Deactivation and Spinel Phase Regeneration of Reduced Nickel Aluminate

Abstract

Nickel aluminate (NiAl2O4) is an attractive material for dry reforming of methane due to selective reduction yielding catalytically active Ni metal particles. However, during this process, the catalyst is subject to mechanisms that ultimately degrade the catalytic activity. In the present work, characterization of reduced NiAl2O4 + 2.5 wt. % ZrO2 (Ni + defect spinel) via novel in-situ low voltage high resolution high temperature scanning electron microscopy (HTSEM) coupled with in-situ high temperature X-ray diffraction (HTXRD) is investigated to address: 1. The feasibility of observing surface behavior during deactivation and regeneration of the catalyst by reduction and reoxidation; 2. Surface and bulk microstructural evolution during thermal exposure; and 3. Stability of surface and bulk morphology over time at operating temperatures.

This work reveals surface microstructural and topographical features as well as their effects on functional behavior that can be observed via in-situ HTSEM. However, challenges in the analysis include local control of pO2, damage to the specimen surface, and the inability to replicate exact service conditions in terms of flowing gasses.

HTXRD performed in air demonstrates that oxidation of Ni occurs between 400-500°C, and concludes by 900°C. Spinel XRD peaks grow upon oxidation, indicating phase regeneration. HTXRD under industrial grade N2 with small amounts of oxygen exhibit spinel, Ni metal, and ZrO2 peaks until 1100°C is reached, no NiO formation is exhibited, even upon cooling. An increase in spinel peak intensity is also observed under N2 with 5-20 ppm pO2.

HTSEM demonstrates that Ni metal surface particles migrate and coalesce upon the support surface between 900-1000°C revealing pit structures upon the surface. The composition of the primary particles is unknown due to differences in pO2 between HTSEM and HTXRD, but is expected to be Ni metal. Mechanisms for particle movement are also unclear; movement is not gravity driven and Ni is a solid at this temperature. Additional reduction of the matrix is observed between 900-1000°C, which is likely due to low pO2 within the SEM chamber. Above 1000°C, all observed particles reincorporate into the matrix, exposing pit like structures on the surface, similar to ones observed post migration. These pits do not heal with time at temperature; instead the pits and crevices mature with time as evidenced by sharpening of edges and corners of crystalline facets. The overall reduction and reoxidation process is expressed as: NiAl2O4 [H2 + heat] → Ni1-xAl2O4-x + xNi + x/2O2 [O2 + heat] → xNiO + Ni1-xAl2O4-x [time + heat] → NiAl2O4.

HTSEM provides direct evidence that particles migrate and coalesce during the initial steps of catalyst regeneration, revealing a roughened surface with pits and crevasses. This surface rearrangement may provide longer distances for particle migration to occur, slowing the coalescence and regeneration of the original spinel to improve the catalyst service time.