Spectroscopic and structural studies of aluminum oxides
Abstract
Al oxides play an active role in soil and industrial processes because of their highly reactive surfaces. However, knowledge about their structures and surfaces is limited due to, in part, their low crystallinity. Water associated with Al oxides has interfered with their characterization. In this study, a methodology was developed to study the properties of water and to use water as a probe to study the surfaces of Al oxides. Poorly crystalline boehmite (PCB) is a model oxide because its particle size approaches the boundary between crystalline and amorphous materials. Traditional methods intrinsically underestimate the surface areas of oxides like PCB. In this study, two independent methods were used to obtain the surface area of PCB. First, the BETH2O surface area of PCB was determined to be 514 ± 36 m2/g using a gravimetric FTIR method. Second, the mean crystallite dimensions were estimated to be 4.5 × 2.2 × 10.0 nm using the X-ray diffraction and Scherrer equation. A surface area value of 504 ± 45 m2/g calculated using the mean crystallite dimensions was in good agreement with the BETH2O surface area. The geometrical structure of PCB was also analyzed using water adsorption and the pores with radii of 0.35 and 0.85 nm were detected. The micropores in PCB were filled with water under ambient conditions and could not be eliminated without heating the sample. XRD, FTIR and NMR spectroscopic analyses were used to study PCB. The spectroscopic differences between PCB and its well crystalline counterpart were assigned to the structural defects (i.e., adsorbed water and surface OH groups). The approach developed to study PCB was then applied to determine the surface of an amorphous aluminum phosphate (APA) whose structure was sensitive to drying. Water molecules were found to interact with the surface of APA through coordinating to the structural Al and H-bonding to the OH and PO4 groups that were exposed to the surfaces in the pores with sizes of 0.46 and 1.19 nm. The small pores are filled with water under ambient conditions. The surface in large pores represents a surface area of 262 m2/g, which would be freely accessible for adsorbates.
Degree
Ph.D.
Advisors
Johnston, Purdue University.
Subject Area
Soil sciences
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