Model Of Selective Energy Transfer Research Articles

An analysis of ammonia synthesis by the model of Selective Energy Transfer (SET)

The SET theory implies that energy is transferred from the catalyst system via infrared radiation to the molecules that are supposed to react. In previous investigations it has been demonstrated that the activation of the reacting species-as long as the molecules are infrared active-can occur at low adsorption strength. However, for molecules that are IR inactive, e.g. dinitrogen, this is not possible.

The effects of external physical fields on the isothermal kinetics of fullerol formation

The isothermal kinetics of fullerol formation under different heating modes: conventional (CH), ultrasonic (USH) and microwave (MWH) was investigated. The isothermal kinetic curves of fullerol formation at different temperatures (293, 298, 303, 308 and 313 K) were measured. The kinetic model of the reaction was determined on the basis of the shape of the dependencies of reaction rate on fullerene concentration and on method of initial rates. The values of isothermal rate constants under different heating conditions and temperatures and their kinetic parameters were calculated. The effect of heating mode on the values of the kinetic parameters was established. The possibility of an existence of overheating and hot spots on the rate of fullerol formation was evaluated and discussed. The model of impact of USH and MWH on the reaction kinetics that is based on the model of selective energy transfer is suggested. The established effects of USH and MWH on the reaction of fullerol formation are successfully explained by that model.

A novel approach to the explanation the effect of microwave heating on isothermal kinetic of crosslinking polymerization of acrylic acid

The effect of microwave heating (MWH) on the isothermal kinetic of crosslinking-polymerization of acrylic acid (CPAA) was investigated. The kinetic curves of CPAA were determined in the temperature range from 303 to 328 K. By applying the model-fitting method it was revealed that the isothermal kinetics of CPAA was described by the first order chemical reaction kinetics model under the MWH and by the second order chemical reaction rate model for the conventionally heated (CH) process. The values of the reaction rate constants of CPAA are about 40 times higher for the microwave heated system than for the conventional heating. The kinetic parameters (activation energy (E a) and pre-exponential factor (lnA)) of the CPAA are significantly lower than the corresponding values for CH process. It was found that the increase in the reaction rate of CPAA for MWH was not a consequence of overheating neither the hot spots in the reaction system. Based on model of selective energy transfer between the reacting molecules and the heating bath a novel explanation of the established effects of MWH on the kinetics of CPAA is given. A quantized nature and value of activation energy was confirmed. The decrease in the value of activation energy of CPAA under microwave heating is explained by the increased value of energy of ground vibration level of resonant oscillator in the AA molecule (v = 1417 cm−1) caused by the absorption of non-thermal energy of MW field.

Concluding remarks on the theory of selective energy transfer and exemplification on a zeolite kinetics study

Some 30 years ago it was suggested that catalytic reactions that showed a 'compensation' effect were activated through a vibrational resonance between that one vibrational mode of the reactant that led to reaction and a comparable vibration mode of the catalyst system that represented the energy input. The isokinetic effect can be calculated from this model, in some cases within less than 1 % from the experimental values. The application of the selective energy transfer model to various kinetic investigations provides some rules: (1) The resonance interaction can occur between the quantum of the catalyst vibration and the quantum of the reactant vibration in a ratio of small integers. (2) The activation energy is built up by a certain number of vibrational quanta relating to the critical vibration. (3) The isokinetic temperature can be expressed as a function of the wavenumber of the catalyst vibrator and the wavenumber of the reactant vibrator. (4) The isokinetic data can be used to establish reaction mechanisms. (5) One can also extract the anharmonicity constant of the critical vibration from the data. One observation is that the reacting molecule is not markedly adsorbed, whereas, of course, the remaining molecules may be strongly adsorbed. (Less)

Isokinetic behavior in the gas phase hydrogenation of nitroarenes over Au/TiO2: application of the selective energy transfer model

The gas phase selective hydrogenation of a series of nitroarenes (nitrobenzene, p-chloronitrobenzene, p-bromonitrobenzene, p-nitroaniline, p-nitrotoluene, p-nitrophenol and p-nitroanisole) has been examined over Au/TiO2 (0.3 % w/w Au, mean Au particle size = 3.9 nm). Compensation behavior is demonstrated with an associated isokinetic temperature (Tiso) of 558 ± 32 K. We account for this response in terms of the selective energy transfer (SET) model where the occurrence of resonance between catalyst and reactant vibrations generates the activated complex. An analysis of the stepwise variation of the activation energies has identified a critical vibrational frequency of 853 cm−1, which is close (±2 cm−1) to the reference value for nitro-group (in-plane symmetric O–N–O bending and stretching) vibrations. Application of SET suggests activation of weakly adsorbed nitroarene (at the support or metal/support interface) by excitation of the nitro-group via IR radiation from a strongly adsorbed surface nitroarene component. The excited nitroarene is then attacked by reactive hydrogen supplied by the Au sites to generate the respective aromatic amine with 100 % selectivity. Agreement of the SET predicted Tiso with the experimental value requires the incorporation of a term due to C–N torsional entropy resulting from distortion of the O–N–O plane.

A comparative kinetics study on the isothermal heterogeneous acid-catalyzed hydrolysis of sucrose under conventional and microwave heating

The isothermal kinetics of sucrose hydrolysis at the acidic ion-exchange resin type IR-120 H under conventional (CH) and microwave heating (MWH) was investigated. Isothermal kinetics curves in the temperature range from 303 to 343K for both CH and MWH were determined. By application the model-fitting method, it was recognized that the kinetics of sucrose hydrolysis can be described by a first-order chemical reaction for both heating modes. The values of the activation energy (Ea) and pre-exponential factor (lnA) for sucrose hydrolysis were found to be lower under MWH than under CH. Application of the differential isoconversional method showed that sucrose hydrolysis was kinetically an elementary reaction. It is found that the increased rate of hydrolysis observed under MWH was not a consequence of overheating. A new explanation of the established effects of microwave heating based on a model of selective energy transfer during the chemical reaction is suggested. The established decreases in the activation energy and in the pre-exponential factor under MWH in comparison to CH is explained by an increase in the energy of the ground vibrational level of the –OH out-of-plane deformation in the sucrose molecule and with a decrease in the anharmonicity factor, which is caused by the selective resonant transfer of energy from the catalyst to the –OH oscillators in the sucrose molecules.

A study of the kinetics of isothermal nicotine desorption from silicon dioxide

The isothermal kinetics of nicotine desorption from silicon dioxide (SiO 2) was investigated. The isothermal thermogravimetric curves of nicotine at temperatures of 115 °C, 130 °C and 152 °C were recorded. The kinetic parameters ( E a, ln A) of desorption of nicotine were calculated using various methods (stationary point, model constants and differential isoconversion method). By applying the “model-fitting” method, it was found that the kinetic model of nicotine desorption from silicon dioxide was a phase boundary controlled reaction (contracting volume). The values of the kinetic parameters, E a,α and ln A α, complexly change with changing degree of desorption and a compensation effect exists. A new mechanism of activation for the desorption of the absorbed molecules of nicotine was suggested in agreement with model of selective energy transfer.

Application of the Selective Energy Transfer Model to Account for an Isokinetic Response in the Gas Phase Reductive Cleavage of Hydroxyl, Carbonyl and Carboxyl Groups from Benzene Over Nickel/Silica

The gas phase hydrodeoxygenation of a series of aromatic alcohols, aldehydes and acids has been examined over Ni/SiO2. Compensation behaviour is established with an isokinetic temperature (518 ± 21 K) that is consistent with with the point of intersection of the Arrhenius lines. This is accounted for using the Selective Energy Transfer model that is based on resonance between the catalytic Ni–H vibration and out-of plane C–H vibrations of the aromatic reactants with a transferral of resonance energy from the catalyst to generate the “activated complex”. The calculated wave number of this vibration mode is 720 ± 29 cm−1 with an associated anhamonicity of −3.3 ± 0.9 cm−1. Our analysis suggests that the oxygenated aromatic is weakly adsorbed on the catalyst and surface mobility facilitates reaction with adsorbed hydrogen atoms.

Compensation Effect of Benzene Hydrogenation on Pt(111) and Pt(100) Analyzed by the Selective Energy Transfer Model

Kinetic measurements at low temperatures (310-360 K) using gas chromatography (GC) for benzene hydrogenation on Pt(100) and Pt(111) single crystal surfaces have been carried out at Torr pressures. These kinetic measurements demonstrated a linear compensation effect for the production of cyclohexane. A detailed application of the model of selective energy transfer to the experimentally obtained results yields the vibrational frequency of the adsorbate leading to reaction. This frequency is attributed to ring distortion modes. The vibrational frequency of the heat bath, or catalyst, is ascribed to a Pt-H mode. An approximate heat of adsorption of the reacting molecule is also calculated from the model.

On the stepwise change of activation energies in the hydrodechlorination of chlorobenzene over supported nickel

Kinetics of chlorobenzene hydrodechlorination have been measured over Ni on SiO2, Al2O3, MgO, activated carbon and graphite. A stepwise variation of E-a is analysed using the selective energy transfer model where E-a is identified as the vibrational energy associated with an excitation of the chlorobenzene out-of-plane C-H bending mode. Variation of E-a with vibrational quantum number yields a vibrational frequency of 749 cm(-1) and a value (-1.1 cm(-1)) for the anharmonicity term, which is characteristic of bending vibrational modes. Our analysis suggests that the reacting species are weakly adsorbed on the catalyst: heat of adsorption = -0.31 kJ mol(-1).

The gas phase reaction between CO2 and lanthanide atoms. A test of a model for the isokinetic effect

Using literature data on the gas phase reaction between free lanthanide atoms and carbon dioxide, we have found an isokinetic relation for seven of the twelve investigated elements with an isokinetic temperature, Tiso, of 2500±330 K. This value is analyzed in the framework of the model of selective energy transfer, SET, indicating that metal-CO2 complexes are formed as pre-reaction states. The result is in accordance with the fundamental concepts of SET.

Compensation effect in trichlorosilane synthesis

In a recent publication [J. Acker, K. Bohmhammel, J. Phys. Chem. B 106 (2002) 5105], the reactions between transition metal silicides and hydrogen chloride were studied by isothermal calorimetric measurements. The obtained apparent activation energies and pre-exponential factors show clearly a linear dependence that is attributed to the compensation effect. An isokinetic temperature of 696.9±22.1 K was determined. According to Larsson's model of selective energy transfer, a characteristic frequency of 969.3±46.5 cm −1 is calculated. The occurrence of the compensation effect is discussed in terms of chemisorption, precursor formation, and the involvement of surface species in essential reaction steps.

Hydrogenolysis of ethane on silica-supported cobalt catalysts

The kinetics of ethane hydrogenolysis over cobalt catalysts supported on silica has been investigated. The results compare well with previous data from Sinfelt et al., Haddad and Goodwin, and Babernics et al. The data were found to be characterized by an isokinetic temperature, T iso=490±40 K and it was therefore possible to apply the model of selective energy transfer (SET). This approach indicates that either there is an energy transfer from the catalyst by full resonance to a vibration mode of the reactant with ν=680 cm −1 corresponding to a metal (M)CH 2 bond, or there is an energy transfer to an M–CH 3 vibration mode in the region of 400 cm −1, most likely at 355 cm −1. In any case, these interpretations indicate that it is the breaking of one or two metal–carbon bonds that determine the reaction, not the cleavage of the carbon–carbon bond of ethane. This is in agreement with recent views of Sinfelt.

Catalysis as a resonance effect, V. Factors that determine the isokinetic temperature in catalytic reactions

When describing isokinetic effects by the model of selective energy transfer by resonance, it is sometimes found that the frequency of the forcing oscillator is quite different to that of the reacting molecule,i.e.‘resonance’ is not a very good concept to use. By noting that the two frequencies thus found are related to each other as the ratios of small integers,e.g. 4∶3, this dilemma is resolved. The investigation covers formic acid decomposition, homogeneous ligand displacement reactions of Ni(DMSO)62+ and the dissociation of CO2 on metal surfaces.

On the isokinetic effect of ligand substitution reactions of Ni(DMSO) 62+ in DMSO solution

A model of selective energy transfer, originally developed for catalysis, is applied to explain the isokinetic temperature found for some substitution (and exchange) reactions of the hexasolvated complex of Ni(II) in dimethyl sulfoxide solutions. An analysis of the stepwise change of activation energies for these and similar reactions indicates that the threefold degenerate Ni-O stretching vibration is involved in the activation process. The reaction coordinate can be described as an expansion of one face of the octahedron, where the reactant enters, and the corresponding contraction of the opposite face.

Isokinetic effects in ethane hydrogenolysis and their relation to the mechanism of the reaction

Literature data on the kinetics of ethane hydrogenolysis over metal catalysts are used to indicate the existence of isokinetic temperatures, Θ. For platinum Θ = 725 K, for Ni Θ is probably in the same temperature range. For Fe and Co on the other hand Θ is found at much lower values, about 330 K for Fe and 320 K for Co. From an analysis of the model of selective energy transfer, these values support the proposition of Sinfelt that the elements of high atomic number in one and the same transition metal series favour the splitting of the carbon-carbon bond as rate determining step, whereas those elements that have lower atomic numbers cause the splitting of the metal-carbon bond to be the rate determining step.

A model of selective energy transfer at the active site of the catalyst

A model is described of catalyst-reactant interaction. It implies that one role of the catalyst is to supply energy into that vibrational mode of the reactant that most effectively takes the system to the activated state. In this way the dissipation of energy from the excited molecule is effectively counteracted. By treating the system as the coupled, damped oscillator of classical mechanics, an expression is derived for the isocatalytic temperature (describing the ‘compensation effect’) in terms of the vibrational frequencies of the reactant and the catalyst system. Examples are given of the application of this model to studies on heterogeneous as well as homogeneous systems.