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Study of Fe–Co mixed metal oxide nanoparticles in the catalytic low-temperature CO oxidation
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 489–494
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Study of Fe–Co mixed metal oxide nanoparticles in the catalytic low-temperature CO oxidation Abolfazl Biabani-Ravandi a , Mehran Rezaei a,b,∗ , Zohreh Fattah a a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran b Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
a b s t r a c t Iron–cobalt mixed metal oxide nanoparticles (Co/Fe molar ratio: 1/5) have been prepared by a simple co-precipitation method and employed as catalyst in low-temperature CO oxidation. The prepared catalysts were characterized by thermal gravimetric and differential thermal gravimetric analyses (TGA/DTG), X-ray diffraction (XRD), temperature programmed reduction (TPR), N2 adsorption (BET) and transmission electron microscopy (TEM) techniques. The results revealed that inexpensive iron–cobalt mixed metal oxide nanoparticles have a high potential as catalyst in low temperature CO oxidation. The results showed that increasing in calcination temperature increased the crystallite and particle size and decreased the specific surface area, which caused a decrease in catalytic activity of prepared catalysts. In addition, the pretreatment conditions affect the catalytic activity and catalyst pretreated under oxidative atmosphere showed the higher activity than those pretreated under reductive and inert atmospheres. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: CO oxidation; Iron; Cobalt; Nanoparticles; Calcination; Pretreatment atmosphere
1.
Introduction
Catalytic low-temperature abatement of carbon monoxide is important in environmental pollution control (Tang et al., 2006; Cheng et al., 2007; Jones et al., 2009). This reaction process has extensive applications in many fields such as automotive exhaust emission control, mine rescue devices, and traceable CO removal in enclosed atmospheres (Kunkalekar and Salker, 2010; Shao et al., 2007). Precious metals work very well with the high catalytic activity and stability on CO oxidation at low-temperature. Because of the price and the limited availability of precious metals, considerable attention has been paid to various transition metal oxides and their mixed metal oxides (Tang et al., 2006). Among the transition metal oxide catalysts, iron oxides are one of the cheapest materials for oxidation of CO. Several studies have shown iron oxides and their composites to be effective catalysts for the oxidation of CO (Abdel Halim et al., 2007; Jozwiak et al., 2007; Kwon et al., 2007; Reddy et al., 2004; Li et al.,
2003), but iron oxides have a relatively low activity at low temperatures (below 200 ◦ C). On the other hand, cobalt oxides such as CoO and Co3 O4 have very high activity at low temperature oxidation reactions (Royer and Duprez, 2011), because of the presence of mobile oxygen in Co3 O4 (Tang et al., 2006). The unique properties of nano particles make them of interest for catalysis applications. In this paper, a nano-scale Fe–Co mixed oxide catalyst with molar ratio of Co/Fe = 1/5 was synthesized by a simple co-precipitation method and the effect of calcination temperature and pretreatment conditions (none, inert, oxidative and reducing atmosphere) on catalytic activity of prepared catalysts was investigated.
2.
Experimental
2.1.
Catalyst preparation
Iron–cobalt mixed metal oxide nanocatalysts were prepared by co-precipitation of Fe and Co salts. Firstly, Co(NO3 )2 ·6H2 O (Merck, 99%) and Fe(NO3 )3 ·9H2 O (Merck, 98%) with a molar ratio of Co/Fe = 1/5 were dissolved in distillated water under
∗ Corresponding author at: Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. Tel.: +98 361 5912469; fax: +98 361 5559930. E-mail addresses: [email protected], [email protected] (M. Rezaei). Received 25 July 2012; Received in revised form 7 October 2012; Accepted 15 October 2012 0957-5820/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.psep.2012.10.015
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vigorous stirring. The resulting solution was heated to 70 ◦ C and an aqueous solution of sodium carbonate (0.3 M) was added to this solution under continuous stirring at a rate of 2–3 ml/min at 70 ◦ C. The resulting precipitate was aged at pH of 7.0 for 2 h and then filtered and washed with warm distilled water several times for removing excess ions. The precipitate was then dried in air at 110 ◦ C for 16 h. Dried catalyst was subjected to different calcination temperatures: 300, 400, 500 and 600 ◦ C (assigned as BR-300, BR-400, BR-500 and BR-600, respectively) in air atmosphere for 6 h with a ramp rate of 3 ◦ C/min.
2.2.
Catalyst characterization
The samples were characterized by powder XRD using an X-ray diffractometer (PANalytical X’Pert-Pro) with a Cu-K␣ monochromatized radiation source and a Ni filter in the range 2 = 20–80◦ . Thermal gravimetric and differential thermal gravimetric analyses (TGA/DTG) were performed using air as oxidant at the heating rate of 10 ◦ C/min in a NETZSCH STA 409 PC/PG system. The N2 adsorption/desorption analysis (BET) was performed at −196 ◦ C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). Temperature programmed reduction (TPR) analysis was conducted for evaluating the reduction properties of prepared catalysts. In the TPR measurement, about 50 mg of samples were loaded and subjected to a heat treatment (10 ◦ C/min) in a gas flow (30 ml/min) containing a mixture of H2 :Ar (10:90). Prior to TPR experiment, the samples were pretreated under an inert atmosphere (Ar) at 200 ◦ C for 1 h and then cooled to room temperature. The amount of H2 uptake during the sample reduction was measured using a thermal conductivity detector (TCD). Transmission electron microscopy (TEM) was performed with JEOL JEM-2100UHR, operated at 200 kV.
2.3.
Catalytic evaluation
The catalytic activity of CO oxidation was performed under atmospheric pressure in a continuous flow reactor made of a 7-mm-i.d. quartz tube. The reactor was charged with 100 mg of the prepared catalyst sieved to 35–70 mesh. The reaction temperature was measured using an Omega K-type thermocouple projecting under catalytic bed. The feed gas for oxidation contained 2% CO, 20% O2 and 10% N2 balanced with He at a gas hourly space velocity (GHSV) of 60,000 ml/g.h. The flow rates were controlled by Bronkhorst High-Tech (EL-FLOW series) mass flow controllers. The activity tests were performed at different temperatures, ranging from 25 to 200 ◦ C in steps of 25 ◦ C that were kept for 30 min at each temperature. Before the reaction the catalysts were pretreated in 20% O2 balanced with He for 2 h at 250 ◦ C. The effluent gases from the reactor were analyzed by a gas chromatograph (Varian, model 3400) equipped with a TCD detector and a molecular sieve 5A column. The CO conversion was defined as the number of moles of CO consumed after the reaction with respect to the amount of inlet CO in feed gas.
3.
Results and discussion
3.1.
Characterization of catalysts
Fig. 1 shows TGA/DTG curves of BR-300 and the XRD patterns as well as H2 -TPR profiles of catalyst calcined under different temperatures. Fig. 1a shows the weight losing behavior of BR300 during thermal calcination process. Three different weight
Fig. 1 – (a) TG/DTG curves of BR-300, (b) XRD patterns and (c) H2 -TPR profiles of Fe–Co catalyst calcined at different temperatures. loss peaks were observed at the temperature ranges below 100, 170–390 and 480–520 ◦ C as seen in Fig. 1a. The first weight loss peak corresponds to the loss of surface absorbed water and the release of loosely bound hydrated water molecules of the iron and cobalt hydrate, whereas the second peak reveals the decomposition of iron and cobalt nitrate to amorphous or weak crystalline phase of iron and cobalt oxide. The third peak is mainly attributed to the transformation of amorphous or weak crystalline phase of iron and cobalt oxide to crystalline phases. It can be seen from Fig. 1b that the major crystallite phase of the prepared samples is Fe2 O3 in good agreement with
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Fig. 2 – FTIR spectra of (a) dried sample and (b) calcined sample at 400 ◦ C.
JCPDS 13-0534. Besides the predominate Fe2 O3 phase, it can be clearly observed that CoFe2 O4 phase and small amount of Co3 O4 (JCPDS 02-1045 and 43-1003, respectively) are presented in catalyst structures, especially at higher calcination temperatures. Moreover the BR-300 and BR-400 maybe contain a small amount of CoO(OH) (JCPDS 02-0214) that be too low to be identified (Tang et al., 2008). As the main intensive peak of Fe2 O3 and CoFe2 O4 is at about 2 = 35.5◦ , a peak overlapping took place at this angle. The amount of CoFe2 O4 and degree of crystallinity of both Fe2 O3 and CoFe2 O4 increases with increasing the calcination temperature from 300 to 600 ◦ C. Furthermore, the catalyst prepared at 300 ◦ C might have unconverted iron and cobalt nitrate precursors (Yang et al., 2009) and previous studies show that higher calcination temperature is required to fully convert metal nitrates to the corresponding metal oxides (Borg et al., 2007), but their quantity in samples was too low to be recognized by XRD analysis. However, TPR profiles of the Fe–Co systems are relatively complex. As it can be seen in Fig. 1c, BR-300 has five apparent reduction peaks at 200, 270, 360, 433, and 520 ◦ C. Based on the XRD results suggesting the presence of unconverted metal nitrate precursors in this catalyst, the peak at 200 ◦ C might have been derived from the reduction of the residual nitrate precursors (Yang et al., 2009), or according to the Tang et al. (2008), the peak at 200 ◦ C was attributed to the reduction of CoO(OH) to Co3 O4 . The hydrogen consumption peaks at 270 and 433 ◦ C were ascribed to reduction of Co3 O4 to CoO and then CoO to metallic cobalt (Co0 ), respectively. The other extensive peaks at 360 and 520 ◦ C are related to transformations of Fe3+ → Fe2+ → Fe0 , respectively (Jozwiak et al., 2007). TPR of other samples are similar to that of BR-300 except the disappearance of the peak located at around 200 ◦ C due to increasing in calcination temperature. The results show that calcination at higher temperatures can change the catalyst structure and thereupon make it more difficult to be reduced. Generation of CoFe2 O4 and other structures even in small amount can reshape the TPR profile (Yang et al., 2009).
The FTIR spectra of the as-prepared and calcined samples are shown in Fig. 2. The broad absorption at low-frequency region (400–600 cm−1 ) was ascribed to Co O stretching and Co OH bending vibrations. The FTIR result is in good agreement with the results of Fig. 1b and possibility of existence of small amount of CoO(OH) in the samples BR-300 and BR-400. Table 1 presents the structural properties of prepared Fe-Co mixed metal oxide catalyst calcined at different temperatures. The average crystallite sizes were calculated using Scherrer’s formula for major XRD peaks of samples from Fig. 1b. The results revealed that with increasing in calcination temperature the average crystallite size increased (Table 1). In addition, with assuming spherical shape for sample particles, the theoretical particle sizes were calculated from following equation: DBET =
6000
(1)
×S
where DBET is the equivalent particle diameter in nanometers, is the density of the material in g/cm3 and S is the specific surface area in m2 /g. The obtained results show an increase in particle size with increasing the calcination temperature. These results are in agreement with those obtained by Scherer formula. The results in Table 1 show that the sample calcined at 300 ◦ C (BR-300) has the highest specific surface area, which could be due to the smallest crystallite and particle sizes. The results also show that increasing in calcination temperature leads to a decrease in specific surface area. The lowest surface area was observed for the sample calcined at 600 ◦ C (BR-600), which might be due to growth of particles or collapse of pores (El-Shobaky and Deraz, 2001). This sample shows the biggest crystallite and particle size. It is seen that increasing in calcination temperature increased the pore size and decreased the pore volume. The pore size distributions and N2 adsorption/desorption profiles for the prepared catalysts are shown in Fig. 3a and
Table 1 – Effect of calcination temperature on the structural properties of prepared Fe–Co mixed oxide catalysts. Catalyst
SBET (m2 /g)
Pore volume (cm3 /g)
Pore size (nm)
Average crystallite size (nm)
DBET (nm)
BR-300 BR-400 BR-500 BR-600
145.03 117.86 25.38 9.04
0.288 0.280 0.187 0.084
5.21 6.18 15.01 24.06
7.67 7.12 23.60 27.04
7.67 9.44 43.84 123.08
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Fig. 3 – Effect of calcination temperature on (a) pore size distributions and (b) N2 adsorption/desorption profiles of Fe–Co mixed oxide catalysts.
b, respectively. These curves reveal that pore size of BR-300 distributes from 2 to 7 nm and this sample has the narrowest pore size distribution. It can be seen, the increase in calcination temperature, induces a decrease in pore sizes and wide in its distribution. The nitrogen adsorption/desorption isotherms of the samples calcined at 300 and 400 ◦ C can be classified as a type IV isotherm, typical of mesoporous materials. According to IUPAC classification, the hysteresis loop is type H2 indicating a complex mesoporous structure. This type of hysteresis is characteristic of solids consisting of particles crossed by nearly cylindrical channels or made by aggregates (consolidated) or agglomerates (unconsolidated) of spherical particles. In this case the pores have nonuniform size or shape (type H2). For the samples calcined at 500 and 600 ◦ C the hysteresis loop is type H3. This hysteresis is usually found on solids consisting of aggregates or agglomerates of particles forming slit shaped pores (plates or edged particles like cubes), with nonuniform size and/or shape. The TEM images (Fig. 4a and b) of sample BR-300 clearly show that the particles are in nano scale and in range of 5–8 nm, which is in good agreement with the crystallite size determined by XRD analysis. The particles are spherical in nature. The electron diffraction pattern (ED) of sample BR-300 (Fig. 4c) shows that rings are continue that reveals signifying polycrystalline in nature.
Fig. 4 – (a) TEM image, (b) HRTEM image and (c) electron diffraction pattern of BR-300.
3.2.
Catalytic performance
3.2.1.
Effect of calcination temperature
Fig. 5 presents the effect of reaction temperature on the CO oxidation over iron–cobalt mixed metal oxide catalysts calcined at different temperatures. It is seen that the catalytic activity of Fe–Co mixed metal oxide catalysts decreases with increasing the calcination temperature and these changes are very intensive from BR-400 to BR-500. It is due to a severe decreasing in BET surface area and increasing in average crystallize size with calcination temperature (El-Shobaky et al., 2007). The BR-300 displays complete CO oxidation at 200 ◦ C but BR-600 has only 28% conversion at this temperature. As it can be seen, the activity of BR-300 and BR-400 are almost similar and temperature rising in calcination from 300 to 400 ◦ C had a small effect on catalytic CO oxidation. Furthermore, with considering the previous investigations on CO oxidation over iron oxides (Kwon et al., 2007; Li et al., 2003), we can conclude adding small amount of Co to iron (Co/Fe = 1/5) has a dramatic effect on decreasing light-off temperature of 100% CO oxidation. The complete conversion of CO oxidation over
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 489–494
493
Fig. 5 – The catalytic activity of Fe–Co mixed oxide catalysts calcined at different temperatures, reaction condition: 2% CO, 20% O2 and 10% N2 balanced with He, GHSV = 60,000 ml/g h, pretreated in oxidative atmosphere. pure iron oxide was reported at 500 ◦ C but this temperature is about 200 ◦ C for BR-300 and BR-400.
3.2.2.
Effect of pretreatment conditions
In order to observe the effect of pretreatment on activity of Fe–Co mixed oxide catalysts during CO oxidation, different pretreatment processes (oxidative, reductive, and inert atmosphere) were carried out. Fig. 6a shows the effect of different pretreatment conditions on activity of Fe–Co mixed oxide catalyst. In a typical test, firstly 100 mg catalyst BR-300 was charged to the reactor. Then catalyst was subjected to different pretreatment atmospheres. For example, to perform the oxidative atmosphere pretreatment, before the reaction a gas mixture with 20% O2 in He was passed over catalyst bed at 250 ◦ C for 2 h. We also used 20% H2 in He for reductive and pure He for inert atmosphere pretreatment. As it can be seen, the pretreatment condition plays a significant role in catalytic performance of catalysts (Fig. 6a). The results show that the oxidative atmosphere has positive effects on increasing activity of Fe–Co mixed oxide catalysts. It might be due to modification of surface oxygen atoms and/or metal-metal interaction (Lashina et al., 2009; Eichler, 2002). Fig. 6a shows a decreasing in activity with applying reductive atmosphere pretreatment. It can be due to partial reduction of Fe–Co mixed oxides to species with lower oxygen capacity (Zhang et al., 2011). The results reveal that the inert atmosphere has an intermediate effect. Moreover we performed a test without any pretreatment and saw its result was almost similar to that of inert pretreatment test, our results show only oxidative pretreatment has an extensive role to increasing activity of CO oxidation on Fe–Co mixed oxide catalyst and applying inert pretreatment has no any important effect on this reaction. The catalytic stability of Fe–Co mixed oxide catalyst after oxidative pretreatment is tested with applying a long-term stability test on the BR-300 at 160 ◦ C for 30 h. As it can be seen in Fig. 5, the highest CO conversion for BR-300 is at 200 ◦ C, but the long-term stability was performed at 160 ◦ C, in order to better observing the deactivation rate of Fe–Co mixed metal oxide catalyst at a condition more far than complete CO conversion. Fig. 6b shows that the catalyst calcined at 300 ◦ C has a relatively high stability under the reaction conditions. The loss in initial CO conversion was around 8%.
Fig. 6 – (a) Effect of different pretreatment conditions on the activity of sample BR-300 and (b) stability study of CO oxidation over BR-300: reaction condition for (a) and (b): 2% CO, 20% O2 and 10% N2 balanced with He, GHSV = 60,000 ml/g h, temperature: 160 ◦ C, oxidative pretreatment at 250 ◦ C for 2 h.
4.
Conclusions
In conclusion, iron–cobalt mixed metal nanoparticles (Co/Fe molar ratio: 1/5) with small particle size and large surface area can be prepared by a co-precipitation technique. The results show that the activity of iron–cobalt mixed metal oxide catalysts in CO oxidation reaction was significantly affected by calcination temperature and pretreatment conditions. Our results indicate that calcination at lower temperatures and/or introducing oxidative pretreatment has an extensive role to increasing activity of CO oxidation on Fe–Co mixed oxide catalysts. In addition, the stability test shows that the BR-300 has a relatively high activity and stability at 160 ◦ C under the reaction conditions.
Acknowledgments The authors gratefully acknowledge the supports from University of Kashan by Grant No. 158426/6.
References Abdel Halim, K.S., Khedr, M.H., Nasr, M.I., El-Mansy, A.M., 2007. Factors affecting CO oxidation over nanosized Fe2 O3 . Mater. Res. Bull. 42, 731–741.
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Borg, Ø., Eri, S., Blekkan, E.A., Storsæter, S., Wigum, H., Rytter, E., 2007. Fischer–Tropsch synthesis over ␥-alumina-supported cobalt catalysts: effect of support variables, A., Holmen. J. Catal. 248, 89–100. Cheng, T., Fang, Z., Hu, Q., Han, K., Yang, X., Zhang, Y., 2007. Low-temperature CO oxidation over CuO/Fe2 O3 catalysts. Catal. Commun. 8, 1167–1171. Eichler, A., 2002. CO oxidation on transition metal surfaces: reaction rates from first principles. Surf. Sci. 498, 314–320. El-Shobaky, G.A., Deraz, N.M., 2001. Surface and catalytic properties of cobaltic oxide supported on an active magnesia. Mater. Lett. 47, 231–240. El-Shobaky, G.A., Radwan Nagi, R.E., El-Shall, M.S., Turky, A.M., Hassan, H.M.A., 2007. The role of method of preparation of CuO–NiO system on its physicochemical surface and catalytic properties. Colloids Surf. A: Physicochem. Eng. Asp. 311, 161–169. Jones, C., Cole, K.J., Taylor, S.H., Crudace, M.J., Hutchings, G.J., 2009. Copper manganese oxide catalysts for ambient temperature carbon monoxide oxidation: effect of calcination on activity. J. Mol. Catal. A: Chem. 305, 121–124. Jozwiak, W.K., Kaczmarek, E., Maniecki, T.P., Ignaczak, W., Maniukiewicz, W., 2007. Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres. Appl. Catal. A: Gen. 326, 17–27. Kunkalekar, R.K., Salker, A.V., 2010. Low temperature carbon monoxide oxidation over nanosized silver doped manganese dioxide catalysts. Catal. Commun. 12, 193–196. Kwon, S.C., Fan, M., Wheelock, T.D., Saha, B., 2007. Nano- and micro-iron oxide catalysts for controlling the emission of carbon monoxide and methane. Sep. Purif. Technol. 58, 40–48.
Lashina, E.A., Chumakova, N.A., Chumakov, G.A., Boronin, A.I., 2009. Chaotic dynamics in the three-variable kinetic model of CO oxidation on platinum group metals. Chem. Eng. J. 154, 82–87. Li, P., Miser, D.E., Rabiei, S., Yadav, R.T., Hajaligol, M.R., 2003. The removal of carbon monoxide by iron oxide nanoparticles. Appl. Catal. B. 43, 151–162. Reddy, B.V., Rasouli, F., Hajaligol, M.R., Khanna, S.N., 2004. Novel mechanism for oxidation of CO by Fe2 O3 clusters. Fuel 83, 1537–1541. Royer, S., Duprez, D., 2011. Catalytic oxidation of carbon monoxide over transition metal oxides. ChemCatChem 3, 24–65. Shao, J., Zhang, P., Tang, X., Zhang, B., Song, W., Xu, Y., Shen, W., 2007. Effect of preparation method and calcination temperature on low-temperature CO oxidation over Co3 O4 /CeO2 catalysts. Chin. J. Catal. 28, 163–196. Tang, C.W., Kuo, C.C., Kuo, M.C., Wang, C.B., Chien, S.H., 2006. Influence of pretreatment conditions on low-temperature carbon monoxide oxidation over CeO2 /Co3 O4 catalysts. Appl. Catal. A: Gen. 309, 37–43. Tang, C.W., Wang, C.B., Chien, S.H., 2008. Characterization of cobalt oxides studied by FT-IR, Raman, TPR and TG–MS. Thermochim. Acta 473, 68–73. Yang, Q., Choi, H., Al-Abed, S.R., Dionysiou, D.D., 2009. Iron–cobalt mixed oxide nanocatalysts: heterogeneous peroxymonosulfate activation, cobalt leaching, and ferromagnetic properties for environmental applications. Appl. Catal. B: Env. 88, 462–469. Zhang, R.R., Ren, L.H., Lu, A.H., Li, W.C., 2011. Influence of pretreatment atmospheres on the activity of Au/CeO2 catalyst for low-temperature CO oxidation. Catal. Commun. 13, 18–21.
Contents lists available at ScienceDirect
Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Study of Fe–Co mixed metal oxide nanoparticles in the catalytic low-temperature CO oxidation Abolfazl Biabani-Ravandi a , Mehran Rezaei a,b,∗ , Zohreh Fattah a a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran b Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
a b s t r a c t Iron–cobalt mixed metal oxide nanoparticles (Co/Fe molar ratio: 1/5) have been prepared by a simple co-precipitation method and employed as catalyst in low-temperature CO oxidation. The prepared catalysts were characterized by thermal gravimetric and differential thermal gravimetric analyses (TGA/DTG), X-ray diffraction (XRD), temperature programmed reduction (TPR), N2 adsorption (BET) and transmission electron microscopy (TEM) techniques. The results revealed that inexpensive iron–cobalt mixed metal oxide nanoparticles have a high potential as catalyst in low temperature CO oxidation. The results showed that increasing in calcination temperature increased the crystallite and particle size and decreased the specific surface area, which caused a decrease in catalytic activity of prepared catalysts. In addition, the pretreatment conditions affect the catalytic activity and catalyst pretreated under oxidative atmosphere showed the higher activity than those pretreated under reductive and inert atmospheres. © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: CO oxidation; Iron; Cobalt; Nanoparticles; Calcination; Pretreatment atmosphere
1.
Introduction
Catalytic low-temperature abatement of carbon monoxide is important in environmental pollution control (Tang et al., 2006; Cheng et al., 2007; Jones et al., 2009). This reaction process has extensive applications in many fields such as automotive exhaust emission control, mine rescue devices, and traceable CO removal in enclosed atmospheres (Kunkalekar and Salker, 2010; Shao et al., 2007). Precious metals work very well with the high catalytic activity and stability on CO oxidation at low-temperature. Because of the price and the limited availability of precious metals, considerable attention has been paid to various transition metal oxides and their mixed metal oxides (Tang et al., 2006). Among the transition metal oxide catalysts, iron oxides are one of the cheapest materials for oxidation of CO. Several studies have shown iron oxides and their composites to be effective catalysts for the oxidation of CO (Abdel Halim et al., 2007; Jozwiak et al., 2007; Kwon et al., 2007; Reddy et al., 2004; Li et al.,
2003), but iron oxides have a relatively low activity at low temperatures (below 200 ◦ C). On the other hand, cobalt oxides such as CoO and Co3 O4 have very high activity at low temperature oxidation reactions (Royer and Duprez, 2011), because of the presence of mobile oxygen in Co3 O4 (Tang et al., 2006). The unique properties of nano particles make them of interest for catalysis applications. In this paper, a nano-scale Fe–Co mixed oxide catalyst with molar ratio of Co/Fe = 1/5 was synthesized by a simple co-precipitation method and the effect of calcination temperature and pretreatment conditions (none, inert, oxidative and reducing atmosphere) on catalytic activity of prepared catalysts was investigated.
2.
Experimental
2.1.
Catalyst preparation
Iron–cobalt mixed metal oxide nanocatalysts were prepared by co-precipitation of Fe and Co salts. Firstly, Co(NO3 )2 ·6H2 O (Merck, 99%) and Fe(NO3 )3 ·9H2 O (Merck, 98%) with a molar ratio of Co/Fe = 1/5 were dissolved in distillated water under
∗ Corresponding author at: Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. Tel.: +98 361 5912469; fax: +98 361 5559930. E-mail addresses: [email protected], [email protected] (M. Rezaei). Received 25 July 2012; Received in revised form 7 October 2012; Accepted 15 October 2012 0957-5820/$ – see front matter © 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.psep.2012.10.015
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Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 489–494
vigorous stirring. The resulting solution was heated to 70 ◦ C and an aqueous solution of sodium carbonate (0.3 M) was added to this solution under continuous stirring at a rate of 2–3 ml/min at 70 ◦ C. The resulting precipitate was aged at pH of 7.0 for 2 h and then filtered and washed with warm distilled water several times for removing excess ions. The precipitate was then dried in air at 110 ◦ C for 16 h. Dried catalyst was subjected to different calcination temperatures: 300, 400, 500 and 600 ◦ C (assigned as BR-300, BR-400, BR-500 and BR-600, respectively) in air atmosphere for 6 h with a ramp rate of 3 ◦ C/min.
2.2.
Catalyst characterization
The samples were characterized by powder XRD using an X-ray diffractometer (PANalytical X’Pert-Pro) with a Cu-K␣ monochromatized radiation source and a Ni filter in the range 2 = 20–80◦ . Thermal gravimetric and differential thermal gravimetric analyses (TGA/DTG) were performed using air as oxidant at the heating rate of 10 ◦ C/min in a NETZSCH STA 409 PC/PG system. The N2 adsorption/desorption analysis (BET) was performed at −196 ◦ C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). Temperature programmed reduction (TPR) analysis was conducted for evaluating the reduction properties of prepared catalysts. In the TPR measurement, about 50 mg of samples were loaded and subjected to a heat treatment (10 ◦ C/min) in a gas flow (30 ml/min) containing a mixture of H2 :Ar (10:90). Prior to TPR experiment, the samples were pretreated under an inert atmosphere (Ar) at 200 ◦ C for 1 h and then cooled to room temperature. The amount of H2 uptake during the sample reduction was measured using a thermal conductivity detector (TCD). Transmission electron microscopy (TEM) was performed with JEOL JEM-2100UHR, operated at 200 kV.
2.3.
Catalytic evaluation
The catalytic activity of CO oxidation was performed under atmospheric pressure in a continuous flow reactor made of a 7-mm-i.d. quartz tube. The reactor was charged with 100 mg of the prepared catalyst sieved to 35–70 mesh. The reaction temperature was measured using an Omega K-type thermocouple projecting under catalytic bed. The feed gas for oxidation contained 2% CO, 20% O2 and 10% N2 balanced with He at a gas hourly space velocity (GHSV) of 60,000 ml/g.h. The flow rates were controlled by Bronkhorst High-Tech (EL-FLOW series) mass flow controllers. The activity tests were performed at different temperatures, ranging from 25 to 200 ◦ C in steps of 25 ◦ C that were kept for 30 min at each temperature. Before the reaction the catalysts were pretreated in 20% O2 balanced with He for 2 h at 250 ◦ C. The effluent gases from the reactor were analyzed by a gas chromatograph (Varian, model 3400) equipped with a TCD detector and a molecular sieve 5A column. The CO conversion was defined as the number of moles of CO consumed after the reaction with respect to the amount of inlet CO in feed gas.
3.
Results and discussion
3.1.
Characterization of catalysts
Fig. 1 shows TGA/DTG curves of BR-300 and the XRD patterns as well as H2 -TPR profiles of catalyst calcined under different temperatures. Fig. 1a shows the weight losing behavior of BR300 during thermal calcination process. Three different weight
Fig. 1 – (a) TG/DTG curves of BR-300, (b) XRD patterns and (c) H2 -TPR profiles of Fe–Co catalyst calcined at different temperatures. loss peaks were observed at the temperature ranges below 100, 170–390 and 480–520 ◦ C as seen in Fig. 1a. The first weight loss peak corresponds to the loss of surface absorbed water and the release of loosely bound hydrated water molecules of the iron and cobalt hydrate, whereas the second peak reveals the decomposition of iron and cobalt nitrate to amorphous or weak crystalline phase of iron and cobalt oxide. The third peak is mainly attributed to the transformation of amorphous or weak crystalline phase of iron and cobalt oxide to crystalline phases. It can be seen from Fig. 1b that the major crystallite phase of the prepared samples is Fe2 O3 in good agreement with
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Fig. 2 – FTIR spectra of (a) dried sample and (b) calcined sample at 400 ◦ C.
JCPDS 13-0534. Besides the predominate Fe2 O3 phase, it can be clearly observed that CoFe2 O4 phase and small amount of Co3 O4 (JCPDS 02-1045 and 43-1003, respectively) are presented in catalyst structures, especially at higher calcination temperatures. Moreover the BR-300 and BR-400 maybe contain a small amount of CoO(OH) (JCPDS 02-0214) that be too low to be identified (Tang et al., 2008). As the main intensive peak of Fe2 O3 and CoFe2 O4 is at about 2 = 35.5◦ , a peak overlapping took place at this angle. The amount of CoFe2 O4 and degree of crystallinity of both Fe2 O3 and CoFe2 O4 increases with increasing the calcination temperature from 300 to 600 ◦ C. Furthermore, the catalyst prepared at 300 ◦ C might have unconverted iron and cobalt nitrate precursors (Yang et al., 2009) and previous studies show that higher calcination temperature is required to fully convert metal nitrates to the corresponding metal oxides (Borg et al., 2007), but their quantity in samples was too low to be recognized by XRD analysis. However, TPR profiles of the Fe–Co systems are relatively complex. As it can be seen in Fig. 1c, BR-300 has five apparent reduction peaks at 200, 270, 360, 433, and 520 ◦ C. Based on the XRD results suggesting the presence of unconverted metal nitrate precursors in this catalyst, the peak at 200 ◦ C might have been derived from the reduction of the residual nitrate precursors (Yang et al., 2009), or according to the Tang et al. (2008), the peak at 200 ◦ C was attributed to the reduction of CoO(OH) to Co3 O4 . The hydrogen consumption peaks at 270 and 433 ◦ C were ascribed to reduction of Co3 O4 to CoO and then CoO to metallic cobalt (Co0 ), respectively. The other extensive peaks at 360 and 520 ◦ C are related to transformations of Fe3+ → Fe2+ → Fe0 , respectively (Jozwiak et al., 2007). TPR of other samples are similar to that of BR-300 except the disappearance of the peak located at around 200 ◦ C due to increasing in calcination temperature. The results show that calcination at higher temperatures can change the catalyst structure and thereupon make it more difficult to be reduced. Generation of CoFe2 O4 and other structures even in small amount can reshape the TPR profile (Yang et al., 2009).
The FTIR spectra of the as-prepared and calcined samples are shown in Fig. 2. The broad absorption at low-frequency region (400–600 cm−1 ) was ascribed to Co O stretching and Co OH bending vibrations. The FTIR result is in good agreement with the results of Fig. 1b and possibility of existence of small amount of CoO(OH) in the samples BR-300 and BR-400. Table 1 presents the structural properties of prepared Fe-Co mixed metal oxide catalyst calcined at different temperatures. The average crystallite sizes were calculated using Scherrer’s formula for major XRD peaks of samples from Fig. 1b. The results revealed that with increasing in calcination temperature the average crystallite size increased (Table 1). In addition, with assuming spherical shape for sample particles, the theoretical particle sizes were calculated from following equation: DBET =
6000
(1)
×S
where DBET is the equivalent particle diameter in nanometers, is the density of the material in g/cm3 and S is the specific surface area in m2 /g. The obtained results show an increase in particle size with increasing the calcination temperature. These results are in agreement with those obtained by Scherer formula. The results in Table 1 show that the sample calcined at 300 ◦ C (BR-300) has the highest specific surface area, which could be due to the smallest crystallite and particle sizes. The results also show that increasing in calcination temperature leads to a decrease in specific surface area. The lowest surface area was observed for the sample calcined at 600 ◦ C (BR-600), which might be due to growth of particles or collapse of pores (El-Shobaky and Deraz, 2001). This sample shows the biggest crystallite and particle size. It is seen that increasing in calcination temperature increased the pore size and decreased the pore volume. The pore size distributions and N2 adsorption/desorption profiles for the prepared catalysts are shown in Fig. 3a and
Table 1 – Effect of calcination temperature on the structural properties of prepared Fe–Co mixed oxide catalysts. Catalyst
SBET (m2 /g)
Pore volume (cm3 /g)
Pore size (nm)
Average crystallite size (nm)
DBET (nm)
BR-300 BR-400 BR-500 BR-600
145.03 117.86 25.38 9.04
0.288 0.280 0.187 0.084
5.21 6.18 15.01 24.06
7.67 7.12 23.60 27.04
7.67 9.44 43.84 123.08
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Fig. 3 – Effect of calcination temperature on (a) pore size distributions and (b) N2 adsorption/desorption profiles of Fe–Co mixed oxide catalysts.
b, respectively. These curves reveal that pore size of BR-300 distributes from 2 to 7 nm and this sample has the narrowest pore size distribution. It can be seen, the increase in calcination temperature, induces a decrease in pore sizes and wide in its distribution. The nitrogen adsorption/desorption isotherms of the samples calcined at 300 and 400 ◦ C can be classified as a type IV isotherm, typical of mesoporous materials. According to IUPAC classification, the hysteresis loop is type H2 indicating a complex mesoporous structure. This type of hysteresis is characteristic of solids consisting of particles crossed by nearly cylindrical channels or made by aggregates (consolidated) or agglomerates (unconsolidated) of spherical particles. In this case the pores have nonuniform size or shape (type H2). For the samples calcined at 500 and 600 ◦ C the hysteresis loop is type H3. This hysteresis is usually found on solids consisting of aggregates or agglomerates of particles forming slit shaped pores (plates or edged particles like cubes), with nonuniform size and/or shape. The TEM images (Fig. 4a and b) of sample BR-300 clearly show that the particles are in nano scale and in range of 5–8 nm, which is in good agreement with the crystallite size determined by XRD analysis. The particles are spherical in nature. The electron diffraction pattern (ED) of sample BR-300 (Fig. 4c) shows that rings are continue that reveals signifying polycrystalline in nature.
Fig. 4 – (a) TEM image, (b) HRTEM image and (c) electron diffraction pattern of BR-300.
3.2.
Catalytic performance
3.2.1.
Effect of calcination temperature
Fig. 5 presents the effect of reaction temperature on the CO oxidation over iron–cobalt mixed metal oxide catalysts calcined at different temperatures. It is seen that the catalytic activity of Fe–Co mixed metal oxide catalysts decreases with increasing the calcination temperature and these changes are very intensive from BR-400 to BR-500. It is due to a severe decreasing in BET surface area and increasing in average crystallize size with calcination temperature (El-Shobaky et al., 2007). The BR-300 displays complete CO oxidation at 200 ◦ C but BR-600 has only 28% conversion at this temperature. As it can be seen, the activity of BR-300 and BR-400 are almost similar and temperature rising in calcination from 300 to 400 ◦ C had a small effect on catalytic CO oxidation. Furthermore, with considering the previous investigations on CO oxidation over iron oxides (Kwon et al., 2007; Li et al., 2003), we can conclude adding small amount of Co to iron (Co/Fe = 1/5) has a dramatic effect on decreasing light-off temperature of 100% CO oxidation. The complete conversion of CO oxidation over
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 489–494
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Fig. 5 – The catalytic activity of Fe–Co mixed oxide catalysts calcined at different temperatures, reaction condition: 2% CO, 20% O2 and 10% N2 balanced with He, GHSV = 60,000 ml/g h, pretreated in oxidative atmosphere. pure iron oxide was reported at 500 ◦ C but this temperature is about 200 ◦ C for BR-300 and BR-400.
3.2.2.
Effect of pretreatment conditions
In order to observe the effect of pretreatment on activity of Fe–Co mixed oxide catalysts during CO oxidation, different pretreatment processes (oxidative, reductive, and inert atmosphere) were carried out. Fig. 6a shows the effect of different pretreatment conditions on activity of Fe–Co mixed oxide catalyst. In a typical test, firstly 100 mg catalyst BR-300 was charged to the reactor. Then catalyst was subjected to different pretreatment atmospheres. For example, to perform the oxidative atmosphere pretreatment, before the reaction a gas mixture with 20% O2 in He was passed over catalyst bed at 250 ◦ C for 2 h. We also used 20% H2 in He for reductive and pure He for inert atmosphere pretreatment. As it can be seen, the pretreatment condition plays a significant role in catalytic performance of catalysts (Fig. 6a). The results show that the oxidative atmosphere has positive effects on increasing activity of Fe–Co mixed oxide catalysts. It might be due to modification of surface oxygen atoms and/or metal-metal interaction (Lashina et al., 2009; Eichler, 2002). Fig. 6a shows a decreasing in activity with applying reductive atmosphere pretreatment. It can be due to partial reduction of Fe–Co mixed oxides to species with lower oxygen capacity (Zhang et al., 2011). The results reveal that the inert atmosphere has an intermediate effect. Moreover we performed a test without any pretreatment and saw its result was almost similar to that of inert pretreatment test, our results show only oxidative pretreatment has an extensive role to increasing activity of CO oxidation on Fe–Co mixed oxide catalyst and applying inert pretreatment has no any important effect on this reaction. The catalytic stability of Fe–Co mixed oxide catalyst after oxidative pretreatment is tested with applying a long-term stability test on the BR-300 at 160 ◦ C for 30 h. As it can be seen in Fig. 5, the highest CO conversion for BR-300 is at 200 ◦ C, but the long-term stability was performed at 160 ◦ C, in order to better observing the deactivation rate of Fe–Co mixed metal oxide catalyst at a condition more far than complete CO conversion. Fig. 6b shows that the catalyst calcined at 300 ◦ C has a relatively high stability under the reaction conditions. The loss in initial CO conversion was around 8%.
Fig. 6 – (a) Effect of different pretreatment conditions on the activity of sample BR-300 and (b) stability study of CO oxidation over BR-300: reaction condition for (a) and (b): 2% CO, 20% O2 and 10% N2 balanced with He, GHSV = 60,000 ml/g h, temperature: 160 ◦ C, oxidative pretreatment at 250 ◦ C for 2 h.
4.
Conclusions
In conclusion, iron–cobalt mixed metal nanoparticles (Co/Fe molar ratio: 1/5) with small particle size and large surface area can be prepared by a co-precipitation technique. The results show that the activity of iron–cobalt mixed metal oxide catalysts in CO oxidation reaction was significantly affected by calcination temperature and pretreatment conditions. Our results indicate that calcination at lower temperatures and/or introducing oxidative pretreatment has an extensive role to increasing activity of CO oxidation on Fe–Co mixed oxide catalysts. In addition, the stability test shows that the BR-300 has a relatively high activity and stability at 160 ◦ C under the reaction conditions.
Acknowledgments The authors gratefully acknowledge the supports from University of Kashan by Grant No. 158426/6.
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