Enhancing the Catalytic Activity and Stability of Noble Metal

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Enhancing the Catalytic Activity and Stability of Noble Metal Nanoparticles by the Strong Interaction of Magnetic Biochar Support Shun-Feng Jiang, Li-Li Ling, Zhuoran Xu, Wu-Jun Liu, and Hong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02777 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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Industrial & Engineering Chemistry Research

Enhancing the Catalytic Activity and Stability of Noble Metal Nanoparticles by the Strong Interaction of Magnetic Biochar Support

Shun-Feng Jiang a, Li-Li Ling a, Zhuoran Xu b, Wu-Jun Liu a, and Hong Jiang a, *

a

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry,

University of Science and Technology of China, Hefei 230026, China b

Department of Chemical and Biological Engineering, University of Wisconsin,

Madison, Wisconsin 53706, United States

* Corresponding authors E-mail:[email protected]

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ABSTRACT In this study, the catalytic activity and stability of Pd nanoparticles during the reduction of 4-nitrophenol (4-NP) were significant enhanced by the interaction of Fe3O4/biochar support prepared by pyrolysis of fir sawdust. Compared with the commercial Pd/C with 5% of Pd content, the as-synthesized Pd@Fe3O4/biochar with only 1.58% of Pd content exhibited twice higher catalytic performance towards the 4-NP reduction. The significant enhancement of catalytic activity was ascribed to the interaction between Fe and Pd NPs which lowers the binding energy of Pd on surface of Pd@Fe3O4/biochar, resulting the fast desorption of the intermediate species during the reaction. Furthermore, the carbon skeleton expedites the electron transfer and facilitates the formation of active hydrogen and deprotonation of 4-NP which are key steps of reduction of 4-NP. The abundant oxygen containing groups in biochar may act as pinchers to tightly fix the Pd NPs and improve the stability of Pd@Fe3O4/biochar.

Keywords: catalyst support, biochar, activity and stability, separability, catalytic reduction, noble metal

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INTRODUCTION Noble metal (e.g., Pd, Pt, Au, Ag, and Ru) nanoparticles (NPs) based catalysts are highly efficient in catalytic hydrogenation reactions that are widely used to remedy the contaminated water/soil, such as reduction of nitroaromatics, hydrodehalogenation polychlorinated biphenyls (PCBs) and perfluorochemicals, and detoxicity of heavy metals.1-4 However, the extremely high cost of noble metal catalysts has obstructed their wide applications. How to reduce the cost of noble metal catalysts has attracted much attention globally. One of the solutions is to enhance the catalytic activity of noble metal NPs in hydrogenation reactions by tuning the particle sizes, morphology, and exposed facets of the NPs, which correspondently decreases the consumption of noble metals. For instance, Wu et al controlled the shape of Pt NPs by varying the growth inhibition agent and probed their structure-activity dependence in hydrogenation reactions.5 However, the bare noble metal NPs are difficult to recover after catalytic reaction, elevating the remediation cost and bringing second pollution to environment.6 Moreover, noble metal NPs prone to aggregate in the reaction due to their high surface energy and Van der Waals forces,7 which counteracts the improvement of performance in a certain extent. Porous supports play a pivotal role in the improvement of performance of noble metal NPs. Depositing the finely controlled noble metal NPs on the surface of porous supports can not only keep the high catalytic activity of noble metal NPs, but also facilitate the recovery as well as inhibit the aggregation of metal NPs. For instance, Wang et al used ZSM-5 confined Pd NPs for hydrogenation of p-nitrophenol, which could prevent the loss and aggregation of Pd NPs.8 Matos and Corma found the 3



Pd-based catalysts supported on hybrid TiO2-carbon materials can significantly improve the catalytic activity and selectivity of hydrogenation of phenol.9 Zhang et al reported the improved catalytic performance and reusability of magnetite supported Pd NPs in hydrogenation reactions.10 Compared with the metal supports, porous carbon materials have large surface area and stable chemical properties which can be used in harsh environments.11-13 Among them, graphene and carbon nanotubes are emerging supports for noble metal NPs and extensively investigated. By using graphene and carbon nanotubes with special structure and large surface area as supports, the dispersity and catalytic activity of noble metal NPs have been significantly enhanced.14,15 The carbonyl and carboxyl groups on the surface of porous carbon are the ideal nucleation sites for noble metal NPs,16 which can significantly improve the dispersity and immobilization of noble metal NPs. Nevertheless, regardless of graphene and carbon nanotubes without deliberate surface modification, there are insufficient binding sites for anchoring the noble metal NPs, which usually leads to poor dispersion and large metal nanoparticles, especially under high loading conditions.17 The preparation of graphene and carbon nanotubes is usually expensive and laborsome, let alone the additional functionalization progress introducing carbonyl and carboxyl groups on carbon supports .18,19 Therefore, how to cost-effectively prepare carbon supports with abundant functional groups is a big challenge. Biochar, a by-product of production of renewable energy by biomass pyrolysis, has abundant surface functional groups (C-O, C=O, COOH, OH, etc.), which could provide enough binding sites for noble metal NPs. On the other hand, if the Fe3+ ions were preloaded in the biomass, they can be reduced to Fe3O4 NPs by the small molecular hydrocarbons produced during the thermochemical decomposition of 4


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biomass and tightly embedded in biochar, which endows the biochar with magnetism and significantly improve the separability of catalysts.20-23 In addition, the synergetic effect between noble metal NPs and Fe3O4 NPs may enhance the catalytic activity of the composites.24,25 Based on abovementioned considerations, we therefore envisage that the Fe3O4 NPs embedded biochar can significantly enhance the catalytic performance of noble metal NPs by improving the dispersity of metal NPs, expediting the electron transfer, and enriching the pollutants and reductants. Meanwhile, the abundant functional groups on the surface of biochar can function as pinchers to tightly grasp the noble metal NPs and improve the stability of the catalyst. In addition, the Fe species can act as magnet to facilitate the separation of catalyst. The main objectives of this study are to verify this hypothesis by preparing the Pd@Fe3O4/biochar catalyst, demonstrating its catalytic activity and stability, and exploring the peculiar role played by the Fe-biochar support by detailedly characterizations and contrast experiments. To this end, the Pd@Fe3O4/biochar catalyst was prepared by fast pyrolysis of FeCl3 preloaded sawdust and subsequent wet chemical reduction and deposition of Pd NPs. 4-nitrophenol (4-NP), as a typical carcinogenic and genotoxic compound and widely appeared in wastewater and surface water,26-28 was chosen as a model pollutant to test the catalytic performance of the catalyst, while NaBH4 was used as a reductant. The catalytic performance of Pd@Fe3O4/biochar was compared with commercial Pd/C and reported catalysts. X-ray photoelectron spectroscopy (XPS), FTIR, and Raman analyses were performed to explain the enhancement mechanism. To further confirm this mechanism, Pt@Fe3O4/biochar and Ag@Fe3O4/biochar were also prepared using same procedure and used in the reduction of 4-NP.

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MATERIALS AND METHODS Materials. The chemicals and reagents used here were in analytical grade. The biomass used in the experiments was fir sawdust gathered from a local timber treatment plant. It has been crushed with a rotary cutting mill and screened to obtain particles smaller than 100 mesh. The sawdust particles were washed with HCl (1 M) for 300 min to remove the inorganic species, and then dried in an oven at 378 K overnight. 10.0 g of dry biomass and 1000 mL of FeCl3 solution (10 mmol/L) were mixed in a flask and shaken in a constant-temperature oscillator at room temperature for 300 min. When the adsorption was completed, the moisture was removed through a rotary evaporator and then dried again at 353 K overnight to obtain the FeCl3 preloaded biomass. Synthesis of the Fe3O4/biochar. The FeCl3 preloaded biomass was pyrolyzed in a self-designed fixed bed reactor to obtain the magnetic biochar (Fe3O4/biochar).29 The reactor was heated to a set temperature of 873 K, while the air in the reactor was purged by a N2 flow (400 mL/min) for 30 min. Then, 5.0 g of FeCl3 preloaded biomass was quickly fed into the reactor and the produced volatiles were swept out by N2 flow (200 mL/min) which was further condensed by a cold ethanol trap at a temperature of 253 K to form bio-oil. When the pyrolysis process was finished, the temperature was maintained at 873 K for another 120 minutes for carbonization. After that, the reactor was removed from the heating zone to cool down in a N2 flow of 200 mL/min, and the Fe3O4/biochar support was prepared. Synthesis of the Fe3O4/biochar supported noble metal NPs. Pd@Fe3O4/biochar was synthesized through a facile wet chemical reduction method. First, 0.2 g of Fe3O4/biochar was placed into a 50-mL flask containing 20 mL of water under 6


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stirring; then Na2PdCl4 solution (mPd:mFe3O4/biochar =1:20) was slowly added into the flask. After stirring for 30 min at room temperature, the ascorbic acid solution (nascorbic acid:nPd=2:1)

was injected into the mixture and reacted for 120 min. Finally, the

mixture was filtered and the obtained solid was washed repeatedly with deionized water, followed by drying at 353 K overnight to obtain Pd@Fe3O4/biochar. Pt@Fe3O4/biochar and Ag@Fe3O4/biochar were prepared using the similar method. Characterizations. The metal contents of Pd@Fe3O4/biochar were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (Optima 7300 DV, Perkin Elmer Corporation, USA) after being digested with concentrated nitric acid (HNO3) and hydrogen peroxide (H2O2, 30%, v/v) at about 493 K. The morphology of Pd@Fe3O4/biochar was studied by scanning electron microscope (SEM) (Zeiss Supra 40, Carl Zeiss AG, Germany) and transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) (H7650, Hitachi Co., Japan). X-ray diffraction (XRD) measurements were performed on a theta rotating anode X-ray diffractometer (TTR-III, Rigaku, Japan) using Cu-Kα radiation with the 2θ range of 20–80o. The chemical state and surface composition of Pd@Fe3O4/biochar were analyzed by XPS. N2 adsorption–desorption isotherms were measured at 77 K using a Micromeritics Gemini apparatus (ASAP 2020 M+C, Micromeritics Co., USA). The surface areas and average pore diameters of the samples were calculated based on the Brunauer–Emmett–Teller (BET) method. The Raman analysis of the catalyst was performed in a Laser Raman spectrometer (LabRamHR, HORIBA Jobin Yvon, France). The surface functional groups of samples were measured by a Fourier transform infrared spectroscopy (FTIR, Bruker EQUIVOX55, Germany). 7



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Catalytic reduction of 4-NP by noble metal NPs@Fe3O4/biochar. The catalytic activity of the catalysts was evaluated in the aqueous phase reduction reaction of the 4-NP. In a typical reaction, a given amount of catalyst was added to 20 mL of 0.1 mM 4-NP solution under stirring. 0.5 mmol of NaBH4 was then added to the mixture under vigorous stirring at room temperature. At each time interval, 1.0 mL of the reaction solution was withdrawn and diluted to 3.0 mL with deionic water to analyze the concentrations of 4-NP with a UV-Vis spectrometer (UV-1800, MAPADA Instrument Co., Ltd. China). For comparison, various control experiments were also carried out under the same conditions. For the recyclability test, the used catalyst was separated from the reaction solution by centrifugation and then drying in an oven for the next reaction.

RESULTS AND DISCUSSION Structure and composition of Pd@Fe3O4/biochar. A schematic illustration of the synthesis of Pd@Fe3O4/biochar is shown in Figure S1 of Supporting Information (SI). Fir sawdust is a typical lignocellulosic biomass, which mainly contains lignin, cellulose, and hemicellulose. During the pyrolysis process, the biomass components were thermochemically decomposed into volatiles and carbon skeleton with oxygen-containing groups (biochar). The volatiles, including oxygenated compounds (e.g., phenols, aldehydes, ketones, and carboxylic acids), can be condensed into bio-oil.30 The preloaded FeCl3 was converted into Fe3O4 (Eqs. 1 and 2) by the reducing species (e.g., CO, H2, and fiery C) produced during the pyrolysis of biomass, which endows the biochar with magnetism (Fe3O4/biochar). FeCl3 + H2O →Fe(OH)3→FeO(OH)→Fe2O3 8


(1)

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3Fe2O3 + 3(H2, CO, C) →2Fe3O4 + 3(H2O, CO2, CO)↑

(2)

The yields of magnetic biochar (Fe3O4/biochar) and bio-oil during pyrolysis are shown in the Figure S2. The results show that the yield of magnetic biochar was 25.1%, indicating the magnetic carbon support can be massively produced. After that, Pd NPs were deposited on the Fe3O4/biochar by a common wet reduction process to obtain the surface dispersed Pd@Fe3O4/biochar. The Pd content in the Pd@Fe3O4/biochar is determined to be 1.58 wt.% by ICP-AES. The morphology of the as-synthesized Pd@Fe3O4/biochar was studied using SEM and TEM. Figure 1a shows the SEM images of the Pd@Fe3O4/biochar, from which two types of NPs with different shapes and sizes uniformly dispersed on the surface of biochar can be seen. The large particles are Fe3O4 NPs and the small ones are Pd NPs. The TEM images (Figure 1b) of the Pd@Fe3O4/biochar indicated that the Pd NPs are monodispersed on the surface of the biochar, with a hexagon shape and an average particle size of about 35 nm. More detailed structural information on Pd NPs can be found in the HRTEM image (Figure 1c), which shows a lattice spacing of fringes of ca. 0.25 nm, and they can be attributed to the metallic Pd facets with a face-centred cubic (FCC) phase (0.249 nm, JCPDS card No. 46-1043). The microstructure of the Pd@Fe3O4/biochar was investigated with TEM-EDX mapping, and the results indicate that C, O, Fe, and Pd are the main elements in the material, all of which are evenly distributed on the surface of Pd@Fe3O4/biochar (Figure 1d). Just as our anticipation, the Pd NPs are formed and monodipersed on the surface of Fe/biochar, which provide the maximum exposure of active sites.

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Figure 1. (a) SEM images of the Pd@Fe3O4/biochar; (b) TEM images of the Pd@Fe3O4/biochar; (c) HRTEM image of the Pd@Fe3O4/biochar; (d) TEM-EDX spectrum of the Pd@Fe3O4/biochar (the element Cu here comes from the Cu substrate used for TEM measurement, not the element in the material) and EDX elements mapping of the Pd@Fe3O4/biochar. (e) XRD patterns of the Pd@Fe3O4/biochar, Fe3O4/biochar, and biochar; (f) nitrogen adsorption-desorption isotherms of the Pd@Fe3O4/biochar. Figure 1e shows the XRD pattern of the Pd@Fe3O4/biochar. The two peaks at 2θ of 40.2o and 45.8o can be assigned to the metallic Pd (111) and Pd (200) planes (JCPDS, 05-0681), suggesting that the Pd on the biochar is in the metallic form with a face-centred cubic crystal. The diffraction peaks at 2θ of 30.2°, 36.4°, 43.2°, 56.7° and 63.2° are characteristic peaks of Fe3O4 crystallites (JCPDS, 26-1136). As a 10


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comparison, the XRD patterns of Fe3O4/biochar and raw biochar are also presented. It can be observed from the XRD patterns that the Fe3O4 crystallites remains unchanged after the introduction of Pd NPs. XRD analysis is in agreement with the results of morphologic images, indicating the Pd@Fe3O4/biochar is successfully synthesized. The surface area and pore structure of the Pd@Fe3O4/biochar were analyzed by nitrogen adsorption-desorption method, and the results are shown in Figure 1f. It can be seen that the adsorption-desorption isotherms exhibit a typical type IV patterns, corresponding to the mesoporous feature. The hysteresis loop of this pattern is type H3 at high relative pressures, suggesting the presence of slit-shaped pores with non-uniform sizes and shapes in the material. Based on the isotherms results, the surface area, pore volume, and pore size were calculated to be 295.4 m2/g, 0.242 cm3/g, and 1.63 nm, respectively. The significantly enhanced surface area compared to original biochar (Table S1) is important for the gathering the pollutants from aqueous solution and expedites the catalytic reduction reaction. Catalytic performance of Pd@Fe3O4/biochar. The Pd@Fe3O4/biochar was used as a catalyst for the aqueous reduction of 4-NP with NaBH4. As shown in Figure 2a, with the catalysis of Pd@Fe3O4/biochar, the intensity of the peak at about 400 nm, which is attributed to the characteristic absorption of 4-NP, decreased quickly, and completely disappeared within 3 min. Alternatively, a new adsorption peak attributing to 4-aminophenol can be found at about 280 nm, indicating that the 4-NP was totally converted into 4-aminophenol within 3 min with the catalysis of Pd@Fe3O4/biochar. The color change during the reaction was shown in Figure S3, and the color became shallow with the time. The catalytic activity of Pd@Fe3O4/biochar is higher than most of the reported noble metal based catalysts (Table S2). Notably, the commercial Pd/C 11



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catalyst with near 3-fold of Pd content (5 wt.% Pd) of Pd@Fe3O4/biochar needs 6 min to reduce 4-NP completely, demonstrating that Pd@Fe3O4/biochar is a low-cost and high-efficient catalyst. Similarly, the catalytic activity of Pt@Fe3O4/biochar (Figure 2b) and Ag@ Fe3O4/biochar (Figure 2c) has a noteworthy improvement compared with the commercial Pt/C and Ag NPs with the same metal content, verifying that the Fe3O4/biochar support plays an important role in the enhancement of catalytic activity of noble metal NPs. Langmuir-Hinshelwood model was employed to illustrate the reaction process of catalytic reduction of 4-NP. The reaction rate (k) can be evaluated with a pseudo-first-order kinetics (Eq. 3) by fitting ln (C/C0) against the reaction time t due to the presence of excessive NaBH4.

ln

C = kt C0

(3)

The Pd@Fe3O4/biochar exhibited a remarkable catalytic reactivity with a rate constant of 1.47 min−1, which was 2 times higher than that of the commercial Pd/C (Figure 2g). The rate constant of Pt@Fe3O4/biochar (Figure 2h) and Ag@Fe3O4/biochar (Figure 2i) were also high than that of commercial Pt/C and Ag NPs, respectively.

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Figure 2. Reduction of 4-NP by NaBH4 with the catalysis of (a) Pd@Fe3O4/biochar, (b)Pt@Fe3O4/biochar, (c)Ag@Fe3O4/biochar，(d)Pd/C, (e)Pt/C and (f)Ag NPs. (d), (e) and (f) ln(C/C0) versus time for the 4-NP reduction over the different catalysts. Reaction conditions: [4-NP]=0.1mM, [NaBH4]=25.0 mM, [Pd]=0.014 mM, [Pt]=0.064 mM, [Ag]=0.467 mM .

As a heterogeneous catalyst, the separability and durability are critical. As shown in Figure 3a, the Pd@Fe3O4/biochar exhibited high durability towards 4-NP reduction for as many as 10 cycles, which is evidenced by the invariant reaction rate and final C/C0 value at the end of each run. The recyclability of

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this catalyst is supreme compared to many state-of-the-art Pd-based catalysts (Table S2). The catalyst is also very easy to separate from the reaction mixture with an external magnet because of its high magnetism endowed by the Fe3O4 (the saturation magnetization of the material reaches 10 emu g−1 at 5000 Oe) (Figure S4). In the practical application of Pd@Fe3O4/biochar catalyst, the leaching of Pd should be considered because the leached Pd is harmful to the environment. As shown in Figure 3b, the Pd concentrations of the reaction solutions in all the 10 times cycles were less than 5.0 ng/mL, indicating that the Pd in the catalyst is highly stable, and suggesting the Pd NPs were tightly fixed on the surface of Fe3O4/biochar. This also suggests that the catalytic activity of the catalyst is not attributed to the Pd leached to the solution, but rather the heterogeneous Pd NPs on the catalysts.

Figure 3. (a) Cycle performance of the Pd@Fe3O4/biochar (C/C0 is the remaining fraction of 4-NP at different reaction times); (b) the Pd leaching during each cycle reuse of the Pd@Fe3O4/biochar. Reaction conditions: [4-NP]=0.1 mM, [NaBH4]=25 mM, [Pd]=0.014 mM.

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Effect of magnetic biochar support on the catalytic activity Pd NPs. From abovementioned results, Pd@Fe3O4/biochar with 1.58% of Pd has 2 times of activity of commercial Pd/C with 5% of Pd in catalytic reduction of 4-NP. To reveal the underlying mechanism, a series of control experiments were conducted. Because the electrochemical potential of 4-NP to 4-aminophenol (E4-NP/4-Aminophenol=0.76 V vs SCE) is remarkably lower than that of BH4− to BO2− (EBH4-/BO2-=1.33 V vs SCE), the reduction of 4-NP to 4-aminophenol with NaBH4 is thermodynamically feasible. However, when 4-NP and NaBH4 were mixed in solution, there is no obvious decrease of the adsorption peak at 400 nm after 60 min, which should be attributed to the high kinetic barrier (Figure 4a). When biochar was added in the solution containing 4-NP and NaBH4, a slight decrease of the adsorption peak at 400 nm after 60 min was observed, indicating the degradation and adsorption of 4-NP by biochar are neglectable (Figure 4b). When the magnetic biochar (Fe3O4/biochar) was added in the solution, a slight decrease of peak at 400 nm was observed while no new peaks appeared, suggesting Fe species improved the adsorption of biochar rather reduction (Figure 4c). Compared these results of contrast tests with that of the addition of Pd@Fe3O4/biochar, it can be concluded that the order of contribution in Pd@Fe3O4/biochar is Pd NPs>Fe species>biochar.

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Figure 4. Reduction of 4-NP by NaBH4 with (a) no catalyst; (b) biochar; (c) Fe3O4/biochar; (d) Pd@biochar; (e) Pd@Fe3O4; and (f) ln(C/C0) versus time for the 4-NP reduction over the different catalysts. Reaction conditions: [4-NP]=0.1 mM, [NaBH4]=25.0 mM, [catalyst]=0.1 g/L.

Pd is well known reduction catalyst, but it is strange that the Pd in Pd@Fe3O4/biochar has much more activity than that in Pd/C, whereas both biochar and Fe3O4/biochar have little catalytic activity for 4-NP reduction, suggesting that a synergistic effect may exist among its components. We also tested the catalytic 16


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performance of Pd@biochar and Pd@Fe3O4 under the same reaction condition (Figure 4d and e). The results show that both of Pd@biochar and Pd@Fe3O4 have lower catalytic performance than that of Pd@Fe3O4/biochar, comfirming the synergistic effect among Pd, Fe, and biochar. Thus, we reasonably inferred that Fe3O4 and biochar both act as “accelerants” to significantly boost the activity of Pd@Fe3O4/biochar. The excellent catalytic performance of Pd@Fe3O4/biochar for the 4-NP reduction may be ascribed to the distinct electronic structure of Pd modified by Fe species and the interface chemistry of biochar with abundant surface functional groups. To elucidate the activity boosting mechanism, chemical state of Pd in Pd@ Fe3O4/biochar and commercial Pd/C were analyzed by XPS. Figure 5a shows the XPS survey of the Pd@Fe3O4/biochar, from which four main elements, including Pd, Fe, O, and C can be seen. From the XPS Pd 3d spectrum shown in Figure 5b, the peaks at 335.6 and 340.9 eV in Pd 3d 5/2 and 3d 3/2 can be assigned to the metallic Pd. The peaks at 336.7 and 342.5 eV can be assigned to Pd(II) oxides.31 The Fe 2p spectrum (Figure 5c) is comprised of five peaks. The peaks at binding energies of 733.0 and 712.6 eV can be attributed to Fe(III), while the peaks at binding energies of 725.1 and 710.9 eV were assigned to Fe(II). A noticeable satellite is found at a binding energy of 718.6 eV, which is a typical peak of Fe3O4 as also confirmed by the XRD patterns.32 The results indicate that metallic Pd, Pd oxides, Fe(II), and Fe(III) are co-existed in the Pd@Fe3O4/biochar. Noteworthy, the XPS spectrum showed that the binding energy for Pd 3d of Pd@Fe3O4/biochar has a slight shift (ca. 0.2 eV) to lower value compared with that of commercial Pd/C (Figure 5d). This shift has also been observed in similar systems 17



involving Pd overlapped on Fe3O4 substrates.33 It is indicative of the bonding interactions between Pd and Fe3O4 which results in a redistribution of charge that affects the core levels.34 The moderate shift of binding energy for Fe 2p and O 1s of Pd@Fe3O4/biochar also confirmed this conclusion (Figure S5). The species and electric structure of Pd in catalyst influence the performance of catalyst. The lower binding energy means that the Pd on surface of Pd@Fe3O4/biochar possesses a reduction (electron gain) state and weak adsorbate binding energies,35,36 resulting the fast desorption of the intermediate species during the reaction and enhance the reaction rate.

Figure 5. (a) XPS survey, (b) Pd 3d, and (c) Fe 2p spectra of the Pd@Fe3O4/biochar. (d) Pd 3d spectra of Pd@Fe3O4/biochar and commercial Pd/C. 18


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In the reduction of 4-NP catalyzed by Pd@Fe3O4/biochar, 4-NP and BH4− were diffused from the bulk solution to catalyst and adsorbed by catalyst (Figure 6a). As a strong nucleophile, the high electron rejection capability of BH4− made it easily lose an electron and form hydrogen via Pd NPs, while at the same time 4-NP deprotonated to form a nitrophenolate ion which is then adsorbed reversibly on the surface of the biochar. Thereafter, the hydrogen atoms and electrons were transferred from the hydrogen

to

the

nitro

group

of

4-NP,

followed

by

several

steps

of

hydrodeoxygenation reactions to produce 4-aminophenol (Eq. 4).37-39

(4) To verify whether the active hydrogen species transfer route played a role in the 4-NP reduction, H2 was directly supplied as an alternative to NaBH4 for the reduction of 4-NP under the same conditions. As shown in Figure 6b, considerable amount of 4-NP was converted into 4-aminophenol within 60 min, suggesting that the H2 could act as a reactant for the conversion of 4-NP into 4-aminophenol with the catalysis of Pd@Fe3O4/biochar, thus demonstrating the feasibility of the H transfer route in the 4-NP reduction. However, compared to NaBH4, the time required for the reduction of 4-NP by H2 was longer than 60 min. This phenomenon suggests the active hydrogen formed in situ in the dehydrogenation of NaBH4 has much higher reactivity than the H2 gas, and could react with 4-NP more readily through the H transfer process.40 We also conducted a comparative experiment using commercial Pd/C and H2 to reduce the 4-NP (Figure 6c and d). The results show that the reduction rate of 4-NP using 19



Pd@Fe3O4/biochar is higher than that using commercial Pd/C, suggesting the Pd@Fe3O4/biochar promotes the production of active hydrogen.

Figure 6. (a) the possible reaction pathway. UV-vis spectra of the 4-NP reduction with H2 under the catalysis of (b) Pd@Fe3O4/biochar and (c) commercial Pd/C. (d) The rate constant of k for Pd@Fe3O4/biochar (1.58% wt. of Pd) and commercial Pd/C (5% wt. of Pd). Reaction conditions: [4-NP]=0.1 mM, [Pd]=0.014 mM, [H2]=20 mL/min.

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Increase exposure of active Pd by second deposition is another contribution to enhanced catalytic active. To confirm this assumption, same concentrations of Pd and Fe salts were preloaded on biomass and pyrolyzed under same conditions as that Pd@Fe3O4/biochar was prepared. The one-pot prepared Pd-Fe3O4/biochar has much lower catalytic activity than that prepared by two-step method (Figure S6). Surface elemental composition of the Pd@Fe3O4/biochar and Pd-Fe3O4/biochar were analyzed by XPS. There is only 0.13% of Pd on the surface of Pd-Fe3O4/biochar, much lower than that of Pd@Fe3O4/biochar (1.28%), confirming the high exposure of active Pd NPs of Pd@Fe3O4/biochar is important to the efficient catalytic reduction of 4-NP. However, high exposure of active Pd NPs means the high loss of Pd during the catalytic reaction, and low stability of catalyst. Pd@Fe3O4/biochar exhibited high stability, suggesting the surface chemistry of biochar play an important role in the fixation Pd NPs on the surface of biochar. The surface chemical properties of biochar were characterized. As the FTIR spectra shown, the functional groups on biochar, Fe3O4/biochar, and Pd@Fe3O4/biochar were mainly -OH, C-O, and C=O, and the introducing the Fe and Pd has not obviously changed the types of groups (Figure 7a). However, Raman spectra showed that the IG/ID (denoting the aromatization degree of carbon materials) of Fe3O4/biochar is 0.7, which is much lower than that of biochar (IG/ID=1.3) and close to that of Pd@Fe3O4/biochar (IG/ID=0.8) (Figure 7b), suggesting that the Fe3O4/biochar has more functional groups than biochar. The C1s spectra of biochar and Fe3O4/biochar were shown in Figure 7c and d, respectively. The XPS spectra of the C1s are comprised of three peaks, assign to C-C at 284.8eV, hydroxyl C-OH at 285.7 eV, and carbonyl C=O at 288.4 eV.41 The total oxygen-containing function groups (C-O and C=O) contents of the Fe3O4/biochar was 28.8%, higher than 21



that of biochar (23.8%). This significant increase of oxygen-containing function groups may be attributed to the catalysis of FeCl3 during pyrolysis.42,43 The oxygen-containing groups have been reported to be the linkers to anchor noble metal NPs onto porous carbon surface.17,44 The surface oxygen-containing groups of the Fe3O4/biochar could increase the surface binding sites, avoid the aggregation of Pd NPs, improve the dispersion and reduce the average size of Pd NPs.

Figure 7. (a) FTIR spectra, (b) Raman specrta of the catalyst ,and C1s XPS spectrum of (c) Biochar, (d) Fe3O4/biochar

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Besides increasing the stability of catalyst, the oxygen-containing groups were also reported to increase the adsorption ability of the metal-carbon materials and expedite the catalytic reduction reaction.45 Figure 4d shows that Fe3O4/biochar has significant higher removal rate than biochar, suggesting the oxygen-containing groups enhanced the adsorption affinity toward 4-NP, which decreases the mass transfer resistance and facilitates the reduction reaction. The pathway of the catalytic reaction in solution is involved diffusion, adsorption, and reaction.46 In the reduction of 4-NP catalyzed by Pd@Fe3O4/biochar, 4-NP and BH4− can be enriched by Fe3O4/biochar, and facilitate their reaction. The biochar is a porous carbon skeleton, which has been reported to enhance the electron transfer, and thus expediting the catalytic reduction reaction.

CONCLUSIONS The Fe3O4/biochar was successfully used as a promising support for noble metal NPs. The as-synthesized Pd@Fe3O4/biochar exhibited high catalytic activity towards the 4-NP reduction compared with commercial Pd/C catalyst. Pd@Fe3O4/biochar also showed high stability and separability. The significant enhancement of catalytic activity may be ascribed to the interaction between Fe and Pd NPs which lowers the binding energy of Pd on surface of Pd@Fe3O4/biochar, resulting the fast desorption of the intermediate species during the reaction and enhancement of the reaction rate. Furthermore, the carbon skeleton expedites the electron transfer and facilitates the formation of active hydrogen and deprotonation of 4-NP which are key steps of reduction of 4-NP. The abundant oxygen containing groups in biochar may act as pinchers to fix the Pd NPs tightly and improve the stability of Pd@Fe3O4/biochar.

ASSOCIATED CONTENT 23



Supporting Information The surface area and pore structure of catalysts (Table S1), comparison of the catalytic performance with other Pd based catalysts (Table S2), schematic illustration, mass balance during the pyrolysis process, colour change，magnetic hysteresis loop, XPS spectra and catalyst performance of one-pot prepared Pd-Fe@biochar (Figure S1-S6).

ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support from National Natural Science Foundation of China (21677138, 21607147), and Fundamental Research Funds for the Central Universities (WK2060190063), for the support of this work

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Enhancing the Catalytic Activity and Stability of Noble Metal

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