The promoting effect of energetic activation of a methane molecular-beam in the direct catalytic partial oxidation of methane

Article abstract of DOI:10.1039/b606763h

Energetic activation of a methane molecular-beam promoted remarkably the direct catalytic partial oxidation on Pt and Rh foils, in particular, hydrogen formation was dramatically enhanced. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606763h

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The promoting effect of energetic activation of a methane molecular-
beam in the direct catalytic partial oxidation of methane{
Toshiaki Sasaki, Kenji Nakao, Keiichi Tomishige and Kimio Kunimori*
Received (in Cambridge, UK) 12th May 2006, Accepted 6th July 2006
First published as an Advance Article on the web 3rd August 2006
DOI: 10.1039/b606763h
Energetic activation of a methane molecular-beam promoted
remarkably the direct catalytic partial oxidation on Pt and Rh
foils, in particular, hydrogen formation was dramatically
enhanced.
making the thermally-activated methane beam. The foil was
positioned at about 15 mm away from the nozzles. The surface
temperature (T
for methane were measured by chromel–alumel thermocouples,
which were calibrated by a pyrometer. Even when T was raised
) of the foil and the temperature (T ) of the nozzle
S
N
N
Many studies on the interaction between methane molecules and
1–9
metal surfaces have been carried out extensively,
up to 1100 K, the pyrolytic products of methane were below the
detection limit of QMS. Furthermore, a molecular-beam of
methane diluted in helium was also used. The helium dilution in
conjunction with a supersonic expansion is regarded as a seeding
technique, and the effect is to increase the translational energy of
because
methane dissociation on a metal surface is a very important
elementary step in methane steam reforming and the catalytic
partial oxidation of methane. These catalytic reactions have been
utilized for the production of hydrogen and synthesis gas (CO +
17
the methane molecules. The rotational energy of the methane
molecule is much lower than translational and vibrational energies.
This is because the molecular-beam is cooled during the initial part
10–12
2
H ) from natural gas.
Fundamental studies on the reaction of
methane on metal surfaces have been carried out using single
crystal metal surfaces with or without the combination of
1
8
of the free-jet expansion. The reaction between methane and
oxygen over metal foils supplied by the molecular-beams under
each reaction condition was observed for 0.5 h at least. The Pt or
1–9
the molecular-beam technique.
In almost all the cases, the
researchers have focused on the elementary step of methane
dissociation and the measurement of the sticking probability. The
gaseous products have often been unclear and the surface
temperature was much lower than that for methane partial
oxidation and steam reformation. In contrast, catalytic partial
oxidation of methane under steady-state conditions and at high
reaction temperatures, with the product distribution, is reported in
this communication. Hickman and Schmidt have reported that a
Rh surface was pretreated at 1400 K by flowing O
of surface contaminants. The amounts of both the reactants and
the products, except H O, were analyzed by the differentially-
pumped QMS, where the background pressure level was about
2
for the removal
2
28
1
6 10 Torr. This makes the accurate determination of the
formation rate of products possible, even under the low conversion
levels. The experimental error bar in the measurement of the
formation rate of each product can be estimated to be ¡0.5 6
Pt- or Rh-coated monolith gave high CH conversion to synthesis
4
2
3
gas with short contact times (10 s) and high spatial velocities
15
10
22 21
cm
s
based on the mass signal fluctuation. The
6
10 h ) through direct catalytic partial oxidation of methane.
21
13
(
production rate of H O was calculated on the basis of the
2
On the other hand, we have reported that the selective formation
of synthesis gas is possible by the partial oxidation of
2 2
production rates of CO, CO and H . The beam flux was
controlled using mass flow controllers, and transformed on the
basis of the surface area that the molecular-beam impinges, which
2
was estimated to be 2 cm .
hydrocarbons such as C
14,15
conditions.
4
H
10 under the molecular-beam reaction
However, methane has very low reactivity
14
compared to other alkanes, therefore, we have investigated the
effect of the energetic activation of methane on Pt or Rh using a
nozzle heating and seeding technique.
Fig. 1 shows the formation rates of the products in methane
oxidation over a Pt foil as a function of surface temperature.
2 2 2
Products were CO, CO , H and H O. Hydrocarbons other than
A molecular-beam reaction apparatus consists of a single
methane were not detected. The activity at each temperature was
almost stable during this measurement; i.e., the steady-state
activities are reported here. When methane without the energetic
2
chamber equipped with a free-jet nozzle for O , a free-jet heatable
nozzle for methane and a differentially-pumped quadrupole mass
spectrometer (QMS, QME200: Pfeiffer Vacuum Technology
activation was used, the main products were CO and H O, and
2
1
5,16
AG).
through the nozzles, and they collided with the Pt or Rh foil
12.5 6 25 mm). The nozzle for methane is made of ceramic and it
Reaction gases were introduced as molecular-beams
the reaction formula is expressed as CH
4
+ O
2
2
A CO + 2H O
(Fig. 1 (a)). In the case of the heated methane (Fig. 1 (b)), the
(
production rate of CO and H O became a little higher, although
2
16
is surrounded by a thin graphite heater. This is utilized for
the product distribution was similar to the case of methane without
the energetic activation. On the other hand, when the heated and
seeded methane was utilized, the product distribution was
drastically changed (Fig. 1 (c)). In both cases, methane without
the energetic activation and the heated methane, hydrogen
formation was very small even at high temperatures. In contrast,
hydrogen production was promoted drastically by the heated and
Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba, Ibaraki, 305-8573, Japan.
E-mail: kunimori@ims.tsukuba.ac.jp; Fax: +81 29 853 4490;
Tel: +81 29 853 5026
{
Electronic supplementary information (ESI) available: Schematic
diagram of molecular-beam reaction apparatus. See DOI: 10.1039/
b606763h
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3821–3823 | 3821

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Table 1 Effect of energetic activation of methane on methane
a
oxidation over a Rh foil
CH molecular-beam
Nozzle Concentration
1
5
22 21
s
4
Production rate/610 cm
N
temp. (T )/K in He (%)
H
2
CO
CO
2
2
H O
(
(
(
a) rt
b) 1100
c) 1100
100
100
10
0.0
4.3
0.8
3.7
9.0
0.0
0.0
0.0
1.5
3.1
0.8
17
= 1400 K, CH
= 200. (a) No activation: 100% CH
methane: 100% CH , T = 1100 K. (c) Heated and seeded methane:
0% CH , T = 1100 K.
a
18
22 21
Reaction conditions; T
CH /O
S
4
= 2.1 6 10 cm
s ,
4
2
4
, T = rt. (b) Heated
N
4
N
1
4
N
which was applied to Pt foil. Hydrogen formation on Rh foil was
negligible even in the oxidation of the heated and seeded methane.
1
9
This can be related to the higher oxygen affinity of Rh than Pt.
Under the same O flux conditions, the oxygen coverage on Rh
2
foil can be much higher than that on Pt. This high oxygen
coverage can suppress hydrogen formation.
In the oxidation of the heated and seeded methane over Pt foil,
the effect of the methane nozzle temperature was investigated and
the results are listed in Table 2. The reaction rate of methane
oxidation increased remarkably with increasing the methane
nozzle temperature. An important point is that the hydrogen is,
among the products, influenced most strongly by the methane
nozzle temperature. Furthermore, we also investigated the effect of
the seeding concentration of methane (Table 2). The addition of
He gas to the methane stream promoted methane oxidation. In
particular, in this case, hydrogen is also influenced most strongly
by the seeding concentration. As a result, it is found that hydrogen
formation via direct partial oxidation of methane is promoted
remarkably by the energetic activation of methane with heating
and seeding.
Fig. 1 Surface temperature dependence of product distribution in
18
22 21
methane oxidation over Pt foil. CH
. (a) Methane without energetic activation: 100% CH
Heated methane: 100% CH , T = 1100 K. (c) Heated and seeded
methane: 10% CH , T = 1100 K.
4
= 1.0 6 10 cm
s
4 2
, CH /O =
5
4
, T = rt. (b)
N
4
N
4
N
seeded methane, and the rate became twenty times higher
compared to the case of methane without the energetic activation
S
at T = 1400 K. It should be noted that the effective pressure of the
gases on the foil surface with the molecular collision is estimated
2
2
approximately to be 5 6 10 Torr, which was calculated using
the relation between collision flux and pressure. On the other hand,
Fig. 2 illustrates the production rate as a function of O
methane oxidation over a Pt foil at T = 1400 K. In the oxidation
of methane without the energetic activation, the production rate
decreased with increasing O flux. As a result, the reaction order of
the methane conversion rate with respect to O flux is determined
to be 20.5. This negative order with respect to O means that the
2
flux in
2
2
the pressure in the reaction chamber was about 1 6 10 Torr
under these flux conditions. Considering the pressure difference
between on the surface and in the chamber, it is expected that the
reactant molecule can have only a chance to collide with the foil
surface under these reaction conditions, therefore it is interpreted
that hydrogen can be formed by direct catalytic partial oxidation
of methane, and the heating and seeding of methane molecules can
open the reaction route for the catalytic partial oxidation. In
addition, the apparent activation energy of the methane conversion
rate can be estimated from the data in the temperature range of
S
2
2
2
adsorbed oxygen species can suppress the dissociative adsorption
of methane in the case of methane without the energetic activation.
In the case of the oxidation of the heated and seeded methane, the
production rate of products except for hydrogen increased with
2
increasing O flux, and this behavior is much different from that
1150–1400 K. The activation energies of the oxidation of methane
the case of methane without the energetic activation. As a result,
without the activation, the heated methane, and the heated and
2
1
seeded methane are 73, 48 and 31 kJ mol , respectively. The
energetic activation of methane can decrease the activation energy
of the reaction. The results suggest that the energy in methane is
transferred to the activation energy, in particular, the step of
methane dissociation.
Table 2 Effect of the methane nozzle temperature on the reaction
rate in the oxidation of heated and seeded methane on a Pt foil
a
15
Production rate/610 cm
22 21
s
CH
4
molecular-beam
Nozzle
temp. (T
Concentration
)/K in He (%)
N
H
2
CO
CO
2
2
H O
The results at T
the case of no activation, the main products were CO and H
On the other hand, the methane heating promoted hydrogen
S
= 1400 K over Rh foil are listed in Table 1. In
6
8
9
1100
1100
00
00
50
10
10
10
10
30
100
0.4
3.0
7.2
15
6.7
1.7
3.5
6.0
11
19
13
0.2
1.4
2.0
3.0
3.8
3.9
7.1
12
18
29
27
22
2
O.
formation, and the heated and seeded methane decreased H O
2
formation and made the H /CO ratio close to 2, where the partial
2
1
100
7.9
oxidation of methane is the main reaction route. The value of
a
18
= 1.0 6 10 cm
22 21
s ,
Reaction conditions: T
CH /O = 5.
s
= 1400 K, CH
4
CH /O
4
2
was 200 on Rh foil, and this is much higher than the case
/O = 5,
4
2
for Pt foil. We also obtained the data on Rh foil at CH
4
2
3
822 | Chem. Commun., 2006, 3821–3823
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From the results, it is found that the energetic activation of
methane promotes hydrogen formation, therefore, the formation
route of hydrogen is discussed here. In the case of methane without
the energetic activation, the main reaction in methane oxidation is
CH
4
+ O
2
2
A CO + 2H O even at high surface temperatures.
Based on the temperature programmed desorption profile of
22,23
oxygen on a Pt metal surface,
it is expected that the coverage
of the adsorbed oxygen species is very small at high surface
temperatures. Therefore, low coverage of oxygen can not explain
the switching of the reaction route by the energetic activation of
methane. One possible explanation is that the energetically
activated methane reacts with adsorbed oxygen species to give a
precursor for hydrogen formation such as methoxy and hydro-
formyl species, although the dissociative adsorption of methane
without the energetic activation can be inhibited by the adsorbed
oxygen. These differences are represented by the reaction schemes
as shown below.
Fig. 2 Production rates as a function of the flux of O
oxidation over a Pt foil at T = 1400 K. (a) Methane without energetic
activation: 100% CH , T = rt. (b) Heated and seeded methane: 10% CH
= 1100 K. Reaction conditions: O
2
in methane
S
4
N
4
,
17
22 21
s
T
N
2
flux = 1.6–2.5 6 10 cm
22 21
at a
18
constant CH
4
flux of 1.0 6 10 cm
s .
CH + O A
/
CH3a + H + O
a
4
a
a
the reaction order of the methane conversion rate with respect to
CH
* + O
4
a
A CH O + H
3 a a
O flux is determined to be 0.3. The heating and seeding changed
2
CH * + O A CH O + 2H
4
a
2
a
a
the reaction order remarkably with regards to O . This positive
2
(
CH
4 4
and CH *: without and with the energetic activation,
order suggests that the adsorbed oxygen species can promote the
dissociative adsorption of methane, and this is another character-
istic feature in the oxidation of the heated and seeded methane. In
addition, the error bar of the reaction order of methane conversion
is determined to be lower than ¡0.08 based on the inaccuracy in
the measurement of formation rates.
respectively.)
According to the results of HREELS (high resolution electron
energy loss spectroscopy) analysis for the reaction of methane with
7
preadsorbed oxygen on Ni{100}, hydroformyl species have been
observed and this supports our interpretation.
There have been some reports on the effect of preadsorbed
oxygen on the dissociative sticking probability of methane over
Notes and references
9
2
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3
1
2
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8
6
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4
5
the dissociation probability of CH
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has been investigated at a
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1
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1
21 4
1
translational energy range 10–25 kJ mol . The reasons for the
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2
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Chem. Commun., 2006, 3821–3823 | 3823