Textural characteristics, surface chemistry and oxidation of activated carbon

Journal of Natural Gas Chemistry 19(2010)267–279

Review

Textural characteristics, surface chemistry and oxidation of activated carbon Wan Mohd Ashri Wan Daud,

Amir Hossein Houshamnd∗

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia [ Manuscript received October 30, 2009; revised December 9, 2009 ]

Abstract Numerous researches were reviewed and interpreted to depict a comprehensive illustration of activated carbon and its behavior towards oxidation. Activated carbon as one of the most important adsorbents is tried to be described in this review paper by terms of its “Textural Characteristics” and “Surface Chemistry”. These two terms, coupled with each other, are responsible for behavior of activated carbon in adsorption processes and in catalytic applications. Although as-prepared activated carbons are usually nonselective and their surfaces suffer from lack of enough reactive groups, their different aspects may be improved and developed by diverse types of modifications. Oxidation is one of the most conventional modifications used for activated carbons. It may be used as a final modification or as a pre-modification followed by further treatment. In this paper, methods of oxidation of activated carbon and other graphene-layer carbon materials are introduced and wet oxidation as an extensively-used category of oxidation is discussed in more detail.

Professor Wan Mohd Ashri Wan Daud was born in 1968 in east coast of Malaysia. He received his bachelor degree in the field of chemical engineering from Leeds University, UK in 1991. He got Master of engineering in the field of combustion science and pollution control and PhD in reaction engineering from University of Sheffield, UK in 1993 and 1996, respectively. Now he is a professor in the Department of chemical engineering at University of Malaya.

Amir Hossein Houshamnd was born in 1969. He received his bachelor’s and Master’s degrees in chemical Engineering from Amirkabir University of Technology (AUT), Iran in 1992 and 1995. He worked for 5 years as an operation engineer in Arak Petrochemical Complex, Iran and 5 years as head of process discipline of HDPE Project in Ilam Petrochemical Company, Iran. Now he is a PhD student of chemical engineering at University of Malaya (UM), Malaysia.

Key words activated carbon; graphene-layer carbon; surface chemistry; textural characteristics; oxidation

1. Introduction

Nowadays activated carbon (AC) is extensively used as catalyst, catalyst support [1−3] and also as adsorbent to capture a variety of species such as organic substances, metal ions and gas/vapour adsorbate from gas/liquid phase. As a catalyst, AC is applicable for catalytic wet air oxidation of, e.g., ammonia [4], aniline [5] and phenol [6], nonoxidative dehydrogenation of light alcohols, catalytic oxidative dehydrogenation of, e.g. cyclohexanol [7] and so on. The high performance of catalytic properties of AC may stem from its high adsorption capacity, as believed for phenol decompo∗

sition [6]. As an adsorbent, activated carbon has some certain advantages over other adsorbents such as zeolite, silica and porous polymers. Generally, activated carbons are relatively cheap, resistant to heat and radiation, stable in acidic and basic solutions [3,4], show good mechanical strength, do not swell or shrink very much by changing pH and are cost-effective from the viewpoint of regeneration, whereas silica degrades in solution of high pH, polymers show swelling or shrinking by change of pH [8,9] and zeolites usually are low-efficient and high-cost in regeneration because of their small pore structure [10]. Besides, surface characteristics of AC may be tailored based on requirements of its different applications [6].

Corresponding author. Tel: +60 12 6069431; Fax: +60 3 79675371; E-mail: [email protected] (A. H. Houshmand)

Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60066-9

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Internal and external surface of activated carbon may be modified using functional groups, small or large molecules/species and also by using macromolecules (polymer chains). In most cases, when we speak about “functional groups” on surface of activated carbon, we considered them covalently and chemically attached or bonded onto the surface. However, the conditions are different for molecules and polymer chains: They may be physically adsorbed (through impregnation [11−14]) or chemically bonded (via diverse grafting methods [15−25]) onto the surface of AC and other supports. Covalently bonded functional groups may be generated on the surface of AC by different ways including heat treatment, chemical treatment, UV, high-energy electron, γirradiation, plasma treatment, ozone exposure, etc. [17]. These attached functional groups either may improve carbon characteristics for a particular application or may be used for further modification, because in some cases AC surface initially suffers from the lack of chemically reactive functional groups appropriate for a desired modification. Oxidation is one of the most conventional methods for modification or pre-modification. Oxygen-containing groups created by oxidation are extensively used for further modification [18−22,26,27]. However, in some cases, oxidation is used as target modification. For instance, acidic groups created by oxidation favor catalytic wet air oxidation of aromatic compounds (e.g. of aniline [5]). As reported in literature, catalytic activity is influenced by surface chemistry [2]. The acidic groups improve metal ion/metallic species adsorption [28−31] from solution by mechanism of ion-exchange and complex formation. This property is used for both pollution control and preparation of supported metal catalyst [1−3,9,32]. In the latter, acidic groups also facilitate dispersion of catalyst [1−3,32] on activated carbon surface. The acidic surface groups also enhance adsorption of some organic compounds such as sulfur compounds (e.g. in fuel desulfurization) [33−35], acetaldehyde (under some conditions, depending on degree of oxidation and acetaldehyde concentration) [36] and aqueous ammonia [4]. Also, reaction with acidic groups, especially carboxylics [37] is one of the postulated mechanisms for phenol adsorption. Dispersion (π-π electron) interaction, donoracceptor complex formation and hydrogen bonding are other possible mechanisms [6,38]. However, it was reported that oxidation of AC decreases phenol adsorption [38−40]. This is attributed to decrease in both basicity of basal planes and surface area imposed by oxidation. In the mentioned works, dispersive interaction is considered as prevailing mechanism for phenol adsorption [41]. Because of similarity between structural units of AC and aromatic compounds, reactions of AC with many reagents are similar to the reaction of aromatic compounds with the same reagents [42]. Similar to AC, carbon black, carbon fibres and carbon nanostructures such as carbon nanotubes (CNT), carbon nanofibers, graphitic carbon nanofibers, carbon nanobeads and so on, which have graphene structure

may be oxidized (or go under other types of reactions and modifications) in similar ways due to the same aromatic components in their structures. In this paper, some researches related to carbon materials other than AC have been paid attention. Generally, CNTs may be oxidized to gain chemically bonded oxygen functional groups for further modification with other functional groups or molecules. Attached functional groups or organic molecules improve CNTs dispersity in a polymer bulk and/or increase attractive forces between CNTs and polymer chains by creating, for instance, hydrogen bonding. This leads to produce a composite with enhanced mechanical properties [43,44]. CNTs are considered as key building block in nanotechnology field [20]. Also, functionalized carbon nanomaterials have found application for adsorption purpose. As the last point in this introduction, every treatment for improving surface chemistry of AC may change textural characteristics such as surface area, pore volume, pore size distribution, etc [28]. 2. Textural characteristic and surface chemistry of AC 2.1. Porous morphology vs. surface chemistry Activated carbon has a wide variety of origins such as coal, wood, petroleum coke and anthracite [45], and it may also be produced from biomass or agricultural solid waste of palm shell, fruit stones, coconut shell, sugar cane bagasee, etc. Compared to coal-based AC, these biomass-based ACs are low-ash content [46,47]. In order to describe activated carbon and especially its adsorption properties, two main concepts are important to be considered: porous morphology (surface area and porous structure) and surface chemistry [12,48−50]. AC is commonly considered as a non-selective adsorbent [26] containing graphene layers of carbon atoms having sp2 hybridization with slit-shaped porosity [27,36,46,51,52]. The origin of precursor, carbonization conditions, activation conditions and agents, and further treatments (post-treatments) are influential in porous morphology and surface characteristics [53−55]. Lahaye [48] believes that porous morphology is significant for application of AC in gas adsorption and its surface nature has a more important role for liquid phase adsorption and catalytic application. Biniak et al. [9] indicate that the role of surface chemistry in adsorption of inorganic compounds from aqueous solution is higher than porosity. However, it is well known that porous structure determines adsorption capacity [34,46,53,56−58] and is important in adsorption mechanism [58]; Chemistry of surface strongly influences hydrophobicity, electronic density of graphene layers and type of interaction of AC with adsorbate [34,36,40], because adsorbate molecules may interact with some surface functional groups. Speaking in a more exact way, on the surface of AC, two distinct regions can be distinguished [59,60]; a basal car-

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bon surface, which is non-polar and hydrophobic, and a region containing diverse functional groups. For a specified adsorbate, there may be a specific interaction between adsorbate species and a particular functional group. Adsorption stemmed from this interaction is recognized as specific adsorption, in which the driving force of enthalpy is involved. Dipole-dipole interaction (electrostatic attraction), Hydrogen bonding, complex formation and ion exchange are possible mechanism of interaction between adsorbate species and surface functional groups [37,41,60,61]. However, adsorption of an adsorbate is not limited to specific adsorption; there is usually an interaction between adsorbate species and non-polar surface, in which hydrophobic dispersive interaction [41] and driving force of entropy are involved. This part of adsorption is important, because non-polar surfaces are a significant fraction of total surface of AC [59]. It is noteworthy that introduction of new functional groups lowers the surface area of non-polar crystallite. Hence, surface chemistry may be deciding that either specific or nonspecific interaction is the prevailing mechanism for adsorption of a given adsorbate [41]. On the other hand, since the number of active sites on surface for activation is limited to some special regions, the number of functional groups cannot exceed a specified value [59]. The work done by Pittman et al [60,62,63] also reveals this limitation; they observed that the rate of generation of functional groups on carbon fibres by nitric acid decreases with time. Decoupling the roles of textural characteristics and surface chemistry in adsorption application may lead to select the better precursor and suitable activation and modification processes as it was carried out by Zhang et al [64] for lead adsorption using empirical modelling. When the surface of an adsorbent is modified, e.g. new surface functional groups are created on surface, physicochemical properties, chemical, thermal and hydrothermal stability and reactivity both in adsorption and catalytic applications will change depending on nature of modifying groups or molecules and their bonding type onto the surface. Consequently, surface modification of activated carbon conditions its applications. For instance, if the adsorbents are used in a Temperature Swing Adsorption (TSA) process, they should have enough thermal and hydrothermal stability [65]. Apart from characteristics of AC, the ratio of adsorbate molecular size to pore sizes of AC is important for adsorption process. Adsorption capacity for a specified adsorbate depends on accessibility of the adsorbate molecules into pores and consequently on the size of pores and of adsorbate [41]. Besides, micropores (<2 nm) located between carbon (graphene) layers, AC may consist of mesopores (2 nm> and <50 nm) and macropores (>50 nm) in different proportions. Existence of mesopores and macropores depends on precursors [51]. Micropores are sometime categorized to two subcategories, for example, narrow micropores (<0.7 nm) and super micropores (0.7−2 nm) [66] or primary micropores (<0.8 nm) and secondary micropores (0.8−2 nm) [67]. ACs with dominantly microporous struc-

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ture are effective for adsorption of small molecules, especially adsorption of gases whose molecular size is usually between 0.4−0.9 nm, whereas adsorption of large molecules are more efficient by using mesoporous ACs [5,13,21,36,46,68−70]. A high capacity of sorption may be achieved, if there is compatibility between molecular size of adsorbate and pore diameter [55]. 2.2. Surface chemistry Although AC surface is primarily non-polar, some surface oxides already exist. AC owes its surface chemistry to the existence of heteroatoms such as oxygen, nitrogen, hydrogen, phosphorus and sulfur [20,37,49,71]. The ability of activated carbon to be modified by heteroatoms is one of its interesting and advantageous characteristics. The amounts of these atoms depend on origin of AC and activation or modification method [49]. Active sites available on surface of carbon including edge carbon and defect carbon rings are exposed to reaction with heteroatoms. Oxygen is the most important heteroatom that is covalently bonded to carbon, however carbon-oxygen bond is less stable than carbon-hydrogen bond [72]. Surface functional groups containing oxygen have a major role in AC properties such as surface behaviour and reaction, hydrophobicity, surface charge, electron density of graphene layers and catalytic properties [41,42] and may be used for further surface modification. It should be noticed that ash or mineral content of AC seems to be important in its surface chemistry, too [42]. The functional groups from heteroatoms may be categorized to acidic, basic and neutral [53,55,56,73]. Basic behaviour of AC surface usually originated from some oxygencontaining groups such as pyrone structure [51,74−76] and superoxide ions (O− 2 ) [9] and also nitrogen-containing groups including pyridinic (N6 ), Quaternary ammonium (NQ ), pyrolic and pyridonic (N5 ), oxides of nitrogen (NX ), nitriles, amine and amide groups depending on the type and conditions of treatment applied on AC [74,75,77]. Nitrogen also can change the charge distribution within graphene layers [70,71]. Some other oxygen-containing groups including chromene and diketone or quinone, inorganic impurity [32,76] and unsaturated valence [69,70] have been reported to contribute to basic character of activated carbon. On the other hand, non-polar surface of AC has basic properties originated from π-electrons of carbon [51,59,61,69,70,75,78,79]. Although some researchers have suggested that basic property stemmed from basal planes is weak compared to that stemmed from basic functional groups [72], Moreno-Castilla et al. [41,79] believe that basicity of AC mainly stemmed from basal planes’ π-electron density. Evidences for the latter are the decrease in basicity of AC and the decrease in net enthalpy of neutralization of basic groups with increasing acidic groups by oxidation. Figure 1 [77] shows how the different nitrogen groups are bonded onto AC surface. Heat treatment at elevated temperature causes mainly the staying of N6 , NQ , N5 and NX on

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surface [52,74,75,77,80,81]. These nitrogen groups are not decomposed lower than 1200 K [75].

Figure 1. Types of nitrogen groups on AC surface. (a) and (d) amide groups, (b) tertiary amine, (c) lactam, (e) pyridine-pyrrole-like, (f) nitrile [77]

Although there are several possible structures of oxygencontaining groups on AC surface, as shown in Figure 2 [48], the most common ones are carboxylic, carbonyl, phenols and lactones [20,26,49,82]. Carboxylic, phenol, acid anhydrides and lactone groups are responsible for acidic behaviour of AC [42,51,53]. The rise of acidic group on surface usually leads to decline in basic groups [78,79]. Guo et al. [46] reported that palm-shell-based AC activated by CO2 has neutral or weakacidic function groups such as ketone, quinone, ethers and phenols. Oxygen content found in AC by elemental analysis

Figure 2. Possible structures of oxygen groups on surfaces [48]. (a) carboxyl, (b) phenolic, (c) carboxylic anhydride, (d) lactal, (e) carbonyl, (f) ether, (g) lactone, (h) quinone

usually is larger than detected functional (acidic/basic) oxygen groups. This oxygen is attributed to ether-type oxygen [51]. There are evidences that surface functional groups may react with one another: carboxylic groups close together may give carboxyl anhydrides. In addition, carbonyl group may condense with carboxyl or phenol in the neighbourhood to form lactone or lactol [51]. Julien et al. [58] reported that basic oxygen-containing groups such as pyrone and chromene are converted to acidic groups by the mechanism of heterocycle opening. Reduction of AC surface by LiAlH4 or sodium borohydride to increase phenol group was carried out by Alvez et al. [83] and Aburub et al. [59], respectively. Probably ketones and aldehyde convert to phenol groups by reduction [59]. Surface functional groups may also receive different kinds of interactions including inductive, mesomeric, steric, tautometric and hydrogen-bonding effects from other groups in their neighbourhood [20,84] that affect their behaviour: The inductive interactions may influence the acidity/basicity of functional groups so that they show a range of acidity or basicity instead of a single value [79,84]. For example, Gorgulho et al. [20] suggested that introduced carboxylic groups (on MWNT) exhibit a range of strength or dissociation constants (Ka). Also, it was reported that neighbouring to carbonyl and hydroxyl groups increases carboxyl acidity [79]. Density of acidic groups and size of the graphene layer affect this phenomenon [20,78]. The above-mentioned facts give a complexity to activated carbon from the viewpoint of behaviour of surface functional groups so that, as stated by Bondosz and Ania [72], “carbon surface should be considered as a unique whole entity rather than as a sum of individual functional groups”. 3. Analysis methods There are a number of experimental techniques to characterize surface chemistry of activated carbon. They can be classified into wet methods and dry methods [73]. Chemical titration method (developed by Boehm [51]) and potentiometric titration (PT), which are valuable quantitative techniques, are of wet methods. Dry methods consist of temperature programmed desorption method (TPD), X-ray photoelectron spectroscopy (XPS), infra red spectroscopy methods (FTIR and DRIFTS), etc. In addition, voltammetry and polarography as electrochemical analysis techniques may be used to study electrochemically active groups available on activated carbon surface in a qualitative manner [72]. Studying catalytic activity of H2 O2 decomposition [40], methanol dehydration [79], isopropanol [85] and so on are indirect techniques to specify acid-base character of activated carbon. For instance, basic groups increase activity in catalytic decomposition of hydrogen peroxide. Each analysis method has some limitations that can be considered as its defects: XPS can only analyze the uppermost layers of AC particles (5−15 nm) [60,66,74,86]. If the

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particles are crushed finely, XPS may assess more functional groups [66], although grinding may cause some changes on the carbon surface [84]. FTIR is a well-known method for analyzing surface chemistry, especially different oxides [73,87] for both qualitative and quantitative measurement [88]. DRIFT also is not a quantitative method of analysis, but shows trend in surface modification [36]. Groups revealed by Boehm titration are limited to those groups having a certain acidity, such as carboxylic, phenol and lactone groups, whereas other oxygen-containing groups (such as aldehydes, ketone, pyrone, ether) and nitrogen-, sulfur- and phosphorous-containing groups are not recognized by this method [73]. Besides, NaOH used in Boehm titration can titrate only phenolic OH and is not strong enough to titrate alcoholic OH [59]. Soluble inorganic compounds in AC can complicate Boehm’s titration [64]. Generally, Boehm titration provides good results, if other heteroatoms in activated carbon are negligible. If not, their acidic groups will be incorporated into oxygen acidic groups’ amount [72]. The same trend of results and also good quantitative overlap between Boehm’s and potentiometric titration have been reported [55,84]. TPD can determine all functional groups by their decomposition, so that amounts of surface groups determined by TPD may be higher than those by PT [20]. It will be more useful, when combined with mass spectrometric analysis [20]. Consequently, a combination of a few analysis methods results in more accurate information due to complexity of activated carbon surface. Salame et al. [73] believe that Boehm and potentiometric titration in combination with FTIR and TPD provides a valuable understanding of AC surface. However, a comparison of the results of the above-mentioned analysis methods on the same samples has rarely been done. Reproducibility of data is a subject that is encountered. El-Sayed and Bandosz [55] state that “It is almost impossible to achieve exact reproducibility of data when oxidation of carbons is carried out with nitric acid”. However, for other types of oxidations, a similar concern may exist. It has been reported that particle size is important to achieve reproducible results for titration methods [84]. 4. Oxidation of AC 4.1. Outlines Oxidation is mainly used to introduce oxygen-containing functional groups on the surface of AC [26,27]. The oxidation reaction is likely to be occurred on the aliphatic side chains of the carbon, i.e. on peripheral carbon atoms at the edge of carbon surface/crystallite or on defect sites of carbon surface, because these sites are highly susceptible to oxidation [20,49,51,54,59,89,90]. These generated groups are usually near the pore opening so that they can block the pores [59]. Oxidation of AC surface also lowers the π-electron density in the graphene layers and subsequently decreases the carbon dispersive (non-specific) adsorption potential [37]. Gen-

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erally, oxidation increases acidic groups and decreases basicity. However, as a rare case, a slight increase in basic groups as well as acidic groups by oxidation with nitric acid has been reported [55]. Oxidation of AC can be categorized [91] to (1) Dry oxidation in which AC is contacted with oxidizing gases such as steam, carbon dioxide, oxygen, ozone and so on usually at elevated temperature (maybe more than 970 K), and (2) Wet oxidation which is defined as reaction of AC with a solution of oxidizing agents under mild reaction conditions. Electrochemical oxidation, which is a possible approach for oxidation [78], may be considered in wet oxidation category. In this section, heat treatment of AC in different gas atmospheres is briefly reviewed, and wet oxidation and oxygen plasma treatment will be discussed in Sections 4.2 and 4.3, respectively. When speaking about oxidation, it is worth mentioning the concept of “aging”: When AC is heated to a specified elevated temperature in an inert atmosphere, surface oxides previously generated or originally available on the surface are decomposed. After cooling to room temperature, some remained reactive sites on AC surface interact with oxygen of air and/or water vapour to create new oxides on surface. This phenomenon is called aging [42,47,51]. The oxygen functional groups created by aging are mainly basic rather than acidic [72]. As well, AC’s external surface is mainly subject to aging oxidation [42]. Oxidation in air or oxygen may increase phenolic and ketone groups [7] or hydroxyl and carbonyl groups [86]. It was reported that oxidation in moist air causes a large decrease in basic groups and a slight increase in acidic groups so that total number of surface functional groups is less than untreated AC [40]. Oxidation by ozone, called ozonation was reported to keep textural characteristics of AC (under some experimental conditions), but creates considerable amounts of carboxylic groups [29]. However, slight decrease in surface area and pore volume under mild condition and matrix destruction under severe conditions has been reported [38]. Besides, AC origin and history, ozonation temperature is determinant for density and type of created oxygen surface groups [38,91]. Alvarez et al. [38] reported that at low temperature (298 K) mainly carboxylic groups and at higher temperature (373 K) carboxylic, lactonic, hydroxyl and carbonyl groups are created. Thus temperature may be optimized based on AC type. For conducting ozonation study, thermal decomposition of ozone, especially taking place at high temperature (>423 K) should be taken into account [91]. It should be noticed that ozonation has more cost compared to air oxidation, although over the years a reduction in its cost has been taken place [91]. Although heat treatment in presence of oxidant gas/vapour (dry oxidation) increases oxygen surface groups, however, heat treatment at an elevated temperature and under inert atmosphere leads to different results. Researches on this process that is called annealing, degassing or pyrolysis in different literature, give mainly the similar results:

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increase in micropore volume without essential change in mesopore and macropore volumes. It is well known that oxygen functional groups significantly decrease by annealing [39,54,58,61,75,81,92] and so basicity of activated carbon rises [93]. These two phenomena (i.e. decrease in oxygen functional groups and increase in micropore volumes) are consistent with the previously-mentioned postulation that oxygen surface groups are located at the opening of pores and may block them. However, no substantial change in surface area and textural properties by annealing in vacuum has been reported [9,40]. When degassing, usually CO2 evolves in the temperature ranging from 670−920 K and CO evolves in the range of 870−1220 K. CO2 evolving usually corresponds to desorption of carboxylic and lactone groups, whereas CO evolving is attributed to quinone, carbonyl and OH groups [94]. A temperature range between 473 and 873 K for CO2 evolution and above 873 K for CO evolution has been reported, too [36,55]. Annealing in temperature above 1370 K may destroy the texture of carbon [39,58]. The study conducted by Norikazu et al. [95] indicates that microwave cold plasma heating of activated carbon leads to results somewhat similar to vacuum heating treatment, i.e. increase in surface area and moderate decrease in total oxygen surface groups. However, according to this work, raised output power and/or extended time of treatment will cause decrease in surface area probably due to pore destruction. “In situ” modification of wood-based AC precursor during its activation to create oxygen surface group has been studied and experienced by de Celis et al. [31] as a combination of activation and post-treatment. As the last point in this section, it is worth mentioning another type of heat treatment here that may change surface chemistry and textural properties of AC: Hydrogen treatment (heating under hydrogen atmosphere) at high temperature is a reduction process [4] and decomposes oxygen groups [35,72,96] with an increase in basicity [96]. Different or opposite effects on surface area and pore structure by hydrogen treatment in different activated carbons have been reported [35]. 4.2. Wet oxidation Numerous researches in literature have been dedicated to wet oxidations and their effects on diverse carbon materials. A number of oxidants are available for wet oxidation including H2 O2 (abbreviated as HP in this paper), HNO3 (abbreviated as NC), H2 SO4 (abbreviated as SA), (NH4)2 S2 O8 (abbreviated as APS), NaOCl, KClO3 , ClO2 , ZnCl2 , KIO4 , KMnO4 , AgNO3 , KNO3 , KBrO3 , K2 Cr2 O7 and H2 PtCl6 used under mild conditions, for instance 293−373 K [6,25,51,54,56,64,97,98]. Among all these oxidants, NC is probably the most used one, because oxidizing specifications can be controlled by concentration and temperature [97]. The first deduction from literature review is different and sometimes contrary results reported for wet oxidation

of activated carbons. Here, we categorize the reasons into two groups: (1) oxidant specifications and oxidation circumstances and (2) activated carbon specifications. We try to discuss these sources of diversity in AC oxidation using evidences from literature. Regarding the oxidant, the first parameter is obviously type of oxidant that determines its power. El-Hendawy [47] believes that power of NC, HP and air as some major oxidants is in the order of NC>HP>air and this is independent of porosity. Although NC may be the strongest oxidant, however, oxidant strength is not the only factor for determining degree of oxidation. Oxidant concentration, oxidation conditions (that determine oxidation severity) and also factors affecting diffusion of oxidant species in pores play roles in oxidation level [6]. The limited diffusion of oxidant into micropores, which is affected by oxidation conditions, may result in introduction of acidic groups mostly inside some wider micropores [47,99]. Severity of oxidation will be discussed later in this section. Different ACs may cause different changes in their texture and surface chemistry by a specified oxidant under the same oxidation conditions. Different origin, history of preparation and activation, and kind of AC texture [6,33,37,55,73,100] are possible justifications for this diversity. For instance, it was reported by some researchers that NC treatment of AC increases surface area, whereas some other researchers reported an opposite result [58,59]. In the study carried out by Salame et al. [37], who oxidized two types of AC, named W- and U-type with APS, phenol groups did not increase in oxidized U-type, whereas it had an increase in oxidized W-type, compared to their own untreated ACs. Oxidation process has more serious effects on AC activated in low temperature than on AC activated in high temperature [73]. AC activated in lower temperature is suspected to suffer destruction upon acidic treatment [77]. On the other hand, Moreno-Castilla et al. [79] stated that higher-activated AC willcause more influence by a specified oxidant, because they have thinner walls which are subject to easier destruction. As an example of the effect of original structure, it was reported that oxygen groups created on wood-based AC by NC is higher than on bituminous-based AC [55]. In order to investigate the effect of mesoporosity and microporosity on oxidation, Tamai et al. [21] treated polymerbased ACs with NC and observed that mesoporous AC and microporous AC show, respectively, increase and decrease in surface area under the same conditions of oxidation. MorenoCastilla et al. [100] and Alvarez-Merino et al. [67] noticed that ACs with more developed textures (developed meso- and macroporosity and greater microvolumes) and greater surface area will gain more functional groups by wet oxidation. It is well known that wet oxidation generally increases different oxygen-containing groups such as carboxylic, phenolics, lactones and carbonyl. However, these oxygen groups do not increase by the same factor. For instance, some researches indicate tendency toward phenol groups in treatment by HP and sodium hypochlorite. There are a lot of reports indicating that NC creates a large amount of carboxylic groups

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on AC surface [2,3,7,37,49,86]. Results of some studies indicate that NC treatment increases carboxylic and phenol groups in higher orders [42,60,62,63]. Strelko et al. [101] oxidized a commercial AC with a 20% NC solution and changed oxidation agent with new one in halfway of oxidation. They did not compare renewing oxidant in halfway with keeping oxidant throughout oxidation. In some cases, the decrease in density of few acidic groups has been reported [73,92]. Although wet oxidants usually remove basic sites and create acidic groups, creation of new basic groups by HP on AC surface has been observed [6]. Some oxidants have their own special features. For example, in case of oxidation with KMnO4 , total oxygen content seems to be higher than acidic groups. This fact is attributed to MnO2 dispersed on surface [98]. Reduction in surface area and pore volume by wet oxidation has been reported by several researchers [20,22,30,37,39,49,57,67,96,102,103]. This trend is attributed to one or a combination of reasons: Pore blockage by oxygen surface groups or by large molecules of humic material (produced by acidic treatment), electrostatic repulsion of surface probe molecules (nitrogen) and wall erosion or destruction of micropore walls (converting to mesopores) by liquid oxidants are possible postulations [20,36,38,42,47,75,77,92,100,101,104]. Although many investigations indicate the decrease in surface area by wet oxidation, there are a significant number of works in conflict, i.e. increased or constant surface area [32,40,56−60,62,63,77,86,98]. Increased surface area and mesopore volume of a commercial AC by APS and HP oxidation have been observed [56]. Also, increase in surface area by NC was detected by Julien et al. [58]. Increase in surface area of carbon fibers by NC oxidation was observed by Pittman et al. [60,62,63]. Xue et al. [32] pointed out an increase in surface area by NC and HP and attributed it to pore widening (e.g. converting micropores to mesopores) and blocked pore openings. Aburub et al. [59] observed an increase in surface area in the first early stages of AC oxidation by a mixture of NC and SA and a decrease at extended oxidation time. Also, it was reported that oxidation of AC by NC will result in increase in micropores and mesopore in case of lower severity and destruction of pore walls in higher severity [39]. The above observations show that type of change in textural characteristics (for example, increase or decrease in surface area) may be seriously affected by severity of oxidation. Severity of wet oxidation for a specified oxidant may be adjusted or controlled by a combination of oxidant concentration, oxidation time and oxidation temperature. This means that concentration is not the only factor for determining severity. For example, severity of a dilute oxidant may increase by increasing oxidation time. Modifications applied on textural characteristics and surface chemistry of a specified AC is changed with change of severity of oxidation. More severity may not result in optimum functionalization of AC. For example, in a work done by Lazaro et al. [105] on Ordered Mesoporous Carbon (OMC) ratio of acidic groups was changed

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with change in severity and density of carboxylic groups was not optimum by the most severe oxidation. The effect of oxidation time as a parameter determining severity has been studied in several works. Generally, investigation of oxidation under the same conditions but with different times allows tracking behaviour of acidic groups on surface versus time. One of the informative studies in this regard was done by Moreno-Castilla et al [100]. Their work shows that density of carboxylic group increases with increasing oxidation time of an AC by APS solution, whereas OH density increases in the first stages of oxidation and decreases after that probably by converting to lactones. This conversion has been also reported by other researchers [6,42,59,105,106]. Strelko et al. [42] reported a constant OH group density by time. Also, there are reports that NC treatment of AC decreases phenol groups [92]. All these observations may be justified by OH converting to lactones. There are some evidences that extended time of oxidation may cause destruction of AC structure (mesopores and/or micropores) [106]. However, it is obvious that the time needed for destruction depends on other factors determining severity, type of oxidant and structure of AC, so that ACs with weak structure are suspected to suffer destruction by acidic treatment to a higher degree [77]. More generated acidic groups by increasing time of electrochemical oxidation have been observed, too [78]. Temperature of oxidation as a severity factor affects oxidation results. Higher temperature may result in more generated acidic groups as detected by Barton et al. [78] for oxidation of porous carbons using NC. Some researchers have compared two or several oxidants, but their results sometimes are in conflict with each other due to previously-mentioned reasons. Lemus-Yegres et al. [22] noticed that APS generates more carboxylic and phenol groups compared to NC. It is not completely consistent with other works such as one done by Salame et al. [73]. In the latter study, for a wood-based AC, different acidic groups’ densities created by APS, NC and HP were in the following orders: Carboxylics: APS>NC>HP>Untreated; Phenols: HP>NC>APS>Untreated; Lactones: NC>HP>Untreated> APS (i.e. APS decreases lactones density); Total acidic: NC>APS>HP>Untreated. Also, El-Sheikh [30] who investigated oxidation of AC using APS indicated that carboxylic groups generated by APS are more acidic than by some other oxidants. As a comparison between dry and wet oxidation, Li et al. [92] investigated oxidation of two bituminous coal-based ACs by air oxidation at 693 K (after decomposition of all oxygen groups by heat treatment under helium atmosphere at 1200 K for 2 h) and wet oxidation with NC of 6N at room temperature for 5 h. Although both methods significantly increased total oxygen surface groups on ACs, air oxidation generated considerably more carboxyl groups in both ACs compared to NC oxidation. In addition, air oxidation increased phenol groups significantly, whereas NC treatment slightly decreased it. For lactone groups, results were on the contrary. Also, Alvez et al. [83] reported that total oxygen groups

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generated by dry oxidation (by 5% O2 at 693 K for 20 h) are higher than wet oxidation (by diluted NC refluxed for 8 h). In our opinion, since the results of experiments can easily change with changing severity of oxidation (concentration, time, temperature and so on), comparison of two different oxidants, each acted on its own severity conditions cannot lead to general rules and may be confusing. The reason is that there are no criteria to compare severity factors of two oxidants. Therefore, when doing these types of comparisons, the conditions should be clearly explained. NC and mixture of NC/SA are of most important oxidants and have been extensively studied. Researchers usually use NC to create acidic groups on AC surface. On the other hand, NC/SA is well known to create nitro groups on AC surface and is often used for this purpose, called nitration. Nitro groups are created on surface of AC through producing nitronium ion (NO+ 2 ) by NC/SA mixture [89,107,108]. Nitronium ion is an electrophile centre suitable to initiate electrophilic aromatic substitution of nitration. However, NC/SA mixture has been used somewhere for generation of acidic groups not for nitration [109]. As well, there are some reports indicating NC alone can generate an amount of nitrogen-containing groups including nitro groups [20,47,73,89], nitrate [47], NO and CONH2 [106]. Also, NO evolution in TPD of an AC oxidized by NC was reported [1,7], although it was attributed to desorption of nitrate [1] and decomposition of NC residue [7]. It has been stated that 40% of oxygen content of an AC oxidized by NC is in the form of nitro groups and the remaining in the form of other oxygen-containing groups [105]. Takaoka et al. [106] suggest that treatment of AC with NC produces a large number of active sites that are proper for further modification and reactions. The density of these active sites increases with increasing oxidation time. Tamai et al. [109] used NC/SA with different ratio for producing acidic groups on surface. Although, they did not report creation of nitro groups, they reported that increasing SA ratio led to more decrease in surface area due to destruc-

tion of surface and more increase in acidic groups. Ratio of SA to NC may be considered as a factor determining oxidation severity in the case of this oxidant. Liquid oxidants may remove ash or inorganic content from AC and subsequently change structural properties [6,53,104]. This process is called “demineralization” or “deashing”. The extent of removed ash increases with increase in acidic strength [32]. Demineralization is usually performed by HCl or HF solution [6,9,40,56,74,81], though these acids are rarely used as oxidant [1,2,53]. However, in such cases, their role in removing ash has been noticed: It was observed that HCl as oxidant, slightly increases oxygen surface groups and also surface area [1]. The latter may be attributed to ash removal [1,2]. A decreased amount of carboxylic and lactonic groups and an increased amount of phenolic and carbonyl ones by HCl oxidation have been indicated [2]. As mentioned earlier, production of humic substances as by-product of wet oxidation has been reported to be a reason of decreasing surface area by oxidation. Some researchers tried to remove these humic substances by washing AC with NaOH [89,101], and a re-increase of surface area has been reported by this procedure [89]. As a general conclusion, information reported in literature about the changes in surface chemistry and textural characteristics of AC by treatment with a specified oxidant are not similar, and in some cases are in conflict. The reasons for this inconsistency may be different original structure of used ACs and different treatment conditions such as temperature, pH of solution and concentration of oxidant. Diversity of precursors and activation processes and variety of oxidants, procedures and conditions in conjunction with diversity of analysis methods used to investigate the results make it almost impossible to reach some crucial rules about oxidation of AC. A summary of researches on wet oxidation and its effect on AC texture is given in Table 1.

Table 1. A summary of researches on wet oxidation Carbon material Commercial AC Commercial AC Commercial AC

Oxidant NC HCl NC

Effect on texture Slight decrease in surface area and pore volume Slight increase in surface area and pore volume Slight decrease in surface area and pore volume

Commercial AC

HCl, HF

Increase in surface area and pore volume

Commercial ACs (Hydraffin and ROX)

NC

Commercial AC

NC

Commercial ACs Commercial ACs Commercial ACs Commercial AC (D43/1) AC

NC APS HP NC NC

AC

NC

Slight increase in meso surface and decrease in W0 for Hydraffin Slight decrease in meso surface area and no change in W0 Essentially no change in surface area and mesopore volume Significant decrease in surface area Slight increase in surface area Increase/decrease in surface area for different ACs Slight decrease in surface area and pore volume A slight increase in surface area and micropore volume A decrease in surface area and pore volume

Application/remarks Enhanced dispersion of Cu catalyst Enhanced dispersion of Cu catalyst Dispersion of Cu catalyst on AC surface for N2 O and NO reduction Dispersion of Cu catalyst on AC surface for N2 O and NO reduction Dispersion of Pt metallic crystallites

Ref. No. [1] [1] [2]

Catalytic wet air oxidation of aqueous ammonia Catalytic wet air oxidation of phenol Catalytic wet air oxidation of phenol Catalytic wet air oxidation of phenol

[4]

Anchoring chiral manganese (II) Salen complex For post treatment with EDA

[2] [3]

[6] [6] [6] [9] [18] [20]

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Table 1. (Continued) Carbon material MWCNT Microporous AC

Oxidant NC NC

Mesoporous AC

NC

AC AC Cherry-stone-based AC Cherry-stone-based AC AC

NC APS NC HP NC

Effect on texture An increase in surface area A significant decrease in surface area and pore volume An increase in surface area and decrease in pore volume A slight decrease in surface area and pore volume A decrease in surface area and pore volume A decrease in surface area A slight decrease in surface area Decrease in surface area

AC AC

APS HP

Decrease in surface area Decrease in surface area

AC

NC, HP, SA

Wood-based and bituminous-coalbased ACs

NC

Wood-based ACs

APS

AC

NC

AC

APS

Commercial AC

NC

Corncob-based AC Wood-based AC Wood-based AC Wood-based AC Coconut-shell-based AC

NC NC APS HP HCl

Commercial AC Commercial AC Commercial AC

APS HP NC, HP

Decrease in surface area, micropore and mesopore volume Decrease in surface area, micropore and mesopore volume Decrease in surface area and slightly in micropore volume A decrease in surface area A decrease in surface area A decrease in surface area A decrease in surface area A slight increase in surface area, pore volume and average pore diameter An increase in surface area and pore volume An increase in surface area and pore volume Decrease in surface area and pore volume

Coconut-, wood-, and coal-based ACs Commercial AC

NC

An increase in surface area

NC/SA

A decrease in surface area in more severe cases

AC fibers

NC

Commercial ACs

APS

Commercial AC Commercial AC

NC NC

Commercial AC Olive-stonebased ACs

NC APS

Olive-stonebased ACs Olive-stonebased ACs Commercial AC

NC HP NC

Increase/decrease in surface area and pore volume for different coal-based ACs and decrease in surface area and pore volume for wood-based AC A decrease in surface area and pore volume

A decrease in surface area and micropore volume A slight decrease in surface area A slight decrease in surface area and pore volume No essential change in surface area Decrease/increase in surface area for different ACs and decrease in mesopore and macropore volume Decrease in surface area and mesopore and macropore volume Decrease in surface area and mesopore volume and increase in macropore volume An increase in surface area and slight increase in pore volume

Application/remarks Oxidized AC was used for diamine anchoring Oxidized AC was used for diamine anchoring Rhodium complex anchoring Rhodium complex anchoring Cu(II) adsorption Cu(II) adsorption Increase adsorption of Cr3+ , Mn2+ , Pb2+ , Cu2+ , Cd2+ and Zn2+ Increase adsorption of Mn2+ and Zn2+ Increase adsorption of Cr3+ , Mn2+ , Pb2+ , Cu2+ , Cd2+ and Zn2+ Improve dispersion of CuO and NO adsorption(catalytic application) Study of valeric acid adsorption

Phenol adsorption from solution

Ref. No. [20] [21] [21] [22] [22] [29] [29] [30] [30] [30] [32] [36,55]

[37] [39] [39] [40]

Water adsorption Water adsorption Water adsorption Cr(VI) adsorption

Study of cyclohexanol conversion (type and amount of oxygen groups determine catalytic activity and selectivity)

[47] [49] [49] [49] [53] [56] [56] [57]

[58] For phenobarbital adsorption

[59]

For postmodification by polyamines (TEPA) for adsorption of AC fibers into polyurethane and epoxy resin matrices For adsorption of Zinc (II)

[60]

[67]

For NOx reduction with ammonia

[74] [74] [77] [79]

[79] [79] [86]

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Table 1. (Continued) Carbon material Commercial AC

Oxidant HP

Coal-based AC (F400) Bituminous-coalbased ACs AC

NC

NC

AC

APS

AC

HP

Commercial AC

NC, HP

Decrease in surface area and pore volume

Coconut- and coal-based ACs

NC

Coconut- and coal-based ACs

APS

Coconut- and coal-based ACs Coconut commercial AC Olive-stone ACs

KMnO4

A slight decrease/increase in surface area and pore volume for two coal-based ACs and a slight decrease in surface area and pore volume for coconut-shell-based AC A decrease/increase in surface area and pore volume for two coal-based ACs and a decrease in surface area and pore volume for coconut-shell-based AC An increase in surface area and pore volume for all ACs An increase in surface area and pore volume

NC

NC

A decrease in surface area and no change/ decrease in pore volume for different cases

NC

No change or slight decrease in surface area for ACs of different burn-off An increase in BET surface area

NC

A decrease in surface area

Stone-fruit-based AC

HP

A decrease in surface area

Nut-shell-based AC Peach stone-based AC Coconut AC

NC/SA NC NC

A decrease in surface area and pore volume A decrease in surface area and pore volume A decrease in surface area for severe cases

Coconut AC

HP

A slight increase in surface area

AC

NC

A significant decrease in surface area and a decrease in pore volume

Coal-based AC (F400) Stone-fruit-based AC

APS

Effect on texture An increase in surface area and slight increase in pore volume A decrease in surface area

4.3. Oxygen plasma treatment Apart from dry and wet oxidation to increase acidic groups on surface of AC, some researches were performed to use oxygen plasma for this purpose. Generally, plasma treatment with different kinds of media and gases may be used to modify surface of activated carbon via oxidation, reduction or inactive reaction. For example, plasma treatment with species containing amine groups has been successfully used for surface modification [110]. Moreover, N2 plasma created by different techniques has been ex-

Application/remarks

Removal of humic substances increases surface area again Hg adsorption Oxidation increase Cr(III) adsorption and decrease CR(VI) adsorption Oxidation increase Cr(III) adsorption and decrease CR(VI) adsorption Oxidation increase Cr(III) adsorption and decrease CR(VI) adsorption Dye adsorption study (surface chemical groups have major role in dye adsorption)

Ref. No. [86] [89] [92] [94] [94] [94] [96] [98]

[98]

[98] [99] [100] Oxidation increase removal of transition metals Preparation of palladium catalysts: Oxygen groups, especially carboxylic groups increase dispersion of cationic or metal complex Pd catalyst Preparation of palladium catalysts: Oxygen groups, especially carboxylic groups increase dispersion of cationic or metal complex Pd catalyst For formaldehyde removal application Oxidation of AC improve pentachloro benzene adsorption Oxidation of AC improve pentachloro benzene adsorption Adsorption of methyl mercaptan in N2

[101] [102]

[102]

[103] [104] [106] [106] [109]

ploited to modify surface chemistry and to some extent textural characteristics of carbon materials [111,112]. Oxygen and oxygen-containing plasma may be used to modify surface chemistry of different kinds of carbon materials such as carbon fibres, meso-carbon microbeads, graphite, glassy carbon, carbon blacks and activated carbon [87,113]. Oxygen radicals, oxygen ions and electrons are common active species created by oxygen plasma reactor [66] and are used as oxidizing agent for oxidation of AC surface. In addition, ozone may be generated in the oxygen plasma device [68]. Ozone is highly-active and can oxidize carbon surface. It is well known that oxygen plasma considerably increases

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oxygen-containing surface groups [56,66,68,87,88,113]. As reported in the above-mentioned researches, different oxygencontaining groups including carboxyl, phenol and carbonyl have are formed via oxygen plasma. Oxidation by plasma seems to occur at aromatic rings and their substitution [66,87]. Mainly external surface of porous materials are subjected to plasma treatment [56,68,87,88,113]. This observed phenomenon is attributed to high reactivity of oxygen radicals that react with outer surface of activated carbon and cannot penetrate into core of particles. On the other hand, there are evidences that some active oxygen species, which are recombination of initial active species or resulted from carbon destruction, may penetrate into core of activated carbon and introduce oxygen groups on its internal surface [66]. However, observations about effects of plasma treatment on textural properties of activated carbon are not so consistent. It may have no significant effect on porous structure and micropores under some adjusted experimental conditions [56,113]. In contrast, it has been reported that plasma treatment is time dependent and causes a decrease in surface area, especially for extended time of plasma treatment [66,68,87,88,113]. Lee et al. [87] and Park et al. [88] attributed this decrease to destruction of wall pores and/or pore blockage by newly-created oxygen functional groups, whereas Boudou et al. [66] and Garcia et al. [113] observed an increased weight-loss of activated carbon with increasing plasma-treatment time due to activated carbon burn-off. In the latter work, minor changes in structural parameters (interlayer spacing and crystallite sizes) of carbon crystals were observed. This work also indicated that more disordered areas of activated carbon are subject to more oxidation by plasma. The differences in the results of these researches may be due to different experimental conditions and specification of parent ACs. It seems that type of plasmagenerating device and plasma gas are important, too. Lee et al. [87] believe that helium-oxygen dielectric barrier discharge (DBD) plasma, generating oxygen radicals in helium plasma acts more uniformly than pure oxygen plasma. In addition, review of researches conducted in this area implies that composition and characteristics of active species created by different plasma devices are not the same. As a comparison between plasma treatment and other oxidation methods, Boudou et al. [66] suggested that total acidity introduced by oxygen plasma is lower than a typical oxidation by nitric acid, whereas Domingo-Garcia et al [56] observed that oxygen plasma introduces more oxygen functional groups on activated carbon surface compared to oxidation by ammonium persulfate and hydrogen peroxide solutions. 5. Summary Activated carbon may be considered as a stack of graphene layers whose characteristics are formed by two main features including porous morphology and surface chemistry. Surface chemistry of activated carbon is seriously affected by already existing or created heteroatoms-containing functional

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groups and may be modified via a variety of methods. However, every treatment intending for modification of AC surface affects more or less its textural characteristics. Oxidation as one of the most important modifications is categorized to dry and wet methods. Wet oxidation may increase or decrease specific surface area and/or pore volume, depending on the type of oxidant, the parameters influencing severity of oxidation (including concentration of liquid oxidant, temperature of oxidation and oxidation time) and characteristics of activated carbon (including origin and history of preparation). Although, increasing the severity of oxidation will increase the intensity of changes in textural characteristics of activated carbon, however, the type and extent of changes in textural characteristics for a specified severity depends on the type of oxidant and activated carbon. For instance, for an AC with non-developed structure (e.g. an AC activated in a low temperature) oxidation with a specified oxidant would increase surface area and/or pore volume until a fully-developed texture is reached. More oxidation (i.e. increase in oxidation severity) has negative effects on textural characteristics. Activated carbon and other graphene-layer carbon materials are usually oxidized to gain different types of oxygencontaining groups. Almost all of the factors determining in changes of textural characteristics are effective in the types and amounts of created oxygen groups. Increase in severity usually increase the total amount of oxygen groups but do not optimize the amount of desired group(s). The hypothesis that created oxygen groups may block the pores makes it difficult to predict or optimize the effects of oxidation on both surface chemistry and textural specification of AC. The capability of tailoring of surface chemistry has increased the number of applications of activated carbon as adsorbent, catalyst and catalyst support over these years. Oxygen groups may play major roles for tailoring of AC surface chemistry, because heteroatoms-containing groups, especially oxygen groups are able to act as linking agents for further modification of surface. Therefore, optimization of desired surface groups created by oxidation and deconvolution of oxidation effects on surface chemistry and textural characteristics are expected to be developed in future. References [1] Tseng H H, Wey M Y. Chemosphere, 2006, 62(5): 756 [2] Zhu Z H, Radovic L R, Lu G Q. Carbon, 2000, 38(3): 451 [3] Aksoylu A E, Madalena M, Freitas A, Pereira M F R, Figueiredo J L. Carbon, 2001, 39(2): 175 [4] Aguilar C, Garcia R, Soto-Garrido G, Arriagada R. Appl Catal B, 2003, 46(2): 229 [5] Gomes H T, Machado B F, Ribeiro A, Moreira I, Rosario M, Silva A M T, Figueiredo J L, Faria J L. J Hazard Mater, 2008, 159(2-3): 420 [6] Santiago M, Stuber F, Fortuny A, Fabregat A, Font J. Carbon, 2005, 43(10): 2134 [7] Silva I F, Vital J, Ramos A M, Valente H, do Rego A M B, Reis M J. Carbon, 1998, 36(7-8): 1159 [8] Yantasee W, Lin Y H, Fryxell G E, Alford K L, Busche B J,

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