Mahmoud M. Khader

Kinetics of the reduction of Fe2O3 with hydrogen

Abstract The kinetics of the heterogeneous reduction of Fe2O3 with gaseous hydrogen was studied over the temperature range of 300–400°C. As Fe2O3 is reduced, the resulting oxygen vacancies act as anion dopants of the remaining Fe2O3, thus increasing its conductivity. The final reaction product is FeO. It reoxidizes quickly upon exposure to air forming Fe3O4, as revealed by X-ray diffraction analysis. The progress of the reaction was followed by measuring the variation of the electrical conductivity of sintered compressed pellets with time. The variation of conductivity with the time of reduction was found to be quite complex, and can be divided into three regions: (a) region I, at short times, where the process is controlled by the chemical reaction at the outer surface between hydrogen and ferric oxide. This is an activated reaction characterized by an activation energy of 56.5 kJ mole−1; (b) region II, which signals the onset of diffusion control on the overall rate of the process, where the reaction starts to penetrate through the solid phase. Within this region, it was found that the conductivity varied linearly with the square root of the time; and (c) region III where the process is controlled by diffusion through the solid phase. The behavior of the system in regions II and III was found to be in agreement with the predictions obtained from solving Ficks second law of diffusion.

Activated nitriles in heterocyclic synthesis: Synthesis of several new coumarin derivatives

Several new benzopyran and benzopyranopyridine derivatives were synthesized via condensation of 2-amino-1,1,3-tricyanopropene (1) and diethyl 2-amino-1-cyanopropene-1,3-dicarboxylate (2) with salicylaldehyde and treatment of the resulted products with active methylene regents. The spectroscopic data of the synthesized compounds are reported and discussed.ZusammenfassungEinige neue Benzopyran- und Benzopyranopyridin-derivate wurden durch Kondensation von 2-Amino-1,1,3-tricyanopropen (1) und 2-Amino-1-cyanopropen-1,3-dicarbonsäuredimethylester (2) mit Salicylaldehyd und Behandlung der Reaktionsprodukte mit aktiven Methylenreagenzien dargestellt. Die spektroskopischen Daten der neuen Verbindungen werden diskutiert.

Photoelectrochemical dissociation of water at silicon doped n-GaAs electrodes

Abstract The photoelectrochemical properties of silicon-doped n-GaAs were studied in aqueous solution of different pH values. The photosensitivity is higher in alkaline than in acidic solutions. However, visual photocorrosion, and deterioration of photocurrent could be observed after few hours of illumination in 0.1 M NaOH solution. The corrosion was decreased by coating the GaAs electrode with a 50 A thick gold layer. If GaAs was used as an anode in an electrochemical cell, and illuminated with visible light ( 4200 A A ), photocurrent was observed due to the generation of H2 over a Pt counter electrode. The photocurrent generated in 0.1 M NaOH solution has an onset potential of −1.1 V (NHE). Furthermore, it saturated at a potential of −0.3 V (NHE). Both the rate of H2-production and photocurrent varied linearly with light intensity. The quantum efficiency for the hydrogen production is 0.4 hydrogen atom per second per photogenerated electron-hole pair. Our results are explained in the light of the band model of the semiconductor-solution interface.

Comparative study between the photoelectrochemical behaviors of metal-loaded n- and p-GaAs

GaAs electrodes are coated electrochemically with thin films of Pt, Ni, Cu, Au, Cd and Pb. The thickness of metal film is estimated from the number of coulombs passes during its electrodeposition. The current-voltage curves of the metal/GaAs systems are registered under illumination in a three-electrode electrochemical cell. Metal additives to n-GaAs causes displacement in the direction of a negative potential in the order Pt, Ni, Cu and in the positive direction in the order Cd, Pb. Correspondingly, for metal/p-GaAs systems the onsetpotential of the photocurrent show behaviors opposite to those of the metal/n-GaAs systems, i.e. the onsetpotential of the photocurrent shifts towards the positive direction in the order Pt, Ni and Cu and in the negative direction in the order Cd and Pb. The results for the metal/n-GaAs systems are explained within view of the Schottky barrier model. According to this model, the extent of the negative shift in the onsetpotential of the photocurrent is proportional to the work function of the deposited metal. On the other hand, the photoelectrochemical behaviors of metal/p-GaAs systems are explained in accordance to the eletrocatalytic model. In this regard deposition of a metal of a high activity towards hydrogen evolution shifts the onsetpotential of p-GaAs towards more positive potential.

Application of porous flow through electrodes: IV. Hydrogen evolution on packed bed electrodes of iron spheres in flowing alkaline solutions

Packed bed electrodes of small iron spheres have been used for the electrolytic production of hydrogen from alkaline solutions at different temperatures under conditions of electrolyte flow. The effects of temperature, electrolyte type, concentration and flow rate on the polarization behavior of the electrode were evaluated and analyzed. It was shown that increases in the conductivity of the electrolyte or the operating temperature decreases the potential required to support the reaction. The generated gas bubbles disperse in the pore electrolyte, resulting in an increase in its resistivity and, subsequently, an increase in the potential. It was shown that some gas bubbles are trapped within the porous electrode. The implications of the trapped gas bubbles on the behaviour of the electrode are discussed.

Catalytic effects for hydrogen photogeneration due to metallic deposition on P-GaAs

Abstract In continuation of our previous work on water photodissociation at semiconductor surfaces, we report on the catalytic effects of hydrogen photogeneration due to metallic deposits on p-GaAs. These effects were investigated by measuring the current-voltage curves in 0.5 M H2SO4 solution. Metallic coating was performed electrochemically by depositing an amount of a metal equivalent to five monolayers on GaAs. Such a coverage has achieved a maximum positive shift in the onset-potential of the photocurrent, Ep. Etching GaAs electrodes in (H2SO4 + H2O2 + H2O) mixture improved the (J-E) characteristics, however temporarily by shifting Ep towards positive potentials and also by increasing the value of the limiting photocurrent density by approximately ~ 15%. The effect of etching is for the surface with an elemental component, possibly, arsenic, due to the preferential dissolution of the other elemental component in the etching mixture. Coating with Pt, Ag and Cu gave a positive shift in Ep, but coating with Pb gave a negative shift. Whereas the shift of the onset potential of the photocurrent was independent of the height of the Schottky barrier, which is expected to be created at the metal/semiconductor interface, the magnitude of this shift, towards positive potential, was proportional to the dark electrocatalytic activity for hydrogen evolution at the deposited metal. This activity has been found to be inversely proportional to the magnitude of the Tafel slope of the metal.

Photoelectrochemical dissociation of water at copper-doped p-GaAs electrodes

Abstract Cu-doped p -GaAs was used as a photocathode in a three-electrode electrochemical cell. It was illuminated with visible light in 0.05M H 2 SO 4 . The onset potential of photocurrent and H 2 O decomposition was about 0.6 V, cathodic with respect to the flat-band potential of p -GaAs (0.3 V vs SCE). The photocurrent anomaly near the flat-band potential ( V fb ) was attributed to the poor catalytic properties of p -GaAs towards H 2 evolution. It was also attributed to the existence of a surface state which can trap the photoexcited electrons from the conduction band. The deposition of five monolayers of Pt over the GaAs surface shifted the onset potential of photocurrent by 0.1 V towards the positive direction. Upon illumination of p -GaAs, H 2 evolved over it and O 2 evolved over the Pt counter-electrode in a 2:1 ratio. Their evolution obeys zero-order kinetics. The photocurrent as well as the water decomposition changed linearly with light intensity.

Hydrogen evolution on metallized plastic packed bed electrodes

Abstract This paper describe the electrolytic production of hydrogen using packed bed electrodes of metallized plastic. Particles or screens of polyvinyl chlorides (PVC) were metallized by chemically deposited copper or silver and used to fabricate packed bed electrodes. These electrodes were used for the electrolysis of flowing alkaline solutions under different conditions of electrolyte concentration and temperature. The matrix resistivity of the packed bed electrodes was found to influence their polarization behaviour. This resistivity decreased by further electroplating of the packed bed with Ag, Ni or Pd. The effects of these electrocatalytic materials on the polarization of the hydrogen evolution reaction were evaluated and analysed. The electrocatalytic activity of the electrodes used in this study was in the order: Pd/Cu/PVC≅Ni/Cu/PVC > Ag/Cu/PVC > Cu/PVC. These electrodes showed excellent metallic conductivity with good chemical and mechanical stabilities.

Adsorption/desorption of H2 and O2 at a p-Cr2O3 surface : Electrical conductivity studies

The behaviour of H2 and O2 flows over a porous p-Cr2O3 pellet have been studied by means of an in situ electrical conductivity technique. Changes in the electrical conductivity associated with such gas flows were attributed to changes in the concentration of positively charged holes in the material studied. In the presence of an H2 flow, the electrical conductivity of the p-Cr2O3 surface first increased and then decreased sharply. The initial increase was attributed to the chemisorption of H2 leading to surface hydroxylation. The formation of O–H bonds during this process resulted in a decrease in the electron density at the surface and thereby increased the contribution of positively charged holes towards the total conductivity. In contrast, the subsequent sharp decrease in conductivity was explained by the elimination of chemisorbed oxygen which had originally been adsorbed dissociatively at the surface. During this process, elimination of chemisorbed oxygen atoms led to the presence of electrons in the chromia lattice which generated a sharp decrease in the hole concentration via an electron–hole recombination process. In an O2 atmosphere, the conductivity of the deoxygenated chromia increased exponentially to a maximum value and then decreased until it reached a value similar to that for stoichiometric chromia. The increase in conductivity observed in this case was assigned to the adsorption of oxygen on the chromia surface while the subsequent decrease was attributed to hydrogen desorption. The kinetics of the four processes observed, i.e. hydrogen adsorption, oxygen desorption, oxygen adsorption and hydrogen desorption, have been analyzed.

Studies of the adsorption of SO2, H2 and O2 on a Cr2O3 surface by an electrical conductivity technique

Electrical conductivity measurements have been used to detect extremely small changes in the oxygen content of oxide semiconductors in contact with reducing or oxidizing gases. In continuation of previous studies, the oxygen loss of chromium oxide in H2 and SO2 flows, as well as the oxygen gain in an oxygen atmosphere, were studied. In SO2, the conductivity dropped instantaneously to minimum values due to its adsorption on adsorbed oxygen sites. The treatment of Cr2O3 in SO2 led to the elimination of chemisorbed oxygen and the covering of the surface with polymeric SO2. In contrast, in an H2 flow, the conductivity of Cr2O3 initially exhibited an induction period during which the value was constant. At the end of the induction period, the conductivity increased rapidly to a maximum value and then dropped sharply to a minimum. The induction period may be regarded as the time necessary to create an oxygen vacancy, the activation energy for such a process being 21.1 kJ/mol. A hydrogen molecule is then adsorbed on to the oxygen vacancy possibly as a hydride ion, and leading to the initial increase in the conductivity. The hydride ion then migrated to a chemisorbed oxygen site, where it formed a surface hydroxy group and caused a consequent decrease in the electrical conductivity. The surface then dehydroxylated due to the interaction of surface hydroxy groups with gaseous hydrogen, leaving coordinatively unsaturated surface chromium ions behind. In an oxygen flow at 400°C and above, either SO2 or H2 treatments led to a sharp increase in conductivity due to oxygen adsorption. In contrast, at temperatures less than 350°C, oxygen adsorption was retarded after an SO2 flow, possibly due to the strong adsorption of a polymeric film of SO2. Correspondingly, after H2 treatments, oxygen was adsorbed instantaneously at temperatures as low as 200°C, presumably because of the weak sorption of H2 on the surface chromium ions. After discontinuing the hydrogen flow, further oxygenation caused a subsequent decrease in the conductivity, possibly due to surface hydroxylation. Hydrogen trapped in the bulk of the Cr2O3 could spill over the surface and cause such a hydroxylation process.