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Short Communication
Pt/CeO2 catalyst synthesized by combustion
method for dehydrogenation of perhydrodibenzyltoluene as liquid organic hydrogen carrier:
Effect of pore size and metal dispersion
Sanghun Lee, Jaemyung Lee, Taehong Kim, Gwangwoo Han, Jaeseok Lee,
Kangyong Lee, Joongmyeon Bae*
Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro,
Yuseong-gu, Daejeon, 305-701, Republic of Korea
highlights
Pt/Al2O3 and Pt/CeO2 catalysts were synthesized by the glycine nitrate process (GNP).
The dehydrogenation reaction of H18-DBT with Pt/Al2O3 and Pt/CeO2 was evaluated.
Pt/Al2O3 showed slow reaction of 3.5%/2.5h due to poor mass transfer by small pores.
Pt/CeO2 showed faster reaction of 80.5%/2.5h than commercial catalyst of 17.8%/2.5h.
article info
abstract
Article history:
Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores
Received 25 September 2020
hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material
Received in revised form
due to its high storage density, low ignitability, and low cost. In this study, Pt/Al2O3 and Pt/CeO2
29 October 2020
catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine
Accepted 5 November 2020
nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-
Available online 2 December 2020
dibenzyltoluene (H18-DBT). Pt/CeO2 exhibits much faster dehydrogenation than Pt/Al2O3,
80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogena-
Keywords:
tion rates, microstructural characterization by N2 physisorption, CO chemisorption and
Liquid organic hydrogen carrier
transmission electron microscopy analysis are conducted, and the catalytic activities are
Dibenzyltoluene
evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation
Dehydrogenation
can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due
Catalyst
to the small pores in Pt/Al2O3, which is a rarely addressed issue for other LOHC materials. This is
Glycine nitrate process
because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO2 synthesized by the GNP is also compared with a
commercial Pt/Al2O3 catalyst. The commercial Pt/Al2O3 catalyst shows a dehydrogenation of
17.8%/2.5 h, which is much slower than that of Pt/CeO2 synthesized by the GNP, at 80.5%/2.5 h.
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author.
E-mail address: jmbae@kaist.ac.kr (J. Bae).
https://doi.org/10.1016/j.ijhydene.2020.11.038
0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction
Hydrogen energy technologies have attracted attention for
various applications. As the portion of renewable energies in
the electricity grids has increased, stationary hydrogen energy
storage systems consisting of an electrolyzer, hydrogen storage, and fuel cells have been developed [1e4]. For the import
and export of large-scale energy, tanker ships for transporting
hydrogen have been developed [5e7]. For these purposes,
various large-scale hydrogen storage technologies such as
compressed hydrogen, liquefied hydrogen, and chemical
hydrogen storage have been proposed [8e11]. Liquid organic
hydrogen carriers (LOHC) represent chemical hydrogen storage technology. LOHCs have the advantages of a high
hydrogen storage density, easy liquid handling, storage at
atmospheric pressure, and possible utilization of conventional oil infrastructures [12e14].
Various kinds of LOHC materials have been proposed and
investigated [15e17]. Dibenzyltoluene (DBT) is a prominent
LOHC material because of the advantages of its liquid phase at
room temperature, low vapor pressure, lack of carcinogenicity, and low ignitability [18] compared to other LOHC materials
such as N-ethylcarbazole (NEC), methylcyclohexane (MCH),
and formic acid [19e21]. Thus, various studies have been
conducted on DBT. To investigate the basic properties of DBT
as an LOHC material, the material property and characterization techniques were used. The thermophysical and thermochemical properties, density, viscosity, refractive index,
and solubility were characterized using ultravioletevisible
(UV-VIS) spectrophotometry, and Raman spectroscopy
[22e26]. Characterization techniques such as nuclear magnetic resonance (NMR) and gas chromatography coupled with
mass spectrometry (GC-MS) were used to investigate DBT
[27,28]. Then, the feasibility of the use of DBT as an LOHC was
evaluated. The hydrogen purities under various reaction
conditions were evaluated [29]. The catalytic reactions were
analyzed based on density functional theory (DFT) [30]. The
feasibility of the connected operation of an LOHC dehydrogenation reactor and polymer electrolyte membrane fuel cell
was evaluated [31]. A hot pressure swing reactor concept for
hydrogenation and dehydrogenation in a single reactor was
proposed [32,33]. Hydrogen purification by selective hydrogenation of LOHCs was suggested [34]. In addition, various LOHC
system configurations were proposed [35,36].
When hydrogen is stored in DBT, perhydrodibenzyltoluene (H18-DBT) is formed, and it stores nine molecules of hydrogen for a molecule of DBT. To release the
hydrogen stored in H18-DBT, noble metals such as Ru and Pt
were evaluated [18]. However, precious metal catalysts are
expensive and increase the LOHC system cost. Therefore, the
development of highly active dehydrogenation catalysts is
necessary to achieve economic competitiveness of LOHCs. For
the development of highly active catalysts to improve dehydrogenation rates, a good dispersion of active metals, large
surface areas, and highly porous structures are necessary [37].
The impregnation method, which is a widely used catalyst
synthesis method for LOHC dehydrogenation, is a simple
synthesis process [38e44]. However, the impregnation
method has the limitation of poor catalyst metal dispersion
5521
because the catalyst metals easily aggregate on the surface of
the support. In addition, only commercial catalyst material
screening with noble metals or catalysts synthesized by the
conventional impregnation method were reported for the
dehydrogenation of H18-DBT, and the catalytic characteristics
of the reaction have rarely been reported [18,45,46].
In this study, a dehydrogenation catalyst was synthesized
through a combustion-based nanocatalyst synthesis method
called the glycine nitrate process (GNP) method. The GNP
enables the synthesis of nanocatalysts with good dispersion
and porous structures by rapid gas formation during combustion. For the GNP method, Al2O3 and CeO2 were investigated as candidates for support materials. Al2O3 is the most
commonly used support material due to its high surface area
and low cost [47]. CeO2 is a commonly used support material
for the GNP method [48e50]. The dehydrogenation with the
synthesized Pt/Al2O3 and Pt/CeO2 catalysts was evaluated.
Then, the material characteristics of the catalysts were
measured using X-ray diffraction (XRD), transmission electron
microscopy (TEM), energy dispersive spectrometry (EDS), N2
physisorption, and CO chemisorption. To investigate the catalytic activities with minimized mass transport effects, the
catalysts were evaluated at high liquid hourly space velocities
(LHSVs) utilizing a fixed-bed reactor. Based on the results, the
effects of the support materials and mass transport of H18DBT were discussed. Finally, the Pt/CeO2 catalysts synthesized by the GNP were compared with commercial Pt/Al2O3
catalysts that were previously reported for the dehydrogenation of H18-DBT.
Experiments
Catalyst synthesis and characterization
To synthesize 5 wt% Pt/Al2O3 and 5 wt% Pt/CeO2 catalysts by
the GNP, precursors of Al(NO3)3$9H2O (99.997%, Sigma Aldrich,
USA), Ce(NO3)3$6H2O (99.99%, Sigma Aldrich, USA) and
Pt(NH3)4(NO3)2 (99.995%, Sigma Aldrich, USA) were prepared.
First, 139.81 g Al(NO3)3$9H2O or 47.93 g Ce(NO3)3$6H2O and
1.98 g Pt(NH3)4(NO3)2 were dissolved in deionized (DI) water
(>15 MU). Glycines of 127.06 g for Pt/Al2O3 and 38.45 g for Pt/
CeO2 were added to the mixture, which is a 1.5 M ratio of
glycine to total nitrate (NO3) as a fuel for the combustion reaction [48,49,51]. The mixture was heated to evaporate the
excess water and maintained until a rapid combustion reaction of the precursors and glycine began. During combustion,
the Pt/Al2O3 and Pt/CeO2 porous nanocatalysts were synthesized. Finally, the powders were calcined in an electric furnace
at a temperature of 600 C for 4 h to calcine the remaining
precursors.
The compositions of the catalysts were examined by
inductively coupled plasma mass spectrometer (ICP-MS
7700S, Agilent, USA). The amount of Pt loading was maintained within 5 ± 0.5 wt%. The crystalline structures of the
catalysts were investigated by powder XRD (SmartLab X-ray
diffractometer, Rigaku, Japan). The microstructures of the
catalysts were observed by TEM (JEM-2100F HR, JEOL Ltd.,
Japan) The BrunauereEmmetteTeller (BET) surface areas
and pore sizes were evaluated by N2 physisorption (3Flex,
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Micromeritics, USA). The catalyst samples were pretreated
at 200 C for 4 h to remove moisture, and N2 physisorption
Degree of dehydrogenationð%Þ ¼
Amount of hydrogen released from LOHCðmolÞ
100ð%Þ
Amount of hydrogen stored in LOHCðmolÞ
was conducted at a temperature of 196 C and pressure of
0e760 mmHg. The metal dispersions and metal surface
areas were examined by CO chemisorption (ASAP 2020,
Micromeritics, USA). The catalyst samples were also pretreated at 200 C for 4 h to remove moisture. Then, the
amount of adsorbed CO was measured at a temperature of
35 C and a pressure of 50e600 mmHg at 50 mmHg intervals.
The metal surface area, metal dispersion, and mean surface
metal particle size [52] were calculated by the following
equations.
Am ¼
D¼
vm ,L,n,am
22414,m,wt
vm ,n,M
100
22414,m,wt
dpPt ¼
113:2
D
where vm is the volume of an adsorbed monolayer; L is Avogadro number of 6.02 1023; n is the chemisorption stoichiometry of 1 for CO [52]; am the is atomic cross-sectional area of
platinum of 0.0841 nm2 [52]; m is the sample mass; wt is the
metal loading of 0.05; M is the atomic mass of the metal of
195.084 g/mol; dpPt is the mean surface metal particle size.
Dehydrogenation test at a batch-type rector
For the dehydrogenation experiments, H18-DBT was prepared
by hydrogenating DBT (Marlotherm SH, Sasol, South Africa).
The hydrogenation of H0-DBT was conducted at a temperature of 150 C and a pressure of 50 bar with Ru/Al2O3 catalysts
(Sigma Aldrich, USA) until the hydrogenation loading reached
100% of the theoretical hydrogen storage in a 600-ml batchtype reactor (Mini reactor 4568, Parr instrument, USA).
For the dehydrogenation tests, a dehydrogenation test
bench was established, as shown in Fig. 1(aeb). For dehydrogenation, a 250 ml reactor composed of a three-neck flask was
installed. A nitrogen gas supply line was connected for purging. The dehydrogenation tests were conducted with 30 ml of
H18-DBT. Commercial 5 wt% Pt/Al2O3 catalyst (Sigma Aldrich,
BET ¼ 96 m2/g), 5 wt% Pt/Al2O3, and 5 wt% Pt/CeO2 synthesized
by the GNP were evaluated with an H18-DBT-to-Pt molar ratio
of 667:1 (0.15 mol%, 0.59 g of catalyst for 0.1 mol of H18-DBT)
under the conditions of 300 C and 1 bar with a stirrer at
250 rpm.
From the dehydrogenation of H18-DBT, hydrogen and intermediates of DBT are formed as products as follows:
C21 H38 ðH18DBTÞ /C21 H32 þ3H2 /C21 H26 þ6H2 /C21 H20 ðDBTÞ
þ 9H2
To evaluate the dehydrogenation rates, the degree of
dehydrogenation was defined as follows:
To evaluate the degree of dehydrogenation, a mass flow meter
(MFM, Bronkhorst, Netherlands) was utilized to measure the gas
flow rates at the outlet of the reactor. In addition, H18-DBT samples were obtained every 30 min. The densities of the samples
were measured, and the degree of the reaction was calculated
based on the densities [24]. The degree of the reaction was
observed during dehydrogenation by NMR analysis (Agilent
400 MHz 54 mm NMR DD2, Agilent Technologies, USA) on the
collected H18-DBT samples, and no side reactions were observed.
Dehydrogenation catalytic activity tests in a fixed-bed type
rector
To evaluate the catalytic activity at various LHSVs, a fixed-bed
type reactor was established, as shown in Fig. 1(ced). The
fixed-bed type reactor system was composed of an LOHC
pump (UI-22-110S, Flom, Japan), LOHC dehydrogenation
reactor, and purification components: a condenser with a
chiller (RW3-2025, Jeiotech, Korea) and an adsorption vessel
with activated carbon (Norit RB 4, Cabot Corporation, USA). An
MFM (Bronkhorst, Netherlands) was used to measure the
hydrogen release rate by measuring the outlet flow rates from
the dehydrogenation reactor.
The dehydrogenation activity of the Pt/Al2O3 and Pt/CeO2
catalysts synthesized by the GNP was evaluated by the established fixed-bed type reactor. Three milliliters of the catalysts
were used for the test. H18-DBT was supplied to the reactor with
LHSVs of 1.0, 5.0, and 10.0. The LHSV was defined as follows:
LHSVð=hÞ ¼
Flow rate of LOHCðml=hÞ
Volume of catalyst bed ðmlÞ
The dehydrogenation temperature was controlled to
300 ± 2 C over the whole catalyst region. To evaluate the catalysts, the conversion and reaction rates were defined as follows:
Conversionð%Þ ¼
Measured hydrogen release rate at the LHSV
Theoretical hydrogen release rate at the LHSV
100ð%Þ
Hydrogen release rateðmol=sÞ
Reaction rate mol gcatal $ s ¼
Catalyst weight gcatal
Results and discussion
Catalyst synthesis and batch-type dehydrogenation test of
H18-DBT
Fig. 2(aeb) shows the XRD results of the synthesized catalysts
of Pt/Al2O3 and Pt/CeO2. Pt/Al2O3 showed amorphous Al2O3
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Fig. 1 e A batch-type reactor for the catalytic dehydrogenation test: (a) schematic diagram; (b) test bench. A fixed-bed reactor
for catalytic activity evaluation: (c) schematic diagram; (d) test bench.
and crystalline Pt peaks. It is known that the full crystallization of Al2O3 is possible after calcination at a high temperature
of 1,100 C [53]. However, catalytically active Pt is crystallized
at a much lower temperature of 600 C, and the surface area of
the Pt would be significantly reduced by sintering at 1,100 C.
Thus, in this study, Pt/Al2O3 was calcined at 600 C. Pt/CeO2
showed only CeO2 peaks, and Pt peaks were not observed. This
could be attributed to the typical XRD spectrum of nanoparticles. It is known that nanoparticles show broader peak
widths because nanoparticles contain a limited number of
atomic planes [54]. In addition, Pt atoms could be doped into
the CeO2 lattice to form a single-phase solid solution [55],
which is rare for Al2O3 because of its amorphous phase and
strong structural stability [56].
The dehydrogenation reaction was conducted in the batchtype reactor with the Pt/Al2O3 and Pt/CeO2 catalysts synthesized by the GNP, and the results are shown in Fig. 2(c). Pt/
Al2O3 showed a slow dehydrogenation rate of 3.5%/2.5 h, while
Pt/CeO2 showed a much faster dehydrogenation rate of 80.5%/
2.5 h. There were two possible causes for the slow
dehydrogenation rate of Pt/Al2O3 synthesized by the GNP, the
low catalytic activity of the catalyst itself or mass transport.
Catalytic activity evaluation by the fixed-bed type reactor
To compare the catalytic activities of Pt/Al2O3 and Pt/CeO2, the
dehydrogenation of H18-DBT was conducted in a fixed-bed
reactor with various LHSVs. Fig. 3(a) shows the conversion of
H18-DBT to DBT with the Pt/Al2O3 and Pt/CeO2 catalysts
depending on the LHSV. At a low LHSV, the conversion of H18DBT to DBT was high, and it decreased as the LHSV increased.
This reduction in conversion occurred because the flow rate of
H18-DBT exceeded the allowable dehydrogenation rate of each
catalyst as the LHSV increased. Fig. 3(b) shows the reaction rates
of Pt/Al2O3 and Pt/CeO2 depending on the LHSV. At a low LHSV,
below 1.0/h, both catalysts showed a small reaction rate difference, while at a high LHSV, 10.0/h, Pt/CeO2 exhibited a higher
reaction rate than Pt/Al2O3. At high LHSVs, where a sufficient
amount of reactants are supplied to the catalysts and hence the
mass transport issues are limited, the reaction rates can be
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Fig. 2 e XRD peaks of (a) Pt/Al2O3 and (b) Pt/CeO2 synthesized by the GNP, and (c) dehydrogenation of H18-DBT at 300 C with
Pt/CeO2 and Pt/Al2O3 synthesized by the GNP.
considered catalytic activities that reflect the adsorption, surface reaction, and desorption of the reactants and products
[57,58]. Thus, catalyst kinetics studies are conducted in high
space velocity regions [59].
The difference in reaction rate at a high LHSV of 10/h implies
a difference between the catalytic activities of the catalysts. Pt/
CeO2 shows a reaction rate of 0.00042 mol/gcatal$s, which was
more than double the reaction rate of Pt/Al2O3 at 0.00018 mol/
gcatal$s. The catalytic activity difference can be attributed to the
interaction of the Pt metal catalysts and support materials of
Al2O3 and CeO2 [60]. Al2O3 possesses strong Lewis acid sites,
which increase the reactant adsorption capacity and decrease
the desorption rates [61]. Thus, the acidity of the support materials can cause trade-off issues between adsorption and
desorption. As a result, Pt/Al2O3 could exhibit lower activity
than Pt/CeO2 due to the slow desorption rate caused by the
acidic support material.
However, the catalytic activity difference could not explain
the more than 20-fold difference in the dehydrogenation rates
of Pt/Al2O3 and Pt/CeO2, at 3.5%/2.5 h and 80.5%/2.5 h,
respectively. There could be mass transport issues associated
with the dehydrogenation of H18-DBT, as shown in Fig. 3(c). As
shown in Table 1, the average pore sizes of the Pt/Al2O3 and Pt/
CeO2 catalysts were approximately 4.5 nm and 12.6 nm,
respectively, which were measured by N2 physisorption. The
exact kinetic diameters of H18-DBT and DBT are unknown;
well-established experimental setup and in-depth analysis
are required to determine the kinetic diameters H18-DBT and
DBT. However, they are presumably longer than those of
cyclohexane and benzene (0.60 nm and 0.58 nm, respectively),
as shown in Fig. 3(d) [62e65]. The average pore sizes of both
catalysts were larger than 0.58 nm; however, the statistical
probability of reactant diffusion into the pores was significantly higher for Pt/CeO2 than Pt/Al2O3. Accordingly, for Pt/
Al2O3 synthesized by the GNP, the mass transport of H18-DBT
to the metal catalysts inside the support materials could be
more difficult than that for Pt/CeO2 synthesized by the GNP
because of the large differences in pore sizes. As a result, the
large differences in dehydrogenation rates of Pt/CeO2 and Pt/
Al2O3 can be obtained. This phenomenon could be more critical for H18-DBT than other LOHC materials because the
dehydrogenation of H18-DBT is conducted in the liquid phase,
while for other materials such as N-ethylcarbazole, toluene,
and naphthalene, it is conducted in the gas phase [66]. However, the pore diffusion issue for the dehydrogenation of H18DBT has rarely been investigated, and only the material
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Fig. 3 e (a) Conversion and (b) reaction rate of the dehydrogenation of H18-DBT at 300 C with Pt/Al2O3 and Pt/CeO2 catalysts
synthesized by the GNP at various LHSVs, (c) a schematic diagram of pore diffusion and (d) the kinetic diameters of H18-DBT
and DBT.
Table 1 e Properties of the Pt/Al2O3 and Pt/CeO2
synthesized by the GNP.
Average pore size (nm)
BET surface area (m2/g)
Pt/Al2O3 (GNP)
Pt/CeO2 (GNP)
4.2
32
12.6
52
properties of the diffusion coefficients have been reported
[67]. Therefore, the effects of the pore diffusion of H18-DBT on
the dehydrogenation reaction must be further studied. The
catalyst kinetics, detailed reaction mechanism, and activation
energy must also be explained to investigate this phenomenon. In addition, the effects of support materials of Pt/Al2O3
and Pt/CeO2 when both catalysts have similar textural properties can be an interesting research topic.
Comparison with commercial catalyst for H18-DBT
dehydrogenation
In this section, the Pt/CeO2 catalyst synthesized by the GNP
was compared with a commercial catalyst that has been
reported in another study [18]. Fig. 4(a) shows the dehydrogenation test results of each catalyst. Pt/CeO2 synthesized by
the GNP yielded 80.5%/2.5 h, while commercial Pt/Al2O3 yielded 17.8%/2.5 h. Through the synthesis of nanocatalysts by
the GNP, the dehydrogenation was faster than that of the
commercial catalyst.
To investigate the reasons why the dehydrogenation rate of
Pt/CeO2 synthesized by the GNP was faster than that of commercial Pt/Al2O3, microstructure analysis was conducted.
Fig. 4(beg) shows the TEM images of commercial Pt/Al2O3 and
Pt/CeO2 synthesized by GNP. Commercial Pt/Al2O3 show
obvious agglomerated particles on the support, while Pt/CeO2
did not. The well-dispersed Pt particles of Pt/CeO2 synthesized
by the GNP can be attributed to the single-step nanocatalyst
synthesis of Pt catalysts and CeO2 support materials by combustion [68], while commercial catalysts are probably synthesized by a conventional synthesis method such as impregnating
Pt catalysts into the presynthesized support materials or precipitation. The mean Pt particle size of commercial catalysts is
3.43 ± 1.11 nm. To confirm the existence of the Pt particles, EDS
mapping was conducted. The agglomerated particles of commercial Pt/Al2O3 were Pt particles. In contrast, EDS mapping of
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Fig. 4 e (a) Dehydrogenation of H18-DBT at 300 C with commercial Pt/Al2O3 and Pt/CeO2 synthesized by the GNP evaluated
in the batch-type reaction, TEM-EDS images of the (bed) commercial Pt/Al2O3 and (eeg) Pt/CeO2 synthesized by the GNP.
Pt/CeO2 showed a well-dispersed Pt distribution over the CeO2
supports.
To characterize the physical properties of each catalyst, N2
physisorption was conducted to measure the BET surface areas
and pore sizes, as listed in Table 2. The Pt/CeO2 synthesized by
the GNP showed a larger average pore size than the commercial
Pt/Al2O3, 12.6 nm versus 9.8 nm, which could enable better pore
diffusion. In contrast, Pt/CeO2 had a smaller BET surface area
than the commercial Pt/Al2O3, 51.8 m2/g versus 97.0 m2/g. For a
fast catalytic reaction, however, the surface area of the catalytically active metals is more significant than the BET surface
area. The metal surface areas and metal dispersions were
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Table 2 e Properties of the commercial Pt/Al2O3 and Pt/
CeO2 synthesized by the GNP.
Pt/Al2O3
(Commercial)
Pt/CeO2
(GNP)
9.8
97.0
54.5
22.1
5.12
12.6
51.8
118.6
48.0
2.36
Average pore size (nm)
BET surface area (m2/g)
Metal surface area (m2/g metal)
Metal dispersion (%)
Average surface metal particle
size (nm)
investigated by CO chemisorption, as listed in Table 2. Pt/CeO2
had a larger metal surface area and better dispersion than
commercial Pt/Al2O3, 118.6 m2/gmetal and 48.0% versus 54.5 m2/
gmetal and 22.1%, respectively. The mean surface metal particle
sizes of Pt/CeO2 and Pt/Al2O3 obtained by CO chemisorption
analysis were 2.36 nm and 5.12 nm. The difference in particle
size of the commercial catalysts between CO chemisorption and
TEM images can be attributed to the assumption of the chemisorption stoichiometry of 1 for CO with Pt and a limitation of
image processing. In conclusion, the Pt/CeO2 synthesized by the
GNP had large pore sizes and metal surface areas and good
dispersion than those of commercial Pt/Al2O3, enabling faster
dehydrogenation of H18-DBT.
Conclusion and future work
To achieve a high dehydrogenation rate of H18-DBT, nanocatalysts of Pt/Al2O3 and Pt/CeO2 were synthesized by the GNP
method. The crystalline structures of the catalysts were
investigated, and the dehydrogenation rates of H18-DBT were
evaluated. Pt/CeO2 showed much faster dehydrogenation
than Pt/Al2O3, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the
causes of the significant difference in the dehydrogenation
rates, the catalytic activities were evaluated by changing the
LHSV. At a high LHSV, 10/h, where the effects of the mass
transport issues were minimized, Pt/CeO2 showed a faster
catalytic reaction rate than Pt/Al2O3, 0.00042 mol/gcatal$s
versus 0.00018 mol/gcatal$s. However, the difference in the
reaction rates at the high LHSV of 10/h was insufficient to
explain the more than 20-fold difference between the dehydrogenation values of 80.5%/2.5 h for Pt/CeO2 and 3.5%/2.5 h
for Pt/Al2O3. Considering the pore sizes of the catalysts and
the kinetic diameter of DBT, it was found that mass transport
issues can be a significant factor affecting the dehydrogenation rate of H18-DBT. This could be caused by the large kinetic
diameter of H18-DBT, the low diffusion coefficient due to the
use of the liquid phase, and the small pore sizes of the catalyst
supports. This mass transport issue should be investigated in
future studies by evaluating the diffusion coefficients, reaction mechanisms, kinetics, and activation energies to develop
highly active dehydrogenation catalysts of H18-DBT.
Compared to the commercial catalyst, the dehydrogenation rate of H18-DBT was improved from the 17.8%/2.5 h using
the commercial Pt/Al2O3 catalysts to 80.5%/2.5 h using the Pt/
CeO2 catalyst synthesized by the GNP. It was found that the
5527
pore size was increased from 9.8 nm to 12.6 nm, that the metal
dispersion was improved from 22% to 48% and that the metal
surface area was enlarged from 54.5 m2/g metal to 118.6 m2/g
metal for the Pt/CeO2 synthesized by the GNP compared with
those of the commercial catalyst.
In future studies, the effects of the pore diffusion of H18DBT, catalyst kinetics, reaction mechanism, and activation
energy must be examined to investigate the detailed mass
transport issues. In addition, the effects of support materials
of Pt/Al2O3 and Pt/CeO2 when both catalysts have similar
textural properties and catalyst durability are interesting
research topics.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) and the Ministry
of Trade, Industry & Energy (MOTIE) of the Republic of Korea
(No. 2019281010007A & 20194030202360).
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