Al2O3 meso phase catalysts (M=Ce, V, Cu)

Materials Letters 113 (2013) 96–99

Contents lists available at ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Synthesis, characterisation, and CO oxidation activity of M/Al2O3 meso phase catalysts (M¼ Ce, V, Cu) N.K. Renuka n, A.V. Shijina 1, A.K. Praveen 1 Department of Chemistry, University of Calicut, Kerala 673635, India

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2013 Accepted 18 September 2013 Available online 25 September 2013

Here we compare the ability of V, Ce, and Cu supported mesoporous alumina catalysts in facilitating the oxidation of carbon monoxide, an identified major air pollutant. The catalyst systems are characterised using physico–chemical methods including SEM, TEM, XRD, BET surface area analysis, porosity studies, TPR, and DR-UV analysis. Cu/Al system has been identified as the most efficient catalyst among the series. The highest activity of copper based system is attributed to the low temperature reducibility of the dispersed copper ions when compared to the other two active metal ion species. & 2013 Elsevier B.V. All rights reserved.

Keywords: Porous materials Texture Supported metal oxide CO oxidation

1. Introduction Today, research on carbon monoxide removal is in full swing universally, as the presence of CO in atmosphere pilots to serious environmental problems. Automotive exhaust gas has been identified as a major source for atmospheric CO. Reduction of CO amount in automotive exhaust gas is one of the greatest challenges due to the strict emission regulations adopted all over the world. Catalytic oxidative removal is an established facile method to minimise the concentration of this hazardous gas in vehicle exhaust gas. Supported noble metals as well as metal oxides have been extensively surveyed to this effect. Among the various metal oxides that serve as active support, alumina with high surface area and fine textural features stands superior in catalytic field. Noble metal supported on alumina and mixed/supported alumina have been investigated by various research groups [1–8]. Other applications of alumina reach out various fields including lithographic pattern designing, bone demineralisation process, chromatography etc. [9,10]. The present study is aimed at comparing the CO oxidation activity of some well acclaimed oxidation catalysts, vis., CuO, V2O5 and CeO2, that are dispersed over meso alumina. The mesoporous phase is marked by higher surface area and desirable pore features than conventional alumina. The only report on the said reaction over meso alumina discusses the activity of dispersed noble metals, Pt, Pd and Ag [11]. In our work, meso alumina support is synthesised by dodecyl amine assisted template route, and is n

Corresponding author. Tel.: þ 91 494 2401414; fax: þ91 494 2400269. E-mail address: [email protected] (N.K. Renuka). 1 Tel.: þ91 494 2401144; fax : þ 91 494 2400269.

0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.09.073

characterised adopting various analytical methods. The exhibited trend in carbon monoxide activity is correlated with the reducibility of the catalyst systems. The study reveals the superiority of dispersed copper oxide in facilitating carbon monoxide oxidation.

2. Materials and methods Mesoporous alumina support has been prepared according to the reported procedure [12]. Wet impregnation procedure using ammonium metavanadae was used to yield V/Al catalyst system. Ce and Cu loaded aluminas were synthesised using deposition precipitation of oxides from the respective nitrate solutions by ammonia precipitation. The weight percentage of the active metal oxide in the support has been selected as the criterion for naming the catalysts. The low and wide angle powder X-ray diffraction (XRD) patterns of the support were obtained by a Brucker Nanostar instrument and a Rigaku D/MAX- diffractometer respectively. (Fourier Transform Infra red) FTIR spectra were recorded using JASCO FTIR-4100 spectrometer. Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) images were achieved with a Philips CM 200 transmission electron microscope, and JEOL Model JSM 6390LV instrument respectively. Diffuse Reflectance UV (DR-UV) spectra were taken with BaSO4 as reference using a Jasco V-550 spectrophotometer. The BET surface area and pore size distribution were obtained using a Micromeritics Gemini Surface Area analyser by the nitrogen adsorption method. Micromeritics Pulse Chemisorb-2705 instrument yielded the temperature programmed reduction (TPR) analysis data. In order to trace the catalytic activity, 0.3 g of the catalyst samples were activated at 300 1C for 1 h, and the reaction was carried out

N.K. Renuka et al. / Materials Letters 113 (2013) 96–99

in a quartz reactor. Gas flow was adjusted to a space velocity of 28,800 h  1, which contained 6% V/V of oxygen, 1% V/V of carbon monoxide and the rest nitrogen. CO and CO2 in the outlet gas are separated using “Pouropack” packed column followed by converting to methane by the “methanator” and finally detected separately by GC-FID.

97

assisted route (Fig. 1(b)). Type IV adsorption isotherm typical for mesoporous materials with H1 type hysteresis loop was displayed by alumina (not included here). Mesopores of size 3–14 nm were also noticed in the support. Absence of bands characteristic of dodecyl amine (2920 and 2850 cm  1) was confirmed from the FTIR spectra, indicating complete template removal from the calcined support material. XRD pattern (Fig. 2(a)) revealed a rather amorphous nature of meso alumina. However, from the relative intensity of the diffraction peaks, the γ-phase was of alumina was identified in the support, that displayed major peaks at 2θ values 36.791, 45.731 and 66.491, corresponding to d values 2.53, 1.93 and 1.39 nm respectively (JCPDS reference number 00-010-0425). The XRD peak characteristic of meso order of alumina, appearing at high d spacing is depicted in Fig. 2(b), which indicated worm hole

3. Results and discussion Identification of texture and mesoporous morphology of alumina are done from the electron microscopy images displayed in Fig. 1, which reveals nanoparticles of size below 50 nm (Fig. 1(a)), and worm hole type mesopores in alumina prepared via surfactant

Fig. 1. SEM image (a) and TEM image (b) of γ-Al2O3 mesophase.

Intensity (a.u.)

16000

Intensity

12000

8000

4000

10

20

30

40

50

60

70

80

0

1

2

3

4

5

2θ (°)

2θ (°)

Fig. 2. XRD patterns of catalysts. Wide angle (a) and low angle pattern of support (b).

1.0

3.0

(αhυ)2

Absorbance

1.0

2Ce/Al 2V/Al 2Cu/Al

0.6

Pore volume (cm3/g)

1.2 0.8

0.4

0.8

2V/Al 2Ce/Al 2Cu/Al

0.6 0.4

0.2

0.2 0.0 200

300

400

500

600

Wavelength (nm)

700

800

2.0

2.5 Al2O3 Cu/Al V/Al Ce/Al

2.0 1.5 1.0 0.5 0.0

2.5

3.0

Energy (eV)

3.5

4.0

0

10

20

30

40

50

60

70

Pore width (nm)

Fig. 3. UV absorbance spectra of the supported catalysts (a); Kubelka–Munk plot (b); pore size distribution of support and 2 wt% metal supported systems (c).

98

N.K. Renuka et al. / Materials Letters 113 (2013) 96–99

type mesopores, marked by a single peak that vanishes in intensity [13]. The diffraction patterns of supported analogues differed in no way from the pure support, indicating that the added metal oxides are well dispersed over alumina, rather than forming crystallites. This also could be a signal for structural changes or some kind of axial growth of active species on alumina lattice. Pure alumina showed no absorption in the UV region, while the active metal oxide loaded systems exhibited strong absorption bands. The spectra of the three supported analogues are provided in Fig. 3(a). It has been established that surface vanadia present in tetrahedral coordination generates UV bands at 240 and 300 nm [14], predicting the absence of crystalline vanadia in V/Al system [15]. In Ce/Al sample, the absorption maxima were located at 242, 280, and 350 nm, which account for O2 Ce3 þ ; O2 Ce4 þ charge transfer transitions, and interband transitions, respectively [16]. The relevant spectrum of Cu analogue exhibited strong absorption bands at 210–270 and 357 nm, and an intense band at 600–800 nm. The band at 210–270 nm indicates ligand-tometal charge-transfer (LMCT) transition of Cu2ohþ cations [17–20]. The bands at 350 nm, and 600–900 nm show copper clusters different from bulk CuO [21], as reported by Mendes and Schmal, thus confirming the dispersed copper oxide species on alumina. It can be noted that all the characterisation tools discussed so far are in harmony in predicting the absence of bulk dopants in the modified meso alumina systems. From the UV absorption edge energy, calculation of indirect band gap energy of the material is possible, which is inversely proportional to the size of the dispersed metal oxide particle in nano regime. The data in this regard are displayed in Table 1. It is evident that the mesoporous character of the support is maintained even after the addition of active species. As expected, the surface area and pore volume showed a decrease, concomitant with a significant reduction in average pore size (Table 1), which is attributed to the filling of mesopores of the support by the addition of metal oxide. The pore size distribution in the supported samples, presented in Fig. 3(c) justifies this observation. Temperature programmed reduction using hydrogen is an appreciated technique in explaining the oxidising ability of metal oxide

catalysts. It was noticed that pure alumina was irreducible at the experimental TPR range. On the other hand, the supported catalysts were reducible in this temperature range, which generated reduction peaks as displayed in Fig. 4(a). As evident from the plot, the reduction temperature followed the order Ce/Al 4 V/Al4 Cu/Al. The peaks are attributed to the reduction of dispersed metal ion species in the supported analogues. Carbon monoxide oxidation activity (experimental error  1%) of the systems is illustrated in Fig. 4(b). It is interesting to note that vanadia and ceria loaded alumina were remarkably less active which affectedo20% conversion of CO, while Cu/Al system with the same wt% of the active species could achieve 68% conversion at the maximum reaction temperature. A comparison of the catalytic activity in terms of T10 (1C) (temperature required to achieve 10% conversion of the reactant) revealed that the parameters were 307, 315 and 280 1C respectively for Cu, V and Ce catalysts. The superiority of Cu/Al system in facilitating CO oxidation is quite clear from this data. The TPR analysis indicated that easily available lattice oxygens, that get reduced at lower temperature is observed in Cu/Al2O3, and the oxygen exchange in V and Ce catalysts takes place only at temperature4450 1C, which makes the oxidation process impossible at the experimental reaction temperatures. This in turn suggests that the Mars and Van Krevelen Mechanism is supposed to operate during the oxidation of CO [22]. The above said mechanism is characterised by an initial oxidation of the reactant by lattice oxygen, which in turn is replenished by the gas phase oxygen. To investigate further on this, copper loading was enhanced to 11 wt% in alumina (11Cu/Al system). The result shows that the conversion of CO reached  100% over this sample. This observation can be justified by the presence of dispersed copper ions which are reducible at further low temperature, as evident from the temperature programmed reduction profile. Hence the role of reducibility of metal oxide species in deciding the oxidation activity has been confirmed from this study.

Table 1 Characterisation data of the catalyst systems.

4. Conclusions

Catalyst

Band gap (eV)

Surface area (m2/g)

Pore volume (cm3/g)

Average pore width (nm)

Al2O3 2Cu/Al 2V/Al 2Ce/Al

– 2.96 2.87 2.93

320.99 243.85 247.6 240.32

0.92 0.58 0.72 0.68

9.9 9.2 9.2 9.1

The CO oxidation activity of V, Ce and Cu loaded meso alumina has been reported here. Mesoporous nature of alumina was maintained even after the incorporation of active ions, and the surface area and other porosity parameters of support are decreased upon their addition. Among the series of active catalysts examined here, easily reducible CuO has been proven to be the most efficient for affecting CO oxidation.

120

H2 Consumption

2Cu/Al

2V/Al

2Ce/Al

Conversion of CO (%)

100 11 Cu/Al

2Ce/Al 2V/Al 2Cu/Al 11Cu/Al

80 60 40 20 0

200

300

400

500

Temperature (°C)

600

700

0

50

100

150

200

250

300

350

Temperature (°C)

Fig. 4. TPR profiles of the impregnated samples (a). Catalytic activity of M/Al systems for CO oxidation (b).

400

N.K. Renuka et al. / Materials Letters 113 (2013) 96–99

Acknowledgement The corresponding author acknowledges the financial assistance received from the Kerala State Council for Science, Technology and Environment. References [1] Bourane A, Bianchi D. Journal of Catalysis 2002;209:126–34. [2] Uysal G, Akin AN, Onsan ZI, Yildirim R. Catalysis Letters 2006;111:173–6. [3] Ko EY, Park ED, Seo KW, Lee HC, Lee D, Kim S. Catalysis Letters 2006;110:275–9. [4] Ozyonum GN, Akin AN, Yildirim R. Turkey Journal Chemistry 2007;31:445–53. [5] Hu YH, Dong L, Wang J, Ding WP, Chen Y. Journal of Molecular Catalysis A: Chemistry 2000;162:307–16. [6] Wan HQ, Li D, Dai Y, Hu YH, Zhang YH, Liu LJ, Zhao B, Liu B, Sun KQ, Dong L, Chen Y. Applied Catalysis A: General 2009;360:26–32. [7] Yu Q, Liu LJ, Dong LH, Li D, Liu B, Gao F, Sun KQ, Dong L, Chen Y. Applied Catalysis B: Environmental 2010;96:350–60. [8] Suhonen S, Valden M, Hietikko M, Laitinen R, Savimäki A, Härkönen M. Applied Catalysis A: General 2001;218:151–60.

99

[9] Banerjee N, Huijben M, Koster G, Rijnders G. Applied Physics Letters 2012;100:041601. [10] Banerjee N, Koster G, Rijnders G. Applied Physics Letters 2013;102:142909. [11] Li ZX, Shi FB, Li LL, Zhang T, Yan CH. Physical Chemistry and Chemical Physics 2011;13:2488–91. [12] Renuka NK, Shijina AV, Praveen AK. Materials Letters 2012;82:42–4. [13] Zimny K, Roques-Carmes T, Carteret C, Stebe MJ, Blin JL. Journal of Physical Chemistry C 2012;116:6585–94. [14] Miao S, Ma D, Zhu Q, Zheng H, Jia G, Zhaou S, Bao X. Journal of Natural Gas Chemistry 2005;14:77–87. [15] Shijina AV, Renuka NK. Reaction Kinetics and Catalysis Letters 2008;94:261–70. [16] Yu Q, Wu X, Yao X, Liu B, Gao F, Wang J, Dong L. Catalysis Communications 2011;12:1311–7. [17] Rao GR, Sahu HR. Proceedings of the Indian Academy of Science (Chemical Sciences) 2001;113(5 and 6):651–8. [18] Martinez-Arias A, Garcıa MF, Salamanca LN, Valenzuela RX, Conesa JC, Soria J. Journal of Physical Chemistry B 2000;104:4038–46. [19] Chen L, Horiuchi T, Osaki T, Mori T. Applied Catalysis 1999;B 23:259–69. [20] Velu S, Suzuki K, Okazaki M, Kapoor MP, Osaki T, Ohashi F. Journal of Catalysis 2000;194:373–84. [21] Mendes FMT, Schmal M. Applied Catalysis :A 1997;151:393–408. [22] Liu Y, Wen C, Guo Y, Lu G, Wang Y. Journal of Physics Chemistry C 2010;114:9889–97.