Studies Of The Model Ni-mo/alumina Catalysts In The Reactions Of Hydrocarbons
STUDIES OF THE MODEL Ni-Mo/ALUMINA CATALYSTS IN THE REACTIONS OF HYDROCARBONS
T. Borowiecki1, R. Dziembaj2, M. Drozdek2, G. Giecko1,
M. Panczyk1, Z. Piwowarska2
1Faculty of Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
2Faculty of Chemistry, Jagiellonian University, 30-060 Krakow, Poland
Steam reforming of hydrocarbons is conducted on nickel catalysts and can be accompanied by the formation of harmful filamentous carbon deposits, which, mechanically destroying the catalyst, make its deactivation irreversible. Borowiecki et al. [1] have shown that the introduction of small amounts (0.1-1 wt. %) of molybdenum to nickel catalysts greatly improved their resistance to coking. On the other hand, in the reaction of hydrogenolysis the same amounts of the promoter had practically no effect on the coking rate [2]. It implies that the surface properties of catalysts Ni‑Mo/Al2O3 change in various reaction mixtures. The decisive factors seem to be hydrogen partial pressure and its ratio to the pressure of water vapour.
The aim of the present studies is to explain the influence of Mo on the properties of nickel catalysts in the hydrogenolysis of n-butane.
Experimental
The model catalysts containing Ni and Mo in the similar ratio as in the catalysts described elsewhere [1] but with enhanced amounts of both elements (70 wt. % of NiO and 0.1-10 wt. % of MoO3) were prepared. Samples were dried, then calcined at 400° C, and reduced at 800° C. After high temperature treatment the catalysts were cooled in the hydrogen stream and passivated in non-oxidised nitrogen.
The properties of the examined catalysts have been set up in Table 1. The introduction of Mo into the nickel catalysts leads to the slight decrease of the total and active surface area. TPR results showed that the presence of Mo enhanced the reduction process of the passivated catalysts [3]. The XPS studies indicated that after depassivation of the catalysts Mo occurred in different oxidation stages including Mo0.
Table 1. Influence of promoter amount on the surface area of nickel, activity and surface species of Mo
Catalyst
Mo/Ni
atomic ratio
Relative
surface area of nickel
Relative reaction ratea
at
XC4=15 %
Amounts of different surface species of Mo before and after depassivation [%]
Binding energy [eV]
Mo+6
[232.5±0.3]
Mo+x
[230.5±0.2]
Mo0
[228.0±0.3]
Ni
0
1.0
1.0
-
-
-
Ni-Mo (1.0)
0.0074
0.972
1.60
Ni-Mo (2.0)
0.0148
0.975
1.45
72/31
3/7
25/62
Ni-Mo (5.0)
0.037
0.958
1.30
54/19
16/15
30/66
Ni-Mo (10.0)
0.074
0.879
1.27
52/23
12/19
36/58
a – at the temperature 260oC; H2:C4H10 = 6:1
x – (4
Addition of Mo to nickel catalysts has a favourable effect on the activity in n-butane hydrogenolysis. In the range of 1.0-2.0 wt. % of Mo there was observed a maximum of the reaction rate. An increase of the activity cannot be explained by changes in the size of the nickel surface area.
The presence of different forms of molybdenum on the catalysts surface was depended on Mo amounts. In the case of the reduced samples, the smallest changes are observed for the Mo0 form. An increase of Mo contents in the catalysts cause decrease of Mo+6 connected with an increase of Mo+x (4
Leclercq et al. [4] have presented a similar effect of Mo additions on the activity of Pt-Mo/SiO2 catalyst in alkane hydrogenolysis.
Acknowledgement
This work was supported by the Grant No. 3 T09B 007 15 of the State Committee for Scientific Research (KBN).
References
1. T. Borowiecki, A. Gołębiowski, B. Stasińska, Appl. Catal., A: General, 153 (1997) 141.
2. T. Borowiecki, G. Giecko, M. Pańczyk, Appl. Catal., A: General, submitted to press.
3. T. Borowiecki et al., EuropaCat-V, Limerick, September 2001, accepted for presentation.
4. G. Leclercq et. al., Ind. Eng. Chem. Res., 36 (1997) 4015.
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