Effect of surface chemical and structural heterogeneity of copper-based MOF/graphite oxide composites on the adsorption of ammonia.

Two graphite oxides (GOs), obtained by oxidation of graphites of different origins, were used as composite components with copper-based metal-organic frameworks, MOFs. Such composites were tested for ammonia adsorption at room temperature, in dry and moist conditions. The materials were characterized by X-ray diffraction, FT-IR spectroscopy, adsorption of nitrogen, and thermal analysis. Generally, the ammonia adsorption capacities of the composites were found to be lower than those calculated for the physical mixture of their components. An involvement of NH3 adsorption sites of MOF in a composite formation was found to be a major factor, lowering the adsorption capacity of a composite in dry conditions. The composites with the smaller amount of GO were found to be better adsorbents of ammonia in the absence of moisture than those, with the higher amount of GO. The adsorption of ammonia in moist conditions resulted in a collapse of MOF structure, accompanied by the release of active groups. These groups contributed to the enhanced adsorption of ammonia via acid-base reactions. Thus, in the presence of moisture, the composites with the higher amounts of GO, and the ones, containing more carboxylic groups than epoxy groups in GO, were found to be the best performing samples.