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Strengthening the catalytic activity for ozonation of Cu/Al2O3 by electroless plating-calcination process Yi Ren, Jun Li, Jiali Peng, Fangzhou Ji, and Bo Lai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04286 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Industrial & Engineering Chemistry Research

Strengthening the catalytic activity for ozonation of Cu/Al2O3 by electroless plating-calcination process Yi Ren, Jun Li, Jiali Peng, Fangzhou Ji, Bo Lai Department of Environmental Science and Engineering, School of Architecture and Environment, Sichuan University, Chengdu 610065, China

Abstract: A new method of electroless plating-calcination was developed to prepare Cu/Al2O3 with high catalytic activity and long operational life for catalytic ozonation. First, effects of key preparation parameters on the catalytic activity of new Cu/Al2O3 were

investigated.

In

addition,

new

Cu/Al2O3

prepared

by

electroless

plating-calcination and conventional Cu/Al2O3 prepared by impregnation-calcination were characterized. The results show that when Cu/Al2O3 were prepared by electroless plating-calcination, copper oxides was uniformly and densely deposited on the surface of Al2O3, and the copper compounds consist of CuO. Furthermore, the catalytic activity and operational life of the two kinds of Cu/Al2O3 was also investigated comparatively. The results confirmed the advantage of the new method for the preparation of Cu/Al2O3 for catalytic ozonation. Finally, the degradation pathway was proposed. It can be concluded that electroless plating-calcination is a promising technology to prepare the robust Cu/Al2O3 with high catalytic activity for the decomposition of ozone. Keywords: catalytic ozonation;

Cu/Al2O3; electroless plating;

calcinations;

p-nitrophenol



Corresponding authors. Tel./fax: +86 18682752302 E-mail address: [email protected] (Bo Lai) 1

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1. Introduction Ozone, an effective oxidant in air for disinfection and purification, is also applied in wastewater treatment. Ozonation can be performed for organic pollutants degradation and mineralization via two routes: (a) direct oxidation by ozone, (b) indirect oxidation by the reactive oxygen species (ROS) generated from the decomposition of ozone1. However, the direct oxidation by the ozone alone is relatively slow and selective, and it could not achieve total mineralization of the organic pollutants. Therefore, it is necessary to develop a promising catalytic ozonation technology, which could generate more radicals to degrade the toxic and refractory pollutants in wastewater. In recent years, heterogeneous catalytic ozonation has been considered as a promising treatment technology which utilizes insoluble catalysts to degrade and mineralize the organic pollutants with high treatment efficiency and low negative effects (e.g., secondary pollution)2. Copper and its oxides, as a kind of cost effective catalysts3-5, are also included. For example, copper deposited on support materials (e.g., Al2O3 and MnO2) has been investigated as heterogeneous catalysts of the ozonation process, which showed satisfactory results in catalytic ozonation due to their catalytic activity and low costs6-8. Impregnation-calcination (e.g., Cu/Al2O3 prepared by impregnation first, and then calcination) is a conventional and most common technology for Cu-loaded ozonation catalysts preparation. For example, Pi et al. reported the oxalic acid removal by CuO/Al2O3.The CuO/Al2O3 was prepared by impregnation(with 2% copper acetate) and calcination (with 600 oC for 3 hours) method. The heterogeneous catalytic ozonation system with CuO/Al2O3 could obtain higher TOC removal the homogeneous catalytic ozonation system with Cu2+. However, the operational life and catalytic activity of the catalysts would be limited 2


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by the abscission of Cu layer during the long-term operation9. Thus, it is necessary to improve the present prepared method (i.e., impregnation-calcination) and overcome its limits. Electroless plating, a developed chemical deposition technology without applying an external electric voltage10, has been widely used in many industrial applications, such as printed circuit board industry11-12, aerospace and automotive industry13, anti-corrosion industry14. Via the redox reaction, metal ions (e.g., Cu2+, Ni2+, Co2+, etc.) are reduced by some reductants (e.g., sodium hypophosphite) in an electrolyte solution, and then deposited on the substrates 15-17. Fe/Cu bimetallic particles prepared by electroless plating have been developed in our previous study, and the results suggest that electroless plating is a promising technology for environmental materials preparation18. If Cu-loaded materials for heterogeneous catalytic ozonation were prepared by electroless plating-calcination, the deposition rate of metal could be controlled through adjusting the preparation conditions or additive agents in electroless plating process. In addition, electroless plating might deposit Cu uniformly and densely on the surface of support materials18, which would obtained better catalytic activity compared with those prepared by impregnation. Furthermore, adhesion between Cu coating and support material obtained by electroless plating would also be much stronger than that of impregnation. Therefore, the electroless plating-calcination might be much better than the impregnation-calcination for the catalytic ozonation Cu-loaded materials preparation. However, there is no report on the preparation of the materials for catalytic ozonation application by using the electroless plating. p-Nitrophenol (PNP), a well-known toxic and refractory industrial wastewater containing aromatic pollutant, has been widely discharged from pesticides, 3



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plasticizers, dyes and explosives industries19-22. The presence of PNP and its derivatives in the aqueous environment is harmful to ecosystems and human health, and it has been listed as a priority pollutant by the US Environmental Protection Agency (USEPA)23. Some studies have documented that the conventional microbiology-based

techniques

cannot

treat

these

industrial

wastewaters

effectively24-25. To decompose PNP and improve its biodegradation, advanced oxidation processes (AOPs), like ozonation, was used to degrade these toxic and recalcitrant wastewaters. Therefore, PNP was chosen as the model pollutant to evaluate the catalytic activity of the prepared Cu/Al2O in catalytic ozonation process. In this study, the electroless plating was used to synthesize Cu-loaded Al2O3 particles (i.e., Cu/Al2O3). A promising electroless plating-calcination for Cu/Al2O3 preparation would be obtained by experiments and analyses. The summarized objective of this study were to (i) investigate the effects of the preparation parameters on the catalytic activity of Cu/Al2O3 prepared by electroless plating-calcination, (ii) compare the two kinds of the Cu/Al2O3 (prepared by impregnation-calcination and electroless plating-calcination) by the analyses of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron

spectroscopy (XPS)

and

Brunauer-Emmet-Teller

(BET),

(iii)

investigate the catalytic activity and operational life of the two kinds of Cu/Al2O3 and (iv) propose the PNP degradation pathway according to the determine intermediates.

2. Experimental 2.1 Reagents In the experiment, the analytical reagents including PNP, γ-Al2O3, CuSO4·5H2O, Cu(NO3)2·3H2O, NaH2PO2·H2O (sodium hypophosphite), H3BO3 (boracic acid), Na3C6H5O7·2H2O (trisodium citrate dehydrate), SnCl2·2H2O, PdCl2 and HCl (37.5%, 4


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w/w) were purchased from Chengdu Kelong chemical reagent factory and of analytical grade. Other chemicals used in the experiment were of analytical grade. Deionized water was used in all experiments. 2.2 Preparation of the Cu/Al2O3 Two kinds of Cu/Al2O3 were prepared in this study. One was prepared by the conventional impregnation-calcination process, and the other was prepared by the new electroless plating-calcination process. (i) Cu/Al2O3 prepared by electroless plating-calcination: the Al2O3 particles were used as the substrates to prepare Cu/Al2O3, and copper was deposited on Al2O3 by electroless plating. First, the Al2O3 (25 g/L) particles were sensitized for 10 min by 200 mL solution with SnCl2·2H2O (10 g/L) and HCl (40 mL/L) at 25±1 oC by water batch heating. And then, they were activated for 10 min by 200 mL solution with PdCl2 (0.02 g/L) and HCl (10 mL/L) at 25±1 oC by water batch heating. After that, the particles were added to the bath (200 mL) for electroless plating with desired plating time (0, 0.5, 1, 3, 5, 10, 15, 20 and 25 min). The bath composition and plating operating conditions are summarized as follows: CuSO4·5H2O of 10 g/L, NaH2PO2·H2O of 30 g/L, H3BO3 of 30 g/L, Na3C6H5O7·2H2O of 15 g/L and water batch heating of 70±1 oC26-27. The slurry was mixed by a mechanical agitator (250 rpm), and the separated particles were rinsed twice with deionized water after each step. The obtained Cu/Al2O3 particles were rinsed twice with deionized water, twice with ethanol, and then they were dried at 70±5 oC for 2 hours. Furthermore, the Cu/Al2O3 would be calcinated (with airing) by muffle furnace (Nabertherm GmbH, Germany) with desired temperature (100, 200, 400, 500, 600, 700, 800 and 1000 oC) and time (0, 1, 1.5, 2, 2.5, 3, 4, 5 and 6 hour(s)). (ii) Cu/Al2O3 prepared by impregnation-calcination: the Al2O3 particles were used as the substrates to prepare Cu/Al2O3, and copper was deposited on Al2O3 by 5



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impregnation. First, the Al2O3 were added to Cu(NO3)2 solution (same Cu addition with optimal that by electroless plating) mixed by a mechanical agitator (250 rpm) for 4 hours at 25±1 oC by water batch heating. And then the slurry was aged for 8 hours at 120±5 oC by oven. Also, the particles were calcinated by muffle furnace (the same calcination temperature and time with optimal conditions by electroless plating). 2.3 Experiment procedure The catalytic activity of Cu/Al2O3 was evaluated indirectly by the obtained COD removal efficiency when PNP aqueous solution was treated by catalytic ozonation with Cu/Al2O3. The PNP aqueous solution (500 mg/L) was prepared by simple dissolution PNP in deionized water. In each experiment, 200 mL PNP aqueous solution was added in a beaker, and the initial pH of PNP aqueous solution was adjusted to 9.0 with 0.1 mol/L NaOH solution. The reaction was initiated with catalysts (10 g/L) added and ozone generated by ozone generator (inlet O2 flow rate of generator=300 mL/min, outlet O3 concentration of generator=64 mg/L). The solution was stirred continuously by a mechanical stirrer (250 rpm) and the experimental temperature was controlled at 25±1 oC by water batch heating. Meanwhile, 1 mL samples were taken out from the reactive beaker at determined intervals, and then filtered through the hydrophilic polyethersulfone (PES) syringe filters (0.45 μm) to remove the particles. The COD, pH, UV–Vis spectra and the concentration of PNP and its intermediates were analyzed by detecting the obtained samples. In addition, the preparation conditions (plating time, calcination temperature and calcination time) of Cu/Al2O3 prepared by electroless plating-calcination were optimized, respectively. Furthermore, five control experiments of (i) O3 alone, (ii) O3+raw Al2O3, (iii) O3+Al2O3 by calcination, (iv) O3+Cu/Al2O3 prepared by electroless plating and (v) O3+Cu/Al2O3 prepared by impregnation-calcination were 6


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setup to confirm the high catalytic activity of Cu/Al2O3 prepared by electroless plating-calcination. To comparatively investigate the operational life of the two kinds of Cu/Al2O3, 10 g/L Cu/Al2O3 (prepared by electroless plating-calcination or by impregnation-calcination) added at the first batch experiment was repeatedly used in the whole 5 times batch experiments, and there were no fresh Cu/Al2O3 to be complemented. The used Cu/Al2O3 was washed by deionized water for three times and by ethanol for three times. After that the Cu/Al2O3 was vacuum dried with the temperature of 50 oC for 2 hours. for next time used. Also, the PNP degradation pathway was proposed according to the UV-Vis spectral analysis and intermediates determination. Besides, characteristics of the two kinds of Cu/Al2O3 before and after employments were observed by SEM, EDS, XRD, XPS and BET, respectively. 2.4 Analytical method The analytical methods of high performance liquid chromatography, atomic absorption spectroscopy, UV–vis, absorption spectra, chemical oxygen demand, pH, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Brunauer-Emmet-Teller are illustrated, and details pertaining to them are provided in the Supporting Information.

3 Results and discussion 3.1

Parameters

optimization

of

Cu/Al2O3

prepared

by

electroless

plating-calcination The catalytic activity of Cu/Al2O3 for the decomposition of ozone would be influenced significantly by the key prepared parameters. To evaluate the catalytic activity of the prepared Cu/Al2O3, the catalytic ozonation system with Cu/Al2O3 was setup to treat PNP aqueous solution. Subsequently, its catalytic activity could be evaluated indirectly according to the obtained COD removal for the PNP aqueous 7



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solution by the catalytic ozonation with the prepared Cu/Al2O3. 3.1.1 Effects of the plating time According to the previous study, the catalytic activity of copper-loaded heterogeneous ozonation catalysts could be affected by Cu mass loading6-7, 28. The deposition of Cu was achieved by the reduction of Cu2+ by hypophosphite (Eq. (1)) in electroless plating process

18

. Since there were excessive Cu2+ and H2PO2- in plating

bath, the Cu mass loading on Al2O3 surface mainly depended on the plating time. Therefore, effect of the plating time on catalytic activity of Cu/Al 2O3 for the decomposition of ozonation should be investigated thoroughly. Cu2+ + 2H2PO2- + 2H2O → Cu0 + 2H2PO3- + 2H+ + H2

(1)

The Cu/Al2O3 prepared by electroless plating-calcination with different plating time (0-25 min) were used to treat 500 mg/L PNP in aqueous solution, respectively Figure 1 (a) shows that COD removal efficiency increased from 66.9% to 76.3% after 20 min treatment with the plating time increase from 0 to 10 min, and then decreased slightly to 73.3% with further increase to 25 min. The theoretical Cu mass loading (TMLCu) was calculated through the copper ion concentration before and after plating process detected by atomic absorption spectroscopy. The results indicated that the optimal TMLCu was about 0.076 g Cu/g Al2O3 (7.6 wt%) when the plating time was 10 min. The results suggest that short or long plating time (i.e., low or high Cu mass loading) would limit the catalytic activity of Cu/Al2O3, which could be explained from two aspects, (i) The lower Cu mass loading (e.g., TMLCu < 0.076 g Cu/g Al2O3 or plating time < 10 min) would cause less catalytic sites formation on the surface of Al2O3 substrate, which would affect the catalytic activity of Cu/Al2O3 significantly. (ii) The excess Cu mass loading (e.g., TMLCu > 0.076 g Cu/g Al2O3 or plating time > 10 8


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min) might cause the incomplete oxidation and Cu2O generation with lower catalytic activity. In fact, the high catalytic activity was mainly attributed to the CuO formed on the surface of the supported materials

29-30

. In addition, the similar phenomenon has

been reported when these catalytic materials were prepared by using the impregnation-calcination

31-33

. Therefore, the optimal plating time of 10 min was

selected in the sequential experiments for further investigation. 3.1.2 Effects of the calcination temperature When Cu0 deposited on Al2O3 substrate by electroless plating was calcinated by muffle furnace in the present of air, copper oxide catalysts were generated on the surface of Al2O3 substrate. Meanwhile characteristics and performance of the generated copper oxide catalysts might be affected significantly by the calcination temperature. Figure 1(b) shows that an increase of calcination temperature from 100 to 600 oC could improve the COD removal efficiency, and it reached the maximum at the calcination temperature of 600 oC (62.9% after 10 min treatment and 77.1% after 20 min treatment). Furthermore, the COD removal efficiency was decreased rapidly when the calcination temperature was above 800 oC. In other words, Cu/Al2O3 prepared at the calcination temperature of 600 oC had the strongest catalytic activity for the decomposition of ozone. The different catalytic activity might be attributed to the different copper oxides formed at different calcination temperature. Thus characteristics of Cu/Al2O3 prepared at different calcination temperature (100, 600 and 1000 oC) were observed by SEM-EDS and XRD. The results are shown in Supporting Information (Section 2.1.2, Figure S1 and Figure S2). As a result, the strong catalytic activity of Cu/Al2O3 prepared at the calcination 9



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temperature of 600 oC was mainly attributed to three aspects: (i) Plenty of CuO generated at 600 oC shows good performance for catalytic ozonation34-35, but there is few study on the catalytic activity of Cu2O or CuAl2O4 for the decomposition of ozone. (ii) Higher calcination temperature would cause greater degree swelling capacity of Cu/Al2O3. Copper and its oxides would be abscised from the surface of Al2O3 substrate due the different thermal expansion coefficient of Cu (1.8×10-5 K-1), and Al2O3 (7×10-6-1×10-5 K-1)36. Thus only a little of Cu (0.71 wt%) was detected on the surface of Cu/Al2O3 prepared at the calcinations temperature of 1000 oC. The low copper mass loading represents the weak catalytic activity for the decomposition of zone. (iii) Al2O3 could obtain best catalytic activity and stability after calcination near 600 to 800 oC due to the formation of γ-Al2O330, 37-38. In addition, the higher (＞800 C) calcination temperature could cause the formation of α-Al2O3 with less specific

o

area, lower catalytic activity and stability37-38. Therefore, the calcination temperature of 600 oC was chosen as the optimal condition with better crystal form of Al2O3, better Cu oxidation and less Cu abscission. 3.1.3 Effects of the calcination time The effect of the calcination time on catalytic activity of Cu/Al 2O3 for the decomposition of ozonation was investigated thoroughly. The results and discussion are shown in Figure 1(c) and Supporting Information (Section 2.1.3). The results indicated that the calcination time of 2.5 h should be chosen as the optimal condition due to the complete oxidation of Cu0 (to CuO) and less Cu abscission. 3.2 Characteristics of the prepared Cu/Al2O3 3.2.1 SEM and EDS analysis Two kinds of Cu/Al2O3 with the same TMLCu of 0.076 g Cu/g Al2O3 (7.6 wt%) were prepared by electroless plating-calcination and impregnation-calcination, 10


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respectively. Meanwhile their surface morphology, element composition and distribution were observed by SEM and EDS. As shown in Figure 2, raw Al2O3 and Cu/Al2O3 prepared by electroless plating-calcination had similar morphology (smooth and clean surface) under low magnification observation (see Figure 2(a), (b), (e) and (f)). Under high magnification observation (see Figure 2(c), (d), (g) and (h)), however, the dense and flat copper oxides film was observed on the surface of Cu/Al2O3 prepared by electroless plating-calcination. The results suggest that the whole surface of the Al2O3 substrate was wrapped by the thin, dense and flat copper oxides film. Meanwhile, plenty of nano-copper oxides particles were deposited on the surface of this copper oxides film, which would significantly increase the catalytic sites. In contrast, lots of micro-scale copper oxides blocks were heterogeneously and loose distributed on the surface of Al2O3 substrate when Cu/Al2O3 was prepared by impregnation-calcination (see Figure 2 (i)-(l)). Figure 2(i) also shows that some copper oxides blocks have dropped off from Al2O3, which could decrease the actual mass Cu loading. The EDS mapping of Cu/Al2O3 prepared by electroless plating-calcination and impregnation-calcination is shown in Figure S3. In particular, Cu element was distributed uniformly on the surface of Cu/Al2O3 prepared by electroless plating-calcination (Figure S3 (c)), while Cu element was distributed heterogeneously on the surface of Cu/Al2O3 prepared by impregnation-calcination (Figure S3 (g)). The results were similar to the SEM images (Figure S3 (a) and (e)). Meanwhile, Al element was also uniform on the surface of Cu/Al2O3 prepared by electroless plating-calcination, while it was not detected at the Cu depositing position on the surface of Cu/Al2O3 prepared by impregnation-calcination. This result suggests that the copper oxides film of the former was so thin that Al element below the film can be 11



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detected by EDS, while the copper oxides blocks of the latter were so thick that Al element cannot be detected. According to the analysis of SEM and EDS, it can be concluded that the dense, thin, uniform and flat copper oxides film was formed on the surface of Cu/Al2O3 prepared by electroless plating-calcination. However, only plenty of micro-scale copper oxides blocks were heterogeneously and loose distributed on the surface of Cu/Al2O3 prepared by impregnation-calcination. Subsequently, the contact of copper oxides (thin, dense, uniform and flat film) of the former was much higher than that (micro-scale blocks) of the latter. In other words, the number of catalytic sites of the former was much more than that of the latter. Therefore, the catalytic activity of the former for the decomposition of ozone would be much stronger than that of the latter. 3.2.2 BET analysis The BET analysis and discussion are shown in Supporting Information (Section 1.1, Figure S4 and Table S1). The results confirmed that many nano-copper particles formed on the surface of dense and flat copper oxides film when Cu/Al 2O3 was prepared by electroless plating-calcination. Meanwhile, copper oxides deposited in the

micropores

of

Al2O3

when

the

Cu/Al2O3

was

prepared

by

impregnation-calcination. 3.2.3 XRD analysis On the basis of above analysis, the two kinds of Cu/Al2O3 were further analyzed by using XRD. As shown in Figure 3(a) and (c), this two kinds of Cu/Al2O3 had same compounds composition. The crystallines including CuO and Al2O3 (mainly of γ- Al2O3) were detected on them by XRD. This result indicated that high catalytic activity CuO and Al2O3 could also be obtained prepared by electroless plating-calcination on Cu/Al2O3, which were similar with that on Cu/Al2O3 prepared 12


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by traditional impregnation-calcination. In addition, according to the full width at the half-maximum of the diffraction peak (characteristic peaks of CuO at 35.6o and 38.7 o

), the average crystallite size of CuO deposited on the Al2O3 by different methods

can be calculated by using the Scherrer equation39:

Dhkl =

Kλ

(2)

βhkl cos𝜃hkl

where Dhkl is the crystallite size perpendicular to the normal line of the (hkl) plane, K is a constant, βhkl is the full width at half maximum (fwhm) of the (hkl) diffraction peak, θhkl is the Bragg angle of the (hkl) peak, and λ is the X-ray wavelength. The average

crystallite

size

of

CuO

on

Cu/Al2O3

prepared

by

electroless

plating-calcination is 8.6 nm, and that of CuO on Cu/Al2O3 prepared by impregnation-calcination is 30.3 nm. The smaller crystallite size could increase the active sites and enlarge the effective contact area between Cu/Al2O3 and the materials in liquid phase (e.g., O3, PNP and its intermediates). Therefore, the catalytic activity of Cu/Al2O3 prepared by electroless plating-calcination was much higher than that prepared by impregnation-calcination. 3.2.4 XPS analysis Details pertaining to XPS analysis are shown in Section 2.2.1 and Figure S5 in Supporting Information. The conclusions could be summarized as follows: (i) the uniform and dense copper oxides film on the surface of Cu/Al2O3 prepared by electroless plating-calcination was very thin, which not only had high catalytic, but also only needed a little copper consumption for catalyst preparation. (ii) Only Cu(II) existed in both of these two kinds of Cu/Al2O3, which were also confirmed by XRD analysis (Figure 3 (a) and (c)). (iii) Only high catalytic activity CuO was formed when Cu/Al2O3 was prepared by electroless plating-calcination, while some by-products (e.g., copper hydroxides, copper peroxides and other copper compounds) 13



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with low catalytic activity or easy to dissolve was formed in the subsequent calcination when Cu/Al2O3 was prepared by impregnation-calcination 3.3. Advantage of the Cu/Al2O3 prepared by electroless plating-calcination To evaluate the advantage of Cu/Al2O3 prepared by electroless plating-calcination, five control experiments, i.e., (i) O3 alone, (ii) O3+raw Al2O3, (iii) O3+Al2O3 prepared by calcination (600 oC), (iv) O3+Cu/Al2O3 prepared by electroless plating without calcination, (v) O3+Cu/Al2O3 prepared by impregnation-calcination, were setup to treat 500 mg/L PNP in aqueous solution under the same conditions. The results and discussion about COD removal and pH of treatment effluent are shown in Section 2.3 in Supporting Information and Figure 4. The highest COD removal efficiency and rapid decrease and increase of pH during the whole treatment process in O3+Cu/Al2O3 system confirmed the advantage of Cu/Al2O3 prepared by impregnation-calcination. As a conclusion, the new catalyst prepared by electroless plating-calcination is a promising material for catalytic ozonation. 3.4 Stability and operation life of the Cu/Al2O3 Figure 5 shows that two different kinds of Cu/Al2O3 were used to remove 500 mg/L PNP in aqueous solution for 5 times under the same conditions, respectively. In particular, COD removal obtained by O3+Cu/Al2O3 prepared by electroless plating-calcination decreased from 77.2% to 71.7%, while that of O3+Cu/Al2O3 prepared by impregnation-calcination system decreased from 70.0% to 61.6% after 5 times used of the catalysts. In addition, COD removal of the former began to become steady after 3 times running, while the decreasing trend of the latter did not slow down. The results indicate that the stability and operational life of Cu/Al2O3 prepared by the new method was much stronger and longer than that prepared by the conventional method. 14


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The denaturation (inactivation) and wastage of Cu/Al2O3 would result in the decline of the catalytic activity for ozonation, which would inhibit the COD removal efficiency of wastewaters38,

40-41

. Thus, changes of the characteristics of the two

different kinds of Cu/Al2O3 after recycled 5 times were observed by using SEM and XRD. Figure 6 (a) and (b) show that after 5 recycling treatment of the 500 mg/L PNP aqueous solution, copper oxides (CuO) film on the surface of Cu/Al2O3 prepared by electroless plating-calcination had no obvious difference compared with that before employment. However, as shown in Figure 6(c) and (d), it is clear that plenty of copper oxides blocks on Cu/Al2O3 prepared by impregnation-calcination were dropped off after 5 recycling treatment. The results suggest that the adhesion between CuO coating and Al2O3 substrate of the Cu/Al2O3 prepared by electroless plating-calcination was much stronger than that of the Cu/Al2O3 prepared by impregnation-calcination. Furthermore, the high adhesion was mainly attributed to the electroless plating technology26. Finally, the stability and operational life of Cu/Al2O3 was mainly attributed to the high adhesion between CuO coating and Al2O3 substrate. In addition, Figure 3 shows that XRD spectra of the two different kinds of Cu/Al2O3 do not change obviously after 5 times recycling employments. The results suggest that the chemical characteristics of these Cu/Al2O3 do not change obviously. Therefore, the low stability and short operational life of Cu/Al2O3 was mainly attributed to the rapid abscission of CuO, which would obviously decrease the number of catalytic activity sites of Cu/Al2O3. 3.5. Proposed reaction pathway for the destruction of PNP 3.5.1 UV-Vis spectral analysis The UV-vis spectra of PNP aqueous solution during the degradation process by O3+Cu/Al2O3 prepared by electroless plating-calcination are shown in Figure S7, and 15



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the discussion is shown in Section 2.4.1 in Supporting Information. The results indicate that benzene ring and -NO2 of PNP in aqueous solution were decomposed completely after only 10 min treatment. In addition, the intermediates (e.g., low-molecular-weight organic acids) could be further degraded in the subsequent 50 min treatment. 3.5.2 Intermediates of PNP degradation In literature, there are plenty of discussions on the degradation mechanism of PNP by ozonation42-46. In particular, the benzene ring structure of the oxidation products of PNP (i.e., HC, BK, ect.) would be further oxidized and the benzene ring would be opened with the generation of small molecular organics (e.g., oxalic acid, maleic acid, acetic acid, etc.). The small molecular organics would be further degraded into CO2 and H2O by O3+catalyst systems. However, it is difficult for O3 alone system to further degrade small molecular organics. The degradation intermediates detected in this study were BK, HC, oxalic acid, maleic acid and acetic acid. The concentration variation of each intermediate and the residual PNP during the 60 min treatment process by O3+Cu/Al2O3 prepared by electroless plating-calcination and O3 alone is presented in Figure 7. It can be seen that PNP had been removed absolutely in the initial 10 min treatment process by ozonation with or without catalyst. Meanwhile, the oxidation product (i.e., HC and BK) was rapidly increased to the maximum at 5-10 min, and then it began to decrease gradually in the following treatment process in both of these two systems. However, the maximum of HC (10.0 mg/L at 5 min) and BK (9.9 mg/L at 10 min) of O3+Cu/Al2O3 was lower than that of O3 alone (HC of 18.8 mg/L at 5 min, and HC of 14.6 mg/L at 5 min). The results suggest that there was a same degradation pathway of denitration and benzene ring opening reaction for PNP in the two treatment system. 16


Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(i.e., O3+Cu/Al2O3 and O3 alone). In other words, this degradation step was not affected by the addition of catalyst. It is also reported in literature that benzene ring could be opened by ozone without catalyst42, 46. In addition, the lower accumulation of HC and BK in the O3+Cu/Al2O3 treatment system suggests that the catalyst could accelerate the rate of this step due to the generated ROS from the catalytic ozonation. Figure 7(b) shows that during the 60 min treatment by O3+Cu/Al2O3 system, the concentration of oxalic acid (88.6 mg/L), acetic acid (33.7 mg/L) and maleic acid (10.8 mg/L) increased to the maximum, and then they gradually reduced to 11.5, 21.1 and 0 mg/L, respectively. However, plenty of oxalic acid (135.8 mg/L) and acetic acid (42.1 mg/L) were accumulated after 60 min treatment by ozone alone (Figure 7(d)). The results could confirm the different effluent pH of O3+Cu/Al2O3 (pH of 6.7) and O3 alone (pH of 2.9) after 60 min treatment (Figure 4(d)). In other words, the generated organic acids only could be further degraded by the generated ROS from the ozonation with high activity catalyst. Therefore, COD removal obtained by Cu/Al2O3 (93.4%) was much higher than that of O3 alone (77.8%) after 60 min treatment (see Figure 4(a)). Finally, according to the measured intermediates (i.e., BK, HC, oxalic acid, maleic acid and acetic acid), the main degradation pathway of PNP by O3+Cu/Al2O3 is proposed in Figure 8. This results also confirmed that Cu/Al2O3 prepared by electroless plating-calcination system showed a good performance for catalytic ozonation.

4. Conclusions In this study, the catalytic activity for ozonation of Cu/Al2O3 was strengthened by electroless plating-calcination process, and the characteristics of Cu/Al2O3 and treatment efficiency of the catalystic ozonation systems were investigated thoroughly.. 17



Page 18 of 35

The optimal key preparation conditions (i.e., plating time=10 min, calcination temperature=600 oC and calcination time=2.5 h) were obtained in this study. Furthermore, characteristics of two different kinds of Cu/Al2O3 (i.e., prepared by new electroless plating-calcination and conventional impregnation-calcination) were observed by SEM, EDS, XRD, XPS and BET. The result indicated that Cu/Al2O3 prepared by electroless plating-calcination has thin, flat, uniform and dense copper oxides film deposited on the surface of Al2O3 substrate, while Cu/Al2O3 prepared by impregnation-calcination has heterogeneous and loose copper oxides blocks. In addition, the catalytic activity, stability and operational life of Cu/Al2O3 prepared by electroless plating-calcination were much better than those of Cu/Al2O3 prepared by conventional impregnation-calcination. High performance of Cu/Al2O3 was mainly attributed to the strong adhesion between CuO coating and Al2O3 substrate, and it was mainly strengthen by the electroless plating technology. In particular, COD removal obtained by O3+Cu/Al2O3 prepared by electroless plating-calcination (71.7%) was still much higher than that of O3+Cu/Al2O3 prepared by impregnation-calcination (61.6%) after 5 times recycling running. According to the SEM images, it can found that

the

decrease

of

catalytic

activity

of

Cu/Al2O3

prepared

by

impregnation-calcination was mainly resulted from the rapid abscission of the loose CuO blocks. Meanwhile, the mechanism of the catalytic ozonation of the new system was investigated according to the free radical quenching experiment. Finally, the PNP degradation pathway was proposed according to the intermediates measured by HPLC and UV-vis. The results suggest that PNP was first oxidized to the benzenes intermediates (i.e., BK, HC, etc.), then to low-molecular-weight organic acids (i.e. oxalic acid, maleic acid, acetic acid, etc.), and finally to CO2 and H2O by the catalytic ozonation with Cu/Al2O3 prepared by electroless plating-calcination. 18


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Therefore, it can be concluded that the developed electroless plating-calcination in this study was much better than the conventional impregnation-calcination for Cu/Al2O3 preparation, and this new Cu/Al2O3 should be used as a promising catalyst for the catalytic ozonation in the field of wastewater treatment.

Supporting Information Specific surface area of different materials (Table S1); SEM-EDS of Cu/Al2O3 prepared by electroless plating-calcination with different calcination temperature (Figure S1); XRD patterns of Cu/Al2O3 prepared by electroless plating-calcination with different calcination temperature (Figure S2); EDS mapping imaging of Cu/Al2O3 prepared by electroless plating-calcination and impregnation-calcination (Figure S3); N2 adsorption and desorption isotherms of Cu/Al2O3 prepared by impregnation-calcination, Cu/Al2O3 prepared by electroless plating-calcination and Al2O3 (Figure S4); XPS spectra of Cu/Al2O3 prepared by electroless plating-calcination and Cu/Al2O3 prepared by impregnation-calcination (Figure S5); Quenching experiment of O3+ Cu/Al2O3 prepared by impregnation-calcination (Figure S6); UV spectra of aqueous solution during treatment process with O3+optimal Cu/Al2O3 prepared by electroless plating-calcination (Figure S7); TPR spectrum of Cu/Al2O3 prepared by electroless plating-calcination (Figure S8); PNP adsorption isotherms of Cu/Al2O3 prepared by impregnation-calcination, Cu/Al2O3 prepared by electroless plating-calcination and raw Al2O3 (Figure S9)

Acknowledgements The authors would like to acknowledge the financial support from Science and 19



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Technology Project of Sichuan Province (2016JY0154), and Fundamental Research Funds for the Central Universities (No. 2015SCU04A09).

20


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80

(a)

COD removal (%)

60

50

(b)

70

10 min 20 min

70

COD removal (%)

80

[PNP]0=500 mg/L

Catalyst dosage=10 g/L

Initial pH =9.0

Gas flow rate (O2)=300 mL/min

10 min 20 min

60

50 [PNP]0=500 mg/L

Calcination time=2 h Calcination temprature=700 º C

40 40

0

5

10

15

20

25

80

Gas flow rate 2(O )=300 mL/min

Plating time=10 min

Calcination time=2 h

0

200

COD removal (%)

10 min 20 min

60

-1

0

[PNP]0=500 mg/L


Initial pH =9.0

Gas flow rate (O )=300 mL/min 2

Plating time=10 min

Calcination tempreture=700 ºC

1

2

3

600

800

1000

(d)

80

50

400

Calcination tempreture (ºC)

(c)

70


Initial pH =9.0

Plating time (min)

COD removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

5

6

7

10 min 20 min

70

60

50

40

2

[PNP]0=500 mg/L


Calcination time=2.5 h

Gas flow rate (O )=300 mL/min 2

Plating time=10 min

Calcination tempreture=700 ºC

3

4

Calcination time (h)

5

6

7

8

9

10

11

12

Initial pH

Figure 1. Effect of (a) plating time, (b) calcination temperature, (c) calcination time, and (d) initial pH on the catalytic activity of Cu/Al2O3 prepared by electroless plating-calcination. (Catalytic activity was evaluated indirectly according to the obtained COD removal for the PNP aqueous solution by the catalytic ozonation with the prepared Cu/Al2O3.)

27



(a)

(b)

(e)

(f)

Page 28 of 35

(i)

(j)

(c)

(g)

(k)

(d)

(h)

(l)

Figure 2. SEM imaging of (a-d) raw Al2O3, (e-h) Cu/Al2O3 prepared by electroless plating-calcination and (i-l) Cu/Al2O3 prepared by impregnation-calcination (calcinations temperature=600 oC, calcination time=2.5 h and plating time=10 min).

28


Page 29 of 35

(d) (c)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) (a)

Al2O3 (46-1215) Al2O3 (46-1212) CuO (48-1548)

10

20

30

40

50

60

70

80

90

2-Theta (Deg.) Figure 3. XRD patterns of (a) Cu/Al2O3 prepared by electroless plating-calcination, (b) Cu/Al2O3 prepared by electroless plating-calcination after five employments, (c) Cu/Al2O3 prepared by impregnation-calcination, (d) Cu/Al2O3 prepared by impregnation-calcination after five employments (conditions same as those in Figure 2).

29



100

(a)

COD removal (%)

80 60 40 20 0 0

10

20

30

40

50

60

Treatment time (min)

0.14

(b)

(c) A: O

B: O3+raw Al2O3

3

0.0

C: O3+Al2O3 prepared by calcination

0.12

D: O3+Cu/Al2O3 prepared by electroless plating E: O3+Cu/Al2O3 prepared by impregnation-calcination

kobs value (min-1)

ln(COD/COD0)

0.10

-0.5

-1.0

F: O3+Cu/Al2O3 preparedby electroless plating-calcination

0.077

0.08 0.06

0.048

0.054

0.060

0.058

C

D

0.063

0.04 0.02

-1.5

0.00

0

5

10

15

20

A

B

E

F

Treatment time (min) 10 9

(d)

O3 (Kobs=0.048 R2=0.96) (Kobs=0.054 R2=0.94)

7

O3+Al2O3 prepared by calcination

6

(Kobs=0.060 R2=0.95)

5

(Kobs=0.058 R2=0.095)

-0.5

O3+Cu/Al2O3 prepared by electroless plating -1.0

O3+Cu/Al2O3 prepraed by impregnation-calcination

4

(Kobs=0.063 R2=0.94)

3 2

0.0

O3+raw Al2O3

8

pH value

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

O3+Cu/Al2O3 prepared by electroless plating-calcination

0

10

20

30

40

50

60

(Kobs=0.077 R2=0.96)

0


Figure 4. (a) COD removal efficiency, (b) COD removal efficiency described by pseudo-first-order equation, (c) kobs for COD removal and (d) pH value during treatment process with different catalysts ([PNP]0=500 mg/L, catalyst dosage=10 g/L, initial pH=9.0 and gas flow rate (O2)=300 mg/L).

30


-1.5 5

Page 31 of 35

80

75

COD removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70

65

60

O3+Cu/Al2O3 prepared by electroless plating-calcination O3+Cu/Al2O3 prepared by impregnation-calcination

55

1

2

3

4

5

Number of cycles for catalyst Figure

5.

Operational

life

of

the

catalyst

(Cu/Al2O3

prepared

by

electroless

plating-calcination and Cu/Al2O3 prepared by impregnation-calcination) ([PNP]0=500 mg/L, catalyst dosage=10 g/L, initial pH=9.0, gas flow rate (O2)=300 mg/L and treatment time=20 min).

31



(a)

(b)

(c)

(d)

Page 32 of 35

Figure 6. SEM imaging of (a) Cu/Al2O3 prepared by electroless plating-calcination, (b) Cu/Al2O3 prepared by electroless plating-calcination after five employments, (c) Cu/Al2O3 prepared

by

impregnation-calcination

and

(d)

Cu/Al2O3

prepared

by

impregnation-calcination after five employments.

32


Page 33 of 35

(a)

6

(b)

400

80

300

PNP

200 100 0

4

0

10

20

30

40

50

60


2

HC BK

Concentration (mg/L)

8


100

500 Concentration (mg/L)

10

0

Oxalic acid Maleic acid Acetic acid

60

40

20

0

0

10

20

30

40

50

60

0

10


(c)

500

16 14 12 10

30

40

50

60

140

(d)

400

120

300

PNP

200 100 0

8

20


0

6

10

20

30

40

50

60


HC BK

4 2


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Oxalic acid Maleic acid Acetic acid

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Figure 7. PNP and its intermediates in the aqueous solution by (a and b) O3+Cu/Al2O3 prepared by electroless plating-calcination system and (c and d) O3 alone system (conditions same as those in Figure 5).

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Figure 8. Proposed reaction pathway for the degradation of PNP by O3+Cu/Al2O3 prepared by electroless plating-calcination system.

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Al2O3 by

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