Observation of Surface and Mass Transport on Selectively Reducible Spinel Oxides

Abstract

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.