Combination of reduction-deposition Pd loading and zeolite dealumination as an effective route for promoting methane combustion over Pd/Beta

Article abstract of DOI:10.1016/j.cattod.2020.07.005

Catalytic removal of trace methane emission over supported Pd catalysts has drawn much attention over the years. For practical application, high activity, excellent thermal stability and outstanding water resistance are always desirable. Herein, we report a facile route for simultaneously improving the activity, stability and water resistance of the zeolite supported Pd catalyst through the combination of reduction-deposition Pd loading and support dealumination. This combination strategy leads to highly dispersed and stable PdO nanoparticles with strong interaction with the Beta support, rich active Pdⁿ⁺ (0 < n < 2), and plentiful surface chemisorbed oxygen. The as-designed Pd/Beta shows a high methane conversion above 99 % at 375 °C with a space velocity of 30,000 mL h^?1 g^?1 and 1 vol.% CH₄ feed concentration, enhanced long-term stability, and greatly improved water resistance with a low 0.5 wt.% Pd loading amount. Such a synthetic strategy is expected to provide a useful approach towards designing efficient zeolite supported noble metal catalysts.

Full text of DOI:10.1016/j.cattod.2020.07.005

Journal Pre-proof
Combination of reduction-deposition Pd loading and zeolite
dealumination as an effective route for promoting methane combustion
over Pd/Beta
Liling Zhang (Investigation) (Formal analysis) (Data curation)
(Methodology) (Writing - original draft), Junfei Chen (Data curation)
(Methodology) (Writing - review and editing), Xiaomin Guo (Formal
analysis) (Writing - review and editing), Shumeng Yin (Formal
analysis) (Conceptualization), Meng Zhang (Methodology) (Formal
analysis) (Resources) (Writing - review and editing), Zebao Rui
(Conceptualization) (Methodology) (Formal analysis) (Resources)
(Supervision) (Project administration) (Funding acquisition) (Writing
- review and editing)
PII:
S0920-5861(20)30464-8
DOI:
https://doi.org/10.1016/j.cattod.2020.07.005
CATTOD 12996
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
6 February 2020
26 June 2020
4 July 2020
Please cite this article as: Zhang L, Chen J, Guo X, Yin S, Zhang M, Rui Z, Combination of
reduction-deposition Pd loading and zeolite dealumination as an effective route for promoting
methane combustion over Pd/Beta, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.07.005

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Combination of reduction-deposition Pd loading and zeolite
dealumination as an effective route for promoting methane
combustion over Pd/Beta
Liling Zhang a, Junfei Chen a, Xiaomin Guo a, Shumeng Yin b, Meng Zhang a, *, Zebao Rui a, *
a School of Chemical Engineering and Technology, The Key Laboratory of Low-carbon
Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University,
Zhuhai 519082, China
b Sinopec Qingdao Safety Engineering Research Institute, Qingdao 266000, China
*Correspondence to the authors can be sent to Z.B. Rui (ruizebao@mail.sysu.edu.cn) and M. Zhang
(zhangm85@mail.sysu.edu.cn)
Graphical abstract
Highlights




Pd/Beta was prepared by combining Pd reduction-deposition and Beta dealumination.
The combination strategy promotes the distribution of PdOx nanoparticles.
The combination strategy affects the chemical state of PdOx.
The combination strategy enhances catalytic performance in methane oxidation.
1

Abstract
Catalytic removal of trace methane emission over supported Pd catalysts has drawn
much attention over the years. For practical application, high activity, excellent thermal
stability and outstanding water resistance are always desirable. Herein, we report a
facile route for simultaneously improving the activity, stability and water resistance of
the zeolite supported Pd catalyst through the combination of reduction-deposition Pd
loading and support dealumination. This combination strategy leads to highly dispersed
and stable PdO nanoparticles with strong interaction with the Beta support, rich active
Pdn+ (0 < n < 2), and plentiful surface chemisorbed oxygen. The as-designed Pd/Beta
shows a high methane conversion above 99 % at 375 oC with a space velocity of 30,000
mL h-1 g-1 and 1 vol.% CH4 feed concentration, enhanced long-term stability, and
greatly improved water resistance with a low 0.5 wt.% Pd loading amount. Such a
synthetic strategy is expected to provide a useful approach towards designing efficient
zeolite supported noble metal catalysts.
Keywords: Catalytic combustion; PdO; Methane; Zeolite dealumination; Reduction-
deposition
1. Introduction
Methane, the main constituent of natural gas, has high energy density as well as
low carbon and nitrogen oxide emissions during combustion, making lean-burn natural
gas vehicles an up-and-coming substitution to petrol and diesel vehicles. Nonetheless,
the emission of unburned methane from natural gas engines could elicit underlying
2

environmental issues due to its 20 times higher potential global warming than CO2 [1,
2]. Great attention has been paid to catalytic combustion of methane exhaust gas, owing
to its lower ignition temperature and air pollutant by-product than that of direct flame
combustion [3-5]. At present, supported catalysts incorporating PdO as active species
are most prevalent for low concentration CH4 abatement [5-9]. However, state-of-the-
art Pd-based CH4 oxidation catalysts still suffer from the requirement for high Pd
loading, stability problem from sintering and water poison, and high CH4 light-off
temperature [6, 10, 11]. With the rapid rise of Pd price, there is an urgent requirement
for improving the performance of Pd-based catalysts.
Over the years, various strategies, such as support selection and modulation [9, 12,
13], promoters addition [7, 14], and tailoring of PdO-support interaction [7, 8, 10], have
been employed to improve the performance of Pd-based catalysts through optimizing
their steric and (or) electronic microenvironment. Unfortunately, there is always trade-
off among these strategies, leading to the unsatisfied promotion for practical application.
For example, zeolite has been widely employed as support of PdO particles for CH4
oxidation taking advantages of its large surface area, adjustable acidity, systematic pore
structure, and high thermal stability [4, 11, 13, 15]. Especially, the anti-water or
hydrophobicity property of the zeolite supported PdO catalysts could be tailored
through regulating the Si/Al ratio [11, 13, 16]. However, the strong metal-support
interaction (SMSI), which is usually reported in CeO2 and TiO2 supported catalysts,
important to stabilize and disperse the noble metal nanoparticles, and beneficial to the
oxidation performance [17-19], is poor in zeolite supported catalysts. Some groups
reported that the metal-support interaction and CH4 oxidation performance in zeolite
supported PdO could be regulated through tailoring the surface Bronsted acid sites [4],
but others reported inconsistent results that the anchoring and promotion effect of
3

Bronsted acid sites on PdO was negligible [15]. Although the co-confinement of PdO
and promoter metal oxide into the zeolite is a promising tailoring route [14], its precise
and complex synthesis procedure limits its practical application.
In this work, we provide a facile route for simultaneously improving the activity,
stability and water resistance of Pd/Beta for low temperature CH4 oxidation through the
combination of reduction-deposition Pd loading and Beta support dealumination.
Reduction-deposition loading method, referring to deposit nanoparticles on support
during reduction process by adding a reducing agent to a solution of metal precursor
and support, is an effective way to obtain well-dispersed metallic nanoparticles [20-22].
Meanwhile, traps and T-atom sites could be generated in zeolite by post-dealumination,
on which metal nanoparticles would be located and stabilized [23-25]. Such a facile
combination strategy leads to highly dispersed and stable PdO nanoparticles, rich Pdn+
(0 < n < 2) and surface chemisorbed oxygen, and remarkably enhanced CH4
mineralization performance.
2. Experimental
2.1. Catalyst preparation
Commercial Beta zeolite (Nankai University Catalyst Co., Si/Al=~12.5) was
employed as the support or the precursor to prepare dealuminated Beta (D-Beta). D-
Beta was prepared by a nitric acid leaching method. Commercial Beta was firstly ion-
exchanged with 1 mol/L ammonium chloride at 80 °C for 2 h followed by drying and
calcination at 450 °C in air for 4 h. Such a process was repeated twice to obtain H-Beta.
The as-prepared H-Beta was calcined at 700 oC for 4 h and then treated with 1 mol/L
nitric acid solution at 80 oC for 3 h followed by drying at 80 oC overnight to obtain D-
Beta.
Both Beta and D-Beta were then employed as the support to load 0.5 wt.%
4

palladium with incipient wetness impregnation (IW) or reduction-deposition method
(RD). In the IW process, the support Beta or D-Beta was added into the Pd(NO3)2·2H2O
(Aldrich Co.) solution with a full agitation. Then, the samples were dried overnight at
80 °C and calcined at 500 °C in air for 4 h to obtain the catalyst IW-Pd/Beta and IW-
Pd/D-Beta, respectively. In the RD experiment, 1 g Beta or D-Beta was first dispersed
into 150 mL distilled water with 1 h ultrasonic stirring, and then the diluted palladium
nitrate solution prepared by dissolving targeted amount of Pd(NO3)2·2H2O in 25 mL
distilled water was added drop by drop. After stirring the mixture for 2 h, 2 mL of
sodium borohydride (0.5 mol/L) solution was added dropwise followed by continuous
stirring for 1 h. The mixture was then filtered and washed with water and ethanol. The
product was dried at 80 °C overnight and then calcined in air at 500 °C for 4 h to obtain
RD-Pd/Beta or RD-Pd/D-Beta.
2.2. Catalyst characterization
The crystal structure of the samples was figured out by X-ray diffraction (XRD)
with Cu Ka radiation (Ultima IV, Rigaku). The contents of Pd and Al in the samples
were determined by inductively coupled plasma-atomic emission spectrometry (ICP,
IRIS(HR), CID). Semi-quantitative determination of the silicon to aluminium ratio was
obtained by energy dispersive spectroscopy (EDS) attached with thermal field emission
environmental scanning electron microscope (TFEE-SEM, Quanta 400F France).
Fourier transform infrared (FTIR) spectra were obtained by using a Perkin Elmer FTIR
spectrometer in the region ranging from 4000 to 450 cm-1. X-ray photoelectron
spectroscopy (XPS) measurement was carried out on ESCALAB 250 (Thermo Fisher
Scientific, Al Ka, hv = 1486.6 eV) spectrometer. N2 adsorption-desorption was
performed at 77 K on a Micromeritics ASAP 2460 instrument to determine the surface
area and pore size of the samples. Prior to adsorption analyses, the catalysts were
5

degassed at 300 °C for 12 h. PdO particle size and Pd dispersion were determined by
high-angle annular dark-field scanning transmission electron microscope (HAADF-
STEM, FEI Tecnai G2 Spirit) at an accelerating voltage of 300 kV or lower. The IR
spectra of pyridine adsorbed samples were recorded on Thermo fisher Nicolet iS50.
The samples were first evacuated in situ at 723 K for 2 h before pyridine adsorption,
then the pyridine vapor (1.3 kPa) was introduced into the cell at 423 K for 0.5 h.
Physically adsorbed pyridine was then removed by evacuation at 423 K for 1 h before
the IR spectra collection at 423 K.
2.3. Performance evaluation
CH4 oxidation test was operated in a quartz fixed-bed reactor (Φ=3 mm) at
atmospheric pressure and about 200 mg of catalyst (40~60 mesh) was used. The feed
gas was composed of 1 vol.% CH4 and 20 vol.% O2 balanced with N2 and passed
through the catalyst bed at a flow rate of 100 mL/min. The temperature of the catalyst
bed was programmed from 150 to 500 °C at a rate of 5 °C/min. The CH4 amount of the
reactant/product mixture was measured at a temperature interval of 25 °C by an online
gas chromatograph (GC 1690) equipped with a flame ionization detector (FID). Under
such CH4-lean conditions and with Pd-based catalysts, the CO2 selectivity was close to
100 % [7, 8]. CH4 conversion was calculated according to the CH4 content change after
the reaction. The performance stability of the sample was conducted for 30 h under the
same reaction conditions at a specific temperature of 80 % CH4 conversion. Water vapor
resistance of the samples was evaluated by introducing 5 % volume steam at intervals
at a temperature of 99 % CH4 conversion.
3. Results
3.1. Performance comparison
6

Fig. 1a displays the light-off curves for methane combustion on Beta and various
supported Pd catalysts. Apparently, the pure support exhibits negligible activity for
methane oxidation below 500 oC in comparison with those Pd-based samples. Notably,
RD-Pd/Beta exhibits a remarkably better activity than IW-Pd/Beta. The temperatures
corresponding to (>) 99 % CH4 conversion are 400 oC for RD-Pd/Beta and 500 oC for
IW-Pd/Beta, respectively. After Beta support dealumination, the activity of IW-Pd/D-
Beta (T99=375 oC) is much higher than IW-Pd/Beta (T99=500 oC), and the activity of
RD-Pd/D-Beta is also slightly higher (T99=375 oC) than that of RD-Pd/Beta (T99=400
oC). The long-term stability of the samples at a temperature of 80 % CH4 conversion is
compared in Fig. 1b. After 30 h on-stream test, the CH4 conversions respectively
declines to 35 % for IW-Pd/Beta, 61 % for RD-Pd/Beta, 68 % for IW-Pd/D-Beta, and
71 % for RD-Pd/D-Beta. Apparently, both reduction-deposition Pd loading and Beta
dealumination can enhance the activity and stability of Pd/Beta, and their combination
can further improve the performance. The catalytic performance comparison for
methane oxidation between state-of-the-art Pd supported on zeolite catalysts [4, 9, 15,
26-29] and the as-developed RD-Pd/D-Beta is listed in Table 1. Notably, RD-Pd/D-Beta
also shows an outstanding catalytic performance among the catalysts regarding the
lower complete CH4 conversion temperature with a lower Pd loading amount.
7

100
80
60
40
20
0
Beta
100
80
60
40
20
0
IW-Pd/Beta
RD-Pd/Beta
IW-Pd/D-Beta
RD-Pd/D-Beta
IW-Pd/Beta
RD-Pd/Beta
IW-Pd/D-Beta
RD-Pd/D-Beta
(a)
(b)
150 200 250 300 350 400 450 500
0
5
10
15
20
25
30
Temperature (oC)
Time on stream (h)
Fig. 1. (a) Light-off curves for methane combustion on IW-Pd/Beta, RD-Pd/Beta, IW-
Pd/D-Beta and RD-Pd/D-Beta, (b) Long-term stability test at a temperature
corresponding to 80 % CH4 conversion on IW-Pd/Beta (440 oC), RD-Pd/Beta (368 oC),
IW-Pd/D-Beta (345 oC) and RD-Pd/D-Beta (348 oC). Reaction conditions: 1 vol.% CH4,
20 vol.% O2 and N2 as a balance gas, GHSV = 30,000 mL h-1 g-1.
The effect of water vapor on the catalytic performance of samples was explored
by intermittently feeding 5 vol.% H2O vapour. As displayed in Fig. 2, the catalytic
activity for all samples falls down upon feeding the water vapor. However, the decline
degree of RD-Pd/D-Beta is greatly lower than other samples. Moreover, when the water
vapor is turned off, the CH4 conversion of IW-Pd/D-Beta and RD-Pd/D-Beta returns to
the original value and the H2O on-off cycles is repeatable, while the dramatic activity
decrease of IW-Pd/Beta and RD-Pd/Beta cannot be fully restored. Therefore, the water
resistance can be remarkably enhanced by the combination of reduction-deposition Pd
loading and zeolite support dealumination.
8

100
80
60
40
20
0
100
80
60
40
20
0
5 vol.% H
O
5 vol.% H
O
2
2
5 vol.% H
O
5 vol.% H O
2
2
IW-Pd/Beta
IW-Pd/D-Beta
RD-Pd/Beta
RD-Pd/D-Beta
(a)
(b)
4Tim6e on strea1m0 (h)
0
2
8
12
14
16
0
2
4
6
8
10
12
14
16
Time on stream (h)
Fig. 2. Methane conversion as a function of time at a temperature upon reaching (>)
99 % CH4 conversion: (a) IW-Pd/Beta (500 oC) and IW-Pd/D-Beta (410 oC), (b) RD-
o
o
Pd/Beta (415 C) and RD-Pd/D-Beta (410 C). Reaction conditions: GHSV = 30,000
mL h-1 g-1, 1 vol.% CH4, 20 vol.% O2, and a balance gas of N2 and intermittently added
5 vol.% H2O vapor.
Table 1. Performance comparison of Pd supported on zeolite for methane oxidation
Activity
Catalyst
CH4 concentration
Space velocity
Ref.
[4]
T50/oC
T100/ oC
(wt % Pd)
1% CH4
1.67% Pd/HZSM-5
1% Pd/Na-MOR
1% Pd/NaZSM-5
1.1% Pd-SSZ-13
2% Pd/HZSM-5
0.12% Pd/S-1-in
-
320
15000 mL h-1g-1
1% CH4
335
-
415
425
402
450
450
450
375
[9]
70000 h-1
0.5% CH4
[15]
[26]
[27]
[28]
[29]
This work
30000 mL h-1g-1
0.15% CH4
100000 h-1
317
-
1% CH4
15000 h-1
1% CH4
329
345
333
16000 mL h-1g-1
0.98%
1%CH4
PdOx/silicalite-1
18750 mL h-1g-1
1% CH4
0.5 % RD-Pd/D-Beta
30000 mL h-1g-1
9

3.2. Structural properties
XRD patterns of the supported Pd catalysts in Fig. 3 indicate that there is only tiny
red shift of the peak around 22.5°, corresponding to d302 spacing of Beta [25], for IW-
Pd/D-Beta (22.56°) and RD-Pd/D-Beta (22.50°) compared to IW-Pd/Beta (22.46°) and
RD-Pd/Beta (22.42°), which is attributed to the contraction of Beta matrix after the
dealumination procedure [25]. No detectable peaks of PdO or Pd are found in all the
supported Pd samples possibly due to the low loading and good dispersion of Pd-related
species. The chemical composition measured by ICP, EDS analysis, and textural
properties characterized by N2 adsorption-desorption are summarized in Table 2. As
expected, the overall silicon to aluminium ratio of the samples drastically rises after
dealumination treatment and the actual Pd loading is close to the nominal amount. Both
the BET surface area and pore volume of the dealuminated samples are slightly larger
than those pristine Beta based samples probably ascribing to the minor defect formation
during the dealumination process [30]. Apparently, Beta support dealumination
performed in this work has little effect on its crystal and textural structure.
Table 2. Physicochemical properties of the samples
Pdn+ / (Pdn++ Pd2+)c Textual properties
a
Al a
(wt %)
Pd
Oads/
Sample
Si/Alb
(wt %) ( Oads+ Olat)c
d
e
SBET
Vtotal
Fresh
Used
-
(m2 g-1)
643
(cm3 g-1)
0.42
Beta
-
-
-
-
-
IW-Pd/Beta
RD-Pd/Beta
1.87
1.78
12
12
57
62
0.51
0.57
0.44
0.51
0.34
0.31
0.75
0.74
0.47
0.49
0.79
0.81
0.29
0.30
0.53
0.79
589
591
657
641
0.41
0.39
0.45
0.50
IW-Pd/D-Beta 0.41
RD-Pd/D-Beta 0.40
a Weight percent, determined by ICP.
b Atom ratio, determined by EDS.
c Calculated by XPS spectra of the samples.
d Verified by N2 adsorption at 77 K.
e Calculated by t-plot method.
10

Beta
IW-Pd/Beta
RD-Pd/Beta
IW-Pd/D-Beta
RD-Pd/D-Beta
5
10
15
20
25
30
35
40
20
25
2-Theta
Fig. 3. XRD patterns of Beta, IW-Pd/Beta, RD-Pd/Beta, IW-Pd/D-Beta, and RD-Pd/D-
Beta
HAADF-STEM images and the corresponding PdO particles size distribution of
the as-prepared samples are compared in Fig. 4. The PdO particles are mostly
aggregated into irregular large particles on IW-Pd/Beta with a large average size of ~17
nm (Fig. 4a&b). Both the reduction-deposition loading method and Beta support
dealumination promote the PdO dispersion. Well-distributed PdO nanoparticles with
small particle size around 5.9 nm are discerned on RD-Pd/Beta (Fig. 4c&d), and the
average particle size of PdO on IW-Pd/D-Beta is around 8.5 nm (Fig. 4e&f). More
uniform PdO nanoparticles with a smaller average particle size around 4.5 nm than
those on the aforementioned samples are calculated on RD-Pd/D-Beta (Fig.4g&h),
suggesting the combination of reduction-deposition Pd loading and Beta support
dealumination benefit for dispersing PdO nanoparticles.
11

Fig. 4. HAADF-STEM images and PdO naoparticle size distribution of IW-Pd/Beta (a,
b), RD-Pd/Beta (c, d), IW-Pd/D-Beta (e, f), and RD-Pd/D-Beta (g, h)
After 30 h of on-stream stability test, the sintering of PdO particles with different
degree is observed, as displayed in Fig. 5. The average PdO particle size respectively
grows to 23.8 nm on IW-Pd/Beta-used (Fig. 5a&b), 19.7 nm on RD-Pd/Beta-used (Fig.
5c&d), 13.5 nm on IW-Pd/D-Beta-used (Fig. 5e&f), and 11.5 nm on RD-Pd/D-Beta-
used (Fig. 5g&h). Obviously, more severe sintering appears on IW-Pd/Beta-used and
RD-Pd/Beta-used in comparison with the corresponding dealuminated samples, and the
combination of reduction-deposition Pd loading and Beta support dealumination can
further alleviate the sintering problem.
12

Fig. 5. HAADF-STEM images and PdO nanoparticle size distribution of the samples
after 30 h stability test: (a&b) IW-Pd/Beta-used, (c&d) RD-Pd/Beta-used, (e&f) IW-
Pd/D-Beta-used, and (g&h) RD-Pd/D-Beta-used
3.3. Surface chemical state
FTIR spectra of the samples in Fig. 6a indicate all the samples exhibit six bands
around 3450, 1648, 1225, 1092, 751, and 543 cm-1. The broad band at about 3450 cm-
1
and the peak around 1648 cm-1 are respectively assigned to the O-H stretching and
bending mode of absorbed water or surface OH on Beta or D-Beta [31, 32]. The weak
peak around 1225 cm-1 and the strong band at 1092 cm-1 are attributed to asymmetric
stretching of the external and internal vibration of SiO4 or AlO4, respectively [32, 33].
The small peak at ~751 cm-1 corresponds to symmetric stretching of SiO4 or AlO4,
while the band observed at ~543 cm-1 is due to double ring vibration [31, 32]. Notably,
there is an extra new peak at ~954 cm-1 on D-Beta in comparison with the spectrum of
Beta, which is assigned to the stretching vibration of Si-O that is deviated SiO4
tetrahedral with a hydroxyl group [25, 34], indicating the generation of T-atom sites
attached with silanol groups are generated during Beta dealumination [23, 25]. Notably,
this peak is almost vanished after loading Pd-related species, and such silanol
13

consumption is related to the interaction of Pd-related species with hydroxyl of silanol
groups, that is, the incorporation of Pd species into D-Beta framework. This
phenomenon is consistent with the reported interaction between the metal species and
the T-atom sites in the literatures [23, 25]. Fig. 6b displays the FTIR spectra of RD-
Pd/Beta, RD-Pd/D-Beta, and the samples after water resistance test, i.e., RD-Pd/Beta-
WR and RD-Pd/D-Beta-WR. As compared, the peak around 1648 cm-1 assigned to O-
H vibration of absorbed water or hydroxyl groups on RD-Pd/Beta-WR is more intensive
than that on RD-Pd/Beta, which is not the case for RD-Pd/D-Beta-WR, indicating the
less H2O/OH- accumulation on RD-Pd/D-Beta-WR.
Beta
D-Beta
IW-Pd/Beta
RD-Pd/Beta
IW-Pd/D-Beta
954
RD-Pd/D-Beta
3450
1648
751
543
954
1092
1225
1648
RD-Pd/D-Beta
RD-Pd/D-Beta-WR
RD-Pd/Beta
RD-Pd/Beta-WR
1700
1650
1600
1550
(a)
(b)
Wavenumber (cm-1
)
1000 800
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 6. (a) FTIR spectra of Beta, D-Beta, IW-Pd/Beta, RD-Pd/Beta, IW-Pd/D-Beta, and
RD-Pd/D-Beta; (b) FTIR spectra of RD-Pd/Beta, RD-Pd/D-Beta, and the used samples
after water resistance test
Pyridine FTIR analysis of the samples in Fig. 7 exhibits the peaks at about 1554
and 1622 cm-1 corresponding to Lewis acid sites (LA), most of which are from extra-
framework aluminium species (EFAL) [30, 35], and the band at 1543 cm-1 being
assigned to Bronsted acid sites (BA), which are mainly related to the framework Al [35,
36]. In addition, the peak around 1490 cm-1 attributed to pyridine species interacting
with these two kinds of acid sites is also detected [4, 37]. The peaks around 1442 and
14

1592 cm-1 on RD-Pd/Beta and RD-Pd/D-Beta are assigned to pyridine adsorbed on
sodium ion, which is introduced during sodium borohydride reduction process [38, 39].
As compared, the amounts of BAS and LAS on the dealuminated samples IW-Pd/D-
Beta and RD-Pd/D-Beta are remarkably lower than those on IW-Pd/Beta and RD-
Pd/Beta, demonstrating that the nitric acid dealumination weakens the acidity by
cleaning out the EFAL species and removing part of framework aluminium species.
IW-Pd/Beta
RD-Pd/Beta
IW-Pd/D-Beta
RD-Pd/D-Beta
1622 LA
1454 LA
1490
1543 BA
1640 1600 1560 1520 1480 1440
Wavenumber (cm-1)
Fig. 7. FTIR spectra after the adsorption of pyridine followed with desorption at 423 K
of IW-Pd/Beta, RD-Pd/Beta, IW-Pd/D-Beta, and RD-Pd/D-Beta. LA and BA refer to
Lewis acid and Bronsted acid related bands, respectively.
The O 1s XPS spectra of the samples are provided in Fig. 8a. The red shift of O 1s
peaks of IW-Pd/D-Beta and RD-Pd/D-Beta compared with IW-Pd/Beta and RD-
Pd/Beta, is closely related to the removal of surface aluminum in consideration with the
higher O 1s peak value in SiO2 (533.2 eV) [40] than that in Beta (532.3eV) [41]. The O
1s peak of the samples was further divided into two peaks. The peak around 533.1-
533.3 eV is assigned to the surface chemisorbed oxygen (Oads) and the other one at
532.3-532.6 eV could is attributed to the lattice oxygen (Olat) [28, 42].The ratio of Oads
15

in the total surface oxygen (Oads+Olat) in each sample is calculated from the
corresponding areas of XPS fitted peaks, giving a value of 0.34 for IW-Pd/Beta, 0.31
for RD-Pd/Beta, 0.75 for IW-Pd/D-Beta, and 0.74 for RD-Pd/D-Beta, as listed in Table
2. The higher Oads ratios for IW-Pd/D-Beta and RD-Pd/D-Beta demonstrate that the
dealuminated samples hold richer surface chemisorbed oxygen.
O
O
Pd 3d5/2
n+
lat
O 1s
ads
2+
Pd
IW-Pd/Beta
RD-Pd/Beta
Pd
Pd 3d3/2
IW-Pd/BeTa
RD-Pd/BeTa
IW-Pd/D-Beta
RD-Pd/D-Beta
IW-Pd/D-BeTa
RD-Pd/D-BeTa
(a)
532 531 530 529
(b)
334
537 536 535 534
Binding5e33nergy (eV)
344
342
340
338
336
Binding energy (eV)
Pd 3d5/2
n+
Pd2+
IW-Pd/Beta-used
Pd
Pd 3d3/2
RD-Pd/Beta-used
IW-Pd/D-Beta-used
RD-Pd/D-Beta-used
(c)
334
344
342
340
338
336
Binding energy (eV)
Fig. 8. (a) O 1s XPS and (b) Pd 3d XPS spectra of IW-Pd/Beta, RD-Pd/Beta, IW-Pd/D-
Beta, and RD-Pd/D-Beta, and (c) Pd 3d XPS spectra of IW-Pd/Beta-used, RD-Pd/Beta-
used, IW-Pd/D-Beta-used, and RD-Pd/D-Beta-used after 30 h stability test
Pd 3d XPS spectra of the samples in Fig. 8b&c can be deconvoluted into two
components. The peak at 337.1-337.3 eV is assigned to Pd2+ (PdO) and the peak located
at 336.3-336.9 eV that is between metallic Pd (335.4 eV) and Pd2+ (337.1 eV) is
attributed to Pdn+ (0 < n < 2) species, demonstrating the presence of partially oxidized
Pd species [4, 43]. Due to the incorporation of Pd species in the vacant T-atom sites
16

(Fig. 6a) and the higher electronegativity of Pd in comparison with Si [44], the
introduced Pd species are less positively charged in Pd-O-Si bonds than in Pd-O-Pd
bonds, leading to higher Pdn+/Pd2+ ratio (0<n<2) in the dealuminated samples. Semi-
quantitative analysis based on the peak area gives the Pdn+/Pd2+ ratio of 0.47 for IW-
Pd/Beta, 0.49 for RD-Pd/Beta, 0.79 for IW-Pd/D-Beta, and 0.81 for RD-Pd/D-Beta. Pd
3d XPS spectra of the samples after 30 h stability test are showed in Fig. 8c. The
Pdn+/Pd2+ ratio declines to 0.29 for IW-Pd/Beta-used, 0.30 for RD-Pd/Beta-used, 0.53
for IW-Pd/D-Beta-used while shows no obvious decrease for RD-Pd/D-Beta-used
(0.79), as compared in Table 2. In addition, as compared in Fig. 8b&c, there is tiny red
shift of the Pd 3d peaks (~0.1 eV) in IW-Pd/D-Beta and RD-Pd/D-Beta compared with
those in the untreated samples (i.e., IW-Pd/Beta and RD-Pd/Beta), reflecting the
difference in the electron cloud density around the Pd cations and their electronegativity
among the samples [45, 46], which is attributed to the less positively charged Pd cations
with a lower electronegativity in the dealuminated samples. Apparently, the catalyst
prepared by the combination of Beta support dealumination and reduction-deposition
Pd loading holds the ability to tailor the chemical state of the PdOx species and
maintains the high ratio of Pdn+/Pd2+ even after 30 h of on-stream stability test.
4. Discussion
Herein, both Beta support dealumination and reduction-deposition Pd loading
were used for promoting the dispersion, activity and stability of Pd/Beta for low
temperature CH4 oxidation. The support structure modification such as nitric acid
treatment generally causes changes in the surface chemical environment, which affects
17

the electronic properties of the active component [23, 45]. Beta support dealumination
process leads to a dramatic rise of surface Si/Al ratio (Table 2), and the formation of
vacant T-atom sites attached with silanol groups (Fig. 6a & 7). It was reported that
mental precursor could interact with hydroxyl of silanol groups and be incorporated
into zeolite framework [23, 25]. Thus, vacant T-atom sites formation benefits for
dispersing and anchoring PdO nanoparticles (Table 2), leading to well-distributed PdO
particles and enhanced anti-sintering property of the dealuminated samples (Fig. 4). It
was reported that the anchoring of PdOx on the T-atom sites, where there have been
Brønsted acid sites [25], was responsible for the electronic structure modulation of the
anchored Pd species [4]. Accordingly, higher Pdn+/Pd2+ (0<n<2) ratio and richer active
surface chemisorbed oxygen amount are detected on the dealuminated samples (IW-
Pd/D-Beta and RD-Pd/D-Beta) than those on IW-Pd/Beta or RD-Pd/Beta (Table 2),
which are important for promoting the redox cycle of PdO-Pd and the enhanced CH4
combustion performance according to Mars-van Krevelen redox mechanism [4, 28, 47].
Additionally, the negative effect of water vapor on the performance of Pd/Beta, which
is mainly attributed to the formation of surface hydroxyl groups and its accumulation
on the active sites surface (Fig. 6b) [11, 48], is alleviated by the support dealumination
(Fig. 2). The zeolite support with high ratio of silicon to aluminium could improve the
hydrophobicity, and inhibit H2O adsorption and Pd(OH)2 formation to some extent [11,
13, 16, 49]. Consequently, the activity, stability, and water resistance of the
dealuminated samples are greatly improved.
In consistent with the finding in the literature [20, 22], reduction-deposition Pd
loading shows advantage in obtaining highly dispersed palladium oxide nanoparticles
(Fig. 4), thus the activity of RD-Pd/Beta is much better than IW-Pd/Beta due to more
exposure of active sites (Fig. 1a). Since the metal-support interaction and its effect on
18

the chemical state and stabilization of metal nanoparticles increase with decreasing the
size of metal particles [19], the small size and homogeneous distribution of Pd particles
(or PdO particles size after calcination) produced by reduction-deposition Pd loading
further enhance the interaction between PdO and D-Beta, i.e., stable PdO nanoparticles
and rich Pdn+ (0 < n < 2) species (Fig. 5&8b). In this regard, the combination of
reduction-deposition Pd loading and Beta support dealumination can further alleviate
the sintering problem and enhance the activity of Pd/Beta.
5. Conclusions
Efficient Pd/Beta catalyst for trace methane oxidation was developed by
combination of reduction-deposition Pd loading and Beta support dealumination. The
as-developed Pd/Beta demonstrated an excellent activity with a high methane
conversion above 99 % at 375 oC, well long-term stability, and greatly enhanced water
resistance with a low 0.5 wt.% Pd loading amount. The superior performance was
attributed to the highly dispersed PdO nanoparticles, plentiful surface chemisorbed
oxygen, stable PdO species with rich Pdn+ (0 < n < 2) species anchored on T-atom sites,
and enhanced hydrophobicity produced by the combined strategy. Briefly, we
demonstrate that the combination of reduction-deposition metal particles loading and
zeolite support dealumfiination is a facile strategy for designing efficient zeolite
supported noble metal catalyst.
Credit Author Statement
Liling Zhang: Investigation, Formal analysis, Data curation, Methodology,
Roles/Writing - original draft.
Junfei Chen: Data curation, Methodology, Writing - review & editing.
19

Xiaomin Guo: Formal analysis, Writing - review & editing.
Shumeng Yin: Formal analysis, Conceptualization.
Meng Zhang: Methodology, Formal analysis, Resources, Writing - review & editing.
Zebao Rui: Conceptualization, Methodology, Formal analysis, Resources, Supervision,
Project administration, Funding acquisition, Writing - review & editing.
Declaration of interests
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.
Acknowledgements
We appreciate the national natural science foundation of China (No. 21576298 and
21776322) for financial support.
References
[1] S.A. Montzka, E.J. Dlugokencky and J.H. Butler, Nature 476 (2011) 43-50.
[2] P. Gélin and M. Primet, Appl. Catal. B-Environ. 39 (2002) 1-37.
[3] J. Chen, X. Zou, Z. Rui, H. Ji, Energy Technol., 2019, 1900641.
[4] Y. Lou, J. Ma, W. Hu, Q. Dai, L. Wang, W. Zhan, Y. Guo, X. Cao, Y. Guo, P. Hu and
G. Lu, ACS Catal. 6 (2016) 8127-8139.
[5] X. Zou, J. Chen, Z. Rui, H. Ji, Appl. Catal. B- Environ., 273(2020) 119071.
[6] M. Cargnello, J.J.D. Jaen, J.C.H. Garrido, K. Bakhmutsky, T. Montini, J.J.C.
Gamez, R.J. Gorte and P. Fornasiero, Science 337 (2012) 713-717.
[7] X. Zou, Z. Rui, S. Song and H. Ji, J. Catal. 338 (2016) 192-201.
[8] X. Zou, Z. Rui and H. Ji, ACS Catal. 7 (2017) 1615-1625.
[9] A.W. Petrov, D. Ferri, F. Krumeich, M. Nachtegaal, J.A. van Bokhoven and O.
20

Kröcher, Nat. Commun. 9 (2018) 2545-2553.
[10] A.W. Petrov, D. Ferri, O. Kröcher and J.A. van Bokhoven, ACS Catal. 9 (2019)
2303-2312.
[11] I. Friberg, N. Sadokhina and L. Olsson, Appl. Catal. B- Environ. 250 (2019) 117-
131.
[12] M. Hoffmann, S. Kreft, G. Georgi, G. Fulda, M. Pohl, D. Seeburg, C. Berger-
Karin, E.V. Kondratenko and S. Wohlrab, Appl. Catal. B-Environ. 179 (2015) 313-320.
[13] K. Okumura, S. Matsumoto, N. Nishiaki and M. Niwa, Appl. Catal. B- Environ.
40 (2003) 151-159.
[14] Q. Dai, S. Bai, Y. Lou, X. Wang, Y. Guo and G. Lu, Nanoscale 8 (2016) 9621-
9628.
[15] Q. Dai, Q. Zhu, Y. Lou and X. Wang. J. Catal. 357 (2018) 29-40.
[16] K. Okumura, E. Shinohara and M. Niwa, Catal. Today 117 (2006) 577-583.
[17] S. Colussi, A. Gayen, M. Farnesi Camellone, M. Boaro, J. Llorca, S. Fabris and A.
Trovarelli, Angew. Chem. Int. Ed. 48 (2009) 8481-8484.
[18] Z. Rui, S. Wu, C. Peng and H. Ji, Chem. Eng. J. 243 (2014) 254-264.
[19] W. Ji, X. Wang, M. Tang, L. Yang, Z. Rui, Y. Tong and J.Y.S. Lin, Chem. Commun.
55 (2019) 6846-6849.
[20] H. Huang and D.Y.C. Leung, ACS Catal. 1 (2011) 348-354.
[21] Y. Wang, H. Arandiyan, J. Scott, M. Akia, H. Dai, J. Deng, K.-F. Aguey-Zinsou
and R. Amal, ACS Catal. 6 (2016) 6935-6947.
[22] J. Ma, Y. Lou, Y. Cai, Z. Zhao, L. Wang, W. Zhan, Y. Guo and Y. Guo, Catal. Sci.
Technol. 8 (2018) 2567-2577.
[23] S. Dzwigaj, Y. Millot, J. Krafft, N. Popovych and P. Kyriienko, J. Phys. Chem. C.
117 (2013) 12552-12559.
21

[24] G. Wang, S. Xu, L. Wang, Z. Liu, X. Dong, L. Wang, A. Zheng, X. Meng and F.
Xiao, Chem. Commun. 54 (2018) 3274-3277.
[25] C. Chen, Z. Hu, J. Ren, S. Zhang, Z. Wang and Z. Yuan, ChemCatChem. 11 (2019)
868-877.
[26] J.B. Lim, D. Jo and S.B. Hong, Appl. Catal. B-Environ. 219 (2017) 155-162.
[27] C. Liu, K. Yu, Y. Zhang, X. Zhu, F. He and B. Eliasson, Appl. Catal. B-Environ.
47 (2004) 95-100.
[28] R. Niu, P. Liu, W. Li, S. Wang and J. Li, Micropor. Mesopor. Mat. 284 (2019) 235-
240.
[29] C. Xiao, Y. Du, L. Dong, X. Zhou, Y. Xie and Z. Tan, Catal. Lett. 147 (2017) 1775-
1782.
[30] Y. Wang, R. Otomo, T. Tatsumi and T. Yokoi, Micropor. Mesopor. Mat. 220 (2016)
275-281.
[31] E. Ng, H. Nur, K. Wong, M.N.M. Muhid and H. Hamdan, Appl. Catal. A- Gen.
323 (2007) 58-65.
[32] H. Nur, G. Lee Kee, H. Hamdan, T.M. Indra Mahlia, J. Efendi and H.S. Cornelis
Metselaar, Int. J. Hydrogen Energ. 37 (2012) 12513-12521.
[33] R.W. Silverstein and G.C. Bassler, Wiley Etextbooks, US, 2014.
[34] S. Dzwigaj, P. Massiani, A. Davidson and M. Che, J. Mol. Catal. A-Chem. 155
(2000) 169-182.
[35] J.P. Marques, I. Gener, P. Ayrault, J.C. Bordado, J.M. Lopes, F. Ramôa Ribeiro
and M. Guisnet, Micropor. Mesopor. Mat. 60 (2003) 251-262.
[36] J.X. Liu, N. He, C.Y. Liu, G.R. Wang, Q. Xin and H.C. Guo, RSC Adv. 8 (2018)
9731-9740.
[37] F. Jin and Y. Li, Catal. Today 145 (2009) 101-107.
22

[38] E. Dumitriu, V. Hulea, S. Kaliaguine and M.M. Huang, Appl. Catal. A- Gen. 135
(1996) 57-81.
[39] Y. Wang, Y. Liu, X. Li, H. Wu, M. He and P. Wu, J. Catal. 266 (2009) 258-267.
[40] M.H. Kibel and P.W. Leech, Surf. Interface Anal. 24 (1996) 605-610.
[41] R.B. Borade and A. Clearfield, J. Phys. Chem. 96 (1992) 6729-6737.
[42] J. Ding, Z. Rui, P. Lyu, Y. Liu, X. Liu and H. Ji, Appl. Surf. Sci. 457 (2018) 670-
675.
[43] Y. Shen, G. Lu, Y. Guo, Y. Wang, Y. Guo and X. Gong, Catal. Today. 175 (2011)
558-567.
[44] J.A.E. Dean, Lange's Handbook of Chemistry, sixteenth ed., McGraw-Hill, New
York, 2005.
[45] D. Wang, J. Luo, Q. Yang, J. Yan, K. Zhang, W. Zhang, Y. Peng, J. Li and J.
Crittenden, Environ. Sci. Technol. 53 (2019) 6937-6944.
[46] D. Wang, Y. Peng, S. Xiong, B. Li, L. Gan, C. Lu, J. Chen, Y. Ma and J. Li, Appl.
Catal. B- Environ. 221 (2018) 556-564.
[47] J. Chen, H. Arandiyan, X. Gao and J. Li, Catal. Surv. Asia. 19 (2015) 140-171.
[48] N. Sadokhina, F. Ghasempour, X. Auvray, G. Smedler, U. Nylén, M. Olofsson
and L. Olsson, Catal. Lett. 147 (2017) 2360-2371.
[49] K. Okumura, E. Shinohara and M. Niwa, Catal. Today 117 (2006) 577-583.
23