D.P. Dissanayake

Partial oxidation of methane to carbon monoxide and hydrogen over a Ni/Al2O3 catalyst

Abstract Partial oxidation of methane occurs in the temperature range 450–900°C by reaction of an oxygen-deficient CH 4 /O 2 mixture over a 25 wt% Ni/Al 2 O 3 catalyst. Carbon monoxide selectivities approaching 95% and virtually complete conversion of the methane feed can be achieved at temperatures >700°C. The oxidation state and phase composition of the catalyst were characterized using X-ray photoelectron spectroscopy and X-ray powder diffractometry. This study revealed that, under operating conditions, the previously calcined catalyst bed consists of three different regions. The first of these, contacting the initial CH 4 /O 2 /He feed mixture, is NiAl 2 O 4 , which has only moderate activity for complete oxidation of methane to CO 2 and H 2 O. The second region is NiO + Al 2 O 3 , over which complete oxidation of methane to CO 2 occurs, resulting in an exotherm in this section of the bed. As a result of complete consumption of O 2 in the second region, the third portion of the catalyst bed consists of a reduced Ni/Al 2 O 3 phase. Formation of the CO and H 2 products, corresponding to thermodynamic equilibrium at the catalyst bed temperature, occurs in this final region, via reforming reactions of CH 4 with the CO 2 and H 2 O produced during the complete oxidation reaction over the NiO/Al 2 O 3 phase.

The direct formation of H2O2 from H2 and O2 over colloidal palladium

Additional evidence is presented for the catalytic role of aqueous colloidal palladium in the direct conversion of H2 and O2 to H2O2. Reactions typically were carried out at an O2/H2 ratio of 2, and 25 °C, by introducing the gases at 1 atm through a frit into the aqueous slurry. The source of palladium was either PdCl2 or reduced palladium supported on silica gel. During the course of the reaction the palladium is distributed among PdCl42−, colloidal palladium, palladium deposited on the frit, and Pd/SiO2 when SiO2 is present. Although the amount of colloidal palladium differs, depending on its source and the time on stream, the rate of H2O2 formation is proportional to the amount of colloid present at a particular time. Maximum rates were observed for colloids prepared from Pd/SiO2; however, the largest H2O2 concentration of 0.7 wt% was attained when PdCl2 was the source of colloidal palladium. The ultimate steady-state concentration of H2O2 is, in part, affected by the rate of H2O2 decomposition, which is relatively large over the Pd/SiO2 sample that was tested. The rate of decomposition also is determined by the H+ concentration. Results obtained with a mixture of 16O2/18O2 confirm that oxygen remains undissociated during the formation of H2O2, which may explain why palladium is uniquely suited as a catalyst for this reaction.

Cyclohexane oxidation and carbon deposition over metal oxide catalysts

Catalytic activity of V, Mn, Ni, Cu, Zn, Mo, Zr and Ce oxides over an alpha-alumina support was evaluated for cyclohexane oxidation under oxygen deficient conditions in order to understand the relation between carbon deposition and catalytic activity/selectivity. Carbon formation over the catalysts during the oxidation reaction was measured by means of Fourier transformed infrared spectroscopy (FTIR). Catalysts Mn/Al2O3 and Ce/Al2O3, which are selective for deep oxidation of cyclohexane, possessed relatively carbon free surfaces. The catalysts with relatively high carbon deposition (V, Ni, Cu, Zn, Mo and Zr) produced CO in addition to CO2. Traces of formaldehyde were produced over the catalysts Mo and V.

Contribution of dissolved sulfates and sulfites in hydrogen sulfide emission from stagnant water bodies in Sri Lanka

Accumulation of sulfur-containing compounds and their bacterial mediated reductions have led to the emission of pungent odors from stagnant water bodies. This study is focused on the contribution of inorganic sulfur compounds in the emission of hydrogen sulfide. The measured dissolved oxygen levels have demonstrated good negative correlations with the dissolved sulfide levels implying the oxygen deficiency is the key for the reduction of sulfate ion and sulfite ion to sulfide ion. Particularly, the dissolved molar fractions of sulfide from the total dissolved sulfur compounds (sulfates, sulfites and sulfides) have a very good correlation with the dissolved oxygen for the stagnant water bodies except the artificially aerated prawn farms. For the stagnant water bodies with significant correlations, linear regressions are reported for them to be utilized in estimating one component of the regression from the measurement of the other. The measured data were further utilized to estimate the levels of hydrogen sulfide gas. The pH of the water bodies has confined much of the dissolved sulfides in the form of bisulfide ion and they can be easily escaped to the atmosphere upon acidification due to industrial discharges and/or acidic precipitations. The estimated levels of hydrogen sulfide just above the water surface were plotted for the most polluted stagnant water body in Sri Lanka for the pH range of 5-10 and temperature range of 25-35 degrees C.

Spectroscopic Characterization of Surface Oxygen Species on Barium-Containing Methane Coupling Catalysts

Abstract Promotion of MgO, CaO, and ZnO with 2 mol% Ba imparts high activity and C 2 selectivity to these oxide supports for the oxidative coupling of CH 4 at 800-850°C. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) of CO 2 have been employed to characterize the nature of surface species on both the fresh and used catalysts. These techniques have established that a direct correlation exists between the oxidative coupling activity of these promoted oxides and their ability to form surface peroxide ions (O 2 2 ) in the presence of gaseous O 2 , but that activity is not directly related to support basicity. Selective poisoning of CH 4 /CD 4 isotopic exchange reactions indicate that at least two different types of sites that are capable of activating C-H bonds in CH 4 exist on these catalysts.

Fluorescence Characteristics of [(Benzoyloxy)methyl]anthracene Donor–Acceptor Systems

A theoretical investigation on fluorescence properties of [(benzoyloxy)methyl]anthracene derivatives containing different groups (OCH3, CH3, H, CF3, F, CN, and NO2) as substituent on the phenyl ring is reported. Electron transfer rate constants for the molecules were calculated using Marcus theory. Theoretical electron transfer rate constants agreed with experimentally observed trend of fluorescence quenching. Large electron transfer rate constants were obtained for molecules containing strongly electron withdrawing groups as the substituent on the phenyl ring. Calculations were conducted at Hartree-Fock and density functional (HF/6-31G(d) and B3LYP/6-31G(d)) levels of theory. Density functional theory predicted spurious charge transfer excited states for molecules containing NO2, CN, and CF3 as the substituent on the phenyl ring.

Biopharmaceutical constants for carbamazepine immediate release tablets in simplifying bioequivalence studies

Current study was undertaken in order to determine model biopharmaceutical constants for carbamazepine immediate release tablets (200 mg) from documented data of plasma concentration vs time curves. The constants and the proposed methodology simplify bioequivalence determinations to blood sampling restricted only to two time points. Twelve volunteer drug plasma concentration (Cp) determinations from a crossover design bioequivalence study were fitted into equations containing two rate processes. The optimized rate constants were used to generate the Cp vs time curves (generated curves). Generated curves were then differentiated (dCp/dt ) to obtain the first derivative curve for each volunteer from which times for highest rate of absorption (TAmaxn) and highest rate of elimination (TEmaxn) were determined. The corresponding highest rate of absorption and the highest rate of elimination for each individual were then obtained from the generated curve and named as Amaxn and Emaxn. Individual Amaxn and Emaxn values were then averaged to obtain the mean Amax and Emax. Out of the 24 determinations, a total of 13 Amaxn and 20 Emaxn values fell within ±20% of the overall mean. Final Amax and Emax values ware arrived at by averaging each set of individual 13 values and 20 values respectively. From these two mean coordinates, the corresponding constants, plasma drug concentration at the point of highest rate of absorption (CpAmax) and corresponding time TAmax, as well as the plasma drug concentration at the point of highest rate of elimination (CpEmax) and the corresponding time TEmax, were determined.