Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and N-containing carbons

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Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and N-containing carbons Juan Matos a b

a,* ,

Magdalena Hofman b, Robert Pietrzak

b

Department of Catalysis and Alternative Energies, Venezuelan Institute for Scientific Research (IVIC), 20632, Caracas 1020-A, Venezuela Faculty of Chemistry, Adam Mickiewicz University in Poznan´, Grunwaldzka 6, 60-780 Poznan´, Poland

A R T I C L E I N F O

A B S T R A C T

Article history:

N-containing carbon materials were obtained from waste plum stones submitted to pyro-

Received 26 August 2012

lysis under Ar flow at 700 C or to activation under steam at 800 C and enriched with nitro-

Accepted 2 December 2012

gen by heating in a NH3/air mixture at 270 C or in NO at 300 C. In situ mixtures of TiO2 and

Available online 8 December 2012

carbons were prepared by the slurry method and methylene blue photodegradation was chosen as a model reaction to verify the influence of N-containing carbons on the photocatalytic activity of TiO2 under artificial visible light irradiation. From the kinetics of methylene blue degradation an important synergy effect between both solids was detected with a remarkable increase up to a factor of 5.3 higher in the photocatalytic activity on TiO2–C than that on TiO2 alone. A mechanism for the photoassisting role of N-containing carbons upon the photoactivity of TiO2 under visible light is discussed.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

An important quantity of the total world production of dyes and azo-dyes is released in textile effluents with concomitant environmental hazards [1]. Different technologies for the removal of these organic molecules are adsorption, bio- and chemical degradation methods including advanced oxidation technologies as heterogeneous photocatalysis by using TiO2. Up to now TiO2 is the best photocatalyst, however, it is only photoactive under UV irradiation which clearly limits its upscale in water photodetoxification under real solar conditions [2]. On the other hand, in previous works we have showed that surface functionalization of activated carbon (AC) with oxygenated functional groups plays a photo-assisting role which enhances TiO2 photoactivity in the degradations of phenol and 4-chlorophenol [3–5], azo-dyes [6,7] and in the hydrogen photoproduction by water splitting [8] under artificial solar conditions. Recently, Ania and co-workers have shown that some activated carbons can be photoactive under

UV irradiation in the phenol photodegradation [9]. Also, Wang and co-workers have showed that graphitic carbon nitrides structures are photoactive under visible light for the hydrogen evolution [10,11] and for the selective oxidation of benzene to phenol [12]. The main objective of the present work is to show the photoassisting role of nitrogen-containing carbon materials. Thus, the photodegradation of methylene blue (MB) as a model dye under artificial visible light of N-containing carbons and of the binary materials TiO2–C was performed. Cooperative synergistic effects between both solids were verified and results were compared against those obtained with a commercial TiO2.

2.

Experimental

2.1.

Materials

Methylene blue (MB) was used as pollutant molecule. For comparative reasons TiO2 P25 (Degussa) was employed as

* Corresponding author: Fax: +58 212 5041166. E-mail addresses: [email protected], [email protected] (J. Matos). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.12.002

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photocatalyst. Waste plum stones (P) were the starting material which was ground and sieved to a uniform size range of 1.5–2.5 mm. The obtained materials were subjected to pyrolysis, modification by nitrogen factor or physical activation. The pyrolysis (K) was carried out in a horizontal furnace under a stream of argon with a flow rate of 170 mL min1. The sample was heated (5C min1) from room temperature to the final pyrolysis temperature 700 C. In the final temperature sample was kept for 60 min and then it was cooled under inert atmosphere. The activation process (A) was performed under steam at 800 C in laboratory furnace, to about 50% of burnoff. Water was fed by two micro-feeding pumps, the steam leaving the reactor was directed to the cooler in which it was liquefied and the gases formed in the reaction after passing the cooler and gas meter were combusted in a gas burner. The samples were heated (10 C min1) from room temperature to the final activation temperature (800C), maintained for 90 min and then cooled to room temperature. Modification by nitrogen factor was performed by ammoxidation (N) or nitrogenation (NO). Ammoxidation was carried out using a mixture of ammonia and air at a volume ratio of 1:3 (250 mL min1:750 mL min1) in a flow reactor at 300 C for 5 h [13]. In the process of nitrogenation, samples were exposed to NO (620 mL min1) in a flow reactor at 300 C for 2 h [14]. As reported elsewhere [13,14], the final product has been subjected to purification by washing and drying until residual ammonia molecules were totally removed from carbons. Binary hybrid materials TiO2–C were prepared at 25 C by the slurry method reported elsewhere [3,8] starting from a homogeneous suspension of both materials in a weight ratio TiO2:C equal to 10:1, then filtered and dry by 2 h at 110 C under static air.

2.2.

Characterization

Characterization of N-containing carbons was performed as follows. The elemental analysis of the starting plum stones and carbons were performed on a CHNS elemental analyzer (Perkin Elmer 2400 Series). The pH was measured using the following procedure: 0.4 g sample of dry carbon powder was added to 20 mL of distilled water and the suspension was stirred overnight to reach equilibrium. Then the pH of the suspension was measured on a pH meter manufactured by Metrohm Ion Analysis equipped in Unitrode Pt1000 (combined glass pH electrode with temperature sensor), calibrated with standards solutions of pH 4, 7 and 9. Surface oxide functional groups were determined by the Boehm method [15] and the nature of the N-containing functional groups on the carbon materials were characterized by X-ray photoelectronic spectroscopy (XPS) and included in Tables S1 and S2 in Supplementary material [16]. Characterization of the pore structure of N-containing carbons was performed by nitrogen adsorption–desorption isotherms [17] measured on an ASAP 2010 (Micromeritics). Scanning electron microscopy (SEM) images were obtained using a scanning electron microscope (SEM) 515 made by Philips (Netherlands) in the following conditions: working distance of 14 mm, accelerating voltage of 15 kV and digital image recording DISS.

2.3.

461

Photoreactor and light source

The batch photoreactor (Fig. S1, Supplementary material) was an open to air cylindrical flask made of Pyrex of 200 mL with a bottom optical window of 6 cm diameter [6]. Irradiation was provided with a metal halide lamp with a total photon flux of 1.44 · 1017 photons cm2 s1 [6] with about 90% visible region. Irradiation was filtered by a circulating water cell (thickness 2.0 cm) to remove IR beams thus preventing any heating of the suspension. The emission spectrum of the lamp is reported elsewhere [6] and showed in Fig. S2 (Supplementary material).

2.4.

Procedure

Previous experiments of adsorption in the dark were performed to achieve the weights of samples giving comparable quantities of MB adsorption that permits correctly compare the photocatalytic activity discussed below. These optima weights were 6.3 mg for carbon samples and 62.5 mg for TiO2. Samples were maintained in the dark for 60 min to complete adsorption in the dark at equilibrium before irradiation. Photocatalytic tests were performed at 25 C with 6.3 mg of the carbons or 62.5 mg TiO2 under stirring in 125 mL of MB, 25 ppm (78.2 lmol L1) initial concentration. The weight ratio of the binary material TiO2:C was constant in all experiments and equal to 10:1. After centrifugation MB aliquots were filtered and analyzed by UV-spectrophotometer at 664 nm (Perkin Elmer, k-35) and kinetic parameters were estimated assuming a first-order reaction mechanism [1,3,6].

3.

Results and discussion

3.1.

Characterization

Elemental composition of the materials investigated is presented in Table 1. PK and PNO samples showed similar carbon content e.g. 87.4 and 84.7 wt.% and low oxygen content about 8.5 and 7.1 wt.%, respectively. PN and PNA samples exhibit lower carbon content about 75 wt.%, accompanied by high oxygen contents about 16.8 and 22.6 wt.%. It is can be note that PN sample reveals the highest value of nitrogen content (Table 1) as a result of ammoxidation process (NH3/air mixture). Although PNO sample has been also exposed to a nitrogen factor (NO), the nitrogen content is lower (4.1 wt.%) that that of PN (5.3 wt.%). This is the consequence of a more aggressive chemical interaction between carbon and nitrogen atoms for the case of the ammoxidation process which additionally leads to the carbon’s surface oxidation. As expected, PNA sample exhibit very low nitrogen content (similar to the pyrolised plum stone, 0.4 wt.%) as a consequence of the activation under steam. This clear decrease in nitrogen composition in PNA sample suggest that nitrogen surface groups on the PN samples are very susceptible to react with the H2O molecules during the activation and/or their thermal stability is low. The speciation of functional groups formed on the surface of ammoxidised carbonaceous materials and the influence of activation upon these groups has been previously reported [16] and summarized as follows. According to the

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Table 1 – Elemental analysis of carbon materials. Sample

C

H

PK PN PNA PNO

87.4 72.3 75.4 84.7

2.9 5.0 1.1 3.8

N

S

Odif

0.8 0.6 0.3 0.3

8.5 16.8 22.6 7.1

(wt.%) 0.4 5.3 0.6 4.1

P: plum stones. K: pyrolysis at 700 C. A: steam activation at 800 C. N: ammoxidation by NH3/air (1:3) mixture at 300 C. NO: nitrogenation by NO2 at 300 C. The sequence of symbols is a sequence of processes.

XPS analysis of pyrolised plum stones (PK) the dominant carbon species were C–C and C–H, while hydroxyl groups bound a remarkable 23.5% total carbon groups among hydroxyl, ether, carboxyl and carbonyl groups (Table S1, Supplementary material). The nitrogen content in the PK sample was about 0.5% and dominant species are the most thermally stable pyrrole and pyridynium groups as well as quaternary structure (Table S2, Supplementary material). The ammoxidised sample (PN) is characterized by high content of carbon (Table S1, Supplementary material) on the surface in the form of C– OH, C–N and carboxyl groups. Moreover, the contribution of ether and hydroxyl groups is prevalent, and the presence of carboxyl groups and carbonate groups is also notable. The total content of the surface nitrogen value in PN sample was about 9.7% (Table S2, Supplementary material). According to elemental analysis discussed above (Table 1) the nitrogen content was about 5.3 wt.%. This significant difference in nitrogen composition between surface and the bulk of material clearly indicates that most of nitrogen species is placed on the surface of the material instead of whole volume. The dominant nitrogen species in PN were the pyrrolic and pyridinium, quaternary nitrogen groups, surface nitrogen oxides as well as imines, amine and imides species. XPS analysis also showed [16] that activation of PN sample (PNA) led to a significant decrease in the carbon surface content as compared with PN (Table S1, Supplementary material). At the same time, the activation process also results in an important decrease in the nitrogen content (Table S2, Supplementary materials) on PNA surface in comparison of PN sample. The imine, amide and imide species and N–Q and nitrogen oxides structures disappeared consequence of their relatively low thermal stability. The presence of these unstable nitrogen groups is in agreement with a high trend of nitrogen surface groups in the PN carbon materials to interact with water during the steam activation discussed above from the decrease in nitrogen composition and a clear increase of surface oxygen after activation (Table 1). The acid–base character of samples is presented in Table 2. In case of PK, PN and PNA samples the majority of surface oxide groups reveal basic character while in the case of PNO sample most of surface oxides exhibit acidic character. It is worth to notice that PNA reveal the lowest content of surface oxides, what is caused by high temperature of activation process. By contrast, PNO is characterized by the highest value of total content of surface oxides and simultaneously by the lowest pH value indicating a clear acidic nature. According to textural parameters of the adsorbents investigated (Table 3) it can be assumed that after pyrolysis PK

developed a moderate surface area of 373 m2 g1 and total pore volume about 0.244 cm3 g1. PK sample showed a framework mainly characterized by micropores as indicate the high rate between micro- to total volume (Vmicro/Vt) about 0.76. The nitrogen modified samples, PN and PNO, are characterized by very small values of both surface area and total pore volume (Table 3). This is the consequence of the low temperature used during the NH3/air mixture or NO processes commonly characterized by a clogging of pores [17]. By contrast, activation of PN material has a beneficial effect on structural parameters as can be seen in the PNA sample with a high surface area (895 m2 g1) and high total pore volume about 0.766 cm3 g1 with an important framework of micropores (Vmicro/Vt about 0.63) but with a more clear rate of mesoand mainly macropores in comparison of PK sample. Fig. 1 shows SEM images of PK (Fig. 1A and B), PN (Fig. 1C and D), and PNA (Fig. 1E and F) carbon materials. SEM images show that carbon particles in a micrometer scale are in agreement with sieving performed. As expected, SEM images suggest that PK and PNA carbons are constituted by a cellular structure remaining from the plum stone, a lignocellulosic material. Also, SEM image of PK carbon in Fig. 1B and PNA sample in Fig. 1F showed a porous framework indicating an effective activation during the pyrolysis. This is in agreement with the fact that the pyrolysis and the physical activation with steam brought in this work were realized in a similar (700 C) or higher temperature (800 C) that the critical temperature required for the spontaneous activation in conditions of pyrolysis of sawdust of a hard wood [18] or under steam flow in [18,19]. Similar trends have been reported for the case of activated fibers obtained from rayon fibers [20] and for activated carbons obtained from almond shells [21].

3.2.

Kinetics of MB adsorption in the dark

The kinetics of MB adsorption in the dark for the case of carbon bare materials and binary materials are shown in the Fig. 2A and B, respectively. Fig. 2B also contains the kinetic of MB adsorption on TiO2 alone for comparative purpose. It can be seen from Fig. 2A and B that after 60 min adsorption in the dark, the change in the MB concentration is practically negligible; indicating the achievement of the equilibrium condition for the adsorption and this period of time will be considered before irradiation. This equilibrium conditions is obtained both for the neat carbon materials (Fig. 2A) as for the binary materials (Fig. 2B). The equilibrium condition detected from kinetics of adsorption in the dark in Fig. 2 suggest that any further changes detected in the MB concentration

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Table 2 – Acid–base properties of carbon materials. Sample

Acidic groups

Basic groups

PK PN PNA PNO

0.81 0.75 0.73 1.52

1.32 1.54 0.95 1.02

Total content of surface oxides

pH

2.13 2.29 1.68 2.54

7.2 7.1 7.2 5.8

(mmol/g)

Table 3 – Textural properties of carbon samples. BET surface (SBET), micropore volume (Vmicro), mesopore volume (Vmeso), macropore volume (Vmacro), total pore volume (Vt), rate of micropores (Vmicro/Vt), mean pore diameter (D). Sample

SBET (m2/g)

Vmicro (cm3/g)

Vmeso (cm3/g)

Vmacro (cm3/g)

Vt (cm3/g)

Vmicro/Vt

D (nm)

PK PN PNA PNO

373 1 895 2

0.185 0.001 0.481 0.001

0.034 0.002 0.140 0.003

0.025 0.002 0.145 0.003

0.244 0.005 0.766 0.007

0.76 0.20 0.63 0.14

1.99 4.29 2.15 5.48

during irradiation can be attributed to MB photodegradation as we discussed below at Section 3.3. Table 4 shows the values of MB adsorption in the dark after equilibrium. It can be seen from values in Table 4 that carbon bare materials (Fig. 2A) adsorbed very low MB in comparison of any of the binary materials or TiO2 alone (Fig. 2B). The decreasing order was PNA > PK > PN = PNO for the carbon materials while the following TiO2–PK > TiO2–PN  TiO2 > TiO2–PNO  TiO2–PNA was found for the binary materials and TiO2 alone. After comparison of the values in Table 4, it can be suggested that most of MB would be adsorbed on the TiO2 surface. This is because the binary materials showed none additive effects in the adsorption of MB with the exception of TiO2–PK which developed a synergy effect in the adsorption after equilibrium with a adsorption value lightly higher (28%) than that obtained in both materials separately (22% and 4% for TiO2 and PK, respectively). The synergy effect between TiO2 and PK carbon is very important because adsorption deals with the Thermodynamics´s third Law or Gibbs´s free energy Law of a process by means of the expression (1): DG ¼ RT LnðKads Þ

ð1Þ

being Kads the adsorption constant from Langmuir´s adsorption isotherms [1,3], described by the expression (2): nads ¼ ½nT Kads Ceq =ð1 þ Kads Ceq Þ

ð2Þ

being nads the adsorption mols in the dark from a specific concentration of MB and the nT the total number of adsorption sites. nT and the adsorption constants Kads can be obtained from the linear transform (1/nads) = f(1/Ceq). Assuming Kads is the adsorption constant relative to that obtained on TiO2 alone, then it can be inferred that the higher the adsorption in the dark the higher the adsorption constant in comparison of that value obtained on TiO2 alone and therefore more negative the DG value indicating a higher thermodynamic trend to adsorb more MB on the binary materials in comparison of that on TiO2 alone. The relationship between the synergy

effect during adsorption and the synergy effect detected during MB photodegradation will be discussed below in the Section 3.3. On the other hand, in agreement with previous results [2,4,5] obtained on O-containing carbons, the loss of adsorption sites detected on TiO2–PNA and TiO2–PNO suggest an important contact interface between TiO2 and carbon samples. This kinetic behavior for the adsorption of MB is in agreement with the well-known hydrophilic behavior of TiO2–P25 [22] which has a high affinity to adsorb basic amines such as methylene blue with a high dissociation constant (pKb) in water and a high half neutralization potential [23]. In addition, in spite of Mb adsorbed in the dark on TiO2 was clearly higher than those on the bare carbons, all carbon materials adsorbed much more MB per mass unit (6.3 mg) than that adsorbed on TiO2 (62.5 mg). This result is in agreement with the high surface areas of PK and mainly PNA (Table 3). The above can be explained from the contribution of the pore size distribution as recently reported by Bandosz and co-workers for the case of the enhancement of dibenzothiophenes adsorption in Sdoped activated carbons [24,25] and previously reported by Pelekani and Snoeyink [26] in non-sulfur doped activated carbons. When a narrow distribution of primary volume of ˚ ) is present, as in the case of micropores (pore width < 10 A PK and PNA samples, the adsorption of MB is increased as in present work, where the ratio Vmicro/Vt is about 76% and 63% for PK and PNA, respectively. In other words, increasing the total pore volume and shifting the pore size distribution into the micropore region the adsorption of MB is thermodynamically induced. This phenomena results from overlapping the pore walls potentials for the case of pores with mean width in the micropore region [26] such as in the present case for PK and ˚ and 21.5 A ˚, PNA with means width of pores about 19.9 A respectively (Table 3). The above phenomena has been reported as a sink effect by Laine and co-workers for the case of thiophene [27] and ethylene [28,29] adsorption and by Matos and co-workers for the case of phenol [30], 4-chlorophenol [31– 33] and 2,4-dichlorophenoxiacetic acid [31].

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Fig. 1 – SEM images of carbon materials. (A and B) PK (C and D) PN (E and F) PNA.

3.3.

Photodegradation of MB

As reported by Vinodgopal et al. [34,35] and Hoffmann et al. [36] dye molecules absorb visible light and photoassist to semiconductor during photodegradations. This mechanism involves charge injection from excited dye into photocatalysts and subsequent degradation of oxidized dyes. However, interesting points of views have been established regarding the use of dyes and azo-dyes in photocatalytic degradation studies. One of the most remarkable works has been published by Ohtani et al. [37] whom established that due to the fact MB absorbs photons from visible light it would not be suitable as an appropriate substrate for the evaluation of the photocatalytic activity under visible light irradiation of semiconductors. On the contrary, several authors [1,38–41] have shown that methylene blue does is an appropriate molecule to verify the pho-

tocatalytic activity of different kinds of photoactive materials. For example, neat TiO2 reported by Herrmann et al. [1] whom in agreement with Ohtani et al. [37] suggest that the color fading of dyes and azo-dyes under visible light irradiation is clearly depending of the experimental conditions, mainly the source of the irradiation. For example, Herrmann et al. [1] suggest that under steady-state conditions of the influence of photon flux upon the first-order rate constant, the color fading of the dyes is not a problem to evaluate the photocataytic activity of semiconductors. Similar approaches have been reported for the photocatalytic degradation of MB by Inagaki et al. [38] on carbon coated TiO2, by Carvalho et al. [39] on visible-light irradiated TiO2–Cu thin films, by Nava et al. [40] using titania-decorated SBA-15, and by our group [41] on a recent study under visible light irradiated S-doped carbons.

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Fig. 2 – Kinetics of MB adsorption in the dark. (A): Carbon materials (B): TiO2 and TiO2–C.

In our case, preliminary experiments of direct photolysis of MB (in absence of carbons or photocatalysts) were previously performed [6,7] in the same experimental conditions of the present work (see the metal halide lamp described in Supplementary material). In these works we have showed that direct photolysis of the methylene blue was negligible under irradiation with the same artificial solar light employed in this work and the presence of a photoactive semiconductor

is required to induce the photooxidation of the dye [7]. We can infer that color fading problem of methylene blue reported by Ohtani et al. [37] during visible irradiation is negligible in the present case because color back to the photoreactor was not detected after turn-off the irradiation. This indicates MB disappearance showed in Fig. 3 is not a consequence of photoelectrons excited in the dye in agreement with Herrmann et al. [1], whom work in similar experimental condition of high photon flux as in the present work about 1.4 · 1017 photons cm2 s1 [6]. Fig. 3 shows the kinetics of MB photodegradation on carbon materials (Fig. 3A) and on the irradiated binary materials and TiO2 for comparison (Fig. 3B). Fig. 4A and B show the linear regression of the kinetic data from Fig. 3A and B, respectively. A summary of the kinetic results for the photodegradation of MB such as adsorption in the dark (Ads), apparent first-order rate constant (kapp), square regression factor (R2), photocatalytic activity relative to TiO2 (/photo) and the synergy effect (Sy) detected between both solids is showed in Table 4. Fig. 3A suggest carbon materials are photoactivity under visible light with apparent first-order rate constants (Table 4) about 1.7 · 103 min1, 2.0 · 103 min1, 2.3·103 min1, and 1.3·103 min1, for PK, PN, PNA and PNO, respectively. The square regression factor were higher than 0.93 with the exception of PK sample, indicating that MB photodegradation is in good agreement with a first-order mechanism [1,3]. It can be seen that apparent first-order rate constants (kapp) obtained on carbon materials are lower than that of TiO2 alone. However, it should be pointed out that only 6.3 mg of carbon samples was employed while 62.5 mg were used for TiO2. PNA and PN samples were the most photoactive bare carbons which can be associated with the speciation of nitrogen surface groups discussed above. Previous results reported for home-made activated carbons [6,7] showed that non-functionalized activated carbons were not photoactive for the degradation of MB under the same experimental conditions of artificial solar light. Ania and co-workers [9] have showed that oxygen functionalized carbon materials showed chemical photoactivity under UV irradiation but to our knowledge, this is the first report of N-doped carbon materials showing photoactivity under visible irradiation. This photoactivity under visible light can be attributed to an electron

Table 4 – Adsorption in the dark of MB (Adsdark), apparent first-order rate constants (kapp), square regression factor (R2), photocatalytic activity relative to TiO2 (/photo), and synergy effect between both solids (Sy). Photocatalyst

Adsa (%)

kapp · 103 (min1)

R2

/photob

Syc

PK PN PNA PNO TiO2 TiO2–PK TiO2–PN TiO2–PNA TiO2–PNO

4 2 5 2 22 28 23 15 16

1.7 2.0 2.3 1.3 4.5 22.5 17.1 23.9 16.2

0.9078 0.9376 0.9840 0.9637 0.9858 0.9871 0.9931 0.9887 0.9914

0.38 0.44 0.51 0.29 1 5.0 3.8 5.3 3.6

– – – – 1 3.6 2.6 3.5 2.8

a After 60 min adsorption. b /photo = (kapp-i/kapp-TiO2). c Sy = (kapp-i/kapp-TiO2 + kapp-AC), with kapp-i and kapp-AC the apparent first-order rate constants on the binary materials and on the bare carbon materials, respectively.

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Fig. 3 – Kinetics of MB photodegradation. (A): Carbon materials (B): TiO2 and TiO2–C.

density transfer to the carbon matrix from excited electrons in p* states formed after irradiation on the photosensible nitrogen functional groups in a similar manner as reported by Lettmann and co-workers [42] and by our group [6,43]. It should be pointed out that the PN and PNO samples are characterized by different content of nitrogen groups (Table S2, Supplementary material) as well as by their different chemical character (Table 2). For PN sample modifying factor was the NH3/air mixture and since it is also oxidizing in its properties the amount of nitrogen introduced is much larger comparing PNO sample. Moreover the chemical character of nitrogen functional groups generated as a result of the modification is more basic for the case of PNA and PH bare carbons (Table 2). In the case of PNO sample, the nitrogen was introduced by NO, and there was no additional oxidizing factor as in the case of PN sample. Thus the amount of nitrogen introduced is lower in PNO than that in PN and the chemical character of surface groups generated as a result of this modification is more acidic with a surface pH about 5.8 against 7.1 (Table 2) for PNO and PN, respectively. Based on our experience and the results of our previous reports [14,16] we are able to indicated the kind of the nitrogen surface functional groups for PNO. XPS analysis would be helpful for this interpretation; however, the thermodynamic non-stable condition of PNO sample under high-vacuum condition, hardly difficult the analysis for that reason, this analysis was not included in

Fig. 4 – Linear regressions of kinetic data. (A): From Fig. 2A. (B): From Fig. 2B.

Table S2 (Supplementary material). In short, we can attribute to the combination of the acid surface pH (Table 2) and to the very low surface area (Table 3) of PNO the lowest photocatalytic activity (Table 4) detected in the bare carbon materials (Fig. 3A). On the other hand, it can be seen from Fig. 3B that the binary materials are more photoactive than TiO2 alone and much more photoactive that carbon bare materials because they require only about 3 h of irradiation to total photodegradation of MB (Fig. 3A) in comparison of about 6 h for TiO2 and about 10 h on carbon materials (Fig. 3A). It can be seen from Table 4 the following kapp values 23.9 · 103 min1, 22.5 · 103 min1, 17.1 · 103 min1, 16.2 · 103 min1 and 4.5 · 103 min1 for TiO2–PNA, TiO2–PK, TiO2–PN, TiO2–PNO, and TiO2, respectively. In other words, the binary materials TiO2–PNA, TiO2–PK, TiO2– PN, TiO2–PNO were 5.3, 5.0, 3.8, and 3.6, respectively, more photoactive (/photo, Table 4) than TiO2 alone. This remarkable increase in the photoactivity of the binary materials relative to that of TiO2 is the consequence of a synergy effect (Sy, Table 4) between both solids which was estimated by factors about 3.5, 3.6, 2.6, and 2.8 for TiO2–PNA, TiO2–PK, TiO2–PN, TiO2– PNO, respectively. It should be pointed out that TiO2–PK showed the highest synergy effect for the MB photodegradation about 3.6 (Table 4) suggesting an interesting relationship with the synergy effect detected for the MB adsorption in the dark on the binary material TiO2–PK discussed above.

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Photoassisting mechanism of N-containing carbon

Results discussed above seem to indicate that MB readily undergoes degradation on the photocatalysts surface under visible light by a photo-assisting mechanism as discussed as follows. Photocatalytic results summarized in Table 4 demonstrated that N-doped carbon materials are photoactive themselves under visible light irradiation. It can be seen that apparent first-order rate constants (kapp) obtained on carbon materials are lower than that of TiO2 alone. However, it should be remind that only 6.3 mg of carbon samples was employed while 62.5 mg were used for TiO2. By contrast, results in Table 4 showed that the binary materials (TiO2–C) are clearly more photoactive up to a factor about 5.3 for the TiO2–PNA material. This increase in the photocatalytic activity is the consequence of a synergy effect between both solids estimated in a factor about 3.5 for the interaction between TiO2 and PNA carbon. A very similar behavior was obtained for TiO2–PK binary material with an increased photoactivity of five times higher than TiO2 alone and with a synergy effect about 3.6. As discussed above, in terms of the speciation of the nitrogen functional groups on the surface of carbon materials, we suggest that nitrogen basic functional organic groups such as pyridine, pyrrolic and pyridinium groups are responsible of the interaction with metallic center in the TiO2 and this interaction enhance the photoactivity of the semiconductor. Fig. 5 shows a schematic mechanism of interaction between N-containing carbons and TiO2. This mechanism suggest that both TiO2 and N-doped carbons can be photoexcited by UV-irradiation and visible light, respectively. Once, nitrogen functional groups in carbon materials are photoexcited to p* levels, electrons can be transferred from this orbital to the conduction band of TiO2. As consequence, it can be suggested that N-doped carbon materials photoassist the TiO2 in the methylene blue photodegradation. However, in spite of nitrogen composition is an important factor, a clear

467

relationship between nitrogen composition in carbon materials and photoactivity was not found, indicating that additional factors must be consider as micropore framework and oxygen composition. In terms of the micropore volume (Table 3), the best binary photocatalyst TiO2–PNA and TiO2– PK showed high for the Vmicro/Vt rate. However, the other two binary materials showed very low Vmicro/Vt rate and they were more photoactive than TiO2 alone. By contrast, Fig. 6 shows two interesting relationships between kapp and oxygen content (Fig. 6A) and as a function of the total acidic groups (Fig. 6B). Fig. 6A shows that the higher the oxygen contents in carbon materials the higher the apparent constants. Moreover, Fig. 6B seems to indicate that those oxygen groups must be basic in nature, because the lower the acidic groups the higher the photoactivity. These relations are in good agreement with results found by Ania et al. [9]. Thus, we suggest that both N-containing and O-containing functional groups in carbon materials are responsible of the enhanced photoactivity of TiO2 under visible light irradiation. This suggestion is being subject of further studies in order to verify the composition CxNyOz with a maxima synergy with TiO2.

3.5.

General discussion

As a final remark about the controversy concerning oxidation of carbon surface by ammoxidation at low temperature it should be point out the following discussion. It has been reported that decomposition temperature of ammonia is around 400 C and the progress of ammoxidation reaction at low temperature as 300 C is difficult [44]. However, many recent reports [45–49] have demonstrated that different kinds of carbon materials such as active carbons [45], single-walled carbon nanotubes [46], mesoporous carbons [47], graphite oxide [48], and carbon black [49], suffered efficient ammoxidation at low temperatures. In addition, the ammoxidation of different carbon materials has been very well studied in

Fig. 5 – Schematic mechanism of interaction between N-containing carbons and TiO2.

468

CARBON

5 4 ( 2 0 1 3 ) 4 6 0 –4 7 1

Fig. 6 – Relationship between the first-order apparent constant rates (kapp) and the oxygen content (A) or the total acidic groups (B) in the carbon samples.

our group [13,14,17] and to our experience we have found that carbon material (plum stones in this case) are degassed in the temperature range employed in this work. Since the NH3/air mixture interacts with the surface of the materials which is degassed in the process condition and, additionally, modifying factor has oxidizing properties, it is easier to introduce nitrogen in high concentration as in the present study (5 wt.%). Moreover, including air in modifying factor allows to decreases the effectiveness temperature of the process and for that reason at low temperatures such as 300 C our process of ammoxidation is efficient. We understand that in some previous papers there are some reports about difficulties in ammoxidation at low temperatures. However some of them concern to amminooxidation process which is different than ammoxidation process. Amminooxidation is the process carried out with the use of amines, which may be used in the form of liquids. In such cases, we agree that the amount of nitrogen introduced into the modified material structure may not be high. Another important factor is the material subjected to modification process (in terms of the degree of coalification). In case of ammoxidation, studied in the present work, we used mixture of NH3 (gas) and air in optimized volume ratio. The air serves as an oxidizing agent. This allows for the pre-oxidation of the modified material structure. Pre-oxidation of the organic material modified activates the surface and the formation of oxygen to the functional groups, which greatly facilitates the formation of stable surface structures containing nitrogen as those shown in Figs. 7 and 8. Fig. 7 shows the ammoxidation mechanism of an aromatic model structure [50,52] while Fig. 8 shows the evolution of nitrogen functionalities on surface of carbonaceous materials as results of their modification under NH3/air mixture [16]. Ammonia can react with

O C O C O

NH 3

O C C

O C

NH 2 OH

NH

- H2 O

C O

O

NH 3

NH 3

O

O C C

C NH 2 ONH 4 O

C

NH2 NH2 O

Fig. 7 – Ammoxidation mechanism of an aromatic model structure.

CARBON

CH3

O

CH3

OH O C

N-6

C O

C

469

5 4 (2 0 1 3) 4 6 0–47 1

C

NH3 /air

NH2 ONH4

O

O

H

N-5

N

N

N N

T

N-X N O

N N

NH2

I

N-Q

II

III

Fig. 8 – Evolution of nitrogen functionalities on surface of carbonaceous materials as results of their modification. (I) Raw material (plum stones), (II) ammoxidized structure under NH3/air flow, (III) N-containing carbons after carbonization at temperature T.

oxygen functional groups present on the material surface. Then, mainly the form of ammonium salts of surface oxygen groups are generated, which are hydrolyzed with small amounts of basic amine and amide whose links in the interaction with the carbonyl groups [50–52]. In case of our studies, the starting material (plum stones), has plenty of surface oxygen groups itself and using the NH3/air mixture can guarantee that the introduction of nitrogen into its structure takes place. Moreover, in our group we examined ammoxidation of different materials characterized by different degree of coalification (lignite, brown coal, subbituminous coal). The process has been also examined in wide temperature range. The results showed that ammoxidation of these materials can generate nitrogen surface functional groups even in lower range of temperatures studied. Moreover, it has been shown that in case of the materials characterized by low degree of coalification, it is enough to carry out the ammoxidation in low temperatures, close to first step of their degasification [13,16,53,54]. Concerning to the difference in nitrogen composition between surface and the bulk of material it should be point out that the date included in Table 1 present the elemental composition performed on a CHNS elemental analyzer. Thus they show the amount of the elements in whole sample. The nitrogen content for the PN sample estimated by this method is 5.3 wt.%. On the other hand, due to the fact that for the same sample the total amount of nitrogen estimated by XPS method (Table S2, Supplementary material) which, as it is well known, can measure only few layers of the sample, is higher, we can conclude that most of nitrogen is situated in those few layers of the sample instead of its whole volume. In consequence, it seems to be logical that these nitrogen surface groups will be able to interact with methylene blue molecules as discussed above. In summary, it can be suggested that most of acid groups in PNA sample are located on the surface in agreement with a much higher BET surface area of this activated carbon in comparison of PN carbon with only external surface area. In other words, in spite of acid surface groups in PNA and PN are practically similar as indicate values in Table 2, the oxygen-containing groups in the surface of PNA carbon are more efficient to interact with Ti atoms in agreement with a higher synergy effect in TiO2–PNA binary material than that detected on TiO2–PN (Table 4).

4.

Conclusions

N-containing carbon materials were prepared by exposing original materials to NH3/air mixture or NO generating chemically different nitrogen surface groups. Kinetic evidences suggest that N-containing carbons are lightly photoactive under visible light. The binary material TiO2–C showed a clear synergy effect between both solids with a remarkable increase up to a factor of 5.3 higher in the photocatalytic activity than that of TiO2. This synergy effect suggests that nitrogen functional groups in carbon materials could play a photoassisting role to enhance the photoactivity of TiO2 under visible light irradiation.

Acknowledgement J. Matos thanks to the Venezuelan Ministry of Science and Technology for the funds to support part of this work.

Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2012.12.002.

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