Browsing by Author "Ladonis, Alec C."

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    Deactivation and Spinel Phase Regeneration of Reduced Nickel Aluminate
    (New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2016-09) Ladonis, Alec C.; Misture, Scott; Payton, Eric; Pilgrim, Steven; Lipke, David
    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.
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    Observation of Surface and Mass Transport on Selectively Reducible Spinel Oxides
    (New York State College of Ceramics at Alfred University. Inamori School of Engineering., 2020-12) Ladonis, Alec C.; Misture, Scott; Cormack, Alastair; Tidrow, Steven; Ding, Junjun
    While many investigations rely on macro- or nanoscale techniques to determine the deactivation mechanisms on selectively reducible oxide catalysts, little attention is paid to the surfaces and near-surface effects. This study addresses the literature gap by investigation of a model NiAl2O4 catalyst by novel low voltage in-situ high temperature scanning electron microscopy and other complementary techniques. Extensions to other compositions of Ni bearing aluminate spinel are presented. Expedited mass transport was identified on a bulk scale, on the surface, and around modifying oxide particles by in-situ techniques at relevant service ρO2 and temperatures. The regeneration pathway was found to depend on system ρO2. Oxidative atmosphere favors NiO formation and reaction with the remnant spinel whereas vacuum favors Ni reincorporation via either reaction at the triple phase boundary or by formation of a core-shell structure. Structural relaxation at the surface is evidenced by Raman spectroscopy, where vacuum reoxidized samples show band sharpening and splitting as compared to the reduced analogue, however, some surface defects are retained. The unusual nature of the surface of the selectively reduced spinel is highlighted by enhanced surface diffusion in the form of metal particle migration and coalescence, which was observed on the spinel above the reduction temperature. Comparison of Ni migration distances on two supports with varied defect content exhibit diffusional distances up to 350 times the predicted value. Additional surface pits are revealed during migration and coalescence, which are crystallographically registered to the spinel. These features do not heal; however, they sharpen with extended time at high temperature. Extensions to technologically relevant Ni-Mg-Co catalysts show that phase assemblages during redox cycling are indeed reversible. morphological changes during reduction and oxidation were proven to be irreversible, creating new features on each reduction. The catalyst upon the second reduction exhibits 50% more exsolved metal, a 4x increase in particle number density, and over a 50% decrease in particle size as compared to the initial reduction. Irreversible changes in the surface metal dispersion and surface area are attributed to microstructural maturation, where surface features promote better metal dispersion and surface area for catalytic reactions. A unique and unusual tunneling phenomenon was identified at the interface of the spinel and modifying oxide at elevated temperatures, implying the process is thermally activated. Changing the modifying oxide from ZrO2 to HfO2 increased the onset temperature by 200°C but had similar rates. Further investigation of the interface revealed non-homogeneously distributed voids form beneath the tunneled ZrO2, where the interface between the spinel and ZrO2 is Ni deficient, reminiscent of a defective spinel structure. A model is proposed based on enhanced interfacial diffusion, where voids occur due to variations in the magnitude of diffusional fluxes arising from the modifying oxide additions.