Aberration-Corrected STEM for Understanding of the Catalytic Mechanisms and Development of New Catalysts

Abstract

Aberration-corrected Z-contrast (HAADF) STEM has proven an invaluable tool for catalysis research due to its superior resolution and high sensitivity. ORNL’s 300 kV VG Microscopes’ HB603U STEM with Nion aberration corrector was shown to resolve lattice spacings as small as 0.78 A [1] and routinely allows us to image single atoms inside the materials and on their surfaces as well . The ability to simultaneously collect EELS spectra provides additional possibilities for characterization. All these aspects have proven instrumental for the studies of catalytic activity and degradation in Cr-doped transition aluminas, which are widely used for dehydrogenation of alkanes [2]. Even though structural difference between γ-Al2O3 and η-Al2O3 is minimal and only amounts to different distribution of cation vacancies within spinel framework, Cr-doped η-Al2O3 catalysts can last several years, while Cr/γ-Al2O3 catalysts degrade in weeks [3]. Our STEM studies revealed that Cr dopant is distributed differently in these two systems. On γ-Al2O3, Cr is segregated into extended “patches” of Cr2O3 (Fig. 1). On η-Al2O3, on the other hand, no segregation was observed. Contrast in the STEM images was mostly uniform, with the thinnest areas of the alumina flakes sometimes displaying faint spots (Fig.2a)). The spots have similar intensity (Figs. 2b)-d)) of about the right level for Cr. EELS studies (Fig. 2e)) also indicate uniform distribution of dopant on η-Al2O3 surface. Our first-principles calculations suggest that catalytic processes primarily happen on isolated CrOx species, and preserving a high density of these active sites is necessary to retain activity. Due to a unique surface reconstruction resulting from its specific distribution of cation vacancies, γ-Al2O3 (110C) surface can accommodate growth of extended Cr2O3 “patches’, while on η-Al2O3 Cr remains isolated, resulting in much slower degradation.