X-ray diffraction study of the hydrogen reduction of NiO/α-Al2O3 steam reforming catalysts Original Research Article

Hydrogen reduction of model steam reforming catalysts, comprising about 10 wt.% NiO on α-Al2O3 with and without the addition of 1–2 wt.% CaO or MgO, was studied with in situ hot-stage X-ray diffraction in the temperature range 175–900 °C. 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 50 μm slab of dispersed 20 μm diameter particles, 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 rates of disappearance of NiO and appearance of metallic Ni matched each other, with no time lag or induction period. The reaction followed pseudo-first-order kinetics with an apparent activation energy that increased from 95 kJ mol−1 for the catalyst with no additive to 122 and 322 kJ mol−1 with added CaO and MgO, respectively. When 2.2×10−2 atm of H2O was added to the reducing gas, activation energies increased by another 23 and 96 kJ mol−1 for the first two catalysts but showed little effect for the third.