Studies Of The Molybdenum Migration From Ni/mo Catalysts In The Presence Of Steam
STUDIES OF THE MOLYBDENUM MIGRATION FROM Ni/Mo CATALYSTS IN THE PRESENCE OF STEAM
W. Grzegorczyk, A. Denis, T. Borowiecki
Department of Chemical Technology, Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
Migration of catalysts constituents in the stream of gaseous reagents is an undesirable phenomena. It causes material loses of valuable components, deterioration of catalysts’ properties (deactivation) and the other disturbances of installation. An introduction of molybdena compounds as a promoter of nickel catalysts for steam reforming of higher hydrocarbons, which decreases coking phenomena [1], state the question about this effect stability.
This is well known, that MoO3 undergo sublimation at higher temperatures. It seems, that under the real process conditions the migration of Mo will depend on the interaction with the support, and will be influenced by the reaction mixture on the chemical form of the promoter. We have found, that in Ni-Mo catalysts under conditions of the steam reforming reaction, MoO3 undergoes reduction [2]. However, it was necessary to verify the influence of the H2O partial pressure on the dynamics of molybdena migration phenomena.
Experimental
Studies were conducted over the model co-precipitated 70 NiO + 30 Al2O3 (wt. %) catalysts promoted by molybdena (5 and 10 wt. %), which was introduced by impregnation from ammonium heptamolybdenate aqueous solution. Experiments were carried out with unreduced and reduced at 800° C samples.
Investigations were maintained by the gravimetric method utilising TG121 (CAHN) microbalance under dynamic conditions in a quartz reactor (under the flow of H2O+He mixture at 800° C). Experiments were conducted with a varied mole fraction of steam in the reaction mixture from 0.1 to 0.8.
It was found, that molybdena migration from the catalyst surface occurs only in the presence of steam. During keeping oxidized form of the catalyst sample (after oxidative treatment at 800° C) in the stream of pure helium, loss of catalyst mass (i.e. molybdena removal) is not observed.
In the stream of gaseous reagents the sublimation ability and migration from the catalyst surface has only molybdena trioxide. Even careful catalyst reduction prevents this process. During heating of the reduced form of the catalyst in the stream of mixture contained hydrogen (pH2 = 0.05) the loss of molybdena was not observed.
In the case of oxidized catalysts samples (pH2O = 0.1) the migration rate of molybdena depends on its contents. Because dispersion degree of MoO3 does not change in the both of the investigated samples, so the factor determining the dynamics phenomena could be the surface area directly accessible for the gas phase.
It was noticed, that migration rate of molybdena depends on the partial pressure of steam, which is in an equilibrium over the catalyst surface.
Under assumption of the formal kinetic equation where the process rate is determined by the stage of steam adsorption and structure destroying of orthomolybdenate [MoO4]2-:
H2O
[MoO42-]s Þ MoO2(OH)2 Þ MoO3 (1)
the following kinetic equation for the investigated catalyst type was introduced:
r = 9.46 · 10-5 · (XH2O)0.6 [mol/h gcat] (2)
where: XH2O – mole fraction of steam in the reaction mixture
References
1. T. Borowiecki, A. Gołębiowski, B. Stasińska, Appl. Catal. A: Gen., 153 (1997) 141.
2. T. Borowiecki, A. Denis, W. Gac, J. Gryglicki, B. Stasińska, Zeszyty Naukowe Polit. Łódzkiej – Chemia, z.47, 1999, 41.
Related articles::