Carbonization of nickel catalysts and its effect on methane dry reforming

Applied Catalysis A: General 453 (2013) 71–79

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Carbonization of nickel catalysts and its effect on methane dry reforming V.Yu. Bychkov ∗ , Yu.P. Tyulenin, A.A. Firsova, E.A. Shafranovsky, A.Ya. Gorenberg, V.N. Korchak Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygina str. 4, GSP-1, Moscow 119991, Russia

a r t i c l e

i n f o

Article history: Received 1 October 2012 Received in revised form 14 November 2012 Accepted 5 December 2012 Available online xxx Keywords: CO2 reforming of methane Nickel catalysts Thermogravimetry Carbon deposition Catalyst deactivation Carbon nanofibers

a b s t r a c t Formation and reactivity of carbon in Ni/Al2 O3 catalysts of methane dry reforming (MDR) have been studied at their interaction with CH4 , CO2 , H2 or CH4 –CO2 mixture using thermogravimetry and massspectrometry. Temperature ranges of carbon accumulation and removal have been established, with a temperature hysteresis of carbon accumulation being observed in CH4 –CO2 mixture. The primary deposited carbon blocks MDR activity, but the deactivation is reversible under MDR conditions. The primary carbon transforms into the secondary nanofiber carbon which is stable under MDR conditions and less blocks MDR activity. Effect of Ni catalysts deactivation because of carbonization can be reduced, if at the initial stage the catalyst does not contact with the reaction mixture at temperatures below 700 ◦ C. The rate of carbon accumulation was increased with increasing Ni particle size from 2 to 5 nm. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

centers (usually Ni, Co, platinum metals) forming the associated carbon and releasing hydrogen (2).

Methane dry reforming (MDR) (1) is a promising process to produce synthesis gas with a low H2 /CO ratio that is suitable for the production of oxygenated chemicals. MDR process is also a CO2 consuming one that may be important for the widely discussed problem of CO2 reduction. CH4 + CO2 = 2CO + 2H2

(1)

Nickel-containing catalysts were more than once reported as efficient catalysts for this process (see reviews [1,2]), but they were found to be sensitive to deactivation because of carbonization during MDR reaction. The carbonization is one of the main reasons preventing industrial application of MDR process. Therefore, detailed investigations of the nature of Ni catalyst carbonization are very important. Most research of MDR reaction [1,2] has shown that in the course of the reaction methane adsorbs dissociatively on the surface active

∗ Corresponding author. Tel.: +7 495 939 75 46. E-mail addresses: [email protected], [email protected] (V.Yu. Bychkov). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.12.006

Me + CH4 = Me–C + 2H2

(2)

Further on, that carbon reacts directly with CO2 according to (3) [3,4] Me–C + CO2 = Me + 2CO

(3)

or indirectly (4) and(5) through the intermediate dissociative adsorption of CO2 [5,6] Me + CO2 = Me–O + CO

(4)

Me–C + Me-O = 2Me + CO

(5)

Thus, the associated carbon acts as an intermediate in the MDR process. At the same time, the associated carbon prevents further methane adsorption by the surface atoms of the active metal (2), so carbon accumulation can result in a decrease of MDR catalytic activity, as mentioned by many authors [1,2,7–15]. However, different papers disclose dissimilar results concerning correlation between the amount of accumulated carbon and the decrease the MDR reaction rate. Sometimes, carbon deposition has led to a total deactivation of Ni/Al2 O3 catalyst [10], but the authors of [13] reported that MDR activity of Ni/Al2 O3 was stable during carbon accumulation. Usually, amount of deposited carbon is measured already after the completion of catalytic experiment [10,11,13–15],

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but not during a variation of catalytic activity. In this work we have applied thermogravimetry combined with mass-spectrometry. This combination allows to control carbon accumulation in situ by a catalyst weight simultaneously with measurements of gas phase composition by mass-spectrometer that can give more information about an effect of carbonization to reactivity. There are many indications that carbonization of Ni catalysts strongly depends on a Ni particle size both during pure CH4 decomposition [16–18] and MDR catalytic reaction [10,14,15,19,20]. It was shown in paper [16] that under the tested conditions a maximum rate of carbon nanofiber growth was observed for Ni particles of 34 nm, and samples with smaller and greater Ni particles demonstrated lower rates of carbon accumulation. Similar results were reported in paper [17], while in article [18] Ni particles with 10–20 nm size worked best in the carbon nanofiber growth. The authors of [19] have shown that with Ni particles size below 10 nm, carbon fibers do not form and there is no catalyst deactivation. According to [10] the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. In [15] the reduction of La2 NiO4 leads to the formation of nickel particles (7 nm), smaller than the particles obtained by reduction of LaNiO3 (15 nm), the presence of such small particles would be responsible for the high activity. In order to study the role of particle size, we prepared a series of Ni catalysts supported at different conditions on alumina supports with various specific surface areas to obtain Ni systems with different size of Ni particles. Redox method that was already applied earlier [21] was used for the estimation of average size of Ni particles in the catalysts.

Table 1 wsurf /wbulk values and average diameters of Ni particles. Sample

wsurf /wbulk (×10)

D (nm)

Ni/␣-Al-3 Ni/␣-Al-5 Ni/␣-Al-6 Ni/␣-Al-7 Ni/␪-Al Ni/␥-Al Aerosol Ni

1.2 1.5 1.7 1.6 2.1 3.4 0.23

5.4 4.3 3.85 4.2 3.1 2.0 29

isothermal mode. The thermobalance allowed to feed CH4 –CO2 mixture as a single flow or as a secondary flow in addition to the primary flow of inert gas (usually He). The last mode enabled not to pump out the inner part of the furnace to change the reactive gas type and to carry out the experiments with a feed of CH4 –CO2 mixture into a sample preheated in He. The mass-spectrometer was applied in the mode of the continuous measurement of ion currents at the selected values m/z equal 2 (H2 ), 15 (CH4 ), 18 (H2 O), 28 (CO, CO2 ), 32 (O2 ), 44 (CO2 ). To calculate the ion current corresponding only to CO, the CO2 contribution defined from the independent experiments was subtracted from the total ion current value, and the value obtained was designated as 28*. Microphotographs were taken using a high-resolution scanning electron microscope (SEM) JSM-7001F (JEOL) whose resolution reached 1.2 nm at 30 kV. 3. Results

2. Experimental 2.1. Catalysts preparation Supported nickel catalysts were prepared by impregnation of ␣Al2 O3 , ␪-Al2 O3 , ␥-Al2 O3 (specific surface areas 6, 80 and 170 m2 /g, respectively) with a water solution of Ni(NO3 )2 with subsequent 4 h drying at 120 ◦ C and 4 h calcination at 800 ◦ C. The amounts of supported nickel correspond to 6%mass of NiO. The samples obtained were designated as Ni/␣-Al, Ni/␪-Al and Ni/␥-Al. To obtain catalysts with different Ni particle size on one support, ␣-Al2 O3 , the latter was impregnated with ordinary water solution of Ni(NO3 )2 (pH = 3.0) or solution (suspension) with partially agglomerated Ni(OH)2 . A set of Ni(OH)2 suspensions visually distinguished by opacity degree was prepared by adding of ammonia solution until a predetermined pH value (5.65, 5.9 or 7.5) has been reached. The corresponding samples obtained were designated as Ni/␣-Al3, Ni/␣-Al-5, Ni/␣-Al-6 and Ni/␣-Al-7. After calcination in air the samples were reduced in H2 flow at 800 ◦ C for 2 h. Aerosol Ni particles were prepared by evaporation of preliminarily vacuum-degassed Ni foil from a tungsten cathode in Ar atmosphere at 3 Torr. Details of the gas evaporation method can be found in [22]. 2.2. Testing technique The thermogravimetric set-up included a thermobalance SETSYS Evolution (Setaram) and a mass-spectrometer OmniStar GSD 301 (Pfeiffer). A catalyst sample (∼30 mg) was loaded into a quartz cap suspended inside a thermobalance furnace. For a detailed description of the setup see [21]. The initial sample was heated to 800 ◦ C in the furnace in H2 flow and kept at this temperature for 10 min to obtain freshly reduced sample. Then the sample was cooled down to 30 ◦ C and examined in CH4 , CO2 , H2 , O2 or CH4 :CO2 = 1:1 flows under heating (usually 10 ◦ C/min) or in the

3.1. Determination of Ni particle size Interaction of metallic nickel with gaseous oxygen at temperature ∼100 ◦ C results only in surface oxidation. A measurement of the amount of oxygen in thus partially oxidized Ni sample and a comparison with the amount of oxygen in a sample totally oxidized to NiO make clear the ratio of Ni particles surface to their volume. To oxidize Ni surface only a sample in thermobalance was kept for 20 min in 5% O2 –He flow (20 ml/min) at 100 ◦ C, then cooled down to 30 ◦ C and then heated again in 5% H2 –He flow (20 ml/min) up to 800 ◦ C (10 ◦ C/min). At 200–250 ◦ C a sample weight drop wsurf was observed, accompanied by H2 consumption and H2 O evolution. Then the reduced sample was cooled down to 30 ◦ C and heated again in 5% O2 –He flow (20 ml/min) up to 800 ◦ C to measure the weight increase from total Ni oxidation with temperature ranging 200–500 ◦ C until a constant weight was attained. After that the sample was cooled again in 5% O2 –He flow down to 30 ◦ C and then heated in 5% H2 –He flow to measure at 300–700 ◦ C the weight drop wbulk due to NiO reduction. In Table 1 measured values of wsurf /wbulk ratios are given for all tested catalysts. With the assumption that Ni particles are spheres of the same diameter and atomic surface density of 1015 Ni at/cm2 , it is possible to calculate the diameter of Ni particles using the following formulae D = 6 × 107 × S /(V × (NS /NV )), where D is particle diameter (nm), S is atomic surface density (at/cm2 ), V is atomic bulk density (at/cm3 ), NS /NV is ratio of atoms at the surface and in the bulk, respectively. At the calculations a NS /NV value was considered to be equal to a measured value wsurf /wbulk . Calculated thus D values are given in Table 1. Table 1 shows that increasing the specific surface area of alumina oxides from ␣-Al2 O3 to ␥-Al2 O3 results in decreasing the average Ni particle diameter from 5.4 to 2 nm. Catalysts prepared by impregnating ␣-Al2 O3 with solutions at different pH have Ni particles in a range 3.85–5.4 nm.

2.5

5.E-06

2

4.E-06

1.5

3.E-06

1

2.E-06

0.5

1.E-06

0 0

200

400

600

800

73

Ion current, A

Weight, mg/mg cat

V.Yu. Bychkov et al. / Applied Catalysis A: General 453 (2013) 71–79

0.E+00 1000

Temperature, C Fig. 1. Weight variation and H2 evolution (ion current at m/z = 2) during the heating of Ni/␣-Al-6 sample in CH4 flow (10 ◦ C/min, 30 ml/min). Fig. 2. Microphotograph of Ni/␣-Al-3 sample after the heating in CH4 flow.

0.16 Ni/α-Al-3

0.14

Weight, mg/mg cat

For independent verification of the applied gravimetric redox method of Ni particle size determination microphotographs of nanosized Ni powder prepared by the gas evaporation method were made using a scanning electron microscope. The powder consisted of rounded Ni particles. Visual diameters of Ni particles were measured and an average diameter was found to be 26 nm. Determination of Ni particle size by the gravimetric redox method described above resulted in 29 nm value that is rather close to the value obtained from microscopy. This gives an argument to believe the results obtained by the gravimetric redox method.

0.12 0.1

Ni/θ-Al

0.08 0.06

Ni/γ-Al

0.04 0.02 0 -0.02 300

3.2. Interaction of the catalysts with pure CH4

400

450

500

550

Temperature, C Fig. 3. Weight variation during the heating of Ni/␣-Al-3, Ni/␪-Al, Ni/␥-Al samples in CH4 flow (10 ◦ C/min, 30 ml/min).

3.3. Interaction of carbonized catalysts with CO2 Fig. 4 shows results of the interaction of carbonized Ni/␣-Al3, Ni/␪-Al and Ni/␥-Al samples with CO2 flow under heating from 30 to 800 ◦ C. During the heating CO2 reacts with the accumulated carbon forming CO (reaction (3)), which is detected by the mass-spectrometer, and a weight decreases because of the carbon removal. No water evolution was detected during the carbon removal indicating the absence of hydrogen in the accumulated carbon. It is clear from Fig. 4 that though the samples contained different amounts of the accumulated carbon, initial parts of the weight variation curves are very similar in shape. Reaction C with

0.1 0

Weight, mg/mg cat

Fig. 1 shows results of the experiment with heating of a Ni/␣Al-6 sample in CH4 flow from 30 to 850 ◦ C. At ∼400 ◦ C the sample weight starts to increase which is accompanied by H2 evolution. Obviously, this is caused by the formation of carbon according to reaction (2). The process gradually accelerates until the temperature reaches 600–650 ◦ C, after which it starts slowing down dramatically and practically stops at 670 ◦ C. By now the sample contained 1.9 mg C/mg cat (200 at. C/at. Ni). During further heating, starting at 780 ◦ C, additional weight increase occurring at a slower pace was observed accompanied by H2 evolution. Other experiments with feeding CH4 under isothermal conditions at 600 and 700 ◦ C have shown that the slowing down of carbon accumulation at temperatures higher than 650 ◦ C is caused exactly by the lower rate of the process at higher temperatures, and not by the ordinary deceleration due to the completion of carbon accumulation. In another experiment Ni/␣-Al-6 sample was heated in CH4 flow up to 600 ◦ C (temperature of maximum rate of carbon accumulation) and then kept at this temperature for 1 h. Under these conditions the carbon accumulation stopped after 45 min with 3.26 mg C/mg cat (340 at. C/at. Ni). For other supported Ni catalysts prepared the methane decomposition and carbon accumulation were found to proceed similarly. Fig. 2 shows a SEM microphotograph of the Ni/␣-Al-3 sample after it’s heating in CH4 . In Fig. 2 one can see carbon nanofibers with bulbs at their ends containing Ni particles (see review [23]). Fig. 3 shows initial parts of the curves representing the weight increase due to the carbon accumulation during CH4 interactions with Ni/␣-Al-3, Ni/␪-Al, Ni/␥-Al catalysts. It is clear that the rate of carbon accumulation increases in the sequence Ni/␣-Al-3 > Ni/␪Al > Ni/␥-Al along with increasing Ni particle size from 2 to 5.4 nm (Table 1). However, during similar experiments with Ni/␣-Al-3, Ni/␣-Al-5, Ni/␣-Al-6 and Ni/␣-Al-7 catalysts with 4–5 nm Ni particles practically the same rates of carbon accumulation were observed.

350

Ni/θ-Al

-0.1

Ni/γ-Al

-0.2 -0.3 -0.4 -0.5

Ni/α-Al-3

-0.6 0

200

400

600

800

1000

Temperature, C Fig. 4. Weight variation during the heating of Ni/␣-Al-3, Ni/␪-Al, Ni/␥-Al samples in CO2 flow (10 ◦ C/min, 30 ml/min).

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V.Yu. Bychkov et al. / Applied Catalysis A: General 453 (2013) 71–79 0.7

6.E-09

5.E-06

b a

4.E-06

c

0.3 3.E-06

d

0.2 0.1

4.E-09 3.E-09 2.E-09 a

1.E-09

0

2.E-06

0

20

40

60

80

0.E+00 100

100

Time, min

CO2 starts at ∼500 ◦ C and completes at 600–750 ◦ C depending on the amount of previously accumulated carbon. Partial oxidation of Ni also proceeds during the CO2 treatment. This results in a time delay of CH4 decomposition during the following heating of the catalyst in CH4 flow where CH4 decomposition starts only at 505 ◦ C. For the Ni/␣-Al catalysts the weight rise due to carbon accumulation in CH4 corresponds rather well to the weight drop during the CO2 treatment. For the Ni/␪-Al and Ni/␥-Al samples only part of accumulated carbon was removed at 600–750 ◦ C during the CO2 treatment. Fig. 5 demonstrates the results of the experiment, in which the Ni/␥-Al sample was sequentially heated in flows of CH4 , CO2 and 5% O2 –He mixture. Carbon accumulation in CH4 had reached 0.562 mg C/mg cat (curve a in Fig. 5), after which the sample was pumped out and cooled down. During repeated heating in CO2 flow a weight drop was observed at 480–700 ◦ C (curve b in Fig. 7), accompanied by CO evolution (curve c in Fig. 5). It is clear that this process finished at 700 ◦ C and 0.415 mg C/mg cat was removed until this temperature had been reached, and that accounts for 74% of all accumulated carbon. At further heating in CO2 flow above 800 ◦ C a beginning of a second process of carbon removal and CO evolution was observed, which was not completed in this experiment. Thus, after the CO2 treatment the Ni/␥-Al sample still contained carbon, not reactive with CO2 at 500–700 ◦ C. During following heating in 5% O2 –He flow this residual carbon was oxidized to CO2 at 550–700 ◦ C with a weight drop of 0.145 mg C/mg cat corresponding to 26% of all accumulated carbon (curve d in Fig. 5). To investigate the reactivity of carbon with CO2 in the absence of Ni a similar experiment was performed with a diesel soot sample preheated in inert gas flow up to 700 ◦ C. During heating of soot in

-0.85

3.E-07

2.E-07 -0.95 1.E-07 c, ×30

b

-1.05 0

100

200

300

Ion current, A

a -0.9

-1

400

200

300

400

500

600

700

Temperature, C

Fig. 5. Weight variation (a, b, d) and CO evolution (c) during the sequential heatings (10 ◦ C/min) of Ni/␥-Al sample in flows of CH4 (a), CO2 (b, c) and 5% O2 –He mixture (d).

Weight, mg/ng cat

Ion current, A

0.5 0.4

b

5.E-09

Ion current, A

Weight, mg/mg cat

0.6

500

600

700

0.E+00 800

Temperature, C Fig. 6. Weight variation (a) and CH4 evolution (b, c) during the heating of the carbonized Ni/␪-Al sample in H2 flow (10 ◦ C/min, 30 ml/min).

Fig. 7. CH4 evolution during the heating of a fresh Ni/␥-Al sample (a) and the Ni/␥-Al sample after CO2 adsorption (b) in H2 flow (10 ◦ C/min, 30 ml/min).

CO2 flow up to 850 ◦ C no significant carbon removal-related weight decrease was observed. 3.4. Interaction of carbonized catalysts with H2 It is known that H2 TPR allows to detect different forms of carbon existing in Ni catalysts after their treatment with CH4 –CO2 mixture [24], CO, C2 H4 [25], CH4 –O2 [21]. It is usually believed that the carbon form that reacts with H2 at a lower temperature is more active in catalysis (i.e. MDR) and is a main intermediate of catalytic reaction. In order to spot such carbon forms in our catalysts the samples after treatment in CH4 were quickly cooled and heated again in H2 flow measuring a weight variation and CH4 evolution. Fig. 6 shows results of such an experiment for a Ni/␪-Al sample. It is clear that carbon methanation proceeds as a single peak with maximum at 610 ◦ C. Curve c in Fig. 6 represents an ion current from CH4 magnified 30 times. Such magnification makes visible an additional low temperature peak of CH4 at 310 ◦ C. This peak was also present during the heating of Ni/␥-Al catalyst in H2 flow, but it was absent during the heating of the Ni/␣-Al samples. So it was suggested that this low temperature peak related not to the carbon methanation, but to the reaction of other carbonaceous compound, adsorbed on the alumina support, such as surface carbonate. In order to check this assumption a fresh Ni/␥-Al sample was heated in H2 flow up to 700 ◦ C (see curve a in Fig. 7), cooled down to 30 ◦ C, heated again in CO2 flow from 30 to 500 ◦ C and slowly (10 ◦ C/min) cooled down to 100 ◦ C in CO2 . After that TPR in H2 was repeated again (curve b in Fig. 7). It is clear from Fig. 7 that the Ni/␥-Al sample after CO2 adsorption really does contain some surface carbonates, which react with H2 under heating forming methane at temperatures ∼230, 300, 400, 600 ◦ C. This fact apparently explains the CH4 formation during the heating of fresh Ni/␥-Al sample in H2 flow. Therefore, it is proven that the small CH4 peak observed at 310 ◦ C during the methanation of carbonized Ni catalysts supported on alumina with high surface area was caused by carbonate species. 3.5. Interaction with CH4 –CO2 mixture Carbon accumulation during MDR process was studied in thermobalance using two modes, with a single flow of CH4 –CO2 mixture or two flows of CH4 –CO2 mixture and He (see Section 2). The singleflow mode is closer to the ordinary conditions of catalytic activity testing, while the two-flow mode in a SETSYS thermogravimetric instrument allows to feed CH4 –CO2 mixture onto a catalyst preheated to a desired high temperature in a flow of inert gas. Fig. 8 shows variations of the Ni/␣-Al-3 sample weight and of the gas phase composition obtained during the heating of the sample in CH4 :CO2 = 1:1 mixture from 30 to 850 ◦ C. It is clear from Fig. 8

V.Yu. Bychkov et al. / Applied Catalysis A: General 453 (2013) 71–79

0.45

0.35

b c d

4.E-06

0.3 0.25

3.E-06

0.2 e

2.E-06

0.15

5.E-06

0.1 1.E-06 0.E+00 200

g 300

400

500

600

700

800

0.06

0.05

0.04 2.E-06

0.E+00

o

4.E-06 tg

0.8 CO

2.E-06

0.6 0.4 H2

Ion current, A

Weight, mg/mg sample

3.E-06

1

1.E-06

H2O

0 50

100

150

200

250

f

b

0 0

600 C

0

0.02

1.E-06

0 900

0.E+00 300

Time, min Fig. 9. Variations of weight (tg) and ion currents corresponding to H2 , *CO and H2 O during the heating of Ni/␣-Al-3 sample in CH4 –CO2 = 1:1 flow (30 ◦ C/min, 20 ml/min) up to 600 ◦ C and keeping 3 h at 600 ◦ C. The vertical line indicates the moment of 600 ◦ C reaching.

50

100

150

200

Time, min

b

700oC

5.E-06

g

4.E-06

Ion current, A

that starting from ∼400 ◦ C H2 , CO and H2 O appear in the effluent gas. CO content (curve d) steadily rises with the temperature, while the rate of H2 evolution (and CH4 consumption) has an intermediate maximum at 620–640 ◦ C. In the range of 500–650 ◦ C one can also see a significant increase of the catalyst weight that ceases at 670 ◦ C. The weight increase deals with carbon accumulation that was proven by a following treatment in CO2 or H2 flow. The temperature dependence of carbon accumulation is very similar to that observed during the heating in pure CH4 (Fig. 1), but the amount accumulated is lower. During the heating in pure CH4 the accumulation of 1.88 mg C/mg cat was observed, and only 0.37 mg C/mg cat was found in the CH4 –CO2 mixture. When we stopped the heating in CH4 –CO2 flow at temperature 600 ◦ C and kept Ni/␣-Al-3 sample for 3 h at this temperature, the carbon accumulation continued for ∼50 min achieving 1.0 mg C/mg cat (Fig. 9). After that the sample weight remained constant. Contents of reaction products H2 , CO and H2 O were at their maximum upon reaching the temperature 600 ◦ C (marked by a vertical line in Fig. 9) and slowly decreased with the progress of carbon accumulation. Here H2 evolution corresponds both to MDR process and CH4 decomposition by reaction (2). On the contrary, CO formation relates to MDR only (we do not consider possible (reversed) water-gas reaction). Fig. 9 demonstrates that the rate of MDR reaction kept steadily decreasing in the course of the carbon accumulation (60–90 min in Fig. 9) before dropping relatively quickly (90–100 min) at the moment of the completion of carbon accumulation, and then remaining almost constant for 2 h until the end of the experiment.

0.2

d

3.E-06

e

Fig. 8. Variations of weight (a) and ion currents corresponding to H2 (b), CO2 (c), *CO (d), CH4 (e), H2 O (g) during the heating of Ni/␣-Al-3 sample in CH4 –CO2 = 1:1 flow (10 ◦ C/min, 20 ml/min).

1.2

0.08 c

4.E-06

Temperature, C

1.4

0.1

Weight, mg/mg cat

0.4 5.E-06

o

700 C

6.E-06

a

Weight, mg/mg cat

6.E-06

Ion current, A

a

0.5 a

Ion current, A

7.E-06

75

h 3.E-06 i 2.E-06 j 1.E-06 k l

0.E+00 0

50

100

150

200

Time, min Fig. 10. ab Variations of weight (a, b) and ion currents corresponding to CO2 (c d), CH4 (e, f), *CO (g, h), H2 (i, j), H2 O (k, l) during the heating of Ni/␣-Al-6 sample in CH4 –CO2 = 1:1 flow (10 ◦ C/min, 16 ml/min) from 30 to 700 ◦ C and keeping 2 h at 700 ◦ C (a, c, e, h, j, k) and during the feeding CH4 –CO2 = 1:1 flow to Ni/␣-Al-6 sample preheated up to 700 ◦ C in He (b, d, f, g, i, l). Vertical lines at 66 min indicate the moment of 700 ◦ C reaching.

The Ni/␣-Al-3 sample that was saturated by carbon to the maximum extent in pure CH4 flow at 600 ◦ C up to 3.26 mg C/mg cat (see Section 3.2) was also heated in CH4 –CO2 mixture from 30 to 600 ◦ C and kept at this temperature for 2 h. During this experiment the catalyst weight remained practically invariable, i.e. neither additional carbon accumulation, nor its consumption was observed. For this precarbonized sample MDR reaction started at ∼470 ◦ C accelerating up to 600 ◦ C, and after the concentrations of the gas phase components remained almost constant. The values of the ion currents obtained for H2 and CO at 600 ◦ C in this experiment were very close to those achieved after heating fresh Ni/␣-Al-3 sample to 600 ◦ C in CH4 –CO2 flow (range of 150–240 min in Fig. 9) (ion current of ∼7 × 10−7 A corresponds to ∼10%vol H2 ). Therefore, it was proven that even maximum saturation of supported Ni catalyst with nanofiber carbon did not result in total loss of MDR catalytic activity. The data obtained indicate that carbon accumulation during MDR process over the supported Ni catalysts impairs the catalytic activity, but this effect occurs mainly in the temperature range of 400–670 ◦ C. This allows to conjecture that MDR catalytic activity after feeding CH4 –CO2 flow onto a catalyst preheated up to 700 ◦ C in inert gas may be higher than after gradual heating in CH4 –CO2 flow from environment temperature. Such comparison was made using the mode feeding two gas flows into the thermobalance furnace. Fig. 10a,b presents the results of two experiments. In the first experiment, a Ni/␣-Al-6 sample was heated from 30 to 700 ◦ C while a He flow (20 ml/min) and a CH4 –CO2 flow (16 ml/min) were being fed simultaneously. Upon reaching 700 ◦ C the temperature was kept for 2 h, then the furnace was switched off causing fast

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0.9 c

pure CH4 pulses

5.E-06

0.7 0.6

Ion current, A

Weight, mg/mg cat

0.8

b

0.5 0.4 0.3 0.2

a

4.E-06 3.E-06 2.E-06 1.E-06

0.1 0.E+00

0 0

100

200

300

400

500

600

700

70

800

80

90

100

cooling of the catalyst. In the second experiment, a Ni/␣-Al-6 sample was preheated up to 700 ◦ C in He flow (36 ml/min). Then at constant temperature of 700 ◦ C the rate of He flow was simultaneously decreased down to 20 ml/min and an additional CH4 –CO2 flow of 16 ml/min was switched on. After the reaction had lasted for 2 h at 700 ◦ C the furnace was switched off. In Fig. 10a,b parts of ion current curves for CH4 and CO2 after the fast catalyst cooling are given for a demonstration of the reproducibility of the massspectrometer data. It is clear from Fig. 10a that in the case of gradual heating in the reaction mixture (experiment 1) the sample weight also increased by 0.09 mg C/mg cat at 500–600 ◦ C (curve a). During further heating and 2 h isotherm at 700 ◦ C a small weight decrease could be observed. On the contrary, during the feeding of CH4 –CO2 flow onto the preheated catalyst (experiment 2) a gradual weight increase was detected (curve b), which however did not reach the value of carbon accumulation in the first experiment. In the second experiment notably greater conversions of CH4 and CO2 (see Fig. 10a), as well as greater concentrations of H2 and CO and water content lower (Fig. 10b) than in the first experiment were observed. Therefore, the experimental data indicate that at 700 ◦ C one can have a relatively greater MDR catalytic activity, if the catalyst was not previously heated in CH4 –CO2 flow from environment temperature. Additional experiments have shown that carbon accumulation during MDR process depends not only on the catalyst temperature, but also on the direction of temperature variation – whether the sample is being heated or cooled down. Fig. 11 demonstrates the weight variation of the Ni/␣-Al-3 sample during its successive heating in CH4 –CO2 flow from 30 to 700 ◦ C (curve a), cooling down to 300 ◦ C (10 ◦ C/min) (curve b) and repeated heating up to 700 ◦ C (curve c). It is clear that during the first heating the sample weight increased in the range of 450–650 ◦ C by 0.39 mg C/mg cat, and the cooling caused practically no weight change in the same temperature range, but during the second heating the weight again started to increase at 500–650 ◦ C by 0.215 mg C/mg cat. Similar behavior is reproducible and was observed also for Ni/␪-Al and Ni/␥-Al samples. 3.6. Interaction with CH4 –CO2 and CH4 pulses To understand the effect produced by smaller amounts of surface carbon on the rate of MDR reaction, a Ni/␣-Al-6 sample (20 mg) was loaded in an ordinary quartz tube reactor, in which He flow (30 ml/min) was fed and pulses of CH4 –CO2 mixture (∼0.5 ml) with periodicity of 2 min. For a predetermined time interval the He flow could be replaced by a CH4 flow. A small fraction of effluent gas from the reactor was sampled by a metallic capillary into

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130

140

150

Fig. 12. H2 evolution during the reaction of Ni/␣-Al-6 sample with pulses of CH4 –CO2 = 1:1 and CH4 at 600 ◦ C.

the mass-spectrometer OmniStar GSD 301. Experiments were performed at 600 ◦ C. Fig. 12 shows a signal of H2 evolution. In Fig. 12 two first peaks of H2 correspond to the catalytic reaction of CH4 –CO2 pulses with a fresh Ni/␣-Al-6 sample. Next followed H2 evolution from a pure CH4 flow for 30 s, five CH4 –CO2 pulses, 1 min CH4 flow, five CH4 –CO2 pulses, 3 min CH4 flow, five CH4 –CO2 pulses, 12 min CH4 flow, five CH4 –CO2 pulses. The values of the ion current allowed to estimate the H2 content in the effluent gas and to calculate the amount of carbon accumulated in the sample during the CH4 treatment, assuming that all the H2 was the product of the reaction (2). The calculated values were 0.016, 0.029, 0.078 and 0.25 mg C/mg cat (1.7, 3.0, 8.1, 26 at. C/at. Ni) for the four CH4 runs, respectively. It is clear from Fig. 12 that the catalytic activity falls sharply after the pure CH4 feeding, but gradually regenerates in a series of CH4 –CO2 pulses. The more significant the H2 evolution (and the corresponding carbon deposition) from CH4 decomposition over the catalyst, the deeper the drop of the catalytic activity in the CH4 –CO2 pulses, but this effect is nonlinear. Fig. 13 shows the values of the relative catalytic activity Wrel calculated as (peak H2 signal in the first CH4 –CO2 pulse after CH4 flow) × 100%/(peak H2 signal in the CH4 –CO2 pulse over the initial catalyst) plotted against the amount of the accumulated carbon. It is clear from Fig. 13 that the catalytic activity falls rapidly after the deposition of ∼3 at. C/at. Ni, but a further increase of carbon accumulation causes a lesser poisoning effect on the catalytic activity.

100 90

Relative activity W rel, %

Fig. 11. Weight variation during the sequential heating-cooling-heating of Ni/␣-Al3 sample in CH4 –CO2 = 1:1 flow (10 ◦ C/min, 20 ml/min).

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Time, min

Temperature, C

80 70 60 50 40 30 20 10 0 0

5

10

15

20

25

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Accumulated carbon, at. C/ at. Ni Fig. 13. Relative catalytic activity Wrel of Ni/␣-Al-6 sample in a pulse mode (see Fig. 12) versus the amounts of accumulated carbon (at. C/at. Ni).

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5.E-06

Ion current, A

4.E-06

a

3.E-06 b 2.E-06

1.E-06

0.E+00 300

400

500

600

700

800

Temperature, C Fig. 14. CO evolution during the heating (10 ◦ C/min) of the initial Ni/␣-Al-3 sample in CH4 –CO2 = 1:1 flow (a) and the carbonized Ni/␣-Al-3 sample in CO2 flow (b).

4. Discussion The findings show that the carbon accumulation in the supported Ni catalysts proceeds in both CH4 and CH4 –CO2 mixture. The accumulated carbon reacts with CO2 by reaction (3), i.e. plays the role of reactive intermediate in the MDR process. Microphotographs show (Fig. 2) that carbon accumulates as nanofibers, which is in good agreement with literature data [23]. The results indicate that if CH4 and CO2 are present in initial gas mixtures in equal concentrations (100% for pure gases, 50% or lower for CH4 –CO2 mixtures), the rate of CH4 decomposition by reaction (2) on a freshly reduced catalyst will be much higher than the rate of CO2 consumption by reaction (3). While the CH4 decomposition and carbon accumulation start at 400 ◦ C, the removal of deposited carbon is only noticeable at 500 ◦ C. So while CO2 presence in the gas phase distinctly reduces the carbon accumulation evidently due to its simultaneous removal (see results of Section 3.5) and can even cause some decrease of the carbon content at temperatures of 700 ◦ C and higher, under stationary MDR catalytic conditions in Ni catalysts there always exists a significant amount of accumulated carbon. The findings of the CH4 –CO2 pulse experiments (Fig. 13) demonstrate that the deposition of relatively small amounts of carbon on Ni catalysts (∼3 at. C/at. Ni) sharply decreases MDR catalytic activity, but this form of carbon is highly reactive and gets rather quickly consumed in CH4 –CO2 mixture resulting in regeneration of the initial catalytic activity. Further carbon deposition ∼8 at. C/at. Ni causes a smaller catalyst deactivation and an additional accumulation of carbon up to 26 at. C/at. Ni in fig. 13 practically does not add to the catalyst deactivation. H2 evolution remained after carbon deposition of 26 at. C/at. Ni (pulse at 141 min in Fig. 13) was close to the H2 evolution in CH4 –CO2 flow after 3 h (see Fig. 9) over the catalyst with 104 at. C/at. Ni. This nanofiber carbon, depending on MDR conditions, very small or practically not consumes during the catalysis. Fig. 14 shows temperature curves for CO evolution during the heating of a fresh Ni/␣-Al-3 sample in CH4 –CO2 flow (curve a) as well as during the heating of a carbonized Ni/␣-Al-3 sample after CH4 treatment, in a CO2 flow (curve b). A comparison between the two curves indicates that at 400–500 ◦ C a rather intensive CO formation occurs in the CH4 –CO2 mixture, however in this temperature range the removal of accumulated carbon in CO2 is practically absent. This means that nanofiber carbon, predominant in the supported Ni catalysts after the CH4 decomposition, has a lower reactivity in reaction (3) than the intermediate carbon existing during the catalytic reaction. Paper [26] studied the mechanism of MDR reaction over Co/Al2 O3 showing that the carbon formed on the metal surface

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during CH4 decomposition did not remain unchanged, but underwent some further transformations. Calorimetric measurements revealed that the first form of carbon produced in the system in small quantities immediately after the reaction of Co/Al2 O3 with a CH4 pulse had a relatively high value of carbon formation enthalpy HCf = 57 kJ/mol. According to literature data on cobalt carbide formation enthalpy this carbon form can be regarded as “carbidic” carbon. But the carbon accumulated in significant quantities in CH4 after 10 min at 700 ◦ C yielded the value HCf = 31 kJ/mol. This second form of carbon corresponds obviously to the nanofiber carbon as its HCf value is still higher than the HCf value for graphitic carbon (0 kJ/mol). These results help to understand the data presented in Fig. 14. Evidently, the high catalytic MDR activity at 400–500 ◦ C deals with the formation and reactions of the “carbidic” carbon existing in relatively small quantities in metallic Ni, and the nanofiber carbon accumulating in much greater amounts has a lower reactivity to CO2 . It was repeatedly mentioned in literature that after interactions of hydrocarbons with Ni systems various types of carbon formed can be found in the catalysts [21,24,25]. These carbon forms can be distinguished through subsequent thermo programmed heating in a flow of H2 . In our case, as described in Section 3.4, only one peak of CH4 evolution was observed during the heating of the catalysts supported on ␣-Al2 O3 and two peaks were found for the samples supported on ␪-Al2 O3 and ␥-Al2 O3 . For the latter systems the small low temperature CH4 peak was shown to correspond to the methanation of carbonate species present on alumina surface, but not to the methanation of the highly reactive form of surface carbon. That there are more than one carbon types in the catalyst can also be inferred from their interaction with CO2 under thermo programmed heating. Similar experiments have been mentioned in papers devoted to MDR catalysts investigations times and again, and as far as we know, only one temperature peak of CO evolution due to reaction (3) has ever been observed in any of them. According to the Section 3.3 data on CO2 treatment of carbonized catalysts supported on ␣-Al2 O3 all the accumulated carbon was also removed as a single peak of CO at 500–700 ◦ C. However, in case of catalysts supported on ␪-Al2 O3 and ␥-Al2 O3 after the completion of reaction (3) at 500–700 ◦ C some quantity of accumulated carbon still remained in the sample. This remaining carbon can slowly react with CO2 at temperatures above 800 ◦ C or else it can be oxidized to CO2 in an O2 flow (see Fig. 5). The portion of this carbon can be significant and reached 26% of total accumulated carbon in the example in Fig. 5. It has been mentioned in Section 3.3 that under similar experimental conditions the soot sample containing no Ni does not react with CO2 before the temperature rises to 800 ◦ C. This allows us to conclude that in order to react with CO2 at 500–700 ◦ C the accumulated carbon must be in contact with nickel (or other active metal). The existence of carbon deposited from CH4 –CO2 mixture that does not react with CO2 was also observed for Ni/SiO2 [14] and Pt/Al2 O3 [27] catalysts. The authors of [14] do not explain the nature of this unreactive carbon. The authors of [27] consider that the unreactive carbon is a carbon deposited on alumina. An argument for that supposition was the detection of two peaks of CO2 evolution (shoulder at 750 K and main peak at >840 K) during the temperature-programmed oxidation (TPO) of the carbonized sample. We also have conducted TPO experiment for a carbonized Ni/␥-Al catalyst and only a single peak of CO2 at 600–700 ◦ C was observed in the range of 30–700 ◦ C. So we consider that the explanation from paper [27] is not acceptable in our case and another explanation for the nature of unreactive deposited carbon is proposed below. Numerous studies of carbon nanofibers (see a review [23]) indicate that during the nanofiber carbon growth there is always a microcrystal of active metal (Ni, Co) present at the growing end of the fiber. This fact has also been

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proven by microphotographs obtained here in which one can see (Fig. 2) that carbon fibers have bulbs of brighter color on their ends indicating the presence of Ni microcrystals. Evidently, in this area the carbon of the nanofiber can react with CO2 by reaction (3) or by reactions (4) and (5), and during the process of carbon removal as CO the Ni microcrystal moves together with the shortening carbon thread always remaining in contact with it. It can be supposed that in case of alumina with a higher specific surface area and a lower pore size steric troubles may more often cause a loss of contact between Ni microcrystal and the carbon thread during its shortening. After the contact loss the carbon fiber becomes similar to ordinary soot and does not react with CO2 until 800 ◦ C. Such “torn” carbon fibers probably compose the form of accumulated carbon not reacting with CO2 at 500–700 ◦ C. The literature on the subjects proposes many ways to save a high catalytic MDR activity against a catalyst deactivation, mostly connected with catalysts modification were proposed [1,2]. It was demonstrated that the introduction of some additives to MDR catalysts could restrain the catalysts carbonization and help to preserve catalytic MDR activity. The results in Section 3.5 show that the decrease in carbon accumulation and preserving the catalytic activity can also be achieved by keeping the catalytic reaction conditions under control. As the most intensive carbonization of the supported Ni catalysts takes place below 700 ◦ C, an exclusion of this stage from the catalyst operation mode (for example, heating of the catalyst in reaction mixture CH4 –CO2 from an environmental temperature to a reaction temperature) can give an essential increase in catalytic MDR activity (see Fig. 10). The data obtained prove that in the studied range of Ni particle sizes the rate of carbon accumulation increases with increasing Ni particle diameter from 2 to 5 nm. This result confirms the literature data [10,14,15,19,20] where Ni catalysts with smaller Ni particles were more active in MDR, and this phenomenon was explained by a lower rate of cocking. The results obtained reveal a rather complicated picture of the carbon accumulation influence on the MDR catalytic activity of Ni catalysts. The primary “carbidic” carbon strongly, but reversibly blocks the MDR catalytic activity. The formation of the secondary nanofiber carbon is also accompanied by a decrease in catalytic MDR activity, but with significantly lower efficiency. Results presented in Fig. 9 demonstrate that Ni/␣-Al-3 sample accumulated 1.0 mg C/mg cat, i.e. ∼100 carbon atoms per one Ni atom still has rather high catalytic activity. Moreover, the sample which accumulated the maximum amount of carbon 3.26 mg C/mg cat (∼340 carbon atoms per one Ni atom) during CH4 decomposition has practically the same catalytic MDR activity. This retaining of the catalytic activity can obviously be explained by the fact that during the growth of a carbon nanofiber an active Ni microcrystal moves at the growing thread end remaining significantly accessible for the catalytic reaction. For all the studied supported Ni catalysts a well-defined limit of carbon nanofiber accumulation was observed which depended on the reaction temperature, partial pressures of CH4 and CO2 . It is considered [23] that the growth of carbon fibers stops once the surface of the Ni particle has been fully coated and blocked by carbon. It would be consistent to expect completion of the catalytic MDR reaction at this moment as well, but it does not take place nonetheless. Experiments with soot revealed that it was practically inactive in the MDR reaction under the given conditions, therefore parts of carbon fibers having no contact with Ni particles can not have also the catalytic MDR activity either. Evidently, in CH4 –CO2 gas mixture evolution of Ni particle does not end up in a state where it is fully blocked by carbon, instead its final state is one where metallic Ni surface is still partly accessible to interaction with the gas phase. The same partial “cleaning” proceeds when feeding CH4 –CO2 mixture to a totally carbonized catalyst at a high temperature. Obviously, the formation of such a Ni particle

state of “the catalysis continues without the carbon fiber growth” explains the unusual observed temperature “hysteresis” of carbon accumulation during the sequential heating-cooling-heating of the supported Ni catalysts in CH4 –CO2 mixture (Fig. 11) when carbon accumulation took place at 500–600 ◦ C during the temperature rise, but was absent during the cooling. It should be noted that during the carbon deposition a significant increase of visible catalyst volume was observed. A portion of the Ni/␣-Al-3 catalyst after the treatment in CH4 –CO2 mixture flow for 3 h at 600 ◦ C had the visible volume which was ∼3 times more than that of the catalyst before the treatment. So if the MDR catalytic activity would be tested not in the open cap of the microbalance, but in an ordinary tube reactor with a finite accessible volume, we would witness catalyst compaction and compression instead of its expansion resulting in an additional decrease of the catalytic activity. 5. Conclusions The results obtained allow to better understand the processes of carbonization of Ni catalysts during MDR as well as the effect of carbon accumulation on the catalytic activity. The carbon formed from CH4 in relatively small quantities is an active intermediate of MDR quickly formed and consumed in the process. Larger amounts of carbon accumulate as carbon nanofibers which take practically no part in the catalysis and only partially decrease the catalytic MDR activity. Most of all the nanofibers accumulate at 500–650 ◦ C. Sometimes, if the MDR performed at 700 ◦ C, i.e. above the specified temperature range, it allows to prevent the intensive accumulation of carbon and keep up a high catalytic MDR activity. It has been shown that the rate of carbon accumulation depends on the Ni particle size in the catalyst and increases with Ni particles diameter from 2 to 5 nm. Acknowledgements Support for this work was provided through the projects (grant Nos. 10-3-00715 and 12-03-00282) funded by the Russian Foundation of Basic Researchs. The equipment of CKP MIPT and REC “Nanotechnology” of MIPT has been used in this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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