Surface morphological characterization of activated carbon-metal (hydr)oxide composites: some insights into the role of the precursor chemistry in aqueous solution

Abstract

Morphological features of metal (hydr)oxide (MO) particles supported on activated carbon (AC) largely influence the performance of these composite materials in most of their applications, particularly in heterogeneous catalysis. Furthermore, the MO precursor as well as the preparation method and conditions strongly determine these morphological features. Thus, the present work is aimed at shedding light on the role of the precursor chemistry on the surface morphology of a series of AC-MO composites prepared by wet impregnation of a commercial AC with Al(NO₃)₃, Fe(NO₃)₃, and Zn(NO₃)₂ in aqueous solution. These materials are characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The microstructure, morphology, size distribution and degree of dispersion of the supported MO (nano)particles strongly depend on the chemical transformations undergone by the precursors not only in the impregnation solutions after their contact with AC but also during the oven-drying step. Al³⁺, Fe³⁺ and Zn²⁺ species in aqueous medium are involved in hydrolysis and polymerization processes, which notably modify the pH of the starting precursor solutions. Upon their contact with AC, pH markedly increases due to the strong basic character of the carbon surface (pH_pzc ≈ 10.50), leading to the precipitation of the metal hydroxides or oxyhydroxides. Both supported bayerite (α-Al(OH)₃) and goethite (α-FeO(OH)) are essentially amorphous; however, the former grows in micrometric particles while the latter does as nanoparticles. By contrast, the higher crystallinity and larger particle size of supported wülfingite (ε-Zn(OH)₂) are connected with an additional transformation of the as-precipitated amorphous hydroxide during the heating step at 120 °C.

Graphical Abstract

Keywords:

• Activated carbon
• aqueous solution
• metal (hydr)oxides
• precursor chemistry
• surface characterization

Acknowledgements

Financial support by Junta de Extremadura and European FEDER Funds is gratefully acknowledged. Adrián Barroso-Bogeat thanks support from the “Juan de la Cierva-Formación” Fellowship Program of Spanish Ministry of Science, Innovation and Universities (FJCI-2015-25999).