Low-concentration methane combustion over a Cu/γ-Al2O3 catalyst: Effects of water

Article abstract of DOI:10.1039/c5ra00633c

The influence of water on low-concentration methane oxidation over a Cu/γ-Al₂O₃ catalyst was investigated in a fixed bed reactor. This paper studied the effect of water on the activity of methane combustion, using parameters such as water reversible adsorption, regeneration of the activity and surface characteristics of the catalyst. Apparent activation energy was found by experiments, and water surface coverage was calculated using the Langmuir equation. It was found that the activity of methane combustion over a Cu/γ-Al₂O₃ catalyst decreased with time due to water adsorption. The inhibitory effect generated by water weakened as the temperature rose above 550°C. Reactivity could be refreshed if the catalyst particles were scavenged by N₂. Kinetic experiments showed that, if water was added into the feed, apparent activation energy (E_a) increased noticeably (81.4 kJ mol^-1 → 153.0 kJ mol^-1) and the reaction order with respect to water was -0.6 to -1. Using the Langmuir equation, it could be concluded that the coverage of water adsorption on catalytic active sites increased noticeably as vapor was introduced into the feed. If the temperature increased, water coverage went down and tended towards 0% above 625°C.

Full text of DOI:10.1039/c5ra00633c

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Low-concentration methane combustion over a
Cu/g-Al O catalyst: effects of water
2
3
Cite this: RSC Adv., 2015, 5, 18915
Haojie Geng, Zhongqing Yang,* Jingyu Ran, Li Zhang, Yunfei Yan and Mingnv Guo
2 3
The influence of water on low-concentration methane oxidation over a Cu/g-Al O catalyst was
investigated in a fixed bed reactor. This paper studied the effect of water on the activity of methane
combustion, using parameters such as water reversible adsorption, regeneration of the activity and
surface characteristics of the catalyst. Apparent activation energy was found by experiments, and water
surface coverage was calculated using the Langmuir equation. It was found that the activity of methane
combustion over a Cu/g-Al
O
2 3
catalyst decreased with time due to water adsorption. The inhibitory
ꢀ
effect generated by water weakened as the temperature rose above 550 C. Reactivity could be refreshed
if the catalyst particles were scavenged by N
2
. Kinetic experiments showed that, if water was added into
ꢁ1
ꢁ1
the feed, apparent activation energy (E ) increased noticeably (81.4 kJ mol / 153.0 kJ mol ) and the
a
Received 12th January 2015
Accepted 3rd February 2015
reaction order with respect to water was ꢁ0.6 to ꢁ1. Using the Langmuir equation, it could be concluded
that the coverage of water adsorption on catalytic active sites increased noticeably as vapor was
introduced into the feed. If the temperature increased, water coverage went down and tended towards
DOI: 10.1039/c5ra00633c
ꢀ
www.rsc.org/advances
0% above 625 C.
to progress the reforming reaction of methane with water.
About the inuence of water, the studies of Ciuparu et al.
1
. Introduction
16,17
There was a large amount of low-concentration methane in the reported that, by means of in situ DR-FTIR investigation, several
coal bed, especially in China. It was hard to make use of the OH adsorption bands were observed on the catalyst surface and
methane using conventional combustion technology and there hydroxyls were found to be related to the PdO phase. OH groups
was no choice but to emit the gas into the atmosphere. Catalytic had bridged and terminal bonds before recombination and
combustion had been considered to be a comparatively ideal desorption as water molecules, suggesting the dehydroxylation
18,19
burning mode so far, which could achieve high-conversion- mechanism proceeds. Stasinska et al.
reported that steam
efficiency and low-pollution-emission with noble metal or could promote the growth of surface grain, resulting in the
1
–4
transition metal catalysts. A large number of studies had been decrease of the surface area of the catalyst. Meanwhile, Qiao
2
0,21
done on the catalytic combustion of methane with noble or et al.
reported that water vapor could severely inhibit the
–CuO solid solutions for CH and
–NiO showed a good resistance to
large amount of steam which could affect the reactivity during water. Using special treatments to ensure differential condi-
transition metal catalysts, and combustion mechanism had catalytic performance of CeO
2
4
5–7
been widely reported. However the feed always contained a CO conversion, while CeO
2
22
the actual reaction. Therefore, the effect of water on methane tions, J. C van Giezen reported that apparent activation energy
ꢁ1
ꢁ1
8
–10
combustion required further research.
of methane (E ) increased from 86 kJ mol to 151 kJ mol if
a
Many catalysts had been investigated for their activity in the water was introduced into the feed and the order with respect to
catalytic combustion of methane with water over noble metal water was found to be ꢁ0.76.
catalysts (such as Pd, Pt) and many conclusions were drawn.
2 4
For methane combustion with water (O /CH > 2), the
Some scholars considered that H O adsorbed on the active inhibitory effects of water had been widely acknowledged,
2
23–26
sites, which formed OH* groups, generated inhibitory effects on especially in Pd or Pt.
However, there were few studies about
Among the combustion products, transition metal catalysts, such as copper. The inhibitory effects
O had a more obvious inuence than CO
on methane of water at low temperature, adsorption characteristics and
combustion. In oxygen-rich conditions (O /CH
> 2), it was hard catalyst surface property were not yet clear for methane
combustion. H O surface coverage with differing temperature
11–15
methane combustion.
H
2
2
2
4
2
required further study. The aim of this work was to investigate
the inhibitory effects of water on the copper based catalyst from
Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of
Education, College of Power Engineering, Chongqing University, Shapingba District, the following aspects: water adsorption, regeneration of activity,
Chongqing 400030, People’s Republic of China. E-mail: zqyang@cqu.edu.cn; Fax:
kinetic parameters of methane and water surface coverage.
+
86-23-65111832; Tel: +86-023-65103114
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2. Experimental
2.1. Catalyst preparation and characterization
2
ꢁ1
2 3
g-Al O particles (specic surface area, 181.4 m g ; pore volume,
3
ꢁ1
˚
0
.46 cm g ; pore diameter, 190.6 A) were used as supports of the
Cu catalyst. The g-Al
2
O
3
particle diameter was lower than 0.1 mm,
ꢀ
and each was calcined at 500 C for 6 h before loading. Alumina
powder was impregnated with the metal using the incipient
wetness technique. 0.294 g Cu(NO ) powder was chosen to load
3
2
0
.1 g Cu on the support (1 g). The mass of distilled water, the
solvent for the Cu(NO powder, was determined by the support
mass and pore volume. Support particles were impregnated with
3 2
)
Cu(NO
3
)
2
solution for 5 h; aer impregnation, the catalyst was
ꢀ
Fig. 1 XRD spectra of the Cu/g-Al
catalyst after reaction (550 C, 3 vol% CH
2
O
3
catalyst. (a) Fresh catalyst; (b)
dried at 150 C in a drying oven for 6 h in air and calcined for 6 h
in a muffle furnace at 500 C in N
ꢀ
4
, 20 vol% O
2
, N
2
balance).
ꢀ
2
. The self-made catalyst was
cooled down to room temperature in a muffle furnace (under N2
atmosphere). During this process, the nitrate precursor and
hydroxyl group were completely decomposed and desorbed fully
from the catalyst. Table 1 and 2 show the content of each element
acquisition and control system which controlled the reaction
temperature. The effluent gas was measured by gas chroma-
tography to determine the concentrations of CH and CO .
4 2
(
XRF) and specic surface area (BET) of the self-made catalyst.
Fig. 1 shows the XRD spectra of the Cu/g-Al catalyst: a,
fresh catalyst; b, catalyst aer reaction. From Fig. 1(a) and (b), the
2 3
O
2.3. Catalytic activity test for methane oxidation
ꢀ
ꢀ
fresh catalyst consisted of CuO and Al
O
2 3
between 30 and 70 . The reactivity of Cu/g-Al was determined by methane
2 3
O
1
+
1ꢁ
Other species (Cu , O , O2 ads) were not found in Fig. 1. This conversion. According to the carbon balance, methane conver-
meant that CuO was the active site that provided O* (chem- sion x is described as:
isorbed O) to methane. In the ow reactor, as O was enough for
2
½
CO
2
ꢂ
CH
4
oxidation (O
2
/CH
4
> 2), the Cu cluster, especially surface Cu,
x ¼
(1)
½
CO
2
ꢂ þ ½CH ꢂ
4
2
+
was able to maintain Cu and provide O*. Fig. 1(b) showed that,
2 4
during the reaction, the catalyst phase did not change much, and Where [CO ] and [CH ] denote volume percentage of carbon
other peaks were not presented aer methane oxidation.
dioxide and methane in the stream.
The microstructure and characterization of the catalyst
surface were investigated by scanning electron microscopy
2.2. Experimental devices and system
(SEM), which magnied the catalyst surface by 1000 times. BET
Fig. 2 presents the experimental device and system. The exper- surface area, pore volume and pore diameter were measured by
iment of methane combustion was conducted in the xed-bed the BET method before or aer the experiments.
reactor which had a ceramic reaction tube with 10 mm inner
diameter and 1200 mm length. The reaction mixture gas (3 vol%
3
. Results and discussion
CH , 20 vol% O and N balance) controlled by a mass ow-
4
2
2
ꢁ1
3.1. Effects of water on methane combustion
meter, was injected into the xed-bed reactor at 200 ml min
.
Water vapor, generated by a vapor generator (W-202A-220-K) The effect of water on methane combustion over a Cu/g-Al
O
2 3
ꢀ
was carried by N
2
at 150 C and mixed with CH
4
and O
2
catalyst is shown in Fig. 3 and 4. Fig. 3 shows the change in
before entering into the reactor. The reactor was heated by a
resistance-heated furnace that was connected to the data
Table 1 The content of each element of the Cu/g-Al
2
O
3
catalyst (XRF)
Cu Al Si
O
K
Other
Mass (wt/%) 9.64% 40.76% 42.98% 1.36% 1.78% 3.84%
2 3
Table 2 BET surface area of the Cu/g-Al O catalyst
Specic surface area Pore volume
Pore diameter
Fig. 2 Flow chart of the experimental system; 1. Flow control pump;
2. pressure relief valve; 3. mass flowmeter; 4. data acquisition and
control system; 5. fixed-bed reactor; 6. resistance stove.
2
ꢁ1
3
ꢁ1
188.363 A˚
1
80.6 m g
0.4571 cm g
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at high steam pressure. For the inhibitory effect of water vapor,
it was generally considered that water species could adsorb on
the catalyst surface, and react with chemisorbed O (O*): H O +
2
O* / OH*. Thus, a portion of O* did not participate in
9,12–14
methane oxidation, making methane conversion lower.
From Fig. 4, if H O ow was turned off, the activity almost
2
returned to the initial level. This indicated there was some water
adsorbed on the catalyst surface at low temperature. When the
water was turned off, water pressure decreased. H
2
O gradually
desorbed from the surface and O* was able to react with CH4.
3.2. Regeneration of activity of the catalyst
Regeneration of the catalytic activity of the Cu/g-Al O catalyst
2
3
was studied by adding water cyclically to the feed gas. By
ꢀ
ꢀ
ꢀ
controlling the reaction temperature (500 C, 550 C, 600 C)
and water concentration (2.5 vol%; 5 vol%; 10 vol%), Fig. 5
2 3 4
Fig. 3 Methane oxidation with water over a Cu/g-Al O catalyst (CH ,
3
vol%; O
2
, 20 vol%; H
2
O, 0–20 vol%; N
2
, balance).
shows the effect when the catalyst was purged with N
stream was as follows: CH , 3 vol%; O , 20 vol%; H O (2.5 vol%;
vol%; 10 vol%); N , balance.
In Fig. 5, CH conversion was measured by gas chromatog-
2
. The feed
4
2
2
5
2
methane conversion with temperature at different water
content values (0 vol%, 2.5 vol%, 5 vol%, 10 vol%, 15 vol%, raphy every 15 minutes, whereas the catalyst was purged with
0 vol%). Fig. 4 shows the inhibitory effect of water on methane nitrogen in the absence of CH for another 10 minutes. Methane
4
2
4
ꢀ
ꢀ
combustion at 500 C and 600 C. From these gures, water conversion was initially high. Then, the activity dropped as water
signicantly affected catalytic activity and CH conversion was introduced into the feed gas. Thereaer, the catalyst was
4
dropped if water was introduced into the feed. When the water purged with N2 at the same temperature; consequently, no
vapor increased from 0 vol% to 10 vol%, the catalyst continued methane was shown in the gure. Aer purging, the activity of
losing activity with time. From Fig. 3, with temperature rising the catalyst almost recovered to its initial level. Comparisons of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
from 475 C to 600 C, CH conversion increased quickly. The 500 C, 550 C and 600 C show high temperature weakened the
4
inhibition effect of H O on methane combustion increased inhibitory effect and activity dropped by a smaller amount
2

rstly and then decreased, and water inhibition among the compared with low temperature. This meant H O or surface OH*
2
ꢀ
curves reached its maximum at 525 C. The process showed that did not adsorb on the catalyst surface stably, and if temperature
water vapor had a signicant inhibitory effect on methane increased (500 C / 600 C), the process of desorption or
ꢀ
ꢀ
combustion, while, with temperature increasing, the inuence decomposition was easy to proceed: OH* / H O + O*.
2
weakened gradually. A considerably higher temperature was
most likely required for the catalytic reaction to proceed.
Different from Pd and Pt catalysts, the Cu catalyst displayed a
stronger resistance to water and could maintain a stable activity
3
.3. Surface characteristics and stability
In order to discover the inuence of water vapor on the catalyst
surface, the SEM pictures (1000 times) for the catalyst surface
Fig.
Cu/g-Al
4
Water inhibitory effect on methane oxidation over the Fig. 5 Regeneration of the catalytic activity of the Cu/g-Al
catalyst (3 vol% CH , 20 vol% O ). (3 vol% CH , 20 vol% O ).
2 3
O catalyst
O
2 3
4
2
4
2
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Fig. 6 Surface characteristics of the Cu/g-Al
O
2 3
catalyst (SEM). (a) Fresh catalyst; (b) catalyst after reaction in the dry feed; (c) catalyst after
reaction with 5 vol% water; (d) catalyst after reaction with 10 vol% water.
are shown in Fig. 6 and BET surface areas (fresh catalyst or the differential conditions. Methane conversion is required to be
catalyst aer the reaction) are shown in Table 3.
limited in a low range (<10%) and the catalyst must be diluted
In Table 3, the BET surface area of the fresh catalyst was to reduce the inuence of mass transfer and temperature uc-
2
ꢁ1
2
27,28
1
88.2 m g , while the catalyst aer the reaction in the dry feed tuation.
Therefore, we tested the intra-pellet dilution (1 : 10,
1 : 15, 1 : 20, 1 : 25) and inter-pellet dilution (1 : 50, 1 : 75,
aer the reaction. When 5 vol% or 10 vol% vapor was added into 1 : 100) to obtain a suitable dilution ratio to reduce these
ꢁ1
2
ꢁ1
was 180.2 m g . The surface area was reduced by 8.0 m g
2
ꢁ1
the feed, the BET surface area decreased to 166.8 m g and inuences. Fig. 8 shows the results of the dilution experiment.
2
ꢁ1
1
52.6 m g respectively. Comparing Fig. 6(a) and (b) shows the From Fig. 8, when the dilution ratios were 1 : 20 for intra-pellet
sizes of catalyst surface grains were uniform aer the reaction. dilution and 1 : 75 for inter-pellet dilution, the reaction rate
However, in Fig. 6(c) and (d), when 5 vol% and 10 vol% water reached stability. For intra-pellet dilution, 0.05 g Cu was chosen
was introduced into the feed stream, the catalyst grains grew to load on 1 g g-Al
signicantly and an agglomerate phenomenon obviously g-Al for inter-pellet dilution. The exact values of Cu loading
occurred.
According to sections 3.1 and 3.2, activity of the Cu/g-Al
2 3
O , and this catalyst was mixed with 3.75 g
2 3
O
on the support for intra-pellet dilution (1 : 20) were tested by
XRF: Cu, 5.13%; Al, 43.64%; O, 45.63%; Si, 1.43%; K, 1.52%;
2 3
O
catalyst did not recover to its initial level, in spite of high other, 2.65%. Reaction rate r can be described as:
temperature or N2 purging. The reason might be that the
morphology of particles was changing very quickly due to this
heat and water treatment, and surface deformation blocked the
active site. Additional water promoted the catalyst surface sin-
tering and some active sites were covered or embedded into the
support. Parts of the active sites did not interact with methane
x
r ¼ FCH ;0
(2)
4
W
F
CH ,0 and W represent methane input and catalyst mass,
4
respectively.
ꢀ
Temperature was controlled between 360–450 C to limit
methane conversion (<10%). The reaction conditions were:
CH , 3 vol%; O , 20 vol%; H O, 0–10 vol%; N balance. It is
2 3
or oxygen, making the activity of the Cu/g-Al O catalyst
decrease.
4
2
2
2
generally believed that, unlike the inuence of O or CO on
2
2
Fig. 7 showed the stability of the activity of the Cu/g-Al O
2
3
catalyst for 42 h with and without water. During the 42 h, when
vol% water was introduced into the feed, the catalyst main-
5
tained high activity for a long time. As the water was turned off,
reactivity could recover and maintain stable for 18 h.
3.4. Apparent activation energy of methane
In order to get the apparent activation energy and kinetic
parameters of methane combustion with water vapor, we
controlled the reaction conditions and the catalyst to meet the
Table 3 BET surface area of the catalyst before or after the reaction
Without water
Before reaction
With water
5 vol%
Aer reaction
10 vol%
2
ꢁ1
2
ꢁ1
2
ꢁ1
2
ꢁ1
1
88.2 (m g
)
180.2 (m g
)
166.8 (m g
)
152.6 (m g )
ꢀ
2 3
Fig. 7 Stability of the activity of the Cu/g-Al O catalyst (600 C).
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ꢀ
4
Fig. 8 Intra-pellet dilution and inter-pellet dilution of catalyst (1 vol% CH , 400 C).
combustion in the oxygen-rich conditions, water has an obvious
1
E
a;Dry
¼
E
a;Wet
:
(3)
effect on activity. The reaction orders with respect to O
2
and CO
2
1 ꢁ g
were zero and to methane was constant, nearly 1.
Fig. 9 shows the Arrhenius point linear tting results of
methane combustion with different water concentrations in the
feed gas. As no water was introduced into the feed, apparent
According to the above equation, we calculated the reaction
order with respect to water: ꢁ0.6, ꢁ0.796 and ꢁ0.911, respec-
tively. Reaction order with respect to water decreased as water
concentration increased, but the value was within the scope of
ꢁ1
a 2
activation energy (E ) was 81.4 kJ mol . When H O was intro-
duced into the feed (2.5 vol%, 5 vol%, 10 vol%), apparent acti-
2
2,23
ꢁ
0.6 to ꢁ1 and in agreement with the related reports.
H O
2
ꢁ
1
ꢁ1
vation energy (E
1
a
) increased: 100.8 kJ mol , 113.8 kJ mol
21 kJ mol . Therefore, water blocked methane oxidation.
Using the method of Arrhenius point linear tting, Van
,
ꢀ
inuence mainly happened at low temperature (<550 C), in
which water adsorbed on O* easily. Apparent activation energy
and reaction order with respect to water changed under certain
conditions.
ꢁ
1
22
Giezen et al. also reported the apparent activation energy of
methane over a Pd catalyst. They controlled the intake ow
ratio: CH
assumed 0 order reactions with respect to oxygen and CO
constant reaction orders with respect to methane (1). Based on
the assumptions above, they demonstrated the relationship To explain the adsorption characteristics of vapor on the active
4
, 1 vol%; O
2
, 4 vol%; N
2
balance, H
2
O, 0 or 2 vol%, and
2
, and
3.5. Water surface coverage
between reaction order with respect to H
2
O and apparent acti- site, we quote the Langmuir equation to describe water
vation energy:
adsorption on the active site. We supposed that the adsorption
of methane molecules on the active site was the rate-limiting
step (oxygen-rich) and methane adsorption on the active site
was dissociative and irreversible. Water molecules adsorbed
onto the active site surface, and formed OH* groups. OH* could
recombine to desorb as H O molecules which returned to
2
gaseous phase reactants, generating surface reduced sites. In
this process, water and methane molecules were competing for
adsorption on the active site. The rate equation can be
expressed as:
r ¼ k[CH ]q
(4)
4
r is the methane reaction rate, k the rate constant, q the fraction
of vacancy sites on the surface, and [CH
concentration.
4
] the methane
Meanwhile, due to the lower partial pressure of methane as a
gaseous reactant and rapid consumption with O aer adsorp-
2
tion, methane surface coverage was much lower compared to
water. Surface active vacancies could be expressed by means of
the Langmuir isotherm:
Fig. 9 Arrhenius plots of reaction rate vs. 1/T for methane combustion
3 vol% CH ; 20 vol% O ; catalyst intra-pellet dilution, 1 : 20; catalyst
inter-pellet dilution, 1 : 75).
(
4
2
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K
H O½H
2
Oꢂ
2
4. Conclusion
q ¼ 1 ꢁ qH O ¼ 1 ꢁ 1
(5)
(6)
2
þ KH O½H
ꢀꢁH
2
Oꢂ
2
2 3
The effect of water on methane combustion over the Cu/g-Al O
catalyst was investigated in a xed bed reactor. From the
experimental results, we concluded the following points:
ꢁ
ads
K
H O ¼ K
0
exp
2
RT
1. Combustion efficiency of methane over the Cu/g-Al
2 3
O
catalyst was inhibited by H O. The inhibitory effect was weak-
2
q
H O is the water surface coverage on the active site, KH O and
2 2
ened with rising temperature. Because of reversible adsorption
of water, reactivity could be recovered by N2 purging which
promoted water molecule desorption from the active site.
H
ads are the water adsorption equilibrium constant and the
water adsorption enthalpy, K the pre-exponential factor, and
0
[
H O] the water concentration. When water was introduced into
2
2. Water vapor promoted the deformation of the catalyst
the feed, the expression is:
surface, leading to the decrease of surface area and other irre-
versible effects.
3. The apparent activation energy of methane rose up
noticeably as vapor was introduced into the feed and the reac-
tion order with respect to water vapor was maintained between
À
Á
K
H O ½H
2
Oꢂ þ 2x½CH
4
in
ꢂ
2
in
q
H O ¼ 1
À
Á
(7)
2
þ KH O ½H
2
Oꢂ þ 2x½CH
4
ꢂ
2
in
in
[H
2
O]in and [CH
4
]
in are the inlet concentration of water and
methane, and x the methane conversion.
According to related reports and the results of density
functional theory calculations,
ꢁ
0.6 and ꢁ1. Water surface coverage had a downtrend with
ꢀ
temperature, which tended towards 0% above 625 C.
10,11
water adsorption enthalpy
ꢁ1
(H
ads) over the Cu catalyst was about ꢁ120 to ꢁ160 kJ mol and
1
0
Acknowledgements
the pre-exponential factor (K
Although this was a rough estimate, it obtained an acceptable
0
) was about a magnitude of 10 .
The authors would like to thank the nancial support of Natural
Science Foundation of China with Project no. 51206200, and the
Fundamental Research Funds for the Central Universities with
Project no. CDJZR12140031, and visiting Scholar Foundation of
Key Lab. of Low-grade Energy Utilization Technologies and
System, MOE of China in Chongqing University (LLEUTS-201301).
ꢁ
1
10
value of qH O as Hads and K
respectively. Calculating using eqn (7) with these parameters,
we obtained water coverage with temperature.
Fig. 10 shows water coverage changing with temperature.
Water concentrations were: 0 vol%, 2.5 vol%, 5 vol%,10 vol%.
Without water in the feed gas, water coverage was below 30% at
0
were ꢁ160 kJ mol and 10 ,
2
ꢀ
4
00 C. As water was introduced into the feed, water coverage
rose above 87% at 400 C. At each concentration (water:
ꢀ
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