Transformation of methane over platinum supported catalysts at moderate temperature

Article abstract of DOI:10.1016/j.molcata.2011.04.011

Non-oxidative methane reactions over solid catalysts are normally regarded as high temperature reactions (700 °C and above). Here we show that methane can be converted into ethane during many hours over two platinum supported catalysts at 370 °C. Analysis

Full text of DOI:10.1016/j.molcata.2011.04.011

Journal of Molecular Catalysis A: Chemical 342–343 (2011) 1–5
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Editor’s Choice paper
Transformation of methane over platinum supported catalysts at moderate
temperature
Dmitry B. Lukyanov∗, Tanya Vazhnova
Catalysis and Reaction Engineering Group, Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Non-oxidative methane reactions over solid catalysts are normally regarded as high temperature reac-
tions (700 ◦C and above). Here we show that methane can be converted into ethane during many hours
over two platinum supported catalysts at 370 ◦C. Analysis of the experimental data and DFT calculations,
available in the literature, suggests that this reaction proceeds via formation and subsequent coupling
of surface carbonaceous species (CHx and C) and that the relatively stable catalyst performance is asso-
ciated with slow coupling of carbon species. Our work indicates that other transition metal (Co, Fe, Ru,
Rh) supported catalysts should be active in methane conversion into ethane at similar and, possibly,
lower temperatures. Thus, further research into these catalytic systems would furnish new experimental
data on formation and transformation of surface CHx species, which are believed to be involved in such
reactions as Fischer–Tropsch synthesis, methanation and methane steam reforming.
Received 27 November 2010
Received in revised form 11 April 2011
Accepted 14 April 2011
Available online 22 April 2011
Keywords:
Methane
Activation
Reaction
DFT
© 2011 Elsevier B.V. All rights reserved.
Platinum
Transition metals
1. Introduction
techniques (papers [7,11–13] and references therein) and, more
recently, was explained by the DFT studies, which were com-
Methane, the principal component of natural gas and an
important greenhouse gas, is nowadays considered as a vast
potential feedstock for production of chemicals [1–6]. Differ-
ent approaches to methane activation and transformation have
been investigated extensively during the last 3 decades, but the
direct efficient methane transformation has yet to be developed
[1–6]. For long time non-oxidative methane reactions have been
regarded as high temperature reactions (700 ◦C and above) due to
high thermodynamic stability of methane and associated unfavor-
able thermodynamics of its transformation at lower temperatures
[3,5,7]. However, it would be a mistake to underestimate the impor-
tance of the research into these reactions at lower temperatures,
since it could improve our understanding of the mechanisms of C–C
bond formation at low/moderate temperatures, which is currently
an area of significant scientific interest.
prehensively reviewed by van Santen et al. [14]. Curiously, very
little data on conversion of pure methane into gaseous products
was reported and discussed in these experimental and theoretical
studies. Of particular relevance to our communication is the work
on methane activation and transformation into hydrocarbons
over transition metal supported catalysts via two-step reaction
sequence [7,15–20], when methane is activated under relatively
mild conditions with formation of C1 surface intermediates (step
1), which produce C2+ hydrocarbons upon hydrogenation (step
2). In these experiments, Amariglio et al. [16–18] and Solymosi
et al. [19] observed formation of ethane during the first step of
the cycle. The ethane formation rate decreased during 5–20 min
of the reaction and no data on ethane formation was reported at
longer time. Obviously, ethane formation was taking place under
transient conditions and it is therefore of interest to investigate
the possibilities of a steady state (or quasi steady state) methane
conversion into ethane (or higher hydrocarbons) over transition
metal supported catalysts. Such an investigation appears to be
important, since it may contribute to our understanding of the
mechanisms of C–C bond formation from C1 surface species at
low/moderate temperatures.
Activation of methane at relatively low temperatures
(150–250 ◦C) was first demonstrated by Kemball [8–10] in
the early 1950s in the experiments on CH4–D2 exchange on
metal films. Since that time methane activation on various metal
catalysts at low and moderate temperatures was confirmed by
many research groups using different catalytic and surface science
Our recent study of conversion of methane/benzene mixture
into toluene has suggested [21] that reasonably stable trans-
formation of pure methane into ethane could be achieved over
Pt-containing H-MFI zeolite catalyst at such low temperate as
370 ◦C. The aim of this work was to test this suggestion and to
∗
Corresponding author. Tel.: +44 1225 383329; fax: +44 1225 385713.
E-mail addresses: D.B.Lukyanov@bath.ac.uk (D.B. Lukyanov),
T.Vazhnova@bath.ac.uk (T. Vazhnova).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.04.011

2
D.B. Lukyanov, T. Vazhnova / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 1–5
TS4
attempt to get new insights into mechanisms of activation and
transformation of pure methane over transition metal supported
catalysts. To achieve this goal, we decided (i) to analyze the recently
reported results of the density functional theory (DFT) studies, rel-
evant to methane activation, and (ii) to consider the results of this
analysis together with the experiments on methane transformation
over Pt-containing catalysts at 370 ◦C.
TS1
TS2
80
40
0
TS3
2. Experimental
-
-
40
80
2.1. Catalyst preparation and characterization
Two Pt-supported catalysts (1 wt% Pt), defined as PtH-MFI and
Pt/Al2O3, were prepared by incipient wetness impregnation of
the H-MFI zeolite (ZEOLYST, Si/Al = 15) and ␥-Al2O3 (Alfa Aesar,
320 m2/g), respectively, with an aqueous solution of tetraammine-
platinum(II) nitrate, Pt(NH3)4(NO3)2. Prior to the reaction studies,
the catalysts were dried at room temperature, calcined in air at
530 ◦C and then reduced in hydrogen at 500 ◦C. The Pt dispersion
in the reduced PtH-MFI and Pt/Al2O3 catalysts was 12 and 33%,
respectively. For kinetic studies, the catalyst powder samples were
pressed into disks, crashed, and sieved to obtain catalyst particle
sizes in the range of 0.25–0.5 mm. The full description of the cata-
lyst preparation and characterization is presented in our previous
publications [21,22].
CH4
CH3+H
CH2+2H
CH+3H
C+4H
Fig. 1. Reaction energy diagrams for transformation of methane into carbon and
hydrogen chemisorbed on flat Pt(1 1 1) terraces and stepped Pt(1 1 0)(1 × 2) surfaces.
All energies are referenced to methane in the gas phase and the clean surfaces. The
top diagram for the flat Pt surface is based on the data in Ref. [33], and the bottom
diagram for the stepped Pt surface is based on the data in Ref. [34]. TS stand for
transition state.
of these species into methane over transition metals have been
studied by different research groups [14,26–35]. However, these
reactions were usually analyzed as important reaction steps in
methanation [14,26,27], Fischer–Tropsch [14,28–31] and methane
steam reforming [32] processes, and were not examined as a part of
transformation of pure methane into larger hydrocarbons. Here we
consider the available theoretical data with the focus on methane
transformation.
2.2. Kinetic studies and thermogravimetric analysis
The catalysts were tested at atmospheric pressure in a continu-
ous flow reactor at 370 ◦C. Experiments were performed with pure
DFT calculations of methane decomposition were reported for
different transition metals [14,27,32–35], and are illustrated in
Fig. 1 by the energy diagrams for this reaction on relatively flat ter-
races [33] and stepped [34] platinum surfaces. Fig. 1 shows that the
adsorption energies for different species as well as the activation
energies of transformation of these species depend on the type of
the platinum surface. In general, the adsorption of carbon (C) and
CHx (x = 1–3) species is stronger on stepped Pt surface, while the
activation energies are higher, as a rule, for flat Pt surface. Fig. 1
reveals that the activation energy of methane transformation into
CH3 + H (that is, in fact, the methane activation step) is ∼65 and
35 kJ/mol for the flat and stepped Pt surfaces, respectively. Inter-
estingly, regardless of the type of Pt surface, the most stable surface
species are CH species and formation of carbon species requires the
highest activation energy. In general, this is true for other transi-
tion metals, such as Co, Ni, Rh, Ru, Pd and Ir [14]. Moreover, for Pt
surfaces, carbon species have the lowest adsorption energy (Fig. 1)
and, as a consequence, should be present in the lowest concen-
tration on the catalyst surface. Thus, the reactions between two
carbon species (leading to coke formation) should be less favorable,
in terms of the concentrations of the reacting species, than coupling
of CHx species with formation of C2 hydrocarbon precursors.
The above conclusion is further enhanced by the DFT calcu-
lations reported by Hu and coworkers [29–31] for the coupling
reactions between carbon and CHx species. Indeed, for the cou-
pling reactions involving C, CH, CH2 and CH3 species, the coupling
C + C reaction was found to have the highest activation energy
(174–283 kJ/mol, depending on the metal). This result was obtained
for stepped surfaces of five transition metals (Rh, Co, Ru, Fe and Re)
and it is therefore likely that it will stand also for platinum. The low-
est activation energies of CHx coupling reactions over five different
metals were found [31] to be 26 kJ/mol for Co and Fe, 83 kJ/mol for
Rh, 89 kJ/mol for Ru and 137 kJ/mol for Re. These estimates can be
compared with 102 kJ/mol for CHx coupling over flat (1 1 1) and
stepped (2 1 1) Pt surfaces [35], and with 102 and 65 kJ/mol for
(CH2 + CH) and (CH3 + CH) reactions, respectively, over Ru (0 0 0 1)
surface [28].
methane as a feed at weight hour space velocity (WHSV) of 1.26 h−1
,
and the reaction mixture was analyzed by on-line gas chromatog-
raphy, as described elsewhere [22].
After completion of the reaction (24 h for Pt/Al2O3 catalyst and
25 h for PtH-MFI catalyst), the reactor was purged by oxygen-free
nitrogen (30 ml/min) for 0.5 h at 370 ◦C and then cooled down to
room temperature in a nitrogen atmosphere. The catalyst samples
were then unloaded from the reactor for further thermogravimetric
analysis (TGA). Similar procedure was used earlier in the study of
coke formation during benzene alkylation with ethane into ethyl-
benzene [23]. TGA of the discharged PtH-MFI and Pt/Al2O3 catalysts
was performed in a Setaram TG-92 thermogravimetric analyzer fol-
lowing the procedure developed by Manos and coworkers [24,25].
This procedure allows one to distinguish between coke precursors,
which can be removed through volatization in N2, and hard coke,
which stays on the catalyst up to 600 ◦C and is removed by burning
in air. In our experiments, a catalyst sample of ∼50 mg was first
purged at room temperature by nitrogen (30 ml/min) for 2 h, then
heated to 250 ◦C at the rate of 10 ◦C/min and kept at this temper-
ature for 1 h in the same N2 flow to remove adsorbed water. Then
the temperature was raised (10 ◦C/min) to 600 ◦C and kept at this
temperature under N2 flow for 30 min in an attempt to remove coke
precursors. Finally, nitrogen was replaced by air at the same flow
rate (30 ml/min) at 600 ◦C, and the sample was kept under these
conditions for 1.5 h.
3. Results and discussion
3.1. Analysis of DFT calculations available in the literature
As a consequence of the significant progress in DFT calculations
[14,26], theoretical descriptions of the realistic energetic profiles
of various reactions have become a reality and, therefore, can be
used to generate better understanding of these reactions. Methane
decomposition into carbonaceous species as well as hydrogenation

D.B. Lukyanov, T. Vazhnova / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 1–5
3
The DFT results discussed above reveal that activation of
A
methane and its further transformation into C2 hydrocarbons
(ethane and/or ethene) via coupling reactions is quite possible at
moderate temperatures for a range of transition metals due to
relatively low activation energies of the reaction steps involved.
Moreover, the reaction is expected to be quite stable, since the steps
of coke formation appear to occur at slow rates. The prediction
regarding methane activation agrees well with the experiments
on CH4–D2 exchange on metal films at 150–250 ◦C [8–10] and
the experiments [16–20] on methane decomposition into car-
bonaceous species and hydrogen over transition metal supported
catalysts (Ru, Rh, Pd, Pt, Co, Ir) at such low temperatures as
100–200 ◦C. However, the prediction of the stable methane con-
version into C2 hydrocarbons has no experimental support. As
noted in Section 1, the formation of ethane was indeed observed
at low temperature (250 ◦C), but the catalysts were loosing their
activity during 5–20 min of the reaction [16,19]. Hence, the steady
state catalyst performance was not achieved. This is an impor-
tant result, since it questions the validity of the DFT calculations
that predict slow catalyst deactivation due to slow coke formation.
Another possibility of the disagreement between the experimental
and theoretical results might be associated with very low equilib-
rium conversion of methane into ethane at 250 ◦C (below 0.1%).
Such low steady state conversion (if it was in truth achieved) could
be simply considered as zero conversion and, therefore, ‘missed’.
0.16
0.12
0.08
0.04
0.00
0
4
4
4
8
8
8
12
12
12
16
16
16
20
20
20
24
24
24
28
28
28
B
0.06
0.04
0.02
0.00
0
3.2. Experimental study of methane transformation on PtH-MFI
and Pt/Al2O3 catalysts
C
0.003
0.002
0.001
0.000
To clarify the reason of the disagreement between the existing
experimental and theoretical results and to examine the possibility
of relatively stable methane conversion, we decided to test two Pt-
supported catalysts in the reaction of pure methane at 370 ◦C. The
choice of the temperature and the first catalyst, PtH-MFI, was based
on our recent work [21] that demonstrated highly selective and sta-
ble conversion of methane and benzene into toluene at 370 ◦C over
this catalyst. The second catalyst, Pt/Al2O3, was chosen as a more
conventional catalyst. Both catalysts contained 1 wt% of platinum,
but the Pt dispersion was different: 12 and 33% on the PtH-MFI
and Pt/Al2O3 catalysts, respectively. The equilibrium conversion of
pure methane in ethane and hydrogen was estimated, using the
thermodynamic data [36], to be 0.25% at 370 ◦C.
Transformation of pure methane over the PtH-MFI and Pt/Al2O3
catalysts was studied in a continuous flow reactor during 24–25 h.
Fig. 2A shows that methane conversion was increasing during the
first 4–6 h of the reaction reaching 0.15 and 0.06% over PtH-MFI
and Pt/Al2O3 catalysts, respectively. With further increase in time,
slow Pt/Al2O3 catalyst deactivation was observed, while the per-
formance of the PtH-MFI catalyst was stable. Ethane was the sole
carbon containing product over the Pt/Al2O3 catalyst and the major
product over the PtH-MFI catalyst (Fig. 2B and C). In the latter
case, benzene and toluene were also produced but their amounts
were much smaller than the amount of ethane. Interestingly, ben-
zene and toluene were not observed even in trace amounts on
the Pt/Al2O3 catalyst, which did not contain Brønsted acid sites
(BAS). Thus, it is possible that formation of benzene and toluene
over PtH-MFI catalyst was following the dehydrocyclodimerization
(aromatization) mechanisms over bifunctional catalysts, which
were first proposed by Csicsery [37] and were later supported by
kinetic modeling studies by Lukyanov et al. [38,39].
0
Time on stream (h)
Fig. 2. Effect of time on stream on methane conversion and product formation over
PtH-MFI (solid symbols) and Pt/Al2O3 (open symbols) catalysts: (A) Methane con-
version (᭹, ꢀ), (B) concentration of ethane (ꢀ, ꢁ), and (C) concentrations of benzene
(ꢂ) and toluene (ꢃ). Reaction was carried out at 370 ◦C with pure methane as a feed
and at weight hour space velocity of 1.26 h−1
.
for methane decomposition on a number of transition metals. One
can expect that the accumulation of these species on Pt active sites
would lead to changes in the catalyst performance that was indeed
observed in the experiments (Fig. 2). Formation of surface carbona-
ceous species should lead to formation of H2 in the gas phase,
as schematically illustrated below by the reactions generating the
surface CH3 and C species.
CH4 + [Pt] → CH3–[Pt] + (1/2)H2
CH4 + [Pt] → C–[Pt] + 2H2
(1)
(2)
Hydrogen released in reactions (1) and (2) cannot be accounted
for by formation of the hydrocarbons detected in the gas phase.
Hence, one would expect to observe hydrogen formation in excess
of formation of the gaseous hydrocarbon products. The experi-
mental data in Fig. 3 support this expectation. Indeed, Fig. 3A
demonstrates that the experimentally determined hydrogen con-
centration is significantly higher during the transient period than
the hydrogen concentration, which corresponds to formation of the
hydrocarbons observed in the gas phase. The latter was calculated
The initial (transient) period of the reaction (Fig. 2) is likely to
be linked to the accumulation of the surface carbonaceous species
(e.g. CH3, CH2, CH, C, C2Hx) on the Pt particles of the catalysts. For-
mation of such species from methane on the surface of transition
metals is predicted by the DFT calculations (see Fig. 1 and discussion
above), and was considered by different authors [7,12,13,16–20,40]

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D.B. Lukyanov, T. Vazhnova / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 1–5
53
A
0.5
0.4
0.3
0.2
0.1
0.0
600
500
400
300
200
A
52
51
50
49
48
47
100
0
0
100
200
300
400
0
4
8
12
16
20
24
28
51
50
49
48
47
46
45
B
600
500
400
300
200
0.5
0.4
0.3
0.2
0.1
0.0
B
100
0
0
100
200
300
400
0
4
8
12
16
20
24
28
Time (min)
Time on stream (h)
Fig. 4. Thermogravimetric profiles for (A) PtH-MFI and (B) Pt/Al2O3 catalysts after
25 and 24 h of the reaction, respectively. Reaction conditions are given in the legend
of Fig. 2. TGA procedure is described in Section 2.2, and the vertical dotted line
corresponds to the switch from the nitrogen to the air flow.
Fig. 3. Effect of time on stream on hydrogen formation over PtH-MFI (solid symbols)
and Pt/Al2O3 (open symbols) catalysts. (A) Experimental (ꢀ, ꢁ) and calculated (ꢂ,
ꢁ) values, and (B) the difference between the experimental and calculated values
(᭹, ꢀ). Calculated values for hydrogen correspond to formation of all hydrocarbon
products observed in the gas phase (see text for explanation). Reaction conditions
are given in the legend of Fig. 2.
conversion (Eq. (3)). This is not the case in our experiments, since
additional hydrogen was produced during formation of the surface
carbonaceous species and, with the PtH-MFI catalyst, during forma-
tion of benzene and toluene as well. With this additional hydrogen
taken into account, we determined that for 4 h on stream the exper-
imental methane conversion into ethane was about 74 and 60% of
the equilibrium methane conversion for the PtH-MFI and Pt/Al2O3
catalysts, respectively. Hence, the methane conversion over the
PtH-MFI catalyst was quite close to the equilibrium conversion, and
one may suggest that this could be the reason for the observed sta-
ble performance of this catalyst. Indeed, in general, the effect of
coke on the conversion would be more pronounced at lower con-
versions than at conversions approaching equilibrium values. To
test this suggestion, we analyzed the coke content on two catalysts
used in this study.
The thermogravimetric profiles for the PtH-MFI and Pt/Al2O3
catalysts are shown in Fig. 4. The obtained data reveal that only
0.3 wt% of carbonaceous species are retained by the PtH-MFI cata-
lyst and that these species are desorbed from the catalyst in the flow
of nitrogen, when the catalyst is heated from 250 to 600 ◦C (Fig. 4A).
Interestingly, no species, which could be considered as hard coke
[24,25], are present on this catalyst (hard coke can be removed by
burning only). In our opinion, these results provide a solid expla-
nation for the observed stable performance of the PtH-MFI catalyst
(Fig. 2) and demonstrate that the retained carbonaceous species
are unlikely to be of a graphitic nature (i.e. built by the reactions of
carbon (C) species). The latter result is in excellent agreement with
the analysis of the DFT studies (see Section 3.1) that indicates slow
rates of C + C coupling reactions.
on the basis of the experimentally determined concentrations of
these hydrocarbons (ethane, benzene and toluene) and the stoi-
chiometric equations for the reactions shown below.
2CH4 = C2H6 + H2
(3)
(4)
(5)
6CH4 = C6H6 + 9H2
7CH4 = C6H5(CH3) + 10H2
The difference between the experimental and calculated hydro-
gen concentrations is shown in Fig. 3B and corresponds to hydrogen
released during formation of the surface carbonaceous species. This
difference is decreasing rapidly during the first 4–6 h of the catalyst
operation and then stabilizes at ∼0.05% and ∼0.15% for the PtH-MFI
and Pt/Al2O3 catalysts, respectively. Such a pattern is consistent
with two possible processes, which may take place at the catalyst
surface and release H2 in the gas phase. The first process (accumu-
lation of surface carbonaceous species) appears to be completed
in about 4–6 h (Fig. 3) and is likely responsible for the transient
catalyst operation during this time (Fig. 2). The second process is
taking place during all 25 h of the experiment and produces H2 in
much smaller amounts than the first one (Fig. 3). Comparison of the
data in Figs. 2 and 3 suggests that this, second process is associated
with formation of inactive surface carbon species that are respon-
sible for the slow deactivation of the Pt/Al2O3 catalyst after 4–6 h
on stream. The deactivation of the PtH-MFI catalyst is not observed
in our experiments and this agrees with the low concentration of
hydrogen formed on this catalyst (Fig. 3B).
Let us now consider the extent of methane transformation into
ethane over two catalysts tested in this work. As noted above,
the equilibrium conversion of methane into ethane and hydro-
gen was estimated to be 0.25% assuming stoichiometric methane
The data in Fig. 4B show that the Pt/Al2O3 catalyst contains
both the coke precursors (0.8 wt%) and hard coke (0.4 wt%). Thus,
this catalyst would be expected to deactivate much faster than the
PtH-MFI one, and this is exactly what is observed in the catalytic

D.B. Lukyanov, T. Vazhnova / Journal of Molecular Catalysis A: Chemical 342–343 (2011) 1–5
5
experiments (Fig. 2). It is also worth to note that the difference in
Acknowledgements
the amounts of the carbonaceous species retained by two catalysts
used in this work agrees well with the difference in the amounts of
hydrogen, which was assumed to be produced over these catalysts
during formation of the surface carbonaceous species (see Fig. 3B).
The data in Fig. 4B reveal that the amount of the hard coke (possibly,
of graphitic nature) is 2 times lower than the amount of the coke
precursors, thus pointing out to the slower rates of C + C coupling
reactions in comparison with the rates of the coupling reactions
between CHx species.
We thank Stan Golunski (Cardiff University) and Paul Millington
(Johnson Matthey) for discussions and the data on Pt dispersion.
This work was supported by EPSRC of the UK (Grant EP/C532554)
and by the Department of Chemical Engineering at the University
of Bath.
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