Intrinsic Activation Energy Research Articles

Multi-temperature indentation creep tests on nanotwinned copper

Abstract: The present work further develops the multi-temperature approach on load, time, and temperature-dependent deformation for indentation creep. Multi-temperature micro-indentation creep tests were carried out on nanotwinned copper (nt-Cu) at five temperatures of 22 °C (RT), 40 °C, 50 °C, 60 °C and 70 °C. In analogy with stress, hardness is used to gauge the indentation creep loading level, while the indentation depth is used to characterize the indentation creep deformation and the creep strain rate is represented by the indentation depth strain rate. The multi-temperature micro-indentation creep tests generate sufficiently large experimental data, which makes the development of a novel formula for indentation creep feasible. There are few intrinsic parameters that characterize the capability of the microstructure of a material against load, time, and temperature dependent deformation and they are the strain rate sensitivity, the athermal hardness exponent, intrinsic activation energy, and activation volume. The strain rate sensitivity is determined from isothermal creep data at one temperature, while the other parameters have to be determined from multi-temperature creep data. The novel formula is validated by the experimental data of the multi-temperature indentation creep tests on the nt-Cu. The creep mechanisms of the nt-Cu are also discussed and analyzed by using the determined values of the intrinsic parameters.

Consideration of Acid Strength of a Single OH Group on Zeolites by Isotope Exchange Reaction with Ethane at High Temperatures

Infrared probe (IR) methods to estimate the acid strength of OH groups on H-form zeolites were compared on mordenites (HM90, Si/Al = 45 and HM20, Si/Al = 10), ZSM-5 (Z5-90H, Si/Al = 45) and Y (HY5.6, Si/Al = 2.8). Heat of adsorption as well as the internal frequency of adsorbed CO was not sensitive enough to evaluate the acid strength of different zeolites. Thus, H/D isotope exchange reaction of acidic OD groups with a small hydrocarbon, C2H6, was employed. For the estimation of intrinsic activation energy, heat of adsorption of C2H6 was first measured. However, C2H6 molecules could not enter the 8 membered ring (MR) of HM20, and the secondary interaction of C2H6 with pore walls of Z5-90H increased the heat of adsorption, both due to the observation at low temperatures. This prohibited the comparison of heat of adsorption on a single Bronsted acid site in different spaces of zeolite with different topologies. Considering that the structural restrictions of such small molecules as C2H6 become negligible at high temperatures as a result of the vibrational excitation of pore mouth-breathing modes of zeolites, and that the topology difference of a single Bronsted acid site in heat of adsorption is relatively small, we proposed that the apparent activation energy for H/D isotope exchange reaction would reflect the acid strength of single sites. The present results are compared and discussed with conclusions led by other methods.

Effects of phosphorus and alkyl substituents on C–H, C–C, and C–O bond rupture within carboxylic acids on Ru(0001)

Quantitative analysis of temperature programmed reaction (TPR) spectra of formic acid, acetic acid, propionic acid, and butyric acid decomposition on Ru(0001) and phosphorus (P)-modified Ru(0001) surfaces show that both the length of carboxylic acids alkyl substituents (i.e., R=H, CH3, CH2CH3, and CH2CH2CH3) and the presence of P atoms alter the intrinsic activation energy for bond rupture. Inversion analysis of TPR spectra reveal that the intrinsic activation energy barriers on Ru(0001) for C–H bond cleavage in formate is 94 kJ mol−1, while C–C bond cleavage (decarboxylation) barriers for acetate, propionate, and butyrate are 67, 49, and 55 kJ mol−1, respectively. The activation energies to decarboxylate all four of these species correlate linearly with differences between the homolytic dissociation energies of the relevant bonds (e.g., H-COOH, CH3-COOH, CH3CH2-COOH, and CH3CH2CH2-COOH), which suggests that longer alkyl substituents (e.g., propyl and butyl) stabilize bond rupture transition states by donating electron density to the R–COOH bond via inductive effects. Simultaneously, longer alkyl chains also promote self-stabilizing lateral interactions between carboxylates via van der Waals forces that increase the barriers for R–COOH bond rupture slightly (by ∼1–5 kJ mol−1) at high surface coverages. The addition of P atoms to Ru(0001) increases the intrinsic activation energies for the rupture of all bonds (i.e., C–O, C–H, and C–C bonds), specifically, by 5–50 kJ mol−1 for C–H and C–C bonds. P atoms change the Ru(0001) surface likely via an electronic effect by decreasing the extent of electron back donation from Ru atoms to the antibonding orbitals of the carboxylate intermediate. These results provide useful information for transition metal phosphide catalyst design and carboxylic acid alkyl substituent selection to tailor selectivity toward C–O, C–H, and C–C bond rupture.

Characterization and controlled combustion of carbonaceous deactivating species deposited on an activated carbon-based catalyst

The composition of the carbonaceous deactivating species (coke) deposited on a Pt and Pd supported P-containing activated carbon catalyst has been studied. These deactivating species were deposited on the catalyst during the hydrocracking of scrap tire pyrolysis oil at 400–500°C, and it has been selectively characterized by means of temperature-programmed oxidation (TPO), temperature-programmed desorption/gas chromatography (TPD/GC) and laser desorption-ionization/mass spectroscopy (LDI/MS). In addition, the evolution of the textural properties and the acidity of the deactivated catalysts have been evaluated. The high thermal and oxidation resistance of the catalytic support has allowed to combust the coke in the TPO and calculate its intrinsic activation energy as a function of the extent of the combustion. Combined TPO and LDI/MS results have shown that an increase in the hydrocracking temperature attenuates the catalyst deactivation due to the hydrocracking of coke precursors. Coke aging, by evolving towards a more condensed structure, is also favored at higher hydrocracking temperatures. The combustion of the most condensed coke requires of higher temperatures than 375°C, which hinders the complete regeneration of the activated carbon-based catalyst.

The coumarin synthesis: a combined experimental and theoretical study

Coumarin, which is an important material class of substrate in agricultural and pharmaceutical industries, can be obtained by the Pechmann condensation reaction. In this work, coumarin was synthesized from ethyl acetoacetate and resorcinol with sulfuric acid as the catalyst with 86% yield under a mild condition. The product was confirmed by IR, 1H NMR, and 13C NMR spectroscopy. Reaction rates at difference temperatures between 273 and 313 K were used to determine the apparent activation energy of the reaction to be 34.7 kJ/mol. Molecular-level understanding about the mechanism of Pechmann condensation reaction was revealed by the complimentary M06-2X density functional theory. The reaction proceeded via a three-step mechanism; transesterification, intramolecular hydroxylalkylation, and dehydration. The intrinsic activation energies of these three steps were calculated to be 136.7, 76.5, and 134.0 kJ/mol, respectively. The calculated apparent activation energy of 40.0 kJ/mol was in agreement with the experimental data. The transesterification was considered as the rate-determining step of the reaction.

Characteristics of high temperature C-CO2 gasification reactivity of Victorian brown coal char and its blends with high ash fusion temperature bituminous coal

This research paper explores the high temperature gasification reactivity of brown coal char and its potential for promoting the gasification of high-ash-fusion temperature bituminous coal under entrained-flow gasification conditions. Using thermogravimetric analysis (TGA), the C-CO2 gasification reactivity for a brown coal char and two bituminous coals with low/high ash contents and high ash fusion temperatures, as well as their blends were investigated over a temperature range of 900–1300°C and atmospheric pressure. The nth order reaction rate equations were fitted based on a random pore model of the gasification reaction. The results show that the brown coal char has a high reactivity compared to the two bituminous coals, which mainly due to its more porous physical structure, whereas the catalytic effect of its ash-forming elements is insignificant above 1100°C. The intrinsic activation energy was determined to be 154.5kJ/mol for the brown coal char, which is lower than the higher activation energy of the two bituminous coals of 189.61kJ/mol and 196.02kJ/mol. Upon increasing the bituminous coal fraction, the activation energy increases stably due to the increased ash amount and increased molten ash that inhibit the gas diffusion. For the low-ash bituminous coal blended with 20–40% brown coal char, its C-CO2 reactivity is maximised at 1200°C when the ash turns soft. Upon further increase of the temperature, its reactivity decreases due to the enhanced melting of the ash. Instead, for the high-ash bituminous coal, its addition of 20% to brown coal char is beneficial in decreasing the overall ash melting temperature. The C-CO2 gasification reactivity of 20% high-ash bituminous coal blended with brown coal char is maximised at 1100°C when the ash turns soft and is minimised at 1200°C when the ash is fully molten.

Ethylene insertion in the presence of new alkoxysilane electron donors for Ziegler-Natta catalyzed polyethylene

Density functional theory (DFT) calculations have been carried out to investigate the ethylene insertion pathway using a Ziegler-Natta catalyst in the absence and presence of electron donor (ED) systems on the (110) MgCl2 surface. The coadsorptions of four different EDs i.e. Si(OEt)mCln (m+n=4) were investigated. The presence of Si(OEt)4 on the (110) MgCl2 surface with the preferential bidentate mode was found to have the strongest adsorption energy (ΔEads). The potential energy surface (PES) map indicated that the reaction mechanism of the ethylene insertion into the TiC bond on the (110) Mg13Cl26.TiCl3-CH2CH3 surface is pseudo-concerted. As the differences in the intrinsic activation energies (ΔEa) obtained from all systems are so small, this energy cannot be used to fully explain the significant changes in the rates of the ethylene insertion reaction observed when an ED is employed. Here, the apparent activation energy (ΔΔEaa) was calculated using the PBE functional and the 6-31G(d, p) basis set for C, H, O, Mg and Cl, and the LANL2DZ basis set with an ECP function for the Ti atom. All EDs presented in this work in the ethylene insertion reaction can significantly reduce the apparent energy barrier when compared to an absence of any ED system. The obtained ΔΔEaa for the four ED complexes were found to decrease in the following order: SiOEtCl3>Si(OEt)2Cl2>Si(OEt)3Cl>Si(OEt)4. The obtained data lead to the conclusion that Si(OEt)4 is the most suitable ED to increase the productivity of PE in the presence of alkoxysilane.

Computational study of the 1,3-dipolar cycloaddition between methyl 2-trifluorobutynoate and substituted azides in terms of reactivity indices and activation parameters

The 1,3-dipolar cycloaddition of methyl 2-trifluorobutynoate with various azides has been studied in terms of several theoretical approaches at DFT/B3LYP/6-311++G(d,p) level of theory. The mechanism of regioselectivity of these reactions was investigated through the evaluation of the potential energy surface of the cycloaddition process calculations and density DFT-based reactivity indices. These approaches were successfully applied to prediction of preferable regio-isomers for various reactions of 1,3-dipolar cycloadditions. The reactions were followed by performing transition state optimization, calculation of Intrinsic Reaction Coordinate and activation energies. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a single elementary step (one-step mechanism) but asynchronous mechanism. The calculation of the activation energies and reaction energies show that the 1,5-regioisomer for substituted phenyl azides as dipoles and the 1,4-regioisomer for substituted benzyl azides as dipoles are thermodynamically in all the cycloadditions reactions. The solvent effect was also studied in the solvent tert-butyl alcohol using self-consistent reaction field model. The observed regioselectivity was explained by using developed DFT-based reactivity descriptors, such as Fukui and Parr functions. The results were compared with experimental data to find a good agreement.

Intrinsic Reduction Kinetics Investigation on a Hematite Oxygen Carrier by CO in Chemical Looping Combustion

In the present work, the intrinsic reduction kinetics of a chemical looping oxygen carrier, hematite, reduced by CO has been investigated. Initially, the pore structure features of the hematite OC were tested to provide the necessary parameters for the kinetics analysis. Then, temperature-programmed reduction experiments were conducted in a thermogravimetric analyzer. The results showed that the weight loss rate appears to be a bimodal distribution and the appropriate experimental temperature range for the intrinsic kinetics testing of the first reduction stage of hematite is 400–650 °C. Additionally, a series of experiments were further conducted in a batch fluidized bed system with different temperatures and different CO concentrations. After elimination of the internal and external diffusion limitations, the intrinsic activation energy and pre-exponential factor were determined as 74.48 kJ/mol and 1.2 × 1012 s–1, respectively. The analysis of the internal and external diffusion effects was beneficial t...

Kinetic study of hydrodeoxygenation of palmitic acid as a model compound for microalgae oil over Pt/γ-Al2O3

The kinetics of hydrodeoxygenation (HDO) of palmitic acid over the 1% Pt/γ-Al2O3 catalyst was investigated in a microreactor using integral method. Kinetic data were collected under process conditions of industrial interest but in the regime with negligible heat and mass transfer limitations. For the first time, rate equations derived from three kinetic models (Power law, Langmuir-Hinshelwood, and Eley-Rideal) were investigated in data fitting through nonlinear regression using Levenberg-Marquardt algorithm, and evaluated based on proposed statistical and thermodynamics criteria. H2 was found to be adsorbed on the catalyst surface without dissociation, which is different from gaseous reaction with consistent dissociative adsorption of H2. The apparent activation energy is 60.3kJ/mol, which is lower than the intrinsic activation energy of 92.9kJ/mol due to the exothermic adsorption of reactants. The proposed rate equations are expected to be applicable to liquid phase HDO of fatty acids, and not limited to palmitic acid. This complete kinetic study (including rate equations with specified kinetic parameters) fills an important knowledge gap in green diesel production from algae, and can be used for rational design of reactors and the determination of optimum operating conditions. Moreover, the presented work develops a systematic procedure for kinetic study by identifying the challenges that need to be overcome to obtain meaningful kinetic data.

Insight into the kinetic mechanism of the first-step dehydrogenation of Mg(AlH4)2

The detailed mechanism of the first-step dehydrogenation of Mg(AlH4)2 is investigated by using a comprehensive kinetic analysis. The reaction mode (geometrical contraction or nucleation-growth-impingement), intrinsic rate constant, dimensionality, rate-controlling step and activation energies of nucleation and growth are obtained from the simultaneous fitting of isothermal and non-isothermal experimental curves. The interpretation and comparison of these kinetic parameters for the pristine, high-energy ball milled and TiF4 doped Mg(AlH4)2 help reveal the mechanism of the dehydrogenation of Mg(AlH4)2, including how Mg(AlH4)2 decomposes to MgH2, Al and H2, and why high-energy ball milling and TiF4 doping can significantly improve the dehydrogenation properties.

Does the DFT Self-Interaction Error Affect Energies Calculated in Proteins with Large QM Systems?

We have examined how the self-interaction error in density-functional theory (DFT) calculations affects energies calculated on large systems (600-1000 atoms) involving several charged groups. We employ 18 different quantum mechanical (QM) methods, including Hartree-Fock, as well as pure, hybrid, and range-separated DFT methods. They are used to calculate reaction and activation energies for three different protein models in vacuum, in a point-charge surrounding, or with a continuum-solvent model. We show that pure DFT functionals give rise to a significant delocalization of the charges in charged groups in the protein, typically by ∼0.1 e, as evidenced from the Mulliken charges. This has a clear effect on how the surroundings affect calculated reaction and activation energies, indicating that these methods should be avoided for DFT calculations on large systems. Fortunately, methods such as CAM-B3LYP, BHLYP, and M06-2X give results that agree within a few kilojoules per mole, especially when the calculations are performed in a point-charge surrounding. Therefore, we recommend these methods to estimate the effect of the surroundings with large QM systems (but other QM methods may be used to study the intrinsic reaction and activation energies).

Acid site densities and reactivity of oxygen-modified transition metal carbide catalysts

Acidic properties of β-Mo2C, α-Mo2C, W2C, and WC were quantified by assessing the kinetics of isopropanol (IPA) dehydration at 415K either (i) under inert He/Ar atmosphere or (ii) with 13kPa O2 co-feed. Dehydration kinetics were zero-order with respect to IPA for all catalysts and under all reaction conditions. Intrinsic activation energies were similar across all catalysts (89–104kJmol−1). Acid site densities calculated via in situ 2,6-di-tert-butylpyridine (DTBP) titration were used to normalize dehydration turnover frequencies (TOF). O2 co-feed increased dehydration rates per gram by an order of magnitude for all catalysts tested, but TOF remained invariant within a factor of ∼2. Mo- and W-based carbides showed similar dehydration kinetics regardless of O2 co-feed, and O2 co-feed did not alter bulk carbidic structure as noted by X-ray diffraction. Brønsted acid site provenance results from the oxophilicity of Mo and W carbides.

Performance of mesoporous silicas and carbon in adsorptive removal of phenanthrene as a typical gaseous polycyclic aromatic hydrocarbon

Adsorptive removal of the predominant polycyclic aromatic hydrocarbons (PAHs), represented by phenanthrene, from hot gases using mesoporous adsorbents (mesosilicas MCM-41 and SBA-15, and mesocarbon CMK-3) has been studied through tests of column adsorption and temperature programmed desorption (TPD). Adsorption equilibria on MCM-41 and CMK-3, and on SBA-15 were well described by Langmuir and Freundlich models, respectively, showing the order of adsorption capacity of CMK-3 > SBA-15 > MCM-41 originated from different geometrical structures and texture properties. Adsorption dynamics based on breakthrough fitting by the constant-pattern wave propagation model showed that mesoporosity contributed to fast overall adsorption kinetics by reducing the internal resistance as being the predominant mass-transfer step, particularly on SBA-15 with a 2-D cross-linked structure. The TPD tests revealed the order of ease of desorption of SBA-15 ≈ MCM-41 > CMK-3, where the structural advantage on SBA-15 dominated the strong site binding as indicated by the intrinsic activation energy and exponential factor obtained by TPD models. Results from different TPD models showed that re-adsorption effect during desorption increased with decreasing dimension of adsorbent structure and that the utilization of the simplified linear TPD model could be violated by the strong sorbate-sorbent binding. Comparisons with traditional adsorbents and with the removal of naphthalene by these mesoporous adsorbents were made, showing higher applicability of CMK-3 for phenanthrene regarding stronger adsorption affinity and capacity as well as decreased disadvantages in regenerability relative to mesosilicas.

Lattice strain driven dielectric insulation response of Sr1−xCaxBi2Nb2O9 ceramics

In this paper, effects of lattice strain on dielectric properties of Sr[Formula: see text]CaxBi2Nb2O9 ([Formula: see text]–0.5 in steps of 0.1) ceramics are reported. High frequency dipolar polarization is observed to increase for 10% calcium addition and suppresses thereafter on higher calcium concentrations. The dielectric loss values for all doped samples were lower compared to undoped one at frequencies beyond 1[Formula: see text]kHz. The sample with [Formula: see text] shows room temperature dielectric constant in the range (130[Formula: see text][Formula: see text][Formula: see text]12) over a wide frequency range from 50[Formula: see text]Hz to 1[Formula: see text]MHz also minimum dielectric loss among all other sample for the same frequency range. Remarkable increase in Curie temperature ([Formula: see text]) by 50[Formula: see text]C is recorded in the present work by increasing 10% calcium content in SBN compostion. All higher doped samples indicate increase in [Formula: see text] by further higher amount. In addition, dc conductivity of all samples is measured for all samples and increase in calcium content is observed to generate more insulating compositions compared to undoped SBN with higher intrinsic activation energies.

Role of Reduced CeO2(110) Surface for CO2 Reduction to CO and Methanol

Density functional theory (DFT) calculations were performed to study the mechanism of carbon dioxide (CO2) reduction to carbon monoxide (CO) and methanol (CH3OH) on CeO2(110) surface. CO2 dissociates to CO on interacting with the oxygen vacancy on reduced ceria surface. The oxygen atom heals the vacancy site and regenerates the stoichiometric surface via a redox mechanism with intrinsic activation and reaction energies of 259.2 and 238.6 kJ/mol, respectively. Lateral interaction of oxygen vacancies were studied by the generation of two oxygen vacancies per unit of CeO2 surface. Compared to a single isolated vacancy, the activation and reaction energies of CO2 dissociation on a divacancy were approximately reduced to half of its value. Hydrogen atom coadsorbed on the surface was observed to assist CO2 dissociation by forming a carboxyl intermediate, CO2+H → COOH (ΔEact = 39.0 kJ/mol, ΔH = −69.2 kJ/mol) which on subsequent dissociation produces CO via the redox mechanism. On hydrogenation, CO is likely to p...

Monomolecular Cracking Rates of Light Alkanes over Zeolites Determined by IR Operando Spectroscopy

The coverage of H-MFI zeolite acid sites by light alkanes (C3–C7) at monomolecular cracking reaction conditions was determined using infrared operando spectroscopy. Under such conditions, alkane adsorption through H-bonding leads to a fully reversible perturbation of the zeolite νOH band at 3600 cm–1. This was used to assess the coverage at various temperatures and pressures, allowing for the determination of the adsorption thermodynamic parameters at reaction conditions. The simultaneous determination of apparent monomolecular cracking rate constants allowed for the direct determination of the intrinsic cracking rate constants, activation energies, and activation entropies. These results show that while the coverage of the active sites increases with the alkane size, the differences tend to decrease at high temperature because of entropic effects. The intrinsic activation energy was constant for all alkanes investigated in this study (∼190 kJ mol–1), lying in the lower range of the values usually derived...

Effects of mass transfer on Fischer-Tropsch kinetics over mesoporous silica-supported Co[sbnd]Mn[sbnd]Ce nano catalysts in a fixed-bed reactor

An impregnated ternary mixed oxide (CoMnCe) on nano silica support was applied, as a novel Fischer-Tropsch catalyst, in a fixed-bed reactor. Using catalysts of various pellet sizes, the Thiele modulus was determined to quantify regions of pore diffusion resistance and surface reaction. Mechanistic studies were conducted via intrinsic kinetic experiments under no pore diffusion conditions (i.e. Thiele modulus<0.4) as follows: T = 483.15–583.15 k, P = 2–6 bar, H2/CO = 1–2, and GHSV = 4500 hr−1. Based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach, the optimum mechanism was found to be associative adsorption of CO and H2 with the irreversible associative hydrogen, with the carbon monoxide adsorption determined as the rate-controlling step. For the optimum model, intrinsic activation energy, R-squared value, and mean absolute relative residual were obtained as 62 kj/mol, 0.97 and 10.41%, respectively. Finally, a promising graphical approach was proposed to predict effective reaction rate, considering the effects of internal mass transfer in catalyst beads.

Acetone Hydrodeoxygenation over Bifunctional Metallic–Acidic Molybdenum Carbide Catalysts

Metallic–acidic bifunctionality of molybdenum carbidic catalytic formulations can be tuned by oxygen cofeed and reductive pretreatments to control carbonyl hydrodeoxygenation (HDO). Acetone is deoxygenated over activated Mo2C via sequential hydrogenation of acetone to equilibrium and subsequent dehydration of isopropyl alcohol (IPA). Dehydration to propylene occurs over Bronsted acid sites with an intrinsic activation energy of 103 ± 1 kJ mol–1 and a rate-determining step of β-hydrogen scission, as inferred from a kinetic isotope effect (KIE) of 1.85. HDO rate-determining dehydration rates were kinetically independent of H2 and oxygenate pressure from 10 to 82 kPa and from 0.05 to 4 kPa, respectively. Both the kinetics and the deoxygenation reaction pathway were shown to be similar for the aldehyde group of propanal. Oxygen treatment of activated carbides via 13.5 kPa O2 cofeed was shown to decrease the catalyst surface area from 68 to 9 m2 g–1 and to suppress metallic hydrogenation of acetone to IPA. IPA...

Activity and kinetics of ruthenium supported catalysts for sodium borohydride hydrolysis to hydrogen

RuO2 with oxygen vacancies in Ru–RuO2/C determines the sodium borohydride hydrolysis activity. Without the mass transfer limitation, intrinsic activation energy of Ru–RuO2/C is lower than that from Ru/Ni foam without oxygen vacancies.