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.

Porous nanographene formation on γ-alumina nanoparticles via transition-metal-free methane activation

“…We analyzed the effects of temperature on the surface structure of g-ANPs by IR and TPD (Fig. 3a and b) up to 900 C. The IR spectra showed desorption of H 2 O and depletion of surfacebound hydroxyl groups (3800-3000 cm À1 ) 65,66 as the operating temperature increased under a steady ow of inert gases. Water desorption became almost negligible aer 30 min at 900 C as shown in the TPD (Fig.…”

Increasing Al-pair abundance in SSZ-13 zeolite via zeolite synthesis in the presence of alkaline earth metal hydroxide produces hydro-thermally stable Co-, Cu- and Pd-SSZ-13 materials

“…This remarkable property of Co/SSZ-13(Sr) material, thus, prompted us to load 1.7 wt% Pd in its micropores and study its PNA properties and hydrothermal stability of the resulting Pd-containing SSZ-13 composite [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Indeed, the sample shows excellent PNA performance after loading Pd in it (Fig.…”

“…Emissions during vehicle cold start, however, remain a challenge. To circumvent this challenge, Pd/zeolite materials (passive NOx adsorbers) have been introduced industrially that adsorb NO strongly as Pd(II)-NO, Pd(I)-NO, Pd(II)(OH)(NO) and Pd(II)(NO)(CO) complexes (depending on Pd speciation in zeolites) at low temperatures (80-130 ⁰C) during cold start and then release them at higher temperatures (>170-180 ⁰C) when downstream SCR catalysts become very active [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. The main challenge for application of Metal/zeolite materials for industrial vehicle use has always been their hydrothermal stability.…”

“…They may be exposed in vehicles to harsh steam treatment (during diesel particular filter regeneration, for example) [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In general, it is desirable to develop materials that can survive hydrothermal aging at temperatures as harsh as 800 ⁰C and higher.…”

“…In general, it is desirable to develop materials that can survive hydrothermal aging at temperatures as harsh as 800 ⁰C and higher. For passive NOx adsorbers, the highest known survival temperatures are 750-820 ⁰C [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. For example, we have been able to engineer Pd/BEA crystals that can survive 750 ⁰C hydrothermal treatment without any deterioration [21].…”