The effect of the iron precursor on the thermal decomposition of iron oxyhydroxides was studied by DSC, DTA and TG in this work. Samples were prepared from iron nitrate, iron sulfate and iron chloride and the thermal curves obtained were analyzed by specific area measurements, X-ray diffraction and Mössbauer spectroscopy. It was found that the iron oxyhydroxide precursors affect the temperatures of the hematite formation as well as the textural properties of the final hematite producing particles with different diameters as following: iron sulfate (3.3 nm)&lt;&lt;iron nitrate (15 nm)&lt;iron chloride (24 nm).Centro de Investigación y Desarrollo en Ciencias Aplicada

Carmo Rangel, M. do

Marchetti, Sergio Gustavo

Oliveira, Alcineia C.

Journal of Thermal Analysis and Calorimetry

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Journal of Thermal Analysis and Calorimetry, Vol. 73 (2003) 233–240THE EFFECT OF THE STARTING MATERIAL ON THETHERMAL DECOMPOSITION OF IRONOXYHYDROXIDESA. C. Oliveira 1, G. S. Marchetti 2 and M. do Carmo Rangel 1*1GECCAT, Instituto de Química, Universidade Federal da Bahia, Campus Universitário de Ondina,40170-290 Salvador, Ba, Brazil2CINDECA, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900, 47 y 115,La Plata, Argentina(Received August 2, 2002; in revised form February 7, 2003)AbstractThe effect of the iron precursor on the thermal decomposition of iron oxyhydroxides was studied byDSC, DTA and TG in this work. Samples were prepared from iron nitrate, iron sulfate and iron chlorideand the thermal curves obtained were analyzed by specific area measurements, X-ray diffraction andMössbauer spectroscopy. It was found that the iron oxyhydroxide precursors affect the temperatures ofthe hematite formation as well as the textural properties of the final hematite producing particles withdifferent diameters as following: iron sulfate (3.3 nm)<<iron nitrate (15 nm)<iron chloride (24 nm).Keywords: Fe(III)hydrite, hematite, iron chloride, iron nitrate, iron oxide, iron sulfateIntroductionIron oxides are an important group of catalysts due to their chemical properties and lowcost. They have been extensively studied for several applications [1–3]. In the catalysisfield, they have been used in many industrial processes as ammonia synthesis [4], wa-ter-gas shift reaction [5, 6] and Fischer–Tropsch synthesis [7] among others.In all cases, the thermal methods can provide useful information about the char-acteristics and properties of the catalysts which in turn determine their applications aswell as the best conditions of their operation. One of the most important study con-cerning the thermal analysis of catalysts is related to the monitoring of the catalystproduction from its precursor. The profile of the thermal curves can often be used toget information about the textural and other characteristics of the catalysts which inturn can be related to catalytic properties. However, the iron precursor can affect thethermal curves and thus make this analysis more difficult.In order to overcome this problem, the effect of the iron precursor on the thermalcurves was studied in this paper. Samples were prepared from iron nitrate, iron sul-1388–6150/2003/ $ 20.00© 2003 Akadémiai Kiadó, BudapestAkadémiai Kiadó, BudapestKluwer Academic Publishers, Dordrecht* Author for correspondence: E-mail: mcarmov@ufba.brfate and iron chloride and the thermal curves were analyzed with the aim of chemicalanalysis, specific area measurements, X-ray diffraction and Mössbauer spectroscopy.ExperimentalSamples were prepared by precipitation techniques at room temperature using ironnitrate (N sample), iron sulfate (S sample) and iron chloride (C sample). Aqueous so-lutions of the iron precursor (1N) and of ammonium hydroxide (25% w/w) wereadded to a beaker with water, under stirring. The final pH was adjusted to 11 and thesystem was kept under stirring for 30 min. The sol produced was centrifuged(2000 rpm, 5 min), rinsed with water to remove the anions from the starting materialand dried at 120°C.The DSC experiments were carried out in a DSC/DTA/TG 50 model Shimadzuequipment, under nitrogen flow (100 mL min–1) at a heating rate of 10° min–1 fromroom temperature up to 600°C, using around 2 mg of the sample in a closed crucible.TG and DTA experiments were performed in the same equipment and in the sameconditions but heating the samples up to 1000°C.In order to identify the peaks of the DSC curves some values of temperatureswere selected, as shown in Fig. 1. The solids were heated, in independent experi-ments, at 10° min–1 and kept at the desired temperature for 2 h, under nitrogen flow(100 mL min–1). The samples were characterized by chemical analysis (N, S, Cl), spe-cific area measurements, X-ray diffraction and Mössbauer spectroscopy.The sulfur and nitrogen contents in the solids were determined using a modelFlashEa 1119 Thermofinnigan equipment. Samples (15 mg) were dried at 90°C for2 h, mixed with vanadium pentoxide (15 mg) and then analyzed. Before the analysis,a calibration curve was obtained using 2 mg of sample in a tin capsule. The chlorideamount was measured by the Volhard method [8]. A silver nitrate solution was addedto the sample previously dissolved in concentrated nitric acid until a white precipi-tated is produced. The solution was titrated with a sodium thiocianate solution.X-ray diffractograms were got at room temperature with a model XD3A Shimadzuinstrument using CuKα radiation generated at 40 kV and 20 mA. The specific area(BET method) were measured in a model TPD/TPO 2900 Micromeritics equipment fromsamples previously heated under nitrogen (150°C, 1 h).The Mössbauer spectra were obtained in transmission geometry, with a512-channel constant acceleration spectrometer at room temperature. A source of57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performedvs. a 12 µm thick α-Fe foil. All isomer shifts mentioned in this paper are referred tothis standard at 298 K. The temperature between 22 and 298 K was varied using aDisplex DE-202 Closed Cycle Cryogenic System. The spectra were evaluated by us-ing a least-squares nonlinear computer fitting program with constraints. Lorentzianlines were considered with equal widths for each spectrum component. The spectrawere folded to minimize the geometric effects. All spectra were fitted with a programincluding hyperfine-parameters distributions to account for the different particlesizes and/or crystallographic sites of the iron phases.J. Therm. Anal. Cal., 73, 2003234 OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDESResults and discussionFigure 1 shows the DSC curves with the selected temperatures, the specific areas andthe results of chemical analysis of the solids heated at each temperature. It can beseen that the specific area of the sample prepared from iron nitrate (N sample) de-creased with the increasing temperature until 310°C; however it increased with ahigher temperature achieving a value of 142 m2 g–1 which is close to the area of thesolid heated at 150°C. The X-ray diffractogram of this sample heated at 150°C dis-played two amorphous halos as shown in Fig. 2. The Mössbauer spectra of N150 con-sist of a quadrupole-split doublet, at 298 K, and a sextet at 22 K. Both signals havebroad lines. Their parameters (Tables 1a and 1b) can be assigned to a ‘poorly-crystallized’ Fe(III)hydrite and/or lepidocrite (γ-FeOOH) [9, 10]. Considering: (i) thepresence of hematite (α-Fe2O3) at a temperature higher than 150°C and (ii) the struc-J. Therm. Anal. Cal., 73, 2003OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDES 235Fig. 1 DSC curves of iron oxyhydroxide prepared with a – iron nitrate (N sample),b – iron sulfate (S sample) and c – iron chloride (C sample). The specific area ofthe solids and the amounts of nitrogen (N), sulfur (S) and chloride (Cl) at se-lected temperatures are also shownture of Fe(III)hydrite is related to that of hematite, with hexagonal close packing ofoxygen and Fe3+ occupying the octahedral interstices, we can conclude that N150sample is Fe(III)hydrite. By comparing these results with the TG profile (Fig. 3a) andwith the Mössabuer spectra of the sample before heating, we can see that there is astrong loss of mass (41.204%) but no phase change. Therefore, the first endothermicpeak can be assigned to the loss of volatile compounds in accordance with otherworks [11, 12]. Similar X-ray pattern, Mössbauer spectra and loss of mass werefound for N250. The increase of the hyperfine magnetic field (Table 1b) and the de-crease of the width lines in the Mössbauer spectrum at 22 K indicate an increase ofthe crystallinity of the sample. Besides, a decrease of specific area was noted, show-ing that the coalescence of particles and/or porous occurred simultaneously with theloss of volatile. At 310°C, the solid produced an X-ray diffractogram with somepeaks typical of hematite. The Mössbauer spectrum of the N310 sample at 298 Kshows a central quadrupole doublet and a magnetic sextet. The parameters of the sex-tet correspond to hematite (Table 1a). This signal represents 12% of the spectrum. At22 K it is not possible to distinguish the presence of α-Fe2O3 due to the broadFe(III)hydrite sextet. Therefore, we can deduce that the shoulder detected by DSC, inthe range of 200–300°C (Fig. 1a), is related to the beginning of the hematite forma-tion. At 510°C, the solid showed the X-ray pattern of hematite and it was confirmedJ. Therm. Anal. Cal., 73, 2003236 OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDESFig. 2 X-ray diffactograms of iron oxyhydroxide prepared with a – iron nitrate (N sam-ple), b – iron sulfate (S sample) and c – iron chloride (C sample) after heating atselected temperaturesby the Mössbauer spectrum at 298 K, as it is shown in Table 1. It means that the peakcentered at 450°C, in DSC pattern, can be assigned to the crystallization of hematite.As we can see from the TG curve, this process is not followed by the loss of mass. Bycomparing the DSC curve with the DTA one (Fig. 3b), we can see that the shoulderdetected in the first case can be better seen in the DTA curve, as a peak at 210°C,showing that DTA is a more sensitive technique than DSC to detect the beginning ofthe hematite formation. The first DTA peak or the shoulder DSC can be assigned tothe production of hematite on the surface of the particles since it is expected that thereaction begins in the border of the particle [13]. To estimate roughly the average sizeof the N410 particles we applied the Collective Magnetic Excitation (CME) model[14], assuming that all the magnetic hyperfine field diminution, in comparison withα-Fe2O3 bulk, is originated in the reduced crystalline size. The average size obtainedfollowing this methodology was about 15 nm.J. Therm. Anal. Cal., 73, 2003OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDES 237Fig. 3a TG curves of iron oxyhydroxide prepared with iron nitrate (N sample), ironsulfate (S sample) and iron chloride (C sample)Fig. 3b DTA curves of iron oxyhydroxide prepared with iron nitrate (N sample), ironsulfate (S sample) and iron chloride (C sample)Table 1a Mössbauer parameter of the sample prepared from iron nitrate (N sample) taken fromthe spectrum recorded at 298 KSpecies Parameters N150 N250 N310 N410Fe(III)hydrite (sp) ∆/mm s–1 0.69±0.01 0.72±0.01 0.75±0.01 –≈Fe5(OH)8⋅4H2O δ/mm s–1 0.30±0.01 0.30±0.01 0.28±0.01 –α-Fe2O3H/T – – 502±1 504±1δ/mm s–1 – – 0.34±0.02 0.31±0.012ε/mm s–1 – – –0.26±0.04 –0.22±0.04H – hyperfine magnetic field (Teslas); δ – isomer shift (all the isomer shifts are referred to α-Fe at298 K); 2ε – electrical quadrupole interaction; ∆ – quadrupole splitting, (sp) – superparamagneticTable 1b Mössbauer parameter of the sample prepared from iron nitrate (N sample) taken fromthe spectrum recorded at 22 KSpecies Parameters N150 N250 N310Magnetic Hmax/T 453±1 477±1 484±1Fe(III)hydrite <H>/T 415±1 459±1 479±1≈Fe5(OH)8⋅4H2O δ/mm s–1 0.46±0.01 0.46±0.02 0.46±0.012ε/mm s–1 –0.01±0.01 –0.02±0.01 –0.02±0.02Hmax – maximum of the distribution of the hyperfine magnetic field (Teslas);<H> – average of the distribution of the hyperfine magnetic field (Teslas);δ – isomer shift (all the isomer shifts are referred to α-Fe at 298 K);2ε – electrical quadrupole interactionThe sample prepared from iron sulfate showed a DSC curve with an endothermicpeak at temperatures below 150°C and a shoulder at around 250°C. No significantchange of surface area was noted in this range of temperature. From the DTA curve onecan detect a peak which can be assigned to the loss of volatile in accordance with the TGcurve. The X-ray diffractograms of the S150 and S310 samples showed two amorphoushalos as shown in Fig. 2. The Mössbauer spectra of these samples showed a quadrupoledoublet at room temperature and a very broad magnetic sextet at 22 K. Analyzing theseresults in a similar way to the N samples, we conclude that S150 and S310 are alsoFe(III)hydrite (Tables 2a and 2b). However, the particle sizes and the crystallinities arelower than that the corresponding N samples. This conclusion is supported by thehyperfine magnetic fields in the S150 and S310 samples which are lower, and whoselines are wider, than in the N150 and N310 samples.Heating the solid at 510°C, one can detect hematite by X-ray technique (Fig. 2).The Mössbauer spectrum of S510 at 298 K shows a central doublet and a magneticsextet. Analyzing the Mössbauer parameters (Table 2a), the sextuplet can be assignedto hematite [10], while the central doublet can be attributed to very small hematiteparticles with a superparamagnetic relaxation. The last assignment was confirmed bythe spectrum at 22 K (Table 2b). It displayed two magnetic signals, assignable to the‘core’ and the ‘shell’ of hematite nanoparticles [15]. Assuming homogeneous spheri-J. Therm. Anal. Cal., 73, 2003238 OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDEScal particles and using the ratio of the areas of the two sextuplets, it was possible toestimate an average nanoparticle diameter of 3.3 nm [15].The sample prepared from iron chloride also showed a DSC curve with an endo-thermic peak at temperatures below 150°C (Fig. 1). The sample heated at this temper-ature showed a specific area of 192 m2 g–1 and the X-ray diffractogram showed twoamorphous halos (Fig. 2). The Mössbauer spectrum of C410 at room temperatureshows a central doublet and a magnetic sextet. The magnetic fraction represents 18%of the total area, and analyzing their Mössbauer parameters this signal can be as-signed to hematite (Table 3). The central doublet would be a very small superpara-magnetic hematite particles or Fe(III)hydrite. The Mössbauer spectrum of C510 atroom temperature only shows a signal magnetically splitted with hyperfine parame-ters typical of hematite [10] (Table 3). Applying the CME model an average particlesize of about 24 nm was obtained. From 150 to 410°C, there was a strong loss of massand at this temperature the solid showed a smaller specific area showing that this pro-cess was followed by coalescence of particles and porous. In the range of 410 to510°C no significant loss of mass and change in the specific area were noted.The amounts of nitrogen, sulfur and chloride in the samples are shown in Fig. 1.It can be seen that very small amounts of nitrate remained in the solid heated at 150°Cand there was no nitrate at 450°C. This can be assigned to the weakness of theFe-NO3– bonds in solids. On the other hand, chloride and sulfate are strongly boundedand will not be easily removed from the solids, as shown the results of chemical anal-ysis (Fig. 1). These results suggest that sulfur and chloride acts as impurities affectingthe thermal decomposition of iron oxides. This can be explained by considering thatthey decrease the mobility of the species for reacting and crystallizing.ConclusionsThe use of different precursors affects the thermal curves of the thermal decomposi-tion of iron oxyhydroxides. When iron nitrate is used, the production of hematite be-gins at around 250°C from Fe(III)hydrite; it would correspond to the oxide producedJ. Therm. Anal. Cal., 73, 2003OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDES 239Table 2 Mössbauer parameter of the sample prepared from iron chloride (C sample) taken fromthe spectrum recorded at 298 KSpecies Parameters C150 C410 C510α-Fe2O3 (sp)∆/mm s–1 – 0.67±0.01 –δ/mm s–1 – 0.34±0.01 –α-Fe2O3H/T 511±1 504±1 514±1δ/mm s–1 0.37±0.01 0.37±0.01 0.37±0.012ε/mm s–1 –0.22±0.01 –0.22±0.02 –0.22±0.01H – hyperfine magnetic field in Teslas; δ – isomer shift (all the isomer shifts are referred to α-Fe at298 K); 2ε – electrical quadrupole interaction; ∆ – quadrupole splitting, (sp) – superparamagneticin the external part of the particle and can be detected as a shoulder in the DSC curveand as a peak in DTA curve. When the precursors of the iron oxyhydroxides are sul-fate or chloride the beginning of hematite formation occurs at higher temperatures.This can be assigned to strong Fe-sulfate and Fe-chloride bonds which decrease themobility of the species for reacting and crystallizing. Besides, there is a pronouncedeffect of the iron precursor on the final particle diameters. Thus:Dsulfate precursor(3.3 nm)<<Dnitrate precursor (15 nm)<Dchloride precursor (24 nm)Therefore, it was demonstrated the influence of the iron oxyhydroxide precur-sors on the textural properties of the final hematite, in accordance with previousworks carried out with other precursors [16, 17]. This conclusion is very important intechnologic applications of the hematite, for example, in some chemical reactions inwhich the particle size plays an essential role when this solid is used as a catalyst.* * *The authors thank the financial support from PADCT/FINEP/CNPq. A.C.O. acknowledges CNPqfor her graduate scholarship.References1 A. Navrotsky, J. Therm. Anal. Cal., 57 (1999) 653.2 J. Orewczk, J. Therm. Anal. Cal., 60 (2000) 265.3 K. Przepiera and A. Przepiera, J. Therm. Anal. Cal., 65 (2001) 497.4 A. Nielsen and H. Bohlbro, An Investigation on Promoted Iron Catalysts for the Synthesis ofAmmonia, Gjellerup Forlag, Copenhagen 1968, p. 221.5 M. V. Twigg, L. Lloyd and D. E. Ridler, Catalyst Handbook, Manson Publishing Ltd.,London 1996, p. 339.6 J. L. R. Costa, G. S. Marchetti and M. C. Rangel, Catal. Today, 77 (2002) 205.7 M. E. Dry, The Sasol Fisher–Tropsch Processes in B. F. Leach (Ed.), Applied Industrial Catal-ysis, Academic Press, New York 1983, p.177.8 J. Bassett, R. C. Denney, G. H. Jeffery and J. Mendhan, Vogel’s A. J.: A Textbook of PracticalInorganic Qualitative Chemistry, Longman, 1978, p. 372, 446.9 E. Murad, L. H. Bowen, G. J. Long and T. G. Quin, Clay Minerals, 23 (1988) 161.10 R. E. Vandenberghe, Mössbauer Spectroscopy and Applications in Geology, InternationalTraining Centre for Post-Graduate Soil Scientists, Belgium 1991.11 M. C. Rangel and F. Galembeck, J. Catal., 145 (1994) 364.12 Y. Ichiyanagi and Y. Kimishima, J. Therm. Anal. Cal., 69 (2002) 919.13 R. Furuichi, M. Hachia and T. Ishii, Thermochim. Acta, 4 (1988) 13.14 S. MÝrup and H. TopsÝe, Appl. Phys., 11 (1976) 63.15 M. Vasquez-Mansilla, R. D. Zysler, C. Arciprete, M. I. Dimitrijewits, C. Saragovi andJ. M. Greneche, J. Magn. Magn. Mater., 204 (1999) 29.16 I. Mitov, D. Paneca and B. Kunev, Thermochim. Acta, 386 (2002) 179.17 N. Koga, S. Takemoto, T. Nakamura and H. Tanaka, Thermochim. Acta, 282 (1996) 81.J. Therm. Anal. Cal., 73, 2003240 OLIVEIRA et al.: DECOMPOSITION OF IRON OXYHYDROXIDES

The effect of the starting material on the thermal decomposition of iron oxyhydroxides

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The effect of the starting material on the thermal decomposition of iron oxyhydroxides

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