Mohamed Elshakre

Electrolytic generation of hydrogen on Pt-loaded porous graphite electrodes from flowing alkaline solutions

The aim of this work was to test the feasibility of using porous graphite electrodes as cathodes for the electrolytic production of hydrogen fro_m flowing alkaline solutions. The platinum loading on the graphite substrate was found to decrease the potential required to sustain a certain rate of hydrogen production, and hence the energy consumed during electrolysis. The experimental i/E relations agreed with those theoretically predicted on the basis of a mathematical model up to a current density of about 200 mA cm−2. The excessive polarization at higher current is attributed to trapping of hydrogen gas bubbles within the porous electrode.

Applications of porous flow-through electrodes. III. Effect of gas evolution on the pore electrolyte resistance

Gas evolution, within the pores of porous flow-through electrodes, was found to have significant effects on the pore electrolyte resistance. An electrical circuit was used whereby the pore electrolyte resistance was measured during hydrogen evolutionin situ within the porous electrode. The resistance was measured for packed bed electrodes of Cu turnings, Cu wool and Ag wool at various rates of hydrogen evolution. The pore electrolyte resistance increased with the rate of hydrogen evolution in the order Cu turnings > Cu wool (shreds) > Ag wool (hair-like fibres). The gas void fraction was calculated, using Bruggemans equations at various rates of hydrogen evolution. It was shown that the mode of variation of pore electrolyte resistance with the rate of hydrogen evolution can be explained on the basis of the variations in the gas void fraction. In all cases, it was found that the volume of gas retained within the porous bed depends on the rate of gas evolution rather nonlinearly.

Hydrogen evolution on stacked copper screen electrodes from flowing alkaline solutions

Abstract The aim of this work was to test the feasibility of using commercially available Cu screens as cathodes for the electrolytic production of hydrogen. The screens were stacked in the form of packed bed flow-through electrodes. The effects of the various operating conditions were studied e.g. screen mesh size, electrode thickness, electrolyte conductivity, flow rate and temperature. The effects of thin metal coatings on the Cu screens were also tested. It was found that increases in the electrolyte conductivity, temperature, electrode thickness and/or mesh size, decrease the potential required to support a certain rate of hydrogen production. The electrolyte flow rate had no significant effect on the polarization behaviour of the electrode. The Cu screens coated with black nickel showed lower potentials than those coated with either Watts nickel or palladium. The above effects were interpreted in the light of the existing electrochemical theory.

Electronic spectra and DFT calculations of some pyrimido[1,2-a]benzimidazole derivatives.

Ground state properties of 2,4-diphenyl-1,4-dihydrobenzo[4,5]imidazo[1,2-a]pyrimidine, compound 1, and its derivatives are investigated experimentally and theoretically in Dioxane and DMF. The calculations show that all the studied compounds (1-7) are non-planar, resulting in a significant impact on the electronic and structural properties. The ground state properties of compounds 1-7 at B3LYP/6-311G (d, p) show that compound 5 has the lowest EHOMO, ELUMO, and ΔE indicating highest reactivity. Compound 7 is found to have the highest polarity. The observed UV spectra in Dioxane and DMF of compounds 1-4 show 2 bands, while compounds 5-7 show 4 bands in both solvents. Band maxima (λmax) and intensities of the spectra are found to have solvent dependence reflected as blue and red shifts. The theoretical spectra computed at TD-B3LYP/6-311G (d, p) in gas phase, Dioxane and DMF indicate a good agreement with the observed spectra.

A DFT study of the ionization and electron attachment of 2-azido pyridine

A DFT study using B3LYP/6-31G(d,p) level of theory is pursued to investigate the energy and geometry changes of 2-azido pyridine isomers in the S0, D0(+), and D0(-). In S0, cis-2 azidopyridine is the most stable, followed by cyclic tetrazole and trans isomer is the least stable. In D0(+), the cis isomer is the most stable, followed by trans isomer, and tetrazole isomer is the least stable. In the D0(-) state, the trans form is the most stable, followed by the cis and the tetrazolo form is the least stable. Single point vertical excitation energy calculations of the cis form gave four excited states at 266.4, 239.4, 199.4, and 196.1nm, all having π-π(∗) character. For trans isomer, five excited states at, 256.7, 250.3, 229.7, 199.0, 197.0nm all resulting from π-π(∗) transitions. For the tetrazole isomer, seven excited states at 269.3, 242.7, 198.0, 187.2, 183.9, 182.1, 178.7nm all resulting from π-π(∗) transitions. The geometries of the 3 isomers show noticeable changes in bond lengths, bond angles and dihedral angles, upon ionization and electron attachment. The ionization results in a remarkable variation of the NBO atomic charges as a result of ionization and electron attachment. where electron density of HOMO of the D0(+) state is found to be much lower than that of the HOMO of the S0 in the case of ionization. The dipole moment calculations show that tetrazole isomer has the largest polarization in the S0 state, followed by cis isomer and trans isomer is the least polarized. The same pattern is found in the D0(+) state. In the D0(-) state, trans isomer is the highest polarized, followed by the cis, and tetrazole isomer is the least polarized. The rotational constants calculations of cis- isomer show that the cation is elongated along the long in-plane axis and compressed along the two short out-of-plane axes while the anion is compressed along the long in-plane axis, and elongated along the two short out-of plane axes. The cation and anion of trans isomer show similar behavior to the cis isomer. The cation of cyclic tetrazole, is compressed along the long in-plane axis and elongated along the two short out-of-plane axes, while the anion elongates along the 3 mutually perpendicular axes.

Strong-field photoionization of O{sub 2} at intermediate light intensity

We investigated by electron spectroscopy the strong-field multiphoton ionization of O-2 molecules with ultrashort laser pulses in the intensity range between the multiphoton and tunneling regimes. The ionization proceeds by at least three different mechanisms, in addition to the eight- and nine-photon nonresonant pathways. Transient multiphoton resonances with vibrational Rydberg levels give rise to direct Freeman-type peaks with sublaser linewidth and spin-orbit splitting. Some resonance levels actually become populated and yield extremely narrow lines because of postpulse vibrational autoionization. When the lowest photon order resonance channel for the Rydberg states is closed, a third contribution becomes dominant with a main peak at 0.4 eV that shares its main properties with the recently discovered universal low-energy structure in the electron spectra of atoms and molecules [C. I. Blaga et al., Nat. Phys. 5, 335 (2009); W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)]. The variation of the Freeman resonance spectrum with the laser peak intensity is well correlated with the vibronic Franck-Condon factors for the overlap of the intermediate Rydberg state with the O-2 ground state. Accordingly, the Freeman peaks could be unambiguously assigned to individual vibronic multiphoton resonances, and the disappearance of the Freeman resonances at a certain laser intensity could be explained. The population of the autoionizing Rydberg states could be assigned similarly to such vibronic resonances.