Negative Activation Energy Research Articles

Preferential Carbon Monoxide Oxidation over Copper‐Based Catalysts under In Situ Ball Milling

AbstractIn situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO‐PROX) under in situ ball milling of Cu‐based catalysts such as Cu/Cr2O3 is presented. At temperatures as low as −40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short‐lived surface defects. In situ ball milling at sub‐zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis.

Preferential Carbon Monoxide Oxidation over Copper‐Based Catalysts under In Situ Ball Milling

In situ ball milling of solid catalysts is a promising yet almost unexplored concept for boosting catalytic performance. The continuous preferential oxidation of CO (CO-PROX) under in situ ball milling of Cu-based catalysts such as Cu/Cr2 O3 is presented. At temperatures as low as -40 °C, considerable activity and more than 95 % selectivity were achieved. A negative apparent activation energy was observed, which is attributed to the mechanically induced generation and subsequent thermal healing of short-lived surface defects. In situ ball milling at sub-zero temperatures resulted in an increase of the CO oxidation rate by roughly 4 orders of magnitude. This drastic and highly selective enhancement of CO oxidation showcases the potential of in situ ball milling in heterogeneous catalysis.

Kinetic Behavior of Solid K2CO3 under Postcombustion CO2 Capture Conditions

Trends in CO2 and H2O uptakes by solid K2CO3 were studied at different carbonation temperatures and flue gas compositions over 50 consecutive regeneration–carbonation cycles. Differences in carbonation behavior between the initial and final cycles evidence changes in the phase composition of K2CO3 during cycling, which stabilize in accordance with the operating conditions. Rates of carbonation and hydration were observed to decrease at higher temperatures, indicating that the forward reactions are limited by adsorption, not chemical reaction. Multiple regression analysis of the uptake data yielded negative apparent activation energies for both forward reactions modeled as simple chemical reactions, thus proving the assumption invalid. A proposed Langmuir–Hinshelwood model yielded consistent apparent activation energies for the forward reactions, showing that adsorption limitations are significant in this chemisorption process. Important process design considerations, for realizing high carbonation rates a...

High activity and negative apparent activation energy in low-temperature CO oxidation – present on Au/Mg(OH)2, absent on Au/TiO2

Aiming at a better understanding of the unusual low-temperature CO oxidation reaction behavior on Au/Mg(OH)2 catalysts, we investigated this reaction mainly by combined kinetic and in situ IR spectroscopy measurements over a wide range of temperatures, from −90 °C to 200 °C.

Reactions of 2-Propanol Radical with Halogenated Organics in Aqueous Solution: Theoretical Evidence for Proton-Coupled Electron Transfer and Competing Mechanisms.

The reactions of α-hydroxyalkyl radicals in aqueous medium are of interest because they exhibit a rich variety of fundamentally important competing mechanisms, such as proton-coupled electron transfer (PCET), hydrogen atom transfer, free radical substitutions, abstractions and additions, etc. We present a theoretical study of the mechanism and kinetics of the aqueous reactions of α-hydroxyisopropyl (2-propanol) radical with four halogenated organic substrates: iodoacetate (IAc), iodoacetamide (IAm), 5-bromouracil (5-BrU), and carbon tetrachloride (CCl4). The reactions are studied using density functional theory (DFT) (M06-2X), and the solvent is modeled as a polarizable continuum, either without the explicit solvent molecules or with one added water molecule. For an additional refinement, the double hybrid DFT B2PLYP energies were calculated at the M06-2X stationary points. Within this framework, for each substrate, we determine the most favorable radical-induced decomposition pathway among the several found and compare the thermochemical predictions against the experimental kinetics. The following dominant decomposition mechanisms are inferred: PCET for IAc, PCET-H2O and the I-atom abstraction for IAm, the ortho-addition to the double bond for 5-BrU, and the Cl-atom abstraction for CCl4. These pathways are invariably characterized by the negative apparent activation energies. Whereas for 5-BrU and CCl4 the transition state theory rate constants are in good agreement with the experiment, the rate constants for IAc and IAm-the two substrates reacting preferably via the PCET-are difficult to predict correctly. Consequently, the corresponding reaction barriers necessitate lowering by 1-3 kcal mol-1 to bring them in accord with experiment. The B2PLYP method provides a worthwhile improvement over the M06-2X energetics although the largest errors remain for the two PCET processes.

Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of “negative activation energy” after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

Negative activation energies for carbene additions to tetramethoxyethylene

Activation energies were determined for the additions of three arylhalocarbenes to tetramethoxyethylene (TMeOE) and tetramethylethylene (TME) in pentane and decane. For addition of 3,5-dinitrophenylchlorocarbene to TMeOE in decane, Ea=−10.4±0.7kcal/mol, the most negative activation energy yet encountered in a carbene–alkene addition reaction. Computational studies parallel and elucidate the experimental results.

Extremely reactive carbenes: electrophiles and nucleophiles

We describe the intermolecular reactivity toward alkenes of several extremely reactive carbenes as determined by experimental and computational methods. Among the examined species are the electrophiles trifluoromethylchlorocarbene and (N‐methyl‐3‐pyridinium)chlorocarbene and the nucleophiles adamantanylidene and dimethylcarbene. Phenomena discussed include negative activation energies for addition reactions and apparent compensation between the enthalpies and entropies of activation. Copyright © 2016 John Wiley & Sons, Ltd.

Gas Flow Rate and Temperature Dependence of the Kinetics of Difluoromethane Clathrate Hydrate Formation from CF2H2 Gas and Ice Particles

The effects of temperature and gas flow rate on the formation kinetics of difluoromethane clathrate hydrate from CF2H2 gas and <38 μm diameter ice particles were investigated. The early stage of the reaction exhibits a negative correlation with temperature that is characterized by an activation energy of −7.2 ± 0.7 kJ/mol(H2O) for the 6 mmol/h flow rate and −6.7 ± 1.5 kJ/mol(H2O) for 19 mmol/h. We surmise that the origin of the negative activation energy involves the existence of an unstable structure or gas adsorption to the ice surface. Although the effect of flow rate on the observed activation energy is modest, other aspects of the kinetics are more dramatically affected. Initially, the reaction is faster for the higher flow rate simply because more gas is available for the reaction. However, the reaction stalls for a lower percent yield for the faster flow rate. We postulate that the fast flow rate creates more uniform hydrate formation over the ice surface, resulting in the onset of stage III kineti...

Experimental and Theoretical Study of the OH-Initiated Photo-oxidation of Formamide.

The kinetics of OH radical reaction with formamide was studied by the relative rate method employing proton transfer reaction-mass spectrometry detection at the European Photochemical Reactor in Valencia, Spain. The rate coefficient was determined to be (4.5 ± 0.4) × 10(-12) cm(3) molecule(-1) s(-1) at 309 ± 3 K and 1013 ± 1 hPa. Isocyanic acid was observed as the sole product. The experimental results are supported by quantum chemical calculations and kinetic simulations using a master equation model. The calculated rate coefficient is independent of pressure at tropospheric conditions and can be accurately described by an Arrhenius expression having negative activation energy. The reaction is predicted to proceed exclusively via C-H abstraction.

Influence of external stimuli on the network properties of cationic poly(N-acryloyl-N’-propyl piperazine) hydrogels

Stimuli-responsive cationic hydrogels based on N-acryloyl-N′-propyl piperazine (AcrNPP) crosslinked with N,N′-methylenebisacrylamide (Mba) and 1,4-butanedioldiacrylate (Bda) were prepared by UV light-initiated free-radical polymerization in bulk. The effect of external stimuli and type of crosslinker on the equilibrium swelling behavior and dynamic swelling was investigated in detail in buffer solution of various pH and temperatures. The equilibrium swelling capacity of the gels was large in swelling medium at pH 3.0 than at pH 10.0 due to ionization of polymer network under acidic conditions. With increase in temperature from 25 °C to 45 °C, the gels exhibited negative temperature-responsive (thermo-shrinking/thermophobic) behavior with negative activation energy for diffusion of water. The thermodynamic parameters such as Gibbs' free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) for the swelling of gels as function of temperature were negative indicating an exothermic swelling process. Water (media) transport mechanism and diffusion process in thin rectangular gels was studied. At pH 3.0, the diffusion process was non-Fickian (anomalous) while at pH 10.0 it was quasi-Fickian. The transport mechanism was partly influenced by the type of crosslinker in the gel. The dynamic swelling data was analyzed using early-time, late-time and Etters diffusion models. From the equilibrium swelling studies the average molecular weight between crosslinks (Mc), the crosslink density (ρc), and the mesh size (ξ) were determined. The Mc was large at pH 3.0 due to ionization of polymer and chain expansion. The experimental Mc was much larger than the theoretical Mc which implied that the gels were loosely crosslinked real networks. The mesh size of gels were between 447 and 786 Å in the swollen (ionized) state (pH 3.0), and between 100 and 231 Å in the collapsed (non-ionized) state (pH 10.0). The mesh size increased between three to four times during the pH-dependent swelling transition. The state of water in fully swollen hydrogels which influences many important biomaterial properties was determined by differential scanning calorimetry. The bound water content of gels increased linearly with increase in pH of the swelling medium while the unbound water decreased. These hydrogels have potential to be used as controlled drug delivery systems and sorbents for removal of pollutants from water.

Thermochemical and Kinetics of Hydrazine Dehydrogenation by an Oxygen Atom in Hydrazine-Rich Systems: A Dimer Model.

The kinetics of the reaction of N2H4 with oxygen depends sensitively on the initial conditions used. In oxygen-rich systems, the rate constant shows a conventional positive temperature dependence, while in hydrazine-rich setups the dependence is negative in certain temperature ranges. In this study, a theoretical model is presented that adequately reproduces the experimental results trend and values for hydrazine-rich environment, consisting of the hydrogen abstraction from the hydrazine (N2H4) dimer by an oxygen atom. The thermochemical properties of the reaction were computed using two quantum chemical approaches, the coupled cluster theory with single, double, and noniterative triple excitations (CCSD(T)) and the M06-2X DFT approach with the aug-cc-pVTZ and the maug-cc-pVTZ basis sets, respectively. The kinetic data were calculated with the improved canonical variational theory (ICVT) using a dual-level methodology to build the reaction path. The tunneling effects were considered by means of the small curvature tunneling (SCT) approximation. Potential wells on both sides of the reaction ((N2H4)2 + O → N2H4···N2H3 + OH) were determined. A reaction path with a negative activation energy was found leading, in the temperature range of 250-423 K, to a negative dependence of the rate constant on the temperature, which is in good agreement with the experimental measurements. Therefore, the consideration of the hydrazine dimer model provides significantly improved agreement with the experimental data and should be included in the mechanism of the global N2H4 combustion process, as it can be particularly important in hydrazine-rich systems.

Theoretical study of the oxidation mechanisms of thiophene initiated by hydroxyl radicals.

The mechanisms for the oxidation of thiophene by OH radicals under inert conditions (Ar) have been studied using density functional theory in conjunction with various exchange-correlation functionals. These results were compared with benchmark CBS-QB3 theoretical results. Kinetic rate constants were estimated by means of variational transition state theory (VTST) and the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. Effective rate constants were calculated via a steady-state analysis based upon a two-step model reaction mechanism. In line with experimental results, the computed branching ratios indicate that the most kinetically efficient process involves OH addition to a carbon atom adjacent to the sulfur atom. Due to the presence of negative activation energies, pressures larger than 10(4)bar are required to reach the high-pressure limit. Nucleus-independent chemical shift indices and natural bond orbital analysis show that the computed activation energies are dictated by changes in aromaticity and charge-transfer effects due to the delocalization of lone pairs from sulfur to empty π(*) orbitals. Graphical Abstract CBS-QB3 energy profiles for the reaction pathways 1-3 characterizing the oxidation of thiophene by hydroxyl radicals into the related products.

Fluctuation of Arabidopsis seed dormancy with relative humidity and temperature during dry storage.

The changes in germination potential of freshly harvested seeds of Arabidopsis thaliana stored in various combinations of temperature and relative humidity were investigated over 63 weeks of storage. Seeds of the wild type Col-0 and of two mutants displaying low and high levels of dormancy, cat2-1 and mtr4-1, respectively, were stored at harvest in 24 different environments including a combination of eight relative humidities, from 1 to 85%, and four temperatures (10, 15, 20, and 25 °C). These mutations did not influence behaviour of seeds during storage. Primary dormant seeds did not germinate in darkness at 25 °C but acquired the potential to germinate at this temperature within 7 weeks when stored in relative humidities close to 50% across all temperatures. Sorption isotherms and Arrhenius plots demonstrated that the seed moisture content of 0.06 g H2O/g dry weight was a critical value below which dormancy release was associated with reactions of negative activation energy and above which dormancy release increased with temperature. Longer storage times when relative humidity did not exceed 75-85% led to decreased germination at 25 °C, corresponding to the induction of secondary dormancy. Dormancy release and induction of secondary dormancy in the dry state were associated with induction or repression of key genes related to abscisic acid and gibberellins biosynthesis and signalling pathways. In high relative humidity, prolonged storage of seeds induced ageing and progressive loss of viability, but this was not related to the initial level of dormancy.

Suppression Mechanism for Enol–Enol Isomerization of 2‐Substituted Dimedones

Abstract2‐Substituted dimedones, described as having a simple chemical structure, are a type of cyclic β‐diketone with enol–enol interconversion. The influence of the enol‐enol isomerism interferes with structure analysis of many dimedones in solution by NMR spectroscopy. It was demonstrated that the isomerism of the 2‐substituted dimedones depends on solvent, time, solute concentration, and temperature. Negative activation energy of the isomerization was determined by the NMR line shape and spin‐spin relaxation analyses, which is an extraordinary phenomenon found for the first time in the molecular dynamics of NMR. Suppression of the isomerization by heating, dilution of the solute, and time control in several low‐polarity solvents enabled analysis of the enol form in detail. It was clarified that deceleration of the intramolecular chemical exchange rate in the isomerization was responsible for the decrease in the intermolecular hydrogen‐bonding effect in the suppression mechanism.

Kinetics of Isothermal Melt Crystallization in CaO-SiO2-CaF2-Based Mold Fluxes

A kinetic study for isothermal melt crystallization of CaO-SiO2-CaF2-based mold fluxes with different basicity of 0.94 and 1.34 has been carried out systematically by DSC measurements. The kinetic parameters were determined by Johnson–Mehl–Avrami equation. The average Avrami exponent of cuspidine (3CaO·2SiO2·CaF2) crystallization for mold flux of lower basicity (0.94) is calculated to be 3.1, implying that the crystallization mode is instantaneous nucleation followed by 3-dimensional growth. For the mold flux of higher basicity (1.34), the average Avrami exponent of cuspidine equals to 3.4, strongly suggesting that the growth is still 3 dimensional but the nucleation should be continuous. It was found that the effective crystallization rate constant for both mold fluxes increases as the crystallization temperature decreases, showing that the crystallization rate could be governed by nucleation rate. The negative effective activation energy indicates an anti-Arrhenius behavior for crystallization of the mold fluxes studied. Therefore, it is concluded that the melt crystallization for the commercial mold fluxes will be determined by thermodynamics of nucleation which is relevant to degree of undercooling. The morphology of cuspidine crystals observed by SEM agreeds well with the isothermal crystallization kinetics results.

On the negative activation energy for limestone calcination at high temperatures nearby equilibrium

The study of limestone calcination under high CO2 pressure and high temperature has gained an extraordinary practical importance due to the recently emerged Ca-looping technology for post-combustion CO2 capture, which uses natural limestone (CaCO3) as precursor of the CO2 solid sorbent (CaO). A critical issue of this promising process is the necessity of producing CaO by calcination of limestone under high CO2 partial pressure. Experimental measurements on the kinetics of limestone calcination usually show that the rate of the endothermic calcination reaction is increased with temperature according to an Arrhenius law with an activation energy similar to the reaction enthalpy change. In situ microscopic observations have demonstrated that the reaction involves a crystallographic structural transformation of CaO. Calcination is started by chemical decomposition leading to the development of metastable CaO⁎ nanocrystals after which CO2 is desorbed while stable CaO cubic crystals grow. In calcination environments with low CO2 partial pressure P (as compared to the equilibrium pressure: P/Peq≪1), desorption of CO2 and the exothermic structural transformation of CaO⁎ to its stable CaO form occur extremely fast and do not play a role on the reaction kinetics, which is just determined by chemical decomposition. However, at high values of P/Peq the reaction rate may be significantly influenced by the structural transformation above a critical temperature. As a main consequence, the reaction mechanism proposed in the present paper shows that, above a critical temperature, limestone calcination is characterized by a negative activation energy in agreement with the experimental results shown.

Stereodynamical Origin of Anti-Arrhenius Kinetics: Negative Activation Energy and Roaming for a Four-Atom Reaction.

The OH + HBr → H2O + Br reaction, prototypical of halogen-atom liberating processes relevant to mechanisms for atmospheric ozone destruction, attracted frequent attention of experimental chemical kinetics: the nature of the unusual reactivity drop from low to high temperatures eluded a variety of theoretical efforts, ranking this one among the most studied four-atom reactions. Here, inspired by oriented molecular-beams experiments, we develop a first-principles stereodynamical approach. Thermalized sets of trajectories, evolving on a multidimensional potential energy surface quantum mechanically generated on-the-fly, provide a map of most visited regions at each temperature. Visualizations of rearrangements of bonds along trajectories and of the role of specific angles of reactants' mutual approach elucidate the mechanistic change from the low kinetic energy regime (where incident reactants reorient to find the propitious alignment leading to reaction) to high temperature (where speed hinders adjustment of directionality and roaming delays reactivity).

Facile Synthesis of Well-Defined MDMO-PPV Containing (Tri)Block—Copolymers via Controlled Radical Polymerization and CuAAC Conjugation

A systematic investigation into the chain transfer polymerization of the so-called radical precursor polymerization of poly(p-phenylene vinylene) (PPV) materials is presented. Polymerizations are characterized by systematic variation of chain transfer agent (CTA) concentration and reaction temperature. For the chain transfer constant, a negative activation energy of −12.8 kJ·mol−1 was deduced. Good control over molecular weight is achieved for both the sulfinyl and the dithiocarbamate route (DTC). PPVs with molecular weights ranging from thousands to ten thousands g·mol−1 were obtained. To allow for a meaningful analysis of the CTA influence, Mark–Houwink–Kuhn–Sakurada (MHKS) parameters were determined for conjugated MDMO-PPV ([2-methoxy-5-(3',7'-dimethyloctyloxy)]-1,4-phenylenevinylene) to α = 0.809 and k = 0.00002 mL·g−1. Further, high-endgroup fidelity of the CBr4-derived PPVs was proven via chain extension experiments. MDMO-PPV-Br was successfully used as macroinitiator in atom transfer radical polymerization (ATRP) with acrylates and styrene. A more polar PPV counterpart was chain extended by an acrylate in single-electron transfer living radical polymerization (SET-LRP). In a last step, copper-catalyzed azide alkyne cycloaddition (CuAAC) was used to synthesize block copolymer structures. Direct azidation followed by macromolecular conjugation showed only partial success, while the successive chain extension via ATRP followed by CuAAC afforded triblock copolymers of the poly(p-phenylene vinylene)-block-poly(tert-butyl acrylate)-block-poly(ethylene glycol) (PPV-b-PtBuA-b-PEG).

High-temperature kinetics of the reaction between chlorine atoms and hydrogen sulfide

The rate constant k1 for Cl+H2S has been measured over 290–914K using the laser flash photolysis/resonance fluorescence technique. Atomic chlorine was generated by pulsed 193nm photolysis of CCl4 and C6H5Cl and monitored by time-resolved resonance fluorescence at 130–140nm. The data are summarized as k1=(2.90±0.10)×10−11exp(2.38±0.11kJmol−1/RT)cm3molecule−1s−1 where the uncertainties in the Arrhenius parameters are ±2σ. 95% confidence limits for k1 are ±6%. These extended range measurements confirm a negative activation energy for the reaction.