Cu3âxNixAl-Layered Double Hydroxide-Reduced Graphene Oxide

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Cu3-xNixAl Layered Double Hydroxide-Reduced Graphene Oxide Nanosheet Array for the Reduction of 4-Nitrophenol Zhuojun Wei, Yangguang Li, Liguang Dou, Muhammad Ahmad, and Hui Zhang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00273 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Nano Materials

Cu3-xNixAl Layered Double Hydroxide-Reduced Graphene Oxide Nanosheet Array for the Reduction of 4-Nitrophenol

Zhuojun Wei, Yangguang Li, Liguang Dou, Muhammad Ahmad, Hui Zhang*

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

*Correspondence should be addressed to Hui Zhang. E-mail: [email protected] Tel.: +8610 64425872 Fax: +8610 64425385

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Abstract A series of bi-transition metals hybrids Cu3-xNixAl-LDH/rGO (x = 2.5, 2.0, 1.5) were synthesized via a facile pre-adjusted pH citric acid-assisted coprecipitation method. Systematic characterizations suggest that the hybrids show honeycomb-like nanosheets array morphology with ultrathin CuNiAl-LDH nanosheets (~72.3×4.2 nm) staggered vertically grown on both sides of rGO substrate. The Cu3-xNixAl-LDH/rGO hybrids present a remarkably superior reduction performance for 4-nitrophenol (4-NP) than pure LDH samples. Particularly, Cu1Ni2Al-LDH/rGO shows the best activity with kapp = 3.4 × 10-2 s-1, ~1.7-fold of Cu1Ni2Al-LDH. It is worth noting that the activity of this bi-transition metals hybrid Cu1Ni2Al-LDH/rGO is 37% higher than our previously reported mono-transition metal hybrid Cu1Mg2Al-LDH/rGO (kapp = 2.5× 10-2 s-1). These findings can be ascribed to 1) highly dispersed ultrasmall Cu2O nanoparticles (~3.8 nm) instantaneously formed via in-situ reduction of atomic-level dispersed Cu2+ ions in LDH layer lattices of the Cu3-xNixAl-LDH hybrids beneficial from clear isolation and stabilization effect of Ni-OH; 2) strong promotion of the doping of atomic-level dispersed Ni2+ ions in LDH and possible Cu2O–Ni-OH(CuNiAl-LDH)–rGO three-phase synergistic effect; 3) enhanced adsorption capacity for reactants upon π-π stacking and the unique honeycomb-like nanosheet array morphology. The series of hybrids exhibit not only excellent reduction properties for varied aromatic nitro compounds, but also good decolorization ability for anionic azo dyes. Further fixed bed experiments reveal that Cu1Ni2Al-LDH/rGO can efficiently decolorize the mixed solution of 4-NP and methyl orange at high flow rate (8 mL/min) with TOF of 1.68 × 10-3 s-1, suggesting great application potential of the as-obtained nanosheet array bi-transition metals hybrids for water remediation.

Keywords: Bi-transition metals CuNiAl-LDH, reduced graphene oxide (rGO), nanosheets array-like, hybrids, catalytic reduction, nitrophenol.

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1. Introduction The study on catalytic reduction of nitro derivatives has been a great challenge due to their pollutant nature for many years.1,2 Specially, the hydrogenation of 4-nitrophenol (4-NP) is crucially significant because 4-NP is toxic, anthropogenic and carcinogenic in nature and exhibits excellent solubility and stability in water.2,3,4 Moreover, the reduction product, 4-aminophenol (4-AP), is a significant industrial raw material and pharmaceutical intermediate, widely used for the productions of corrosion inhibitor, hair-dyeing agent, photographic developer, antipyretics, and analgesic drugs.2,4,5 Consequently, the catalytic hydrogenation of 4-NP has especially crucial environmental and industrial significances. The most common and efficient method for the reduction of 4-NP is to utilize NaBH4 as a reductant and a noble metal as a catalyst,2,6,7 for instance, Au25 nanoclusters,8 Pd nanoclusters,9 Pt-CeO2 nanocrystalline,10 Ag nanoparticles,11 Au-Ag bimetallic nanoparticles,12 Pd/Au bimetallic nanocrystals13 and Au-Pt nanoparticles.14,15 However, the high cost and less abundance of the above noble metals or their bimetallic counterparts greatly restrict their practical applications. Recently, transition metals such as Cu,16 Co,17 Ni17 and their oxides18,19 have been widely used for the hydrogenation of 4-NP. Especially, Cu-based catalysts have been considered as promising candidates for 4-NP hydrogenation owing to their high activity, high abundance and low cost, such as hollow porous Cu nanoparticles,16 ternary nanocomposites Cu2O-Cu-CuO,20 and CuO nanostructures with various morphologies (the rod-, spherical-, star- and flower- shaped morphologies).21 However, the preparations of the above examples suffer from multi-steps and the unavoidable use of organic solvents. The common methods for synthesizing Cu-based catalysts, such as precursor route, wet chemical reduction, hydrothermal process and pyrolysis, ordinarily lead to the aggregation of metals and their oxide particles, which result in a large size (50-100 nm) and relatively poor stability. Therefore, it is greatly attractive to develop a mild and green strategy to synthesize hierarchically structured catalysts with stable structure and high performance. Layered double hydroxides (LDHs), an important two-dimensional (2D) anionic layered 3



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compound with the common formula [M2+1-xM3+x(OH)2]x+[(An-)x/n·mH2O] (M2+: Cu2+, Ni2+, Mg2+ and M3+: Al3+, Fe3+ etc.),22-24 have been increasingly used in catalysis due to their unique 2D lamellar structure involving di- and trivalent cations orderly distributed on the edge-sharing MO6 octahedral layers23-26 and alteration of layered metal cations and interlayer anions.27,28 Actually, the nanoscaled LDHs tend to agglomerate in the form of nanoplatelets due to the strong particle-particle interaction.29 The hybridization of LDH nanosheets and reduced graphene oxide (rGO), being of the large surface area, high conductivity, superior mechanical flexibility, high chemical and thermal stability,30,31 can effectively improve the catalytic activity of 2D LDH nanosheets.32-35 Dou et al.26 obtained a series of nanosheets array-like hybrids CuxMg3-xAl-LDH/rGO via a citric acid-assisted aqueous-phase coprecipitation method, showing much higher 4-NP hydrogenation activity and stability than pure Cu-LDH. Considering the single contribution of Mg2+ cations in CuxMg3-xAl-LDH/rGO as stabilizers in LDH layer lattice, the incorporation of the second transition metal in catalyst system is highly desired for greatly improved reduction activity. Bi-transition metals catalysts have gained lots of attention recently due to their higher activity than single transition metal catalysts because of their special geometrical structure and composition, specific electronic structures and bimetallic synergistic effect.17,36-40 Particularly, nickel is often used as the second transition metal in bimetallic Cu-based catalysts because of the great improvement in activity of Cu-based catalysts. For example, Su et al.37 reported hollow nanospheres (HNSs) of metal oxides (NiO, CuO and NiO/CuO) coated with a porous carbon shell based on a nanoscale Kirkendall effect, and the NiO/CuO showed 1.5 and 2.6-fold of activity than single CuO and NiO in the reduction of 4-NP, respectively. Sun et al.38 reported a series of CuNi alloy nanoparticles assembled on graphene via co-reduction followed adsorption, showing Cu/Ni composition-dependent catalytic performance for methanolysis of ammonia borane and hydrogenation of aromatic nitro compounds. In addition, there are some reports that also demonstrate the enhancement in performance of Cu species by the transition metal element Ni, such as core-shell Cu@Ni/RGO,36 Cu/Ni nanoparticles 4


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supported on graphene39 and CuxNi100−x-CeO2 nanocomposites.40 Nevertheless, most of the reported CuNi bi-transition metals catalysts are nanoparticles36,40 or alloys,38,39 and the above synthesis procedures are complicated and time-consuming as well as the use of organic solvents, which inevitably limit their practical applications. Hence, it is extremely desirable to develop a novelly hierarchical CuNi bi-transition metals catalyst with facile and green synthesis, smallsize and high activity. In this study, a series of bi-transition metal-based nanohybrids Cu3-xNixAl-LDH/rGO (x = 2.5, 2.0, 1.5) with different nickel doping were designed and synthesized, systematically characterized by mainly using XRD, SEM, AFM, HRTEM and XPS techniques. Cu3-xNixAl-LDH/rGO exhibited much more excellent activity than single transition metal-based LDH/rGO hybrids, in which the Cu1Ni2Al-LDH/rGO possessed the best activity, indicating the possible strong synergistic effect between Cu and Ni. Characterizations of the catalysts treated with NaBH4 revealed that the active center of the catalysts is the highly-dispersed and ultrasmall Cu2O nanoparticles (~3.8 nm). This study provides a new idea for the design and synthesis of bi-transition metal-based nanohybrid catalysts for water remediation.

2. Experimental 2.1 Materials Natural graphite powder (325 mesh) was obtained from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. Cu(NO3)2∙3H2O, Ni(NO3)2∙6H2O and Al(NO3)3∙9H2O were bought from Xilong Chemical Co. Ltd. 2-Nitrophenol (2-NP), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), 4-nitrobenzaldehyde and 2,4-dinitrotoluene were bought from Aladdin Chemistry Co. Ltd (Shanghai China). NaBH4, Rhodamine B (RhB), and Methylene blue (MB) were bought from Sinopharm Chemical Reagent Co. Ltd. Citric acid (CA), NaOH and Na2CO3 were bought from Beijing Chemical Works. The chemical reagents utilized in our experiments were all of analytical grade and without any other purification process. The resistivity of the deionized water used all through our experiments is > 18.25 MΩ cm 5



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(25 oC). 2.2 Synthesis of hierarchical Cu3-xNixAl-LDH/rGO hybrids Graphite oxide (GO) was first prepared based on a modified Hummers method via the chemical oxidation of crystalline flake graphite.41 A series of hierarchical hybrids Cu3-xNixAl-LDH/rGO (x = 2.5, 2.0, 1.5) were synthesized through a CA-assisted coprecipitation strategy in aqueous media. Taking Cu1Ni2Al-LDH/rGO as an example, 50 mg GO (6.8 mg/mL, 5.88 mL) was dispersed in 100 mL deionized water firstly and exfoliated by sonication for about 25 min. Then, 25 mg CA was added into the aforementioned GO suspension during the sonication for a further 5 min to obtain the CA-GO suspension. 5 mmol of Cu(NO3)2∙3H2O, 10 mmol of Ni(NO3)2∙6H2O, and 5 mmol Al(NO3)3∙9H2O were together dissolved into 100 mL deionized water to obtain a mixed metal salt solution (molar ratio Cu/Ni/Al = 1/2/1 and mass ratio of total salts to GO is 113:1). A mixed alkaline solution (100 mL) of Na2CO3 and NaOH with [CO32-]/[Al3+] = 2 and [OH-]/[CO32-] = 3.2 (based on Al content) was gradually added dropwise into the aforementioned CA-GO suspension under vigorous stirring to adjust the pH value to 10.0 and remained for 10 min for stabilization. After that, the above mixed salt and alkaline solution were concurrently added dropwise into the aforementioned CA-GO suspension with vigorous stirring, maintaining the stable pH at 10.0 ± 0.1 until the mixed salt solution is added totally (within 40 min). Finally, the precipitates were placed in a water bath at 65 oC for 4 h for further crystallized growth, then centrifuged and washed with deionized water thoroughly, and finally freeze-dried in a LGJ-12 freeze dryer (Beijing Songyuan Huaxing

Technology

Development

Co.

Ltd.),

obtaining

the

gray

hierarchical

hybrid

Cu1Ni2Al-LDH/rGO. Cu0.5Ni2.5Al-LDH/rGO and Cu1.5Ni1.5Al-LDH/rGO with different Ni doping were obtained through the same route. Pure Cu1Ni2Al-LDH without GO addition, CuAl-LDH/rGO (Cu/Al = 2) and NiAl-LDH/rGO (Ni/Al = 3) were prepared for comparison. 2.3 Characterizations X-ray diffraction (XRD) patterns were documented on a Shimadzu XRD-6000 diffractometer 6


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with a graphite-filtered Cu-Kα source (0.15418 nm) at 30 mA and 40 kV. The samples were step-scanned in a step of 0.02° (2θ) in the range of 3-70° with a count time of 4 s each step. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses were performed on an Oxford Instruments INCAx-act EDS detector fixed to a Zeiss Supra 55 field emission scanning electron microscope using a 20 kV electron beam and 60 s acquisition time. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a JEM-2010 with an accelerating voltage of 200 kV. Atomic force microscopy (AFM) micrographs were achieved by a Bruker Dimension Fast Scan 2-SYS in tapping mode at a scan rate of 1.49 Hz to evaluate the surface characteristics of the obtained LDH/rGO hybrid and GO. The FT-IR spectra were recorded on a Bruker Vector-22 FT-IR spectrophotometer from 500 to 4000 cm-1 (resolution: 4 cm-1) based on the KBr pellet method (sample/KBr = 1/100). The Raman spectra were taken with a Jobin Yvon Hodiba Raman spectrometer model HR800 by using a 532 nm line of Ar+ ion laser as the excitation source. The content of metal components was obtained via inductively coupled (ICP) emission spectroscopy on a Shinadzu ICPS-7500 instrument. Textural parameters were obtained from low temperature N2 adsorption-desorption isotherms by using a Quanta chrome Autosorb-1C-VP system at 77 K. The samples were outgassed in vacuum at 80 oC for 8 h before the surface area measurements. The specific surface area (SBET) was determined by the Brunauer-Emmett-Teller (BET) method upon the adsorption branch and the pore size distribution by the Barrett-Joyner-Halenda (BJH) method upon the desorption branch of the isotherms. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB250 X-ray photoelectron spectrometer at a base pressure in the analysis chamber of 2×10-9 Pa using Al Kα radiation (1486.6 eV), and the obtained binding energies were referenced to the C 1s line set at 284.9 eV. The ultraviolet-visible (UV-vis) absorbance spectra were documented on a Perkin-Elmer Lambda 35 double beam spectrophotometer. Fluorescence tests were performed on a Shimadzu RF-5301 PC fluorospectrophotometer to show the evidence of the firm π-π stacking between LDH/rGO and 7



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pyrene (λex = 335 nm). 2.4 Evaluation of catalytic activity 2.4.1 Catalytic 4-NP hydrogenation activity of hybrids The reduction of 4-NP with an excess amount of NaBH4 was regarded as a probe reaction to evaluate the catalytic activity of the Cu3-xNixAl-LDH/rGO hybrids and pure Cu1Ni2Al-LDH. Specifically, a freshly prepared 1 mM 4-NP solution (200 μL) and 10 mM NaBH4 (2.5 mL) were added into a standard quartz cuvette. Subsequently, 10 μL of catalyst suspension (2.5 mg mL-1) was added, and the solution was fast undergoing the UV-vis measurements to take the consecutive variation of the reaction in the scanning range of 250-500 nm. Kinetic measurements were carried out in time course form by monitoring the absorbance change at 400 nm. After each cycle of reaction, an extra 10 μL of 20 mM 4-NP and 10 μL of 2.5 M NaBH4 solution were straight added to the reaction solution, and this process was repeated 10 cycles to study the reusability of the catalysts. 2.4.2. Catalytic reduction of various nitroarenes and degradation of organic dyes The catalytic reduction of 2-NP, 3-NP, 4-nitrobenzaldehyde and 2,4-dinitrotoluene, as well as the decolorization of rhodamine B (RhB) and methylene blue (MB) were tested respectively in order to investigate the catalytic performance of Cu1Ni2Al-LDH/rGO for nitroarenes with different kinds of functional groups and various organic dyes. The specific experimental procedure is the same as the catalytic reduction of 4-NP. The disappearance of the absorption peak of the reactant indicates the complete degradation. Then, record the time of complete degradation and calculate the TOF value accordingly. (2-NP, 418 nm; 3-NP, 256 nm; 2,4-dinitrotoluene, 252 nm; 4-nitrobenzaldehyde, 275 nm; MB, shoulder peaks at 665 nm and 610 nm; RhB, 553 nm.) 2.4.3 Experiment of fixed bed system The quartz sand and was first calcined in an air atmosphere at 600 oC for 6 h, then immersed in a 5% HCl for 12 h and washed with deionized water to neutrality. Afterwards, it was placed at 60 oC for 24 h to gain the dry and clean quartz sand. Take 8 g clean and dry quartz sand and 35 mg 8


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Cu1Ni2Al-LDH/rGO catalyst in an agate mortar and mix them well for at least 30 min. The mixture was placed in an acid burette with an internal diameter of 1 cm. (Both ends are sealed with quartz wool and the bed is about 6 cm). Prepare 1 mM 4-NP, 1 mM methyl orange (MO) solution and the mixed solution of 4-NP and MO (molar ratio of 4-NP/MO = 1/1) and add from the top of the burette with 10 mg NaBH4 respectively, then, turn on the switch immediately and start timing, keeping the flow rate about 8 mL min-1 by applying pressure. The liquid from the outlet of the burette was collected and tested for UV-vis absorption spectra (test range 200-600) to determine the concentration of reactants in the effluent. 2.5 Exploration of Catalytic reduction mechanism 2.5.1 The adsorption of pyrene over Cu1Ni2Al-LDH/rGO Different concentrations of the catalyst suspension (46.8-750 μg mL-1, 0.3 mL) and pyrene solution (100 ppb, 2.7 mL) were added to the quartz cuvette and mixed for 0.5 min. Fluorescence quenching (λex = 335 nm) in the range of 350-500 nm was then recorded and the final concentration of pyrene is calculated by the standard curve. Eliminate the effects of intensity signals from different concentrations of catalyst to minimize interference. 2.5.2 The adsorption of RhB over Cu1Ni2Al-LDH/rGO RhB solution (700 ppb, 2 mL) and catalyst suspension (2.5 mg mL-1) were added to the quartz cuvette, mixed for 0.5 min. Then, the above mixture was exposed to a UV light with a wavelength of 365 nm and the ability of the adsorption of RhB over the catalyst was detected by the fluorescence characteristics of RhB. Afterwards, the mixture was centrifuged and the UV-vis absorption spectrum of the supernatant was recorded (in the range of 300-700 nm at 25 oC), monitoring the absorbance change of RhB at 553 nm.

3. Results and discussion 3.1 Synthesis strategy, structure, composition and morphology 9



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Scheme 1. (A) Schematic illustration of the synthesis of nanosheets array-like bi-transition metals Cu3-xNixAl-LDH/rGO hybrids via a citric acid-assisted coprecipitation method. (B) Illustration of the reaction route for catalytic reduction of 4-NP to 4-AP catalyzed by Cu3-xNixAl-LDH/rGO.

The synthesis strategy of the bi-transition metals Cu3-xNixAl-LDH/rGO nanosheets array hybrids is shown in Scheme 1(A). First, a suitable amount of pre-prepared GO colloid was dispersed in water and exfoliated for 25 min by sonication, forming a stable GO suspension. Then, a suitable amount of citric acid (CA) was added into the above GO suspension, attaching onto the GO sheets through hydrogen bonds (O… H…O) between carboxyl and hydroxyl groups on both citrate ions and exfoliated GO layers. Next, a mixed alkaline solution of NaOH and Na2CO3 were added dropwise until pH~10, leading to the deprotonation of the -COOH groups in CA (pKa1 = 3.13, pKa2 = 4.76, and pKa3 = 5.40 at 25 oC). After that, a mixed salt solution containing Cu2+, Ni2+ and Al3+ and the mixed 10


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basic solution were concurrently added dropwise into the aforementioned suspension with vigorous stirring. The metal cations were instantly arrested by the negatively charged citrate ions on GO via electrostatic attraction and coordination function, resulting in the located nucleation of Cu3-xNixAl-LDH on the surface of CA-functionalized GO, which effectively inhibited the homogeneous nucleation and followed self-aggregation of Cu3-xNixAl-LDH in the aqueous phase. Finally, to minimize the surface energy, the formed Cu3-xNixAl-LDH nuclei were uniformly attached to the GO surface with vertically staggered growth, because of a fast growth rate of the (110) plane while compared to the (003) one,28 which effectively inhibited the re-stacking of GO during the synthesis and simultaneously along with the reduction of GO to rGO. Therefore, the present pre-adjusted pH citric acid-assisted coprecipitation method is beneficial to the formation of bi-transition metals Cu3-xNixAl-LDH/rGO nanosheets array hybrids.

Figure 1. XRD patterns (A), Raman spectra (B), Cu 2p 3/2 (C) and Ni 2p 3/2 (D) XPS spectra of

Cu3-xNixAl-LDH/rGO hybrids (a, b and c corresponding to x = 2.5, 2.0 and 1.5, respectively) compared with Cu1Ni2Al-LDH (d) and GO.

Figure 1(A) depicts the XRD patterns of the three nanosheets array-like bi-transition metal 11



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hybrids Cu3-xNixAl-LDH/rGO compared with pure Cu1Ni2Al-LDH and GO, and the XRD patterns of Cu0.5Ni2.5Al-LDH and Cu1.5Ni2.5Al-LDH are shown in Figure S1. The GO shows a strong peak corresponding to the (001) plane at ca. 10.9o with an interlayer spacing of 0.85 nm larger than graphite (0.34 nm), indicating that oxygen-containing functional groups are successfully introduced.42 The three Cu3-xNixAl-LDH/rGO hybrids all show clear peaks at ca. 11.6o (003), 23.3o (006), 34.7o (012), 39.2o (015), 46.7o (018), 60.6o (110) and 61.7o (113) without any other peaks, similar to typical hexagonal Cu6Al2CO3(OH)16·4H2O phase (JCPDS 37-0630), and the peak intensity is significantly weakened. However, no diffraction peak of GO (001) is observed in the hybrids, probably due to the effective reduction of GO during the synthesis of alkaline conditions (pH = 10), which proved that the uniformly dispersed LDH and rGO organic combinations were obtained. The parameter a of the hybrids Cu3-xNixAl-LDH/rGO decreased from 0.3058 nm to 0.3042 nm with the increase of Ni2+ doping (Table S1), owing to the smaller ionic radius of Ni2+ (0.069 nm) than that of Cu2+ (0.072 nm). As for the gradually increases of c may be attributed to the slightly higher electronegativity of Ni (1.91) than Cu (1.90), which enhances the electron-withdrawing ability of the LDH layers and leads to an increase in the negative charge of it. The mutual repulsion between adjacent layers increases, resulting in an increase of interlayer distance. These trends are consistent with pure Cu3-xNixAl-LDH. D003 (~3.4 nm) and D110 (~7.5 nm) (details are enlarged in Figure S2) of the hybrids are significantly smaller than those of the corresponding pure LDH on account of the multiple interactions between Cu3-xNixAl-LDH and rGO such as van der Waals forces and electrostatic attraction, which greatly affect the nucleation of Cu3-xNixAl-LDH nanosheets, further inhibiting the agglomeration of LDH nanosheets.26 It is worth noting that the (110) and (003) diffraction peak intensity ratios I110/I003 for the hybrids Cu3-xNixAl-LDH/rGO (0.18 ~ 0.23) are much higher than the corresponding pure Cu3-xNixAl-LDH (0.13 ~ 0.16), implying the probably oriented growth of CuNiAl-LDH nanosheets with ab-plane perpendicularly staggered on both sides of the rGO layer, 12


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thereby effectively inhibiting the re-stacking of GO. FT-IR results (Figure S3) further indicate that the peaks of C=O (carboxylic acid) at 1731 cm-1, C-OH (carboxyl) at 1414 cm-1, C-O-C (epoxy) at 1225 cm-1 and C-O (alkoxy) at 1052 cm-1 in the hybrids Cu3-xNixAl-LDH/rGO almost disappear compared to GO, and the stretching vibration peak of carbon skeleton (C=C/C-C) at 1621 cm-1 can be obviously observed in GO, which proves GO is effectively reduced during the synthesis,43 in line with XRD results. In addition, the peaks of the hybrids at ~3486 cm-1, 1370 cm-1 and 852 cm-1 can be attributed to the typical υ(O-H), υ3(symmetric stretching) and υ2(out-of-plane deformation) of CO32-, respectively, implying the presence of H2O and CO32- in the interlayer spaces of the Cu3-xNixAl-LDH. And the bands below 800 cm-1 are assigned to the vibrations of M-OH and M-O modes in the LDH layer lattice.44 It is worth noting that with the amount of Ni2+ doping increasing, the peak assigned to the vibrations of M-OH modes gradually shifts from 770 cm-1 to 779 cm-1, suggesting the possible electronic interactions between Cu and Ni. The Raman spectra of Cu3-xNixAl-LDH/rGO hybrids (x = 2.5, 2.0, 1.5), pure Cu1Ni2Al-LDH and GO are shown in Figure 1(B). The band of pure LDH at 1062 cm-1 can be attributed to the υ1(CO32-), frequently observed in CO32--LDH materials,26,44,45 and two bands at 483 and 550 cm-1 can be attributed to the M-O modes.45 However, these bands were not observed in the hybrids, demonstrating the high dispersion of Cu3-xNixAl-LDH nanosheets on the surface of rGO. And two clear bands for the hybrids Cu3-xNixAl-LDH/rGO and GO at 1351 and 1586 cm-1 correspond to the D and G bands for the carbon materials, respectively. The ID/IG values of ~1.06 for the hybrids are significantly higher than that of GO (0.94), illustrating the existence of a large number of unrepaired defects along with a reduction of the size of the sp2 regions in the hybrids.42,43 These defect sites can promote the nucleation of Cu3-xNixAl-LDH on the rGO surface and inhibit the aggregation of LDH nanocrystals, which plays an important role in the in-situ anchoring of Cu3-xNixAl-LDH nanosheets on rGO surface forming a highly uniform dispersed nanosheets array-like structure as above XRD 13



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results implied. ICP results (Table S2) indicate that the molar ratios Cu/Ni/Al of the hybrids are very closed to their raw material ratios, indicating that all metal ions can effectively participate in the formation of Cu3-xNixAl-LDH during the synthesis process, in line with the design concept of green chemistry. 10%, 33% and 50% (molar ratio) of Cu2+ in the CuAl-LDH layer lattice can be isomorphously substituted

by

Ni2+,

obtaining

the

highly

crystallized

hybrids

Cu0.5Ni2.5Al-LDH/rGO,

Cu1Ni2Al-LDH/rGO and Cu1.5Ni1.5Al-LDH/rGO, respectively. Figure 1(C,D) is the XPS spectrum of Cu 2p3/2 and Ni 2p3/2 of series hybrids Cu3-xNixAl-LDH/rGO and pure Cu1Ni2Al-LDH. The surface element compositions and valence state are analyzed to further reveal the metal-metal and metal-support interactions among Cu2+, Ni2+ and rGO. The C 1s spectrum of Cu3-xNixAl-LDH/rGO hybrids and rGO (Figure S4) can be deconvoluted into five types bonds: O-C=O (289.6 eV), C=O (287.8 eV), C-O (286.5 eV), sp3C-C (285.6 eV) and sp2 C=C (284.6 eV).46 The peaks of oxygen containing function groups of the Cu3-xNixAl-LDH/rGO hybrids are alike to those of rGO (Table S3), confirming the effective reduction of GO through the preparation procedure, in line with FTIR and Raman data. The primary peaks with binding energy (BE) values of ~935.3 eV for Cu 2p3/2 for all the hybrids and obvious satellites at ca. 944.5 eV can be assigned to Cu2+ ions in LDH layers.46,47 Compared with pure Cu1Ni2Al-LDH (935.0 eV), the BE values of the hybrids shift slightly towards high binding energy (ΔBE = 0.1 ~ 0.5 eV), strongly suggesting the electron transfer from Cu3-xNixAl-LDHtorGO, which may be related to the hybridization of Cu3-xNixAl-LDH and rGO. Simultaneously, the primary peaks with BE values of ~856.8 eV for Ni 2p3/2 of all the hybrids and obvious satellites at ~862 eV can be assigned to Ni2+ ions48,49, also shift towards high binding energy, further verifying the electron transfer from Cu3-xNixAl-LDH to rGO, which indicates a strong interaction between Cu3-xNixAl-LDH and rGO. These results are consistent with the negative shifts of the BE values of C 1s in the series of hybrids (Figure S4) compared with pure Cu1Ni2Al-LDH and rGO. The electron transfer effect induced by the 14


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substrate leads to the electrophilicity of the surface of Cu3-xNixAl-LDH in the series of hybrids, favorable to contact with the nucleophilic BH4- ions and the negatively charged reactant 4-nitrophenol anions, thereby promoting the catalytic reduction of 4-NP. It’s worth noting that with increasing of the doping of Ni2+, the BE values for Cu 2p3/2 and Ni 2p3/2 show a regular variation with increase first and then decrease, suggesting the possible electronic interaction between Cu2+ and Ni2+. Among them, the ΔBE values of Cu 2p3/2 (0.5 eV) and Ni 2p3/2 (0.8 eV) for Cu1Ni2Al-LDH/rGO are both largest, implying that it has the relatively strongest electrophilicity to enrich more BH4- and 4-nitrophenol anions, playing a crucial role in the activity of the catalyst. Compared with Cu1Mg2Al-LDH/rGO26 (934.7 eV), the ΔBE value of the primary peaks of Cu 2p3/2 for the hybrid Cu1Ni2Al-LDH/rGO is 0.8 eV, suggesting the electron transfer from Cu2+ to Ni2+, further indicating that the doping of Ni2+ significantly changes the surface electronic state of Cu2+, which implies the possible synergistic effect between the two transition metals of Cu and Ni. The

surface

Cu

contents

of

Cu0.5Ni2.5Al-LDH/rGO,

Cu1Ni2Al-LDH/rGO

and

Cu1.5Ni1.5Al-LDH/rGO are 5.09%、10.29% and 14.30%, respectively, clearly higher than those of the corresponding pure Cu3-xNixAl-LDH (Table S2), while the bulk Cu contents of the hybrids are slightly lowered due to the incorporation of rGO. Here the honeycombed nanosheet array structure and much higher SBET values of the hybrids may be beneficial to the migration of Cu2+ ions to the surface of the sample thus contributing to the exposure of the active ingredients. The analysis of surface oxygen upon the curve fittings of O 1s show three oxygen species of the hybrids Cu3-xNixAl-LDH/rGO (Figure S4), containing lattice O2- (OIII, 529-531 eV) species, surface OH (OII, 531-532 eV) and adsorbed H2O or intercalated carbonate (OI, 533-534 eV).50 The BE values of OII in the Cu3-xNixAl-LDH/rGO hybrids show a positive shift compared to pure Cu1Ni2Al-LDH/rGO, suggesting the decline of the electron density after hybridizing with rGO. In the three hybrids, the order of the percentage for OII is Cu0.5Ni2.5Al-LDH/rGO (57.1%) > Cu1Ni2Al-LDH/rGO (52.2%) > Cu1.5Ni1.5Al-LDH/rGO (51.8%), which increases with the increase of the amount of the doping of 15



Page 16 of 39

Ni2+, suggesting the possible modification of Ni2+. The observed strong electron interactions between atomic-level dispersed Cu2+, Ni2+ on LDH layers and rGO of the as-obtained nanosheet array hybrids Cu3-xNixAl-LDH/rGO may be in favor of the electron transportation during the catalytic reduction of 4-nitrophenol.

Figure 2. SEM (A), AFM (B), TEM (C) and HRTEM (D) images of Cu1Ni2Al-LDH/rGO hybrid.

The SEM (A), AFM (B), TEM (C) and HRTEM (D) images of typical hybrid Cu1Ni2Al-LDH/rGO are shown in Figure 2. The SEM image (Figure 2A) clearly shows that the well-crystallized Cu1Ni2Al-LDH nanosheets are vertically staggered on both sides of rGO densely (as shown by the arrows in Figure 2A) to form a uniform honeycombed (many interstitial caves: 25-45 nm) nanosheets array-like bi-transition metals hybrid. The average height of the nanosheets array is ~145.5 nm, equivalent to the length of two nanosheets and a rGO layer, the average size of the nanosheets is ~72.3×4.2 nm, slightly thinner than that of the single transition metal hybrid Cu1Mg2Al-LDH/rGO (~ 70.0×4.5 nm) prepared under similar conditions, which is possibly related 16


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to the second transition metal Ni2+. EDS (Figure S5) shows that the elements Cu, Ni, Al, C and O are all uniformly distributed in the hybrid, indicating that the metal elements in the Cu1Ni2Al-LDH layer are highly dispersed on the one hand and the high dispersion of LDH nanosheets on the other, which clearly demonstrates the effective hybridization between Cu1Ni2Al-LDH nanosheets and the rGO. The AFM phase image (Figure 2B) shows two distinct phase features with upper bright Cu1Ni2Al-LDH nanosheets grown vertically and the bottom dark areas attributed to the rGO substrate. Since the Cu1Ni2Al-LDH nanosheets are vertically staggered on the rGO layer, many dark interstitial pores with a diameter of ~35 nm can be observed in the 2D phase image (Figure S6), consistent with the SEM results. The TEM image (Figure 2C) clearly shows that the slightly curved ultrathin Cu1Ni2Al-LDH nanosheets (~66.6×4.3 nm) are vertically staggered on the surface of rGO substrate. The light gray area (the blue arrow in Figure 2C inset) corresponds to the rGO substrate, and a large number of dark lines and light gray lines correspond to Cu1Ni2Al-LDH nanosheets grown on both sides of the rGO substrate, respectively, consistent with SEM and AFM results. A clear lattice fringe can be observed in the HRTEM image (Figure 2D) with the inter-planar distance of 0.258 nm and 0.194 nm, corresponding to the d012 and d018 of rhombohedral LDH crystals, respectively. The corresponding FFT pattern further shows the clear diffraction spots indexed to the (012) and (018) plane of LDH, suggesting good crystallinity of the Cu1Ni2Al-LDH nanosheets in the hybrid. The other two hybrids Cu0.5Ni2.5Al-LDH/rGO (Figure S7A and A') and Cu1.5Ni1.5Al-LDH/rGO (Figure S7B and B') both show similar honeycombed nanosheets array-like morphology. The morphology of the hybrids is quite different from that of pure Cu1Ni2Al-LDH (~135.8×11.8 nm) with typical “desert rose” morphology (Figure S7C and C'), indicating that the rGO layer significantly weaken the strong particle-particle interaction often occurring in LDH nanosheets, resulting in honeycombed nanosheets array-like hybrids Cu3-xNixAl-LDH/rGO. The 3D hierarchical structure promotes the diffusion and transport of reactants, thereby in favor of the catalytic reduction reaction of 4-NP. 17



Page 18 of 39

The N2 adsorption isotherms and the corresponding BJH pore size distributions of the hybrids Cu3-xNixAl-LDH/rGO (Figure S8) present similar IV type characteristics with broad H3 hysteresis loops (p/p0 > 0.45), indicating the presence of predominant mesopores in the hybrids, consistent with the pore size distribution shown in the insets. The BET specific surface area (SBET) of the Cu3-xNixAl-LDH/rGO hybrids (~151.8 m2/g) are much higher than that of the pure Cu1Ni2Al-LDH (83.4 m2/g) due to the greatly increased dispersion of Cu3-xNixAl-LDH nanosheets by rGO. Such a hierarchical structure not only prevents the agglomeration of Cu3-xNixAl-LDH nanosheets and the re-stacking of rGO themselves, but also improves the contact efficiency between the active center and reactants. The high specific surface area of the hybrids and a large number of mesopores (about 3.5 ~ 4.3 nm) provide many transport channels, greatly improving the catalytic reduction activity.

18


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3.2 Catalytic activity of hierarchical Cu3-xNixAl-LDH/rGO hybrids

Figure 3. UV-vis spectra of 4-NP solution before and after addition of NaBH4 (A) and 4-NP reduction without catalyst (B).UV-vis spectra of 4-NP reduction catalyzed by hybrids Cu3-xNixAl-LDH/rGO (C: x = 2.5, D: x = 2.0, E: x = 1.5) and Cu1Ni2Al-LDH (F). Pseudo-first order kinetic plots of 4-NP reduction catalyzed by hybrids

Cu3-xNixAl-LDH/rGO, Cu1Ni2Al-LDH and without catalyst: Ct/C0 - t (G) and ln Ct/C0 - t (H).

Inspired by the atomic-level dispersed Cu2+ and Ni2+ ions, nanosheets array-like morphology and large surface area, we chose hydrogenation of 4-nitrophenol (4-NP) with an excess amount of NaBH4 as a probe reaction to evaluate the catalytic activity of the as-obtained hybrids Cu3-xNixAl-LDH/rGO. As shown in Figure 3A, the 4-NP solution shows a strong absorption peak at 19



Page 20 of 39

317 nm and the peak shifts to 400 nm quickly after addition of NaBH4 because of the formation of 4-nitrophenolate anions,51,52 along with the solution turning from light-yellow to yellow-green. The absorption peak at 400 nm remains unchanged for 1 h in the absence of catalysts, shown in Figure 3B. However, the peak weakens rapidly and the solution changes from yellow-green to colorless fast after the addition of a small amount of hybrids Cu3-xNixAl-LDH/rGO (Figure 3C-F). In detail, for Cu1Ni2Al-LDH/rGO (Figure 3D), the absorption peak of the 4-nitrophenol anion at 400 nm weakens rapidly during the reduction procedure and eventually disappears at 1.5 min, along with a concomitant increase of the peak of 4-aminophenol (4-AP) at 300 nm, clearly revealing the conversion of 4-NP to 4-AP without any other by-products.53 For Cu0.5Ni2.5Al-LDH/rGO (Figure 3C) and Cu1.5Ni1.5Al-LDH/rGO (Figure 3E), 4-NP converts to 4-AP completely at 2.5 and 2.0 min, respectively. It is worth noting that the three hybrids Cu3-xNixAl-LDH/rGO all show a much higher reaction rate than pure Cu1Ni2Al-LDH (Figure 3F, 3 min) for entire conversion of 4-NP to 4-AP. Inspiringly, Cu1Ni2Al-LDH/rGO shows a greatly higher reaction rate than the single transition metal hybrid Cu1Mg2Al-LDH/rGO under the same reaction conditions.26 These results, as well as the fact that Ni3Al-LDH and rGO are inactive for this reaction, strongly suggest that the Cu-related species may be the actual active sites in the 4-NP hydrogenation reaction. And the doping of the second transition metal Ni2+ in the honeycombed nanosheets array-like hybrids Cu3-xNixAl-LDH/rGO as well as the possible synergistic effect between the LDH nanosheets and rGO substrate play a key role in the reduction of 4-NP. The catalytic reaction rate is independent of the BH4- concentration because the amount of NaBH4 is more than 100 times (125:1) higher than 4-NP in the reaction system. Therefore, pseudo-first-order kinetics is utilized to evaluate the rate of the catalytic reduction reaction.15, 53, 54 In the current reaction system, the ratio of (Ct/C0) can be calculated by the absorbance ratio (At/A0) of the peak at 400 nm. (Ct and C0 are the concentrations of 4-nitrophenolate anions at t moment and 0 moment, respectively.)The change of Ct/C0 with time (t) is shown in Figure 3G. As expected, a good 20


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linear relationship between ln (Ct/C0) and t is shown (Figure 3H) and the apparent rate constant (kapp) can be calculated according to the following equation: ln (Ct/C0) = –kappt

(1)

As shown in Table 1, the kapp of bi-transition metals Cu1Ni2Al-LDH is 20.20×10-3 s-1, which is comparable to the recently reported Cu@Ni/RGO nanocomposite36 and NiO/CuO HNSs@C hollow nanospheres37 catalysts. More excitingly, the kapp values of the three hybrids Cu3-xNixAl-LDH/rGO are higher (at least 1.3-fold) than that of Cu3-xNixAl-LDH, suggesting that the atomic-level dispersed Cu active center in the LDH interacting with the rGO substrate greatly increases the catalytic efficiency. Clearly, the kapp (34.37×10-3·s-1) of Cu1Ni2Al-LDH/rGO is 1.7-fold than that of Cu0.5Ni2.5Al-LDH/rGO (19.67×10-3·s-1), probably because of the higher surface Cu content (10.29 wt%) of the former (Table S2). That is, the more Cu2+ species in the LDH lattice, the more potential active sites in the hybrids. However, Cu1.5Ni1.5Al-LDH/rGO with a higher Cu surface content (14.30 wt%) shows a lower kapp value (22.96×10-3·s-1) , probably due to the decrease of Ni2+ doping, causing a decrease in the electron transfer ability between Cu and Ni, which leads to a weakening of the synergistic effect between the double transition metals Cu and Ni. However, the decrease of the activity caused by the weakening synergistic effect exceeds the increase of the activity caused by the Cu surface content increase, comprehensively showing that the catalytic activity of Cu1.5Ni1.5Al-LDH/rGO is lower than that of Cu1Ni2Al-LDH/rGO. The activity of the catalysts is closely connected to the amount of active component and the concentration of the reactants.26 Therefore, we further calculate the normalized apparent rate constant knor (knor = kapp/ mmetal) and the conversion frequency (TOF) of the hybrids. The knor and TOF values of Cu0.5Ni2.5Al-LDH/rGO (15460 g-1·s-1, 239.7 h-1) are slightly higher than those of Cu1Ni2Al-LDH/rGO (13350 g-1·s-1, 197.6 h-1) because of the relatively lower surface Cu content of the former, while those of Cu1.5Ni1.5Al-LDH/rGO (6420 g-1·s-1,106.6 h-1) are significantly lower than Cu1Ni2Al-LDH/rGO. Considering the reaction rate and active Cu content comprehensively, the 21



Page 22 of 39

hybrid Cu1Ni2Al-LDH/rGO is believed to be the relatively most optimal catalyst. It is worth noting that the activity of this bi-transition metals hybrid Cu1Ni2Al-LDH/rGO is 37% higher than that of the single transition metal hybrid Cu1Mg2Al-LDH/rGO (kapp = 2.5× 10-2 s-1)26 under the same reaction conditions, suggesting that the doping of Ni2+ greatly increases the activity of the Cu component due to the synergistic effect between the double transition metals Cu and Ni. The comparison of the activity between the as-obtained hybrids and the recently reported catalysts shows that the bi-transition metals hybrids Cu3-xNixAl-LDH/rGO (Table 1), especially Cu1Ni2Al-LDH/rGO, present significantly higher catalytic reduction efficiency of 4-NP than the recently reported ternary nanocomposite Cu2O-Cu-CuO (15.6 × 10-3·s-1),20 star-shaped CuO (1.95 × 10-3·s-1),21 Cu@Ni/RGO nanocomposite (23 × 10-3·s-1)36 and NiO/CuO HNSs@C hollow nanosphere (25.05 × 10-3·s-1),37 and even polydopamine-rGO supported Pt-Au dendrimer-like alloy nanoparticles (9.58 × 10-3·s-1, 200 h-1)15 and graphdiyne oxide loaded Pd nanoparticles (5.37 × 10-3·s-1),54 further indicating the excellent catalytic activity of the present hybrids Cu3-xNixAl-LDH/rGO.

22


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Table 1. Comparison of the catalytic activity of the Cu3-xNixAl-LDH/rGO hybrids and related catalysts for the reduction of 4-NP. Catalysts

Type

4-NP

Catalyst

/×10-3mmol

used/mg

t /min

kapp/×

knor/

aTOF

10-3·s-1

g-1·s-1

/h-1

Cu0.5Ni2.5Al-LDH/rGO

Nanosheets array

0.2

0.025

2.5

19.67

15460

239.7

Cu1Ni2Al-LDH/rGO

Nanosheets array

0.2

0.025

1.5

34.37

13350

197.6

Cu1.5Ni1.5Al-LDH/rGO

Nanosheets array

0.2

0.025

2.0

22.96

6420

106.6

Cu0.5Ni2.5Al-LDH

Sand-Rose-like

0.2

0.025

3.5

15.25

13760

196.6

Cu1Ni2Al-LDH

Sand-Rose-like

0.2

0.025

3.0

20.20

8610

108.3

Cu1.5Ni1.5Al-LDH

Sand-Rose-like

0.2

0.025

3.5

15.40

4340

61.4

Ni3Al-LDH/rGO

Nanosheets array

0.2

0.025

--

--

--

--

Cu1Mg2Al-LDH/rGO35

Nanosheet array

0.2

0.025

2.0

25.11

6090

161.9

Cu2O-Cu-CuO20

Nanocomposite

0.3

1.0

3.0

15.6

--

--

CuO21

Star-shaped

0.2

0.01

20.0

1.95

--

--

Cu@Ni/RGO36

Nanoparticle

1.0

5

1.5

23

--

--

NiO/CuO HNSs@C37

Hollow

0.3

0.05

4.0

25.05

--

--

0.27

0.06

3.0

9.58

--

200

0.067

0.00125

12.0

5.37

--

--

porous

nanospheres Pt3Au1-PDA/RGO15

Dendrimer-like

Pd/GDYO54

Nanoparticle

aTOF:

the turnover frequency, the moles of 4-NP reduced per mole of activity component (Cu, Au etc.) per hour.

23



Page 24 of 39

3.3 Catalytic reduction mechanism over Cu3-xNixAl-LDH/rGO

Figure 4. TEM (A), HRTEM (B), Ni 2p (C) and Cu 2p (D) XPS spectra of Cu1Ni2Al-LDH/rGO treated by NaBH4 (a) compared with fresh Cu1Ni2Al-LDH/rGO (b).

For sake of explaining the highly efficient catalytic reduction activity of the present bi-transition metals hybrids Cu3-xNixAl-LDH/rGO, it is necessary to study deeply the changes of atomic-level dispersed Cu species in the Ni-containing LDH layer during the hydrogenation reaction to reveal the real Cu-related active centers in the hybrids. Thus the typical Cu1Ni2Al-LDH/rGO was treated merely with NaBH4 for 90 s (short as Cu1Ni2Al-LDH/rGO-tr) with 480-fold catalyst usage (30 mL, 0.4 mg mL-1) to obtain a suitable amount of treated sample for convincible XPS and (HR)TEM 24


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characterizations. Apparently, the light earthy yellow suspension of Cu1Ni2Al-LDH/rGO in water immediately changes to dark brown after the addition of NaBH4 (Figure S9), implying the formation of new Cu species in treated sample. The TEM of the Cu1Ni2Al-LDH/rGO-tr (Figure 4A) clearly shows spherical nanoparticles of ~3.8 nm mainly located in the edges of LDH layer or the LDH-rGO interface regions, which can be identified by HRTEM and FFT images (Figure 4B) revealing the inter-planar distances of 0.242 and 0.213 nm of the Cu2O (111) and (200) planes (JCPDS 78-2076), respectively. While the other two NaBH4 treated samples Cu0.5Ni2.5Al-LDH/rGO-tr and Cu1.5Ni1.5Al-LDH/rGO-tr (Figure S10) show a little larger Cu2O size of ~4.4 nm and quite larger Cu2O size of ~8.2 nm, respectively. The highly dispersed ultrasmall Cu2O (~3.8 nm), much smaller than previously reported Cu1Mg2Al-LDH/rGO (~6.8 nm),26 may be resulted from instantaneously in-situ reduction of atomic-level dispersed Cu2+ ions in LDH layer lattices which are most probably beneficial from remarkable isolation and stabilization effect of Ni-OH groups on the LDH layers. The Ni 2p XPS spectra of Cu1Ni2Al-LDH/rGO-tr (Figure 4C(a)) clearly shows two main peaks at 873.1 and 855.4 eV for Ni 2p1/2 and Ni 2p3/2, respectively, along with an obvious satellite at ca. 862 eV, corresponding to the Ni2+ in LDH layers, which confirms the existence of the atomic-level dispersed Ni-OH groups in the LDH layer lattice. Meanwhile, the Cu 2p XPS (Figure 4D(a)) depicts a broad asymmetric peak which can be resolved into two components. The main peaks at 954.9 and 935.0 eV for Cu 2p1/2 and Cu 2p3/2, respectively, along with an obvious satellite at ca. 943 eV, can be ascribed to the atomic-level dispersed Cu2+in LDH layers,20,26 while the weak ones at 932.8 and 952.9 eV for Cu 2p3/2 and Cu 2p1/2, respectively, can be ascribed to Cu+ in Cu2O,19,55 clearly suggesting the partial reduction of Cu2+ ions in the ultrathin Cu1Ni2Al-LDH nanosheets, which causes an obvious increase of surface Cu content (Table S2). Clear downshifts of 0.6 and 0.5 eV for Ni 2p3/2 and Cu 2p3/2 of Cu1Ni2Al-LDH/rGO-tr, respectively, compared to fresh one, indicating the increase in electron density around Ni and Cu core-level after treated with NaBH4, which is exactly 25



Page 26 of 39

desired for the catalytic reduction process of 4-NP. Given that the standard oxidation-reduction potentials of (BO2,H2O)/BH4-, Cu(OH)2/Cu and Cu(OH)2/Cu2O are -1.24, -0.222 and -0.08 V, respectively, the more positive reduction potential of the Cu(OH)2/Cu2O couple designates the feasible reduction by NaBH4. Therefore, Cu2O nanoparticles may be preferentially produced in the presence of the strong reductant NaBH4. And NaBH4 dissociates in aqueous solution with the evolution of H2, which reacts with OH- ions of water and releases electrons to Ni2+ and Cu2+ species.55 And a part of Cu2+ ions in the LDH layer lattice (~22.2% of surface Cu2+ (XPS), i.e.15.9% of total Cu) was instantaneously in-situ reduced to Cu2O and mainly located in the edges of LDH layer or the LDH-rGO interface regions, in line with TEM result, due to more available reaction sites and possible synergy. The Cu2O accounts for 42.5% of the surface Cu content of Cu1Ni2Al-LDH/rGO-tr. In this case, the large quantity of Cu2+ ions in LDH layer lattice of the hybrids Cu3-xNixAl-LDH/rGO can be regarded as an efficient reservoir providing the highly active Cu2O nanoparticles continuously without any additional prereduction step before 4-NP reduction with excess NaBH4, very different from traditional supported or self-assembled Cu-based catalysts,16,18-21,52 implying the high efficiency of the present hybrids. It is also noted that the main peak of C1s of Cu1Ni2Al-LDH/rGO-tr just downshifts 0.07 eV compared to fresh one (Table S3) because of the electron transfer from CuNiAl-LDH to the rGO substrate (Figure 1) obscuring the electron shift between Cu and Ni to some extent. However, the main peak at ~855.5 eV for Ni 2p3/2 of Cu1Ni2Al-LDH shows 0.4 eV downshift, compared to that in Ni3Al-LDH,49 indicating an increase in surface electron density of Ni due to the electron transfer from Cu to Ni. Moreover, the Cu+ 2p3/2 for Cu1Ni2Al-LDH/rGO-tr shows a positive BE shift of 0.3 eV compared with Cu1Mg2Al-LDH/rGO treated by BH4-26 under the same conditions, possibly because of the electron transfer from Cu to Ni upon a slightly larger electronegativity of Ni than Cu, indicating that the surface of Cu2O in present system is more electrophilic, therefore more strongly interacting with the OH groups from LDH further stabilizing Cu2O nanoparticles on LDH/rGO. In 26


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fact, the distinguishing interface between the Cu1Ni2Al-LDH domains and the single-layer rGO can be clearly observed in Figure 4B. Thus, the Cu2O–Ni-OH(CuNiAl-LDH)–rGO three-phase interfaces in the hybrid exist during the reaction process (red dotted line in Figure 4D). These ubiquitous three-phase interfaces are more likely to be the most active catalytic areas considering the 1.7-fold higher activity of the ultrathin nanosheets array-like Cu1Ni2Al- -LDH/rGO compared to pure Cu1Ni2Al-LDH with large and thick nanoplates (only a few Cu2O/LDH domains in Figure S11). Considering the clear isolation and stabilization effect of Ni-OH for Cu2O, we tentatively propose the strong synergistic effect among the in situ reduced Cu2O, Ni-OH(CuNiAl-LDH) and the conductive rGO substrate leading to the significantly enhanced electron transfer ability of the catalysts Cu3-xNixAl-LDH/rGO, resulting in the greatly improved activity for 4-NP hydrogenation. Meantime, the more electrophilic surface of such Cu2O is being more favorable for the adsorption of nucleophilic reagents BH4- and 4-NP anions. Therefore, a plausible catalytic mechanism on Cu3-xNixAl-LDH/rGO was tentatively proposed and shown in Scheme 1B. Firstly, BH4- and 4-NP molecules simultaneously diffuse into the Cu2O-Ni-OH(CuNiAl-LDH)-rGO three-phase interface region. Then, BH4- reacts with Cu2O to form surface active hydrogen (H) species, and transfer H and e- to the nitro functional group (-NO2) of 4-NP through Cu2O to reduce -NO2 to amino functional group (-NH2) rapidly, generating the final product 4-AP.52 Finally, the product 4-AP molecules are desorbed from the catalyst surface into the solution to regenerate the active center. As a result, the Cu3-xNixAl-LDH/rGO catalyst regenerated, being reused in the next cycle. To prove the π-π stacking between the hybrids and nitroarenes, we studied the adsorption of rhodamine B (RhB) and pyrene molecules with Cu1Ni2Al-LDH/rGO via fluorescence quenching test. The adsorption between RhB and the Cu1Ni2Al-LDH/rGO hybrid are shown in Figure 5. RhB solution showed strong orange fluorescence under UV light, and the orange fluorescence completely disappeared after the Cu1Ni2Al-LDH/rGO hybrid was added (Figure 5A). After centrifugation of the hybrid, the peak intensity of the UV absorption spectrum for RhB molecule in supernatant is 27



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significantly lower than that at the initial moment (Figure 5B). However, fluorescence can still be observed after adding pure Cu1Ni2Al-LDH suspension, and the color of the fluorescent changes to pink, possibly caused by the presence of Cu1Ni2Al-LDH26, and the peak intensity is as well as that at primary stage. What’s more, pyrene solution was mixed with the hybrid suspension with different concentrations for 1 min. And subsequently the fluorescence quenching can be observed obviously, suggesting the strong adsorption between pyrene molecules and the hybrid (Figure 5C). For comparison, no significant fluorescence quenching was observed when pure LDH suspension with different concentrations were added (Figure S12). According to fluorescence resonance energy transfer (FRET), only donor and acceptor molecules are spatially close to each other can the fluorescence quenching produce, proving the strong π-π stacking effect between reactant and rGO with a conjugated structure in the hybrid.56 The curve of adsorption percentage change with time shows that 91.8% of pyrene molecules can be quickly captured within 1 min, and the amount of adsorption does not increase significantly with time, indicating pyrene molecules can be adsorbed on the surface of the hybrid rapidly (Figure 5D). Considering the fluorescence quenching of pyrene and adsorption of RhB, the Cu1Ni2Al-LDH/rGO hybrid possesses strong π-π stacking with aromatic compound molecules via the rGO substrate with abundant π electron. The rGO substrate with abundant π electron greatly enhances the adsorption of reactants in the reaction, so that the 4-NP molecules rapidly gather around the active center, thereby improving the catalytic performance of the hybrid.

28


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Figure 5. Photos of Cu1Ni2Al-LDH/rGO or Cu1Ni2Al-LDH suspension mixed with RhB solution under UV light (A), UV-vis spectra of the centrifuged supernatants (B). Fluorenscence quenching of pyrene solution by adding Cu1Ni2Al-LDH/rGO suspensions with different concentrations (C) and time-dependent adsorption percentage of pyrene by Cu1Ni2Al-LDH/rGO (D).

3.4 Cycling catalytic performance of the hybrid Recyclability is one of the most significant properties of the catalyst in practical applications. The results in Figure 6A show that the Cu1Ni2Al-LDH/rGO hybrids sustains a high performance for 4-NP hydrogenation with conversion rate of 90.3% and kapp = 31.05 ×10-3 s-1 in 90 s even in the 10th cycle. Since the product 4-AP was not removed during the whole process, the catalytic activity of the hybrids was not affected by the product accumulation (Figure 6B), suggesting the rapid separation of the 4-AP molecule from the active centre, which is possible one of the most important reasons why the hybrid catalyst possesses excellent activity. The (HR)TEM image of the hybrid after 10th cycles is shown in Figure 6C and D, and it can be obviously observed that the catalyst can still maintain the nanosheets array-like morphology. The average size of active component Cu2O nanoparticles increased from ~ 3.8 nm after once reaction to ~ 9.8 nm after 10th cycles, which is the main reason for the slight decrease in activity. 29



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Figure 6. Reusability of Cu1Ni2Al-LDH/rGO nanohybrid as catalyst in successive 10 cycles (A), time-dependent UV-vis spectra of reduction of 4-NP by catalyst in the 10th cycle (B), TEM (C) and HRTEM (D) of Cu1Ni2Al-LDH/rGO nanohybrid after 10 successive cycles.

3.5 The universality of the catalyst In order to further research the versatility of the present Cu3-xNixAl-LDH/rGO hybrid catalysts, we continue to explore the catalytic reduction properties of the typical Cu1Ni2Al-LDH/rGO to a variety of aromatic nitro compounds and anionic azo dyes. For a series of aromatic nitro compounds, 2-NP, 3-NP, 2,4-dinitrotoluene, 4-nitroaniline and 4-nitrobenzaldehyde all were fast reduced to the corresponding aniline products within 90 s (Figure S13A-E) no matter of the types and location of substituents of the reactants. The catalytic activities for the reduction of the three nitrophenols follow an order of 3-NP > 2-NP > 4-NP (Table S4). Actually, the nitrophenols take part in the catalytic reaction with their ionic form (nitrophenol anion), and the negative charge in the O atom of 4-NP 30


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anion can be effectively delocalized by the benzene ring, leading to the formation of a stable resonance system making 4-NP most stable among the three isomers. The 2-NP shows weaker resonance effect than 4-NP, and simultaneously suffers from the steric hindrance effect of the functional group, making it more active than 4-NP, while 3-NP has no resonance effect at all explaining its worst stability.57 Therefore the hybrid exhibits the relatively fastest reaction rate on the catalytic reduction of 3-NP. Then for the catalytic decolorization of varied anionic azo dyes, the absorption peak of MO (465 nm), AO7 (483 nm), MB (663 nm) and RhB (554 nm) decreased rapidly within 7 min (Figure S13F-I, Table S4). These findings strongly imply that the hybrid catalyst Cu1Ni2Al-LDH/rGO exhibits not only excellent reduction properties for varied aromatic nitro compounds, but also good decolorization ability for anionic azo dyes, which is much better than recently reported mesoporous Cu2O-CeO2 composite nanospheres,19 ternary nanocomposites Cu2O-Cu-CuO,20 CuO-MnO2 nanocomposite.58

3.6 Nanocatalysis in fixed-bed system In industrial application of catalysts, fixed bed system are often used to charge the catalyst to achieve a continuous reaction for maximum recyclable ability, which are suitable for automated procedure.59 In this study, we used an acid burette with Teflon switch to simulate a fixed bed device to evaluate the catalyst to degrade 4-NP, MO and their mixed solutions in a continuous flow reaction (Figure 7). The reaction solution maintain in the original color before passing through the catalyst-quartz sand bed. Turning on the switch and keeping a certain applied pressure to control the flow rate to ~ 8.0 mL min-1, 10 mL of the reaction solution takes about 75 s to pass through the bed and the solution collected at the outlet is completely clarified with TOF = 6.048 h-1. It can be observed that the corresponding ultraviolet absorption peaks of the solution collected at the outlet completely disappear and after the 4-NP flowing through the fixed bed, the absorption peak at 300 nm in the ultraviolet spectra significantly enhanced, confirming the formation of 4-AP. In summary, 31



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the present hybrid catalysts have great application prospective for the handling of manufacturing effluents containing both the nitrophenols and organic dyes.

Figure 7. Photographs showing the catalytic reduction of 4-NP (A), MO (B) and 4-NP + MO mixed solution (C) as well as the UV-vis spectra of corresponding effluent.

4. Conclusions In summary, a series of honeycomb-like bi-transition metals nanosheets array hybrids Cu3-xNixAl-LDH/rGO (x = 2.5, 2.0, 1.5) were synthesized via a pre-adjusted pH citric acid-assisted coprecipitation method. The hybrids consist of ultrathin CuNiAl-LDH nanosheets (~72.3×4.2 nm) staggered vertically grown on both sides of rGO substrate, having large surface area (~151 m2/g) and abundant mesopores (~3.76 nm). The obtained Cu3-xNixAl-LDH/rGO hybrids can be regarded as a potential efficient reservoir of in situ formed ultrasmall Cu2O nanoparticles (~ 3.8 nm) upon the reduction of atomic-level dispersed Cu2+ ions in LDH layer lattices. Notably, the obtained 32


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Cu3-xNixAl-LDH/rGO hybrids exhibit highly efficient catalytic performance for 4-NP hydrogenation, especially Cu1Ni2Al-LDH/rGO with kapp = 3.4×10-3, knor = 13350 g-1·s-1 and TOF = 197.6 h-1, ascribed to highly dispersed ultrasmall Cu2O nanoparticles (~3.8 nm) instantaneously formed by in-situ reduction of atomic-level dispersed Cu2+ ions beneficial from clear isolation and stabilization effect of Ni-OH, possible Cu2O–Ni-OH(CuNiAl-LDH)–rGO three-phase synergistic effect, enhanced adsorption capacity for reactants upon π-π stacking and the unique honeycomb-like nanosheets array morphology. The hybrids can be recycled more than 10 times without significant decrease of activity and well used to the reduction of diverse nitroarenes and the decolorization for anionic azo dyes. The hybrids also exhibit outstanding efficiency in the fixed bed system with simulated manufacturing effluents containing nitrophenols and organic dyes. The present facile synthesis route of bi-transition metal-based LDH/rGO hybrids are believed to be significantly beneficial to the design and construction of many other multi-transition metal-based LDH/rGO hybrids with excellent structural robustness and versatility for a wide range of important applications in environmental catalysis, electrochemistry, water splitting, and drug delivery system.

Associated Content Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI Figure S1-S13; Table S1-S4

Author Information Corresponding Author *Phone: +8610-6442 5872; fax: +8610-6442 5385; e-mail: [email protected]. ORCID Hui Zhang: 0000-0002-5512-1171 33



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Notes No competing financial interest.

Acknowledgements The authors greatly appreciate the financial support by the National Natural Science Foundation of China (21576013, 21878007 and 21838007).

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