Luis A. Pérez-Maqueda

Kinetic Analysis of Complex Solid-State Reactions. A New Deconvolution Procedure

The kinetic analysis of complex solid-state reactions that involve simultaneous overlapping processes is challenging. A method that involves the deconvolution of the individual processes from the overall differential kinetic curves obtained under linear heating rate conditions, followed by the kinetic analysis of the discrete processes using combined kinetic analysis, is proposed. Different conventional mathematical fitting functions have been tested for deconvolution, paying special attention to the shape analysis of the kinetic curves. It has been shown that many conventional mathematical curves such as the Gaussian and Lorentzian ones fit kinetic curves inaccurately and the subsequent kinetic analysis yields incorrect kinetic parameters. Alternatively, other fitting functions such as the Fraser-Suzuki one properly fit the kinetic curves independently of the kinetic model followed by the reaction and their kinetic parameters, and moreover, the subsequent kinetic analysis yields the correct kinetic parameters. The method has been tested with the kinetic analysis of complex processes, both simulated and experimental.

Enhancement of Fast CO2 Capture by a Nano-SiO2/CaO Composite at Ca-Looping Conditions

In this paper we show the performance of a new CO(2) sorbent consisting of a dry physical mixture of a Ca-based sorbent and a SiO(2) nanostructured powder. Thermo-gravimetric analysis (TGA) performed at conditions close to the Ca-looping process demonstrate that the rate of CO(2) capture by the mixture is enhanced during the fast carbonation stage of practical interest in applications. Moreover, the residual capture capacity of the mixture is increased. SEM/EDX, physisorption, and XRD analyses indicate that there is a relevant interaction between the nanostructured SiO(2) skeleton and CaO at high temperatures, which serves to improve the efficiency of the transfer of CO(2) to small reactive pores as well as the stability of the sorbent pore structure.

A unified theory for the kinetic analysis of solid state reactions under any thermal pathway

The Ozawa concept of generalized time has been used for developing master plots for the different kinetic models describing solid state reactions. These plots can be indistinctly used for analysing isothermal or non-isothermal experimental data. It is demonstrated that it is not possible to discriminate the kinetic model from a single non-isothermal curve without a previous knowledge of the activation energy. However, it has been shown that the ln [(da/dt)/f(a)] data taken from a set of DTG curves obtained at different heating rates lie on a single straight line when represented as a function of 1/T only if the kinetic model really obeyed by the reaction is considered. Moreover, the true values of E and A are obtained from the slope and the intercept of this straight line.

Talc from Puebla de Lillo, Spain. II. Effect of dry grinding on particle size and shape

Abstract The effect of dry grinding on the structure, particle size and shape of talc from Puebla de Lillo (Leon, Spain), was studied by means of nitrogen adsorption, XRD, DTA-TG and by SEM-EDS methods. Grinding of talc produces an increase of the starting BET surface area value progressively up to a maximum of 110 M 2 /g at 30 min. A subsequent decrease of surface area with increasing grinding time was observed. The ground, activated particles with high surface energy start a reaggregation process recovering the original surface area value at 210 min. The values of coherent scattering thickness (crystallite size) indicated a reduction of thickness due to crystalline degradation along [00 l ], with initial delamination accompanied by a lesser degree of crystal structure breakdown and amorphization of the structure. The latter increases with increasing grinding time. A profound structural alteration, with progressive smearing of 00 l basal reflections, have been observed. Structural changes have been correlated with particles size, shape and coefficient of texture. The tendency of particles to preferred orientate decreases with grinding. A breakdown of the original particles due to mechanochemical effect produced more rounded particles, aggregates and agglomerates, as proved by SEM. With prolonged grinding, the rate of size reduction decreases with time, but reduction continues up to about 30 min. At this stage, particle size was reduced to about 0.02 μm in diameter and 0.009 μm in thickness, with aspect ratio 2.6, producing an increased degree of amorphism and a decrease in the surface area by reaggregation of the ground powder. After 30 min, the mechanical reduction of the original particles appears to have a limit. The thermal behavior is strongly dependent on the structural state of the material. Grinding produces a slight shift in the main endothermal DTA peak associated to talc dehydroxylation, as indicated also by the TG and DTG curves. An increase of weight loss on heating at lower temperatures than those observed in unground talc have also been detected. The exothermic DTA effect appears on grinding of talc. It has been detected from 10 min grinding time. This effect increases markedly in intensity during grinding, especially when structural breakdown is very important and the amorphous reactive solid is produced.

The Accuracy of Senum and Yang's Approximations to the Arrhenius Integral

The accuracy of the integral of the Arrhenius equation, as determined from the 1st to the 4th degree rational approximation proposed by Senum and Yang, has been calculated. The precision of the 5th to 8th rational approximations, here proposed for the first time, has also been analyzed. It has been concluded that the accuracy increases by increasing the order of the rational approximation. It has been shown that these approximations to the Arrhenius equation integral would allow an accuracy better than 10−8 % in the E/RT range generally observed for solid state reactions. Moreover, it has been demonstrated that errors closed to 10−2 % can be obtained even for E/RT=1, provided that high enough degrees of rational approximation have been used. Thus, it would be reasonable to assume that high degree rational approximations for the Arrhenius integral could be used for the kinetic analysis of processes, like adsorption or desorption of gases on solid surfaces, which can take place at low temperatures with very low values of E/RT.

Combined kinetic analysis for crystallization kinetics of non-crystalline solids

A new method of kinetic analysis that allows the combined analysis of data obtained under different experimental conditions, i.e. heating pathway, is presented for the crystallization of non-crystalline solids. The method consists of a two-stage procedure. Firstly, an optimization procedure is applied for the determination of the Johnson, Mehl and Avrami (JMA). This optimization procedure is based on the linearization of the rate equation throw a logarithmic transformation; where the correlation coefficient function is the objective function, and the JMA order is the adjustable parameter. The JMA order obtained from this optimization procedure allows one to determine in a second stage the other kinetic parameters, i.e. activation energy and pre-exponential Arrhenius factor. The method proposed here allows one to analyze in a combined way data obtained under any heating profile, because no assumptions are made on the heating profile for the derivation of the rate equation. The method is tested with computed and experimental curves obtained under different heating conditions. The crystallization kinetics of (GeS2)0.3(Sb2S3)0.7 glass are studied from the combined analysis of several curves.

Sonication as a tool for preparing nanometric vermiculite particles

We have studied the effect of ultrasound on two different natural macroscopic vermiculite samples from Ojen and Santa Olalla (Spain). We describe the conditions for the preparation of nanometric particles by sonochemistry. Sonication produces not only a delamination effect in the [00l] direction, but also a breaking of layers in the other directions, while the crystalline character is retained. Some differences have been observed in the behaviour of both vermiculites. Thus, the Santa Olalla sample suffers an important decrease in crystallite size with up to 30 h treatment time; longer treatment times do not produce any effect on crystallite size. On the other hand, the Ojen sample suffers a decrease in its crystallite size with up to 80 h treatment time, remaining unchanged for longer treatments. The crystallite sizes in the [00l] direction are much smaller for the Ojen sample than for the Santa Olalla sample. In addition, the Santa Olalla sample maintains the typical laminar shape after sonication, while the Ojen sample suffers a morphological change. The different behaviours seem to be related to the different compositions and, therefore, the different layer charges of both vermiculite samples.

Kinetic approach to partially overlapped thermal decomposition processes

Practical usefulness of the kinetic deconvolution for partially overlapped thermal decomposition processes of solids was examined by applying to the co-precipitated basic zinc carbonate and zinc carbonate. Comparing with the experimental deconvolutions by thermoanalytical techniques and mathematical deconvolutions using different statistical fitting functions, performance of the kinetic deconvolution based on an accumulative kinetic equation for the independent processes overlapped partially was evaluated in views of the peak deconvolution and kinetic evaluation. Two-independent kinetic processes of thermal decompositions of basic zinc carbonate and zinc carbonate were successfully deconvoluted by means of the thermoanalytical measurements in flowing CO2 and by applying sample controlled thermal analysis (SCTA). The deconvolutions by the mathematical curve fittings using different fitting functions and subsequent formal kinetic analysis provide acceptable values of the mass-loss fractions and apparent activation energies of the respective reaction processes, but the estimated kinetic model function changes depending on the fitting functions employed for the peak deconvolution. The mass-loss fractions and apparent kinetic parameters of the respective reaction processes can be optimized simultaneously by the kinetic deconvolution based on the kinetic equation through nonlinear least square analysis, where all the parameters indicated acceptable correspondences to those estimated through the experimental and mathematical deconvolutions. As long as the reaction processes overlapped are independent kinetically, the simple and rapid procedure of kinetic deconvolution is useful as a tool for characterizing the partially overlapped kinetic processes of the thermal decomposition of solids.

Particle size distribution in sonicated high- and low-charge vermiculites

Abstract High- and low-charge vermiculites from Spain (Santa Olalla and Ojen) were subjected to treatment in ultrasonic liquid processor in an attempt to reveal changes in particle size, morphology, and structure ordering. Sonication caused a rise of the S BET from the initial value below 1 m 2 /g for the raw samples to 36 and 54 m 2 /g at the final stage of processing of Santa Olalla and Ojen samples, respectively. These values, converted into equivalent spherical diameter (e.s.d.), prove that vermiculite from Ojen has finer particle size (0.042 μm) than the one from Santa Olalla (0.063 μm ). The low-charge vermiculite (Ojen) suffered an important delaminating effect. Some scrolling at the borders of particles was observed after first cycle of sonication. After eight cycles, the scrolling increased, yielding triangular-shaped particles that finally converted into tubes. The high-charge vermiculite (Santa Olalla) also showed delamination, but no scrolling was observed. The effect of sonication on the crystal structure of the high-charge vermiculite is negligible. The low-charge vermiculite shows broadening and smearing of the diffraction lines. It also shows wider spectrum of thickness of domains of coherent scattering, ranging from the initial value of 0.053 μm to the final one of 0.019 μm, as compared to 0.041 and 0.026 μm for the high-charge vermiculite. It was proved that, contrary to grinding, sonication produces particle division without important structural damages, and that this process is dependent on a layer charge of given vermiculite.

Generalized kinetic master plots for the thermal degradation of polymers following a random scission mechanism.

In this paper, the f(alpha) conversion functions for random scission mechanisms have been proposed to allow for the construction of generalized master plots suitable for these kinds of mechanisms. The master plots have been validated by their application to simulated data and to the thermal degradation of poly(butylene terephthalate), polyethylene, and poly(tetrafluoroethylene).