Synthesis of Zeolitic Mo-Doped Vanadotungstates and Their Catalytic Activity for Low-Temperature NH3-SCR

Article abstract of DOI:10.1021/acs.inorgchem.1c00107

Mo was successfully introduced into a vanadotungstate (VT-1), which is a crystalline microporous zeolitic transition-metal oxide based on cubane clusters [W4O16]8- and VO2+ linkers (MoxW4-x. x: number of Mo in VT-1 unit cell determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)). It was confirmed that W in the cubane units was substituted by Mo. The resulting materials showed higher microporosity compared with VT-1. The surface area and the micropore volume increased to 296 m2·g-1 and 0.097 cm3·g-1, respectively, for Mo0.6W3.4 compared with the those values for VT-1 (249 m2·g-1 and 0.078 cm3·g-1, respectively). The introduction of Mo changed the acid properties including the acid amount (VT-1: 1.06 mmol g-1, Mo0.6W3.4: 2.18 mmol·g-1) and its strength because of the changes of the chemical bonding in the framework structure. MoxW4-x showed substantial catalytic activity for the selective catalytic reduction of NO with NH3 (NH3-selective catalytic reduction (SCR)) at a temperature as low as 150 °C.

Full text of DOI:10.1021/acs.inorgchem.1c00107

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Article
Synthesis of Zeolitic Mo-Doped Vanadotungstates and Their
Catalytic Activity for Low-Temperature NH3‑SCR
Meilin Tao, Satoshi Ishikawa,* Toru Murayama, Yusuke Inomata, Akiho Kamiyama, and Wataru Ueda*
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ABSTRACT: Mo was successfully introduced into a vanadotungstate (VT-1), which is a
crystalline microporous zeolitic transition-metal oxide based on cubane clusters [W4O16]8− and
VO2+ linkers (MoxW4−x. x: number of Mo in VT-1 unit cell determined by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES)). It was confirmed that W in the cubane
units was substituted by Mo. The resulting materials showed higher microporosity compared
with VT-1. The surface area and the micropore volume increased to 296 m2·g−1 and 0.097 cm3·
g−1, respectively, for Mo0.6W3.4 compared with the those values for VT-1 (249 m2·g−1 and
0.078 cm3·g−1, respectively). The introduction of Mo changed the acid properties including the
acid amount (VT-1: 1.06 mmol g−1, Mo0.6W3.4: 2.18 mmol·g−1) and its strength because of the changes of the chemical bonding in
the framework structure. MoxW4−x showed substantial catalytic activity for the selective catalytic reduction of NO with NH3 (NH3-
selective catalytic reduction (SCR)) at a temperature as low as 150 °C.
ment based on the pentagonal [Mo6O21]6− units and MO6 (M
= Mo, V) octahedra that form a crystalline structure with the
micropores of hexagonal and heptagonal channels.
Recently, we reported a new microporous zeolitic transition-
INTRODUCTION
Microporous materials can be exploited for extensive
applications because the open space in the micropores
provides confined chemical reaction fields with various
■
m e t a l o x i d e b a s e d o n v a n a d o t u n g s t a t e ,
(NH4)0.25K1.5H0.25[WVI4VIV3O19]·9.5H2O (denoted as VT-1),
which is composed of [W4O16]8− cubane units and VO2+
linkers.9 We call this material ZOMO (zeolitic octahedral
metal oxide) in short. This material has an open space
accessible for small molecules with the molecular size less than
0.43 nm, which results in the considerably high Brunauer−
Emmett−Teller (BET) surface area (∼300 m2 g−1) despite the
use of heavy atoms as constituent elements.
VT-1-based materials exhibit substantial catalytic activity for
the selective catalytic reduction of NO with NH3 (NH3-SCR)
at low temperatures because of the combination of acidity,
redox properties, and high surface area.9 NH3-SCR is a
significant reaction because NO is the chief inducer of ozone
depletion, acid rain, and global photo-oxidation pollution.10,11
Vanadium (V) oxide-based catalysts (V2O5/TiO2, V2O5-WO3/
TiO2) have been widely used in industries because of the high
N2 selectivity and good thermal stability.12 However, such
catalysts require a working temperature as high as 300 °C.
Therefore, the SCR system has to be placed just after the boiler
followed by the electric precipitator and the deSOx system.
functions such as molecular sieving, molecular recognition,
and shape-selective catalysis. Zeolites are representative
microporous materials, which are thermally stable crystalline
metal oxides with ordered microporous structures and have
high surface areas with solid-acid properties. Zeolites are,
therefore, widely used as catalysts in oil-refining and
petrochemical industries.1−3 Although there are many
structural variants and elemental diversity in the zeolite family,
it is still very hard to introduce redox properties on the basis of
transition elements in the zeolitic porous frameworks. If redox
properties are intrinsically introduced into the porous
framework, the resulting materials can be further utilized in
oxidation catalysis, photochemical reactions, and electro-
chemical reactions. Thus, the discovery and synthesis of
ordered porous crystalline transition-metal oxides is an
interesting and important topic to create novel functional
materials.
Transition-metal elements generally adopt an octahedral
coordination state. The use of octahedral building blocks to
form porous materials is an interesting approach. However, it is
difficult to realize high-dimensional assembling of these
polyoctahedra units.4 Nevertheless, a few microporous
transition-metal oxides have been reported. The most well-
known materials are microporous manganese oxides, so-called
“octahedral molecular sieves” (OMS).5−7 The OMS con-
stitutes infinite chains of edge-shared MnO6 octahedra that
form a one-dimensional (1D) channel in the crystal. Another
typical example is crystalline Mo−V mixed-metal oxides
(Mo3VOx).8 These materials comprise a structural arrange-
Received: January 12, 2021
Published: March 17, 2021
https://doi.org/10.1021/acs.inorgchem.1c00107
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 5081−5086
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Mo0.6W3.4 with a yield of 62.2%, APT (7.5 mmol of W) to AHM (2.5
mmol of Mo) for Mo0.7W3.3 with a yield of 53.4%, APT (7.0 mmol of
W) to AHM (3.0 mmol of Mo) for Mo0.8W3.2 with a yield of 45.1%,
and APT (6.0 mmol of W) to AHM (4.0 mmol of Mo) for Mo1.0W3.0
with a yield of 29.3%. The atomic ratios obtained by ICP and energy-
dispersive X-ray (EDX) analyses are presented in Table S1. It was
assumed that the occupation of W in the cubane unit is four in VT-1.
The total amount of W + Mo should be four. Hereafter, the data were
calculated based on this rule.
The catalyst is directly exposed to SOx and ash, and it is easily
deactivated by the sulfates and ash.13 This problem can be
solved if a catalyst is effective at a temperature lower than 150
°C. Up to now, various catalysts have been prepared and
applied in low-temperature-selective catalytic reduction (LT-
SCR) such as transition-metal oxides (Fe, V, Cr, Cu, Co, and
Mn)14−17 and Cu/Fe ion-exchanged zeolite (Cu-ZSM-5 and
Fe-ZSM-5).18,19 Particularly, manganese-containing catalysts
have attracted much attention because of their variable valence
states and excellent redox ability.20,21 However, these catalysts
exhibit lower N2 selectivity, and they usually produce N2O as a
byproduct.22 Hence, the development of catalysts that show
both good low-temperature activity and high N2 selectivity is of
great importance for the NH3-SCR reaction. VT-1 is exactly
one of the excellent candidates that show high efficiency for
NH3-SCR at low temperatures with 100% N2 selectivity.9
It is known that the properties of transition-metal oxides,
such as redox property, acidity, and magnetic properties are
tunable by combining component elements.23,24 We attempted
to expand the elemental diversity of VT-1 to realize
Characterization. X-ray diffraction (XRD) measurements were
carried out with an Ultima III X-ray diffractometer (Rigaku, Japan)
with Cu-Kα radiation (tube voltage: 40 kV, tube current: 40 mA).
Fourier-transform infrared (FT-IR) spectra were obtained on a
JASCO-FT/IR-4100 instrument (JASCO. Japan). Diffuse reflectance-
ultraviolet−visible (DR-UV−vis) spectra were recorded using a
JASCO V-550 UV−vis spectrophotometer equipped with an ISN-
470 reflectance spectroscopy accessory (JASCO, Japan), and using
BaSO4 as a diluter. X-ray photoelectron spectroscopy (XPS) was
performed on a JPS-9010MC (JEOL) instrument after Au coating
(SC-701 MkII, Sanyu Electron) the specimen. The spectrometer
energies were calibrated using the Au 4f 7/2 peak at 84.0 eV. Raman
spectra (inVia Reflex Raman spectrometer, JASCO) were obtained in
air using a static sample, with an Ar laser (532 nm). The peak position
was calibrated relative to the Si peak at 520 cm−1. The elemental
composition of the bulk sample was measured by ICP-AES (ICPE-
9820, Shimadzu). Temperature-programmed desorption (TPD)
measurements were carried out using a TPD apparatus (BEL Japan,
Inc., Japan) equipped with a quadrupole mass spectrometer. Samples
(50 mg) were placed between two layers of quartz wool. The TPD of
NH3 (m/z = 16) and H2O (m/z = 18) was measured by increasing
the temperature from 40 to 500 °C at a heating rate of 10 °C min−1
under He flow (flow rate: 50 mL min−1). Thermal gravimetry−
differential thermal analysis (TG−DTA) was carried out from room
temperature to 500 °C at a heating rate of 10 °C min−1 under
nitrogen flow (flow rate: 50 mL min−1) on a Thermo Plus EVO2 TG-
DTA8122 (Rigaku, Japan) instrument. Gas adsorption experiments
were carried out on a BELsorp Max system (BEL, Japan). Before the
adsorption experiments, the materials were heat-treated at 175 °C for
2.5 h under vacuum. N2 adsorption measurements were conducted at
the liquid N2 temperature (−196 °C). The surface area and pore
volume were estimated using the Brunauer−Emmett−Teller (BET)
method and t-plot method, respectively. The structures of MoxW4−x
were refined by powder XRD Rietveld refinement using Materials
Studio 7.1 (Accelrys Software Inc.).
multifunctionality. In the present study, we report a series of
IV
Mo-doped vanadotungstates [MoVIxWVI
V
3O19]2−
4−x
(MoxW4−x. x means the substitution amount of W by Mo
determined by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES)) (Figure 1). The redox property,
Figure 1. Structural models of VT-1 and Mo-substituted VT-1. Blue:
W; gray: V; pink: a mixture of Mo and W; red: O; purple: K.
porosity, and acidity of VT-1 were enhanced by the
introduction of Mo, and the Mo-substituted VT-1 materials
exhibited higher efficiency for NH3-SCR at low temperatures.
NH3-SCR. NH3-SCR was conducted over MoxW4−x catalysts using
a fixed-bed reactor at atmospheric pressure. Before the reactions, the
MoxW4−x catalysts were treated at 220 °C under a 250 mL min−1 flow
of Ar for 1 h. After the system cooled to a set temperature, the
reaction gas was introduced at a flow rate of 250 mL min−1. The gas
comprised NO (250 ppm), NH3 (250 ppm), and O2 (4 vol %), with
Ar as a diluter. The catalytic reaction was carried out at set
temperatures of 120 °C. The reactants and products were analyzed
with an FT-IR instrument (FT/IR-4700ST, JASCO) equipped with a
gas cell (LPC-12 M-S, light path length 12 m, JASCO). The standard
NH3-SCR reaction is represented by eq 1. No byproducts such as NO
and N2O were observed during the NH3-SCR, and the selectivity for
N2 was 100%. The respective catalytic parameters were calculated as
follows:
EXPERIMENTAL SECTION
■
Synthesis of [W4V3O19]2− (VT-1). Ammonium paratungstate
hydrate (abbreviated as APT, 2.5533 g, 10 mmol of W) was dissolved
in 15 mL of KOH (2.1784 g, 33 mmol) aqueous solution. After
acidification with 6 mL of H2SO4 (2 M), 10 mL of VOSO4 (2.0571 g,
8 mmol) aqueous solution was added. After adjusting the pH of the
solution to 4.0 by adding NH4OH (28%), the mixed solution was
subjected to the hydrothermal synthesis at 175 °C for 24 h. The crude
solid formed in the hydrothermal reaction was separated by
centrifugation (5000 rpm, 4 min), dispersed in 20 mL of deionized
water, and centrifuged (2000 rpm, 2 min) several times to remove
undesired solids. Green solids were obtained from the upper turbid
liquid by high-speed centrifugation (8000 rpm, 15 min) and dried at
80 °C overnight. The yield of VT-1 was 86.5% based on W. The
yields of these materials were calculated on the basis as the ratio of the
solid weights after centrifugation and the theoretical value based on
the amount of W used.
4NO + 4NH3 + O2 → 4N2 + H2O
(1)
NO conversion [%] = (NO − NOout)/NO × 100
(2)
(3)
in
in
Synthesis of [MoxW4−xV3O19]2− (MoxW4−x). The MoxW4−x
materials were prepared using the same method as that for preparing
VT-1, with different ratios of APT to ammonium heptamolybdate
hydrate (abbreviated as AHM). The W/Mo mole ratios and the
corresponding yields are as follows: APT (9.5 mmol of W) to AHM
(0.5 mmol of Mo) for Mo0.2W3.8 with a yield of 82.5%, APT (9.0
mmol of W) to AHM (1.0 mmol of Mo) for Mo0.4W3.6 with a yield of
74.3%, APT (8.0 mmol of W) to AHM (2.0 mmol of Mo) for
NH3 conversion [%] = (NH3in − NH3out)/NH3in × 100
N2 selectivity [%] = 2 × N2out/[(NO + NH3in
)
in
− (NO + NH3out)] × 100
(4)
out
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RESULTS AND DISCUSSION
Synthesis and Characterization of MoxW4−x. A series of
MoxW4−x catalysts with varying x were synthesized. Figure 2
and further addition of Mo resulted in the formation of
unidentified materials.
■
The lattice parameters of MoxW4−x calculated by the
Reitveld refinement (Figure S4) are shown in Figure 3b. The
lattice parameters decreased continuously with increasing x,
suggesting the introduction of Mo into the VT-1 structure. In
fact, the ionic radius of Mo6+ (0.59 Å) is smaller than that of
W6+ (0.60 Å) in octahedral coordination; thus, the decrease in
the lattice parameters with the introduction of Mo is
reasonable. This result, as well as the results of elemental
analysis, clearly reveal that W in the cubane units is partially
replaced by Mo.
FT-IR, Raman, and DR UV−vis analyses were carried out to
investigate the state of the chemical bonds in the VT-1
structure. The IR band at 975 cm−1 is attributable to the
weakly bound VO in square pyramidal coordination in the
linker, and the band at 900 cm−1 is assigned to the V−Ob−
W(Mo)of bridging oxygen between the cubane and the linker;
the signals at 810 and 590 cm−1 were assigned to corner-
sharing W(Mo)−Oc−W in the cubane (Figure S5a).25−27 The
IR bands at 810 and 590 cm−1 were continuously red-shifted
with x (Figure S5b), clearly supporting the replacement of W
with Mo in the cubane unit. Figure S6a shows the Raman
spectra of MoxW4−x. The peaks at 960 and 910 cm−1 are
assigned to the symmetrical stretching vibration and
asymmetric stretching vibration of the W−Oc−W bond,
respectively.28 The peak at 1005 cm−1 is attributed to V
O.29 A shoulder peak of the Mo−O−W band at 880 cm−1 was
observed in the Raman spectra of MoxW4−x, suggesting the
incorporation of Mo into the VT-1 structure.30−32 The
intensity of the peak attributed to W−Oc−W decreased as
the amount of the Mo dopant increased, particularly when x
exceeded 0.6. Simultaneously, the intensity of the Mo−O−W
band increased with x (Figure S6b).
Figure 2. Changes in the amount of W and V as a function of the
amount of Mo in the MoxW4−x unit.
shows the changes in the amount of W and V as a function of
x. The W/V ratio of pristine VT-1 was 4.0/3.0, which matches
well with the reported value.9 The introduction of Mo was
accompanied by a continuous decrease in the amount of W.
However, the amount of V in the bulk was constant despite the
increase in x, up to x = 1.0, and the (Mo + W)/V ratio of
MoxW4−x remained consistent at 4.0/3.0. When the prepara-
tive composition of Mo to W was increased to 2.0/2.0 (x =
2.0), the elemental composition changed drastically. The
elemental composition became Mo/W/V = 2.0/2.0/2.0,
implying that the obtained material has no longer the VT-1
structure.
The color of MoxW4−x changed by changing the amount of
Mo in solids (Figure S1). VT-1 was green, MoxW4−x (x = 0.6−
0.8) was light brown, Mo1.0W3.0 was dark brown, and the color
of Mo0.2W3.8 and Mo0.4W3.6 was yellow-green, which was the
transition color between VT-1 and Mo0.6W3.4. The morphology
and size of VT-1 and MoxW4−x were studied by scanning
electron microscopy (SEM) (Figure S2). It showed that they
were cube-shaped materials with the diameter of ca. 50 nm. It
showed that the morphology of the materials did not
significantly change after Mo was introduced.
In the case of the DR UV−vis analyses (Figure 4a), the
absorption bands at 390 nm are attributable to the intervalence
XRD measurements were carried out for MoxW4−x. The
XRD peaks at 2θ = 10.3, 14.6, and 17.1° and 2θ = 21.0−36.0°
were attributable to the VT-1 structure,9 and the patterns of
MoxW4−x (x ≤ 1.0) were almost the same as that of pristine
VT-1, except for the small peak shifts and peak broadening,
particularly at x = 1.0 (Figure S3). No XRD peaks attributable
to the VT-1 structure were observed when the elemental
composition of Mo was higher than 1.0, and new XRD peaks
that are not identified appeared (Figure 3a). On the basis of
the ICP and XRD results, we concluded that less than one W
among four W in the cubane unit could be replaced by Mo,
Figure 4. UV−vis spectra of MoxW4−x (a) and absorbance at 390 and
530 nm (b): black: VT-1, red: Mo0.2W3.8, blue: Mo0.4W3.6, green:
Mo0.6W3.4, pink: Mo0.7W3.3, brown: Mo0.8W3.2, and dark-blue:
Mo1.0W3.0.
charge transfer (IVCT) in V4+−O−W6+. After the introduction
of Mo, the adsorption of V4+ at 660 and 860 nm was
constant.9,33 The absorption of the IVCT of the V4+−O−Mo6+
unit clearly increased with x, accompanied by a decrease in the
IVCT derived from V4+−O−W6+ (Figure 4b). This observa-
tion is consistent with the FT-IR data, which further confirmed
that Mo was indeed introduced into the VT-1 structure by
replacing W in the cubane unit.
It was confirmed that the amount of W in VT-1 decreased,
while the amount of Mo increased. There are two possibilities
of the Mo position: (1) W in the cubane units is randomly
Figure 3. XRD patterns of MoxW4−x (a) and lattice parameters after
Rietveld refinement (b).
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replaced by Mo and (2) Mo atoms exist in one cubane unit,
while they do not exist in another cubane unit. In order to
figure out the location site of Mo in the VT-1 structure, we
have built two structure models and carried out XRD
simulation experiments (Figure S7). The XRD patterns
simulated from the structural model (1) were almost the
same with the experimentally obtained XRD patterns.
However, a new peak at 7.3° appeared in the XRD patterns
simulated from the structural model (2), which was not
observed in the experimental samples. The simulated XRD
patterns strongly suggested that Mo atoms just statistically
locate throughout the cubane unit. The redshift observed using
FT-IR, Raman, and UV−vis spectroscopy techniques further
implied that the Mo atoms were statistically introduced in the
W cubane unit, and W−O−V or W−O−W bonds were
affected by Mo. While if Mo was located independently as
cubane, the chemical bonds of W−O−W are considered to be
changed significantly.
formation energies of MoxW4−x were obtained and plotted, as
shown in Figure S14. The lowest formation energy of −711.36
kJ·mol·atom was observed for VT-1. The formation energy
increased gradually as the amount of Mo increased, indicating
that the stability of MoxW4−x decreased continuously with
increasing x. This observation may account for why the
substitution limit is one in four W in the cubane.
N2 Adsorption Properties. The microporosity of
MoxW4−x was investigated by N2 adsorption−desorption
measurements at liquid N2 temperature (Figure 5). All the
MoxW4−x was subjected to XPS in order to investigate the
oxidation states of the constituent elements (Figures S8−10).
The Mo XPS profile showed two peaks at 235.6 and 232.3 eV
assignable to the 3d3/2 and 3d5/2 states of MoVI, respectively.34
The two peaks at 37.8 and 35.8 eV are attributed to the 4f5/2
and 4f7/2 states of WVI, respectively.35 The peak at 517.0 is
derived from VIV (2p3/2).36 The oxidation states of Mo, W, and
V in the cubane unit were constant regardless of the increase in
x, which indicated that the introduction of Mo did not change
the oxidation states of the other constituent elements of VT-1.
The composition of each element calculated from the XPS data
is presented in Table S1. The Mo and W compositions were
consistent with the results of ICP and EDX, which suggests
that the elemental composition of Mo and W in the bulk and
on the surface of the materials is the same. The amount of V
determined by XPS was less, possibly due to the fact that V did
not occupy all the linker on the surface.
Figure 5. N2 adsorption isotherm of MoxW4−x; blank: VT-1, red:
Mo0.2W3.8, blue: Mo0.4W3.6, dark cyan: Mo0.6W3.4, pink: Mo0.7W3.3,
dark yellow: Mo0.8W3.2, and navy: Mo1.0W3.0.
materials showed N2 uptake in the quite low-pressure range
(P/P0 < 10−5), indicating the microporosity of these materials.
The external surface area and the micropore volume of the
materials estimated from the t-plot are shown in Table 1. The
Table 1. External Surface Area and Micropore Volume of
MoxW4−x Based on N2 Adsorption Isotherms and Acid Site
Distribution Based on NH3-TPD
external
micropore
volume
total
surface area
acidity
peak area ratio
samples
(m2 g−1
)
(cm3 g−1
)
(mmol/g) 250 °C/180 °C
Thermal Stability. The TPD analysis of the materials
demonstrated that the removal of H2O occurred at ∼100 °C.
Two peaks corresponding to NH3 desorption were observed at
100 and 300 °C (Figure S11). The TG-DTA indicated about
10% weight loss around 100−200 °C (Figure S12), which is
attributed to the removal of water molecules and NH3 from
MoxW4−x. According to the fact that the weight loss around
100−200 °C can be almost attributable to the desorption of
water (Figure S11), we calculated the chemical formulas of
MoxW4−x based on the results of ICP, TPD, and TG as
follows: Mo0.2W3.8: (NH4)0.25K1.5H0.25[Mo0.2W3.8V3O19]·
6.8H2O; Mo0.4W3.6: (NH4)0.25K1.5H0.25[Mo0.4W3.6V3O19]·
6.1H2O; Mo0.6W3.4: (NH4)0.25K1.5H0.25[Mo0.6W3.4V3O19]·
6.5H2O; Mo0.7W3.3: (NH4)0.25K1.5H0.25[Mo0.7W3.3V3O19]·
5.9H2O; Mo0.8W3.2: (NH4)0.25K1.5H0.25[Mo0.8W3.2V3O19]·
6.2H2O; Mo1.0W3.0: (NH4)0.25K1.5H0.25[Mo1.0W3.0V3O19]·
6.4H2O. The exothermic peak around 300 °C was assigned
to the desorption of NH3, because the MoxW4−x materials
began to collapse above 300 °C. The endothermic peak around
400−450 °C corresponds to the phase transformation between
the two unknown phases. The materials were stable up to 250
°C in N2 without changes in the XRD and FTIR spectra
(Figure S13). Therefore, water can be removed by calcination
without decomposing the material structure at this temper-
ature.
VT-1
49
52
57
60
61
62
64
0.078
0.082
0.084
0.097
0.083
0.072
0.061
1.06
1.24
1.69
2.18
2.02
0.95
0.84
0.51
0.53
0.64
0.82
0.90
1.07
1.23
Mo0.2W3.8
Mo0.4W3.6
Mo0.6W3.4
Mo0.7W3.3
Mo0.8W3.2
Mo1.0W3.0
micropore volume of MoxW4−x increased as x increased up to
0.7. The increase in the micropore volume cannot be explained
by the change in the amount of K+ in the micropores because
the amount of K+ in the structure was almost the same
regardless of x. It is proposed that the slight change in the
location site of K+ in the pore may account for the observed
increase in the microporosity. As implied by geometry
optimization calculations (Figure S15), the distance between
K+ and W, V atoms was 3.614 and 3.514 Å for x = 0.6, which is
smaller than those in VT-1 (3.717 and 3.538 Å). The position
of K+ in Mo0.6W3.4 was closer to that in the cubane unit and
that in VT-1, which left more space in the micropore of
Mo0.6W3.4 to absorb more N2 molecules. Undoubtedly, further
analyses are necessary to clearly understand this improvement.
Further increasing the amount of Mo resulted in a decrease in
the micropore volume. When x exceeded 0.8, the crystallinity
decreased, as indicated by the XRD measurements. This
observation suggests poor micropore texture when x exceeds
The stability of the MoxW4−x materials can also be calculated
theoretically by applying density functional theory (DFT). The
structures of MoxW4−x were optimized by DFT, and the
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0.8, which may be the reason for the decreased microporosity
(Table 1).
acidity increased from 1.06 mmol/g for VT-1 to 2.18 mmol/g
for Mo0.6W3.4. The introduced Mo also showed catalytic
activity for the NH3-SCR at a temperature as low as 150 °C,
with 60% NO conversion and 100% N2 selectivity.
Acidity Characterization. The temperature-programmed
desorption of ammonia (NH3-TPD) is one of the most
conventional methods for characterizing acidity in porous
materials because the small size of ammonia allows the
quantitative analysis of almost all the acid sites in micro-,
meso-, and macroporous oxides.37 The NH3-TPD data for VT-
1 and MoxW4−x are presented in Figures S16−17. The
materials showed two NH3 desorption peaks at 180 and 250
°C after the adsorption of NH3 at 100 °C; the area ratios of the
two peaks are summarized in Table 1. Compared with the
conventional VT-1, the introduction of Mo increased the peak
area for NH3 desorption at 250 °C, which indicates that the
amount of stronger acid sites increased with the incorporation
of Mo. Some chemical bonds in MoxW4−x changed from W−
O−V to Mo−O−V, which may lead to variations in the
electron density of the Mo, W, and V atoms; thus, the total
acidity and acid strength also changed. In addition, the
microporosity of the materials was adjusted by the
introduction of Mo, which exhibited the same trend with the
total acidity. Therefore, the microporosity also made
contributions to the adsorption of NH3.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00107.
■
sı
Color of solids, crystal morphology, XRD patterns,
results of the Rietveld refinement, FT-IR, Raman, XPS,
TPD-MS, TG-DTA, and the elemental analysis of
MoxW4−x (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Satoshi Ishikawa − Faculty of Engineering, Kanagawa
University, Yokohama-shi, Kanagawa 221-8686, Japan;
orcid.org/0000-0003-4372-4108; Email: sishikawa@
kanagawa-u.ac.jp
Wataru Ueda − Faculty of Engineering, Kanagawa University,
Yokohama-shi, Kanagawa 221-8686, Japan;
Email: uedaw@kanagawa-u.ac.jp
Catalytic Activity. NO conversion in the NH3-SCR over
the VT-1 and MoxW4−x (x = 0.4, 0.6) catalysts was measured
at the reaction temperature of 150 °C (Figure 6). The catalytic
Authors
Meilin Tao − Faculty of Engineering, Kanagawa University,
Yokohama-shi, Kanagawa 221-8686, Japan
Toru Murayama − Research Center for Gold Chemistry,
Graduate School of Urban Environmental Sciences, Tokyo
Metropolitan University, Hachioji, Tokyo 192-0397, Japan;
orcid.org/0000-0001-5105-3290
Yusuke Inomata − Research Center for Gold Chemistry,
Graduate School of Urban Environmental Sciences, Tokyo
Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Akiho Kamiyama − Faculty of Engineering, Kanagawa
University, Yokohama-shi, Kanagawa 221-8686, Japan
Figure 6. Catalytic activity of VT-1 and MoxW4−x (x = 0.4, 0.6) for
NH3-SCR at 150 °C. Reaction conditions: NO: 250 ppm, NH3: 250
ppm, 4 vol % of O2 in Ar, flow rate: 250 mL/min, catalysts: 375 mg.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00107
Author Contributions
activity of the materials was constant over the entire reaction
time studied, and the XRD patterns did not change
significantly after the reactions (Figure S18). No formation
of byproducts such as NO and N2O during the NH3-SCR was
observed; thus, the selectivity for N2 was 100%. The NO
conversion over VT-1 was around 50%. MoxW4−x (x = 0.4,
0.6) also promoted NO conversion at 150 °C, and the activity
was slightly higher than that of VT-1 (NO conversion = ca.
60%). There are many considerable reasons about the
improvement of the catalytic activity, including the change of
the acid properties, microporosity as catalysis field, oxidation
state during the reaction, and so forth. The critical factors for
the improvement of the catalytic activity are currently under
investigation based on the study of the reaction mechanism.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Number
19H00843.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Number
19H00843.
REFERENCES
■
CONCLUSIONS
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