Changes in graphite oxide texture and chemistry upon oxidation and reduction and their effect on adsorption of ammonia

Two graphite oxides were prepared by a subsequent two-step oxidation of commercial graphite. The samples were then reduced with hydrazine hydrate. Surface characterization before and after exposure to ammonia was carried out using X-ray diffraction, Fourier-transform infrared spectroscopy, potentiometric titration, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy, thermal analysis and nitrogen adsorption. The results indicated significant differences in the texture and chemistry of the materials, which had effects on their ammonia adsorption capacity. Strong oxidation led to relatively amorphous samples with a developed porosity owing to the treatment. Acidity is governed by the presence of sulfonic and carboxylic groups. Even though these groups react with ammonia, the significant adsorption capacity is linked to the presence of surface epoxides, which can react with ammonia leading to the formation of amines. While treatment with hydrazine expanded the surface area of the mildly oxidized sample, in the case of strongly oxidized graphite only surface chemistry was affected and a significant amount of nitrogen was introduced to it. Creation of porosity after reduction did not enhance ammonia adsorption due to the absence of reactive oxygen (epoxides) sites.