Correlation between Fischer-Tropsch catalytic activity and composition of catalysts

Ali et al. Chemistry Central Journal 2011, 5:68 http://journal.chemistrycentral.com/content/5/1/68 RESEARCH ARTICLE Open Access Correlation between Fischer-Tropsch catalytic activity and composition of catalysts Sardar Ali1, Noor Asmawati Mohd Zabidi2* and Duvvuri Subbarao1 Abstract This paper presents the synthesis and characterization of monometallic and bimetallic cobalt and iron nanoparticles supported on alumina. The catalysts were prepared by a wet impregnation method. Samples were characterized using temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), COchemisorption, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM-EDX) and N2-adsorption analysis. Fischer-Tropsch synthesis (FTS) was carried out in a fixed-bed microreactor at 543 K and 1 atm, with H2/CO = 2 v/v and space velocity, SV = 12L/g.h. The physicochemical properties and the FTS activity of the bimetallic catalysts were analyzed and compared with those of monometallic cobalt and iron catalysts at similar operating conditions. H2-TPR analysis of cobalt catalyst indicated three temperature regions at 506°C (low), 650°C (medium) and 731°C (high). The incorporation of iron up to 30% into cobalt catalysts increased the reduction, CO chemisorption and number of cobalt active sites of the catalyst while an opposite trend was observed for the iron-riched bimetallic catalysts. The CO conversion was 6.3% and 4.6%, over the monometallic cobalt and iron catalysts, respectively. Bimetallic catalysts enhanced the CO conversion. Amongst the catalysts studied, bimetallic catalyst with the composition of 70Co30Fe showed the highest CO conversion (8.1%) while exhibiting the same product selectivity as that of monometallic Co catalyst. Monometallic iron catalyst showed the lowest selectivity for C5+ hydrocarbons (1.6%). 1. Background Fischer-Tropsch synthesis (FTS) is a process which deals with the conversion of syngas derived from coal, biomass and natural gas into hydrocarbons consisting of paraffins, olefins, alcohols and aldehydes with a high cetane number and is environmentally friendly [1]. Due to limited petroleum reserves and environmental restrictions, Fischer-Tropsch synthesis (FTS) is gaining more attention nowadays than ever. FTS is considered as a surface-catalyzed polymerization reaction. During this process, CO is adsorbed on the surface of the transition metal and hydrogenated producing CH x monomers which consequently propagate to produce hydrocarbons and oxygenates with a broad range of functionalities and chain lengths [2]. All the group VIII elements show considerable activity for this process. Among them Co, Fe and Ru present * Correspondence: 2 Department of Fundamental and Applied Sciences, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia Full list of author information is available at the end of the article © 2011 Ali et al the highest activity [3]. Due to high activity for FischerTropsch synthesis, high selectivity to linear products, more stability towards deactivation, low activity towards water-gas shift (WGS) reaction and low cost compared to Ru, cobalt-based catalysts are the preferred catalyst for Fischer-Tropsch synthesis [4,5]. In order to enhance the catalytic activity and stability, different combinations of these active metals have been reported such as Co-Fe [6], Co-Mn [7] and Fe-Mn [8]. It has been reported that the addition of two active FTS metals gave additional properties which are quite different than the one expected for monometallic catalysts. The physical and chemical properties of Co/Fe systems have been discussed by Guerrero-Ruiz et al. [9]. In many heterogeneous reactions, these active phases are dispersed on a support which not only acts as a carrier but may also contribute to the catalytic activity. Al 2 O 3 , SiO 2 and TiO2 are the commonly used supports for cobalt-based catalysts [10,11]. The incorporation of a second metal component into the catalyst may result in the geometric or electronic Ali et al. Chemistry Central Journal 2011, 5:68 http://journal.chemistrycentral.com/content/5/1/68 modifications of the catalyst which may result in the modification of adsorption characteristics of the catalysts’ surface and in some cases alter the reduction and deactivation behavior. Bimetallic catalysts possess different physicochemical properties than that of the monometallic catalysts. Pena O’Shea et al. [12] studied the activity of Co-Fe/SiO2 in a fixed-bed and slurry reactors. They reported that bimetallic catalysts were more active and stable than the monometallic catalysts in the fixedbed reactor. An opposite behavior was observed for the slurry reactor. The performance of Co-Fe/TiO2 [13] catalyst has also been reported. The performance of the bimetallic on alumina has not that much been reported in the literature. In this paper we wish to report our preliminary study on the synthesis of mono- and bimetallic catalysts of Co and Fe. The effects of incorporating Fe into Co on the physicochemical properties of the alumina-supported catalysts in terms of degree of reduction, CO and H2 chemisorptions, metal particle size, textural properties and their selectivities and activities in the FTS are presented. 2. Results and Discussion 2.1 Morphology of catalysts Figure 1 depicts the representative FESEM image of calcined Co/Al2O3 catalyst. The metal particles are indicated by arrows. FESEM analysis revealed that the metal Figure 1 Representative FESEM image of the 5wt%Co/Al2O3 catalyst. Page 2 of 8 particles were evenly distributed on the alumina support. Addition of iron in lower amounts (up to 30%) did not change the morphology of the catalysts. Figure 2 shows the TEM images of monometallic and bimetallic catalysts on Al2O3 support. The average size of metaloxide particles was calculated from the TEM images. The average particle size of Co and Fe was found to be 13 nm and 10 nm, respectively. Incorporation of iron into Co resulted in an increase in the average metal particle size where for the 50Co50Fe on Al2O3, the average size of metal particles was found to be 18 nm. 2.2 Textural properties of the catalysts BET specific surface area, total pore volume and average pore diameter of the samples are listed in Table 1. All the samples exhibited type IV isotherm at high relative pressures (p/po), which is typical of mesoporous materials. There was a significant decrease in BET specific surface area and pore volume for alumina after metal impregnation. The specific surface area of alumina was found to be 190 m 2 /g while its pore volume was 0.1 cm3/g with a pore diameter of 9.8 nm. The surface area of Co/Al2O3 and Fe/Al2O3 was 180 m2/g and 165 m2/g, respectively. Pore volume and pore size of these catalyst samples were found to be less than those of the alumina. The decrease in BET specific surface area, total pore volume and pore diameter after the impregnation A (...truncated)