Selective hydrogen production from formic acid decomposition over Mo carbides supported on carbon materials

Article abstract of DOI:10.1039/d0cy01088j

The decomposition of formic acid to obtain hydrogen has been studied using molybdenum carbides supported on activated carbon and two high surface area graphites, H200 (200 m2 g-1) and H400 (400 m2 g-1). Particular attention is paid to the effect of Mo loading. The catalysts were prepared in situ using a mixture of CH4 and H2 at a temperature of up to 700 °C. Under these conditions, carburization was mostly complete. We observed that the support influenced the MoxC phase obtained so that it seems that the ratio of defective carbon influences the phase. However, for these materials the C/Mo ratio did not influence the obtained crystal phase. Characterization by XRD showed that while the β-Mo2C phase was obtained over activated carbon and over H200, in contrast, MoOxCy was obtained over H400. These catalysts reached 100% conversion on formic acid decomposition at temperatures in the range of 190-250 °C and were also highly selective under these mild conditions, with values for CO2 selectivity in the range of 85.0-96.5%. The best results were achieved over a 10 wt% Mo loading on activated carbon that reached 96.5% selectivity to H2. Also, changes in the molybdenum phases were observed on the spent catalyst. Some redox transformations during reaction were responsible for the transformation of β-Mo2C into oxycarbide MoOxCy. In summary, the results of the catalytic performance indicated that the β-Mo2C phase was more active, selective and stable than MoOxCy under the studied conditions.

Full text of DOI:10.1039/d0cy01088j

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D0CY01088J
ARTICLE
Selective hydrogen production from formic acid decomposition
over Mo carbides supported on carbon materials
Received 00th January 20xx,
Accepted 00th January 20xx
D.H Carrales-Alvaradoa, A.B. Dongil*a, J.M. Fernández-Moralesb, M. Fernández-Garcíaa,c,
A.Guerrero-Ruizb,c, I. Rodríguez-Ramos*a,c
DOI: 10.1039/x0xx00000x
The decomposition of formic acid to obtain hydrogen has been studied using molybdenum carbides supported on an
activated carbon and two high surface area graphites, H200 (200 m2/g) and H400 (400 m2/g). Particular attention is paid to the
effect of Mo loading. The catalysts were prepared in situ using a mixture of CH4 and H2 up to 700°C. Under these conditions
carburization was mostly complete. We observed, that the support influenced the MoxC phase obtained so that it seems
that the ratio of defective carbon influences the phase. However, for these materials the C/Mo ratio did not influence the
obtained crystal phase. The characterization by XRD showed that while -Mo2C phase was obtained over activated carbon
and over H200. In contrast MoOxCy was obtained over H400. These catalysts reached 100% conversion on the formic acid
decomposition at temperatures in the range 190-250C and were also highly selective under these mild conditions with
values for CO2 selectivity in the range 85.0-96.5%. The best results were achieved over a 10 wt% Mo loading on activated
carbon that reached 96.5% selectivity to H2. Also, changes in the molybdenum phases were observed on the spent catalyst.
Some redox transformations during reaction were resposible of the transformation of -Mo2C into oxycarbide MoOxCy. In
summary, the results of catalytic performance indicated that -Mo2C phase was more active, selective and stable than
MoOxCy under the studied conditions.
dioxide in the presence of a catalyst. However, upon reaction
conditions the catalyst may also promote the dehydration
reaction of formic acid, producing carbon monoxide and water
1. Introduction
which is an undesirable reaction path, not only because of the
lower hydrogen production but also because carbon monoxide
is a poison of catalysts, specially those most commonly
employed in fuel cells such as Pt [3].
The need of substituting fossil sources with other more
environmentally friendly alternatives that match current energy
schemes, has prompted research on more sustainable energy
sources. One of the possibilities is to use hydrogen as energy
vector as it is well known to own a high energy density per mass,
and it only produces water upon combustion. However,
hydrogen actually holds a low energy per unit volume in gas
phase, so it would occupy a large volume which limits its
widespread application. Alternatively, hydrogen can be stored
in other molecules that are more easily handled and that
decompose in hydrogen when required [1]. Among them,
formic acid, HCOOH, represents an interesting alternative since
it offers a high content of hydrogen 4.3 wt%, it is safe and it is
produced in large quantities in biorefineries as a subproduct of
the Biofine process [2]. Its attractiveness is also due to the soft
conditions required to decompose in hydrogen and carbon
Formic acid decomposition over heterogeneous catalysts has
been studied over metals in their reduced state [4], metal
oxides and more recently using immobilized noble metal
complexes [5]. Despite the clear potential of these latter
systems that could provide a high atom efficiency, their high
cost and low stability complicates industrial application.
Moreover, an attractive catalyst should also be easy to
synthetize using an environmentally safe process and cost
effective. Some works using oxides such as -Fe2O3, pure Al2O3
and MgO doped Al2O3 appeared, however their activity and
selectivity is low even working at temperatures above 200°C [6
7].
In this context, the use of transition metal carbides emerges as
an interesting alternative since they have proven to be very
active and selective in several reactions. The reason for these
results seems to be their structural similarity to Pt group metals.
The potential of molybdenum carbide on the production of
hydrogen by decomposition of several starting molecules such
as methanol or formic acid has already been assessed [8,9, 10].
It was demonstrated that decomposition of formic acid over
molybdenum carbide surfaces was enhanced compared to
a.
Instituto de Catálisis y Petroleoquímica, CSIC, c/Marie Curie No. 2, Cantoblanco,
28049 Madrid, Spain
b. Dpto. Química Inorgánica y Técnica, Facultad de Ciencias UNED, Senda del Rey 9,
28040 Madrid Spain.
c. UA UNED-ICP(CSIC), Grupo de Diseño y Aplicación de Catalizadores Heterogéneos,
Madrid, Spain.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Table 1. Textural, physical-chemical properties of the catalysts and intrinsic activity.
metallic molybdenum even at low temperatures [9]. Indeed, the
selectivity obtained over C-Mo (110) was 15 times higher than
over Mo (110).
Other authors studied the effect of carbon source on
unsupported molybdenum carbide structure and its relation to
formic acid decomposition [11]. One of the challenges of
molybdenum carbide to be used in heterogeneous catalysis is
obtaining a high surface area material to maximize the activity.
In this respect, some alternative synthetic procedures such as
using other carbon precursors have been studied, but still
View Article Online
DOI: 10.1039/D0CY01088J
dTEM
(nm)
2.0
2.2
3.8
na
Activity (mol
Catalyst
SBET(m2/g)
Mo/C XPS
CO2/mol Mo h-1)
5MoxC/H400
10MoxC/H400
20MoxC/H400
10MoxC/H200
20MoxC/H200
10MoxC/AC
217
213
204
107
72
0,0049
0,0103
0,0105
-
-
-
85
112
73
61
10
8.1
2.1
377
240
surface areas below 40 m2/g were obtained [12]. Another Diffractograms of molybdenum carbides samples, in Fig.1, differ
possibility is to support the carbide onto a high surface area depending mainly on the type of support.
material. In this sense, the use of carbon supports can be The XRD patterns of 10MoxC/AC and 20MoxC/H200 show quite
beneficial for several reasons. The carbon support may favour sharp diffractions at 2θ of 34.4°, 37.7°, 39.4° and 61.5°. On the
the formation of the carbide by providing additional carbon other hand, the catalysts prepared over H400, 5-20 wt% Mo and
source for the synthesis. Also, since water might be produced 10MoxC/H200, display similar XRD profiles showing wide
during the reaction, a hydrophobic and stable support under diffractions with maxima shifted to lower angles, ca. 2 of
such conditions is preferred.
36.7°-36.9°, compared to 20MoxC/H200 and 10MoxC/AC, a fact
The effect of the nature of carbon support and its influence on suggesting that the contribution of several phases and/or the
the catalytic performance on reactions like steam reforming of amorphous structure of the resulting supported nanoparticles.
methanol or dry reforming of methane has been reported [13]. Moreover, none of the XRD of the fresh catalysts prepared over
Still, the catalytic behaviour of carbon-supported molybdenum H400 and AC display diffractions of the oxide phases. This
carbide on formic acid decomposition has been scarcely confirming that the carburization treatment was mostly
investigated and assessing the effect of its structure is highly effective. Nonetheless, the XRD pattern of the catalyst
challenging and not fully understood.
20MoxC/H200 shows two small diffraction peaks at 2θ of 23.7°
Hence, we have studied the synthesis of molybdenum carbide and 49.1° corresponding to MoO3 (JCPDS-PDF 05-0508 and 76-
using a mixture of CH4 and H2 over two commercial high surface 1003).
area graphites and we have compared their performance with
the activated carbon supported counterpart to evaluate the
effect of the graphitic structure.
5MoxC/H400
x2
10MoxC/H400
20MoxC/H400
2. Results and discussion
External surface area of the catalysts was measured for each
sample and results are summarized on Table 1. The SBET of the
parent supports is 950, 400 and 200 m2/g for AC, H400 and H200
respectively. It can be observed that SBET decreases as the
amount of carbide increases. The reduction of SBET for the high
surface area graphite-based catalysts is higher than the
expected value due to the weight percentage of carbide on the
surface. The loss of surface area in carbon supports upon
carburization has been well reported and ascribed to the pore-
blocking due to the carbon growth during the carburization
and/or the metal nanoparticles or to partial gasification of the
support [14]. However, considering the structure of high surface
area graphites, the observed decrease on surface area is likely
due to the agglomeration of the graphite particles upon thermal
treatment. The higher loss of surface area on active carbon
could be ascribed either to the easier gasification of carbon on
that material and particularly, to its large proportion of porous
in the microporous range which are easily blocked. The external
surface area of the activated carbon without microporous is 400
m2/g.
10MoxC/H200
20MoxC/H200
10MoxC/AC
20
30
40
50
60
70
80
90
2
Fig. 1 XRD patterns of the catalysts. Dotted lines represent Mo2C (red) graphite
There are several reports with different assignations for the XRD
and no clear consensus exists. In order to assess for the
carburisation mechanism taking place and the obtained
structures, we followed the synthesis of 10MoxC/H400 using in
situ Mo K-edge X-ray absorption near-edge spectroscopy
(XANES). The Mo K-edge spectra, in Fig. SI 1, show the evolution
of molybdenum species under the CH4/H2/He atmosphere
during the temperature treatment. Application of principal
component analysis (PCA) [15] to this set of spectra evaluates
the number of Mo chemical species involved during the
synthesis process and their concentration evolution. This
information is summarized in Fig. 2. The absorption edges
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(obtained using the derivatives of the spectra) were used to Mo2C on the selected 10MoxC/H400 sample than what is
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DOI: 10.1039/D0CY01088J
estimate the oxidation state of each species by using the linear observed by XRD. This can be attributed to the small size of the
correlation obtained with the reference Mo compounds (see supported carbide particles which would be below the
Figures SI2 and SI3). In figure 2B the two Mo6+ species spectra detection limit of XRD, i.e. < 5 nm, as also the TEM images, in
exhibit similar characteristics to those of the AHM reference Fig. 3 and Fig. SI4, confirmed. On the contrary for 10MoxC/AC,
with a pre-edge feature in the spectra that does not appear in both large and small particles are carburised as the XRD and
the other Mo species. So, the sample initially contains the HRTEM images showed. For this sample, the well-resolved
supported ammonium heptamolybdate (AHM), in which Mo+6 is lattice fringes of 0.23 nm that correspond to the -Mo2C (101)
in six-fold coordinated sites (see reference spectra in Figure planes are observed in particles as small as 2 nm. Similar
SI3). It is possible to observe three characteristics XANES findings are observed for 10MoxC/H400 for which also -Mo2C
resonances whose relative intensity is influenced by the local (101) is observed in small particles.
order around Mo atoms in octahedral symmetry [16]. Upon Hence, it can as well be inferred that larger particles are more
heating, the supported AHM is transformed into another Mo6+ difficult to carburise. This is agreement with the diffraction
species alsoin octahedralcoordination, molybdenum oxide type peaks ascribed to MoO3 observed on 20MoxC/H200 which is the
structure, whose XANES spectrum shows higher intensity of the sample with the largest particle size, ca. 8.1 nm as estimated by
second XANES resonance relative to the first and third. This TEM in Table 1.
indicates changes in the specific surrounding environment of
Mo atoms, likely an increase in the cluster size of the
molybdenum oxide [16]. This process takes up to 350°C when
the first Mo6+ is mostly absent. Meanwhile, a Mo+2 species
ascribed to molybdenum oxycarbide [16], MoOxCy, starts to
appear at 300°C while the second Mo6+ species decreases. In the
temperature range of ca. 400-600°C, the fraction of MoOxCy is
-Mo2C, (101)
mostly constant indicating that simultaneous formation and
consumption of MoOxCy takes place with parallel formation of a
Mo1+ species with XANES spectrum similar to that
corresponding to the molybdenum carbide phase -Mo2C
(Figure SI2). Upon heating up to 700°C, it resulted in further
carburisation with a final proportion of ca. 90% of -Mo2C and
10% of MoOxCy.
-Mo2C, (101)
-Mo2C, (101)
Figure 2. Concentration profiles (A) and Mo K-edge XANES spectra (B) of the different
pure Mo species observed during the synthesis of 10MoxC/H400
.
Based on XANES experiments we can now assign the XRD peaks
as follows. The diffraction peaks observed for the catalyst
10MoxC/AC correspond to the (100), (002), (101) and (110)
planes of the -Mo2C hcp phase (JCPDS-PDF 77-0720). The
higher angle at which the maximum is observed for the other
catalysts, would be in agreement with the presence of MoOxCy
phase that holds a face-centered-cubic (fcc) structure with
diffraction peaks at 37.1°, 44.1° and 62.9° [17, 18]. It must be
noted, however, that XANES showed a higher proportion of -
Fig. 3 Typical HRTEM images of a)10MoxC/H400; b) 10MoxC/AC and c) 20MoxC/H200 and
the corresponding Fast-Fourier transform (FFT) pattern.
The XRD patterns of the samples with different Mo loading also
seem to indicate that under the studied conditions the Mo/C
loading did not influence the obtained carbide phase.
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It has been reported that the synthesis of molybdenum carbides 10MoxC/H400 and 10MoxC/AC to assess for differences among
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through the TPR method may follow one of these paths: a) the supports. The masses corresponding DtoOtI:h1e0.m10a39in/Df0raCgYm010e8n8tJs
firstly, the decomposition of the molybdate precursor into of H2O (m/z 18), CO (m/z 28), CO2 (m/z 44) and NH3 (m/z 17) and
MoO3 takes places, which is then reduced to MoO2 to finally the secondary of CH4 (m/z 14) were followed along with other
result in the -Mo2C. b) Alternatively, after MoO3 formation, secondary fragments of H2O and NH3 (m/z 16, 15) and shown in
this oxide may suffer a partial carburization to an oxycarbide Fig. 5
MoOxCy, which is eventually transformed into -Mo2C [19]. This
latter path proposed in literature, agrees quite well with the
XANES results obtained in the present work, where both
oxycarbide and  phases are observed.
With the aim of gaining more information on the synthesis
mechanism and the effect of the support, additional XANES
experiments were performed with 20MoxC/H400 sample but
using a H2/He atmosphere. The analysis of the in situ Mo k-edge
XANES spectra (Figure SI5) showed that the same MoO3,
MoOxCy and -Mo2C species are obtained as when CH4/H2/He
is employed over the 10MoxC/H400 sample (see Figures 2 and 4).
These results suggest that under the studied conditions, the
carbon source to obtain the carbide is the support itself
independently of the reaction atmosphere. Similar results were
reported previously for temperatures below 600°C [19]. The
slight differences between samples, i.e. the 20MoxC/H400 is
somewhat more resistant to reduction in agreement with XRD
experiments, may be attributed to effects of the different
particle size in the samples (Table 1). The 20MoxC/H400 sample
with larger particles is more difficult to carburize.
Figure 4. Concentration profiles (A) and Mo K-edge XANES spectra (B) of the different
pure Mo species observed during the treatment of Mo20/H400 H2/He atmosphere.
Also, considering these stages it is reasonable to presume that
the availability of more reactive carbon atoms from the support
would aid on the formation of -Mo2C. In this sense, the carbon
supports are characterized by their different proportion of
edges to basal planes, i.e. the size of the graphitic layers, so that,
a higher proportion of edges means more reactive carbon. This
feature is given by the Raman spectra that allows comparison
by using the intensity of the so-called D and G bands [20]. The
Raman spectra of the supports, in Fig. SI6, used in the present
work show that ID/IG ratio follows the trend AC >H400 >H200. Since
external CH4 does not seem to be involved on the carburisation
mechanism, we performed H2-TPR experiments coupled to
mass spectrometry of the impregnated samples 10MoxC/H200,
Fig 5. MS profiles during H2-TPR of impregnated samples. m/z: (
) 14, (
) 16,
 
(
) 17, (
) 18, (
) 28, (
) 44.
   
The three samples, displayed evolution of m/z 18-16 and 17
with maxima at 200-210°C and 290°C in a proportion that
indicates they correspond to NH3 and H2O. These gases would
be generated from the decomposition of the precursor, AHM,
and the temperature range agrees with that observed in XANES
for this transformation.
At higher temperatures, the profile is different for 10MoxC/AC.
This sample, shows a maximum at 400°C in which contributions
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of m/z 18-16 are observed in a proportion that suggest that they carbon is the support itself. Also, the fact that the support is
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DOI: 10.1039/D0CY01088J
correspond to H2O. The same masses are observed at ca. 600°C the source of carbon is in agreement with the easiness
but the intensity of these latter peaks is lower. Small intense carburisation of smaller particles that would be in closer contact
signals for m/z 28 and 44 at ca. 290°C and 415°C are also with the support.
observed. The samples 10MoxC/H200 and 10MoxC/H400 also
display contributions of m/z 18-16 which can be ascribed to 3.2 Reaction results
H2O. However, the maxima appeared at 400°C and 660°C, this
latter being the most intense peak. Also, simultaneous The conversion of formic acid decomposition measured at each
evolution of m/z 28 due to CO is also observed, although its temperature is given in Fig 6A for all the tested catalysts. As the
intensity is very low.
Figure shows, all the catalysts were active, however the
The XANES results suggested that the source of carbon is the temperature at which 100% conversion was reached varied
support itself and that carburisation already starts at 300°C, greatly in the range 190°C to 250°C. Among the catalysts, the
with the formation of the oxycarbide which is then transformed conversion curve was very similar for the samples 10MoxC/H400
,
into -Mo2C (reaction 1).
20MoxC/H400 and 10MoxC/AC, while those of 5MoxC/H400,
10MoxC/H200 and 20MoxC/H200 shifted to higher temperatures.
MoO3+C*+ H2→MoOxCY + H2O -Mo2C + CO (1)
In this range of temperatures, carburisation must be preceded
by the formation of gaseous carbon species, most likely CH4
coming from the reaction with hydrogen and releasing H2O.
However, the intensity of m/z 16-14 in the profiles up to 700°°C
does not suggest the evolution of CH4 and only above that
temperature, an intense peak started to appear. We believe this
is due to the consumption of the evolved CH4 on the
carburisation process.
A)
Also, the different intensity of the MS-TPD peaks at 400°C and
600°C for AC and the two H samples seem to indicate that the
extent in which the first step occurs in AC is greater than on H200
and H400 samples. This is reasonable since in this first step the
incorporation of carbon takes place and, as already pointed out,
activated carbon holds a larger proportion of reactive carbon
atoms. In turn, it also suggests that carburisation took place in
a larger extent, as also the XRD showed.
B)
Regarding previous literature results, it has been reported that
the synthesis of molybdenum carbide over carbon nanofibers
and carbon nanotubes using the TPR method (CH4/H2) also led
to the formation of the -Mo2C and that above 700°C no
molybdenum oxide was observed [21, 22]. Other authors also
analysed the effect of surface chemistry and structural
properties of the support using an oxidized graphite and
oxidized activated carbon [23]. Although a clear correlation of
surface chemistry and structural properties with the formed
molybdenum phase was difficult to obtain, the authors
suggested that the defective carbon was somehow responsible
for the formation of the Mo2C and that the controlled
reduction of the molybdenum precursor was a critical factor to
determine the formed crystal phase.
Some authors proposed that Mo/C ratio on an ordered
mesoporous carbon support controls the MoxC phase [24]. We
observed that samples with different loading, mainly
10MoxC/H200 and 20MoxC/H200 displayed different XRD profile.
However, this seems to be more related to the contribution of
larger particles to the XRD profile, which are present in a higher
ratio on samples with greater loading.
Fig 6. A) Conversion of FA vs T and b) CO2 selectivity vs T. Reaction conditions: 1 bar,
GHSV= 20000 h-1.
In order to minimize the effect of Mo loading, the activities at
150 °C were estimated and the results are displayed in Table 1.
The activity follows the trend 10MoxC/AC > 10MoxC/H400
>
5MoxC/H400 > 20MoxC/H400 > 10MoxC/H200 > 20MoxC/H200. The
activity values and the similar particle size of the carbide on
10MoxC/H400 and 10MoxC/AC, seem to indicate that the catalyst
presenting exclusively Mo2C phase is more active than those
in which MoOxCy appeared as well. Regarding the effect of
metal carbide loading, the activities for the sample with 10 wt%
Mo supported on H400 was significantly higher than those of 5
Our results are in agreement with some literature, that
reported oxycarbide and Mo2C as the only detected phases.
We can add that even in a CH4/H2 atmosphere the source of
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wt% and 20 wt%. According to the microscopy, carbide 7, remain stable with time for all the catalysts, eVxicewepArtticfloerOntlhinee
DOI: 10.1039/D0CY01088J
nanoparticles are larger on the sample with higher loading, conversion achieved with 20MoxC/H
that showed a
200
20MoxC/H400, in agreement with the similar Mo/C surface ratio continuous decrease with time, while selectivity was constant.
obtained by XPS for 10MoxC/H400 and 20MoxC/H400, shown in It is known that the decomposition of formic acid may take
Table 1. This can explain straight forward the lower activity of place through two paths: dehydrogenation (HCOOH H2 + CO2)
20MoxC/H400 and suggest that, in the studied system, above and dehydration (HCOOH  CO + H2O). Also, water gas shift
10% wt. Mo there is an optimum loading before particles tend and/or the reverse reaction may take place producing CO2+H2
to form large agglomerates that diminished the active area, as or CO and H2O respectively [1].
observed in other carbon supported systems [25]. Similar and
even more clear effect is observed for the catalysts supported
on H200.
100
A)
90
The selectivity to CO2 at each temperature, Fig 6B, was
80
estimated considering the total concentration of CO2 and CO as
the only detected products and revealed significant differences.
70
The catalysts displayed maximum selectivity in the range 85.0-
60
96.5%, the catalyst 10MoxC/AC offering the best selectivity
among the tested systems. This also points to a beneficial effect
50
of -Mo2C phase compared to MoOxCy.
40
Indeed, if we compare the selectivity achieved with the
catalysts 10MoxC/AC and 10MoxC/H400, it is clear that the
different phase influences the concentration of H2 in the
100
B)
95
90
products. While with 10MoxC/AC selectivity to CO2 reached
96.5% at conversions above 50%, with 10MoxC/H400 the
85
selectivity for conversions over 50% was 91%.
Regarding molybdenum-based catalysts, previous literature
80
75
10Mo C/AC
reported that 100% formic acid conversion was achieved at
temperatures above 250°C for unsupported molybdenum
carbide systems [11] and over 200°C for supported systems over
activated carbon with selectivity around 98% [26]. However, the
catalysts employed in Ref. 11 and 26, were prepared using liquid
phase mixture with organic compounds that is less attractive
from an industrial point of view and may leave impurities on the
catalysts.
20Mo C/H400
x
x
70
65
60
5Mo C/H400
x
20Mo C/H
x
200
10Mo C/H400
x
10Mo C/H200
x
100
200
300
400
500
600
700
Time (min)
Fig 7. Time on stream A) Conversion of FA and B) CO2 Selectivity, Reaction conditions: 1
bar, T: 180-190 º C (220 ºC for 20MoxC/H200) GHSV= 20000 h-1.
Nevertheless, the reported catalytic performance using
molybdenum carbide and our own results are better than other
reported for non-noble metal systems like metal oxides, -
Fe2O3 for which maximum conversion was around 24% at
temperatures of 200°C with low selectivity to H2 [27,7] and
other systems where Ag and Mg were used as dopants for MoxC
that reached 90% selectivity [28].
Interestingly, the selectivity profiles with temperature shown in
Fig 6B are similar. All the catalysts prepared over high surface
area graphite, showed an initial selectivity decrease up to 140-
180°C which corresponds to formic acid conversions in the
range 10-12 % followed by a selectivity increase, up to
temperatures above 200-260°C, i.e. when conversion reached
90%. These results are in agreement with previously reported
selectivity profiles where other authors observed that CO2
selectivity decreased with temperature in the temperature
range 100-150°C [6]. However, the evaluation of the catalysts at
higher temperatures performed in the present work shows that
selectivity increased from conversions above 15%, and at 95%
conversion the selectivity decreased again.
According to DFT studies described in literature, the surface of
molybdenum carbide leads exclusively to the formation of CO2
and H2 since both Mo and C sites can chemisorb oxygen and
eliminate it as CO2 [8]. Also, the simulations performed in that
investigation suggested that molybdenum carbide decreased
the rate of dehydration (undesired reaction to form CO+H2O).
Regarding WGS reaction, theoretical studies reported that
neither Mo0 or Mo-terminated Mo2C are active on the WGS
reaction [29]. However, oxygen covering C-terminated surfaces,
O-C-Mo2C, were active on WGS apparently due to the formation
of oxycarbide species as a consequence of the presence of H2O.
Moreover, we performed additional reaction tests using
molybdenum oxide supported on H400 and tested in the FA
decomposition. The results indicated that molybdenum oxide is
active above 190°C when conversion reached 16% but
selectivity to CO2 was 22% (see Fig SI7).
Thus, considering the potential reactions and the selectivity
profiles we could explain the results as follows. -Mo2C would
be selective to the dehydrogenation of FA producing mainly
CO2+H2. The presence of small amounts of a different phase
other than -Mo2C produced during the reaction on
10MoxC/AC, i.e. MoOxCy and/or molybdenum oxide, could
explain the 96.5% selectivity to CO2 obtained with that catalyst.
As long as the stability is concerned, additional experiments in
time on stream at temperatures of 180-190°C for all the
catalysts and at 220°C for the 20MoxC/H200 were performed
during 12 hours. Both conversion and selectivity profiles, in Fig.
6 | J. Name., 2012, 00, 1-3
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Catalysis Science & Technology
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Journal Name
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On the other hand, for the catalysts presenting also MoOxCy, it oxycarbides species which are active in WGS. However, the
View Article Online
is plausible that both FA dehydrogenation and dehydration larger particle size inferred from the sharDpOXIR: 1D0.1p0e3a9k/Ds0wCoY0u1ld08b8eJ
occur at the beginning of the experiment and when conversion responsible of the conversion decrease with time.
reaches 10% the formed CO and H2O through dehydration, In contrast, the XRD pattern of the 10MoxC/AC sample after
react to produce CO2 and H2 via the water gas shift reaction, this reaction also corresponds to the -Mo2C and no other
increasing the selectivity. If this happens it is plausible that only diffraction is envisaged, in agreement with the higher selectivity
limited oxidation of molybdenum carbide occurs since water to CO2 and better stability. Nonetheless, the selectivity below
would be consumed by WGS. Hence, we performed the XRD of 100% could also suggest that small undetected amounts of
the spent catalysts to assess for potential changes and are another phase are present. So, even in the presence of oxidants,
shown in Fig 8 labelled as “catalyst-PR”.
H2O or CO2, this catalyst resulted to be highly stable under the
reaction conditions.
The change of molybdenum phase under reaction conditions
have already been reported. For example, during dry reforming
of methane reaction, deactivation of Mo2C catalysts due to
oxidation to MoO2 has been reported [32]. Ledoux et al. also
found that Mo2C changed to MoO2, while -MoxC was
transformed into MoO2 and eventually to Mo2C [19].
Similarly, phase changes were also observed in the spent
catalysts after steam reforming of methanol reaction [33].
Some authors, have reported that other phase such as -MoC
is more active on formic acid decomposition than -Mo2C [5].
However, chloro and nitrogen containing compounds were
used as carbon source to prepare the catalysts and the effect of
those elements should have been considered. Indeed, in the
same report it is also shown that a conventional -Mo2C
prepared from a CH4/H2 mixture was the most active and
selective catalyst. In line with our results, molybdenum carbide
prepared over activated carbon was more active and selective
than when carbon nanotubes were used as support [6]. Even
though the crystallographic phase of the molybdenum carbides
were not shown, it is likely that the -Mo2C phase was formed
over activated carbon as already shown in several literature and
in the present work [23,26]. As demonstrated by XRD, in the
present work the main phase on the catalysts was Mo2C,
although MoOxCy could be found on H400 and H200 based
catalysts, so our results are in quite good agreement with
literature, showing that -Mo2C is more active and selective.
Very recently, the reaction mechanisms of both dehydration
and dehydrogenation reaction of formic acid on molybdenum
carbide have been studied [34]. The authors proposed that the
target reaction leading to CO2 and H2 takes place through
bridged formate species that evolve into monodentate formate,
and transformed to CO2 and H* following a Langmuir-
Hinshelwood mechanism. On the other hand, the dehydration
reaction follows an Eley-Rideal path in which gaseous HCOOH
reacts with adsorbed H* to form H2O and CO. Considering these
paths, the rate in which dehydrogenation occurs might depend
on the easiness of the bridge type adsorption of the formate.
The carbon/molybdenum ratio influences the physico-chemical
properties of the carbides and so the catalytic properties. In this
sense, it has already been reported that a higher C/Mo ratio has
a negative impact on the reactivity. In this respect, the MoOxCy
surface would be more saturated and less metallic than the
unsaturated Mo2C surface [35]. Following this argument, it is
plausible that the cleavage of the O-H bond and adsorption of
the formate on the less saturated surface of -Mo2C would be
energetically favoured than over MoOxCy.
5MoxC/H400-PR
10MoxC/H400-PR
20MoxC/H400-PR
20MoxC/H200-PR
10MoxC/AC-PR
30
40
50
60
70
80
90
2
Fig. 8 XRD patterns of the spent catalysts.
However, different results were observed. On the one hand,
10MoxC/H400-PR displayed diffractions at 2θ of 34.4°, 37.7°,
39.6° and 61.5° and sharper than those of the fresh catalyst that
correspond to sintered particles of the -Mo2C phase. On the
other hand, 20MoxC/H400-PR shows diffractions at 2θ of 35.2°,
36.7° and at 2θ of 53.5° which can be ascribed to MoO2 [30, 31].
For 5MoxC/H400-PR, no diffractions other than those of the
support were observed. This is probably due to the small
particle size and/or the low concentration of species, below the
detection limit.
In any case, the detected changes on the catalysts phase, either
conversion to -Mo2C or to oxycarbide agree with the selectivity
profiles observed for 10MoxC/H400-PR and 20MoxC/H400-PR
since both new phases may promote the CO2 selectivity either
by direct transformation of FA to CO2 or through WGS.
A different case is that of the catalyst 20MoxC/H200. This catalyst
initially presented both carbide and oxide phases and the
selectivity to CO2 is below the other tested catalysts but it also
showed the selectivity increase at around 10% conversion. The
XRD of the spent catalyst, shows diffractions at 2θ of 34.4°,
37.8°, and 39.8° from the -Mo2C along with peaks at 35.7°,
38.2° and 60.1°. The position of these latter peaks seems to
indicate that they correspond to oxycarbide. Again, it seems
that oxycarbide is formed during the reaction and, although -
Mo2C phase is still detected, the intense and sharp diffraction
ascribed to oxycarbide seems to indicate that large particles of
these species have been formed on the surface. Despite these
new nanoparticles appeared, the selectivity remained stable
with time probably due to the positive effect of the new
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Journal Name
3. Experimental
View Article Online
DOI: 10.1039/D0CY01088J
3
∑
∑
ni di
3.1 Synthesis of materials
dTEM
=
2
Metal carbides were prepared by wetness impregnation
followed by carburization treatment. Activated carbon (Type
CO-850 from Petrochil S.A) was used as support and was first
grounded and sieved to < 150 µm particle size, and then dried
at 110°C for 2 h before impregnation. Commercial high surface
area graphite (H400 and H200, from Timcal Graphite).
An aqueous solution of the precursor (NH4)6Mo7O24 (99%
from Aldrich) was impregnated on the support using the
corresponding amount to obtain the metal loading, left for
maturation for 6 h, and dried overnight at 80°C. The
carburization was carried out in situ prior to the reaction, under
the mixture composition at 80/20 of H2/CH4 (%vol) at 700 °C,
(5°C.min-1) for 2 h. Resulting catalysts will be labelled according
to the metal composition, loading and support.
ni di
Mo K-edge (20.000 eV) X-ray absorption near edge spectra
(XANES) were recorded in dispersive mode at the BM23
beamline at the European Synchrotron Radiation Facility (ESRF,
Grenoble, France). The catalysts were pressed into pellets and
sieved to a size between 0.090 and 0.140 mm. The samples (ca.
15 mg) were loaded in a quartz plug-flow microreactor system
developed at BM23. The reactor was continuously fed with: a)
30 mL min−1 of the mixture composition 20/5/75 of H2/CH4/He
for the temperature-programmed carburization experiment or
b) 30 mL min−1 of 20% hydrogen in helium during temperature-
programmed reduction experiment. The temperature was
raised by 3 °C min−1 up to 700 °C. XANES spectra were collected
every 30 °C or 10 min during the heating.
3.2 Characterization
3.3 Reaction
Textural properties were measured from the adsorption
isotherm of N2 at -196°C using a 3Flex instrument from
Micromeritics. Around 100 mg were previously degassed at 4 h
at 110°C under vacuum using a SmartVacPrep instrument from
Micromeritics. The surface area was calculated from the
adsorption branch in the range 0.02 ≤ p/p0 ≤ 0.25 using the
Brunauer-Emmett-Teller (BET) theory. Total pore volume was
defined as the single-point pore volume at p/p0=0.99.
X-ray diffraction (XRD) patterns of the passivated catalysts were
acquired in the 2Ɵ range between 4° and 90° with a step of
0.04°/s using a Polycristal X’Pert Pro PANalytical diffractometer
with Ni-filtered Cu Kα radiation (λ = 1.54 Å) operating at 45 kV
and 40 mA.
XPS measurements were performed with an energy analyser
(PHOIBOS 150 9MCD, SPECS GmbH) using non-monochromatic
Al radiation (200 W, 1486.61 eV). The samples were pelletized
and transferred to the outgassed chamber. Prior to the
experiments, samples were outgassed in-situ for 24 h to achieve
a dynamic vacuum below 10-10 mbar. The binding energy (BE)
was measured by reference to the C 1s peak at 284.6 eV, with
The measurements of the catalyst activity in vapor phase formic
acid decomposition were carried out in a fixed-bed flow reactor.
The catalysts (0.075 g) were placed in a U-tube reactor with an
internal diameter of 4 mm. All the samples were in situ
carburized in CH4/H2 (20:80 vol) at 700 °C for 2 h and cooled in
N2 to a reaction temperature prior to testing (in situ reduction).
The mixture of 5.5 vol % formic acid/N2 at a total flow rate of 25
cm3 (STP)/ min was fed to the reactor by a saturator. The
reactants and products were analysed by a gas chromatograph
(Varian 3400) fitted with a 60/80 Carboxen TM 1000 column
and a thermal conductivity detector. At each temperature, a
few measurements were performed to be sure in reaching of
steady-state activity. During the test, the unique products
determined were CO, CO2 and H2. The concentrations of these
compounds were calculated by following equations.
Area H2
RF
Area CO2
RF
Area CO
RF
[
]
[
]
CO2 =
[
]
CO =
H2
=
an equipment error of less than 0.01 eV in the energy As the formation of products different from those indicated was
determinations.
negligible, the total conversion of formic acid was determined
Information about the supported metal particles was acquired as the sum of CO and CO2 concentrations related to the initial
by TEM in a JEOL 2100F field emission gun electron microscope concentration of formic acid.
[
[퐶푂]푚 + 퐶푂2
=
]
[
]
HCOOH
operated at 200 kV and equipped with an Energy-Dispersive X-
at Tmax to obtain a complete
0
푚
Ray detector. The sample was ground until powder and a small conversion of HCOOH
amount was suspended in ethanol solution using an ultrasonic To determinate the conversion of formic acid, it was necessary
bath. Some drops were added to the copper grid (Aname, Lacey to calculate the concentration of CO and CO2 and using the
carbon 200 mesh) and the ethanol was evaporated at room equation (1).
temperature before introduce in the microscope. The Scanning
[
]
[
]
CO + CO2
XHCOOH
=
× 100
(1)
Transmission Electron Microscopy (STEM) was done using a
spot size of 1 nm. Average particle size area (dTEM) was
calculated as
[
]
HCOOH
0
In addition, the selectivity to CO2 was calculated. The catalysts
were studied in two heating cycles. The stability of the catalyst
was evaluated during 18 hours at a selected temperature.
8 | J. Name., 2012, 00, 1-3
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4
5
a) Sustainable Energy Fuels, 2019, 3, 1042. b) VWiew. AGrtaicnle,OPn.linJe.
Dyson and G. Laurenczy, ChemCatChem, 2013, 5, 3124. c) W.
Gan, P. J. Dyson and G. Laurenczy, ReDaOctI.: 1K0i.n10e3t9. /CDa0tCaYl.01L0e8t8t.J,
2009, 98 , 205. d) Y. Zhao, L. Deng , S.-Y. Tang , D.-M. Lai , B.
Liao , Y. Fu and Q.-X. Guo , Energy Fuels, 2011, 25 , 3693.
G. H. Gunasekar, H. Kim and S. Yoon, Sustain. Energy Fuels,
2019, 3, 1042–1047.
Tests with the bare supports proved that conversion was
negligible.
Conclusions
The synthesis of molybdenum carbide by the TPR method using
a CH4/H2 feed was performed successfully over activated carbon
and two high surface area graphites, H200 and H400, of different
graphitic layer dimension. The formed carbide nanoparticles
were between 2.0 and 8.1 nm size which depended on the
surface area and metal loading. Also, the characterization
showed that the carbide phase was influenced by the support.
Accordingly, when activated carbon was used, the Mo2C
phase was obtained mostly exclusively. However, over H400 and
H200 the oxycarbide phase, MoOxCy was also observed. We
explained the differences based on the graphitic layer size. This
is related to the availability of defective carbon which in turn
favours the carburisation. Interestingly, XANES experiments
showed that even under a CH4/H2 atmosphere the main source
of carbon is the support itself. This also implies that smaller
particles, which are in closer contact with the support, are more
easily carburised.
The different molybdenum carbide phase influences the
catalytic performance. The results showed that the Mo2C
phase is more active and selective than the MoOxCy. However,
since MoOxCy is also active on the WGS reaction, the selectivity
to CO2 increases during the reaction. Moreover, Mo2C
supported on activated carbon proved to be stable under
reaction conditions.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We acknowledge financial support from the Spanish Agencia
Estatal de Investigación (AEI) y el Fondo Europeo de Desarrollo
Regional (FEDER), UE (projects CTQ2017-89443-C3-1-R and
CTQ2017-89443-C3-3-R). The ESRF synchrotron is thanked for
granting beamtime at BM23. A.B.Dongil acknowledges financial
support from Fundación General CSIC (Programa ComFuturo).
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