Quantitative determination of nickel speciation for the presence of free oxide in commercial fluid catalytic cracking catalysts

Abstract

Motivated by the potential over-catalysis and hazardousness of free nickel oxide (NiO) in the equilibrated catalysts (Ecats) collected from fluid catalytic cracking (FCC) refineries, this study aims to establish a simple bulk method, namely chemical leaching method to rapidly quantify the NiO content within thirteen commercial Ecats collected from different units. The synchrotron-–based, X–ray absorption near–edge structure (XANES) was performed to assess the validity of this method, as well as to examine the coordination structure of Ni by the extended X–ray absorption fine structure (EXAFS) analysis. Additionally, thermodynamics calculation was attempted to validate the affiliation of Ni with other metal oxides. The optimum leaching condition was validated as 60 °C in 60 min using 1 M sulfuric acid. Accordingly, the NiO was determined to reach 70–439 ppm in all Ecats, which warrants a non–hazardousness of these samples. Additionally, for the Ecats with a remarkably higher Ni content, the XANES results are biased by the enrichment of NiO on the catalyst surface. The majority of Ni was found to affiliate with Al in the form of aluminate, with the remaining 11%–15% Ni passivated effectively by lanthanum (La). The oxygen–rich regeneration condition could be beneficial for promoting the affiliation of Ni by La.