Christine A. Philip

Adsorption Characteristics of Microporous Carbons from Apricot Stones Activated by Phosphoric Acid

Crushed apricot stone shells were impregnated with varying H 3 PO 4 acid concentrations (20-50 wt%), followed by carbonisation at 573-773 K. The products were characterised by nitrogen gas adsorption. Analysis of the nitrogen isotherms by the DR and α s methods proved that most of the obtained carbons are highly microporous, with high surface areas (≥1000 m 2 /g) and very low mesoporosity. Increasing acid concentration, at 573 and 673 K, increases surface area and pore volume, whereas at 733 K a small decrease in both parameters appears at higher H 3 PO 4 concentrations. Whole apricot stones produce activated carbon of inferior porous characteristics. Development of the extensive pore structure was described in light of the effect of H 3 PO 4 on the lignocellulosic material during carbonisation.

Artichoke as a non-conventional precursor for activated carbon: Role of the activation process

Abstract Artichoke peels were used to produce activated carbon using chemical activation methods. Two activation protocols were compared: a two-step method A and a one-step method B. As newly used activating agents, KCl, CrCl3 and TiCl4 were compared. The results show that method B is superior to A. KOH with method B had an area of 2321 m2/g and a total pore volume 1.0071 cm3/g, of which 0.9794 cm3/g was confined to micropores. The corresponding values for KCl are 1731, 0.6925 and 0.6718. TiCl4 had lower but comparable values with those of KCl. CrCl3 appeared to be the least successful among the three newly used activating agents. The post-activation washing step strongly affects the characteristics of the final product. The differences among the effects of Zn, Cr and Ti are discussed in terms of the differences in polarizing power.

Effect of mechanically mixed ZnO on the evolution of pore structure of SiO2 upon thermal treatment

N2 adsorption at 77 K was used to follow the change in the pore structure of silica (mesoporous) produced on heating at 300 and 600‡C in the presence of different contents of mechanically mixed ZnO (15–85 mol%).Heating of pure silica at 300‡C caused pore narrowing. This proceeded differently in the presence of ZnO, the pore system being split into two ranges of limited sizes. The pore widening effect upon heating silica at 600‡C was greatly reduced upon the addition of ZnO, to the extent of the evolution of some micropores at low levels of ZnO.At both heating temperatures, the composition 30% ZnO represented a transient state through the flux of pore system change. The IR spectra indicated that at this composition the silica particles are more oblate.The pore structure variations due to heating at the examined temperatures correlated with the solid-state reactions that take place at 1000‡C, where the spinel Zn2SiO4 is detected.

Solid Acids from Persulphated and Perchlorated Physical Mixtures of Zirconium and Titanium Hydroxides

Calcination at 650°C of a physical mixture of zirconium and titanium hydroxides led to the formation of the corresponding oxides, monoclinic zirconia and anatase. The adsorption of perchlorate or persulphate anions (as 0.05, 0.1, 0.2 or 0.4 M aqueous solutions) before calcination did not inhibit crystallization; however, perchlorate anions activated the formation of rutile in addition to the predominant anatase phase. Indeed, the adsorption of perchlorated anions prior to calcination allowed the thermodynamically less stable tetragonal phase of zirconia to be detected in addition to monoclinic zirconia at ambient temperature. In contrast, the adsorption of persulphate anions before calcination stabilized the tetragonal phase with no rutile phase being detected in this case. Infrared spectroscopy showed that adsorbed S2O82– anions were held more strongly by the solid than ClO4− anions which tended to decompose when the solid was calcined. The acidities of the solid acids produced because of S2O82– or ClO4−anion adsorption were studied via the adsorption of pyridine (pKa = 5.3) from cyclohexane solution. The amounts and strengths of the acid sites formed during persulphate treatment were higher than those resulting from perchlorate adsorption. The strength of the acid sites formed on samples calcined before loading with S2O82– or ClO4− anions showed no significant differences. Variations detected in the structural aspects arising from S2O82– or ClO4− anion adsorption were reflected in the texture as assessed by nitrogen adsorption at −196°C.

Structural and textural studies of ZnO modified with dodecyl-trimethylammonium chloride

The effect of dodecyltrimethylammonium chloride — a cationic surfactant — on the texture and structure of ZnO, which is commonly used in the photocatalytic degradation of surfactants, has been investigated together with the effect of thermal treatment of the resulting materials at temperatures ≤350°C. Structural changes were followed by combining diffraction and infrared (IR) spectroscopic measurements while textural variations were monitored via low-temperature nitrogen adsorption studies. In addition, the effect of soaking the original sample of ZnO in water for 5 d has been monitored together with the effect of heat treatment on the resulting products. Bands corresponding to a carbonate complex were observed in the IR spectrum of all samples subjected to thermal treatment below 275°C. This complex gave rise to new bands in the XRD patterns of such samples situated at d-distances of 2.339, 2.023 and 1.431 Å, respectively, which disappeared when the samples were subjected to heat treatment above 275°C. The intensities of such bands decreased when increasing surfactant concentrations were employed in the initial treatment of the ZnO solid. Acid OH groups on the surface of the solid appear to provide sites for the formation of the carbonate complex. Such sites are attacked either by the aqua complex [Zn(H2O)6]2+ formed from dissolved ZnO in the presence of water alone or by the cationic surfactant when the latter is present in the system. Attack of the surfactant on the surface probably occurs through exchange with an H+ ion. The cationic surfactant was also adsorbed on to basic sites present on the surface of the solid since the concentration of such sites diminished as the surfactant concentration was increased. Measurements of the specific surface areas and pore structural analyses for all the samples studied indicated the presence of micro- and meso-pores in most cases, the former being absent from the parent ZnO employed. A comparison between the application of the V l versus t plot and the α s -plot for such pore structural analyses is presented together with a discussion of the criteria necessary for the application of the αs-method.

Effect of halide impregnation on the structure and surface characteristics of rutile

Titanium dioxide (TiO2, rutile) subjected to fluoride (1 M and 2 M NH4F) and iodide (1 M KI) treatment has been investigated by combining several approaches: structural and textural characterization as well as surface acidity and basicity. Fluorination reduces the crystallinity at both concentrations studied, the higher concentration being more effective. Iodination has the same effect. Fluoride ions replace all basic OH groups whereas iodination does not lead to full replacement due to competition with the Ti4+ substrate. A distinct exotherm is observed at 228°C and 235°C, respectively, for the F− and I−treated samples, resulting from energetic changes leading to the activated dissociation of water and restoration of the basic OH groups. A surface compound is apparently formed on fluorination and iodination, and this decomposes endothermally at 270–275°C. In addition, impregnation with I− appears to increase the photosensitivity of the TiO2 leading to the formation of a carbonate with atmospheric CO2 which generates an endotherm at 285°C and whose existence has been confirmed by IR spectral analysis. Specific surface areas and pore structure analyses demonstrate differences not only upon increasing the F− concentration but also between the F− and I− treated samples, where the addition of the latter markedly reduced the surface parameters. Two groups of pore sizes could be recognized with most samples. Increasing the F− concentration increased the sintering temperature to 550°C, the value for the low F− sample being 400°C.