A review on surface modification of activated carbon for carbon dioxide adsorption

Abstract

The influence of surface modification of activated carbon with gaseous ammonia on adsorption properties toward carbon dioxide (CO₂) was reviewed. It was apparent from the literature survey that the surface chemistry of activated carbon strongly affects the adsorption capacity. In general, CO₂ adsorption capacity of activated carbon can be increased by the introduction of basic nitrogen functionalities into the carbon surface. Accordingly, in this review the impact of changes in surface chemistry and formation of specific surface groups on adsorption properties of activated carbon were studied. Two different methods, heat treatment and ammonia treatment (amination) for producing activated carbon with basic surface were compared. Amination was found to be suitable modification technique for obtaining efficient CO₂ adsorbents. Finally, the common characterization methods were also mentioned.

Introduction

Global warming is widely attributed to an increase in atmospheric level of greenhouse gases. Carbon dioxide (CO₂) is considered as the most important greenhouse gas with the largest impact on climate change [1]. The gradual increase in the atmospheric concentration of CO₂ due to burning of fossil fuels is becoming a serious environmental problem [2]. CO₂ capture and sequestration from point source emissions has been recognized as a potential way to stabilize CO₂ in the atmosphere [3], [4], [5]. Various processes, such as liquid solvent absorption, cryogenic techniques, membrane separation, solid sorbents, and pressure (and/or temperature) swing adsorption have been proposed for the separation and recovery of CO₂ emitted by power plants [6], [7], [8], [9], [10], [11]. To date, most of commercial CO₂ capture plants use amine-based processes and wet scrubbing systems [12], but they are energy intensive due to the large amount of water needed in these systems [2], [3]. Other problems associated with these processes are amine degradation by oxidation leading to corrosion of process equipments [13], [14], [15].

The goal in reducing CO₂ emissions on an industrial scale is the development of cost-effective techniques for the separation and capture of CO₂ [16]. Adsorption is considered as one of the potential options because of the low energy requirement, cost advantage, and ease of applicability over a relatively wide range of temperatures and pressures [2], [17]. However, the success of this approach is dependent on the development of easily regenerable and durable adsorbent with a high CO₂ selectivity and adsorption capacity [4], [18]. Zeolites have shown promising results for separating CO₂ from gas mixtures. However, the presence of water inhibits the CO₂ adsorption capacity of zeolites [19], [20], [21]. Activated carbons are being proposed as suitable candidates for CO₂ capture: they do not require any moisture removal, present a high CO₂ adsorption capacity at ambient pressure and, moreover, they are easy to regenerate [22]. It has been recognized that the surface chemistry of activated carbon can strongly affect the adsorption capacity [7]. Due to acidic role of CO₂ (weak Lewis acid), it is expected that the introduction of Lewis bases onto the activated carbon surfaces may favour the CO₂ capture performance of these materials [18].

One of the popular ways used for the preparation of activated carbon with increased basicity is to remove or neutralize the acidic functionalities, and other way is to replace acidic groups with proper basic groups (e.g., basic nitrogen functionalities). It has been shown that introduction of nitrogen functional groups into the carbon surface can increase the capacity of activated carbon to adsorb CO₂ [1], [3], [7], [15], [16], [17], [23], [24], [25], [26]. Nitrogen containing functionalities can be introduced through either reaction with nitrogen containing reagents (such as NH₃, nitric acid, and amines) or activation with nitrogen containing precursors [17], [27], [28], [29], [30], [31], [32], [33], [34]. The objective of ammonia treatment is to increase the basicity of activated carbon by introducing basic nitrogen functionalities to the carbon surface [31], [32], [35], [36], [37]. When the carbon materials are treated with ammonia at high temperatures, ammonia will decompose to free radicals such as NH₂, NH, and atomic hydrogen and nitrogen. Those free radicals attack the carbon to form nitrogen containing functional groups [38], [39], [40]. Several authors have studied thermal treatment of carbons in an ammonia atmosphere [30], [31], [32], [38], [39], [40], [41], [42]. Economy et al. [43] treated activated carbon fibers (ACF) using NH₃ at elevated temperature to obtain a product with basic characteristics. Plaza et al. [18], [22] proposed the modification of activated carbon with gaseous ammonia as a suitable technique to produce efficient CO₂ adsorbents. Various characterization methods have been used to detect and verify the existence of surface functional groups of the activated carbons [44], [45]. The most common of these methods are chemical titration, temperature programmed desorption (TPD), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR).

The main objective of this review is how activated carbon should be modified to enhance surface basicity for carbon dioxide adsorption.