ZrGP nanocomposites with enhanced catalytic activity for catalytic reduction of 4-nitrophenol

Journal Pre-proofs Full Length Article Preparation of Ag/ZrGP nanocomposites with enhanced catalytic activity for catalytic reduction of 4-nitrophenol Annan Zhou, Jinzhou Li, Guohui Wang, Qinghong Xu PII: DOI: Reference:

S0169-4332(19)33386-0 https://doi.org/10.1016/j.apsusc.2019.144570 APSUSC 144570

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

8 June 2019 30 October 2019 31 October 2019

Please cite this article as: A. Zhou, J. Li, G. Wang, Q. Xu, Preparation of Ag/ZrGP nanocomposites with enhanced catalytic activity for catalytic reduction of 4-nitrophenol, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.144570

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Preparation of Ag/ZrGP nanocomposites with enhanced catalytic activity for catalytic reduction of 4-nitrophenol

Annan Zhou, Jinzhou Li, Guohui Wang, Qinghong Xu*

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

Corresponding author: Tel.: +86-10-64425037; Fax: +86-10-64425385. Email address: [email protected]

1

Abstract Silver nanoparticles (Ag NPs), generated in situ by silver nitrate reduction under some conditions, were supported on Zirconium Glyphosate (ZrGP, a kind of layered zirconium phosphonate). It was found that the Ag NPs were dispersed on outside and inner surface of ZrGP nanosheets uniformly. The Ag/ZrGP composite was found to have excellent catalytic efficiency to the reduction of 4-nitrophenol by using NaBH4 as a reducing agent, and more than 99% of 4-nitrophenol was reduced to 4-aminophenol within 10 min. The composite could remain more than 95% activity after it was used seven times Keywords: Zirconium Glyphosate; silver nanoparticles; 4-nitrophenol reduction

2

1. Introduction 4-nitrophenol (4-NP) is a kind of toxic pollutant, which could result in some diseases (such as inflammation, allergies, cyanosis, etc) after it is dissolved in blood [1]. However, 4-NP can be reduced to 4-aminophenol (4-AP), a valuable chemical for producing some drugs, photographic developer, corrosion inhibitor, dyeing agent and so on [2-4]. Thus, removing 4-NP from environment, for example in water, and reusing it again is indispensable. Metal nanoparticles, especially Ag nanoparticles (Ag NPs), have attracted great interest for their excellent catalytic performances in the past decades [5-9]. Many reactions, including the reduction from 4-NP to 4-AP, can be catalyzed by Ag NPs [10,11]. But the catalytic activities of Ag NPs are influenced by size, shape and dispersion, for exposure of active sites of the catalysts increases with the decrease of particle size. However, Ag NPs with small scale are easily aggregated into large scale due to their higher surface area and surface energy, which prevents the use of Ag NPs [9]. Therefore, some improvements have already been taken to solve the problem. Dispersing Ag NPs on different supports to form various composites is one of applicable

strategies,

such

as

Ag/(carbon

materials),

Ag/(metallic

oxide),

Ag/(nonmetallic oxide), Ag/(metal hydroxide) and Ag/(layer double hydroxide materials) [12-17]. With the supports of the carrier, Ag NPs get highly desperation and more active centers are exposed in reaction. The composites have been successfully used for reduction of 4-NP. In the past years, multifunctional nanocomposite has attracted considerable attention in the field of nanotechnology for their potential applications [18,19]. Among them, Zirconium glyphosate [Zr(O3PCH2NHCH2COOH)2·0.5H2O, denoted as ZrGP], a layered compound with long lamella morphology, was reported in our earlier study. Many advantages of ZrGP were also reported, such as high surface area, strong coordinative ability with metallic ions, applications as support to metallocene catalyst in ethylene polymerization and as support to immobilize lipase [20-23]. In this work, a new kind of composite material composed of Ag and ZrGP was successfully synthesized. ZrGP nanosheets were introduced as carrier, and Ag/ZrGP 3

composites were prepared by mixing AgNO3 and ZrGP under magnetic stirring. Ag+ ions in the composite were reduced to Ag NPs. The catalytic properties of the obtained Ag/ZrGP composites were investigated by the reduction of 4-NP to 4-AP by NaBH4 as a model reaction, and the Ag/ZrGP composites showed excellent catalytic activity. 2. Materials and methods 2.1 Materials and characterizations All chemicals used are of analytical reagent grade available from a commercial supplier without further purification. AgNO3 (≥99%), NaBH4 (≥99%), 4-nitrophenol (≥99%), HF (40%) and sodium citrate (Na3C6H5O7·2H2O, ≥99%) were purchased from Beijing Chemical Works. Glyphosate (≥95%) was bought from Heibei Qifeng Chemical Engineering Company. ZrOCl2·8H2O (≥98%) was purchased from Sinopharm Group Chemical Reagent Co. Ltd. Nicolet 8700 Fourier transform infrared spectrometer (FTIR, Thermo Electron, USA) was used to characterize the functional groups in the samples. FTIR spectra were acquired at a resolution of 4 cm–1 in the wave number ranging from 4000 cm-1 to 400 cm-1. The changes on ultraviolet absorption of the catalyzed solution were obtained on Shimadzu UV-3600 ultraviolet and visible spectrophotometer (UV-vis, Shimadzu, Japan) in range of 250-500 cm-1. Powder X-ray diffraction (PXRD) data were collected on a DX-1000 X-ray diffractometer (XRD, Bruker D8 Advance, Germany), using Cu Ka X-ray source (40 kV and 40 mA) between 3° and 90° with a scanning rate of 10° min-1. HT-7700 transmission electron microscopy (TEM, Hitachi, Japan) and J-3010 high resolution transmission electron microscopy (HRTEM, Hitachi, Japan) were used to observe the morphology of the synthesized products. X-ray photoelectron spectroscopy (XPS) of the samples was collected using a Kratos Axis Supra (XPS, Shimadzu, Japan). The calibration peak is C1s at 284.6 eV. Surface areas of the samples were obtained by the Brunauer−Emmett−Teller (BET) method and calculated using the Barrett−Joyner−Halenda (BJH) model. The photographs of 4

the samples were taken by a LYA-AL10 (Digital mobile phone, Huawei, China). 2.2 Preparation of Ag/ZrGP composites Ag/ZrGP composites were prepared in two methods. The first method. 30 mL of ZrOCl2·8H2O solution (0.3 M) and 4 mL HF (40%) was added into a 400 mL mixture that contained Glyphosate (0.05 M) and deionized water. The mixture was heated at 70 °C for 8 h under a continuous stirring. The white product was then obtained by centrifuging the mixture at 5000 r/min for several times and dried at 70 °C for 12 h. 100 mL of a mixture containing 0.44 g of ZrGP, AgNO3 (0.05 M) and deionized water was heated at 80 °C for 6 h under continuous stirring. The mixture was then centrifuged and the solid components were dispersed in a flask containing 100 mL deionized water. After the above mixture was heated to 98 °C, 100 mL of sodium citrate (0.05 M) solution was added into the mixture drop by drop. All the steps above were protected by nitrogen gas. After the sodium citrate solution was added, the mixture was then centrifuged at 5000 r/min for several times and the solid product (noted as S1) was dried at 70 °C for 12 h. The second method. 30 mL of ZrOCl2·8H2O solution (0.3 M) and 4 ml of HF (40%) were mixed with 1.52 g of AgNO3. The mixture was then centrifuged at 5000 r/min and the supernatant was reserved to eliminate chloride ions. A mixture (400 mL), containing Glyphosate (0.05 M), AgNO3 (0.1 M) and deionized water, was heated at 70 °C for 6 h under a continuous stirring and the above Zr4+ solution was then added to obtain sol production. After solid component from the above mixture by centrifugation (5000 r/min) was dispersed into 100 mL deionized water and heated to 98 °C, 100 mL solution of Na3C6H5O7·2H2O (0.5 M) was added to realize reduction of Ag+ in the solid. The reaction was protected under nitrogen gas and stirred continuously. The Ag/ ZrGP solid composite (noted as S2) was finally obtained by centrifuging at 5000 r/min and drying (at 70 °C for 12 h). 2.3 Tests on catalytic activity and stability of Ag/ ZrGP composites Catalytic activities of the synthesized Ag/ ZrGP composites were evaluated by reduction of 4-NP to 4-AP using NaBH4 as reducing agent at 25 °C. 2.5 mL of 4-NP 5

solution (0.1 mmol/L) and 0.5 mL of freshly prepared NaBH4 solution (0.5 M) were mixed in a standard quartz cuvette. Subsequently, 10 mg of Ag/ZrGP nanocomposites was added to start the reaction. Conversion rate of 4-NP was checked with UV-Vis absorption spectra from 250 nm to 500 nm. In-situ FTIR analysis was used to verify the findings during the reaction. As a comparison, UV-Vis absorption spectra (from 250 nm to 500 nm) were collected with 1 mg pure Ag NPs or 10 mg pure ZrGP as catalysts. The synthesis of pure Ag NPs was according to the reference [24]. In order to find the influences of different factors on the catalytic activity, the UV-Vis absorption spectra (from 250 nm to 500 nm) were obtained under different operating conditions with S2 as catalysts, including the amount of the catalysts added (from 2.5 mg to 12.5 mg) and reaction temperature (from 25 °C to 45 °C). The catalytic activity stability of Ag/ZrGP was investigated by monitoring the successive cycles of the reduction reactions. 10 mL of NaBH4 solution (freshly in water, 0.5 M), 100 mL of 4-NP (0.1 mmol/L) water solution and 10 mg of Ag/ZrGP composites were mixed together under stirring. After 10 min, 3 mL of the mixture was taken out to measure its UV-Vis absorption at 400 nm [21]. Meanwhile, 1 mL of 4-NP (0.1 mmol/L) solution and 2 mL of NaBH4 (0.5 M) solution were added for another catalytic cycle. The process was repeated for 10 times. 3. Results and discussion 3.1 Characterization on Ag/ ZrGP composites FTIR spectra of S1 and S2 are shown in Fig. 1. Difference is hardly found between the two products. The absorptions of νC−H, νC=O, δ−NH, ν−CH2 and νP−O−Zr in the samples are found at 2924 cm−1 (νC−H), 2854 cm−1 (νC−H), 1733 cm−1 (νC=O), 1632 cm−1 (δ−NH), 1247 cm−1 (ν−CH2), 1485 cm−1 (ν−CH2), 996 cm−1 (νP−O−Zr), 1069 cm−1 (νP−O−Zr), and 1154 cm−1 (νP−O−Zr) respectively, identifying with the spectrum of ZrGP reported in the literature [18]. EDS analysis (in Fig. 2) indicates that the atomic ratios of P to Zr in S1 and S2 are all about 2:1, similar to the results in ref 20. Also the weight percentages of Ag in S1 and S2 are about 0.91% and 1.51% respectively from the 6

figure. (Position of Fig. 1) (Position of Fig. 2) XRD patterns of S1 and S2 with different synthesis process are shown in Fig. 3. Both the samples have strong diffractions at 5.37°, which well matched the main principal diffraction plane (002) of ZrGP from ref 18. For the low contents and high dispersion of Ag in S1 and S2, the diffraction intensities of Ag NPs in the two materials are too weak to be observed. The interlayer spacings are calculated to be 1.64 nm, 1.93 nm and 2.02 nm for ZrGP, S1 and S2 according to the XRD results. (Position of Fig. 3) The BET analysis shows that the surface area of the pure ZrGP is about 10.46 m²/g, while the surface area of S1 and S2 is about 23.66 m²/g or 28.42 m²/g, respectively. Compared to pure ZrGP, the surface area of S1 and S2 has increased obviously. While the surface area of S2 has more increment than S1, which indicates that S2 has more content of Ag NPs. TEM image indicates that the morphology of ZrGP is manifested as long sheets (Fig. 4A) and Ag NPs were found to be aggregated and adhered inhomogeneously on the surface of ZrGP nanosheets with an average diameter about 20 nm (product S1, Fig. 4B), for most of Ag+ was aborted and aggregated on outside surface of the nanosheets and a little entered the interlayer. When sodium citrate was added, the Ag+ ions in the aggregates were reduced and nanoparticles formed. However, when Ag+ ions were firstly coordinated with N and O elements in glyphosates (noted as Gly) and Ag+/Gly reacted with ZrOCl2 to form product S2, Ag+ ions distributed on the inner board evenly and were reduced in-situ. TEM image of S2 indicates that the Ag NPs with an average diameter about 3 nm (some Ag+ ions in the solution also entered into the interlayer and were absorbed by the organic chains to form the nanoparticles) were formed and dispersed on the inner board of ZrGP homogeneously (Fig. 4C). Due to the interactions (including chemical and physical interactions) between the Ag+ ions and the inner board of layered material, more Ag NPs in S2 were found. Meanwhile, the HRTEM image shown in Fig. 4D suggests that the ZrGP nanosheets contain of 7

small sliver nanoparticles. The attached silver nanoparticle has a lattice distance of 0.237 nm, corresponding to the (111) crystal plane of metallic silver [24]. Schematic diagram on synthesis of the product S1 and S2 is shown in Scheme 1. (Position of Fig. 4) (Position of Scheme 1) The typical results of X-ray photoelectron spectroscopy (XPS) of N1s and O1s obtained in ZrGP, Ag/ZrGP (S2) and the intermediate product Ag+/ZrGP (S2) are shown in Fig. 5. Compared to the binding energies of N1s and O1s in ZrGP, the change of NN-H (from 400.4 eV to 400.6 eV) and NN-COOH (from 402.2 eV to 402.6 eV) was clearly observed in the presence of Ag+/ZrGP, and the binding energies of OO=C (from 531.4 eV to 531.6 eV) and OO-H (from 532.1 eV to 532.6 eV) had similar results as that of N1s in Ag+/ZrGP. Meanwhile, the binding energy of Ag3d was decreased by about 0.2 eV in Ag+/ZrGP than that in AgNO3 (shown in Fig. 5 and Table 1). It proves that there exist actions among Ag, O and N elements in Ag+/ZrGP. The coordination between Ag+ and C=O and =NH groups should be happened in the layers. (Position of Fig. 5) (Position of Table 1) 3.2 Catalytic activities of the synthesized products in reduction reaction of 4-nitrophenol The reduction reaction of 4-NP to 4-AP under the existence of NaBH4 and synthesized

Ag/ZrGP

composites

as

catalysts

was

studied,

and

UV-vis

spectrophotometer was used to check the conversion rate of 4-NP in range of 250-500 cm-1. Typically, the original absorption peak of 4-NP shifted from 317 nm to 400 nm due to the formation of 4-NP ions upon the addition of freshly prepared NaBH4 solution [21]. The absorption intensities of 4-NP at 400 nm were found decreasing and the absorption intensities of 4-AP were found increasing gradually when the reduction proceeded after addition of Ag/ZrGP composites (shown in Fig. 6A and 6B), and 4-NP was transformed into 4-AP with color of the solution turned from deep yellow to colorless rapidly (Fig. 6D). The reduction reactions were nearly completed 8

after 16 min and 10 min for S1 and S2, respectively. The in-situ FTIR analysis was used to verify the results of UV-Vis [25]. The attenuation of νsNO2 (found at 1637 cm-1) and enhancement of δ−NH2 (found at 1370 cm-1) indicates that the NO2 groups were transformed to NH2 groups under the catalysis of S2 (Fig. 7), which supports the results in Fig.6. It is obviously that the catalyst of S2, with Ag NPs distributed homogeneously, smaller size and higher content, has higher catalytic activity in the reduction reaction. (Position of Fig. 6) (Position of Fig. 7) The concentration of NaBH4 in the reactive system was in large excess to 4-NP, and it could be considered as a constant during the reaction period. So the reduction rate can be evaluated by the pseudo-first-order kinetics with respect to 4-NP. Fig. 6C shows ln(At/A0) (At and A0 correspond to the absorbance of 4-NP at 400 nm at time t and the beginning) versus t (reaction time) [26]. Apparently, they display an almost linear evolution, slopes of which give the rate constants (k). ln [At/A0] = kt The rate constant k of catalytic activity is calculated to be 0.0959 min-1 and 0.1750 min-1 for S1 and S2, respectively. However, k is independent of the amount of catalysts. To evaluate the catalytic activity more objectively, another key indicator, the normalized rate constant K’ is defined as the activity factor k over the weight of catalysts (K’ = k/m). K’ are 1053.84 min-1 g-1 and 1158.94 min-1 g-1 for S1 and S2, which are much higher than those of Ag NPs deposited on the surface of micron silica spheres (37.16 min-1 g-1) and Ag/Ni(OH)2 (841.61 min-1 g-1) compared to the previous findings in literature for the silver nanoparticles [15,27]. The effect of catalyst dosage was studied and the result is shown in Fig. 8A. Certain amount of S2 (from 0 mg to 12.5 mg) was added to the reaction system, and the absorbance was measured after 10 minutes. The absorbance at 400 nm was found decreasing when the catalyst dosage was increased from 0 mg to 10 mg. While a further increase of the catalyst (12.5 mg) did not get more decrease of absorbance at 400 nm, which indicates that 4-NP was almost reduced to 4-AP when the catalyst 9

dosage was 10 mg. Thus 10 mg is considered as the best dosage of the catalyst for the reaction. The influence of reaction temperature is shown in Fig. 8B, while catalyst dosage was 10 mg and the UV-Vis spectra were obtained 10 minutes later. The 4-NP was reduced nearly completely at 25 °C. The higher reaction temperature did not show superiority in reaction speed. 25°C is the most suitable condition for this transformation. As a comparison, pure Ag NPs or pure ZrGP was used as a catalyst for the reaction, and the results are shown in Fig. 8C and Fig. 8D, respectively. The spectra show little difference in Fig. 8D, which indicates that ZrGP has no benefits on the reaction. 4-NP was reduced to 4-AP in 8 min with 1mg pure Ag NPs as catalyst (Fig. 8C), which is much more inefficient than S1 and S2. (Position of Fig. 8) 3.3 Catalytic activity stability of S2 in reduction reaction of 4-nitrophenol The stability of catalytic activity of S2 was studied carefully and the results are shown in Fig. 9. The reduction rate of 4-NP was found to be over 95% after the catalyst S2 was recycled 7 times, but it got a remarkable decrease since the eighth time. The conversion rate was calculated as follow: Conversion rate = (A0+At1 -At2)/(A0+At1) Where A0, At1 and At2 represent original absorbance of the system, absorbance of the system in the previous recycle test and absorbance of the system in the next recycle test, respectively. Owing to the existence of oxygen in the system, part of Ag NPs was oxidized gradually and its catalytic activity was decreased step by step. Conversion rate of 4-NP decreased to less than 50% after S2 was reused tenth time, indicating more than half of the active ingredient had been lost. (Position of Fig. 9) 4. Conclusions In this work, a new kind composite of Ag/ZrGP has been developed and Ag NPs 10

with diameter about 3 nm were found to be dispersed on inner boards of the ZrGP uniformly. The obtained Ag/ZrGP composites show the high catalytic activity in reduction reaction of 4-NP under the existence of NaBH4 in a few minutes, and the catalysts can keep high activity (over 95%) after it was used for many times. The new composite (Ag/ZrGP) with high catalytic activity shows a good prospect for industrial application. Notes The authors declare no competing financial interest. Acknowledgments The present work is supported by the NSFC (No. 21521005 and U1362113) and the “Double Leaders” of university teachers in the Ministry of Education of the People’s Republic of China. References [1] Toxicological Profile For Nitrophenols, Agency for Toxic Substances and Disease Registry. U.S. Public Health Service, 1992, July. [2] Y. Li, Y. Cao, J. Xie, D. Jia, H. Qin, Z. Liang, Facile solid-state synthesis of Ag/graphene oxide nanocomposites as highly active and stable catalyst for the reduction of 4-nitrophenol, Catal. Commun. 58 (2015) 21−25. [3] X. Zhang, H. Xiang, Q. Wang, H. Cao, J. Xue, Carbon Encapsulated FeCu4 Alloy Nanoparticles Modified Glass Carbon Electrode to Promote the Degradation for P–Nitrophenol, Integr. Ferroelectr. 147 (2013) 97−102. [4] D.S. Patil, S.A. Pawar, R.S. Devan, M.G. Gang, Y.R. Ma, J.H. Kim, P.S. Patil, Electrochemical

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Figures and Scheme: Fig. 1. The FTIR spectra of S1 and S2 Fig. 2. EDS of Ag/ZrGP composites: S1 (A) and S2 (B) Fig. 3. PXRD pattern of ZrGP, S1, S2. Fig. 4. TEM images of ZrGP (A), S1 (B) and S2 (C); HRTEM image of S2 (D). Fig. 5. The XPS analysis results of N1S (A) and O1s (B) in ZrGP, Ag+/ZrGP and Ag/ZrGP; Ag3d (C) in AgNO3, Ag+/ZrGP and Ag/ZrGP. Fig. 6. UV-Vis spectra of the 4-NP and NaBH4 solution using S1 (A) and S2 (B) as a catalyst; plots of ln (At/A0) versus time for catalytic reduction of 4-NP (C); the photograph of the system before and after the catalytic reaction (D). Fig. 7. The in-situ FTIR spectra of catalytic reaction with S2 as a catalyst. Fig. 8. UV-Vis spectra of the 4-NP and NaBH4 solution: the influence of catalyst dosage (A) and reaction temperature (B) with S2 as a catalyst; with pure Ag NPs as a catalyst (C) and with pure ZrGP as a catalyst (D). Fig. 9. The cyclic stability of Ag/ZrGP. Scheme 1. The synthesis process of S1 and S2.

15

Figures:

Fig. 1

Fig. 2

16

Fig. 3

Fig. 4

17

Fig. 5

Fig. 6

18

Fig. 7

Fig. 8

19

Fig. 9

20

Scheme:

Scheme 1

21

Table: Table 1. Binding energies in the N1s, O1s and Ag3d of the samples. Binding energy (eV)

Sample

N1s

O1s

Ag3d 5/2

Ag3d 3/2

-

-

368.3

374.4

AgNO3 ZrGP

400.4 (N-H)

401.7 (N-C)

402.2 (N-COOH)

531.4 (O-C)

532.1 (O-H)

-

-

Ag+/ZrGP(S2)

400.6

401.7

402.6

531.6

532.6

368.1

374.2

Ag/ZrGP(S2)

401.1

401.9

402.8

531.8

533.0

367.9

374.1

22

Highlights 1. A kind of catalyst with Ag nanoparticles attached in interlayer of ZrGP was synthesized. 2. Ag/ZrGP catalyst showed high catalytic activity (᧺99%) in the reduction of 4-NP to 4-AP. 3. Ag/ZrGP catalyst can remain activity (≥95%) at least 7 times in reuse test.

23

Graphical Abstract

24