Probing the effect of nanotubes on N-nitrosodimethylamine photocatalytic degradation efficiency and reaction pathway

Author’s Accepted Manuscript Probing the effect of nanotubes on Nnitrosodimethylamine photocatalytic degradation efficiency and reaction pathway Xiaoyan Guo, Huaiqi Shao, Lulu Kong, Mingce Long, Man Zhang, Qixing Zhou, Wanli Hu www.elsevier.com/locate/ces

PII: DOI: Reference:

S0009-2509(16)00028-2 http://dx.doi.org/10.1016/j.ces.2016.01.018 CES12751

To appear in: Chemical Engineering Science Received date: 13 June 2015 Revised date: 6 January 2016 Accepted date: 7 January 2016 Cite this article as: Xiaoyan Guo, Huaiqi Shao, Lulu Kong, Mingce Long, Man Zhang, Qixing Zhou and Wanli Hu, Probing the effect of nanotubes on Nnitrosodimethylamine photocatalytic degradation efficiency and reaction p a t h w a y , Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2016.01.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Probing the effect of nanotubes on N-nitrosodimethylamine photocatalytic degradation efficiency and reaction pathway Xiaoyan Guoa*, Huaiqi Shaob, Lulu Konga, Mingce Longc,Man Zhanga, Qixing Zhoua*, Wanli Hua a: College of Environmental Science and Engineering/Tianjin Key Laboratory of

Environmental Remediation and Pollution Control/Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Nankai University, Tianjin 300071, China; b: College of Materials Science and Chemical Engineering, Tianjin University of Science & Technology, Tianjin 300457, China; c: School of Environmental Science and Engineering, Shanghai Jiao Tong University,

Shanghai 200240, China. *To whom correspondence should be addressed.

Highlights ·TiO2 nanotube and Au-modified TiO2 nanotube were prepared to evaluate tube effect. ·Tubular morphology of TiO2 nanotube is kept after Au modification. ·Tubular morphology of TiO2 nanotube plays a key role in enhancing NDMA degradation. ·Tubular TiO2 initiates new NDMA photocatalytic path, generating DMA as main product.

1

Abstract Degradation

efficiency,

N-nitrosodimethylamine

product (NDMA)

distribution photocatalytic

and

reaction

degradation

pathway over

of TiO2

nanotube(TiNT), Au-modified TiO2 nanotube(Au/TiNT) and TiO2 nanopowder(TiO2) were investigated to evaluate the tube effect. The results show that TiO2 nanotube was prepared and tubular morphology was kept after Au modification. The degradation efficiencies of NDMA over TiNT and Au/TiNT are significantly higher than that over TiO2 powder, DMA becomes to be the major product, and NO2- production is also detectable, which all stem from the tubular morphology of TiNT and Au/TiNT. In the presence of TiNT and Au/TiNT, the small NDMA molecules enter into the nanotube and lead to a higher concentration and a closer contact in a nanotube structure space, resulting in remarkable high degradation efficiencies of NDMA, and also initiating new photocatalytic reaction pathway derived from effectively contacts between radical intermediate and NO2- products in the nanotube structure. The nanotubular morphology of TiNT and Au/TiNT plays an important role in enhancing NDMA photocatalytic performances. Keywords:TiO2 nanotube; tubular morphology; N-nitrosodimethylamine; photocatalytic degradation

1. Introduction Photocatalytic degradation of environmental pollutants over TiO2 has always 2

been a hot issue during the past decades. Various strategies such as morphological modifications have been adopted for improving the phocatalytic efficiency of TiO2 (Pelaez et al., 2012). Among these, TiO2 with one-dimensional (1D) morphology, such as nanotube (Bavykin et al., 2006; Liu et al., 2011; Macak et al.,2007) and nanowire (Feng et al., 2012; Wu et al., 2009), was received growing attention due to their unique structures and improved performances. Among those materials, the TiO2 nanotube(TiNT) synthesized by a one-pot alkaline hydrothermal process provides a wide variety of possible applications due to their unique physiochemical properties and the capability of cheaply mass production (Bavykin et al., 2006). Enhanced photocatalytic activity of TiNT was reported due to the larger specific surface area (Wu et al., 2010; Riss et al., 2009), unfortunately, there are still very few studies focused on the peculiar photodegradation pathways over the nanotubes. According to previous studies (Riss et al., 2009; Yoshida et al., 2005; Pang et al.,2010), nanotube structure of TiNT is not stable and could be destroyed after further modifications or calcination. Surface modification with noble metals on TiO2 is a traditional method to improve the photocatalytic activity (Li and Gray, 2007). Au(or Pt)-modified TiNT obtained by photodeposition method was investigated by Zhao et al. (2009), and a moderately enhanced activity was observed for methyl orange degradation. It should also be noted that the nanotube structure is mostly destroyed after the noble metal deposition. Therefore, Au-modified TiNT prepared by a deposit-precipitate urea (DP-urea) method will further be investigated and the nanotube structure and morphology of the as-prepared Au-modified TiNT, as well as its photocatalytic activity, will also be assessed in this paper. N-nitrosodimethylamine (NDMA), as a kind of emerging nitrogenous disinfect byproduct, is aroused a concern on drinking water safety. Photocatalytic degradation of NDMA presents a good potential among many methods, which is tried to remove or eliminate NDMA from water, including air stripping, adsorption, reverse osmosis (Mitch et al., 2003; Kong et al., 2010a; Fleming et al., 1996; Kommineni et al., 2003), nanofiltration (Fujioka et al., 2013a, 2013b), biodegradation (Tate and Alexander, 1976; Mallik and Tesfai, 1981), UV photolysis (Mitch et al., 2003), advanced oxidation (Yang et al., 2009), photocatalytic degradation (Lee et al., 2005a) and electrochemical treatment (Chaplin et al., 2009, 2010). Efficiently photocatalytic degradation of NDMA over TiO2 nanopowders was reported by Lee et al. (2005a). Moreover, enhanced efficiencies and altered product distributions have been observed 3

over various surface-modified and morphology-modified TiO2 (namely platinum deposition, silica loading, Nafion coating, and surface fluorination), suggesting that the modified TiO2 should be expected to be an effective method to increase degradation efficiency of NDMA (Henderson, 2011). In this paper, we prepared and characterized TiO2 nanotube (TiNT) and gold nanoparticles modified TiO2 nanotube (Au/TiNT), and analyzed their photocatalytic performances and products distribution in the degradation of NDMA. According to the comparison with TiO2 nanopowder, various degradation pathways were proposed and the effect of nanotube was discussed. 2. Materials and methods 2.1. Chemicals and materials All chemicals were used as received. The nano-sized anatase TiO2 powder was obtained from Hehai Nanometer Science and Technology Co., China. HAuCl4 aqueous solution (wt.49% Au) and urea (99.0% purity) were purchased from J&K Scientific Co., China. NDMA (99.5% purity) was purchased from Wako Pure Chemical, Japan. Methanol (HPLC grade) and methanesulfonic acid (MSA, analytical grade) were obtained from Tianjin Concord Co., China. Sodium carbonate, sodium hydrogen carbonate, sodium hydroxide, hydrochloride acid and perchloric acid were analytical grade reagents procured from Tianjin Kermel Chemical Reagent Co., China. All solutions were prepared in ultrapure water (18 MΩ·cm). 2.2. Preparation of TiNT and Au/TiNT The TiO2 nanotube (TiNT) was synthesized by a modified hydrothermal method from the previous report (Kasuga, 2006). 1 g of anatase TiO2 was dispersed in an aqueous solution of 50 mL 10 M NaOH, then the suspension was transferred into a Teflon-lined stainless steel autoclave and statically kept at 150℃ for 24 h. After hydrothermal treatment, the precipitate was separated by filter, and washing with 0.1 M HCl solution. The resulting suspension with a pH value of 2 was stirred for 24 h, and repeated by filtration and washing three times. The precipitate was filtered, washed with distilled water, and dried at 80 ℃. After grinded, the resultant powder was calcined at 380 ℃ for 2 h to obtain TiO2 nanotube, designated as TiNT. The immobilization of Au onto TiNT was carried out by a deposit-precipitate urea (DP-urea) method, as described by Zanella et al. (2002). The TiNT powder (0.5 g) 4

were dispersed in 25 mL of distilled water dissolved with 0.78 g urea, and then a certain volume of HAuCl4 solution was added to make the content of Au is 0.5 wt%. After dispersed by supersonic for 15 min, the suspension was heated to 90 ℃ in the thermostatic water bath for 4 h. The urea was slowly hydrolyzed to release hydroxyl, which acted with HAuCl4 to produce Au(OH)3 precipitate. The resultant suspension solution was filtered and dried. After grinded, the precipitate was calcined at 300 ℃ for 2 h to obtain the Au nanoparticle modified TiO2 nanotube, designated as Au/TiNT. 2.3. Characterization of TiNT and Au/TiNT The supported content of Au on the TiNT was 0.38% determined through an Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Elan drc-e, PerkinElmer) by analyzing the Au content in the residue solution after deposition and followed filtration. Gold exists as a mixture of Au0 and Au3+ (Au2O3), according to XPS results in Figure S1 of the supplementary (Park and Lee, 1999). Nitrogen adsorption and desorption isotherms were measured using an autosorb apparatus (iQ, Quantachrome). Specific surface areas (SBET) were calculated using the Brunauer-Emmett-Teller (BET) method (P/P0 = 0.05-0.35). The pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH method) using desorption branch. The pore diameter (DP) was determined by peak value of pore size distribution curve, and the total pore volume (VP) was determined from the adsorption volume at a value of P/P0 of 0.995. The morphology characteristics and crystal phase structure of the as-prepared catalysts were characterized by transmission electron microscopy (TEM, JEM-2100F, JEOL) and X-ray diffraction (XRD, D/MAX 2200PC, Rigaku), respectively. Zeta potential measurements were obtained with a ZS90 Zeta Potential Analyzer (Malvern). 2.4. Photocatalytic process and products Analysis Photocatalytic degradation of NDMA (1mmol/L) was performed in a 400 ml photocatalytic reactor consisting of two screwed cylindrical parts sealed by a gasket and placed horizontally in a constant temperature bath. According to UV-vis diffuse reflectance spectra of TiNT and Au/TiNT, 332nm was selected as experimental frequency for degradation of NDMA (see Figure S2 of the supplementary). Irradiated directly by a 500 W xenon arc lamp, all suspensions were prepared at a catalyst concentration of 0.5 g/L and dispersed continuously by magnetic stirrer, the initial pH 5

of the suspension was adjusted to a desired value with 1 mol/L HClO4 or 1 mol/L NaOH, and then kept in dark for 30 minutes to evaluate the impact of adsorption before irradiation (see Figure S3 of the supplementary). Sample aliquots of 10 mL were withdrawn from the illuminated reactor with a syringe at regular time intervals, filtered through a 0.45 μm filter to remove catalysts, and injected into a 10 mL glass vial. A set of triplicate experiments was carried out, and the average values and the standard deviations are presented. A blank test of NDMA photolysis without catalysts under the same illumination showed that after 240min irradiation, about 32% of NDMA was mainly converted to dimethylamine (DMA) and NO2- at neutral pH. This photolysis can be attributed to the weak absorption of NDMA centered at 332 nm (n→π* transition band) (Lee et al., 2005b; Plumlee and Reinhard, 2007; Lee, et al., 2005c). Quantitative analysis of NDMA was performed by high-performance liquid chromatography (HPLC, Waters 1525, UV detector) equipped with an XTerra RP18 column (Waters, 250 mm × 4.6 mm, 5 μm packing material). The mobile phase was an isocratic 80/20 water/methanol mixture with flow rate of 1 mL/min and monitored at 228 nm. The main products of the NDMA photocatalysis (methylamine (MA), dimethylamine (DMA), NO3-, and NO2-) were analyzed by ion chromatograph (IC, Dionex, DX 120) with a conductivity detector (Kong et al., 2010b). Methanesulfonic acid (MSA, 3 mM) or Na2CO3/NaHCO3 (1.8 mM/1.7 mM) at a flow rate of 1.2 mL/min were served as eluent for cation or anion measurement, respectively. 3. Results and discussion 3.1. Characterization of TiO2 nanotubes Figure 1 and 2 show the TEM images and XRD patterns of the TiNT and Au/TiNT, respectively. The obvious tubular morphology and well-defined structure of TiNT samples transformed from TiO2 particle can be observed. The nanotube sample has an average diameter of 10 nm, with a tube wall of about 2 nm and a tube length of several tens of nm, which indicates that TiNT has been successfully obtained by hydrothermal synthesis. On the other hand, for the Au/TiNT samples, some small dark spots of Au nanoparticles with a size of 6-10 nm appear on the external and internal surface of TiO2 nanotube. Moreover, after the DP-urea process of Au deposition and the subsequent calcination, TiO2 nanotubes become shorter (also see SEM images in

6

Figure S4). Similar to TiO2 powder, the crystal phase of TiNT is anatase according to the characteristic peak at 2θ=25.28° (see Figure 2). The crystallinity of Au/TiNT prepared by DP-urea method was improved, and slight rutile turned up in accordance with the characteristic peak at 2θ=27.4°. Besides, there is no obvious peak of Au, since Au nanoparticles with a small content are homogeneously dispersed on the surface of TiO2 nanoparticles.

Fig. 1. TEM images of TiNT (a, b), Au/TiNT(c) and TiO2(d). A

A: Anatase R: Rutile

Intensity (a. u.)

TiO2

A

A

A

Au/TiNT R R

R

TiNT

20

30

40

50

60

2() Fig. 2. XRD patterns of TiNT, Au/TiNT and TiO2. Table 1 presents the pore properties of the TiO2 powder and the TiO2 nanotubes. 7

When the TiO2 powder was prepared to nanotube structure (TiNT), the surface areas are decreased, which may be caused by hydrothermal treatment and sinter of powders. After supporting Au, the surface area of TiO2 nanotube is further decreased, accompanied by decrease of pore volume and pore size. According to comparison of shape of the adsorption isotherms assigned to Type IV isotherms with Type H3 hysteresis loops (Abida et al, 2011) and the pore size distributions between TiNT and Au/TiNT, presented in Figure S5 and S6 of the supplementary, it can determine that the pore structure of TiO2 nanotube does not be changed by supported Au, but pore size decreases and pore size distribution becomes narrower. Additionally, the patterns of TiO2 nanotubes do not be changed after supporting Au, according to SEM images presented in Figure S4. Above results illustrate that Au is mainly supported on the channel wall to decrease the surface area and the pore size, but the tube structure does not be changed. Table 1 Pore properties of TiO2 powder and TiO2 nanotubes. SBET (m2/g)

VP (cm3/g)

DP (nm)

TiNT

99.073

0.602

4.3

Au/TiNT

51.170

0.407

3.4

TiO2

145.310

0.539

5.6

3.2. Influences of nanotubes on NDMA photocatalytic degradation efficiency, product distribution and reaction pathway Figure 3 presents the degradation efficiencies and product distributions of NDMA over various photocatalysts (TiO2, TiNT and Au/TiNT) at solution pH of 7. There exist obvious differences of NDMA degradation efficiencies and product distributions between TiO2 nanopowder (TiO2) and nanotubes (TiNT and Au/TiNT), although they are of similar crystal structure (see Figure 2). The degradation efficiencies of NDMA over TiNT and Au/TiNT are significantly higher than that over TiO2, which can be rationalized by that the small NDMA molecules might enter into the nanotube and lead to a higher concentration and a closer contact in a nanotube structure space. This can be further supported by the higher adsorption capability of TiNT and Au/TiNT than that of TiO2 (see Figure S3), although the latter has higher surface area. In a previous report on NDMA photocatalytic degradation (Lee et al., 2005a), 8

Lee proposed that hydroxyl radicals generated on the UV-illuminated TiO2 surface initiated the photocatalytic reaction of NDMA by attacking one of three positions: the methyl group, the amine nitrogen and the nitrosyl nitrogen to produce MA, DMA, NO2- and NO3-, etc. In our present research, according to the results of hydroxyl radical scavenge tests, hydroxyl radical is also confirmed as major oxidizer during the process of NDMA photocatalytic degradation (see Figure 4). In the mean time, the products of NDMA photocatalytic degradation are found to be mainly composed of MA, DMA, NO2- and NO3- (see Figure 3). Therefore, we can also infer that hydroxyl radical presumably initiates the following three reaction pathways in the process of NDMA photocatalytic degradation (Lee et al., 2005a), as presented in Scheme1. Path A generates a carbon-centered radical (I) upon H-atom abstraction, which further reacts with O2, generating peroxy and alkoxyl radical intermediates to result in a demethylated product, MA. Path B, the reaction of the hydroxyl radical with the lone pair electron on the amine nitrogen of NDMA generates cationic radical (II) that subsequently decomposes into a NO radical and an iminium ion, CH3NH+=CH2, which hydrolyzes to produce MA and formaldehyde. The generated NO radical reacts with O2.- transformed from the reduced oxygen and detected by quantifying XTT sodium salt by a UV-Vis spectrophotometer to form peroxynitrite (ONOO-), which is finally transformed to NO3- by spontaneous isomerization or its reaction with NO2- in aqueous solution (Chow et al., 1972). Path C produces the NDMA-OH adduct (radical (III) caused by attacking the nitrosyl nitrogen of NDMA by hydroxyl radical, and the NDMA-OH adduct decomposes into nitrite and the dimethylaminyl radical, which subsequently is transformed to DMA through a reaction with e-/H+ or an H-atom abstraction from NDMA.

9

Fig.3. Time profiles of NDMA photocatalytic degradation on TiO2, TiNT and Au/TiNT at pH of 7.

10

Fig.4. Degradation of NDMA in presence of TiO2, TiNT and Au/TiNT with hydroxyl radical quenching reagent(tert-butyl alcohol 10mM) or without addition

By comparison with the products distribution on three photocatalysts (TiO2, TiNT and Au/TiNT) shown in Figure 3, we notice that the total N balance is slightly lower than 100% and NO2- is not detectable on TiO2 nanopowder, which implies that there exists missing product. It is presumably that N2O3 is easily formed rather than NO2- (Chow et al., 1972), when hydroxyl radical attacks the nitroso group of NDMA to initiate Path C. In contrast to TiO2 nanopowder, over two TiO2 nanotubes of TiNT and Au/TiNT, the total N balance was in the range of 100±20%, which was considered as satisfactory(Lee et al., 2005a). DMA becomes to be the major product with the yields increased dramatically to 70% of the initial NDMA concentration after 4 h irradiation, and NO2- production is also detectable, as shown in Figure 3. It is probably thought that pathway C is greatly favored. According to the results of zeta potential (see Figure S7), tubular TiNT and Au/TiNT are negatively charged over the most pH region, the protons aiming to counterbalance the negative surface charges are accumulated at the interface between catalyst and water, resulting in the formation of protonated NDMA and the decrease of the reactivity between electrophilic hydroxyl radical and amine nitrogen, thus facilitating the reaction of hydroxyl radical with nitrosyl nitrogen (Lee et al., 2005a). Moreover, as shown in Figure 3, it should also be noted that the production of NO2- increases at the first beginning in the process of NDMA photocatalytic degradation and then gradually decreases. On the one hand, the decrease of NO2- ion can be attributed to the formation of NO3- ion by photocatalytic oxidization. On the 11

other hand, according to previous study (Chow et al., 1972), NO2- ion produced by pathway C can act as nucleophile and react with the radical (II) to form the dimethylammonium ion, represented as pathway D shown in Scheme 2. This is probably attributed to shorter diffusion path in the straight tubes of TiNT and Au/TiNT to make the intermediate products NO2- and radical (II) react to initiate a novel pathway. The previous research reported that the straight tubular geometry of nanotubular TiO2 favored for shorting the diffusion path of carrier and reaction matter from the solution to the active surface site (Macak et al., 2007), resulting in the enhanced photocatalytic activity on the nanotubular TiO2. Our present work indicates the tubular structures of TiNT and Au/TiNT also have some influences on the diffusion path of intermediates and products, further resulting in the contact and reaction between carrier and intermediate or product. That is, the product NO2- from Path C reacts with the intermediate radical (II) from Path B to produce more amounts of DMA. This will broaden and deepen the understanding of photocatalysis over nanotubular TiO2 with straight tubular geometry. On the basis of above analysis, considering the nanotube structure and space-restricted contact, cationic radical (II) can be formed by the attack of hydroxyl radical, and makes pathway B and pathway D take place. The later one also contributes much for the NDMA degradation over TiNT and Au/TiNT, leading to a sharply enhanced photodegradation efficiency. In addition, it should be noticed that the NDMA photocatalytic performances are fairly similar on TiNT and Au/TiNT, although both have obviously different surface area and pore size. Such results imply that the nanotubes structures of TiNT and Au/TiNT play more important role than the surface area in the process of NDMA photocatalytic degradation, even if the nanotubes of Au/TiNT are kind of short. As for Au/TiNT, Au actually facilitates photocatalytic oxidation, because the work function of Au is higher than that of TiO2, which results in accumulation of reductant negative charges on Au and increasing the rate of photogenerated electrons transferred to oxygen adsorbed on the surface of catalysts, furthermore, the recombination of electron and hole is effectively suppressed by the schottky barrier formed between Au and TiO2. But the slight rutile crystal phase turned up in Au/TiNT may result in lower catalytic performance than that in TiNT in the anatase form (Augustynski, 1993; Hoffmann et al., 1995; Lai et al., 2006; Linsebigler et al., 1995; Mills and Hunte, 1997). As a result, the nanotube of Au/TiNT can contribute much to higher photocatalytic efficiency in the process of NDMA photocatalytic degradation. 12

Scheme1. Three proposed degradation pathways of NDMA. A

OOCH2

H 2C N

N

O

B N

O

-O2

N

N

N

O

N

O

CH3NH2

MA

-HCHO

H 3C

H 3C

NO O2

C N

N

×2

(I)

OH

A

N H 3C

H 3C

H3 C

O2

2ecb-/2H+

OCH3

B

H3 C

O

H3 C

+ N

+ CH3NH=CH2 N

O

H3 C

C

O2

NO

(II)

NDMA

H 2O

CH3NH2 + HCHO

-

ONOO-

MA

-

ONOO-

NO3-

NO3-

HNO2 (N2O3)

H 3C N

N

OH

H3 C

O

H 3C

ecb-/H+ N

(III)

or H-abstraction

(H3C)2NH

DMA

H3 C

Scheme 2. Pathway D proposed for the NDMA photocatalytic degradation over TiNT or Au/TiNT. OH H3 C

H3 C N

N

H3 C

O

+ N

NO2-/H+ N

H3 C NH2+ + N2O3

O

2HNO2

H3 C

H3 C

(II)

4. Conclusions Nanotubular morphology plays an important role in enhancing NDMA photocatalytic performances, not only significantly improving NDMA photocatalytic efficiency, but also initiating new photocatalytic reaction pathway derived from effectively contacting between intermediate products and products in the nanotubular structure. Such results would bring new insight into the applications of nanotube TiO2 for removal of other organic pollutants and also intrigue broad interests in the effect of specific morphologies on the photocatalytic reactions. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 50808102 and Grant No. 20907031) and the Natural Science Foundation of Tianjin (14JCYBJC23100 and 15JCYBJC48100). We gratefully acknowledge the valuable suggestion from Prof. Chen Wei of Nankai University. Appendix A. Supplementary data The Au XPS spectrum, UV-vis diffuse reflectance spectra, adsorption data of NDMA over TiO2 nanotubes, SEM images, adsorption isotherms and pore size distribution curves of TiO2 nanotubes, and zeta potentials can be found in supporting

13

information file.

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Highlights ·TiO2 nanotube and Au-modified TiO2 nanotube were prepared to evaluate tube effect. ·Tubular morphology of TiO2 nanotube is kept after Au modification. ·Tubular morphology of TiO2 nanotube plays a key role in enhancing NDMA degradation. ·Tubular TiO2 initiates new NDMA photocatalytic path, generating DMA as main product. 16