New Insights On the Role of YSZ in a NiAl2O4/Al2O3-YSZ Diesel Steam Reforming Catalyst

A recently patented Ni-Al spinel supported on alumina (Al2O3) and Y2O3-stabilized ZrO2 (YSZ) catalyst “NiAl2O4/Al2O3-YSZ” demonstrated high activity and high resistance to coke deposition when used in commercial diesel and biodiesel steam reforming. In a previous publication it has been shown that the Ni-Al spinel (NiAl2O4) is deposited preferentially at the alumina phase. Nevertheless , the presence of YSZ had proven instrumental in terms of catalyst efficiency and stability , mainly associated with a decrease of surface carbon formation.In order to explain the YSZ role in the spinel catalyst performance , a wet-impregnation-based protocol was used to prepare catalysts formulations with different Al2O3/YSZ mass ratios as well as YSZ with different Y2O3/ZrO2 molar ratios. Inductively coupled plasma mass spectrometry (ICP-MS) was used to label precisely the prepared catalysts. In all tested formulations the Ni load was kept constant at 5% w/w. The morphology and metal dispersion of the catalysts as well as their crystalline phases , composition and dispersion were examined by scanning electron microscopy coupled with Energy-Dispersive-X-Ray Spectroscopy (SEM-EDXS) and X-ray diffraction (XRD). Temperature-programmed-oxidation and reduction (TPO and TPD) has been used in fresh and used catalytic formulations to evaluate the role of the YSZ as an atomic oxygen-vehicle during the reaction. The experimental runs are performed in a differential fixed-bed reactor set-up. A proprietary diesel-water emulsion mixture , at low C/H2O molar ratio of 2 is vaporized in the preheating area of the set-up before entering the steam reforming reaction zone operated at the relatively low -for steam reforming- temperature level of 730°C and mass space velocities ranging between 15 000 and 55 000 cm3 h-1 g-1. Gaseous products were analyzed using on-line Gas Chromatography (GC). The results show that diesel conversion and products selectivity are influenced by the presence of YSZ and its Y (as yttria) content. Moreover , catalyst efficiency over time-on-stream seems being also influenced. More details regarding the mechanism of these phenomena will be presented; they are used to optimize the catalyst formulation.