Facile Route Fabrication of Steam Exploded Poplar Loaded with Non-metal Doped Ni-Fe Nanoparticles: Catalytic Activities in 4-nitrophenol Reduction and Electrocatalytic Reaction

A simple and effective method for the preparation of non-metallic ion-doped nickel-supported catalyst is reported. Using economical and recyclable ber raw materials as carriers, nickel-supported catalysts are prepared by adsorption and reduction at room temperature. The nanoparticles dispersedly anchored on a rational support can eciently inhibit the aggregation and thus enhance the catalytic activity. For the model catalytic hydrogenation of 4-NP by NaBH 4 , the N-B-Ni 2 P/SEP and N-B-NiFeP/SEP catalysts exhibited much better catalytic performances than other catalysts recently reported in terms of the catalytic activity (reaction completed within 5 min), reaction rate constant (1.617 and 0.765 min − 1 ) and the activity factor K (539 and 255 min − 1 ·g − 1 ), respectively. The catalyst showed activities for electrocatalytic HER and OER under ambient conditions. In general, the reported preparation method of nickel-supported catalyst is convenient, economical and environmentally friendly, which is in line with many green chemistry and sustainable development principles and widely available starting materials.


Introduction
The energy crisis and environmental pollution require clean and renewable energy to replace fossil fuels widely used by electricity. As an alternative energy carrier, hydrogen is becoming an important part of the future energy system because of its high energy density and environment-friendly. In recent years, hydrogen production by hydrolysis is an economical and feasible method. At present, the development of catalysts to improve hydrolysis e ciency is an urgent problem to be solved, so a variety of rare earth-rich non-precious metal catalysts have been developed. it contains transition metal compounds such as sul des, phosphates, carbides, nitrides, oxides and selenides to solve the problem. The addition of nonmetallic elements (O, S or N) to the transition metal-based electrocatalyst can also adjust the kinetics of the reaction and improve its catalytic activity. (

2018)
Nickel-supported catalysts have attracted wide attention in the hydrogenation, oxygen reduction and ole n oxidation of nitrobenzene and nitrophenol because of their low price and excellent catalytic performance. In order to facilitate the recovery of catalysts, nickel particles are usually dispersed on solid matrix to prepare heterogeneous nickel catalysts. Therefore, various materials are used as carriers, including silica, alumina, titanium dioxide, zirconia, magnesia, carbon and so on. Among them, porous carbon carrier is the most commonly used economic carrier, and carbon-based carriers such as cellulose paper with chemical deposition of metallic nickel particles on the surface (Sahasrabudhe A et al. 2018), nickel based mesoporous carbons (Yang Y et al. 2014), In-situ preparation of Ru nanoclusters and porous carbon (Ding R et al. 2020) have many advantages over other carriers because of their chemical inertia and good stability. Compared with other carbon materials, the advantages of carbonized ber obtained from biomass include easier availability, easier regeneration and lower cost (Lai C et al. 2019).
Meanwhile, as the emphasis of technology is gradually shifting towards green synthetic strategy, the utilization of nontoxic, renewable and environmentally benign chemicals are required (C. ; Ming YuKuo et al. 2019). How to design high-value products with long life, reusability, cost-effectiveness and high e ciency is particularly important. The products of agricultural, industrial or forestry wastes are complex and di cult to separate, so how to transform them into speci c products of a speci c nature and complexity is particularly critical. How to transform it into a speci c product of a speci c nature and complexity is particularly critical. Other complex factors include excessive chemicals accumulated during the use of the product, natural aging, the recycling process itself and the ow of materials and products associated with it (Kümmerer K et al. 2020). Steam explosion is an optional pretreatment technology in the eld of biomass conversion. The particle sizes of different types of agricultural wastes are different after steam blasting. In the process of steam explosion treatment of biomass raw materials, a large amount of water vapor permeates into biomass raw materials and forms hydrogen bonds with some hydroxyl groups on the cellulose molecular chain. At the same time, the condition of high temperature and high pressure aggravates the fracture of hydrogen bond in cellulose, releases new hydroxyl groups, increases the speci c surface area of cellulose and increases the adsorption capacity of blasting products. Lignocellulose is an ideal carbon source for the preparation of carbon carriers because of its renewability and rich hydroxyl groups. In the process of preparation, it is of great signi cance to try to embed nickel nanoparticles directly into carbon materials to prepare "embedded" catalysts.
Here, we report a simple and effective method for the preparation of non-metallic ion-doped nickelsupported catalyst. using economical and recyclable ber raw materials as carriers, nickel-supported catalysts are prepared by adsorption and reduction at room temperature. among them, non-metallic ions and Ni-Fe metal particles are highly dispersed. The nanoparticles dispersedly anchored on a rational support can e ciently inhibit the aggregation and thus enhance the catalytic activity. (Fu Y.K et al. 2019) Non-metallic ion-doped nickel-supported catalysts have catalytic activity and durability, and can be used in various catalytic reactions, such as electrochemical reaction, 4-nitrophenol reduction and so on. In general, the reported preparation method of nickel-supported catalyst is convenient, economical and environmentally friendly, which is in line with many green chemistry and sustainable development principles and widely available starting materials.

Materials
Poplar was steam exploded at 213°C for 5 min. The compositional analysis of steam exploded poplar (SEP) on a dry basis was carried out. Analytical grade hydrogen sodium borohydride (NaBH 4 ), 4nitrophenol (4-NP), were procured from Sigma-Aldrich (Shanghai, China). Nickel nitrate hexahydrate, Iron nitrate nonahydrate, ethanol, were analytical grade and procured from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used without further puri cation.

Nickel Supported Catalyst Preparation
Steam-exploded poplar (SEP) prepared at 213°C for 5 min by steam explosion. 2g steam-exploded poplar (SEP) and Nickel nitrate hexahydrate (5mmol) were dispersed to 100 mL of deionized water for 15 minutes under ultrasonic treatment, and stirred for 40 minutes to completely dissolve. After completion of Ni 2+ and SEP, 5 mL of NaBH 4 (0.5 mol/L) solution was added, and the boron-containing metal oxide was grown vertically in situ in SEP at room temperature. The resulting product was collected by centrifugation and washing with deionized water and ethanol, and then dried in vacuo to dry overnight. The carbon material was hereafter referred to as N-B-Ni/SEP. Thereafter, the obtained product was used to pyrolyze the feedstock as follows: the precursor and 200 mg NaH 2 PO 2 will be prepared in both ends of the porcelain. Temperature was increased at a rate of 5°C/min under nitrogen protection, and the N-B-Ni/SEP was held for 1.5 hours at 350°C, followed by cooling to room temperature inside the furnace. The carbon material was hereafter referred to as N-B-Ni 2 P/SEP. N-B-NiFeP/SEP was prepared by adding Nickel nitrate hexahydrate (5mmol) and Iron nitrate nonahydrate (5mmol), under the same conditions.

Characterization
The Fourier transform infrared spectrometer (Karlsruebrook, Germany) used KBr pellet technology to measure FT-IR. The ZEISS Merlin roller was observed by scanning electron microscope (SEM) under 10kV voltage. Energy dispersive spectroscopy (EDS) was used to determine the element composition. The crystal structure of the sample was analyzed by Ultima IV X-ray diffractometer. The working voltage of the X-ray diffractometer was 40 kV, and the current density was 30 mA. X-ray photoelectron spectroscopy (XPS) analysis was carried out by using ESCALAB 250 analyzer (Thermo Science) and monochromatic Al Ka X-ray source. The adsorption-desorption isotherm of nitrogen was determined by BELSORP-mini II instrument and Brunauer-Emmett-Teller (BET) method. The UV-vis absorption spectra were recorded via a UV-2900 spectrophotometer (Hitachi, Japan). Inductive coupling plasma emission spectrometer (ICP-OES) was carried out by using PerkinElmer 8300 analyzer.

Catalytic Reduction Of 4-np
The reduction of 4-nitrophenol (4-NP) was carried out in a quartz cuvette and monitored using UV-vis spectroscopy (Hitachi UV-2900) at room temperature. For comparison, the aqueous 4-NP solution (0.01 M) was prepared and measured prior to monitoring the change of absorption. Then a total of 25 µl of aqueous 4-NP solution was mixed with 2.5 ml of a fresh NaBH 4 (0.01 M) solution. Subsequently, a given amount of nickel catalyst was added to start the reaction, and the UV spectrometry was employed to in situ monitor the reduction by measuring the absorbance of the solution at 400 nm over time.

Electrochemical Characterization
The electrochemical measurement was carried out at room temperature using a three-electrode device using CHI760E electrochemical workstation. The glassy carbon electrode was the working electrode (opposite electrode) and the Ag/AgCl electrode is the reference electrode. The linear sweep voltammogram (LSV) was recorded at the scanning rate of 5mV/s in 1.0M KOH electrolyte for OER, and in 0.5M H 2 SO 4 for HER. The scanning range was 1.0-1.8vs.RHE. The linear sweep voltammetry curve was obtained by the scanning rate of 5mV/s, and the linear sweep voltammetry curve was corrected by 90% IR compensation method. According to the Nernst equation (ERHE = EAg/AgCl + 0.059·pH + 0.197), the measured potential was converted into the corresponding reversible hydrogen electrode potential. The current density (J) was normalized to the geometric surface area, and the measured potential EAppl (vs.Ag/AgCl) was converted into reversible hydrogen electrode (RHE). The overpotential (η) of OER when the current density was 10mA/cm 2 is calculated by using the equation (η = E RHE -1.23V). The overpotential (η) of HER when the current density was 10mA/cm 2 is calculated by using the equation (η = E RHE ). NiFe coating deposited at room temperature did not change the ber structure of steam-exploded poplar ( Fig. 2a-b). It can be seen from nickel-plated iron or nickel scanning electron microscope (SEM) that there are a large number of voids in the bracket. SEM images ( Fig. 2a-b) show the growth of NiFe layer Ni-P vertically arranged nano-thin sheets, with interconnected macroporous morphology, will not hinder the underlying macroporous structure. This interesting morphology is bene cial to electrocatalysis because it provides a large number of exposed catalytic active sites and enables electrons to travel rapidly along vertical nano akes. Energy dispersive X-ray (EDX) spectroscopy was used to further characterize the elemental composition and distribution of the Nmai-B-talk NiFeP-hand EDS samples by EDS surface scan ( Fig. 2d-g). The results show that Ni, Fe, P, B and N are uniformly distributed in the sample, and the atomic ratio is 1.28 (Ni): 1.21 (Fe). It is further con rmed that B and N atoms have been successfully entered into SEP. The above results further prove that the NMagi B talk NiFeP hand SEP is successfully realized by introducing zero valent NMagi B atoms.
The detailed structural features of the obtained sample were rstly investigated by X-ray diffraction On the basis of these results, we come to the conclusion that the metal particles can be well dispersed on the surface of the ber, and the doping of Fe will affect the crystallinity of the alloy particles. The above results showed that the addition of Ni could effectively promote the miniaturization of Fe grains (Mansouriieh N et al. 2016). The XRD pattern of NiFe/Ni-P electrode (Fig. 3) further con rms the amorphous nature of NiFe catalyst layer as no new peaks are observed besides those from the catalyst. In fact, it has been proposed that amorphous NiFe electrocatalysts are much more active than their crystalline counterparts due to their structural exibility and a high density of co-ordinatively unsaturated sites that help in the adsorption of oxidized intermediates. The XPS survey scan spectrum (Fig. 4a) clearly con rmed that all Ni, Fe, B, P, N, O, and C elements were in the samples. According to XPS analysis, the contents of Ni and Fe in N-B-NiFeP/SEP were measured to be 11.14 and 14.84 wt% (Table 1) As shown in Fig. 4b, the four components of C1s spectrum (284.77, 286.36, 288.49 and 291.54 eV) were attributed to sp2 C-C, sp3 C-C, C-O and carboxylic groups, respectively. In the high-resolution XPS spectra of P 2p exhibits three contributions, P 2p3/2 and P 2p1/2, located at respectively 129.49 and 130.46 eV (Fig. 4e), which can be assigned to Ni 2 P, and the peak at 133.72 eV can be caused by oxidized P species.
The B 1s spectrum (Fig. 4f)  As for the high-resolution N 1s spectrum, in addition to the characteristics related to pyrrolic-N (402.07 eV) and pyridinic-N (400.00 eV), a characteristic peak with a 397.10 eV binding energy is observed in the N regions, that is ascribed to metal-nitrogen bonds, indicating zero valent N(0) atoms in the N-B-NiFeP/SEP and N, B-Ni 2 P/ SEP (Fig. 4d). The presence of N dopant in the sample will inherently improve the interaction ability with the reactants and produce a higher positive charge density on its adjacent carbon atoms, which may also contribute to the high activity of the sample. The speci c surface area and porosity of the obtained materials have been investigated by N 2 adsorption-desorption experiments. In the curves of N-B-NiFeP/SEP and N-B-Ni 2 P/SEP (Fig. 6a), the type IV adsorption branches were corresponding to the mesoporous structure. According to IUPAC classi cation, the isotherms (Fig. 6) of the mixed oxides were classi ed as type IV with an H3 hysteresis loop, suggesting the existence of mesoporous materials with an incision-like pore geometry. The speci c surface area of N-B-NiFeP/SEP and N-B-NiP/SEP were calculated to be 55.44 and 57.18 m 2 ·g − 1 , respectively. The pore size distributions are shown in Fig. 6b. The average pore size of N-B-NiFeP/SEP was about 11.86 nm, while those of N-B-Ni 2 P/SEP was around 8.42 nm. It was clear that N-B-NiFeP/SEP and N-B-Ni 2 P/SEP were mainly composed of micropores and mesopores around 10 nm. As shown in Fig. 6d, the average pore widths of three samples follow the order of N-B-NiFeP/SEP > N-B-Ni 2 P/SEP and the pore volumes of N-B-NiFeP/SEP and N-B-Ni 2 P/SEP were 0.19 and 0.11 cm 3 ·g − 1 , respectively. The pore structure of materials played an important and even decisive role in many properties of materials. Carbon materials as carriers, their porous properties were conducive to the diffusion of substrates and products, and can expose more active sites, thus improving the overall activity of the catalyst.

Oxygen evolution reaction
The electrocatalytic OER performance of N-B-NiFeP/SEP and N-B-Ni 2 P/SEP were studied in O 2 -saturated 1 M KOH. The linear sweep voltammograms (LSVs) data ( Fig. 7a-b) was recorded with the scan rate of 5 mV•s − 1 . N-B-Ni 2 P/SEP showed that the Ni 2+ /Ni 3+ was oxidized in the potential range of 1.35-1.5 V (all potentials were versus reversible hydrogen electrode (RHE)) (Fig. 7b). The existence of the oxidation peak indicated that the insu cient oxidation may not form a fully protected NiO shell outside the Ni nanoparticles, leading to corrosion of metal Ni and the formation of NiOOH during OER in alkaline solution (Sivanantham A et al. 2016). The curves of polarization ( Fig. 7a-b) showed that the N-B-NiFeP/SEP exhibited excellent OER performance with a overpotential of 395mV at 10 mA•cm − 2 and 488mV at 30 mA•cm − 2 current density, compared to N-B-Ni 2 P/SEP (431 and 579 mV).
In addition, to investigate the kinetics of these catalysts, the Tafel slopes obtained from the LSV polarization curves were shown in Fig. 7c.

Hydrogen evolution reaction
To assess the electrocatalytic HER activity of the N-B-NiFeP/SEP and N-B-Ni 2 P/SEP, the related electrochemical measurements were performed using a three-electrode system. Figure 7d showed the polarization curves of the N-B-NiFeP/SEP and N-B-Ni 2 P/SEP in N 2 -saturated 0.5 M H 2 SO 4 solution. In comparison to N-B-Ni 2 P/SEP, which shows a η10 value of 397 mV, the N-B-NiFeP/SEP requires 392 mV to reach 10 mA cm − 2 , which means that the Fe trace in N-B-NiFeP/SEP do not contribute to the electrochemical activities and remain as mere spectator species. Tafel slopes were drawn to evaluate HER kinetics (Fig. 7e). The Tafel slope is 122 mV dec − 1 for N-B-NiFeP/SEP, which is much smaller than that N-B-Ni 2 P/SEP (119 mV dec − 1 ). In the study of the mechanism of electrocatalytic hydrogen evolution in acidic media, it is generally believed that the reaction process is divided into the following three steps: the rst step is the electrochemical reaction process, the second step is the electrochemical desorption process, and the third step is the compound desorption process. The general hydrogen evolution reaction mechanism includes at least an electrochemical process and a desorption process, so it can be divided into Volmer-Heyrovsky mechanism or Volmer-Tafel mechanism according to different rate steps. As can be seen from the diagram, the Tafel slope of the N-B-NiFeP/SEP and N-B-Ni 2 P/SEP is 122 and 119 mV dec − 1 , respectively. So the hydrogen evolution process of the catalyst in acidic medium is a slow discharge mechanism, and the Volmer reaction process is a rate control step, which is Volmer-Heyrovsky mechanism (B. Therefore, the reduction of 4-NP toward 4aminophenol (4-AP) in the presence of NaBH 4 was selected as a model reaction to further con rm the generality of N-B-NiFeP/SEP and N-B-Ni 2 P/SEP. As shown in Fig. 8a, the adsorption peak of 4-NP was red-shifted from 317 to 400 nm immediately upon the addition of NaBH 4 solution which corresponds to a color change from light yellow to yellow green, due to the formation of the 4-nitrophenolate ion under alkaline conditions. When catalyst was added, the intensity of characteristic peak at 400 nm quickly declined. The reduction of 4-NP was completed within 10 min over 3 mg N-B-Ni 2 P/SEP and N-B-NiFeP/SEP (Fig. 8c). Considering that the reductant concentration is much higher than that of 4-NP (C NaBH4 /C 4 − NP = 100) in the reaction mixture, the pseudo-rst-order rate kinetics with respect to 4-NP concentration could be used to evaluate the catalytic rate. The reaction kinetics can be described as − ln(C t /C 0 ) = kt, where k is the rate constant at a given temperature and t is the reaction time. C 0 and C t are the 4-NP concentration at the beginning and at time t, respectively. As expected, a good linear correlation of ln(C t /C 0 ) vs. reaction time t was obtained (Fig. 8b), whereby the kinetic rate constant k was estimated to be 1.617 (R 2 = 0.99) and 0.765 (R 2 = 0.99) min − 1 for N-B-Ni 2 P/SEP and N-B-NiFeP/SEP. To compare different catalysts, we calculated the ratio of rate constant K over total weight of nickel catalyst, where K = k/m. Thus the activity factor K was calculated to be 539 and 255 min − 1 ·g − 1 for N-B-Ni 2 P/SEP and N-B-NiFeP/SEP, respectively. It is clear that N-B-Ni 2 P/SEP shows the largest activity factor.
The excellent catalytic performances of N-B-Ni 2 P/SEP for 4-nitrophenol reduction can be contributed to the following featured advantages. From the point of view of catalysis, steam explosion poplar is an ideal substrate for the growth of active catalyst layer. Because there are abundant coordination hydroxyl groups and epoxy functional groups on the cellulose micro ber, the ultra-ne and clean metal nanoparticles formed in situ are uniformly dispersed on the surface of the carrier rather than embedded in the carrier. Together, these two functions can lead to stronger binding and faster mass transfer kinetics.

Conclusions
In summary, using economical and recyclable ber raw materials as carriers, nickel-supported catalysts are prepared by adsorption and reduction at room temperature. For the model catalytic hydrogenation of 4-NP by NaBH 4 , the N-B-Ni 2 P/SEP and N-B-NiFeP/SEP catalysts exhibited much better catalytic performances than other catalysts recently reported in terms of the catalytic activity (reaction completed within 5 min) and reaction rate constant (1.617 and 0.765 min − 1 ). The catalyst showed activities for electrocatalytic HER and OER under ambient conditions. In general, the reported preparation method of nickel-supported catalyst is convenient, economical and environmentally friendly, which is in line with many green chemistry and sustainable development principles and widely available starting materials.

Declarations
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