Showing papers in "Physical Chemistry Chemical Physics in 2001"

Molecular states of water in room temperature ionic liquids

Abstract: ATR and transmission IR spectroscopy have been used to investigate the state of water in room temperature ionic liquids (RTILs) based on the 1-alkyl-3-methylimidazolium cation with the anions: [PF6]−, [SbF6]−, [BF4]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]−, [NO3]− and [CF3CO2]−. It has been shown that in these RTILs water molecules absorbed from the air are present mostly in the “free” (not self-associated) state, bound via H-bonding with [PF6]−, [BF4]−, [SbF6]−, [ClO4]−, [CF3SO3]−, [(CF3SO2)2N]− with the concentrations of dissolved water in the range 0.2–1.0 mol dm−3. It has been concluded that most of the water molecules at these concentrations exist in symmetric 1 : 2 type H-bonded complexes: anion...HOH...anion. Additional evidence that the preferred sites of interaction with water molecules are the anions has been obtained from the experiments with RTILs of the 1-butyl-2,3-dimethylimidazolium and 1-butyl-2,3,4,5-tetramethylimidazolium cations. Water molecules can also form associated liquid-like formations in RTILs with anions of stronger basicity such as [NO3]− and [CF3CO2]−. When these RTILs are exposed to air the water concentrations exceed 1.0 mol dm−3. The strength of H-bonding between water molecules and anions increases in the order [PF6]− < [SbF6]− < [BF4]− < [(CF3SO2)2N]− < [ClO4]− < [CF3SO3]− < [NO3]− < [CF3CO2]−. The energies of this H-bonding were estimated from spectral shifts, with the resulting enthalpies being in the range 8–13 kJ mol−1. ATR-IR spectroscopy has also been used to study H-bonding between methanol and RTILs.

Ligand and ensemble effects in adsorption on alloy surfaces

Abstract: Density functional theory is used to study the adsorption of carbon monoxide, oxygen and nitrogen on various Au/Pd(111) bimetallic alloy surfaces. By varying the Au content in the surface we are able to make a clear separation into geometrical (or ensemble) effects and electronic (or ligand) effects determining the adsorption properties.

Melting and freezing of water in ordered mesoporous silica materials

Abstract: The melting and freezing of water in a series of mesoporous silica materials with a hexagonal arrangement of unidimensional cylindrical pores and narrow pore-size distribution (MCM-41 with pore diameters from 2.9 to 3.7 nm, and SBA-15 with pore diameters from 4.4 to 11.7 nm) was studied by differential scanning calorimetry (DSC). A lowering of the melting temperature ΔTm = Tmb − Tm(R) up to 50 K was found for water in pores of decreasing radius R. The melting point data can be represented by a modified Gibbs–Thomson equation, ΔTm(R) = K/(R-t), with K = 52 K nm and t = 0.4 nm, in agreement with an earlier study of water in MCM-41 materials of pore width 2 to 4 nm. The value of K agrees with an estimate of the Gibbs–Thomson constant based on thermodynamic data for the normal melting point of ice, and the parameter t can be taken as the thickness of a surface layer of non-frozen water at the pore wall. DSC scans of the freezing of H2O and D2O in partially filled pores of SBA-15 reveal a peak pattern depending on the degree of pore filling ϕ. The different peaks are attributed to different states of the liquid in the pore space, iz. pore water in completely filled regions, and water as an adsorbed film at the pore wall. These two states coexist in a pore filling range ϕmin<ϕ<1, but only the film state exists at ϕ<ϕmin. The freezing peak of pore water appears to be nucleated by external bulk ice (at ϕ>1) but exhibits substantial supercooling at ϕ<1. On the other hand, the peak attributed to the freezing of film water exhibits no significant supercooling and is found at temperatures near 237 K, almost independent of ϕ and the pore width of the SBA-15 sample. The nature of a further (small) peak near 233 K is not yet understood. Contrary to cooling scans, only one DSC peak is observed in heating scans, independent of the pore filling. This finding indicates that the adsorbed liquid film is metastable relative to the frozen pore liquid.

Generalised equivalent circuits for mass and charge transport: chemical capacitance and its implications

Abstract: An exact equivalent circuit including terminal parts, which takes account of electrical and chemical control parameters in a unified way, is derived for a cell with a mixed conductor (or electrolyte) without internal sources or sinks. In one-dimensional problems electrochemical kinetics can be mapped by two-dimensional circuits exhibiting the spatial and the thermodynamic displacement as two independent coordinates. One main advantage of the exact circuits with respect to the underlying differential equations is the ability to simplify the description according to specific situations. As we show in several examples in the second part of the paper, it is straightforward to select the elements relevant for the particular experimental conditions and so to make appropriate approximations. This is most helpful for the description of electrochemical systems, such as fuel cells, membranes, pumps and batteries.

The thermal decomposition of silver (I, III) oxide: A combined XRD, FT-IR and Raman spectroscopic study

Abstract: FT-IR and Raman spectra for polycrystalline powders of silver (I, III) oxide, AgO, and silver (I) oxide, Ag2O, are reported. The vibrational spectra for each oxide are discussed in relation to its crystal structure, and were found to be consistent with factor group analysis predictions. Infrared and Raman spectroscopy, in conjunction with powder XRD, were also used to follow the thermal decomposition of AgO powder in air. Supplementary studies employing differential scanning calorimetry (DSC) and temperature programmed reaction (TPR), provided additional information relevant to the decomposition process. In agreement with mechanisms previously reported, AgO was thermally reduced to metallic silver ia two non-reversible steps, with the intermediate formation of Ag2O. The transformation of AgO to Ag2O occurred with heating in the 373–473 K region, while the product of this reaction remained stable to temperatures in excess of 623 K. Complete thermal decomposition of the Ag2O intermediate to Ag and O2 occurred at 673 K.

High-level multireference methods in the quantum-chemistry program system COLUMBUS: Analytic MR-CISD and MR-AQCC gradients and MR-AQCC-LRT for excited states, GUGA spin–orbit CI and parallel CI density

Abstract: Development of several new computational approaches within the framework of multi-reference ab initio molecular electronic structure methodology and their implementation in the COLUMBUS program system are reported. These new features are: calculation of the analytical MR-CI gradient for excited states based on state-averaged MCSCF orbitals, the extension of the MR-ACPF/AQCC methods to excited states in the framework of linear-response theory, spin–orbit CI for molecules containing heavy atoms and the development of a massively-parallel code for the computation of the one- and two-particle density matrix elements. Illustrative examples are given for each of these cases.

Getting into shape: Conformational and supramolecular landscapes in small biomolecules and their hydrated clusters

Abstract: The last few years have seen a very rapid growth in understanding the influence of non-bonded, particularly hydrogen-bonded interactions, on the shapes and conformations of flexible molecules, including those of pharmacological or biological importance, and of the supramolecular structures of their hydrated clusters. This has come about through the combination of a wide range of newly developed spectroscopic strategies, many of which are laser-based, coupled with powerful and widely available ab initio codes for structural computation. The consequent rapid growth of a new link between the worlds of chemistry and biophysics is surveyed in a review which introduces the range of present strategies, their origins, and their application to studies of neutrotransmitters, amides and peptides, amino acids and nucleic acid bases. It concludes with a prospectus for the future.

CO Tolerance of Pt alloy electrocatalysts for polymer electrolyte fuel cells and the detoxification mechanism

Abstract: The electrocatalytic activity for H2 oxidation in the presence of 100 ppm CO has been investigated on a series of binary Pt alloy electrocatalysts with non-precious metals of various compositions. Regardless of the composition, Pt–Fe, Pt–Ni, Pt–Co and Pt–Mo alloys have been found to exhibit excellent CO tolerance in H2 oxidation, similar to that of the Pt–Ru alloy. At these CO-tolerant electrodes, the equilibrium coverage of CO was suppressed to values less than ca. 0.6. Based on X-ray photoelectron spectroscopy (XPS) data, it was found that the surfaces of all non-precious metal alloys are composed of a thin Pt layer with an electronic structure different from that of pure Pt, indicating an increased 5d vacancy of Pt in the layers of the CO-tolerant alloys. The CO coverage, particularly with multi-bonding, was lowered due to decreased electron donation from the Pt band to the 2π* orbital of CO. A weakening of bond strength between the Pt skin layer and CO was also indicated by in situ FTIR, suggesting that the H2 oxidation sites are not blocked by CO due to its enhanced mobility. Thus, the mechanism of CO tolerance described above at the Pt skin on alloy surfaces is proposed as a “detoxification mechanism”.

Conversion of hydrocarbons and alcohols for fuel cells

Abstract: Fuel conversion to hydrogen is an important part of most fuel cell systems. The paper describes the available technologies for conversion of hydrocarbons and alcohols. The endothermic steam reforming catalysts and processes as well as autothermal reforming are proven technologies. Recent developments include catalytic partial oxidation. The integration of the fuel processing with the fuel cell represents a task with requirements depending on each type of fuel cell and application. The automotive use of fuel cells is at present a special challenge. The optimum fuel for stationary plants is natural gas (if available), whereas light naphtha appears to be the choice for automotive use.

A second distinct structural “state” of high-density amorphous ice at 77 K and 1 bar

Abstract: High-density amorphous ice (HDA), further densified on isobaric heating from 77 K to 165 (177) K at 1.1 (1.9) GPa, relaxes at 77 K and 1 bar to the same structural “state” with a density of 1.25 ± 0.01 g cm−3. Its density is higher by ≈9% than that of HDA, and thus it is called very-high-density amorphous ice (VHDA). X-ray diffractogram and Raman spectrum of VHDA clearly differs from that of HDA, and the hydrogen-bonded O–O distance increases from 2.82 A in HDA to 2.85 A in VHDA. Implications for the polyamorphism of the amorphous forms of water are discussed.

Size selective protein adsorption on thiol-functionalised SBA-15 mesoporous molecular sieve

Abstract: The mesoporous silica SBA-15, functionalised with propylthiol groups during synthesis and rendered porous (mean pore diameter 51 A) by extraction of surfactant template molecules, shows strong and size selective adsorption of proteins, selectively excluding those with molecular weights of ca. 40000 u and above. A model for the adsorption process is proposed, in which reversible physisorption is followed by irreversible chemisorption. Adsorption of proteins on an unfunctionalised SBA-15 from which the template has been removed by calcination, (mean pore diameter 56 A) shows shape selective and reversible adsorption of proteins with molecular weights of ca. 43000 u and below.

CO2 reforming of methane by the combination of dielectric-barrier discharges and catalysis

Abstract: The combination of solid state catalysts with a dielectric-barrier discharge was studied for the CO2 reforming of methane, including the methane and carbon dioxide decomposition reactions. Nickel and rhodium coatings were investigated for their influence on the plasma chemical reactions. It is shown that the use of a catalytic coating in the discharge can have a promoting effect on the plasma chemistry. Secondly, the use of the ceramic foams improved the temperature control in the discharge and lowered the undesired temperature increase in the discharge.

Phthalocyanines: structure and vibrations

Abstract: Raman scattering for a range of metal phthalocyanines using excitation frequencies from 457.9 to 1064 nm have been interpreted in the light of the recent DFT calculation on zinc phthalocyanine. The intention was to determine the features of the spectra that might be used for in situ analysis of specific phthalocyanines. B1 bands were found to be the most intense. The frequency of one band with a large C–N–C ring displacement is particularly sensitive to the metal ion size. The effect is determined not only by the size of the ion but also by the effect it has on ring shape. First and second order overtone bands of zinc and copper phthalocyanines are broadly similar, with some coupling differences at about 2000 cm−1. The region between 1350 and 1550 cm−1 has been little studied previously. It shows a remarkable sensitivity to the metal ion present and provides a specific signature for each phthalocyanine studied. In contrast, a study of α-, β-, γ- and e-copper phthalocyanines using 514.5 nm excitation showed very few differences, suggesting that intra- rather than intermolecular markers are most efficiently determined by Raman scattering. The study enables the interpretation of the Raman spectra of the phthalocyanines in terms of molecular structure and due to the resonant enhancement involved will enable the in situ identification of specific phthalocyanines in matrices.

Conformational landscapes of aromatic amino acids in the gas phase : infrared and ultraviolet ion dip spectroscopy of tryptophan

Abstract: The conformational structures of tryptophan, isolated in the gas phase, have been assigned by combining the results of ultraviolet hole-burning and infrared ion dip spectroscopy with the predictions of ab initio calculations conducted at the MP2/6-311 + G(d,p)//B3LYP/6-31 + G(d) levels of theory. As in phenylalanine, the most strongly populated, and lowest energy conformer presents a folded alanyl side chain that is stabilised by a ‘daisy chain’ of hydrogen-bonded interactions. These link the acidic proton, the amino group and the indole ring. There is a further interaction between the carbonyl oxygen and the neighbouring CH group on the pyrrole ring. A quantitative evaluation of the dipole–dipole interactions between the alanyl side chain and the indole ring in the 1La and 1Lb electronic states does not support the suggestion of electronic state mixing. In particular it casts doubt on the assignment of the fluorescence of the most stable, ‘special’ conformer to emission from the 1La state.

Photocatalytic activity of transition-metal-loaded titanium(IV) oxide powders suspended in aqueous solutions: Correlation with electron hole recombination kinetics

Abstract: Photocatalytic reactions by transition-metal (V, Cr, Fe, Co, Cu, Mo, or W) loaded TiO2 (M-TiO2) powders suspended in aqueous solutions of methanol, (S)-lysine (Lys), or acetic acid were investigated. The photoactivities of various samples were compared with the rate constant (kr) of recombination of photoexcited electrons and positive holes determined by femtosecond pump–probe diffuse reflection spectroscopy (PP-DRS). As a general trend, increased loading decreased the rate of formation of the main products (H2 , pipecolinic acid (PCA), and CO2) under UV (>300 nm) irradiation, and the effect became more intense on increasing the loading. In PP-DRS, these M-TiO2 gave similar decays of absorption at 620 nm arising from excitation by a 310 nm pulse (<100 fs). The second-order rate constant (kr) markedly increased with loading, even at a low level (0.3%) and further increased with an increase in loading up to 5%. The photocatalytic activity of platinized M-TiO2 for H2 and PCA production under deaerated conditions depended strongly on kr, but the relation between kr and the rate of CO2 production by unplatinized M-TiO2 under aerated conditions was ambiguous; other factor(s) might control the rate of the latter. These different kr dependences of photoactivity on the reaction kinetics governed by e−–h+ recombination were attributed to the presence of O2 and Pt deposits. A simple kinetic model to explain the overall rate of these photocatalytic reactions is proposed, and the effect of recombination kinetics on photoactivity is discussed.

Bulk and surface phases of iron oxides in an oxygen and water atmosphere at low pressure

Abstract: Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p⩽1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe–H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4–Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5 × 10−5 mbar oxygen at 920–1000 K results in a new γ-Fe2O3(111)-like intermediate surface phase during the oxidation to α-Fe2O3(0001). A model is suggested for the growth of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.

Iron nanoparticles: Synthesis and applications in surface enhanced Raman scattering and electrocatalysis

Abstract: The microemulsion method is employed to synthesize iron nanoparticles with an average size of ∽3 nm using trioctyl phosphine oxide (TOPO) as a stabilizing agent. The morphology, structure, and composition of the nanoparticles are studied by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–VIS spectroscopy. The iron nanoparticles show a remarkable surface-enhanced Raman scattering (SERS) activity, and the scheme of iron nanoparticle-on-electrode is successfully used in the in situ SERS study of adsorbed molecules. Electrocatalysis over the iron nanoparticles is demonstrated in the highly efficient and selective reduction of H2O2 in the presence of oxygen.

Interaction of bovine serum albumin with anionic surfactants

Abstract: The effect of binding and conformational changes induced by anionic surfactants sodium dodecyl sulfate (SDS) and sodium octyl sulfate (SOS) on bovine serum albumin (BSA) have been studied using differential scanning calorimetry (DSC), circular dichroism (CD), fluorescence and UV spectroscopic methods. The denaturation temperature, van't Hoff enthalpy and calorimetric enthalpy of BSA in the presence of SDS and SOS and urea at pH 7 have been determined. The results indicate that SDS plays two opposite roles in the folding and stability of BSA. It acts as a structure stabiliser at a low molar concentration ratio of SDS/BSA and as a destabilizer at a higher concentration ratio as a result of binding of SDS to denatured BSA. The Brandts and Lin model has been used to simulate the results.

Photochromism of nitrospiropyrans: effects of structure, solvent and temperature

Abstract: The thermal and photochemical ring opening and ring closure of the spiropyran/merocyanine couple of three nitro-substituted spiropyrans (6-NO2: 6, 7 and 8-NO2: 8) were studied and the results were compared with those of other 6-NO2 spirobenzopyranindolines (BIPS) (1–5). The photocolouration, which occurs in the triplet manifold throughout, and the photochemical conversion and thermal relaxation of the two observable photomerocyanines into the closed spiropyran (Sp) form (decolouration) were quantified as a function of solvent polarity and temperature. The relaxation time (τt-Sp) at 25°C ranges from 2 s for 5 in methylcyclohexane to 104 s for 7 in ethanol. This large variation in τt-Sp is due to changes in both the activation energy (Ea = 75–105 kJ mol−1, increasing with polarity) and the pre-exponential factor (A = 1012–1015 s−1). The quantum yield of colouration with 308 nm pulses is substantial in solvents of low polarity (Φcol = 0.3–0.8) and decreases (<0.2) with increasing polarity. The observed merocyanine triplet state is rather short-lived (<10 μs) and the precursor of the trans- and a cis-merocyanine. The same intermediates, the triplet state and the cis-isomer, were observed when the trans-merocyanine was excited at 530 nm. The thermal cis → trans isomerization takes place in the μs–ms range and has an activation energy of Ec → t = 30–40 kJ mol−1. The effects of substituents and medium properties are described and the mechanism of photochromism is discussed.

Molecular composition and orientation at the surface of room-temperature ionic liquids: Effect of molecular structure

Abstract: We have used direct recoil spectrometry (DRS) to investigate the composition and molecular orientation at the surfaces of a variety of room-temperature ionic liquids of the general type [Cnmim]X where [Cnmim] = 1-CnH2n+1-3-methylimidazolium cation (n = 4 (bmim), 8 (omim), 12 (C12mim)) and X is [PF6]−, [BF4]−, Cl− or Br−. Comparing experimental atomic F/C and H/C ratios for [Cnmim][PF6] (n = 4, 8, 12) or [Cnmim][BF4] (n = 4, 8) with predicted ratios for various surface compositions suggests that the surface is populated by both anions and cations with no segregation. The data best fit the cation ring being perpendicular rather than parallel to the surface. The data for the cations of fluorine-containing liquids support a common orientation with the N atoms of the ring uppermost. The cations of the shorter chain hexafluorophosphate liquids appears to stay within a rotation angular spread of about ± 30° from this orientation but increasing the alkyl chain length to 12 carbon atoms favors a tilt of ∼45° to bring the Me group closer to the surface. Changing to the smaller [BF4]− anion has a similar effect. For [omim][Cl], the data fit best for a cation orientation with either N-up or N-down and the methyl group tilted towards the surface in a similar manner to [omim][BF4]. In the case of [omim][Br], a number of cation orientations are compatible with the data.

Isobaric thermal expansivity and thermophysical characterization of liquids and liquid mixtures

Abstract: A methodology for the thermophysical characterization of liquids and liquid mixtures based on measurements of density, sound speed and isobaric heat capacity per unit volume at atmospheric pressure as a function of temperature is proposed. Density and sound speed data are used to determine the isentropic compressibility from the Laplace equation. The precision in density measurements allows one to obtain the isobaric thermal expansivity at different temperatures using an incremental procedure with quite acceptable accuracy and precision. The isothermal compressibility and isochoric molar heat capacity are both obtained from the previous properties, using well-known thermodynamic relations. The accuracy of the proposed methodology was checked by determining the above-mentioned properties for liquid n-hexane, n-heptane, n-octane, n-dodecane, n-hexadecane, cyclohexane, and toluene over the temperature range (288.15–333.15) K and comparing the results with selected reported data. The average absolute deviations from the latter showed data obtained with the proposed methodology to be reasonably accurate. The excess quantities for nine binary mixtures of the cyclohexane + n-dodecane system were also determined with a view to assessing precision, which was found to be quite good as regards the dependence on both composition and temperature.

Bimetallic PtSn catalyst for selective CO oxidation in H2-rich gases at low temperatures

Abstract: Kinetic measurements on preferential CO oxidation in a H2-rich atmosphere (PROX) over a bimetallic, carbon supported PtSn catalyst reveal a high activity and selectivity already at low temperatures (0–80°C), superior to a commercial Pt/Al2O3 system The selectivity, though steadily decreasing with temperature, is remarkably high, 85% at low temperatures around 0–20°C, and even at 120°C it is, at 45%, still higher than that of standard Pt catalysts The observation that CO desorption is not rate limiting and that the selectivity decreases with increasing temperature, can be explained in a mechanistic model involving separation of the reactant adsorption sites (bifunctional surface), with competing CO and hydrogen adsorption on Pt sites/areas and oxygen adsorption predominantly on Sn sites and SnOx islands on/adjacent to the active PtSn particles The reaction takes place in a bifunctional way at the perimeter of these islands or by invoking a spill-over process This model is supported by CO temperature-programmed desorption (TPD), in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), and x-ray photon spectroscopy (XPS) measurements, which indicate that under reaction conditions the surface CO coverage on the metallic particles is high, but decreases with temperature, and that only part of the Sn is reduced, included in PtSn alloy particles, while another part is in an oxidic state, forming SnOx islands on and presumably also beside the active particles Its excellent performance makes PtSn an interesting catalyst for fuel gas purification in low temperature polymer electrolyte membrane fuel cell technology (PEM-FC)

Sum frequency generation (SFG) study of the pH-dependent water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) monolayer

Abstract: The interface-sensitive spectroscopic method, sum frequency generation (SFG), has been used to investigate the interfacial water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) self-assembled monolayer in phosphate buffered solutions at various pHs. The experimental results demonstrate that the water molecules at the quartz/OTS surface flip while the water molecules at the OTS surface maintain their orientation when the solution pH is changed from neutral to acidic. The results show that most of the silanol groups still exist on the fused quartz surface even after a silane coupling reaction of OTS under the reported experimental conditions.

Kinetic origin of the ozone isotope effect: a critical analysis of enrichments and rate coefficients

Abstract: The oxygen isotope anomaly in ozone was first identified in isotopomers 49O3 and 50O3 with enrichments of over 10%. Recent rate coefficient measurements imply a new interpretation of the anomaly. The large variability of the rate coefficients of ozone formation O + O2 → O3 of about 50% characterizes the ozone isotope effect and demonstrates its kinetic origin. Surprisingly, the rate coefficients correlate with the zero-point energy change of the oxygen molecules participating in the isotope exchange reactions but not with the mass of the ozone molecules. Rate coefficients of symmetric molecules depart from this linear relationship by about 20%.

Bioelectrocatalysis-based dihydrogen/dioxygen fuel cell operating at physiological pH

Abstract: A biochemical fuel cell was constructed using H2 as fuel to produce H2O in the reaction with O2 at neutral pH and ambient temperature. The cell uses carbon felt as an electrode material for both the anode and the cathode and an anion exchange membrane as a separator. The anodic oxidation of H2 was accelerated by methyl viologen-mediated electrocatalysis with bacterial cells Desulfoibrio ulgaris (Hildenborough) as catalysts, and the cathodic reduction of O2 was accelerated by 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)-mediated electrocatalysis with bilirubin oxidase as a catalyst. The bioelectrocatalytic systems allowed the cell to operate at 1.0 V with current 0.9 mA at an electrode of size 1.5 × 1.5 × 0.1 cm3. The cell voltage attained 1.17 V at open circuit, which is close to the standard electromotive force 1.23 V. The cell voltage–current behavior is interpretable by linear sweep voltammetry using the same electrode system. On this basis, the electrochemistry behind the performance of the biochemical fuel cell is discussed.

Pt–Ru model catalysts for anodic methanol oxidation: Influence of structure and composition on the reactivity

Abstract: The activity of different types of PtRu surfaces towards anodic methanol oxidation has been investigated. As expected the activity of Pt(111) modified with Ru and analyzed in a UHV environment depends on the total number of Pt–Ru pair sites. Their population can be increased by artificially creating additional surface defects before or after ruthenium deposition. Ruthenium alloyed into smooth Pt(111) terraces in turn does not lead to comparable electrocatalytic activity, moreover the current density under potentiostatic conditions undergoes an exponential decline towards zero. Other model surfaces are also found to present a continuous loss in activity during chronoamperometric tests, which consists of a fast initial current decrease during the first 5–10 min followed by a slower one over several hours. The latter decay exhibits hyperbolic behavior which we can explain kinetically as being caused by a second-order process. The first current decay can be repeatedly observed by re-starting the experiment after setting the potential back to the initial value, thus indicating a certain degree of reversibility. The slow loss in activity cannot be recovered at low potentials. However, the original surface activity can be restored by applying a potential step to higher anodic values, e.g. up to 1.2 V for a few seconds. Structure optimized porous PtRu surfaces, on the other hand, do not show any current decrease during the chronoamperometric experiment.

Autothermal methanol reforming for hydrogen production in fuel cell applications

Abstract: Fuel cell powered electric cars using on-board methanol reforming to produce a hydrogen-rich gas represent a low-emissions alternative to gasoline internal combustion engines (ICE). In order to exceed the well-to-wheel efficiencies of 17% for the gasoline ICE, high-efficiency fuel cells and methanol reformers must be developed. Catalytic autothermal reforming of methanol offers advantages over endothermic steam-reforming and exothermic partial oxidation. Microreactor testing of copper-containing catalysts was carried out in the temperature range between 250 and 330°C showing nearly complete methanol conversion at 85% hydrogen yield. For the overall process a simplified model of the reaction network, consisting of the total oxidation of methanol, the reverse water-gas shift reaction, and the steam-reforming of methanol, is proposed. Individual kinetic measurements for the latter two reactions on a commercial Cu/ZnO/Al2O3 catalyst are presented.

Investigation of the structures of MgO clusters using a genetic algorithm

Abstract: The application of a genetic algorithm, for optimizing the geometries of stoichiometric and non-stoichiometric MgO clusters, bound by a simple Coulomb-plus-Born–Mayer potential, is investigated. The genetic algorithm is shown to be efficient and reliable for finding, reproducibly the global minima for these clusters. The variation of the structures of MgO clusters are investigated as a function of the formal charges (±q) on the ions—ranging from q = 1 to q = 2. In agreement with previous studies, lower charges are found to favour compact, rocksalt-like cuboidal clusters, while the higher formal charges favour hollow pseudo-spherical structures. Hexagonal stacks are also found to be stable for small (MgO)N clusters with N = 3n. Comparisons are made with experimental mass spectral abundances and the results of previous empirical calculations, as well as with more sophisticated model potential and ab initio calculations. Finally, possible ways in which the genetic algorithm search method could be coupled with more accurate calculation methods are discussed.

Excited state spectroscopy and dynamics of isolated adenine and 9-methyladenine

Abstract: We report electronic spectra of the DNA base adenine and its derivative 9-methyladenine in a molecular beam, using [1 + 1] resonantly enhanced multiphoton ionisation. In addition an excited state lifetime of adenine was determined by picosecond time-resolved [1 + 1′] photoionisation. The spectrum of adenine is dominated by a band at 36105 cm−1, with several low-frequency bands nearby. A comparison with the spectrum of 9-methyladenine with its origin at 36136 cm−1 confirms that the electronic spectrum of adenine is dominated by the π → π* transition of the N9–H tautomer. Several bands observed before and assigned to a n → π* transition were not observed. For the most intense band of adenine at 36105 cm−1 we determined an excited state lifetime of 9 ps.

Simulation of powder diffraction patterns of modified ordered mesoporous materials

Abstract: Powder diffraction patterns of ordered mesoporous materials are simulated with a newly developed program, which allows investigation of the influence of any desired matter distribution in the unit cell on the diffraction pattern. The simulation process can be subdivided into two major steps. First, a unit cell is generated from SiO2 and, optionally, other building units. A weighted random placement of atoms is used to simulate the distribution of different atoms in different parts of the unit cell. This is done by a Fermi-type function, by which the probability of finding an atom on a site depends on the distance of a point from the center of a pore, leading to a smooth, continuous transition from wall to pore. Secondly, structure factors and then intensities of reflections are calculated, using the Lorentz correction and a geometric correction for powder data. The use of this program is demonstrated by the simulation of diffraction patterns, mainly for unmodified and modified SBA-15 as well as for MCM-41. Good agreement of simulated and experimental data is observed.