INFLUENCE OF PREPARATION METHOD OF Fe3O4-Cr2O3 CATALYSTS FOR WATER GAS SHIFT REACTION
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: Water gas shift, Membrane reactor, Catalysts
Water gas shift reaction (CO + H2 ↔ CO2 + H2O) is an important step in the industrial manufacture of high-purity hydrogen. The reaction is exothermic and reversible, and is usually carried out in two stages to overcome equilibrium limitation. The first stage involves a high temperature (HTWGS ) catalyst based on Fe3O4/Cr2O3 while the second one uses a low temperature catalyst based on Cu/ZnO/Al2O3. Another approach is to use a membrane reactor, so hydrogen can be continuously removed. This allows carrying out this process in a single stage using a high temperature water gas shift (WGS) catalyst.
The active phase in HTWGS catalyst is magnetite (Fe3O4), which in absence of chromium oxide rapidly loses its activity due to sintering. There is some controversy about the role of Cr2O3 in the stabilisation of the catalyst structure. Some researchers have proposed that the reduced catalyst forms an inverse spinel-type structure with Cr3+ ions in solid solution within the Fe3O4 lattice [1]. On the other hand, several authors suggest that Cr2O3 appears as fine particles that physically block Fe3O4 sintering [2].
In this work, we study the oxiprecipitation method to obtain WGS catalysts directly in active phase. These materials are compared to those obtained by the usual method which consists of coprecipitation, thermal treatment and reduction.
The solids obtained by oxiprecipitation were prepared through a previously reported procedure [3]. The variables studied were pH, air flowrate and temperature. The materials obtained by coprecipitation were prepared from FeCl3 solution or FeCl3 and FeCl2 solution and NaOH as precipitating agent. The solid was filtered and washed out with demineralised water. Then it was dried at 110ºC and calcined at 250 ºC or 600 ºC depending on the experiment. Catalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), nitrogen adsorption/desorption at 77 K, Raman spectroscopy and temperature programmed reduction (TPR). From these analyse, the best oxiprecipitation conditions were pH = 7, air flowrate = 1,1 L/min and T = 70ºC.
The solids obtained by coprecipitation were synthesized as α-Fe2O3 or γ-Fe2O3 and partially reduced to active phase (Fe3O4). The samples prepared by oxiprecipitation and coprecipitation-reduction showed the typical IR band of magnetite (565 cm-1). TEM and N2 adsorption results showed that oxiprecipitation materials were composed of Fe3O4 crystals surrounded by smaller high chromium containing particles and high BET surface (>100 m2/g). In the case of coprecipitated samples, Cr2O3 was in solid solution and BET surface was lower as result of thermal treatment.
[1] H., Topsöe; M., Boudart. “Moessbauer spectroscopy of carbon monoxide shift catalysts promoted with lead”. Journal of Catalysis (1973), 31(3), 346-59.
[2] G. C., Chinchen; R. H., Logan; M. S., Spencer. “Water gas shift reaction over an iron oxide/chromium oxide catalyst. I: Mass Transport Effects”. Applied Catalysis, 12 (1984) 69-88.
[3] J. Dufour; C. Martos; A. Ruiz; A. Carrasco; M. Maroño. “Synthesis of Fe3O4-Cr2O3 catalysts by oxy-coprecipitation for water gas shift reaction”. 10th Mediterranean Congress of Chemical Engineering (2005).
ACKNOWLEDGEMENT: The Ministry of Education and Science of Spain is acknowledged for funding through project ENE2004-07758-C02-02.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
