Tsair-Wang Chung

In situ DRIFT and kinetic studies of photocatalytic degradation on benzene vapor with visible-light-driven silver vanadates.

The visible-light active silver vanadates with different types of crystallines (Ag(4)V(2)O(7) and Ag(3)VO(4) phases) were synthesized by an environmentally friendly aqueous process. The parameters of hydrothermal temperature and hydrothermal time were tuned to maximize the photocatalytic efficiency for the decomposition of benzene vapor under visible-light irradiation. The quantum efficiencies of the photocatalysts are compared on the basis of the crystalline phases, surface area, intensity of surface hydroxyl groups, and Brönsted acid sites. From the results of DRIFTS studies, the photocatalytic activities strongly depend on the intensities of the Brönsted acidity and hydroxyl groups presented on the silver vanadates. The sample synthesized at 140 degrees C and 4h (HM140) exhibits the best photocatalytic activity; it has a reaction rate constant (k(app)) of 1.42 min(-1), much higher than that of P25 (k(app)=0.13 min(-1)). For an irradiation time of 720 min, the mineralization yields of benzene were 48% and 11% for HM140 and P25, respectively. Based on the short-term decrease of benzene concentration and the long-term increase of CO(2) concentration, the photocatalytic ability of the HM140 sample is significantly superior to that of P25. The highest activity can be attributed to the synergetic effects of the richest Brönsted acid sites, and a favorable crystalline phase combined with abundant surface hydroxyl groups.

Influence of manufacturing variables on surface properties and dynamic adsorption properties of silica gels

The surface properties of silica gel such as surface area, pore volume, and average pore diameter are affected by the manufacturing variables including pH values, gelling and dehydration temperatures. The dynamic adsorption properties of silica gel are also affected by their preparation conditions. The mass transfer zone and the amount of adsorption were discussed using the experimental data of breakthrough curve. The influence of the manufacturing variables on the trends of breakthrough curves is important in most of the adsorbent modification. It was found in this study that the manufacturing variables affected the adsorption performance significantly. Therefore, the relationship between the manufacturing variables and the corresponding adsorbent properties conducted by the dynamic adsorption apparatus was examined in this study. This discussion is rare in most of the studies of preparing silica gels, which are usually focused on the relationship between the manufacturing variables and the surface properties of silica gels only. However, it is not necessary that a silica gel with larger surface area will give a smaller mass transfer zone or mass transfer resistance. The modification of adsorbents should consider not only the improvement of their surface properties but also their adsorption performance.

SURFACE PROPERTIES AND ADSORPTION BREAKTHROUGH CURVES OF PLASMA-TREATED SILICA GELS

Plasma treatments are used widely in surface modification of thin films and membranes. A similar treatment method was used for granular silica gel by using both argon and oxygen plasma. The surface properties, such as contact angle of water, pore diameter, and BET surface area, and the adsorption breakthrough curves for water were obtained on the plasma-treated silica gel. The contact angle of water decreased rapidly and then remained approximately constant during the plasma treatment. This indicated that argon- or oxygen-plasma treatment made the silica gel surface more hydrophilic. The increase in BET surface area of the plasma-treated silica gels was 13 to 15%. The experimental breakthrough curves showed that modification not only increased the effective surface area and active sites but also reduced the mass transfer resistance. The time for breakthrough increased by about 4 minutes when either argon- or oxygen-plasma treated silica gels were compared to the untreated one. The amount of moisture adsorbed by the modified silica gel as calculated from the breakthrough curve was increased by 18% compared to the untreated sample.

DETERMINATION OF THE OPTIMUM PREPARATION CONDITION OF THE SOL-GEL SiO2 USING THE RESPONSE SURFAC METHODOLOGY

A typical sol - gel process consists of the liquid reaction, the solution gelation, and followed by the dehydration. The surface properties of silica gel such as surface area, pore volumes, and the pore diameter were affected by the manufacturing variables including pH values, gelation and dehydration temperatures. The objective of this study is to determine the optimum preparation conditions to maximize a response of surface area, or minimize its pore diameter. In addition, interactions between process variables were studied and their significance to the surface properties was also weighted. It was found that the surface area of silica gels increased with an increasing amount of NH4OH to a maximum value and then decreased. As a drying temperature kept constant, the surface area and the pore volume increased with an increasing gelation temperature. However, the pore diameter was not influenced by this factor and the pore size was almost uniform at a low NH4OH concentration. For a higher NH4OH concentration, the pore volume and the pore diameter became larger but the surface area became smaller as the gelation temperature increased. By means of the response surface methodology analysis, the optimum processing condition was found to be 0.0155 mole of NH4OH, 80,3°C for gelation temperature, and 63.2°C for the dehydration. As a result the maximum surface area corresponding to the optimum preparation conditions was 818.9 (m2/g) as expected.

EXPERIMENTAL DETERMINATION AND PREDICTIONS OF BREAKTHROUGH CURVES IN A FIXED BED OF SILICA GEL USING THE TRANSFER UNIT APPROACH

Abstract A semi-empirical model was developed to predict the breakthrough curves of moisture adsorption on silica gel under different operating conditions. The model requires only the basic physical and transfer properties of the flowing gas stream. Experimental breakthrough curves were determined in this study for correlating some model parameters. These dynamic experiments were carried out with different values of the gas velocity, the inlet humidity, and the properties of the adsorbent. The breakthrough curves of the modified silica gel with neutron flux in a previous study were used to verify the proposed model. It is feasible to develop a single adsorption model to simulate the fixed-bed breakthrough curves for original and modified silica gels. The results show that the complex breakthrough curves can be predicted and are in very good agreement with the experimental data.