Thermally-activated Reactions Research Articles

Structure of graphene and a surface carbide grown on the (0001) surface of rhenium

Transition metal surfaces catalyse a broad range of thermally-activated reactions involving carbon-containing-species -- from atomic carbon to small hydrocarbons or organic molecules, and polymers. These reactions yield well-separated phases, for instance graphene and the metal surface, or, on the contrary, alloyed phases, such as metal carbides. Here, we investigate carbon phases on a rhenium (0001) surface, where the former kind of phase can transform into the latter. We find that this transformation occurs with increasing annealing time, which is hence not suitable to increase the quality of graphene. Our scanning tunneling spectroscopy and reflection high-energy electron diffraction analysis reveal that repeated short annealing cycles are best suited to increase the lateral extension of the structurally coherent graphene domains. Using the same techniques and with the support of density functional theory calculations, we next unveil, in real space, the symmetry of the many variants (two six-fold families) of a rhenium surface carbide observed with diffraction since the 1970s, and finally propose models of the atomic details. One of these models, which nicely matches the microscopy observations, consists of parallel rows of eight aligned carbon trimers with a so-called $(7\times\sqrt{\mathrm{19}})$ unit cell with respect to Re(0001).

Quantum charge transport and conformational dynamics of macromolecules

We study the dynamics of quantum excitations inside macromolecules which can undergo conformational transitions. In the first part of the paper, we use the path integral formalism to rigorously derive a set of coupled equations of motion which simultaneously describe the molecular and quantum transport dynamics, and obey the fluctuation/dissipation relationship. We also introduce an algorithm which yields the most probable molecular and quantum transport pathways in rare, thermally activated reactions. In the second part of the paper, we apply this formalism to simulate the propagation of a quantum charge during the collapse of a polymer from an initial stretched conformation to a final globular state. We find that the charge dynamics is quenched when the chain reaches a molten globule state. Using random matrix theory we show that this transition is due to an increase of quantum localization driven by dynamical disorder. We argue that collapsing conducting polymers may represent a physical realization of quantum small-world networks with dynamical rewiring probability.

Isomerization of Eugenol Under Ultrasound Activation Catalyzed by Alkali Modified Mesoporous NbMCM-41

The present paper discloses a new alternative method for the isomerization of eugenol to isoeugenol where mesoporous materials containing niobium are used as solid base catalysts, affording good yields of isomerized product and being environmentally friendly. Nb-containing MCM-41 mesoporous materials were modified with alkaline cations (Li, Na, K, Rb and Cs) and characterized by BET, FTIR, XRD, and UV–vis techniques. The prepared materials were tested in the isomerization of eugenol to isoeugenol by sonochemical and thermally-activated reactions. Li, Na, K, Rb, and Cs/NbMCM-41 catalysts are active catalyst for the preparation of isoeugenol, and selective to the trans-isomer. Rb/NbMCM-41 sample is the optimum catalysts for the reaction. This result is in accordance with the calculated basicity observed during the Knoevenagel probe reaction of basicity. Conversions around 90% are obtained when using Rb-NbMCM-41 catalysts, with 90% selectivity to trans-isoeugenol, under ultrasound activation. In contrast, conversion around 72% with 90% selectivity is attained under classical thermal activation. The catalysts were reused four times without a substantial decrease of activity.

Nonenergetic reactions between atomic hydrogen and molecules on interstellar grain surfaces

Reactions of atomic hydrogen with CO and O2 on amorphous solid water (ASW), relevant to chemical evolution on cosmic ice dust, were experimentally investigated at around 10 K. Successive addition of hydrogen atoms to CO and O2 produces H2CO, CH3OH, and H2O2, H2O, respectively. At such low temperatures, some of hydrogen additions such as H + CO → HCO proceed via tunneling reactions rather than thermally-activated reactions. Effective reaction rates and isotope effect of the tunneling reactions to produce HCO and H2O were measured. The surface of ASW was found to enhance the effective rate of hydrogen addition to CO at relatively higher temperatures, namely around 20 K.

Modeling high pressure ethane oxidation and pyrolysis

A previously developed detailed chemical kinetic model for ethane oxidation and pyrolysis has been extended by considering the impact of much higher pressures on the chemically- and thermally-activated reactions in the mechanism. The resulting mechanism is now compared to data at much higher pressures. These data include both ethane oxidation and pyrolysis shock tube experiments up to pressures of 986atm as well as autoignition and ethane dissociation data near 10atm. The temperature and pressure dependencies of the rate coefficients in the model are represented by Chebyshev polynomials. This model, with no adjustments, describes the varied experimental data very well. A first order sensitivity analysis identified the most important reactions in each of the kinetic regimes. These results, coupled with the earlier validation studies at lower pressures, suggest this model is broadly applicable to analysis of ethane pyrolysis and oxidation over very wide temperature and pressure ranges.

Sonocatalysis in solvent free conditions: An efficient eco-friendly methodology to prepare chalcones using a new type of amino grafted zeolites

The synthesis of chalcones via Claisen-Schmidt condensation between benzaldehyde and acetophenone by sonochemical and thermally-activated reactions over a new type of zeolite as catalyst is reported. The catalysts were prepared by grafting amino groups on sodium and cesium exchanged X zeolite. In this green, solvent free procedure, chalcones are selectively produced in very high yields (>95%) when cesium exchanged X-NH 2 is employed under ultrasound activation

Three-Day Prediction of Concrete Compressive Strength Evolution

When cement is first produced, its mechanical properties are time-dependent and involve hydration. The hydration process is a thermally-activated reaction that can be described by the Arrhenius equation. Predictions of the mechanical properties of concrete, particularly compressive strength, are based on maturity indexes. This article presents a simple and efficient procedure to evaluate the activation energy and the rate constant of concrete. The authors contend that these two parameters can be used for a rapid prediction of the mechanical properties of concrete and particularly the evolution of compressive strength. They also allow the separation of effects due to physical phenomena such as humidity loss. The procedure uses an experimentally-determined parameter called “hardening time” as an indicator of equivalent maturity when comparing two hardening profiles. The authors provide the test results from specimens of six concrete types; results validate the proposed procedure. The methodology allows a fast and accurate prediction of compressive strength on site; 72 hours are sufficient to gather the data and provide accuracy of less than 8% error. The resulting improvements in understanding of compressive strength evolution can lead to savings during construction and improved safety.

Ultrasound-promoted N-alkylation of imidazole. Catalysis by solid-base, alkali-metal doped carbons

The synthesis of N-substituted imidazoles via alkylation of imidazole with 1-bromobutane by sonochemical and thermally-activated reactions over two alkali-metal promoted carbons (Na+- and Cs+-Norit) as catalysts is reported. In this green, solvent free procedure, N-substituted imidazoles are produced exclusively in very high yields (>80%) when the Cs+-Norit carbon is employed under ultrasound activation. The basicity of the alkali metal cation increases the conversion and ultrasound activation affords a remarkable increase in the yields.

Single-point blowup for nonlocal parabolic problems

The formation of shear bands in materials has important implications to a variety of physical processes. These bands are observed in very thin zones and are generally regarded as a precursor to material failure. Shear band formation is caused by the heat generated in regions with highest strain rate. With insufficient time for diffusion of this heat, a localized thermal softening of the material occurs which enhances plastic flow in a thin zone. This adiabatic strain localization can be modelled as nonlinear thermally-activated reaction–diffusion equations. This leads to a class of nonlocal parabolic problems and their associated time-independent steady-state counterparts. A discussion of the models and an analysis of the problems will be presented.

NEXAFS studies of the reaction of SO2 with TiO2(100)-(1 × 1) and -(1 × 3)

Abstract S K-edge NEXAFS has been used to monitor the thermally-activated reaction of SO2 with TiO2(100)-(1 × 3) and TiO2(100)-(1 × 1) in order to investigate the influence of surface structure and oxygen vacancies on the reactivity. In each case SO2 chemisorbs at 110 K, reacting further to form a sulphite-like intermediate species at higher temperatures (110–;400 K). This intermediate transforms to a sulphate-like species as the temperature is further increased (130–;500 K). The polarisation dependence of the NEXAFS for the sulphate-like species on both substrates at 130–;220 K evidences a lowering of the expected Td molecular symmetry. Overall, the data indicate that the mesoscopic morphology of the substrate has little influence on the adsorption site or reaction pathway for SO2. In addition, the only discernable effect of oxygen vacancies on the reaction is that 0.03 ML of sulphide is formed on TiO2(100)-(1 × 3) following SO2 exposure.

NEXAFS fingerprinting of TiO2(110)-SO2

Polarisation-dependent S K-edge NEXAFS has been used to monitor the progression of the thermally-activated reaction between TiO2(110) and SO2.SO2 chemisorbs at 105 K, reacting to form a surface sulphate-like species as the substrate is heated to ⩾ 135 K. This is the final reaction product, having a coverage of 0.1 ML up to at least 450 K. A sulphite-like species is identified as an intermediate in this reaction, having a maximum concentration of ∼ 0.2 ML at 155 K. Structural models are suggested in which SO2 initially bonds to Ti sites, subsequently forming complex anion-like species by incorporation into the raised row of bridging O atoms.

Chemisorption and decomposition reactions of oxygen-containing organic molecules on clean Pd surfaces studied by UV photoemission

We have used uv photoeinission (primarily at a photon energy hv = 40.8 eV) to study chemisorption and decomposition reactions of small oxygen-containing organic molecules on clean polycrystalline Pd surfaces at 120 and 300 K. These molecules include methanol (CH 3OH), dimethyl ether (CH 3OCH 3) formaldehyde (H 2CO), acetaldehyde [H(CH 3)CO], and acetone [(CH 3) 2CO]. Chemisorption bonding of these molecules to the Pd surface occurs primarily through the lone-pair orbitais associated with the oxygen atoms, as evidenced by chemical bonding shifts of these orbitais toward larger electron binding energy relative to the other adsorbate valence orbitals. At 300 K all the molecules studied decompose on the surface, resulting in chemisorbed CO. Since chemisorbed (as well as condensed) phases of some of these molecules (CH 3OH and H(CH 3)CO) are observed at low temperature, the decomposition to CO is a thermally-activated reaction. The observed orbital shifts associated with chemisorption bonding are used to make rough estimates of interaction strengths and chemisorption bond energies (within the framework of Mulliken's theory of electron donor-acceptor complexes as applied to chemisorption by Grimley). The resulting heats of chemisorption are consistent with the observed surface reactions.