Dehydrogenative Coupling of Methane Catalyzed by Platinum-Added Sulfated Zirconia and Characterization of the Catalyst Surface

Article abstract of DOI:10.1006/jcat.1998.2209

An active and stable catalyst of 3 wt%Pt-added sulfated-zirconia for the dehydrogenative coupling of methane without oxidant was obtained by sulfating zirconia gel with 0.5 M H₂SO₄, followed by drying, impregnating the sulfated gel with a solution of H₂PtCl₆, calcining in air at 600°C, and finally reducing in H₂ at 500°C. The catalyst gave conversion of 0.23%, ca 40% of the equilibrium value, at 500°C and was steady during the run of 5 h in a flow reactor (0.5 g catalyst, 10 ml/min methane). The products were ethane and ethylene in a ratio of 9:1;CO₂ was negligible. A remarkable loss of activity was observed with the catalysts sulfated with ammonium sulfate instead of sulfuric acid. XRD analysis showed that the sulfation of zirconia gel with sulfuric acid led to high dispersion of the Pt matter onto the support; the observations were confirmed by the amount of CO chemisorbed on the surface Pt atom. The adsorption measurement of CO showed the ratio of CO/Pt to be 0.09. Using TG-DTA the sulfur quantity on the catalyst was estimated from a weight loss at 600-1000°C, caused by decomposition of the sulfate species, and the ratio of Pt to S was determined to be 1. XPS showed SO^2-₄ for the S state and 10% of the platinate to be in the metallic state. Measurements of TG-DTA and XPS, in addition to the CO chemisorption, showed an active site (in the steady state) to be Pt⁰SO₄XZrO₂, surrounded by 10 times platinate-SO₄/ZrO₂ with various oxidation numbers of Pt except 0, whose species protect the active one from aggregation. The turnover frequencies were estimated to be 0.18 min^-1.

Full text of DOI:10.1006/jcat.1998.2209

JOURNAL OF CATALYSIS 179, 28–35 (1998)
ARTICLE NO. CA982209
Dehydrogenative Coupling of Methane Catalyzed by Platinum-Added
Sulfated Zirconia and Characterization of the Catalyst Surface
Tadahiro Kurosaka, Hiromi Matsuhashi, and Kazushi Arata1
Department of Science, Hokkaido University of Education, Hachiman-cho 1-2, Hakodate, Japan 040-8567
Received December 2, 1997; revised June 15, 1998; accepted June 15, 1998
Work in our laboratory has demonstrated that on the
oxide surfaces of Fe, Ti, Zr, Hf, Sn, Si, and Al superacids
with an acid stronger than that of 100% H2SO4 are gener-
ated by addition of the sulfate species. Solid acids with an
acid strength of up to the Hammett acidity function Ho be-
low ꢁ16.04 were synthesized by adsorbing sulfate ion onto
the amorphous oxides followed by calcination in air above
500ꢀC (10–12). The superacids were active in a heteroge-
neous system for reactions of hydrocarbons such as butanes
and pentanes.
We have also demonstrated that impregnation of the sul-
fated zirconia (SO4/ZrO2) with Fe, as well as noble metals
such as Pt and Pd, leads to pronounced activity for the re-
action of butane to isobutane, much more active than the
SO4/ZrO2 and the highest in acidity among the superacids
prepared by the addition of sulfate ion to metal oxides, the
Pt concentration being 7–8 wt% ; the catalyst with Pt was
highest in activity for the reaction, more active than any
solid catalyst yet reported (13–15).
We also studied the zirconia-based catalysts for the re-
action of methane; sodium doped ZrO2 was found to be
active for the selective formation of C3 hydrocarbons at
the relatively low temperature range, ꢃ600ꢀC (16), and or-
ganic compounds containing oxygen, mainly acetaldehyde,
were formed over SO4/ZrO2 (17). Following these obser-
vations, application of the noble metal-doped SO4/ZrO2 to
methane activation is now of interest. In the present paper
we report the selective coupling of methane to ethane over
Pt-SO4/ZrO2 without using O2 as the oxidant.
An active and stable catalyst of 3 wt% Pt-added sulfated-zirconia
for the dehydrogenative coupling of methane without oxidant was
obtained by sulfating zirconia gel with 0.5 M H2SO4, followed by
drying, impregnating the sulfated gel with a solution of H2PtCl6,
calcining in air at 600ꢀC, and finally reducing in H2 at 500ꢀC. The
catalyst gave conversion of 0.23%, ca 40% of the equilibrium value,
at 500ꢀC and was steady during the run of 5 h in a flow reactor
(0.5 g catalyst, 10 ml/min methane). The products were ethane and
ethylene in a ratio of 9 : 1; CO2 was negligible. A remarkable loss of
activity was observed with the catalysts sulfated with ammonium
sulfate instead of sulfuric acid. XRD analysis showed that the sul-
fation of zirconia gel with sulfuric acid led to high dispersion of the
Pt matter onto the support; the observations were confirmed by the
amount of CO chemisorbed on the surface Pt atom. The adsorption
measurement of CO showed the ratio of CO/Pt to be 0.09. Using
TG-DTA the sulfur quantity on the catalyst was estimated from a
weight loss at 600–1000ꢀC, caused by decomposition of the sulfate
species, and the ratio of Pt to S was determined to be 1. XPS showed
SO24ꢁ for the S state and 10% of the platinate to be in the metal-
lic state. Measurements of TG-DTA and XPS, in addition to the
CO chemisorption, showed an active site (in the steady state) to be
Pt0-SO4/ZrO2, surrounded by 10 times platinate-SO4/ZrO2 with var-
ious oxidation numbers of Pt except 0, whose species protect the ac-
tive one from aggregation. The turnover frequencies were estimated
to be 0.18 minꢁ1
.
ꢂ 1998 Academic Press
c
INTRODUCTION
The partial oxidation of methane to form C2 and higher
hydrocarbons is an alternative to petrochemical feedstocks.
A number of catalytic systems that show methane conver-
sions and C2 hydrocarbon selectivities have been reported
(1–6), several reviews being also found (7–9). The mecha-
nism for this reaction is believed to initiate the homolytic
cleavage of a C-H bond, resulting in the formation of CH3
radicals which undergo coupling to form ethane; therefore
hydrogen atom abstraction becomes a critical step in the
activation process. The reaction usually needs high temper-
atures, more than 700ꢀC.
EXPERIMENTAL
First zirconia gel was obtained by hydrolyzing ZrOCl2
with aqueous ammonia to pH 8 at a temperature of 50–
60ꢀC; aqueous ammonia solution (25% , ꢄ15 ml) was added
dropwise with stirring into 25 g of ZrOCl2 ꢅ 8H2O dissolved
in 500 ml of distilled hot water, followed by keeping the
precipitated solution in a water bath warmed at 50–60ꢀC
for 6 h, washing the precipitate 2–3 times with 250 ml
of hot water for each, and drying at 110ꢀC for a yield
of 10 g.
1 Corresponding author.
0021-9517/98 $25.00
28
c
Copyright ꢂ 1998 by Academic Press
All rights of reproduction in any form reserved.

DEHYDROGENATIVE COUPLING OF METHANE
29
Pt-SO4/ZrO2(S)
a programmed rate of 10ꢀC/min. X-ray powder diffraction
(XRD) spectra were obtained with an X-ray diffractometer
(Rigaku RAD-RB) by using Cu radiation (100 mA, 30 kV).
Infrared (IR) spectra were recorded on a Perkin Elmer 1600
FTIR spectrometer using a tablet mixed with 2 mg of cata-
lyst and 50 mg of KBr. X-ray photoelectron spectra (XPS)
were measured with Ulvac-Phi XPS5600ci using Al K radi-
ation (300 W).
Two grams of the gel (20–30 mesh) were immersed in
30 ml of 0.5 M H2SO4 for 0.5 h, followed by filtering and
drying at 100ꢀC for 1 h. The resulting sulfated gel was im-
pregnated with 7 ml of aqueous H2PtCl6 ꢅ 6H2O (to obtain
1, 3, 5, and 7.5 wt% Pt in the final catalyst), followed by
evaporating the residual water, drying, and finally calcining
in air at 600ꢀC for 3 h.
RESULTS AND DISCUSSION
Pt-SO4 /ZrO2(A)
Much attention has been focused on the oxidative cou-
pling of methane because of the thermodynamically ad-
vantageous reaction with 1G = ꢁ30 Kcal/mol at 600ꢀC.
However, many kinds of catalyst systems still need high
temperatures to get the satisfactory activity and selectiv-
ity, 600ꢀC or higher. On the other hand, the dehydrogena-
tive coupling of methane is thermodynamically disadvan-
tageous (1G = 17 Kcal/mol at 600ꢀC), but it is one of the
attractive catalytic reactions to be developped.
Four gramsofthe gelwere impregnated with 7mlofaque-
ous H2PtCl6 ꢅ 6H2O, followed by evaporating the water and
drying; the Pt quantity was 1, 3, 5, and 7.5 wt% based on Pt-
ZrO2. The dried Pt-doped gel was then kneaded well with
ammonium sulfate using a mortar and pestle; the quantity
of (NH4)2SO4 was 15 wt% (3.6 wt% S) based on Pt-ZrO2.
The kneaded material(20–30mesh) waspreheated at 150ꢀC
for 1.5 h and calcined at 600ꢀC for 3 h (13).
The dehydrogenative reaction without any oxidant was
carried out over the Pt-SO4/ZrO2(S) catalysts after reduc-
tion in hydrogen at 500ꢀC. The results with 3 wt% Pt at
500ꢀC are shown in Fig. 1 as a function of time on stream.
A steady conversion of 0.2% was observed between 100
and 180 min after a rapid increase of conversion in the
first 20 min, and this high conversion was maintained over
360 min. The products were ethane and ethylene in the ratio
9 :1;no CO2 was detected. Since the equilibrium conversion
of methane into ethane and dihydrogen is estimated ther-
modynamically to be 0.6% , this conversion is satisfactory
judging from the reaction temperature of 500ꢀC.
Pt-ZrO2
Platinum-added zirconia without the sulfation was pre-
pared by impregnation of the gel with the aqueous platinate
following the above procedures: 3 wt% Pt and 600ꢀC, 3 h
for calcination.
The coupling of methane (research grade) was carried
out at atmospheric pressure in a single-pass fixed-bed Pyrex
reactor; 0.5 g of catalyst powder (20–30 mesh) was placed
between glass wool plugs. The reduction of catalyst was
performed in hydrogen at a flow rate of 10 ml/min at 500ꢀC
for 1 h before reaction. After the pretreatment methane
was introduced at a rate of 10 ml/min without any carrier
gas, and effluent products were analyzed with an on-line gas
chromatograph (Porapack R-2 m, 30ꢀC). Calculation of the
low conversion of methane was based on the appearance
of products; the GC system used detected all the products.
The amount of surface Pt atoms was measured volumetri-
cally by CO chemisorption at room temperature on samples
reduced in hydrogen. A conventional pulse reactor with a
fixed-bed catalyst, equipped with calibrated bulbs, was used
for the volumetric measurements. After a 0.5 g sample (0.3 g
after the reaction of methane) was heated in flowing hy-
drogen (10 ml/min) at 500ꢀC for 1 h and cooled to room
temperature, CO was adsorbed passing through the sample
bed several times while the remaining amount of CO was
monitored with the gas chromatograph. When the g.c. area
reached a steady point the amount of CO adsorbed was
quantified by comparing the area under the TCD curve to
peak areas of CO calibration pulses injected. An adsorp-
tion stoichiometry of CO (adsorbed) : Pt (surface atom) = 1
was used in the calculations.
Thermogravimetric and differential thermal analysis
(TG-DTA) was carried out using a Rigaku model with
FIG. 1. Reaction of methane over 3% Pt-SO4/ZrO2(S) at 500ꢀC:
an operating range from room temperature to 1000ꢀC at (᭿) conversion; (᭹) ethane; (᭡) ethylene.

30
KUROSAKA, MATSUHASHI, AND ARATA
FIG. 2. Reaction of methane over 7.5% Pt-SO4/ZrO2(S) at 500ꢀC:
(᭿) conversion; (᭹) ethane; (᭡) ethylene.
A highly active superacid of SO4/ZrO2 for the low tem-
perature conversion of butane was obtained by the addi-
tion of a large amount of Pt; the maximum activity was
observed with 7.5 wt% Pt (based on the gel) (14). Thus,
the 7.5% Pt-SO4/ZrO2(S) catalyst was examined in the re-
action, the results for longer time on stream being shown
in Fig. 2. No effect on the conversion increase was ob-
served, although the induction period was shorter. The
conversion was relatively constant after 180 min, but the
product ratio of ethylene/ethane gradually increased with
time. It is proposed that loading a large amount of Pt
leads to this behavior. It is seen, together with the results
in Fig. 1, that this catalyst system is quite steady during
the run.
Hydrogen was detected in the products by the negative
peak with TCD of detector in the GC chart. When H2 was
mixed in the reactant stream, the conversion dropped, ow-
ing to the influence of the reverse reaction, while the con-
version recovered its normal steady state activity when hy-
drogen was released. The effect of modifying the period
of reduction before reaction by more than 1 h was stud-
ied; the catalytic action was not altered, the unit hour being
sufficient.
FIG. 3. Catalytic activities of 5% Pt-SO4/ZrO2(S) (᭹) and 5% Pt-
SO4/ZrO2(A) (᭿) at 500ꢀC.
The remarkable loss of activity was also observed with
5% Pt-SO4/ZrO2(A); the results are shown in Fig. 3, to-
gether with those of 5% Pt-SO4/ZrO2(S) for comparison.
Figure 3 shows how the sulfation method controls the cata-
lytic activity, as well as the life of the catalyst; the sulfation
with sulfuric acid is superior beyond comparison.
Table 1 summarizes the highest activities and the selectiv-
ities in several runs over the Pt-SO4/ZrO2(S) with various
quantitiesofPt. The maximum conversion, 0.23% , obtained
by the materials with 3 and 5% of Pt is ca 40% of the equi-
librium value, 0.6% . Although no CO2 was detected in all
the runs over the present catalysts, only CO2 was produced
when the Pt substance was not loaded. The formation of
hydrogen was confirmed in all runs. Analysis of TG-DTA
showed carbon deposit on the catalyst to be at most 0.25%
of the methane reacted, and the deposition was not con-
cerned with the catalytic activity.
TABLE 1
Activities and Turnover Frequencies of Pt-SO4/ZrO2(S)
with Various Amounts of Pt at 500ꢀC
Highly active 7.5% Pt-SO4/ZrO2 catalyst for the bu-
tane conversion was also obtained by sulfation with ammo-
nium sulfate in place of sulfuric acid. Thus, the preparation
method was examined; that is, the Zr gel was impregnated
with the platinate (7.5 wt% Pt) followed by kneading well
with ammonium sulfate, heating at 150ꢀC to melt homo-
geneously the sulfate, and finally calcining at 600ꢀC, 7.5%
Pt-SO4/ZrO2(A). The matter gave 0.1% conversion at most,
and a continual loss of activity was observed with time af-
ter 60 min, down to half at 120 min. The catalyst caused a
sulfurous odor after the reaction, suggesting desorption of
the sulfate species from the catalyst surface.
Selectivity (% )
Quantity of Pt
(wt% )
Conversion
(% )
TOF
(minꢁ1
)
C2H6
C2H4
CO2
0
0.08
0.05a
0.12
0.23
0.23
0.19
0
0
88
90
91
90
0
0
12
10
9
100
100a
0
0
0
0a
1
0.16
0.18
0.16
0.13
3
5
7.5
10
0
a Prepared by sulfation with ammonium sulfate.

DEHYDROGENATIVE COUPLING OF METHANE
31
It isindicative that the active site for the couplingismetal-
lic platinum. Thus, Pt-loaded zirconia without the sulfation
was examined in the reaction. The 3% Pt-ZrO2 catalyst
showed the highest conversion of 0.20% at 60 min, but a
large decrease of the conversion, afterward, was observed
just like the cases of the catalysts sulfated with ammonium
sulfate.
Experiments using XRD were carried out in order to elu-
cidate the surface property of 3% Pt-SO4/ZrO2(S), whose
activity was highest; the spectra are shown in Fig. 4, to-
gether with those of the samples before reduction and after
reaction for comparison. Zirconium oxide heat-treated at
500ꢀC consists of a mixture of tetragonal and monoclinic
forms, and the former converts into the latter form by cal-
cination over 500ꢀC, with only the monoclinic system being
developed after calcination over 700ꢀC. The figure shows
all of the samples to be 100% tetragonal forms, indicating
retardation of the crystallographic phase transformation by
sulfation, and no influence on the crystal structure at all is
brought about by the reduction nor the reaction.
Although relatively large amounts of platinate were
added, any peak based on the Pt substances was not de-
tected (Fig. 4). However, remarkable peaks owing to the Pt
metal were found when sulfated with (NH4)2SO4 as shown
in Fig. 5. The Pt exists on a condensed state, while on a
state of dispersion when sulfated with H2SO4. The results
FIG. 4. XRD profiles of 3% Pt-SO4/ZrO2(S) after calcination (A),
after reduction (B), and after reaction (C): (ᮀ) tetragonal; (♦) monoclinic.
FIG. 5. XRD profiles of 3% Pt-SO4/ZrO2 sulfated with ammonium sulfate and sulfuric acid.

32
KUROSAKA, MATSUHASHI, AND ARATA
FIG. 6. XRD profiles of 7.5% Pt-SO4/ZrO2 sulfated with ammonium sulfate and sulfuric acid.
show how the sulfation method controls the dispersion of
The adsorption measurement was also performed with
Pt on the catalyst surface. As far as the XRD analysis was the samples with sulfuric acid after reaction (Fig. 8). The
concerned, the apparent dispersion was kept at an almost results were, however, different from those before reac-
constant state up to the addition of 7.5 wt% of Pt as shown tion; the amount stayed constant from 3 to 7.5% of the Pt
in Fig. 6. It is of interest that impregnating zirconia gel with
sulfuric acid leads to quite high dispersion of the Pt matter
onto the support.
Relative peak intensities of Pt to ZrO2 using the catalysts
after reaction were estimated, and the values are shown in
Fig. 7. The estimation was based on the peaks of (111) plane
at 30.2 and 39.7ꢀ for ZrO2 and Pt, respectively. The peak
intensity of Pt for the case of ammonium sulfate increases
relatively with the Pt quantity, while not for the case of
sulfuric acid, indicating the good dispersion of Pt in the
case of the latter.
The dispersion, or fraction of surface Pt atoms, was de-
termined by CO chemisorption. The amount of CO ad-
sorption for the catalysts sulfated with both sulfuric acid
and ammonium sulfate, followed by the reduction with H2,
was measured as shown in Fig. 8. The amount of CO in-
creased relative to the Pt quantity for the matter sulfated
with sulfuric acid, while the volume remained steady as the
Pt concentration was increased more than 3% for the cases
with ammonium sulfate. The observations are in agreement
with those of XRD; the Pt particles begin to grow rapidly
as the deposition values exceed 3% for the sulfation with
ammonium sulfate.
FIG. 7. Peak intensity of Pt(111) in the XRD profiles of Pt-SO4/ZrO2
after reaction.

DEHYDROGENATIVE COUPLING OF METHANE
33
FIG. 8. Amount of CO adsorption over Pt-SO4/ZrO2(S) after re-
duction (᭹) and reaction (᭡) and Pt-SO4/ZrO2(A) after reduction (᭿);
Pt-ZrO2 after reaction (ᮀ).
FIG. 9. IR spectra of 3% Pt-SO4/ZrO2(S) after calcination (A), after
reduction (B), and after reaction (C).
vanish after reaction. The catalyst was deeply colored after
reaction, and the sulfate materials were removed by reduc-
tion during the induction period as shown in Fig. 1, although
the reduction did not occur while pretreating with H2 at the
same temperature as the coupling reaction. It is notewor-
thy that the catalyst cannot be reduced to desulfurize the
surface in flowing hydrogen in the presence of Pt at 500ꢀC.
Since investigation of the relation between sulfate and Pt
at the steady state of reaction was found difficult using the ir
analysis, experiments using TG-DTA were performed. The
TGA in air of the sulfated zirconia matter shows a weight
decrease at 640–1000ꢀC, which is caused by the decompo-
sition of sulfate on the surface to form SO2 (18–20). The
weight decrease leads to the sulfur content, whose values
agree well with those of the elemental analysis. Quantities
of the loaded sulfur estimated from a weight loss at 600–
1000ꢀC are shown in Table 2, a large decrease being seen
addition, although it was in proportion to the catalytic activ-
ity for the coupling of methane. It is indicative of the grow-
ing aggregation of the metal during reaction, but in com-
parison with the samples before reaction an increase of the
peak strength of Pt in the XRD curve is not observed. Pre-
treatment with H2 at 500ꢀC was done before the measure-
ment was carried out to remove the carbon materials after
reaction.
Platinum-doped ZrO2 without sulfation showed the
chemisorbed value close to that of the sample sulfated with
sulfuric acid (Fig. 8). Since the sample is active for the cou-
pling (0.2% in conversion), it is assumed that the value is
equivalent to the number of active surface atoms of Pt.
Thus, based on an adsorption stoichiometry of unity of CO-
chemisorbed/Pt-surface atom, calculation of the CO value
of 3% Pt-SO4/ZrO2(S), highest in activity, leads to the ra-
tio of CO/Pt = 0.09, where 9 atoms per 100 adsorb CO and
convert methane into C2 species. The turnover frequencies
(TOF) for the reaction are also estimated from the conver-
sion of 0.23% to be 0.18 minꢁ1, those for other catalysts
being 0.13–0.16 minꢁ1 as shown in Table 1.
TABLE 2
Thermal Gravimetrical Analysis of Sulfur
in the Pt-SO4/ZrO2(S) Catalysts
Experiments using an IR instrument were carried out in
order to elucidate the relation between the sulfate species
and Pt; the spectra of 3% Pt-SO4/ZrO2(S) are shown in
Fig. 9. The superacid of SO4/ZrO2 shows characteristic ad-
sorption bands at 980–1230 cmꢁ1, which are assigned to the
bidentate sulfate coordinated to metal elements (10–12).
The adsorptions at 1050, 1140, and 1240 cmꢁ1, which are
analogous to those bands, are observed with the samples
after calcination and reduction, but the adsorption peaks
Quantity of S (wt% )
Quantity of
Pt (% )
Before reduction
After reaction
0
1
3
5
3.2
2.0
2.2
3.2
2.5
0.4
0.5
0.6
0.8
1.5
7.5

34
KUROSAKA, MATSUHASHI, AND ARATA
a Pt metal with a sulfated zirconia, Pt-SO4-ZrO2, which is
obtained by sulfation of the zirconia gel with sulfuric acid,
followed by the addition of platinum. This species is sur-
rounded by 10 times platinate-SO4/ZrO2 with various oxi-
dation numbers of Pt, except 0, when 3 wt% Pt is added and
protected from aggregation. Platinums without the sulfate
or with the matter sulfated with ammonium sulfate gather
together during reaction to result in deactivation. As for
superacidity, the sulfate-metal oxides obtained by the treat-
ment with sulfuric acid are usually higher in activity, as well
as acidity, than those with ammonium sulfate (10,11). The
latter substances are gradually decomposed during reaction
at the present high temperature because of weak interac-
tion of the sulfate species with the ZrO2 support, resulting
in the condensation of Pt. The fact that the sulfation with
FIG. 10. XPS spectra of S(2p) for 3% Pt-SO4/ZrO2(S) after calcina-
tion (A) and after reaction (B).
for the samples after reaction. The values determined can
lead to the calculation of a Pt number per the surface S
atom. The ratios of Pt/S were 0.4, 0.9, 1.0, and 0.8 mol/mol
for 1, 3, 5, and 7.5% Pt-SO4/ZrO2(S) after reaction, respec-
tively; the values before reduction were 0.1, 0.2, and 0.4 for
1, 3, and 7.5% Pt-sample, respectively. Thus, it is concluded
that the ratio of Pt to S on the active catalysts of 3–7.5%
Pt-SO4/ZrO2(S) is unity during reaction, the S amount per
Pt being more than 2.5 before reaction.
Measurements of XPS were carried out in order to char-
acterize S and Pt, the spectra of S and Pt of 3% Pt-SO4/
ZO2(S) being shown in Figs. 10 and 11, respectively. The
binding energy of S (2p) after reaction was 168.3 eV, quite
similar to that before reduction, 168.9 eV, whose value
agreed with that of metal sulfates (10,11). The state of S
is SO24ꢁ. A new peak at 162 eV after reaction shows the
state of S2ꢁ (21), indicating the reduction of part of the
SO24ꢁ during the reaction.
The binding energy of Pt (4f) is known to be 71.7, 72.6,
and 74.5 eV for Pt0, Pt+2, Pt+4, respectively (22,23). The
spectra showed the platinate to be reduced from calcination
(A), through reduction (B), to reaction (C). The binding en-
ergy of 71.4 eV after reaction showed platinum to be in the
metallic state, and the peak area was estimated to be around
10% , which was close to the chemisorbed value of CO.
In consideration of the results, it is proposed that an ac-
FIG. 11. XPS spectra of Pt(4f) for 3% Pt-SO4/ZrO2(S) after calcina-
tive site (in the steady state) is created by interaction of tion (A), after reduction (B), and after reaction (C).

DEHYDROGENATIVE COUPLING OF METHANE
35
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