One-step prepared cobalt-based nanosheet as an efficient heterogeneous catalyst for activating peroxymonosulfate to degrade caffeine in water

Accepted Manuscript One-step prepared Cobalt-based Nanosheet as an Efficient Heterogeneous Catalyst for Activating Peroxymonosulfate to Degrade Caffeine in Water Kun-Yi Andrew Lin, Hong-Kai Lai, Shaoping Tong PII: DOI: Reference:

S0021-9797(17)31430-3 https://doi.org/10.1016/j.jcis.2017.12.040 YJCIS 23112

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

21 September 2017 15 December 2017 15 December 2017

Please cite this article as: K-Y. Andrew Lin, H-K. Lai, S. Tong, One-step prepared Cobalt-based Nanosheet as an Efficient Heterogeneous Catalyst for Activating Peroxymonosulfate to Degrade Caffeine in Water, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis.2017.12.040

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One-step prepared Cobalt-based Nanosheet as an Efficient Heterogeneous Catalyst for Activating Peroxymonosulfate to Degrade Caffeine in Water

Kun-Yi Andrew Lina,*, Hong-Kai Laia and Shaoping Tong b,* a

Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung, Taiwan

b

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China

*Corresponding Author. E-mail address: [email protected] (Kun-Yi Andrew Lin); E-mail address: [email protected] (Shaoping Tong).

1

Abstract Two-dimensional (2D) planar cobalt-containing materials are promising catalysts for activating peroxymonosulfate (PMS) to degrade contaminants because 2D sheet-like morphology provides large reactive surfaces. However, preparation of these sheet-supported cobaltic materials typically involves multiple steps and complex reagents, making them less practical for PMS activation. In this study, a cobalt-based nanosheet (CoNS) is particularly developed using a one-step hydrothermal process with a single reagent in water. The resulting CoNS can exhibit a thickness as thin as a few nanometers and 2-D morphology. CoNS is also primarily comprised of cobalt species in a coordinated form of Prussian Blue analogue, which consists of both Co3+ and Co2+. These features make CoNS promising for activating PMS in aqueous systems. As degradation of an emerging contaminant, caffeine, is selected as a representative reaction, CoNS not only successfully activates PMS to fully degrade caffeine in 20 min but also exhibits a much higher catalytic activity than the most common PMS activator, Co 3O4. Via studying inhibitive effects of radical scavengers, caffeine degradation by CoNS-activated PMS is primarily attributed to sulfate radicals and hydroxyl radicals to a lesser extent. The degradation products of caffeine by CoNS-activated PMS are also identified and a potential degradation pathway is proposed. Moreover, CoNS could be also re-used to activate PMS for caffeine degradation without activity loss. These results indicate that CoNS is a conveniently prepared and highly effective and stable 2-D catalyst for aqueous chemical oxidation reactions.

Keywords: cobalt, nanosheet, caffeine, peroxymonosulfate, sulfate radicals 2

1. Introduction Wet chemical oxidation represents one of the most widely-used reactions for treating aqueous contamination [1-3], and synthesizing organic compounds [4-9]. Especially, wet chemical oxidation for wastewater treatments is categorized as Advanced Oxidation Processes (AOPs), which usually involve highly reactive radicals, such as hydroxyl (OH•) and sulfate (SO4•−). Although OH•-involved AOPs have been intensively employed (such as Fenton’s reaction), recently SO4•−-involved AOPs are increasingly studied owing to several advantages. For instance, SO4•− shows a similar or even higher oxidation potential than OH• [10]. SO4•− also exhibits a higher selectivity for unsaturated aromatics [10, 11], and a longer half-life than OH• [12, 13]. To obtain SO4•−, PMS (peroxymonosulfate salt) is an attractive reagent because it is inexpensive, and environmentally friendly [14]. However, self-generation of SO4•− from PMS is very slow and “activation” of PMS is required to accelerate generation of SO4•−. To date, many approaches have been developed to “activate” PMS; however activation by transition metals is considered as the most practical approach [15-18]. Among various transition metals (e.g., Fe, Mn, Co, etc.), Co appears to be the most effective metal for PMS activation [16, 18-20]. Therefore, many efforts have been made to develop different cobaltic catalysts for PMS activation, especially heterogeneous Co-based materials. Despite the fact that Co 3O4 has been proven to successfully activate PMS [15-20], several studies attempted to enhance catalytic activities of cobalt materials by fabricating Co3O4 nanoparticles (NPs) which can exhibit much higher surface areas theoretically. Nevertheless, NPs in water tend to aggregate, decreasing substrate-supported

their

Co-NP

catalytic

activities

[21-23].

composites

were

developed,

Therefore, including

several metal

oxide-supported Co NPs [24, 25] and carbon material-supported Co NPs [26-31]. As 3

catalysis primarily involves surface characteristics, two-dimensional (2D) planar substrates (e.g., graphene) are favorably employed to support Co NPs because the 2D structure provides large planes to immobilize Co NPs [28, 29]. Nevertheless, fabrication of these 2D composites typically involves multiple steps, long preparation time and complex reagents. More importantly, Co NPs immobilized on 2D substrates may fall off from substrates during reactions. These issues make these 2D cobaltic composites less practical for PMS activation in water. However, there is still a demand to develop 2D-structured cobalt-bearing nanomaterials via simple and convenient fabrications for activating PMS. To this end, we propose to develop a one-step prepared cobalt-containing nanosheet (NS) using a single reagent for PMS activation. Through one-step hydrothermal process, an aqueous of a single reagent, hexacyanocobaltate, solution can result in CoNSs with a primary constituent of Co3[Co(CN)6]2 which is a Co-based Prussian Blue analogue (PBA). As PBA exhibits a unique coordination structure, different valence states of metal species can co-exist in PBA [32]. PBA also consists of vacancies for balancing charges [32], making it an intriguing catalysts for many applications [33-35]. Thus, this resulting CoNS, with a planar structure and coordinated PBA constituent, can be an advantageous catalyst for PMS activation. To our knowledge, only a very few studies have reported this kind of Co-bearing NSs [36, 37] but preparation of those NSs required multiple reagents and procedures. More importantly, CoNS has not been explored for its catalytic activity for PMS activation in literatures. Thus, this study aims to investigate usage of this one-step prepared, and single-reagent employed CoNS for activating PMS to degrade organic contaminants. In particular, an emerging contaminant, caffeine, is selected in this study because caffeine has been increasingly detected in influents and effluents of wastewater 4

treatment plants, and even drinking water [38]. Chemical oxidation has been recognized as a promising method to treat caffeine-containing wastewater [39, 40].

2. Experimental 2.1 Materials and preparation of CoNS All chemicals involved in this study were purchased from Sigma-Aldrich (USA) and used directly without purification,

including

potassium

hexacyanocobaltate

(K3Co(CN)6) (≥97.0%), tert-butyl alcohol (TBA) (≥99.0%), caffeine (ReagentPlus® grade), and cobalt nitrate hexahydrate (ACS reagent, ≥98%), urea (ACS reagent, ≥99.0%)

were obtained from Sigma-Aldrich (USA). Deionized (DI) water was

prepared to less than 18 Megohm-cm. CoNS was prepared via an one-step hydrothermal process as illustrated in Fig. 1 by dissolving 0.5 g of potassium hexacyanocobaltate to 40 mL in a Teflon-lined autoclave. After hexacyanocobaltate was fully dissolved in water, the homogeneous solution was then heated up to 180 °C for 20 hours. When the solution cooled down, the resulting precipitate was collected by centrifugation, washed thoroughly by DI water and ethanol, dried at 85 °C to afford CoNS. Co3O4, the most common catalyst for PMS activation, was prepared according to the reported protocols [41] for comparison. Briefly, 0.5 g of cobalt nitrate hexahydrate was dissolved in 50 mL of DI water and 0.1 g of urea was then also added. This as-prepared mixture was transferred into a Teflon-lined autoclave and heated at 150 °C for 4 h. The precipitate was then collected, washed with DI water, dried at 85 °C for 12 h and then then calcined at 550 °C for 4 h to afford Co3O4, which exhibited an average size of 70±20 nm. The morphology of CoNS was observed by scanning electronic microscopy (SEM) (JEOL JSM-6700, Japan) and transmission electronic microscopy (TEM) (JEOL 5

JEM-2010, Japan). The chemical composition of CoNS was determined by Energy Dispersive Spectroscopy (EDS) (Oxford Instrument, UK). The crystalline structure of CoNS was measured by an X-ray diffractometer (XRD) (Bruker D8, USA). Atomic force microscopic (AFM) analysis of CoNS was conducted using a scanning probe microscope system (Bruker Dimension Icon, USA). Thermal stability of CoNS was determined by a thermogravimetric (TG) analyzer (ISI TGA i1000, USA) in N2. Zeta potential of CoNS was also measured by a zetasizer (Nano-ZS, Malvern Instruments Ltd, Malvern, UK).

2.2 PMS activation by CoNS for degradation of caffeine Catalytic activities of CoNS for activating PMS were evaluated by degrading an emerging contaminant, caffeine, via batch-type experiments. In a typical experiment, 30 mg of PMS was added to 0.2 L of caffeine solution with an initial concentration (C0) of 50 mg L−1. After PMS was dissolved, 10 mg of CoNS was immediately added to the caffeine solution. After desired intervals, sample aliquots were withdrawn from the caffeine solution and analyzed for remaining concentrations (Ct) of caffeine at a reaction time t by UV-Vis spectrophotometry (275 nm). The effect of initial pH was examined by changing initial pH of solution to 3, 5, 7, 9 and 11 using 0.1 M of HNO3 and NaOH. To determine the effect of radical inhibitors, methanol or TBA (0.1 M) was added to caffeine solutions. Recyclability of CoNS for activating PMS to degrade caffeine was evaluated by testing used CoNS for caffeine degradation over multiple cycles. The degradation products of caffeine were identified by a liquid chromatography-mass spectrometry-mass spectrometry (LC-MS-MS) (TSQ Quantum, Thermo Scientific, USA). Each experiment of caffeine degradation was repeated at least two times to obtain average values and standard deviations as error bars. 6

3 Results and discussion 3.1 Characterizations of CoNS Fig. 2(a) shows a SEM image of the resulting material from one-step hydrothermal process of hexacyanocobaltate. There were many sheet-like materials closely-packed; however, the very thin feature of these sheets, especially on edges, can be still observed. A TEM image (Fig. 2(b)) shows that the sheet was semi-transparent under the electronic beam of TEM, demonstrating that the sheet was very thin. To determine the particle size and thickness of NS, AFM analysis was adopted. The inset in Fig. 2(a) shows an AFM image of a piece of NS and the cross-section profile of the right-upper section of this piece is displayed in Fig. 2(b). The thickness of CoNS of the selected section was ca. 7 nm, indicating that CoNS exhibited a nanoscale sheet-like morphology. Furthermore, the size and thickness distributions of CoNS with statistical analyses can be seen in Fig. S1. In particular, the mean size of CoNS was 140 nm and the corresponding FWHM was 92 nm. The mean thickness of CoNS was 7.6 nm and its corresponding FWHM was 3.9 nm. These dimensional values validate that CoNS can be considered as a nano-sheet like material. While the NS structure was successfully obtained, it was essential to determine chemical composition of these NS sheets. Fig. 2(d) shows a result of EDS, indicating that these NSs were comprised of Co, O, C and N. A noticeable fraction of cobalt validates that these NSs were Co-based NSs (CoNSs). To further specify what cobalt species comprised in the NS, its XRD pattern was measured and is shown in Fig. 3(a). As many sharp peaks can be detected, the resulting NS was proven to be highly crystallized. These peaks, particularly at 14.5, 17.3, 24.6, 35.1, 39.4, 50.1, 53.6, 56.6 and 59.4°, can be attributed to the (111), (200), (220), (400), (420), (440), (600), (620) and (622) planes of cobalt cyanide hydrate (Co3[Co(CN)6]2), (JCPDS#22-0215) which 7

can be considered as a Prussian Blue Analogue (Fig. S2). In addition, a few minor peaks can be also observed at 23.8, 30.1 and 45.0°; these peaks could be assigned to cobalt ammine nitrate nitrite (Co(NH3)5(ONO)(NO3)2) (JCPDS#49-1125), which has been also detected in some reported cobalt-based NSs [37]. Additionally, since there might be a chance that Co3O4 was formed during the preparation of CoNS, the XRD pattern of CoNS was particularly compared with that of Co 3O4 based on Powder Diffraction Files (PDF) as shown in Fig. S3. However, all XRD patterns of Co3O4 species appeared obviously different from the XRD pattern of CoNS, suggesting that almost no or very insignificant amount of Co3O4 was formed during the preparation of CoNS. The surface chemistry of CoNS was further investigated using XPS analysis. In particular, since the major component of CoNS was Co 3[Co(CN)6]2, the Co2p core-level spectrum of was obtained as Fig. 3(b), in which the spectrum was deconvoluted to exhibit several peaks located at 781.8 and 783.5 eV correspond to Co3+ and Co2+ of Co 2p 3/2, respectively [42, 43]. The peaks at 797.3 and 798.9 eV can be assigned to Co3+ and Co 2+ of Co 2p1/2, respectively. As PBA is formulated as MII3[MIII(CN)6]2 (M: transition metals), the presence of Co3+ and Co2+ confirms the formation of Co2+3[Co 3+(CN)6]2. On the other hand, the N1s core-level spectrum is also shown in Fig. 3(c) and the peak can be attributed to C-N [44], which might be derived from Co(CN)6 in the PBA. To evaluate the thermal stability of CoNS, a TG curve of CoNS was measured and is shown in Fig. 3(d). CoNS exhibited a significant weight loss occurred starting at 300 °C because of carbonization of Co(CN)6. This result suggests that CoNS can remain quite stable without significant weight loss up to 200 °C, making CoNS suitable for applications at relatively high temperatures. 8

3.2 Activation of PMS by CoNS for caffeine degradation Before examining PMS activation by CoNS for degrading caffeine, it was critical to determine whether CoNS can remove caffeine via adsorption. Fig. 4(a) shows that almost no caffeine was removed via adsorption to CoNS, demonstrating that CoNS did not exhibit any affinity to caffeine. In addition, as PMS alone was added to the caffeine solution, Ct/C0 did not decrease below 0.95, indicating that PMS alone was very

incapable

of

degrading

caffeine

possibly

due

to

extremely

slow

self-decomposition of PMS without activation. Nevertheless, when CoNS and PMS were both added to a caffeine solution, caffeine was quickly degraded and Ct/C0 reached zero within 20 min. Since CoNS and PMS were inefficient for degrading caffeine separately, this indicates that PMS was activated in the presence of CoNS. As the primary component in CoNS was Co3[Co(CN)6]2, which was constituted of Co 2+ and Co 3+ [45], these cobalt species can react with PMS for producing sulfate radicals as follows (Eqs. (1),(2)) [45, 46] : Co2+@CoNS + HSO5‒ → Co 3+@CoNS + SO4•‒ + OH‒

(1)

Co3+@CoNS + HSO5‒ → Co 2+@CoNS + SO5•‒ + H+

(2)

The resulting peroxymonosulfate radical (SO5•‒) further becomes SO4•‒ as follows (Eq.(3)): 2 SO5•‒ →2 SO4•‒ + O2

(3)

While CoNS was demonstrated to activate PMS, it was important to compare CoNS with a typical PMS catalyst, Co3O4. The caffeine degradation by Co3O4-activated PMS can be also seen in Fig. 4(a) and caffeine was indeed degraded. However, the corresponding Ct/C0 only reached 0.7, indicating the much lower caffeine degradation extent. The corresponding degradation kinetics also appeared to 9

be significantly slower than that obtained by CoNS. For quantitative comparing kinetics between Co 3O4 and CoNS, the most typical rate law, pseudo first order equation, was adopted as follows (Eq. (4)): Ct = C0 exp( −kappt )

(4)

where kapp is the “apparent” pseudo first order rate constant. kapp values for degrading caffeine by CoNS and Co3O4-actvated PMS were calculated as 0.272 and 0.019 min−1 (Table S1). In comparison with a previous study of caffeine degradation using a comparable dosage of PMS [47], CoNS could exhibit a significantly faster rate constant (up to ten times). These comparisons validate that CoNS is an advantageous and promising catalyst for activating PMS to degrade organic contaminants.

3.3 Effects of CoNS and PMS dosages on caffeine degradation While the combination of CoNS and PMS successfully degraded caffeine, it was critical to elucidate respective contributions of CoNS and PMS. Therefore, the effects of CoNS and PMS dosages were further investigated. Fig. 4(b) first shows that caffeine degradation using different dosages of CoNS. Regardless of CoNS dosages, caffeine in the three cases was completely degraded This indicates that although a much lower dosage of CoNS was used, caffeine was still completely degraded, showing the high catalytic activity of CoNS. However the degradation kinetic results were noticeably different. When CoNS increased from 25 to 50 and 100 mg L−1, kapp correspondingly increased from 0.211 to 0.274 and 0.420 min−1, validating the enhancing effect on kinetics at higher CoNS dosages. On the other hand, the effect of PMS dosage is shown in Fig. 5(a). When PMS dosage was changed, the degradation extent and kinetics were greatly influenced. Particularly, Ct/C0 merely reached 0.9 at PMS = 25 mg L−1 and 0.4 at PMS = 100 mg 10

L−1 with much lower kapp values (Table S1). This suggests that caffeine degradation was primarily decided by PMS dosages as sulfate radicals were produced from PMS instead of CoNS. Nevertheless, these results also approved that CoNS played a catalytic role and controlled the kinetics.

3.4 Effects of temperature and pH on caffeine degradation Since temperature is an important parameter for PMS activation, the effect of temperature was also studied in Fig. 5(b). As temperature increased from 30 to 60 °C, degradation extents remained almost the same but the kinetics became substantially faster. k app increased from 0.274 min−1 at 30 °C to 0.426, 1.585 and 3.276 min−1 at 40, 50 and 60 °C, respectively, indicating the enhancing effect of higher temperatures particularly on caffeine degradation kinetics. As kapp increased alone with the temperature, kapp values were further correlated to temperatures via the Arrhenius equation using the following equation (Eq.(5)): Ln k app = Ln A – Ea/RT

(5)

where A denotes the pre-exponential factor (min–1); R is the universal gas constant; and T is the solution temperature in Kelvin (K). Based on Eq. (5), a plot of 1/T versus Ln kapp is shown as an inset in Fig. 5(b) and the data points could be well fitted by a linear regression with R2 = 0.970, demonstrating that the relationship between the caffeine degradation kinetics and temperature can be described by the Arrhenius equation. The slope of the fitting line is then used to calculate the activation energy as 72 kJ mol–1. Additionally, the effect of initial pH (pHin) of caffeine solution was also investigated by changing pHin values of caffeine solutions to 3, 5, 7, 9 and 11 (Fig. 11

6(a)). When pHin decreased from 7 to 5, the degradation extent remained almost the same but the kinetics was noticeably slowed as kapp dropped to 0.194 min−1. When pH decreased further to 3, the degradation extent remained comparable but kapp continued to decrease to 0.143 min−1. These results suggest that caffeine degradation was slightly interfered with the acidic conditions probably due to the fact that PMS is relatively stable and less easily activatable under acidic conditions [23]. While pH was increased from 7 to 9, the caffeine degradation extent was also slightly affected as kapp decreased to 0.232 min−1. As pH was elevated to 11, caffeine degradation became almost insignificant, showing that the highly alkaline condition appeared to be very unfavorable for caffeine degradation. This was possibly because PMS tends to decompose without generating SO4•− under alkaline conditions [23, 48, 49], leading ineffective degradation. Moreover, zeta potentials of CoNA in water under various pH values were measured as shown in Fig. S4. The surface charge of CoNA appeared to be relatively negative as it exhibited negative charges even under acidic conditions. This result agreed with the literature [50] which indicates that Prussian Blue analogous materials are highly negatively-charged in water. As pH increased, the surface charge of CoNS became even more negative because of accumulation of OH− [50]. As the surface of CoNS was relatively negatively charged, the interaction between SO5•− and CoNS surface was restrained, inhibiting the formation of SO4•− (Eq. (3)) and leading to ineffective caffeine degradation [51].

12

Another possibility was that sulfate radicals may react with water to form hydroxyl radicals as follows (Eq. (6)) [52-54]: SO4•− + H2O → SO42− + OH• + H+

(6)

It has been reported that a relatively high amount of OH• could be produced at pH = 9, thereby leading to the higher extent of decolorization at pH = 9 [55]. However, once pH increased to 11, caffeine was barely degraded possibly because of radical inter-scavenging and self-recombination [56]. In addition, caffeine degradation using Co3O4-activated PMS was also evaluated for comparison as shown in Fig. S5. Similar to caffeine degradation by CoNS-activated PMS, the neutral condition (i.e., pH = 7) seemed to be the most favorable condition for caffeine degradation by Co 3O4. Degradation of caffeine was less effective under acidic conditions, possibly because PMS is much more stable and relatively difficult to be activated at low pH values. On the other hand, the highly alkaline condition might cause decomposition of PMS without generation of sulfate radicals, thereby causing ineffective degradation of caffeine. The comparison between Co3O4 and CoNS for caffeine degradation validates that Co 3O4 was less effective than CoNS for activating PMS to degrade caffeine irrespective of pH values.

3.5 Effects of inhibitors on caffeine degradation by CoNS-activated PMS While CoNS was quite effective to activate PMS for degrading caffeine, it was important to explore the underlying mechanism of caffeine degradation by PMS activated by CoNS. Typically, PMS activation leads to the formation of SO4•−, SO4•− might react with H2O to generate OH• as shown in Eq.(6). 13

Since OH• is also able to degrade caffeine, it was critical to distinguish radical species contributing to the caffeine degradation using PMS activation by CoNS. To this end, two special radical inhibitors, TBA and methanol, were adopted to test their respective inhibitive effects on caffeine degradation. This is because TBA, without

α -hydrogen, can react with OH• very quickly, making it a suitable probe reagent for OH•. In contrast, methanol, with α -hydrogen, can react with both OH• and SO4•− rapidly [57], enabling it to be a typical probe reagent for OH• and SO4•−. As TBA was added, caffeine degradation by CoNS-activated PMS was noticeably influenced as kapp became 0.104 min−1 and the corresponding Ct/C0 just reached 0.3 (Fig. 6(b)). This demonstrates that TBA slightly inhibited caffeine degradation and OH• was partially involved in CoNS-activated PMS. Nevertheless, when methanol was present, caffeine degradation was almost completely inhibited, indicating that SO4•− certainly contributed to caffeine degradation. These results reveal that caffeine degradation by CoNS-activated PMS can be attributed primarily to SO4•− and OH• to a lesser extent.

3.6 A possible degradation pathway for caffeine by CoNS-activated PMS As caffeine was quickly degraded by CoNS-activated PMS, it was useful to investigate the corresponding degradation pathway of caffeine. Fig. S6 shows intermediate products of caffeine degradation determined by LC-MS-MS. Based on these products, a possible degradation pathway was proposed and is shown in Fig. 7. When caffeine was present in water, a part of caffeine molecules might be hydroxylated to form an intermediate product as P1[58]. Caffeine and P1 might be attacked by OH• and SO4•− and then the C-N bond of 5-member ring [58, 59] and the C-C bond of 6-member ring were sequentially broken, leading to the formation of P2. Primary amine groups might be further oxidized to result in P3, which has been also 14

reported in other studies of caffeine degradation [58-60]. With continuous oxidizing, P3 was transformed to P4 and then P5, which, however, might evolve into different intermediates (P6 and P7). These intermediates were subsequently oxidized into low-molecule-weight compounds (e.g., NH3, N2 and CO2).

3.7 The recyclability of CoNS to activate PMS As proposed as a heterogeneous catalyst, CoNS should be re-usable for activating PMS. Thus, its recyclability was also evaluated. Fig. 6(c) shows a multiple-cycle caffeine degradation using PMS activated by used CoNS. Even though used CoNS was not regenerated by any means, it was still able to activate PMS and to degrade caffeine efficiently over 5 cycles. This demonstrates that CoNS can be a durable and re-usable heterogeneous catalyst for PMS activation to degrade caffeine.

4. Conclusions: In this study, a cobalt-containing 2D structured material, CoNS, was successfully prepared via a one-step hydrothermal reaction of hexacyanocobaltate and water. Unlike graphene-supported cobalt oxide, the preparation of CoNS was much more convenient. Moreover, as CoNS was comprised of cobaltic PBA, Co3[Co(CN)6]2, cobalt ions (Co2+ and Co3+) can be evenly distributed over CoNS, making CoNS highly promising for PMS activation to degrade contaminants. As caffeine degradation was selected as a representative reaction, CoNS not only showed a much higher catalytic activity than Co 3O4, but also exhibited a much faster rate constant (up to ten times) than a reported catalyst at the same PMS dosage. Moreover, CoNS could be also re-used to activate PMS for caffeine degradation without activity loss. These results indicate that CoNS is a conveniently prepared and highly effective and stable 15

2-D catalyst for activating PMS to oxidize contaminants in water. The preparation scheme reported here can be also applied to fabricate other 2D-structured metal coordination materials for adsorption and catalysis applications.

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generated

by ozonation,

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21

Graphical abstract

22

Fig. 1. A scheme showing preparation of CoNS and its activation mechanism to degrade caffeine.

23

(b)

(a)

500 nm

100 nm C

(c) 30

(d) O

Intensity (a.u.)

Height (nm)

25 20 15

100 nm

10 5

Co Co

N C

0 0

20

40

60

0

80 100 120

Co

2

4

6

8

Energy (KeV)

Distance (nm)

Fig. 2. Characteristics of CoNS: (a) SEM, (b) TEM images, (c) AFM analysis and (d) EDS analysis.

24

(a)

(b)

20

30

40

Intensity (a.u.)

(440) (600) (620) (622)

(033)

(420)

(022)

(020) (220)

(111)

(400)

Intensity (a.u.) 10

50

60

Co2+ Sat.

800

Sat. 3+

Co

790

780

Binding Energy (eV)

(d)

(c)

Co3+

Co2+

Co 2p1/2

70

2 theta (degree)

Co 2p3/2

Co2p

(200)

Blue bracket: Cobalt Ammine Nitrate Nitrite Black bracket: Cobalt Cyanide Hydrate

80

N-C

Weight (%)

Intensity (a.u.)

100

60 40 20 0

406 404 402 400 398 396 394 392

Binding Energy (eV)

0

100 200 300 400 500 600 ο

Temperature ( C)

Fig. 3 (a) XRD pattern, (b) Co2p core-level spectrum, (c) N1s core-level spectrum and thermogravimetric analysis of CoNS in N2.

25

(a)

(b)

Fig. 4. PMS activation by CoNS for caffeine degradation: (a) comparison of degradation extent by PMS alone, adsorption to CoNS, CoNS-activated PMS and Co3O4-activated PMS (T = 30 °C); and (b) effect of CoNS dosage (Caffeine = 50mg L−1, CoNS= 50 mg L−1, PMS = 150 mg L−1, T = 30 °C).

26

(a)

(b)

Fig. 5. Effects of (a) PMS dosage (T = 30 °C) and (b) temperature on caffeine degradation using CoNS-activated PMS (Caffeine = 50 mg L−1, CoNS= 50 mg L−1, PMS = 150 mg L−1).

27

(a)

(b)

(c)

Fig. 6. PMS activation by CoNS for caffeine degradation: (a) effect of initial pH; (b) effect of salt; and (c) recyclability of CoNS for activating PMS (Caffeine = 50mg L−1, CoNS = 50 mg L−1, PMS = 150 mg L−1, T = 30 °C).

28

+ H2O

Caffeine, m/z = 194

P1, m/z = 212

P2, m/z = 174

P5, m/z = 131

P3, m/z = 143

P6, m/z = 121

P4, m/z = 131

P8, m/z = 105

Low molecular-weight compounds P7, m/z = 114

P9, m/z = 100

Fig. 7. A possible degradation pathway of caffeine by CoNS-activated PMS based on intermediate products detected by LC-MS-MS.

29