Experimental investigation on the ammonia adsorption and heat transfer characteristics of the packed multi-walled carbon nanotubes

Carbon nanotube (CNT) is considered as a kind of potential adsorbent because of its large surface area, uniform micropores and unique physicochemical properties. The adsorption performance of ammonia on the packed multi-walled carbon nanotubes (MWCNTs) were studied in this paper. Firstly, the packed MWCNTs were characterized by the scanning electron microscope (SEM) and transmission electron microscopy (TEM). Secondly, the effect of working pressure and adsorbent temperature on the adsorption capacity of ammonia by the packed MWCNTs was investigated. Thirdly, Langmuir and Freundlich models were used to analyze the equilibrium adsorption data and the adsorption kinetics was also discussed using a classical gas diffusion model. The Freundlich isotherm model could elucidate well the adsorption of ammonia on the MWCNTs when compared to the Langmuir equation. The research results showed that the equilibrium adsorption amount of ammonia by the MWCNTs varied between 22.69 and 90.05 mg/g_CNT at the adsorbent temperature of 25-35 °C and working pressure of 0.368-0.744 MPa. It seems that the pure MWCNT is not appropriate to act as the adsorbent for the solid-gas adsorption refrigeration due to its low adsorption capacity. However, our research indicates that the MWCNTs can be used as additive to some other chemical adsorbents to improve their heat transfer characteristics.