Insight into the Enhanced Selectivity of Phosphate-Modified Annealed Nanodiamond for Oxidative Dehydrogenation Reactions

Article abstract of DOI:10.1021/acscatal.5b00042

Due to a lack of fundamental understanding of the surface properties of nanocarbons, tuning their surface active sites for higher selectivity of oxidative dehydrogenation reactions has always been a great challenge for carbon catalysis. In this contribution, annealed nanodiamond was controllably grafted by phosphate, which was demonstrated to be an efficient way to adjust the nanodiamond surface and significantly improve the propene selectivity in the oxidative dehydrogenation reaction. We conducted an in-depth study to explore the role of phosphate modification in the reaction in terms of the interactions between phosphate and carbon surface, the evolution and preferential location of phosphorus species, the promotion mechanism, and the impact on the reaction pathway. The results revealed that phosphate preferentially reacts with the phenol groups initially present on the nanodiamond surface, and then it selectively blocks the defect sites that lead to COx formation with an increased propene selectivity. During this process, the catalyst active sites (ketonic carbonyl groups) were not affected. Such effects originated from the formation of covalent C-O-P bonds on the carbon surface, which was estimated as 15 wt % loading. (Chemical Equation Presented)

Full text of DOI:10.1021/acscatal.5b00042

Research Article
pubs.acs.org/acscatalysis
Insight into the Enhanced Selectivity of Phosphate-Modified
Annealed Nanodiamond for Oxidative Dehydrogenation Reactions
†
†
†
†
‡
,†
Xiaoyan Sun, Yuxiao Ding, Bingsen Zhang, Rui Huang, De Chen, and Dang Sheng Su*
†Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016,
People’s Republic of China
‡
Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway
*
S Supporting Information
ABSTRACT: Due to a lack of fundamental understanding of
the surface properties of nanocarbons, tuning their surface
active sites for higher selectivity of oxidative dehydrogenation
reactions has always been a great challenge for carbon catalysis.
In this contribution, annealed nanodiamond was controllably
grafted by phosphate, which was demonstrated to be an
efficient way to adjust the nanodiamond surface and
significantly improve the propene selectivity in the oxidative
dehydrogenation reaction. We conducted an in-depth study to
explore the role of phosphate modification in the reaction in
terms of the interactions between phosphate and carbon
surface, the evolution and preferential location of phosphorus
species, the promotion mechanism, and the impact on the
reaction pathway. The results revealed that phosphate
preferentially reacts with the phenol groups initially present on the nanodiamond surface, and then it selectively blocks the
defect sites that lead to COx formation with an increased propene selectivity. During this process, the catalyst active sites
(
ketonic carbonyl groups) were not affected. Such effects originated from the formation of covalent C−O−P bonds on the
carbon surface, which was estimated as 15 wt % loading.
KEYWORDS: nanodiamond, phosphate, high selectivity, oxidative dehydrogenation, mechanism
1
3,14
INTRODUCTION
identified as the active sites for alkene production.
■
However, under oxidative reaction conditions, the inevitable
presence of acidic oxygen species could ultimately lead to the
deep oxidation of both the alkanes and alkenes by attack of the
carbon chain through these oxygen species, thus significantly
With the increasing development of metal-free carbon-based
1
catalysis in recent years, detonation nanodiamond (ND) as a
3
new sp -hybridized carbon material has found applications in
catalysis not only as a support but also as an effective catalyst.
ND nanoparticles are composed of a crystalline diamond core
with a curved graphene-like surface, and the structural defects
on the surface are saturated by a large number of oxygen
groups. A distinct feature of NDs among the graphitic
nanocarbons is that they possess more dangling bonds and
higher surface energy due to the high ratio of surface atoms,
2
3
15
reducing the product selectivity. Therefore, regulating the
surface chemical properties of nanocarbon catalysts, especially
the nature of oxygen functional groups and defective sites, plays
a crucial role for improving the selectivity of the oxidative
dehydrogenation reaction. Incorporation and modification of
heteroatoms, such as nitrogen, boron, sulfur, and/or
phosphorus, into carbon materials have been proven to be an
effective method for tailoring the electronic structure and
4
5
thus endowing them with superior surface reactivity. It has
been reported that, in the direct dehydrogenation (DH) of
ethylbenzene, ND showed comparable catalytic performance at
a temperature lower than that for the conventional metal-based
16
surface properties of carbon materials. As for the ODH
reaction, the positive impacts on boosting selectivity by
phosphorus have been described on CNTs and CNFs in the
6
catalysts.
7,17−19
ODH of ethane, propane, and n-butane.
Kinetic studies
For the industrially relevant oxidative dehydrogenation
reaction (ODH), nanocarbons are regarded as promising
alternative metal-free catalysts owing to their unique controll-
ability of physical and chemical properties. It is found that
carbon nanofibers (CNFs), carbon nanotubes (CNTs),
graphene, and nanodiamond can all catalyze ODH reactions
without heavy carbon deposition or coke formation. The
ketonic carbonyl groups on the carbon surface have been
demonstrated that phosphorus can increase the selectivity by
suppressing the combustion rate, rather than enhancing the
formation rate of alkenes. However, no direct evidence was
7
,8
9
10
1
1
12
Received: September 19, 2014
Revised: March 5, 2015
©
XXXX American Chemical Society
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given in previous studies about the chemical nature of
phosphorus and the exact promotion mechanism for the
selectivity. The exploration of these issues is crucial for
understanding the role of heteroatoms in the complex surface
reactions, which could provide principles for rational design of
catalysts to achieve higher selectivity. Herein, a series of
phosphate-modified annealed nanodiamonds were prepared
and thoroughly probed as catalysts for ODH of propane. In the
ODH reaction, we focused on revealing the most important
mechanistic aspects of the phosphate-interfered ODH process,
including the nature of promotion effects by phosphate, the
evolution of phosphorus species, and the interaction between
phosphate and the carbon surface.
AND surface, in which N resulted from the impregnation
precursor. Even for a relatively high loading, such as 20 wt %
(Figure S1 in the Supporting Information), no phosphate
clusters or agglomerations were observed.
Thermogravimetric analysis (TGA) is given in Figure 2a. The
initial AND shows 6% weight loss, which is attributed to the
desorption of surface oxygen groups. The weight loss of
phosphate-modified samples correspondingly increases with
increasing impregnation ratio. Meanwhile, the amounts of water
and ammonia desorbed below 500 °C are consistent with the
calculated weight percentages in the precursor of diammonium
phosphate, confirming no phosphate loss during the prepara-
tion. Temperature-programmed oxidation (TPO) profiles
(
Figure 2b) reveal an increasing resistance to oxidation with
RESULTS AND DISCUSSION
higher phosphate content, and the residuals may result from the
incombustible impurities of AND and the precursor salt. The
oxidation temperature can be improved by 70−170 °C (T50%
in comparison to the temperature needed for the unmodified
sample. This provides the possibility of performing the ODH
reaction at relatively higher temperatures without carbon
combustion.
■
Structure and Surface Chemistry of Phosphate-
Modified Annealed Nanodiamond. It was reported that
the fresh ND surface covered by an unstable disordered carbon
layer tended to transform into onionlike carbon under oxidative
)
12
reaction conditions. Herein, ND was annealed at 1000 °C
(
referred to as AND) as the initial sample for phosphate
modification, the surface of which has been formed as one or
two curved graphitic layers (see Figure 1a). The obtained
The chemical state of phosphorus species was characterized
by diffuse-reflectance infrared Fourier transform (DRIFT)
spectroscopy (Figure 2c). With increasing phosphate loading,
−1
the broad bands in the region 800−1300 cm become more
predominant and slightly shift to higher wavenumbers in
comparison to those of diammonium phosphate. The reason
could be the contribution of the stretching vibration of
phosphate-containing groups, including C−O−P (1250
−
1
−1
−1 21
cm ), PO (1090 cm ), and P−O−P (975 cm ). This
indicates that a chemical bond was formed between phosphate
and the carbon surface during the preparation process.
Accordingly, the vibration modes of N−H groups (1400−
−1
1
500 cm ) can be attributed to the undecomposed precursor
adsorbed on the AND surface. Raman spectra (Figure S2 in the
Supporting Information) show negligible changes in the peak
3
−1
position and intensity of the sp -diamond band at 1320 cm
−1
and G band centered at 1580 cm , indicating no structural
3
1
impact by the modification process. The solid-state P magic-
angle spinning (MAS) NMR spectra of phosphate-modified
samples show broad peaks at around 1 ppm similar to those for
the precursor salt (Figure 2d), which are attributable to the
orthophosphate sites P(O)(OH) derived from diammonium
3
phosphate. A new resonance signal centered at −11 ppm for
phosphate-modified ANDs suggests the presence of surface
21
polyphosphate units linked together by a P−O−P bond. The
ratio of the −11 and 1 ppm peaks is constant when the loading
amount of phosphate was less than 15 wt % but decreased for
Figure 1. TEM images of (a) initial AND and (b) 12P-AND and (c)
STEM image and (d) EDX elemental mapping of 12P-AND. The
mapping was obtained in the boxed area in (c).
2
0P-AND. It seems that, when a certain value is exceeded, the
excess phosphorus could not form relatively more polyphos-
phate structures that are covalently linked by P−O−P.
conjugated graphitic structure can also favor the covalent
Catalytic Properties and the Role of Phosphate.
Propane ODH was performed to investigate the modification
effect on the catalytic activity of ANDs. By impregnation of an
identical phosphorus precursor on SBA-15, no catalytic activity
of phosphorus for the ODH reaction was demonstrated under
the given conditions. In order to directly compare the catalytic
activity derived from nanodiamonds, the catalyst weight used in
the reaction was normalized to the same mass of AND. With an
increase in phosphate loading, the selectivity to propene
significantly improved from 45% to 70%; concomitantly, the
formation of carbon oxides (COx) remarkably decreased
(Figure 3a). This demonstrates that phosphate modification
20
surface functionalization. Phosphate-modified samples were
synthesized by the incipient-wetness impregnation method
using diammonium phosphate as precursor. The loading
amount ranged from 1 to 20 wt %, which is calculated as the
weight of P O (denoted as xP-AND, with x = 1−20). The
2
5
subsequent drying at 120 °C did not involve any loss of
phosphate content. High-resolution transmission electron
microscopy (HRTEM) image (Figure 1b) shows that
phosphate treatment did not affect the AND morphology.
Energy-dispersive X-ray spectroscopy (EDX) elemental maps
(
Figures 1c,d) show P, N, and O uniformly dispersed on the
2
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Figure 2. (a) TGA and (b) dynamic TPO curves of unmodified and phosphate-modified ANDs. Treatment conditions: from room temperature to
−1
31
9
50 °C under argon and air flow, respectively, heating rate 10 °C min . (c) DRIFT and (d) P NMR spectra of unmodified and phosphate-
modified ANDs, in which the spectrum of diammonium phosphate is shown as reference.
is an efficient method to adjust the AND surface for higher
selectivity. Figure 3b shows the isothermal TPO curves
recorded at 500 °C. Initial AND was found to be completely
burned under the oxidizing atmosphere, thus severely limiting
its application to high-temperature reactions. However, after
phosphate modification, the sample shows a high oxidative
stability at 500 °C. Therefore, in a long-term ODH reaction
performed at 500 °C (Figure 3c), the phosphate-modified
catalyst can provide a stable catalytic performance for a time on
stream of 20 h with a conversion higher than 20%, the
selectivity remaining above 45%. It should be mentioned that
this selectivity shows a significant decrease in comparison to
that of the reaction performed at relatively low temperature
comparison to that of alkene formation from reactant alkanes,
raising the reaction temperature would be more sensitive for
the pathway with higher activation energy. This results in a
significant increase in the oxidation reaction rate with a
decreased selectivity. However, in comparison to the metal
oxide catalysts, which need to be used at elevated temper-
24
atures, the performance of phosphate-modified AND catalyst
was rather moderate and stable with the advantage of no carbon
deposition or catalyst deactivation during reaction.
Since phosphate modification can efficiently adjust the AND
surface to significantly improve propene selectivity, the
important question to be asked is as follows: what form of
interactions take place between AND and phosphate? From the
TGA analysis (Figure 2a), it can be determined that the
precursor phosphate salt was completely decomposed by
releasing water and ammonia below the reaction temperature
(450 °C), resulting in mainly phosphorus oxide adsorbed on
the carbon surface. TGA profiles in Figure 3d show almost no
weight change during the isothermal stage of 450 °C in air until
phosphate loading was up to 15 wt %, suggesting a highly stable
interaction between phosphate and the AND surface. In
contrast, in the case of 20P-AND, an obvious weight loss during
the isothermal stage can be observed. This suggests that 15 wt
% phosphate may be exactly sufficient to occupy all of the
available combustion sites on the AND surface. For 20P-AND,
due to the limited accessible attachment sites, the excess
phosphorus is unstable and could desorb or decompose during
(
450 °C, Figure 3a), although the carbon mass balance was still
within 100 ± 0.5%. To check the structural integrity of the
catalysts after the reaction, two samples reacted at 450 and 500
°
C were characterized by temperature-programmed desorption
(
TPD) and NMR (Figure S3 in the Supporting Information).
There are only small changes in the surface properties between
the two catalysts, regardless of a slight decrease in the CO2
desorption after the reaction at 500 °C (the attribution of each
peak in the TPD and NMR will be discussed in detail below).
Therefore, the possible reason for the decrease in selectivity at a
higher temperature could be the parallel and consecutive
reaction pathway of ODH, which has been reported for both
22,23
carbon catalysts and metal oxide catalysts.
That is, due to
the higher activation energy for COx formation from alkenes, in
2
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Figure 3. (a) Propene and COx selectivity as a function of P O loading (W ). Reaction conditions: 450 °C, catalyst weight 150−180 mg, C H /
2
5
P
2
O
5
3
8
O /He = 1/1/31.3. (b) Isothermal TPO curves recorded at 500 °C. (c) Long-term catalytic activity of 15P-AND conducted at 500 °C. Reaction
2
conditions: catalyst weight 250 mg, C H /O /He = 1/1/31.3. (d) TGA profiles from room temperature to 450 °C (under argon flow) and at 450 °C
3
8
2
for 8 h (the flowing gas was switched from argon to air).
the reaction. This also explains why 20P-AND showed catalytic
activity similar to that of 15P-AND (Figure 3a).
were negligible structural changes during the catalytic process
which can be evidenced by TEM and Raman spectroscopy
(Figure S4 in the Supporting Information). Phosphorus is well
dispersed throughout the AND surface after the reaction, and
Raman spectra show similar peak positions and intensities of
It should be that the low sublimation temperature of
phosphorus oxide makes it unlikely that it would exist as a
separate phase under the reaction conditions. Therefore, the
stable interaction, up to 15 wt % of phosphorus oxide, may
come from the formation of chemical bonds with the carbon
3
sp diamond band and G band in comparison to those before
the reaction. Therefore, it can be concluded that forming a
stable C−O−P chemical bond between the phosphate and
AND may result in the selective coverage of the combustion
sites, thus leading to an increased selectivity to propene.
In order to gain further insight into the role of phosphorus in
the ODH reaction, the propene formation rates for different
phosphate loadings are given in Figure 4d. Interestingly, it can
be found that the formation rate of propene slightly decreased
to a certain extent at a low phosphate content (around 1 wt %)
and stayed at the same level with increasing phosphate loading.
This suggests that the phosphate should preferentially interact
with a portion of ODH active sites on the AND surface under
the reaction conditions, and this is followed by the blockage of
the combustion sites. This interaction sequence can be further
confirmed by normalizing the difference of the formation rate
between propene and COx on per unit mass of phosphorus
species. As shown in Figure 4d, at a low loading amount, each
phosphate causes significant changes in the reaction rate due to
the coverage of catalytic sites, while after these sites are
occupied, the additional phosphate starts to interact with
combustion sites that lead to COx formation with the same
efficiency. The catalytically active sites for the ODH reaction
2
5,26
surface, resulting from C−O−P or C−P−O bonding,
which can evolve at the heating stage to 450 °C or during the
reaction. Characterization of the catalysts after the reaction can
reveal the chemical nature and evolution of phosphorus-
containing groups. Figure 4a displays similar TGA thermo-
grams for all used phosphate-modified catalysts, indicating that
the same chemical properties were formed on the AND surface
during reaction. A significant weight loss between 550 and 800
°
C was found in the initial AND in comparison to that of
phosphate-modified samples. This is attributed to the
desorption of C−O groups on the unmodified surface, while
it disappears on the used phosphate-modified catalysts due to
the formation of the stable C−O−P bond with the AND
surface. NMR (Figure 4b) and IR (Figure 4c) spectroscopy can
further confirm this chemical interaction. NMR spectra of the
catalysts used display the same resonance signals at around −24
ppm, which suggests the presence of the phosphorus species
21
featured as P(O)(OC)(OP)2. Correspondingly, IR spectra
−1
show the similar C−O−P vibration band at 1252 cm and P
−1
O band at 1097 cm . No evidence for the formation of C−P−
O species was observed after the reaction. In addition, there
2
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Figure 4. (a) TG curves of the catalysts used under argon flow. (b) 31P NMR of phosphate-modified ANDs after the ODH reaction at 450 °C
asterisks indicate spinning side bands). (c) DRIFT spectra of ANDs after the ODH reaction at 450 °C. (d) Propene formation rate and the
difference between the formation rates of propene and COx at 450 °C on per unit mass of phosphorus as a function of P O loading (W O5).
(
2
5
P
2
have been known to be the ketonic carbonyl groups, which can
play the role of electron donor to activate the alkane fragment
by forming phenol-like intermediates. Considering the
Oh and Rodriguez found that the armchair faces of graphite
were oxidized at a rate slower than that for the zigzag faces,
suggesting that phosphorus species were preferentially located
at armchair sites. Considering the surface of annealed
nanodiamond has already been formed as one or two curved
graphitic layers, it is likely that the selective adsorption of
7
28
interaction between phosphate and AND, which is linked by
the chemical C−O−P bond, we propose the active sites that are
most likely preferentially covered by phosphate are the phenol
groups, which might not be the dominating active site but could
be involved in the catalytic redox process during the ODH
reaction. Obviously, the phenol groups could easily form C−
O−P with phosphate through the dehydration reaction (Figure
S5 in the Supporting Information). In order to confirm this
assumption, we quantified the amount of surface oxygen groups
present on the initial AND by deconvolution of TPD spectra
phosphate observed on the AND surface follows the same rule.
−
On the other hand, electrophilic oxygen species such as O
2
(superoxide), and O22 (peroxide) that are responsible for COx
−
formation were found to favor attachment to armchair sites, for
29
steric reasons. Therefore, it is reasonable to assume that the
interaction between phosphate and such sites would reduce the
generation of the electrophilic oxygen groups on these defects
during the reaction, leading to an increased selectivity of
propene.
(
Figure S5). The phenol groups can release CO upon
27
decomposition in the temperature range 550−750 °C. On
this basis, the calculated amount of phenol groups is estimated
to be 151 μmol/g. Assuming that one phenol group could react
with one phosphate, occupying these sites would need an
approximate loading of 1.1 wt % P O , which is consistent with
TPD can give quantitative evidence about the evolution of
surface groups after phosphate modification and further reveal
the interaction between AND and phosphate. In general, CO
desorption is mainly attributed to carboxylic anhydride, phenol,
2
5
the catalytic behavior in Figure 4d. This means that, as long as
these phenol groups are covered, the reactivity could be
reduced to an extent and stay at the same level with increasing
phosphate content without affecting the structure of the main
active sites (Figure 4d).
After some active sites were occupied, the increasing
adsorption of phosphate gradually blocked certain sites for
COx formation, leading to a higher selectivity toward propene
and carbonyl/quinone species, whereas CO desorption results
2
from carboxylic anhydride at low temperatures and lactones at
2
7,30−33
higher temperatures.
On this basis, CO and CO2
desorption profiles of the catalysts used were deconvoluted to
separate those distinct peaks from the decomposition of various
functionalities (Figures 5a,b and Tables S1 and S2 in the
Supporting Information). It is noteworthy that, after phosphate
modification, the new evolution of CO and CO appears in the
2
(
Figure 3a). The selective adsorption of phosphorus has been
range 800−900 °C and the desorption peaks shift to higher
temperature with higher intensity with an increase in the
28
reported in the literature for graphite. By using in situ TEM,
2
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Figure 5. Deconvolution of TPD profiles of ANDs after the ODH reaction at 450 °C for (a) CO desorption and (b) CO desorption. The black
2
lines represent the original TPD curves, and the red lines represent the fitted curves. Detailed parameters for the deconvolution are given in Tables
S1 and S2 in the Supporting Information. (c) Variations in the amounts of different oxygen-containing groups on the basis of the deconvoluted
spectra.
phosphate loading. This phenomenon was also observed by
others when using phosphoric acid to activate carbon
are responsible for COx formation. Indeed, phosphate
modification generated a significant amount of groups with
high thermal stability during TPD, and the cleavage of the
2
2,34,35
materials.
Evolutions of CO and CO at these high
2
temperatures can be associated with the decomposition of the
highly stable C−O−PO groups makes the greatest contribu-
3
−
COO−PO and C−O−PO groups, respectively, in which the
tion. This can probably be attributed to the strong interaction
of phosphate with the oxygen groups and defects on the AND
surface, which can happen under the reaction conditions or
could be induced by a high-temperature treatment of the TPD
process. On the other hand, by comparison of the TPD spectra
of phosphate-modified samples before and after the reaction
(Figure S6 in the Supporting Information), the desorbed
amounts of −COO−PO and C−O−PO groups increased
3
3
O−P bond may be broken first at high temperature due to the
relatively lower bond energy, leaving the O atom bonded to the
carbon site further desorbed in the form of CO and CO2.
34
This corroborates the formation of a C−O−P bond at the
surface. The shift for those peaks is probably due to the
formation of a polyphosphate structure that hinders the
extraction of the phosphorus species. The amounts of different
functional groups on the basis of the deconvoluted spectra are
given in Figure 5c. The amounts of carbonyl groups show
negligible changes, confirming that phosphate modification did
not affect the active sites during the reaction. However, a
decrease in the desorption of the phenol group was observed
after modification due to its interaction with phosphate. More
importantly, in comparison to unmodified AND, the desorption
of anhydride groups was significantly reduced after modification
and stayed at the same level when the loading exceeded 15 wt
3
3
after the ODH. This might be due to reactions between
phosphate and electrophilic oxygen species during the ODH to
34
form new −COO−PO and C−O−PO groups. This can
3
3
inhibit the negative effect of electrophilic oxygen species on the
reaction, thus providing a higher selectivity. Meanwhile, the
slightly increased desorptions of lactone and phenol groups
were also observed after ODH in comparison to those before
the reaction, which were formed in situ during the reaction and
considered to have no contribution to the catalytic activity.
To obtain more detailed information about the reaction
pathway, Figure 6a shows the linear fit of selectivity of COx and
%
. This demonstrates that phosphate mainly suppressed the
formation of electrophilic oxygen species during reaction, which
2
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Figure 6. (a) C H and COx selectivity at variable contact times (W/F) for AND and 15P-AND catalysts. Reaction conditions: 450 °C, catalyst
3
6
weight 50 mg, 3% C H , and 3% O , He balance, total flow rate ranging from 10 to 50 mL/min. (b) Reaction network in the ODH of propane.
3
8
2
propene at variable contact times (W/F) performed on the two
under the reaction conditions. Phosphate preferentially
interacts with the phenol groups initially present on the AND
surface by dehydration. As the phosphorus content increases, it
gradually occupies the defects to suppress the in situ formation
of electrophilic oxygen species, thus resulting in a higher
selectivity toward propene production, during which the active
sites (ketonic carbonyl groups) for dehydrogenation were not
affected. The reaction pathway analysis indicates that phosphate
selected catalysts. It can be found that C H6 selectivity
3
decreases while COx selectivity increases with contact time.
By examination of the zero W/F intercept of the selectivity, the
36
primary products can be confirmed. The selectivities of
propene and COx at zero contact time are much larger than
zero (Figure 6a), indicating that both propene and COx are
primary products from propane. In addition, the selectivity of
propene remarkably decreases and selectivity of COx increases
almost linearly with increasing contact time. This suggests that
propene is the unstable primary product and COx species are
the primary and secondary stable products. On the basis of the
product analysis, a reaction network of ODH is proposed as
both parallel and consecutive reaction steps (Figure 6b), which
modification mainly suppresses CO formation from propane
2
and CO formation from the direct combustion and deep
oxidation reaction. The results give a fundamental under-
standing of phosphate-modified nanocarbon for the ODH
reaction. This work also provides a new direction to improve
the selectivity by controllable synthesis on the termination of
nanocarbon defect sites.
37
is in good agreement with the previous reported network.
More specifically, it can be found that the selectivity of CO is a
2
weak function of contact time, indicating that CO is mostly a
primary product from propane. Phosphate modification of 15
METHODS
2
■
Preparation of Phosphate-Modified ANDs. The ND
used in this study (high-purity grade) was supplied by Beijing
Grish Hitech Co. China; it was purified from the black powder
produced by explosive detonation using a nitric acid/sulfuric
acid/fuming sulfuric acid mixture. The diamond powder thus
obtained has a light brown color with particle sizes of ca. 3−10
nm and with a phase purity of powder >98%. In order to
remove the unstable disordered carbon on the surface, ND was
then thermally annealed in a furnace at 1000 °C for 4 h under a
helium flow (defined as AND). The modified catalysts were
prepared by the incipient-wetness impregnation method. AND
was soaked in a controlled amount of (NH ) HPO aqueous
wt % did not change the slope of the CO selectivity curve but
2
reduced more than 50% of the selectivity at zero contact time.
This suggests that phosphate significantly suppressed CO2
formation from the direct combustion of propane (k2).
However, CO is a primary product and a stable secondary
product, since the selectivity concurrently increases with an
increase in the contact time. Adding phosphate changes both
the intersection and slope of the CO curve, which indicates that
both k and k pathways for CO formation were efficiently
2
3
inhibited by phosphate selective blockage.
4
2
4
CONCLUSION
■
solution and stirred ultrasonically to a near-dryness state. The
impregnated samples were then dried in air at 120 °C
overnight.
The annealed nanodiamond was controllably modified by
phosphate and used as a catalyst for the ODH reaction. The
significantly enhanced oxidative stability by phosphate mod-
ification allows AND to act as an efficient metal-free catalyst for
the ODH reaction at elevated temperature with a high
selectivity. Furthermore, we gave a clear picture about the
promotion mechanism and interaction between phosphate and
AND for the first time. On the basis of combined character-
izations, it is demonstrated that this promotion effect originated
from the formation of a covalent C−O−P bond with the AND
surface, which was highly stable during the reaction. The
location of phosphate on the carbon matrix is quite selective
Characterizations. HRTEM was performed using a FEI
Cs-corrected Titan 80-300 microscope and a FEI Tecnai G2
F20 microscope. Thermogravimetric (TG) analysis was
performed on a NETZSCH STA 449 F3 instrument under a
−1
flow of argon or air (50 mL min ) with a heating rate of 10 °C
−1
min . IR studies were conducted with a Thermo Nicolet iZ10
FTIR system using a diffuse reflectance infrared Fourier
transform (DRIFT) cell that has been extensively modified to
allow in situ treatments up to 500 °C under flowing gases. The
spectra were recorded in the 650−4000 cm− range with 128
1
2
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−
1
scans at a resolution of 4 cm . Temperature-programmed
desorption (TPD) experiments were performed under a He
atmosphere. The temperature program involved heating at 10
REFERENCES
■
(
5
1) Su, D. S.; Perathoner, S.; Centi, G. Chem. Rev. 2013, 113, 5782−
816.
2) Vershinin, N. N.; Efimov, O. N.; Bakaev, V. A.; Aleksenskii, A. E.;
°
C/min from ambient temperature to 120 °C, at which it was
kept for 40 min, and then the temperature was raised to 1000
C at 5 °C/min. The product gases were analyzed by online
MS (Pfeiffer-Balzer Omnistar); the peaks at m/z 28 and 44
(
Baidakova, M. V.; Sitnikova, A. A.; Vul’, A. Y. Fullerenes, Nanotubes,
Carbon Nanostruct. 2011, 19, 63−68.
°
(3) Zhang, J.; Wang, R.; Su, D. S. Zhongguo Kexue: Huaxue 2012, 42,
406−414.
were monitored for the detection of CO and CO with a time
2
(
4) Aleksenskii, A. E.; Baidakova, M. V.; Vul’, A. Y.; Siklitskii, V. I.
resolution of 0.1 s. The integration of the peak area of evolved
gases was quantified on the basis of the calibration curve built
by using calcium oxalate (CaC O ·H O) as a standard sample.
Phys. Solid State 1999, 41, 668−671.
5) Ozawa, M.; Inaguma, M.; Takahashi, M.; Kataoka, F.; Kruger, A.;
(
2
4
2
Osawa, E. Adv. Mater. 2007, 19, 1201−1206.
Raman spectroscopy was performed on a LabRam HR 800
3
1
(6) Zhang, J.; Su, D. S.; Blume, R.; Schlogl, R.; Wang, R.; Yang, X. G.;
Gajovic, A. Angew. Chem., Int. Ed. 2010, 49, 8640−8644.
instrument using a 325 nm laser. Solid-state P magic angle
spinning (MAS) NMR characterization was performed on a
Varian Infinity-plus 400 spectrometer using a 5 mm probe with
a recycle delay of 5 s (ensuring a complete relaxation of all the
(
7) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schlogl, R.; Su, D. S.
Science 2008, 322, 73−77.
8) Su, D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.;
(
3
1
P signals).
Paraknowitsch, J.; Schlogl, R. ChemSusChem 2010, 3, 169−180.
(9) Zhao, T. J.; Sun, W. Z.; Gu, X. Y.; Ronning, M.; Chen, D.; Dai, Y.
C.; Yuan, W. K.; Holmen, A. Appl. Catal., A 2007, 323, 135−146.
10) Liu, X.; Su, D. S.; Schlogl, R. Carbon 2008, 46, 547−549.
11) Schwartz, V.; Fu, W. J.; Tsai, Y. T.; Meyer, H. M.; Rondinone, A.
J.; Chen, J. H.; Wu, Z. L.; Overbury, S. H.; Liang, C. D. ChemSusChem
013, 6, 840−846.
12) Liu, X.; Frank, B.; Zhang, W.; Cotter, T. P.; Schlogl, R.; Su, D. S.
Angew. Chem., Int. Ed. 2011, 50, 3318−3322.
13) Pereira, M. F. R.; Orfao, J. J. M.; Figueiredo, J. L. Appl. Catal., A
999, 184, 153−160.
14) Macia-Agullo, J. A.; Cazorla-Amoros, D.; Linares-Solano, A.;
Catalytic Tests. Oxidative dehydrogenation of propane
ODH) was carried out in a quartz fixed-bed reactor. Carbon
(
(
(
catalysts were fixed between two layers of quartz wool. The gas
flows were monitored by a mass controller system calibrated for
each gas. The reaction temperature was set at 450 °C and
controlled by a thermocouple located inside the reactor bed.
The reaction products were analyzed by an Agilent 7890A gas
chromatograph equipped with a flame ionization detector
2
(
(
1
(
(
(
FID) for hydrocarbon and a thermal conductivity detector
TCD) for inorganic components. Experiments with blanks
and SBA-15 modified with phosphate showed that the reaction
rates were negligible without carbon catalyst. In all tests, carbon
mass balances were within 100 ± 0.5%. The reaction network
analysis was performed under differential reaction conditions,
and the conversion level was less than 10%, being free of heat
and mass transfer limitation.
Wild, U.; Su, D. S.; Schlogl, R. Catal. Today 2005, 102, 248−253.
(15) Pelech, I.; Soares, O. S. G. P.; Pereira, M. F. R.; Figueiredo, J. L.
Catal. Today 2014, DOI: 10.1016/j.cattod.2014.10.007.
(16) Serp, P.; Figueiredo, J. L. Carbon Materials for Catalysis; Wiley:
Hoboken, NJ, 2009.
(
17) Frank, B.; Zhang, J.; Blume, R.; Schlogl, R.; Su, D. S. Angew.
Chem., Int. Ed. 2009, 48, 6913−6917.
18) Frank, B.; Morassutto, M.; Schomac
ChemCatChem 2010, 2, 644−648.
19) Sui, Z. J.; Zhou, J. H.; Dai, Y. C.; Yuan, W. K. Catal. Today
005, 106, 90−94.
20) Liang, Y. J.; Meinhardt, T.; Jarre, G.; Ozawa, M.; Vrdoljak, P.;
(
̈ ̈
ker, R.; Schlogl, R.; Su, D. S.
ASSOCIATED CONTENT
■
S
Supporting Information
(
2
(
*
The following file is available free of charge on the ACS
Publications website at DOI: 10.1021/acscatal.5b00042.
Scholl, A.; Reinert, F.; Krueger, A. J. Colloid Interface Sci. 2011, 354,
23−30.
EDX mapping, TEM, Raman spectra, and TPD (PDF)
(21) Bourbigot, S.; Lebras, M.; Delobel, R.; Breant, P.; Tremillon, J.
M. Carbon 1995, 33, 283−294.
22) Schwartz, V.; Xie, H.; Meyer, H. M.; Overbury, S. H.; Liang, C.
D. Carbon 2011, 49, 659−668.
23) Tellez, C.; Menendez, M.; Santamaria, J. J. Catal. 1999, 183,
10−221.
24) Cavani, F.; Ballarini, N.; Cericola, A. Catal. Today 2007, 127,
AUTHOR INFORMATION
(
■
*
cn.
Corresponding Author
D.S.S.: tel and fax, (+86)24-23971577; e-mail, dssu@imr.ac.
(
2
(
Notes
113−131.
(25) Mckee, D. W. Carbon 1972, 10, 491−492.
(26) Imamura, R.; Matsui, K.; Takeda, S.; Ozaki, J.; Oya, A. Carbon
The authors declare no competing financial interest.
1
999, 37, 261−267.
ACKNOWLEDGMENTS
■
(27) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J.
The authors sincerely acknowledge the assistances by Xiaoli
Pan for TEM characterizations and Xianchun Liu from the
Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, for NMR tests. The authors also thank Prof.
Figueiredo from the University of Porto (Portugal), Dr. Rui
Wang from the National Institute of Clean-and-Low-Carbon
Energy, and Dr. Wei Qi and Guodong Wen for fruitful
discussions. This work was supported by the Foundation from
MOST (Grant Nos. 2011CBA00504), NSFC of China (Grant
Nos. 21133010, 21103203, 51221264, 21261160487,
M. Carbon 1999, 37, 1379−1389.
(28) Oh, S. G.; Rodriguez, N. M. J. Mater. Res. 1993, 8, 2879−2888.
(
(
(
29) Sherman, A.; Eyring, H. J. Am. Chem. Soc. 1932, 54, 2661−2675.
30) Boehm, H. P. Carbon 2002, 40, 145−149.
31) Zhou, J. H.; Sui, Z. J.; Zhu, J.; Li, P.; De, C.; Dai, Y. C.; Yuan, W.
K. Carbon 2007, 45, 785−796.
32) Li, N.; Ma, X. L.; Zha, Q. F.; Kim, K.; Chen, Y. S.; Song, C. S.
Carbon 2011, 49, 5002−5013.
33) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J.
M. Ind. Eng. Chem. Res. 2007, 46, 4110−4115.
(
(
(
34) Wu, X. X.; Radovic, L. R. Carbon 2006, 44, 141−151.
2
1203215), and “Strategic Priority Research Program” of the
(35) Bedia, J.; Rosas, J. M.; Marquez, J.; Rodriguez-Mirasol, J.;
Chinese Academy of Sciences (Grant No. XDA09030103).
Cordero, T. Carbon 2009, 47, 286−294.
2
443
DOI: 10.1021/acscatal.5b00042
ACS Catal. 2015, 5, 2436−2444

ACS Catalysis
Research Article
(
36) Michaels, J. N.; Stern, D. L.; Grasselli, R. K. Catal. Lett. 1996,
2, 135−137.
37) Dinse, A.; Khennache, S.; Frank, B.; Hess, C.; Herbert, R.;
Wrabetz, S.; Schlogl, R.; Schomacker, R. J. Mol. Catal. A: Chem. 2009,
07, 43−50.
4
(
3
2
444
DOI: 10.1021/acscatal.5b00042
ACS Catal. 2015, 5, 2436−2444