Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation
Graphical abstract
Research highlights
▶ Decomposition of oxygen functionalities was an intermediate stage to development of active sites before amination for the formation of nitrogen surface groups. ▶ Oxidation followed by high temperature amination considerably enhanced the CO2 uptake at higher temperatures. ▶ Compared to the textural characteristics, formation of basic nitrogen functionalities plays a more active role on CO2 adsorption at higher temperatures. ▶ CO2 capture capacity depends on both the type and amount of the nitrogen surface groups introduced onto the carbon surface.
Introduction
It is widely accepted that carbon dioxide (CO2) is the most important greenhouse gas with the largest impact on climate change [1], [2]. The increase in the atmospheric level of CO2, has led to the search for technologies designed to capture CO2 from point source emissions and stabilize its concentration in the atmosphere [3], [4], [5]. To date, most of commercial CO2 capture plants use amine-based processes and wet scrubbing systems [6], [7], but they have serious drawbacks, such as high energy requirements and corrosion of process equipment [1], [2], [8], [9], [10], [11]. Adsorption is considered as one of the most viable options that can be applied to carry out the separation of CO2 as it could reduce the cost associated with the capture step [4], [10], [11], [12], [13]. Among all adsorbents, activated carbons are being proposed as suitable candidates for CO2 capture: they are inexpensive, less sensitive to moisture, present a high CO2 adsorption capacity at ambient pressure and, moreover, they are easy to regenerate [14], [15].
The CO2 adsorption performance of activated carbon is strongly influenced by the modification of surface chemistry [16]. The basic nature of the sorbents is expected to be favorable for their application in the adsorption of an acidic gas, such as CO2 [17], [18]. It has been recognized that introduction of basic nitrogen functionalities into the carbon surface can increase the capacity of activated carbon to adsorb CO2 [1], [3], [4], [5], [9], [10], [11], [17], [18], [19], [20], [21], [22]. One of the most common procedures used for creation of nitrogen surface groups is the reaction with nitrogen containing reagents (such as NH3 and amines) [19], [23], [24], [25], [26], [27], [28]. The objective of ammonia treatment is to increase the basicity of activated carbon by introducing basic nitrogen functionalities to the carbon surface [27], [28], [29], [30], [31]. Several authors have studied thermal treatment of carbons in an ammonia atmosphere [24], [26], [27], [28], [32]. Plaza et al. [16], [18] proposed the modification of activated carbon with gaseous ammonia as a suitable technique to produce efficient CO2 adsorbents.
Although the presence of acidic oxygen functionalities individually is undesirable for CO2 adsorption, before introduction of nitrogen functionalities onto the carbon surface the carbon is usually oxidized. The main reason for surface oxidation and development of oxygen surface groups is the role of these functionalities as an intermediate stage to develop some oxygenated anchoring sites before introducing nitrogen functionalities to the carbon surface [24], [33]. When the oxidized carbons are treated with ammonia at high temperatures, the free radicals (such as NH2, NH, and atomic hydrogen) which were created during ammonia decomposition may attack to the surface oxides and active sites present on the carbon surface to form nitrogen containing functional groups [26], [27], [29], [32]. Mangum et al. [25] modified activated carbon fiber (ACF) with dry ammonia and demonstrated that reactivity of ammonia gas with carbon surface and consequent formation of N-containing functionalities increase with the oxygen content of precursor carbon.
It has been reported that CO2 adsorption capacity of activated carbon decreases with increasing adsorption temperature due to the physical nature of the adsorption process [4], [11], [15], [16], [18], [19], [21], [22]. As in CO2 capture plants, generally separation from the flue gas streams should be carried out at relatively high temperature (up to 100 °C) [1], [12], [22], the main objective of this work is to develop an adsorbent with high adsorption capacity at higher temperatures. To develop such an adsorbent, previous works have mainly focused on zeolite-based adsorbents due to their promising results in CO2 separation from gas mixtures. However, the presence of water inhibits the CO2 adsorption capacity of these materials [12], [34]. Accordingly, in this study oxidation preceding high temperature amination is proposed as a suitable modification technique for improving the capture performance of activated carbons at relatively high temperature.
Section snippets
Activated carbon sample
A commercial palm shell-based granular activated carbon (GAC) was used as starting material for the preparation of CO2 adsorbent. The precursor was ground and sieved to the US mesh size 20–35 (850–500 μm) for all further treatments. In order to eliminate fines, it was then thoroughly washed with distilled-deionized water (DDW), dried at 105 °C for 24 h to remove moisture, and stored in a vacuumed desiccator until use. The following is a brief outline of the process used for the modification of
Ultimate analysis, proximate analysis and point of zero charge
Elemental analysis and pHPZC results of the virgin and ammonia modified carbons are shown in Table 1. As can be seen from the table, reactions between the carbon surface and the radicals that were created during ammonia decomposition successfully incorporated nitrogen into the carbon structure. For instance, nitrogen content increased from 0.3 wt.% for virgin carbon to 3.1 wt.% and 4.6 wt.% for HTA-800 and OXA-800 samples, respectively. The differences in nitrogen content between the pre oxidized
Conclusion
In this work, the impact of changes in surface chemistry on CO2 adsorption performance of the modified activated carbon samples was studied. Two methods for producing activated carbon with basic surface were considered: ammonia treatment without preliminary oxidation and amination of oxidized samples. In summary, modification of the surface chemistry had different effects on CO2 capture capacity depending on the methods employed. It was found that decomposition of oxygen containing
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