Negative Apparent Activation Energy Research Articles

The interpretation of diverging hydrogen and carbon dioxide permeations with temperature across silica-based membranes

Hydrogen (H2) permeance across silica-based membranes increases with temperature, but carbon dioxide (CO2) permeance decreases. Because of the opposite signs of activation energies for H2 and CO2 permeation, silica-based membranes are widely used in separating H2 and CO2 at high temperatures due to the elevated difference between H2 and CO2 permeabilities. However, there has not been an explanation of the diverging permeance between H2 and CO2 with temperature. This study developed a gas permeation model encompassing gas penetration through silica matrix, Knudsen diffusion, and viscous flow. An Oscillator model with an effective medium approach was used to screen all possible pore size distributions and calculate the gas penetration through the silica matrix. However, no pore size distribution could allow positive activation energy for hydrogen and negative activation energy for carbon dioxide at the same time. After including Knudsen diffusion and viscous flow, the diverging permeance between H2 and CO2 with temperature was successfully interpreted. The pore size distribution and the fractional contribution from Knudsen diffusion and viscous flow, which could best fit experimental activation energies for H2 and CO2 permeation, were identified. In the silica matrix that formed the membranes, 5-membered rings were the dominant structures in pores, which was responsible for the positive apparent activation energy for H2. The negative CO2 apparent activation energy indicated that it was inevitable to have some Knudsen diffusion and viscous flow through some imperfections of the membrane. Our model demonstrated the importance of high-temperature gas separation, as high temperature could minimize the undesirable gas flow through imperfections. This study also implied that further improvement of H2/CO2 separation by silica-based membranes could be achieved by reducing imperfections.

Very Low Temperature CO Oxidation over Atomically Precise Au25 Nanoclusters on MnO2.

Atomically precise Au25 nanoclusters have garnered significant interest in the field of heterogeneous catalysis due to their remarkable activity and selectivity. However, for the extensively studied reaction of low-temperature CO oxidation, their performance has not been competitive compared to other known gold nanocatalysts. To address this, we deposited Au25(SR)18 (R = CH2CH2Ph) nanoclusters onto a manganese oxide support (Au25/MnO2), resulting in a very stable and highly active catalyst. By optimizing the pretreatment temperature, we were able to significantly enhance the performance of the Au25/MnO2 catalyst, which outperformed most other gold catalysts. Impressively, 100% conversion of CO was achieved at temperatures as low as -50 °C, with 50% conversion being reached below -70 °C. Furthermore, the existence of ligands could also influence the negative apparent activation energy observed at intermediate temperatures. Analysis using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD) techniques indicated that the Au25 nanoclusters remained stable on the catalyst surface even after pretreatment at high temperatures. In-situ modulation excitation spectroscopy (MES) spectra also confirmed that the Au cluster was the active site for CO oxidation, highlighting the potential of atomically precise Au25 nanoclusters as primary active sites at very low temperatures.

Mechanistic Insights on the Low-Temperature Oxidation of CO Catalyzed by Isolated Co Ions in N-Doped Carbon

Isolated cobalt ions on nitrogen-doped carbon (Co–N–C) catalyze CO oxidation at temperatures as low as 196 K, but the active site and mechanism for this reaction remain elusive. In this work, steady-state CO oxidation around 273 K over Co–N–C revealed nearly first-order behavior in both CO and O2 as well as a negative apparent activation energy. Isotopic transient analysis of the reaction confirmed a rapid turnover frequency and low surface coverage of adsorbed intermediates leading to CO2 (<10% of the Co). Results from kinetics experiments combined with quantum chemical calculations and molecular dynamics simulations are consistent with a reaction path involving weak adsorption of CO onto Co ions followed by a low barrier for the CO-assisted activation of weakly adsorbed O2. This proposed mechanism for dioxygen activation does not involve a redox cycle with the transition-metal ion and may be important in other low-temperature catalytic reactions involving O2.

Surface protonic conductivity in chemisorbed water in porous nanoscopic CeO2

CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O1/2, pH2O1, and pH2O3/2 dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

Measuring RNA UNCG Tetraloop Refolding Dynamics Using Temperature-Jump/Drop Infrared Spectroscopy.

Determining the structural dynamics of RNA and DNA is essential to understanding their cellular function, but direct measurement of strand association or folding remains experimentally challenging. Here we illustrate a temperature-jump/drop method able to reveal refolding dynamics. Time-resolved temperature-jump/drop infrared spectroscopy is used to measure the melting and refolding dynamics of a 12-nucleotide RNA sequence comprising a UACG tetraloop and a four-base-pair double-stranded GC stem, comparing them to an equivalent DNA (TACG) sequence. Stem-loop melting occurred an order of magnitude more slowly in RNA than DNA (6.0 ± 0.1 μs versus 0.8 ± 0.1 μs at 70 °C). In contrast, the refolding dynamics of both sequences occurred on similar time scales (200 μs). While the melting and refolding dynamics of RNA and DNA hairpins both followed Arrhenius temperature dependences, refolding was characterized by an apparent negative activation energy, consistent with a mechanism involving multiple misfolded intermediates prior to zipping of the stem base pairs.

Biochar Synthesis from Mineral- and Ash-Rich Waste Biomass, Part 1: Investigation of Thermal Decomposition Mechanism during Slow Pyrolysis.

Synthesizing biochar from mineral- and ash-rich waste biomass (MWB), a by-product of human activities in urban areas, can result in renewable and versatile multi-functional materials, which can also cater to the need of solid waste management. Hybridizing biochar with minerals, silicates, and metals is widely investigated to improve parent functionalities. MWB intrinsically possesses such foreign materials. The pyrolysis of such MWB is kinetically complex and requires detailed investigation. Using TGA-FTIR, this study investigates and compares the kinetics and decomposition mechanism during pyrolysis of three types of MWB: (i) mineral-rich banana peduncle (BP), (ii) ash-rich sewage sludge (SS), and (iii) mineral and ash-rich anaerobic digestate (AD). The results show that the pyrolysis of BP, SS, and AD is exothermic, catalyzed by its mineral content, with heat of pyrolysis 5480, 4066, and 1286 kJ/kg, respectively. The pyrolysis favors char formation kinetics mainly releasing CO2 and H2O. The secondary tar reactions initiate from ≈318 °C (BP), 481 °C (SS), and 376 °C (AD). Moreover, negative apparent activation energies are intrinsic to their kinetics after 313 °C (BP), 448 °C (SS), and 339 °C (AD). The results can support in tailoring and controlling sustainable biochar synthesis from slow pyrolysis of MWB.

On the Physical Meaning of Single-Value Activation Energies for BTI in Si and SiC MOSFET Devices

Bias temperature instability (BTI) has been shown to be the collective response of an ensemble of defects to a gate voltage signal. Each defect can be described using first-order reactions for either charge trapping or defect creation with a broad distribution of time constants. Consequently, the temperature dependence of BTI has to be determined by a distribution of activation energies reflecting the variety of available defects. Commonly employed single-value apparent activation energy-based modeling of BTI can therefore be at best a very rough approximation and its apparent activation energy is consequently a model parameter without physical justification. This study investigates the impact of measurement parameters, such as the extraction point in the drift curve and the recovery time, on these apparent activation energies that can be extracted either by threshold voltage shift increase (vertical method) or by stress time acceleration (horizontal method). The framework of activation energy maps enables an analytical mathematical approach to calculate the dependence of extracted apparent activation energies on the measurement parameters. A comparison between Si and SiC MOSFET devices is employed, and nonphysical negative apparent activation energies are explained. Since apparent activation energies have been repeatedly employed to justify physics-based descriptions of BTI, their physical (in)significance is discussed.

Temperature dependence of rate coefficients for the gas phase reaction of OH with 3-chloropropene. A theoretical and experimental study

This work studies the mechanism steps of the OH radical + 3-chloropropene gas phase reaction that could explain the apparent negative activation energy experimentally observed. A reinvestigation of the rate coefficients (k) temperature dependence, using a PLP-LIF technique, between 253 and 371 K, was performed to provide new data for kinetic parameters critical revisions. A canonical Variational Transition State Theory study was performed to obtain the k temperature dependence considering four additions and one H atom abstraction pathways. The theoretical results can explain the experimental Arrhenius behavior, being an OH addition channel not described in previous literature the main reaction pathway.

Linking low and high temperature NO oxidation mechanisms over Brønsted acidic chabazite to dynamic changes of the active site

The oxidation of NO over Brønsted acid sites in chabazite (CHA) zeolites shows an atypical temperature dependence; at low temperature the apparent activation energy is negative, but it becomes positive as the temperature exceeds a transition temperature. To explain this behavior we used density functional theory and statistical mechanics to investigate high and low temperature mechanisms for this reaction and propose a dynamic active site change in response to temperature variation. Our simulations show that the apparent activation barrier in the low temperature regime is more negative over Brønsted acidic CHA as compared to the siliceous zeolite framework. This effect is attributed to further enthalpic stabilization of the transition states by physical interaction with the H-CHA Brønsted acid sites. At elevated temperature, our calculations support both the existence and the significant catalytic role of NO+ in providing a modified active site. The temperature dependent transformation of the active site from H-CHA to NO-CHA sites may occur via two plausible ion-exchange mechanisms that define a transition temperature for the reaction. This transition temperature can be tuned by incorporating different trivalent metal atoms (B, Al, Ga or In) within the CHA framework. We found the lowest transition temperature for H-[In]CHA and H-[Ga]CHA. The ability to control the dynamic response of the active site and the associated switch between low and high temperature mechanism with negative and positive apparent activation energy, respectively, is of fundamental interest for the design of zeolite catalysts operating in the presence of NO.

A stepwise mechanism for aqueous two-phase system formation in concentrated antibody solutions

Aqueous two-phase system (ATPS) formation is the macroscopic completion of liquid-liquid phase separation (LLPS), a process by which aqueous solutions demix into 2 distinct phases. We report the temperature-dependent kinetics of ATPS formation for solutions containing a monoclonal antibody and polyethylene glycol. Measurements are made by capturing dark-field images of protein-rich droplet suspensions as a function of time along a linear temperature gradient. The rate constants for ATPS formation fall into 3 kinetically distinct categories that are directly visualized along the temperature gradient. In the metastable region, just below the phase separation temperature, Tph , ATPS formation is slow and has a large negative apparent activation energy. By contrast, ATPS formation proceeds more rapidly in the spinodal region, below the metastable temperature, Tmeta , and a small positive apparent activation energy is observed. These region-specific apparent activation energies suggest that ATPS formation involves 2 steps with opposite temperature dependencies. Droplet growth is the first step, which accelerates with decreasing temperature as the solution becomes increasingly supersaturated. The second step, however, involves droplet coalescence and is proportional to temperature. It becomes the rate-limiting step in the spinodal region. At even colder temperatures, below a gelation temperature, Tgel , the proteins assemble into a kinetically trapped gel state that arrests ATPS formation. The kinetics of ATPS formation near Tgel is associated with a remarkably fragile solid-like gel structure, which can form below either the metastable or the spinodal region of the phase diagram.

Explorations on the continuous oxidation kinetics of diesel PM from heavy-duty vehicles using a single ramp rate method

Particulate matter (PM) emitted by internal combustion engines has brought about serious environmental problems. Investigations of the PM oxidation behaviors and kinetics make the foundations of decreasing PM emission and working out the regeneration problems of PM capture devices. In this paper, the oxidation kinetics and microstructure evolutions of diesel PM in the oxidation process were researched. The results showed that thermogravimetric analysis (TGA) temperature history had a limited influence on the tendency of the oxidation kinetics. The volatilization of the organic compounds dominated the mass loss without any heat release at the initial stage of PM oxidation. The abnormal phenomenon (negative apparent activation energy) was observed in the oxidation process when the oxidation temperature was in the range of 200 °C–340 °C, which was ubiquitous during PM oxidation. However, the abnormal phenomenon disappeared if the PM sample was pre-treated to remove the volatile organic compounds (VOC), with the result that the phenomenon was caused by the VOC. The hydrogen bonding complex was formed in that temperature range, where the energy of the hydrogen bonding complex was lower than the value of the reactants, which caused the negative apparent activation energy. The apparent activation energy increased gradually when the temperature was higher than 340 °C.

New insights into the mechanism of NH3-SCR over Cu- and Fe-zeolite catalyst: Apparent negative activation energy at high temperature and catalyst unit design consequences

We demonstrate an unusual behavior of a practically-important reaction of selective catalytic reduction (SCR) of NOx with NH3 over a state-of-the-art Cu-SSZ-13 catalyst. In response to increasing temperature, the rate of SCR reaction increases initially, attains a maximum around 300 °C, and then declines, resulting in a negative apparent activation energy at higher temperatures. This behavior has never been previously reported because the side reaction of NH3 oxidation obfuscated the kinetic analysis of SCR reaction at high temperatures. We were able to discover this phenomenon by performing severe hydrothermal aging of the catalyst that suppressed the NH3 oxidation activity without a significant change in the SCR activity. Further, we show that the phenomenon is more general and also found in other Cu- and Fe-exchanged zeolite and V2O5-(WO3)/TiO2 catalysts. The behavior is explained based on the fact that the activation energy for NH3 desorption is higher than that for standard SCR reaction. Therefore, at higher temperatures, the increase in the SCR reaction rate constant with temperature gets outpaced by the decline in the NH3 coverage, resulting in the overall decline of the reaction rate.One of the implications of the finding is that the studied Cu-SSZ-13 SCR catalyst never operates in a purely external mass transfer limited regime, under the range of conditions relevant to the practical SCR applications. In other words, there is always a certain contribution of slower reaction rate to the performance of the SCR catalyst, even after increasing the reaction rates by increasing catalyst amount, using the “fast” SCR reaction, or operating at elevated temperatures. This has significant consequences for the design of practical SCR catalysts, in particular related to the choices of the active material loading and channel hydraulic diameter, as empirically and analytically demonstrated in this paper.

First-principles theoretical assessment of catalysis by confinement: NO-O2 reactions within voids of molecular dimensions in siliceous crystalline frameworks.

Density functional theory methods that include dispersive forces are used to show how voids of molecular dimensions enhance reaction rates by the mere confinement of transition states analogous to those involved in homogeneous routes and without requiring specific binding sites or structural defects within confining voids. These van der Waals interactions account for the observed large rate enhancements for NO oxidation in the presence of purely siliceous crystalline frameworks. The minimum free energy paths for NO oxidation within chabazite (CHA) and silicalite (SIL) frameworks involve intermediates similar in stoichiometry, geometry, and kinetic relevance to those involved in the homogeneous route. The termolecular transition state for the kinetically-relevant cis-NOO2NO isomerization to trans-NOO2NO is strongly stabilized by confinement within CHA (by 36.3 kJ mol-1 in enthalpy) and SIL (by 39.2 kJ mol-1); such enthalpic stabilization is compensated, in part, by concomitant entropy losses brought forth by confinement (CHA: 44.9; SIL: 45.3, J mol-1 K-1 at 298 K). These enthalpy and entropy changes upon confinement agree well with those measured and combine to significantly decrease activation free energies and are consistent with the rate enhancements that become larger as temperature decreases because of the more negative apparent activation energies in confined systems compared with homogeneous routes. Calculated free energies of confinement are in quantitative agreement with measured rate enhancements and with their temperature sensitivity. Such quantitative agreements reflect preeminent effects of geometry in determining the van der Waals contributions from contacts between the transition states (TS) and the confining walls and the weak effects of the level of theory on TS geometries. NO oxidation reactions are chosen here to illustrate these remarkable effects of confinement because detailed kinetic analysis of rate data are available, but also because of their critical role in the treatment of combustion effluents and in the synthesis of nitric acid and nitrates. Similar effects are evident from rate enhancements by confinement observed for Diels-Alder and alkyne oligomerization reactions. These reactions also occur in gaseous media at near ambient temperatures, for which enthalpic stabilization upon confinement of their homogeneous transition states becomes the preeminent component of activation free energies.

Molecular recognition and binding of beta-lactamase II from Bacillus cereus with penicillin V and sulbactam by spectroscopic analysis in combination with docking simulation.

The molecular recognition and binding interaction of beta-lactamase II from Bacillus cereus (Bc II) with penicillin V (PV) and sulbactam (Sul) at 277K were studied by spectroscopic analysis and molecular docking. The results showed that a non-fluorescence static complex was separately formed between Bc II and two ligands, the molecular ratio of Bc II to PV or Sul was both 1:1 in the binding and the binding constants were 2.00×106 and 3.98×105 (L/mol), respectively. The negative free energy changes and apparent activation energies indicated that both the binding processes were spontaneous. Molecular docking showed that in the binding process, the whole Sul molecule entered into the binding pocket of Bc II while only part of the whole PV molecule entered into the pocket due to a long side chain, and electrostatic interactions were the major contribution to the binding processes. In addition, a weak conformational change of Bc II was also observed in the molecular recognition and binding process of Bc II with PV or Sul. This study may provide some valuable information for exploring the recognition and binding of proteins with ligands in the binding process and for the design of novel super-antibiotics.

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.

Polyplex Formation Influences Release Mechanism of Mono- and Di-Valent Ions from Phosphorylcholine Group Bearing Hydrogels

The release of monovalent potassium and divalent calcium ions from zwitterionic phosphorylcholine containing poly(2-hydroxyethyl methacrylate) (pHEMA)-based hydrogels was studied and the effects of polymer swelling, ion valence and temperature were investigated. For comparison, ions were loaded during hydrogel formulation or loaded by partitioning following construct synthesis. Using the Koshmeyer-Peppas release model, the apparent diffusion coefficient, Dapp, and diffusional exponents, n, were Dapp (pre-K+) = 2.03 × 10−5, n = 0.4 and Dapp (post-K+) = 1.86 × 10−5, n = 0.33 respectively, indicative of Fickian transport. The Dapp (pre-Ca2+) = 3.90 × 10−6, n = 0.60 and Dapp (post-Ca2+) = 2.85 × 10−6, n = 0.85, respectively, indicative of case II and anomalous transport. Results indicate that divalent cations form cation-polyelectrolyte anion polymer complexes while monovalent ions do not. Temperature dependence of potassium ion release was shown to follow an Arrhenius-type relation with negative apparent activation energy of −19 ± 15 while calcium ion release was temperature independent over the physiologically relevant range (25–45 °C) studied. The negative apparent activation energy may be due to temperature dependent polymer swelling. No effect of polymer swelling on the diffusional exponent or rate constant was found suggesting polymer relaxation occurs independent of polymer swelling.