Adsorption of Chloroaromatic Models for Dioxins on Porous Carbons

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Adsorption of Chloroaromatic Models for Dioxins on Porous Carbons: The Influence of Adsorbate Structure and Surface Functional Groups on Surface Interactions and Adsorption Kinetics Jon G. Bell, Xuebo Zhao, Yaprak Uygur, and K. Mark Thomas* Northern Carbon Research Laboratories, Joseph Swan Institute for Energy Research, School of Chemical Engineering and Advanced Materials, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, U.K.

bS Supporting Information ABSTRACT: Polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran species are classes of extremely toxic compounds generated in very low concentrations in postcombustion gases and these may be removed by adsorption on porous carbons. Their extreme toxicity and very low volatility prevent detailed study of their adsorption characteristics, and therefore, models for dioxins have been used in this study. Chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole were used as models to investigate factors influencing the adsorption characteristics of dioxins on porous carbons. Adsorption studies were carried out under conditions of very low concentration and temperatures up to 453 K, which simulate those found in dioxin abatement systems. Adsorption of 2-chloroanisole on three carbons with various micro/ mesoporous structures showed that microporous structure was a critical adsorbent characteristic under these conditions. A microporous activated carbon was selected for detailed thermodynamic and kinetic studies of adsorption of chloroaromatic species in relation to adsorbate structure and adsorbent surface functional groups. Virial equation analysis of adsorption isotherms was used to determine the Henry’s Law constants and isosteric enthalpies of adsorption at zero surface coverage to compare adsorbate-adsorbent interactions. The van’t Hoff equation was used to determine the enthalpy of adsorption as a function of surface coverage. The role of surface functional groups on adsorption thermodynamics was investigated by oxidizing and reducing the carbon in nitric acid and hydrogen, respectively. The important factor influencing adsorption at very low concentrations is the adsorbate adsorbent interaction. Oxidation of the carbon adsorbent only has a small effect on the isosteric enthalpy of adsorption. The adsorption kinetics for each isotherm pressure increment were described by the stretched exponential equation. The activation energies and enthalpies of activation were calculated as a function of surface coverage for adsorption kinetics of chloroaromatic species. The planar molecules studied had lower activation energies and enthalpies of activation than isosteric enthalpy of adsorption indicating that a site-to-site surface hopping mechanism is the main factor in determining the adsorption kinetics. In comparison, 2-chloroanisole is nonplanar with a methoxy group giving rise to a larger minimum cross-section size and higher barrier to diffusion than isosteric enthalpy of adsorption at low surface coverage leading to the adsorption kinetics being mainly determined by diffusion through constrictions in the porous structure under these conditions. The isosteric enthalpies of adsorption initially increase with increasing surface coverage and this is attributed to π-π interactions of planar aromatic molecules confined in microporosity. The trends in the kinetic barriers and isosteric enthalpies of adsorption with surface coverage for 2-chlorotoluene are similar irrespective of adsorbent oxidation/reduction, indicating that surface functional groups only have a relatively small effect on adsorption characteristics.

1. INTRODUCTION Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are classes of aromatic compounds that form in low concentrations when chlorine species are present in postcombustion gases.1 These gases are complex mixtures of gases and particles and hence both homogeneous and heterogeneous formation and decomposition mechanisms are observed for PCDDs and PCDFs.2-8 The homogeneous formation mechanism involves reactions of many precursors such as chlorobenzenes and chlorophenols in the temperature range 400-800 °C. Heterogeneous reactions mechanisms occur in the temperature range 200-400 °C and include (1) the de novo r 2011 American Chemical Society

mechanism3,9,10 involving the burn off, oxidation, and chlorination of carbonaceous materials and (2) reactions of precursors catalyzed by fly ash particles.2-6 The complexity of dioxin formation in combustion systems has led to the use of quantum chemical calculations and modeling reaction mechanisms and adsorption of precursor species on surfaces to understand the relationship of PCCD/PCDF formation to operational conditions.11-18 Although emissions of dioxin species are very low, they are extremely Received: October 18, 2010 Revised: December 22, 2010 Published: January 25, 2011 2776

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The Journal of Physical Chemistry C toxic and tend to accumulate in biosystems, causing damage and interference with biological processes.19-21 Therefore, emissions must be controlled to minimize the environmental impact of combustion processes.8 PCDD/PCDFs, and their precursors (chlorobenzenes, chlorinated phenols etc.), that result in the formation of dioxins can be removed from the postcombustion flue gases by adsorption on carbons, which are injected into flue gases where the temperature is 150-180 °C,22-31 and by catalytic methods.32 In flue gases, dioxin concentrations are very low and adsorption is far away from equilibrium in the real situation. Hence, in the carbon injection method, it is unclear which aspects of pore structure and surface chemistry characteristics are important for optimizing the adsorbent for removal of dioxins. To understand the mechanism of adsorption of PCDDs and PCDFs on porous carbons, realistic simulation of the conditions under which they are adsorbed in the emission abatement system is essential for investigation of the factors that control the adsorption of dioxins. Porous activated carbons have very high surface areas and pore volumes combined with a pore size distribution. These materials comprise hydrophobic graphene layer surfaces and hydrophilic functional groups located mainly on the edges of the graphene layers. Adsorption is enhanced by confinement in microporosity where the overlap of potential energy fields occurs due to proximity of pore walls.33 The adsorption characteristics of aromatic molecules on activated carbons at low partial pressure is influenced by both the surface chemistry34 and microporous structure.35,36 The effect of systematic modification of the pore size distribution by gasification on gas/vapor adsorption has been investigated.35 However, the role of adsorptive structure and surface chemistry on the mechanism of adsorption of aromatic vapors at very low partial pressures is less clear. The toxicity of PCDDs/PCDFs prevents direct study of their adsorption isotherms on activated carbons.37 Therefore, to understand adsorption of dioxin species, chloroaromatic compounds with similar structural characteristics and low toxicity must be used as models for PCDDs/PCDFs adsorption. The use of model compounds such as chlorobenzenes, chlorinated phenols, and related species, which are also precursors in dioxin formation, to replicate dioxin adsorption is limited by their low volatility. Studies of the breakthrough curves for adsorption of 1,2,3,4-tetrachlorobenzene on activated carbons and cokes showed that micropore volume (0.001 mbar) conditions

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to simulate these conditions. This required the development of enhanced experimental protocols to study adsorption under conditions that simulate abatement conditions because of the very small amounts of adsorption under these conditions. An activated carbon with >95% microporosity was used in this study and the surface chemistry was modified by both chemical and gas phase treatment procedures to modify the functional groups without markedly changing the porous structure. This allowed effects due to surface chemistry and confinement in pores to be separated. The Henry’s Law constants were determined and the isosteric enthalpies of adsorption at zero surface coverage calculated to determine the strength of the interactions of species with surfaces. The enthalpies of adsorption at increasing surface coverage has also been calculated using the van’t Hoff equation. Barriers to diffusion of chloroaromatic species in carbon porous structures have been calculated as a function of surface coverage and a comparison is made with the isosteric enthalpies of adsorption. The results are discussed in terms of adsorptive structure, oxygen surface functional groups, adsorbate-surface interactions, thermodynamics, and kinetic mechanisms.

2. EXPERIMENTAL METHODS 2.1. Materials Used. Carbon G209, a commercially available steam activated coconut shell derived carbon was obtained from Pica, Vierzon, France. The particle size fraction used was 6  12 mesh (3.35  1.7 mm). The gases used were nitrogen (99.9995% purity), carbon dioxide (99.999% purity), and hydrogen (99.9995%), and these were supplied by BOC. Chlorobenzene (99%), 2chlorotoluene (99%), 1,3-dichlorobenzene (99%), 2-chloroanisole (99%), and nitric acid (70 wt %) were supplied by Sigma Aldrich. 2.2. Nitric Acid Oxidation of Carbon G209. Carbon G209 was refluxed in 7.5 M HNO3 solution for 24 h at 353 K, followed by Soxhlet extraction with water until a constant pH was achieved, to remove residual HNO3 and any water-soluble materials. The resulting material was then dried at 373 K and designated code GOX24. 2.3. Hydrogen reduction of Carbon G209. Carbon G209 was treated in a flow of hydrogen (50 mL min-1) at 250 mbar for 1 h at 673 K using Hiden Isochema Intelligent Gravimetric Analyzer (IGA) with flow control. The resulting carbon was designated code GH2. 2.4. Carbon Characterization. Elemental Analysis. Carbon, hydrogen, nitrogen, and oxygen analyses were performed by Elemental Micro-Analysis Ltd., Okehampton, Devon, U.K. 2.5. Determination of Surface Oxygen Functional Groups. Fourier Transform Infrared Spectroscopy (FTIR). Infrared spectra were obtained using a Nicolet 20-PCIR Fourier transform infrared spectrometer with CsI optics DTGS detector. The spectral range used was 600-4000 cm-1. The sample chamber was placed under a flow of nitrogen (250 mL min-1) to prevent interference by atmospheric CO2. Sample disks were prepared by compressing mixtures of 0.5% finely ground carbon in KBr. Titration Studies. Oxygen surface functional group concentrations were evaluated by the method developed by Boehm.34,38 Approximately, 0.2 g of carbon sample was placed in 25 mL of the following solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The mixtures were sealed in an atmosphere of nitrogen for 72 h at room temperature. The remaining excess base and acid were back-titrated with 0.1 N HCl and 0.1 N NaOH, respectively. The concentration of 2777

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acidic sites was calculated by assuming that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2CO3 neutralizes carboxylic and lactonic groups, and NaHCO3 neutralizes only carboxylic groups. The concentration of surface basic sites was obtained from titration with hydrochloric acid.39 Temperature Programmed Desorption (TPD). TPD studies were performed using a Thermal Science STA 1500 thermogravimetric analyzer (TGA) connected to a VG Quadrupole 300 amu mass spectrometer by a heated stainless steel capillary, lined with deactivated fused silica. Approximately ∼5 mg of activated carbon was placed in a ceramic bucket. A flow of nitrogen was introduced into the sample chamber at a flow rate of 50 mL min-1. The sample was heated to 1273 K at a heating rate of 15 K min-1. The concentrations of desorbed species were monitored by mass spectrometry. 2.6. Adsorption Studies. Adsorption characteristics of nitrogen, carbon dioxide, and chloroaromatic adsorptives on activated carbons used in this study were investigated using an Intelligent Gravimetric Analyzer (IGA), supplied by Hiden Isochema Ltd., Warrington, U.K. The system is an ultrahigh vacuum (UHV) system comprising a fully computer controlled microbalance and pressure admit and temperature regulation systems. The mass was recorded using a microbalance, and the temperature and pressure were computer-controlled. The microbalance has a longterm stability of (1 μg with a weighing resolution of 0.2 μg. The carbon sample (100 ( 1 mg) was outgassed to a constant weight, at chlorobenzene The order is consistent with increasing substitution in the aromatic ring increasing KH. Adsorption of 2-chlorotoluene on G209 and Oxidized and Reduced Carbons. 2-Chlorotoluene was chosen to study the effect

The trends obtained from the comparison of uptakes at specific pressures (Figure S11a,b, Supporting Information) and the virial parameters (and Henry’s Law constants) show that G209 and GH2 are very similar. Since the porous structure characteristics only change to a small extent, changes in the A0 and KH values are attributed to differences in functional group concentrations, which influence the electronic structure of the graphene layers rather than to small changes in the porosity. Enthalpies of Adsorption i. Isosteric Enthalpies at Zero Surface Coverage. The LF equation fitted the isotherms well at uptakes >0.7 mmol g-1. However, this equation cannot be used at low uptakes because it does not reduce to Henry’s Law. Therefore, a virial equation was used for analysis of the adsorption data in the low pressure region. The A0 virial parameters were used to calculate the isosteric enthalpies of adsorption at zero surface coverage (Qst,n=0) for chlorobenzene, 2-chlorotoluene, 2-chloroanisole, 1,3-dichlorobenzene, and 2-chloroanisole on G209. Graphs of A0 versus 1/T are shown in Figure 6a,b, and the Qst,n=0 values are given in Table 6. The Qst,n=0 are similar, showing only small differences in interaction energy with the carbon surface for the adsorbates studied. The enthalpies of vaporization of 1,3-dichlorobenzene, chlorobenzene, 2-chlorotoluene, and 2-chloroanisole reported in the literature are in the ranges 44.1-53.9 (348-513 K), 35.4-43.9 (313-597 K), 41.6-45.3 (338-493 K), and 48.349.4 kJ mol-1 (388-460 K), respectively.62 The isosteric enthalpies of adsorption at zero surface coverage should be greater than the corresponding enthalpies of vaporization. Clearly there are significant ranges of values reported in the literature for enthalpies of vaporization for the adsorptives used, and these ranges limit this comparison. However, results show that there are only relatively small differences in the enthalpies of adsorption at zero surface coverage and vaporization for the various species studied. The effect of varying carbon surface functional group chemistry was studied by comparing Qst,n=0 values for 2-chlorotoluene 2782

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Figure 6. (a) Graphs of A0 vs 1/T for adsorption of chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole on G209. (b) Graphs of A0 (ln(mol g-1 Pa-1)) vs 1/T (1/K) for 2-chlorotoluene adsorption on G209, GOX24, and GH2 in the temperature range 353-453 K.

Figure 5. Adsorption isotherms for 2-chlorotoluene on GOX24 (a, b) and GH2 (c, d) on a pressure (mbar) and relative pressure basis (p/p0). (a) Pressure (mbar) range p 0-11 mbar (temperature range: 352.77-452.67 K). (b) At 352.77-423.09 K, p/p0 0-0.01, and at 452.67 K, p/p0 0-0.007 (at 352.77 K, p 0-0.760 mbar; at 373.30 K, p 0-1.676 mbar; at 393.31 K, p 0-3.311 mbar; at 423.09 K, p 0-7.970 mbar; at 452.67 K, p 0-11.069 mbar). (c) Adsorption isotherms for 2-chlorotoluene on GH2 in the pressure (mbar) range p 0-11 mbar (temperature range: 353.77-452.97 K). (d) At 352.77-422.77 K, p/p0 0-0.01, and at 452.97 K, p/p0 0-0.007 (at 352.77 K, p 0-0.759 mbar; at 373.29 K, p 0-1.675 mbar; at 392.67 K, p 0-3.244 mbar; at 422.77 K, p 0-7.900 mbar; at 452.97 K, p 0-10.978 mbar). (e) Comparison of 2-chlorotoluene adsorption on G209, GOX24, and GH2 at ∼453 K.

adsorption on G209, GOX24, and GH2. Table 6 shows that GH2, G209, and GOX24, have Qst,n=0 values of 53.22 ( 3.50, 46.00 ( 1.27, and 42.26 ( 2.48 kJ mol-1, respectively. This trend is similar to that observed on the basis of amounts adsorbed and KH. The range of Qst,n=0 values cover a narrow range (40-54 kJ mol-1) for all the adsorptives studied, and it is apparent extensive surface oxidation of the carbon has only a small effect on adsorbate-adsorbent interactions. ii. Enthalpy of Adsorption As Function of Amount Adsorbed. The differential enthalpies (ΔHn) and entropies of adsorption (ΔSn) were calculated as a function of amount adsorbed (n) from the isotherms measured over a range of temperatures using the van’t Hoff isochore, which is given by the following equation. lnðpÞn ¼

ΔHn ΔSn RT R

ð5Þ

A graph of ln(p) versus 1/T at constant amount adsorbed allows the differential enthalpy and entropy of adsorption and also, the isosteric 2783

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Table 6. Isosteric Enthalpies of Adsorption (Qst,n=0), Enthalpies of Activation (ΔH‡n=0), and Activation Energies (Ea,n=0) at Zero Surface Coverage for Chlorobenzene, 2-Chlorotoluene, 1,3-Dichlorobenzene, and 2-Chloroanisole on G209 and 2-Chlorotoluene on GOX24 and GH2 (Temperature Range 353-453 K) carbon

a

adsorptive

isosteric enthalpy of adsorptiona

enthalpy of activation at zero surface coverage

activation energy at zero surface coverage

(Qst,n=0)/kJ mol-1

(ΔH‡n=0)/kJ mol-1

(Ea)/kJmol-1

G209

chlorobenzene

45.04 ( 2.43

9.78 ( 1.33

13.09 ( 1.15

G209

1,3-dichlorobenzene

45.52 ( 3.12

32.61 ( 3.41

35.91 ( 3.40

G209

2-chloroanisole

38.36 ( 14.32

66.15 ( 0.39

69.73 ( 0.44

G209 GOX24

2-chlorotoluene 2-chlorotoluene

46.00 ( 1.27 42.26 ( 2.48

32.52 ( 4.10 10.99 ( 1.66

35.84 ( 4.05 14.31 ( 1.65

GH2

2-chlorotoluene

53.22 ( 3.50

20.40 ( 1.83

23.72 ( 1.82

Determined by virial equation analysis.

enthalpy of adsorption (Q st,n) to be determined. Typical graphs of ln(p) versus 1/T for adsorption of 2-chlorotoluene on GOX24 at various amounts adsorbed are shown in Figure 7a,b. The pressure values for a specific amount adsorbed were calculated from the adsorption isotherms by the following methods (1) assuming a linear relationship between adjacent isotherm points starting from the first isotherm point, and (2) fitting the LF isotherm equation to the experimental data. The standard deviations for the points were typically in the range up to (3 kJ mol-1 and the highest uncertainties were observed on the steepest part of the isotherm at low pressure (Supporting Information, Table S5). Good agreement was obtained for the two methods and the trends are consistent with the values obtained from the virial method (Table 6, Figure 8, and Supporting Information Table S5). The trend in the enthalpies of adsorption as a function of the amount adsorbed for chlorobenzene, 2-chlrotoluene, 1,3-dichlorobenzene, and 2-chloroanisole on G209 are shown in Figure 8a,b. The Qst,n values increase with increasing amounts adsorbed for all the adsorptives studied. The trend in enthalpies of adsorption between 1.0 and 1.5 mmol g-1 is 2-chlorotoluene > chlorobenzene > 1; 3-dichlorobenzene > 2-chloroanisole The isosteric enthalpies of adsorption at higher surface amounts adsorbed were significantly larger than the corresponding isosteric enthalpies at zero surface coverage and the enthalpies of vaporization. Adsorption of benzene on G209 was studied over the temperature range 293-323 K to show if the increase was related to the aromatic character of the adsorptives. The results showed a similar trend of increasing enthalpy of adsorption with increasing surface coverage (Supporting Information, Table S5g and Figure S12a (isotherms) and S12b (enthalpies)). There have been several studies of the adsorption of benzene on porous carbons,63,64 carbon blacks,65-67 and carbon nanotubes.68 The adsorption of benzene on graphitized carbon blacks showed that the isosteric enthalpy of adsorption varied in the range 40-44 kJ mol-1 with surface coverage.69,70 The values obtained in this study were in the range 30-53 kJ mol-1. Therefore, it is proposed that the increase in isosteric enthalpies of adsorption of benzene and chloroaromatic species with increasing surface coverage is due to confinement in micropores increasing attractive intermolecular π-π dispersion interactions leading to structural stacking and ordering of planar aromatic adsorbate molecules.71 The ΔS values decreases with increasing surface coverage (Table S5). 3.6. Adsorption Kinetics. Adsorption Kinetics for Chlorobenzene, 2-Chlorotoluene, 1,3-Dichlorobenzene, and 2-Chloroanisole on G209. Double exponential(DE),72,73 stretched exponential (SE)74-76 and linear driving force (LDF)77-85 models, which are

Figure 7. (a) Van’t Hoff Isochores for adsorption of chlorobenzene, 2-chloroanisole, 2-chlorotoluene, and 1,3-dichlorobenzene on G209 at 1.5 mmol g-1. (b) Van’t Hoff Isochores for 2-chlorotoluene adsorption on G209, GOX24, and GH2 at 1.2 mmol g-1.

nested models of a double stretched exponential model have been used to describe the adsorption kinetics of gases and vapors on carbon molecular sieves,77,79-81,83,84 activated carbon,74,75,79,82 2784

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Figure 9. Typical fitting of a normalized mass uptake profile obtained for 1,3-dichlorobenzene adsorption on G209 for p 0.006-0.011 mbar (p/p0: 1.41  10-4 to 2.50  10-4) using the stretched exponential model at 349 K.

Figure 8. (a) Isosteric enthalpies of adsorption for chlorobenzene, 2-chloroanisole, and 1,3-dichlorobenzene on G209. (b) Isosteric enthalpies of adsorption for 2-chlorotoluene adsorption on G209, GOX24, and GH2.

silicas,85 and metal organic framework materials72,73,86 with the choice of model depending on both pore and adsorbate structures. The stretched exponential model has been used to study the adsorption and desorption of a wide range of hydrophilic to hydrophobic adsorptives on activated carbons.73,75,76 Klafter and Shlesinger showed87 that the stretched exponential model is a common underlying mathematical structure relating the Forster direct-transfer mechanism,88 which is an example of relaxation via parallel channels and the serial hierarchically constrained dynamics89 and defect-diffusion models.90-92 The unifying mathematical feature is the presence of a scale-invariant distribution of relaxation times. The stretched exponential (SE) model is described by the following equation: β Mt ¼ 1 - e - ðktÞ Me

ð6Þ

where Mt is the mass at time, Me is the mass at equilibrium, k is the rate constant (s-1), and t is the time(s). The quantity β is a

material dependent parameter reflecting the width of the distribution of relaxation times. The SE model is 1-dimensonal with a distribution of relaxation times when β = 0.5, and 3-dimensional with a single relaxation times when β = 1 (linear driving force (LDF) model).87 Adsorption of chloroaromatic species on activated carbons followed the SE model over the temperature range studied. A typical SE fit of the normalized mass uptake profile for adsorption of 1,3-dichlorobenzene on G209 for isotherm pressure increment 0.006-0.011 mbar, at 349 K, is shown in Figure 9. The residuals show that the kinetic model fits within (2% over the entire uptake range. Similar fitting characteristics were obtained for the SE model for kinetic normalized profiles for 2-chlorotoluene adsorption on GH2, G209, and GOX24 at 353 K and these are shown in Supporting Information (Figure S13). Figure 10 shows graphs of ln(k) vs amount adsorbed (mmol g-1) for 2-chlorotoluene adsorption on G209 for temperatures range 353-453 K. Corresponding graphs for chlorobenzene, 1,3dichlorobenzene, and 2-chloroanisole adsorption on G209 and 2-chlorotoluene adsorption on GOX24 and GH2 are shown in the Supporting Information (Figure S14a-e). These parameters typically correspond to diffusion coefficients in the range ∼(2-30)  10-6 cm2 s-1 at 453 K. The rate constants increase linearly with increasing surface coverage and this is related to the change in chemical potential over the isotherm. The graphs of ln(k) versus amounts adsorbed were used to calculate ln(kn=0) values for specific amounts adsorbed. Extrapolation to n = 0 using linear regression analysis allowed determination of the rate constant at zero surface coverage for each temperature. ln(kn=0) values at each temperature are listed in the Supporting Information (Table S7). The activation energies at various amounts adsorbed (Ea,n) were calculated from the Arrhenius equation and enthalpies of activation (ΔH‡n) were calculated from graphs of ln(k,n=0/T) versus 1/T. The values of Ea,n=0 and ΔH‡n=0 are listed in Table 6. The enthalpies of activation at zero surface coverage (ΔH‡n=0) are lower than the corresponding activation energies (Ea,n=0) by RT (3.3 kJ mol-1), which is consistent with a unimolecular adsorption process93 (Table 6). 2785

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Figure 10. Graphs of ln(k) versus amount adsorbed for 2-chlorotoluene adsorption on G209 in the temperature range 353-453 K.

In all cases the adsorption rate constant increases with increasing surface coverage as the chemical potential gradient increases over the isotherm as shown by linear graphs of ln(k) versus amount adsorbed for all adsorbate-adsorbent systems investigated (Figure 10 and Figures S14a-e, Supporting Information). The variation of ΔH‡ with amount adsorbed is shown in Figure 11a,b. In the case of 2-chloroanisole, initially Ea,n=0 and ΔH‡n=0 are higher than Qst,n=0 due to the presence of the relatively large methoxy group (Table 6). Ea,n and ΔH‡n decrease while Qst,n increases with increasing amount adsorbed (Figures 8 and 11). When the adsorption kinetics are slow at low surface coverage, where the chemical potential gradient is lowest, the kinetics are determined by diffusion of the 2-chloroanisole through constrictions in the porous structure. As the chemical potential gradient and rate of adsorption increase with increasing surface coverage, surface diffusion becomes the increasingly dominant mechanism as Qst,n becomes larger than ΔH‡n. When Qst,n is greater than ΔH‡n, then the probability that desorption into the gas phase occurs is lower than the surface diffusion and hence the latter is the rate controlling step. In the case of the other adsorbateadsorbent systems studied, Ea,n and ΔH‡n are always much lower than Qst,n so that surface diffusion by a site-to-site hopping mechanism involving an activated transition state, between physisorption potential energy wells on the surface, is the dominant kinetic mecahnism.93 Effect of Surface Functional Groups in Porous Carbons on Kinetics of 2-Chlorotoluene Adsorption. The kinetics and thermodynamic parameters for adsorption of 2-chlorotoluene on carbons G209, GOX24, and GH2 at zero surface coverage are shown in Table 6, and the SE kinetic parameters are given in Supporting Information, Table S7. The isosteric enthalpy of adsorption (Qst,n=0) at zero surface coverage is in the order GH2 > G209 > GOX24, showing the effect of increasing surface oxygen oxidation. GOX24 also has the lowest values for ΔH‡n=0 and Ea,n=0. The SE kinetic parameters for 2-chlorotoluene adsorption at zero surface coverage on GH2, G209, and GOX24 are given in Table S7. The SE kinetic parameters at zero surface coverage are in the order GOX24 . G209 > GH2 at 353 K while the SE kinetic parameters are in the order G209 > GOX24 > GH2 at 453 K. However, at higher surface coverage at 453 K, for

Figure 11. (a) The variation of ΔH‡ with amount adsorbed for chlorobenzene, 2-chlorotoluene, 1,3-dichlorobenzene, and 2-chloroanisole on G209. (b) The variation of ΔH‡ with amount adsorbed for 2-chlorotoluene on G209, GOX24, and GH2.

example, 0.5 mmol g-1, the order is GOX24 > G209 > GH2 (Figure 10 and Supporting Information, Figures S14d,e). The presence of surface oxygen functional groups decrease both the barrier to diffusion and isosteric enthalpy of adsorption at zero surface coverage (Table 6). This is consistent with electron withdrawing carboxyl surface groups at the edges of graphene layers decreasing the electron π-π donation and the adsorbateadsorbent interaction strength, and this is expected to have the largest effect at low surface coverage.94 The results are consistent with the surface hopping mechanism on the graphene layer hydrophobic sites, since the isosteric enthalpy of adsorption is much greater than the barrier to diffusion.93 At high surface coverage, π-π adsorbate-adsorbate interactions71 will have increasing influence on adsorption kinetics. Effect of Adsorbate Structure on the Kinetics and Mechanism of Adsorption. Diffusion into porous materials is influenced by the smallest molecular dimension for slit shaped pores and the two minimum dimensions for spherical shaped pores. Molecular dimensions and kinetic diameters for the chloroaromatic species 2786

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The Journal of Physical Chemistry C studied have not, as far as we are aware, been reported in the literature with the exception of chlorobenzene, which has a kinetic diameter of 7.28 Å.95 The molar volumes show that the molecular size is in the order: 2-chloroanisole > 2-chlorotoluene > 1.3-dichlorobenzene > chlorobenzene. The SE rate constants at zero surface coverage differ by a factor of 10 at 353 K and 4 at 453 K (Supporting Information, Table S7). No clear trend with molecular volume was observed. Adsorption of planar chlorobenzene, 2-chlorotoluene, and 1,3-dichlorobenzene on G209 showed that Ea,n=0 and ΔH‡n=0 were significantly lower than the corresponding enthalpy of adsorption at zero surface coverage (Qst,n=0). When Ea,n=0 and ΔH‡n=0 are much smaller than Qst,n=0, a site-to-site surface diffusion mechanism controls the adsorption kinetics.93 The activation energy (Ea,n=0) and enthalpy of activation (ΔH‡n=0) for 2-chloroanisole adsorption on G209 are significantly larger than the isosteric enthalpy of adsorption, as shown in Table 6. The large kinetic barrier is attributed to increased diffusion resistance of 2-chloroanisole, with larger minimum cross-sectional area, in ultramicroporosity. The stretched exponential equation parameter β lies between 0.75 and 1 for nonplanar 2-chloroanisole. Hence, for 2-chloroanisole there is a narrower distribution of relaxation times as the rate determining step is close to the LDF mechanism. The adsorption kinetics for a wide range of gases on carbon molecular sieves used for air separation follow the LDF model (β = 1).77-85 3.7. Interactions of Models for Dioxins and Carbon Surfaces. Dioxin abatement by adsorption on carbons injected into flue gases occurs under conditions where the adsorption is small and not close to equilibrium. Physisorption is small under high temperature and low partial pressure conditions where adsorption in ultramicropores and surface chemistry are most important. In contrast, with adsorption under low temperature and high relative pressure conditions, the available pore volume is a limiting factor. Oxygen functional groups on the edges of graphene layers provide surface sites for adsorption of hydrophilic species and may also influence adsorption on hydrophobic graphene layers. The narrower the pores, the greater the overlap of the potential energy fields of the pore walls and, hence, increased adsorption. Therefore, changes in the ultramicroporous structure and surface chemistry due to chemical treatments must be considered if the role of adsorptive structure and surface chemistry are to be studied independently. Nitric acid treatment of carbons incorporates larger amounts of oxygen functional groups on the carbon surface compared to other liquid oxidants.96 The porous structure does not change greatly, and therefore, the trends indicated above are due to changes in the oxygen surface groups.43,44,97 The trends in thermodynamic and kinetic parameters show that GOX24 has weaker adsorbate-adsorbent interactions at zero surface coverage but stronger interactions at higher surface coverage. The latter is attributed to π-π dispersion interactions of planar aromatic molecules due to confinement effects in ultramicropores. The enthalpies of adsorption for adsorption of 2-chlorotoluene on GH2 are similar to those for G209. This is reasonable as G209 and GH2 have similar oxygen contents. Gas phase adsorption of aromatic species on carbon surfaces is governed by π-π dispersion interactions with the graphene layers. The isosteric enthalpy of adsorption at zero surface coverage is a fundamental measure of the adsorbate adsorbent interaction. Carboxyl functional groups on the edges of the graphene layers withdraw electrons from the π density of the graphene basal planes. At low surface coverage the π-π interactions involving

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electron donation from the carbon basal plane to the aromatic adsorbate are weakened due to the electron withdrawing carboxyl functional groups at the edges of graphene layers as shown by the following trend: GH2 > G209 > GOX24 for 2-chlorotoluene. However, chlorine also withdraws electron density in the aromatic adsorptive, but this does not have a significant effect on adsorbate-adsorbent interaction. The enthalpy of adsorption increases with increasing surface coverage, and this is due to the influence of π-π adsorbate-adsorbate interactions. An increase in adsorbate size increases the interaction strength as has been shown for benzene and naphthalene adsorption on graphite.98

4. CONCLUSIONS This study has involved an investigation of the adsorption characteristics of chloroaromatic species on activated carbon surfaces in the temperature range 353-453 K as models to simulate dioxin adsorption under conditions used in abatement systems. The microporous structure of the carbon was the most important adsorbent characteristic with diffusion into the microporous structure being the rate determining process. The isosteric enthalpies of adsorption of chloroaromatic species on carbons are a function of adsorptive structure and surface functional groups in the carbon. The isosteric enthalpies of adsorption, activation energies, and enthalpies of activation at zero surface coverage are quite similar for G209 and GH2. Extreme oxidation of carbon decreases the enthalpy of adsorption at zero surface coverage, indicating weaker adsorbent-adsorbate interactions. Also, extreme oxidation increases the barriers to diffusion into the porous structure to a small extent. Therefore, this study of the adsorption of models for dioxins indicates that mild oxidation of carbon occurring in the flue gases should not be a major issue for dioxin abatement. Adsorption kinetics for all systems studied follows a stretched exponential model. A linear relationship between ln(k) and the amount adsorbed was observed. The controlling factors in the diffusion of species into porous structures are related to the relative magnitudes of the enthalpies of adsorption and activation. The isosteric enthalpy of adsorption increases with increasing surface coverage for adsorption of all chloroaromatic adsorptive and carbon adsorbent systems. The same trend is observed for benzene adsorption on G209, and this is attributed to enhanced structural ordering due to increased π-π dispersion interactions between planar aromatic molecules confined in ultramicropores. The activation energies and enthalpies of activation are typically lower than the isosteric enthalpies of adsorption for all surface coverages for planar species. This is consistent with a surface hopping diffusion process being the main rate determining process for diffusion into the porous material. At low surface coverage, for adsorption of nonplanar 2-chloranisole, the activation energy is greater than the enthalpy of adsorption but the activation energy decreases with increasing surface coverage. The different mechanism controlling diffusion is due to the larger minimum cross-section area associated with the methoxy group of 2-chloroanisole. Surface interactions are the most significant factor; however, the presence of side groups may cause kinetic limitations for low amounts adsorbed where the chemical potential gradient is low. ’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of additional carbon characterization (pore volumes, surface areas), adsorption thermodynamic

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The Journal of Physical Chemistry C data (Langmuir-Freundlich fitting parameters, virial parameters, enthalpies), and kinetic data (adsorption isotherm grandients, ln(kn=0) values). Figures of FTIR spectra, TPD profiles, adsorprtion isotherms, Dubinin-Radushkevich graphs, Langmuir isotherms, experimental fitting graphics, adsorption comparisons, mass uptake profiles, and ln(k) vs adsoprption). This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] Tel: þ44 191 222 6839 Fax: þ44 191 222 6929.

’ ACKNOWLEDGMENT This work was supported by the European Union Research Programme of the Research Fund for Coal and Steel under projects entitled: Production and application of special coke for environmental purposes; contract 7220-PR/067 and Zero “dioxin” releases in coal combustion and coal/organic waste cocombustion processes; contract RFC-CR-04007. ’ REFERENCES (1) Olie, K.; Vermeulen, P. L.; Hutzinger, O. Chemosphere 1977, 6, 455–459. (2) Karasek, F. W.; Dickson, L. C. Science 1987, 237, 754–756. (3) Dickson, L. C.; Lenoir, D.; Hutzinger, O. Environ. Sci. Technol. 1992, 26, 1822–1828. (4) Huang, H.; Buekens, A. Chemosphere 1995, 31, 4099–4117. (5) Sidhu, S. S.; Maqsud, L.; Dellinger, B.; Mascolo, G. In 25th International Symposium on Combustion; Elsevier Science Publ Co Inc: Irvine, CA, 1994; pp 11-20. (6) McKay, G. Chem. Eng. J 2002, 86, 343–368. (7) Everaert, K.; Baeyens, J. Chemosphere 2002, 46, 439–448. (8) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Prog. Energ. Combust. 2009, 35, 245–274. (9) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17, 721–730. (10) Luijk, R.; Akkerman, D. M.; Slot, P.; Olie, K.; Kapteijn, F. Environ. Sci. Technol. 1994, 28, 312–321. (11) Okamoto, Y.; Tomonari, M. J. Phys. Chem. A 1999, 103, 7686– 7691. (12) Lee, J. E.; Choi, W.; Mhin, B. J. J. Phys. Chem. A 2003, 107, 2693–2699. (13) Khachatryan, L.; Asatryan, R.; Dellinger, B. J. Phys. Chem. A 2004, 108, 9567–9572. (14) Asatryan, R.; Davtyan, A.; Khachatryan, L.; Dellinger, B. J. Phys. Chem. A 2005, 109, 11198–11205. (15) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. J. Phys. Chem. A 2007, 111, 2563–2573. (16) Lomnicki, S.; Dellinger, B. J. Phys. Chem. A 2003, 107, 4387– 4395. (17) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. J. Phys. Chem. A 2007, 111, 7133–7140. (18) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. J. Phys. Chem. A 2008, 112, 3680–3692. (19) Baccarelli, A.; Pesatori, A. C.; Masten, S. A.; Patterson, D. G.; Needham, L. L.; Mocarelli, P.; Caporaso, N. E.; Consonni, D.; Grassman, J. A.; Bertazzi, P. A.; Landi, M. T. Toxicol. Lett. 2004, 149, 287. (20) Stephens, R. D.; Petreas, M. X.; Hayward, D. G. Sci. Total Environ. 1995, 175, 253. (21) Ryan, J. J.; Schecter, A.; Lizotte, R.; Sun, W.-F.; Miller, L. Chemosphere 1985, 14, 929.

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