Steam reforming of methane to syngas over NiAl 2O 4 spinel catalysts

Abstract

A series of several mixed oxides close to NiAl 2O 4 (NiAl 2O 4, 5 wt% NiO/NiAl 2O 4, 10 wt% NiO/NiAl 2O 4 and 15 wt% NiO/NiAl 2O 4) were prepared by a pseudo sol gel method using propionic acid then calcinated at 900 °C during 4 h. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), surface areas (BET) and temperature-programmed reduction (TPR). The nature of the crystalline structure, the surface area and the reduction temperature depend on the Ni/Al ratio. For the stoichiometric ratio Ni/Al exactly equal to 0.5, homogeneous crystalline spinel phase was formed after temperature of calcination was equal or higher than 725 °C. The spinel structure being a non-tolerant structure for an excess of Ni, when Ni/Al is higher than 0.5, free NiO in strong interaction with the spinel was present and favoured the reduction of the nickel of the structure at lower temperature compared to the spinel. Till 700 °C, the spinel structure was well conserved even if it was partially reduced. For a temperature higher than 700 °C, the structure leads to Ni species formation on γ-Al 2O 3. Comparative steam reforming of methane was conducted to control the formation of Ni and its sintering. The stoichiometric spinel showed relevant catalytic performances: high CH 4 conversion, high selectivities of CO and H 2 and low carbon formation. The presence of nickel inside the spinel structure of NiAl 2O 4 catalyst confers a high stability and strong dispersion for the metallic particles. The low formation of carbon was due to the high dispersion and the limited growing of the Ni particles on γ-Al 2O 3. Over 5 wt% of NiO excess, a rough decrease of the CH 4 conversion, CO and H 2 selectivities were observed. The x wt% NiO/NiAl 2O 4 systems give an important carbon formation due to a large Ni particle formation after rapid sintering.