Chung-Kung Lee

Multifractal scaling analysis of reactions over fractal surfaces

Abstract Simulations of the Eley-Rideal diffusion-limited reaction mechanism and its modified versions over surfaces of different fractal objects having different fractal dimensions were performed using the Monte Carlo random walk algorithm. Effects on the reaction probability distribution (RPD) were examined by employing various sticking probability functions. Other effects also studied included cluster size effects and noise reduction. Multifractal analyses were then carried out on the reaction probability distribution to study the effects of those factors on model chemical reactions.

Multifractal characteristics in air pollutant concentration time series

One-year series of hourly average air pollutant concentration(APC) observations, including O3, CO, SO2 and NO2,were analyzed by means of statistical tools: histogram, variancemethod, and multifractal analysis, to examine the possible scale-invariant behavior and the clustering characteristics. It was found that all APC measurements exhibit the characteristicright-skewed unimodal frequency distribution and long term memory. A monodimensional fractal analysis was performed by thebox counting method. Scale invariance was found in these time series and the box dimension was shown to be a decreasing function of the threshold APC level, implying multifractal characteristics, i.e., the weak and intense regions scaledifferently. To test this hypothesis, the APC time series weretransferred into a useful compact form through the multifractalformalism, namely, the τ(q)-q and f(α)-α plots. The analysis confirmed the existence of multifractal characteristics in the investigated APC time series. It wasconcluded that the origin of both the pronounced right-skewnessand multifractal phenomena in APC time series may be interpretedin terms of the Ott (1995) proposed successive random dilution (SRD) theory and the dynamics of APC distribution process can bedescribed as a random multiplicative process.

Adsorption and diffusion of aromatics in AIPO4-5

The packing arrangements of selected hydrocarbons in AlPO4-5 molecular sieves were proposed. These molecules were of sizes comparable to the pore diameter of AlPO4-5. These packing arrangements at least qualitatively explained the results on vapor adsorption, liquid-phase counter diffusion, and binary liquid adsorption. Ortho-xylene behaved differently from its isomers in adsorption and diffusion processes. Its packing arrangement also reflected this fact.

Adsorption of aromatic compounds in large MFI zeolite crystals

Very large (180 × 40 × 40 µ3) and well defined crystals have been used in a gravimetric system to investigate the adsorption of aromatic compounds in MFI (silicalite) zeolite. The isotherms and isosteric heats of adsorption (Qst) were reported for future comparison with various adsorption models. Three types of phase transition were found. Isotherms of p-xylene showed a sharp step rise from ca. 4 to 8 molecules uc–1. A clear phase boundary could be outlined. Adsorbed benzene behaved as a dual-phase system in the range between 4.6 and 6 molecules uc–1. There were more phase transitions above 6 molecules uc–1 but it was not possible to outline the phase boundary. Molar entropy changes of 210 and 180 J mol–1 K–1 were found for the observed p-xylene–MFI and benzene–MFI phase transitions. For ethylbenzene and toluene, a dual-phase region was also observed but the transition was less pronounced. Isosteric heats, calculated from isotherms, showed a complex variation with loading and a strong dependence on temperature. It was also found that the adsorption kinetics are strongly influenced by the previous adsorption history. For freshly calcined samples the uptake rate was relatively fast. However, re-calcination after the adsorption of p-xylene created a diffusion barrier inside the crystalline material and subsequent adsorption of other aromatic compounds became much slower and displayed two-step kinetics.

Fractal analysis of temporal variation of air pollutant concentration by box counting

Abstract The scale-invariant behavior of air pollutant concentration (APC) time structure was investigated by applying the box counting method to APC time series. One-year series of hourly average APC observations, including O3, CO, SO2, NO, NO2, and PM10 which were obtained from urban, traffic, and national park air monitoring station at Taipei (Taiwan), were transferred into a useful compact form through this method, namely, the box-dimension (DB)-threshold (Th) and critical scale (CS)-threshold (Th) plots. The validity of this approach was supported with the result that the practical implications of DB-Th (or CS-Th) plots could be interpreted in terms of traditional statistical parameters. Since the dependences of both DB and CS on the Th values were closely related to the variation of APC in time, they were used to characterize the temporal distribution of APC. The analysis confirmed the existence of scale invariance in those investigated APC time series. Moreover, the DB (CS) was shown to be a decreasing (increasing) function of the threshold level, implying multifractal characteristics, i.e. the weak and intense regions scale differently. Some practical applications based on the box counting method were also discussed.

PORE CONNECTIVITY OF ALUMINA AND ALUMINIUM BORATE FROM NITROGEN ISOTHERMS

The effects of calcination temperature on the connectivity of pore networks of alumina and aluminium borate samples, prepared by coprecipitation methods and low-water sol–gel processes, have been investigated based on the percolation analysis of their nitrogen adsorption isotherms. The mean coordination number (Z) was determined with the aid of the recently proposed Seaton theory. Values of Z obtained with this method cover the range 2–3, implying a low connection of pores in all the examined porous samples. Thermal effects may increase or decrease Z values depending on the samples. On the other hand, Z values of aluminium borate samples obtained from the coprecipitation method were slightly larger than those from the sol–gel process. It was also found that for sol–gel processed samples, Z values of alumina were slightly larger than that of aluminium borate.

The fractal and percolation analysis of a polymeric Al2O3 gel

Abstract Fractal and percolation analysis have been performed on nitrogen adsorption isotherms of polymeric Al 2 O 3 xerogel, prepared from a low-water non-aqueous solution of aluminum alkoxide. These xerogels showed a distinctively low pore connectivity compared to that reported in the literature on more conventional alumina gels. The alumina xerogel exhibited a surface fractal behavior in the scale range from nitrogen size to a few nanometers. The mean pore coordination number and the surface fractal dimension of the xerogel were rather stable upon calcination, until the appearance of the α-alumina crystalline phase, despite of a large decrease in BET surface area and pore volume.

Application of Hydrothermal Method Derived Titanate Nanotubes as Adsorbents

Titanate nanotubes (TNT) derived from alkaline hydrothermal method are characterized by high specific surface area, specific pore volume, and ion-exchange capacity. They may be a promising and important adsorbent in the environmental protection. Although their applications in the fields of lithium ion batteries, dye-sensitized solar cell, photocatalysis, catalysts support, gas and humidity sensors, and ion exchange have been intensely studied during recent years, however, the researches concerning their potential application as an adsorbent are seldom reported. In this mini-review, we first highlight the effects of hydrothermal temperature and sodium content on the microstructures of hydrothermal method derived TNT, because the morphology and microstructure of TNT are highly dependent on the preparation conditions. Effects of the alterations of microstructures induced by the variation of hydrothermal temperature and sodium content on the dyes, heavy metal ions, and organic vapors adsorption characteristics of TNT are then introduced citing recent patents.

Effects of soil properties on surfactant adsorption

Abstract The effects of surface area, soil organic matter (SOM) content, and cation exchange capacity (CEC) of natural soils and clays on the adsorption capacity of cationic, anionic, and nonionic surfactants in water‐solid systems were investigated based on the adsorption isotherm analysis. The sorption capacity for a cationic surfactant was proportional to the CEC of the solids. For both anionic and nonionic surfactants, the sorption capacity was related to the soil mineral fraction. However, other soil properties probably affect the practical sorption. The investigated soil properties were treated case by case.

Multifractal scaling analysis of autocatalytic and autopoisoning reactions over DLA surfaces

Abstract Monte Carlo simulations of decay- and enhance-type Eley—Rideal mechanisms over surfaces of diffusion-limited aggregates (DLA) were performed to examine the morphological effect on autopoisoning and autocatalytic surface reactions. Effects of decay and enhancing profiles on the reaction probability distribution (RPD) were then analyzed by multifractal scaling techniques. It is concluded that RPD tends to be uniform at fast decay and relatively concentrated at faster enhancing rate.