Designed Pt promoted 3D mesoporous Co3O4 catalyst in CO2 hydrogenation

Article abstract of DOI:10.1166/jnn.2019.15779

Controlled size Pt nanoparticles were anchored onto the surface of 3D mesoporous cobalt-oxide support and was tested in CO₂ hydrogenation reactions compared to commercial cobalt-oxide supported Pt nanoparticles prepared by the wet impregnation

Full text of DOI:10.1166/jnn.2019.15779

Article
Journal of
Nanoscience and Nanotechnology
Vol. 19, 436–441, 2019
Copyright © 2019 American Scientific Publishers
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Designed Pt Promoted 3D Mesoporous Co3O4
Catalyst in CO2 Hydrogenation
1ꢀ†
2
3
1
András Sápi1ꢀ†, Gyula Halasi1ꢀ∗ꢀ†, András Grósz , János Kiss , Albert Kéri , Gergo Ballai ,
˝
Gábor Galbács3, Ákos Kukovecz1, and Zoltán Kónya1ꢀ2
1Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Hungary
2MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, University of Szeged, H-6720 Szeged, Hungary
3Department of Inorganic and Analytical Chemistry, University of Szeged, H-6720 Szeged, Hungary
Controlled size Pt nanoparticles were anchored onto the surface of 3D mesoporous cobalt-oxide
support and was tested in CO2 hydrogenation reactions compared to commercial cobalt-oxide sup-
ported Pt nanoparticles prepared by the wet impregnation method as well as SBA-15 silica sup-
ported nanoparticles. Designed Pt/mesoporous cobalt-oxide catalysts showed the highest activity
as well as the highest methane selectivity. Such catalyst was active at 573 K, while other catalysts
showed activity >673 K.
Keywords: CO2 Hydrogenation, Mesoporous Co3O4, Controlled Size Pt Nanoparticles.
IP: 91.243.93.135 On: Wed, 19 Dec 2018 01:46:37
Copyright: American Scientific Publishers
1. INTRODUCTION
Delivered bypIrnegseennctea of hydrogen on metal supported oxide is more
favorable.11
The catalytic conversion of carbon dioxide to more valu-
able compounds has again become of great interest since
CO2 is one of the most dangerous greenhouse gases. As an
economical safe and renewable carbon source, CO2 turns
out to be an attractive C1 building block for making other
chemicals, or to reduce and store the energy as hydro-
carbons. The hydrogenation of CO2 is a promising possi-
bility for realization these processes. Number of excellent
reviews have summarized the results and discussed them in
detail.1ꢀ2 The Sabatier reaction (CO2 +4H2 = CH4 +2H2O)
for synthesizing methane from CO2 is also in the main
focus of research and of industry.3–6
It was an important finding that the catalytic perfor-
mance of Pt metals in the hydrogenation of CO2 is strongly
affected by the nature of the supports. TiO2 was found to
be the most effective one, which was attributed to the elec-
tronic interaction between the metals and TiO2.12 Using
both SiO2 and defected TiO2 with oxygen vacancy as sup-
port, Pt nanoparticles (NP) is able to enhance the overall
CO2 conversion via increased CO2 binding. The enhance-
ment is more significant for Pt/TiO2 than Pt/SiO2 and
therefore higher activity is expected for TiO2.13
Recently enhanced CO oxidation rates at interface of
mesoporous oxide and Pt nanoparticles were observed.14
Nanoporous Co3O4, NiO, MnO2, Fe2O3 and CeO2 with
loading Pt nanoparticles were synthesized and utilized in
CO oxidation reaction. Much higher reaction rate was
observed when Co3O4 was the support.15 Using this anal-
ogy, we investigated the CO2 hydrogenation in the pres-
ence of Pt/Co3O4 and Pt/SBA-15 catalysts. In this project,
size controlled Pt nanoparticles with 3.6 nm diameter were
anchored onto the surface of designed 3D mesoporous
cobalt oxide support. Pt/Co3O4 (commercial), as well as
Pt supported on SBA-15 catalysts were also tested. The
catalytic results pointed out the importance of nature of
metal-interface.
The chemical activation of CO2 is a quite popular term
nowadays in catalysis and surface science when discussing
CO2 chemistry, which involves the activation and cleavage
of the strong carbon-oxygen bonds and subsequently an
establishment of new chemistry with higher enthalpy.7ꢀ8
CO2 in the gas-phase is non-reactive. One possible way
to reduce CO2 is to generate CO∗2−, the electron attached
state of CO2, which is controlled by the reduction potential
of the CO−2 /CO2 redox couple, although the probability of
this mechanism under mild conditions is very low because
of the high redox potential.9ꢀ10 The activation of CO2 in
∗Author to whom correspondence should be addressed.
†These three authors contributed equally to this work.
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doi:10.1166/jnn.2019.15779

Sápi et al.
Designed Pt Promoted 3D Mesoporous Co3O4 Catalyst in CO2 Hydrogenation
2. EXPERIMENTAL DETAILS
2.1. Synthesis of 3.6 nm Pt Nanoparticles
H2PtCl6 ×6H2O (Reanal) to yield the desired amount of
Pt content (1%). The suspension was stirred for 3 h at
∼353 K, after that it was dried at 373 K for 2 h under
infrared lamp. The prepared catalyst pellets (∼200 mg)
were loaded into the catalytic reactor and was pretreated
in situ before the catalytic experiments.
20 mg of H2PtCl6 · 6H2O and 110 mg of PVP was dis-
solved in 5 ml ethylene-glycol, separately. The solutions
were mixed in a 25 ml round bottom flask and refluxed
ꢀ
at 160 C for 60 min under Ar purging. The Pt nanoparti-
cles were collected by precipitation with 40 ml of acetone,
followed by several washing cycles based on ethanol dis-
persion and hexane precipitation. The product was finally
redispersed in 10 ml ethanol.
2.6. Hydrogenation of Carbon-Dioxide in a
Continuous Flow Reactor
The catalytic reactions were carried out at 1 atm pres-
sure in a fixed-bed, continuous-flow reactor (8 mm i.d.),
made from quartz. The dead volume of the reactor was
filled with quartz beads. The operating temperature was
controlled by a thermocouple placed inside the oven close
to the reactor wall. For catalytic studies small fragments
(about 1 mm) of slightly compressed pellets were used.
Typically, the reactor filling contained 200 mg of catalyst.
In the reacting gas mixture, the CO2:H2 molar ratio was
1:4. The CO2:H2 mixture was introduced to the reactor
with mass flow controllers (Aalborg), the flow rate was,
in general, 40 mLmin−1. The reacting gas mixture flow
entered and left the reactor through an externally heated
tube to avoid condensation. Analysis of the gases was per-
formed with an Agilent 4890 gas chromatograph using
Equity-1 capillary and Porapak QS packed columns to
allow the complete separation of the reactants and prod-
ucts. The gases were detected simultaneously by thermal
2.2. Synthesis of Mesoporous Cobalt-Oxide
For the preparation of mesoporous cobalt-oxide support
(Co3O4ꢁ, KIT-6 silica was used as template. The synthe-
sis of KIT-6 was described before.16 Co3O4 was prepared
through the hard template method using the as-prepared
KIT-6. In a typical synthesis, 4.65 g Co(NO3ꢁ2 · 6H2O
was dissolved in 8 ml water and mixed with a suspen-
sion of 4 g KIT-6 in 50 ml toluene. Vigorous stirring was
ꢀ
applied to the mixture at 65 C until the total evaporation
of toluene. After the evaporation,ꢀthe precipitated prod-
uct was collected andꢀdried at 60 C overnight, followed
by calcination at 300 C for 6 h. The silica template was
completely removed by several washing steps using 2 M
aqueous NaOH solution. The filtered product was dried at
ꢀ
60 C overnight.
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conductivity (TCD) and flame ionization (FID) detectors.
Before the catalytic experiments either in a continuous-
2.3. Synthesis of SBA-15 Mesoporous Silica Support
Copyright: American Scientific Publishers
SBA-15 mesoporous silica support was synthesized from
tetraethyl orthosilicate (TEOS) by a soft template method.
Here, 8 g of pre-melted Pluronic-123, 60 ml distilledꢀwater
and 240 ml of 2 M HCl solution were mixed at 40 C for
2 hours. After the dissolution of P-123,ꢀ17 g of TEOS was
added dropwise to the mixture at 40 C and continuous
stirring was applied for 20 hours. Stirring was continued
for 36 hours at 60 ꢀC. After the synthesis, the product was
filtered and washedꢀwith distilled water. Then, the product
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flow reactor the as received catalysts were oxidized in O2
atmosphere at 573 K for 30 min to remove the surface con-
taminants as well as the PVP capping agent2ꢀ3 and there-
after were reduced in H2 at 573 K for 60 min to reduce
the Pt on the surface.
2.7. High Resolution Transmission Electron
Microscopy (HR-TEM)
ꢀ
was heated to 100 C with a heating rate of 2 C · min−1
Imaging of the different supports, Pt nanoparticles and
derived catalysts, as well as the size distribution of the
nanoparticles before and after the catalytic reaction, was
performed by an FEI TECNAI G2 20 X-Twin high-
resolution transmission electron microscope (equipped
with electron diffraction) operating at an accelerating volt-
age of 200 kV. The samples were drop-cast onto carbon
film coated copper grids from ethanol suspension.
and aged for 5 hours; then the temperature was elevated to
ꢀ
ꢀ
550 C with a heating rate of 1 C·min−1. After 4 hours
of calcination, the product was ready for further use.
2.4. Synthesis of Mesoporous Cobalt-Oxide and
SBA-15 Supported Pt Catalysts
To fabricate supported catalysts, the ethanol suspension
of Pt nanoparticles and Co3O4 or SBA-15 were mixed in
ethanol and sonicated in an ultrasonic bath (40 kHz, 80 W)
for 3 hrs. The supported nanoparticles were collected by
centrifugation. The products were washed with ethanol
2.8. Powder X-ray Diffraction (XRD)
XRD studies of the silica supports were performed on a
Rigaku MiniFlex II instrument with a Ni-filtered CuKꢂ
source in the range of 2ꢃ = 10–80ꢀ.
ꢀ
three times before they were dried at 80 C overnight.
2.5. Synthesis of Bulk Co3O4 Supported Pt
2.9. Gas Sorption Measurements
Catalyst by Wet Impregnation Method
Cobalt-oxide support (product of Merck) was impreg-
nated at room temperature in aqueous solution of
The specific surface area (BET method), the pore size
distribution and the total pore volume were determined
by the BJH method using a Quantachrome NOVA 2200
J. Nanosci. Nanotechnol. 19, 436–441, 2019
437

Designed Pt Promoted 3D Mesoporous Co3O4 Catalyst in CO2 Hydrogenation
Sápi et al.
gas sorption analyzer by N2 gas adsorption/desorption at
−196 ꢀC. Before the measurements, the samples were pre-
treated in vacuum (<∼0.1 mbar) at 473 K for 2 hours.
Replica method using KIT-6 as a hard template resulted
in mesoporous cobalt-oxide with ordered pore structure
(Fig. 1(C)). The N2 adsorption/desorption studies of the
Co3O4 support show isotherms with hysteresis loop char-
acteristic for mesoporous materials (Fig. 2(A)). The shapes
of the isotherm grouped into the mesoporous materials
with type V isotherms. The specific surface areas of the
mesoporous cobalt-oxide, the commercial cobalt-oxide and
the SBA-15 silica support are 51 m2 ·g−1, 16 m2 ·g−1 and
799 m2 ·g−1, respectively (Table I). The crystal structure of
mesoporous cobalt-oxide was Co3O4 similar to the com-
mercial cobalt-oxide catalyst support (Fig. 2(B)).
The sonication method for catalyst prepration were suc-
cesful showing the presence of controlled size Pt nanopar-
ticles dispersed on the surface of mesoporous cobalt-oxide
and SBA-15 silica (Figs. 1(C–D)). The concentration of Pt
on the surface of the catalysts were 0.82 wt%, 0.57 wt%
and 0.55 wt% for Pt/Co3O4 (impregnated), Pt/Co3O4
(mesoporous) and Pt/SBA-15, respectively (Table I).
2.10. Inductively Coupled Plasma Mass
Spectrometry (ICP-MS)
The loading of the Pt on the supported catalysts was
determined by ICP-MS technique. The measurements were
performed using an Agilent 7700× type ICP-MS spectrom-
eter. Typically, 10 mg of Pt/silica catalysts were dispersed
in 10 ml of cc. HCl solution and the mixture were heated
at elevated temperatures for 30 min before the tests. The
as-prepared samples were washed into a 100 ml flask than
it was connected to the sample introduction system con-
sisted of an I-AS type autosampler, a MicroMist pneu-
matic micronebulizer and a Peltier-cooled (2 ꢀC) Scott type
double-pass spray chamber. The sample uptake rate was
200 ꢄL · min−1. The ICP plasma and interface parameters
were set according to the standard hot plasma configuration
(e.g., RF forward power: 1550 W, argon carrier gas flow
rate: 1.05 L/min, sampling depth: 8.0 mm). Fine tuning was
performed before the analytical measurements using tun-
ing solutions supplied by the manufacturer (G1820-60410,
5185-5959 Agilent Technologies). All measurements were
carried out using the He mode of the ORS3 collision cell, by
monitoring the signal of 195Pt isotope. For calibration pur-
poses, six standard samples were prepared from a certified,
3.2. Reaction of CO2 and H2
The catalytic measurements between CO2 and H2 were
carried out with stoichiometric ratio of 1:4 in all cases.
The nominal Pt content was 1 w%, the actual metal con-
centration was determined by inductively coupled plasma
mass spectrometry (ICP-MS), the obtained Pt contents are
presented on Table I. The catalytic conversions and the
product distributions were determined on three different
catalysts. The conversion of CO2 was calculated taking
into account the amount of carbon-dioxide consumed. It
was also determined basis of the sum carbon balance.
Selectivity values were defined as
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multielement ICP-MS standard stock solution (8500-6948
Copyright: American Scientific Publishers
Agilent Technologies) via dilution with trace quality deion-
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ized water (Synergy+Elix 3, Millipore). QC samples were
also used to ensure the accuracy of determinations. All
labware (PE autosampler vials, PMP certified volumetric
flasks, etc.) were thoroughly cleaned before use with dilute
ultratrace quality nitric and hydrochloric acid (Ultrapure
Normatom, VWR).
xini
Si = ꢀ
xini
where xi is the mol fractions of product (i) and ni is the
number of carbon atoms in each molecule of product (i).
Experiments were executed in the temperature range of
573–973 K.
3. RESULTS AND DISCUSSION
3.1. Characterization of the Catalysts
Controlled size Pt nanoparticles were synthesized by a
polyol method using ethylene glycol as a media and PVP
as a capping agent from Pt-salts. 3ꢅ6 1ꢅ25 nm Pt nanopar-
ticles were obtained where the particles have narrow size
distributions as well as spherical shape (Fig. 1(A)). Com-
mercial cobalt-oxide support has no controlled structure.
TEM image show particles of 5–100 nm size with no
controlled shape (Fig. 1(B)). The crystal structure was
determined as Co3O4 by powder XRD technique (not
shown here). Wet impregnation of cobalt-oxide with Pt-
salt resulted in tiny Pt nanoparticles hardly observable with
TEM (white arrows at Fig. 1(B)).
However, ICP-MS results (Table I) proof the presence of
platinum in the catalysts. Electron Diffraction patterns (not
shown here) confirmed the presence of Pt (111), Pt (200),
and Pt (220) crystallite planes characteristic for metallic
face-centered cubic (fcc) platinum.
On Figure 3(A) the conversions of the CO2 hydrogena-
tion are plotted obtained on Pt supported by mesoporous
Co3O4, on commercial (Merck) Co3O4 and on SBA-15 as
a function of temperature. Significant differences can be
concluded based on the results.
Very low activities were observed at 570–600 K on
both Pt/Co3O4 (where the Pt was loaded by impregnation
method) and Pt/SBA-15. An increasing with temperature
was found on both catalysts. Outstanding activity (∼60%)
was observed when mesoporous oxide was used even at
573 K (Fig. 3(A)). Catalytic stability experiments were
also performed at 973 K, and the high activity remained
stable at all time under the reaction (5 hours). The conver-
sion values were between 69–70% in the case of Pt/Co3O4
(imp.), and 68–64% range in the matter of mesoporous
sample.
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Sápi et al.
Designed Pt Promoted 3D Mesoporous Co3O4 Catalyst in CO2 Hydrogenation
IP: 91.243.93.135 On: Wed, 19 Dec 2018 01:46:37
Copyright: American Scientific Publishers
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Figure 1. Typical TEM images of 3.6 nm Pt nanoparticles (A), Pt/cobalt-oxide support made by impregnation (B), mesoporous cobalt-oxide supported
Pt nanoparticles (C), SBA-15 supported Pt nanoparticle catalyst (D).
The product distributions were also different as can be
seen on Figures 3(B), (C). The CO production mono-
tonically increased with temperature and approximately
the same values were measured in every temperature
on the three samples (Fig. 3(C)). Exclusively almost
CO was formed on Pt/SBA-15 during CO2 dehydrogena-
tion. In contrast, on Pt supported on mesoporous Co3O4
large amount of methane was formed at low tempera-
ture (570–600 K). When the temperature was raised, the
methane formation decreased and CO production increased
(Fig. 3(B)).
When the Pt was loaded by impregnation on commercial
Co3O4, 80% methane and only 20% CO was the product
ratio at 673 K. In the case of Pt/SBA-15 CO was the main
product, only the trace amount of methane was observed.
This large difference in the activity and selectivity can be
explained by significant support effects. It is well known
that the electronic interaction is very weak between noble
metal and silica-based oxide.13 The interaction is signifi-
cant on Pt/Co3O4. X-ray photoelectron spectroscopy (XPS)
and near-edge X-ray absorption fine structure spectroscopy
(NEXAFS) showed that cobalt oxide reduction occurs in
presence of Pt.14 Powder XRD analysis of the mesoporous
cobalt-oxide supported Pt sample after CO2 hydrogenation
reaction (not shown here) showed the presence of pure
fcc phase metallic cobalt as well as CoO. In addition, it
is interesting to acknowledge the presence of cubic CoCx
which can also contribute to the high activity of the meso-
porous cobalt supported catalysts. We may assume that a
special interface is more effective in the case of meso-
porous Co3O4 during the interaction with Pt. The different
product distributions on Pt/SBA-15 and Pt/Co3O4 suppose
different reaction mechanisms.
The differences in selectivity are well demonstrated on
the three samples at 673 K in Figure 4. When meso-
porous Co3O4 was used as support for uniform size of
platinum, only methane was detected with traces of CO.
Table I. Specific surface area and Pt content of the supports and
catalysts.
Support
SBET (m2 ∗g−1
)
Catalyst
Pt content (wt%)
Co3O4 (bulk)
Co3O4
16
51
Pt/Co3 O4 (bulk)
Pt/Co3 O4
0.82
0.57
(mesoporous)
SBA-15
(mesoporous)
Pt/SBA-15
Based on the reaction products (CO and water) on
Pt/SBA-15 we propose a direct C–O bond cleavage
799
0.55
J. Nanosci. Nanotechnol. 19, 436–441, 2019
439

Designed Pt Promoted 3D Mesoporous Co3O4 Catalyst in CO2 Hydrogenation
Sápi et al.
Figure 2. Adsorption–desorption isotherm (A) as well as powder XRD pattern (B) of mesoporous cobalt-oxide.
pathway and a reverse water-gas shift reaction mechanism
in first approximation. It is generally accepted in CO2
transformation reactions the formation of activated CO2
(vibrationally or electronically; CO∗2−ꢁ.7 The activated car-
bon dioxide dissociates to CO and O on the surface of the
Pt nanoparticle (Eq. (1)).
extent. Although detail DRIFTS measurements are planned
in the future, we propose a formate pathway which was
established in the case of Pt/TiO123 and for Pd/Al2O3.18ꢀ19
In this case the carbon dioxide can be activated on oxy-
gen vacancies of oxide support nearby the platinum with
the mediation of hydrogen atoms. The formed CO2 radical
anion (CO∗2−ꢁ captures a nearby hydrogen atoms forming
formate via carboxylate:
CO2ꢆgꢁ → CO∗2ꢆ−Ptꢁ → COꢆPtꢁ +OꢆPtꢁ
COꢆPtꢁ → COꢆgꢁ
(1)
(2)
∗−
CO
+HꢆPtꢁ → HCOO−ꢆPt/CoOꢁ
(4)
2ꢆCoOꢁ
The presence of adsorbed hydrogen, which is formed on
Pt sites during dissociation of molecular hydrogen, makes
this process more easily.4ꢀ10 The adsorbed hydrogen may
react with oxygen forming water:
This reaction step was supposed on supported noble
metals, probably it occurs at metal/oxide interface.17ꢀ18
This formate species may reacts and decomposes further
12
18ꢀ20
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forming methane or CO.
Copyright: American Scientific Publishers
2Hꢆgꢁ +OꢆPtꢁ → H2Oꢆgꢁ
(3)
−
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HCOOꢆPt/COꢁ +HꢆPtꢁ → CH4ꢆgꢁ +H2OꢆCoOꢁ
HCOO−ꢆPt/COꢁ +HꢆPtꢁ → COꢆPtꢁ +H2OꢆCoOꢁ
HCOO−ꢆPt/COꢁ → COꢆPtꢁ +OHꢆ−CoOꢁ
(5)
(6)
(7)
When the reducible oxide is the catalysts support,
the reaction mechanism could be different, although the
above-mentioned mechanism could be also valid in certain
Figure 3. Effect of support in the hydrogenation of CO2 on Pt anchored Merck Co3O4, mesoporous Co3O4 and SBA-15 catalysts. Conversion (A),
formation rate of methane (B) and carbon-monoxide (C).
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Sápi et al.
Designed Pt Promoted 3D Mesoporous Co3O4 Catalyst in CO2 Hydrogenation
supported noble metals, probably it occurs at metal/oxide
interface, at Pt-CoO in present case. Finally, it was proved
that the controlled size Pt nanoparticles on a mesoporous
material as a designed catalyst, was more efficient in the
same catalytic reaction, than the prepared by the classic
method. The authors assume that the catalyst design is an
essential step in the future of the heterogeneous catalysis.
Acknowledgment: This work was supported by the
János Bolyai Research Scholarship of the Hungarian
Academy of Sciences and the ÚNKP-ÚNKP-16-4 New
National Excellence Program of the Ministry of Human
Capacities as well as the Hungarian Research Development
and Innovation Office through grants NKFIH OTKA PD
120877 of András Sápi, Ákos Kukovecz and Gyula Halasi
are grateful for the fund of NKFIH (OTKA) K112531 and
PD 115769, respectively. This collaborative research was
partially supported by the “Széchenyi 2020” program in
the framework of GINOP-2.3.2-15-2016-00013“Intelligent
materials based on functional surfaces—from syntheses to
applications” project. The authors would like to express
their thanks to Balázs Nagy for his help and they are also
grateful for Koppány L. Juhász and Dorina G. Dobó for
their work considering the catalysts preparation.
Figure 4. Selectivity values of the products in the CO2 + H2 reaction
on different Pt promoted catalysts at 673 K.
The product distribution obtained on Pt/Co3O4 suggests
that Step 5 is dominant at low temperatures while the
CO formation mechanism (Steps 6 and 7) are favorable
at higher temperatures. The CH4 formation via formate
hydrogenation may involve the hydrogenation of adsorbed
carbonyl.
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Received: 30 September 2017. Accepted: 11 January 2018.
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