Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D

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Energy, Environmental, and Catalysis Applications

Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D Mesoporous Carbon Derived from Cu-MOF for Efficient Methanol Oxidation and Nitrophenol Reduction Xue-Qian Wu, Jun Zhao, Ya-Pan Wu, Wen-Wen Dong, Dongsheng Li, Jian-Rong Li, and Qichun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01970 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Materials & Interfaces

Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D Mesoporous Carbon Derived from Cu-MOF for Efficient Methanol Oxidation and Nitrophenol Reduction Xue-Qian Wu,1,2‡ Jun Zhao,1‡ Ya-Pan Wu,1 Wen-wen Dong,1 Dong-Sheng Li,*1 Jian-Rong Li,2 Qichun Zhang*3 1

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. 2

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. 3

School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore. ‡These authors contributed equally to the work.

Keywords: Metal-organic frameworks, hard template, nanoporous carbon, methanol oxidation, nitrophenol reduction 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 (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 mass-specific 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 the different integrated form 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.

1. Introduction Nanoporous carbons (NPCs) including CNTs (carbon

species and carriers. Moreover, these carbon materials can be

nanotubes), carbon nanofibers, graphene, and mesoporous

doped by heteroatoms (N, S, P, B, Fe, Co and Ni etc.) to

carbon, show a lot of energy-related applications including

approach high performance in several electrochemical

conversion/storage, mixture separation, and catalysis due to

reactions.3 Thus, reasonable porous systems (such as

their good surface functionality, large surface area, high

microstructures or nanostructures) could not only allow the

conductivity, multiple pore size distribution, and notable

establishment of confinement and selection effect, but also be

chemical stability.1,2 Among all kinds of carbon structures,

helpful to the mass transfer processes. Nanoporous carbons are

NPCs

for

easily fabricated through many different methods including the

loading/capturing catalysts, restraining the agglomeration of

high-temperature decomposition of organic compounds or

nanoparticles, and improving the interaction between guest

polymers,

are

widely

employed

as

solid

carriers

template

1

ACS Paragon Plus Environment

synthesis,

chemical

and

physical

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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activation.4 Among them, mesoporous silica, zeolites and other

conductivity with excellent mass transfer together, which are

inorganic solid materials are normally used as promising

significant for MOR reaction.

5

templates to fabricate NPCs. Nevertheless, these methods

Herein, for the first time, we present an effective route for

generally require independent carbon sources or scaffolds and

the fabrication of nonoporous carbon materials (NPCs) with

the as-resulted products usually possess narrow pore size

hierarchical pores based on the famous Cu-MOF (HKUST-1).

distribution. Thus, exploring an efficient way to prepare

Subsequently, the recombination and structural evolution for

distinctive carbon materials is highly desirable.

NPCs were carried out by using chloroplatinic acid, nickel

As a novel type of crystalline porous materials, MOFs

nitrate and hydrochloric acid with the aid of ultrasonication,

(referring to metal-organic frameworks) with highly ordered

resulting in the formation of composite catalysts (Pt/NPC and

permanent pore structures and diverse compositions have been

Ni/NPC). The representative materials Pt/NPC-900 show

used as suitable precursors for the construction of NPCs

outstanding properties in the catalytic reduction of nitrophenol

materials.

6,7

In 2008, Xu and co-workers successfully

with KBH4, and an enhanced efficiency for the oxidation of

synthesized porous carbon for the first time through the

CH3OH comparing to the commercially-available Pt/C (20%).

impregnation of a secondary carbon source within pores of

The as-prepared Ni/NPC-900 also exhibits prominent catalytic

8

MOF-5. After that, various carbon materials have been

activities for above two kinds of reactions, whereas the

directly made from several famous MOFs, such as ZIF-8,

selectivity to

ZIF-67, ZIF-68, MOF-5, MOF-74 and PCN-244. The

Pt/NPC-900, attributing to the different integrated forms

as-prepared products have become a new family of novel

between active species and carbon matrix.

the

nitrophenol

substrates

differs

from

carbon structures with certain morphologies (decided by the MOF precursors).9 Comparing

to

conventional

carbon

2. Experimental Section

materials, MOF-derived carbons can have a precise control in

No

the porous architecture, pore volumes and surface area,

further

purification

is

required

for

all

commercially-available reagents (Alfa Aesar and Aladdin).

originating from the inherent diversity of MOFs. Meanwhile, many researches have been conducted to construct diverse

2.1 Materials preparation

composite catalysts with MOFs as precursors. These

2.1.1 Synthesis of nanoporous carbon materials

as-prepared composites have been widely used for energy conversion-related

reactions

through

Original HKUST-1 was prepared according to the previous report.19 Typically, 250 mg trimesic acid (H3BTC) was

electrochemistry

including ORR (oxygen reduction reaction), HER (hydrogen

dissolved into 120 ml mixed solvents (DMF/ethanol/H2O

evolution reaction), OER (oxygen evolution reaction), and as

1/1/1 in V/V). Under vigorous stirring, 430 mg Cu(AC)2 was

well as Li-air batteries and electrochemical reduction of

slowly added. The blue flocculent precipitate was harvested by

carbon dioxide.

10-13

Although tremendous efforts have been

centrifugal operation (8000 rpm for 10min), and washed five

witnessed in past decades, the research relate to the application

times with ethanol, and ultrapure water. Finally, the blue

of MOF-derived carbons for DMFCs (direct methanol fuel

powder product was vacuumed to dry at 80oC for 3 h.

cells) is very slow.

To prepare carbon structures, a ceramic boat containing the

Owing to their high energy conversion efficiency, low operating

temperature,

and

friendliness,

inside of a quartz tube. The calcination was carried out at

scientists believe that DMFCs should be one of the cleanest

550oC for 6 h inside a furnace. After natural cooling to room

and renewable energy sources.

environmental

as-obtained HKUST-1 sample (2 g) was firstly put into the

14,15

temperature, the resulting dull-red powder (donated as C550)

Due to the sluggish anode reaction),

was etched three times in 6 M HCl solution at 80 oC under

electrocatalysts have become bottlenecks in enhancing the

vigorously stirring for the removal of metal species. After

kinetics

of

MOR

performance of DMFCs.

(methanol 16,17

oxidation

Previous endeavors focused on

completely washed with ultrapure water and dried in vacuum,

using metal/nonmetal oxides (for instance, TiO2, CeO2, SiO2,

a black powder product was acquired in a 25% yield. Secondly,

18

SnO2 ) and carbon materials as electrocatalyst carriers.

a further pyrolysis operation was conducted at 800, 900 and

Among them, nanoporous carbons can integrate perfect

1000oC for 6 h to improve its crystalline degree (referred to as 2


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NPC-800, -900, or -1000). All carbonization treatments were

Cyclic voltammetry (CV) and chronoamperometric (I-T)

performed under a stream of N2 (0.5 s.c.c.m) at a heating rate

experiments

o

of 5 C/min.

have

been

conducted

on

a

CHI660E

electrochemical analyzer to observe the electrochemical activity of the as-prepared composite materials. A traditional three-electrodes system containing either a Ag/AgCl (saturated

2.1.2 Preparation of composite catalyst materials A certain amount of NPC-900 and aqueous chloroplatinic

KCl) electrode or a saturated calomel electrode (SCE) as a

-1

acid solution (10 mg L ) was stirring for 1 h. Then a

reference electrode, a modified glassy carbon electrode as a

-1

freshly-prepared potassium borohydride solution (2 mg ml )

working electrode, and a platinum wire as a counter electrode,

was added dropwisely under ultrasonication. Then, the

was applied. Before electrochemical measurements, the GCE

as-prepared Pt/NPC powder was obtained through filtration

that coated with as-produced samples was electrochemically

and washed with CH3CH2OH for six times. The same

activated with a potential cycling window ranging from -0.2 to

procedure was also employed to fabricate Pt/NPC-800 and

1.2 V (vs. SCE) in 0.5 M H2SO4 (-0.2 V - 1.0 V vs. Ag/AgCl in

Pt/NPC-1000 catalysts.

1.0 M NaOH for Ni/NPC-900) until the as-obtained CV curves

The preparation of Ni/NPC-900 was similar to that of

tend to coincide.

Pt/NPC-900 by using nickel nitrate and sodium borohydride in

Methanol electro-oxidation test for Pt/NPC-800/900/1000

place of chloroplatinic acid and potassium borohydride.

were 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

2.2 Materials characterization A Rigaku Ultima Ⅳ diffractometer have been employed to

1.0 M NaOH + 1.0 M methanol solution.

measure PXRD (powder X-ray diffraction) patterns of all as-prepared products (Cu Kα radiation, λ = 1.5406 Å). An ESCALABMKLL

X-ray

photoelectron

2.4 Procedure to reduce nitrophenol

spectrometer

The catalytic performance of Pt/NPC-900 and Ni/NPC-900

equipping with an Al Kα source were employed to perform

for the reduction of 4-NP and its homologous series (2-NP and

XPS (X-ray photoelectron spectroscopy) measurements. An

3-NP) were conducted at r.t. Generally, 1 mg of catalyst was

ASAP 2020 surface area and pore size analyzer was used to

placed into an aqueous solution containing nitrophenol. The

maesure N2 adsorption/desorption isotherms. A LabRAM Ara

concentrations normally are 20 mg L-1 (4-NP), 100 mg L-1

mis Raman Spectrometer was used to record Raman spectra.

(2-NP) and 100 mg L-1 (3-NP) for Pt/NPC-900 (60 mg L-1

The morphology of composite materials and their particle size

(2-NP), 60 mg L-1 (3-NP) for Ni/NPC-900). Then, the reaction

were investigated by HRTEM (high-resolution transmission

was initiated upon the addition of 3 mg KBH4 (18 mg for

electron microscopy) and SEM (scanning electron microscopy)

Pt/NPC-900) into the system. At certain time periods, the

equipped with an EDS (energy dispersive X-ray spectroscopy).

absorbance was recorded using a UV-vis equipment.

A Shimadzu UV 2550 spectrometer was employed to measure the UV−vis spectra for all samples.

3. Results and discussion 3.1. Synthesis and optimization of nanoporous carbon The route for stepwise structural revolution from HKUST-1

2.3. Procedure for the oxidation of methanol A commercially-available GCE (glassy carbon electrode, d

to nanoporous carbon materials is schematically shown in

= 3 mm) was employed to carry catalyst materials powder.

Figure 1. Generally, MOFs were employed as both precursors

After polished with Al2O3 particles, carbon electrode was

and sacrificial templates to prepare nanoporous carbon

cleaned with ultrapure H2O. Catalytic solution was prepared

structures via the pyrolyzation at high temperatures under a

through dispersing catalyst particles (10 mg) into 0.5 ml

flow of inert gases. After that, the as-obtained composite

anhydrous ethanol and sonicating for 3 min. The electrode was

products were reformed by chemical etching, accompanying

then coated with 0.5µl above suspension and sealed by 1µl

with the formation of pore structure in the locations that ever

Nafion solution (0.5 wt% from Aldrich). The as-prepared

belong to the metal species.20 It is known that the pore

electrode was air-dried and kept in a desiccator.

structure and graphitization degree of carbon materials are 3



Page 4 of 15

closely correlated to their treatment temperature. Thus, the

sorption isotherm at 77K of C550 can be classified as a typical

NPCs from different carbonization temperature were also

Ⅳ isotherm, corresponding to nonporous solid materials

investigated.

21

(surface area: 118.4 m2 g-1) while the sorption isotherm of NPC-900 reveals 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

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

3.2. Characterization and MOR properties of composite materials

The typical SEM images of HKUST-1 and resultant NPCs have provided in Supporting Information (Figure S2 and S3).

Guided by these observations, it is logical to apply NPCs as

The parent HKUST-1 is uniform octahedron-like block

supporters to construct composite catalysts on account of high

samples, while both products are presented non-uniform

surface

dispersion states, resulting from the removal of Cu species and

crystallographic structure, chemical environment, elemental

thermal treatment. Phase structures of the transition states and

composition and morphology of composite materials were

products were characterized by PXRD. As shown in Figure 2a,

investigated by pXRD, XPS, EDS and TEM analysis,

NPC-800/900/1000 displayed two main peaks at around 2θ =

respectively. pXRD patterns of Pt/NPC-800/900/1000 display

area

and

confinement

effect.

Similarly,

the

24.4 and 44.0 , which can be assigned to (002) and (101)

four diffraction peaks centered at 39.9o, 46.5o, 67.7o, 82.2o,

planes of carbon. Comparing to the PXRD patterns of C550

which can be attributed to the planes of (111), (200), (220) and

o

o

o

(Figure S4, the carbonization at 550 C without etching

(311) in fcc Pt particles. As presented in Figure 4, the Pt and C

treatment), NPC products didn’t show any copper peaks,

peaks are clearly observed in XPS spectrum. Two peaks at

implying that all copper ions have been washed out and the

around 71.0 and 74.4 eV in XPS spectrum come from Pt 4f7/2

as-obtained carbon materials should be metal-free. Raman

and Pt 4f5/2.24,25 Meanwhile, three peaks (284.6 eV, 285.5 eV,

spectra of NPCs are provided in Figure 2b. All three samples

and 286.7 eV) deconvoluted from C1s spectrum are attributed to C=C, C-C and C=O, respectively.26,27

-1

display two typical peaks at 1345 and 1595 cm , 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 BET method. The N2 4


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

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

SEM/TEM were further employed to characterize the

Cyclic voltammetry (CV) measurements of Pt/NPC-900 in

morphology and detailed structures of the materials. Figure 5

0.5 M H2SO4 solution show that a clear cathodic peak located

shows a typical TEM image of Pt/NPC-900, suggesting the

near 0.4 V comes from the reduction of PtO. For comparison,

presence of Pt nanoparticles uniformly distributed on the

the catalysts with the optimized carbon matrixes that were

surface or embedded within the carbon matrix with an average

carbonized at different temperatures and commercial Pt/C

particle size of 2-3 nm (SEM images, Figure S5).

(20%) were also studied empolying the same conditions. The

Notably, Figure 5a reveals that Pt/NPC-900 possesses a

CVs of all catalysts in 0.5 M H2SO4 solution were initially

remarkable cellular mesoporous structure, which may provide

recorded to measure the electrochemical activity surface area

stable and active sites during further electrochemical tests. The

and activate the catalyst. Owing to the poor definition of the

corresponding

as

hydrogen adsorption and desorption regions, derived from the

demonstrated by the presence of diffraction rings (selected

double layer capacitor effect, the final results are normalized

area electron diffraction pattern in Figure 5e). Elemental

to mass activity.28

Pt

nanoparticles

are

polycrystalline

mapping indicates that Pt/NPC-900 is manly composed of C and Pt, where Pt are 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. 5



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

Figure 5. (a-e) HR-TEM images of Pt/NPC-900 with different magnifications (The mesoporous structure is marked by the white dotted line), (e insert) SAED pattern, (f) EDS spectrum and elemental mappings of sample. 6


Page 6 of 15

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ACS Applied Materials & Interfaces As displayed in Figure 6b, the peak at backward is

and poison resistance. Yang and co-workers developed a new

attributed to the oxidation of accumulated intermediates (such

way to fabricate highly-dispersed Pt NPs on the surface of

as carbon monoxide and formaldehyde) while the one at

reduced

forward scan attributes to the oxidation of CH3OH

as-synthesized catalyst demonstrated superior improvements

29

GO/phenyl

formaldehyde

polymer,

and

the

molecules. Although all the composites display a prominent

to MOR with a current density of 404 mA mg-1

catalytic behavior for the MOR, the higher current density of

Hierarchical-carbon-coated molybdenum dioxide nanotubes

-1

1195 mA mg

Pt for

31

can also be constructed as nanostructured supports for MOR

Pt/NPC-900 suggests its superior activity, -1

Pt.

Pt),

electrocatalyst and exhibited an improved activity (570 mA

1.22 times than Pt/NPC-1000 (980 mA mg-1 Pt) and 3.4 times

mg-1 Pt).32 In order to deeply understand the behaviors of the

than commercially-available Pt/C (350 mA mg-1

as-prepared

which is about 1.51 times than Pt/NPC-800 (790 mA mg Pt).

The

some

other

previously-reported

materials are summarized in Table S1.

tolerance of the catalysts for CO-poisoning can be monitored

In addition, the durability of the catalysts has also been

by IF (the ratio of the forward scan peak current) versus IB (the 30

catalysts,

A higher IF/IB ratio means

studied through chronoamperometric tests. Figure 6c presents

better oxidation conversion of CH3OH into CO2 during the

the chronoamperometric curves for CH3OH at 0.7 V for 2000 s.

anodic scan and the excessive accumulation of CO on the

The current densities of four curves declined rapidly, which is

surface of catalyst. The corresponding IF/IB ratios were

probably due to the accumulation of toxic substances and the

calculated to be 1.25 (Pt/NPC-800), 1.58 (Pt/NPC-900), 0.98

aggregation of nanoparticles.33 Overall, Pt/NPC-800/900/1000

(Pt/NPC-1000) and 0.82 for commercial Pt/C, indicating that

displayed prominent electrocatalytic activities due to the

Pt/NPC-900 also shows better poison resistance. The

appropriate vector, including good electronic conductivity,

Pt/NPC-900 sample gave a higher MOR activity comparing to

high surface area, and the suitable pore size distribution.

backward scan peak current).

the commercially-available Pt/C in terms of current density

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. 7



Page 8 of 15

It is noteworthy that previous reports point out that carbon

spectrum in Figure 7f demonstrates the deposition of nickel

structures containing pore sizes of about 25 nm displayed the

metal on NPC-900 powder. To accurately prove the structure

best activity for ORR among all porous-carbon-supported

and chemical composition of the electrocatalyst, XPS was

PtRu catalysts.

34

Based on these results, we believe that

further undertaken. Generally, amorphous nickel nanoparticles

hierarchical pore structure and mesopore size (~17 nm) for

are prepared by chemical reduction of nickle salt with

Pt/NPC-900 were helpful to the even dispersion of Pt and

NaBH4,36 however, the XPS spectra peak of Ni0 was not found,

convenient transport of reactants (mesopores across the

which might be due to the formation of a thin layer of NiO by

micro-porous matrix provide the required accessibility to

exposing the sample in air.37 Further analysis shows that the

reactant molecule for quick diffusion while micropores

peak of Ni 2p1/2 at 874.1 eV is the divalent Ni2+ in NiO, while

contribute to the high surface area). Furthermore, a plausible

the peaks of Ni 2p3/2 at 865.2 and 861.6 eV are the Ni2+ in Ni

but reasonable reaction mechanism for the electrooxidation

species, suggesting that O2 from air can easily oxidize the Ni

process has been concluded as the following two main steps:

element. The detailed morphologies and structure information

(1) Hydroxymethyl/methoxy adspecies are formed after

were observed through TEM and SEM (Figure S9) analysis.

CH3OH molecule attaches onto the surface of Pt, and (2)

Figure 7a shows that NPC-900 are uniformly covered by these

Hydroxymethyl

Ni species. The distribution of Ni species on the carbon matrix

radical/CO2/

is

further

HCOOH

converted

while

becomes HCHO and CH2(OH).

into

methoxy

CO/HCOO

was further confirmed by elemental mapping (Figure 7f).

dehydrogenates

35

The Ni/NPC-900 exhibits a typical cyclic voltammetry (CV)

Inspiring by the success in constructing Pt/NPC-900, it is

behavior in 1.0 M NaOH solution at 50 mV s-1 (Figure 8). The

logical to anticipate that NPC-900 should be a good supporter

current density starts to increase at 407 mV and reaches an

to

effective

oxidation peak at 566 mV. This result corresponds to the

electrocatalysts. As expected, X-ray diffraction confirms an

conversion of Ni(OH)2 species into NiOOH through the

amorphous character of the catalyst (without any diffraction

following electro-oxidation mechanism:38

peaks of nickel species shown in Figure S7) and EDS

Νi(ΟΗ) 2 + ΟΗ − → ΝiΟΟΗ + Η 2Ο + e −

load

Ni

species

for

the

formation

of

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

8


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

1 M NaOH solution was employed to enrich OH- anions

the scan rate, suggesting that the oxidation of methanol is

-

and OOH species onto the surface of the electrocatalyst,

determined by its diffusion speed.41 Such an observation is in

leading to the thicker catalytic layers.39 The addition of 1.0 M

accordance with the results for Ni/C samples.42 In recent years,

methanol leads to a notable change in the CV curve.

nickel nanoparticles, nickel foam, nickel alloy nanostructures

Accompanying with the formation of NiOOH, an oxidation

(such as Ni/Cu, Ni/Co and Ni/Mn, etc)43 and other Ni-based

peak was observed at a potential value of 806 mV with a

species have attracted increasing attentions owing to their

-1

current density of 449.8 mA mg . The electrocatalytic active

excellent activities (current densities from 20 to 65 mA cm-2),

component toward CH3OH is believed to be NiOOH, arising

cost, and potential as alternative catalysts to the noble metals.

from its empty d-orbitals or unpaired-d-electrons and

As Ni/NPC-900 produces a current density of 449.8 mA mg-1

convenient conditions for the bond formation with absorbed

around 800 mV (vs. Ag/AgCl), a further comparison of

species.

40

catalytic

properties

between

Ni/NPC-900

and

known

Ni-containing catalysts is listed in Table S2.

ΟΗ − + 4 NiOOH + CH 3OH → 4 Νi(ΟΗ) 2 + ΗCOO -

Chronoamperometry was also used to further determine

The CVs of Ni/NPC-900 have been conducted in solutions

the stability and long-term activity of Ni/NPC-900 for MOR.

containing 1.0 M NaOH and 1.0 M CH3OH at different scan

Figure 8d compares the chronoamperograms obtained from

rates (Figure 8b). Obviously, the current densities of methanol

the different voltage values. Apparently, the catalytic activity

oxidation and NiOOH reduction increases with the increasing

was decreased gradually for all three situations. At the early

scan rate, associating with more negative potential values for

stage of the reaction, all these active sites are free to contact

cathodic reduction peak. Furthermore, as shown in Figure 8c,

with methanol molecules, where carbonaceous species

there is a linear relationship between the current density and

subsequently formed, leading to decay phenomenon. 9



reaction rate for above reactions, which can be attributed to

3.3. Performance for the reduction of nitrophenol Since the degradation of aqueous nitrophenol is widely

the different integrated form between active species and

used as a good model reaction to assess the behavior of the as-fabricated catalysts,

44

Page 10 of 15

carbon matrix. (Table 1).

we also use this type of reactions to

The Langmuir-Hinshelwood mechanism emphasized by

assess our catalysts (Pt/NPC-900 and Ni/NPC-900). Generally,

Ballauff et al indicates that the metal-catalyzed nitrophenol

strong absorption peaks at 317, 277 or 272 nm can be seen in

reduction is strongly affected by the surface environment of

the solutions of 4-NP, 2-NP or 3-NP. Upon the addition of

catalysts.48 Thus, the catalytic efficiency is strongly decided by

KBH4, yellow-green color will be observed, suggesting the

the contacting opportunity between the catalytic sites and

generation of nitrophenolate ions.45 After adding catalysts, the

reactants. For Pt/NPC-900, Pt NPs evenly dispersed onto the

obvious change in the absorption spectra and solution color

surface or embedded within the hierarchical pores and many

can be seen in all samples. Figure 9 showed UV absorption

nitrophenol molecules contact with the catalyst surface

during catalytic reduction of nitrophenol over two different

through diffusion processes, in which molecular geometry

catalysts.

complete

plays a vital role in the final catalytic behavior. Since the

decomposition of all three nitroaromatics in the presence of

geometry of 4-NP is linear, the rate diffusion of 4-NP is much

KBH4 within the time range of 6 to 40 min, while Ni/NPC-900

higher than 2-NP/3-NP, resulting in a larger reaction rate

only took 6-8 min. The recyclability tests of both catalysts

(Figure S12). However, nickel species mostly deposited on the

were conducted by removing the catalysts from reaction

surface of NPCs, creating an equal probability for collision

through filtration, washing them with DI water, and reusing

between 2/3/4-NP and the catalyst surface. Therefore, two

them in the next run with the similar reaction conditions. The

composite materials displayed uneven selectivities towards the

above-mentioned two catalysts can be reused at least 3 times

reduction of nitrophenol due to the hierarchical pore feature of

(Figure S10).

supporters (Figure 10).49

Pt/NPC-900

can

catalyze

the

We also investigated the kinetics of each reaction in order

Table 1 Catalytic behaviors of composite catalysts in

to understand the decomposition speed of nitrophenol on reduction reaction

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

reaction substrate

a

Catalysts

pseudo-first-order reaction,46 from where the rate constant k is

4-NP

2-NP

3-NP

Pt/NPC-900

0.200 s-1 g-1 L

0.020 s-1 g-1 L

0.022 s-1 g-1 L

Ni/NPC-900

0.030 s-1 g-1 L

0.023 s-1 g-1 L

0.015 s-1 g-1 L

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 rates k’ were 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 composites outperformed many other Ni/Pt-based catalysts, as judged from the higher activity parameter. The detailed comparison is presented in Table S3. Most impressively, Pt/NPC-900 shows diverse selectivities for different substrates, whereas Ni/NPC-900 appears similar

10


Page 11 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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). Insert: The relationship between ln(Ct/C0) and reaction time (t).

current density and poison-resistant capability. Ni/NPC-900 composites was 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, Figure 10. Illustration of the diffusion processes for 2/3/4-NP

component, structure and so on) would endow them more

molecules within the hierarchical pore structure and adsorption

opportunities to be applied in heterogeneous organic catalysis

state on the surface of amorphous nickel.

and energy electrocatalysis.

4. Conclusions

ASSOCIATED CONTENT Supporting Information. Additional X-ray diffraction data,

In summary, a commercially-available Cu-MOF (HKUST-1) have been successfully used as templates/precursors to

SEM/TEM images, EDS spectrum and catalytic test data can be

construct nanostructured porous carbons (NPCs) through

found in the supporting information. This material is available

pyrolysis treatment, followed by chemical etching. NPCs were

free of charge via the Internet at http://pubs.acs.org.

used as supporters for capturing Pt nanoparticles and

AUTHOR INFORMATION

amorphous nickel through chemical reduction and the as-prepared nanoparticles were evenly dispersed onto the

Corresponding Author

surface or embedded within the carbon matrix. Compared with *Dong-sheng Li:e-mail: [email protected].

the commercial Pt/C (20%) catalyst, the as-synthesized Pt/NPC-900

shows

an

enhanced

activity

for

*Qichun Zhang:e-mail: [email protected].

the

Author Contributions

electrooxidation of methanol in terms of both oxidation 11



The manuscript was written through contributions of all authors.

Page 12 of 15

(9) Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A

All authors have given approval to the final version of the

metal-organic

manuscript. ‡These authors contributed equally.

electrocatalyst. Nature Energy 2016, 1, 15006.

framework-derived

bifunctional

oxygen

(10) Liao, P. Q.; Shen, J. Q.; Zhang, J. P. Metal-organic frameworks

Notes

for electrocatalysis. Coord. Chem. Rev. https: //doi.org/10.1016/

The authors declare no competing financial interest.

j.ccr.2017.09.001. (11) Guo, Y. Y.; Zeng, X. Q.; Zhang, Y.; Dai, Z. F.; Fan, H. S.;

ACKNOWLEDGMENT

Huang, Y.; Zhang, W. N.; Zhang, H.; Lu, J.; Huo, F. W.; Yan, Q.

This work was supported by the NSF of China (Nos.: 21673127,

Y. Sn Nanoparticles Encapsulated in 3D Nanoporous Carbon

21373122, 21671119, 51572152 and 51502155).

Derived from a Metal-Organic Framework for Anode Material in

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Table of Contents


15

Ultrafine Pt Nanoparticles and Amorphous Nickel Supported on 3D

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