Compensation effect in heterogeneous non-isothermal kinetics

Zsakó, J.

doi:10.1007/bf01980913

Cited by 24 publications

(3 citation statements)

References 46 publications

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“…Examples of this association have been reported for many diverse reaction systems under kinetic control, including TGA carbon pyrolysis and combustion, − but hitherto it is not understood if this phenomenon is mechanistically important or just statistically significant. As such, it has even been implied that the behavior might arise from the TGA methodology and be an artifact of the data processing. − Figure shows that a similar compensation effect can occur during the regime II combustion of different particle sizes of the same coal. Mechanistically this could be from mass transfer restrictions or surface area availability, although an underlying autocorrelation cannot be fully dismissed .…”

Section: Resultsmentioning

confidence: 95%

Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

et al. 2009

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Although thermogravimetric analysis (TGA) is a widely accepted technique for assessing coal combustion, conflicting trends have often been reported when relating apparent TGA reactivities to pulverized fuel (PF) burner conditions. Therefore, this paper compares the reactivity of chars generated in a drop tube furnace (DTF) to those from TGA. The implications of devolatilization temperature, heating rate and residence time are considered. For the smaller particle size ranges of the bituminous coal investigated (ATC), optimized devolatilization procedures were used to generate corresponding TGA burnout rates between the two char types. However, with fractions of >75 μm, the DTF chars showed an increased burnout propensity when moving from combustion regime II to combustion regime III. Scanning electron microscope (SEM) images and internal surface areas indicate that this is because of incompatible char morphologies. Thus, while chars produced under the conditions of TGA pyrolysis strongly resemble raw coal and display an undeveloped pore network; the DTF chars are highly porous, extensively swollen and possess considerably larger internal surface areas. Subsequently, char burnout variability was quantified, with the reactivity distribution for the DTF samples found to be up to an order of magnitude more significant than for the TGA chars. This is attributed to a fluctuating devolatilization environment on the DTF. Finally, a TGA study observed a robust particle size based compensation effect for the TGA chars, with the relative reaction rates and activation energies demonstrating the presence of internal diffusion control. However this phenomenon was partly alleviated for the DTF chars, since their higher porosities reduce mass transfer restrictions. Moreover, it should be realized that DTF char fractions of <38 μm, including those required to ensure true intrinsic control under the investigated burnout conditions, cannot be produced directly. This is because of bridging and sloughing in the DTF's screw-feeder. Instead, such samples must be created by grinding larger particles, which destroys the char's existing porosity.

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Section: Resultsmentioning

confidence: 95%

Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

et al. 2009

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“…Several theories and explanations accounting for such compensation behavior have also been put forward [9][10][11][12]. Evidence of such a compensation effect has since been reported in many publications [13][14][15][16][17], even though some of these reports have subsequently been disputed [18][19][20][21].…”

Section: Introductionmentioning

confidence: 99%

Kinetics of Nonisothermal Degradation of Some Polymer Composites: Change of Entropy at the Formation of the Activated Complex from the Reagents

Turmanova

Genieva

Vlaev

2011

Journal of Thermodynamics

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Studying the nonisothermal kinetics of degradation of rice husks in air or nitrogen atmosphere, polypropylene and tetrafluoroethylene-ethylene copolymer filled with different quantities of rice husks flour or the products of its thermal degradation, namely "white" or "black" rice husks ash, a linear dependence was observed between the ln A and E A , known as the kinetic compensation effect or theta rule. A linear relationship was also established between E A and the change of the entropy ΔS / = for the formation of the activated complex from the reagents. These dependences are related to the assumption of identical kinetic mechanisms of thermal degradation of the composites studied. The negative values of ΔS / = obtained show that the activated complex is a "more organized" structure than the initial reactants and that these reactions may be classified as "slow" ones. It may be concluded that the products of the thermal degradation of rice husks in a fluidized bed reactor can successfully replace the more expensive synthetic fillers to obtain different polymer composites. These polymer composites can lead to the futuristic "organic-inorganic hybrid materials" with specific properties.

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“…The linear relationship of ln A versus E a , shown in Figure B, clearly demonstrates the compensation effect in our catalytic system. Although the compensation effect is very common in heterogeneous catalysis and a large number of homogeneous systems, − the mechanism is still not completely understood. According to the switching theory proposed by Bligaard and co-workers, we believe this effect should be related to a switching in kinetics, from a regime where the overall rate is dominated by the rate of activation of the reactant (i.e., p -nitrophenolate) to a regime where the stability of the reaction product (i.e., p -aminophenol) adsorbed on the surface of the catalyst becomes more important.…”

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confidence: 99%

A Comparison Study of the Catalytic Properties of Au-Based Nanocages, Nanoboxes, and Nanoparticles

et al. 2009

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We have evaluated the catalytic properties of Au-based nanostructures (including nanocages, nanoboxes, and solid nanoparticles) using a model reaction based on the reduction of p-nitrophenol by NaBH(4). From the average reaction rate constants at three different temperatures, we determined the activation energy, the entropy of activation, and the pre-exponential factor for each type of Au nanostructure. The kinetic data indicate that the Au-based nanocages are catalytically more active than both the nanoboxes and nanoparticles probably due to their extremely thin but electrically continuous walls, the high content of Au, and the accessibility of both inner and outer surfaces through the pores in the walls. In addition, a compensation effect was observed in this Au-based catalytic system, which can be primarily interpreted by a model based on kinetic regime switching.

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Compensation effect in heterogeneous non-isothermal kinetics

Cited by 24 publications

References 46 publications

Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

Kinetics of Nonisothermal Degradation of Some Polymer Composites: Change of Entropy at the Formation of the Activated Complex from the Reagents

A Comparison Study of the Catalytic Properties of Au-Based Nanocages, Nanoboxes, and Nanoparticles

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