J. Song et al.
Applied Catalysis A, General 621 (2021) 118211
catalytic activity [20].
pulse method. Typically, 100 mg catalyst was pretreated under 10 % H2/
He (30 mL/min) at 300 ◦C for 2 h. Pulses of 10 % CO/He were intro-
duced to the catalyst at room temperature until uptake saturation was
obtained.
Due to the peculiarity of the desired crystalline structure and the
need of maintaining a high surface area for Pd loading, the preparation
method is crucially important [2]. Li et al. [21] found that spinel syn-
thesized via layered double hydroxides (LDHs) precursor by controlled
thermal decomposition possesses higher surface area and
well-crystallized lattice. LDHs are composed of positively charged hy-
droxide layers and negatively charged interlayer galleries where anions
and water molecules exist [22]. LDHs have two attractive features that
impact catalytic properties: basic sites and uniform distribution of metal
cations. Mixed metal oxides can be formed by removing the interlayer
anion and water through heat treatment [23]. Therefore Mg-Al LDHs are
considered to be beneficial for preparing MgAl2O4 spinel support.
Herein, in consideration of the promotion effect of MgAl2O4 on the
performance of PdO and the necessity of a high surface area of support,
the spinel-supported palladium catalysts prepared via LDHs precursors
were designed for methane combustion with outstanding performance.
A series of Pd/MgxAly catalysts with various Mg/Al molar ratios (x:
y = 0:1, 1:4, 1:3, 1:2, 1:1, 2:1, 1:0) were investigated. Furthermore, the
interaction between MgAl2O4 spinel and PdO, surface oxygen mobility
as well as the acid-base property of supports were discussed in detail.
The NH3 and CO2 adsorption sites and binding energies were ob-
tained through NH3-TPD and CO2-TPD experiments on a Micromeritics
Autochem II 2920 connected with a TCD detector. The supports of
◦
100 mg were treated under He at 200 C for 1 h, cooled down to the
room temperature. In case of NH3-TPD measurement, NH3 was adsorbed
on metal oxides for 30 min. In the case of CO2-TPD, CO2 was injected to
the samples for 1 h. And then, temperature was raised to 700 ◦C under
He with a rate of 10 ◦C/min.
The CH4-TPR experiment was carried out on a fixed-bed flow reactor
equipped with an FID detector. Prior to the test, the catalysts were
treated in air at 400 ◦C for 1 h. The TPR tests were carried out by heating
the catalysts from 50 ◦C to 500 ◦C with a rate of 5 ◦C /min in 1 vol%
CH4/N2.
X-ray photoelectron spectra (XPS) were performed on an ESCALAB
Xi+ spectrometer (ThermoFisher Scientific) using Al Kα (hυ = 1486.6ev)
radiation as the X-ray source. The C 1s peak at 284.8 eV was used to
calibrate the XPS spectra. The catalysts were pre-treated at 300◦C for 2 h
under the high-vacuum environment and then transferred into the test
chamber without exposure to external environment.
2. Experimental
Raman spectra of the catalysts were recorded on a Laser Micro-
Raman spectrometer. An Ar-laser excitation source at λ 532 nm with a
2.1. Materials
power of 10 mW was used. The resolution was 1 cmꢀ 1
.
Al(NO3)3⋅9H2O (99 %), Na2CO3⋅10H2O (99 %), and NaOH (99 %)
were purchased from Kemiou Chemical Reagent Co. Ltd. Mg
(NO3)2⋅6H2O (99 %) was purchased from Tianjin Jiangtian Chemical Co
Ltd. Pd(NO3)2⋅2H2O (≥39 % Pd) was purchased from Shanghai Aladdin
Bio-Chem Technology Co. Ltd.
2.4. Catalytic performance
A fixed-bed tubular quartz reactor (length 240 mm and inner
diameter 6 mm) loaded with 100 mg of catalyst (sieve fraction of
0.25ꢀ 0.4 mm) was used to investigate the methane combustion under
atmospheric pressure. The feed gas is a mixture of 1 vol% CH4 in air at a
total flow rate of 180 mL/min. The gas hourly space velocity is
108,000 mL/g h The water vapor was supplied by passing air through a
saturation evaporator. Temperature was ramped at 1 ◦C /min from 200
to 500 ◦C. The effluent gas was analyzed online by a SP-7890A GC
equipped with a FID detector. The selectivity of CO2 was regarded as 100
% over the Pd-based catalysts under CH4-lean conditions (molar ratio of
CH4/O2 = 1/20) [24], therefore the CH4 conversion was used as the
index for the performance of the catalysts. Turnover frequency (TOF)
and the apparent activation energy (Ea) was performed at a methane
conversion below 20 %. Each data point was taken for 5 times at each
temperature.
2.2. Catalyst preparation
Mg-Al LDHs-containing precursor was synthesized under the co-
precipitation method. Typically, Mg(NO3)2⋅6H2O and Al(NO3)3⋅9H2O
with different molar ratio were dissolved in 100 mL de-ionized water,
and then was dropwise added into 100 mL Na2CO3 aqueous solution
under stirring at 40 ◦C. NaOH aqueous solution was used to adjust the
pH of the solution to 10 ± 0.5. After stirring for 35 min, the as-formed
◦
suspension was heated to 70 C and aged for 12 h. The precipitation
was filtrated, dried and calcined at 700 ◦C for 5 h in static air. The
palladium catalysts with 0.5 wt% Pd loading were prepared by incipient
wetness method, and followed by drying at 80 ◦C overnight in a vacuum
oven and calcining at 550 ◦C for 3 h. The obtained catalysts were
denoted as Pd/MgxAly, where x and y represent the molar ratios of Mg
and Al.
3. Result
3.1. Catalytic activity in methane combustion
2.3. Catalyst characterization
The light-off curves of Pd/MgxAly for methane combustion are pre-
sented in Fig. 1a. The activity profiles demonstrate that the catalytic
activity varies with the Mg/Al ratios. Apparently, adding Mg into the
Al2O3 support significantly improves the activity of Pd/Al2O3 catalyst,
but the promotion effect strongly depends on the amount of Mg. Among
these catalysts, Pd/MgAl3 and Pd/MgAl4 show better apparent activity
than Pd/Al2O3. However, further increasing the amount of Mg leads to a
loss in catalytic activity. As listed in Table 1, T10, T50 and T90 are re-
action temperature corresponding to CH4 conversion of 10 %, 50 % and
90 % in activity test. All of them decrease in Pd/MgAl3 catalyst and the
lowest ones are at 275, 336 and 378 ◦C, respectively, which are lower
than most of the previously reported catalysts (Table S1).
The specific surface areas and pore size distribution were obtained by
the static nitrogen physical adsorption-desorption. A Micromeritics
Tristar II 3000 analyzer instrument was used. The samples were pre-
treated at 300 ◦C for 3 h to remove water.
Inductively coupled plasma optical emission spectrometer (ICP-OES,
Varian Vista-MPX) was employed to measure the actual Pd loadings.
High-resolution TEM (HRTEM) and STEM energy dispersive spec-
troscopy (STEM-EDS) micrographs were measured on a JEM-200 F
electron microscope at 200 kV.
X-ray powder diffraction (XRD) patterns of the catalysts were con-
ducted on a Rigaku C/MAX-2500 diffractometer with a Cu Kα radiation
(γ =1.5406 Å). Data were obtained by scanning from 20◦ to 80◦ at the
According to the calculated results of Mears and the Weise-Prater
criterions, the external and internal diffusion effects on the kinetic test
can be excluded (Table S2). Fig. 1b. shows the Arrhenius result of Pd/
MgxAly catalysts and Ea were calculated and summarized in Table 1.
Obviously, Ea values decrease in the following order: Pd/MgO (85.9 kJ/
scanning speed of 6◦/min.
The amount of CO chemisorption of the catalyst, which was used to
calculate the dispersion of Pd, was measured by Micromeritics
Autochem II 2920 equipped with a TCD detector using the dynamic
2