Studies Of The Promoted Nickel Catalysts For Steam Reforming Of Hydrocarbons
STUDIES OF THE PROMOTED NICKEL CATALYSTS FOR STEAM REFORMING OF HYDROCARBONS
W. Gac, M. Czubryt-Idzik, J. Sikorska, B. Wal, M. Pielach, T. Borowiecki
Department of Chemical Technology, Faculty of Chemistry,
Maria Curie-Skłodowska University,
Maria Curie-Skłodowska sq. 3, 20-031 Lublin, Poland
Steam reforming of hydrocarbons is conducted on nickel catalysts and can be accompanied by formation of carbon deposits. Particularly dangerous are filamentary carbon deposits which, by mechanical destroying the catalyst, make its deactivation irreversible. The simplest manner of improving the quality of the nickel catalysts is an introduction of certain additions into them, which increase their resistance to coking. Borowiecki et al. [1,2] has shown that the introduction of small amounts (0.5-2 wt. %) of molybdenum or tungsten compounds to nickel catalysts greatly improved their resistance to coking. In an other paper Borowiecki et al. [3] has shown that the cerium promoter has very favourable influence on the nickel dispersion and its resistance for sintering.
The aim of the present research was to examine the influence of two promoters (Mo and Ce) on the properties of catalysts for the steam reforming of hydrocarbons.
Experimental
Studies were carried out on a series of catalysts containing various amounts of CeO2 (0.2 –2.0 wt. %) and a constant amount of MoO3 (0.5 wt. %). Samples were prepared by the impregnation of the commercial Ni/a-Al2O3 catalyst with a solution of appropriate promoter salts. The samples were dried at 105° C and calcined at 450° C for 4h. Investigations were conducted after catalysts reduction at 1073 K for 2 h in deoxidised and dried hydrogen.
Studies of the coking rates in the steam reforming of n-butane were carried out by the gravimetric method in a flow reactor, at constant temperature and partial pressure of n-butane (500° C and 6.1 kPa), and different regents ratios, namely, H2O : C = 0.7 or 1.5.
The properties and the coking rate of the examined catalysts have been set up in Table 1.
Table 1. Surface area and coking rate of the promoted catalysts
Catalysts
Surface areaa
(m2 g-1)
Coking rate at various reagent ratios (H2O:C)
0.7
1.5
Total
Active
rg
rNi
rg
rNi
Ni
3.8
2.0
8499
4250
2762
1381
Ni-Ce (0.2)
6.4
2.9
-
-
2294
791
Ni-Ce (0.5)
6.2
3.4
-
-
1994
586
Ni-Ce (1.0)
6.5
3.9
7010
1797
1931
495
Ni-Ce (2.0)
5.5
3.3
8478
2569
1868
566
Ni-Ce (0.2)-Mo
5.2
2.4
-
-
38
16
Ni-Ce (1.0)-Mo
5.2
2.75
3643
1325
89
32
Ni-Ce (2.0)-Mo
5.2
2.5
4854
1941
41
16
a – determined after reduction at 800° C
rg – coking rate [mg /gcat min]
rNi – coking rate [mg /m2Ni min]
The presence of Ce promoter increases both the total and active nickel areas. On the other handit decreases the coking rate, however, only in excess of the steam in the reaction mixture. An increasing amount of cerium above 0.5wt. % does not have a noticeable influence on the decreasing of the coking rate. For the nickel dispersion the optimal amount of promoter should be around 1 wt. %. Addition of Mo deteriorates the development of Ni area in comparison to Ni-Ce catalysts. However, the sample containing 1 wt. % of Ce is still the best. The presence of Mo limits the coking rate. The effectiveness of Mo is noticeable at the reagents ratio (H2O:C) equal to 1.5.
The most significant properties improvement of the typical, industrial catalyst were achieved by introducing 1 wt. % of Ce and ~0.5 wt. % of Mo.
References
1. T. Borowiecki, A. Gołębiowski, Catal.Lett., 25(1994)309.
2. T. Borowiecki, A. Gołębiowski, B. Stasińska, Appl. Catal., A: General, 153(1997)141.
3. T. Borowiecki, A. Gołębiowski, J. Ryczkowski, B.Stasińska, in Natural Gas Conversion V, A. Parmaliana et al., Eds., Stud.Surf.Sci.Catal., v.119, Elsevier, Amsterdam, 1998, 711.
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