RKCL4193 CATALYTIC ACTIVITY OF ALUMINIUM AND COPPER-DOPED MAGNETITE IN THE HIGH TEMPERATURE SHIFT REACTION

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1 Jointly published by React.Kinet.Catal.Lett. Akadémiai Kiadó, Budapest Vol. 79, No. 1, and Kluwer Academic Publishers, Dordrecht (2003) RKCL4193 CATALYTIC ACTIVITY OF ALUMINIUM AND COPPER-DOPED MAGNETITE IN THE HIGH TEMPERATURE SHIFT REACTION Alexilda Oliveira de Souza and Maria do Carmo Rangel GECCAT- Instituto de Química, UFBA, Campus Universitário de Ondina, Federação Salvador, Ba, Brazil. Received August 21, 2003 In revised form January 6, 2003 Accepted January 13, 2003 Abstract Aluminium and copper-doped magnetite was evaluated as high temperature shift catalyst and compared with a hematite-based sample. The first one is less active but can save energy in industrial processes. Keywords: Hematite, magnetite, hydrogen production INTRODUCTION The demand for high-purity hydrogen for industrial applications is largely met by the water-gas shift reaction (WGSR). The reaction is often performed in two steps to achieve rates for commercial purposes. The first stage (high temperature shift, HTS) is carried out in the range of K while the other (low temperature shift, LTS) is performed at K [1,2]. The classical industrial HTS catalysts contain iron oxide (hematite) and chromium oxide which acts as a stabilizer, retarding sintering and the loss of surface area [1-3]. Before the HTS catalysts can be used, hematite (α-fe 2 O 3 ) is converted to magnetite (Fe 3 O 4 ), the active phase. This reaction is carried out with the process gas (a mixture with composition around 10%CO, 10%CO 2, 60%H 2 and 20%N 2 ) and should be controlled to avoid further reduction of magnetite to metallic iron. It would promote undesirable reactions such as /2003/US$ Akadémiai Kiadó, Budapest. All rights reserved.

2 176 SOUZA, RANGEL: HEMATITE methanation and carbon monoxide disproportionation [3]. In order to ensure the magnetite stability in industrial processes, large amounts of steam are used. However, this procedure increases the operational costs and thus generates the need of developing catalysts in the active phase [2]. The production of a catalyst in magnetite form may increase the process efficiency not only because of the energy saved but also due to the increase of the life of the catalyst. With this goal in mind, this work deals with the evaluation of aluminium and copperdoped magnetite in the HTS reaction. Aluminium is an efficient promoter in iron oxide as shown in previous works [4] and copper is a new promoter, recently added to commercial catalysts [5]. The samples were compared to a hematite-based solid, which has proven to be a promising catalyst to HTS reaction [4]. EXPERIMENTAL All reagents used were of analytical grade. The catalysts were prepared by coprecipitation techniques at room temperature, followed by heating at 500 o C for 2 h under nitrogen flow (to produce magnetite) or air flow (to produce hematite). The following samples were prepared as hematite (I=H) or magnetite (I = M), using an iron to dopant molar ratio of 10: (i) with aluminium and copper (IAC sample); (ii) with only aluminium (IA sample); (iii) with only copper (IC sample) and (iv) without any dopant (I sample). The aluminium and copper-based catalysts were prepared by adding aqueous solutions of Fe(NO 3 ) 3.9H 2 0 (1N) and Al(NO 3 ) 3.9H 2 O (0.1 N) and a concentrated (25 wt.%) aqueous solution of ammonium hydroxide to a beaker with water, under stirring. The final ph was adjusted to 11. After 30 min, the sol was centrifuged and rinsed with a 5 wt.% ammonium acetate solution and centrifuged again. This procedure was repeated until the nitrate ion was not noted anymore. The gel was then impregnated with an aqueous solution of Cu(NO 3 ) 2.3H 2 O (0.06N) for 24 h under stirring, centrifuged again and dried in an oven at 120 o C. The same procedure was used to prepare the other samples. After each centrifugation, the supernatant was analyzed to nitrate, by adding about 1 ml of concentrated sulfuric acid to 10 ml of the supernatant. The formation of [Fe(NO)] 2+ was detected by a brown ring [6]. The absence of nitrate in the solids was confirmed by infrared spectroscopy ( cm -1 ) using a model IR-430 Shimadzu spectrometer and KBr discs. The metal contents were determined by inductively coupled plasma atomic emission spectrometry (ICP/AES) in an Arl 3410 model equipment. X-ray diffractograms were recorded with a Shimadzu model XD3A instrument using CuKα radiation generated at 30 kv and 20 ma. The surface areas (BET

3 SOUZA, RANGEL: HEMATITE 177 method) were measured in a Micromeritics model TPD/TPO 2900 equipment on samples previously heated under nitrogen (150 o C, 2 h). The surface areas of the reduced catalysts were measured after heating the solids under a 50%H 2 /N 2 mixture (a composition close to the reaction atmosphere) at 370 o C, for 2 h. X- ray microanalyses (XRM) were carried out in a Noran microprobe coupled to a model JSM-T300 microscope operating at kv. The catalyst performance was evaluated using 0.2 cm 3 of powder within -50 and +325 mesh size and a fixed bed microreactor, avoiding any diffusion effect. The experiments were carried out for 6 h using a WHSV= h -1, under isothermal condition (370 o C) and atmospheric pressure. A gas mixture with composition around 10%CO, 10%CO 2, 50% H 2 and 30% N 2 and a steam to gas molar ratio of 0.6 were used. The conversion was kept less than 10%. The gaseous effluent was analyzed by on line gas chromatography, using a CG- 35 instrument. After each experiment, the Fe(II) content in the catalysts was determined to follow the iron reduction under reaction atmosphere. Samples were dissolved in concentrated hydrochloric acid, under carbon dioxide atmosphere and then titrated with potassium dichromate [6]. RESULTS AND DISCUSSION The Fe/Cu and Fe/Al molar ratios are shown in Table 1. It can be seen that both Fe/Al and Fe/Cu ratios are close to the expected one, showing that the experimental conditions promoted the simultaneous precipitation of iron and aluminium compounds since they precipitate in the same ph range and have similar hydrolysis constants [7]. Copper was mostly sorbed on the gel rather than complexed by the residual ammonium ions. This suggests that adsorption forces stronger than complexion forces were generated in the gel. Magnetite was detected in the samples heated under nitrogen flow and hematite was found in the solids heated under air flow, in accordance with previous works [4,5]. No other phase was detected. As the dopants have ionic radii similar to the iron atom, they are expected to go into the iron oxide lattice [8] rather than to segregate as another phase. The presence of aluminium or the two dopants increased the surface area of the magnetite-based solids (Table 1),which were not affected by doping the solid with only copper. Both dopants increased the surface area of the spent catalysts. A similar behavior was shown by the hematite-based samples. As a general tendency, the catalyst reduction decreased the surface area, showing that the catalysts went on sintering during the HTS reaction.

4 178 SOUZA, RANGEL: HEMATITE Table 1 Surface area (Sg) of aluminium and copper-doped iron oxides previously heated under nitrogen (fresh catalysts) and under a reducing gaseous mixture (reduced catalysts). M= magnetite; H= hematite. I sample: aluminium-doped iron oxide; IC sample: copper-doped iron oxide and IAC sample: aluminium and copper-doped iron oxide Sample Fe/Al (molar) Fe/Cu (molar) Sg (m 2 g -1 ) Sg (m 2 g -1 ) (+ 0.5) (+0.5) (fresh catalysts) (reduced catalysts) I=M I= H I= M I= H I= M I= H I= M I= H I IA IC IAC These results show that aluminium is able to delay sintering both in magnetite and hematite samples, in accordance with previous works [4,9,10] about the role of aluminium as a textural promoter in several catalysts. By comparing the analysis of the bulk of the solids (ICP/AES) and of the layers near the surface (XRM), in Table 2, one can see that most of aluminium is present in the bulk of the magnetite samples and thus its action cannot be related to the traditional role of spacer on the surface [10]. In this case, Table 2 XRM and ICP/AES results of the magnetite-based catalysts (MAC sample) and hematite-based catalyst (HAC) doped with aluminium and copper Sample Fe/Al Fe/Cu ICP/AES XRM ICP/AES XRM MAC HAC aluminium is supposed to cause strains in the lattice, shifting the equilibrium particle size toward smaller particles, since the ratio of strain to the surface effects becomes greater for larger particles [11]. However, in the hematite samples most of aluminium is present near the surface where it probably acts as a spacer. Also, copper tends to go inside the solid.

5 SOUZA, RANGEL: HEMATITE 179 All catalysts were active towards the HTS reaction (Table 3) and they showed different performances. Both aluminium and copper decreased the activity per area of magnetite and increased its surface area; these effects tend to compensate themselves and, as a result, the most active catalysts are those doped with both metals and with copper alone. In hematite-based catalysts, copper increased the activity per area while aluminium did the opposite. Both dopants increased the activity. In a general tendency hematite-based catalysts are more active than the magnetite-based ones. The Fe(II)/Fe(III) molar ratio remained close to the stoichiometric value of magnetite (0.5) showing that the catalysts were stable during the reaction. Copper is easier to reduce than iron and then the Fe(II)/Fe(III) ratio is higher in the samples doped with copper alone. In the aluminium and copper doped-samples aluminium tends to delay iron reduction as found previously [4]. Table 3 Catalytic activity (a), catalytic activity per area (a*) and Fe(II) to Fe(III) molar ratio of plain iron oxide (I sample), aluminium-doped iron oxide (IA sample), copper-doped iron oxide (IC sample) and aluminium and copper-doped iron oxide (IAC sample). M= magnetite; H= hematite Sample a x 10 4 a* x 10 5 Fe(II)/Fe(III) (mol g -1 h -1 ) (mol m -2 h -1 ) I= M I= H I= M I= H I= M I= H I IA IC IAC The most active catalyst was the aluminium and copper-doped hematite. This shows that hematite is more convenient than magnetite to produce these catalysts. However, the magnetite-based catalyst is only 27% less active than the hematite-based one and thus may be compensated by the saved energy related to the steam used in the reduction step. CONCLUSIONS Aluminium and copper-doped iron oxide can be prepared as hematite or magnetite which are both active in the HTS reaction. The hematite-based catalyst is 27% more active than the magnetite-based one and thus hematite is

6 180 SOUZA, RANGEL: HEMATITE the phase recommended to prepare these catalysts. However, magnetite has the advantage of avoiding the reduction step of the catalyst and thus can save energy in industrial processes. REFERENCES 1. D. S. Newsome: Catal. Rev.- Sci. Eng., 21, 275 (1980). 2. H. Bohlbro: An Investigation on the Kinetics of the Conversion of Carbon Monoxide with Water Vapor over Iron Oxide based Catalysts, p. 6. The Haldor Topsoe Laboratory, Copenhagen L. Lloyd, D. E. Ridler, M. V. Twigg: Catalysis Handbook, p.283. Manson Publishing Ltd, London G. C. Araújo, M.C. Rangel: Catal. Today, 62, 201 (2000). 5. J. L. Rangel; S. Marchetti, M. C. Rangel: Catal. Today, 77, 205 (2002). 6. A.I. Vogel: Quantitative Inorganic Analysis, p Longman, London J. Burgess: Metal Ions in Solution, p. 269, Wiley, New York M. E. Dry, L. C. Ferreira: J. Catal., 7, 352 (1967). 9. J. M. T. Souza, M. C. Rangel: React. Kinet. Catal. Lett., 77, 29 (2002). 10. A. Nielsen, H. Bohlbro: An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia, p Gjellerup Forlag, Copenhagen H. Topsoe, J. A. Dumesic, M. Boudart: J. Catal., 28, 477 (1978).