Photoassisted solid-catalysed reduction of nitrogen by water and decomposition of water over partially reduced ferric oxide
Khader, Mahmoud M.
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Partially reduced ferric oxide was found to be an active photocatalyst for the production of H2 from water and NH3 from N2 and H2O. The catalyst was prepared by reducing α-Fe2O3 in a mixture of H2 and H2O vapor, and then oxidizing it in air. It was used in the form of powder, sintered pellets and sputtered thin films. X-ray diffraction and oxidimetry showed that the active samples contain about 5 mole% Fe(II). Surface characterization of thin films of iron oxide deposited by d.c. sputtering on Pt substrates was carried out in an ultrahigh vacuum (UHV) chamber using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). Water desorption from thin films of stoichiometric ferric oxide exhibited zero order kinetics and yielded a single peak at 170 K. By contrast, TDS of H2O from partially reduced films showed two H2O desorption peaks at 170 and 225 K. Ordered overlayers of iron oxide on Pt(111) were detected by LEED. In a photoelectrochemical (PEC) cell, partially reduced iron oxide served as a photoanode. The material proved to be an n-type semiconductor with low dark currents and high anodic photocurrents when irradiated with light of energy greater than 2.2 eV. Photolysis of water to H2 and O2 over the partially reduced ferric oxide was shown to be a catalytic process with H2 yields higher than the stoichiometric reducing capacity of the catalyst observed. In a photoelectrochemical cell, the threshold photon energy for H2 formation was 2.2-2.3 eV. The rate of evolution of H2 and photocurrent increased with biasing voltage with no indication of any saturation, and also increased linearly with light intensity. At voltages below +0.7 V versus a saturated calomel electrode (SCE) the rate of hydrogen production and photocurrent varied linearly with each other. NH3 was formed when N2 at atmospheric pressure, saturated with water vapor, was bubbled through a suspension of α-Fe2O3 powder in H2O illuminated by 20 mW/cm^2 of light of wavelengths from 4200 Aº to 27500 Aº. The estimated maximum turnover frequency (TOF) was 1.3 x 10^-5 molecules/site.sec. In the presence of formaldehyde or glucose the TOF of ammonia formation was 1.7 x 10^-4 and 1.3 x 10^-4 molecule/site.sec, respectively. These rates declined when the reaction proceeded for longer time. Ammonia synthesis over partially reduced iron oxide was also shown to be catalytic, with a threshold for production at a photon energy of 2.2 eV. The rate of ammonia formation over partially reduced ferric oxide increased to a maximum at a light intensity of 20 mW/cm^2, after which it decreased at higher light intensities. The rate of ammonia formation was also found to increase linearly with volume of water per unit weight of catalyst. Pure ammonia photodecomposed to N2 and H2 over the partially reduced ferric oxide, but in the presence of O2 it photodecomposed to N2 and H2O.
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