Effects of metal cation doping in CeO2 support on catalytic methane steam reforming at low temperature in an electric field

Article abstract of DOI:10.1039/d0ra01721c

Catalytic methane steam reforming was conducted at low temperature using a Pd catalyst supported on Ce_1-xM_xO₂ (x = 0 or 0.1, M = Ca, Ba, La, Y or Al) oxides with or without an electric field (EF). The effects of the catalyst support on catalytic activity and surface proton hopping were investigated. Results show that Pd/Al-CeO₂ (Pd/Ce_0.9Al_0.1O₂) showed higher activity than Pd/CeO₂ with EF, although their activity was identical without EF. Thermogravimetry revealed a larger amount of H₂O adsorbed onto Pd/Al-CeO₂ than onto Pd/CeO₂, so Al doping to CeO₂ contributes to greater H₂O adsorption. Furthermore, electrochemical conduction measurements of Pd/Al-CeO₂ revealed a larger contribution of surface proton hopping than that for Pd/CeO₂. This promotes the surface proton conductivity and catalytic activity during EF application.

Full text of DOI:10.1039/d0ra01721c

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PAPER
Effects of metal cation doping in CeO2 support on
catalytic methane steam reforming at low
temperature in an electric field†
Cite this: RSC Adv., 2020, 10, 14487
Ayako Takahashi,a Reona Inagaki,a Maki Torimoto,a Yudai Hisai,a Taku Matsuda,a
c
a
Quanbao Ma,b Jeong Gil Seo,
Shuhei Ogo,
Takuma Higo,
Hideaki Tsuneki,a
*
Truls Norbyb and Yasushi Sekine
a
a
Catalytic methane steam reforming was conducted at low temperature using a Pd catalyst supported on
Ce1ꢀxMxO2 (x ¼ 0 or 0.1, M ¼ Ca, Ba, La, Y or Al) oxides with or without an electric field (EF). The effects
of the catalyst support on catalytic activity and surface proton hopping were investigated. Results show
that Pd/Al-CeO2 (Pd/Ce0.9Al0.1O2) showed higher activity than Pd/CeO2 with EF, although their activity
was identical without EF. Thermogravimetry revealed a larger amount of H2O adsorbed onto Pd/Al-CeO2
than onto Pd/CeO2, so Al doping to CeO2 contributes to greater H2O adsorption. Furthermore,
electrochemical conduction measurements of Pd/Al-CeO2 revealed a larger contribution of surface
proton hopping than that for Pd/CeO2. This promotes the surface proton conductivity and catalytic
activity during EF application.
Received 22nd February 2020
Accepted 31st March 2020
DOI: 10.1039/d0ra01721c
rsc.li/rsc-advances
resistant materials and multiple heat-exchange processes. To
resolve these difficulties, various catalytic research efforts
1 Introduction
related to MSR have been investigated to decrease the reaction
temperature. Several researchers have reported that MSR can
proceed at 873 K over Ni-based catalysts,10,11 but further inves-
tigation must be undertaken to establish new catalytic
processes for obtaining high conversion rates, even at low
temperatures.
Currently, H2 is anticipated as an alternative energy source to
fossil fuel, and as an important resource for chemical indus-
tries. The most widely used process for H2 production is
methane steam reforming (MSR),1,2 the reaction of which
proceeds as shown in eqn (1). The water gas shi reaction
proceeds sequentially as shown in eqn (2). The total reaction
can be presented as eqn (3).
Our earlier studies12–19 revealed that application of an electric
eld (EF) to Pd, Pt, or Rh catalyst supported on CeO2 enabled
MSR to proceed even at low temperature such as 473 K. Espe-
cially in a lower temperature region (T < 600 K), CH4 conversion
exceeded the thermodynamic equilibrium.15 Furthermore,
various experiments were conducted to investigate the reaction
mechanism with EF, and the following results were obtained.
Firstly, to observe changes of the surface species with EF,
operando – diffuse reectance infrared Fourier transform
(DRIFT) measurements using Pd/CeO2 catalyst were conduct-
ed.15 When EF was applied, the peak at 855 cmꢀ1 was observed,
which strongly relates to proton conduction via H bond of
water, so-called the Grotthuss mechanism. Furthermore, peak
of the O–H stretching bond shied to lower wavenumber, which
indicated that the water is activated by applying EF. These
results revealed applying the EF promotes surface proton
hopping via adsorbed H2O on the catalyst surface. Kinetics
analysis using isotope were also conducted, and the inverse
kinetic isotope effect (inverse KIE) was observed only when EF
was applied.16 Considering inverse KIE theory and surface
proton conduction in EF, the reaction mechanism is considered
that proton collides with CH4. Second, to elucidate the active
CH4 + H2O / CO + 3H2 DH0298 ¼ 206.1 kJ molꢀ1
CO + H2O / CO2 + H2 DH0298 ¼ ꢀ41.2 kJ molꢀ1
(1)
(2)
CH4 + 2H2O / CO2 + 4H2 DH0298 ¼ 164.9 kJ molꢀ1 (3)
In general, this reaction is conducted industrially at high
temperatures over 973 K with Ni catalysts to achieve high H2
yield because it is a highly endothermic reaction and because
CH4 has a solid C–H bond.1–9 Nevertheless, such a severe
condition entails many severe shortcomings such as carbon
deposition, catalyst deactivation, and the need for heat-
aDepartment of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo,
169-8555, Japan. E-mail: ysekine@waseda.jp
b
´
Department of Chemistry, University of Oslo, FERMiO, Gaustadalleen 21, NO-0349,
Oslo, Norway
cDepartment of Energy Science and Technology, Myongji University, Nam-dong,
Cheoin-gu, Yongin-si, Gyeonggi-do 449-728, South Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01721c
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site, turn over frequency (TOF) normalized by the number of Pd bed. As DC current (3–9 mA) was applied to the catalyst bed
on both two sites, perimeter and surface, were calculated through these electrodes, the response voltage was recorded
respectively. Results revealed that TOF of the reaction with EF using a digital phosphor oscilloscope (TDS 2001C; Tektronix
depends on the number of Pd at perimeter, although that of Inc.). The actual catalyst bed temperature was monitored using
conventional thermal reaction depends on the number of a thermocouple inserted into the reactor. The furnace temper-
surface Pd.15 Hence, the active site is metal-support interface in ature was xed at 473 K for the reaction in EF. The reaction gas
the reaction with EF, while that is the metal surface in the composition was CH4 : H2O : Ar ¼ 1 : 2 : 7 (total ow rate: 120
conventional reaction. To summarize these works, it is revealed SCCM). The product gases were analyzed aer the reaction
that the hopping proton collides with CH4 at the metal–support using a gas chromatograph (GC-2014; Shimadzu Corp.) with
interface, which promotes dissociative adsorption of CH4, the a ame ionization detector (FID) and a handmade methanizer
rate-determining step of MSR.15,16 These results demonstrated (Ru/Al2O3). CH4, CO, and CO2 were assessed using GC-FID with
that the reaction mechanism with EF differs completely from a Porapak QS packed column. The reaction rate was calculated
the conventional one.
using the amount of generated CO and CO2 (rCO+CO ).
2
Consequently, surface proton hopping has a fundamentally
important effect for obtaining high catalytic activity in MSR
with EF. Because surface proton hopping occurs mainly over
catalyst support,20 the surface proton hopping and MSR activity
2.3. Evaluation of the Pd loading state
2.3.1. CO-pulse adsorption measurement. With a catalyst
in the EF can presumably be controlled by doping various analyzer (BELCAT II; Microtrac BEL Corp.), we took CO-pulse
cations to the catalyst support. This study was aimed at measurements to evaluate the particle size of the supported
increasing the catalytic activity with EF by doping various Pd. About 50 mg of each catalyst was placed in the cell. First, the
cations to the catalyst support, anticipating the improvement of catalyst was heated to 473 K for 60 min under He atmosphere as
surface proton conduction. To investigate the doping effects on a pre-treatment. Then, the temperature was cooled to 323 K and
activity and surface ion conductivity, various Pd/Ce1ꢀxMxO2 kept for a while. Aer these operations, a series of CO pulses
(x ¼ 0 or 0.1; M ¼ Ca, Ba, La, Y or Al) catalysts were prepared. was injected until the detected CO amount reached a steady
Then their MSR activity and surface ion conductivity were state. The amount of adsorbed CO on Pd was measured
evaluated.
assuming adsorption stoichiometry of CO/Pd ¼ 1.
2.3.2. Diffuse reectance infrared Fourier transform spec-
troscopy (DRIFTS) measurement. We conducted DRIFTS
measurements with resolution of 2 cmꢀ1 and 50 scans using an
FT-IR spectrometer (FT/IR-6200; Jasco Corp.) equipped with an
MCT detector and ZnSe window. About 80 mg of catalysts sieved
to 355–500 mm were used. As a pre-treatment, the catalyst was
heated at 723 K for 30 min and was then cooled to 323 K,
supplying only Ar (65 SCCM ow rate). Subsequently, the
background spectra (BKG) were recorded. The spectra of
adsorbed CO on Pd over the respective supports were recorded
aer purging with CO + Ar gas (CO : Ar ¼ 1 : 12; 65 SCCM total
ow rate) for 10 min.
2.3.3. X-ray photoelectron spectroscopy (XPS) measure-
ment. The electronic state of Pd over catalyst was investigated
by X-ray photoelectron spectroscopy (XPS; VersaProbe 2;
ULVAC-PHI Inc.). The measurements were conducted with an Al
Ka X-ray source. The binding energies were calibrated to C 1s
peak at 284.8 eV. The distribution of Pd0 and Pd2+ was estimated
2 Experimental
2.1. Catalyst preparation
For this study, CeO2 and Ce0.9M0.1O2 (M ¼ Ca, Ba, La, Y or Al)
supports were prepared using a complex polymerization
method with each metal nitrate, e.g. Ce(NO3)3$6H2O,
Ca(NO3)2$4H2O, Ba(NO3)2, La(NO3)3$6H2O, Y(NO3)3$6H2O and
Al(NO3)3$9H2O (Kanto Chemical Co. Inc.). Aer ethylene glycol
and citric acid were dissolved in distilled water, metal nitrates
were dissolved in the solution. The resultant mixed solution was
heated at 343 K for 24 h with stirring. Then the solution was
heated on a hot plate to remove water completely. The obtained
powders were calcined at 773 K for 5 h, with a ramping rate of
10 K minꢀ1
.
As an active metal, Pd (1.0 wt%) was loaded on the prepared
CeO2 or Ce0.9M0.1O2 (M-CeO2) support using an impregnation
method with Pd(OCOCH3)2 (Kanto Chemical Co. Inc.) as a metal
precursor. Aer drying catalysts at 393 K for 24 h, they were
by each peak area of 3d5/2 and 3d3/2
.
calcined at 723 K for 6 h, with a ramping rate of 10 K minꢀ1
.
2.4. Other characterizations
2.2. Catalytic activity test
Powder X-ray diffraction (XRD; SmartLab III; Rigaku Corp.) for
Catalytic activity tests were conducted using a xed-bed ow- each sample was observed at 40 kV and 40 mA with Cu-Ka
type reactor at atmospheric pressure. A quartz tube (8.0 mm radiation in order to investigate the crystalline structure of Pd
o.d., 6.0 mm i.d.) was used as the reactor tube. Catalyst (80 mg) loaded catalysts and supports. Results are shown in Fig. S1 and
was placed in the reactor. The catalyst bed height was 1.6 mm, S6 (ESI).† Raman spectra were recorded with a Raman spec-
and we conrmed that the condition is in the kinetic region and troscopy (NRS-4500; JASCO Corp.). Moreover, we took induc-
that diffusion is not a rate determining factor. Two stainless tively coupled plasma optical emission spectrometry (ICP-OES;
steel electrodes (2.0 mm o.d.) were inserted into the reactor and 5100 ICP-OES, Agilent Inc.) to detect the molar ratio of the
were attached to both the top and bottom sides of the catalyst support. About 20 mg of the catalyst was dissolved in nitric acid.
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Calibration curves were performed by Ce and Al solutions in the following eqn. (5)) was measured using N2 adsorption by
ranging from approximately 0–20 ppm.
the Brunauer–Emmett–Teller (BET) method using an auto-
mated specic surface area analyzer (Gemini VII; Micromeritics
Instrument Corp.).
2.5. Measurement of electron/ion conductivity
W473 K ꢀ W723 K
2.5.1. Electrochemical impedance spectroscopy (EIS)
measurement. Electrochemical impedance spectroscopy (EIS)
measurements were taken under dry (Ar) atmosphere in a two-
electrode four-wire cell connected to an impedance spectrom-
eter (alpha-Al; Novocontrol Technologies) with a ZG4 interface.
The measurement sample preparation procedures are
described in ESI.† Only Ar gas (50 SCCM total ow rate) was
supplied into the measurement cell, at temperatures of
373–673 K. All AC impedance spectra were recorded at
frequencies of 106 to 10ꢀ3 Hz with amplitude of 0.5 V RMS. The
obtained data were analyzed using equivalent circuit tting
soware (ZView ver. 3.5a; Scribner Associates Inc.). The equiv-
alent circuit model is a simple RC parallel circuit of bulk
component. Electrical conductivity was calculated using eqn (4),
where L represents the pellet thickness, S denotes the Pt elec-
trode area, and R stands for the tted resistance value.
Wwater
¼
(5)
Acatalyst ꢁ m
Wwater/mg mꢀ2: the amount of adsorbed H2O per specic
surface area at 473 K, W473
K)/g: weight loss at 473 K
(723
K
(723 K), Acatalyst/m2 gꢀ1: specic surface area of catalysts, m/g:
initial weight of catalysts.
3 Results and discussion
3.1. Catalytic activity tests
Catalytic activity tests were performed using 1.0 wt%Pd/M-CeO2
(M ¼ Ca, Ba, Y, La or Al) with and without an electric eld (EF) to
evaluate the metal doping effect on catalytic activity. Results are
presented in Fig. 1. In addition, the temperature dependence of
CH4 conversion without EF and its related data are presented
respectively in Fig. S2 and Table S1.† Without application of EF
(Fig. 1(A)), although little difference of catalytic activity at higher
temperature than 700 K was observed from Fig. S2 (ESI),† the
apparent activation energies were almost identical among all
catalysts (Table S1).† Considering that the catalytic activity is
known to depend on the metal dispersion and electronic state
of the supported metal, the metal dispersion and the electronic
state of the supported metal have little difference among these
catalysts. With EF (Fig. 1(B)), however, the activity varied
markedly by doping different metal cations. Particularly, as
presented in Fig. 1(C), Pd/Al-CeO2 showed higher activity than
Pd/CeO2 with EF at the same electric power (EP) input (0.8 W).
L
S
1
L
S
I
s ¼
ꢁ
¼
ꢁ
(4)
R
V
2.5.2. Measurement of apparent conductivity using
a reactor. The apparent conductivity including electrons and
ions of each sample was assessed under the reaction atmo-
sphere using a xed-bed ow-type reactor with conditions
identical to those described in Section 2.2. The furnace
temperature was changed in the range of 333–573 K. DC current
(9 mA) was applied at 10 min aer the reactant gases were
provided. The response voltages at 5 and 10 min aer applica-
tion of current were recorded with an oscilloscope at each
temperature. The apparent electron/ion conductivity calculated
using eqn (4), where I denotes the applied current and V
signies the response voltage.
2.6. Characterization for adsorbed H2O
2.6.1. In situ DRIFTS measurements. To evaluate surface
adsorbed H2O on the catalyst, in situ DRIFTS measurements
were performed under the same conditions and using the
catalysts as those described in 2.3.1. The temperature was
increased to 473 K under only Ar atmosphere (65 SCCM). The
BKG spectra were recorded aer the temperature was stabilized.
The spectra were recorded aer supplying the reactant gases
(CH4 : H2O : Ar ¼ 1 : 2 : 62; 65 SCCM total ow rate) for 10 min.
2.6.2. Thermogravimetry (TG) measurements. Using
a thermogravimetric analyzer (TGA-50; Shimadzu Corp.) we
took TG measurements. Aer about 40 mg of each catalyst was
heated at 723 K for over 60 min under Ar atmosphere (100
SCCM ow rate), the sample was cooled to 323 K and heated to
723 K with a ramping rate of 5 K minꢀ1 under wet gas (H2O : Ar
¼ 1 : 49; 100 SCCM total ow rate). During these procedures,
the weight change of sample was attributed to H2O adsorption/
desorption. The amount of adsorbed H2O was calculated using
Fig. 1 Catalytic activity of 1.0 wt%Pd/M-CeO2 (M ¼ Ca, Ba, La, Y or Al).
(A) Arrhenius plots without EF at 523–723 K, (B) electric power (EP)
dependences of activity with EF (3–9 mA) at 473 K, and (C) catalytic
eqn (5). The specic surface area of catalysts (denoting Acatalyst activity with EF (EP ¼ 0.8 W) at 473 K.
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Table 1 The results of XPS analysis for Pd 3d5/2 and 3d3/2
To clarify the positive effect of Al-doping on the catalytic activity
with EF, various characterizations were conducted for both Pd/
CeO2 and Pd/Al-CeO2 in the following sections.
Pd distribution at
surface/%
Binding energy/eV
Catalyst
Pd0
Pd2+
Pd0
Pd2+
3.2. Pd loading state
Pd/CeO2
Pd/Al-CeO2
335.3
335.4
336.5
336.7
42.2
38.4
57.8
61.6
Catalytic activity is well known to depend on the metal particle
size and electronic state of the supported metal. To investigate
the particle size and electronic state of the supported Pd, CO-
pulse chemisorption and DRIFTS measurements were con-
ducted with CO adsorption. Results of CO-pulse measurements
show that the Pd particle size of Pd/Al-CeO2 was similar to that
of Pd/CeO2 (Pd/CeO2: 1.12 nm, Pd/Al-CeO2: 1.60 nm). The
results of XPS is shown in Fig. 2 and Table 1. The peaks at
335.30 eV for Pd/CeO2 and 335.39 eV for Pd/Al-CeO2 were
assigned to Pd 3d5/2, while the peaks at 336.52 eV for Pd/CeO2
and 336.75 eV for Pd/Al-CeO2 were assigned to 3d3/2, respec-
tively.21,22 As shown in Table 1, the Pd0 distribution for Pd/CeO2
was similar to that of Pd/Al-CeO2 (Pd/CeO2; 42.2%, Pd/Al-CeO2;
38.4%). The apparent activation energies Ea for these catalysts
were also similar without the electric eld. Results of DRIFTS
measurements aer CO adsorption are shown in Fig. S3.† Both
catalysts showed a peak at 2087 cmꢀ1, which is assignable to the
stretching vibration of liner-adsorbed CO on Pd
(2100–2000 cmꢀ1).23,24 Because the wavenumber of this peak on
Pd/CeO2 was equal to that on Pd/Al-CeO2, the electronic states
of the supported Pd over these catalysts are estimated as the
same. By CO-pulse, XPS and DRIFTS measurements, results
suggest that the Pd loading state over each support was almost
identical. Therefore, we inferred that the difference in catalytic
activity with EF between Pd/CeO2 and Pd/Al-CeO2 was not
derived from the difference in Pd loading state (i.e. the particle
size and electronic state).
over the catalyst was investigated qualitatively and quantita-
tively based respectively on DRIFTS and TG measurements.
Fig. S4† shows the DRIFT spectra of Pd/CeO2 and Pd/Al-CeO2
under owing CH4 + H2O + Ar gas at 473 K. A peak at 3675 cmꢀ1
,
which is assignable to the O–H stretching bond of adsorbed
H2O,26,27 was observed over both catalysts. In other words, H2O
molecules surely adsorbed over the catalysts even at 473 K. This
result indicated that the strengths of O–H stretching bond of
adsorbed H2O over both samples were identical, and that the
proton mobility would be almost identical.
Then, TG measurements were taken to quantify the amount
of adsorbed H2O on the catalysts. Fig. 3 depicts the temperature
dependence of the weight loss per specic surface area (SSA). As
shown in Fig. 3, the amount of adsorbed H2O depended on the
temperature drastically. Therefore, the amount of adsorbed
H2O at 473 K for both catalysts was considered, so as to compare
the effect of the amount of H2O on the surface proton hopping
at the reaction temperature. Table 2 presents the calculated
amount of H2O over each catalyst. As depicted in Table 2, the
adsorbed amount of adsorbed H2O per unit area of Pd/Al-CeO2
was greater than that of Pd/CeO2. K. Murakami et al. revealed
that the adsorption energy of H2O becomes lower with doping
smaller cation like Al into CeO2 by DFT calculation.28 By doping
Al to CeO2, the adsorption energy of H2O on several Ce cations
which are adjacent to Al decreases. Therefore, the amount of
adsorbed H2O improved drastically. In other words, doping Al
to CeO2 caused that more H2O molecules adsorbed compared to
Pd/CeO2, which contributed to the promotion of surface proton
conduction.
3.3. Characterization for adsorbed H2O
In principle, proton conduction depends on the strength of the
O–H stretching bond of adsorbed H2O or the adsorbed amount
of H2O on the surface.15,25 Therefore, the adsorbed H2O value
3.4. Evaluation of proton conductivity
The surface electronic/ionic conductivity plays an important
role in the reaction in the electric eld.15–18 The electronic/ionic
conductivity was assessed under dry (Ar) and wet (reaction
condition i.e. CH4 : H2O : Ar ¼ 1 : 2 : 7) atmospheres to evaluate
the surface proton conduction. Fig. S5† shows the temperature
dependence of conductivity for CeO2 and Al-CeO2 under a dry
condition. In fact, both supports showed typical Arrhenius
behavior: the conductivity decreased with decreasing tempera-
ture. In this temperature region (373 K < T < 673 K), the domi-
nant conductive carrier mechanism was estimated as electron
diffusion in the inner bulk,15,25 because there is no surface
adsorbate. Comparison of the behaviors of CeO2 and Al-CeO2
reveals that the apparent activation energy and the conductive
magnitude were almost equal. Fig. 4 depicts the temperature
dependence of conductivity of Pd/CeO2 and Pd/Al-CeO2 under
Fig. 2 Pd 3d spectra for Pd/CeO2 and Pd/Al-CeO2.
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Fig. 4 Temperature dependence of electron/ion conductivity of Pd/
CeO2 and Pd/Al-CeO2 under a wet condition (CH4 : H2O : Ar ¼
1 : 2 : 7) with application of 9 mA current.
Fig. 3 Temperature dependence of weight loss on specific surface
area (SSA). Inset is a flow chart of the experiment.
The apparent activation energy and conductive magnitude of
electrochemical conductivity are almost equal between CeO2
and Al-CeO2 under a dry atmosphere. From the measured
temperature region (373 K < T < 673 K), the dominant conduc-
tion carrier was estimated as electron, which diffuses
throughout the inner bulk for both samples. From this result, it
was inferred that Al doping to CeO2 does not affect bulk electron
conduction. As presented in Fig. 4, however, the apparent
conductivity of Pd/Al-CeO2 was higher than that of Pd/CeO2
under a wet atmosphere, especially at low temperatures. Under
such conditions, the dominant conduction carrier is expected to
be proton generated from adsorbed H2O over surface. Consid-
ering that the bulk electron conductivity of both samples was
identical, the difference of apparent conductivity under a wet
atmosphere between these samples can be attributed to the
difference in surface proton conductivity. Accordingly, the
contribution of proton conduction over Pd/Al-CeO2 was greater
than that over Pd/CeO2.
Generally, the factors governing proton conductivity are
regarded as the strength of the O–H stretching bond of the
adsorbed H2O and the amount of H2O adsorbed on the
surface.15,25 From results of DRIFTS and TG measurements, it
was presumed that enhancement of proton conductivity by Al
doping was attributable to an increase of the adsorbed H2O
amount, not to a change in the O–H stretching bond strength.
Consequently, the Al doping effect to CeO2 was identied as the
increase of H2O amount over the catalyst surface, which
contributes to the promotion of surface proton hopping. That
would induce higher catalytic activity with the EF application.
a wet condition. Results show that both supports exhibit anti-
Arrhenius behavior: the conductivity increased at the lower
temperature region (T < 423 K). Such a specic trend is derived
from surface proton conduction at lower temperatures.15,25,29–36
Under a humid condition, H2O physisorption on grain surface
is feasible at lower temperatures. Then, proton conductivity is
enhanced by the formed water layer. Comparison of Pd/CeO2
and Pd/Al-CeO2, Pd/Al-CeO2 shows a larger contribution of
surface proton hopping than Pd/CeO2 at lower temperatures.
These results indicate that the bulk conductivity of electron is
the same for CeO2 and Al-CeO2. However, the surface proton
hopping is promoted by Al doping into CeO2 under a wet
condition.
3.5. Discussion
Result of catalytic activity tests show that the activity with EF of
Pd/Al-CeO2 was higher than that of Pd/CeO2, although no
difference was found between them without application of EF.
These results revealed that enhancement of activity by Al doping
can be exactly attributed to the EF application.
With EF, the activity is reportedly inuenced by three factors:
(i) the electronic state of the active metal, (ii) the perimeter of
the metal–support interface, and (iii) surface proton conduc-
tivity.15,18 The results of CO-pulse, XPS and DRIFTS measure-
ments demonstrated that the Pd loading state over each support
was almost identical, indicating that the increase of activity is
attributable to the difference in catalyst support, not in the Pd
loading state. Therefore, in this case, factors (i) and (ii) are
negligible. We therefore specically examined the surface
proton conductive property.
4 Conclusions
Catalytic methane steam reforming was investigated with and
without application of an electric eld (EF) using Pd/Ce1ꢀxMxO2
(x ¼ 0 or 0.1, M ¼ Ca, Ba, La, Y or Al). Results show that activity
with EF was different among Pd/Ce1ꢀxMxO2 (Pd/M-CeO2) cata-
lysts, although it was almost identical to that obtained without
EF. It is particularly interesting that Pd/Al-CeO2 showed higher
activity than Pd/CeO2 with EF. That result is attributable to
enhancement of surface proton conduction over the Al-CeO2
support surface because Pd loading state on CeO2 and Al-CeO2
Table 2 Calculated H2O amount per unit area on Pd/CeO2 and Pd/Al-
CeO2
W473 KꢀW723
Catalyst
Acat/m2 gꢀ1
m/mg
K/mg
Wwater/mg mꢀ2
Pd/CeO2
Pd/Al-CeO2
41.5
32.0
41.2
42.1
0.0786
0.208
0.0460
0.154
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was conrmed to be almost identical. In fact, electronic/ionic 17 M. Torimoto, S. Ogo, D. Harjowinoto, T. Higo, J. G. Seo,
conduction measurements conrmed a larger contribution of
surface proton hopping for Al-CeO2. In this case, the proton
S. Furukawa and Y. Sekine, Chem. Commun., 2019, 55,
6693–6695.
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amount because the absorbed H2O amount on Pd/Al-CeO2 was Jpn., 2019, 92, 1785–1792.
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Conflicts of interest
23 J. A. Wang, J. M. Dominguez, A. Montoya, S. Castillo,
J. Navarrete, M. Moran-Pineda, J. Reyes-Gasga and
X. Bokhimi, Chem. Mater., 2002, 14, 4676–4683.
24 J. R. Gallagher, D. J. Childers, H. Zhao, R. E. Winans,
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There are no conicts to declare.
Acknowledgements
This research was supported as a JST CREST JPMJCR1423 and
COLD project.
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