Low-temperature CO oxidation over CuO/Fe 2 O 3 catalysts Tao Cheng, Zhiyong Fang, Qixiu Hu, Kaidong Han, Xiaozhi Yang, Youjin Zhang * Department of Chemistry, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, PR China Received 31 May 2006; received in revised form 2 November 2006; accepted 3 November 2006 Available online 9 November 2006 Abstract Copper iron composite oxide catalysts have been prepared by co-precipitation method. The catalytic activity and stability of the cat- alysts on CO oxidation were evaluated by using a microreactor-GC system. The results indicated that the copper iron composite oxide catalysts exhibited obviously high stability and catalytic activity on CO oxidation at low temperature. The effect of the calcination tem- perature, the molar ratios of copper to iron, the specific surface areas and the particle sizes on the catalytic activity of the catalysts was investigated in this paper. Ó 2006 Elsevier B.V. All rights reserved. Keywords: CuO/Fe 2 O 3 catalysts; CO oxidation; Catalytic activity; Stability 1. Introduction CO oxidation at low temperature remains an intense and important research topic at present. In particular, the process is widely adopted by mining industries, vehicle exhaust catalysts, CO 2 lasers and gas sensors [1–6]. Pre- cious metals work very well with the high catalytic activity and stability on CO oxidation at low temperature [7–9]. However, the high cost, the need for complex pre-treat- ment and the limited availability of precious metals moti- vated the search for the substitutes. Transition metals and their oxides have attracted much considerable atten- tion due to their significant catalytic activities and low cost. Among the transition metal and their oxides, copper and copper oxides have been explored as possible substitutes for precious metal catalysts on CO oxidation at low tem- perature [10–20]. Iron oxides and their composite oxides are also used as the catalysts and the catalyst carriers on CO oxidation. Supported iron oxide catalysts are especially attractive candidates in certain special applications, such as the removal of CO in a burning cigarette, where the poten- tial toxicity of other catalysts is a concern [8,20]. Due to the synergistic effect, the composite oxides of transition metals exhibit higher catalytic activity on CO oxidation than those of individual transition metal and their oxides [21–27]. The copper–iron single crystals and supported copper–iron catalysts have been studied by several groups [28–30]. Nano-scale transition metals and their oxides may provide significantly improved catalytic activities over non-nano- catalysts, due to their small particle size, high specific sur- face area and more densely populated surface activity sites. To the best of our knowledge, there are few reports on the study of copper iron composite oxide catalysts prepared by co-precipitation on CO oxidation at low temperature. In this paper, the synthesis of nano-scale copper iron compos- ite oxide catalysts by co-precipitation was reported. The catalytic activity and stability of the catalysts on CO oxida- tion were evaluated. The effect of the calcination tempera- ture, the molar ratios of copper to iron, the specific surface areas and particle sizes on the catalytic activity of the cat- alysts was investigated. 2. Experiment All chemicals obtained from Shanghai Chemical Reagent Ltd. Co. of China are analytical grade and are used as received without further purification. 1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.11.002 * Corresponding author. Tel.: +86 551 3492145; fax: +86 551 3492083. E-mail address: [email protected](Y. Zhang). www.elsevier.com/locate/catcom Catalysis Communications 8 (2007) 1167–1171
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www.elsevier.com/locate/catcom
Catalysis Communications 8 (2007) 1167–1171
Low-temperature CO oxidation over CuO/Fe2O3 catalysts
Department of Chemistry, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, PR China
Received 31 May 2006; received in revised form 2 November 2006; accepted 3 November 2006Available online 9 November 2006
Abstract
Copper iron composite oxide catalysts have been prepared by co-precipitation method. The catalytic activity and stability of the cat-alysts on CO oxidation were evaluated by using a microreactor-GC system. The results indicated that the copper iron composite oxidecatalysts exhibited obviously high stability and catalytic activity on CO oxidation at low temperature. The effect of the calcination tem-perature, the molar ratios of copper to iron, the specific surface areas and the particle sizes on the catalytic activity of the catalysts wasinvestigated in this paper.� 2006 Elsevier B.V. All rights reserved.
Keywords: CuO/Fe2O3 catalysts; CO oxidation; Catalytic activity; Stability
1. Introduction
CO oxidation at low temperature remains an intenseand important research topic at present. In particular, theprocess is widely adopted by mining industries, vehicleexhaust catalysts, CO2 lasers and gas sensors [1–6]. Pre-cious metals work very well with the high catalytic activityand stability on CO oxidation at low temperature [7–9].However, the high cost, the need for complex pre-treat-ment and the limited availability of precious metals moti-vated the search for the substitutes. Transition metalsand their oxides have attracted much considerable atten-tion due to their significant catalytic activities and low cost.Among the transition metal and their oxides, copper andcopper oxides have been explored as possible substitutesfor precious metal catalysts on CO oxidation at low tem-perature [10–20]. Iron oxides and their composite oxidesare also used as the catalysts and the catalyst carriers onCO oxidation. Supported iron oxide catalysts are especiallyattractive candidates in certain special applications, such asthe removal of CO in a burning cigarette, where the poten-tial toxicity of other catalysts is a concern [8,20]. Due to the
1566-7367/$ - see front matter � 2006 Elsevier B.V. All rights reserved.
synergistic effect, the composite oxides of transition metalsexhibit higher catalytic activity on CO oxidation than thoseof individual transition metal and their oxides [21–27]. Thecopper–iron single crystals and supported copper–ironcatalysts have been studied by several groups [28–30].Nano-scale transition metals and their oxides may providesignificantly improved catalytic activities over non-nano-catalysts, due to their small particle size, high specific sur-face area and more densely populated surface activity sites.To the best of our knowledge, there are few reports on thestudy of copper iron composite oxide catalysts prepared byco-precipitation on CO oxidation at low temperature. Inthis paper, the synthesis of nano-scale copper iron compos-ite oxide catalysts by co-precipitation was reported. Thecatalytic activity and stability of the catalysts on CO oxida-tion were evaluated. The effect of the calcination tempera-ture, the molar ratios of copper to iron, the specific surfaceareas and particle sizes on the catalytic activity of the cat-alysts was investigated.
2. Experiment
All chemicals obtained from Shanghai ChemicalReagent Ltd. Co. of China are analytical grade and areused as received without further purification.
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2.1. Preparation
Copper iron composite oxide catalysts were prepared byco-precipitation method. In a typical procedure, the aque-ous solutions of Cu(NO3)2 Æ 3H2O (0.25 mol/L) andFe(NO3)3 Æ 9H2O (0.25 mol/L) were pre-mixed in a 1:2ratio. An aqueous solution of NaOH (25 wt%) was addedto the pre-mixed solution under vigorous stirring until itspH was in the range 8.8–9.0. After the reaction completed,the precipitate was then filtered, washed several times withdeionized water, and dried at 110 �C for 12 h in air. Subse-quently, the samples were calcined at 200, 300, 400, 500and 600 �C for 5 h in air, respectively. The catalysts withdifferent catalytic activity were obtained by changing the
Fig. 1. XRD patterns of the catalysts calcined at different temperature.
Fig. 2. TEM images of the catalysts calcined at different tempera
calcination temperature and the molar ratio of copper toiron according to the above preparation method.
2.2. Characterization
The phase purity and structure parameters of catalystswere examined by power X-ray diffraction (XRD), usinga Rigaku Dmax r-A X-ray diffractometer with graphite-monochromatized Cu Ka radiation (k = 0.15418 nm), bya scanning rate of 0.05 �/s in the 2h range from 10� to70�, the operation voltage and current maintained at40 kV and 40 mA. The transmission electron microscopy(TEM) images were performed on a Hitachi Model H-800 instrument with a tungsten filament, using an acceler-ating voltage of 200 kV. The X-ray photoelectron spectra(XPS) measurements were carried out on an ESCALABMK II X-ray photoelectron spectrometer, using Mg Karadiation as the excitation source. The XPS spectra werecorrected by adjusting the C 1s peak to a position of284.6 eV. The thermogravimetry–differential thermal anal-ysis (TG–DTA) of the sample was conducted on a RigakuStandard Model thermal analyzer (in air atmosphere, flowrate: 90 ml min�1; heat rate: 10 �C min�1). The specific sur-face areas (SBET) of the catalysts were calculated from amultipoint Braunauer–Emmett–Teller (BET) analysis ofthe nitrogen adsorption isotherms at 77 K recorded on anOmnisorp 100CX instrument. The catalytic activity andstability of the catalysts were evaluated on a small fixed-bed microreactor operating under atmospheric pressureand an online GC using 100 mg sample of 40–80 meshes.
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The flow rate of the feed gas was 30 mL min�1. The analy-sis of the effluent gas was tested with an online FuLi9790model gas chromatograph with a Molecular Sieve 5 A col-umn and a thermal conductivity detector (TCD). The cat-alysts were directly exposed to reaction gas containing2.5% (v/v) CO, 10% (v/v) O2, and 87.5% (v/v) N2.
3. Results and discussion
Copper iron composite oxide catalysts calcined at differ-ent temperature are examined by XRD and the results areshown in Fig. 1. From Fig. 1, the catalysts are amorphouswhen the calcination temperature is below 300 �C, and thecatalysts become more crystalline when the calcinationtemperature increases from 400 to 600 �C. The crystallinityof the catalysts became better with the increase of the cal-cination temperature.
The general morphologies and microstructure of the cat-alysts calcined at different temperature were investigated bytransmission electron microscopy (TEM) and shown inFig. 2. The TEM images reveal that most of the nanopar-ticle catalysts are uniform in size (with a narrow distribut-ing). The average nanoparticle sizes obtained from theTEM images were shown in Table 1. From Table 1, itcan be seen that the particle size increased with increasingthe calcination temperature from 300 to 600 �C, and it isnoted that the particle sizes of the samples calcined at200 and 300 �C are almost the same.
Fig. 3 depicts the typical TG–DTA curves of the samplecalcined at 300 �C. The sharp endothermic peak below100 �C could be attributed to the loss of surface absorbedwater, which was confirmed by a dramatic weight loss with
Table 1SBET, TEM particle size and T100% of the catalysts calcined at differenttemperature
Fig. 3. TG–DTA curves of the catalyst calcined at 300 �C.
4 wt% in the TG curve over the corresponding temperaturerange. The weight loss with 7 wt% between 100 �C and500 �C on the TG curve could be explained that there isthe removal of residual hydroxyl groups and crystallizationprocess in this region. Correspondingly, there is no obviousendothermic peak on the DTA curve. The exothermicpeaks at 540 �C and 610 �C on the DTA curve can beattributed to the phase transformation from c-Fe2O3 toa-Fe2O3 and the form of copper ferrite from the producedoxides [30]. Correspondingly, the TG curve does not showany noticeable mass loss. Based on the TG–DTA results, itis can be safely concluded that the major phase composi-tion of the catalyst calcined at 300 �C was CuO and
Fig. 4. XPS analysis of the catalysts calcined at 300 �C: (a) O 1 s spectrum(b) Cu 2p spectrum (c) Fe 2p spectrum.
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Fe2O3, and some of Cu(OH)2 and Fe(OH)3 is present in thecatalyst. On the basis of the above analysis, we proposethat the active phase of the catalysts is metal oxides.Although the catalyst calcined at 300 �C is not pure CuOand Fe2O3, it still exhibits the highest catalytic activity.This attributes to the related influential factors such asthe molar ratios of copper to iron, the specific surfaceareas, particle sizes and so forth.
Investigations by means of XPS were performed inorder to illuminate the surface composition of the studiedmetal oxides, but also in order to acquire detailed informa-tion on the chemical state of the cations and anions. Thespectra of the O 1s, Cu 2p and Fe 2p binding energies ofthe catalysts calcined at 300 �C are shown in Fig. 4. InFig. 4a, the binding energy of the O 1s shows a main peakat 530.2 eV, which is assigned to lattice oxygen of Fe2O3
and Cu2O phase. The peaks of Cu 2p3/2 and Cu 2p1/2 werecentered at 932.5 eV and 952.2 eV, respectively (Fig. 4b).The binding energy of Fe 2p3/2 and Fe 2p1/2 are about710.8 eV and 723.8 eV, respectively (Fig. 4c). This suggeststhe presence of Fe2O3 in the CuO/Fe2O3 catalysts [31].
The catalytic activity and stability of the catalysts cal-cined at different temperature are investigated and theresults are presented in Fig. 5. From Fig. 5a, the catalyticactivity of the catalysts increased by increasing the calcina-tion temperature from 200 to 300 �C and decreased from300 to 600 �C. The catalyst calcined at 300 �C exhibitedthe highest catalytic activity on CO oxidation with COtotal conversion at 100 �C. The catalytic activity of the cat-alysts increased with increasing the catalytic reaction tem-perature measured in the catalyst bed for all of thecatalysts. In Fig. 5b, the reactor temperature was main-tained at 90 �C, all of the catalysts showed an initialdecrease in catalytic activity over first 40 min on line, butafter this steady state period the catalytic activity wasmaintained over 500 min test period. The initial deactiva-tion at the start of the catalyst evaluation may be resultfrom the interaction between the reactants and the oxides,through which redox balance and stable surface composi-
Fig. 5. The catalytic activity (a) and stability (b) testing of the catalysts calcine(s) 600 �C.
tion were established in the initial stage. The steady statewas attained after the initial stage and maintained over500 min. On the basis of the above experiment results, itcan be firmly concluded that the catalytic activity and thestability of the copper iron composite oxide catalysts onCO oxidation at low temperature is quite high.
The molar ratios of copper to iron also have importanteffect on the catalytic activity of the catalysts, this effectwas investigated and the results were presented in Fig. 6.From Fig. 6, the catalytic activity increased with increasingthe molar ratios form 2:1 to 1:2 and decreased from 1:2 to1:4. The catalyst with molar ratio of 1:2 exhibited highestcatalytic activity on CO oxidation with lowest T100%. Theseresults indicated that increasing the copper content in thecatalysts can promote the catalytic activity of the catalyststo some extent, but it is not always so. These results prob-ably result from copper species having more chance toaggregation after their content attains a certain value. AsTang et al. [12] suggest that the catalytic activity of the cat-alysts is strongly influenced by the dispersion of the copperspecies on the catalysts, we propose that the observedchanges of the catalytic activity were probably due to theinfluence of different copper content in the catalysts, whichresults in the different dispersion of copper on the catalystsand probably changes the active sites on the surface of thecatalysts and the catalytically active components of the cat-alysts [32]. Therefore, the optimal molar ratio should befixed at 1:2 to obtain the catalyst with high catalyticactivity.
The specific surface areas (SBET), the TEM particle sizesand T100% of the catalysts calcined at different temperatureare presented in Table 1. The T100% is here defined as thetemperature at which the conversion of CO reaches100%. From Table 1, it can be seen that the catalysts cal-cined at 300 �C exhibited the highest catalytic activity onCO oxidation with the lowest T100% and possessed the larg-est SBET and least particle size, which is in accordance withthe results of catalysis for CO conversion and further con-firm our conclusion. Similarly, the SBET of the catalysts
d at different temperature (d) 200 �C, (n) 300 �C, (m) 400 �C, (.) 500 �C,
Fig. 6. Catalytic activity of the catalysts with different molar ratios(Cu:Fe).
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increases slowly by increasing the calcination temperaturefrom 200 to 300 �C and decreases rapidly from 300 to600 �C. The observed modifications in the catalytic activityof the catalysts are result from the modifications of theSBET of the catalysts, which are dependent on their differ-ent calcination temperature. Based on the above experi-mental results, it can be safely concluded that thecatalytic activity of the catalysts is strongly dependent onthe calcination temperature, the molar ratio of copper toiron, the particle sizes and the specific surface areas (SBET).The lager specific surface areas and smaller particle sizeprobably create more uniform dispersion of copper onthe catalysts, which result in more active sites on the sur-face of the catalysts and the promoting of the catalyticactivity.
4. Conclusions
In summary, the copper iron composite oxide catalystswere prepared by a co-precipitated technique. The resultsindicated that they exhibited obviously high catalytic activ-ity and stability on CO oxidation at low temperature, andthat their catalytic activity was greatly related to the calci-nation temperature, the molar ratios of copper to iron, thespecific surface areas (SBET) and the particle size. This sys-tem is now worthy of further investigation.
References
[1] Y.Z. Yuan, A.P. Kozlova, K. A sakura, H. Wan, K. Tsai, Y.Iwasawa, J. Catal. 170 (1997) 191.
