ACS Symposium Series - ACS Publications - American Chemical

Chapter 13

A Highly Active and Carbon-Resistant Catalyst for C H Reforming with CO : Nickel Supported on an Ultra-Fine ZrO

Downloaded by NORTH CAROLINA STATE UNIV on May 3, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch013

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Jun-Mei Wei, Bo-Qing Xu, Jin-Lu Li, Zhen-Xing Cheng, and Qi-Ming Zhu State Key Laboratory of C Chemistry & Technology, Department of Chemistry, Tsinghua University, Beijing 100084, China 1

N i supported on a specially prepared ultra-fine Z r O is studied for activity and carbon-resistant ability in catalytic reforming of CH with CO to synthesis gas. This catalyst provides a space time yield of 46.7 g CO/h·g-cat at 1030 Κ and a catalyst life longer than 600 h without detectable deactivation. Possible reasons for the extremely high activity and stability of the catalyst are discussed. 2

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Introduction

The catalytic reforming of C H with C 0 for producing synthesis gas ( C H + C 0 -> 2CO + 2H ) is one of the attractive routes for utilization of the two 4

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© 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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198 greenhouse gases. Moreover, this process can produce synthesis gas with a H /CO ratio less than a unity, which is more desirable for Oxo syntheses, and for the syntheses of oxygenates as well as long chain hydrocarbons. Numerous papers have been documented on the catalysis of this reaction in recent years, as were reviewed by Bradford and Vannice in 1999 [1]. It is commonly recognized that Ni-based catalysts are active for this reaction, but they deactivate rapidly due to carbon deposition via CO disproportionation and/or C H decomposition [2]. Supported noble metal catalysts were found less sensitive to coking [3,4], but their practical utilization was restricted by the high cost and limited resource of the noble metals. In the search for highly active and carbon-resistant nickelbased catalysts, we showed very recently that an ultra-fine Z r 0 (u-Zr0 ) supported Ni catalyst was very active and extremely stable for the reforming reaction [5-8]. The Ni loading in the catalyst system can be as high as 27% without degrading the catalyst. Besides, we have also found that u-Zr0 itself is somewhat active for this reaction. This presentation reports on the behavior of the catalysis. 2


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Experimental

The ultra-fine Zr0 with particle sizes around 6 nm and a specific surface area of 160 mVg was prepared with a precursor of drying an alcogel of ZrO(OH) under conditions for supercritical ethanol (543 K, 8.0 MPa). The alcogel was obtained by washing with anhydrous ethanol of a ZrO(OH) hydrogel prepared by addition of 0.17 M ZrOCl solution into a 2.5 M ammonia water solution with careful control of pH=9~ll. X-ray diffraction (XRD) measurement showed that the crystals of u-Zr0 were 22% monoclinic and 78% tetragonal. Ni/u-Zr0 , Ni/Al 0 , Ni/Ti0 , and Ni/Si0 catalysts were prepared by impregnating the u-Zr0 , A1 0 , Ti0 , and Si0 with an aqueous solution of nickel nitrate, followed by drying (at 383 Κ for 12 h) and calcining (at 923 Κ for 5 h). The three conventional supports were purchasedfromTianjing Institute of Chemical Industry, China. The catalytic reaction was performed under atmospheric pressure at 1030 Κ with a feed of 1:1 C H / C 0 (GHSV=2.4xl0 ml/h-g-cat). 200 mg catalysts diluted with 500 mg alfa-alumina or 200 mg supports with sizes ranging in 2040 meshes were loaded into a fixed bed quartz tubular reactor for the activity and stability measurements. Before reaction, the samples were reduced in situ at 973 Κ with flowing 10% H / N for 3 h. Ni loading in the catalysts was determined with XRF and expressed by weight percent. 2

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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Results and Discussions Figure 1 shows C H conversion versus reaction time on stream (TOS) over 5%Ni catalysts supported by different supports. The conversions of C H and C 0 and the selectivity to CO and H at TOS=T0 h are showed in Table 1. From figure 1 it can be seen that the initial activities over N1/AI2O3, Ni/Ti0 , and Ni/uZr0 are all quite high and only Ni/Si0 shows poor activity. Although the initial activity over Ni/u-Zr0 is not the highest, it maintains its initial activity for longer than 200 h TOS without any deactivation. In contrast, the activity of the other catalysts declines rapidly. All these observations show that supports exert significant influence on activity and stability of the Ni-based catalyst. Figure 2 shows C H conversion versus TOS over Ni/u-Zr0 catalysts with different Ni loadings. It can be seen that the catalyst activity increases with the increase of Ni loadings and that the C H conversion is close to the equilibrium value (87%) over the catalyst with a nickel loading of 27%. It can also be seen that all the Ni/u-Zr0 catalysts with different nickel loadings are very stable and that no deactivation occurs on the 27%Ni/u-Zr0 catalyst even after 600 h TOS. Besides, an obvious oscillation of C H conversion on the three catalysts takes place. Figure 3 shows C H and C 0 conversions versus TOS over u-Zr0 under conditions of atmospheric pressure, 1073 K, GHSV=2.4xl0 ml/h-g-cat. Around 10% C H conversion is obtained after an induction period and no deactivation happens after 50 h TOS. Besides, oscillation also occurs on the u-Zr0 sample. The conventional Si0 , Ti0 , and A1 0 supports with no Ni were also tested and all of them showed very low activity with C H conversion being less than 1%. The influence of space velocity (GHSV) on C H conversion over the 27%Ni/u-Zr0 catalyst was also investigated under the conditions mentioned above except with varying GHSV. Table 2 shows that increasing the GHSV results in a decrease in C H conversion, but the decreasing rate is so slow that the C H conversion can still be near the equilibrium with a GHSV as high as 4.8xl0 ml/h-g-cat indicating very high activity of the catalyst. The present 27%Ni/u-Zr0 catalyst is more active than the Nio.03Mgo.97O catalyst reported by Fujimoto et al. [9]. C H conversion over the latter catalyst is 82% at 1123 Κ and GHSV=1.8xl0 ml/h-g-cat (W/F=1.2 hg-cat/mol), corresponding to a STY of 11.9 g CO/hg-cat. For comparison, the present catalyst provides 85% C H conversion at 1030 Κ (ca.100 Κ lower than over Nio.03Mgo.97O) and GHSV=4.8xl0 ml/h-g-cat, corresponding to a STY of 46.7 g CO/h-g-cat. The extremely high activity and stability of the Ni/u-Zr0 catalysts may be accounted for with the unique character of the u-Zr0 and the structure of the catalyst. On one hand, the u-Zr0 support is able to accommodate as high as near 30% Ni without degrading the catalyst. This is the reason why our catalyst is much more active than Fujimoto's, in which only 4.26 wt%Ni was contained. 4

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ο

fi ο

0

-—-ο— —


Μ

u > fi ο

—^—Ni/u-Zr02 —*--Ni/AI203 —•—-Ni/Si02

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-A—Nî/Ti02 •1

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Time on Stream / h Figure 1: CH conversion versus reaction time over Ni supported on different support 4

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- 5%Ni/u-Zr02

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ta u

- 125%Ni/u-Zr02 60t -27%Ni/u-Zr02 5 0

H10 0

J

1 20 0

1

1 300

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1 400

500

6 00

Time on stieam / h Figure 2: CH conversion versus reaction time over Ni/u-Zr0 catalysts with different Ni loadings 4

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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002. 4

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79.5 87.9 83.2 91.1 86.6

95.4 96.7 97.3 98.4 95.2

1.20 1.10 1.23 1.08 1.10

1.0 1.5 2.0 2.8 3.7

88.3 86.7 86.4 81.2 77.5

86.2

85.9

85.8

79.7

79.1

2.4

3.6

4.8

7.2

9.6

Reaction temperature: 1030 K; The data represent the results at TOS=50 h.

83.4 97.6 1.17

H Sel. /% CO Sel. /% CO/H2

0.5

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100.0

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CH TOF / s"

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C 0 Conv. /%

88.7

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C H Conv. /%

1.2

GHSV / 10 ml-h-g-cat

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Table 2 Effect of GHSV on the activity and selectivity over 27%Ni/u-Zr0 catalyst

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85.9

98.7

1.18

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67.8

Reaction conditions: T=1030 K , P=0.1 M P a , G H S V = 2 . 4 x l 0 ml/g-cat-h,C0 /CH =l

': The data represent the results at TOS=10 h

Ni/u-Zr0

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65.0

95.7

97.8

1.02

0.5

16.4

11.6

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Ni/Si0

84.2

87.7

1.05

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2.5

66.5

67.0

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Ni/Ti0

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85.5

CO/H 90.6

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1.06

2.9

71.3

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70.9

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Νί/γ-Α1 0

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TOF C H /s-

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C 0 Conv. /%

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CH Conv. /%

Catalyst

H Sel. /%

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C O Sel. /%

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Table 1 Activity and selectivity of Ni-based catalysts for C H reforming with C 0 over different Ni-based catalysts



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Figure 3: CH and C0 conversions versus reaction time over u-Zr0 catalysts 4

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In our Ni/u-Zr0 catalyst, Ni particles are dispersed among u-Zr0 , there is no limitation to the loading of Ni, though there may be some interaction between Ni and u-Zr0 . In fact, TEM and other characterization data show that the Ni/uZ r 0 catalyst can better be described as a nano-composite composed of nano-Ni metal and nano-Zr0 crystals. It is assumed that the nano-composite nature of the catalyst is essential for the long-lasting anti-carbon property of the catalyst. On the other hand, the fact that u-Zr0 itself possesses some activity for the desired reaction implies that, though being a support, it can activate both C H and C 0 . This would appreciably increase the number of the active sites and therefore enhance the activity. In addition, surface oxygen formed by the dissociation of C 0 on u-Zr0 at the interface between metal and support [5] facilitates the elimination of carbon, thereby the stability of the catalyst is enhanced. This mechanism could be effective for the present Ni/u-Zr0 catalyst because Ni and u-Zr0 particles are very tiny and therefore they are intimately dispersed and contacted. The large area of the interface and the close distance between Ni and u-Zr0 allow the surface oxygen to migrate onto the Ni surface and react with carbon easily. Oscillation may stem from the alternative generation and elimination of carbon. Further investigations on this phenomenon are being made. 2

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Acknowledgement

This work was financially supported by the grants from the Fundamental Research Foundation of Tsinghua University and National Natural Science Foundation of China (NSFC, grant 0094 ).

References

1. 2. 3. 4.

Bradford, M.C.J., Vannice, M.A., Catal. Rev.-Sci. Eng., 1999, 41, 1. Claridge, J.B., Green, M.L.H., Tsang, S.C., York A.P.E., Ashcroft A.T., Catal. Lett., 1 9 9 3 , 22, 299. Ashcroft, T., Cheetham, A.K., Greenand, M.L.H., Vernon, P.D.F., Nature, 1 9 9 1 , 352, 225. Bitter, J.H., Seshan, K., Lercher, J.A., J. Catal., 1 9 9 8 , 176, 93.


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Wei, J.M., Xu, B.Q., Li, J.L., Cheng, Z.X., Zhu, Q.M., Appl. Catal. A: General, 2000, 196,L167. Wei, J.M., Xu, B.Q., Cheng, Z.X., Li, J.L., Zhu, Q.M., Stud. Surf. Sci. Catal., 2000, 130D, 3687 Wei, J.M., Xu, B.Q., Li, J.L., Cheng, Z.X., Zhu, Q.M., Fuel Chemistry Division preprint, 2001, 46, xx Xu, B.Q., Wei, J.M., Wang H.Y., Sun K.Q., Zhu, Q.M., Catal. Today, in press Tomishige, K., Yamazaki, O., Chen, Y., Yokoyama, K., Li, X., Fujimoto K., Catal. Today, 1998, 45, 35.


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