Phenolic resin as a carbon source for the synthesis of monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route

Article abstract of DOI:10.1080/10426507.2017.1418740

It was the first time that phenolic resin (PR) was used as a carbon source for the synthesis of nanostructured monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route. The results showed that phase-pure β-Mo₂C can be formed under an Ar atmosphere at 900°C or a H₂ atmosphere above 700°C. However, almost pure CoMo carbides (Co₃Mo₃C and Co₆Mo₆C) can be obtained only under a H₂ atmosphere at a low temperature of 630°C for 24 and 48?h, respectively. The role of PR in the preparation process has been investigated and a detailed formation mechanism was proposed based on the experimental results.

Full text of DOI:10.1080/10426507.2017.1418740

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Phenolic resin as a carbon source for the synthesis
of monometallic Mo and bimetallic CoMo carbides
via carbothermal reduction route
Liang Gao, Yan Shi, Zhiwei Yao, Haifeng Gao, Yue Sun, Feixue liang &
Baojiang Jiang
To cite this article: Liang Gao, Yan Shi, Zhiwei Yao, Haifeng Gao, Yue Sun, Feixue liang &
Baojiang Jiang (2017): Phenolic resin as a carbon source for the synthesis of monometallic Mo and
bimetallic CoMo carbides via carbothermal reduction route, Phosphorus, Sulfur, and Silicon and the
Related Elements, DOI: 10.1080/10426507.2017.1418740
To link to this article: https://doi.org/10.1080/10426507.2017.1418740
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PHOSPHORUS, SULFUR, AND SILICON
https://doi.org/./..
Phenolic resin as a carbon source for the synthesis of monometallic Mo and bimetallic
CoMo carbides via carbothermal reduction route
Liang Gaoa, Yan Shia, Zhiwei Yaoa, Haifeng Gaoa, Yue Suna, Feixue lianga, and Baojiang Jiangb
aCollege of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun, P. R. China; bKey Laboratory of
Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin, P. R. China
ABSTRACT
ARTICLE HISTORY
It was the first time that phenolic resin (PR) was used as a carbon source for the synthesis of nanostructured
Received  May 
monometallic Mo and bimetallic CoMo carbides via carbothermal reduction route. The results showed that
Accepted  December 
phase-pure β-Mo2C can be formed under an Ar atmosphere at 900°C or a H2 atmosphere above 700°C.
However, almost pure CoMo carbides (Co3Mo3C and Co6Mo6C) can be obtained only under a H2 atmosphere
at a low temperature of 630°C for 24 and 48 h, respectively. The role of PR in the preparation process has been
investigated and a detailed formation mechanism was proposed based on the experimental results.
KEYWORDS
Phenolic resin;
nanostructured carbides;
carbothermal reduction;
monometallic Mo carbide;
bimetallic CoMo carbide
GRAPHICAL ABSTRACT
Introduction
reaction[8] and dry reforming of methane.[9,10] Among these
carbides, Mo2C and Co3Mo3C/Co6Mo6C have been studied
extensively in recent years.[11,12] Monometallic Mo2C was most
commonly synthesized by temperature programmed reduction
(TPR) of MoO3 precursor under mixtures of hydrocarbons
(such as CH4, C2H6, C3H8 and C4H10) and hydrogen.[13–15]
Unfortunately, the CoMo oxide precursor was carburized
to unstoichiometric CoMoCx carbide and CoMoCxOy oxy-
Carbon and phosphorus were one of the most common Group
14 and Group 15 elements. The chemistry of carbon and
phosphorus was very important in the field of catalysis. Both
carbon and phosphorus atoms can be inserted into transition
metal lattices to form metal carbides and phosphides, and
they were known as transition metal interstitial compounds.
Transition metal carbides show unique catalytic performances
similar to phosphides, and thus they have attracted consider-
able attention in recent years.[1] Various kinds of transition
metal carbides such as monometallic Mo2C, WC, Fe2C and
Co2C, and bimetallic Co3Mo3C/Co6Mo6C, Ni2Mo3C and
Fe3Mo3C have been prepared and used as catalysts in vari-
carbide rather than Co3Mo3C/Co6Mo6C in the TPR process
[7–16]
mentioned above.
Thus, this simple TPR method was
limited to the synthesis of monometallic carbide. In recent
decades, Co3Mo3C/Co6Mo6C can be synthesized using more
involved or complex procedures, including nitridation of CoMo
oxide by ammonia, followed by carburization of the nitride
precursor, [17,18] successive nitridation and carburization reac-
tions via decomposition of Co,Mo-hexamethylenetetramine
[2]
ous reactions, including hydrogenation and hydrotreating,
hydrogen evolution reaction,[3] Fischer-Tropsch synthesis,[4,5]
N2H4 decomposition,[6] CH4 decomposition, [7] water-gas shift
CONTACT Zhiwei Yao
mezhiwei@.com
College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun,
, P.R. China; Baojiang Jiang
jiangbaojiang@sina.com Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s
Republic of China, Heilongjiang University, Harbin , P. R. China.
Color versions of one or more of thei figures in the article can be found online at www.tandfonline.com/gpss.
Supplemental data for this article can be accessed on the publisher’s website
©  Liang Gao, Yan Shi, Zhiwei Yao, Haifeng Gao, Yue Sun, Feixue liang, and Baojiang Jiang

2
L. GAO ET AL.
(HMT) complex,[12] and carburization of an oxidic precursor
followed by heating in hydrogen.[19] On the other hand, a
facile carbothermal reduction route was developed for the
preparation of bimetallic CoMo carbides.[20,21] Liang et al.
prepared Co3Mo3C carbide using activated carbon as a car-
bon source via this route.[20] In addition, Regmi and Leonard
reported that dispersed Co3Mo3C and Co6Mo6C particles were
obtained using decolorizing carbon as a carbon source during
the carbothermal reduction process.[21] To our knowledge,
there are no reports on the carbothermal reduction route to the
synthesis of CoMo carbides other than the two cases mentioned
above. Nevertheless, up to now the carbon source was limited
to the type of solid carbon. Therefore, it was still worthwhile
to explore novel carbon sources to prepare bimetallic carbides
with modified microstructures.
In the present study, it was the first time that phenolic resin
(PR) was used in the carbothermal reduction method for the
preparation of carbides, and nanosized monometallic Mo2C,
bimetallic Co3Mo3C and Co6Mo6C carbides were successfully
synthesized. It was found that the as-prepared bimetallic CoMo
carbides showed much smaller sizes than those submicrometer-
sized bimetallic CoMo carbides prepared using decolorizing
carbon as carbon source mentioned above.[21] Techniques of
XRD, TEM and TG-MS were adopted for the investigation of
solid and gas products, and to give insights into the formation
mechanism of these carbides during carbothermal reduction.
Results and discussion
Figure. XRDpatternsofthesamplespreparedfromMo-containingresinprecursor
at different temperatures in Ar (a) or H (b).
Microstructural characterization
Figure 1 shows the XRD pattern of the phenolic resin used in the
study, and the result indicated that the raw resin was completely
amorphous, which was similar to the result of previous study.[22]
Carburization of Mo- and CoMo-containing resin precursors
was studied as a function of treatment conditions in the temper-
ature range 500–900°C. Figure 2 shows the XRD patterns of the
samples prepared from Mo-containing resin precursor under
Ar or H2 at different temperatures. It can be seen that the Mo-
Ar-700-1 demonstrated the main diffraction peaks at 2θ of 26.0,
37.0, 53.5, 60.2 and 66.6° for MoO2 (JCPDS 78–1069) when
the Mo-containing resin precursor was heated at 700°C in Ar
flow. With increase of the temperature to 800°C, the peaks for
MoO2 became very weak but β-Mo2C phase (JCPDS 35–0787)
was observed, with the main peaks at 2θ of 34.4, 38.0, 39.4,
52.1, 61.5 and 69.6°, corresponding respectively to (100), (002),
(101), (102), (110) and (103) planes. With a further increase
of the temperature to 900°C, the MoO2 phase disappeared
completely, and the β-Mo2C was obtained as a single phase.
As for the products obtained in H2 gas, the MoO2 phase was
minor and poorly crystalline β-Mo2C can be observed even
at 600°C. The pure β-Mo2C phase can be obtained when the
temperature increased up to above 700°C. The results indicated
that H2 atmosphere facilitated the carburization of MoO2.[23]
Figure 3 shows the XRD patterns of the samples prepared
from CoMo-containing resin precursor under Ar or H2 at
different temperatures. In the case of CoMo sample treated
under an Ar atmosphere (see Figure 3a), at 500°C some main
diffraction peaks at 2θ of 19.1, 23.3, 25.5, 26.5 and 27.3° ascribed
to CoMoO4 phase (JCPDS 21–0868) and a weak peak at 2θ
of 37.0° due to Co3O4 phase (JCPDS 65–3103) were detected
besides some weak peaks assigned to the MoO2 phase. The
results were in agreement with the results reported in the
literature.[20] For the sample reduced at 630°C or 650°C, the
peaks due to CoMoO4 and Co3O4 phases disappeared, and new
Co2Mo3O8 (main diffraction peaks at 2θ = 17.9, 25.3, 32.5,
36.0, 37.1 and 45.5°, JCPDS 71–1423) and Co phases (main
diffraction peaks at 2θ = 44.2, 51.5 and 75.9°, JCPDS 15–0806)
were observed. The results indicated that the CoMoO4 and
Co3O4 phases were reduced by carbon to form Co2Mo3O8
and Co phases, as suggested before.[24] When the temperature
Figure . XRD pattern of the phenolic resin used in the study.

PHOSPHORUS, SULFUR, AND SILICON
3
Figure . XRD patterns of the samples prepared from CoMo-containing resin pre-
cursor under H at °C with different calcination times.
completely decomposed to β-Mo2C and Co at 800°C. It can
be concluded from Figure 3 that a H2 atmosphere and a low
temperature of 630°C were the most appropriate conditions to
prepare Co3Mo3C with minor impurities.
Subsequently, the influence of calcination time on the growth
of Co3Mo3C crystallites was investigated. Figure 4 shows the
XRD patterns of the samples prepared from CoMo-containing
resin precursor under H2 at 630°C with different calcination
times. The results clearly showed that the Co3Mo3C crystallites
grew with calcination time and they appeared to be of good
quality at 24 h. However, the β-Mo2C and Co impurities were
always very minor and they remained unchanged in quantity
with calcination time. It was interesting to note that when the
calcination time was increased to 48 h, only little shifts of 2θ
values can be observed for all reflection peaks in comparison
with those obtained at 24 h. These reflection peaks located at
2θ = 35.8, 40.5, 43.1, 47.1 and 73.7° matched with the standard
Figure . XRD patterns of the samples prepared from CoMo-containing resin pre-
cursor at different temperatures in Ar (a) or H (b).
increased up to 700°C, there were new diffraction peaks at 2θ of data of Co6Mo6C phase (JCPDS 80–0338). The result indi-
35.3, 39.9, 42.4, 46.4 and 72.4°, which was assigned to Co3Mo3C cated that Co3Mo3C phase can be transformed into Co6Mo6C
carbide (JCPDS 65–7128). It was clear that the CoMo bimetal- phase via long calcination time probably due to the fact that
lic carbide was obtained from Co2Mo3O8. In addition, some their metal atom arrangement and the type of crystal lattice
β-Mo2C and Co impurities were found in the material. With (Fd3m) were the same as each other.[24] Therefore, it confirmed
a further increase of the temperature to 800°C, the diffraction that Co6Mo6C carbide had been prepared by this resin-based
peaks for Co3Mo3C became weaker but those for β-Mo2C and carbothermal reduction route.
Co became much sharper, indicating that phase separation
The morphologies of as-prepared β-Mo2C (Mo-Ar-900-1),
occurred in the Co3Mo3C carbide at a higher temperature. The Co3Mo3C (CoMo-H2-630-24) and Co6Mo6C (CoMo-H2-630-
result was similar to the observation in previous study.[20] It 48) samples were characterized by TEM. It can be seen from
was therefore reasonable to deduce that the β-Mo2C and Co TEM images (Figure 5) that the morphologies of these samples
impurities were obtained via two pathways of (i) the carboth- were composed of dispersed nanoparticles with the sizes of
ermal reduction of MoO2, CoMoO4 and Co3O4 phases at lower <100 nm. Some particles gathered closer to each other and
temperatures and (ii) the decomposition of Co3Mo3C phase at formed large aggregates. Note that there was a halo around the
higher temperatures. In the case of CoMo sample treated under agglomerate in Figs. 5b and c, and the EDX results (Fig. S 1
H2 atmosphere (see Figure 3b), the resulting materials were Supplemental Materials) revealed that only carbon could be
similar to those described above. However, H2 atmosphere pro- detected in this microregion. Therefore, this halo might be a
moted the formation, growth and decomposition of Co3Mo3C. carbon platelet originated from the carburization of resin. The
It was found that the Co2Mo3O8 began to be converted into result agreed with our previous study in which resin-based
Co3Mo3C phase even at 600°C. The complete transformation carbothermal reduction route can produce carbon platelet-
of the CoMo oxide occurred at 630°C, and at this temperature wrapped particles.[22] According to the XRD data (Figs. 2
a very small amount of β-Mo2C and Co impurities were found and 4), the crystal sizes of β-Mo2C, Co3Mo3C and Co6Mo6C
in the Co3Mo3C product. With the increase in temperature, samples estimated by the Scherrer method were 30, 53 and
the Co3Mo3C crystallites grew, at the same time they began to 54 nm, respectively. These values were in the size range observed
decompose with temperature. The Co3Mo3C phase was almost by TEM. Note that the as-prepared bimetallic CoMo carbides

4
L. GAO ET AL.
Figure. LowandhighresolutionTEMimagesof(a)Mo-Ar--,(b)CoMo-H--
 and (c) CoMo-H-- samples.
showed much smaller sizes than those submicrometer-sized
bimetallic CoMo carbides prepared using decolorizing carbon
as a carbon source.[21] Additionally, the HRTEM images clearly
showed that the measured spacings between lattice fringes
of 0.228, 0.213 and 0.209 nm can be found, and they were
consistent with the d-spacings of the (101) plane of β-Mo2C,
the (511) plane of Co3Mo3C and the (511) plane of Co6Mo6C,
respectively.[25] For catalysis, one of the main characteristics
was the specific surface area. The specific surface areas of the
Mo-Ar-900-1, CoMo-H2-630-24 and CoMo-H2-630-48 sam-
ples measured by surface area analyzer were 387.5, 405.4 and
400.2 m2 g−1, respectively.
Figure. TG-MSprofilesof(a)PR, (b)Mo-and(c)CoMo-containingresinprecursors.
C2H2, with a comparatively low level of benzene and toluene.
In this work, only the results for masses (m/z) of 2 (H2), 16
(CH4), 18 (H2O), 28 (C2H2 or CO) and 44 (CO2) were shown
for understanding the formation mechanism, as the masses of
78 (C6H6) and 91 (C7H8) were almost featureless or provided
little additional information. Figure 6 shows the TG-MS profiles
of PR and Mo- and CoMo-containing resin precursors, and
the corresponding results are listed in Table 1. It can be seen
that the carburization process of PR involved two temperature
regions: low-temperature (ࣘ440°C) and high-temperature
Formation mechanism
The carburization processes of the Mo- and CoMo-containing (>440°C) stages (see Figure 6a). In the low-temperature range
resin precursors were followed by TG-MS in Ar to investigate a successive sharp weight loss (40.16%) was observed on the
the formation mechanism of monometallic and bimetallic car- TG profile, which should be due to the dehydration reactions
bides. As suggested by previous studies,[26,27] when the resins of resin,[27] because a large quantity of H2 (m/z = 2) and H2O
were carburized in inert atmosphere or vacuum, the volatile (m/z = 18) gases were detected by MS. The TG profile showed
products should be mainly composed of H2O, CH4, H2 and a sharp weight loss (20.21%) in the high-temperature range,

PHOSPHORUS, SULFUR, AND SILICON
5
Table . The TG-MS results for PR, Mo- and CoMo-containing resin precursors.
Weight loss (%)
Assignment
Sample
ࣘ°C
>°C
ࣘ°C
Resin dehydration
Resin dehydration and MoO formation
Resin dehydration and CoMoOx formation
>°C
PR
.
.
.
.
.
.
Resin carburization
Resin carburization and MoC formation
Resin carburization and CoMoCx formation
Mo precursor
CoMo precursor
which was probably due to the carburization of resin to produce raw resin was dried at 110°C for 12 h, and then ground to a
carbon.[27] A large quantity of multi-component gases were fine powder. After that, the resin powder was incipient-wetness
detected by MS and the simultaneous signals of m/z = 2, 16, 18 impregnated by an aqueous solution of (NH4)6Mo7O24·4H2O,
and 28 corresponded to H2, CH4, H2O and C2H4, respectively. or an aqueous solution of (NH4)6Mo7O24·4H2O and
In comparison to the PR, the TG-MS profiles of Mo- and CoMo- Co(NO3)2·6H2O mixture (1:1 Mo:Co ratio) with a 30 wt%
containing resin precursors showed several different features in Mo loading. The slurries were evaporated at 110°C for 12 h
both the low- and the high-temperature stages. On the one hand, to obtain Mo- and CoMo-containing resin precursors. Finally,
we observed that in the low-temperature regions (see Figs. 6b about 1.0 g of the dried resin precursor was packed in the mid-
and c), the difficulty in the dehydration of resin ranked in the dle of a 4.2 cm ID, quartz tubular reactor. The quartz reactor
order of CoMo-containing resin (215°C) < Mo-containing was heated with a tubular electric resistance furnace equipped
resin (360°C) < PR (400°C). The results indicated that Mo and with a temperature controller (purchased from Xiamen Yudian
Co metal species (especially for Co) can facilitate the dehydra- Automation Technology Co. Ltd.) and the temperature was mea-
tion of resin. And the weight losses of Mo and CoMo precursors sured with a thermocouple placed adjacent to the sample inside
(25.89 and 22.58%) were less than that of PR (see Table 1), the reactor. The carrier gas (Ar or H2) was controlled using mass
which suggested the formation of MoOx and CoMoOx.[27] On flow controllers (purchased from Beijing Sevenstar Electronics
the other hand, in the high-temperature ranges (Figs. 6b and c), Co. Ltd.) and was introduced at a flow rate of 50 mL min−1. The
the TG curves showed another strong weight losses around 830 precursor was heated from room temperature (RT) to a given
and 790°C for Mo and CoMo precursors, respectively, but the value at a rate of 10°C min−1 and maintained at this temperature
weight losses (26.87 and 26.36%) were more than that of PR (see for different times from 1 to 48 h, followed by cooling to RT
Table 1). In addition, MS curves showed that a large amount under Ar or H2 flow, and then passivated in a 1% O2/Ar flow
of CO was evolved, accompanied with a small amount of CO2. for 2 h. A series of products were obtained from heating at 500,
The facts indicated that the high-temperature carburization 600, 630, 650, 700, 800 and 900°C, designated as Mo-Ar/H2-T-t
stages of Mo- and CoMo-containing resin precursors involved and CoMo-Ar/H2-T-t for Mo and CoMo samples, respectively
strong redox reactions, leading to the formation of carbides. (T = 500, 600, 630, 650, 700, 800 and 900, t = 1, 6, 12, 24
It was therefore reasonable to conclude that the solid product or 48).
(i.e. carbon) from thermal carburization of resin should serve
as a reducing agent in these redox reactions. In addition, there
was a consumption of the H2 generated at about 600°C during
Sample characterizations
carburization processes, which contributed more or less to the
reduction of oxide precursors. It was obvious from Figs. 6b and
c that the final carbide products were formed at 830 and 790°C
for Mo- and CoMo-containing resin precursors, respectively.
The results also indicated that the addition of Co to resin
precursor facilitated the formation of carbide, which was in
accord with the XRD analyses (Figs. 2 and 3).
The formation processes of carbides under H2 atmo-
sphere via carbothermal reduction method were proposed as
following.[20] Reduction of metal oxides by hydrogen; reaction
between partially reduced oxides and surface carbon atoms
of carbon materials under the hydrogen atmosphere. The
processes were similar to those under Ar mentioned above.
However, it can be seen from Figs. 2 and 3 that H2 atmosphere
facilitated the formation of carbides at lower temperatures than
Ar atmosphere.
X-ray diffraction (XRD) measurements were carried out using
Cu Kα source with a X’Pert Pro MPD diffractometer. BET
surface area and pore structure of the sample were measured
through nitrogen adsorption at liquid-nitrogen tempera-
ture (77 K) by a surface area analyzer (NOVA4200). The
morphologies and microstructure of the products were char-
acterized by transmission electron microscopy (TEM, Philips
Tecnal 10) equipped with an EDX system. Thermogravimetry-
mass spectrometry (TG-MS) experiments were performed
using thermoanalyzer STA 449 F5 Jupiter coupled with
quadrupolar mass-spectrometer QMS 403 D Aeolos. The
samples were heated in a stream of pure Ar gas at a rate of 10°C
Table . The major indexes and properties of the phenolic resin.
Index/Property
Value
Softening pointa (°C)
Polymerization ratea (s)
Free phenola (%)

-


.
.
.
Experimental
Fugitive constituenta (%)
BET surface areab (m g−
)
Sample preparation
Average pore diameterb (nm)
Total pore volumeb (cm− g−
)
The raw resin used was a phenolic resin (PR, [C6H6O·CH2O]x,
purchased from Wuxi Xinyehao Chemical Co. Ltd.) and the
major indexes and properties were listed in Table 2. Firstly, the
aProvided by the company.
bObtained by BET surface area measurement.

6
L. GAO ET AL.
min−1 from 50 to 1000°C. The traces of masses (m/z) with
the increase of temperature were recorded as follows: m/z = 2,
m/z = 16, m/z = 18, m/z = 28, m/z = 40, m/z = 44, m/z = 78
and m/z = 91.
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Conclusions
In summary, a phenolic resin (PR) was reported firstly as a car-
bon source for the synthesis of nanostructured monometallic
Mo and bimetallic CoMo carbides via carbothermal reduc-
tion route. The results showed that phase-pure β-Mo2C can
be formed under an Ar atmosphere at 900°C or a H2 atmo-
sphere above 700°C. However, almost pure CoMo carbides
(Co3Mo3C and Co6Mo6C) can be obtained only under a
H2 atmosphere at a low temperature of 630°C for 24 and 48 h,
respectively. The carburization processes of the Mo- and CoMo-
containing resin precursors involved two temperature regions:
low-temperature (ࣘ440°C) and high-temperature (>440°C)
stages. The formation of Mo and CoMo carbides was observed
at the high-temperature ranges, in which oxide precursors were
reduced by the carburization products (C and H2) to yield
carbides, with the release of COx and H2O. Additionally, the
replacement of Ar atmosphere with H2 atmosphere facilitated
the formation of carbides in the carbothermal reduction route.
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