Sequentially prepared Mo-V-Based SCR catalyst for simultaneous Hg0 oxidation and NO reduction

Article abstract of DOI:10.1016/j.apcata.2021.118032

Molybdenum (Mo)-vanadium (V)-based selective catalytic reduction (SCR) catalyst synthesized by the sequential impregnation of Mo and W followed by V was investigated for simultaneous elemental mercury (Hg⁰) oxidation and nitrogen oxide (NO) red

Full text of DOI:10.1016/j.apcata.2021.118032

Applied Catalysis A, General 614 (2021) 118032
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Sequentially prepared Mo-V-Based SCR catalyst for simultaneous Hg0
oxidation and NO reduction
a
a
a
b
a,
Can Li , Vishnu Sriram , Zhouyang Liu , Dale Brewe , Joo-Youp Lee *
a
Chemical Engineering Program, Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221-0012, United States
b
X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Molybdenum (Mo)-vanadium (V)-based selective catalytic reduction (SCR) catalyst synthesized by the sequential
Mercury oxidation
Nitrogen oxide reduction
SCR catalyst
0
impregnation of Mo and W followed by V was investigated for simultaneous elemental mercury (Hg ) oxidation
and nitrogen oxide (NO) reduction in an existing SCR unit with respect to different TiO
2
phases, calcination
and the
temperatures, flue gas constituents, gas velocities, and reaction temperatures. Anatase phase TiO
2
Calcination temperature
Bimetallic catalyst
calcination temperatures of 400 and 500 C resulted in ~69% Hg0 oxidation at 10 ppmv HCl and 350 C. The
◦
◦
◦
high calcination temperature of 700 C resulted in TiO
2
phase transformation from anatase to rutile and
agglomeration. The modified SCR catalyst prepared with the impregnation sequence of Mo and W followed by V
◦
using anatase TiO
conversions at an NH
bituminous and lignite coal simulated flue gases. The effects of these parameters and conditions were further
investigated using various characterization techniques including BET, TEM, XRD, NH TPD and XAFS.
2
and calcination temperature 500 C showed ~99% Hg0 oxidation and 87% NO reduction
◦
ꢀ 1
/NO molar ratio of 0.9 under 350 C and 5,000 hr space velocity in typical sub-
3
3
1
. Introduction
injection, fuel additives, selective catalytic reduction (SCR) optimiza-
tion, and Hg2 separation from wet FGD system. Sorbent injection uses
+
0
2+
Coal-fired power plants are considered to be responsible for over
raw or chemically-modified activated carbon to collect Hg and Hg
3
0% (>44 tonnes) mercury emissions from all kinds of anthropogenic
[2–4]. However, the cost is expensive [5]. Fuel additives including
chlorine and bromine salts can be added to the power plant boiler to
improve the Hg0 oxidation capability [6–8]. However, it generally re-
quires high halogen addition and can also cause a corrosion problem in
the air preheater. Previous studies demonstrated that ~15% to 68% of
sources every year in the United States [1]. There exist three different
types of mercury in coal combustion exhaust gas including
p
2+
0
p
particulate-bound (Hg ), oxidized (Hg ), and elemental (Hg ) [2]. Hg
2
+
and Hg can be easily separated using current conventional air pollu-
tion control systems, such as baghouse filter, electrostatic precipitator
0
2+
Hg vapor could be converted into Hg form over SCR catalyst with
HCl. [9–11] Currently, SCR units and wet FGD systems have been
equipped in most coal-fired power plants. The number of SCR and wet
FGD unit installations has increased due to the Cross State Air Pollution
(
ESP) and wet flue gas desulfurization (FGD) [3]. Nevertheless, it is very
0
challenging to separate Hg species because it is highly volatile, very low
in concentrations, and nearly insoluble in water. The Mercury and Air
Toxics Standards (MATS) rule issued in 2012 the mercury emissions
limits of 1.2 and 0.015 lb/TBtu (or 0.013 and 0.0002 lb/GWh) from
existing and new coal-fired power plants, respectively. These regulation
limits for existing and new coal-fired power plants are equivalent to
x 2
Rule (CSAPR) regulating NO and SO emissions more stringently from
fossil fuel-fired power plants in 27 states from 2012 in the United States.
0
2+
+
The Hg to Hg conversion over the SCR unit provides an opportunity
to capture Hg2 in a downstream wet FGD system to lower both mercury
and NO emissions.
-WO or V
commercial SCR catalysts to reduce NO
~
90+ and ~99+% on a monthly rolling average basis, respectively.
x
0
Therefore, the separation of Hg is imperative to successfully comply
with the current stringent regulations.
V
2
O
5
3
2
O
5
-MoO
3
supported on TiO
2
are widely used as
x
[12–14]. Previous studies on
0
0
Existing mercury control technologies in the market include sorbent
Hg oxidation over V
2
O
5
-WO
3 2
/TiO reported limited Hg oxidation
*
Corresponding author at: Chemical Engineering Program, Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio,
5221-0012, United States.
E-mail addresses: li2c2@mail.uc.edu (C. Li), sriramvu@mail.uc.edu (V. Sriram), liuzy@mail.uc.edu (Z. Liu), brewe@aps.anl.gov (D. Brewe), joo.lee@uc.edu
J.-Y. Lee).
4
(
https://doi.org/10.1016/j.apcata.2021.118032
Received 17 June 2020; Received in revised form 30 January 2021; Accepted 1 February 2021
Available online 5 February 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
capability under the condition of low HCl concentrations in
sub-bituminous and lignite coal combustion flue gases. [9,15,16] To
enhance the Hg0 oxidation capability under low HCl concentrations,
additional active metals such as Mn, Ce, Ru, Cu, and Fe have been
attempted to optimize conventional SCR catalyst formulas [17–24].
However, all the above modified catalysts need additional equipment
supported metal oxide catalysts due to its simplicity and the ease of
scalability [30]. Anatase TiO
2
(DT-51, from Cristal), mixed anatase/r-
(STR-100W, from
utile TiO (P25, from Sigma Aldrich) and rutile TiO
2
2
Sakai Chemical) were selected as the catalyst supports, respectively.
◦
Four different temperatures 400, 500, 600 and 700 C were chosen as
the calcination temperatures, respectively. The detailed synthesis pro-
cedures are shown in our previous publication [28]. The previous work
demonstrated that the Mo-V-based SCR catalyst containing 0.5 wt.%
installations or changes in different operating temperatures. A few
0
studies have been taken to explore Hg oxidation over V
2
O
5
-MoO
3
/TiO
2
.
A V
2
O
5
(5%)-MoO
3
(5%)/TiO
2
catalyst synthesized using a sol-gel tech-
V
2
O
5
, 7 wt.% MoO
3
and 3 wt.% WO
3
with the impregnation sequence of
(3)→V (0.5)/TiO ) had
0
nique reported an up to 28% improvement in Hg oxidation activity
Mo and W followed by V (i.e. MoO
3
(7)+WO
3
2
O
5
2
relative to an in-house V
2
O
5
(5%)/TiO
2
catalyst under 6%(v) O
2
and no
the best activity. Therefore, the same impregnation sequence and
loading percentages of the metals have been used to prepare the modi-
◦
HCl at 350 C [25]. A previous study using V
2
O
5
(5%)-MoO (5%)/TiO
3
2
0
reported that HCl and NO gases could positively affect Hg oxidation
activity while H O, SO , and NH gases showed an inhibitory effect [26].
The Hg oxidation activity of the catalyst was higher than that of a fresh
commercial SCR catalyst (V (3.30%)-WO (2.27%)/TiO ) regardless
of NH under actual flue gas conditions [27].
fied SCR catalyst samples with different TiO
temperatures. The synthesized samples were designated as
MoO /TiO A B, where A and B represented TiO support
2
supports and calcination
2
2
3
0
3
+WO
3
→V
2
O
5
2
2
2
O
5
3
2
and calcination temperature, respectively. Co-impregnation method was
utilized to synthesize V-based conventional SCR catalyst with 0.5 wt.%
3
Our previous study reported that the modified SCR catalyst prepared
using the impregnation sequence of Mo and W followed by V exhibited
V
2
O
5
and 9 wt.% WO
3
. This sample was denoted as V
2
O
5
+WO
3 2
/TiO
DT-51 CT500. The pelletized catalyst was employed to conduct the
catalytic activity test. The powdered catalyst was employed to carry out
characterization analysis after grinding the pelletized catalyst.
~
69% Hg0 oxidation activity at 10 ppmv HCl and 350 C [28]. Mean-
◦
while, the NO reduction activity was ~80% with respect to various
metal loadings and impregnation sequences. The objective of this study
0
is to examine the simultaneous Hg oxidation and NO reduction in terms
2
.2. Activity tests
2
of different TiO phases, calcination temperatures, flue gas constituents,
gas velocities, and reaction temperatures over our Mo-V-based SCR
catalyst. Different characterization techniques were employed to un-
derstand potential relationships between the different preparation
conditions and catalytic activities. The study results can provide
fundamental understanding and scientific insights for simultaneous
The catalytic activity tests of the Mo-V-based SCR catalyst in terms of
2
different TiO supports and calcination temperatures were conducted in
0
a fixed-bed reactor system as illustrated in Fig. 1 to evaluate their Hg
0
oxidation and NO reduction efficiencies. Hg vapor was brought into the
system by introducing 100 mL/min N
tube (VICI Metronics) located in a calibration gas generator oven
Dynacalibrator, VICI Metronics) and ~15 ppbv Hg0 was used as a
2
through a mercury permeation
x
mercury and NO emissions control in an existing SCR system.
(
2
. Experimental
typical inlet concentration. All of the simulated flue gas constituents
regulated through mass flow controllers were produced using gas cyl-
inders and specified in Results and Discussions part. Water vapor was fed
into the system through controlling the temperature of a water bubbler.
The total flow rate of the simulated coal combustion flue gas was fixed at
2
.1. Catalyst synthesis
In this work, our Mo-V-based SCR catalyst was synthesized using
incipient wetness impregnation with different TiO
2
supports and calci-
1
L/min. Typically, a quartz reactor with a 16 mm inner diameter
nation temperatures [29]. It is the most widely cited method for
containing ~1.72 g of catalyst pellets was used to conduct the activity
Fig. 1. Schematic of lab-scale fixed-bed reactor set-up.
2

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
tests. The quartz reactor was placed in the center of a vertical oven
equipped with a temperature controller. These tests were run under a
tested in fluorescence mode. Each sample was run at least 8 scans to
enhance the signal-to-noise ratio.
ꢀ
1
◦
gas hourly space velocity (GHSV) of 40,000 hr at 350 C to examine
the catalytic activities of Hg0 oxidation and NO reduction in terms of
3. Results and Discussions
2
different TiO supports and calcination temperatures [16]. When the
0
effect of GHSV on the activities was investigated, a different catalyst
amount was used to set the GHSV inside the reactor.
3.1. Effects of preparation parameters on activities of Hg oxidation and
NO reduction
During all the catalytic activity tests, a cold vapor atomic absorption
spectrophotometer (CVAAS, Model 400A, Buck Scientific Inc.) com-
bined with Ontario Hydro Method was used to measure mercury con-
centrations at the inlet and outlet of the system. Two different impinger
3.1.1. Effects of TiO2 supports
TiO is a widely used substrate resistant to metal sulfate formation in
2
coal combustion flue gas containing SO . TiO -based catalysts are stable
2
2
solutions: 1 M KCl and 4%(w/v) KMnO
4
/10% (v/v) H
2
SO
4
were
and has a resistance to sulfation in the SCR process [31,32]. To explore
employed to collect oxidized mercury and Hg0 vapor, respectively. A
Micro Emission Analyzer (Model 500, Enerac LLC) was employed to
determine NO inlet and outlet concentrations. The activities of Hg0
oxidation and NO reduction were calculated based on the inlet and
0
the effects of different TiO phases on simultaneous Hg oxidation and
2
NO reduction, the three different TiO supports of anatase DT-51, 75 wt.
2
% anatase/25 wt.% rutile P25, and rutile STR-100W were selected.
0
The activities of simultaneous Hg oxidation and NO reduction over
0
outlet Hg and NO concentrations as shown below.
the Mo-V-based SCR catalysts with different TiO2 phases were tested
under a simulated lignite and sub-bituminous coal combustion flue gas
0
0
Hg -Hg
0
in
out
0
Hg oxidation activity =
× 100%
(1)
(2)
condition and the activity results are exhibited in Fig. 2. For Hg
0
Hg
in
2
oxidation, among these three catalysts, anatase TiO DT-51 supported
Mo-V-based SCR catalyst exhibited the highest Hg0 oxidation activity
(~53% at 5 ppmv HCl and ~69% at 10 ppmv HCl) and NO reduction
NOin-NOout
NOin
NO reduction activity =
× 100%
0
activity (~80%). When P25 was employed as a support, Hg oxidation
activity decreased to ~32% at 5 ppmv HCl and ~56% at 10 ppmv HCl,
2
.3. Catalyst characterization
The Brunauer-Emmett-Teller (BET) surface area, pore volume and
pore diameter of the Mo-V-based SCR catalysts were determined at 77 K
by nitrogen adsorption using Micromeritics ASAP 2020. The catalyst
morphology was analyzed using a FEI CM 20 transmission electron
microscope (TEM). Isopropyl alcohol (IPA) was utilized to spread the
ground catalyst to ensure that the catalyst powder was uniformly
dispersed over a copper grid for TEM analysis.
The X-ray powder diffraction (XRD) analysis was conducted under an
X’Pert Pro MPD X-ray diffractometer to measure the crystalline structure
of the samples. The XRD diffractometer was equipped with Cu K ra-
α
diation (λ = 0.1543 nm). The ground catalyst powder was packed into a
sample holder. The sample was scanned under a step time of 0.5 s in a
◦
◦
◦
range of 10 to 60 (2θ) with a step size of 0.02 .
Temperature-programmed desorption of NH
3
3
(NH -TPD) was con-
ducted using AutoChem 2910 to determine the surface acidity of the
sequentially modified SCR catalysts on different substrates. In a typical
◦
pretreatment process, ~100 mg catalyst was heated up to 500 C with a
◦
ramp rate of 20 C/min and held at this temperature for 1 hr under 20
mL/min He. Then, the sample was cooled down to room temperature
and saturated with 4% NH
At the end of the saturation procedure, pure He at 20 mL/min was used
to flush over the sample for 30 min to remove weakly adsorbed NH . The
thermal conductivity detector (TCD) signal was recorded while heating
3
balanced with He at 30 mL/min for 30 min.
3
◦
◦
the sample up to 500 C at a ramp rate of 15 C /min in pure He for NH
desorption.
3
X-ray absorption near-edge structure (XANES) spectroscopy was
conducted at Argonne National Laboratory (ANL, Chicago, IL). The data
were taken on the 20-BM-B Beamline from the Advanced Photon Source
(
APS) with a Si (111) monochromator. The energy scale was calibrated
at 5,465 eV by a V metal foil. Three different vanadium references
containing V , VO and V were used to carry out the energy scale
2
O
5
2
2 3
O
calibration. These references were finely pulverized using a mortar and
evenly dispersed over Kapton tape. Then the folded tape was analyzed
under transmission mode. Each sample was run at least three times to
enhance the signal-to-noise ratio. Since the V concentration on synthe-
sized SCR catalysts was low and the interference between V and Ti, a
larger amount of sample was packed in a sample holder. To analyze V in
0
Fig. 2. Activity test results of Mo-V-based SCR catalysts for Hg oxidation (a)
2
and NO reduction (b) in terms of different TiO supports (DT-51*, P25 and STR-
fresh modified SCR catalysts with different TiO
2
supports and calcina-
0
1
00W). Experimental conditions: 15 ppbv Hg , 300 ppmv NO, 270 ppmv NH
3
tion temperatures, these modified SCR samples were finely pulverized
using a mortar and put into a Teflon holder. Then, the samples were
(molar ratio of NH3 to NO = 0.9), 200 ppmv SO , 3%(v) O , 10%(v) H O, 12%
2
2
2
at T = 350 ◦C and GHSV = 40,000 hr . Note: * [28].
ꢀ 1
2 2
(v) CO balanced with N
3

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
◦
respectively. In the meantime, the NO reduction activity was kept the
same at ~80% for both supports. When rutile STR-100W was used, the
catalyst almost lost its Hg0 oxidation ability and also showed the
significantly decreased NO reduction activity of ~49%. This result
activity of ~80% at all calcination temperatures except at 700
C
showing the lowest NO reduction activity of ~54%. Therefore, lower
◦
temperature of 400 or 500 C would be an optimal calcination tem-
perature for synthesizing the Mo-V-based SCR catalyst. The effects of
calcination temperatures were studied using multiple characterization
techniques in Section 3.3.
shows that anatase phase TiO
2
should be used as a support for simul-
0
taneous Hg and NO removal.
0
3
.1.2. Effects of calcination temperatures
Calcination temperature is an important factor during SCR catalyst
3.2. Effects of experimental conditions on activities of Hg oxidation and
NO reduction
preparation. Because calcination temperature can affect phase trans-
formation and active metal dispersion, it can influence SCR catalyst
activity [33–35]. To probe the effect of calcination temperatures on
The activities of MoO +WO →V O /TiO DT-51 CT500 prepared
3
3
2
5
2
◦
using anatase TiO2 DT-51 as the support and 500 C as the calcination
temperature were investigated in terms of flue gas constituents, gas
velocities, and reaction temperatures using a fixed-bed reactor system.
0
simultaneous Hg oxidation and NO reduction, four different calcination
temperatures were selected. The activities of simultaneous Hg0 oxida-
tion and NO reduction over the Mo-V-based SCR catalysts with different
calcination temperatures were tested under a simulated flue gas and the
test results are exhibited in Fig. 3. For Hg0 oxidation, when the calci-
3.2.1. Effect of O2
O plays a significant part during the catalytic process. The effects of
2
nation temperatures were 400 and 500 C, it showed the highest Hg0
◦
0
O on simultaneous Hg oxidation and NO reduction are shown in Fig.4.
2
0
oxidation activity (~53% at 5 ppmv HCl and ~69% at 10 ppmv HCl).
With an O2 concentration increasing from 3 to 9%(v), Hg oxidation
With calcination temperature increasing to 600 C, the Hg0 oxidation
◦
activity enhanced from 69% to 81%. It demonstrated that O was crucial
2
0
conversion was reduced to 39% at 5 ppmv HCl and 55% at 10 ppmv HCl,
to Hg oxidation and had a positive effect. This result clearly shows that
◦
0
respectively. Once the calcination temperature was enhanced to 700 C,
O is required for Hg oxidation. However, considering a typical range of
2
0
the modified catalyst almost lost the capability to oxidize Hg . For NO
~3ꢀ 6% O2 concentrations existing in coal combustion flue gas, the
reduction, all of the catalysts showed the comparable NO reduction
impact of O2 concentration was not significant. Meanwhile, NO reduc-
2
tion activity was ~80% with respect to different O concentrations.
0
0
Fig. 3. Activity test results of Mo-V-based SCR catalysts for Hg oxidation (a)
Fig. 4. Effect of O
2
on simultaneous Hg oxidation (a) and NO reduction (b)
and NO reduction (b) in terms of different calcination temperatures (400, 500,
over Mo-V-based SCR catalyst MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT500. Experi-
(molar ratio of
O, 12%(v) CO
2
◦
0
0
6
00 and 700 C). Experimental conditions: 15 ppbv Hg , 300 ppmv NO, 270
mental conditions: 15 ppbv Hg , 300 ppmv NO, 270 ppmv NH
3
ppmv NH
3
(molar ratio of NH
3
to NO = 0.9), 200 ppmv SO
2
, 3%(v) O
at T = 350 ◦C and GHSV = 40,000 hr
2
, 10%(v)
NH
3
to NO = 0.9), 200 ppmv SO
2
, 10 ppmv HCl, 10%(v) H
2
ꢀ 1
at T = 350 ◦C and GHSV = 40,000 hrꢀ 1
H
2
O, 12%(v) CO
2
balanced with N
2
.
balanced with N
2
.
4

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
3
.2.2. Effect of SO
When SO concentration was raised from 200 to 2000 ppmv, the
2
2
0
catalytic activities of Hg oxidation and NO reduction did not signifi-
cantly change as shown in Fig.5. This means that the modified SCR
catalyst is very robust and can be used in a wide range of SO
concentrations.
2
3
.2.3. Effect of molar ratio of NH
NH
/NO molar ratio is an important factor affecting Hg0 oxidation
and NO reduction activities because NH can competitively adsorb with
HCl. A ratio between 0 and 1.1 is used in the field to minimize an NH
slip of <5 ppmv to avoid the formation of ammonium sulfate [36,37].
The effects of NH /NO molar ratio on the simultaneous removal activ-
ities of Hg and NO over MoO →V /TiO DT-51 CT500 catalyst
+WO
were tested under a simulated flue gas and the test results are exhibited
in Fig. 6. When an NH /NO molar ratio increased over the range of 0.3 to
.9, the Hg oxidation activity was almost constant or slightly decreased
3
to NO
3
3
3
3
0
3
3
2
O
5
2
3
0
0
from 73% to 69% (Fig. 6 (a)). However, with a further increase from 0.9
0
to 1.1, the Hg oxidation activity significantly started to decrease from
6
0
9
3
9% to 59%. On the other hand, as NH /NO molar ratio increased from
to 1.1, the NO reduction activity almost linearly enhanced from 0 to
5%. The NO reduction activity is found to be almost primarily depen-
3
dent on the NH /NO molar ratio and to be insensitive to the rest of the
parameters studied above.
0
Fig. 6. Effect of NH
3
/NO molar ratios on simultaneous Hg oxidation (a) and
NO reduction (b) over MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT500. Experimental
, 3%(v) O , 10 ppmv
0
conditions: 15 ppbv Hg , 300 ppmv NO, 200 ppmv SO
2
2
HCl, 10%(v) H
2
O, 12%(v) CO
2
balanced with N
2
at T = 350 ◦C and GHSV =
ꢀ
1
4
0,000 hr .
3
.2.4. Effect of GHSV
The effects of GHSV on the simultaneous removal activities of Hg0
+WO →V /TiO DT-51 CT500 catalyst were
tested under the same combustion flue gas conditions and the test results
and NO over MoO
3
3
2
O
5
2
ꢀ 1
are exhibited in Fig. 7. With a decrease in the GHSV from 40,000 hr to
ꢀ 1
0
5
,000 hr , the Hg oxidation activities enhanced from 53% to 93% at 5
ppmv HCl and from 69% to 99% at 10 ppmv HCl under the flue gas
conditions. On the other hand, the NO reduction activities slightly
increased from ~80% to 87% over a range of 5–10 ppmv HCl. It shows
0
that the space velocity has an obvious impact on Hg oxidation.
3
.2.5. Effect of temperature
A typical operating temperature window of the SCR unit is between
◦
3
00ꢀ 400 C in coal-fired power plants. The activities of simultaneous
0
Hg oxidation and NO reduction over the Mo-V-based SCR catalyst were
◦
tested between 300 and 400 C and the test results are exhibited in
◦
◦
0
Fig. 8. With an increase in temperature from 300 C to 400 C, Hg
oxidation activities was reduced from 81% to 38%. It indicated that in
the typical SCR operating temperature window, lower temperature was
0
beneficial to oxidize Hg . Meantime, the NO reduction activities of this
0
Mo-V-based SCR catalyst was quite constant at ~80% within the same
Fig. 5. Effect of SO
over MoO →V
+WO
Hg , 300 ppmv NO, 270 ppmv NH
2
on simultaneous Hg oxidation (a) and NO reduction (b)
/TiO DT-51 CT500. Experimental conditions: 15 ppbv
(molar ratio of NH
O, 12%(v) CO balanced with N
temperature range. An in-house V-based SCR catalyst V
2
O
5
+WO
3 2
/TiO
3
3
2
O
5
2
0
DT-51 CT500 was also evaluated to compare with the Mo-V-based SCR
3
3
to NO = 0.9), 3%(v) O
2
,
0
at T = 350 ◦C and
catalyst. The conventional V-based SCR catalyst showed lower Hg
1
0 ppmv HCl, 10%(v) H
2
2
2
GHSV = 40,000 hrꢀ
1
.
oxidation activities within a typical SCR operating temperature window
5

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
0
Fig. 7. Effect of GHSV on simultaneous Hg oxidation (a) and NO reduction (b)
over MoO →V /TiO DT-51 CT500. Experimental conditions: 15 ppbv
+WO
Hg , 300 ppmv NO, 270 ppmv NH (molar ratio of NH
SO , 3%(v) O , 10%(v) H O, 12%(v) CO balanced with N
0
Fig. 8. Effect of reaction temperatures on simultaneous Hg oxidation (a) and
3
3
2
O
5
2
0
NO reduction (b) over Mo-V-based SCR catalyst MoO +WO →V O /TiO DT-
3
3
2
5
2
3
3
to NO = 0.9), 200 ppmv
at T = 350 ◦C.
51 CT500 and V
2
O
5
+WO
3 2
/TiO DT-51 CT500. Experimental conditions: 15
2
2
2
2
2
0
ppbv Hg , 300 ppmv NO, 270 ppmv NH
3
(molar ratio of NH
O, 12%(v) CO
3
to NO = 0.9), 200
balanced with N
ppmv SO , 3%(v) O , 10 ppmv HCl, 10%(v) H
2
2
2
2
2
◦
0
between 300 and 400 C. The activities difference in Hg oxidation for
these two catalyst samples decreased with an increase in temperature.
Lower temperature could facilitate more HCl adsorption onto the cata-
ꢀ 1
.
and GHSV = 40,000 hr
0
lyst surface and thus Hg oxidation could be kinetically limited by HCl
Table 1
adsorption [24]. The conventional V-based SCR catalyst also showed a
comparable NO reduction activity at ~80%.
BET surface area, pore volume and average pore diameter measurements for Mo-
V-based SCR catalysts.
Catalyst
BET surface
Pore volume
Average pore
diameter (nm)
2
3
area (m /g)
(cm /g)
3
3
.3. Characterization
TiO
TiO
TiO
2
2
2
(DT-51)*
(P25)
72
51
67
65
0.38
0.31
0.37
0.26
20.2
24.8
22.4
15.7
.3.1. BET
(STR-100W)
Since the activities were significantly different with respect to
MoO
3
+WO
3
→V
2
O
O
O
5
5
5
/TiO
/TiO
/TiO
2
2
2
2
2
2
DT-51 CT500*
2
different TiO supports and calcination temperatures, the BET surface
MoO
3
+WO
3
→V
2
2
42
62
69
48
5
0.27
0.33
0.25
0.21
0.01
26.2
27.7
14.3
17.7
8.9
area, pore volume and average pore diameter were examined by nitro-
gen adsorption-desorption isotherms as listed in Table 1. The catalyst
P25 CT500
MoO →V
+WO
3
3
samples with different TiO
2
supports showed slightly lower surface areas
STR-100W CT500
and pore volumes after doping metals. The loadings of V, Mo and W
metals only insignificantly vary the surface areas and pore values of the
MoO
DT-51 CT400
MoO →V
+WO
DT-51 CT600
MoO →V
+WO
DT-51 CT700
3
+WO
3
→V
2
2
2
O
5
O
5
O
5
/TiO
/TiO
/TiO
3
3
2
Mo-V-based SCR catalysts with different TiO supports. These BET sur-
2
face area and pore volume values were still in the range of 36ꢀ 120 m /g
3
3
3
and 0.17ꢀ 0.31 cm /g of typical SCR catalysts, respectively [11,15,38].
The SCR catalyst sample prepared onto P25 had a surface area lower
than that onto DT-51. The SCR catalyst sample onto rutile STR-100W
had surface area, pore volume and size comparable to that onto
anatase DT-51 although its activities were much lower (Fig. 2). There-
Note:
*
[
28]; CT: calcination temperature.
2
fore, anatase phase TiO was selected as a support for our Mo-V-based
6

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
SCR catalyst.
particles formed over the samples. With the calcination temperature
◦
When the calcination temperature of the samples onto anatase DT-51
increasing to 700 C, the TiO
2
support particle dimension dramatically
◦
was 400 and 500 C, the surface areas, pore volumes, and pore sizes
increased from ~30 nm to ~180 nm. Meanwhile, the small grain par-
ticles on the catalyst surface disappeared, suggesting that the calcination
were not significantly reduced. With the calcination temperature
◦
◦
increasing to 600 C, they started to significantly decrease. At the
temperature of 700 C led to the agglomeration of the support and the
◦
calcination temperature of 700 C, they were almost close to zero. These
active metals. This is consistent with the very low surface area and pore
◦
results lead to additional examinations by images and X-ray-based
techniques.
volume of the sample prepared at 700 C. The poor catalytic activities
are very likely attributed to this agglomeration phenomenon taking
◦
place at 700 C.
3
.3.2. TEM
The images of TEM from the raw TiO
2
supports and Mo-V-based SCR
3.3.3. XRD
catalysts based on different TiO
2
supports are shown in Fig. 9. After
2
The XRD patterns of the raw TiO (anatase DT-51, 75% anatase/25%
doping onto the support, the modified SCR catalyst samples retained
almost the same physical structure as the raw supports, which was
consistent with BET results. From the TEM images, some grain particles
were found from the catalyst surfaces for all the three modified SCR
catalysts (b, d, f). Since the loading quantity of V or W metals were
rather lower than those needed to generate the theoretical monolayers,
these grain particles are most likely derived from Mo as observed from
previous studies. [28,39,40]
rutile P25, and rutile STR-100W) supports and the catalyst samples
◦
synthesized at 500 C are exhibited in Fig. 11. All the TiO
2
supports
demonstrated their XRD patterns consistent with the information pro-
vided from the manufacture. All of the Mo-V-based SCR catalysts did not
show any visible peaks for the active metals, suggesting that they were
likely in the amorphous phase or evenly spread over the TiO
2
support
beyond the detection limit of XRD technique. [12]
The XRD patterns of the Mo-V-based SCR catalysts prepared on the
anatase DT-51 TiO support with respect to different calcination tem-
The TEM images of raw anatase DT-51 TiO
2
support and Mo-V-based
2
SCR catalysts in terms of different calcination temperatures are exhibi-
ted in Fig. 10. The Mo-V-based SCR catalysts showed the morphological
peratures are exhibited in Fig. 12. For the Mo-V-based SCR catalysts
◦
synthesized at 400, 500 and 600 C, no visible peaks for V
2
O
5
, MoO
3
,
structure similar to the raw TiO
2
support when the calcination tem-
and WO
3
were detected, suggesting the amorphous phase or uniform
◦
perature was 400, 500 or 600 C. The only difference was visible grain
dispersion. Nevertheless, with the calcination temperature increasing to
Fig. 9. TEM images of Mo-V-based SCR catalysts in terms of different TiO
d) MoO →V /TiO P25 CT500; (e) TiO (STR-100W); (f) MoO
+WO
2
supports: (a) TiO
2
(DT-51)*; (b) MoO
3
+WO
3
→V
2 5 2 2
O /TiO DT-51 CT500*; (c) TiO (P25);
(
3
3
2
O
5
2
2
3
+WO
3
2 5 2
→V O /TiO STR-100W CT500. Scale bars: 20 nm. Note: * [28].
7

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
Fig. 10. TEM images of Mo-V-based SCR catalysts in terms of different calcination temperatures: (a) TiO
2
(DT-51)*; (b) MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT400; (c)
/TiO DT-51
MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT500*; (d) MoO
3
+WO
3
→V
2
O
5
/TiO
2
3
DT-51 CT600; (e) MoO +WO
3
→V
2
O
5
/TiO DT-51 CT700; (f) MoO
2
3
+WO →V
3
2
O
5
2
CT700. All the scale bars are 20 nm except for (e) with 200 nm. Note: * [28].
◦
◦
◦
◦
◦
◦
◦
7
00 C, the peaks at 27 , 36 , 41 , 44 , 54 and 57 belonging to the
the one with STR-100W was thermally unstable within a typical SCR
◦
rutile phase were visible, indicating the transition from the anatase to
rutile phase happened due to the high calcination temperature [33–35].
The phase transformation from anatase to rutile seems to lead to the
operating temperature range of ~300ꢀ 400 C and only a slight amount
of NH
broader NH
A total amount of NH
3
was desorbed in this temperature range. When DT-51 was used, a
3
desorption spectrum at higher temperatures was obtained.
desorbed increased to ~0.23 mmol/g catalyst.
◦
major activities difference shown in Fig. 3. A peak at 23 was from
3
◦
MoO
3
. At 700 C, small MoO
3
grain particles agglomerated into larger
More importantly, the sample showed higher thermal stability for
◦
sizes, which was detectable by XRD. This could also lead to a negative
adsorbed NH
3
between 300 and 400 C, which could be used to reduce
0
impact on Hg oxidation and NO reduction.
gaseous NO in the typical SCR operating temperature range. When P25
TiO with 75% anatase and 25% rutile phases was used, the desorption
spectrum showed a characteristic between anatase DT-51 and rutile
STR-100W substrates with an estimated amount of NH desorbed of
2
3
.3.4. NH
The adsorption of NH
during SCR process. Therefore, the surface acidity of the samples with
different substrates was investigated using NH -TPD as shown in Fig. 13.
3
TPD
3
onto the catalyst surface is an important step
3
~0.18 mmol/g catalyst. More specifically, the desorbed amount of NH
3
◦
3
at 350 C were in the order of MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT500 >
/TiO STR-
For all of the three modified SCR catalysts with different TiO
2
supports,
MoO
3
+WO
3
→V
2
O
5
/TiO
2
P25 CT500 > MoO
3
+WO →V
3
2
O
5
2
◦
they all started to desorb NH
the NH
decreased. Overall, the desorption of NH
3
from ~60 C. As temperature increased,
100W CT500. These results are consistent with our NO reduction per-
formance differences shown in Fig. 2.
3
desorption peak reached a maximum and then gradually
3
occurred with a broad shape
◦
within a wide temperature range of ~60ꢀ 450 C. It indicated that
3.3.5. XANES
several adsorbed NH
present on the catalyst surface.
For the MoO
3
species with different thermal stability were
Previous studies reported inconsistent results for V oxidation state
changes using X-ray photoelectron spectroscopy (XPS) with respect to
different calcination temperature. [41–44] In this study, XANES was
employed to investigate the oxidation states and speciations of V from
3
+WO
3
→V
2
O
5
/TiO
2
STR-100W CT500 catalyst, it
/g catalyst with a maximum NH
species adsorbed onto
showed ~0.18 mmol desorbed NH
3
3
◦
desorption peak at ~168 C. However, the NH
3
2
the fresh catalyst samples on different TiO supports. Due to the
8

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
3 2
Fig. 13. NH -TPD spectra of Mo-V-based SCR catalysts with different TiO
supports (DT-51, P25 and STR-100W).
Fig. 11. XRD patterns of Mo-V-based SCR catalysts on different TiO
2
supports
P25; (c)
◦
calcined at 500 C: (a) Anatase TiO
2
DT-51*; (b) Anatase/rutile TiO
2
Rutile TiO
MoO
+WO
2
STR-100W; (d) MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT500*; (e)
/TiO STR-100W
3
3
→V /TiO P25 CT500; (f) MoO
2
O
5
2
3
+WO
3
→V
2
O
5
2
CT500. Note: * [28].
Fig. 14. Vanadium K-edge XANES spectra of Mo-V-based SCR catalyst samples
with different TiO supports (DT-51*, P25 and STR-100W) and standard
reference vanadium oxide samples. Note: * [28].
2
TiO
2
DT-51 CT500 spectrum (Fig. 15(a)) was fitted with those of the
Fig. 12. XRD patterns of MoO
different calcination temperatures: (a) Anatase TiO
STR-100W; (c) MoO →V /TiO
+WO
MoO /TiO DT-51 CT500*; (e) MoO
+WO
CT600; (f) MoO →V /TiO DT-51 CT700. Note: * [28].
+WO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 with respect to
5+
references, the vanadium oxides could comprise 53.1% V and 46.9%
2
DT-51*; (b) Rutile TiO
2
4
+
V
. When P25 and STR-100W were chosen as the supports, the content
+
3
3
2
O
5
2
DT-51
+WO →V
CT400;
/TiO
(d)
5
of V slightly decreased to 49.3% and 47.1%, respectively. It seems that
TiO supports could not lead to significant differences on vanadium
3
3
→V
2
O
5
2
3
3
2
O
5
2
DT-51
2
3
3
2
O
5
2
0
oxidation state. The significant Hg oxidation and NO reduction activity
differences may not be derived from the vanadium oxidation state
interference between V and Ti, 0.5 wt.% V
SCR formula was too low to conduct XAFS characterization. There-
fore, V loading was raised from 0.5 to 2 wt.%. Fig. 14 exhibits V
K-edge XANES spectra of the Mo-V-based SCR catalyst in terms of
different TiO supports with 2 wt.% V and vanadium oxide refer-
2 5
O used in the Mo-V-based
2
change with respect to different TiO supports.
Fig. 16 exhibits V K-edge XANES spectra of the Mo-V-based SCR
catalyst samples based on different calcination temperatures with 2 wt.
2 5
O
%
2 5
V O and vanadium oxide references. LCF was employed to quantify
2
2 5
O
the V oxidation states from the fresh Mo-V-based SCR catalysts with
different calcination temperatures. The derivative XANES spectra and
LCF results are shown in Fig. 17, and the quantitative speciation data
ences. All the fresh catalyst samples had distinct pre-edge peaks, but the
post-edge part was not differentiable. A comparison of the spectra of the
catalyst samples with those of the V
2 3 2 2 5
O , VO and V O standard samples
5
+
acquired are summarized in Table 3. The analysis for the
suggests that the oxidation state of vanadium oxide consists of V and
◦
4
+
MoO
3
+WO
3
→V
2
O
5
/TiO
2
DT-51 CT 400 C sample (Fig. 17(a)) showed
V
.
5
+ 4+
5
2.2% V and 47.8% V . When the calcination temperature increased
To quantify the V oxidation states in the Mo-V-based SCR catalysts,
◦
5+
to 500 C, the content of V slightly increased to 53.1%. However, as
linear combination fitting (LCF) analysis was employed. The derivative
XANES spectra and LCF results are exhibited in Fig. 15, and the quan-
titative speciation data acquired are summarized in Table 2. The spec-
◦
◦
the calcination temperatures increased to 600 C and 700 C, the content
5
+
of V dramatically decreased to 43.9% and 34.3%, respectively. This is
in agreement with the catalytic activity differences (Fig. 3). As the
standard with V3+ was completely deviated from those
trum of the V
of the catalyst samples during the LCF analysis, and thus the V
standard was ruled out from the analysis. When the MoO →V
+WO
2 3
O
◦
◦
0
calcination temperature increased from 400 or 500 C to 600 C, the Hg
oxidation activities decreased from ~53% to ~39% at 5 ppmv HCl and
2
O
3
3
3
2
O
5
/
9

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
Fig. 15. LCF results for vanadium species of Mo-V-based SCR catalysts with different TiO
2
supports: (a) Fresh MoO
3
+WO
3 2 5 2
→V O /TiO DT-51 CT500*; (b) Fresh
MoO
3
+WO
3
→V
2
O
5
/TiO
2
P25 CT500; (c) Fresh MoO
3
+WO
3
2 5 2
→V O /TiO STR-100W CT500. Note: * [28].
Table 2
LCF results for vanadium species of Mo-V-based SCR catalysts with different
TiO supports.
2
5
+
4+
5+
Samples
V
2
O
5
(V
)
VO
(%)
2
(V
)
Ratio of V
4
+
(
%)
to V
Fresh MoO
3
+WO
+WO
+WO
3
→V
3
→V
3
→V
O
2 5
O
2 5
O
2 5
/TiO
/TiO
/TiO
2
2
2
DT-
53.1 ± 2.0
49.3 ± 2.2
47.1 ± 2.3
46.9 ± 2.0
50.7 ± 2.2
52.9 ± 2.3
1.13
0.97
0.89
5
1 CT500*
Fresh MoO
CT500
3
P25
STR-
Fresh MoO
3
1
00W CT500
Note: * [28].
from ~69% to ~55% at 10 ppmv HCl. When the calcination tempera-
ture was 700 C, the catalyst almost completely lost its Hg0 oxidation
◦
activity and decreased NO reduction activity from ~80% to ~54%.
0
5+
During the Hg oxidation and NO reduction process, V was reduced to
4
+
5+
4+
V
based on our previous proposed mechanism [28]. Low V /V
ratio correlates with low catalytic activity with an increase in calcina-
tion temperature. In addition to the physical agglomeration (Fig. 10)
Fig. 16. Vanadium K-edge XANES spectra of Mo-V-based SCR catalyst samples
◦
with respect to different calcination temperatures (400, 500*, 600 and 700 C)
and the anatase to rutile phase change in TiO
2
(Fig. 12) under different
and standard reference vanadium oxide samples. Note: * [28].
calcination temperatures, the significant activity difference could also
be attributed to a change in the V oxidation state. Throughout this study,
role of rutile TiO
. Conclusions
2
.
rutile-phase TiO
2
was found to degrade both Hg0 oxidation and NO
reduction activities. Previous studies reported that degraded NO
reduction activity of V-based SCR catalyst over the rutile phase is very
4
likely attributed to crystallographic misfit between V
2
O
5
and rutile TiO
45–48]. Although low V /V ratio can help account for the degraded
activities over rutile TiO , it is not currently well understood about the
2
In this study, the effects of preparation parameters including TiO
supports and calcination temperatures for Mo-V-based SCR catalysts on
2
5
+
4+
[
2
1
0

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
Fig. 17. LCF results for vanadium species of Mo-V-based SCR catalysts with respect to different calcination temperatures: (a) Fresh MoO
CT400; (b) Fresh MoO →V /TiO DT-51 CT500*; (c) Fresh MoO +WO →V /TiO DT-51 CT600; (d) Fresh MoO +WO →V
+WO
[28].
3
+WO →V
3
2 5 2
O /TiO DT-51
3
3
2
O
5
2
3
3
2
O
5
2
3
3
2
O
5
/TiO DT-51 CT700. Note:
2
*
under a simulated sub-bituminous and lignite coal combustion flue gas.
Table 3
0
O
2
had a positive impact on Hg oxidation and NH
3
had a negative effect
LCF fitting results for vanadium species of Mo-V-based SCR catalysts with
respect to different calcination temperatures.
0
on Hg oxidation. With the molar ratio of NH
the Hg0 oxidation activity decreased and NO reduction activity
3
/NO in a range of 0 to 1.1,
5
+
4+
(V )
Samples
V
2
O
5
(V
)
VO
(%)
2
Ratio of
0
increased. An increase in GHSV could significantly enhance Hg oxida-
5
+
4+
(
%)
V
to V
tion, but slightly improve NO reduction. When the low space velocity of
ꢀ
1
0
Fresh MoO
3
+WO
3
→V
3
→V
3
→V
3
→V
O
2 5
O
2 5
O
2 5
O
2 5
/TiO
/TiO
/TiO
/TiO
2
2
2
2
DT-
DT-
DT-
DT-
52.2 ± 2.8
53.1 ± 2.0
43.9 ± 2.6
34.3 ± 1.8
47.8 ± 2.8
46.9 ± 2.0
56.1 ± 2.6
65.7 ± 1.8
1.09
1.13
0.78
0.52
5
,000 hr was used, the Hg oxidation activity could achieve up to 99%
5
1 CT400
under a typical sub-bituminous and lignite coal combustion flue gas
condition. Compared to an in-house conventional V-based SCR catalyst,
our Mo-V-based SCR catalyst showed much higher Hg0 oxidation ac-
tivity and comparable NO reduction activity. It has the potential to meet
the stringent regulations for mercury and NO emissions in coal com-
bustion flue gas using Mo-V-based SCR catalyst in an existing SCR unit.
Fresh MoO
3
+WO
5
1 CT500*
Fresh MoO
3
+WO
5
1 CT600
Fresh MoO
1 CT700
3
+WO
5
Note: * [28].
CRediT authorship contribution statement
simultaneous Hg0 oxidation and NO reduction were systematically
investigated for the first time. Among different TiO supports (anatase
2
Can Li: Validation, Formal analysis, Investigation, Data curation,
Writing - original draft, Visualization. Vishnu Sriram: Validation,
Formal analysis, Investigation, Data curation. Zhouyang Liu: Valida-
tion, Formal analysis, Investigation, Data curation. Dale Brewe: Vali-
dation, Formal analysis. Joo-Youp Lee: Conceptualization,
Methodology, Validation, Investigation, Resources, Writing - review &
editing, Supervision, Project administration, Funding acquisition.
DT-51, mixed anatase/rutile phase P25 and rutile STR-100W) and four
◦
different calcination temperatures (400, 500, 600 and 700 C), a com-
2
bination of anatase phase TiO and the calcination temperatures of 400
◦
◦
0
C and 500 C showed the best activities of simultaneous Hg oxidation
and NO reduction. When rutile phase or the high calcination tempera-
◦
ture of 700 C was used to prepare the Mo-V-based SCR catalyst, the
catalyst lost almost its capability to oxidize Hg0 and decreased NO
reduction activity by ~30%. Different TiO
2
phase showed noticeable
Declaration of Competing Interest
◦
differences in the morphology. The calcination temperature of 700
could lead to the TiO
the agglomeration of active metals with TiO
C
2
phase transformation from anatase to rutile and
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.
. The content of V5
+
2
decreased with an increase in rutile TiO
temperature.
2
content or calcination
Our Mo-V-based SCR catalyst using anatase TiO
2
as the support and
Acknowledgements
◦
5
00 C as the calcination temperature preferred lower reaction tem-
◦
perature in a typical SCR operating temperature of 300ꢀ 400 C. It
This study was supported by the National Science Foundation, NSF
CAREER Grant # 1151017. The authors greatly appreciate their
2 2
showed good SO resistance in a wide range tested up to 2,000 ppmv SO
1
1

C. Li et al.
Applied Catalysis A, General 614 (2021) 118032
financial support. This research used resources of the Advanced Photon
Source, an Office of Science User Facility operated for the U.S. Depart-
ment of Energy (DOE) Office of Science by Argonne National Labora-
tory, and was supported by the U.S. DOE under Contract No. DE-AC02-
[20] N. Yan, W. Chen, J. Chen, Z. Qu, Y. Guo, S. Yang, J. Jia, Environ. Sci. Technol. 45
(
2011) 5725–5730.
21] Z. Liu, C. Li, V. Sriram, J.-Y. Lee, D. Brewe, Fuel Process. Technol. 153 (2016)
56–162.
[
1
[22] Z. Liu, V. Sriram, C. Li, J.-Y. Lee, Catal. Sci. Technol. 7 (2017) 4669–4679.
[
[
[
23] V. Sriram, C. Li, Z. Liu, M. Jafari, J.-Y. Lee, Chem. Eng. J. 343 (2018) 244–257.
24] H. Wang, B. Wang, Q. Sun, Y. Li, W.Q. Xu, J. Li, Catal. Commun. (2017).
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