Studies on the nature of salicyl hydroxamate adsorption at the surface of neodymium oxide

Abstract

Adsorption of salicyl hydroxamic acid (SHA) on neodymium oxide (Nd₂O₃) surfaces was studied at different SHA concentrations. The adsorbed surfaces were examined using X-Ray Diffraction (XRD) and Laser Raman Spectroscopy (LRS). XRD results show that the oxide remains intact until stoichiometric amounts equivalent to Nd(SHA)₃ were exceeded. At lower concentrations, the behavior appeared to be an ion exchange of SHA with hydroxide as well as carbonates and chlorides. Results are attributed to a monolayer of chemisorbed SHA at low concentrations and multilayers of surface precipitated Nd(SHA)₃ at higher concentrations. LRS corroborated these findings. Peaks from the C-H stretching band and C = N vibration bands were identified and areas under the peaks were quantified. Formation of the hydroxamate compound was ascertained from the C = N bands around 1597 cm⁻¹. It was concluded that chelation occurs through chemical bonding of SHA with surface Nd confirming chemisorption and with trivalent Nd confirming surface precipitation. Thermodynamic calculations in the form of solubility speciation diagrams suggest that chemisorption predominates at moderate pH between 6 and 9 and surface precipitation predominates at higher pH between 9 and 12. Adsorption isotherms were determined for the monolayer and found to yield free energy values between physisorption and chemisorption but, due to the reaction being ion exchange, it was concluded to be chemisorption.

Graphical Abstract

Keywords:

• Chemisorption
• ion exchange
• salicyl hydroxamic acid
• chelation
• surface precipitation
• neodymium oxide

Acknowledgements

Research was sponsored by the Army Research Laboratory and was accomplished under Cooperative Agreement Number W911NF-15-2-0020. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.