Precise Identification and Characterization of Catalytically Active Sites on the Surface of γ‐Alumina**

Abstract

g-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability.These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of g-alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopya nd solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O) 5 Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet g-alumina as well as the (100) segments of irrational surfaces (invariably always present in all g-alumina samples) responsible for its unique catalytic activity.Such (O) 5 Al(VI)-OH sites are also present on the macroscopically defined (100) facets of g-alumina with elongated/rod-like geometry.T he mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al +3 O 5 sites is clarified. These coordinatively unsaturated penta-coordinate Al sites produce well-defined thermally stable Al-carbonyl complexes.O ur findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of g-alumina and their role as catalytically active sites.

Dramatic improvement of NO reduction activity via reversible re-dispersion of CeO2 nanoparticles into Ce+3 atoms on alumina under high temperature reactive treatment

“…CO adsorption at room temperature on thermally activated sample does not lead to any discernible CO signatures. This indicates that no penta Al sites are available on the surface since these are the only sites that can adsorb CO strongly under ambient conditions on alumina surfaces [43]. This result is further consistent with high-field solidstate 27Al NMR measurements (Fig.…”

“…Furthermore, a new C-O stretching band appears ~2,220 cm -1 . We have previously shown that this band corresponds to CO directly adsorbed on tetra-Al sites [43]: indeed, Ce+3 dispersed on these sites prevent CO adsorption on them. Once they are removed by O2 treatment, these sites become available for CO adsorption.…”

Pd Species Variation and CH4 Reaction Path on Pd–Pt/Al2O3 from Rich to Lean Burn Conditions

“…On γ-Al 2 O 3, the IR bands of the surface hydroxyl groups were 3837, 3700, 3604, 3549, and 3501 cm –1 , and a wide band was less than 3500 cm –1 . These hydroxyl groups represented the −OH in the forms of terminated, bridged, or multiple coordination on different Al 2 O 3 crystal planes . In contrast, new bands at ∼3730 and 3783 cm –1 were observed for Pd–Pt/Al 2 O 3 .…”

Application of 4D STEM and DPC Techniques to Study Surface Reconstruction of Transition Aluminas

“…Kovarik * , K. Khivantsev * , J. Szanyi * * Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington, USA Transition aluminas are used in catalytic applications as catalysts and catalytic supports due to their excellent thermal stability, ability to maintain high surface area, and most importantly their unique surface properties [1]. Understanding the surface properties of transition aluminas has been the subject of extensive research, and while much progress has been made, the actual surface structure of transition aluminas remains only conceptually understood [1,2,3]. A detailed nature of bonding environment has been mostly inaccessible to surface science techniques because of high degree of structural disorder, as well as inherent difficulties associated with studying high surface-area materials.…”