J Sol-Gel Sci Techn (2006) 37: 169–174 DOI 10.1007/s10971-005-6623-0
ORIGINAL ARTICLE
Catalytic combustion of ethyl acetate over ceria-promoted platinum supported on Al2 O3 and ZrO2 catalysts Gina Pecchi · Patricio Reyes · M. Graciela Jiliberto · T. L´opez · J. L. G. Fierro
Published online: 24 February 2006 C Springer Science + Business Media, Inc. 2006
Abstract Platinum-supported catalysts prepared by impregnation of mixed CeO2 /Al2 O3 and CeO2 /ZrO2 oxides using the sol-gel method were characterized and used in the combustion of ethyl acetate. In each series, the effect of CeO2 loading (3 and 5 wt%) was studied. Characterization data from the studied catalysts (specific area measurements, hydrogen chemisorption, programmed temperature reduction (TPR), X-ray diffraction (XRD), photoelectron spectra (XPS) and transmission electron microscopy (TEM)) revealed significant changes in porosity and metal dispersion in each series. The catalytic activity of the solids, evaluated in the total combustion of ethyl acetate, exhibited a positive effect with the addition of Ce in the zirconia series whereas no significant changes was observed in the alumina series. Keywords Platinum . Ceria . Zirconia . Combustion . Ethyl acetate.
1. Introduction Catalytic oxidation of hydrocarbons, volatile organic compounds (VOC) is a widely used method in connection with G. Pecchi () · P. Reyes · M. G. Jiliberto Departamento de F´ısico-Qu´ımica, Facultad de Ciencias Qu´ımicas, Universidad de Concepci´on, Casilla 160-C Concepci´on, Chile e-mail:
[email protected] T. L´opez Universidad Aut´onoma Metropolitana-Iztapalapa, 09340 M´exico, D.F.A.P., 55-534 J. L. G. Fierro Instituto de Cat´alisis y Petroleoqu´ımica, CSIC, Madrid, Spain
the protection of the atmospheric environment. The aim is to convert VOCs to carbon dioxide and water. Catalysts able to perform combustion are divided in two groups: noble metals and transition metal oxides which are less efficient but more resistant towards high temperatures. With regard to noble metal-supported catalysts, most of the studies have been focused on Pt and Pd [1, 2]. The nature of the support and the promoters [3] has also been investigated because using partially reducible supports or promotors can increase the paired Pt◦ -Ptδ+ species responsible for the catalytic activity [4, 5]. During recent years, cerium oxide has become one of the most studied and applied promoters in heterogeneous catalytic reactions because of its ability to form oxygen vacancies, which promote catalytic oxidation reactions [6]. In other reactions such as hydrogenation, the enhancement in catalytic activity has been attributed to metal-ceria interactions [7, 8]. The ability of CeO2 -based promoters to act as an oxygen buffer by releasing/absorbing oxygen through redox processes involving the Ce4+ /Ce3+ couple (i.e. the oxygen storage capacity) provides an interesting way to increase the efficiency of three-way catalysts by enlarging the air-tofuel operating window. Improved stability against thermal sintering as well as enhanced redox properties with respect to CeO2 have been suggested as the main advantages of mixed oxides [9, 10]. The preparation of CeO2 /Al2 O3 or CeO2 /ZrO2 supports is currently performed by cerium impregnation of the supports [11] or by firing mixtures of oxides [12]. Since ZrO2 is able to strongly interact with the metal components leading to an enhancement in the stabilization of metal clusters, it may provide an alternative support for catalysts used in total VOC combustion. The aim of this work is to study the effect of the CeO2 loading and the nature of the support on the catalytic properties in the combustion of ethyl acetate on ceria-promoted platinum-supported Al2 O3 Springer
170
and ZrO2 catalysts, prepared by cogelation of the metal precursors.
2. Experimental Sol-gel CeO2 /Al2 O3 and CeO2 /ZrO3 (3 and 5 wt.%) were prepared by refluxing the respective alkoxides in absolute ethanol and water at 363 K and a gelation pH of 7. The gelling reaction was accomplished in 1 h under reflux, and the gels obtained were dried in air at 373 K and then calcined at 673 K for 4 h. The Pt was incorporated by impregnation of the sol-gel supports (calcined at 673 K in air for 2 h) with a toluene solution containing the desired amount of Pt(acac)2 to obtain 1.0 wt% of Pt.
3. Characterization To evaluate the specific surface area and porosity, nitrogen adsorption at 77 K was performed in an automated Micromeritics ASAP 2010 apparatus in the 0.05–0.995 relative pressure range. The metal dispersion was measured in the same apparatus at 298 K in the pressure range of 1 to 100 mmHg. TEM was used to observe the supported platinum particles. These experiments were performed in a JEOL Model JEM-1200 EX II instrument. The samples were prepared using the extractive replica procedure. TPR experiments were carried out in a TPR/TPD 2900 Micromeritics system with a thermal-conductivity detector. The reducing gas was a mixture of 5%H2 /Ar (40 cm3 min−1 ), and a heating rate of 10 Kmin−1 was employed. X-ray diffraction patterns were obtained on a Rigaku diffractometer using a Ni-filter and CuKα 1 radiation. XPS spectra were recorded using an Escalab 200 R spectrometer with a hemispherical analyser operated in a constant pass energy mode and MgK X-ray radiation (hv = 1253.6 eV, operated at 10 mA and 12 kV). The system was configured with a reaction cell that allows pretreatment at high temperatures. The samples were pressed in a hydraulic die to form thin, smooth discs and placed in the cell. The catalysts were reduced in situ in hydrogen at 573 K for 1 h, and then transported to the analysis chamber without air contact. The surface Pt/M and Ce/M (M = Al or Zr) ratios were estimated from the integrated intensities of the Pt 4d5/2 (M = Al) or 4f572 (M = Zr), Zr 3d5/2 , Ce 3d5/2 and Al 2p3/2 peaks, after background subtraction, and correction for the atomic sensitivity factors [14]. The Al 2p line at 74.5 eV was taken as an internal standard for binding energy (BE) measurements. Platinum, cerium and aluminium or zirconia peaks were decomposed into several components, assuming that the peaks had Gaussian-Lorentzian shapes. The catalytic activity in the combustion of ethyl acetate was evaluated in a conventional flow reactor at atmospheric pressure. In each Springer
J Sol-Gel Sci Techn (2006) 37: 169–174
experiment, 100 mg of the catalyst was diluted with 100 mg of an inert material. The calcined samples were reduced in situ in flowing H2 (50 cm3 g−1 ) up to 773 K for 1 h. Then the samples were cooled to 473 K, and the reducing gas was switched to He as the carrier. After 30 min of stabilisation at this temperature, the carrier gas was switched to the reactant C4 H8 O2 :O2 :He = 1:7:92 (molar) mixture and the combustion cycle was performed. The activity was measured at different temperatures, by increasing the temperature isothermally at a heating rate of 1◦ /min from 423 K until reaching complete conversion. Reactor effluents were analysed using an on-line Hewlett Packard gas chromatograph model HP 4890 D, with a thermal conductivity detector that works at a temperature of 423 K and a current of 150 mA. The column used was a 30-m Supelco 25462 capillary. The carrier gas (He) flow through the column was 4 mL/min and the injector temperature 423 K. In some experiments, a Quadrupole Mass Spectrometer Hiden HPT 20 was used to detect small traces of products.
4. Results and discussion 4.1. Specific area The nitrogen adsorption isotherms exhibit significant differences. In the alumina-supported series, the nitrogen uptakes are higher than in the zirconia-supported materials and the shape of the isotherms indicate an essentially mesoporous solid with a fraction of micropores whose volume oscillates between 20 and 30%. The addition of cerium increases both the specific area and the microporosity. However, at higher loadings, both values decreased due to surface coverage. With respect to the ZrO2 and the CeO2 /ZrO2 isotherms, the samples exhibit essentially type IV isotherms with only large mesopores and an increase in the specific area due to the presence of cerium during the gelling step. This behaviour is attributed to the insertion of cerium in the zirconia lattice, leading to an increase in the surface area coverage by cerium, as was confirmed by XPS. 4.2. Hydrogen chemisorption Hydrogen chemisorption isotherms at 298 K were obtained to estimate platinum dispersion by assuming an adsorption stoichiometry of H/Pts = 1, and the results are shown in Table 1. In the alumina series, the highest coverage of alumina by cerium species does not affect the platinum dispersion; therefore, after the calcination and reduction treatment, high Pt dispersion was obtained. With regard to the zirconium series, due to a lower surface area (in the range 12 to 23 m2 /g) and lower dispersion, larger particle sizes of platinum were detected. The increase in dispersion in the
J Sol-Gel Sci Techn (2006) 37: 169–174 Table 1 Specific surface area, abundance of micropores and reduction, metal dispersion, and mean particle size of Pt/CeO2 /supported catalysts
171
Catalyst
S BET , m2 g−1
Micropore (%) Extent of reduction
D
d, nm ChemH2 TEM
1.0%Pt/Al2 O3 1.0%Pt/3CeO2 -Al2 O3 1.0%Pt/5CeO2 -Al2 O3 1.0%Pt/ZrO2 1.0%Pt/3CeO2 -ZrO2 1.0%Pt/5CeO2 -ZrO2
330 357 343 14 25 23
20 30 22 0 0 0
0.90 0.90 0.90 0.24 0.27 0.38
10 10 10 40 30 25
catalysts doped with cerium suggests that the presence of this promoter favours the interaction of Pt with the support. 4.3. TEM The TEM micrograps show a very narrow platinum particle size distribution in the alumina series. Conversely, in the zirconia-supported catalysts, the distribution was more heterogeneous, showing small particles, comparable in size to those observed in the alumina-supported catalysts as well as a significant fraction of larger particles. A summary of these results is given in Table 2. The H/Pt ratio and the metal particle size obtained from hydrogen chemisorption are also summarised in Table 2, and are in good agreement with the TEM results. 4.4. XRD The XRD patterns of the samples are shown in Figure 1. It was found that the addition of a cerium precursor during the gelation step leads to a rather amorphous solid for the alumina-supported samples, whereas in the zirconia systems, lines attributed to tetragonal zirconia appear. The absence of cerium oxide diffraction lines in the catalysts supported on alumina indicates a high dispersion of this species. Unfortunately in the zirconia-supported series, the principal diffraction lines of CeO2 coincide with those of the monoclinic phase of zirconia, and consequently the crystalline phase cannot be identified by this technique. In the catalysts with lower dispersions, platinum diffraction lines are detected in
Table 2 Binding energy (eV), atomic surface ratios, and ignition temperature in the combustion of ethyl acetate for Pt/CeO2 /supported catalysts
Catalyst 1.0%Pt/Al2 O3 1.0%Pt/3CeO2 -Al2 O3 1.0%Pt/5CeO2 -Al2 O3 1.0%Pt/ZrO2 1.0%Pt/3CeO2 -ZrO2 1.0%Pt/5CeO2 -ZrO2
– 2 2 – 6 7
10 10 10 38 34 24
the case of the zirconia−supported catalysts, but not in the corresponding alumina−supported series. This is attributed to the larger crystallite sizes in the former catalysts, in agreement with the TEM and hydrogen chemisorption results. 4.5. TPR Figure 2 shows the TPR profiles for the cerium-promoted catalysts. Two reduction peaks, attributed to partial reduction of cerium oxides, can be observed. For the Pt/Al2 O3 catalyst, the peaks are centred at 475 and 650 K, corresponding to the reduction of Pt oxides interacting with different support sites. The addition of increasing quantities of cerium leads to an increase in the intensity of the first reduction peak. This process corresponds to the partial reduction of Ce(IV) to Ce(III), which undergoes a significant shift in its reduction temperature due to the spillover of hydrogen on platinum. It should be noted that the reduction of bulk cerium oxide begins at temperatures higher than 800 K. A similar behaviour is observed in the Pt/ZrO2 catalysts. The principal difference is that the reduction peaks are better defined and shifted towards lower temperatures, with reduction maxima at 430 and 580 K, as expected for a support with smaller surface area. The addition of Ce produces a greater shift in the reduction temperature; 410 K for the catalyst with 3% CeO2 . However, it is only in this first peak that there is a superposition with the partial reduction of CeO2 since the second peak is virtually unmodified. This behaviour is more significant in the catalyst with greater cerium content. In fact, the first peak is split into two component, one at 360 and the second at 530 K,
B.E. of Pt, eV a: 4d5/2 b: 4f7/2
Pt/Me Me = Al or Zr
Ce/Me Me = Al or Zr
Tign.50 , K
315.3a 315.4a 315.2a 71.2 (68)b 73.0 (32)b 71.7 (57)b 73.7 (43)b 71.7 (63)b 73.7 (37)b
0.0017 0.0024 0.0028 0.051
– 0.022 0.029 –
487 483 493 490
0.059
0.040
476
0.057
0.229
470
Springer
172
J Sol-Gel Sci Techn (2006) 37: 169–174
a)
H2 Consumption, a.u.
Fig. 2 Temperature-programmed reduction profiles of (a) 1.0%Pt/Al2 O3 , (b) 1.0%Pt/3CeO2 -Al2 O3 , (c) 1.0%Pt/5CeO2 -Al2 O3 , (d) Al2 O3 , (e) 1.0%Pt/ZrO2 , (f) 1.0%Pt/3CeO2 -ZrO2 , (g) 1.0%Pt/5CeO2 -ZrO2 , (h) ZrO2 .
b)
c)
H2 consumption, a.u.
Fig. 1 X-ray diffraction patterns of (a) 1.0%Pt/ZrO2 , (b) 1.0%Pt/3CeO2 -ZrO2 , (c) 1.0%Pt/5CeO2 -ZrO2 .
f)
g) h)
d) 300
400
500
600
700
800
Temperature, K
(a)
which appears as a shoulder to the platinum reduction peak. As expected, due to the high surface area of the alumina, the extent of cerium oxide reduction for the CeO2 /Al2 O3 samples is very low, as shown in Table 1. 4.6. XPS Figure 3 illustrates the Pt 4f7/2 and Pt 4d5/2 core-level spectra of the catalysts studied. The core level peak appears to be broadened, and the deconvolution clearly indicates the presence of both Pt and Ptδ+ species, with the abundance of the Pt◦ species being close to 60%. In the Pt alumina series, the Pt 4d5/2 peaks were analysed instead, because the Pt 4f7/2 peaks are in the same region as Al 2p. The Pt 4d5/2 peaks appear around 315.3 eV, a value slightly higher than that expected for Pt◦ (314.2 eV). This may indicate the possible presence of a Ptδ+ species; however, it should be noted that such shifts usually occur with highly dispersed metal particles. With regard to Pt/ZrO2 and Pt/CeO2 -ZrO2 series, the presence of Pt in two different states was observed, with approximately 67% at 71.2 eV (corresponding to Pt◦ species). The apparent Pt/Al ratio increases with the addition of Ce. This result can be explained by postulating that that the Pt species remain highly dispersed on the support (Al2 O3 ), while the latter is partially covered by cerium oxide species. As expected, the Springer
e)
300
400
500
600
700
800
Temperature, K
(b)
Ce/Al ratio increases with Ce loading because the support is covered by highly dispersed oxide species. With regard to the ZrO2 series, a significant increase in the Ce/Zr ratio occurs as the Ce loading increases, which is attributed to the low surface area of the support. The fivefold increase in the Ce/Zr ratio in the Pt/5wt%CeO2 -ZrO2 catalyst may reflect a cerium re-dispersion on the support. The Pt/Zr ratio increases slightly with the addition of cerium. This is a consequence of two effects: (a) the coverage of the zirconia surface by cerium oxides; and (b) the preferential interaction of Pt with cerium species. The measured isoelectric points of the Al2 O3 (8.0), CeO2 (6.8) and ZrO2 (4.0) support this explanation. 4.7. Catalytic Activity The activity for ethyl acetate oxidation under stoichiometric conditions (C4 H8 :O2 mole ratio, 1:5) was measured as a function of temperature up to complete combustion. The activity is reported as ignition temperatures (Ti 50 ), defined as the temperatures required to obtain a 50% conversion of ethyl acetate. Water and CO2 were the observed products, and only traces of CO were detected at conversions higher than 70%. Typical sigmoidal curves were obtained for all catalysts studied and representative data for Pt-Ce/ZrO2 catalysts is shown in Fig. 4. The reaction starts around 450 K,
308 312
a)
316
320
173
Pt 4d 5/2
100
b)
90 80
Conversion, %
Pt 4d 5/2
counts per second, au
counts per second, au
J Sol-Gel Sci Techn (2006) 37: 169–174
308 312 316 320 324
BE, eV
70 60 50 40 30 20
BE, eV
68
c)
72
76
BE, eV
80
counts per second, au
counts per second, au
10
Pt 4f
Pt 4f
0 450
d)
460
470
480
490
500
Temperature, K Fig. 4 Catalytic activity in the combustion of ethyl acetate. () 1.0%Pt/ZrO2 , (!) 1.0%Pt/3CeO2 -ZrO2 , (7) 1.0%Pt/5CeO2 -ZrO2 .
68
72
76
80
BE, eV
Fig. 3 XP spectra of Pt 4f7/2 and 4d5/2 for selected catalysts. (a) 1.0%Pt/Al2 O3 , (b) 1.0%Pt/5CeO2 -Al2 O3 , (c) 1.0%Pt/ZrO2 , (d) 1.0%Pt/5CeO2 -ZrO2 .
platinum particles. Thus, two different sites exist on the surface of the catalysts, (a) the sites that chemisorbs organic molecules (platinum sites); and (b) those that mediate the oxygen transfer (CeO2−x species), which remain in intimate contact, hence favouring the catalytic reaction.
5. Conclusions and the conversion increases as the temperature increases until reaching complete conversion. No significant differences were found in the Pt/Al2 O3 series, with samples showing an ignition temperature close to 490 K. This result is explained by considering that the metal is in the same oxidation state and possesses the same metal dispersion, and Ce does not significantly affect the properties because it remains strongly associated with the alumina lattice. Conversely, the activity in the Pt/ZrO2 catalyst is slightly higher, and is affected by the ceria addition. In fact, the activity is enhanced as cerium loading increases. This behaviour may be explained by considering that ethyl acetate combustion is favoured in catalysis with larger metal particle sizes as has already been discussed. As shown previously, in the zirconia series the addition of cerium lead to changes in the metal dispersion. Consequently, a decrease in the activity would be expected upon the addition of cerium. However, the observed behaviour is just the opposite. As demonstrated above, the addition of cerium to Pt/ZrO2 catalysts lead to an increases in the metal dispersion. This is likely to occur because platinum species interact preferentially with CeO2 compared to ZrO2 . The evaluation of the catalytic activity in the combustion of ethyl acetate has revealed that the platinum catalysts with higher metal dispersion (or smaller particle size) exhibit higher activity. The observed behaviour suggests that the cerium oxides species induce a partial oxidation of the
The results obtained have shown that the combustion of ethyl acetate can be efficiently performed in the presence of supported platinum catalysts. The effect of the support was analysed by studying alumina and zirconia supported systems, where zirconia produces larger changes. It was found that ceria addition enhances the activity observed in the zirconia series, whereas almost no effect is observed in the alumina series. Acknowledgments The authors thank CONICYT, Grant Fondecyt 1020461 for financial support.
References 1. D. Ciuparu, A. Bensalem, L. Pfefferle, Appl. Catal. B. 26, 241 (2002). 2. J. Silvestre-Albero, F. Rodr´ıguez-Reinoso, A. Sep´ulveda– Escribano, J. Catal. 210, 127 (2002) 3. G. Pecchi, P. Reyes, T. L´opez, R. G´omez, J. Non Cryst. Solids 345, 624 (2004) 4. P.O. Larsson, H. Berggren, A. Anderson, O. Augustsson., Catal. Today 35, 137 (1997). 5. J. Kirchnerova, M. Alifanti, B. Delmon. Appl. Catal. A. 231, 65 (2002). 6. C. de Leitenburg, A. Trovarelli, J. Llorca, F. Cavani, G. Bini, Appl. Catal. A 139, 161 (1996).
Springer
174 7. M. Fern´andez-Garc´ıa, E. G´omez-Rebollo, A. Guerrero-Ruiz, J.C. Conesa, J. Soria, J. Catal. 172, 146 (1997). 8. E. Rocchini, A. Trovarelli, J. Lorca, G.W. Graham, H.W. Weber, M. Maciejewski, A. Baiker, J. Catal. 194, 461 (2000). 9. T. L´opez, J. Hern´andez, E. Mu˜noz, R. G´omez, X. Bokhimi, J.L. Bold´u, E. Mu˜noz, O. Novaro, A. Grac´ıa-Ruiz, Langmuir, 15, 5689 (1999).
Springer
J Sol-Gel Sci Techn (2006) 37: 169–174 10. H. Vidal, J. Kaspar, M. Pijolat, G. Coln, S. Bernal, A. Cordon, V. Perrichon, F. Fally, Appl. Catal. B, 27, 493 (2000). 11. S. Rossignol, C. Micheaud-Especel, D. Duprez D, Stud. Surf. Sci. and Catal. 130, 3327 (2000). 12. G. Ranga Rao, P. Fornasiero, R. di Monte, J. Kaspar, G. Vlaic, G. Balducci, S. Meriani, G. Gubitosa, A. Gremona, M. Graziani. J. Catal. 162, 1 (1996).
