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Environmental Science & Technology
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Ammonia removal using activated carbons: Effect of the

surface chemistry in dry and moist conditions

Journal: Environmental Science & Technology

Manuscript ID: es-2011-03093v.R1

Manuscript Type: Article

Date Submitted by the

n/a
Author:

Complete List of Authors: Gonçalves, Maraisa; Facultad de Ciencias, Universidad de Alicante

Sánchez-García, Laura; Facultad de Ciencias, Universidad de Alicante
Oliveira Jardim, Erika; Facultad de Ciencias, Universidad de Alicante
Silvestre-Albero, Joaquin; University of Alicante, Inorganic Chemistry
Department
Rodriguez-Reinoso, Francisco; Facultad de Ciencias, Universidad de
Alicante

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Ammonia removal using activated carbons: Effect of the
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surface chemistry in dry and moist conditions
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13 Maraisa Gonçalves, Laura Sánchez-García, Erika Oliveira-Jardim, Joaquín Silvestre-Albero*,
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16 Francisco Rodríguez-Reinoso
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19 Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica-Instituto Universitario de
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Materiales, Universidad de Alicante, Ap. 99, E-03080 Alicante, Spain
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56 Corresponding author: Joaquín Silvestre-Albero (joaquin.silvestre@ua.es)
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59 Tel./Fax.: +34965909350/3454
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6 ABSTRACT
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11 The effect of surface chemistry (nature and amount of oxygen groups) in the removal of ammonia was
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14 studied using a modified resin-based activated carbon. NH3 breakthrough column experiments show that
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16 the modification of the original activated carbon with nitric acid, i.e. the incorporation of oxygen surface
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18 groups, highly improves the adsorption behaviour at room temperature. Apparently, there is a linear
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21 relationship between the total adsorption capacity and the amount of the more acidic and less stable
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23 oxygen surface groups. Similar experiments using moist air clearly show that the effect of humidity
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25 highly depends on the surface chemistry of the carbon used. Moisture highly improves the adsorption
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28 behaviour for samples with a low concentration of oxygen functionalities, probably due to the
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30 preferential adsorption of ammonia via dissolution into water. On the contrary, moisture exhibits a small
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32 effect on samples with a rich surface chemistry due to the preferential adsorption pathway via Brønsted
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35 and Lewis acid centers from the carbon surface. FTIR analyses of the exhausted oxidized samples
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37 confirm both the formation of NH4+ species interacting with the Brønsted acid sites, together with the
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39 presence of NH3 species coordinated, through the lone pair electron, to Lewis acid sites on the graphene
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42 layers.
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58 KEYWORDS: Activated carbon, ammonia, surface chemistry.
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Introduction
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3 New air quality emission standards are forcing the different governments to adopt new regulations in
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5 order to control pollutant emissions. Among them, ammonia is considered an important health hazard
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7 because it is poisonous if inhaled in great quantities (breathing levels above 50-100 ppm) while it can
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10 cause eyes, throat and nose irritation in lesser concentrations.1
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12 Actual ammonia emissions are generated mainly from the fertilizer manufacture industry, coke
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manufacture, fossil fuel combustion, livestock and poultry management, and refrigeration methods.
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17 Among all these sources, livestock waste management and fertilizers production account for about 90%
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19 of total ammonia emissions.1,2 Many techniques have been proposed in the literature for the removal of
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NH3 in industrial effluents. These include absorption by solution, reaction with other gases, ion
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24 exchange using polymeric resins, separation using membranes, thermal treatment, catalytic
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26 decomposition and adsorption by porous solids.3-5 Some of these techniques (e.g. thermal combustion)
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are economically convenient for high concentration pollutants, whereas they become economically
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31 unviable for diluted waste streams. For these special cases adsorption on porous solids (e.g. activated
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33 carbons, zeolites, and so on) can be an excellent approach.6-12 Among the different porous solids
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described in the literature, activated carbons exhibit certain advantages such as a high “apparent” surface
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38 area, a highly developed porous structure and, most importantly, the possibility to tailor the porous
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40 structure and surface chemistry in order to adapt it for a special application.13 These modifications using
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43 pre- and post-synthesis treatments are very convenient when trying to adsorb polar gases (e.g. NH3),
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45 because activated carbons usually exhibit non-polar surfaces and surface modifications are mandatory.
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47 Previous studies described in the literature indicated that the surface chemistry is probably the most
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50 critical parameter defining the total adsorption capacity of activated carbons for a basic molecule such as
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52 ammonia.6,7,9 Kim et al. showed that not only the amount but also the nature of the oxygen surface
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54 groups present on the carbon surface, i.e. acidic groups, are responsible for the total amount adsorbed.7
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57 Furthermore, Huang et al. reported a linear correlation between NH3 breakthrough capacity and the total
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amount of acidic groups.6 A similar beneficial effect for the ammonia removal has been described in the
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3 literature for metal-modified (Fe, Co, Cr, Mo and W) activated carbons.8,10,14
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5 Different reaction mechanisms have been proposed for the adsorption process involving mainly
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7 Brønsted acid sites, but the true pathway is still under debate. Unfortunately, many of these studies
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10 described in the literature deal with a wide variety of carbon samples, i.e. different textural and chemical
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12 properties, together with additional effects due to the presence of metal species, which makes difficult a
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clear correlation between the inherent parameters of the carbon support and the adsorption
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17 mechanism.8,10,14 Furthermore, only few studies have paid attention to the role of moisture usually
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19 present in industrial streams.
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The objective of this paper is to evaluate the real role of the oxygen surface groups present on the
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24 surface of activated carbons in the removal of ammonia, either in the presence or absence of moisture.
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26 For this purpose, an activated carbon prepared from a resin precursor was modified by an oxidation
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treatment with HNO3, followed by a subsequent thermal treatment at different temperatures in order to
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31 selectively remove some of the oxygen surface groups. Compared to previous analysis described in the
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33 literature, the sole modification of the surface chemistry in a controlled way will allow to analyze the
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effect of the surface chemistry in the ammonia adsorption process, under dry or moist conditions,
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38 avoiding the influence of additional parameters (e.g. porous structure or inorganic matter).
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43 2. Experimental Section
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45 2.1. Materials
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47 A spherical activated carbon (MA2, supplied by MAST Carbon International, UK) with particle size
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50 around 0.32 mm was prepared by carbonization and subsequent activation using CO2 (43 % burn-off) of
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52 a porous resin obtained by cross-linking of phenol-formaldehyde Novolac precursor with
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54 hexamethylenetetramine and using ethylene glycol as solvent-pore former. For the oxidation treatment,
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57 25 g of carbon were treated with 250 ml HNO3 solution (6M) at 90ºC for 1h. The oxidized sample
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59 (MA2ox) was washed until neutral pH and dried overnight at 85ºC. In order to selectively remove the
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surface functional groups, sample MA2ox was submitted to a subsequent thermal treatment at 300, 500
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3 and 700ºC under a helium flow (50 ml/min) for 1h. Samples were labelled MA2ox300, MA2ox500 and
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5 MA2ox700, respectively.
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10 2.2. Characterization
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12 N2 adsorption/desorption isotherms were performed at -195ºC in a home-made high precision
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volumetric equipment. Before any experiment samples were degassed under vacuum (10-3 Pa) at 150ºC
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17 for 4h. “Apparent” surface area was calculated from the nitrogen adsorption data after application of the
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19 BET equation while the micropore volume (V0) was obtained after application of the Dubinin-
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Radushkevich (DR) equation. CO2 adsorption isotherms were performed in the same apparatus at 0ºC
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24 following the same protocol. Application of the DR equation to the CO2 adsorption data was used to
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26 calculate the volume of narrow micropores (Vn), i.e. those below 0.7 nm.15
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Temperature programmed decomposition experiments (TPD) were performed in order to evaluate the
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31 amount and nature of oxygen surface functionalities for the different samples. 100 mg of carbon were
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33 placed in a quartz reactor under a helium flow (50 ml/min) and submitted a heat treatment up to 1000ºC
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(10ºC/min). The different gas species evolved as a result of surface groups decomposition (mainly CO
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38 and CO2) were analyzed using an on-line mass spectrometer (Omnistar TM, Balzers). Quantitative
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40 analyses were performed after calibration using CaC2O4·H2O as a reference material.
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43 FTIR spectra of the exhausted samples were obtained in the range 4000-700 cm-1, by adding 100 scans
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45 at a resolution of 5 cm-1, using a Mattson Infinity Gold FTIR spectrometer in the diffuse reflectance
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47 method. Before any experiment all samples were diluted with KBr.
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52 2.3. Breakthrough column experiments
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54 The adsorption capacity under dynamic conditions for ammonia removal at room temperature (23ºC)
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57 was measured using a 1 cm i.d. column containing 2 cm carbon bed height (~ 0.6 g of sorbent). A total
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59 flow of 300 ml/min of air containing 1000 ppm of ammonia was passed through the column containing
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the fresh sample (without any pre-treatment). The concentration of ammonia at the exit was followed
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3 using a Polytron 3000 (Drager) detector. Adsorption experiments were performed using either dry or
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5 moist air (70% humidity). For moist conditions, humidity was introduced into the NH3/air flow using a
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7 calibrated syringe. The breakthrough experiment was arbitrarily stopped when the ammonia
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10 concentration at the outlet reached a concentration of 100 ppm. The adsorption capacity was determined
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12 by integration of the area above the breakthrough curve and considering the inlet concentration, the total
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flow rate and the amount of sorbent used. To further clarify the role of humidity, additional experiments
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17 were performed using pre-humidified samples (20%, 40% and 70% relative humidity). For this purpose,
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19 the carbon samples were first submitted to a thermal treatment at 100ºC overnight in order to remove the
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residual humidity and, in a subsequent step, samples were exposed to a given H2O/H2SO4 mixture for
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24 24h at room temperature under controlled humidity conditions. Breakthrough column experiments using
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26 the pre-humidified samples were always performed under dry conditions.
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31 3. Results and discussion
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33 3.1. Sample characterization
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Figure 1 shows the nitrogen adsorption/desorption isotherms for the original and the modified
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38 activated carbon. As it can be observed, the resin-based activated carbon (MA2) exhibits a highly
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40 developed microporous structure, i.e. closed knee in the nitrogen adsorption isotherm at low relative
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43 pressure, together with capillary condensation processes at p/p0 ~ 0.8, corresponding to the filling of
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45 large mesopores. The oxidation treatment with HNO3 produces slight changes in the porous structure,
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47 the main effect being the partial blockage of the microporosity due to the creation of polar oxygen
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50 surface groups at the entrance of narrow pores; this slightly decreases the adsorption of a molecule such
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52 as nitrogen, due to steric effects at the pore mouth.16,17 A subsequent thermal treatment at high
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54 temperature (700ºC) under a helium flow produces the widening of the existing micro/mesopores due to
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57 the partial gasification of the carbon structure.13, 18
The textural properties of the modified activated
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carbon obtained from the N2 and CO2 adsorption data, after application of the corresponding equations,
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3 are reported in Table 1.
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27 Figure 1. Nitrogen adsorption/desorption isotherms at -195ºC for the different activated carbons.
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32 Table 1. Textural parameters for the different activated carbons obtained from the N2 and CO2
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34 adsorption data at -195ºC and 0ºC, respectively.
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39 SBET V0 Vmeso Vt(0.97) Vn*
Samples
40 (m2/g) (cm3/g) (cm3/g) (cm3/g) (cm3/g)
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42 MA2 1550 0.61 0.68 1.29 0.55
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44 MA2ox 1470 0.61 0.63 1.24 0.53
45 MA2ox300 1570 0.63 0.67 1.30 0.54
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47 MA2ox500 1580 0.64 0.67 1.31 0.54
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49 MA2ox700 1700 0.69 0.74 1.43 0.61
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50 Obtained after application of the D-R equation to the CO2 adsorption data at 0ºC
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54 The surface chemistry of the original and modified activated carbons was evaluated using
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56 temperature-programmed decomposition (TPD) experiments. The amount of CO2 and CO evolved in the
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59 temperature range from room temperature up to 1000ºC was analyzed as described in the experimental
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section. Quantitative data for the different samples are compiled in Table 2. CO2 groups usually evolve
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3 at low temperatures and they correspond to the decomposition of the more acidic and less stable oxygen
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5 surface groups (mainly carboxylic and lactone groups), while CO evolution is due to the decomposition
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7 of the more stable and less acidic oxygen groups (mainly phenolic, carbonyl and quinone groups) and
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10 takes place at a higher temperature.19
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Table 2. Total amount of CO2 and CO groups evolved in the TPD experiment.
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19 CO CO2
20 Carbons
(mmol/g) (mmol/g)
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22 MA2 0.837 0.18
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24 MA2ox 2.800 1.770
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26 MA2ox300 2.820 0.862
27 MA2ox500 1.822 0.245
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29 MA2ox700 1.048 0.093
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34 The as-received activated carbon (MA2) shows a relatively poor surface chemistry, i.e. it has a small
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36 amount of oxygen surface groups, with a larger content of the less acidic groups compared to those
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evolved as CO2 (CO/CO2 ratio of 4.6). As expected, the oxidation treatment with nitric acid produces a
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41 large increase in the amount of both types of oxygen surface groups, this enhancement being larger for
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43 the CO2 groups (CO/CO2 ratio of 1.6), in accordance with previous observations.17 A subsequent
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thermal treatment at low temperature (300ºC) produces an important decrease in the amount of the less
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48 stable and more acidic oxygen groups, i.e. those evolved as CO2, while the more stable groups remain
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50 mainly unaffected. Temperatures higher than 300ºC are required to selectively remove not only the more
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53 acidic but also the majority of the more stable oxygen surface groups, i.e. those evolved as CO in the
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55 TPD experiment.
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3.2. Breakthrough column experiments

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3 Breakthrough column experiments were performed in a glass tubular reactor using a total flow of 300
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5 ml/min of either dry or moist air (70% humidity) containing 1000 ppm NH3. Figure 2 shows the
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7 corresponding curves obtained using (a) dry air and (b) moist conditions. The total adsorption capacity
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10 (mg/g) for the different samples is reported in Table 3.
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28 Figure 2. Breakthrough curves of ammonia adsorbed on the different activated carbons (1000 ppm,
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300 ml/min, 23ºC) using (a) dry and (b) moist (70% relative humidity) air.
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35 Dynamic adsorption experiments show important differences among the different samples either in the
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38 presence or absence of moisture. In general all samples exhibit a sharp breakthrough saturation profile
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40 that proves the absence of kinetic restrictions in the adsorption process. This observation must be
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42 attributed to fast adsorption kinetics of NH3 through the mesoporous network on these carbon materials,
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45 since the mesopores constitute the channels to access the inner microporosity. Under dry conditions (see
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47 Figure 2a) the total adsorption capacity largely improves (close to five fold increase) after an oxidation
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49 treatment of the original carbon with HNO3. The total adsorption capacity for the oxidized sample
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52 (MA2ox) achieves a value as high as 17.5 mg/g, in close agreement with previous measurements
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54 reported in the literature for oxidized activated carbons.6,8,20 A subsequent thermal treatment at low and
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56 high temperature produces a large decrease in the uptake down to a residual value even lower than the
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59 one of the original sample. Interestingly, the decline in the adsorption capacity is very drastic after the
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thermal treatment at low temperature (sample MA2ox300), whereas a small decrease is observed
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3 thereafter (up to 700ºC). This observation anticipates the crucial role of the oxygen surface groups in the
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5 gas phase removal of ammonia and, more specifically, the important effect of the more acidic and less
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7 stable (those decomposing at low temperature) oxygen surface groups (mainly carboxylic groups) in the
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10 adsorption process under dry conditions.6,7 Incorporation of 70 % relative humidity in the inlet stream
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12 has a different effect depending on the surface chemistry (Figure 2b). The oxidized sample (MA2ox)
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exhibits a moderate improvement in the adsorption behaviour after moisture incorporation, this clearly
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17 suggesting the predominant role of the oxygen surface groups in the adsorption process, i.e. water must
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19 exhibit a marginal effect in the adsorption mechanism. Concerning the thermally treated samples, Figure
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2b shows that the improvement in the adsorption capacity due to moisture incorporation is larger
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24 compared to the oxidized sample and it remains mainly constant for all samples, independently of the
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26 temperature of the thermal treatment applied (see Table 3). Apparently on these thermally treated
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samples, i.e. these samples were the more acidic and less stable oxygen surface groups have been
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31 removed, moisture exhibits a crucial role in the adsorption mechanism, independently of the amount and
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33 nature of the remaining surface functionalities. On the contrary, on carbons with a rich surface chemistry
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the preferential interaction of ammonia with the carbon surface, assisted by the oxygen surface groups
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38 and, more specifically, by the more acidic groups, could be the key factor defining the total adsorption
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40 capacity, independently of the presence or absence of moisture. The large adsorption capacity achieved
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43 with the oxidized sample (MAox) under dry conditions, together with the slight improvement observed
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45 after moisture incorporation is somehow in contradiction with previous studies described in the
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47 literature, which proposed a crucial role of water in the formation of NH4+ ions, necessary for the
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50 adsorption process via interaction with the Brønsted acid groups of the carbon surface.8
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Table 3. Total adsorption capacity for ammonia under dry and moist (70% relative humidity) air
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3 conditions.
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6 Carbons NH3 dried NH3 70% RH ∆m (mg/g)
7 (mg/g) (mg/g) (0-70% RH)
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9 MA2 4.7 5.3 0.6
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MA2ox 17.5 20.1 2.6
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12 MA2ox300 7.6 12.3 4.7
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14 MA2ox500 3.4 7.6 4.2
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16 MA2ox700 1.9 6.1 4.2
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To further clarify the real role of the oxygen surface groups in the adsorption process Figure 3a
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24 compares the total adsorption capacity, either under dry or moist conditions, and the total amount of
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26 acidic oxygen surface groups, i.e. those decomposing as CO2 in a typical TPD run. As described above,
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the total adsorption capacity is larger in the presence of moist air for all samples, independently of the
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31 surface chemistry. However, contrary to previous analysis described in the literature, the presence of
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33 moisture does not seem to be so critical for the ammonia adsorption process when oxygen surface
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groups, mainly acidic, are present (e.g. sample MAox); in other words, the adsorption capacity of the
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38 oxidized sample under dry conditions is rather high and the improvement after moisture incorporation is
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40 rather small.8 Interestingly, there is a linear correlation between the amount of ammonia adsorbed and
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43 the total amount of acidic groups on the carbon surface, either in the presence or absence of humidity.
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45 Furthermore, a closer look to Figure 3a shows that the beneficial effect of moisture slightly decreases
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47 when the amount of oxygen surface groups increases, thus suggesting a different adsorption mechanism
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50 in the presence or absence of surface functionalities (mainly acidic oxygen surface groups). While
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52 moisture highly improves the adsorption behaviour of the carbon materials with a poor surface
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54 chemistry (69% improvement on sample MA2ox700), moisture seems to exhibit a minor effect (13%
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57 improvement on sample MA2ox) in the presence of oxygen functionalities (mainly acidic groups),
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59 probably due to the preferential adsorption mechanism involving these oxygen surface groups.
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Incorporation of oxygen functionalities to the carbon surface can modify the adsorption behaviour
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3 through two different mechanisms: i) the creation of active sites, preferentially acidic oxygen groups, at
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5 the periphery of the basal planes, which will promote NH3 adsorption via a Brønsted acid-base process
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7 through ammonium ion formation (NH4+) or ii) the creation of Lewis acid centers on the graphene layers
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10 due to the withdraw of electron density by the more electronegative oxygen surface groups on the
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12 periphery, thus promoting the interaction of ammonia (Lewis base) via the lone pair electron. Taking
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into account that the incorporation of the oxygen functionalities to the carbon surface promotes both
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17 processes at the same time, the presence of a linear correlation as observed in Figure 3a does not provide
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19 enough information to exclude one mechanism against the other.
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Previous studies described in the literature using a large variety of activated carbons have shown that
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24 when the adsorption process is governed by a pure Brønsted acid-base process, the adsorption behaviour
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26 increases exponentially below a certain pH (~4.5), which corresponds to the pKa for dissociation of
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carboxylic acid groups on the carbon surface.8 In this sense, Figure 3b shows the relationship between
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31 the total amount adsorbed and the surface pH of the different samples used. At this point it is important
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33 to highlight that at the pH of all activated carbons used in this manuscript (pH<6.5), NH3 will be
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protonated to NH4+ in a proportion close to 100% (pka for ammonia is 9.3). As it can be observed, the
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38 absence of an exponential increase in the amount adsorbed below a certain pH value, either in the
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40 presence or absence of moisture, clearly suggests that the NH3 adsorption mechanism on activated
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43 carbons is not exclusively governed by the Brønsted acid sites on the surface, as proposed in the
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45 literature.8 Most probably, both adsorption mechanisms, i.e. Lewis and Brønsted mechanisms, take place
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47 on activated carbons even in the presence or absence of moisture.
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18 Figure 3. Dependence of the total breakthrough column capacity (a) as a function of the amount of
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acidic oxygen surface groups and (b) as a function of surface pH, in the presence or absence of moisture.
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25 To further clarify the role of humidity, a new series of experiments were performed using pre-
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humidified samples. In a first step, carbon materials were heat treated at 100ºC overnight in order to
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30 remove the residual humidity; in a second step, activated carbons were submitted to a pre-humidification
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32 treatment at different percentages (20%, 40% and 70% relative humidity) using a H2O/H2SO4 mixture
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35 and, in a final step, carbon samples were analyzed in ammonia breakthrough column experiments using
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37 dry air. Figure 4 reports the amount of NH3 adsorbed for samples MA2ox and MA2ox700, i.e. samples
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39 with a completely different surface chemistry, under different pre-humidification conditions and as a
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42 function of the weight gain after the pre-humidification step, i.e. the amount of humidity retained by the
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44 sample. As expected, the oxidized sample (MA2ox) is more hydrophilic and, consequently, it is able to
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46 absorb a larger amount of water under the pre-humidification conditions used. Figure 4 shows that for
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49 the sample with a poor surface chemistry (sample MA2ox700) there is a linear relationship between the
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51 amount of water adsorbed by the carbon sample and the subsequent adsorption capacity for a basic
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53 molecule such as ammonia. This linear relationship confirms that moisture plays a crucial role in the
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56 adsorption behaviour on this sample. Most probably in samples with a poor surface chemistry (absence
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58 of acidic groups), adsorption of ammonia takes place via dissolution into water, in such a way that the
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amount of ammonia dissolved increases with the amount of solvent. The total adsorption capacity ranges
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3 from 1.1 mg/g for the dried sample to 5.3 mg/g for the sample pre-humidified with 70% RH, this
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5 meaning a 4.8 fold increase. On the contrary, the oxidized sample exhibits a completely different
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7 behaviour. As described above, the adsorption capacity of the untreated sample, i.e. in the absence of
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10 humidity, is as large as 15.6 mg/g, thus confirming the crucial role of the oxygen surface groups in the
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12 adsorption of a basic molecule such as ammonia. At this point it must be highlight that this value is
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slightly smaller than the one reported in Table 3 (17.5 mg/g vs 15.6 mg/g) for the same sample, thus
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17 confirming the beneficial effect of the inherent humidity on the fresh sample. Pre-humidification with
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19 20% RH produces a slight increase in the adsorption capacity up to a value of 17.5 mg/g, remaining
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constant thereafter (up to 70% RH). Taking into account that the amount of water adsorbed highly
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24 increases with the pre-humidification step (from 0 to 70% RH) on sample MA2ox, the absence of
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26 significant changes in the adsorption capacity for this sample above 20 % RH clearly suggests that the
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adsorption mechanism in carbons with a rich surface chemistry follows a different pathway. Under these
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31 conditions moisture exhibits a scarce effect, the main contribution to the adsorption process coming
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33 from the oxygen groups, mainly acidic groups, present on the carbon surface as suggested in Figure 3a.
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As described above, the adsorption mechanism on oxidized samples can proceed following two
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38 approaches: i) by interaction of NH3 through the lone pair electron with the Lewis acidic sites on the
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40 graphene planes; or ii) by interaction of NH3 with acidic oxygen surface groups via NH4+ ions
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43 formation. Apparently, both adsorption mechanisms take place in the oxidized samples either in the
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45 presence and absence of moisture, although with a certain prevalence for the Lewis acid-base
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47 mechanism, according to Figure 3b.
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21 Figure 4. Dependence of ammonia adsorption capacity on the amount of water adsorbed under
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24 different humidity conditions (RH 0-70 %) for samples MA2ox and MA2ox700.
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28 In a final step, the nature of the active species after the adsorption process has been analyzed on the
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31 exhausted samples using FTIR. As it can be observed in Figure 5, the heat-treated sample MA2ox700
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33 exhibits a poor FTIR profile, i.e. the amount of surface species after the adsorption process is very small,
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35 in accordance with the above breakthrough column tests. FTIR spectra of the oxidized sample, MA2ox,
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38 after the adsorption process show intense bands, mainly when the experiment is performed in the
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40 presence of moisture. Mutually overlapping bands of OH and NH stretching vibration appear as a broad
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42 contribution at 3271 cm-1 as a result of NH3 adsorption. Furthermore, bands at 1637 cm-1 and 1082 cm-1,
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45 δas NH3 and δsym NH3, respectively, denote the presence of NH3 coordinated to Lewis acid sites on the
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47 carbon surface (graphene layers).11 Formation of surface ammonium salts of carboxylic acid appears as a
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50 band at 1439 cm-1 (δsym NH4+ ion), as well as bands at 1521 cm-1 (νas COO-) and 1393-1236 cm-1 (νsym
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52 COO-). Consequently, FTIR analysis of the exhausted samples confirms the presence of two different
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adsorption mechanisms for ammonia adsorption on oxidized activated carbons: adsorption on Brønsted
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57 acid sites (carboxylic acid sites), via ammonium ion (NH4+) formation, and ii) adsorption on Lewis acid
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sites on the graphene layers, via the lone pair electron of the ammonia molecule. Interestingly, both
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3 mechanisms dominate in oxidized carbon samples either in the presence or absence of moisture.
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24 Figure 5. FTIR spectra of activated carbons (a) MA2ox700, (b)MA2ox700-70% RH, (c) MA2ox, (d)
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27 MA2ox-70% RH, after NH3 breakthrough column experiments.
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31 ACKNOWLEDGMENT
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Authors acknowledge financial support from MEC (projects MAT2007-61734 FEDER) and Generalitat
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37 Valenciana (PROMETEO/2009/002). European Commission is also acknowledge (project FRESP CA,
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39 contract 218138). J.S.A. acknowledges support from MEC, GV and UA (RyC2137/06).
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42 REFERENCES
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46 (1) Phillips, J. Control and pollution prevention options for ammonia emissions, EPA-456/R-95-
47 002, 1995.
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(8) Le Leuch, L. M.; Bandosz, T. J. The role of water and surface acidity on the reactive adsorption
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(14) Bandosz, T. J.; Petit, C. On the reactive adsorption of ammonia on activated carbons modified by
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18 F.; Torregrosa, R. Use of nitrogen vs. carbon dioxide in the characterization of activated carbons.
19 Langmuir 1987, 3, 76-81.
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(16) Rodríguez-Reinoso, F.; Molina-Sabio, M.; Muñecas-Vidal, M. A. Effect of microporosity and
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SYNOPSIS TOC
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15 H2O
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18 O OH O OH
O OH O
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21 ACTIVATED CARBON
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