabs) and emission (νN*) maxima, two new parameters: the position of the emission maximum of the ESIPT product T* state (νT*) and the intensity ratio of the two emission bands (IN*/IT*). An attempt was made to find a correlation between these parameters and physicochemical characteristics of microenvironment: polarity f(e), electronic polarizability f(n) and H-bond donor ability. A detailed spectroscopic study of 4′-diethylamino-3-hydroxyflavone in a set of 21 representative solvents demonstrates that the Stokes shift of the N* band (νabs − νN*) correlates strongly with the Lippert function L = f(e) − f(n), and this correlation does not depend on the effects of intermolecular H-bonding, while the correlation of log(IN*/IT*) with polarity f(e) can be represented by linear functions that are different for protic and aprotic environments. Cross-correlation analysis of the spectroscopic parameters provides criteria to distinguish specific (H-bonding and other) from universal probe interactions with the environment. We suggest an algorithm, which uses four spectroscopic parameters νabs, νN*, νT* and log(IN*/IT*) to provide a simultaneous estimation of three microenvironment characteristics: f(e), f(n) and H-bond donor ability. An application of this algorithm in the studies of binary solvent mixtures, reverse micelles and binding sites of proteins demonstrates the power of this approach and suggests a unique possibility to develop a new generation of fluorescence probes and labels in the 3-hydroxyflavone family for studying complex microheterogeneous systems in physical chemistry, colloid chemistry and the biological sciences.

Refractive indices, molar volumes and molar refractions of binary liquid mixtures: concepts and correlations

Abstract: Prompted by the observation that recent literature displays marked disagreement as to the proper treatment of data on the refractive indices of binary liquid mixtures, this paper clarifies the relationships among refractive index, molar volume and molar refraction and the corresponding mixing properties. It is shown that the molar refraction deviation function must be calculated on a mole fraction basis and the refractive index deviation function on a volume fraction basis, which makes it directly interpretable as a sign-reversed measure of the deviation of reduced free volume from ideality.

Linear scaling local coupled cluster theory with density fitting. Part I: 4-external integrals

Abstract: The density fitting approximation is applied to the most expensive class of 2-electron integrals in local CCSD, i.e., to those integrals that involve four virtual orbitals (or projected AOs). The fitting error in the correlation energy is systematic and considerably smaller than the deviation between the local and the canonical CCSD energy. In order to restore () scaling locality must be exploited for the fitting functions as well as for orbitals. Local fitting domains specified for individual centre pairs provide an adequate basis for such a local description, however, Dunlap's robust formula for the approximate integrals then no longer simplifies to the usual expression known as the V approximation. A symmetric formula is proposed as an alternative, which, although formally non-robust, yields virtually the same results as the robust formalism. The additional fitting error due to the introduction of local fitting domains is considerably smaller than the original fitting error itself (by at least an order of magnitude). Test calculations demonstrate () scaling for the new LDF-LCCSD method. The approximate calculation of the 4-external integrals via density fitting in LDF-LCCSD is 10–100 times faster than the exact calculation via the () 4-index transformation in LCCSD.

Temperature-dependent rate constants for hydroxyl radical reactions with organic compounds in aqueous solutions

Abstract: The OH radical is the most important oxidant in both the tropospheric gas and aqueous phase. Its main sink processes in clouds appear to be reactions with organics but due to the lack of appropriate kinetic data current cloud chemistry models consider only reactions with C1 and C2 compounds. Therefore, in this study temperature dependent rate constants for the reactions of the OH radical with organic compounds (≥C2) were determined. These investigations were performed by competition kinetics (reference substance: SCN−). Initially the experimental system was checked reinvestigating kinetic data for OH reactions with formate (R-1) and tert-butanol (R-2) available from literature. For the reactions (R-1) and (R-2) the following results were obtained: k1(298 K)=(2.4±0.4)×109 M−1 s−1; A1=(7.9±0.7)×1010 M−1 s−1; EA,1=(9±5) kJ mol−1 and k2(298 K)=(5.0±0.6)×108 M−1 s−1; A2=(3.3±0.1)×1010 M−1 s−1; EA,2=(10±3) kJ mol−1 for formate and tert-butanol, respectively. Temperature dependent rate constants for the reactions of OH with ethanol (k3(298 K)=(2.1±0.1)×109 M−1 s−1; A3=(1.0±0.1)×1011 M−1 s−1; EA,3=(10±5) kJ mol−1), 1-propanol (k4(298 K)=(3.2±0.2)×109 M−1 s−1; A4=(5.6±0.6)×1010 M−1 s−1; EA,4=(8±6) kJ mol−1), acetone (k5(298 K)=(2.1±0.6)×108 M−1 s−1; A5=(3.4±0.4)×1011 M−1 s−1; EA,5=(18±11) kJ mol−1) and methylglyoxal (k6(298 K)=(1.1±0.1)×109 M−1 s−1; A6=(2.9±0.3)×1011 M−1 s−1; EA,6=(13±6) kJ mol−1), Propionic acid: k7(298 K)=(3.2±0.5)×108 M−1 s−1; A7=(7.6±0.9)×1011 M−1 s−1; EA,7=(19±8) kJ mol−1; propionate: k8(298 K)=(7.2±0.4)×108 M−1 s−1; A8=(3.2±0.2)×1011 M−1 s−1; EA,8=(15±4) kJ mol−1; glyoxylic acid: k9(298 K)=(3.6±0.2)×108 M−1 s−1, A9=(8.1±0.4)×109 M−1 s−1, EA,9=(8±3) kJ mol−1, glyoxylate: k10(298 K)=(2.6±0.9)×109 M−1 s−1, A10=(6.0±0.4)×1015 M−1 s−1; EA,10=(36±8) kJ mol−1; pyruvic acid k11(298 K)=(1.2±0.4)×108 M−1 s−1; A11=(1.0±0.1)×1012 M−1 s−1; EA,11=(23±4) kJ mol−1; pyruvate: k12(298 K)=(7±2)×108 M−1 s−1; A12=(1.3±0.1)×1012 M−1 s−1; EA,12=(19±4) kJ mol−1; oxalate (monoanion): k13(298 K)=(1.9±0.6)×108 M−1 s−1; A13=(2.5±0.1)×1012 M−1 s−1; EA,13=(23±4) kJ mol−1; oxalate (dianion): k14(298 K)=(1.6±0.6)×108 M−1 s−1, A14=(4.6±0.5)×1014 M−1 s−1; EA,14=(36±10) kJ mol−1; malonate (monoanion): k15(298 K)=(6±1)×107 M−1 s−1, A15=(3.2±0.4)×109 M−1 s−1; EA,15=(11±5) kJ mol−1; succinic acid k18(298 K)=(1.1±0.1)×108 M−1 s−1, A18=(8±1)×109 M−1 s−1; EA,18=(11±6) kJ mol−1 and succinate (dianion): k19(298 K)=(5.0±0.5)×108 M−1 s−1, A19=(5.0±0.4)×1010 M−1 s−1; EA,19=(11±5) kJ mol−1 were determined.

Adsorption of proteins on spherical polyelectrolyte brushes in aqueous solution

Abstract: We consider the adsorption of bovine serum albumin (BSA) on spherical polyelectrolyte brushes (SPB). The SPB consist of a solid polystyrene core of 100 nm diameter onto which linear polyelectrolyte chains [poly(acrylic acid), (PAA)] are grafted. The adsorption of BSA is studied at a pH of 6.1 at different concentrations of added salt and buffer. We observe strong adsorption of BSA onto the SPB despite the effect that the particles as well as the dissolved BSA are charged negatively. The adsorption of BSA is strongest at low salt concentration and decreases drastically with increasing amounts of added salt. Virtually no adsorption takes place at salt concentration of 0.1 M. Moreover, the adsorbed protein can be washed out again by raising the ionic strength from low to high values. A major driving force for the adsorption is located at a lower pH within the brush at low ionic strength. Thus, the isoelectric point of the protein may be approached or even reached. In this case strong interaction between the SPB and the protein results. Moreover, the negative charge of the polyelectrolyte interacts with the patches of positive charges on the protein. In this way the protein becomes a multivalent counterion within the brush and monovalent counterions will be released. All results demonstrate that the SPB present a new class of colloidal carrier particles whose interaction with proteins can be tuned in a well-defined fashion.

Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders

Abstract: The molar amounts of defective sites (Md) in several titanium(IV) oxide (TiO2) powders were determined using photoinduced reactions of electron accumulation in deaerated aqueous solutions containing sacrificial hole scavengers and subsequent reduction of methylviologen to its cation radical. Measurements of pH dependence of typical anatase and rutile TiO2 powders showed that these defective sites were of electronic energy just below the conduction band edge of TiO2 in ranges of 0–0.35 V for anatase and 0–0.25 V for rutile. A linear relation of Md with the rate constant of electron-hole recombination determined by femtosecond pump-probe diffuse reflection spectroscopy revealed that Md could be a quantitative parameter of recombination between a photoexcited electron and a positive hole. The fact that there was no linear relation between Md and the specific surface area suggests that the surface area was not directly reflected on Md. A reciprocal correlation between photocatalytic activity for water oxidation in aqueous silver sulfate solution and Md revealed that the rate of recombination is one of the predominant physical properties governing the activities of TiO2 powders in this reaction system.

Photocatalytic ?OH radical formation in TiO2 aqueous suspension studied by several detection methods

Abstract: Photoinduced reaction with TiO2 semiconductor photocatalysts was investigated by using nitroxide radicals as spin probes for ˙OH radicals. The effects of some additives such as I−, Cl−, ClO4−, methanol and 2-propanol on the photocatalytic decay of nitroxide radicals, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidine-1-oxyl and 4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl, were investigated. Among the additives, only iodide ions that can be oxidized by trapped holes prohibited significantly the decay of the nitroxide radicals, indicating that trapped holes oxidize nitroxide probe radicals. For several TiO2 photocatalysts, the apparent quantum efficiency of ˙OH radical formation was calculated and compared with those obtained by the other detection methods, such as DMPO spin trapping and terephthalic acid fluorescence methods. The experimental observations suggest that the photocatalytic oxidation should be caused more preferably by trapped holes or adsorbed ˙OH radicals, rather than the photoinduced valence band holes in semiconductor and the free ˙OH radicals in solution.

The electro-oxidation of formic acid on Pt–Pd single crystal bimetallic surfaces

Abstract: The interrelationship between the macroscopic kinetic rate of HCOOH oxidation in 0.1 M HClO4 solution and the morphology/composition of the electrode is studied on Pt(111) modified by Pd (denoted hereafter as the Pt(111)–PdxML system, 0 < x < 1) and on Pt–Pd bulk single crystal alloy surfaces (denoted hereafter as the PtPdxat%(111) system, x = 6 and x = 25). The Pd surface composition of the Pt(111)–PdxML and PtPdxat%(111) electrodes was established previously ex-situ by low energy ion scattering (LEIS) measurements. The nature of adsorbed intermediates (COad) and the electrocatalytic properties (the onset of CO2 formation) at the Pt(111)–PdxML and the PtPdxat%(111) interface were studied by FTIR spectroscopy. The results show that Pd atoms either on the surface or in the surface have an unique catalytic activity for HCOOH oxidation, with Pd atoms being three (bulk alloys) or five times (Pd films) more active than Pt atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO2 is observed at low potentials, indicating that the reaction can proceed on Pd at low potentials without the Pt typical “poison” formation.

The helium dimer potential from a combined density functional theory and symmetry-adapted perturbation theory approach using an exact exchange–correlation potential

Abstract: Starting with an analytic representation of the electron density from a Hylleraas wavefunction we have obtained an analytic representation of the exchange–correlation potential of the helium atom. This, essentially exact, exchange–correlation potential has been employed to test a recently developed approach, named DFT-SAPT, which combines symmetry-adapted perturbation theory of intermolecular interaction energies with a density functional theory description of the interacting monomers. In DFT-SAPT all of the second-order contributions including the exchange corrections are determined from coupled-perturbed density functional theory. Comparison of the results for the helium dimer to previous high-quality supermolecular and intermolecular perturbation theory results demonstrates the success of the new approach.

Oxygen diffusion in yttria stabilised zirconia—experimental results and molecular dynamics calculations

Abstract: Bulk oxygen self-diffusion in yttria-stabilised zirconia (YSZ) was investigated using tracer diffusion experiments and molecular dynamics (MD) simulation as a function of the yttria content. Experimentally, 18O tracer diffusion was measured as a function of temperature (650–1200 K) and yttria content (8–24 mol% Y2O3) using gas-phase exchange of the stable isotope 18O and SIMS analysis. For a given temperature, the diffusivity was highest for YSZ containing 10 mol% yttria. The activation enthalpy of diffusion was 0.8 to 1.0 eV, independent of the yttria content. The diffusion process was simulated with molecular dynamics using the program DL_POLY and comparing different potential sets. The oxygen diffusion coefficient was found to be of similar magnitude to the experimental value, and also showed similar concentration dependence with a maximum for YSZ containing 10 mol% yttria. The calculated activation enthalpies of oxygen transport are close to the values observed experimentally.

Doping of single-walled carbon nanotube bundles by Brønsted acids

Abstract: Using X-ray induced photoelectron spectroscopy, the influence of Bronsted acids, namely sulfuric, nitric, and hydrochloric acid on the electronic properties of single-walled carbon nanotubes (SWCNTs) was investigated. Doping effects were monitored by changes in binding energy of the C 1s core level of the nanotubes. For all three acids, an acceptor type doping of the SWCNTs was observed by a shift of the C 1s core level towards lower binding energies. The inferred change of the Fermi-level position was 0.5 eV in the case of H2SO4, 0.2 eV in the case of HNO3, and 0.1 eV in the case of HCl. For HNO3 and HCl the doping was found to be unstable. The S 2p, N 1s, and Cl 2p core level spectra of the corresponding acid showed spectral features which can be attributed to the respective oxidation state of these anions in the acid, indicating that doping was induced by intercalation.

N2O5 hydrolysis on sub-micron organic aerosols: the effect of relative humidity, particle phase, and particle size

Abstract: Measurements of the reactive uptake coefficient for N2O5 hydrolysis, γm, on sub-micron organic aerosols were performed in an entrained aerosol flow tube as a function of relative humidity (RH), aerosol phase, N2O5 partial pressure, and mean aerosol size. Aerosol phase and relative humidity were determined simultaneously, and chemical ionization mass spectrometry was used to detect the decay rate of N2O5 in the presence of malonic acid or azelaic acid aerosol. The γm on solid malonic acid was determined to be less than 0.001 (RH = 10–50%), and on solid azelaic acid, γm was 0.0005 ± 0.0003. Aqueous malonic acid aerosol yielded γm = 0.0020 ± 0.0005 at 10% RH and increased with RH to ∼0.03 at RH = 50–70%. We report the first evidence of an inverse dependence of the γm on the initial partial pressure of N2O5 in the flow reactor, and a dependence on particle size for aerosol with surface area-weighted radii less than ∼100 nm at 50% RH. We find that the super-saturated malonic acid aerosol results are consistent with N2O5 hydrolysis being both aerosol volume-limited where, for RH < 50%, water is the limiting reagent, and also with a surface-specific process.

Water structure as a function of temperature from X-ray scattering experiments and ab initio molecular dynamics

Abstract: We present high-quality X-ray scattering experiments on pure water taken over a temperature range of 2 to 77 °C using a synchrotron beam line at the advanced light source (ALS) at Lawrence Berkeley National Laboratory. The ALS X-ray scattering intensities are qualitatively different in trend of maximum intensity over this temperature range compared to older X-ray experiments. While the common procedure is to report both the intensity curve and radial distribution function(s), the proper extraction of the real-space pair correlation functions from the experimental scattering is very difficult due to uncertainty introduced in the experimental corrections, the proper weighting of OO, OH, and HH contributions, and numerical problems of Fourier transforming truncated data in Q-space. Instead, we consider the direct calculation of X-ray scattering spectra using electron densities derived from density functional theory based on real-space configurations generated with classical water models. The simulation of the experimental intensity is therefore definitive for determining radial distribution functions over a smaller Q-range. We find that the TIP4P, TIP5P and polarizable TIP4P-Pol2 water models, with DFT-LDA densities, show very good agreement with the experimental intensities, and TIP4P-Pol2 in particular shows quantitative agreement over the full temperature range. The resulting radial distribution functions from TIP4P-Pol2 provide the current best benchmarks for real-space water structure over the biologically relevant temperature range studied here.

Preparation of alumina through a sol–gel process. Synthesis, characterization, thermal evolution and model of intermediate boehmite

Abstract: Hydrated boehmite with nano-sized crystallites AlO(OH)·08H2O are obtained through a very reproducible sol–gel procedure and characterized using powder X-ray diffraction, in-situ IR spectroscopy, thermal analysis, BET surface area and photoelectron spectroscopy The unit cell parameters, obtained after deleting the first shifted (020) diffraction peak, are a = 3686 A, b = 12179 A and c = 2855 A They are consistent with well-crystallized boehmite and with the position of the harmonic (080) diffraction peak For the (020) peak, the correlation between position and peak width is confirmed for nano-crystallites The average shape of the crystallites, determined from three peak widths, corresponds to slabs with dimensions 79 × 27 × 87 nm Based on this crystallite shape, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH)·055H2OThe thermal evolution of hydrated boehmite leads first to hydrated γ-alumina Al2O3·033H2O = Al2O267(OH)066 and the number of remaining hydroxyl groups is critical for the porosity Between Al2O3·033H2O and Al2O3·02H2O the surface area remains approximately constant (300 m2 g−1) and the hydroxyl density decreases deeply For calcination temperature higher than 800 K, the loss of the remainder of the water leads to a strong decrease of this area The limiting value corresponds to about 9–10 OH-groups nm−2 and can be related to the hydrogen spinel HAl5O8 The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm−1) A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters

Polymeric solid-state dye lasers: Recent developments

Abstract: An up-to-date overview of the results obtained by our group on the development of polymeric solid-state dye lasers based on dipyrromethene·BF2 complexes is presented. It is shown that appropriate chemical modifications in the dye molecules can yield dyes that lase efficiently and with remarkable photostability when properly incorporated into adequate polymeric matrices. Our results compare favourably with those reported by other authors with the dyes incorporated into organic materials and open the way to the development of solid-state dye lasers competitive with their liquid counterparts.

Raman spectroscopic investigation of speciation in MgSO4(aq)

Abstract: Careful measurements have been made of the Raman spectra of aqueous solutions of Mg(ClO4)2, MgCl2, (NH4)2SO4 and MgSO4 down to 50 cm−1 and, in some cases, to extremely low concentrations (≥0.06 mmol kg−1) and high temperatures (≤200 °C). In MgSO4(aq), the well known asymmetry in the ν1-SO42− mode at ∼980 cm−1 that develops with increasing concentration has been assigned to a mode at 993 cm−1 associated with the formation of an MgOSO3 contact ion pair (CIP). Confirmation of this assignment is provided by the simultaneous and quantitative appearance of stretching modes for the Mg–OSO3 bond of the ligated SO42− at 245 cm−1 and for the (H2O)5MgOSO3 unit at 328 cm−1. The CIP becomes the dominant species at higher temperatures. Alternative explanations of the broadening of the ν1-SO42− mode are shown to be inconsistent with this and other Raman spectral evidence such as the similarity of the ν1-SO42− mode for MgSO4 in H2O and D2O. After subtraction of the CIP component at 993 cm−1, the ν1-SO42− band in MgSO4(aq) showed systematic differences from that in (NH4)2SO4(aq). This is consistent with a previously undetected ν1-SO42− mode at 982.2 cm−1 that can be assigned to the presence of solvent-shared ion pairs (SIPs). In solutions with high Mg2+/SO42− concentration ratios, a further ν1-SO42− mode was observed at 1005 cm−1, which has been tentatively assigned to a Mg2SO42+(aq) triple ion. All of these observations are shown to be in excellent agreement with recent dielectric relaxation spectroscopy measurements. In addition, the correct relationship between the Mg2+/SO42− association constant determined by Raman spectroscopic measurements and those obtained by other techniques is derived. It is shown that thermodynamic data measured by Raman spectroscopy for systems involving other (Raman-undetected) ion-pair types in addition to CIPs, cannot and should not be compared directly with those obtained by traditional techniques.

Electrochemistry of immobilised redox droplets: Concepts and applications

Abstract: The use of microdroplet modified electrodes provides a simple methodology with which to study the biphasic electrochemistry of a plethora of species, encouraging the use of such techniques to mimic emulsion media. Furthermore, since the droplets may be miniaturised, this approach may assist in the field of biomimetic electrochemistry. For these reasons, this paper reviews the voltammetry of electrodes modified with electrochemically active droplets. The primary focus of the review is of unsupported droplets, where electron transfer processes occur at the three phase boundary, the base circumference of the individual droplets (of a volume range spanning nine orders of magnitude, going from microlitre to femtolitre volumes). The voltammetry of such systems is categorised via a semi-quantitative appreciation of the voltammetric characteristics. Finally, several topical examples illustrating the potential of application of this technology are described.

Calculations of the F?+?HD reaction on three potential energy surfaces

Abstract: In this paper, the three-dimensional time-dependent quantum wave packet calculation has been employed to study the non-adiabatic reaction dynamics of F + HD on three, electronically non-adiabatic potential energy surfaces fitted by Alexander, Stark and Werner (J. Chem. Phys., 2000, 113, 11 084; named ASW). The integral cross-sections and branching ratio of the reaction as a function of collision energy are calculated. The integral cross-sections are compared with the experimental measurements and other theoretical results. For the reaction of F in its excited state with HD, the calculated function of integral cross sections as collision energy shows that a steplike feature near 0.9 kcal mol−1 probably arises from the collinear FHD geometry.

Space charge conduction: Simple analytical solutions for ionic and mixed conductors and application to nanocrystalline ceria

Abstract: In this paper we present a set of simple analytical solutions for the parallel and the serial contributions of space charge zones to the overall conduction in bicrystals and (brick layered) ceramics for a variety of situations. The different situations are characterized by signs of the space charge potential, by different charge numbers of the carriers and by the presence or absence of acceptor or donor dopants. We can give solutions for arbitrary charge numbers as long as we restrict to sufficiently high space charge potentials and can presuppose equilibrium for the mobile carriers. A recent analysis of impedance measurements of nanocrystalline ceria based on a space charge model is briefly mentioned as a specific example, as it reflects the occurrence of highly n-conducting accumulation layers in addition to strongly blocking oxygen vacancy depletion layers.

Determination of the oxidation and coordination state of copper on different Cu-based catalysts by XANES spectroscopy in situ or in operando conditions

Abstract: The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.