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

Abstract

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.

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1. Introduction

There was a large amount of low-concentration methane in the coal bed, especially in China. It was hard to make use of the methane using conventional combustion technology and there was no choice but to emit the gas into the atmosphere. Catalytic combustion had been considered to be a comparatively ideal burning mode so far, which could achieve high-conversion-efficiency and low-pollution-emission with noble metal or transition metal catalysts.^1–4 A large number of studies had been done on the catalytic combustion of methane with noble or transition metal catalysts, and combustion mechanism had been widely reported.^5–7 However the feed always contained a large amount of steam which could affect the reactivity during the actual reaction. Therefore, the effect of water on methane combustion required further research.^8–10

Many catalysts had been investigated for their activity in the catalytic combustion of methane with water over noble metal catalysts (such as Pd, Pt) and many conclusions were drawn. Some scholars considered that H₂O adsorbed on the active sites, which formed OH* groups, generated inhibitory effects on methane combustion.^11–15 Among the combustion products, H₂O had a more obvious influence than CO₂ on methane combustion. In oxygen-rich conditions (O₂/CH₄ > 2), it was hard to progress the reforming reaction of methane with water. About the influence of water, the studies of Ciuparu et al.^16,17 reported that, by means of in situ DR-FTIR investigation, several OH adsorption bands were observed on the catalyst surface and hydroxyls were found to be related to the PdO phase. OH groups had bridged and terminal bonds before recombination and desorption as water molecules, suggesting the dehydroxylation mechanism proceeds. Stasinska et al.^18,19 reported that steam could promote the growth of surface grain, resulting in the decrease of the surface area of the catalyst. Meanwhile, Qiao et al.^20,21 reported that water vapor could severely inhibit the catalytic performance of CeO₂–CuO solid solutions for CH₄ and CO conversion, while CeO₂–NiO showed a good resistance to water. Using special treatments to ensure differential conditions, J. C van Giezen²² reported that apparent activation energy of methane (E_a) increased from 86 kJ mol⁻¹ to 151 kJ mol⁻¹ if water was introduced into the feed and the order with respect to water was found to be −0.76.

For methane combustion with water (O₂/CH₄ > 2), the inhibitory effects of water had been widely acknowledged, especially in Pd or Pt.^23–26 However, there were few studies about transition metal catalysts, such as copper. The inhibitory effects of water at low temperature, adsorption characteristics and catalyst surface property were not yet clear for methane combustion. H₂O surface coverage with differing temperature required further study. The aim of this work was to investigate the inhibitory effects of water on the copper based catalyst from the following aspects: water adsorption, regeneration of activity, kinetic parameters of methane and water surface coverage.

2. Experimental

2.1. Catalyst preparation and characterization

γ-Al₂O₃ particles (specific surface area, 181.4 m² g⁻¹; pore volume, 0.46 cm³ g⁻¹; pore diameter, 190.6 Å) were used as supports of the Cu catalyst. The γ-Al₂O₃ 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(NO₃)₂ powder was chosen to load 0.1 g Cu on the support (1 g). The mass of distilled water, the solvent for the Cu(NO₃)₂ powder, was determined by the support mass and pore volume. Support particles were impregnated with Cu(NO₃)₂ solution for 5 h; after impregnation, the catalyst was 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 N₂. 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 (XRF) and specific surface area (BET) of the self-made catalyst.

Table 1 The content of each element of the Cu/γ-Al₂O₃ catalyst (XRF)

	Cu	Al	O	Si	K	Other
Mass (wt/%)	9.64%	40.76%	42.98%	1.36%	1.78%	3.84%

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Table 2 BET surface area of the Cu/γ-Al₂O₃ catalyst

Specific surface area	Pore volume	Pore diameter
180.6 m^2 g^−1	0.4571 cm^3 g^−1	188.363 Å

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Fig. 1 shows the XRD spectra of the Cu/γ-Al₂O₃ catalyst: a, fresh catalyst; b, catalyst after reaction. From Fig. 1(a) and (b), the fresh catalyst consisted of CuO and Al₂O₃ between 30° and 70°. Other species (Cu¹⁺, O¹⁻, O_{2 ads}) were not found in Fig. 1. This meant that CuO was the active site that provided O* (chemisorbed O) to methane. In the flow reactor, as O₂ was enough for CH₄ oxidation (O₂/CH₄ > 2), the Cu cluster, especially surface Cu, was able to maintain Cu²⁺ and provide O*. Fig. 1(b) showed that, during the reaction, the catalyst phase did not change much, and other peaks were not presented after methane oxidation.

Fig. 1 XRD spectra of the Cu/γ-Al₂O₃ catalyst. (a) Fresh catalyst; (b) catalyst after reaction (550 °C, 3 vol% CH₄, 20 vol% O₂, N₂ balance).

2.2. Experimental devices and system

Fig. 2 presents the experimental device and system. The experiment of methane combustion was conducted in the fixed-bed reactor which had a ceramic reaction tube with 10 mm inner diameter and 1200 mm length. The reaction mixture gas (3 vol% CH₄, 20 vol% O₂ and N₂ balance) controlled by a mass flowmeter, was injected into the fixed-bed reactor at 200 ml min⁻¹. Water vapor, generated by a vapor generator (W-202A-220-K) was carried by N₂ at 150 °C and mixed with CH₄ and O₂ before entering into the reactor. The reactor was heated by a resistance-heated furnace that was connected to the data acquisition and control system which controlled the reaction temperature. The effluent gas was measured by gas chromatography to determine the concentrations of CH₄ and CO₂.

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.

2.3. Catalytic activity test for methane oxidation

The reactivity of Cu/γ-Al₂O₃ was determined by methane conversion. According to the carbon balance, methane conversion x is described as:Where [CO₂] and [CH₄] denote volume percentage of carbon dioxide and methane in the stream.

The microstructure and characterization of the catalyst surface were investigated by scanning electron microscopy (SEM), which magnified the catalyst surface by 1000 times. BET surface area, pore volume and pore diameter were measured by the BET method before or after the experiments.

3. Results and discussion

3.1. Effects of water on methane combustion

The effect of water on methane combustion over a Cu/γ-Al₂O₃ catalyst is shown in Fig. 3 and 4. Fig. 3 shows the change in methane conversion with temperature at different water content values (0 vol%, 2.5 vol%, 5 vol%, 10 vol%, 15 vol%, 20 vol%). Fig. 4 shows the inhibitory effect of water on methane combustion at 500 °C and 600 °C. From these figures, water significantly affected catalytic activity and CH₄ conversion dropped if water was introduced into the feed. When the water vapor increased from 0 vol% to 10 vol%, the catalyst continued losing activity with time. From Fig. 3, with temperature rising from 475 °C to 600 °C, CH₄ conversion increased quickly. The inhibition effect of H₂O on methane combustion increased firstly and then decreased, and water inhibition among the curves reached its maximum at 525 °C. The process showed that water vapor had a significant inhibitory effect on methane combustion, while, with temperature increasing, the influence weakened gradually. A considerably higher temperature was most likely required for the catalytic reaction to proceed.

Fig. 3 Methane oxidation with water over a Cu/γ-Al₂O₃ catalyst (CH₄, 3 vol%; O₂, 20 vol%; H₂O, 0–20 vol%; N₂, balance).

Fig. 4 Water inhibitory effect on methane oxidation over the Cu/γ-Al₂O₃ catalyst (3 vol% CH₄, 20 vol% O₂).

Different from Pd and Pt catalysts, the Cu catalyst displayed a stronger resistance to water and could maintain a stable activity 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*): H₂O + O* → OH*. Thus, a portion of O* did not participate in methane oxidation, making methane conversion lower.^9,12–14

From Fig. 4, if H₂O flow was turned off, the activity almost 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. H₂O 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/γ-Al₂O₃ catalyst 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 shows the effect when the catalyst was purged with N₂. The feed stream was as follows: CH₄, 3 vol%; O₂, 20 vol%; H₂O (2.5 vol%; 5 vol%; 10 vol%); N₂, balance.

Fig. 5 Regeneration of the catalytic activity of the Cu/γ-Al₂O₃ catalyst (3 vol% CH₄, 20 vol% O₂).

In Fig. 5, CH₄ conversion was measured by gas chromatography every 15 minutes, whereas the catalyst was purged with nitrogen in the absence of CH₄ for another 10 minutes. Methane conversion was initially high. Then, the activity dropped as water was introduced into the feed gas. Thereafter, the catalyst was purged with N₂ at the same temperature; consequently, no methane was shown in the figure. After purging, the activity of the catalyst almost recovered to its initial level. Comparisons of 500 °C, 550 °C and 600 °C show high temperature weakened the inhibitory effect and activity dropped by a smaller amount compared with low temperature. This meant H₂O or surface OH* did not adsorb on the catalyst surface stably, and if temperature increased (500 °C → 600 °C), the process of desorption or decomposition was easy to proceed: OH* → H₂O + O*.

3.3. Surface characteristics and stability

In order to discover the influence of water vapor on the catalyst surface, the SEM pictures (1000 times) for the catalyst surface are shown in Fig. 6 and BET surface areas (fresh catalyst or the catalyst after the reaction) are shown in Table 3.

Fig. 6 Surface characteristics of the Cu/γ-Al₂O₃ 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.

Table 3 BET surface area of the catalyst before or after the reaction

Without water		With water
Before reaction	After reaction	5 vol%	10 vol%
188.2 (m^2 g^−1)	180.2 (m^2 g^−1)	166.8 (m^2 g^−1)	152.6 (m^2 g^−1)

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In Table 3, the BET surface area of the fresh catalyst was 188.2 m² g⁻¹, while the catalyst after the reaction in the dry feed was 180.2 m² g⁻¹. The surface area was reduced by 8.0 m² g⁻¹ after the reaction. When 5 vol% or 10 vol% vapor was added into the feed, the BET surface area decreased to 166.8 m² g⁻¹ and 152.6 m² g⁻¹ respectively. Comparing Fig. 6(a) and (b) shows the sizes of catalyst surface grains were uniform after the reaction. However, in Fig. 6(c) and (d), when 5 vol% and 10 vol% water was introduced into the feed stream, the catalyst grains grew significantly and an agglomerate phenomenon obviously occurred.

According to sections 3.1 and 3.2, activity of the Cu/γ-Al₂O₃ catalyst did not recover to its initial level, in spite of high 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 sintering and some active sites were covered or embedded into the support. Parts of the active sites did not interact with methane or oxygen, making the activity of the Cu/γ-Al₂O₃ catalyst decrease.

Fig. 7 showed the stability of the activity of the Cu/γ-Al₂O₃ catalyst for 42 h with and without water. During the 42 h, when 5 vol% water was introduced into the feed, the catalyst maintained high activity for a long time. As the water was turned off, reactivity could recover and maintain stable for 18 h.

Fig. 7 Stability of the activity of the Cu/γ-Al₂O₃ catalyst (600 °C).

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 differential conditions. Methane conversion is required to be limited in a low range (<10%) and the catalyst must be diluted to reduce the influence of mass transfer and temperature fluctuation.^27,28 Therefore, we tested the intra-pellet dilution (1 : 10, 1 : 15, 1 : 20, 1 : 25) and inter-pellet dilution (1 : 50, 1 : 75, 1 : 100) to obtain a suitable dilution ratio to reduce these influences. Fig. 8 shows the results of the dilution experiment. From Fig. 8, when the dilution ratios were 1 : 20 for intra-pellet dilution and 1 : 75 for inter-pellet dilution, the reaction rate reached stability. For intra-pellet dilution, 0.05 g Cu was chosen to load on 1 g γ-Al₂O₃, and this catalyst was mixed with 3.75 g γ-Al₂O₃ for inter-pellet dilution. The exact values of Cu loading 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%; other, 2.65%. Reaction rate r can be described as:F_CH4,0 and W represent methane input and catalyst mass, respectively.

Fig. 8 Intra-pellet dilution and inter-pellet dilution of catalyst (1 vol% CH₄, 400 °C).

Temperature was controlled between 360–450 °C to limit methane conversion (<10%). The reaction conditions were: CH₄, 3 vol%; O₂, 20 vol%; H₂O, 0–10 vol%; N₂ balance. It is generally believed that, unlike the influence of O₂ or CO₂ on combustion in the oxygen-rich conditions, water has an obvious effect on activity. The reaction orders with respect to O₂ and CO₂ were zero and to methane was constant, nearly 1.

Fig. 9 shows the Arrhenius point linear fitting results of methane combustion with different water concentrations in the feed gas. As no water was introduced into the feed, apparent activation energy (E_a) was 81.4 kJ mol⁻¹. When H₂O was introduced into the feed (2.5 vol%, 5 vol%, 10 vol%), apparent activation energy (E_a) increased: 100.8 kJ mol⁻¹, 113.8 kJ mol⁻¹, 121 kJ mol⁻¹. Therefore, water blocked methane oxidation.

Fig. 9 Arrhenius plots of reaction rate vs. 1/T for methane combustion (3 vol% CH₄; 20 vol% O₂; catalyst intra-pellet dilution, 1 : 20; catalyst inter-pellet dilution, 1 : 75).

Using the method of Arrhenius point linear fitting, Van Giezen et al.²² also reported the apparent activation energy of methane over a Pd catalyst. They controlled the intake flow ratio: CH₄, 1 vol%; O₂, 4 vol%; N₂ balance, H₂O, 0 or 2 vol%, and assumed 0 order reactions with respect to oxygen and CO₂, and constant reaction orders with respect to methane (1). Based on the assumptions above, they demonstrated the relationship between reaction order with respect to H₂O and apparent activation energy:

According to the above equation, we calculated the reaction order with respect to water: −0.6, −0.796 and −0.911, respectively. Reaction order with respect to water decreased as water concentration increased, but the value was within the scope of −0.6 to −1 and in agreement with the related reports.^22,23 H₂O influence 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.

3.5. Water surface coverage

To explain the adsorption characteristics of vapor on the active site, we quote the Langmuir equation to describe water 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 H₂O molecules which returned to 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 = k[CH₄]θ (4)

r is the methane reaction rate, k the rate constant, θ the fraction of vacancy sites on the surface, and [CH₄] the methane concentration.

Meanwhile, due to the lower partial pressure of methane as a gaseous reactant and rapid consumption with O₂ after adsorption, methane surface coverage was much lower compared to water. Surface active vacancies could be expressed by means of the Langmuir isotherm:

θ_H2O is the water surface coverage on the active site, K_H2O and H_ads are the water adsorption equilibrium constant and the water adsorption enthalpy, K₀ the pre-exponential factor, and [H₂O] the water concentration. When water was introduced into the feed, the expression is:

[H₂O]_in and [CH₄]_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,^10,11 water adsorption enthalpy (H_ads) over the Cu catalyst was about −120 to −160 kJ mol⁻¹ and the pre-exponential factor (K₀) was about a magnitude of 10¹⁰. Although this was a rough estimate, it obtained an acceptable value of θ_H2O as H_ads and K₀ were −160 kJ mol⁻¹ and 10¹⁰, 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 400 °C. As water was introduced into the feed, water coverage rose above 87% at 400 °C. At each concentration (water: 2.5 vol%, 5 vol%, 10 vol%), θ_H2O had the same downtrend which tended towards 0% when the temperature rose up. At low temperature, there was an obvious difference between water coverage with or without water in the feed gas. But at high temperature, the coverage curve tended to the same. High temperature enhanced the desorption of water, making water coverage lower.

Fig. 10 Water coverage vs. temperature (reaction conditions: CH₄, 3 vol%; O₂, 20 vol%; H₂O, 0–10 vol%; N₂ balance).

4. Conclusion

The effect of water on methane combustion over the Cu/γ-Al₂O₃ catalyst was investigated in a fixed bed reactor. From the experimental results, we concluded the following points:

1. Combustion efficiency of methane over the Cu/γ-Al₂O₃ catalyst was inhibited by H₂O. The inhibitory effect was weakened 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.

2. Water vapor promoted the deformation of the catalyst surface, leading to the decrease of surface area and other irreversible effects.

3. The apparent activation energy of methane rose up noticeably as vapor was introduced into the feed and the reaction order with respect to water vapor was maintained between −0.6 and −1. Water surface coverage had a downtrend with temperature, which tended towards 0% above 625 °C.

Acknowledgements

The authors would like to thank the financial 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).

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