Environ Sci Pollut Res
DOI 10.1007/s11356-017-9191-2
RESEARCH ARTICLE
ZIF-8 derived nitrogen-doped porous carbon as metal-free
catalyst of peroxymonosulfate activation
Wenjie Ma 1 & Yunchen Du 1 & Na Wang 1 & Peng Miao 1
Received: 13 January 2017 / Accepted: 2 May 2017
# Springer-Verlag Berlin Heidelberg 2017
Abstract Nitrogen-doped porous carbon (NPC) is synthesized through a direct pyrolysis of zeolitic imidazolate
framework (ZIF)-8 under N2 flow followed by acid washing
process. It is found that NPC-800 pyrolyzed at 800 °C can
inherit the perfect rhombic dodecahedron morphology of ZIF8 crystals and achieve the considerable nitrogen-doping
content of 15.20%. When NPC-800 is applied as the heterogeneous catalyst in peroxymonosulfate (PMS) activation for
the degradation of Rhodamine B (RhB) and phenol, NPC-800
will exhibit its better performance than some conventional
transition metal-based oxides and common carbon materials.
The active sites can be primarily ascribed to nitrogen modification and sp2-hybridized carbon frameworks. Besides, the
influence of several parameters such as the dosage of catalyst,
the concentration of oxidant, and reaction temperature is
conducted systematically. More importantly, NPC-800 can
maintain its considerable degradation in the presence of some
anions and natural organic matters, even under some actual
water background conditions. Although NPC-800 displays
mild deactivation in repeated experiments, its catalytic performance can be easily recovered through heat treatment at
350 °C in air. Radical quenching tests reveal that both sulfate
and hydroxyl radicals are responsible for the removal of
Responsible editor: Vítor Pais Vilar
Electronic supplementary material The online version of this article
(doi:10.1007/s11356-017-9191-2) contains supplementary material,
which is available to authorized users.
* Yunchen Du
yunchendu@hit.edu.cn
1
MIIT Key Laboratory of Critical Materials Technology for New
Energy Conversion and Storage, School of Chemistry and Chemical
Engineering, Harbin Institute of Technology, Harbin 150001, China
organic pollutants. This research may provide a new way for
the application of novel metal-free carbocatalysts in terms of
PMS activation.
Keywords ZIF-8 . Nitrogen-doped porous carbon .
Peroxymonosulfate . Metal-free catalysis . Organic
degradation . Advanced oxidation technology
Introduction
In the past decades, advanced oxidation processes (AOPs)
have been regarded as one of the most promising strategies for disposal of various recalcitrant organics due to
their high efficiency and nearly nonselective degradation
(Anipsitakis and Dionysiou 2004; Tušar et al. 2012).
However, it is unfortunate that classic Fenton reaction
employing H2O2 always suffers from the shortages of
pH adjustment, instability of peroxide, and sludge disposal,
which offer it a dim prospect for large-scale applications
(Neyens and Baeyens 2003). As an advanced alternative
method, sulfate radical (SO4·−)-based AOPs are receiving tremendous attention as they usually demonstrate more costeffective and environmentally friendly processes (Oh et al.
2016; Hu and Long 2016). It is well known that SO4·− can
be released from peroxymonosulfate (PMS) or persulfate (PS)
by heating, UV irradiation, ultrasonication, and catalytic
activation, where catalytic activation has its own advantages without the assistance of necessary equipment and
high-energy input (Oh et al. 2016; Wang et al. 2016b; Lin
and Chen 2017). Compared with PS, PMS [commercially
available as Oxone (2KHSO5·KHSO4·K2SO4)] is a preferable candidate for the generation of SO4·− in catalytic
system, because its asymmetrical molecular structure and
relatively large O-O bond length in free molecular will
Environ Sci Pollut Res
facilitate the activation process, and meanwhile, the redox
cycle of active sites in PMS activation of will also contribute to longer lifespan of catalysts than the irreversible
conversion of active sites in PS activation (Oh et al. 2016;
Duan et al. 2015b). Literature review reveals that catalytic
activation of PMS has exhibited its versatile functionality
in the removal of various types of recalcitrant contaminants, including volatile organic compounds, endocrine
disruptors, pharmaceuticals and their metabolites,
cyanotoxins, synthetic dyes, and perfluorinated compounds, and it could even be utilized for the disintegration
of activated sludge, disinfection, and decontamination of
pool water (Ghanbari and Moradi 2017). Although homogeneous catalytic activation of PMS are very effective, the
toxicity caused by the overload of transition metal ions
(e.g., Co2+, Ni2+, Fe3+) severely hinders the application
of this method in water treatment (Du et al. 2016).
Therefore, there are numerous interests in developing various kinds of heterogeneous catalysts, such as zerovalence metal, metal oxides, and supported composites,
to alleviate the potential secondary contamination from
the homogeneous systems (Lin and Chen 2016a; Zhang
et al. 2013b; Zeng et al. 2015; Wang et al. 2014; Du et al.
2016; Zhao et al. 2016; Yao et al. 2014; Hu et al. 2011;
Guan et al. 2013; Ren et al. 2015), while the negative
effects of leaching ions from heterogeneous catalysts cannot be fully neglected.
More recently, some metal-free materials appeared as a new
kind of heterogeneous catalysts with excellent performance for
PMS activation (Sun et al. 2012; Sun et al. 2014; Indrawirawan
et al. 2015a, b). For example, Zhang et al. demonstrated that
granular activated carbon could reinforce the decolorization
efficiency of Acid Orange 7 by catalytic decomposition of
PMS (Zhang et al. 2013a); Shao et al. employed amorphous
boron (A-boron) as a metal-free catalyst for PMS activation to
produce free radicals for effective degradation of bisphenol S,
and they found that A-boron not only exhibited outstanding
catalytic activity and superior stability in the degradation of
bisphenol S as compared with metal-based and metal-free carbon-based catalysts but also worked for the degradation of
many other typical organic pollutants in water (Shao et al.
2017); Lin et al. and Dong et al. reported that α-sulfur and gC3N4 were also eligible activators for the generation of powerful radicals from PMS (Lin and Zhang 2016b; Dong et al.
2016). Among these metal-free catalysts, carbon materials are
always the most attractive candidates due to their large surface
area, chemical stability, tunable properties, and diverse forms.
Various carbon materials, e.g., carbon nanotubes (CNTs), carbon nanofibers (CNF), nanodiamonds, and reduced graphene
oxide (rGO), have been extensively studied as highperformance PMS activators (Chen et al. 2016; Yang et al.
2016b; Lee et al. 2016; Peng et al. 2013). The active sites of
these carbocatalysts were primarily ascribed to their zigzag
edges and some oxygen-containing groups, where the conjugation of sp2 carbon network, defective sites, and proper amounts
of functional groups (especially the C=O groups) played vital
roles for the electron transfer efficiency in activation processes
(Sun et al. 2012, 2014; Duan et al. 2015e, 2016a, b; Wang et al.
2016a). In the subsequent research, Wang’s group adopted a
heteroatom-substitution (e.g., N, P, S, B) strategy to disrupt the sp2-hybridized carbon configuration, modulate
the electrical properties, and create more active sites in
the carbon framework (Duan et al. 2015f; Wang et al.
2016c). It turned out that after doping modifications, NCNT, N-rGO B-rGO, P-rGO, and S, N-rGO demonstrated
dramatically enhanced activities for phenol removal as
compared with those pristine nanocarbons (Duan et al.
2015c, d). Density functional theory (DFT) calculations
further indicated that with the smaller atom radius and
higher electronegativity, nitrogen atoms could act as the
preferable Lewis basic sites to break the inertness of sp2hybridized carbon configuration by inducing the electron
transfer from the neighboring carbon to nitrogen sites
(Liang et al. 2012). Although many efforts have been
devoted to constructing various N-doped carbocatalysts
with considerable activities (Liang et al. 2016, 2017; Lin
and Zhang 2016a; Yao et al. 2017), most of them are still
stuck with tedious procedures and low N content.
In the recent advances of carbon-based functional materials, in situ pyrolysis of metal-organic frameworks
(MOFs) becomes a fascinating route, since the periodic
arrangement of different atoms in crystalline MOFs provides congenital advantages for homogeneous chemical
distribution and heteroatom substitution (Qiang et al.
2015; Torad et al. 2013; Wu et al. 2015). As a classical
type of MOFs, zeolitic imidazolate frameworks (ZIFs)
with ZIF-8 and ZIF-67 as representative examples [ZIF-8
for Zn(MeIm)2 and ZIF-67 for Co(MeIm)2, MeIm = 2methylimidazole], characterized by uniform polyhedral
morphology, ultrahigh surface areas, and large pore volumes, have distinguished themselves in gas separation,
environmental remediation, and catalysis (Song et al.
2012; Jiang et al. 2013; Jung et al. 2015; Lin and Chang
2015a). Thanks to their thermally stable carbon frameworks and nitrogenous ligand of 2-methylimidazole,
ZIFs have been taken as the ideal precursors for porous
N-doped carbon with excellent performance in different
fields (Bai et al. 2014; Bhadra et al. 2016; Lin et al.
2015c; Zhang et al. 2014). Lin et al. fabricated Co3O4/C
composite for PMS activation in the removal of
Rhodamine B (RhB) and caffeine through one-step pyrolysis of ZIF-67, whereas highly catalytic efficiency could
not conceal the critical drawbacks from the involvement of
high-loading metal oxides (Lin et al. 2015b; Lin and Chen
2016a). When ZIF-8 is chosen as the precursor, metal-free
N-doped carbon will be produced due to the low boiling
Environ Sci Pollut Res
point and easy etching of metal zinc, and the latest developments also validate that the carbocatalyst from ZIF-8
can also perform better efficiency in the degradation of
some organic pollutants by activating PMS than those
from other MOFs (Li et al. 2017; Wang et al. 2017).
However, it is found that the specific relationship between the nitrogen doping and catalytic effectiveness,
as well as the extended applications of this new kind of
carbocatalysts, should be further addressed. In this article, we prepare N-doped porous carbon (NPC) by choosing ZIF-8 as the self-sacrificial template and evaluate
their catalytic performance in the degradation of RhB
and phenol. Compared with nitrogen-containing porous
carbon prepared from tedious procedures, this MOFderived strategy not only endows carbon materials with
rich pore structure properties but also facilitates the process of doping nitrogen into the carbon frameworks,
which can alter the electronegativity of adjacent carbon
atoms, thus boosting the reaction rates. The effects of the
content and existence forms of doping nitrogen species
on the catalytic efficiency are carefully investigated.
Several influential parameters, such as catalyst dosage,
oxidant concentration, and reaction temperature, are also
systematically conducted during the degradation processes. Finally, the performance of the as-prepared NPC was
evaluated under some actual water background conditions, and radical quenching tests are carried out to uncover the catalytic mechanism.
Experimental section
Chemical reagents
Zinc nitrate (Zn(NO3)2·6H2O), sodium thiosulfate (Na2S2O3·
5H 2 O), sodium chloride (NaCl), sodium bicarbonate
(NaHCO 3 ), sodium acetate (NaAc), sodium carbonate
(Na2CO3), sodium nitrate (NaNO3), nitric acid (HNO3), sodium
dihydrogen phosphate (NaH2PO4), activated carbon (AC), and
hydrochloric acid (HCl) were purchased from Tianjin
Fengchuan Chemical Reagent Technologies Co., Ltd.
Graphene oxide and carbon nanotubes were supplied from
Jiangsu Daopeng Technology and Shenzhen Nanotechnologies
Port Co., Ltd., respectively. RhB and phenol were obtained from
Tianjin Guangfu Fine Chemical Research Institute and Dongli
Chemical Reagent Factory, respectively. Methanol was supplied
by Sinopharm Chemical Reagent Co., Ltd. Oxone (KHSO5·
0.5KHSO 4·0.5K 2SO4, PMS), humic acid (HA), and 2methylimidazole were of ACS reagent grade and purchased
from Sigma-Aldrich. Commercial MnO2 and Fe3O4 were supplied by Tianjin Fengchuan Chemical Reagent Technologies
Co., Ltd. All chemicals were used as received without
any purification.
Preparation
ZIF-8 powders were prepared according to previous literature
(Torad et al. 2013). Briefly, Zn(NO3)2·6H2O (0.5160 g) was
dissolved into methanol (40 mL) to form a clear solution, which
was named as solution A. Similarly, 2-methylimidazole
(0.5260 g) was added to another methanol (40 mL) to form
solution B. Then, solutions A and B were mixed together and
stirred for 10 min. The mixed solution was aged for 24 h under
room temperature, and the product was collected by centrifugation, washed by methanol for 3 times, and dried at 60 °C.
As shown in Scheme 1, the as-synthesized ZIF-8 powders were placed into a tubular furnace under flowing N2
atmosphere, and calcined at given temperatures (700, 800,
and 900 °C) for 300 min with the heating rate of 5 °C/
min. Subsequently, the obtained products were mixed
with concentrated hydrochloric acid under vigorous stirring for 24 h to thoroughly remove the residual zinc species. Finally, the product was purified by water for several
times and collected for further use. The samples were denoted
as NPC-X, in which X represents the pyrolysis temperature of
700, 800, and 900 °C, respectively.
For homemade CuO and Co3O4, typically, Cu(NO3)2 or
Co(NO3)2 was put into a crucible boat, and then heated to
450 °C for 1 h in air at a ramp of 5 °C/min; the product was
finally washed for three times and dried for use.
For mesoporous carbon (MPC), 0.44 g of resorcinol, 1.50 g
of F127, and 0.56 g of p-phthalaldehyde were mixed together
thoroughly, and then sealed in an autoclave and heated at
250 °C for 12 h. The obtained brown product was washed
with water for 3 times, dried, and then carbonized at 800 °C
for 5 h under nitrogen atmosphere at a ramp of 5 °C/min
(Zhang et al. 2015b).
For purification of carbon nanotubes, the as-obtained carbon nanotubes was dispersed into the mixture of nitric acid
and sulfuric acid (volume ratio 3:1) and heated to 80 °C for 3 h
according to the previous literature (Huang et al. 2015). Then,
it was collected, washed with water for several times, and
dried at 60 °C.
Characterization
Powder X-ray diffraction (XRD) data were acquired on a
Rigaku D/MAXRC X-ray diffractometer with a Cu Ka radiation source (45.0 kV, 50.0 mA). Nitrogen adsorption isotherms
were recorded on a QUADRASORB SI-KR/MP (degassing at
120 °C). JEM-3000F from JEOL was utilized to obtain the
transmission electron micrographs and high-resolution fieldemission TEM (HRTEM) images. The thermogravimetric
(TG) analysis was carried out on a SDT Q600 TGA (TA
Instruments) in the temperature range of room temperature to
800 °C at a heating rate of 10 °C/min. Total organic carbon
(TOC) was determined by Analytik Jena AG MultiN/C 2100
Environ Sci Pollut Res
Scheme 1 Schematic illustration
of synthesizing ZIF-8 and
nitrogen-doped porous carbon
(NPC)
TOC analyzer. The concentration of several metal elements
was made with an Optima 8300 (PerkinElmer, USA) ICPOES. X-ray photoelectron microscopy (XPS) was conducted
on PHI 5400 ESCA System using Al Kα (1486.6 eV) to irradiate samples. Scanning electron microscopy (SEM) data was
collected by a Quanta 200S (FEI). The confocal Raman spectroscopic system (Renishaw, In Via) with a 633-nm laser was
employed to obtain Raman spectra. Fourier transform infrared
(FTIR) spectra were performed on an Avatar 360 FTIR using
KBr for tableting.
Evaluation of catalytic oxidation
A series of catalytic tests were performed to investigate the
performance of NPC-X for RhB/phenol degradation.
Typically, in a beaker (100 mL) containing of pollutant solution at 20 °C, a fixed amount of catalyst was added to the
solution under constant stirring for about 30 min to reach
absorption-desorption equilibrium. After that, Oxone was
added to initiate the oxidation process. During each time interval, the predetermined amount (1.0 mL) of solution was
injected into a 20-mL vial, in which excessive saturated
Na2S2O3 was used as quenching agent. UV spectrophotometer was employed to obtain the concentration of RhB. The
concentration of phenol was determined using a highperformance liquid chromatography (HPLC) with a UV
detector set at 270 nm, where AC-18 column was used
to separate the organics. The mobile phase with a flow
rate of 1.5 mL/min was made of 40% CH3OH and 60%
water. Every catalytic test was repeated for three times
and the relative standard errors were less than 5%.
Results and discussion
Characterization of nitrogen-doped porous carbon
The structure information of pure ZIF-8 and NPC-X is
primarily investigated by XRD, and as shown in
Fig. S1, all diffraction peaks of ZIF-8 match well with
the simulated data reported in previous literature, implying the formation of high-purity zeolite-type structure
with cubic crystal system (I43m ) (Torad et al. 2013).
However, these characteristic peaks disappear in NPC-X
(Fig. 1a), and only one broad peak centered at ca. 22.6°
can be detected, which indicates that ZIF-8 has been successfully converted into amorphous carbon materials after
high-temperature pyrolysis. The absence of characteristic
peaks and almost 100% of weight loss in air confirm that
Zn species have been completely removed during the processes of high-temperature pyrolysis and subsequent acidic treatment (Fig. S2). With increasing the pyrolysis temperature, it is found that this diffraction peak slightly
shifts toward higher 2θ degree and an additional peak at
43.5° will be identified in NPC-900. These phenomena
are attributed to the fact that higher pyrolysis temperature
can promote the graphitization degree of these carbon
materials. Raman spectroscopy is widely used for the
characterization of the structure and crystallization of carbon materials by examining the bonding state of carbon
atoms within given materials. It is clear that all three samples display two distinguishable peaks assigned to D band
and G band at about 1350 and 1580/cm, respectively. In
general, G band corresponds to the E2g mode induced by
stretching vibrations of sp2 bond, and it occurs at all sp2
sites and not only in crystalline graphite; D band originates from the breathing mode of A1g symmetry involving
phonons near the K zone boundary, and it becomes active
in the presence of defects/disorders (Chu and Li 2006;
Ferrari and Robertson 2000). As a result, the relative intensity ratio of these two bands, ID/IG, always presents
regular variation with the changes of graphitization degree. However, when the pyrolysis temperature increases
from 700 to 900 °C, the ID/IG ratio of NPC-X displays an
abnormal fluctuation, where the maximum is achieved in
NPC-800 (0.91). It is possible that the introduction of
nitrogen species in carbon frameworks upsets the
established change of ID/IG ratio, because the nitrogen
content and nitrogen bonding configurations are different
in NPC-700, NPC-800, and NPC-900. Similar phenomena
Environ Sci Pollut Res
Fig. 1 XRD patterns (a) and
Raman spectra (b) of NPC-700,
800, and 900
have also been observed in some previous reports on
nitrogen-doped carbon materials (Lin et al. 2013; Tao
et al. 2015; Zheng et al. 2014). This kind of carbon
materials with abundant defects and heterogeneous
atoms is highly desirable for the activation of PMS
(Indrawirawan et al. 2015b).
Figure S3 shows FTIR spectra of ZIF-8 and NPC-X.
Pristine ZIF-8 displays a series of regular absorption bands
at 600–1500/cm, which are associated with the ring
stretching and in-plane/out of plane bending of the imidazole. However, for NPC-X, the bands of several functional
groups appear instead, such as the bands at 3300/cm for OH group, 1594/cm for C=C groups, and 1300/cm for
COOH or CHx groups. Besides, it is found that the signals
of oxygen-containing groups become more and more weak
as the pyrolysis temperature is raised from 700 to 900 °C.
These results again verify the gradual enhancement of
graphitization degree. The information of porous structure
in ZIF-8 and NPC-X is determined by N 2 adsorptiondesorption isotherms (Fig. 2a). As observed, ZIF-8 performs standard type-I isotherm according to the IUPAC
classification (Kruk and Jaroniec 2001), indicating its typical microporous characters (Torad et al. 2013). The small
hysteresis hoop at about P/P0 = 0.9–1.0 is attributed to the
interspaces among ZIF-8 particles. In contrast, NPC-800
shows isotherm between type-II and type-IV with a long
and narrow hysteresis hoop at relative pressure (P/P0) from
0.45 to 1.0, which means that some disordered mesopores
are created during the process of high-temperature pyrolysis. NPC-700 and NPC-900 display similar adsorptiondesorption isotherms to NPC-800 (Fig. S4). Based on these
isotherms, their corresponding textural parameters, including specific surface areas and pore volumes, can be further
obtained (Fig. 2b). The specific surface areas of NPC-700,
NPC-800, and NPC-900 are deduced as 694.4, 676.9, and
676.4 m2/g, respectively, while these values are still much
inferior to that of ZIF-8 (1164.3 m2/g). Similarly, the pore
volumes of NPC-X also exhibit more or less decrease as
compared with ZIF-8. The shrinkage of carbon frameworks and the destruction of well-ordered channel system
at high temperature should be responsible for the decreased
textural parameters. However, it is worth noting that the
pore volumes of NPC-700 (0.82 cm 3 /g) and NPC-800
(0.86 cm 3 /g) are much larger than that of NPC-900
(0.72 cm3/g), suggesting that there may be a tremendous
change in the microstructure of NPC-900.
Figure 3 shows SEM and TEM images of ZIF-8 and NPC800. It can be observed that ZIF-8 crystals possess typical
rhombic dodecahedral shape with smooth surface and good
dispersion, and the average size is about 90 nm (Fig. 3a, c).
Very interestingly, NPC-800 can inherit the basic polyhedral
profiles of ZIF-8 and give a visible shrinkage and aggregation
after high-temperature pyrolysis (Fig. 3b, d). The inset in
Fig. 3d also revealed that there are many disordered
mesopores in the polyhedral of NPC-800, in good agreement
with the results of N2 adsorption-desorption isotherms
(Fig. 2). NPC-700 demonstrates similar results to NPC-800
(Fig. S5a, c). However, for NPC-900, some dispersed particles
are sintered and the original polyhedrons are completely torn
up into pieces, even including many dense carbon agglomerates (Fig. S5b, d), which can directly explain its drastically
decreased pore volume (Fig. 2b). It is well known that the
boiling point of zinc is around 800 °C, and thus when the
pyrolysis temperature reaches 900 °C, the removal of residual
zinc species will inevitably cause a severe damage to the integrity of the carbon framework. Besides, element mapping
indicates that carbon and nitrogen species are dispersed homogeneously throughout the whole polyhedral structures,
which predicts great benefits to PMS activation because most
carbon sites with catalytic inertness may be activated by the
sufficient doping with nitrogen species (Fig. S6).
XPS measurements are carried to analyze the elemental
composition and nitrogen bonding configurations in NPC-X.
The summary of element composition validates that carbon,
nitrogen, and oxygen elements are the main compositions for
NPC-X (Fig. 4a), and negligible zinc content can be detected
in NPC-700 (0.67%) and NPC-800 (0.87%). The comparable
oxygen content in NPC-700 (5.44%), NPC-800 (4.31%), and
NPC-900 (5.17%) implies that the oxygen-containing groups
may be involved by the surface oxidation of NPC-X. The
carbon content presents a monotonous increase from 72.52%
for NPC-700 to 90.99% for NPC-900 along with the
Environ Sci Pollut Res
Fig. 2 N2 adsorption-desorption
isotherm of ZIF-8 and NPC-800
(a) and the comparison of BET
specific surface areas and pore
volumes (b)
corresponding loss of nitrogen content from 21.37 to 3.85%,
indicating that high pyrolysis temperature is unfavorable for
the doping of nitrogen species. Of great significance, compared with other conventional nitrogen modifying methods
employing melamine or urea as additives, this MOF-derived
in situ doping strategy can easily obtain the considerable doping content, e.g., 15.20% for NPC-800 vs 8.23% for NGmelamine and 6.94% for NG-urea (Duan et al. 2015a; Yang
et al. 2016a). To access bonding configurations of Ncontaining groups, the deconvolution results of highresolution XPS N1 s spectra have been shown in Fig. 4b–d,
where four main peaks centered at around 398.27, 399.81,
400.86, and 403.93 eV can be assigned to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively (Jiang et al.
2016). Remarkably, with the raise of pyrolysis temperature,
the contents of graphitic N and oxidized N go up sharply,
accompanying with the reduction of pyridinic N and pyrrolic
N, because the latter ones possess a relatively low thermal
Fig. 3 SEM and TEM images of
ZIF-8 (a, c) and NPC-800 (b, d)
stability and are susceptible to be transformed into graphitic N. Furthermore, it is well established that graphitic
N has a higher electronegativity and smaller atom radius
to accelerate the electron flow from neighboring carbon
atom, promoting the charge density of adjacent carbon
(Zhang and Xia 2011). Therefore, PMS activation may
benefit from high graphitic N content. In view of total
nitrogen content and different proportion for graphitic N,
the performance of NPC-700 and NPC-800 may be expected due to their relatively high density of surface graphitic N (total nitrogen content × graphitic N proportion).
Catalytic performance of NPC for RhB/phenol
degradation
Intrigued by their structural features, the catalytic tests of various carbocatalysts in activation of PMS for RhB degradation
are conducted. As shown in Fig. 5a, sole ZIF-8 or NPC-800
Environ Sci Pollut Res
Fig. 4 XPS survey of NPC-700,
800, and 900 (a). XPS N 1s
spectra of NPC-700 (b), NPC-800
(c), and NPC-900 (d)
cannot promise any visible RhB removal, suggesting that the
contribution from physical adsorption is negligible. In contrast, sole PMS accounts for a moderate RhB degradation at
48.5% in 60 min due to the direct oxidation. When both PMS
and NPC-X are present, the degradation efficiency of RhB will
be significantly enhanced, and it is found that the pyrolysis
temperature has an obvious impact on the catalytic performance of NPC-X. The optimal degradation efficiency of about
85.0% can be realized by NPC-800, which is totally consistent
with the prediction of XPS results. However, NPC-700 only
performs RhB degradation at 60.3%, even less than that over
NPC-900 (68.3%). According to the previous literature, sp2hybridized carbon configuration can be ideal platform for the
catalytic efficiency of graphitic N (Duan et al. 2015e; Zhang
and Xia 2011). Although NPC-700 contains similar density of
surface graphitic N to NPC-800, its graphitization degree is
relatively low, and thus there will be less sp2-hybridized
Fig. 5 Degradation of RhB under
various catalytic PMS systems
(a); conditions:
[RhB]0 = 100 mg/L,
[Oxone] = 1.40 g/L,
[catalyst] = 0.20 g/L,
temperature = 20 °C. Phenol
degradation at different
conditions (b); reaction
conditions: [phenol]0 = 25 mg/
L, [Oxone] = 0.80 g/L,
[catalyst] = 0.20 g/L,
temperature = 20 °C
carbon sites in NPC-700, resulting in its inferior catalytic performance. Figure 5b presents the degradation of phenol over
NPC-X obtained at different pyrolysis temperatures. It is a
little different from the degradation of RhB that sole PMS is
unable to degrade phenol in the absence of catalysts and NPC800 removes ca. 14% of phenol by physical adsorption. These
results may be linked with the hydroxyl group and low molecular weight of phenol, where the former makes phenol as a
typical radical scavenger that cannot activate PMS (Liang and
Su 2009; Ghanbari and Moradi 2017), and the latter can facilitate its physical adsorption inside the micropores and
mesopores of NPC-800. The considerable degradation of phenol cannot be observed unless both PMS and NPC-X are introduced into the catalytic system, and the degradation efficiencies of phenol over NPC-700, NPC-800, and NPC-900
are 73.0, 86.1, and 76.4%, respectively. Interestingly, the rich
graphitic N content still endows NPC-800 with the best
Environ Sci Pollut Res
performance in phenol degradation among these three catalysts. Total organic carbon (TOC) measurement shows that the
capacities of mineralization of NPC-800/PMS system for RhB
and phenol degradation are 47.3 and 53.6%, respectively. In
general, the degradations of organic dye and phenol obey the
pseudo-first order kinetics (Wang et al. 2014), and thus the
kinetic rate constant over various catalysts can be calculated
by the following equation,
. ln C C 0 ¼ −kt
ð1Þ
where C and C0 represent the concentration of RhB/phenol at
certain time intervals and at the beginning, respectively, and t
is the reaction time and k means the kinetic rate constant. The
results show that the kinetic rate constants over NPC-700,
NPC-800, and NPC-900 in RhB degradation are 0.0190,
0.0427, and 0.0227/min, respectively, and the corresponding
kinetic rate constants in phenol degradation are 0.0250,
0.0418, and 0.0355/min, respectively. To demonstrate the superiority of nitrogen modification in terms of PMS activation,
we employ homemade mesoporous carbon (MPC), commercial activated carbon (AC), graphene oxide (GO), and carbon
nanotubes (CNTs) as control samples. It is found that MPC
does not show any significant enhancement in RhB degradation, and AC, GO, and CNTs can stimulate RhB degradation
as compared with sole PMS to a certain degree, while their
performances are still less than that of NPC-800 (Fig. S7a),
validating that nitrogen modification is an effective method to
produce active sites in carbon framework. The gap between
these carbon materials and NPC-800 will be further widened
in the degradation of phenol due to its relatively low reactivity
(Fig. S7b). In addition, we also compare the catalytic activity
of NPC-800 in RhB degradation with those of several popular
transition metal-based oxides, including commercial Fe3O4
and MnO2, and homemade CuO and Co3O4 (their XRD
patterns are showed in Fig. S8). It is exciting that NPC-800
can perform superior RhB removal to those transition metalbased oxides that have been widely applied in previous studies
(Fig. S9), which indicates that NPC-800 will be a promising
heterogeneous catalyst for PMS activation.
Effects of different parameters on RhB and phenol
degradation
Several potential influential factors, including catalyst dosage,
oxidant concentration, and reaction temperature, related to
catalytic performance, are extensively studied. Figure 6a
shows the effect of NPC-800 dosage on the degradation of
RhB. With increasing the dosage of NPC-800 from 0.10,
0.15, 0.20 to 0.30 g/L, 66.8, 72.3, 85.0, and 92.5% of RhB
removal can be achieved, which may be attributed to the fact
that more active sites can accelerate the generation of radical
species. Similarly, thanks to the abundant HSO5− absorbed to
the surface of carbocatalyst, when the concentration of Oxone
is adjusted from 0.60 to 3.00 g/L, the degradation efficiency of
RhB will be increased from 65.2 to 90.1% (Fig. 6b). In contrast, it is found that reaction temperature has limited stimulation effect on RhB degradation, where the degradation efficiencies of RhB at 20 and 30 °C are very close, and the removal efficiency of RhB is merely promoted to 92.0% as the
reaction temperature is ascended to 40 °C, implying that a
marginal effect appears between the temperature and the catalytic activity. The kinetic rate constants at different reaction
temperature are further fitted to address the differences
(Fig. S10), and the kinetic rate constants for the degradation
of RhB at 20 and 30 °C are 0.0427/min (R2 = 0.98) and
0.0400/min (R2 = 0.98), respectively, and the degradation of
RhB at 40 °C may account for a little higher kinetic rate
constant [0.0543/min (R2 = 0.99)]. These results indicate that
the positive effect of reaction temperature cannot be presented
until it is obviously higher than room temperature, which may
be attributed to the weak temperature dependence of carrier
mobility in N-doped carbocatalysts. Similar phenomena have
also been reported in the cases of N-doped graphene and Ndoped CNTs (Sun et al. 2014; Wang et al. 2016c).
Moreover, the effects of these influential parameters are
also examined in phenol degradation. As shown in
Fig. S11a, b, the catalyst dosage and oxidant concentration
still play positive roles in promoting the degradation of phenol. For example, when the catalyst dosage is 0.08 g/L, only
44.9% phenol can be removed in 60 min, far less than the
70.1% at 0.14 g/L and 86.1% at 0.20 g/L, and complete phenol degradation can be achieved in 30 min at the catalyst
dosage of 0.30 g/L; meanwhile, the increase of oxidant concentration from 0.20 to 2.00 g/L will induce boost phenol
degradation from 60.0% in 60 min to almost 100% in
20 min. As for the reaction temperature, it also fails to promise
obvious impacts on the degradation of phenol, while the kinetic rate constants at 20 °C [0.0418/min (R2 = 0.96)], 30 °C
[0.0533/min (R2 = 0.99)], and 40 °C [0.0705/min (R2 = 0.97)]
indeed identify the modest contribution from reaction temperature (Fig. S11c, d). Based on these results, it can be concluded that the stimulation effects from catalyst dosage, oxidant
concentration, and reaction temperature are a little stronger
than those in RhB degradation, which may be ascribed to
the variation in the organic molecular structures and degradation pathways (Duan et al. 2015e; Zhang et al. 2013a).
However, the similar functions of these operational parameters suggest that NPC-X has same catalytic mechanism in
these two degradation systems.
Influence of common anions, NOM, and actual water
for RhB degradation
It is well known that some common anions and nature organic
matter (NOM) can affect the degradation of organic pollutants
Environ Sci Pollut Res
Fig. 6 The influence of catalyst
dosage (a), Oxone concentration
(b), reaction temperature (c) to the
RhB degradation. Stability tests
of NPC-800 for RhB removal (d).
Unless explicitly stated, conditions are as follows:
[RhB]0 = 100 mg/L,
[Oxone] = 1.40 g/L,
[NPC-800] = 0.20 g/L, and
temperature = 20 °C
greatly in AOPs (Guan et al. 2013). To testify the potential
application of NPC/PMS system, we also conduct the degradation of RhB in the presence of some typical anions (e.g.,
Cl−, HCO3−, CO32−, CH3COO−, H2PO4−, NO3−) and humic
acid (HA). The influence of chloride with different concentrations (0–5.0 mM) is presented (Fig. S12a). In general, chloride
will compete with organic pollutants to consume SO4·− and
generate less reactive chloride radicals species like Cl· and
Cl2·− (redox potentials for Cl·/Cl− and 2Cl−/Cl2·− are 2.41 and
2.09 V vs NHE, respectively) via Eqs. (2) to (5), while the
active Cl2 and HOCl derived from the directly nonradical
interaction between chloride ion and PMS can also cause fairly positive effect for the proceeding of the degradation through
Eqs. (6) and (7) (Jaafarzadeh et al. 2017; Huie et al. 1991;
Zhou et al. 2015). Thus, the dual roles of chloride could be
finally reflected as the stimulation result at very low concentration and the inhibition effect for the degradation at a relatively high concentration (Wang et al. 2016b). The similar
effect of chloride can be also observed
SO4 − þ Cl− → SO4 2− þ Cl
Cl þ Cl− → Cl2 −
ð2Þ
ð3Þ
Cl2 − þ Cl2 − →2Cl− þ Cl2
ð4Þ
Cl þ Cl →Cl2
ð5Þ
−
−
Cl þHSO5 → SO4
2−
þ HOCl
2Cl− þHSO5 − þHþ →SO4 2− þ Cl2 þH2 O
ð6Þ
ð7Þ
in our case, where the degradation of RhB is slightly promoted
at 0.5 mM of chloride and gradually weakened with higher
chloride concentration. The moderate inhibition effect of chloride in RhB degradation is well consistent with those results
on the degradation of organic dyes in previous studies (Lin
and Zhang 2016b, Li et al. 2017; Zhang et al. 2015a).
Likewise, some other anions, including CO32−, HCO3−,
CH3COO−, H2PO4−, and NO3−, are also able to interact with
SO4·− and produce different counterparts with lower redox
potential compared to SO4·−, for instance, CO3− · and HCO3·
own the redox potential of 1.59 and 1.65 V (vs NHE), which
are inferior to the approximately 2.00 V for CH3COO· and
2.30–2.50 V (vs NHE) for NO 3 · . It was reported that
H2PO4·could even achieve somewhat similar reactivity to
SO4·− (Neta et al. 1988; Liang et al. 2006). In view of the
distinguished oxidation capability of anions radicals, the addition of various anions will decrease the efficiency to a certain extent. Figure S12b, c describes the effect of some other
common anions. Bicarbonate, as a recognized radical scavenger (Laat et al. 2011), displays a monotonously increased inhibition, and when its concentration is changed from 0 to
5.0 mM, the degradation efficiency of RhB will decrease from
85.0 to 72.4% (Fig. S12b). At the same concentration
(3.0 mM), it can be concluded that the adverse effect of these
anions on RhB degradation follows the order of CO32− >
HCO3− > CH3COO− > Cl− > H2PO4− > NO3−. Humic acid
(HA) is always utilized as a kind of characteristic NOM in the
model reaction of AOPs, and its abundant phenolic hydroxyl
and carboxyl groups may absorb onto the surface of heterogeneous catalysts and block the most active sites (Yao et al.
2015; Guan et al. 2013; Wang et al. 2016b; Oh et al. 2015).
However, it is worth noting that HA in the studied concentration interval (2.0–10.0 mg/L) only causes insignificant
Environ Sci Pollut Res
detrimental effect on RhB degradation, which may be possibly
attributed to the fact that high concentration of RhB (100 ppm)
dilute the adsorption of HA on the surface of NPC-800 and
primarily consume radicals released from the active sites.
Three actual water bodies, tap water, surface water
(Songhua River in Harbin), and mining sewage, are further
employed to investigate the integrated influence of backgrounds on the degradation of RhB catalyzed by NPC-800/
PMS system. Several parameters of these actual water bodies
are listed in Table S1. Although these three actual water bodies
possess different compositions, especially that mining sewage
and surface water have much higher total organic carbon
(TOC) concentration than tap water, the degradation efficiencies of RhB in these actual water bodies are still close to that in
Milli-Q water (Fig. S13). The possible reason may be that the
impurities, such as NOMs, dissolved organic matters
(DOMs), and other trace irons in actual water bodies, can only
hinder the interaction between PMS and NPC-800 to a lesser
extent (Gong et al. 2017), which validates the potential of
NPC-800/PMS system for the removal of organic dyes in
the practical applications.
Reusability of NPC-800 for RhB degradation
To investigate the stability and reusability of NPC-800, three
recycling tests are conducted after washing the used catalyst
for 24 h with deionized water. In Fig. 6d, it can be seen that
about 68.8% of RhB is removed in the second and third run,
indicating a slight decline compared with 85.0% of removal
efficiency over the fresh catalyst. This phenomenon seems
that carbocatalysts herein exhibit a poor stability as compared
with commonly used transition metal-based heterogeneous
catalysts. Nevertheless, it is found that the microstructural
morphology of NPC-800 is completely maintained during
the repeated catalytic processes (Fig. S14), and its catalytic
activity can be entirely recovered when carbocatalyst after
third run is subjected to heat treatment at 350 °C for 1 h in
air. XPS results reveal that the proportion of oxygen greatly
increases to 23.35% and the relative amount of nitrogen is
accordingly reduced to 7.20% for NPC-800 after three runs,
Fig. 7 Degradation of RhB by
NPC-800 coupled with various
oxidants (a), radical quenching
tests using methanol and tert-butyl
alcohol (TBA) as the quenching
agents (b), and the molar ratio of
TBA or methanol/PMS is 500 and
1000, respectively; conditions:
[RhB]0 = 100 mg/L, [PMS/PS/
H2O2] = 4.56 mmol/L,
[NPC-800] = 0.20 g/L,
temperature = 20 °C
and thus the deactivation of NPC-800 may be ascribed to the
fact that the intermediates from the decomposition of RhB
molecules susceptibly absorb to the delicate surface of carbon
materials, and then poison the high-activity functional groups
and hinder the access to the inner sp2-hybridized carbon
networks, alleviating the robust performance of catalysts
(Duan et al. 2015a). Fortunately, the oxygen-containing
organic molecules on the surface can be removed by the
high-temperature treatment in air, and the catalytic sites
will be exposed again.
Radical quenching tests and possible active sites for PMS
activation
In this study, two kinds of commonly used oxidants such as PS
and H2O2 are also applied for degradation of RhB to investigate the universality of NPC-800 (Fig. 7a). Compared with
PMS, it is often believed that PS is more difficult to be activated due to its symmetric molecular structure and pretty
higher O-O bonding energy (Oh et al. 2016; Duan et al.
2016b). However, in our case, one can find that NPC-800
coupled with PS also accounts for the removal percentage of
RhB at 73.0%, which is a little inferior to that in PMS system,
indicating that NPC-800 is also effective for PS activation.
When H2O2 is applied as the oxidant, the degradation efficiency of RhB is only 7.1%, and this insufficient RhB degradation
may be attributed to the low adsorption energy and near zero
charge transfer of H2O2 (Duan et al. 2015b).
Different from the PMS activation mechanism of transition
metal-based catalysts, carbocatalysts can perform both radical
and nonradical pathways in terms of their structures. Duan
et al. reported that the intact sp2-conjugated carbon frameworks and rich Lewis basic sites could stimulate PMS dissociation to generate SO4·− and ·OH, and the nonradical oxidations usually occurred at the defective edges of the boundary
in carbon network (Duan et al. 2016a). Therefore, the classical
quenching tests are carried out to figure out the specific reaction mechanism in the system of RhB degradation. It is widely
accepted that tert-butyl alcohol (TBA) may be evaluated as the
typical scavenger for ·OH, because the kinetic rate constant
Environ Sci Pollut Res
from the reaction of TBA and ·OH (3.8–7.6 × 108 /M/s) is
almost three orders of magnitude higher than that from the
reaction of TBA and SO4·− (4–9.1 × 105/M/s) (Liang and
Su 2009; Ghanbari and Moradi 2017). In contrast, methanol is always utilized as a universal scavenger because it
can rapidly react with both SO4·− (3.2 × 106/M/s) and ·OH
(9.7 × 108/M/s). As shown in Fig. 7b, when the molar
ratio of TBA to PMS reaches 500:1, the degradation efficiency will decrease from 85.0 to 71.6%, and this value
will further decline to 65.7% with the molar ratio of TBA
to PMS at 1000:1. It is obvious that methanol always plays a
more aggressive role in inhibiting RhB degradation as compared with TBA, e.g., the degradation efficiencies of RhB are
58.0 and 50.7% at the molar ratios of 500:1 and 1000:1, respectively. These results indicate that both sulfate and hydroxyl radicals are involved in the degradation of RhB, which is
consistent with many studies about PMS activation (Ji et al.
2013; Lin et al. 2015c). In order to rule out the contribution of
radicals completely, we also perform the degradation of RhB
in absolute methanol solution, and it can be found that only
6.7% of RhB is removed, confirming that radical pathway is
dominant in the system of NPC/PMS. Based on the characterization and experimental data, it can be deduced that in our
case, the presence of nitrogen atoms, especially graphitic N,
should be responsible for breaking the inertness of sp2hybridized carbon frameworks and modulating the electronic structures of carbon materials to release the powerful
radicals, and some defective sites and zigzag edges may contribute to the nonradical oxidations moderately.
Conclusions
In conclusion, nitrogen-doped porous carbon (NPC)
with comparable nitrogen content and the regular polyhedral morphologies can be prepared via a facile ZIF-8
derived method. It is found that the as-prepared carbon
materials possess high specific surface area and show
good catalytic efficiency in PMS activation for degradation of RhB and phenol. In particular, NPC-800 displays
better performance than some conventional transition
metal-based oxides and common carbon materials.
Some operational parameters, including the dosage of
catalyst, the concentration of oxidant, and reaction temperature, can impact on the degradation of RhB and
phenol effectively. NPC-800 can be regenerated through
simple heat treatment at 350 °C in air. Radical
quenching tests reveal that both sulfate and hydroxyl
radicals will be released from PMS activation. It is very
interesting that, in the degradation of RhB, NPC-800
can maintain good performance in the presence of some
anions and natural organic matters, even under some
actual water background conditions, indicating that it
is a promising heterogeneous catalyst for PMS activation in the practical applications.
Acknowledgements We appreciate the financial support from Natural
Science Foundation of China (21676065) and the Natural Science
Foundation of Heilongjiang Province (B201405).
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