Graphite Oxide/Polyoxometalate Nanocomposites as Adsorbents of Ammonia

Abstract

Adsorbents for ammonia were prepared by modifications of graphite oxide with addition of a polymer followed by impregnation with either one of two different Keggin polyanions: H3PW12O40 or H3PMo12O40. The high acidity of those polyoxometalates combined with the functionalities and acidity of graphite oxide are expected to initiate strong interactions with NH3. The materials were characterized before and after exposure to ammonia by X-ray diffraction, transmission electron microscopy, sorption of nitrogen, potentiometric titration, thermal analyses, and Fourier transform infrared spectroscopy. Even though the nanocomposites prepared have a disordered structure, an improvement in ammonia retention compared to the parent graphite oxide was observed owing to the formation of complexes between ammonium and polyanions. It was proposed that, in the process of nanocomposite formation, the negatively charged carboxylic groups, located at the edges of graphene-like layers, first interact with the positively charged polymer. Then, the introduced polyacids are attracted via polar interactions to the polymer and oxygen-containing groups, mainly epoxy type, attached to the GO layers forming the house-of-card structure. Interaction of polyacids with the groups located at the edges of the carbon layers causes formation of aggregates and thus limits the accessibility of the interlayer space which results in a decrease in the amount of ammonia adsorbed.