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H2 assisted decomposition of cerium nitrate to ceria with enhanced catalytic properties
Catalysis Communications 11 (2010) 848–852
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a t c o m
H2 assisted decomposition of cerium nitrate to ceria with enhanced catalytic properties I. Miguel-García, S. Parres-Esclapez, D. Lozano-Castelló, A. Bueno-López ⁎ Inorganic Chemistry Department, University of Alicante, Ap. 99 E-03080, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 5 March 2010 Accepted 9 March 2010 Available online 15 March 2010 Keywords: CeO2 CO oxidation Ceria catalyst H2 Cerium nitrate decomposition Cerium oxide
a b s t r a c t CeO2 samples were prepared by Ce(NO3)3·6H2O decomposition under synthetic air or 5%H2/N2 at 500 °C. The samples were characterized by XRD, N2 adsorption at − 196 °C, Temperature Programmed Reduction with H2 (H2–TPR) and DRIFTS. The catalytic activity for CO oxidation was also tested. The main conclusion is that H2 accelerates the decomposition of cerium nitrate to CeO2 with regard to air decomposition, allowing preparation of ceria with enhanced catalytic behavior. The gas atmosphere used in the decomposition of the nitrates has no effect in the crystal size and surface area of the ceria samples obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction CeO2-based materials are widely used in important catalytic applications such us Three Way Catalysts (TWC) [1] and fluid catalytic cracking (FCC) [2]. In addition, their potential application in some other catalytic reactions is being studied, including soot combustion [3], selective catalytic reduction of NOx [4], partial oxidation of methane [5], combustion of Volatile Organic Compounds (VOC) [6] and N2O decomposition [7], among others. CeO2 and CeO2-based solid solutions can be prepared by different methods [8]: calcination of nitrates, microemulsion, solution combustion synthesis (SCS), synthesis with templates, sol–gel, etc. Among these methods, calcination of nitrates is the simplest, cheapest and the most environmental-friendly one, since solvents, surfactants, templates or chemical reactants are not required. The decomposition of cerium nitrate is usually carried out by air calcination, and there is not much information about the preparation of ceria catalysts by nitrate decomposition under some other gas atmospheres. However, it is well established that the physic-chemical properties of cerium oxide-based catalysts change significantly upon thermal treatment under oxidizing and reducing conditions [9–11]. Recently, Kamruddin et al [12] have studied the decomposition of cerium nitrate under O2, vacuum, He and H2, but information about the effect of the decomposition conditions in the catalytic behavior of ceria has not been reported.
⁎ Corresponding author. Tel.: +34 965 90 34 00x2226; fax: +34 965 90 34 54. E-mail address: [email protected] (A. Bueno-López). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.007
In this study, the decomposition of cerium nitrate under a H2containing gas stream has been explored. The physic-chemical and catalytic properties of the resulting ceria samples have been studied and compared with those of the counterpart ceria samples prepared by air calcination. 2. Experimental 2.1. Cerium nitrate decomposition in a TG–MS set-up under H2/N2 or air 20 mg of Ce(NO3)3·6H2O (Aldrich, 99.99% purity) was decomposed in a thermobalance TG–DTA (METTLER TOLEDO model TGA/ SDTA851e/LF/1600) coupled to a mass spectrometer (PFEIFFER VACUUM model THERMOSTAR GSD301T) under 60 ml/min flow of synthetic air or 5%H2/N2, with a heating rate of 10 °C/min. 2.2. Preparation of CeO2 samples Six CeO2 samples were prepared by Ce(NO3)3·6H2O (Aldrich, 99.99% purity) decomposition (2.5 g) in a quart sintered reactor under a stream (100 ml/min) of synthetic air or 5%H2/N2. The heating rate was 10 °C/ min and the temperature was held at 500 °C for 60, 90 or 120 min. These six samples are referred to as “H2/time” or “O2/time”, for samples prepared under H2/N2 and air flow, respectively, and “time” being the duration of the heat treatment. Two additional samples, denoted by “H2/ 90′ + calc” and “O2/90′ + calc”, were prepared by air calcination (500 °C/180 min) of the ceria samples previously prepared by cerium nitrate decomposition under 5%H2/N2 or synthetic air, respectively, for 90 min.
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2.3. Characterization of CeO2 samples X-ray difractograms of the CeO2 samples were recorded in a Seifert powder diffractometer, using CuKα radiation (λ = 0.15418 nm). The diffractograms were registered between 2θ angles from 20 to 60°, with a step of 0.02° and a time per step of 3 s. The average crystal sizes were determined applying the Scherrer's equation to the peak (111) of the cubic phase. The BET surface area of the samples was calculated from N2 adsorption data at −196 °C. The adsorption isotherms were carried out in a volumetric set-up (Autosorb 1, Quantachrome) after degassing the samples at 150 °C for 4 h. Temperature Programmed Reduction experiments were carried out with H2 (H2–TPR) in a Micromeritics device, model Pulse ChemiSorb 2705. 20 mg of sample was heated at 10 °C/min from 25 to 900 °C under a 5% H2/Ar flow (35 ml/min), and the H2 consumption was monitored with a TCD. DRIFT spectra were recorded from 400 to 4000 cm−1, with a resolution of 2 cm−1, in an ATI-Mattson Infinity Series FTIR spectrometer. 64 scans were averaged to obtain each spectrum. 2.4. Catalytic oxidation of CO The catalytic oxidation of CO was selected as test reaction in order to evaluate the catalytic performance of the CeO2 samples [13]. CO oxidation experiments were performed in a cylindrical fixed-bed reactor of 5-mm i.d. at atmospheric pressure. 50 mg of catalyst was used, and the total flow rate was 100 ml/min (1000 ppm CO + 5%O2 + He balance; GHSV = 12,000 h−1). The temperature was raised from 25 to 500 °C with a heating rate of 1 °C/min. The gas composition was analysed by a HP 6890 gas chromatograph equipped with a thermal conductivity detector and two columns (Porapak Q and Molecular Sieve 13X). 3. Results and discussion 3.1. TG–MS study of cerium nitrate decomposition under H2/N2 and air flow Fig. 1a compiles the TG and DTG profiles and Fig. 1b the MS profiles of NO (MS 30), NO2 (MS 46) and H2O (MS 18). The water molecules of the cerium precursor evolve below 230 °C — not shown in Fig. 1 for clarity — and differences in the dehydration profiles between the H2/ N2 and air streams are minor. Nitrates decompose above 230 °C and both the TG–DTG profiles and the MS signals of NO and NO2 suggest that, under air, this process takes place in two consecutive steps since bimodal profiles are shown. On the contrary, single profiles are obtained under H2/N2, and the decomposition takes place faster in comparison to air decomposition. This is deduced from the more negative slope of the TG curve obtained with H2/N2 and is even better observed in the DTG profiles. The MS signals point out that H2 reduces part of the nitrates and/or the just evolved nitrogen oxides, since the NO and NO2 peaks centred at 310 °C for the air stream disappear under H2/N2. Instead of these missing NO and NO2 peaks, a H2O peak appears at a little lower temperature due to H2 oxidation. It has to be underlined that, in both experiments, the final weights of the TG curves are those expected for the CeO2 formation, that is, differences in the stoichiometry of cerium oxide obtained under air and H2/N2 cannot be inferred. This is in agreement with the XPS analysis of all the samples prepared in this study (not shown for the sake of brevity). All of them presented the same proportion of Ce3+ cations with regard to total cerium cations (36–39% of Ce3+), regardless of the gas used in the decomposition of the nitrate and the duration of the decomposition treatment.
Fig. 1. TG–MS analysis of cerium nitrate decomposition under 5%H2/N2 and air (in legends, H2 and O2 respectively). (a) TG–DTG profiles and (b) MS 18 (H2O), 30 (NO) and 46 (NO2) profiles.
3.2. Characterization of CeO2 samples by XRD and N2 adsorption at −196 °C Taking the TG–MS results into account, CeO2 samples were prepared by Ce(NO3)3·6H2O decomposition at 500 °C under synthetic air or 5%H2/N2, and the temperature was held at 500 °C for 60, 90 or 120 min. Table 1 compiles the different treatments performed along with some features of the CeO2 samples obtained. The XRD patterns of all the CeO2 samples, which have not been included for the sake of brevity, showed the typical reflexions of the fluorite structure of ceria, and no other phase but fluorite was identified in any sample. The fluorite structure, with fcc unit cell, shows reflexions at 28.5, 33.1, 47.6, and 56.5°, corresponding to the (111), (200), (220), and (311) planes [14]. The crystal size, estimated with the Scherrer's
Table 1 Preparation conditions and some features of CeO2 samples. Preparation conditionsa
BET (m2/g)
Crystal size from XRD (nm)
Evidences of nitrates?
H2/60′ H2/90′ H2/120′ H2/90′ + calcb O2/60′ O2/90′ O2/120′ O2/90′ + calc
76 80 83 72 84 80 76 75
10.1 10.8 10.6 11.3 11.0 10.1 10.1 12.6
No No No No Yes Yes Yes No
a b
The temperature of all heat treatments was 500 °C. Calc = heat treatment in air at 500 °C for 180 min.
850
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Fig. 3. Temperature Programmed Reduction with H2 of the CeO2 samples prepared under different conditions. Fig. 2. N2 adsorption and desorption isotherms of samples obtained under: (a) H2/N2 and (b) air.
equation, is quite similar for all samples and range from 10.1 to 12.6 nm (see data in Table 1), being highest for samples calcined for 3 h, as expected (samples “H2/90′ + calc” and “O2/90′ + calc”). The N2 adsorption–desorption isotherms compiled in Fig. 2 provide additional information about the ceria samples. The shape of the N2 adsorption–desorption isotherms is the same for all samples. This kind of isotherm is classified by IUPAC as type IV and the hysteresis loop is a type H1 loop, indicating that the adsorption is governed by condensation in mesopores [15–17]. The adsorption isotherms for all the ceria samples prepared are equal at low relative pressure, giving very similar values of BET surface areas for all the CeO2 samples (72–84 m2/g) (see Table 1). Few differences are observed among the N2 adsorption isotherms of some ceria samples in the range of higher relative pressures, that is, in the hysteresis loop. These differences mainly affect the calcined samples “H2/90′ + calc” and “O2/90′ + calc”, whose adsorption capacity is lowest, suggesting that the three-hours calcination step affects the mesoporosity. It is reasonable to think that this calcination treatment promotes particle sintering in a certain extent, which would be on line with the highest particle sizes determined by XRD for these two sample (Table 1). The pore size distributions calculated from the desorption branch by the Barret–Joyner–Halenda (BJH) method were very similar for all the samples, showing a very wide pore distribution covering the range of 40–150 Å with a maximum at around, 60 Å, which is not dependent on the preparation conditions. 3.3. Characterization of CeO2 samples by H2–TPR and DRIFTS The H2–TPR (Fig. 3) and DRIFTS (Fig. 4) characterization showed important differences between samples decomposed under air and H2/N2. Evidences of the presence of nitrogen-containing species left in samples decomposed under air have been obtained, but not in those decomposed under H2/N2. This observation is on line with the
conclusion of the TG–MS experiments about the faster decomposition of nitrates in the presence of H2. In order to ensure total decomposition, the three-hours calcination step was carried out in air with selected samples (denoted by calc in Table 1 and in figures). It has to be also mentioned that several samples were prepared under air and H2/N2 at 400 °C instead of 500 °C, and that all samples prepared at such lower temperature showed evidences of nitrogen species left, regardless of the duration of the heat treatment and the nature of the gas stream used. Information about the samples reducibility is obtained from H2– TPR characterisation (Fig. 3). It is generally accepted that two peaks characterise the reduction profile of pure CeO2 [18], and this kind of profile is obtained with all the CeO2 samples prepared with the H2/N2 stream. The first peak, at ca 570 °C, is attributed to the reduction of the uppermost layers of Ce4+ and the second peak, at ca 825 °C, is related with the reduction of the bulk. Both surface and bulk peaks are also observed in the samples prepared under air, but a third peak is shown at ca 450 °C, which is attributed to the decomposition or reduction of the nitrogen species left on the samples. This peak disappears after the 3 h calcination step, but this thermal treatment has a negative effect on the ceria reducibility, decreasing the area under the curves with regard to shorter treatments. For air decomposition of cerium nitrate there is no other option but to extend the thermal treatment until complete removal of the nitrogen species, with the consequent decrease in the reducibility of the oxide obtained. However, H2assisted decomposition avoids this requirement and also the loss of reducibility. DRIFT spectra of samples, compiled in Fig. 4, confirms that the H2/N2 atmosphere accelerates nitrates decomposition. This is deduced from the analysis of the region 500–1700 cm-1, where nitrite and nitrate modes appear. Bands assignment is complicated in this region because several species can contribute to absorption, including monodentate, bidentate and bridged nitrates, monodentate and bridged nitrites and nitro compounds [19,20]. On the other hand, the analysis of the region 3600–3800 cm−1, where O–H stretching modes are expected to appear
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Fig. 4. DRIFT spectra of the CeO2 samples obtained under (a) H2/N2 and (b) air.
[21], points out that the H2/N2 atmosphere favours the formation of hydroxyl groups on ceria with regard of the air calcination. The complicated spectral signature is the result of OH being present on different defect sites [21]. As a summary, it can be concluded that H2 accelerates the decomposition of cerium nitrate to CeO2 without effect neither in the stoichiometry, crystal size nor in the BET surface area of the ceria obtained. However, the faster decomposition in the presence of H2 avoids the decrease of ceria reducibility associated to long thermal treatments (deduced from the H2–TPR characterisation; Fig. 3).
3.4. Catalytic oxidation of CO Fig. 5 compiles the CO oxidation profiles obtained with all the nitrate-free samples. Catalytic tests have not been performed with the nitrate-containing samples because these nitrates would react with CO and the behavior of the nitrate-containing samples would not be that of a true catalyst. CeO2 samples prepared by H2-assisted decomposition of cerium nitrate are able to catalyse the oxidation of CO from 250 °C and reach the complete oxidation at 410 °C. The calcined samples are less active and the temperature must rise until
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References
Fig. 5. Catalytic oxidation of CO with CeO2 samples prepared under different conditions.
475 °C for total CO oxidation. This behavior is consistent with the better reducibility of the ceria samples prepared under H2/N2. However, this benefit disappears after calcinations of the sample. 4. Conclusions This study demonstrates that H2 accelerates the decomposition of cerium nitrate to CeO2 with regard to air decomposition. This has no effect in the crystal size and surface area of the ceria obtained but allows obtaining ceria with better reducibility and enhanced catalytic properties. Acknowledgments The financial support of the Spanish Ministry of Science and Innovation (project CIT-420000-2009-48) and Generalitat Valenciana (PROMETEO/2009/047) is acknowledged.
[1] J. Kaspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285–298. [2] A. Trovarelli, C. Leitenburg, M. De Boaro, G. Dolcetti, Catal. Today 50 (1999) 353–367. [3] I. Atribak, I. Such-Basáñez, A. Bueno-López, A. García-García, J. Catal. 250 (2007) 75–84. [4] M. Adamowska, S. Muller, P. Da Costa, A. Krzton, P. Burg, Appl. Catal. B 74 (2007) 278–289. [5] S. Eriksson, S. Rojas, M. Boutonnet, J.L.G. Fierro, Appl. Catal. A 326 (2007) 8–16. [6] J.I. Gutiérrez-Ortiz, B. de Rivas, R. López-Fonseca, J.R. González-Velasco, Appl. Catal. B 65 (2006) 191–200. [7] A. Bueno-López, I. Such-Basáñez, C. Salinas-Martínez de Lecea, J. Catal. 244 (2006) 102–112. [8] A. Trovarelli, Catalysis by Ceria and Related Materials, Catalytic Science Series, vol. 2, Imperial College Press, London, 2002. [9] F. Fally, V. Perrichon, H. Vidal, J. Kaspar, G. Blanco, J.M. Pintado, S. Bernal, G. Colon, M. Daturi, J.C. Lavalley, Catal. Today 59 (2000) 373–386. [10] R.T. Baker, S. Bernal, G. Blanco, A.M. Cordón, J.M. Pintado, J.M. RodríguezIzquierdo, F. Fally, V. Perrichon, Chem. Commun. 2 (1999) 149–150. [11] H. Vidal, S. Bernal, J. Kaspar, M. Pijolat, V. Perrichon, G. Blanco, J.M. Pintado, R.T. Bakera, G. Colon, F. Fally, Catal. Today 54 (1999) 93–100. [12] M. Kamruddin, P.K. Ajikumar, G. Mangamma, V.K. Mittal, S.V. Narasimhan, A.K. Tyagi, J. Nanosci. Nanotechnol. 9 (2009) 1–5. [13] K. Min, M.W. Song, C.H. Lee, Appl. Catal. A 251 (2003) 143–156. [14] D. Terribile, A. Trovarelli, J. Llorca, C. de Leitenburg, G. Dolcetti, Catal. Today 43 (1998) 79–88. [15] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure & Appl. Chem. 57 (1985) 603–619. [16] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.M. Haynes, N. Pernicone, J.D. F. Ramsay, K.S.W. Sing, K.K. Unger, Pure & Appl. Chem. 66 (1994) 1739–1758. [17] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by Powders and Porous Solids. Principles, Methodology and Applications, Academic Press, 1999. [18] G.L. Markaryan, L.N. Ikryannikova, G.P. Muravieva, A.O. Turakulova, B.G. Kostyuk, E.V. Lunina, V.V. Lunin, E. Zhilinskaya, A. Aboukais, Colloids Surf. A 151 (1999) 435–447. [19] K.I. Hadjiivanov, Catal. Rev. Sci. Eng. 42 (2000) 71–144. [20] I. Atribak, B. Azambre, A. Bueno López, A. García-García, Appl. Catal. B 92 (2009) 126–137. [21] P. Du, A. Bueno-López, M. Verbaas, A.R. Almeida, M. Makkee, J.A. Moulijn, G. Mul, J. Catal. 260 (2008) 75–80.
Contents lists available at ScienceDirect
Catalysis Communications j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / c a t c o m
H2 assisted decomposition of cerium nitrate to ceria with enhanced catalytic properties I. Miguel-García, S. Parres-Esclapez, D. Lozano-Castelló, A. Bueno-López ⁎ Inorganic Chemistry Department, University of Alicante, Ap. 99 E-03080, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 2 February 2010 Received in revised form 5 March 2010 Accepted 9 March 2010 Available online 15 March 2010 Keywords: CeO2 CO oxidation Ceria catalyst H2 Cerium nitrate decomposition Cerium oxide
a b s t r a c t CeO2 samples were prepared by Ce(NO3)3·6H2O decomposition under synthetic air or 5%H2/N2 at 500 °C. The samples were characterized by XRD, N2 adsorption at − 196 °C, Temperature Programmed Reduction with H2 (H2–TPR) and DRIFTS. The catalytic activity for CO oxidation was also tested. The main conclusion is that H2 accelerates the decomposition of cerium nitrate to CeO2 with regard to air decomposition, allowing preparation of ceria with enhanced catalytic behavior. The gas atmosphere used in the decomposition of the nitrates has no effect in the crystal size and surface area of the ceria samples obtained. © 2010 Elsevier B.V. All rights reserved.
1. Introduction CeO2-based materials are widely used in important catalytic applications such us Three Way Catalysts (TWC) [1] and fluid catalytic cracking (FCC) [2]. In addition, their potential application in some other catalytic reactions is being studied, including soot combustion [3], selective catalytic reduction of NOx [4], partial oxidation of methane [5], combustion of Volatile Organic Compounds (VOC) [6] and N2O decomposition [7], among others. CeO2 and CeO2-based solid solutions can be prepared by different methods [8]: calcination of nitrates, microemulsion, solution combustion synthesis (SCS), synthesis with templates, sol–gel, etc. Among these methods, calcination of nitrates is the simplest, cheapest and the most environmental-friendly one, since solvents, surfactants, templates or chemical reactants are not required. The decomposition of cerium nitrate is usually carried out by air calcination, and there is not much information about the preparation of ceria catalysts by nitrate decomposition under some other gas atmospheres. However, it is well established that the physic-chemical properties of cerium oxide-based catalysts change significantly upon thermal treatment under oxidizing and reducing conditions [9–11]. Recently, Kamruddin et al [12] have studied the decomposition of cerium nitrate under O2, vacuum, He and H2, but information about the effect of the decomposition conditions in the catalytic behavior of ceria has not been reported.
⁎ Corresponding author. Tel.: +34 965 90 34 00x2226; fax: +34 965 90 34 54. E-mail address: [email protected] (A. Bueno-López). 1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2010.03.007
In this study, the decomposition of cerium nitrate under a H2containing gas stream has been explored. The physic-chemical and catalytic properties of the resulting ceria samples have been studied and compared with those of the counterpart ceria samples prepared by air calcination. 2. Experimental 2.1. Cerium nitrate decomposition in a TG–MS set-up under H2/N2 or air 20 mg of Ce(NO3)3·6H2O (Aldrich, 99.99% purity) was decomposed in a thermobalance TG–DTA (METTLER TOLEDO model TGA/ SDTA851e/LF/1600) coupled to a mass spectrometer (PFEIFFER VACUUM model THERMOSTAR GSD301T) under 60 ml/min flow of synthetic air or 5%H2/N2, with a heating rate of 10 °C/min. 2.2. Preparation of CeO2 samples Six CeO2 samples were prepared by Ce(NO3)3·6H2O (Aldrich, 99.99% purity) decomposition (2.5 g) in a quart sintered reactor under a stream (100 ml/min) of synthetic air or 5%H2/N2. The heating rate was 10 °C/ min and the temperature was held at 500 °C for 60, 90 or 120 min. These six samples are referred to as “H2/time” or “O2/time”, for samples prepared under H2/N2 and air flow, respectively, and “time” being the duration of the heat treatment. Two additional samples, denoted by “H2/ 90′ + calc” and “O2/90′ + calc”, were prepared by air calcination (500 °C/180 min) of the ceria samples previously prepared by cerium nitrate decomposition under 5%H2/N2 or synthetic air, respectively, for 90 min.
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2.3. Characterization of CeO2 samples X-ray difractograms of the CeO2 samples were recorded in a Seifert powder diffractometer, using CuKα radiation (λ = 0.15418 nm). The diffractograms were registered between 2θ angles from 20 to 60°, with a step of 0.02° and a time per step of 3 s. The average crystal sizes were determined applying the Scherrer's equation to the peak (111) of the cubic phase. The BET surface area of the samples was calculated from N2 adsorption data at −196 °C. The adsorption isotherms were carried out in a volumetric set-up (Autosorb 1, Quantachrome) after degassing the samples at 150 °C for 4 h. Temperature Programmed Reduction experiments were carried out with H2 (H2–TPR) in a Micromeritics device, model Pulse ChemiSorb 2705. 20 mg of sample was heated at 10 °C/min from 25 to 900 °C under a 5% H2/Ar flow (35 ml/min), and the H2 consumption was monitored with a TCD. DRIFT spectra were recorded from 400 to 4000 cm−1, with a resolution of 2 cm−1, in an ATI-Mattson Infinity Series FTIR spectrometer. 64 scans were averaged to obtain each spectrum. 2.4. Catalytic oxidation of CO The catalytic oxidation of CO was selected as test reaction in order to evaluate the catalytic performance of the CeO2 samples [13]. CO oxidation experiments were performed in a cylindrical fixed-bed reactor of 5-mm i.d. at atmospheric pressure. 50 mg of catalyst was used, and the total flow rate was 100 ml/min (1000 ppm CO + 5%O2 + He balance; GHSV = 12,000 h−1). The temperature was raised from 25 to 500 °C with a heating rate of 1 °C/min. The gas composition was analysed by a HP 6890 gas chromatograph equipped with a thermal conductivity detector and two columns (Porapak Q and Molecular Sieve 13X). 3. Results and discussion 3.1. TG–MS study of cerium nitrate decomposition under H2/N2 and air flow Fig. 1a compiles the TG and DTG profiles and Fig. 1b the MS profiles of NO (MS 30), NO2 (MS 46) and H2O (MS 18). The water molecules of the cerium precursor evolve below 230 °C — not shown in Fig. 1 for clarity — and differences in the dehydration profiles between the H2/ N2 and air streams are minor. Nitrates decompose above 230 °C and both the TG–DTG profiles and the MS signals of NO and NO2 suggest that, under air, this process takes place in two consecutive steps since bimodal profiles are shown. On the contrary, single profiles are obtained under H2/N2, and the decomposition takes place faster in comparison to air decomposition. This is deduced from the more negative slope of the TG curve obtained with H2/N2 and is even better observed in the DTG profiles. The MS signals point out that H2 reduces part of the nitrates and/or the just evolved nitrogen oxides, since the NO and NO2 peaks centred at 310 °C for the air stream disappear under H2/N2. Instead of these missing NO and NO2 peaks, a H2O peak appears at a little lower temperature due to H2 oxidation. It has to be underlined that, in both experiments, the final weights of the TG curves are those expected for the CeO2 formation, that is, differences in the stoichiometry of cerium oxide obtained under air and H2/N2 cannot be inferred. This is in agreement with the XPS analysis of all the samples prepared in this study (not shown for the sake of brevity). All of them presented the same proportion of Ce3+ cations with regard to total cerium cations (36–39% of Ce3+), regardless of the gas used in the decomposition of the nitrate and the duration of the decomposition treatment.
Fig. 1. TG–MS analysis of cerium nitrate decomposition under 5%H2/N2 and air (in legends, H2 and O2 respectively). (a) TG–DTG profiles and (b) MS 18 (H2O), 30 (NO) and 46 (NO2) profiles.
3.2. Characterization of CeO2 samples by XRD and N2 adsorption at −196 °C Taking the TG–MS results into account, CeO2 samples were prepared by Ce(NO3)3·6H2O decomposition at 500 °C under synthetic air or 5%H2/N2, and the temperature was held at 500 °C for 60, 90 or 120 min. Table 1 compiles the different treatments performed along with some features of the CeO2 samples obtained. The XRD patterns of all the CeO2 samples, which have not been included for the sake of brevity, showed the typical reflexions of the fluorite structure of ceria, and no other phase but fluorite was identified in any sample. The fluorite structure, with fcc unit cell, shows reflexions at 28.5, 33.1, 47.6, and 56.5°, corresponding to the (111), (200), (220), and (311) planes [14]. The crystal size, estimated with the Scherrer's
Table 1 Preparation conditions and some features of CeO2 samples. Preparation conditionsa
BET (m2/g)
Crystal size from XRD (nm)
Evidences of nitrates?
H2/60′ H2/90′ H2/120′ H2/90′ + calcb O2/60′ O2/90′ O2/120′ O2/90′ + calc
76 80 83 72 84 80 76 75
10.1 10.8 10.6 11.3 11.0 10.1 10.1 12.6
No No No No Yes Yes Yes No
a b
The temperature of all heat treatments was 500 °C. Calc = heat treatment in air at 500 °C for 180 min.
850
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Fig. 3. Temperature Programmed Reduction with H2 of the CeO2 samples prepared under different conditions. Fig. 2. N2 adsorption and desorption isotherms of samples obtained under: (a) H2/N2 and (b) air.
equation, is quite similar for all samples and range from 10.1 to 12.6 nm (see data in Table 1), being highest for samples calcined for 3 h, as expected (samples “H2/90′ + calc” and “O2/90′ + calc”). The N2 adsorption–desorption isotherms compiled in Fig. 2 provide additional information about the ceria samples. The shape of the N2 adsorption–desorption isotherms is the same for all samples. This kind of isotherm is classified by IUPAC as type IV and the hysteresis loop is a type H1 loop, indicating that the adsorption is governed by condensation in mesopores [15–17]. The adsorption isotherms for all the ceria samples prepared are equal at low relative pressure, giving very similar values of BET surface areas for all the CeO2 samples (72–84 m2/g) (see Table 1). Few differences are observed among the N2 adsorption isotherms of some ceria samples in the range of higher relative pressures, that is, in the hysteresis loop. These differences mainly affect the calcined samples “H2/90′ + calc” and “O2/90′ + calc”, whose adsorption capacity is lowest, suggesting that the three-hours calcination step affects the mesoporosity. It is reasonable to think that this calcination treatment promotes particle sintering in a certain extent, which would be on line with the highest particle sizes determined by XRD for these two sample (Table 1). The pore size distributions calculated from the desorption branch by the Barret–Joyner–Halenda (BJH) method were very similar for all the samples, showing a very wide pore distribution covering the range of 40–150 Å with a maximum at around, 60 Å, which is not dependent on the preparation conditions. 3.3. Characterization of CeO2 samples by H2–TPR and DRIFTS The H2–TPR (Fig. 3) and DRIFTS (Fig. 4) characterization showed important differences between samples decomposed under air and H2/N2. Evidences of the presence of nitrogen-containing species left in samples decomposed under air have been obtained, but not in those decomposed under H2/N2. This observation is on line with the
conclusion of the TG–MS experiments about the faster decomposition of nitrates in the presence of H2. In order to ensure total decomposition, the three-hours calcination step was carried out in air with selected samples (denoted by calc in Table 1 and in figures). It has to be also mentioned that several samples were prepared under air and H2/N2 at 400 °C instead of 500 °C, and that all samples prepared at such lower temperature showed evidences of nitrogen species left, regardless of the duration of the heat treatment and the nature of the gas stream used. Information about the samples reducibility is obtained from H2– TPR characterisation (Fig. 3). It is generally accepted that two peaks characterise the reduction profile of pure CeO2 [18], and this kind of profile is obtained with all the CeO2 samples prepared with the H2/N2 stream. The first peak, at ca 570 °C, is attributed to the reduction of the uppermost layers of Ce4+ and the second peak, at ca 825 °C, is related with the reduction of the bulk. Both surface and bulk peaks are also observed in the samples prepared under air, but a third peak is shown at ca 450 °C, which is attributed to the decomposition or reduction of the nitrogen species left on the samples. This peak disappears after the 3 h calcination step, but this thermal treatment has a negative effect on the ceria reducibility, decreasing the area under the curves with regard to shorter treatments. For air decomposition of cerium nitrate there is no other option but to extend the thermal treatment until complete removal of the nitrogen species, with the consequent decrease in the reducibility of the oxide obtained. However, H2assisted decomposition avoids this requirement and also the loss of reducibility. DRIFT spectra of samples, compiled in Fig. 4, confirms that the H2/N2 atmosphere accelerates nitrates decomposition. This is deduced from the analysis of the region 500–1700 cm-1, where nitrite and nitrate modes appear. Bands assignment is complicated in this region because several species can contribute to absorption, including monodentate, bidentate and bridged nitrates, monodentate and bridged nitrites and nitro compounds [19,20]. On the other hand, the analysis of the region 3600–3800 cm−1, where O–H stretching modes are expected to appear
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Fig. 4. DRIFT spectra of the CeO2 samples obtained under (a) H2/N2 and (b) air.
[21], points out that the H2/N2 atmosphere favours the formation of hydroxyl groups on ceria with regard of the air calcination. The complicated spectral signature is the result of OH being present on different defect sites [21]. As a summary, it can be concluded that H2 accelerates the decomposition of cerium nitrate to CeO2 without effect neither in the stoichiometry, crystal size nor in the BET surface area of the ceria obtained. However, the faster decomposition in the presence of H2 avoids the decrease of ceria reducibility associated to long thermal treatments (deduced from the H2–TPR characterisation; Fig. 3).
3.4. Catalytic oxidation of CO Fig. 5 compiles the CO oxidation profiles obtained with all the nitrate-free samples. Catalytic tests have not been performed with the nitrate-containing samples because these nitrates would react with CO and the behavior of the nitrate-containing samples would not be that of a true catalyst. CeO2 samples prepared by H2-assisted decomposition of cerium nitrate are able to catalyse the oxidation of CO from 250 °C and reach the complete oxidation at 410 °C. The calcined samples are less active and the temperature must rise until
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References
Fig. 5. Catalytic oxidation of CO with CeO2 samples prepared under different conditions.
475 °C for total CO oxidation. This behavior is consistent with the better reducibility of the ceria samples prepared under H2/N2. However, this benefit disappears after calcinations of the sample. 4. Conclusions This study demonstrates that H2 accelerates the decomposition of cerium nitrate to CeO2 with regard to air decomposition. This has no effect in the crystal size and surface area of the ceria obtained but allows obtaining ceria with better reducibility and enhanced catalytic properties. Acknowledgments The financial support of the Spanish Ministry of Science and Innovation (project CIT-420000-2009-48) and Generalitat Valenciana (PROMETEO/2009/047) is acknowledged.
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