Properties Of Nickel Steam Reforming Catalysts
PROPERTIES OF NICKEL STEAM REFORMING CATALYSTS
WITH SMALL MOLYBDENUM ADDITIVES
B. Stasińska, W. Gac, G. Giecko, M. Pańczyk, T.Borowiecki
Maria Curie-Skłodowska University, Faculty of Chemistry,
3 M.Curie-Skłodowska Sq., 20-031 Lublin, Poland
e-mail: beata@ hermes.umcs.lublin.pl
Steam reforming of hydrocarbons is one of the most important methods of commercial production of hydrogen or synthesis gas. One of the significant properties of a good catalyst of hydrocarbons steam reforming is its resistance to coking. 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 [1-3]. In the industrial practice, nickel catalyst with certain amounts of potassium were applied (in spite of a significant decrease of catalysts activity) [1-4]. Borowiecki et al. [5,6] have shown that the introduction of small amounts (0.5-2.0 wt. %) of molybdenum or tungsten compounds into the nickel catalysts greatly improved their resistance to coking.
The aim of the present paper is to study the influence of presence of molybdenum promoter on the reduction and adsorptive or catalytic properties of nickel catalysts of the steam reforming of hydrocarbons.
Experimental
Investigation were carried out on the two series of nickel catalysts with small amounts of Mo: series A - Ni/a-Al2O3, after reduction at 1073 K for 2h and series B - Ni/g- Al2O3, after reduction at 973 K for 3h.
For examined catalysts the following were determined:
– the total surface area by physical adsorption of argon at the temperature of liquid nitrogen
– the surface area of nickel by hydrogen chemisorption at 293 K and 100 mm Hg pressure
– mean size of nickel crystallites by hydrogen chemisorption and x-ray diffraction
– reduction of catalysts by gravimetric and temperature-programmed methods in TGA-121 System (Cahn) and Altamira AMI-1, respectively
– desorption of ammonia by temperature-programmed method in Altamira AMI-1
– adsorption of steam by gravimetric method in TGA-121 System (Cahn),
– rate of coking by gravimetric method in steam reforming of n-butane
– activity of catalysts in gradientless reactor in steam reforming of n-butane.
Results
The introduction of molybdenum did not practically change the total or active surface area of catalysts and the mean size of nickel crystallites. Molybdenum additives modified the process of the reduction of catalysts, and that is noticed by an analysis of both the shape of TPReduction curves and the range of reduction temperature. An increase in Mo contents makes the NiO reduction difficult. However, the gravimetric investigations showed that in the Ni-Mo catalysts at 1073K NiO and MoO3 were totally reduced. This means that the reduction of MoO3 in presence of nickel is easier. An increase in the surface area of the support and their higher reactivity (aÞg), causes a significant increase of hardly reducible fraction of NiO bounded with alumina.
TPDesorption of NH3 does not show influence of the molybdenum additions on the acidity of catalysts surface.
Small quantities of Mo cause a significant increases in resistance to coking of Ni catalysts for steam reforming reaction of hydrocarbons. The effect of decreasing of the coking rate of Ni-Mo catalysts depends on the reagent ratio (H2O/C) [5,6]. It suggests noticeable interaction of these catalysts with steam. For unreduced catalysts of series A, it was found out that the amount of desorbed steam increases for Ni-Mo samples at temperatures higher than 773K. For reduced catalysts of series B, it was found out, by the grawimetric method at 773K, in the mixture H2O+H2 in hellium, that the weight increases in time. These increases depend on Mo contents.
There is no significant influence of the presence and amount of the promoter on the nickel catalysts activity in the n-butane steam reforming at temperature 500°C and 600°C, respectively. Study of the catalysts activity at temperature 500°C, in the mixture of steam reforming with various amounts of hydrogen added, show the influence of hydrogen on the selectivity of the reaction.
Obtained results were employed for the interpretation of the influence of small Mo additions on nickel catalysts properties in the reaction of steam reforming of hydrocarbons.
References
1. J.R. Rostrup-Nielsen, Catal.Today, 18 (1993) 305
2. I. Alstrup, B.S. Clausen, C. Olsen, R.H.H. Smits, J.R. Rostrup-Nielsen, Natural Gas Conversion V, (A.Parmaliana et al. Eds.), Elsevier, 1998, 5
3. D.L. Trimm, Catal.Today, 49 (1999) 3
4. S.P.S. Andrew, Ind.Eng.Chem. Prod.Res.Dev., 8 (1969) 321
5. T. Borowiecki, A. Gołębiowski, Catal.Lett., 25 (1994) 309
6. T. Borowiecki, A. Gołębiowski, B. Stasińska, Appl.Catal., A General, 153 (1997) 141
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