Supports materialization of Pd based catalysts for NOx removal by hydrogen assisted selective catalytic reduction in the presence of oxygen

Article abstract of DOI:10.1002/cctc.202000756

The Pt and Pd based bimetallic catalyst are developed to address the NO_x reduction activity. For this, a series of supports containing three different amounts of CeO₂?MgO (9 : 1, 3 : 1, and 3 : 2) are prepared by the co-precipitation method, followed by impregnation of Pd metal precursors. The physicochemical properties of the catalytic system are investigated using various techniques such as Brunauer-Emmett-Teller (BET), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy, Transmission Electron Microscopy (TEM), Raman spectroscopy, and H₂-TPR. Among all, Pd/CeO₂?MgO (3 : 1) gave the best NO_x reduction activity. Furthermore, the effects of hydrogen concentration (0.2–0.8 vol %) on NO_x reduction were tested. It was found that 0.8 vol % of H₂ shows the highest NO_x conversion. Subsequently, the successive impregnation of Pt metal over Pd based catalyst enhanced the catalytic properties. The bimetallic Pt?Pd/CeO₂?MgO (3 : 1) catalyst showed an 89 % NO_x reduction with about 97 % N₂ selectivity in broad temperature windows (100–300 °C).

Full text of DOI:10.1002/cctc.202000756

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Supports materialization of Pd based catalysts for NOx
removal by hydrogen assisted selective catalytic reduction
in the presence of oxygen
Vivek Kumar Patel[a] and Sweta Sharma*[a]
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The Pt and Pd based bimetallic catalyst are developed to
address the NOx reduction activity. For this, a series of supports
containing three different amounts of CeO2À MgO (9:1, 3:1, and
3:2) are prepared by the co-precipitation method, followed by
impregnation of Pd metal precursors. The physicochemical
properties of the catalytic system are investigated using various
techniques such as Brunauer-Emmett-Teller (BET), X-ray Diffrac-
tion (XRD), X-ray photoelectron spectroscopy, Transmission
Electron Microscopy (TEM), Raman spectroscopy, and H2-TPR.
Among all, Pd/CeO2À MgO (3:1) gave the best NOx reduction
activity. Furthermore, the effects of hydrogen concentration
(0.2–0.8 vol%) on NOx reduction were tested. It was found that
0.8 vol% of H2 shows the highest NOx conversion. Subsequently,
the successive impregnation of Pt metal over Pd based catalyst
enhanced the catalytic properties. The bimetallic PtÀ Pd/
CeO2À MgO (3:1) catalyst showed an 89% NOx reduction with
about 97% N2 selectivity in broad temperature windows (100–
°
300 C).
1. Introduction
various metal oxides, CeO2 has shown excellent results in the
$
past. Ceria can switch its normal oxidation state, Ce3+ Ce4+
,
Increase of automobile vehicles with the prosperous symbol of
people leads to exponential emissions of nitrogen oxides (NOx)
from mobile sources (transportation) in recent year, which
greatly influence environment and health due to acid rain,
photochemical smog, and ozone layer depletion.[1–4] In the
transportation sector, diesel vehicles are one of the primary
sources of NOx emissions.[5,6]
Nowadays, various techniques have developed to reduce
NOx emissions; these include NOx storage reduction (NSR),
selective catalytic reduction (SCR), direct decomposition of NOx,
and selective NOx recirculation (SNR).[7–9] Amongst all, NH3-SCR
is a preferred technology for NOx reduction from medium and
heavy-duty vehicles.[10] However, the commercial applicability of
this technique suffers several drawbacks, including NH3 slip and
deactivation of catalysts.[11,12] Recently, H2-SCR has attracted
great attention as a potential technology alternative to NH3-
SCR.[13–15] H2 can be produced on board by fuel reforming.
Further, the combustion of H2 produces only water which
makes this technique environment-friendly.[16,17]
which is very crucial for the NOx reduction reaction. Many
researchers have reported, ceria as good automotive support,
due to its oxygen storage capacity[28–33] and MgO is important
for its adsorption sites and high stability.[34–37] It is recognized
that the structure and surface properties of support exert a
significant role in the activity of catalysts.[38] So to enhance the
physicochemical properties of the support, we had prepared
three different combinations of CeO2 and MgO as support.
Some groups have presented that Pt-based catalysts have
shown high conversion at low temperatures for H2-SCR.
However, Pt-based catalysts suffer two major drawbacks, the
formation of N2O and the narrow temperature window.[39–43]
Side reactions occur (Eqs. [1–4]) over mixed oxide supported
catalysts for the formation of N2, N2O, and NH3 according to the
following reactions:[44–46]
2NO þ 4H2 þ O2 ! N2 þ 4H2O
(1)
2NO þ 3H2 þ O2 ! N2Oþ
(2)
In H2-SCR, noble metals (Pt, Pd, Ir, and Rh) supported on
transition metal oxides dissociates NO and H2 at relatively low
O2 þ 2H2 ! 2H2O
(3)
[18–20]
°
temperatures (T<200 C).
The combination of the noble
2NO þ 4H2 ! 2NH3 þ H2O
(4)
metal with transition metal oxide has shown improved perform-
ance for NOx reduction by H2-SCR.[21,22] Various transition metal
oxides (CeO2, Al2O3, ZrO2, MgO, and so on) are studied for NOx
removal owing to their good redox properties.[23–27] Among
Pd based catalysts are also extensively used in literature for
NOx reduction because of their high activity and N2 selectivity
for broad temperature range.[47–49] The Pt/MgOÀ CeO2 (1:1)
°
catalyst showed good NOx reduction activity below 200 C. It
[a] V. K. Patel, Dr. S. Sharma
Department of Chemical Engineering & Technology
Indian Institute Of Technology
Banaras Hindu University,
showed a clear mechanism of NOx reduction over the surface of
catalysts.[50–51] Therefore in the present study, the synergy effect
of Pt and Pd noble metals supported on MgOÀ CeO2 support
has been studied for NOx abatement using H2 as a reductant in
the presence of excess oxygen to increase NOx reduction in
Varanasi
221005, Uttar Pradesh (India)
E-mail: shweta.che@itbhu.ac.in
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The surface analysis was performed with a working pressure of less
than 2×10À 7 Pa. The spectrometer was adjusted by assuming the
binding energy (BE) of the Au 4f7/2 line to lie at 84.0 eV to the Fermi
energy level. The binding energies of Ce 3d and O 1s were
calibrated using the C 1s peak (BE=284.8 eV) as standard. TEM
images were obtained using (Tecnai G2 20 TWIN, FEI Company of
USA), an electron microscope equipped with a field emission source
at an accelerating voltage of 200 kV. Drops of the suspension were
applied, and after drying, the fine particles were well dispersed on
a copper grid coated with carbon. The Raman spectra were
measured using (LabRAM HR Evolution Raman spectrometer, HOIBA
scientific) Laser type instrument at a wavelength of 633 nm, and
the calibration of the instrument was tested using 520 cmÀ 1 silicon
band standard. LabSpec 6 Spectroscopy Suite Software was used to
perform baseline adjustment, smoothing, and normalizing Raman
spectra. The Raman shifts were noted in the range of 100–
1000 cmÀ 1. To make a comparison Raman spectra of Pd/CeO2
powder was also obtained in the same condition as employed for
mixed oxides. Temperature programmed reduction (TPR) was
directed to examine the reduction behavior of the catalysts. The
experiments were performed on a Micromeritics Pulse Chemi Sorb
2705 TPD/TPR apparatus equipped with Micromeritics Furnace
Controller using 0.1 g of catalyst and a temperature range from
broad temperature windows and minimization of N2O as a
greenhouse gas.
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Experimental
Synthesis of the catalysts
A
series of cerium and magnesium mixed oxide supported
(CeO2À MgO (wt.%); 9:1, 3:1, and 3:2) catalysts were prepared by
the co-precipitation method. According to this method, precursors
of (Ce(NO3)3 ·6H2O) and (Mg(NO3)2 ·6H2O) were first dissolved in
distilled water in stoichiometric amounts, and ammonia solution
was slowly added to the solution for complete precipitation (pH~
9). Upon filtering and washing several times with distilled water,
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°
the resulting solids were dried in an oven at 105 C and calcined in
°
a muffle furnace at 500 C for 4 h.
The resulting oxides were then impregnated with (0.5%; w/w) a
solution of PdCl2 for Pd supported catalysts and Pt(NH3)4(NO3)2 for
°
Pt supported catalysts The catalysts were dried at 110 C for 12 h
°
and then calcined at 550 C for 5 hr. The resulting catalysts were
denoted as Pd/CeO2, Pd/CeO2À MgO (9:1, 3:1, and 3:2), for the
different weight ratio of MgO and CeO2 support and Pt/CeO2À MgO
(3:1), and PtÀ Pd/CeO2À MgO (3:1), for mono and bimetallic
catalysts.
°
°
100–700 C at a rate of 7 C/min. A flow rate of 30 mL/min of
10 vol% H2/Ar was used for the reduction. A thermal conductivity
detector (TCD) was employed to determine the amount of hydro-
gen consumed.
Catalyst characterization techniques
Catalytic activity tests
The BrunauerÀ EmmettÀ Teller (BET) surface areas and pore volume
of the synthesized catalysts were obtained by N2 adsorption and
desorption (Micromeritics ASAP 2020). Before analysis, catalysts
The activities of the catalysts were determined at steady state in an
Inconel tubular reactor (i.d. 12 mm) using 0.5 g of catalyst (40–
60 mesh) at atmospheric pressure as shown in Figure 1 The
reactant gases were fed at a total flow of 200 mLminÀ 1, correspond-
ing to a space velocity of 20000 hÀ 1. A thermocouple was inserted
inside the reactor connected through a programmable temperature
controller to monitor the reaction conditions. Gas flow was
monitored and controlled through the Bronkhorst Mass flow
controllers. The experimental conditions are shown in Table 1.
Before reaction, the catalyst bed was exposed to a flow of 5 vol%
°
were degassed at 280 C for 10 h. The X-ray diffraction pattern was
recorded, using Cu Kα radiation (0.154 nm), operated at 100 mA
and 40 kV (Rigaku Miniflex 600 X-Ray Diffraction System, RIGAKU
Corporation). The diffractometer values of 2d and 2θwere docu-
À 1
°
°
°
mented at a scanning rate of 5 min from 5 -100 . The crystalline
phases were recognized by matching with ICDD files. X-ray photo-
electron spectroscopy (XPS) was featured using a standard Al K
source (1486.6 eV) working at 350 W (AMICUS, Kratos Analytical).
Figure 1. Experimental setup for NOx reduction.
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which has a smaller cationic radius in the lattice location. The
pore size of Pd/CeO2À MgO (3:2) is larger than Pd/CeO2À MgO
(3:1) but more extensive than Pd/CeO2. The bimetallic catalyst
had a greater pore diameter and smaller BET surface area than
the mixed oxide-based catalyst. It could be due to fewer pores
in a specific amount of catalyst, which had signified no
systematic arrangement according to the composition of
oxides.
Table 1. Reaction conditions for catalytic activity measurement.
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9
°
Temperature [ C]
50–400
Inlet gas concentration (Ar balance)
NOx [ppm]
1000
H2 [ppm]
O2 [%]
2000–8000
6
H2O [%] (when used)
5
SO2 [ppm] (when used)
100
20000
200
GHSV [hÀ 1
]
Total flow [ml/min]
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2.1.2. XRD
°
H2 gas at 400 C for 3 h. The inlet and outlet reactor gas
concentration of NOx were analyzed by a chemiluminescence
technique (UNIPHOS; NO/NO2 analyzer). The exhaust gas composi-
tion of N2, H2, O2, and N2O was monitored by Gas Chromatography
(CENTURION SCIENTIFIC; 5800), and NH3 gas was measured by gas
detector tubes (UNIPHOS). The NOx conversion, N2 selectivity, and
TOF values were calculated based on the following equations
(Eqs. [5–7]):
X-ray diffraction patterns of the examined catalysts are shown
in Figure 2, and their parameters (e.g., crystal size, lattice
parameter) are listed in Table 3. The individual phases of
MgOÀ CeO2 were identified without producing a new com-
pound. The diffraction peaks of CeO2 were observed at 2θ=
28.56, 33.09, 47.49, 56.37, and 59.18 degrees, analogous to the
Miller in-dices of (111), (200), (220), (311), and (222), respec-
ðNOxÞinlet À ðNOxÞoutlet
NOx Conversion ¼
N2 Selectivity ¼
� 100 ð%Þ
(5)
(6)
ðNOxÞinlet
ðN2Þ
ðNOxÞinlet À ðNOxÞoutlet
� 100 ð%Þ
TOF ½sÀ 1� ¼
FNO ½L minÀ 1� � NOConversion ½%�
(7)
m½g� � H2Consumption ½mol gÀ 1� � 2 � 22:4 ½L molÀ 1 � � 60 ½s minÀ 1
�
2. Results and discussions
2.1. Physicochemical Characterization
2.1.1. BET analysis
The textural properties of catalysts were determined from the
N2 adsorption-desorption isotherm at 77 K of the calcined
catalysts. Table 2 summarized the BET specific surface area,
pore-volume, and pore diameter results for the mixed oxides
support over monometallic and bimetallic catalysts. The
addition of MgO improved the BET surface area, pore-volume,
and pore diameter of catalysts. The Pd/CeO2 catalyst showed
the lowest BET surface area compared to other mixed oxide
supported catalysts, thus an explicit dependency on MgO
addition, which corresponded to reported literature.[52–54] One
reason might be due to the swap of Mg2+ ions with Ce4+ ion,
Figure 2. X-ray diffraction patterns of synthesized catalysts: (a) Pd/CeO2 (b)
Pd/CeO2À MgO (9:1) (c) Pd/CeO2À MgO (3:1) (d) Pd/CeO2À MgO (3:2) (e) Pt/
CeO2À MgO (3:1) (f) (Pt-Pd)/CeO2À MgO (3:1) (g) Pd/MgO
Table 3. XRD parameter of catalysts.
Catalysts
Crystal Lattice Parameter Lattice
Particle
size
Size
[nm]
CeO2
MgO
strain
[Δd/
d][a]
[nm][b]
Pd/CeO2
8.42
8.87
9.88
10.53
9.78
11.91
5.41100
5.40700 4.21700 0.033
5.40400 0.128
5.39200 3.79300 0.028
5.38900 3.79300 0.022
5.38900 4.21500 0.028
–
0.013
10.70
11.82
12.58
13.36
12.26
15.96
Table 2. Textural property of catalysts.
Pd/CeO2À MgO (9:1)
Pd/CeO2À MgO (3:1)
Pd/CeO2À MgO (3:2)
Pt/CeO2À MgO (3:1)
PtÀ Pd/CeO2À MgO
(3:1)
Catalysts
SBET
Pore Volume
[cm3/g]
Pore diameter
[nm]
[m2/g]
Pd/CeO2
78.72
82.16
85.09
89.20
84.91
81.36
96.22
0.037
0.057
0.062
0.068
0.061
0.065
0.12
4.38
5.78
7.10
7.80
7.16
8.72
3.8
Pd/CeO2À MgO (9:1)
Pd/CeO2À MgO (3:1)
Pd/CeO2À MgO (3:2)
Pt/CeO2À MgO (3:1)
PtÀ Pd/CeO2À MgO (3:1)
Pd/MgO
[a] (Williamson-Hall method, βCosθ=CsSinθ+K�), where Cs is the lattice
strain, K is shape factor, λ is X-ray wavelength, β is the line broadening at
half the maximum intensity (FWHM) and θ is the Bragg angle) [b] Particle
size calculated from TEM image data
L
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tively. MgO peaks were observed at 2θ=37.02, 42.88 and 62.38
degrees to the Miller indices of (111), (200), and (220),
respectively.[55–57] Two sets of diffraction peaks could be
classified: the peaks marked with ‘‘circle’’ are indexed to cubic
CeO2, while those with ‘‘triangle’’ are indexed to cubic MgO. It is
visible that the diffraction peaks of MgO are weaker and
broader than those of CeO2; this implies that MgO is poorly
crystallized or highly dispersed. Table 3 shows that the crystal
size of MgOÀ CeO2 is larger than that of pure ceria, due to a
combination of MgO with CeO2 during the calcination process.
According to the Rietveld analysis, the presence of MgO makes
a minor shrinkage in the lattice parameter and a larger micro-
strain of ceria in MgOÀ CeO2, as matched with those of only
ceria. The crystal sizes of catalysts are calculated using Scherrer
equation (Γ=Kλ/βCosθ, where Γ is the mean crystal size, K is
shape factor, λ is X-ray wavelength, β is the line broadening at
half the maximum intensity (FWHM), and θ is the Bragg angle),
and the lattice parameter is obtained by X’Pert High Score
software.
spectrum comprises 8 pairs of doublets with spin-orbit splitting,
four corresponding of trivalent cerium, and six correspondings
of tetravalent cerium.[58] The “parent” (Koop man) line, used for
each ionization state corresponds to the “shake-down” process
and the “shake-up” line, in order of increasing binding
energy.[59] The XPS Ce 3d5/2 and 3d3/2 doublets are commonly
extended in the energy range of 875–925 eV. By a peak-fitting
deconvolution, the Ce 3d core-level spectra displayed in
Figure 3 (i) corresponding to 3d3/2 peaks at 882.36, 888.76,
897.19 eV, 904.31, 907.18 and 916.45 eV whereas 3d5/2 peaks at
903.63, and 884.76 respectively.[60,61] In general, the presence of
Ce3+ is related to the formation of oxygen vacancies conferring
to charge compensation. Therefore, the relatively high Ce3+
content in CeO2À MgO specifies the formation of more oxygen
vacancies.[62–63] Figure 3 (ii) showed the two deconvolutions of O
1s peak at 528.97 eV and 532.08 eV, corresponding to lattice
oxygen and adsorbed oxygen. It is found that the addition of
Mg2+ enhanced the lattice oxygen contents of mixed oxide
catalysts.
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The crystal size, particle size, and lattice strain property of
catalyst showed a regular increment on MgO addition into CeO2
in mixed oxide catalysts as shown in Table 3. The lattice
parameter values had not followed by any particular order
corresponding to composition. Still, it had observed that the
lattice parameter of the Pd/CeO2 catalyst was greater than other
mixed oxide-based catalysts.
The Pd 3d spectra of mono and bimetallic catalyst, as shown
in Figure 4 (i), presented two peaks at the binding energy of
336.61 eV and 338.72 eV for 3d3/2, whereas spectra peaks at
[64]
339.89 eV and 340.77 eV for 3d5/2
.
The Pd5/2 binding energy
value of 336.61, which is very close to the specific of Pd2+ [65]
.
The catalyst’s Pt 4f XPS spectra showed in Figure 4 (ii), 4f7/2 and
4f5/2 peaks are observed at 71.20 eV, and 74.34 eV, respectively.
XPS spectra of Pt could be de-convoluted into two sets of the
spin orbit doublet. Accordingly, Pt (4f 7/2, 5/2) peaks at 71.1,
2.1.3. XPS
74.3, and 72.4, 75.7 eV could be assigned to Pt0 and Pt2+
,
respectively. The Pt 4f7/2 XPS peak’s binding energy was in the
range of 70.8–71.5 eV, confirming platinum particle in a metallic
state.[66] Pt is found to be dispersed mostly in the metallic state
on CeO2À MgO crystallites with 30% Pt in the Pt2+ state. In the
XPS spectra of catalyst Pt 4f peaks are observed at 71.20, 71.96;
The XPS investigations were further examined to find out the
chemical state and surface composition. The Ce 3d, O 1s, Pd 3d,
and Pt 4f, X-ray photoelectron spectroscopy (XPS) spectra of all
catalysts are given in Figure 3, and 4.
Ce 3d XPS spectra are characterized by the presence of
“shake-up” and “shake-down” satellites, which obscure the
analysis of the identification of different ionization states. This
and 73.20, 74.34 eV corresponding to Pt0, Pt2+, and Pt4+
,
respectively.[67] In general, two O 1s peaks of catalyst are found
(Figure 3(ii)), one around 529.27 eV, which relates to lattice
Figure 3. XPS spectra of Ce 3d and O1s of catalysts.
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Figure 4. XPS spectra of Pd 3d and Pt 4f of catalysts.
oxygen, and peaks around 532.36 eV can be ascribed to surface
oxygen. This surface oxygen can be due to oxygen binding of
cerium and magnesium element with oxygen atoms.[68] Many
authors also have described the probability of residual chlorine
inducing the metal support interaction of catalyst[69] because
the chlorine atoms are hard to be eliminated from the surface
dispersion on the CeO2À MgO support. Pd/CeO2 catalyst ex-
hibited smaller particle sizes than all reported incentives like
BET data, which justified the MgO addition increased the
particle size with pore size. TEM histogram plots showed the
mean particle sizes of catalysts existed in the range of (10–16)
nm. The particle size marginally increases with MgO concen-
tration. The average particle sizes of all catalysts are shown in
Table 3. According to TEM images, few palladium particles are
agglomerate over the support due to micropores of irregular
particles, which leads to the large surface area while other areas
are partially Pd metals free. The agglomeration of metal was
observed more in bimetallic loading against mono-metallic. This
could be understood from XRD results that both CeO2 and MgO
exist without forming any strong metal complex.
°
even after calcination at 500 C. From the XPS analysis, Table 4
showed the Cl/Pd ratio, which varied in the range of about
2.86–2.45. This value is larger than the expected value of 2,
which is designative of surface chlorine. It was observed that
increasing the MgO contents in the CeO2 support went to
reduce the Cl/Pd ratio. The Pd/ CeO2À MgO (3:1) and Pd/
CeO2À MgO (3:2) showed the almost nearby ratio means further
addition was not much influence the Cl/Pd ratio.
2.1.4. TEM Micrographs
2.1.5. Raman Spectroscopy
Figure 5a, b, c, and d shows the TEM micrographs of Pd loaded
with CeO2À MgO (9:1, 3:1, 3:2) catalyst and Figure 5e,f
corresponding to Pt and PtÀ Pd loaded over CeO2À MgO (3:1)
catalyst. All catalysts exhibit similar morphology by appearance
except change in particle size with irregular shape and micro-
pores. The TEM images showed the particle size and distribu-
tion of active metal with an uneven platinum and palladium
Raman spectra of CeO2 and three compositions of CeO2À MgO
(9:1, 3:1, and 3:2) over Pd based catalyst and are shown in
Figure 6. The peak at about 466 cmÀ 1 corresponded to the
symmetric CeÀ O vibration of the CeO2 lattice, where Ce and O
are 8- fold and 4-fold coordinated, respectively.[70–71] It is found
that the increase in MgO concentration caused the Raman
peaks to shifts to lower wavenumbers with changes in intensity.
For the 10 wt.% MgO in CeO2 specimen, the magnitude of the
Raman shifts was approximately 7.14 cmÀ 1, which is increases to
18.57 cmÀ 1 with the addition of 25 wt.% MgO; however,
4.60 cmÀ 1 decreases in Raman shift was observed with the
addition of 40 wt.% MgO. This spectroscopy suggests that the
maximum shift is not only solely depends upon the addition of
MgO. This could be due to partial reduction of surface Ce4+ to
Ce3+ leads to the formation of an oxygen vacancy, which alters
Table 4. Atomic concentration and atomic ratio by XPS.
Sample
Atomic Concentration [%]
Cl/Pd ratio
obtained by
XPS analysis
Ce3+[Ce]
Oα[Oα +Oβ]
Pd/CeO2
28.31
31.56
34.78
29.12
40.37
74.1
78.13
72.76
2.86
2.65
2.47
2.58
Pd/CeO2À MgO (9:1)
Pd/ CeO2À MgO (3:1)
Pd/ CeO2À MgO (3:2)
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Figure 5. TEM image of synthesized catalysts: (a) Pd/CeO2 (b) Pd/CeO2À MgO (9:1) (c) Pd/CeO2À MgO (3:1) (d) Pd/CeO2À MgO (3:2) (e) Pt/CeO2À MgO (3:1) (f)
(PtÀ Pd)/ CeO2À MgO (3:1).
the geometrical parameters and shifts in the vibrational
frequency of adsorbed oxygen.[72–74]
2.1.6. H2-TPR analysis
It could be concluded that the broadening of Raman peaks
mostly reliable on the oxygen vacancy of the catalyst,
corresponding to adsorbed oxygen concentration.
H2-TPR analysis of all catalysts was performed in the temper-
°
ature range of 60–700 C, which were assigned to the reduction
of surface and bulk oxygen of catalysts. Figure 7, showed the
°
H2-TPR peaks between 100–200 C, which confirms the presence
of PdO over the surface of catalysts.[75]
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Figure 6. Raman spectra of catalysts.
Furthermore, the quantitative results of H2 consumption
(Table 5) indicated that the actual H2 consumption of mixed
oxides based catalyst was more than the corresponding
estimated H2 consumption of the only CeO2 supported catalyst.
It was found that the introduction of MgO promotes the
reduction temperature of bulk CeO2 and more interaction of Pd
with mixed oxide at the lower temperature corresponding to
more H2 consumption.
Figure 7. H2-TPR of all catalysts (a) Pd/CeO2 (b) Pd/CeO2À MgO (9:1) (c) Pd/
CeO2À MgO (3:1) (d) Pd/CeO2À MgO (3:2) (e) Pt/CeO2À MgO (3:1) (f) (PtÀ Pd)/
CeO2À MgO (3:1).
H2 consumption was credited to the reduction of surface
and bulk catalysts since Mg2+ is an irreducible cation in this
temperature range (Figure 7). The broad reduction peaks of
°
CeO2À MgO at around 400 C relates to the reduction of the
uppermost layers of Ce3+, while the reduction temperature for
[76]
bulk CeO2 is above 800 C. With the further addition of Mg,
reduction peak of surface Ce3+, forming two take up peaks
°
°
two central reduction peaks in place of one for surface
reduction were observed. The shifting of reduction peaks to
lower temperature shows the oxides are easier to be reduced.
For CeO2À MgO, peak belongs to bulk CeO2 reduction shifted
significantly to a lower temperature, which was close to the
below 500 C. Pd impregnated oxides were synthesized to
evaluate the interaction between Pd and oxides. For ceria
°
promoted catalyst, the peak at 100–200 C could be assigned to
°
the reduction of surface PdO species, while peaks at 150 C
could be assigned to the reduction of Pd2+ species in
CeO2À MgO. The consumption of hydrogen in the low-temper-
ature region is due to the reduction of Pd. For Mg-doped
°
catalysts, negative peaks at 100 C were attributed to the
Table 5. H2 consumption and Raman shift of catalysts.
decomposition product of Pd hydride formed during H2-TPR,
indicating the existence of metallic Pd in catalysts.[77–78] With
Catalysts
Peak temp.
Total H2
consumption
[mmol/gcat
FWHM of
Raman
line
°
[ C]
°
increased doping of Mg, peaks below 200 C decreased for
]
[cmÀ 1
]
hydrogen consumption, indicating an increase in the Pd/PdO
ratio of the catalysts.[79] In the bimetallic catalyst, both peaks are
Pd/CeO2
Pd/CeO2À MgO (9:1)
197, 546
165, 281,
403
3.95
4.26
221.08
263.28
°
°
observed very close at 418 C, 476 C exhibits the more amount
of lattice oxygen generated corresponded to XPS data.
Pd/CeO2À MgO (3:1)
Pd/CeO2À MgO (3:2)
Pt/CeO2À MgO (3:1)
154, 334,
436
132, 395,
490
134, 396,
512
4.37
4.20
4.47
4.34
404.87
218.36
–
–
PtÀ Pd/CeO2À MgO
135, 418,
476
(3:1)
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2.2. Catalytic performance for H2-SCR
MgO crystallite size and adsorbed oxygen content may also
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affect the NOx reduction ability. From Table 4 the Pd/CeO2À MgO
(3:1) catalyst showed the lowest value of the Cl/Pd ratio but
exhibited the maximum NOx reduction activity. This could be
due to the presence of residual chlorine, which inhibit the
dispersion of palladium over the surface of the support. The
turnover frequency (TOF) is defined as the number of NOx
conversion per active sites present in the catalyst per second.
Here, it was assumed that all the catalyst elements are
participating in the NOx conversion. The TOF value is a function
of conversion, molar flow rate, and catalyst amount. However,
this would not provide a good prediction of activity if the
dispersion of the element is uneven.[85] The TOF value reported
by Pd/CeO2 is 1.459×10-3 sÀ 1, which is smaller than other
materialization based catalysts prepared in current work
(Table 6). The Pd/CeO2À MgO (3:1) catalyst showed higher TOF
value in all the reported temperature ranges which proved it a
better combination of the supported catalyst.
2.2.1. Effect of supports materialization
In literature, single metal oxide catalysts were used to analyze
the NOx reduction but could not found satisfactory results.[80–81]
So the various combinations of mixed oxide supports were
synthesized for maximum dispersion of active metals to obtain
the highest activity of the catalysts. Figure 8 illustrates the
effect of the composition of CeO2À MgO supports on NOx
reduction and N2 selectivity over palladium-based catalysts for
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°
temperatures ranging from 75–400 C. The maximum, 46% NOx
°
conversion was obtained over Pd/ CeO2À MgO (3:1) at 200 C,
but N2 selectivity was slightly lower than Pd/CeO2À MgO (9:1).
Overall, the NOx conversion over palladium catalysts was in the
sequence of Pd/CeO2À MgO (3:1)>Pd/CeO2À MgO (9:1)>Pd/
CeO2À MgO (3:2).
The activity and selectivity showed the strongest depend-
ency over the temperature. It was found that there was no
systematic trend for NOx reduction based on textural properties
as shown in Table 1. The NOx reduction was also disorganized
based on crystal size, lattice parameters, and lattice strain data
of the catalyst displayed in Table 3. From the TEM micrograph,
NOx conversion is not much affected by the particle size of
catalysts, but Pd/CeO2À MgO (3:1) showed significant dispersion
and stabilization of metal over support, which contribute
maximum conversion beyond others. In various literature, the
effect of support on metal dispersion are shown, which
corresponds to the performance of catalysts.[82–83] It is known
through XPS results that the combination of MgO with CeO2
could lead to the creation of oxygen vacancies, which promotes
the NO removal and provide additional active sites for NO
dissociation. The H2-TPR profile shows the broad range of H2
consumption corresponds to lattice oxygen, which contributes
to maximum conversion. Results also showed that the presence
of Mg content in mixed oxide support increased the N2
selectivity compare to only CeO2 based support.[84] This
indicates that besides the pore properties, other factors such as
2.2.2. Effect of hydrogen concentration
Figure 9.a presents the effect of hydrogen concentration (0.2,
0.4 and 0.8 vol%) in the reductant gas stream containing
1000 ppm NO and 6 vol% O2/Ar on the catalytic performance,
in terms of NOx conversion, N2-selectivity, and hydrogen
°
conversion for the temperature range of 50–400 C at a GHSV of
20,000 hÀ 1. The maximum NOx conversion is found when
hydrogen feed concentration is about 0.8 vol%. However, N2-
selectivities for all H2 concentration are in the range of 80–90%
(Figure 9.b). The same is also reported in literature that NOx
conversion increases with increasing H2 concentration.[86–87] It is
useful to find the optimum value of hydrogen concentration,
which are economical and shows NOx concentration and N2
selectivity values above 90%. Ideally, the lower H2 concen-
tration is economical and suitable for industrial application.
For the 0.8 vol% H2 concentration, a very high NOx
conversion (>86%) and N2-selectivities in the range of 80–95%
°
are observed in the temperatures ranges of 150–250 C with
6 vol% O2 in the feed stream. With elaborating the effect of
hydrogen on NOx conversion, it is also necessary to know the
effect of hydrogen dissociation behavior over the Pd surface of
catalysts. Usually, dissociative adsorption of hydrogen mole-
cules on Pd metal surfaces are reported.[88] Upon dissociation,
two H atoms are formed, which then interact independently
with the catalyst surface, this process is referred as hydrogen
chemisorption, and it happens only on selected elements
surface.[89] The extent of hydrogen dissociation step (H2(g)+2 s
Table 6. Activity of catalyst in term of Turnover frequency.
Catalysts
TOF (sÀ 1)×10À 3 TOF (sÀ 1)×10À 3 TOF (sÀ 1)×10À 3
°
°
°
at 200 C
at 250 C
at 300 C
Pd/CeO2
1.459
1.695
1.004
1.702
1.063
0.847
0.873
1.447
0.442
Pd/CeO2À MgO (9:1) 1.571
Pd/CeO2À MgO (3:1) 1.915
Pd/CeO2À MgO (3:2) 1.638
Figure 8. NOx conversion and N2 selectivity; Feed gas composition: 0.2 vol%
H2, 1000 ppm NO, 6 vol% O2, rest Ar, and GHSV ~20000 hÀ 1
.
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Figure 9. Effect of H2 concentration on PtÀ Pd/CeO2À MgO (3:1); (a) NOx conversion (b) N2 selectivity (c) H2 conversion, Feed gas condition: 0.2 vol% H2,
1000 ppm NOx, 6 vol% O2, rest Ar, and GHSV=20000 hÀ 1
.
$
2 H-s) on the Pd surface is to be determined by the available
concentration of Pd2+ surface sites, the gas-phase hydrogen
concentration, and the reaction temperature.
hydrogen concentration (0.2–0.8 vol%), increases the NOx
conversion from 46 to 88% with a slightly 6% increase in N2
selectivity.
The H2 gas concentration, along with the catalyst bed, is
strongly influenced by the rate of hydrogen consumption in the
reaction, which is proportional to the reaction temperature, and
the hydrogen and oxygen gas composition. Because the active
NOx is trapped on the PtÀ MgO, and PtÀ CeO2 interface, the rate
of H spillover from Pt to the Pt-support interface becomes
important.[90] In Figure 9.c, H2 conversions (utilization) with
temperature are plotted for varying H2 feed % and no H2 is
detected at the outlet stream. The results clearly show that
2.2.3. Effect of bi-metallic catalyst
Figure 10.a shows that the highest 76–93% NOx conversion is
°
observed in 80–280 C broad temperature range. Pt/CeO2À MgO
°
catalyst shows 88% NOx conversion below 200 C, whereas Pd
°
based catalyst shows about 90% conversion at 200 C. In
Figure 10.a, the bimetallic catalyst shows the synergetic effect
of Pt and Pd active metal toward NOx reduction. The BET
analysis has not shown any relevant information regarding NOx
conversion. However, pore diameter might have played a
crucial role by providing sufficient volume for NOx reduction.
The XRD and TEM data of bimetallic catalysts showed the
greater crystal size and particle size which corresponding to
maximum NOx conversion. In the bimetallic catalyst, the TEM
image clearly showed the effective dispersion of Pt and Pd
active metals over the CeO2À MgO (3:1) support with some
agglomeration, which provided more active species for NOx
reduction. The XPS demonstrates more availability of Pt (4f)
peaks at 71.96 and binding energy value of 336.61 corresponds
°
other than the lower temperatures (below 130 C), 100%
conversion of H2 is obtained for all the feed %. In the presence
of PdÀ Pt catalyst, H2 reacts with excess O2 in the feed (5%) and
forms water. Similar results are also reported in the literature for
H2-SCR catalytic systems.[91] In Figure 9.b, it is found that the N2-
°
selectivity is reduced by 6% in the 300–400 C temperature
range for 0.8 vol%, however for NOx conversion no reduction is
°
observed in the temperature range of 50–300 C. In the
Figure 9.a, the hydrogen (0.2, 0.4, and 0.8 vol%) consumed in
°
°
the reaction above 125 C temperature range, but at 100 C,
maximum conversion reported by 0.2 vol% H2, where least NOx
conversion was obtained. It is significant that increasing the
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Figure 10. Effect of catalyst on (a) NOx conversion, (b) N2 selectivity and (c) H2 conversion for H2-SCR, and (d) Hydrothermal stability and SO2 tolerance over
PtÀ Pd/CeO2À MgO (3:1) catalyst. Feed gas composition: 1000 ppm NO, 0.4 vol% H2, 6 vol% O2/Ar, 5 vol% H2O (when used), 100 ppm SO2 (when used), rest Ar
and GHSV=20,000 hÀ 1
.
to Pt2+ and Pd2+ contributes oxidation of NOx to NO2 and
further reduction of NO2 to N2 with H+ ions. In literature it is
reported that Pt is the best metal for NOx oxidation whereas Pd
shows a higher reducing activity.[92] One of the important
reasons is the presence of about 80% lattice oxygen in the
catalyst which leads the creation of more oxygen vacancies,
directly related to NOx reduction.
Figure 10.b shows the N2 selectivity obtained from the Pt,
Pd, and PtÀ Pd based catalyst support on CeO2À MgO (3:1), the
N2 selectivity increases with the increase of reaction temper-
ature in all cases except bimetallic. The bimetallic catalyst
shows 97% N2 selectivity in the temperature range of 200–
catalyst showed the insignificant formation of NH3, as it is more
inclined to N2 formation.
2.2.4. Effect of the presence of H2O and SO2
The hydrothermal stability and SO2 tolerance of PtÀ Pd/
CeO2À MgO (3:1) catalyst toward NOx reduction are studied by
supplying 5 vol% H2O (when used) and 100 ppm SO2 (when
used) to the feed stream, as shown in Figure 10 (d). NOx
°
conversion over PtÀ Pd/CeO2À MgO (3:1) catalyst below 225 C
decreases markedly after the addition of 5 vol% H2O while it is
°
°
275 C, after that a slight decrease in N2 selectivity is observed.
increased slightly above 250 C. However, in the reported
These results indicates that the presence of Pt2+ and Pd2+ in
the catalyst promotes N2 selectivity of the reaction. Figure 10.c
temperature range, NOx conversion is less affected by the
introduction of 100 ppm SO2. The activity of catalysts in the
presence and absence of SO2 shows the almost constant
behavior, which established that the current catalysts is stable
under SO2 environment present in the diesel. However, when
SO2 and H2O are simultaneously introduced, NOx conversion
over reported temperature range is slightly reduced. It shows
that SO2 had a synergetic effect in the presence of H2O, possibly
due to sulfation of (3:1) catalyst which covered the active sites
in the presence of H2O.
°
shows the H2 conversion with temperature over 50–400 C. It is
clear that a hundred percent H2 conversions are obtained above
°
°
150 C. Below 150 C, the bimetallic catalyst shows the highest
H2 conversion compared to the mono metallic catalyst. The
75% H2 conversion is sufficient for maximum NOx conversion by
Pt/CeO2À MgO (3:1), which shows the significant dissociation of
°
hydrogen. But at a temperature above 200 C, it contributes to
the small amount of NH3 formation. The PtÀ Pd based bimetallic
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Acknowledgment
I would like to thank Central Institute Facility Centre (IIT BHU) for
their help in research (XPS, XRD, and TEM). This work has been
assisted by the Ministry of Department of Science and Technology
under a fast track project [SB/FTP/ETA-0144/2014].
Conflict of Interest
The authors declare no conflict of interest.
Keywords: H2-SCR · Catalyst · Activity · NOx reduction
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Manuscript received: May 3, 2020
Revised manuscript received: July 1, 2020
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The Catalyst Pd/CeO2À MgO (3:1)
prepared by precipitation followed
by impregnation method shows the
high NOx conversion and reasonable
N2 selectivity with feed gas composi-
tion of 0.2 vol% H2, 1000 ppm NO,
6 vol% O2, rest Ar, and GHSV
~20000 hÀ 1.
V. K. Patel, Dr. S. Sharma*
1 – 13
Supports materialization of Pd
based catalysts for NOx removal by
hydrogen assisted selective
catalytic reduction in the presence
of oxygen
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