Reactivity of carbon species formed on supported noble metal catalysts in methane conversion reactions

Article abstract of DOI:10.1006/jcat.2000.2920

The reactivity of carbon species formed from methane decomposition over supported noble metal catalysts was studied. In the Ru-loaded case, the formation rate of CH_x species in the decomposition of methane and the amount of CH_x species over ruthenium were independent of the type of support. However, the carbon species on Ru/La₂O₃ appeared to be more uniform and reactive than that on Ru/Al₂O₃. After the decomposition of methane over Ru-loaded catalysts at 700°C, a temperature-programmed reaction with D₂ was performed. The major product on all the supports was CD₄ and only little amounts of CH₃D, CH₂D₂, and CHD₃ were detected over La₂O₃, ZrO₂, and Y₂O₃ supports. The reactivity of CH_x species on ruthenium strongly depended on the supports. The reactivity of CH_x species on ruthenium and rhodium loaded on La₂O₃ can be divided into two categories, which are not based on the differences in the number of x in CH_x species. On ruthenium, x in the CH_x species was equal to nearly 0, with a very small amount having x = 2. On the other hand, x was almost exclusively 0 on rhodium. Rhodium seems to lead to a higher rate of decomposition for methane than does ruthenium.

Full text of DOI:10.1006/jcat.2000.2920

Journal of Catalysis 194, 115–121 (2000)
doi:10.1006/jcat.2000.2920, available online at http://www.idealibrary.com on
Reactivity of Carbon Species Formed on Supported Noble Metal
Catalysts in Methane Conversion Reactions
,
,
, ,1
Na-oko Matsui, Kiyoharu Nakagawa, † Na-oki Ikenaga, † and Toshimitsu Suzuki †
Department of Chemical Engineering, Faculty of Engineering and †High Technology Research Center,
Kansai University, Suita, Osaka 564-8680, Japan
Received February 9, 2000; revised April 6, 2000; accepted May 3, 2000
such as Ni (6), Ru (7), Rh (7–9), Pd (7, 10), Pt (7), and Ir (7),
as shown by
The reactivity of carbon species formed from methane decom-
position over supported noble metal catalysts was examined. In the
Ru-loaded case, the rate of formation of CHx species in the de-
composition of CH4 and the amount of CHx species over Ru are
independent of the type of support. However, the carbon species
on Ru/La2O3 seems to be more uniform and reactive than that on
Ru/Al2O3. After the decomposition of CH4 over Ru-loaded catalysts
was carried out at 700 C, a temperature-programmed reaction with
D2 was carried out. CD4 was the major product on all the supports,
and only slight amounts of CH3D, CH2D2, and CHD3 were de-
tected over La2O3, ZrO2, and Y2O3 supports. The reactivity of CHx
species on Ru strongly depends on the supports. The reactivity of
CHx species on Ru and Rh loaded on La2O3 can be divided into two
categories. These categories are not based on the differences in the
number of x in CHx species. On Ru, x in the CHx species is equal to
nearly 0, with a very small portion having x = 2. On the other hand,
x is almost exclusively 0 on Rh. It seems that Rh leads to a higher
c
CH4 ad. → CHx ad. + (4 x)/2H2.
[3]
Osaki et al. have confirmed through a pulse reaction-
rate analysis the dissociative adsorption of CH4, which re-
sults in the release of H2 on supported Ni or Co catalysts
(11, 12).
Solymosi and co-workers have reported that a sup-
ported Rh catalyst is active in the decomposition of
CH4 to give H2, together with a small amount of C2H6
and carbonaceous residues. Three different types of car-
bon having different reactivities on Rh have been distin-
guished, following CH4 decomposition, by means of the H2
temperature-programmed reaction technique (8). Ad-
sorbed CH3 reacted with CO2 over Rh/SiO2 at and above
100 C to give CO. The authors concluded that CH4 frag-
ments formed in the decomposition of CH4 afforded CHx
species, and it reacted with CO2 during CO2 reforming of
CH4 over supported Rh catalysts (9).
rate of decomposition for methane than does Ru.
2000 Academic
Press
Key Words: temperature-programmed reaction; temperature-
programmed oxidation; ruthenium; rhodium; iridium; lanthanum
oxide.
Mark et al. have reported that the activity of Ru, Rh, and
Ir catalysts in the CO2 reforming of CH4 could be corre-
lated with the accessible metal surface area and that there
is no detectable effect of the pore structure or nature of the
support on the reaction rate (13).
Methane can be activated thermally over supported
Group VIII metals to produce carbonaceous species that
react with hydrogen to produce C2 (14, 15). Similar studies
have been reported by Amariglio and co-workers (16, 17)
and Solymosi et al. (18, 19). The dissociative adsorption of
methane occurs at temperatures above 100 C and produces
three forms of carbon with different reactivities.
1. INTRODUCTION
Recently, increasing attention has been paid to the con-
version of methane to synthesis gas. The partial oxidation
and CO2 reforming of methane have attracted much atten-
tion (1–5):
1
CH4 + 2 O2 → CO + 2H2 1H2098
=
36 kJ/mol
[1]
CH4 + CO2 → 2CO + 2H2 1H0298 = +247 kJ/mol. [2]
Carstens and Bell have reported that three types of car-
bon are formed similarly over a Ru/SiO2 catalyst and that
the proportion among these species depends on the total
carbon coverage and the length of time that the carbon has
been aged at a given temperature after deposition (20).
CHx species formed on the metal surface vary with the
support used even if the same metal is employed (4, 9).
It is of interest to shed light on the mechanistic aspects
of the partial oxidation and CO2 reforming of CH4 on the
supported transition-metal catalyst. Methane is believed to
be adsorbed and dissociated on transition metal surfaces
1 To whom correspondence should be addressed. E-mail: tsuzuki@
ipcku.kansai-u.ac.jp.
115
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

116
MATSUI ET AL.
We have reported in previous papers that the perfor-
3. RESULTS AND DISCUSSION
mance of supported iridium catalysts depends strongly
upon the support materials in the partial oxidation of
methane to synthesis gas (21) and that the interaction be-
tween CO2 and the support strongly affects the reactivity of
ruthenium catalysts in the CO2 reforming of methane (22).
In the pulsed reaction of partial oxidation and CO2 reform-
ing of methane to synthesis gas, the mechanism of synthe-
sis gas formation over group VIII transition metal loaded
catalysts was strongly affected by loaded metal species and
support materials (23–25).
This paper deals with temperature-programmed reac-
tions with H2 and O2 as well as the transient response re-
actions of CH4, in order to examine the activation of the
C–H bond and the reactivity of CHx species on noble met-
als loaded on various supports.
In the CO2 reforming of methane, the interaction be-
tween CO2 and the support, as well as the activation of
methane on the Ru surface, has been discussed in a pre-
vious paper (23). The results are summarized as follows:
La2O3, Y2O3, and ZrO2, which showed a high level of ac-
tivity, and Al2O3, commonly used in the reforming reac-
tion, were compared when they were loaded with Ru. Af-
ter CO2 was fed at 600 C, a pulse of CH4 under a steady
Ar flow was introduced over a Ru/La2O3 catalyst. We ob-
served the response of CO that was generated from the re-
action with CHx on the ruthenium and the Ru-Ox formed
during CO2 treatment or during the reaction of Ru-CHx
with adsorbed CO2 onto the La2O3. Over Ru/Al2O3 cata-
lyst, however, a very small response of CO was observed.
A pulse of 13CO2 was introduced under steady CH4 flow
over Ru/La2O3, Ru/Y2O3, and Ru/ZrO2 catalysts. Symmet-
rical 13CO responses were observed, but a small response
of 12CO from 12CHx continued to evolve after generation of
13CO from 13CO2 ceased. The following reaction cycle is be-
lieved to occur in the CO2 reforming of methane on active
supports: A part of metallic ruthenium reacted with CH4
to give Ru-CHx; simultaneously ruthenium metal could be
oxidized with CO2 to give Ru–Ox and CO, and then, oxygen
transfer from Ru–O to Ru–CHx took place to give CO and
metallic ruthenium.
2. EXPERIMENTAL
2.1. Materials
As a support for -Al2O3, ALO-4 (supplied from the
Catalyst Society of Japan), ZrO2 (Japan Aerosil Co.), Y2O3
(Nacalai Tesque, Inc.), and La2O3 (Wako Pure Chemical
Industries Ltd.) were used. All chemicals were used without
further purification.
Noble-metal-loaded catalysts were prepared by impreg-
nating an acetone solution of RuCl3 nH2O, RhCl3 nH2O,
IrCl4 nH2O (Mitsuwa Kagaku Co.) to yield 0.5 or 5.0 wt%
on various supports. All the catalysts were dried under re-
duced pressure, calcined in air at 550 C for 5 h, and pel-
letized and sieved to a size between 60 and 100 mesh.
The effect of the support on the reactivities of CHx
species formed on noble-metal-loaded catalysts is another
important factor controlling the catalytic activity. This fac-
tor was examined by temperature-programmed reactions.
3.1. Responses of H2 Production
to the Decomposition of CH4
2.2. Apparatus and Procedure for the
Pulse–Response Reaction
Figure 1 shows the transient response reaction after
All reactions were performed using a fixed-bed quartz switching from Ar to CH4 for 5 min at temperatures of
reactor having an internal diameter of 4 mm and a length 600, 700, and 800 C on Ru (5.0 wt% )/La2O3 and Ru
of 200 mm, and the reactor was set in a horizontal posi- (5.0 wt% )/Al2O3 catalysts. When CH4 was supplied, the
tion in an electric furnace. Quartz wool plugs were packed response of H2 formation to the decomposition of methane
in the center part of the reactor and 50 mg of the catalyst was immediately observed, indicating that the dissociation
was charged. Before the reaction, the catalysts were re- to hydrogen and carbonaceous species (CHx) is rapid. How-
duced under H2 flow for 60 min at 600 C. CH4, H2, D2, and ever, the concentration of H2 decreased gradually within
O2 were used without purification. The reaction tempera- 5 min. A similar result has been reported by Verykios and
ture was controlled by monitoring the outside temperature co-workers (26) over Rh/Al2O3 catalyst at 650 C. They con-
of the reactor with chromel alumel thermocouples using a cluded that after 10 min of a stream of CH4, the rate of
programmable controller.
H2 production decreased to one-seventh of the initial rate
The transient response reaction, temperature-progra- (26).
mmed reduction (TPR), and oxidation (TPO) were per-
The amount of H2 produced at 600 C in 5 min is al-
formed with an on-line quadrupole mass spectrometer most the same as the amount of loaded ruthenium species
(HAL201, Hiden Analytical Ltd.). The mass spectrometer (Table 1), indicating that x in the CHx species formed over
scanned corresponding parent peaks of 11 components, H2, the ruthenium active site is probably large. The surface area
D2, HD, CH4, CH3D, CH2D2, CHD3, CD4, H2O, CO, and of Ru/La2O3 (SA = 3.6 m2/g) is smaller than that of Al2O3
CO2, within 1 s, and repeated scans were collected with a (SA = 174 m2/g), and dispersion of ruthenium on La2O3
personal computer.
would probably be poor compared to that on Ru/Al2O3

REACTIVITY OF CARBON SPECIES ON NOBLE METAL CATALYSTS
117
FIG. 2. Transient response of O2 flow after CH4 decomposition on
Ru/La2O3 at three different temperatures. Reaction conditions: catalyst,
Ru (5 wt% )/La2O3 (50 mg), flow rate of CH4, 10 mL/min, of O2 10 mL/min;
1 Torr = 133.3 Nm 2. CH4 flow (600–800 C, 5 min) → Ar → O2 steady flow
(600–800 C).
temperature, a transient response reaction was carried out
with oxygen at the same temperature as CH4.
Figures 2 and 3 show profiles of the transient responses
to a steady flow of O2 after CHx species formed on Ru cata-
lysts at 600, 700, and 800 C, respectively. CH4 was swept off
by Ar before O2 was introduced. On the Ru/La2O3 catalyst,
CO was generated immediately when O2 was introduced,
and CO2 was detected after a slight delay (Fig. 2). Such a
delay in CO2 production might be caused by the adsorp-
tion of CO2 on basic sites of La2O3. In contrast, with the
Ru/Al2O3 catalyst, CO2 was the major product with only a
small amount of CO (Fig. 3). Such differences in the oxida-
tion products on Ru between La2O3 and Al2O3 seem to be
correlated with differences in the catalytic activities of these
two catalysts (23). In the oxidation of Ru-CHx species, CO
seems to be produced from highly reactive carbon species.
In contrast, less reactive carbon species would be oxidized
to give CO2. Since at a lower temperature the equilibrium
between CO and CO2 shifted to the CO2 side, at 600 C the
FIG. 1. Response of H2 production. (—) Ru/La2O3: ( ) Ru/Al2O3.
Reaction conditions: catalyst 50 mg, flow rate of CH4, 10 mL/min, 1 Torr =
2
133.3 Nm
.
catalyst. However, a large amount of H2 was generated at
800 C over the Ru/La2O3 catalyst, and the amount of H2
was more than four times that of the Ru species. Although
the dispersion of Ru was not determined, the formation
of such a large amount of hydrogen seems to indicate that
carbon growth does occur on the Ru center.
3.2. Transient Response Reactions and Temperature-
Programmed Reactions with O2
To evaluate the amounts of carbon species formed on
Ru during the treatment of CH4 from 600 to 800 C; at each
TABLE 1
Amount of H2 Production in CHx Formation
Reaction temperature
( C)
Amount of H2
(
Run
Catalyst
10 2 mmol)
1
2
3
4
5
6
Ru/La2O3
600
700
800
600
700
800
2.94
6.33
19.41
3.75
6.08
9.62
Ru/Al2O3
FIG. 3. Transient response of O2 flow after CH4 decomposition on
Ru/Al2O3 at three different temperatures. Reaction conditions: catalyst,
Ru (5 wt% )/Al2O3 (50 mg); flow rate of CH4 = 10 mL/min, of O2 10 mL/
min; 1 Torr = 133.3 Nm 2. CH4 flow (600–800 C, 5 min) → Ar → O2 steady
flow (600–800 C).
Note. Catalyst, 50 mg; loading level of Ru = 5.0 wt% (2.47
10 2 mmol); flow rate of CH4 = 10 ml/min for 5 min.

118
MATSUI ET AL.
ences between the transient response reaction of O2 and
the TPO after the reaction with CH4 in the amount of CO2
formed cannot be interpreted at present, since the amounts
of H2 observed during the reaction with CH4 did not differ
significantly. Carbon species on Ru/La2O3 seem to be more
uniform and reactive, since the oxidation occurs at a lower
temperature and in a narrow temperature range. At least
two types of carbon species would be formed on Ru/Al2O3,
with one being reactive and the other less reactive. Neither
of them is uniform due to the oxidation occurring in a broad
temperature range.
3.3. Temperature-Programmed Reaction with H2
Figure 5 shows temperature-programmed reductions of
CHx on Ru after the formation of CHx species at tempera-
tures between 600 and 800 C. As shown in Fig. 5a, over the
Ru/La2O3 catalyst (reaction of CH4 at 600 C), the reduc-
tion of CHx to give CH4 was observed from about 500 to
700 C. This temperature is the lowest temperature at which
the CO2 reforming of CH4 with Ru catalysts proceeded. In
the case of the Ru/Al2O3 catalyst, no CH4 was observed.
The same reaction sequences were carried out with the
catalysts at 700 and 800 C to form CHx from CH4. Over
the Ru/La2O3 catalyst, a very large amount of CH4 formed
FIG. 4. Temperature-programmed oxidation of surface carbon on the
Ru-loaded catalysts. Reaction conditions: reaction temperature of CH4 on
the catalysts: 700 C, catalyst 50 mg; loading level of Ru, 5.0 wt% ; flow rate
of O2, 10 mL/min; heating rate, 20 C/min; 1 Torr = 133.3 Nm 2. Decom-
position of CH4 (700 C, 20 min) → Ar → O2 TPR (10 mL/min).
amount of CO formed from Ru/La2O3 was smaller than that
of CO2. Because of the higher background levels of H2O,
the formation of H2O was not detected.
In the abundant O2 flow the partial oxidation of CHx
to give CO2 and H2O does not readily occur. The most
plausible paths are as follows:
CHx + O2 → CO2 + H2O
[4]
[5]
→
CO2 + C
2CO.
To prove the above assumption, temperature-
programmed oxidation was carried out. At first, CH4 was
allowed to react with the catalysts at 700 C for 20 min
to form CHx over the catalysts. The catalyst bed temper-
ature was lowered to room temperature under a steady
flow of Ar, and temperature-programmed (heating rate,
20 C/min) oxidation was carried out in a neat steady flow
of O2 (Fig. 4).
Over the Ru/La2O3 catalyst, a sharp CO2 response was
observed between 300 and 500 C, and the maximum rate
of CO2 formation was observed at 350 C. The trace of CO
(m/z = 28) was noisy and no significant formation of CO
was observed in the range between 100 and 800 C. In con-
trast, with the Ru/Al2O3 catalyst, a broad CO2 peak was
observed between 250 and 800 C. Even at 800 C, unburned
carbon species remained. The broad CO2 response was di-
vided into two broad peaks at 500 C, with the lower temper-
ature response showing its maximum rate at 400 C and the
higher temperature response exhibiting its peak at 620 C.
A much larger amount of CO2 was obtained for Ru/Al2O3
FIG. 5. Temperature-programmed reaction of surface carbon with
H2 on the Ru-loaded catalysts. (—) Ru/La2O3; ( ) Ru/Al2O3, reaction
conditions: reaction temperature of CH4 on the catalysts, 700 C; catalyst
50 mg; loading level of Ru, 5.0 wt% ; flow rate of H2 = 10 mL/min; heating
rate, 20 C/min; 1 Torr = 133.3 Nm 2. Decomposition of CH4 (600–800 C,
in the TPO compared to Ru/La2O3. However, such differ- 20 min) → Ar → H2 TPR (10 mL/min, 20 C/min).

REACTIVITY OF CARBON SPECIES ON NOBLE METAL CATALYSTS
119
from 565 to 800 C. In contrast, with the Ru/Al2O3 catalyst, were not detected. These results indicate that the value of x
the total amount of CH4 was smaller than that formed with is almost 0. With the Ru/Al2O3 catalyst, CD4 was again ob-
Ru/La2O3. The temperature of CH4 formation in the TPR tained at 600 to 900 C. CH3D, CH2D2, and CHD3 were not
with H2 can be classified into three ranges depending on detected. The amount of CD4 was much smaller than that
the temperature of the reaction of CH4 with the catalyst: obtained with Ru/La2O3, similar to the results observed in
500 to 700 C, 600 to 780 C, and 750 to 900 C. Although, as the TPR with H2. HD was observed in wide temperature
seen in Fig. 1, the amounts of the Ru–CHx species formed at ranges between 100 and 150 C and between 150 and 500 C.
the same temperature were of the same order of magnitude The formation of HD was observed at a lower temperature
irrespective of the La2O3 or Al2O3 support, the reactivity in both La2O3- and Al2O3-loaded cases. The most plausible
of the CHx species varied greatly according to the nature origin of this species is Ru–H. Judging from the tempera-
of the support. Ru–CHx species formed at a higher tem- ture at which the Ru–CHx species was formed, the forma-
perature seemed to be less reactive, although the amount tion of a hydride species is suspected. An alternate possible
of CHx species was larger than that formed at the lower source of the labile hydrogen could be Ru–CH or Ru–CH3
temperature.
species, since CHD3 and CH3D were not detected in the
When a La2O3 support is chosen for the Ru catalyst show- TPR. Even if Ru–CH or Ru–CH3 species were formed,
ing high activity in the CO2 reforming of CH4, it is impor- they would be transformed into Ru–CD or Ru–CD3 at a
tant to consider the resulting reactivity of the carbon species low temperatures. Consequently no CHD3 or CH3D was
formed over the Ru metal.
observed.
Over the Ru/Al2O3 catalyst, when an O2 pulse was intro-
duced at 600 C under a steady flow of Ar after the surface
carbon species was removed by the H2 TPR reaction, the
generation of CO2 was observed. This result indicates that
in the case of the Ru/Al2O3 catalyst, a certain amount of
CHx species formed on the Ru metal would be very sta-
ble and the activation of CHx with CO2 or H2 is very dif-
ficult. Mark and Maier reported that the number of H in
CHx species from decomposition of CH4 over the Rh/Al2O3
catalyst was almost nil (27).
3.4. Temperature-Programmed Reaction with D2
Temperature-programmed reactions with D2 were car-
ried out in order to determine the value of x in the CHx
species on the Ru-loaded catalysts (Fig. 6). CH4 was re-
acted at 700 C, resulting in the formation of a large amount
of CH4 in the TPR with H2. Over the Ru/La2O3 catalyst,
a large amount of CD4 (m/z = 20) was detected at a tem-
perature range of 550 to 800 C, and HD was obtained at
temperatures from 100 to 150 C. CH2D2, CH3D, and CHD3
The same reaction sequences were carried out on
Ru/Y2O3 and on Ru/ZrO2, both of which showed a high
level of activity in the CO2 reforming of methane (Fig. 7)
(23). CH4 was reacted at 700 C. Similar to the results
with the Ru/La2O3 catalyst, CD4 was detected in the TPR
with D2. These results indicate that reactive carbon species
(Ru–CHx) were formed over Ru/Y2O3 and Ru/ZrO2. Over
Ru/Y2O3, between 600 and 750 C, a much larger amount
of CD4 was produced. In addition, very small amounts of
CHD3 and CH3D were observed at the higher temperature
range of 450 to 700 C.
The results of the XRD analyses of the Ru/La2O3 catalyst
after CO2 reforming exhibited diffraction peaks assignable
to RuO2 and Ru metal. However, the Ru/Y2O3 catalyst
showed only diffraction peaks assigned to Ru metal. These
results indicate that the decomposition of methane is pro-
moted on Ru metal supported on Y2O3.
To determine the value of x in the CHx species on the
Ru/La2O3 catalyst (Fig. 8), CD4 instead of CH4 was reacted
at 700 C to give CDx species on the Ru/La2O3 catalyst.
A large amount of CH4 was formed at a higher temper-
ature range of 500 to 800 C in the TPR with H2. A very
small amount of CH2D2 was detected at a low tempera-
ture of about 300 C. This phenomenon is closely related to
the temperature-programmed reaction with D2 of surface
carbon after CH4 decomposition (Fig. 6). Due to fragment
ions from CD4 (m/z = 20), CD2H+2 (m/z = 18) and CD3+
FIG. 6. Temperature-programmed reaction of surface carbon with D2
on the Ru-loaded catalysts. Reaction conditions: reaction temperature
of CH4 on the catalysts: 700 C, catalyst = 50 mg, loading level of Ru =
5.0 wt% , flow rate: D2 = 10 mL/min, heating rate = 20 C/min, 1 Torr =
133.3 Nm 2. Decomposition of CH4 (700 C, 20 min) → Ar → D2 TPR
(10 mL/min, 20 C/min).

120
MATSUI ET AL.
FIG. 9. Temperature-programmed reaction of surface carbon with
D2 on Ir- and Rh-loaded La2O3 catalysts. Reaction conditions: reaction
temperature: 700 C, catalyst = 50 mg, loading level = 5.0 wt% , flow rate:
D2 = 10 mL/min, heating rate = 20 C/min, 1 Torr = 133.3 Nm 2. Decom-
position of CH4 (700 C, 20 min) → Ar → D2 TPR (10 mL/min, 20 C/min).
Ru/La2O3 catalysts is almost exclusively 0, with a very small
number of species with x = 2.
FIG. 7. Temperature-programmed reaction of surface carbon with D2
on the Ru-loaded catalysts. Reaction conditions: reaction temperature
of CH4 on the catalysts: 700 C, catalyst = 50 mg, loading level of Ru =
5.0 wt% , flow rate: D2 = 10 mL/min, heating rate = 20 C/min, 1 Torr =
The temperature-programmed reaction with D2 was car-
ried out over Ir/La2O3 and Rh/La2O3 catalysts in order to
133.3 Nm 2. Decomposition of CH4 (700 C, 20 min) → Ar → D2 TPR compare the differences in noble metals (Fig. 9). In the par-
(10 mL/min, 20 C/min).
tial oxidation and CO2 reforming of methane to synthesis
gas, Ir-, Ru-, and Rh-loaded catalysts exhibited high ac-
tivities (20). As discussed in a previous paper, the partial
oxidation of methane on Ir-, Ru-, and Rh-loaded catalysts
proceeds by different paths (24, 25). Over the Ir/La2O3 cata-
(m/z = 18) cannot be distinguished. If the CD2 species was
formed on Ru as a result of the decomposition of CD4,
TPR with H2 would produce CD2H2 (m/z = 18). It there-
lyst, a small amount of CD4 (m/z = 20) was detected in the
fore can easily be determined that the CD2 species exists
temperature range from 600 to 800 C. Such a result is con-
on Ru. The above results indicate that the value of x in CHx
sistent with the fact that the use of an Ir-loaded catalyst does
species formed as a result of the decomposition of CH4 over
not result in carbon deposition in the partial oxidation and
CO2 reforming of methane (21, 22). The Ir-loaded catalyst
exhibited poor activity for the decomposition of methane
to CHx species.
In contrast, Rh/La2O3 afforded a much larger amount
of CD4. In addition, a very small amount of CHD3 was
observed at a higher temperature range above 600 C. In
the methane decomposition reaction, the Rh/TiO2 catalyst
is reported to produce a large amount of hydrogen with
carbon deposition (24), indicating that the Rh metal has a
higher level of activity in breaking the C–H bond of CH4.
Mark et al. have reported that there is no detectable ef-
fect of the pore structure or the nature of the support on
the reaction rate for the CO2 reforming of CH4 over Ru,
Rh, and Ir catalysts (13). In our studies, however, methane
activation on noble metals to give CHx species depended
strongly upon the characteristics of the noble metal species
FIG. 8. Temperature-programmed reaction of surface carbon from
CD4 decomposition with H2 over Ru(5 wt% )/La2O3 catalyst. Reaction
conditions: decomposition of CD4: 650 C, 5 mL/min, 20 min, catalyst =
50 mg, flow rate: H2 = 10 mL/min, heating rate = 20 C/min, 1 Torr =
2
133.3 Nm
.

REACTIVITY OF CARBON SPECIES ON NOBLE METAL CATALYSTS
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ACKNOWLEDGMENTS
25. Nakagawa, K., Ikenaga, N., Kobayashi, T., and Suzuki, T., in prepara-
tion.
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31 (1996).
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(1994).
This work has been carried out as a research project of The Japan
Petroleum Institute commissioned by the Petroleum Energy Center with
a subsidy from the Ministry of International Trade and Industry, Japan.
K. Nakagawa is grateful for a fellowship from the Japan Society for the
Promotion of Science (JSPS) for Young Scientists.