Abstract
Nitrogen-doped activated carbons with high surface areas obtained from resorcinol and formaldehyde resins were evaluated as CO2 adsorbents in a simulated flue gas stream under anhydrous and humid conditions. These carbons were prepared using two approaches, namely ammonia treatment without nitric acid pre-oxidation and amination after pre-oxidation. The pre-oxidation of activated carbons considerably enhanced the nitrogen incorporation during the amination process. The amination temperature affects the content and type of nitrogen incorporated onto the carbon surface, as determined by X-ray photoelectron spectroscopy, which enhances the specific adsorbent-adsorbate interaction for CO2 in humid conditions. The presence of H2O in the feed gas significantly decreased CO2 adsorption for a very low nitrogen content of virgin activated carbon. A sample prepared via the amination of pre-oxidized carbon at 700°C (NORF700) exhibited excellent tolerance to moisture and the highest CO2 capacity of 2.10 mmol/g in a 7% CO2/83% N2/10% H2O wet stream at 50°C and 130 kPa. The high performance of NORF700 was ascribed to its high surface area, adequate micropore volume, and high amounts of pyrindinic-like and pyrrole-like nitrogen species. The results indicate that nitric acid pre-oxidation followed by ammonia treatment at 700°C is an appropriate process for preparing adsorbents for CO2 separation in post-combustion applications.
Highlights
Due to global climate change, the increased concentrations of greenhouse gases have become a concern
The results indicate that nitric acid preoxidation followed by ammonia treatment at 700°C is an appropriate process for preparing adsorbents for CO2 separation in post-combustion applications
FTIR Spectra of Initial and Modified Samples To investigate the effect of pre-oxidation on the functional groups present on the surface of the samples, a carbon denoted as ORFAC was prepared by the oxidation of nitric acid
Summary
Due to global climate change, the increased concentrations of greenhouse gases have become a concern. Carbon dioxide (CO2), released by the combustion of fuels and from certain industrial and resource extraction processes, makes the largest contribution to anthropogenic climate change (Plaza et al, 2008). The basic surface of carbon materials ensures a good adsorption for CO2 since CO2 is a weak Lewis acid (electron acceptor) that can interact with electron donors, such as nitrogen surface groups. Plaza et al (2010) modified AC with gaseous ammonia at temperatures from 400 to 900°C and found that the optimum temperature was 800°C, at which CO2 adsorption capacity and nitrogen incorporation reached their maximum values. Due to the small size (3 Å) and basic chemistry of ammonia gas, effective adsorption of NH3 requires an acidic adsorbate surface and specific interactions between NH3 molecules and the adsorbate surface, including hydrogen bonding, acid-base reactions, and complexation. Using an oxidizing agent such as nitric acid is a convenient way to form acidic groups on AC, since nitric acid has been shown to be a very effective oxidizing agent for the introduction of a significant number of oxygenated acidic functionalities in the form of carboxylic and phenolic hydroxyl groups onto the carbon surface (Puri, 1970)
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