Some tentative explanations for the enthalpy–entropy compensation effect in chemical kinetics: from experimental errors to the Hinshelwood-like model

Compensation Effect Exhibited by Gold Bimetallic Nanoparticles during CO Oxidation

While CO oxidation catalyzed by gold nanoparticles has been practiced academically for several decades, there are still important discoveries to be made. One area of current interest is to pair Au with another alloying metal and observe the catalytic consequences of the presence of the other metal. In this work, TiO₂-supported bimetallic Au nanoparticles are alloyed with Cu, Co, Ni, Pd, and Ru and used as catalysts for CO oxidation. Two synthetic methods for the alloys are presented: a strong electrostatic adsorption (SEA) method and a sterically demanding ligand synthesis (SDLS) method which uses triphenylphosphine (TPP) as the ligand. The catalytic performance of the materials synthesized with the SEA and SDLS methods is compared in CO oxidation. The results indicate that the materials tested present an enthalpy-entropy compensation effect. Interestingly, both the enthalpy of activation, ΔH ^‡, and the entropy of activation, ΔS ^‡, generally decrease with particle size. AuCo and AuRu materials exhibit a decrease in the overall activity as compared to Au and the other Au alloys when synthesized via SEA. Au face-centered-cubic alloys AuCu, AuNi, and AuPd prepared via SEA show an improvement in activity compared to monometallic Au in our reaction conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy presents two distinct regions for Au bimetallics where AuCo and AuRu show peak positions in the region of 2070-2050 cm^-1, indicating a weaker interaction for AuCo and AuRu with CO when compared to that of the other alloys. For the SDLS method samples, the hypothesis is that TPP would enhance the CO oxidation rate by enhancing the charge transfer to the metallic surface. The results indicate that SDLS samples have lower CO oxidation rates and if any charge transfer occurs, it is masked by the lateral interactions of the CO π bonds and the phenyl groups of TPP.

Kinetic compensation effect of isoconversional methods

For experimental data obtained under different reaction/process conditions over time or temperature, the kinetic compensation effect (KCE) can be expected. Under dynamic (nonisothermal) conditions, at least two analytical pathways forming the KCE were found. Constant heating rate (q = const) and variable conversion degrees (α = var) lead to a vertical source of the KCE, called the isochronal effect. In turn, for a variable heating rate (q = var) and constant conversion degree (α = const), we can obtain an isoconversional compensation effect. In isothermal conditions (analyzed as polyisothermal), the KCE appears only as an isoconversional source of the compensating effect. The scattering of values for the determined isokinetic temperatures is evidence of a strong influence of the experimental conditions and the calculation methodology. The parameters of the Arrhenius law have been shown to allow the determination of the KCE and further the isokinetic temperature. In turn, using the Eyring equations for the same parameters, we can determine the enthalpy–entropy compensation (EEC) for the activation process and the compensation temperature, which is often treated as an isokinetic temperature. KCE effects have also been shown to be able to be amplified or dissipated, but isokinetic temperature is not a compensating quantity in the literal sense in isoconversional methods because $${T}_{iso}\to \infty .$$ T iso → ∞ . Thus, in isoconversional methods, isoconversional KCE values are characterized by strong variability of activation energy corresponding to the weak variation of the pre-exponential factor, which in practice means that $${\text{ln}}\mathit {A}\to {\text{const}}.$$ ln A → const . This is completely in line with the classical Arrhenius law.

An efficient and eco-friendly method for the thiol-Michael addition in aqueous solutions using amino acid ionic liquids (AAILs) as organocatalysts

A series of amino-acid based ionic liquids (Bmim[AA]s) have been synthesized and evaluated as catalysts, in aqueous solution. The results of a kinetic study of the thiol-Michael reaction of L-Cysteine with trans-β-nitrostyrene demonstrated the advantages of using (Bmim[AA]s) as organocatalysts. The benefits include high rate constants; mild reaction conditions; and, a reusable catalyst, which leads to a simple and efficient method for these important kinds of reactions.

Kinetic compensation effect in logistic distributed activation energy model for lignocellulosic biomass pyrolysis

The kinetic compensation effect in the logistic distributed activation energy model (DAEM) for lignocellulosic biomass pyrolysis was investigated. The sum of square error (SSE) surface tool was used to analyze two theoretically simulated logistic DAEM processes for cellulose and xylan pyrolysis. The logistic DAEM coupled with the pattern search method for parameter estimation was used to analyze the experimental data of cellulose pyrolysis. The results showed that many parameter sets of the logistic DAEM could fit the data at different heating rates very well for both simulated and experimental processes, and a perfect linear relationship between the logarithm of the frequency factor and the mean value of the activation energy distribution was found. The parameters of the logistic DAEM can be estimated by coupling the optimization method and isoconversional kinetic methods. The results would be helpful for chemical kinetic analysis using DAEM.

Radical Cage Effects: The Prediction of Radical Cage Pair Recombination Efficiencies Using Microviscosity Across a Range of Solvent Types

This study reports a method for correlating the radical recombination efficiencies (F_cP) of geminate radical cage pairs to the properties of the solvent. Although bulk viscosity (macroviscosity) is typically used to predict or interpret radical recombination efficiencies, the work reported here shows that microviscosity is a much better parameter. The use of microviscosity is valid over a range of different solvent system types, including nonpolar, aromatic, polar, and hydrogen bonding solvents. In addition, the relationship of F_cP to microviscosity holds for solvent systems containing mixtures of these solvent types. The microviscosities of the solvent systems were straightforwardly determined by measuring the diffusion coefficient of an appropriate probe by NMR DOSY spectroscopy. By using solvent mixtures, selective solvation was shown to not affect the correlation between F_cP and microviscosity. In addition, neither solvent polarity nor radical rotation affects the correlation between F_cP and the microviscosity.

Compensation effects and relation between the activation energy of spin transition and the hysteresis loop width for an iron(ii) complex

Wide thermal hysteresis loops for iron(ii) spin crossover complexes are associated with high activation barriers: the higher the activation barrier, the wider the hysteresis loop for a series of related spin crossover systems.

Enthalpy-Entropy Compensation (EEC) Effect: A Revisit

A short account of the developments and perspectives of IKR (iso-kinetic relation) and EEC (enthalpy (H) - entropy (S) compensation) has been presented. The IKR and EEC are known to be extra thermodynamic or empirical correlations though linear H-S correlation can be thermodynamically deduced. Attempt has also been made to explain the phenomena in terms of statistical thermodynamics. In this study, we have briefly revisited the fundamentals of both IKR and EEC from kinetic and thermodynamic grounds. A detailed revisit of the EEC phenomenon on varied kinetic and equilibrium processes has been also presented. Possible correlations among the free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) changes of different similar and nonsimilar chemical processes under varied conditions have been discussed with possible future projections.

Evaluation of the three-phase equilibrium method for measuring temperature dependence of internally consistent partition coefficients (K(OW), K(OA), and K(AW)) for volatile methylsiloxanes and trimethylsilanol

Partitioning equilibria and their temperature dependence of chemicals between different environmental media are important in determining the fate, transport, and distribution of contaminants. Unfortunately, internally consistent air/water (K(AW)), 1-octanol/air (K(OA)), and 1-octanol/water (K(OW)) partition coefficients, as well as information on their temperature dependence, are scarce for organosilicon compounds because of the reactivity of these compounds in water and octanol and their extreme partition coefficients. A newly published 3-phase equilibrium method was evaluated for simultaneous determination of the temperature dependence of (K(OW), K(OA), and K(AW)) of 5 volatile methylsiloxanes (VMS) and trimethylsilanol (TMS) in a temperature range from 4 °C to 35 °C. The measured partition coefficients at the different temperatures for any given compound, and the enthalpy and entropy changes for the corresponding partition processes, were all internally consistent, suggesting that the 3-phase equilibrium method is suitable for this type of measurement. Compared with common environmental contaminants reported in the literature, VMS have enthalpy and entropy relationships similar to those of alkanes for air/water partitioning and similar to those of polyfluorinated compounds for octanol/air partitioning, but more like those for benzoates and phenolic compounds for octanol/water partitioning. The temperature dependence of the partition coefficients of TMS is different from those of VMS and is more like that of alcohols, phenols, and sulfonamides.

Critical review and interpretation of environmental data for volatile methylsiloxanes: partition properties

Volatile methylsiloxanes (VMS) enter the environment through industrial activities and the use of various consumer products. Reliable measurements of environmental partition properties for these compounds are critical for accurate prediction of their environmental fate, distribution, transport, exposure and potential effects. In this study, the measured partition properties including air/water (K(AW)), octanol/water (K(OW)), and octanol/air partitioning coefficients (K(OA)), soil organic carbon/water distribution coefficient (K(OC)), and biological medium/fluid partition coefficients, and their temperature dependence were critically reviewed. Based on these results, organosilicon compounds such as methylsiloxanes are expected to behave differently in the environment compared to conventional hydrophobic environmental contaminants, as a result of their inherent characteristics related to molecular size and capacity for different types of molecular interactions that control partitioning. The differences are critical and need to be taken into consideration in environmental exposure and risk analyses of these compounds.