Support effects on catalysis of low temperature methane steam reforming

Article abstract of DOI:10.1039/d0ra04717a

Low temperature (<500 K) methane steam reforming in an electric field was investigated over various catalysts. To elucidate the factors governing catalytic activity, activity tests and various characterization methods were conducted over various oxides including CeO2, Nb2O5, and Ta2O5 as supports. Activities of Pd catalysts loaded on these oxides showed the order of CeO2 > Nb2O5 > Ta2O5. Surface proton conductivity has a key role for the activation of methane in an electric field. Proton hopping ability on the oxide surface was estimated using electrochemical impedance measurements. Proton transport ability on the oxide surface at 473 K was in the order of CeO2 > Nb2O5 > Ta2O5. The OH group amounts on the oxide surface were evaluated by measuring pyridine adsorption with and without H2O pretreatment. Results indicate that the surface OH group concentrations on the oxide surface were in the order of CeO2 > Nb2O5 > Ta2O5. These results demonstrate that the surface concentrations of OH groups are related to the proton hopping ability on the oxide surface. The concentrations reflect the catalytic activity of low-temperature methane steam reforming in the electric field.

Full text of DOI:10.1039/d0ra04717a

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PAPER
Support effects on catalysis of low temperature
methane steam reforming†
Cite this: RSC Adv., 2020, 10, 26418
a
a
a
a
a
Maki Torimoto, Shuhei Ogo,
Yudai Hisai, Naoya Nakano, Ayako Takahashi,
b
c
a
b
Quanbao Ma, Jeong Gil Seo,
and Yasushi Sekine
Hideaki Tsuneki, Truls Norby
a
*
Low temperature (<500 K) methane steam reforming in an electric field was investigated over various
catalysts. To elucidate the factors governing catalytic activity, activity tests and various characterization
methods were conducted over various oxides including CeO
2 2 5
, Nb O , and Ta
O
2 5
as supports. Activities
of Pd catalysts loaded on these oxides showed the order of CeO
2
2
> Nb O
5
> Ta 5. Surface proton
2
O
conductivity has a key role for the activation of methane in an electric field. Proton hopping ability on
the oxide surface was estimated using electrochemical impedance measurements. Proton transport
ability on the oxide surface at 473 K was in the order of CeO
on the oxide surface were evaluated by measuring pyridine adsorption with and without H
pretreatment. Results indicate that the surface OH group concentrations on the oxide surface were in
the order of CeO > Nb > Ta 5. These results demonstrate that the surface concentrations of OH
2 2 5 2
> Nb O > Ta O5. The OH group amounts
2
O
Received 28th May 2020
Accepted 8th July 2020
2
O
2 5
2
O
DOI: 10.1039/d0ra04717a
groups are related to the proton hopping ability on the oxide surface. The concentrations reflect the
catalytic activity of low-temperature methane steam reforming in the electric field.
rsc.li/rsc-advances
development of a process with high activity at lower tempera-
tures are important.
1
. Introduction
Recently, H2 demand has been increasing as an alternative
To secure progress in this area, we have proposed a non-
energy source. About half of the worldwide demand for conventional low-temperature catalytic system: steam reform-
hydrogen is met by steam reforming of methane (MSR), ing of methane in an electric eld (denoted as ER). In this
1–5
described by eqn (1). The water gas shi reaction (WGS), as system, applying a few milliamperes of weak direct current to
shown in eqn (2), proceeds sequentially. Because MSR is the catalyst bed can achieve high degrees of methane conver-
a highly endothermic reaction that is controlled by thermody- sion at considerably lower temperatures (below 500 K), at which
6
–13
namic equilibrium, conventional MSR processes require harsh conventional MSR proceeds only slightly.
To elucidate the
1
conditions such as 1000–1200 K.
mechanism of MSR in an electric eld, kinetic analyses were
conducted. Manabe et al. found that the water pressure
(1) dependency on the reaction rate increased from 0.25 to 0.79 and
7
1
CH
4
2 2
+ H O / CO + 3H
, DH298 ¼ 206 kJ molꢀ
ꢀ1
the apparent activation energy decreased from 54.4 kJ mol to
, DH298 ¼ ꢀ41.2 kJ molꢀ
1
(2)
2
CO + H O / CO
2
+ H
2
ꢀ1
14.3 kJ mol
by applying an electric eld. Obviously, the
different reaction mechanism occurred by applying an electric
eld. To summarize the additional investigations, including
operando-DRIFTS measurements and kinetic analyses using
Under such harsh conditions, multiple heat exchangers and
a reaction tube with high heat resistance are necessary, leading
to higher costs. Furthermore, considering the use of hydrogen
as a future energy resource, on-site and on-demand hydrogen
production processes are desirable. Therefore, study and

7,8,11
isotope,
the reaction mechanism in an electric eld was
concluded as below. First, the application of an electric eld
promotes surface proton conduction via adsorbed water on the
14–17
catalyst surface, known as the Grotthuss mechanism.
The
hopping protons collide with physisorbed methane and
promote methane activation at the metal-support interface.
Consequently, the ER reaction mechanism is completely
different from that of MSR. It is necessary to establish a strategy
a
Department of Applied Chemistry, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo,
1
b
69-8555, Japan. E-mail: ysekine@waseda.jp
Department of Chemistry, University of Oslo, FERMiO, Gaustadall ´e en 21, Oslo, NO-
349, Norway
0
c
Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, for designing highly active catalyst on ER.
Seongdong-gu, Seoul 04763, Republic of Korea
For this purpose, it is important to optimize the catalyst
support, which plays an important role for surface proton
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra04717a
1
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4
conduction. Some reports have described the effect of support using a cold-trap. We used eqn (3) to calculate CH conversion
6,9,18
in an electric eld,
but no report to date explains a study of and eqn (4) to calculate carbon balance.
catalytic activity with surface proton conductivity, or claries
catalytic activity or factors of support affecting surface proton
conductivity. Thus, the reason of high activity for some catalysts
were still unclear. By clarifying the support property that
contributes to the high proton conductivity and high ER
activity, we would be able to design catalysts with superior
performance.
In this study, activity tests and various characterizations
including surface proton conductivity were conducted to clarify
the relations among physical properties and surface ion
conductivity, and to clarify ER catalytic activity effects on low
temperature (<500 K) MSR.
CH
4
conv: ð%Þ ¼
Carbon moles of products ðCO and CO Þ
2
ꢁ 100
(3)
Carbon moles of input methane
C balance ð%Þ ¼
Carbon moles of output products ðCH ; CO and CO2Þ
4
ꢁ 100
Carbon moles of input methane
(4)
2
.3 XPS measurement
Pd electronic states of Pd-loaded catalysts were evaluated using
X-ray photoelectron spectroscopy (XPS; Versa Probe II; ULVAC-
PHI, Inc., X-ray source was Al Ka). To evaluate the Pd elec-
tronic state during ER, activity tests were conducted before the
measurement. The activity tests were conducted at 473 K for
0 min while imposing 9 mA of electrical current. Aer that, the
catalysts were purged with Ar, set on the sample table using
a transfer vessel in a gas barrier bag with degassing. Orbitals
2. Experimental
2.1 Catalyst preparation
We employed various oxide supports with loading of 3 wt% Pd
as an active metal. The results are presented in Table S1.†
3
Among them, we chose CeO for additional
2
, Nb
2
O
5
and Ta
2
O
5
investigation. CeO (JRC-CEO-1) and Nb O (JRC-NBO-1) were
2
2 5
3d
5/2 and 3d3/2 of Pd were measured and the binding energies
were calibrated with the C1s peak 284.8 eV.
supplied by the Catalysis Society of Japan. They are used aer
calcination at 773 K for 3 h. A Ta O support with high surface
2
5
area was prepared using a solvothermal method based on the
2
.4 Electrochemical impedance spectroscopy (EIS)
.4.1 Preparation of pellets for EIS. To prepare sample
pellets for EIS measurements, Nb or Ta powder (Kanto
18
reported procedure. Aer 2.5 g of TaCl was dissolved into
2
2
ethanol (250 mL) and was stirred for 2 h, it was heated at 463 K
for 72 h. Then, aer cooling to room temperature, the white
precipitate was ltrated and washed with distilled water. It was
then dried at 333 K for 12 h, and was calcined at 973 K for 3 h.
On these supports, Pd (3 wt%) was loaded using an impregna-
tion method with Pd(OCOCH ) as a precursor. As impregnating
2
O
5
2 5
O
Chemical Co. Inc.) was crushed into ne particles using
a planetary ball mill (Pulverisette 6; Fritsch GmbH). Aer the
obtained powder was pressed to make a pellet (20 mm f), it was
calcined at high temperatures. For Nb
powder was pressed at 110 kN for 30 min and was sintered at
2 5
O pellets, 1.0 g of
3
2
solvents, we used acetone for CeO and Nb O , and ethanol for
2
2 5
1473 K for 12 h. For Ta
2 5
O pellets, 1.1 g of powder was pressed at
Ta O . Aer impregnation and drying at 393 K, it was calcined
2
5
110 kN for 30 min while degasied by an aspirator. It was then
at 773 K for 3 h. The metallic surface area and dispersion of
loaded Pd among these three catalysts is almost identical.
sintered at 1473 K for 12 h. The relative density of pellets was
calculated as 61.1% for Nb O and 61.6% for Ta O . For CeO ,
2
5
2
5
2
19
data from our earlier work were used. As the electrode, Pt ink
Pt ink number 356010; the Nilaco Corp.) was painted on both
Catalytic activity tests were conducted using a xed-bed ow- sides of each pellet and was annealed at 1173 K for 1 h.
type reactor with a quartz tube (6.0 mm i.d. and 8.0 mm o.d.). 2.4.2 EIS measurements. EIS measurements were taken
2.2 Catalytic activity test
(
A schematic image of an apparatus is shown in Fig. S1.† The using a two-electrode four-wire cell (ProboStat; NorECs AS),
catalyst was pressed and crushed to obtain particles of 355–500 which was connected to an impedance spectrometer (Novo-
mm. Then 100 mg of it was used. A thermocouple was set in the control alpha-A) with a ZG4 interface. The frequency range was
6
ꢀ3
bottom of the catalyst bed to measure the catalyst bed 10 to 10 Hz. The amplitude was 0.1 V RMS. The feed gas was
temperature. For ER, two stainless steel rods (2 mm o.d.) were Ar only for a dry condition and Ar + H O (P
¼ 0.026 atm) for
2
2
H O
inserted into the catalyst bed as electrodes. A direct current a wet condition. The temperature range was 373–673 K for the
electric eld (9 mA) was imposed on the catalyst bed using dry condition and 323–673 K for the wet condition. The ob-
a high voltage power supply. The response voltage wave was tained data were analyzed using equivalent circuit tting so-
observed using an oscilloscope (TDS 2001C with a voltage probe ware (ZView ver. 3.5a; Scribner Associates Inc.). The
P6015A; Tektronix Inc.). Activity tests were done under a reac- conductivity was calculated using eqn (5). Here, s represents
tion gas ow (CH
4
: H
2
O : Ar ¼ 1 : 2 : 7, total ow rate 100 electrical conductivity, l denotes the pellet thickness, S stands
ꢀ
1
mL min ) in a kinetic condition (i.e. low conversion, and for the surface area of the painted electrode, and R expresses the
diffusion is not a rate-determining factor conrmed by our resistance. The activation energy (E ) of the electrical conduc-
a
preliminary tests) at different temperatures (423–773 K for ER tivity was calculated using the Arrhenius expression presented
and 473–773 K for MSR). A GC-FID and a GC-TCD (Shimadzu as eqn (6), where A represents the pre-exponential factor and k
B
Corp.) were used to analyze product gases aer removal of H
2
O
is Boltzmann's constant.
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s ¼ (l/S) ꢁ (1/R)
sT ¼ A exp(ꢀE /k
(5)
(6)
a
B
T)
2.5 Transmission FT-IR measurements
To evaluate the Lewis acidity of supports, Fourier transform
infrared (FT-IR) spectroscopy measurements were conducted
using pyridine as a basic probe molecule. An FT-IR spectrom-
Fig. 1 (A) Temperature dependence of catalytic MSR activity in the
electric field and (B) specific activity at 473 K. Reaction conditions: pre-
set temperature, 323–773 K (ER); catalyst weight, 100 mg; flow,
CH : H O : Ar ¼ 1 : 2 : 7, total 100 SCCM; current, 9 mA.
eter (FT/IR-6200; Jasco Corp.) with an MCT detector and CaF
window was used. All spectra were recorded using a trans-
2
ꢀ
1
4
2
mittance mode with 2 cm resolution and 100 scans. The
sample (20–30 mg) was shaped to a very thin disk (10 mm f).
Measurement ows of two types, with and without H
ment, are presented in Fig. S2.† First, the sample was outgassed
in vacuo (p < 5 Pa) and heated at 673 K for 1 h. For H
treatment, the sample was cooled to 473 K. Then H
introduced for 30 min and subsequently outgassed in vacuo.
The sample was then cooled to 323 K and background spectra
were recorded. Then, pyridine was introduced for 10 min and
outgassed in vacuo (p < 5 torr). Spectra of the adsorbed pyridine
on the support were recorded.
2
O treat-
the electric eld, CH
intermediate [CH –H–H] by the collision of hopping H and
CH at the metal-support interface (CH + H / CH + H ). In
4
is known to be activated via a three-atom
+
+
3
O pre-
O was
+
+
3
7,8
2
4
4
2
+
2
this step, the activation energy of its reverse reaction of CH3 + H2
+
/
CH + H is very high, making the step almost irreversible and
4
8
exceeding the thermodynamic equilibrium.
The specic activity based on the CH
4
conversion of Pd/CeO
2
around 473 K was calculated (Fig. 1(B), and Table S2†). At 473 K,
the activity without the electric eld over these catalysts was
sufficiently small to be ignored (0.074% for Pd/CeO
2
, 0.023% for
2.6 Other characterizations
Pd/Nb O , and 0.023% for Pd/Ta O ). The electric eld promoted
2
5
2 5
The crystalline structure of catalysts and prepared pellets for EIS the activity drastically even at such low temperatures at around 473
were characterized using powder X-ray diffraction (XRD; Smar- K. The effects of Joule heating by the electric eld on the reaction
tLab III; Rigaku Corp.) at 40 kV and 40 mA with Cu-Ka radiation rate are negligibly small in this case and no plasma has caused.
(
(
Fig. S3, and S4†). Field emission-scanning electron microscope The specic activities of Pd/Nb
2 5 2 5
O and Pd/Ta O normalized by
FE-SEM; S400S; Hitachi Ltd.) images were taken to observe the activity of Pd/CeO were, respectively, 0.66 and 0.53. Although
2
morphology of the prepared pellets (Fig. S5†). The specic these three catalysts showed almost equal furnace temperature
surface area of each support was measured by N
adsorption and response voltage (i.e. the same input power), conversions of
using Brunauer–Emmett–Teller (BET) method with an auto- these catalysts in the electric eld were clearly different.
2
mated specic surface area analyzer (Gemini VII; Micromeritics
Instrument Corp.).
Furthermore, XPS measurement of these three catalysts were
conducted to evaluate Pd electronic states during ER. In the
case of MSR, the catalytic activity depends on the electronic
state of active metal, and it is considered that ER activity is
affected by the metal state. Thus, before considering the effect
of support for ER activity, it should be conrmed that the
3. Results
3.1 Evaluating catalytic activity over three catalysts
To evaluate the effects of metal oxide supports on the catalytic electronic state of Pd during ER is the same or not. Fig. 2 shows
activity for MSR with and without the electric eld, catalytic results of XPS measurement for Pd 3d5/2 and 3d3/2 for Pd/CeO
activity tests were conducted using Pd catalysts supported on Pd/Nb , and Pd/Ta aer the ER activity tests. In this
various metal oxide supports. We selected CeO , Nb
, and experiment, we used the gas barrier bag, not to be exposed to
Ta
supports for comparison because the Pd catalysts loaded air. As shown in Fig. 2, the binding energy of Pd 3d5/3 and 3d3/2
,
2
O
5
O
2 5
2
2
2 5
O
2 5
O
on these three supports only showed both MSR and ER activi- of these three catalysts are almost identical. Thus, the differ-
ties, and other Pd-supported catalysts showed no ER or MSR ence among the ER activity at lower temperature region (Fig. 1)
activities or electric eld was unstable because of their insu- were not caused by the difference of Pd electronic state.
lation characteristic (details are presented in ESI Table S1†).
Considering that collision with hopping protons in the electric
Fig. 1(A) and S6(A)† present results of activity tests over Pd- eld activates methane molecules, the differences of activities
loaded catalysts. In these experiments, coke formation on these over these catalysts might derive from the proton conduction
catalysts was negligible for both ER and SR, because carbon ability on the catalyst support surface.
balance were almost 100%. These three catalysts showed higher
activity at lower temperatures in the electric eld (i.e. ER),
exceeding the thermodynamic equilibrium. From Fig. S6(b),†
apparent activation energies for these catalysts on MSR (i.e.
3
.2 Evaluating surface proton hopping capability using
electrochemical impedance spectroscopy (EIS) measurement
heated catalyst) were almost identical. However, the trend is To evaluate the proton conductivity on the oxide support, EIS
completely different on ER from MSR, as shown in Fig. S6(C).† In measurements were taken under dry and wet conditions. It has
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no effect of gas phase exists on electron conductivity in bulk),
the contribution of proton conduction under wet conditions
can be estimated as eqn (7). Furthermore, the proton transport
number (TH
calculated proton transport numbers at 473 K were 1.0 (CeO
.74 (Nb ), and 0.34 (Ta ).
+
) is calculable as eqn (8). As shown in Fig. 3(B), the
2
),
0
2
O
5
2 5
O
s
H
+
¼ swet ꢀ sdry
(7)
(8)
T
H
+
¼ s
H
+
/(s
H
+
+ s
e
ꢀ
) ¼ (swet ꢀ sdry)/swet
Therefore, from the EIS measurement results, we inferred
Fig. 2 XPS results for Pd 3d region after the ER activity tests of Pd/ that these three oxides showed different proton transport
2 2 5 2 5
CeO , Pd/Nb O , and Pd/Ta O .
numbers at the same temperature because of the surface proton
transport ability.
been widely reported that a porous pellet with lower relative
density (R. D. ¼ 50–60%) can be used feasibly for extracting
surface ion conductivity because such samples have plenty of
3
.3 Evaluating the amount of surface OH groups using
pyridine IR
vacancy sites for H
surface.
2
O molecules to adsorb on the oxide
Fig. 3(A) presents the temperature dependences of
conductivity among three samples (CeO , Nb O and Ta O )
We sought to clarify the oxide support property which affects
surface proton conductivity. The Grotthuss mechanism is
19–26
2
2
5
2
5
known as sequential proton hopping via the H-bonds of water
under dry and wet conditions. Under a dry condition, all
samples showed typical Arrhenius behaviour: conductivity
decreased concomitantly with decreasing temperature. In this
temperature range (373 K < T < 673 K), the main conduction
pathway might be attributed to the electron conduction in
14–17
molecules.
Consequently, water adsorption on the oxide
surface might have a strong relation with the proton conduc-
tion. Generally, for H O adsorption on metal oxides, the metal
2
cation and oxygen species act respectively as Lewis acid and
base sites, forming a hydroxyl group on the oxide surface (eqn
1
9,21
bulk.
(
The calculated apparent activation energy was 1.27 eV
23,31
(
9) and (10)).
Such mechanisms are also well discussed by ab
ꢂ
ꢂ
300 C < T < 500 C) for CeO
2
, 0.89 eV for Nb
2
O
5
and 0.71 eV for
32–34
initio molecular dynamics (AIMD) simulation.
Ta O , respectively. However, all samples showed anti-
2
5
2
7
Arrhenius behaviour at lower temperature regions under wet
conditions. Such conductive increases at low temperatures
under a wet atmosphere are attributable to the increase in the
water adsorption amount or relative humidity with decreasing
temperature, resulting in enhancement of surface proton
Msurf + H O / Msurf–OH2
(9)
(10)
2
Msurf–OH + O
/ Msurf–OH + Osurf–H
2
surf
To evaluate the Lewis acidity of CeO
2
, Nb
2
O
5
, and Ta
2
O
5
, the
19–30
adsorbed pyridine species on three oxides were measured using
transmission IR. Considering that Lewis acid sites (metal
cation) were able to serve as adsorption sites for hydroxyl group,
the exposed Lewis acid amount would decrease aer pretreat-
conduction via the Grotthuss mechanism.
Consequently, the measured conductivity under dry condi-
tions includes the electron conductivity in bulk and that under
wet conditions includes not only the electron conductivity in
bulk but also the proton conductivity on the oxide surface.
Assuming that the contributions of electron conduction under
both dry and wet conditions are approximately equal (because
ment with H O. Therefore, we conducted pretreatments of two
2
types, with and without H
Fig. 4 presents transmission FT-IR spectra of supports aer
exposure to pyridine with or without H O pretreatment. In this
gure, the solid line shows spectra without H O treatment and
the dotted line shows the spectra with H O treatment. All
2
O.
2

2
2
ꢀ
1
35–37
samples show peaks at 1455–1438 cm
,
which is assigned
ꢀ1
to the 19b mode of pyridine, and 1632–1580 cm , which is
assigned to the 8a mode of pyridine bonded on the Lewis acid
site. However, pyridine adsorbed onto Brønsted acid, of which
ꢀ
1
peaks appear around 1540 cm , was not observed for any
sample. The peak area was quantied using the peak at 1455–
ꢀ1
1
438 cm , normalized by the weight or surface area of catalyst
and presented in Table 1. As shown in Fig. 4 and Table 1, the
exposed Lewis acid amount decreased because of the H
(with H O)],
which was calculated as presented in Table 1, represents the
decrement of Lewis acidity and formation of OH groups per
2
O
Fig. 3 (A) Comparison of apparent electrical conductivity under dry or
wet condition by electrochemical impedance spectroscopy
measurement and (B) calculated transport number of protons at 473 K.
Measurement conditions: temperature, 323–773 K; partial pressure of
treatment. The value [Larea (without H O) ꢀ L
2
area
2
H
2
O, PH O ¼ 0.026.
unit area by H
O treatment. The order of surface OH group
2
2
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proton conductivity via the Grotthuss mechanism. Further-
more, the calculated transport numbers of protons at 473 K
were 1.0 for CeO , 0.74 for Nb O and 0.34 for Ta O . The order
2
2
5
2 5
of transport number (CeO > Nb O > Ta O ) was identical to
2
2
5
2 5
that of the ER activity. The anti-Arrhenius trend is also the
same, as reected in a comparison of Fig. S6(C)† and 3.
Consequently, results showed that proton conductivity on the
surface strongly affects the ER activity. The transport numbers
2
of protons differ among the oxides even if the same H O
amount is introduced. Therefore, the proton hopping ability
among these oxides is probably different. For proton conduc-
tion by the Grotthuss mechanism, the proton hops to the
neighbour site via H-bond of adsorbed water (Fig. 5). The
density of OH groups might be important: if the interval of OH
groups is long, then it is difficult for a proton to hop to the
neighbouring site. The amount of formed OH groups on Lewis
acid was evaluated by estimating the Lewis acid amount (metal
Fig. 4 Transmission FT-IR spectra of supports after exposure to
pyridine and in vacuo with or without H O treatment.
2
concentration is in the order of CeO > Nb O >Ta O . This
2
2
5
2 5
order is identical to that of the proton transport numbers
shown in Fig. 3(B). Considering the proton hopping via H-
bond of water molecule, the OH group distance is important:
if the OH group density is low, then protons have difficulty
cation) on the oxide surface with or without H O pretreatment.
2
Furthermore, the order of formed OH groups per unit area was
CeO2 > Nb O > Ta O , reecting the same trend at that of
2
5
2 5
catalytic activity in the electric eld and of proton hopping
ability by EIS measurements.
By combining the results of activity tests in the electric eld
with the EIS and IR results, the factors governing catalytic
3
8
hopping to the neighbouring site. Consequently, the OH
group concentration has a strong relation with the proton
conduction ability.
2
activity can be considered. For CeO , great amounts of water
dissociated on its surface. Moreover, the density of OH groups
was high, thereby producing the high proton-hopping ability.
However, the H O only slightly dissociates on the Ta O surface.
4
. Discussion
Results of activity tests demonstrated that the activities in the
electric eld for Pd/CeO , Pd/Nb , and Pd/Ta were clearly
different. The order was Pd/CeO > Pd/Nb > Pd/Ta . These
three catalysts showed almost identical activity by heat, as
presented in Fig. S6(A) and S6(b),† but exhibited completely
different trends in the electric eld, as shown in Fig. S6(C).† XPS
revealed that Pd electronic states aer ER activity tests were
almost identical for the three catalysts, and the difference of ER
activity were not caused by the difference of Pd loading state. In
the electric eld, methane is activated by the collision of
2
2 5
2
2
O
5
2
O
5
The proton conduction barrier becomes much higher; also, the
proton hopping ability is low on Ta . Results show that Nb
2 5
has intermediate properties between those of CeO and Ta O .
2
2
O
5
2 5
O
2
O
2 5
O
5
2
In the electric eld, methane is activated by hopping protons.
Thereby, oxides with high proton hopping ability on the surface
were able to achieve high activity.
7,8
hopping protons from the oxide surface. Therefore, the
difference in activity is regarded as the proton conductivity on
the oxide surface. The EIS measurements for CeO , Nb O , and
2
2 5
Ta O under dry and wet conditions showed that the oxide
2
5
conductivity increased at low temperatures with decreasing
Fig. 5 Schematic image of relationship between formed OH group
temperature under wet conditions, indicating an increase of and proton conduction on surface.
a
Table 1 Comparison of Lewis acid amount among CeO
2 2
, Nb O
5 2 5
, Ta O
L
area
SSA (specic
surface area)/m g
(without H
2
O) ꢀ Larea
2
ꢀ1
Sample
CeO
Nb
Condition
Without H
W/mg
L/a.u.
Lweight/a.u.
Larea/a.u.
(with H
2
O)/a.u.
2
2
2
2
O
O
O
19.9
20.9
20.0
20.0
27.8
27.2
122.9
71.29
36.32
5.77
0.88
1.58
1.23
0.71
0.48
290
2.36
0.34
1.11
0.86
0.70
0.49
2.02
With H
Without H
With H
Without H
With H
2
O
42.0
79.2
61.4
25.4
17.7
2
O
5
0.25
2
O
Ta
2
O
5
0.21
2
O
a
W: sample weight [mg], L: area value of Lewis acid peak, Lweight: L per unit weight, Larea: L per unit area.
26422 | RSC Adv., 2020, 10, 26418–26424
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9
Y. Sekine, M. Haraguchi, M. Matsukata and E. Kikuchi,
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2
H
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E. Vøllestad, Phys. Chem. Chem. Phys., 2018, 20, 15653–
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2
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2
the amount of formed OH groups are related strongly to the
proton hopping ability. To summarize this work, as the adsor- 22 B. Scherrer, M. V. F. Schlupp, D. Stender, J. Martynczuk,
121, 12817–12825.
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2
O become larger, the proton
J. G. Grolig, H. Ma, P. Kocher, T. Lippert, M. Prestat and
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L. J. Gauckler, Adv. Funct. Mater., 2013, 23, 1957–1964.
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Conflicts of interest
5
146.
5 S. Raz, K. Sasaki, J. Maier and I. Riess, Solid State Ionics,
001, 143, 181–204.
There are no conicts to declare.
2
2
Acknowledgements
26 Y. Hisai, K. Murakami, Y. Kamite, Q. Ma, E. Vøllestad,
R. Manabe, T. Matsuda, S. Ogo, T. Norby and Y. Sekine,
Chem. Commun., 2020, 56, 2699–2702.
This work was supported by JST CREST JPMJCR1423, Japan.
2
7 K. Murakami, Y. Tanaka, R. Sakai, Y. Hisai, S. Hayashi,
Y. Mizutani, T. Higo, S. Ogo, J.-G. Seo, H. Tsuneki and
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