Dynamics of surface catalyzed reactions; the roles of surface defects, surface diffusion, and hot electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.     

Guest Editors' Foreword

This issue of Discrete & Computational Geometry contains the detailed proof by T. Hales and S.P. Ferguson of the Kepler conjecture that the densest packing of three-dimensional Euclidean space by equal spheres is attained by "cannonball" packing. This is a landmark result. This conjecture, formulated by Kepler in 1611, was stated in Hilbert's formulation of his 18th problem [8]. The proof consists of mathematical arguments and a massive computer verification of many inequalities.     

Synthesis and application of a fluorous Lawesson's reagent: convenient chromatography-free product purification

[reaction: see text] A fluorous analogue of Lawesson's reagent for thionation of carbonyl compounds has been developed and its use demonstrated on a series of amides, esters, and ketones. The separation of the Lawesson's reagent-derived byproducts can be achieved by a simple fluorous solid-phase extraction.     

How ∗ not ∗ to solve a Sudoku

We prove the NP-hardness of a consistency checking problem that arises in certain elimination strategies for solving Sudoku-type problems.     

A global bifurcation index for set-valued perturbations of Fredholm operators

In the paper the homotopy invariant detecting global bifurcations of solutions to multi-parameter equations involving general set-valued perturbations of linear Fredholm operators of non-negative index is introduced. Some applications to the existence problems for differential inclusions are provided.     

Endor for characterizing transition metal centres in LiNbO3

Abstract

ENDOR investigations show that in nearly stoichiometric LiNbO₃ Mn²⁺ substitutes for Li with a Li vacancy in the first Li shell for most centres. In heavily Mg codoped LiNbO₃ the Cr³⁺ dopant, characterized by a new isotropic ESR signal, is shown to substitute for Nb. Information on local disorder is obtained.     

Local Central Limit Theorem for Multi-group Curie–Weiss Models

Journal of Theoretical Probability - We study a multi-group version of the mean-field Ising model, also called Curie–Weiss model. It is known that, in the high-temperature regime of this...     

Development of a validated liquid chromatography/tandem mass spectrometry method for the distinction of thyronine and thyronamine constitutional isomers and for the identification of new deiodinase substrates

Thyronines (THs) and thyronamines (TAMs) are two groups of endogenous iodine-containing signaling molecules whose representatives differ from each other only regarding the number and/or the position of the iodine atoms. Both groups of compounds are substrates of three deiodinase isozymes, which catalyze the sequential reductive removal of iodine from the respective precursor molecule. In this study, a novel analytical method applying liquid chromatography/tandem mass spectrometry (LC-MS/MS) was developed. This method permitted the unequivocal, simultaneous identification and quantification of all THs and TAMs in the same biological sample. Furthermore, a liquid-liquid extraction procedure permitting the concurrent isolation of all THs and TAMs from biological matrices, namely deiodinase (Dio) reaction mixtures, was established. Method validation experiments with extracted TH and TAM analytes demonstrated that the method was selective, devoid of matrix effects, sensitive, linear over a wide range of analyte concentrations and robust in terms of reproducible recoveries, process efficiencies as well as intra-assay and inter-assay stability parameters. The method was applied to study the deiodination reactions of iodinated THs catalyzed by the three deiodinase isozymes. With the HPLC protocol developed herein, sufficient chromatographic separation of all constitutional TH and TAM isomers was achieved. Accordingly, the position of each iodine atom removed from a TH substrate in a Dio-catalyzed reaction was backtracked unequivocally. While several established deiodination reactions were verified, two as yet unknown reactions, namely the phenolic ring deiodination of 3',5'-diiodothyronine (3',5'-T2) by Dio2 and the tyrosyl ring deiodination of 3-monoiodothyronine (3-T1) by Dio3, were newly identified.     

Highly selective synthesis of catalytically active monodisperse rhodium nanocubes

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br- ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.     

Use of 6-<Emphasis Type="Italic">O</Emphasis>-mono-substituted derivatives of β-cyclodextrin-bearing substituent with two permanent positive charges in capillary electrophoresis

This study details the use of two permanently positively charged mono-substituted β-cyclodextrin derivatives, 6^I -deoxy-6^I-(N,N N′,N′,N^t’-pentamethyl-ethylene-1,2-diammonio)-cyclomaltoheptose dichloride (PEMEDA-β-CD) and the newly synthesised 6^I-deoxy-6^I-(N,N,N′,N′,N′-pentamethyl-propylene-1,3-diammonio)-cyclomaltoheptose (PEMPDA-β-CD) as chiral selectors in capillary electrophoresis. Cyclodextrin (CD) derivatives were tested as additives in various buffers at various pH values with the optional addition of an organic modifier. Fourteen anionogenic analytes were tested, including native amino acids, N-blocked amino acids and profens, which were detected using a UV-VIS detector at optimal wavelengths of 214 nm, 254 nm or 280 nm. A borate buffer (15 mmol L^?1) at pH 9.5 without the addition of an organic modifier was chosen as a suitable background electrolyte. In addition, the effect of the concentration of the chiral selector on the separation and enantioseparation of selected analytes was monitored. The additions of cyclodextrin derivatives varied within the concentration range of 0.0–5.0 mmol L^?1. Both chiral selectors were suitable for the enantioseparation of N-Boc-d,l-tryptophan, which was already separated on the baseline at 0.5 mmol L^?1 concentration of the chiral selector.