ZrO2 Nanocrystals As Catalyst for Synthesis of Dimethylcarbonate from Methanol and Carbon Dioxide: Catalytic Activity and Elucidation of Active Sites

Article abstract of DOI:10.1021/acs.langmuir.7b01294

The catalytic activity of zirconium oxide (ZrO₂) nanocrystals for the reaction of carbon dioxide (CO₂) with methanol to form dimethylcarbonate (DMC) was investigated. ZrO₂ nanocrystals prepared by hydrothermal synthesis at various temperatures were compared. The size of the ZrO₂ nanocrystals monotonically increased with the hydrothermal temperature, according to specific surface area, transmission electron microscope measurements, and their X-ray diffraction peak widths. The ZrO₂ nanocrystals prepared by hydrothermal synthesis were found to exhibit high catalytic activity owing to their high surface area and catalytically active surfaces arising from their high crystallinity. Next, adsorbed species generated from CO₂ on the ZrO₂ surfaces were measured using CO₂ temperature-programmed desorption (TPD) and in situ FT-IR spectroscopy. The results confirmed the presence of several kinds of adsorbed species including bidentate bicarbonate (b-HCO₃^-), bidentate carbonate (b-CO₃^2-), and monodentate carbonate (m-CO₃^2-). The relationship between the amounts of these surface species and the catalytic activity of the ZrO₂ was investigated for the first time. The amount of the bidentate species (b-HCO₃^- and b-CO₃^2-) was found to correlate well with the catalytic activity, demonstrating that the surface sites that afford these species contribute to the catalytic activity for this reaction.

Full text of DOI:10.1021/acs.langmuir.7b01294

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Article
2
ZrO Nanocrystals as Catalyst for Synthesis of
Dimethylcarbonate from Methanol and Carbon Dioxide:
Catalytic Activity and Elucidation of Active Sites
Takayuki Akune, Yusuke Morita, Shinya Shirakawa, Kiyofumi Katagiri, and Kei Inumaru
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01294 • Publication Date (Web): 03 Oct 2017
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Clear Copy of Revised Manuscript: REVISION2
ZrO2 Nanocrystals as Catalyst for Synthesis of Dimethylcarbonate from Methanol and
Carbon Dioxide: Catalytic Activity and Elucidation of Active Sites
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Takayuki Akune, Yusuke Morita, Shinya Shirakawa, Kiyofumi Katagiri*, and Kei
Inumaru*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University
1ꢀ4ꢀ1 Kagamiyama, Higashihiroshima, Hiroshima 739ꢀ8527, Japan.
Eꢀmail: inumaru@hiroshimaꢀu.ac.jp
The catalytic activity of zirconium oxide (ZrO2) nanocrystals for the reaction of carbon
dioxide (CO2) with methanol to form dimethylcarbonate (DMC) was investigated. ZrO2
nanocrystals prepared by hydrothermal synthesis at various temperatures were
compared. The size of the ZrO2 nanocrystals monotonically increased with the
hydrothermal temperature, according to specific surface area, transmission electron
microscope measurements, and their Xꢀray diffraction peak widths. The ZrO2
nanocrystals prepared by hydrothermal synthesis were found to exhibit high catalytic
activity owing to their high surface area and catalytically active surfaces arising from
their high crystallinity. Next, adsorbed species generated from CO2 on the ZrO2 surfaces
were measured using CO2 temperature programmed desorption (TPD) and inꢀsitu
FTꢀIR spectroscopy. The results confirmed the presence of several kinds of adsorbed
−
species including bidentate bicarbonate (bꢀHCO3 ), bidentate carbonate (bꢀCO3 −) and
2
monodentate carbonate (mꢀCO3 −). The relationship between the amounts of these
2
surface species and the catalytic activity of the ZrO2 was investigated for the first time.
The amount of the bidentate species (bꢀHCO3− and bꢀCO3 −) was found to well correlate
2
with the catalytic activity, demonstrating that the surface sites which afford these
species contribute the catalytic activity for this reaction.
Keywords: nanoparticle, CO2 TPD, in situ IR, XRD
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Introduction
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Metal oxide nanocrystals have attracted substantial attention as important materials
for the realization of novel functions in various fields1,2). One advantage of nanocrystals
is that their properties, such as shape, size, crystal structure, and crystallinity, can be
controlled by tuning their preparation conditions. Nanocrystals typically have a large
surface area because of their small size, and particular crystal planes exposed, so novel
surface functions are an important objective in nanocrystal science and technology1).
Heterogeneous catalysis is one of the most promising fields for nanocrystals.
The conversion of carbon dioxide (CO2) is a current challenge in chemical technology.
Several methodologies for CO2 conversion have been proposed, including a shift reaction
to produce hydrogen (H2) and carbon monoxide3), and chemical reduction with reducing
reagents such as H2. The reduction of CO2 is the goal of a proposed future energy system
using methanol as an energy carrier4). Another way to use CO2 is the production of
carbonates. Diphenylcarbonate is a major chemical source for polycarbonate resin and
can be formed from dimethylcarbonate (DMC) by the ester exchange reaction shown in
Eq. 1 5). Thus, DMC is a useful chemical and a suitable target product for the chemical
conversion of CO2 Ref. 6).
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CO2
+
2 CH3OH
⇄
(CH3O)2CO
+
H2O.
(1)
Tomishige et al.7ꢀ11) reported that zirconium oxide (ZrO2) is an effective catalyst for
the formation of DMC from CO2 and methanol. They also reported cerium oxides as
active catalysts for this reaction.9ꢀ12) Their works stimulated many studies on this
reaction.13ꢀ21) Bell and coꢀworkers13ꢀ15) investigated the mechanism of this catalytic
reaction in detail using in situ infrared (IR) spectroscopy, and proposed that
coordinately unsaturated Zr sites and neighboring hydroxyl groups are important in
this reaction.
In the present study, the use of ZrO2 nanocrystals as catalysts for the formation of
DMC from CO2 and methanol is investigated. The nanocrystals and reaction system
were characterized by transmission electron microscopy (TEM), Xꢀray diffraction (XRD),
CO2ꢀtemperatureꢀprogrammed desorption (TPD) measurements, and in situ FTꢀIR
spectroscopy. Particularly, it is the first time that the relationship between the amounts
of surface adsorbed species and the actual catalytic activity have been
semiꢀquantitatively investigated. Thus, this study highlights the important
contribution of specific adsorption sites to the catalytic reaction.
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Experimental
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Preparation of ZrO2 nanocrystals
ZrO2 nanocrystals were prepared according to the literature22) as follows. ZrO(NO3)2·2
H2O (1.60 g) and urea (3.46 g) were dissolved in pure water (30 mL, Millipore). The
solution was charged into an autoclave and heated at a selected temperature (393, 433,
473, 493, or 513 K) for 20 h. The resulting solid was separated by centrifugation and
then washed several times by repeated dispersion in water and centrifugation. The
sample was dried overnight at 323 K. The obtained ZrO2 nanocrystal samples were
denoted ZrO2ꢀHT(XXX), where XXX represents the reaction temperature.
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For comparison, ZrO2 was prepared by calcination of zirconium hydroxide
(ZrO2·xH2O, Nacalai Tesque, Inc., Kyoto, Japan) according to the procedure reported by
Tomishige et al 7)
This catalyst was denoted ZrO2ꢀHXꢀC. Mesoporous ZrO2 was also
.
prepared as follows.23) ZrO(NO3)2·2H2O (0.48 g) and Pꢀ123 surfactant template (BASF,
0.21 g) were dissolved in pure water (60 mL). The pH of the stirred solution was then
adjusted to 11 with 28% NH4OH. The solution was aged at 361 K for 24 h to give a white
precipitate. This solid was filtered, washed with water and acetone, and then dried at
383 K overnight. The sample was calcined at 873 K in air for 4 h to remove the template.
The obtained mesoporous ZrO2 after calcination was denoted MesoꢀZrO2.
Characterization
XRD patterns were recorded on a diffractometer (Dꢀ8 Advance, Bruker)
equipped with a positionꢀsensitive detector (Vantec) and Ni filter. In situ IR spectra
were measured with an IR spectrometer (FT/IRꢀ4200, JASCO) equipped with a mercury
cadmium telluride detector and glass in situ cell with NaCl windows. CO2 TPD
experiments were carried out with a customꢀmade flow system. After pretreatment at
423 K (ZrO2ꢀHT(XXX)) or 673 K (ZrO2ꢀHXꢀC, MesoꢀZrO2) for 1 h under an Ar flow, the
sample was exposed to 4000 ppm of CO2 at room temperature. After reversibly adsorbed
species were removed under an Ar flow, the sample was heated under an Ar flow at a
−
1
temperature ramp rate of 10 K min . The gas flow from the outlet was introduced into
the QꢀMass detector of a gas chromatographꢀmass spectrometer (GCꢀMS) apparatus
(JMSꢀQ1050GC, JEOL, Tokyo, Japan). The signal with m/z = 44 was recorded to
monitor the desorption rate of CO2. The desorption rate of CO2 rd is obtained by the
following equation:
rd
= CCO2 · F
where CCO2 is CO2 concentration at the outlet calculated from the mass spectrometer
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signal intensity,
F
is a total flow rate of outlet gas (~ flow rate of the carrier gas, in mol
minꢀ1). The mass spectrometer intensity was calibrated by using standard dilute CO2
gas (Ar balance) with known concentration.
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Catalytic reaction
ZrO2 catalyst (50 mg, after pretreatment in vacuo at 423 K (ZrO2ꢀHT(XXX)) or 673 K
(ZrO2ꢀHXꢀC, MesoꢀZrO2)) and methanol (6.2 mL) were added to an autoclave (190 cm3)
and CO2 gas was introduced. The autoclave was heated to 433 K at a pressure of 4.8
MPa. After a certain reaction time, the autoclave was cooled by soaking in iceꢀwater.
The product in methanol was analyzed with a GC (GCꢀ14B, Shimadzu, Kyoto, Japan)
equipped with a flame ionization detector. The catalytic activities of all the prepared
catalysts were evaluated using the amount of DMC produced within a reaction time of 5
h. At first the time course of DMC formation was measured with a large amount of ZrO2
catalyst (0.5 g) to examine the effect of chemical equilibrium. As shown in Fig. S1, the
DMC amount was almost proportional to the reaction time before DMC reached ca 0.2
mmol. Therefore we used much smaller amount of catalyst (50 mg) and compared the
activity by using the DMC amount at 5 h, so that the DMC amount did not reach 0.2
mmol.
Results and Discussion
Particle size, surface area, and structure of catalysts
Figure 1 depicts TEM images of the ZrO2 nanocrystal catalysts. ZrO2ꢀHT(393)
consisted of aggregated particles of less than 5 nm in size (Fig. 1a). The particle sizes
observed in the TEM images increased to ca. 10–20 nm as the hydrothermal
temperature was increased from 393 to 513 K (Fig. 1a–e). A darkꢀfield image of
ZrO2ꢀHT(513) is given in Fig. 1f. The crystallite size almost coincides with the particle
size observed in the brightꢀfield image (Fig. 1e).
The BrunauerꢀEmmettꢀTeller (BET) surface areas of the nanoparticles are
listed in Table 1. ZrO2ꢀHT(393) had the highest surface area (168 m2 g ) except for that
−
1
−
of MesoꢀZrO2 of 217 m2 g . The surface area of ZrO2ꢀHT(XXX) monotonically decreased
1
as the hydrothermal temperature was increased, consistent with the change in particle
size observed in the TEM images.
Figure 2 shows XRD patterns of the samples. ZrO2ꢀHT(XXX) samples exhibited
clear diffraction peaks indicative of the monoclinic crystal structure. The integrated
intensities of the diffraction patterns in the range of 2θ = 20–45° were almost constant
for all of ZrO2ꢀHT(XXX) samples. This fact directly showed that the amount of
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crystalline moiety in all samples was similar despite their preparation at different
temperatures. If the samples prepared at low temperatures contain amorphous moiety,
the fraction of amorphous moiety should be smaller and the amount of crystalline
moiety should be larger when the samples are prepared at higher temperatures. This
contradict the experimental result. Thus, it was suggested that all the samples
consisted of only crystalline moiety. It is obvious that the diffraction peaks became
sharper as the hydrothermal temperature was increased: the crystallite size increased
with reaction temperature. The crystalline sizes derived from XRD peak half width
were 6.0, 6.6, 14, 15, 16, and 16 nm for the samples prepared at 393, 433, 473, 493 and
513 K. This trend is consistent with the TEM images and changes in surface area.
MesoꢀZrO2 exhibited very broad diffraction peaks, even following calcination at 873 K
(Fig. 2f). The main peak centered at ~30° suggests that this sample contained poorly
crystallized tetragonal ZrO2. ZrO2ꢀHXꢀC prepared by calcination of zirconium hydroxide
showed diffraction peaks mainly attributable to the tetragonal ZrO2 phase as well as
weaker diffraction signals from the monoclinic phase. The peaks were sharp, indicating
that these phases were wellꢀcrystallized in this sample.
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To further study the structure of MesoꢀZrO2, its nitrogen adsorption isotherm
was measured as shown in Fig. 3. An obvious step caused by capillary condensation was
observed around
P / Po = 0.5–0.7, indicating the presence of wellꢀdeveloped mesopores.
BarrettꢀJoynerꢀHalenda (BJH) analysis of this isotherm gave a single peak at around
5.2 nm (not shown). The inset in Fig. 3 shows the lowꢀangle region of the XRD pattern of
MesoꢀZrO2. The peak at 1.8° suggests the presence of an ordered nanostructure. These
results reveal that MesoꢀZrO2 possessed a high surface area with wellꢀdeveloped
mesopores and pore walls of poorly crystallized, mostly tetragonal phase ZrO2.
Catalytic activity of catalysts
The catalytic activity of the ZrO2 catalysts for producing DMC was
investigated; the results are listed in Table 1. The reaction rates were normalized by
surface area, as also listed in Table 1. It is obvious that ZrO2ꢀHXꢀC and the
ZrO2ꢀHT(XXX) samples showed much higher activity per unit surface area than
MesoꢀZrO2. The low catalytic activity of MesoꢀZrO2 may be attributed to its poor
crystallinity, as indicated by XRD. The catalyst ZrO2ꢀHXꢀC, which adopted a tetragonal
crystal structure, showed catalytic activity comparable to that of the ZrO2ꢀHT(XXX)
catalysts, which had a monoclinic structure. This shows that the crystal structure of the
ZrO2 phase did not have a large effect on the catalytic activity in this study. Some
reports described that catalytic activity for this reaction or dynamics of adsorbed species
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were different between the two crystal phases.15) Another influential factor is
preparation conditions and preparation method. It is possible that hydrothermal
synthesis in this study affected the influential factors such as surface crystallinity.
As shown in Table 1, there were moderate differences in the surface
areaꢀnormalized catalytic activity of the samples prepared by hydrothermal synthesis
(ZrO2ꢀHT(XXX)). Samples prepared at lower temperature (ZrO2ꢀHT(393) and
ZrO2ꢀHT(433)) showed relatively lower activity, while those prepared at higher
temperature (493 and 513 K) showed higher activity. More precisely, the surface
areaꢀnormalized activity showed a trend to increase as the hydrothermal temperature
increased to 493 K, while further temperature increase to 513 K brought about a
decrease in the activity. This result reflects the nature of the surfaces of the catalysts
because these activities were per unit surface area. It is possible that the surface
crystallinity affects the nature of the catalysts’ surfaces. For example, it is possible that
the number of active sites per unit surface area of the catalysts were different
depending on the surface crystallinity. To prove this point, we next obtained further
insight into the adsorbed species and their amounts by measuring CO2 TPD and inꢀsitu
IR spectra of these catalysts.
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In terms of total catalytic activity, ZrO2ꢀHT(393) produced the highest amount
of DMC, more than that of ZrO2ꢀHXꢀC prepared by calcination of zirconium hydroxide.
The high surface area of the ZrO2ꢀHT(XXX) samples contributed to their high total
activity. According to these results, we conclude that the present nanocrystal catalysts
prepared by hydrothermal synthesis were advantageous because their high surface area
and high reaction rate per unit surface area yielded a high overall catalytic activity,
especially ZrO2ꢀHT(393).
CO2ꢀTPD and in situ IR spectra
Figure 4 shows CO2ꢀTPD profiles measured for the catalysts. For clarity, Fig.
4a shows data for three selected samples, ZrO2ꢀHT(393), ZrO2ꢀHT(493) and ZrO2ꢀHXꢀC.
The TPD data of all samples are shown in Fig. 4b. The TPD profile of ZrO2ꢀHXꢀC
exhibited an intense peak at 375 K accompanied by long tale spreading to higher
temperature (Fig. 4a). In contrast, the profile of ZrO2ꢀHT(393) showed distinct two
peaks at 375 and 525 K. As for ZrO2ꢀHT(493), The TPD profile gave an intense peak at
375 K and smaller desorption signals beginning at around 490 K. The same location of
the lower temperature peak of ZrO2ꢀHT(393) and ZrO2ꢀHT(493) as the single peak of
ZrO2ꢀHXꢀC suggested that they were associated with the same adsorbed species.
The in situ IR spectra of the three samples are given in Figure 5. The top,
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middle and bottom panels correspond to ZrO2ꢀHXꢀC, ZrO2ꢀHT(493) and ZrO2ꢀHT(393),
respectively. After the pretreatment by evacuation at appropriate temperature (423 K
for ZrO2ꢀHT(XXX) and 673 K for ZrO2ꢀHXꢀC), each sample was dosed to CO2 (1 atm) at
room temperature and an IR spectrum was measured in the presence of CO2. This
spectrum contains the signals from reversiblyꢀadsorbed species. Then the IR cell was
evacuated at room temperature (rt) for 30 min and then an IR spectrum was measured
at rt. The evacuation removes reversiblyꢀadsorbed species from the sample and the
spectrum is associated with only irreversibly adsorbed species. Next, the sample in the
IR cell was evacuated at each elevated temperature for 20 min followed by an IR
spectrum recording. In the TPD measurements the samples were placed in an Ar flow to
remove reversiblyꢀadsorbed species before the heating started. Thus TPD
measurements detect only irreversiblyꢀadsorbed species.
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Table 2 summarizes the literature data of IR bands of surface species formed by
adsorption of CO2 on ZrO2.13ꢀ15,24ꢀ27) The structure of some surface species was
illustrated in Scheme 1. In the IR results of ZrO2ꢀHXꢀC (Fig. 5 top panel), the spectrum
2ꢀ
2ꢀ
taken in the presence of gaseous CO2 gave signals from bidentate CO3 (bꢀCO3 , 1580
ꢀ
ꢀ
(shoulder), 1276, and 1042 cmꢀ1) and bidentate HCO3 (bꢀHCO3 , 1620, 1430 and 1227
ꢀ
ꢀ
cmꢀ1). The signal at 1430 cmꢀ1 is also assignable to ionic HCO3 (iꢀHCO3 , 1430 cmꢀ1 ref
14). After evacuation at rt, all peaks decreased but still existed. The decrease of the
signal at 1430 cmꢀ1 was more significant, probably because the superimposed signal
ꢀ
from ionic HCO3 decreased to large extent. At the same time, small new peaks
appeared at 1454 and 1415 cmꢀ1. According to Table 2 these two peaks correspond to
2ꢀ
2ꢀ
ꢀ
polydentate CO3 (pꢀCO3 ). After evacuation at 375 K for 20 min, signals from bꢀHCO3
2ꢀ
2ꢀ
and bꢀCO3 drastically decreased but signals from pꢀCO3 almost remained. These
results demonstrate that the TPD peak observed at 375 K corresponds to the two
ꢀ
2ꢀ
bidentate species (bꢀHCO3 and bꢀCO3 ). After evacuation at 450 K the signals from
2ꢀ
pꢀCO3 unchanged but disappeared by evacuation at 525 K.
IR spectra of ZrO2ꢀHT(493) (Fig. 5, middle panel) also give useful information
about the surface species. The spectrum taken in the presence of 1 atm CO2 gave signals
ꢀ
2ꢀ
from bꢀHCO3 (1632, 1420 and 1227 cmꢀ1) and bꢀCO3 (1590, 1290, and 1038 cmꢀ1).
These species were observed after evacuation at rt. In the spectrum after 375 K
ꢀ
2ꢀ
evacuation, bꢀHCO3 and bꢀCO3 decreased drastically. It should be noted that a new
2ꢀ
signal appeared at 1330 cmꢀ1. This signal is close to that of mꢀCO3 (Table 2)
.
In the case of ZrO2ꢀHT(393) (Fig. 5c), the discussion is slightly more difficult. In
2ꢀ
ꢀ
the presence of gaseous CO2, bꢀCO3 and bꢀHCO3 were detected. These species
remained after evacuation at rt. In the spectrum after evacuation at 375 K, the signals
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from these bidentate species drastically decreased and very broad bands was observed
2ꢀ
at ca 1600 and 1292 cmꢀ1. These positions are also close to that of bꢀCO3 but at present
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we cannot give a clear assignment to these broad IR signals. A possibility is a
2ꢀ
bꢀCO3 ꢀlike species interacting strongly with the surfaces.
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By combining the TPD and IR results, we can conclude that the TPD peaks at
375 K represented the bidentate species (bꢀHCO3 and bꢀCO3 −). The TPD signals
−
2
−
2
observed at higher temperature around 525 K represented mꢀCO3 (ZrO2ꢀHT(493))
and/or unidentified species for ZrO2ꢀHT(393) (possibly strongly bound bꢀCO3 −ꢀlike
2
species ). Fig. 4b shows TPD profiles for all samples. All the data except for that of
ZrO2ꢀHXꢀC and MesoꢀZrO2 showed both TPD peaks. Our interpretation of the TPD
peaks is consistent with the previous report in the literature.14)
−
Scheme 1. bꢀHCO3 , bꢀCO3 −, and mꢀCO3 − species on the catalysts.
2
2
Correlation between catalytic activity and catalyst properties
Here, we discuss the catalytic activity per unit area of the catalysts in further
detail. To obtain deep insight into a catalytic reaction, it is important to obtain
information on the adsorption sites which contribute to the rate determining step of the
reaction. For this purpose, it is effective to investigate the relationship between the
amounts of certain surface species and the catalytic reaction rate. If any of these
amounts are strongly correlated to the reaction rate, it is strong evidence that the
corresponding adsorption species or related surface sites are involved in the rate
determining step. In this study, we have obtained catalytic reaction rates for ZrO2
catalysts prepared using various methods and conditions. Thus, we next investigated
the relationships between the reaction rate and the amounts of surface species detected
in the CO2 TPD and in situ IR experiments.
The CO2 TPD data were used to obtain information about the amounts of these
surface species. One method is to separate the TPD profiles into two peaks and use the
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integrated areas to calculate the amount of each surface species. However, it is difficult
to exclude unexpected error and artefact due to poor fitting between theoretical curve
and observed data. Thus, we adopted a simpler method; the intensity of each peak (375
and 525 K) in the TPD profile (i.e., desorption rate) was taken as a measure of the
relative amount of each surface species among the different samples. This method
works if the TPD peak shape does not change. In fact, the shape of TPD peaks observed
in Fig. 4 is similar for many samples, especially for the peak at lower temperature (375
K). Of course, this method cannot be used to compare amounts of different species, but
usable to extract essential trend of the sample dependence of each adsorbed species’
amount. This treatment is also valid from the viewpoint of theoretical consideration: the
TPD peak height for each surface species is proportional to the relative amount of each
surface species when the distribution of activation energy for desorption of the species
and other experimental conditions are the same among different samples.28) The
unchanged shape of the TPD peaks implies that the above condition was met in our
samples and experiments.
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Figure 6 shows the relationship between the catalytic reaction rate and the
relative amounts of each surface species derived from the TPD profiles. The vertical
axes of the panels show the catalytic reaction rate per unit surface area. The horizontal
axes show the CO2 desorption rate per unit surface area at 375 K (Fig. 6a) and 525 K
(Fig. 6b). Therefore, the horizontal axis of Fig. 6a can be regarded as the relative
−
amount of the bidentate species (bꢀHCO3 and bꢀCO3 −) per unit surface area of the
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different samples. The horizontal axis of Fig. 6b represents the relative amount of
−
2
mꢀCO3 (ZrO2ꢀHT(493)) and/or the unidentified species (ZrO2ꢀHT(393)). Because the
amount of CO2 adsorbed on MesoꢀZrO2 (per unit surface area) was very small compared
with that on the other samples (Fig. 4b), the plots for MesoꢀZrO2 are isolated at the
lower left in the panels in Fig. 6a and 6b. The plots for the other samples are located
from center to higher right in the panels. Fig. 6a demonstrates that the relative amount
of the bidentate species was reasonably well correlated with the catalytic reaction rate.
That is, the catalytic reaction rate increased with the amount of the bidentate species.
The correlation factor between the plots in Fig. 6a and the linear line was as high as
0.95. In contrast, as shown in Fig. 6b, the plots for all samples except MesoꢀZrO2 and
ZrO2ꢀHXꢀC, located at the upper right in the panel, showed no positive correlation. The
catalytic activity rather decreased as the amount of mꢀCO3 − (ZrO2ꢀHT(493)) and/or the
2
unidentified species (ZrO2ꢀHT(393)) increased.
These findings demonstrate that the surface sites on which the bidentate
species form play a very important role in the rate determining step of the catalytic
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reaction. A bꢀCO3 species forms on a coordinately unsaturated Zr4+ site and
neighboring oxide ion,26, 27) and a bꢀHCO3− may form from an unsaturated Zr4+ site and
a OH group.
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This is the first time that the real catalytic activity of ZrO2 has been linked to
the amount of surface sites or surface adsorption species for this reaction. Bell et al.
previously proposed a reaction scheme for this reaction over ZrO2 based on in situ FTꢀIR
and TPD experiments.13ꢀ15) According to them, the key step is the adsorption of CO2
molecules on the coordinately unsaturated Zr4+ sites, followed by reaction of the
adsorbed species with neighboring methoxy groups. Bell et al. claimed that Lewis
acidity of coordinately unsaturated Zr4+ sites and Lewis basicity of neighboring oxide
ions play important roles in the catalytic formation of DMC from CO2 and methanol. A
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bꢀCO3 species forms on an unsaturated Zr4+ site and its neighboring oxide ion.
−
−
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bꢀHCO3 may form from an unsaturated Zr4+ site and a OH group. The bꢀCO3 and
bꢀHCO3− species detected by TPD were adsorbed irreversibly on the ZrO2 surface, which
reflects the fact that the adsorption sites have strong interaction. The amount of
−
−
2
bꢀHCO3 and bꢀCO3 species detected by TPD must represent the amount of catalytic
active sites contributing to the key reaction step. Our findings have evidently revealed
these active sites by comparing the real catalytic activity of various catalysts and the
amounts of species adsorbed on them for the first time.
Among the catalysts prepared in this study, MesoꢀZrO2 had very low
crystallinity, and ZrO2ꢀHT(XXX) had increasing crystallinity with the increasing
hydrothermal temperature. As a general trend, catalytic activity per surface area
increased as the crystallinity increased (Table 1, except ZrO2ꢀHT(513), the catalyst
prepared at the highest hydrothermal temperature). For all catalysts, a good correlation
was found in Fig. 6a. This demonstrates that the surface sites which afford the
−
bidentate species (bꢀHCO3 and bꢀCO3 −) contribute the catalytic activity for this
2
reaction.
Conclusions
ZrO2 nanocrystals formed by hydrothermal synthesis showed high catalytic activity
for the formation of DMC. Nanocrystal catalysts prepared by hydrothermal synthesis
were advantageous because their high surface area and high reaction rate per unit
surface area gave a high catalytic activity overall.
CO2 TPD measurements and inꢀsitu FTꢀIR spectroscopy confirmed the presence of
several kinds of adsorbed species on the ZrO2 surfaces. It was demonstrated for the first
time that the amount of surface bidentate species shows a good correlation with the
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observed catalytic activity of these catalysts. Our findings show that that surface sites
which afford the species play an important role in the rate determining step of this
catalytic reaction.
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Acknowledgments
This work was supported by the Advanced Catalytic Transformation Program for Carbon Utilization
(JST ACT–C Grant Number JPMJCR12Y2) of the Japan Science and Technology Agency (JST), and
a JSPS KAKENHI Grant Number JP25288108. This work was also partially supported by a
KAKENHI Grant Number JP22107011 in the Innovative Area: “Fusion Materials: Creative
Development of Materials and Exploration of Their Function through Molecular Control” (Area No.
2206) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
This work is also partly supported by the Centre for Smart Materials, the Centre for Functional Nano
Oxide at Hiroshima University. The authors thank Dr. M. Maeda for TEM measurements.
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Table 1. Activity of ZrO2 catalysts.
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BET
surface
area /
m2 gꢀ1
Product (DMC) Reaction rate
formed for 5 h
per unit
Sample
surface area
/ ꢁmol
/ ꢁmol hꢀ1 mꢀ2
ZrO2ꢀHXꢀC
71
168
148
115
96
78
163
140
116
115
90
4.4
3.9
3.8
4.1
4.7
4.4
1.5
ZrO2ꢀHTꢀ(393)
ZrO2ꢀHTꢀ(433)
ZrO2ꢀHTꢀ(473)
ZrO2ꢀHTꢀ(493)
ZrO2ꢀHTꢀ(513)
MesoꢀZrO2
82
218
83
.
Table 2. IR peak assignments in the literatures.
Wavenumber of
Species
References
absorption peaks
/ cmꢀ1
Bidentate carbonate
2ꢀ
1556, 1304, 1061
Kondo et al24)
(tꢀZrO2) Pokrovski et al14)
(mꢀZrO2) Pokrovski et al14)
(mꢀZrO2) Jung et al13)
(bꢀCO3
)
1595, 1315
1575, 1555, 1335, 1325
1325
Bidentate bicarbonate
1600, 1470, 1225
1622, 1465 (wk)
Kondo et al24)
ꢀ
(bꢀHCO3 )
(ZrO2/SiO2) Fisher et al25)
1620,
1225 (tꢀZrO2, mꢀZrO2) Pokrovski et al14)
1625, 1437, 1225
(mꢀZrO2) Jung et al13)
Monodentate carbonate
2ꢀ
1385
(ZrO2/SiO2) Fisher et al25)
Pokrovski et al14)
(mꢀCO3
)
1375, 1355
Polydentate carbonate
1450, 1430
1695, 1435
(tꢀZrO2)
Pokrovski et al14)
2ꢀ
(pꢀCO3
)
Ionic bicarbonate
(mꢀZrO2) Pokrovski et al14)
ꢀ
(iꢀHCO3 )
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Fig. 1. TEM images of ZrO2 nanoparticle catalysts. a) ZrO2ꢀHT(393), b) ZrO2ꢀHT(433), c)
ZrO2ꢀHT(473), d) ZrO2ꢀHT(493), e) ZrO2ꢀHT(513), and f) dark field image of ZrO2ꢀ
HT(513).
Fig. 2. XRD patterns of ZrO2 nanoparticle catalysts. a) ZrO2ꢀHT(393), b) ZrO2ꢀHT(433),
c) ZrO2ꢀHT(473), d) ZrO2ꢀHT(493), e) ZrO2ꢀHT(513), f) MesoꢀZrO2, and g) ZrO2ꢀHXꢀC. h)
and i) are calculated patterns of tetragonal and monoclinic ZrO2, respectively.
Fig. 3. Nitrogen adsorption isotherm of MesoꢀZrO2. The inset shows the lowꢀangle
region of the XRD pattern of MesoꢀZrO2.
Fig. 4. CO2 TPD profiles of catalysts. (a) ZrO2ꢀHT(393), ZrO2ꢀHT(493) and ZrO2ꢀHXꢀC,
b) all samples.
Fig. 5. In situ IR spectra of CO2 adsorbed on ZrO2ꢀHXꢀC (top), ZrO2ꢀHT(493) (middle),
and ZrO2ꢀHT(393) (bottom). Spectra were taken firstly in the presence of 1 atm CO2,
and then taken after evacuation for 20 min at each temperature. The spectrum of the
ZrO2 catalyst after pretreatment has been subtracted from each spectrum.
Fig. 6 Relationship between catalytic activity per unit surface area and CO2 TPD peak
height (per unit surface area) at (a) 375 K, (b) 525 K.
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Fig. 1. TEM images of ZrO2 nanoparticle catalysts. a) ZrO2ꢀHT(393), b) ZrO2ꢀHT(433), c)
ZrO2ꢀHT(473), d) ZrO2ꢀHT(493), e) ZrO2ꢀHT(513), and f) dark field image of ZrO2ꢀ
HT(513).
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Fig. 2. XRD patterns of ZrO2 nanoparticle catalysts. a) ZrO2ꢀHT(393), b) ZrO2ꢀHT(433),
c) ZrO2ꢀHT(473), d) ZrO2ꢀHT(493), e) ZrO2ꢀHT(513), f) MesoꢀZrO2, and g) ZrO2ꢀHXꢀC. h)
and i) are calculated patterns of tetragonal and monoclinic ZrO2, respectively.
Figure 2
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0
2
4
6
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10
2Theta/degree
0
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0.2
0.4
0.6
0.8
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P / P0
Fig. 3. Nitrogen adsorption isotherm of MesoꢀZrO2. The inset shows the lowꢀangle
region of the XRD pattern of MesoꢀZrO2.
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Fig. 4. CO2 TPD profiles of catalysts. (a) ZrO2ꢀHT(393), ZrO2ꢀHT(493) and ZrO2ꢀHXꢀC,
b) all samples.
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Fig. 5. In situ IR spectra of CO2 adsorbed on ZrO2ꢀHXꢀC (top), ZrO2ꢀHT(493) (middle),
and ZrO2ꢀHT(393) (bottom). Spectra were taken firstly in the presence of 1 atm CO2,
and then taken after evacuation for 20 min at each temperature. The spectrum of the
ZrO2 catalyst after pretreatment has been subtracted from each spectrum.
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Fig. 6 Relationship between catalytic activity per unit surface area and CO2 TPD peak
height (per unit surface area) at (a) 375 K, (b) 525 K.
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