INTERFACIAL APPLICATIONS IN ENVIRONMENTAL ENGINEERING - CHAPTER 10 pptx

Catalytic Production of Carbon Nanofibers The catalytic growth of fibrous carbon adsorbents was carried out using bothunsupported and supported Ni and Cu/Ni catalysts.. The catalyst/carbon

Effectiveness of Carbon Nanofibers

in the Removal of Phenol-Based

Organics from Aqueous Media

I BACKGROUND: THE ENVIRONMENTAL

DIMENSION

A significant increase in public awareness and concern over global and localpollution has been prompted, at least in part, by the ever-growing evidence ofenvironmental degradation Air and water pollution constitute the two most prev-alent forms, and volatile organic compounds (VOCs) have been identified asmajor contributors to the decline in air and water quality [1,2] Volatile organiccompounds enter the environment as a result of vehicle exhaust and industrialprocess emissions (oil refining, solvent usage in painting and printing, etc.) [3].Phenol and chlorophenol(s) epitomize a class of particularly hazardous chemicalsthat are commonly found in industrial wastewater, notably from herbicide andbiocide plants [3] The proliferation of phenolic waste has meant that the respon-sible handling/treatment of such toxic material is now of high priority Chemicalspills may be much smaller than oil spills, but they can still be devastating intheir impact Such was the case in June 2001 with a phenol spill in Singapore’sJahor Strait, both one of the busiest seaways in the world and home to many

commercial fish farms An Indonesian-registered ship, the Endah Lestari,

cap-sized in the strait between Malaysia and Singapore, releasing its cargo of 630tons of phenol While salvage activities took effect immediately to pump phenolfrom the damaged vessel, the phenol that had been leaked killed most marinelife within 2 km of the ship Phenol, a corrosive and severe skin irritant on land,also attacks gill tissues of fish when dispersed in water

There are numerous methodologies in operation at this time to combat theproblem of VOC pollution The most frequently applied techniques are centered

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on incineration, absorption/adsorption, condensation, and biological treatment[1–7] Incineration, which is the most widespread strategy for waste disposal (asopposed to treatment) has been heavily criticized in terms of cost and dioxin/furan formation downstream of the oxidation zone Combustion, as a destructivemethodology, does not demonstrate an efficient management of resources and,even if fully effective, releases unwanted carbon dioxide into the environment.Although biological oxidation can be effective when dealing with biodegradableorganics, chloroarenes are used in the production of herbicides and pesticidesand, as such, are very resistant to biodegradation Conversion of halogenatedfeedstock, where feasible, is in any case very slow, necessitating the construction

of oversized and expensive bioreactors

II POLLUTANT ABATEMENT USING CARBON

ADSORBENTS

Adsorption is perhaps the most widely employed nondestructive strategy, ing the possibility of VOC recovery The adsorption of phenol, and chlorophe-nol(s) to a lesser extent, from aqueous media on various forms of amorphouscarbon has been the focus of a number of studies published in the open literature[8–13] Regeneration of the adsorbent, i.e., desorption of the organic pollutant,

offer-is usually carried out either by heating the adsorbent or by stripping with steam[6,14–17] The uptake of VOCs, in general, from gas or liquid streams can, how-ever, call on a variety of solid adsorbents, ranging from macroporous polymericresins [18–22], mesoporous silica–based MCM-41 materials [23–25], and micro-porous zeolites [20,26,27] to carbons [28–35] Currently, carbon is by far thepreferred adsorbent, and it is generally derived from either a selection of naturalproducts, e.g., coal, wood, peanut shells, and fruit stones or can be generatedfrom a catalytic decomposition of a range of organics [10,36–41] Carbon adsor-bents find widespread use because they can be readily and precisely function-alized, often by simple yet effective chemical treatments, to meet various de-mands, e.g., surface oxidation by a gentle thermal oxygen treatment to aid mixing

in aqueous media [42–45] The importance of parameters such as solute tration, solution pH, and adsorbent porosity/surface area in governing ultimateVOC uptake has been established [9,10,28,32,33,35,46] The standard activated(amorphous) carbons do not perform well under “wet” conditions or when treat-ing aqueous streams, and they exhibit indiscriminate adsorption The uptake ofboth the contaminant and water molecules decreases the available volume foradsorption, limiting uptake effectiveness [47–57] The adsorption of water onthe surface is driven mainly by hydrogen binding interactions, e.g., the presence

concen-of certain surface functionalities: O, OH, and Cl can act as nucleation sites and/

or adsorption sites, resulting in the formation of adsorbed water clusters Phillips

and co-workers, in a series of studies [47,58–60], highlighted the complex tionship between the nature of the adsorbent surface and the uptake capacity andmechanism of adsorption These authors, using a combination of microcalorime-try and adsorption techniques, demonstrated that hydrophobic carbon surfacesadsorb very small amounts of water, primarily by physisorption In contrast, oxy-genated carbon surfaces exhibit a significant capacity for water uptake [52–54,58–60] The adsorption of methanol/water mixtures in activated carbon poreswas studied using Monte Carlo simulations by Shevade and co-workers at ambi-ent temperature [51] The findings of this work suggest that water is preferentiallyadsorbed over methanol in the pores of a carbon surface functionalized by car-boxyl groups The hydrophilic nature of the carbon results in a complexation ofboth the water and methanol and a nonselective uptake [47–55] Nevskaia andco-workers, using a commercially available activated carbon, found that an indis-criminate adsorption capacity could be inhibited somewhat by a HNO3treatment[61].

rela-Moreover, recovery of the “loaded” carbon from the treated water can beproblematic Activated carbon is typically supplied in the form of a powder, andloss of fine particulates is often unavoidable but can be circumvented by addi-tional (membrane) filtration The major advantage of the activated/amorphouscarbon that overrides such drawbacks is the high overall uptake that is synony-mous with this material [62] Indeed, a fibrous form of activated carbon hasbeen manufactured that exhibits a greater adsorption capacity than the granu-lated form for the removal of liquid pollutants [39,63,64] It has been claimedthat the fibrous material is particularly selective for the adsorption of low-molecular-weight compounds, a feature that is linked to the molecular size ofthe organic adsorbate [32] Graphite, on the other hand, the highly uniform andordered form of carbon possesses delocalised π-electrons on the basal planes.This property imparts a weakly basic character that, in consort with its hy-drophobic nature, allows selective VOC adsorption, but the characteristic lowsurface area/mass ratios (⬍20 m2g⫺1) results in lower overall uptake values[47,65–68] One significant disadvantage of using activated carbon (or graphite)

is the difficulty associated with separation from the solute; the fine carbon cles require a prolonged settling period to facilitate phase separation Con-versely, operation of a continuous-flow separation process, employing a fixedbed of activated carbon, although highly effective, is hampered by the associ-ated high back-pressures Maintenance of a constant flow is energy demanding,and flow disruptions/plugging can impair an effective processing of contami-nant streams A significant improvement in existing activated carbon–basedVOC treatments would result from the development of an adsorbent that: (1) isreadily separated from the solute, (2) exhibits high mechanical strength, (3) isresistant to crushing/attrition, and (4) delivers uptake values comparable withthose of activated carbon

parti-168 Park and Keane

III APPLICATION OF CARBON NANOFIBERS

An ideal carbon adsorbent is one that encompasses the favorable aspects of bothgraphite (selective adsorption) and amorphous carbon (high uptake) combinedwith a facile separation from the treated phase One possible material that mayfall into this category is the catalytically generated carbon nanofiber Carbon isunique in that it can bond in different ways to create structures with quite dissimi-lar properties Carbon fibers are generally classified as graphitic structures, char-acterized by a series of ordered parallel graphene layers arranged in specific con-formations with an interlayer distance of ca 0.34 nm [69] The direct synthesis

of graphitic carbon fibers/filaments is possible by arc discharge and plasma composition, but such methodologies also yield polyhedron carbon nanoparticles(low aspect ratio) and an appreciable amorphous carbon component [70,71] Thelatter necessitates an additional involved, cumbersome, and costly purificationstage in order to extract the desired structured carbon The generation of orderedcarbon structures with different mechanical/chemical/electrical properties undermilder conditions by catalytic means is now emerging as a viable lower-costroute [72] The carbon product can be tailor-made to desired specifications bythe judicious choice of both catalyst and reaction conditions The pioneering stud-ies by Baker, Rodriguez and co-workers [73–80] and Geus et al [81–86] haveestablished conditions and catalysts by which structured carbon with specific lat-tice orientations and properties can be prepared with a high degree of control.Much of the pertinent literature on the catalytic growth of carbon nanofibers,from its beginnings to the present day, has been the subject of five detailed reviewarticles [73,77,87–89] that summarize the various aspects associated with thegrowth phenomena

de-The applicability of these novel carbon materials as VOC adsorbents has yet

to be established In this chapter, we present the results of an evaluation of theperformance of highly ordered carbon nanofibers to remove phenol and chloro-phenol(s), as established VOC pollutants, from water We adopted the decompo-sition of ethylene over supported and unsupported nickel catalysts as the synthesisroute to generate carbon nanofibers of varying overall dimension and lattice orien-tations The uptake measurements on commercially available activated carbonand graphite serve as a basis against which to assess the adequacy of the variousforms of catalytically generated carbon nanofibers

IV EXPERIMENTAL PROCEDURES

A Catalytic Production of Carbon Nanofibers

The catalytic growth of fibrous carbon adsorbents was carried out using bothunsupported and supported Ni and Cu/Ni catalysts The unsupported Ni and Cu/

Ni catalysts were prepared by standard precipitation/deposition [90], where theprecipitate was thoroughly washed with deionized water and oven-dried at 383

K overnight The precursor was calcined in air at 673 K for 4 h, reduced at 723

K in 20% v/v H2/He for 20 h, cooled to ambient temperature, and passivated in a2% v/v O2/He mixture for 1 h The supported Ni catalysts were prepared byimpregnating a range of supports to incipient wetness with a 2-butanolic solution

of Ni(NO3)2to realize a 10% w/w Ni loading; the catalyst precursor was dried,activated and passivated as described previously The substrates employed in thisstudy include commercially available SiO2, Ta2O5, and activated carbon Therange of metal carriers used provides a range of Ni/support interaction(s) thatgenerate a variety of uniquely structured carbon materials The Ni content wasdetermined to within⫾2% by atomic absorption spectrophotometry (VarianSpec-tra AA-10), where the samples were digested in HF (37% conc.) overnight atambient temperature prior to analysis

The procedure for the catalytic growth of carbon fibers has been discussed insome detail elsewhere [38,91], but specific features that are pertinent to this studyare given here Samples of the passivated catalysts were reduced in flowing 20%v/v H2/He (100 cm3 min⫺1) in a fixed-bed vertically mounted silica reactor tothe reaction temperature (798–873 K) and flushed in dry He before introducingthe C2H4/H2mixture (1/4 to 4/1 v/v mixtures) The production of fibers with thedesired dimensions/morphology and a particular predominant lattice orientation

is strongly dependent on the nature of the catalyst and reaction conditions, asidentified in Table 1 The catalyst/carbon was cooled to ambient temperature andpassivated in 2% v/v O2/He, and the gravimetric carbon yield was determined.Graphite (Sigma-Aldrich, synthetic powder) and activated carbon (Darco G-60,

100 mesh) were used as benchmarks with which to assess the performance ofthe catalytically generated carbon nanofibers The carbonaceous adsorbents weresubjected to acid washing (HCl and HNO3) in order to remove the residual Ni

TABLE 1 Compilation of Catalysts and Reaction Conditions Used to GenerateCarbon Nanofibers of Varying Conformation and Average Diameter

Reaction Carbon NanofiberNanofiber C2H4/H2 temperature yield diameterCatalyst conformation v/v (K) (gcgcat ⫺1) (nm)

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content This acid treatment also served to introduce functional groups to thecarbon surface Oda and Yokokawa reported that the adsorption capacity of anactivated carbon was intimately linked to the surface acidity of the adsorbent [92].Carbon materials in their pristine form are hydrophobic in nature but, followingoxidative treatment, can develop some hydrophilic character [92–94] The car-bonaceous materials (treated with HNO3) were also subjected to a gentle oxida-tive treatment by heating in 5% v/v O2/He (5 K min⫺1to 723–973 K); up to 5%w/w carbon was oxidized/gasified in this step In the case of the carbon nano-fibers, an amorphous layer deposited during the cool-down stage of the reaction,and this was removed in the secondary oxidation step The latter should allowgreater access of the phenolic solutes to the ordered carbon layers/edge sites

B Characterization of Adsorbent Materials

The pertinent characteristics of the carbon adsorbents used in this study (fibrous,graphite and activated carbon) were established using a variety of complementarytechniques Tap bulk densities of the carbonaceous materials (as supplied/grown)were calculated by weighing a known volume of gently compacted samples Ni-trogen BET surface area measurements (Omnisorb 100) were carried out at 77

K Temperature-programmed oxidation (TPO) profiles were obtained from oughly washed, demineralized samples to avoid any possible catalyzed gasifica-tion of carbon by residual metals A known quantity (ca 100 mg) of a demineral-ized sample was ramped (25 K min⫺1) from room temperature to 1233 K in a5% v/v O2/He mixture with on-line TCD analysis of the exhaust gas; the sampletemperature was independently monitored using a TC-08 data logger The associ-

thor-ated Tmaxvalues corresponding to the major oxidation peaks are given inTable

2.High-resolution transmission electron microscopy (HRTEM) analysis was ried out using a Philips CM200 FEGTEM microscope operated at an acceleratingvoltage of 200 keV The specimens were prepared by ultrasonic dispersion inbutan-2-ol, evaporating a drop of the resultant suspension onto a holey carbonsupport grid All gases [He (99.99%), C2H4(99.95%), H2(99.99%), and 5% v/v

car-O2/He (99.9%)] were dried by passage through activated molecular sieves beforeuse

C Uptake of Volatile Organic Compounds

1 Batch Adsorption Studies

Phenol and chlorophenol adsorption studies were conducted batchwise (298 K⫾

3 K) in 100-cm3-capacity polyethylene bottles, kept under constant agitation lenkamp gyratory shaker) at 100 rpm The solutes were of high purity (Sigma-Aldrich, 99⫹%), and stock solutions were used to prepare the test samples by

(Gal-TABLE 2 Tap Densities, N2BET Surface Areas, and Characteristic TPO TmaxValuesAssociated With “As-Grown”/Supplied (Catalytically Generated/Commercial) CarbonAdsorbents

N2BETAdsorbent Density Surface area TPO Tmax

of any impurities in the feed was detected Solute detection was by UV (HitachiModel L-4700 UV detector), with the optimum wavelength set at 280 nm Dataacquisition and analysis were performed using the JCL 6000 (for Windows)chromatography data package Peak area was converted to concentration usingdetailed calibration plots, with standards spanning the concentration range em-ployed in this investigation To ensure that adsorption on the polyethylene bottlewalls or adsorbate volatilization did not contribute to the overall uptake, solutions

of phenol and chlorophenol (in the absence of any adsorbent) were employed

as blanks under the same adsorption conditions Solutions pH was monitoredcontinually for selected adsorbate/adsorbent systems by means of a data-logging

pH probe (Hanna Instrument programmable pH meter) The pH probe was

cali-172 Park and Keane

brated in the pH range 4–11 before the adsorption run and checked for ibility after the analysis period A blank run was employed that involved pHmonitoring of the carbon in deionized water

Phenol removal as a function of time was investigated using a differential columnreactor A stainless steel tube (1/4inch o.d.) was packed with adsorbent, and thephenol solution (1.2 mmol dm⫺3) was fed from a reservoir (1 L) using a HitachiModel L-7100 pump operating in the constant-flow mode; the pump delivered aflow of 10 cm3 min⫺1, regardless of the back-pressure The adsorbent bed wasinitially packed using compressed air to minimize the voidage and to facilitatepacking: adsorbent bed length ⫽ 80 mm, bed volume ⫽ 1.83 cm3, adsorbentweight⫽ 0.2–0.9 g Deionized water was first passed through the system andthe packed adsorbent bed to wet the adsorbent before the aqueous solution ofphenol was introduced The exit stream was regularly sampled, using an on-linesampling valve, to monitor phenol concentration as a function of time; analysiswas by HPLC, as described earlier

V RESULTS AND DISCUSSION

A Characteristic Features of the Carbon Adsorbents

Representative transmission electron microscopy (TEM) images that illustratethe structural characteristics of the catalytically generated carbon nanofibers areshown inFigures 1(unsupported catalyst) and 2 (supported catalysts) A simpleschematic representation of the “ribbon” and “fishbone” fiber structure is shown

configu-ration, the carbon platelets are parallel and oriented at an angle to the fiber axis[75,83,86] This particular arrangement can lead to deviations in the interlayerspacing toward the outer edges of the graphitic platelets, making this particularstructure a strong candidate as an effective adsorbent The fishbone fiber canpossess a narrow hollow channel that runs between the series of angled carbonplatelets [86] The so-called “ribbon” form is quite distinct, in that the carbonplatelets are oriented solely in an arrangement that is parallel to the fiber axis[95] The observed variations in carbon morphology and lattice structure are due

to the differences in the nature of the catalytic metal site The choice of bothcatalyst and reactant is critical when generating carbon nanofibers, because themetal particles can adopt well-defined geometries during the hydrocarbon de-composition step, thereby influencing the nature of the carbon precipitated anddeposited at the rear face of the particle For example, platelet nanofibers aregenerated from metal particles that are typically “rectangular” in shape, whilerhombohedral/diamond-shaped particles produce nanofibers with a fishbone type

FIG 1 Representative TEM images of (a) a fishbone and (b) ribbon nanofibers grownfrom unsupported (a) Ni/Cu and (b) Ni catalysts.

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FIG 2 Representative TEM images of fibrous carbon grown from supported Ni catalysts(details given inTable 1):(a) fishbone structures with platelets arrayed at an angle to thefilament axis; (b) ribbon structures with platelets aligned parallel to the filament axis;(c) spiral structures with platelets oriented parallel to the filament axis; (d) “branched”fibers generated from Ni/activated carbon

(a) (b)

FIG 3 Simplified schematic representations of two forms of catalytically generatednanofibers employed as adsorbents in the current studies: (a) ribbon form, (b) fishboneform

of configuration As a means of aiding a visualization of this phenomenon, TEMimages of an assortment of carbon nanofiber structures are given inFigure 4,

where the relationship between the metal particle shape and the nanofilamentstructural characteristics can be seen Three distinct growths are represented inFigures 4a–4d The first (Fig 4a) is monodirectional in nature, where the carbon

is precipitated at the rear edge of the metal particle in a whiskerlike mode Thesecond is a bidirectional growth (Figs 4b and 4c), where the carbon is precipi-tated at two opposite faces of the particle; the metal component remains entrappedwithin the body of the nanofiber during the growth process The entrapped parti-cle depicted in Figure 4b has assumed a diamond-like morphology, and the bidi-rectional growth of carbon platelets are arrayed around what appears to be a

FIG 4 TEM images illustrating the relationship between Ni particle shape and the nature

of the associated carbon nanofiber growth: (a) pentagonal-shaped particle, monodirectionalfiber growth; (b) diamond-shaped particle, bidirectional fiber growth; (c) rectangular-shaped particle, bidirectional spiral fiber growth; (d) rectangular-shaped particle, multidi-rectional spiral fiber growth

hollow central core On closer examination by HRTEM, distinct parallel plateletswere found in this central core and aligned parallel to the fiber axis Diffusion/precipitation in this core region differs from that associated with the adjacentfaces of the restructured Ni particle Finally, a relatively uncommon type, a multi-directional growth, can be seen in Figure 4d,where two fibers are associatedwith two distinct sets of metal faces: The metal particle is locked at the hub ofthe four filamentous arms From a consideration of these TEM images it becomesclear that the characteristics of the nanofiber are largely determined by the struc-ture adopted by the metal particle The dimensions of the metal face at whichthe carbon is precipitated govern the fiber width This effect is particularly evident

in Figure 4d, where two distinct fiber diameters are generated that match thedimensions of the two sets of metal faces from which these fibers have beengrown By use of controlled-atmosphere electron microscopy, Baker and co-workers (96) demonstrated that the growth of each fibrous arm was identical andthat the fiber grew in a symmetrical manner

The commonly accepted fibrous carbon growth mechanism [73] involves actant (carbon source) decomposition on the top surface of a metal particle, fol-lowed by a diffusion of carbon atoms into the metal, with precipitation at otherfacets of the particle to yield the fiber, which continues to grow until the metalparticle becomes poisoned or completely encapsulated by carbon The growth ofcarbon nanofibers with a spiral (sometimes denoted helical) structure occurs due

re-to an unequal diffusion of carbon through the metal particle, leading re-to the tropic growth; see Figures 4c and 4d Zaikovskii and co-workers [97], using anMgO-supported bimetallic Ni-Cu catalyst, generated symmetrical spiral nanofi-bers These authors proposed that a carbide mechanism was in operation, where

aniso-Ni3C, metastable at 723 K, exists during the hydrocarbon transformation beforedecomposing to metal and carbon It was proposed that the different diffusionalpathways taken by the carbon atoms through the carbide phase led to differentrates of carbon growth, resulting in a “twisted,” or spiral, growth The generation

of fibrous carbon with a spiral structure was also noted by Park and Keane [38,98]using alkali bromide–doped Ni/SiO2catalysts to generate substantial quantities

of carbon with relatively small diameters It was observed that the choice of alkalimetal (from Li to Cs) had a direct impact on the degree of fiber curvature Thespiral growth was again assigned to an anisotropic diffusion of carbon atomsthrough the metal, generating a helical fiber Moreover, doping the catalyst withalkali bromide enhanced both the carbon yield and overall structural order [99–102]

The diameters of the individual carbon nanofibers generated from unsupportedcatalysts are appreciably greater than those grown from supported systems; see

particle size that can be stabilized on the support [74–76,80,86,91,95] The degree

of crystalline order of the carbon product is controlled by various factors,

includ-178 Park and Keane

ing the wetting properties of the metal with graphite and the crystallographicorientation of the metal faces that are in contact with the carbon deposit, featuresthat are ultimately reliant upon the choice of catalyst [75,86] The arrangement

of the metal atoms at the face where the carbon is deposited ultimately regulatesthe nature of the precipitated carbon If the atoms are arranged in such a mannerthat they are consistent with those of the basal plane structure of graphite, thenthe carbon that dissolves in and diffuses through the particle will be precipitated

as an ordered structure Conversely, if there is little or no match between theatomic arrangements of the depositing face and graphite, a more disordered car-bon will be generated The bulk densities of the carbon materials used as adsor-bents are given inTable 2.There is a significant variation (fourfold) in the densi-ties of the catalytically generated carbon Those fibers that display a fishbonestructure exhibit the lowest densities but possess the highest surface areas due

to the large number of accessible edge sites in this more open structure By parison, the fibers that display a predominant ribbon or spiral shape are signifi-cantly denser, with a lower BET surface area The nature of the carbon nanofibersgrown from Ni supported on activated carbon (which also serves in this study

com-as a model adsorbent) is shown in the micrograph given inFigure 2d.There is

no discernible structural order, and the nanofibers exhibit a roughened (or

“branched”) exterior The latter feature can be of benefit in terms of enhancedsites for solute attachment Indeed, it is to be expected that carbon nanofibersgrown from an activated carbon substrate should exhibit uptake characteristicsthat draw on the action of both carbonaceous species, i.e., original amorphous

Ni support and catalytically grown fibers Indeed, the associated surface areameasurement (Table 2) is intermediate between the highly oriented nanofibersand the amorphous carbon

High-resolution TEM (HRTEM) proved to be an invaluable aid in screeningcarbon nanofibers as potential adsorbents and linking uptake data with structuralcharacteristics The presence of an amorphous carbon layer on the filament edges(Fig 2) is an artifact of the cooling stage, upon completion of the catalytic step.This layer may hinder uptake by blocking filament edge sites as potential points

of solute attachment A careful oxidation treatment was employed to remove thisamorphous carbon overlayer, allowing access to the underlying adsorption sites,without disrupting the overall lattice structural order; a weight loss of ca 5%was typically associated with this mild oxidative step Similar oxidative treat-ments have been used by Baker and co-workers [78] to enhance the surface area

of nanofibers, but it should be noted that a gasification of a significant filamentouscomponent accompanied any substantial increase in area Surface areas of up to

700 m2 g⫺1 have, however, been quoted (with a 40% w/w burn-off ), with noapparent damage to the overall structural integrity of the remaining carbon spe-cies [78]