CH4 ratios in the present of water: Combustion or reforming

Energy Conversion and Management 132 (2017) 339–346

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Methane oxidation with low O2/CH4 ratios in the present of water: Combustion or reforming Haojie Geng, Zhongqing Yang ⇑, Li Zhang, Jingyu Ran, Yunfei Yan Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, and College of Power Engineering, Chongqing University, Shapingba District, Chongqing 400030, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 14 August 2016 Received in revised form 15 November 2016 Accepted 15 November 2016

Keywords: Methane reaction Water Combustion Reforming Cu/Co catalyst

a b s t r a c t This paper investigates the reaction of methane over copper and cobalt catalysts under oxygen-deficient conditions with added water. A fixed-bed reactor, TPD analysis, in situ DRIFTS study, and temperature detection were used to test the activity of the methane reaction, water adsorption on the metal surface, OH group behavior, and the endothermic and exothermic processes of the reaction. The results show that the inhibitory effect of water mainly occurs at a low temperature and methane conversion decreases when water is introduced into the feed. Water easily adsorbs on metal clusters and forms OH groups at low temperatures. Copper tends to adsorb more water than cobalt and shows a stronger inhibitory effect. The DRIFTS spectra of the Cu catalyst show strong OH peaks during the reaction, of which the magnitudes increase with the water pressure. When the reaction temperature rises (750 °C), water begins to serve as an oxidant and participates in the reforming reaction. Both catalysts show a transition process between the oxidation and reforming reactions as the temperature increases. Co displays a better catalytic performance in the reforming reaction. Oxidation precedes reforming; water does not participate in the reaction if the oxygen is not fully consumed. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Methane is considered the preferred clean energy source for this century, and many countries such as the USA and China have explored huge natural gas reserves [1–3]. Catalytic combustion is a highly efficient and low-pollution method to obtain thermal energy from natural gas [4–6]. Noble metal catalysts such as Pt, Pd, Au, and Ag have been widely studied for their catalytic performance, especially at low temperatures. The reaction mechanism, reaction intermediates, and structures of metal clusters have been studied by DFT, Raman spectroscopy, DRIFTS, HRTEM, etc. [7,11]. Oxidation and reforming reactions have both been reported on these noble metal catalysts, and a high reactivity is achieved at a low temperature. The chemisorption of methane on noble metal atoms or metal clusters is considered the first step of the process of activating methane on the active sites [8–10]. After adsorption, dehydrogenation begins, in which hydrogen combines with the chemisorbed oxygen (oxidation process) or adsorbs on the metal surface (reforming or decomposition process). Some papers [11–13] (using DFT methods) report that because of the low activation energy, oxygen reacts with CAH (from CH4) to form a CAOAH ⇑ Corresponding author. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.enconman.2016.11.033 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

bond (CH3OH). However, the formation of the CAOAH bond is a relatively complex process because it goes through the scission of the CAH bond and the formation of CAO and OAH bonds. Many CAH activation reactions go through a three-member (CAH-Metal) intermediate. Scission of the CAH on a metal surface is a high energy process only occurs on certain metals (like Pt). The reaction in other cases likely proceeds through a cooperative mechanism. The partial pressures of reactants (the ratio of O2/CH4) have a distinct effect on methane oxidation. In most cases, the methane catalytic reaction proceeds under oxygen-rich conditions, in which the focus is on the complete oxidation of methane [14,15]. Researcher [16] concludes methane reaction in many oxygen-rich cases on Pd catalyst, and the reaction kinetic parameters vary according to different ratio of O2/CH4. When the O2/CH4 ratio decreases, CO and H2 are detected in the products. Researcher [17] divides the reaction regions for methane reaction on Pt catalyst, and reports the maximum ratio of O2/CH4 in the stream that allows the partial oxidation to proceed. Several comments [18,19] report that under oxygen-deficient conditions CO and H2—which are directly produced by the oxidation of CH4 and O2—can be generated in short adiabatic reactors under extreme gradients of temperature and concentration. However, these claims are still inconclusive because we cannot exclude the influence of the reforming reaction under experimental conditions. In

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this work, we assume that oxygen participates in the complete oxidation reaction; partial oxidation is a complex process that experiences two processes: complete oxidation and reforming. The effect of water has been investigated many times, especially on noble metal catalysts [20–24]. Noble metal catalysts such as Pd and Pt easily adsorb gas-phase substances on the surface. Water adsorbs on metal atoms or clusters to form OH groups, which block the interaction between methane and active sites. Some researchers [25–28] have used in situ DR-FTIR spectra to investigate the formation and accumulation of hydroxyl groups on Pd clusters. Several well-resolved OH absorption bands that represent bridged or terminal OH groups have been identified. There are few studies that discuss the inhibitory effect of water on transition metal surfaces (Cr, Mn, Fe, Co, Ni, Cu, and Zn). Cu, Fe, and Zn display the inhibitory effect on methane oxidation with added water, while Cr, Mn, Co, and Ni do not. These metals may have several key properties— such as adsorption characteristics—that lead to the appearance of the water inhibitory effect during oxidation. Aguila [12,30] has investigated the catalytic methane combustion over a copper catalyst and points out that OH groups can adsorb on the copper surface (dynamic adsorption), which inhibits methane conversion. This paper investigates the catalytic performance of copper and cobalt catalysts for the methane reaction. These two elements are close to each other in the periodic table of chemical elements, and they have similar catalytic properties. Both of them show catalytic performance for the oxidation and reforming reactions of methane with a similar reaction trend, transition of oxidation and reforming, and increasing effect of water at high temperatures. However, each element has its specific properties. Co catalyst shows a better catalytic performance compared with Cu catalyst in both the oxidation and reforming processes and has a lower characteristic temperature. These two metals have different reactivity changes when water is injected into the feed for the methane reaction. In this paper, we try to discuss: 1. the water inhibitory effect that tends to appear on the metal that is more likely to adsorb water on its surface; 2. partial oxidation, which is a complex process that includes oxidation and reforming reactions.

2. Experiment

changes little. The main peaks of the cobalt catalyst contained both Co2+ and Co3+, and the satellite peak mainly consisted of Co2+. For the copper catalyst, the high-energy component mainly includes Cu2+ and Cu+ phases, and the satellite peak denotes the Cu2+ phase. Co2+ and Cu2+ are the main phases of the working catalysts. Fig. 1 displays the stable catalytic performance during the reaction. Table 1 shows the property of working catalyst (Cu and Co). Surface Area is measured by BET method. About the metal dispersion, metal particle size and metal surface area, we use irreversible H2 chemisorption to measure them at 50 °C after reduction of the catalysts at 700 °C. This experiment was carried out in a Micromeritics 2000 unit. 2.2. Experimental devices and system The methane catalytic reaction was carried out in a fixed-bed reactor that was a quartz tube with 10 mm inner diameter. The feed gas was controlled by mass flow meters, and well mixed before into the reactor. In this study, we used a steam generator (W-202A-220-K) that is equipped with 2 flow meters to control the flow rate of N2 and water. One controller was used for N2 (volumetric flow rate), and the other one was for water (mass flow rate). Water and N2 were heated in the steam generator from room temperature to 150 °C, and then the gas mixture entered the reactor. Water mass flow rate is determined by the CH4 volumetric flow rate (CH4 mL/min ? CH4 mol/s ? H2O mol/s ? H2O g/s). All experiments were conducted at atmospheric pressure. The data acquisition and control system controls the resistance heater to heat the reactor to working temperature. In order to measure the temperature along the catalyst bed, we installed a thermowell (1.5 mm diameter) in the center of the catalyst bed. An annular tube that has a good thermal conductivity passed through the catalyst bed, allowing the movable K-type thermocouple (0.5 mm diameter) to move slowly in the tube. In Section 3.4, the catalyst bed was 8 cm in length, and the measurement interval was 5 mm. Finally, the exit gas was analyzed by gas chromatography (GC7900); the gas concentrations of CH4, CO, CO2, and H2 were measured. The catalytic reactivity of copper and cobalt catalysts was determined by the methane conversion. According to the carbon balance in the reaction, methane conversion x can be described as:

2.1. Catalyst preparation and characterization

c-Al2O3 particles were used as support for the copper and cobalt

catalysts in this study. The fresh particles were treated at 550 °C for 4 h before introducing copper or cobalt onto the support. The diameter of the support was lower than 0.1 mm. Copper and cobalt were introduced into the catalyst system by incipient aqueous impregnation. The Cu/c-Al2O3 and Co/c-Al2O3 catalysts were prepared using aqueous solutions of Cu(NO3)2 and Co(NO3)2 to obtain a suitable metal loading on the support. The Cu(NO3)2 and Co (NO3)2 solutions were impregnated into the c-Al2O3 particles for 3 h at room temperature, and the catalyst was dried at 150 °C in air for 5 h. The dried samples were calcined at 550 °C in a muffle furnace for 5 h under N2. Catalyst calcination was performed to remove the free radicals such OH
, NO
3 , and other volatile matter from the metallic cluster surfaces. In this study, the loading amounts of the copper and cobalt catalysts were fixed at 0.05 g. After calcination, the catalyst goes through a H2-reduction process and is stored under N2 atmosphere. H2-reduction can transform the metallic oxide to active clusters, but the reactivity is not stable at initial time. Therefore, we run the experiment under oxygendeficient conditions for a long time to achieve steady state [17,29]. XPS spectra of the Co and Cu catalysts before and after the reaction at 750 °C are shown in Fig. 1. The spectra obtained before and after the reaction show that the working phase of each catalyst

x¼

½CH4 in
½CH4 out ½CH4 in

ð1Þ

where ½CH4 in and ½CH4 out represent inlet flow rate and outlet flow rate, respectively. The in situ DRIFTS spectrum in the reaction was obtained by a Spectrum 100 FTIR instrument equipped with a diffuse reflectance accessory that contained a reaction cell for surface reactions. The catalyst was placed in the diffuse reflectance accessory, where the reaction temperature and pressure were controlled by the instrument. DRIFTS spectra were recorded in the range of 2200– 4100 cm
1 (OH spectra) at a resolution of 16 cm
1 and a mirror velocity of 1.67 cm/s. Temperature-programmed desorption analysis was conducted by a purpose-built TPD instrument (PCA-1200) with a TCD detector and a U-pipe reaction cell. The heating rate of the TPD apparatus was controlled in the range 1–30 °C/min. 3. Results and discussion 3.1. Effect of water on Cu and Co catalyst Methane catalytic combustion with water involves complete oxidation, partial oxidation, and reforming. As water is injected in the feed, water can serve as the oxidant in the reforming reac-

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Co 2p

a

Cu 2p

b

2+

After Reaction

Cu

3+

2+

Co

Co

+

Cu

Intensity (a.u.)

Intensity (a.u.)

After Reaction

Before Reaction

800

Before Reaction

790

780

945

770

940

Binding Energy (eV)

935

930

Binding Energy (eV)

Fig. 1. XPS patterns of Co and Cu catalysts before and after reaction (750 °C, 5 kPa CH4, O2/CH4 = 0.8, N2 balance).

Table 1 Property of the working catalyst. Category

BET surface area (m2/g)

Metal dispersion (%)

Metal particle (nm)

Metal surface area (m2/gcat)

Support Cu catalyst Co catalyst

193.6 180.3 174.8

– 18.6 25.8

– 12 20

– 8.3 10.1

tion. Table 2 shows the main reactions that may proceed in the reactor. From Table 2, the two oxidation reactions (O1 and O2, oxygen serves as the oxidant) are exothermic, with the heat released by partial oxidation far less than that from complete oxidation. All the reforming reactions (R1, R2, and R3; water and CO2 serve as the oxidant) are endothermic. Therefore, methane oxidation with water is a complex thermochemical process, including endothermic and exothermic processes [31–33]. In Fig. 1, we see that Co2+ and Cu2+ are the main working phases of the Co and Cu catalysts. During the high-temperature reaction (750 °C), the phase of each catalyst does not change. At this point, we point out that methane mainly consumes the surface oxygen, even at high temperature. Lattice oxygen still maintains the inner structure of active atom groups. As the oxygen concentration goes down to zero, a weak oxidant (water or CO2) starts to participate in the reaction. Fig. 2 shows the methane reaction over the copper catalyst with and without water in the temperature range 400–750 °C. As seen in Fig. 2a, as water is injected into the feed (H2O/CH4 = 0.6), inhibitory effect on the reactivity of methane is presented only over the Cu catalyst under oxygen-rich condition (O2/CH4 = 2). From 475 to 550 °C, the inhibitory effect of methane becomes stronger, while at higher reaction temperatures (>600 °C) the inhibitory effect tends to disappear. We performed the experiment under oxygendeficient conditions (O2/CH4 < 0.8) (Fig. 2b). Below 600 °C, it shows an inhibitory effect as shown in Fig. 2a, while as the temperature increases to >600 °C an increasing effect is seen in the reaction. When the ratio of O2/CH4 is controlled at 0.8, the maximum conversion during the complete oxidation of methane is 40%. As temperature increases (>600 °C) in the dry condition, methane gradually reacts with water (product water) via the reforming reaction, which is able to increase the methane reaction and increase the conversion beyond the maximum conversion from complete oxidation. In addition, with added water (H2O/CH4 = 0.6), the rate curve has a higher conversion compared with the dry feed, which means

Table 2 Main reaction of methane in the present of water. Reaction

Reaction process

Reaction heat (kJ/mol)

Oxidation

O1 O2

CH4 þ 0:5O2 ! CO þ 2H2 CH4 þ 2O2 ! CO2 þ 2H2 O


36
802

Reforming

R1 R2 R3

CH4 þ CO2 ! 2CO þ 2H2 CH4 þ 2H2 O ! CO2 þ 4H2 CH4 þ H2 O ! CO þ 3H2

247 165 206

Water-gas reaction

WG

CO þ H2 O ! CO2 þ H2


41

added water helps to promote methane conversion at high temperature. Fig. 2c and d shows the outlet flow rate of O2, H2, CO, and CO2 from the methane reaction with and without water, respectively. Comparing Fig. 2c and d, water does not change the temperature point that separates the combustion region from the reforming region. As additional water is injected into the feed, amount of CO2 does not decrease as much as in the dry feed, which means water participates in the reforming reaction and reduces the consumption of CO2. As a result, flow rates of both H2 and CO increase. Fig. 3 shows the methane reaction over the cobalt catalyst with and without water in the temperature range 350–650 °C. As seen in Fig. 3a, as water is injected into the feed (O2/CH4 = 2, H2O/ CH4 = 0.6), there is no obvious inhibitory effect on the methane conversion at low temperatures, which is in contrast to the reaction over the Cu catalyst. In Fig. 3b, the experiment is conducted under oxygen-deficient conditions (O2/CH4 = 0.8), which allows the conversion of the complete oxidation of methane to reach 40% at low temperature. When the temperature increases (>500 °C), methane starts to react with product water via the reforming reaction, which increases the conversion in the dry feed. Above 40% methane conversion, Co catalyst displays a similar

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100

a

b 60

CH4 Conversion [%]

60

40

Cu catalyst 5 kPa CH4 ; O2/CH4= 2

40

H2O/CH4= 0

20

H2O/CH4= 0

5 kPa CH4 ; O2/CH4= 2

5 kPa CH4 ; O2/CH4= 0.8

H2O/CH4=0.6

400

Temperature [ oC] 10

Combustion

c

10

550

600

Combustion

d

650

700

Flow Rate r×10-5/(mol·g-1·s-1)

H2O/CH4= 0

H2

4

Cu catalyst 5 kPa CH4 O2/CH4 = 0.8 H2O/CH4= 0.6

O2

6

H2

4

2

2 CO

CO2

450

500

550

600

Temperature

)

400

650 o

750

Reforming

8

O2/CH4= 0.8

6

0 350

500

Temperature [ C]

Cu catalyst 5 kPa CH4

O2

450

o

Reforming

8

H2O/CH4= 0.6

0

0 400 425 450 475 500 525 550 575 600 625 650

Flow Rate r ×10 -5/(mol·g-1·s-1)

Cu catalyst 5 kPa CH4 ; O2/CH4= 0.8

20

700

750

CO CO2

800

C)

0 350

400

450

500

550

600

Temperature

650

)

CH4 Conversion [%]

80

o

700

750

800

C)

Fig. 2. Reactivity of methane over Cu catalyst with and without water ((a) inhibitory effect at low temperature; (b) increasing effect at high temperature; (c) outlet flow rate without water; (d) outlet flow rate with additional water).

reactivity to the Cu catalyst at high temperatures; the reactivity with added water (H2O/CH4 = 0.6) is higher than that in the dry feed. Fig. 3c and d shows the methane reaction over the Co catalyst under oxygen-deficient conditions. As the oxygen concentration goes down to nearly zero, H2, and CO start to appear in the outlet gas. As water is introduced into the feed, amounts of H2 and CO clearly increase and the amount of CO2 decreases to less than the amount shown in Fig. 3c. Water plays an increasing effect on the methane reaction. Figs. 2c, d and 3c, d show similar trends of each species over Cu or Co catalysts: 1. when oxygen is fully consumed, H2 and CO start to form; 2. H2 yield increases to a higher than that of CO; and 3. CO2 production goes through a process that initially increases, and then decreases. The copper catalyst displays an inhibitory effect at low temperature, while the cobalt catalyst does not. Both metals show catalytic property toward the reforming reaction at high temperatures. With additional water, the reactivity of methane is higher than that with the dry feed. The difference of these reactions may be caused by the different properties of the catalytic metal, which we will discuss in a following paper.

3.2. TPD profiles of water on catalysts There must be some property to explain the catalytic performance of copper and cobalt catalysts based on their location close to each other in the periodic table such as complete oxidation, reforming reaction, and apparent activation energy. Meanwhile, there exist several differences in metallic properties, such as the adsorption gaseous products (water, CH4, and O2), reactivity, and reaction temperature [34,35]. In order to find the main difference between copper and cobalt on the effect of water, TPD experiments were conducted to investigate the adsorption characteristics of these metals and the reason for the inhibitory effect of water. Fig. 4 shows the temperature-programmed desorption experiments on the copper (a) and cobalt (b) catalysts to examine their adsorption characteristics for water. The reactor was heated from 20 °C to 700 °C at 10 °C/min and water was added into the reactor and adsorbed at 140 °C. Deconvolution of the profiles displays 4 desorption peaks of water on both catalysts at 189.3, 221.6, 329.5, and 507.4 in Fig. 4a (Cu TPD analysis), and 205.3, 259.6, 334.2, and 468.7 in Fig. 4b (Co TPD analysis). Three peaks located below 350 °C are related to the desorption of surface-adsorbed water and the weak interaction between water and the metal

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100

80

a

60

CH4 Conversion [%]

CH4 Conversion [%]

80

b

60 40 Co catalyst 5 kPa CH4 ; O2/CH4= 2

20

40 Co catalyst 5 kPa CH4 ; O2/CH4= 0.8

20

H2O/CH4= 0

H2O/CH4= 0

5 kPa CH4 ; O2/CH4= 2

0

5 kPa CH4 ; O2/CH4= 0.8

H2O/CH4= 0.6

350

400

450

500

H2O/CH4= 0.6

0

550

350

400

Temperature [ oC] 10

Combustion

c

Reforming

8

450

500

550

600

10 Co catalyst 5 kPa CH4

Combustion

d

Reforming Co catalyst 5 kPa CH4

8

O2/CH4 = 0.8

O2/CH4 = 0.8

O2

Flow Rate r ×10-5/(mol·g-1·s-1)

Flow Rate r×10 -5/( mol·g-1·s-1 )

H2O/CH4= 0

6

650

Temperature [ oC]

H2

4

2

H2O/CH4= 0.6

O2

6

H2

4

2

0 300

350

CO

CO

CO2

400

450

500

550

600

CO2

650

0 300

700

350

400

450

500

550

600

650

700

Temperature ( oC)

Temperature ( o C)

Fig. 3. Reactivity of methane over Co catalyst with and without water ((a) inhibitory effect at low temperature; (b) increasing effect at high temperature; (c) outlet flow rate without water; (d) outlet flow rate with additional water).

1.0 1.2

Cu TPD profile

a

0.8

Intensity [ a.u.]

1.0

Intensity [ a.u.]

Co TPD profile

b

0.8 0.6 0.4

0.6

0.4

0.2 0.2 0.0 100

200

300

400

500 o

Temperature [ C]

600

700

100

200

300

400

500

o

Temperature [ C]

Fig. 4. TPD profiles of water on Cu and Co catalyst (heating rate: 10 °C/min; (a) copper TPD profile; (b) cobalt TPD profile).

600

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0.04 Cu catalyst 5 kPa CH4 ; O2/CH4= 0.4

a

5 kPa CH4 ; O2/CH4= 0.4

0.04

0.02

3800

3600

3400

3200

Wave numbers /cm

3000

2800

-1

Absorbance /a.u.

5 kPa CH4 ; O2/CH4= 0.4 H2O/CH4= 0

Absorbance /a.u.

H2O/CH4= 0.3

H2O/CH4= 0.3

0.06

0.00

Co catalyst 5 kPa CH4 ; O2/CH4= 0.4

b

H2O/CH4= 0

0.02

0.00

3800

3600

3400

3200

Wave numbers /cm

3000

2800

-1

Fig. 5. DRIFTS spectra of methane reaction with or without water over Cu (a) and Co (b) catalysts (400 °C).

cluster. These adsorbed substances do not have a distinct influence on the reaction over these two catalysts because the working temperature of the catalysts is higher than 400 °C. However, the fourth peak is significantly different. Both catalysts show desorption profiles of water in the temperature of 400–600 °C, which is the usual working temperature in the reactor. The peak in the Cu TPD profile is stronger than the peak in the Co TPD profile, which means copper has a stronger adsorbance toward water at the working temperature. Compared with the reaction shown in Figs. 2 and 3, the reactivity over copper catalyst shows a distinct inhibitory effect of water, while the reactivity over cobalt catalyst shows little or no effect as water is added into the feed. Therefore, the adsorption characteristics for water may cause the inhibitory effect during the reaction. As water vapor flows over the catalyst surface, it will adsorb on the active sites in the form of hydroxyl groups, as has been reported many times. The adsorption of surface hydroxyls is a reversible reaction that maintains a dynamic equilibrium on the active sites. Methane adsorption on active sites is inhibited by the competitive adsorption of water, leading to the restriction of CAH bond dissociation and a decrease in methane conversion.

3800 cm
1 and 3450 cm
1, which are considered terminal OH peaks, bridged OH peaks, and multiple OH absorption peaks [12,16]. In Fig. 5a, the red1 curve represents the methane reaction over the Cu catalyst in a dry feed. As water vapor is added to the feed, the absorption peaks rise rapidly. However, in Fig. 5b, when water vapor is injected into the feed, the intensity of the peak over the cobalt catalyst does not increase as much as over the copper catalyst. Meanwhile, compared with the absorbance over Cu and Co, it can be observed that the intensity of OH peaks over the copper catalyst is higher than over the cobalt catalyst. OH species from methane combustion or additional water are shown in Fig. 5. Water is more likely to adsorb on the copper surface and form hydroxyl groups while the adsorption is relatively weaker on the cobalt surface, which is consistent with TPD experiments in Section 3.2. Although OH may be suitable for CAH bond dissociation during methane oxidation, OH⁄ groups are not suitable for the oxidation process [14,29]. Therefore, with increasing OH coverage on the surface, the reactivity of methane combustion decreases.

3.3. DRIFTS study of methane combustion

Thermal fluctuation of the catalyst bed can reflect the endothermic and exothermic processes during the reaction. In this section, we examine the temperature profiles along the catalyst bed, aiming to prove that oxidation reaction precedes the reforming reaction, and the reforming reaction will not proceed if oxygen is not fully consumed by the reaction. Fig. 6 shows the temperature profiles of copper (a) and cobalt (b) catalyst beds during the methane reaction with and without water at high temperature. The experiment was controlled under oxygen-deficient conditions, and the gas pressures were: CH4 = 5 kPa, O2/CH4 = 0.4, H2O/CH4 = 0 or 0.3 (with or without additional water), N2 balance, temperature = 750 °C. Both catalyst beds were controlled at a length of 8 cm, and the thermometer probes were laid out every 5 mm along the catalyst bed. In Fig. 6a and b, there are two similar reaction processes for the two catalysts. These are the exothermic reactions of the combustion process in the first part of the catalyst bed, and endothermic reactions of reforming reaction in the second part of the catalyst bed. Comparing a and b,

In order to examine the differences of hydroxyl adsorption with or without added water vapor during the reaction, in situ DRIFTS studies were conducted to investigate the surface behavior of hydroxyls over copper (a) and cobalt (b) catalysts. These two experiments were conducted under the same reaction conditions and the feed gas concentration was: CH4 = 5 kPa, O2/CH4 = 0.4, H2O/CH4 = 0 or 0.3 (with or without water), temperature = 400 °C. These experiments were conducted under oxygen-deficient conditions and no reforming reaction proceeded in the reactor at low temperatures. Fig. 5a and b shows the in situ DRIFTS spectra of the methane reaction over the Cu and Co catalysts in a broad band covering the region between 2800 cm
1 and 3900 cm
1. Peaks corresponding to the hydroxyls adsorbed on the support are not observed in the catalytic reaction due to their lower intensity and widths that obscure their position. OH peaks corresponding to the Cu and Co catalysts are between 3800 cm
1 and 3450 cm
1, and the peaks around 3000 cm
1 are the peaks of methane adsorption on the metal surface. In Fig. 5a and b, several peaks are seen between

3.4. Temperature profiles of catalyst bed

1 For interpretation of color in Figs. 5 and 7, the reader is referred to the web version of this article.

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780 770

5 kPa CH4 ; O2/CH4= 0.4

a

H2O/CH4= 0.3

°C

°C

H2O/CH4= 0

770

5 kPa CH4 ; O2/CH4= 0.4 760

5 kPa CH4 ; O2/CH4= 0.4

b

H2O/CH4= 0

5 kPa CH4 ; O2/CH4= 0.4 H2O/CH4= 0.3

760 750

750

740 740

730 Cu Catalyst

730

0

2

4

Co Catalyst

6

8

720

10

0

2

Reactor axial position [cm]

4

6

8

10

Reactor axial position [cm]

Fig. 6. Temperature profiles of catalyst bed ((a) Cu catalyst; (b) Co catalyst) during methane reaction with or without water at high temperature (750 °C).

water has little influence on the exothermic process, while in the section of 4–8 cm, the wet feed has a lower temperature than the dry feed. In the second part of the reactor, the additional water served as the oxidant and participated in the reforming reaction. This consumes thermal energy leading to a decrease in the temperature profile. Fig. 7 shows the temperature profiles of copper (a) and cobalt (b) catalyst beds during the methane reaction with or without water at low temperature. This experiment is conducted at 500 °C, and the gas pressure is the same as shown in Fig. 6. In Fig. 7a, because of the low temperature, only the exothermic reaction proceeds in the reactor. An inhibitory effect of water is shown in the combustion process over the copper catalyst. The red profile represents the reaction with the wet feed, which is below the blue profile between 2 and 5 cm. At low temperature, the inhibitory effect restricts methane conversion and decreases the heat released from the combustion. On the other hand, the reaction on the Co catalyst still maintains its high reactivity regardless of the presence of additional water in the feed. The effect of water is not shown in Fig. 7b, and cobalt maintains its catalytic activity steadily through the whole catalyst bed. In this part, we test the temperature profiles of the methane reaction along the catalyst bed. At low temperatures, the reactivity of copper is restricted by water; while at high temperatures, water

serves as an oxidant that helps to increase methane conversion. In Fig. 6, the oxidation reaction runs first and releases a large amount of heat to the reactor. When oxygen is fully consumed, the endothermic reaction (reforming) begins to proceed. With more water in the feed, the endothermic reaction becomes stronger, making the temperature profile go down continually. Water easily adsorbs on metal clusters via surface chemisorption. O and the generated OH groups form a dynamic equilibrium of adsorption and desorption on the catalyst surface, which inhibits methane catalytic reactivity. Some researchers claim that OH helps to improve methane conversion, especially under flame-combustion conditions [23,28]. These studies are based on theoretical calculations and confirm that OH improves CAH bond dissociation. However, OH groups block the oxidation process on catalyst surfaces. After the dehydrogenation of methane, chemisorbed oxygen has a higher reactivity toward methane than surface hydroxyl. Methane does not react with hydroxyl before oxygen is fully depleted. Therefore, with additional water in the feed, OH groups occupy active sites (surface chemisorption O, H2 O þ O þ  2OH ) and block the reaction. The copper catalyst tends to adsorb more water on its surface and presents a distinct inhibitory effect of water, while it is not observed for the cobalt catalyst. In addition, the increasing effect of water at high temperature is due to the presence of the reforming reaction. After

520 5 kPa CH4 ; O2/CH4= 0.4

520

H2O/CH4= 0

5 kPa CH4 ; O2/CH4= 0.4

b

H2O/CH4= 0

5 kPa CH4 ; O2/CH4= 0.4

5 kPa CH4 ; O2/CH4= 0.4

H2O/CH4= 0.3

H2O/CH4= 0.3

510

°C

°C

a

500

510

500

Co Catalyst

Cu Catalyst

490

0

2

4

6

Reactor axial position [cm]

8

10

490

0

2

4

6

8

Reactor axial position [cm]

Fig. 7. Temperature profiles of catalyst bed ((a) Cu catalyst; (b) Co catalyst) during methane reaction with or without water at low temperature (500 °C).

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dehydrogenation, high activity carbon radicals prefer to capture the oxygen in OH, and then form the partial oxidation products CO and H2 [20,22]. Therefore, water shows different effects in methane reaction under different reaction conditions in the reactor. 4. Conclusion This paper investigates methane oxidation over copper and cobalt catalysts with low O2/CH4 ratios in the presence of water in terms of metal property, transition of oxidation and reforming, and the effect of water at low temperatures. The conclusions are listed below: 1. Copper has a stronger tendency to adsorb water on its surface than cobalt. At low temperatures, the reactivity of methane over copper is restricted by water, which forms a large number of OH groups on the catalyst surface. Water has little influence on reactivity over the Co catalyst. 2. When the temperature increases, water serves as an oxidant and participates in the reforming reaction. Oxidation reaction precedes the reforming reaction. Water does not participate in the reaction if there is oxygen in the feed.

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