The role of chemisorbed oxygen on diamond surfaces for the dehydrogenation of ethane in the presence of carbon dioxide

Article abstract of DOI:10.1021/jp022173b

Oxidized diamond played a significant role as an efficient support for a Cr₂O₃-loaded catalyst during the dehydrogenation of C₂H₆ to C₂H₄ in the presence of carbon dioxide at 923 K, giving a 22.5% C₂H₄ yield with a 87.7% C₂H₄ selectivity. The activity of the oxidized diamond-supported Cr₂O₃ catalyst in the presence of CO₂ was about 3 times higher than that in the absence of CO₂. The activity of the oxidized diamond-supported Cr₂O₃ catalyst increased with increasing CO₂ partial pressures. XPS analyses of the fresh and the reacted catalysts in the presence and absence of CO₂ revealed that CO₂ maintained the surface Cr₂O₃ at a higher-oxidation state than the reaction in the absence of CO₂. On the other hand, bulk of Cr₂O₃ was not reduced in either atmosphere.

Full text of DOI:10.1021/jp022173b

4048
J. Phys. Chem. B 2003, 107, 4048-4056
The Role of Chemisorbed Oxygen on Diamond Surfaces for the Dehydrogenation of Ethane
in the Presence of Carbon Dioxide
Kiyoharu Nakagawa,†,‡ Chiaki Kajita,‡ Na-oki Ikenaga,‡ Toshimitsu Suzuki,*,‡
Tetsuhiko Kobayashi,§ Mikka Nishitani-Gamo,| and Toshihiro Ando,*,
National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 3054-0044, Japan,
Department of Chemical Engineering, Faculty of Engineering, and High Technology Research Center,
Kansai UniVersity, Suita, Osaka 564-8680, Japan, Special DiVision for Green Life Technology,
National Institute of AdVanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan,
Institute of Applied Physics and Center for Tsukuba AdVanced Research Alliance (TARA), UniVersity of
Tsukuba, Tsukuba, Ibaraki 305-8577, Japan, Core Research for EVolutional Science and Technology (CREST)
of Japan Science and Technology Corporation (JST) and National Institute for Materials Science (NIMS),
1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan
ReceiVed: October 1, 2002; In Final Form: February 6, 2003
Oxidized diamond played a significant role as an efficient support for a Cr2O3-loaded catalyst during the
dehydrogenation of C2H6 to C2H4 in the presence of carbon dioxide at 923 K, giving a 22.5% C2H4 yield with
a 87.7% C2H4 selectivity. The activity of the oxidized diamond-supported Cr2O3 catalyst in the presence of
CO2 was about 3 times higher than that in the absence of CO2. The activity of the oxidized diamond-supported
Cr2O3 catalyst increased with increasing CO2 partial pressures. XPS analyses of the fresh and the reacted
catalysts in the presence and absence of CO2 revealed that CO2 maintained the surface Cr2O3 at a higher-
oxidation state than the reaction in the absence of CO2. On the other hand, bulk of Cr2O3 was not reduced in
either atmosphere.
1. Introduction
is considered to be a pseudo-carbon-oxide solid phase. The
oxidized diamond surface is expected to behave as a carbon
oxide for specified surface reactions such as catalytic, electro-
chemical reactions. Because such reactions must be controlled
by the interaction between molecules and surfaces, the bulk
structures of solids are less important. Silicon oxide (called
silica) is widely used as a neutral support for many catalytic
reactions. Silica support has very weak interactions with some
metals and metal oxides and plays an important role in yielding
active reaction sites due to its high surface area.
Recently, the surface chemistry of diamond has received
much attention, and several functional groups have been
introduced onto the diamond surface.1-3 However, the limit and
scope of diamond or oxidized diamond-supported catalyst have
not been yet revealed. Further applications of the diamond-
supported catalyst need to be explored. Diamond has long been
considered an inert material. One of the present authors
demonstrated that the diamond surface behaved in the manner
of an organic substances.1 In addition, the diamond surface is
expected to express possible chemical reactivities as organic
molecules.3 This led us to develop the novel utilization of
diamond as a catalyst or catalyst-support material for chemical
reactions.
Oxidized diamond has recently drawn attention as a new
unique material phase as a solid carbon oxide. The diamond
surface is easily oxidized by oxidative acids such as HNO3,
HClO, H2O2, etc. and is also oxidized with oxygen at an elevated
temperature to form C-O-C ether-type structures and CdO
carbonyl-type structures.1 Until now, no carbon-oxide solid
phase has been found; however, the surface of oxidized diamond
Activated carbon has been widely utilized as a support
material for metal or metal oxide-loaded catalysts. Activated
carbon is composed of amorphous sp2 carbon atoms. Recently,
carbon nanofilaments have been reported as a novel carbon
material that can be used for catalytic processes.4 No carbon
material constituted of pure sp3 carbon atoms (diamond) has
been examined as a catalyst-support material. However, our
recent study of oxidized diamond-supported chromium oxide
(III) or nickel exhibited excellent catalytic activity for the
dehydrogenation of light alkanes,5,6 the partial oxidation of
CH4,7,8 and the decomposition of CH3OH to synthesis gas,9
respectively. In these reactions, weak but significant interactions
between loaded metal catalysts and support surfaces seemed to
play important roles in enhancing and prolonging the activity
of the respective catalysts. Oxidized diamond behaved as solid
carbon oxide material and presents an excellent support material
for catalysts.
* Corresponding authors. T. Suzuki, telephone +81-6-6368-0865; fax:
+81-6-6388-8869; e-mail: tsuzuki@ipcku.kansai-u.ac.jp. T. Ando, tele-
phone: +81-298-51-3354 ext. 2530; fax: +81-298-51-4005; e-mail:
c-diamond@md.neweb.ne.jp.
† National Institute for Materials Science (NIMS).
‡ Kansai University.
§ National Institute of Advanced Industrial Science and Technology
(AIST).
C2H4 is predominantly produced by steam cracking of
naphtha, C2H6, or liquefied petroleum gas at high temperatures
at a short residence time. To reduce the energy consumption
of C2H4 production, oxidative dehydrogenation required by
| University of Tsukuba.
Core Research for Evolutional Science and Technology (CREST) of
Japan Science and Technology Corporation (JST) and National Institute
for Materials Science (NIMS).
10.1021/jp022173b CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/04/2003

Diamond as a Catalyst or Catalyst-Support
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4049
C2H6 is proposed (reaction 2).
graph (FID detector) equipped with a 3 mm × 3 m glass column
packed with Porapack Q in N2 carrier. Analyses of CO, CO2,
and CH4 were carried out using a Shimadzu GC8AIT gas
chromatograph (TCD detector) equipped with a 3 mm × 3 m
stainless steel column packed with activated carbon (30/60
mesh) using He as a carrier gas. Analyses of H2 were carried
out using a Shimadzu GC8AIT gas chromatograph (TCD
detector) equipped with a 3 mm × 3 m stainless steel column
packed with an activated carbon (30/60 mesh) and with N2
carrier. The surface area of the catalyst was measured by the
BET method using N2 at 77 K using an automatic Micromeritics
Gemini model 2375.
Diffuse reflectance IR spectra were recorded on an FT-IR
instrument (JIR-7000, JEOL) equipped with a cooled MCT
detector in the region of 400-4000 cm-1 at a resolution of 2
cm-1. A KBr standard powder was used for the reference
spectrum. Diffuse reflectance was converted into Kubelka-
Munk function units. Powder X-ray diffraction (XRD) patterns
were measured using a Simadzu XRD-6000 utilizing mono-
chromatized Cu KR radiation. X-ray photoelectron spectroscopy
(XPS) results were obtained using an X-ray photoelectron
spectrometer (JPS-9000MX, JEOL) using Mg KR radiation as
the energy source. Raman spectroscopy with a 1-µm depth
resolution by Ar+ laser excitation (NR-1800MB, Jasco) was
used.
Transient response measurements of the catalyst bed tem-
perature during the pulsed reactions were carried out using a
fixed-bed quartz reactor (i.d. 4 mm × 200 mm) which was set
in a horizontal position in an electric furnace. Two thin-wall
sheathed thermocouples (o.d. 0.6 mm) were fixed at the front
and the rear end of the catalyst bed, and 100 mg of the catalyst
was charged. The reaction system has been described elsewhere
in detail.24 A pulse of C2H6 was introduced by means of a six-
port gas-sampling valve equipped with measuring tubes, under
a stream of CO2 or Ar carrier gas. The reaction temperature
was controlled using a programmable controller that monitored
the outside temperature of the reactor wall.
C2H6 a C2H4 + H2
∆H0923 ) +141 kJ/mol (1)
∆H0923 ) -149 kJ/mol
C2H6 + 1/2O2 f C2H4 + H2O
(2)
The reaction becomes exothermic and could be thermody-
namically possible at a lower temperature. However, it is
necessary to remove heat from the reaction and to control over-
oxidation to COx to keep high selectivity to C2H4. Recently, a
great variety of catalysts have been developed and tested to
achieve this reaction.10-13 In the oxidative dehydrogenation of
C2H6 and C3H8, some reports14,15 have pointed out that thermal
noncatalytic oxidative dehydrogenation of C2H6 and C3H8
proceeded to give alkene in the same yield as compared to
catalyzed runs which were operated at higher temperatures than
those in noncatalyzed runs. This suggests that the catalyzed
oxidative dehydrogenation of lower alkanes is not highly
superior to thermal nonoxidative pyrolysis. Recently, CO2 has
been investigated as a mild oxidant for the dehydrogenation of
ethylbenzene,16-18 C2H6,19,20 or C3H8,21,22 since many investiga-
tors have suggested that the catalyzed oxidative reactions of
lower alkanes with oxygen have a lesser advantage. We first
found that CO2 markedly promoted dehydrogenation of C2H6
over Ga2O3-based catalysts23-25 and the oxidized diamond-
5
supported Cr2O3 catalyst with high selectivity (reaction 3).
C2H6 + CO2 f C2H4 + CO + H2O
∆H0923 ) +135 kJ/mol (3)
In this work, we dealt with the performance of oxidized
diamond as a new support material for the Cr2O3-loaded catalyst
in the dehydrogenation of C2H6 to C2H4 and found that the
oxidized diamond-supported Cr2O3 catalyst markedly promoted
dehydrogenation of C2H6 to C2H4 in the presence of CO2. We
applied oxidized diamond surfaces as a new catalytic support
for the dehydrogenation of C2H6 to yield C2H4 and elucidated
the roles of CO2 in the dehydrogenation of C2H6 over the
oxidized diamond-supported catalysts.
3. Results and Discussion
3.1. Diffuse Reflectance FT-IR Spectroscopic Analysis of
Hydrogenated and Oxidized Diamonds. Figure 1 shows the
diffuse reflectance FT-IR spectra of hydrogenated and oxidized
diamonds. The spectrum of hydrogenated diamond contained
bands in the region of 2800-2970 cm-1 (Figure 1a), which were
ascribed to C-H stretching vibrations (νC-H) of sp3 hybridized
bonding.1,27,28 Oxidized diamond was obtained by the oxidation
of hydrogenated diamond at 723 K for 1 h under a stream of
O2-Ar (1:4 mixture). No significant C-H stretching vibrations
were observed on the oxidized diamond (Figure 1b). The
spectrum of oxidized diamond contained bands in the region
of 1650-1850 cm-1 (Figure 1b), which were ascribed to CdO
stretching vibrations (νCdO), and absorptions at 1150 and 1250
cm-1 were assigned to C-O-C stretching vibrations (νC-O).26
These results indicated that oxygen species were introduced into
the diamond surface.
3.2. Dehydrogenation of C2H6 in the Presence of CO2 over
Oxidized Diamond-Supported Metal Oxide Catalysts. Figure
2 shows C2H4 yields on various oxidized diamond-supported
metal oxide catalysts in the presence or absence of CO2. Thermal
dehydrogenation gave only a c.a. 2.5% of C2H4 yield in both
CO2 and Ar atmospheres. Equilibrium conversion of C2H6 to
C2H4 is ca. 50% at 923 K at a C2H6-CO2 ratio of 1:5. The
order of the activity of various oxidized diamond-supported
metal oxide catalysts in the presence of CO2 at the reaction
2. Experimental Section
To have a reactivity for a catalytic reaction and a hydrophi-
licity for impregnating metal oxides as an active material on
the diamond surface, oxidized diamond was prepared by the
oxidation of commercial fine-powdered diamond at 723 K for
1 h under a stream of O2-Ar (1:4) mixed gases. Before
oxidation, to eliminate impurities and activate the diamond
surface, diamond powder was hydrogenated at 1173 K for 1 h.
Catalysts were prepared by impregnating aqueous solutions of
Fe(NO3)3‚9H2O, Cr(NO3)3‚9H2O (Wako Pure Chemical Indus-
tries Ltd.), Ga(NO3)3‚8H2O (Kishida Chemicals), Zn(NO3)2‚
6H2O, Ce(NO3)3‚6H2O, (CH3COO)2Mn‚4H2O (Wako Pure
Chemical Industries Ltd.), (NH4)4VO3 and (NH4)6Mo7O24‚4H2O
(Nacalai tesque, Inc.) to Al2O3 (JRC-ALO-4, the reference
catalyst provided by the Catalyst Society of Japan), SiO2
(Merck), activated carbon (Wako Pure Chemical Industries Ltd.),
and oxidized diamond (B.E.T. surface area ) 12.6 m2‚g-1), and
they were evaporated to dryness. Supported catalysts were
calcined at 723 K for 3 h in air prior to the reaction. The reaction
was carried out using a fixed-bed flow-type quartz reactor (i.d.
10 mm × 350 mm) at atmospheric pressure. Using 200 mg of
a catalyst, 5 mL/min C2H6 and 25 mL/min of CO2 or Ar were
introduced. Analyses of the C1, C2, and C3 hydrocarbon gases
were carried out using a Shimadzu GC14BPF gas chromato-

4050 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Nakagawa et al.
TABLE 1: Dehydrogenation of Ethane over Oxidized
Diamond-Supported Catalystsa
selectivity, %
surface
conv.,
% C2H6
yield,
% C2H4
catalysts
area m2/g
C2H4
CH4
Cr2O3(CO2)
Cr2O3(Ar)
V2O5(CO2)
V2O5(Ar)
MoO3(CO2)
MoO3(Ar)
Ga2O3(CO2)
Ga2O3(Ar)
12.0
12.0
10.9
10.9
11.7
11.7
11.6
11.6
27.4
7.0
9.3
5.9
6.8
7.9
5.8
5.2
23.8
6.8
8.3
5.7
5.9
3.3
5.7
5.1
86.7
97.1
89.2
96.0
87.4
57.9
99.7
99.6
13.3
2.9
10.8
4.0
5.7
42.1
0.3
0.4
a Reaction conditions: 923 K, SV ) 9000 h-1 mL/g-cat. Composition
of the feed gas: C2H6/CO2(Ar) ) 5/25.
diamond alone and Sm-, Ca-, Tb-, Y-, Sn-, Zn-, La-, and Pr-
loaded on oxidized diamond catalysts did not show catalytic
activity. Oxidized diamond-supported Mo, Ga, Ce, Zr, Mn, and
Fe oxide catalysts exhibited slight catalytic activity. Mo-, Ga-,
Ce-, and Fe-containing catalysts were known to be active
catalysts for the dehydrogenation of alkane in the presence of
O2 or steam. The oxidized diamond-supported Cr2O3 catalyst
afforded the highest yield of C2H4 (22.5%) among various
oxidized diamond-supported metal oxide catalysts.
Figure 1. Diffuse reflectance FT-IR spectra of hydrogenated diamond
and oxidized diamond reaction conditions: (a) Hydrogenated dia-
mond: 1173 K for 1 h in H2, (b) oxidized diamond: 723 K for 1 h in
air.
The promoting effects of CO2 on C2H4 yield were observed
over oxidized diamond-supported Cr, V, Mo, Ga, and Ce
catalysts (Table 1). In particular, the activity of the oxidized
diamond-supported Cr2O3 catalyst in the presence of CO2 was
3 times higher than that in the absence of CO2. Dehydrogenation
of C2H6 in the presence of CO2 over the oxidized diamond-
supported Cr2O3 catalyst produced mainly C2H4, CO, H2, and
H2O. The promoting effect of CO2 in the dehydrogenation of
C2H6 on the Ga2O3 was twice that under Ar atomosphere.23 In
the case of Ga2O3/TiO2, the conversion of C2H6 in the presence
of CO2 increased twice that in the presence of Ar; however, a
secondary reaction such as the decomposition of C2H6 or C2H4
occurred to give CH4 decreasing C2H4 selectivity.24,25 In the
oxidized diamond-supported Cr2O3 catalyst, the increase in C2H4
yield was about 17% in the presence of CO2. One of the
characteristic features of diamond support is a higher C2H4
selectivity. By using this support, the absence of secondary
reactions with reactants or products could probably be prevented.
3.3. Effect of Support on the Dehydrogenation of C2H6
over Cr2O3-Loaded Catalysts. Oxidized diamond-supported
metal oxide catalysts were compared to unsupported metal oxide
or Cr2O3-loaded catalysts on various support materials. Figure
3 shows C2H4 yields on bulk metal oxide and Cr2O3-loaded on
various support materials. The order of the activity compared
by unit Cr and by unit surface area at a reaction temperature of
923 K was as follows: Oxidized diamond > unsupported Cr2O3
> Al2O3 > SiO2 > activated carbon. The C2H4 selectivities in
all the metal oxide catalysts were higher than 90% in the
dehydrogenation of C2H6 in the presence of CO2. However, the
unsupported Cr2O3 catalyst showed a lower C2H4 yield than
did the oxidized diamond-supported Cr2O3 catalyst. This
indicated that oxidized diamond significantly supported the
Cr2O3-loaded catalyst in the dehydrogenation of C2H6 to C2H4
in the presence of CO2.
Figure 2. Dehydrogenation of C2H6 in the presence of CO2 or Ar over
various oxidized diamond-supported metal oxide catalysts. Reaction
conditions: temperature: 923 K, reaction time: 0.5 h, C2H6:CO2 (Ar)
) 5:25 (mL/mL), catalyst ) 200 mg, SV ) 9000 h-1 mL/g-cat,
numerals in parentheses: surface area (m2/g), M:oxidized diamond )
5:95 (wt %).
3.4. Effect of Temperature on the Dehydrogenation of
C2H6 in the Presence and Absence of CO2 over Oxidized
Diamond-Supported Cr2O3 Catalyst. Figure 4 shows the
temperature dependence of catalytic activity of the oxidized
diamond-supported Cr2O3 catalyst in the presence and the
absence of CO2. C2H4 yields in the noncatalytic runs did not
temperature of 923 K was as follows: Cr2O3 > V2O5 > MoO3
> Ga2O3 > CeO2 > Mn3O4 > Fe2O3 > Sm2O3 > CaO > Tb4O6
> Y2O3 > SnO2 > ZnO > La2O3 > Pr6O11. The C2H4
selectivities in all the metal oxide catalysts were higher than
85% in the dehydrogenation under CO2 atmosphere. Oxidized

Diamond as a Catalyst or Catalyst-Support
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4051
Figure 5. Effect of loading level of oxidized diamond-supported
chromium oxide on C2H4 yield in the presence of CO2 or Ar. Reaction
conditions: temperature: 923 K, reaction time: 0.5 h, C2H6:CO2 (Ar)
) 5:25 (mL/mL), catalyst ) 200 mg, SV ) 9000 h-1 mL/g-cat.
Figure 3. Dehydrogenation of C2H6 in the presence of CO2 or Ar over
various Cr2O3-loaded catalysts. Reaction conditions are the same as
those shown in Figure 2: C2H4 yield under CO2 flow/C2H4 yield under
Ar flow.
Figure 6. Effect of space velocity on the C2H6 conversion, and C2H4
yield and selectivity over oxidized diamond-supported Cr2O3 catalyst.
Reaction conditions: temperature: 923 K, catalyst ) 200 mg, loading
level of Cr ) 2.5 wt %, C2H6:CO2 ) 1:5.
Figure 4. Effect of temperature on the conversion of C2H6 over
oxidized diamond-supported Cr2O3 catalyst. Reaction conditions: the
same as those shown in Figure 2, except for temperature.
depend on atmosphere. In the presence of CO2 over the oxidized
diamond-supported Cr2O3 catalyst, C2H4 yields increased re-
markably with increasing reaction temperature, and a 27.4%
C2H4 yield was obtained at 973 K. In contrast, C2H4 yield in
the absence of CO2 decreased with increasing reaction temper-
ature, and above 948 K the same yield as that of the uncatalyzed
run was obtained. In the absence of CO2, this inverse relation,
i.e., the decrease the C2H4 yield with an increase in the reaction
temperature, is probably caused by rapid carbon deposition on
the catalyst surface. CO was detected over the catalyst during
dehydrogenation of C2H6 in the presence of CO2. In addition,
CO was formed during the reaction between CO2 and used
catalyst (oxidized diamond-supported Cr2O3 catalyst) after the
dehydrogenation of C2H6 in the absence of CO2. Therefore, in
the presence of CO2, the elimination of deposited carbon
(reaction 5) may be a possible case of the promoting effects on
the dehydrogenation of C2H6.
3.5. Effect of Space Velocity on the Dehydrogenation of
C2H6 over Oxidized Diamond-Supported Cr2O3 Catalyst. To
obtain more detailed information regarding the role of CO2 on
the dehydrogenation of C2H6 over the oxidized diamond-
supported Cr2O3 catalyst, the space velocity was varied by
increasing the feed rate at a reaction temperature of 923 K.
Figure 6 shows the effect of space velocity on the dehydroge-
nation of C2H6 over the oxidized diamond-supported Cr2O3
catalyst. In the presence of CO2, C2H6 conversion and C2H4
yield decreased with increasing space velocity. In contrast, C2H4
selectivity increased. Selectivity of CH4 which was produced
by hydrocracking of C2H6 and C2H4 decreased with increasing
space velocity. Probably a secondary reaction of C2H6 or C2H4,
such as hydrocracking, decreased. Under a CO2 atmosphere,
the yield of C2H4 on the oxidized diamond-supported Cr2O3
catalyst did not significantly decrease even at a high space
velocity.
Figure 5 shows effect of the Cr2O3-loading level on C2H4
yield in the dehydrogenation of C2H6 under CO2 atmosphere.
The highest C2H4 yield (23.8%) was obtained at 2.5 wt % Cr2O3,
and C2H4 yield remained constant over a Cr2O3-loading level
of 1.0-7.5 wt %, at a reaction temperature of 923 K.
Figure 7 shows the effect of apparent contact time (1/SV)
on C2H6 conversion over the oxidized diamond-supported Cr2O3
catalyst. Instead of reaction rates, C2H6 conversion at the initial
stage was used to evaluate reaction rates. The good linear
relation between the natural logarithms of conversion against

4052 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Nakagawa et al.
Figure 9. Effect of partial pressure of CO2 on C2H6 conversion, and
C2H4 yield and selectivity over oxidized diamond-supported Cr2O3
catalyst. Reaction conditions: temperature: 923 K, reaction time: 0.5
h, catalyst ) 200 mg, loading level of Cr ) 2.5 wt %, SV ) 9000 h-1
mL/g-cat, C2H6: 5 mL/min, total flow rate ) 30 mL/min.
Figure 7. Effect of contact time on C2H6 conversion over oxidized
diamond-supported Cr2O3 catalyst.
Figure 8. Arrhenius plots showing the dehydrogenation of ethane in
the presence of CO2 over oxidized diamond-supported Cr2O3 catalyst.
Figure 10. The relationship between ln[C2H6 conversion] and ln[CO2
partial pressure] over oxidized diamond-supported Cr2O3 catalyst.
Reaction conditions: temperature: 923 K, reaction time: 0.5 h, catalyst:
200 mg, loading level of Cr ) 2.5 wt %, SV ) 9000 h-1 mL/ g-cat,
C2H6: 5 mL/min, total flow rate ) 30 mL/min.
the inverse of SV clearly shows that the reaction is ap-
proximately first-order to C2H6. In this relation CO2, contact
time was changed simultaneously; thus, the reaction rate
evaluated involves the effect of reaction order against CO2. Since
the reaction was carried out in excess CO2 (CO2/C2H6 ) 5),
the effect of CO2 on the reaction rates was not seen, indicating
that reaction order to CO2 is nearly zero.
Figure 8 shows Arrhenius plots of the dehydrogenation of
ethane in the presence of CO2. The apparent activation energy
over the oxidized diamond-supported Cr2O3 catalyst is ca. 66
kJ/mol and is much lower than that of the thermal reaction of
246 kJ/mol. The large advantage of catalytic dehydrogenation
was semiquantitatively clarified. As shown in Figure 4, however,
due to the rapid deactivation of the dehydrogenation of C2H6
under an Ar atmosphere, a comparison of activation energy
between the run in CO2 and Ar could not be performed.
3.6. Effect of CO2 and Steam Pressure on the Dehydro-
genation of C2H6 over Oxidized Diamond-Supported Cr2O3
Catalyst. Figure 9 shows the effect of CO2 partial pressure on
the dehydrogenation of C2H6 over the oxidized diamond-
supported Cr2O3 catalyst. C2H6 conversion and C2H4 yield
markedly increased with the addition of a small portion of CO2
and increased gradually with further increases in the CO2 partial
pressure. The C2H4 selectivity only slightly decreased with
increases in the CO2 partial pressure. These results indicate that
only a small amount of CO2 played an important role in
increasing C2H4 yield over the oxidized diamond-supported
Cr2O3 catalyst. Figure 10 shows relationship between ln[C2H6
conversion] and ln[partial pressure of CO2] over the oxidized
diamond-supported Cr2O3 catalyst. The good linear relation
between ln[C2H6 conversion] and ln[partial pressure of CO2]
was obtained. It was indicated that C2H6 conversion strongly
depended upon partial pressure of CO2.
In the dehydrogenation of C2H6 under a CO2 atmosphere,
the roles of CO2 would be considered as follows:
CO2 + H2 a CO + H2O
CO2 + C a 2CO
(4)
(5)
In the dehydrogenation of C2H6 under a CO2 atmosphere,
H2O was produced during the reaction (reaction 4). Cr2O3 has

Diamond as a Catalyst or Catalyst-Support
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4053
TABLE 2: Effect of Steam on the Products Yield in the
Dehydrogenation of Ethanea
yield (%)
steam pressure
(Pa)
C2H6 conv.
(%)
atmosphere
CO2
C2H4
CH4
0
6
106
0
6
106
27.4
25.4
20.3
7.0
14.9
15.5
23.8
21.7
18.0
6.8
13.8
16.1
3.6
3.7
2.3
0.2
1.2
1.5
Ar
a Reaction conditions: 923 K, Cr2O3 loading level ) 2.5 wt.%,
catalyst ) 200 mg SV ) 9000 h-1 mL/g-cat, ratio of C2H6/CO2 (or
AR) ) 1/5, flow rate ) 30 mL/min.
been reported to be a solid acid catalyst.29 Therefore, there is
the possibility of carbon deposition on the Cr2O3-loaded catalyst
in the dehydrogenation of ethane. As mentioned above over the
oxidized diamond-supported Cr2O3 catalyst, CO2 acts to elimi-
nate deposited carbon and to remove H2 from the reaction site,
thus shifting the equilibrium favorably to the product side.
Another possibility of the role of CO2 and formed H2O might
be the modification the acidity or the chemical structure of the
surface of the oxidized diamond-supported Cr2O3 catalysts. A
similar result was obtained using Ga2O3-loaded catalysts.24,25
In the dehydrogenation of C2H6 under CO2, water was
produced during the reaction (reaction 3). Carbon deposition is
one of major problems in the dehydrogenation of C2H6. The
role of water in the dehydrogenation of ethane under CO2 would
be considered as follows:
Figure 11. Effect of time on stream on C2H4 yield over oxidized
diamond-supported Cr2O3 catalyst. Reaction conditions: temperature:
923 K, C2H6:CO2 (Ar) ) 5:25 (mL/min), catalyst ) 200 mg, loading
level of Cr ) 2.5 wt %, SV ) 9000 h-1 mL/g-cat, C2H6: H2O:CO2 )
1:1:5, total flow rate ) 30 mL/min.
C + H2O a CO, CO2, H2
(6)
To obtain information regarding the effect of steam on the
dehydrogenation of C2H6, steam was introduced in the dehy-
drogenation of C2H6 over the oxidized diamond-supported Cr2O3
catalyst in the presence and in the absence of CO2. Using the
oxidized diamond-supported Cr2O3 catalyst, C2H4 yield in the
absence of CO2 was about 5.9%. C2H4 yield was markedly
increased by the introduction of steam, and a product distribution
similar to that in the run under CO2 was obtained (Table 2).
These results suggest that steam produced in the presence of
CO2 might also have promoted the dehydrogenation of C2H6.
In this reaction, COx was detected and CH4 selectivity increased.
Therefore, steam could eliminate deposited carbon on the
catalyst’s surface.
3.7. Effect of Time-on-Stream on the Dehydrogenation of
C2H6 over Oxidized Diamond-Supported Cr2O3 Catalyst.
Figure 11 shows effect of time-on-stream on the dehydrogena-
tion of C2H6 over the oxidized diamond-supported Cr2O3
catalyst. Both in the presence and in the absence of CO2 over
this catalyst, C2H4 yields decreased markedly with increasing
reaction time, due to carbon deposition. Using the oxidized
diamond-supported Cr2O3 catalyst, deactivation of the catalyst
in the presence and absence of CO2 without steam decreased
from 22.5 to 11.5% and from 7.4 to 2.7% within 3 h,
respectively.
To improve the stability of the catalyst’s activity, steam was
introduced in the dehydrogenation of C2H6 over the oxidized
diamond-supported Cr2O3 catalyst in the presence and in the
absence of CO2. As described above, the introduction of steam
together with CO2 improved the catalyst’s stability over a long
run.
Figure 12. Temperature profile at the front edges of the catalyst bed
and response to pulse over oxidized diamond-supported Cr2O3 catalyst.
Reaction conditions: Ar or CO2 carrier ) 30 mL/min, mixed gas, C2H6
) 1 mL, furnace temperature, 923 K.
over the oxidized diamond-supported Cr2O3 catalyst, a pulsed
reaction technique25,26 was employed to measure transient
temperature changes in the catalyst beds.
Figure 12 shows the transient temperature responses of the
oxidized diamond-supported Cr2O3 catalyst against a pulsed
injection of C2H6 in the presence and the absence of CO2. The
temperature at the front edge of the catalyst bed decreased with
the introduction of the C2H6 pulse both in the presence and
absence of CO2 at 923 K. In the absence of CO2, a large decrease
in the temperature (c.a. 10 K: 3 mL pulse) of the front edge of
3.8. Role of Oxidized Diamond and CO2 in the Dehydro-
genation of C2H6 over Cr2O3-Loaded Catalyst. To examine
the promoting effect of CO2 on the dehydrogenation of C2H6

4054 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Nakagawa et al.
Figure 14. X-ray photoelectron spectra of oxidized diamond-supported
Cr2O3 catalyst. Reaction conditions: loading level of Cr ) 7.5 wt %,
(a) fresh, (b) after reaction with CO2 flow, (c) after reaction with Ar
flow.
The role of CO2 in the dehydrogenation of C2H6 over the
oxidized diamond-supported Cr2O3 catalyst might be considered
as follows: in the Ga2O3 catalyst, a visible amount of carbon
was deposited;25 however, visible carbon deposition was not
observed over the oxidized diamond-supported Cr2O3 catalyst.
CO was detected at nearly twice the amount of C2H4 over the
oxidized diamond-supported Cr2O3 catalyst during the dehy-
drogenation of C2H6 in the presence of CO2. One possible cause
of this increased dehydrogenation in the presence of CO2 might
be a utilization of CO2 as oxidant for the dehydrogenation of
C2H6. A second possibility would be the elimination of deposited
carbon.
Figure 13. Raman spectra of oxidized diamond-supported Cr2O3
catalyst. Reaction conditions: temperature: 923 K, reaction time: 0.5
h, C2H6:CO2 (Ar))5:25 (mL/mL), Catalyst)200 mg, Loading level of
Cr ) 5 wt %, SV ) 9000 h-1 mL/g-cat.
In the XRD patterns of the oxidized diamond-supported Cr2O3
catalyst, Cr2O3 was observed both in the presence and absence
of CO2 for the fresh and used catalysts. However, XPS analyses
revealed that the binding energy of the chromium species of
the oxidized diamond-supported Cr2O3 catalyst reacted at 923
K in the presence of CO2 shifted to a slightly higher binding
energy than that of fresh catalyst or that reacted at 923 K in the
presence of Ar (Figure 14). This result could show the oxidation
of chromium oxide to maintain a higher-oxidation state. Wang
et al. investigated the dehydrogenation of C2H6 in the presence
of CO2 over a Cr2O3/SiO2 catalyst and, on the basis of the results
of XPS analyses, found Cr3+ and Cr6+ coexist on the catalyst’s
surface. They proposed that the redox property played an
important role in dehydrogenation in the presence of the CO2.19
We consider that maintaining a higher-oxidation state of the
chromium species might be an important factor indicating high
catalytic activities for the dehydrogenation of C2H6 in the
presence of CO2.
the catalyst bed was observed as compared to the temperature
drop in the presence of CO2. Observation of the catalyst-bed
temperature by means of injecting a pulse of reactants might
provide detailed information regarding the reaction being taking
place on the catalyst’s surface. Both reactions 1 and 3 are
endothermic reactions, the value of ∆H of both reactions are
also almost the same. Reaction 1 is more endothermic than
reaction 3. The temperature drop in the catalyst bed might have
indicated that the advantage of the dehydrogenation of C2H6 in
the presence of CO2 is much larger than that in the absence of
CO2. In the dehydrogenation of C2H6 to C2H4, the advantages
in using CO2 instead of Ar (without CO2) would be expressed
on the basis of thermodynamic considerations.
To determine the types of deposited carbons during the
dehydrogenation of C2H6 in the presence and absence of CO2
over the oxidized diamond-supported Cr2O3 catalyst, used
catalysts were examined using Raman spectroscopy. The Raman
spectra of the used oxidized diamond-supported Cr2O3 catalysts
in the presence and the absence of CO2 are shown in Figure
13. Two peaks were observed for the spectra of deposited carbon
both in the presence and the absence of CO2: one appears
around 1330 cm,-1 and the other, at 1600 cm-1. The peak at
1330 cm-1 is ascribed to diamond. In addition, the peak at 1600
cm-1 was assigned to the in-plane carbon-carbon stretching
vibration of graphite layers.30 It is considered that deposited
carbons both in the presence and absence of CO2 might be same
types.
Figure 15 shows diffuse reflectance FT-IR spectra of the
oxidized diamond-supported Cr2O3 catalyst. After the reaction
in the absence of CO2, the spectra from 1650 to 1850 cm-1
and at 1150-1250 cm-1 which were ascribed to CdO and
C-O-C stretching vibrations decreased compared with those
in the presence of CO2. From these results, it is suggested that
CO2 also played an important role in maintaining the oxygen
species on the diamond surface. To obtain a high C2H4 yield in
the dehydrogenation of C2H6, it would be necessary to keep

Diamond as a Catalyst or Catalyst-Support
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4055
SCHEME 2: Proposed Oxidation of Diamond Surface
by CO2
Figure 15. Diffuse reflectance FT-IR spectra of oxidized diamond-
supported Cr2O3 catalyst. Reaction conditions: temperature: 923 K,
C2H6:CO2 (Ar) ) 5:25 (mL/min), catalyst ) 200 mg, loading level of
Cr ) 7.5 wt %, SV ) 9000 h-1 mL/g-cat.
SCHEME 1: Proposed Reaction Pathway for the
Dehydrogenation of Ethane over the Oxidized Diamond-
Supported Cr2O3 Catalyst in the Presence of CO2
the threshold level would be considered based on the total
amount of oxygen species on the diamond surface. The oxygen
species on the oxidized diamond surface were supplied from
CO2 (Scheme 2). Such a mutual promotion effect on chromium
oxide phase formation is believed to be the reason for the
observed apparent synergism or promoting effect of CO2 on
the dehydrogenation of C2H6.
4. Conclusions
Among Cr2O3 loaded-catalysts, the oxidized diamond-sup-
ported Cr2O3 catalyst exhibited the highest C2H4 yield for the
dehydrogenation of C2H6 in the presence of CO2. C2H4 yields
over this catalyst increased with increasing CO2 partial pressure.
To maintain catalytic activity, steam which was produced in
the presence of CO2 also played an important role in the
dehydrogenation of C2H6 over the catalyst.
From the results of XRD and XPS, to obtain a high C2H4
yield in the dehydrogenation of C2H6, it was suggested that a
higher-oxidation state of chromium species might be required.
From the result of FT-IR, the presence of oxygen species on
the diamond surface would also play the important role for the
dehydrogenation of C2H6 to C2H4 in the presence of CO2 using
Cr2O3-loaded oxidized diamond catalyst.
both the highly oxidized state of chromium species and the
presence of oxygen species on diamond surface.
Scheme 1 shows the most plausible interpretation of such a
synergism between Cr2O3 and oxidized diamond regarding the
dehydrogenation of C2H6 in the presence of CO2. To produce
C2H4, Cr2O3 or a higher-oxidation state of chromium oxides
are required. If chromium oxides are dispersed on the oxidized
diamond surface, the chromium oxides might stabilize and
become difficult to reduce. Even if the chromium oxides are
reduced with C2H6 or H2, they will be oxidized by CO2 or
oxygen species of the diamond surface. The threshold loading
level of Cr2O3 on oxidized diamond might be ca. 1.25-2.5 wt
%. In the case of the oxidized diamond-supported Cr2O3 catalyst,

4056 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Nakagawa et al.
(12) Ueda, W.; Lin, S. W.; Tohmoto, I. Catal. Lett. 1997, 44, 241.
(13) Bhasin, M. M.; McCain, J. H.; Vora, B. V.; Imai, T.; Pujado, P. R.
Appl. Catal. A 2001, 221, 397.
(14) Burch, R.; Crabb, E. M. Appl. Catal. A 1993, 100, 111.
(15) Mulla, S. A. R.; Buyevskaya, O. V.; Baerns, M. Appl. Catal. A
2002, 226, 73.
The role of CO2 for the dehydrogenation of C2H6 on the
oxidized diamond-supported Cr2O3 catalyst might be to elimi-
nate deposited carbon of catalyst surface. Furthermore, to
maintain the higher-oxidation state of the chromium species and
oxygen species on the diamond surface, CO2 would be neces-
sary.
(16) Sugino, M.; Shimada, H.; Turuda, T.; Miura, H.; Ikenaga, N.;
Suzuki, T. Appl. Catal. A 1995, 121, 125.
(17) Mimura, N.; Takahara, I.; Saito, M.; Hattori, T.; Okuma, K.; Ando,
M. Catal. Today 1998, 45, 61.
(18) Badstube, T.; Papp, H.; Kustrowski, P.; Dziembaj, R. Catal. Lett.
Acknowledgment. K.N. is grateful to the Japan Society for
the Promotion of Science (JSPS) for his young scientist
fellowship.
1998, 55, 169.
(19) Wang, S.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Sukuki, K.
Appl. Catal. A, 2000, 196, 1.
(20) Valenzuela, R. X.; Bueno, G.; Solbes, A.; Sapin˜a, F.; Martn`ı ez,
E.; Corte´s Corbera`n. V. Top. Catal. 2001, 15, 181.
(21) Takahara, I.; Chang, W. C.; Mimura, N.; Saito, M. Catal. Today
1998, 45, 55.
(22) Hattori, T.; Komai, M.; Satsuma, A.; Murakami, Y. Nippon.
Kagakukaishi 1991, 5, 648. (in Japanese)
(23) Nakagawa, K.; Okamura, M.; Ikenaga, N.; Suzuki, T.; Kobayashi,
T. Chem. Commun 1998, 1025.
(24) Nakagawa, K.; Kajita, C,; Ide, Y.; Okamura, M.; Kato, S.; Kasuya,
H.; Ikenaga, N.; Kobayashi, T.; Suzuki, T. Catal. Lett. 2000, 64, 215.
(25) Nakagawa, K.; Kajita, C.; Okumura, K.; Ikenaga, N.; Gamo, M.
N.; Ando, T.; Kobayashi, T.; Suzuki, T. J. Catal. 2001, 203, 87.
(26) Nakagawa, K., Ikenaga, N.; Teng, Y.; Kobayashi, T.; Suzuki, T.
J. Catal. 1999, 186, 405.
(27) Ando, T.; Inoue, S.; Ishii, M.; Kamo, M.; Sato, Y.; Yamada, O.;
Nakano, T. J. Chem. Soc., Faraday Trans. 1993, 89, 749.
(28) Ando, T.; Ishii, M.; Kamo, M.; Sato, Y. J. Chem. Soc., Faraday
Trans. 1993, 89, 1783.
(29) Tanabe, K. Solid Acid and Base Catalysts. In Catalysis, Science
and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1981;
Vol. 2, p 233.
(30) Hiura, H.; Ebbesen, T. W.; Tanigaki, K. Chem. Phys. Lett. 1993,
202, 509.
References and Notes
(1) Ando, T.; Yamamoto, K.; Ishii, M.; Kamo, M.; Sato, Y.. J. Chem.
Soc., Faraday Trans. 1993, 89, 3635.
(2) D’Evelyn, M. P. Surface Properties of Diamond. In Handbook of
Industrial Diamonds and Diamond Film; Prelas, M. A., Popovici, G.,
Bigelow, L. K., Eds.; Marcel Dekker: New York, 1994; Chapter 4, pp
89-146.
(3) Buriak, J. M.; Angew. Chem., Int. Ed, 2001, 40, 532.
(4) Mestl, G.; Maksimova, N. I.; Keller, N.; Roddatis, V. V.; Schlo¨gl,
R. Angew. Chem., Int. Ed. 2001, 40, 2066.
(5) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Kobayashi, T.; Gamo, M.
N.; Ando, T.; Suzuki, T. Chem. Lett. 2000, 1100.
(6) Nakagawa, K.; Kajita, C.; Ikenaga, N.; Gamo, M. N.; Ando, T.;
Suzuki, T. Catal. Today. Manuscript to be submitted.
(7) Suzuki, T.; Nakagawa, K.; Ikenaga, N.; Ando, T. Stud. Surf. Sci.
Catal. 2002, 143, 1073.
(8) Nakagawa, K.; Nishimoto, H.; Enoki, Y.; Egashira, S.; Ikenaga,
N.; Kobayashi, T.; Gamo, M. N.; Ando, T.; Suzuki, T. Chem. Lett. 2001,
460.
(9) Nakagawa, K.; Hashida, T.; Kajita, C.; Ikenaga, N.; Kobayashi,
T.; Gamo, M. N.; Suzuki, T.; Ando, T. Catal. Lett. 2002, 80, 161.
(10) Cavani, F.; Trifiro`, F. Catal. Today 1995, 24, 307.
(11) Kung, H. H. AdV. Catal. 1994, 40, 1.