X-ray diffraction study of nickel oxide reduction by hydrogen

Hydrogen reduction of porous bulk NiO particles has been studied with in situ hot-stage X-ray diffraction (XRD) in the temperature range 175-300degreesC. This technique has the ability to measure NiO disappearance and Ni appearance simultaneously, together with the crystallite size of each. Since the sample was a very thin, 50-mum slab of dispersed 20-mum diameter grains, textural and morphological features normally encountered during studies with fixed beds of NiO particles were absent and measurements reflected only the chemical mechanism and kinetics. The results indicated that reduction in the absence of water added to the reducing gas followed a series of steps: (1) an induction period associated with the initial reduction of NiO and the appearance of Ni metal clusters; (2) acceleration of the reduction rate as the size of the clusters increase; and (3) a pseudo-first-order (excess H-2) process in which NiO disappeared and Ni appeared in concert until reduction slowed at a fractional conversion of about 0.8. Crystallite size measurements showed NiO crystallites of about 3 nm in size were transformed into Ni crystallites of more than 20 nm, implying that Ni-0 ion transport following reduction was very fast due to the close proximity of the NiO crystallites being reduced. When 2.2 x 10(-2) atm of H2O was added to the reducing gas, induction times increased by approximately a factor of two and reduction rates decreased (increasingly at lower temperatures) with an apparent activation energy of 126 +/- 27 kJ mol(-1) compared to 85 +/- 6 kJ mol(-1) without added water. The lag between NiO reduction and Ni growth observed in previous studies was not seen, indicating that textural and morphological factors are very important in establishing the role of water vapor in the reduction process. (C) 2003 Elsevier Science B.V All rights reserved.