Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D

Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges. University, Yichang 443002, China...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 12740−12749

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Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D Mesoporous Carbon Derived from Cu-Metal−Organic Framework for Efficient Methanol Oxidation and Nitrophenol Reduction Xue-Qian Wu,†,‡ Jun Zhao,† Ya-Pan Wu,† Wen-wen Dong,† Dong-Sheng Li,*,† Jian-Rong Li,‡ and Qichun Zhang*,§ †

College of Material and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, China ‡ Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China § School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore S Supporting Information *

ABSTRACT: The development of novel strategy to produce new porous carbon materials is extremely important because these materials have wide applications in energy storage/ conversion, mixture separation, and catalysis. Herein, for the first time, a novel 3D carbon substrate with hierarchical pores derived from commercially available Cu-MOF (metal−organic framework) (HKUST-1) through carbonization and chemical etching has been employed as the catalysts’ support. Highly dispersed Pt nanoparticles and amorphous nickel were evenly dispersed on the surface or embedded within carbon matrix. The corresponding optimal composite catalyst exhibits a high massspecific peak current of 1195 mA mg−1 Pt and excellent poison resistance capacity (IF/IB = 1.58) for methanol oxidation compared to commercial Pt/C (20%). Moreover, both composite catalysts manifest outstanding properties in the reduction of nitrophenol and demonstrate diverse selectivities for 2/3/4-nitrophenol, which can be attributed to different integrated forms between active species and carbon matrix. This attractive route offers broad prospects for the usage of a large number of available MOFs in fabricating functional carbon materials as well as highly active carbon-based electrocatalysts and heterogeneous organic catalysts. KEYWORDS: metal−organic frameworks, hard template, nanoporous carbon, methanol oxidation, nitrophenol reduction

1. INTRODUCTION

easily fabricated through many different methods including the high-temperature decomposition of organic compounds or polymers, template synthesis, and chemical and physical activation.4 Among them, mesoporous silica, zeolites, and other inorganic solid materials are normally used as promising templates to fabricate NPCs.5 Nevertheless, these methods generally require independent carbon sources or scaffolds, and the as-resulted products usually possess narrow pore-size distribution. Thus, exploring an efficient way to prepare distinctive carbon materials is highly desirable. As a novel type of crystalline porous materials, metal− organic frameworks (referring to MOFs) with highly ordered permanent pore structures and diverse compositions have been

Nanoporous carbons (NPCs) including carbon nanotubes, carbon nanofibers, graphene, and mesoporous carbon show a lot of energy-related applications including conversion/storage, mixture separation, and catalysis because of their good surface functionality, large surface area, high conductivity, multiple pore size distribution, and notable chemical stability.1,2 Among all kinds of carbon structures, NPCs are widely employed as solid carriers for loading/capturing catalysts, restraining the agglomeration of nanoparticles, and improving the interaction between guest species and carriers. Moreover, these carbon materials can be doped by heteroatoms (N, S, P, B, Fe, Co, and Ni) to approach high performance in several electrochemical reactions.3 Thus, reasonable porous systems (such as microstructures or nanostructures) could not only allow the establishment of confinement and selection effect but also be helpful to the mass-transfer processes. Nanoporous carbons are © 2018 American Chemical Society

Received: February 2, 2018 Accepted: March 30, 2018 Published: March 30, 2018 12740

DOI: 10.1021/acsami.8b01970 ACS Appl. Mater. Interfaces 2018, 10, 12740−12749

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To prepare carbon structures, a ceramic boat containing the asobtained HKUST-1 sample (2 g) was first put into the inside of a quartz tube. The calcination was carried out at 550 °C for 6 h inside a furnace. After natural cooling to room temperature, the resulting dullred powder (donated as C550) was etched 3 times in 6 M HCl solution at 80 °C under vigorous stirring for the removal of metal species. After being completely washed with ultrapure water and dried in vacuum, a black powder product was acquired in a 25% yield. Second, a further pyrolysis operation was conducted at 800, 900, and 1000 °C for 6 h to improve its crystalline degree (referred to as NPC-800, -900, or -1000). All carbonization treatments were performed under a stream of N2 (0.5 sccm) at a heating rate of 5 °C/min. 2.1.2. Preparation of Composite Catalyst Materials. A certain amount of NPC-900 and aqueous chloroplatinic acid solution (10 mg L−1) was stirred for 1 h. Then, a freshly prepared potassium borohydride solution (2 mg mL−1) was added dropwisely under ultrasonication. Then, the as-prepared Pt/NPC powder was obtained through filtration and washed with CH3CH2OH 6 times. The same procedure was also employed to fabricate Pt/NPC-800 and Pt/NPC1000 catalysts. The preparation of Ni/NPC-900 was similar to that of Pt/NPC-900 by using nickel nitrate and sodium borohydride in the place of chloroplatinic acid and potassium borohydride. 2.2. Materials Characterization. A Rigaku Ultima IV diffractometer has been employed to measure powder X-ray diffraction (PXRD) patterns of all as-prepared products (Cu Kα radiation, λ = 1.5406 Å). An ESCALAB MKII X-ray photoelectron spectrometer equipped with an Al Kα source was employed to perform X-ray photoelectron spectroscopy (XPS) measurements. An ASAP 2020 surface area and a pore size analyzer were used to measure N2 adsorption/desorption isotherms. A LabRAM Aramis Raman spectrometer was used to record Raman spectra. The morphology of composite materials and their particle size was investigated by highresolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM), equipped with an energy-dispersive X-ray spectroscopy (EDXS). A Shimadzu UV 2550 spectrometer was employed to measure the UV−vis spectra for all samples. 2.3. Procedure for the Oxidation of Methanol. A commercially available glassy carbon electrode (GCE, d = 3 mm) was employed to carry the catalyst materials’ powder. After being polished with Al2O3 particles, carbon electrode was cleaned with ultrapure H2O. Catalytic solution was prepared through dispersing catalyst particles (10 mg) into 0.5 mL of anhydrous ethanol and sonicating for 3 min. The electrode was then coated with 0.5 μL above the suspension and sealed by 1 μL of Nafion solution (0.5 wt % from Aldrich). The as-prepared electrode was air-dried and kept in a desiccator. Cyclic voltammetry (CV) and chronoamperometric (I−T) experiments have been conducted on a CHI660E electrochemical analyzer to observe the electrochemical activity of the as-prepared composite materials. A traditional three-electrode system containing either a Ag/ AgCl (saturated KCl) electrode or a saturated calomel electrode (SCE) as a reference electrode, a modified GCE as a working electrode, and a platinum wire as a counter electrode was applied. Before electrochemical measurements, the GCE coated with the asproduced samples was electrochemically activated with a potential cycling window ranging from −0.2 to 1.2 V (vs SCE) in 0.5 M H2SO4 (−0.2−1.0 V vs Ag/AgCl in 1.0 M NaOH for Ni/NPC-900) until the as-obtained CV curves tend to coincide. Methanol electro-oxidation test for Pt/NPC-800/900/1000 was performed in a 0.5 M H2SO4 solution containing 1.0 M CH3OH with a scanning speed of 50 mV s−1. The electrocatalytic properties for Ni/ NPC-900 were measured in a 1.0 M NaOH + 1.0 M methanol solution. 2.4. Procedure to Reduce Nitrophenol. The catalytic performance of Pt/NPC-900 and Ni/NPC-900 for the reduction of 4-NP and its homologous series (2-NP and 3-NP) was conducted at rt. Generally, 1 mg of catalyst was placed into an aqueous solution containing nitrophenol. The concentrations normally are 20 mg L−1 (4-NP), 100 mg L−1 (2-NP), and 100 mg L−1 (3-NP) for Pt/NPC-900 [60 mg L−1 (2-NP) and 60 mg L−1 (3-NP) for Ni/NPC-900]. Then,

used as suitable precursors for the construction of NPC materials. 6,7 In 2008, Xu and co-workers successfully synthesized porous carbon for the first time through the impregnation of a secondary carbon source within pores of MOF-5.8 After that, various carbon materials have been directly made from several famous MOFs, such as ZIF-8, ZIF-67, ZIF68, MOF-5, MOF-74, and PCN-244. The as-prepared products have become a new family of novel carbon structures with certain morphologies (decided by the MOF precursors).9 Compared to conventional carbon materials, MOF-derived carbons can have a precise control in the porous architecture, pore volumes, and surface area, originating from the inherent diversity of MOFs. Meanwhile, many research studies have been conducted to construct diverse composite catalysts with MOFs as precursors. These as-prepared composites have been widely used for energy conversion-related reactions through electrochemistry including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and Li−air batteries and electrochemical reduction of carbon dioxide.10−13 Although tremendous efforts have been witnessed in the past decades, the research related to the application of MOF-derived carbons for direct methanol fuel cells (DMFCs) is very slow. Owing to their high energy conversion efficiency, low operating temperature, and environmental friendliness, scientists believe that DMFCs should be one of the cleanest and renewable energy sources.14,15 Because of the sluggish anode kinetics of methanol oxidation reaction (MOR), electrocatalysts have become bottlenecks in enhancing the performance of DMFCs.16,17 Previous endeavors focused on using metal/ nonmetal oxides (for instance, TiO2, CeO2, SiO2, and SnO218) and carbon materials as electrocatalyst carriers. Among them, nanoporous carbons can integrate perfect conductivity with excellent mass transfer together, which are significant for the MOR reaction. Herein, for the first time, we present an effective route for the fabrication of nanoporous carbon materials (NPCs) with hierarchical pores based on the famous Cu-MOF (HKUST1). Subsequently, the recombination and structural evolution for NPCs were carried out by using chloroplatinic acid, nickel nitrate, and hydrochloric acid with the aid of ultrasonication, resulting in the formation of composite catalysts (Pt/NPC and Ni/NPC). The representative materials Pt/NPC-900 show outstanding properties in the catalytic reduction of nitrophenol with KBH4 and an enhanced efficiency for the oxidation of CH3OH compared to the commercially available Pt/C (20%). The as-prepared Ni/NPC-900 also exhibits prominent catalytic activities for the above two kinds of reactions, whereas the selectivity to the nitrophenol substrates differs from Pt/NPC900, attributed to different integrated forms between active species and carbon matrix.

2. EXPERIMENTAL SECTION No further purification is required for all commercially available reagents (Alfa Aesar and Aladdin). 2.1. Materials Preparation. 2.1.1. Synthesis of Nanoporous Carbon Materials. Original HKUST-1 was prepared according to the previous report.19 Typically, 250 mg of trimesic acid (H3BTC) was dissolved into 120 mL of mixed solvents (dimethylformamide/ ethanol/H2O 1/1/1 in V/V). Under vigorous stirring, 430 mg of Cu(AC)2 was slowly added. The blue flocculent precipitate was harvested by centrifugal operation (8000 rpm for 10 min) and washed 5 times with ethanol and ultrapure water. Finally, the blue powder product was vacuumed to dry at 80 °C for 3 h. 12741

DOI: 10.1021/acsami.8b01970 ACS Appl. Mater. Interfaces 2018, 10, 12740−12749

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ACS Applied Materials & Interfaces the reaction was initiated upon the addition of 3 mg of KBH4 (18 mg for Pt/NPC-900) into the system. At certain time periods, the absorbance was recorded using UV−vis equipment.

Raman spectra of NPCs are provided in Figure 2b. All three samples display two typical peaks at 1345 and 1595 cm−1, arising from D and G bands. The degree of graphitization of carbon could be evaluated by ID/IG.22 As expected, NPC-1000 has the lowest R value (R = 0.94), which indicates a higher degree of graphitization relative to NPC-800 and NPC-900. Overall, the related porous carbon exhibited partial graphitic crystallites, which could contribute to the enhancement of the catalytic activity and electron transmission. The surface areas of both C550 precursors and NPC-900 products were measured through a Brunauer−Emmett−Teller (BET) method. The N2 sorption isotherm at 77 K of C550 can be classified as a typical II isotherm, corresponding to nonporous solid materials (surface area: 118.4 m2 g−1), whereas the sorption isotherm of NPC-900 reveals a typical reversible type-IV sorption behavior with an obvious hysteresis loop, which represents the characteristic of mesoporous microstructure (BET surface area: 678.3 m2 g−1) (Figure 3). Our experimental results reveal that NPC-900 can provide plenty of spaces and active sites for the deposition of guest species. It is worthy to note that the pore-size distribution of NPC-900 possesses both micropores and mesopores in a certain scope, indicating that chemical etching can be considered as an effective structural evolution route compared with C550. On the basis of the original micropore structure, some new mesopore structures are formed during the acid treatment process; thus, copper species could be defined as hard templates.23 3.2. Characterization and MOR Properties of Composite Materials. Guided by these observations, it is logical to apply NPCs as supporters to construct composite catalysts on account of high surface area and confinement effect. Similarly, the crystallographic structure, chemical environment, elemental composition, and morphology of composite materials were investigated by PXRD, XPS, EDXS, and TEM analysis, respectively. PXRD patterns of Pt/NPC-800/900/1000 display four diffraction peaks centered at 39.9°, 46.5°, 67.7°, and 82.2°, which can be attributed to the planes of (111), (200), (220), and (311) in face-centered cubic Pt particles, respectively. As presented in Figure 4, Pt and C peaks are clearly observed in the XPS spectrum. Two peaks at around 71.0 and 74.4 eV in the XPS spectrum come from Pt 4f7/2 and Pt 4f5/2.24,25 Meanwhile, three peaks (284.6, 285.5, and 286.7 eV) deconvoluted from C 1s spectrum are attributed to CC, C−C, and CO, respectively.26,27 SEM/TEM was further employed to characterize the morphology and detailed structures of the materials. Figure 5

3. RESULTS AND DISCUSSION 3.1. Synthesis and Optimization of Nanoporous Carbon. The route for stepwise structural revolution from HKUST-1 to nanoporous carbon materials is schematically shown in Figure 1. Generally, MOFs were employed as both

Figure 1. Synthetic processes to composite catalytic materials. NPC: nanoporous carbon.

precursors and sacrificial templates to prepare nanoporous carbon structures via the pyrolyzation at high temperatures under a flow of inert gases. After that, the as-obtained composite products were reformed by chemical etching, accompanying the formation of pore structure in the locations that ever belong to the metal species.20 It is known that the pore structure and graphitization degree of carbon materials are closely correlated with their treatment temperature. Thus, the NPCs from different carbonization temperatures were also investigated.21 The typical SEM images of HKUST-1 and the resultant NPCs have provided in the Supporting Information (Figures S2 and S3). The parent HKUST-1 is uniform octahedron-like block samples, whereas both products are presented in nonuniform dispersion states, resulting from the removal of Cu species and thermal treatment. Phase structures of the transition states and products were characterized by PXRD. As shown in Figure 2a, NPC-800/900/1000 displayed two main peaks at around 2θ = 24.4° and 44.0°, which can be assigned to (002) and (101) planes of carbon. Compared to the PXRD patterns of C550 (Figure S4, the carbonization at 550 °C without etching treatment), NPC products did not show any copper peaks, implying that all copper ions have been washed out, and the as-obtained carbon materials should be metal-free.

Figure 2. (a) PXRD patterns of carbons derived from HKUST-1. (b) Raman spectra of carbon materials. 12742

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Figure 3. (a) N2 adsorption/desorption isotherms of NPC-900 and C550 and pore size distribution of the samples (b).

Figure 4. (a) PXRD patterns of Pt/NPC-800/900/1000. (b−d) XPS spectrum for composite materials and the corresponding high-resolution spectrum (taking Pt/NPC-900, for example).

employing the same conditions. The CVs of all catalysts in 0.5 M H2SO4 solution were initially recorded to measure the electrochemical activity surface area and activate the catalyst. Owing to the poor definition of the hydrogen adsorption and desorption regions, derived from the double-layer capacitor effect, the final results are normalized to mass activity.28 As displayed in Figure 6b, the peak at the backward scan is attributed to the oxidation of accumulated intermediates (such as carbon monoxide and formaldehyde), whereas the one at the forward scan attributes to the oxidation of CH 3 OH molecules.29 Although all of the composites display a prominent catalytic behavior for the MOR, the higher current density of 1195 mA mg−1 Pt for Pt/NPC-900 suggests its superior activity, which is about 1.51 times than that of Pt/ NPC-800 (790 mA mg−1 Pt), 1.22 times than that of Pt/NPC1000 (980 mA mg−1 Pt), and 3.4 times than that of commercially available Pt/C (350 mA mg−1 Pt). The tolerance of the catalysts for CO poisoning can be monitored by IF (the ratio of the forward scan peak current) versus IB (the backward scan peak current).30 A higher IF/IB ratio means better

shows a typical TEM image of Pt/NPC-900, suggesting the presence of Pt nanoparticles uniformly distributed on the surface or embedded within the carbon matrix with an average particle size of 2−3 nm (SEM images, Figure S5). Notably, Figure 5a reveals that Pt/NPC-900 possesses a remarkable cellular mesoporous structure, which may provide stable and active sites during further electrochemical tests. The corresponding Pt nanoparticles are polycrystalline as demonstrated by the presence of diffraction rings (selected area electron diffraction pattern in Figure 5e). Elemental mapping indicates that Pt/NPC-900 is manly composed of C and Pt, where Pt is dispersed uniformly throughout the carbon matrix. The above-analyzed results match very well with the results of N2 sorption, PXRD, and XPS, proving the formation of composite catalysts. CV measurements of Pt/NPC-900 in 0.5 M H2SO4 solution show that a clear cathodic peak located near 0.4 V comes from the reduction of PtO. For comparison, the catalysts with the optimized carbon matrixes that were carbonized at different temperatures and commercial Pt/C (20%) were also studied 12743

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Figure 5. (a−e) HRTEM images of Pt/NPC-900 with different magnifications (the mesoporous structure is marked by the white dotted line), [(e) inset] SAED pattern, and (f) EDXS spectrum and elemental mappings of sample.

Figure 6. (a) CV curves of the catalysts in aqueous solution of 0.5 M H2SO4 at a scan rate of 50 mV s−1. (b) Mass-normalized CV curves for CH3OH electro-oxidation of the catalysts in the 0.5 M H2SO4 + 1.0 M CH3OH solution. (c) Chronoamperometric curves at a fixed potential of 0.7 V (vs SCE). (d) Mass-normalized oxidation peak current densities for the catalysts.

fabricate highly dispersed Pt NPs on the surface of reduced graphene oxide/phenyl formaldehyde polymer, and the assynthesized catalyst demonstrated superior improvements to MOR with a current density of 404 mA mg−1 Pt.31 Hierarchical carbon-coated molybdenum dioxide nanotubes can also be constructed as nanostructured supports for the MOR electrocatalyst and exhibited an improved activity (570 mA mg−1 Pt).32 To deeply understand the behaviors of the as-prepared

oxidation conversion of CH3OH into CO2 during the anodic scan and the excessive accumulation of CO on the surface of catalyst. The corresponding IF/IB ratios were calculated to be 1.25 (Pt/NPC-800), 1.58 (Pt/NPC-900), 0.98 (Pt/NPC1000), and 0.82 for commercial Pt/C, indicating that Pt/ NPC-900 also shows a better poison resistance. The Pt/NPC900 sample gave a higher MOR activity compared to the commercially available Pt/C in terms of current density and poison resistance. Yang and co-workers developed a new way to 12744

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Figure 7. (a−e) HRTEM images of Ni/NPC-900 with different magnifications. (c) High-resolution XPS spectrum for nickel species. (f) EDXS spectrum and elemental mappings of sample.

Figure 8. (a) Cyclic voltammograms of CH3OH oxidation at Ni/NPC-900. (b) Cyclic voltammograms for Ni/NPC-900 at different scan rates. (c) Linear relationship between scan rate and current densities. (d) Chronoamperometric curves at different fixed potential.

the appropriate vector, including good electronic conductivity, high surface area, and suitable pore-size distribution. It is noteworthy that previous reports point out that carbon structures containing pore sizes of about 25 nm displayed the best activity for ORR among all porous carbon-supported PtRu catalysts.34 On the basis of these results, we believe that the hierarchical pore structure and the mesopore size (∼17 nm) for Pt/NPC-900 were helpful for the even dispersion of Pt and convenient transport of reactants (mesopores across the microporous matrix provide the required accessibility to

catalysts, some other previously reported materials are summarized in Table S1. In addition, the durability of the catalysts has also been studied through chronoamperometric tests. Figure 6c presents the chronoamperometric curves for CH3OH at 0.7 V for 2000 s. The current densities of four curves declined rapidly, which is probably because of the accumulation of toxic substances and the aggregation of nanoparticles.33 Overall, Pt/NPC-800/900/ 1000 displayed prominent electrocatalytic activities because of 12745

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Figure 9. (a−c) UV−vis spectra showing gradual reduction of 4-NP, 2-NP, and 3-NP over Pt/NPC-900 [(d−f) for Ni/NPC-900]. Inset: Relationship between ln(Ct/C0) and reaction time (t).

reactant molecule for quick diffusion whereas micropores contribute to the high surface area). Furthermore, a plausible but reasonable reaction mechanism for the electro-oxidation process has been concluded as the following two main steps: (1) hydroxymethyl/methoxy adspecies are formed after CH3OH molecule attaches onto the surface of Pt and (2) hydroxymethyl is further converted into CO/HCOO radical/ CO2/HCOOH, whereas methoxy dehydrogenates become HCHO and CH2(OH).35 Inspired by the success in constructing Pt/NPC-900, it is logical to anticipate that NPC-900 should be a good supporter to load Ni species for the formation of effective electrocatalysts. As expected, XRD confirms an amorphous character of the catalyst (without any diffraction peaks of nickel species, as shown in Figure S7), and EDXS spectrum in Figure 7f demonstrates the deposition of nickel metal on NPC-900 powder. To accurately prove the structure and chemical composition of the electrocatalyst, XPS was further undertaken. Generally, amorphous nickel nanoparticles are prepared by the chemical reduction of nickle salt with NaBH4,36 however, the XPS spectra peak of Ni0 was not found, which might be because of the formation of a thin layer of NiO by exposing the sample in air.37 Further analysis shows that the peak of Ni 2p1/2 at 874.1 eV is the divalent Ni2+ in NiO, whereas the peaks of Ni 2p3/2 at 865.2 and 861.6 eV are the Ni2+ in Ni species, suggesting that O2 from air can easily oxidize the Ni element. The detailed morphologies and structure information were observed through TEM and SEM (Figure S9) analysis. Figure 7a shows that NPC-900 is uniformly covered by these Ni species. The distribution of Ni species on the carbon matrix was further confirmed by elemental mapping (Figure 7f). The Ni/NPC-900 exhibits a typical CV behavior in 1.0 M NaOH solution at 50 mV s−1 (Figure 8). The current density starts to increase at 407 mV and reaches an oxidation peak at 566 mV. This result corresponds to the conversion of Ni(OH)2

species into NiOOH through the following electro-oxidation mechanism38 Ni(OH)2 + OH− → NiOOH + H 2O + e−

NaOH solution (1 M) was employed to enrich OH− anions and OOH− species onto the surface of the electrocatalyst, leading to the thicker catalytic layers.39 The addition of 1.0 M methanol leads to a notable change in the CV curve. Accompanying the formation of NiOOH, an oxidation peak was observed at a potential value of 806 mV with a current density of 449.8 mA mg−1. The electrocatalytic active component toward CH3OH is believed to be NiOOH, arising from its empty d-orbitals or unpaired d-electrons and convenient conditions for the bond formation with absorbed species.40 OH− + 4NiOOH + CH3OH → 4Ni(OH)2 + HCOO−

The CVs of Ni/NPC-900 have been conducted in solutions containing 1.0 M NaOH and 1.0 M CH3OH at different scan rates (Figure 8b). Obviously, the current densities of methanol oxidation and NiOOH reduction increase with the increasing scan rate, associated with more negative potential values for cathodic reduction peak. Furthermore, as shown in Figure 8c, there is a linear relationship between the current density and the scan rate, suggesting that the oxidation of methanol is determined by its diffusion speed.41 Such an observation is in accordance with the results for Ni/C samples.42 In recent years, nickel nanoparticles, nickel foam, nickel alloy nanostructures (such as Ni/Cu, Ni/Co and Ni/Mn),43 and other Ni-based species have attracted increasing attentions owing to their excellent activities (current densities from 20 to 65 mA cm−2), cost, and potential as alternative catalysts to the noble metals. As Ni/NPC-900 produces a current density of 449.8 mA mg−1 around 800 mV (vs Ag/AgCl), a further comparison of catalytic 12746

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reduction is strongly affected by the surface environment of catalysts.48 Thus, the catalytic efficiency is strongly decided by the contacting opportunity between the catalytic sites and reactants. For Pt/NPC-900, Pt NPs evenly dispersed onto the surface or embedded within the hierarchical pores and many nitrophenol molecules contact with the catalyst surface through diffusion processes, in which molecular geometry plays a vital role in the final catalytic behavior. Because the geometry of 4NP is linear, the rate diffusion of 4-NP is much higher than 2NP/3-NP, resulting in a larger reaction rate (Figure S12). However, nickel species mostly deposited on the surface of NPCs, creating an equal probability for collision between 2/3/ 4-NP and the catalyst surface. Therefore, two composite materials displayed uneven selectivities toward the reduction of nitrophenol because of the hierarchical pore feature of supporters (Figure 10).49

properties between Ni/NPC-900 and known Ni-containing catalysts is listed in Table S2. Chronoamperometry was also used to further determine the stability and long-term activity of Ni/NPC-900 for MOR. Figure 8d compares the chronoamperograms obtained from the different voltage values. Apparently, the catalytic activity was decreased gradually for all three situations. At the early stage of the reaction, all of these active sites are free to contact with methanol molecules, where carbonaceous species are subsequently formed, leading to decay phenomenon. 3.3. Performance for the Reduction of Nitrophenol. Because the degradation of aqueous nitrophenol is widely used as a good model reaction to assess the behavior of the asfabricated catalysts,44 we also use this type of reactions to assess our catalysts (Pt/NPC-900 and Ni/NPC-900). Generally, strong absorption peaks at 317, 277, or 272 nm can be seen in the solutions of 4-NP, 2-NP, or 3-NP. Upon the addition of KBH4, a yellow-green color will be observed, suggesting the generation of nitrophenolate ions.45 After adding catalysts, the obvious change in the absorption spectra and solution color can be seen in all samples. Figure 9 showed UV absorption during the catalytic reduction of nitrophenol over two different catalysts. Pt/NPC-900 can catalyze the complete decomposition of all three nitroaromatics in the presence of KBH4 within the time range of 6−40 min, whereas Ni/NPC-900 only took 6−8 min. The recyclability tests of both catalysts were conducted by removing the catalysts from reaction through filtration, washing them with deionized water, and reusing them in the next run with similar reaction conditions. The abovementioned two catalysts can be reused at least 3 times (Figure S10). We also investigated the kinetics of each reaction to understand the decomposition speed of nitrophenol on different catalysts (Figure 9). In each reaction, a linear relationship was observed between ln(Ct/C0) and t (reaction time), suggesting that all reactions should be a pseudo-firstorder reaction,46 from which the rate constant k is calculated through the rate equation ln(Ct/C0) = −kt. In general, the apparent rate constant is proportional to the concentration (M, g/L) of the catalysts. To make a quantitative comparison, k′ = k/M was introduced, where k′ is the activity parameter and M is used to exclude the effect of reactant volume change.47 The reaction rate k′ was calculated to be 0.200 s−1 g−1 L (Pt/ NPC-900) and 0.030 s−1 g−1 L (Ni/NPC-900), respectively. Both of the composites outperformed many other Ni/Pt-based catalysts, as judged from the higher activity parameter. A detailed comparison is presented in Table S3. Most impressively, Pt/NPC-900 shows diverse selectivities for different substrates, whereas Ni/NPC-900 shows a similar reaction rate for the above reactions, which can be attributed to different integrated forms between active species and carbon matrix (Table 1). The Langmuir−Hinshelwood mechanism emphasized by Ballauff et al. indicates that the metal-catalyzed nitrophenol

Figure 10. Illustration of the diffusion processes for 2/3/4-NP molecules within the hierarchical pore structure and adsorption state on the surface of amorphous nickel.

4. CONCLUSIONS In summary, a commercially available Cu-MOF (HKUST-1) has been successfully used as templates/precursors to construct nanostructured porous carbons (NPCs) through pyrolysis treatment, followed by chemical etching. NPCs were used as supporters for capturing Pt nanoparticles and amorphous nickel through chemical reduction, and the as-prepared nanoparticles were evenly dispersed onto the surface or embedded within the carbon matrix. Compared with the commercial Pt/C (20%) catalyst, the as-synthesized Pt/NPC-900 shows an enhanced activity for the electro-oxidation of methanol in terms of both oxidation current density and poison-resistant capability. Ni/ NPC-900 composites were also exploited as an electrocatalyst for MOR with remarkable properties. Moreover, both catalysts have been demonstrated to show superior performances in the reduction of nitrophenol with KBH4; however, they displayed different selectivities toward 2/3/4-NP, originating from the hierarchical pore feature of catalyst supporters. Our results clearly indicate that MOF-converted porous carbons with tunable properties (including surface area, pore size, component, structure, and so on) would endow them more opportunities to be applied in heterogeneous organic catalysis and energy electrocatalysis.

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Table 1. Catalytic Behaviors of Composite Catalysts in Reduction Reaction

ASSOCIATED CONTENT

S Supporting Information *

reaction substrate catalysts

4-NP

2-NP

3-NP

Pt/NPC-900 Ni/NPC-900

0.200 s−1 g−1 L 0.030 s−1 g−1 L

0.020 s−1 g−1 L 0.023 s−1 g−1 L

0.022 s−1 g−1 L 0.015 s−1 g−1 L

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01970. Additional XRD data, SEM/TEM images, EDXS spectrum, and catalytic test data (PDF) 12747

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-S.L.). *E-mail: [email protected] (Q.Z.). ORCID

Dong-Sheng Li: 0000-0003-1283-6334 Jian-Rong Li: 0000-0002-8101-8493 Qichun Zhang: 0000-0003-1854-8659 Author Contributions

X.-Q.W. and J.Z. contributed equally to the work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (nos. 21673127, 21373122, 21671119, 51572152, and 51502155). REFERENCES

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.8b01970 ACS Appl. Mater. Interfaces 2018, 10, 12740−12749