Ammonia modification of activated carbon to enhance carbon dioxide adsorption: Effect of pre-oxidation

Abstract

A commercial granular activated carbon (GAC) was subjected to thermal treatment with ammonia for obtaining an efficient carbon dioxide (CO₂) adsorbent. In general, CO₂ adsorption capacity of activated carbon can be increased by introduction of basic nitrogen functionalities onto the carbon surface. In this work, the effect of oxygen surface groups before introduction of basic nitrogen functionalities to the carbon surface on CO₂ adsorption capacity was investigated. For this purpose two different approaches of ammonia treatment without preliminary oxidation and amination of oxidized samples were studied. Modified carbons were characterized by elemental analysis and Fourier Transform Infrared spectroscopy (FT-IR) to study the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties. The texture of the samples was characterized by conducting N₂ adsorption/desorption at −196 °C. CO₂ capture performance of the samples was investigated using a thermogravimetric analysis (TGA). It was found that in both modification techniques, the presence of nitrogen functionalities on carbon surface generally increased the CO₂ adsorption capacity. The results indicated that oxidation followed by high temperature ammonia treatment (800 °C) considerably enhanced the CO₂ uptake at higher temperatures.

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.