Effect of Ni/Al molar ratio on the performance of substoichiometric NiAl2O4 spinel-based catalysts for partial oxidation of methane

Abstract

The viability of a series of nickel aluminate-based spinel catalysts with varying Ni deficiency (corresponding to a Ni/Al molar ratio in the 0.13–0.50 range) was explored for the partial oxidation of methane under different operation conditions in terms of temperature, volume hourly space velocity, O/C molar ratio and time on stream. Thus spinel-type catalysts with a Ni loading between 11 and 31wt.% were prepared by coprecipitation. A wide number of techniques including WDXRF, XRD, N2 physisorption, Raman spectroscopy, XPS, UV–vis-NIR DRS, H2-TPR, TEM and TGA-MS were used to characterise the calcined, reduced and post-run samples.With respect to the reference stoichiometric sample (Ni/Al=0.50) alumina excess in the precursor oxide provoked notable changes in the surface area, structural properties connected with the relative cation distribution between tetrahedral and octahedral coordination and reducibility of the resultant spinel phase. It was found that the catalytic performance of these non-stoichiometric samples could be optimised for a Ni/Al molar ratio of 0.25, which corresponded to a metal loading of 19wt.%Ni. The oxidation activity was associated with the remarkable intrinsic activity of nickel particles derived from Ni2+ cations with a preferential occupancy of octahedral sites in the lattice of the oxide. The promising catalytic behaviour of this sample was further proven by the notable activity and stability shown under severe reaction conditions with a reduced loss of yield of hydrogen with time on stream.