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cerium oxide catalysts
Applied Catalysis B: Environmental 40 (2003) 43–49
Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts Salvatore Scirè a,∗ , Simona Minicò a , Carmelo Crisafulli a , Cristina Satriano a , Alessandro Pistone b a
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy b Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, 98166 Messina, Italy Received 3 March 2002; received in revised form 29 May 2002; accepted 29 May 2002
Abstract Catalytic combustion of some representative volatile organic compounds (VOCs) (2-propanol, methanol and toluene) was investigated on gold/cerium oxide catalysts prepared by coprecipitation (CP) and deposition–precipitation (DP). The presence of gold has been found to enhance the activity of cerium oxide towards the oxidation of the selected volatile organic compounds, the extent of this effect depending on the preparation method of gold catalysts. On the basis of characterisation data (H2 -TPR, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), BET surface area) it has been suggested that the catalytic activity of the Au/CeO2 system is related to the capacity of gold nanoparticles to weaken the surface Ce–O bonds adjacent to Au atoms, thus enhancing the reactivity of the CeO2 surface capping oxygen which is involved in the volatile organic compounds oxidation through a Mars-van Krevelen reaction mechanism. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Volatile organic compounds; Catalytic combustion; Gold catalysts; Cerium oxide; Methanol; 2-Propanol; Toluene
1. Introduction Volatile organic compounds (VOCs), which are emitted from many industrial processes and transportation activities, are considered as an important class of air pollutants [1]. Catalytic combustion is one of the most promising technology for VOCs abatement, due to its definitive character and save of energy [1]. The major advantages of this approach are that it can operate with dilute effluent streams (<1% VOCs) and at lower temperatures than thermal oxidation. Supported noble metals (Pt, Pd, Rh) or metal ∗ Corresponding author. E-mail address: [email protected] (S. Scir`e).
oxides (Cu, Cr, Mn) are typical catalysts for such an application [1]. Recently, some of us have shown that Au/iron oxide catalysts present a high activity towards the catalytic oxidation of VOCs [2]. The catalytic behaviour of this system has been found to be dependent on preparation method and pre-treatment conditions used [3]. The high activity of Au/Fe2 O3 catalysts has been explained on the basis of the capacity of gold to increase the mobility of the iron oxide lattice oxygen which is involved in the VOCs oxidation through a Mars-van Krevelen mechanism [2,4]. Ceria (CeO2 ) is a crucial component in the automobile three-way catalysts primarily for its role in oxygen
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storage, taking up oxygen under oxidising conditions and releasing it under reducing ones [5]. Nowadays, the importance of ceria in catalysis is rapidly growing with new applications in different fields [6]. Potential uses of CeO2 for the removal of soot from diesel engine exhaust, for the removal of organics from wastewaters and as an additive for combustion processes have been described [6]. Noble metal-modified (Pt, Pd, Rh) cerium oxide catalysts have been reported [5–8] to exhibit a higher oxygen storage capacity and reducibility than pure ceria, thus resulting in better catalytic performance. Gold supported on ceria has been also described as a very good catalyst for redox reactions, such as CO [9,10] and CH4 oxidation [10]. More recently, Au/CeO2 was found to be very active for low temperature water–gas shift reaction [11]. Following these considerations in this paper, we report a study on the catalytic oxidation of some representative VOCs (2-propanol, methanol and toluene) on Au/cerium oxide catalysts. The influence of two different preparation methods (coprecipitation (CP) and deposition–precipitation (DP)) on the performance of Au/CeO2 catalysts has been investigated.
2. Experimental Au/cerium oxide catalysts were prepared by coprecipitation or deposition–precipitation, using HAuCl4 (Fluka) and Ce(NO3 )3 ·6H2 O (Aldrich) as precursors. In the case of the CP method, an aqueous mixture of the precursors was poured at 7.5 ml/min rate into an aqueous solution of Na2 CO3 (1 M and pH = 11.9) maintained at 70 ◦ C under vigorous stirring (500 rpm). In the DP method, after the pH of the aqueous solution of HAuCl4 was adjusted to the value of 8 using 0.1 M NaOH, cerium oxide, prepared as later described, was added under vigorous stirring (500 rpm) to the gold solution, keeping the slurry at 70 ◦ C for 2 h. Both in CP and DP methods the obtained slurries were kept digesting for 24 h, washed several times (until disappearance of nitrates and chlorides), then dried under vacuum at 70 ◦ C and finally ground before use. Cerium oxide was prepared by precipitation from Ce(NO3 )3 ·6H2 O following preparation conditions similar to those employed for the coprecipitated Au/CeO2 catalyst.
Before catalytic activity tests and characterisation measurements, all samples were calcined at 450 ◦ C for 1 h by flowing a 10% O2 stream diluted in He. Samples were coded as follows: AuCeCP for the catalyst prepared by coprecipitation, AuCeDP for that prepared by deposition–precipitation and CeO2 for the pure cerium oxide. The gold content of the Au/cerium oxide catalysts used in this work, measured by atomic absorption, was 5.0 wt.% for AuCeDP and 4.7 wt.% for AuCeCP. Catalytic activity tests were performed in a continuous-flow fixed-bed microreactor, using 0.1 g of catalyst (80–140 mesh) diluted with an inert glass powder. The reactant mixture was fed to the reactor by flowing a part of the He stream through a saturator containing the VOC and then mixing with O2 and He before reaching the catalyst. The reactant mixture was 0.7 vol.% VOC and 10 vol.% O2 , the remainder being helium. A space velocity (GHSV) of 7.6 × 10−3 mol/(h gcat ) was always used. The effluent gases were analysed on-line by a gas chromatograph, equipped with a packed column with 10% FFAP on Chromosorb W and FID detector, and by a quadrupole mass spectrometer (VG quadrupoles). The carbon balance was always higher than 95%. Surface area measurements were carried out using the BET nitrogen adsorption method with a Sorptomatic series 1990 (Thermo Quest). Before the tests all samples were outgassed (10−3 Torr) at 120 ◦ C. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Perkin-Elmer PHI 5600 ESCA/SAM spectrophotometer equipped with a hemispherical analyser both a dual Al/Mg anode and monochromatic Al source. The operating conditions were kept constant at 13 keV and 300 W. Spectra were obtained by using the Mg K␣1,2 radiation (1253.6 eV) with a pass energy of 23.5 eV. The pressure in the analysis chamber was about 10−8 Pa during the analysis. In order to subtract the surface charging effect, the C 1s hydrocarbon peak has been fixed, in agreement with the literature, at a binding energy of 285 eV. X-ray powder diffraction (XRD) analysis of the samples was performed with an APD 2000 (Italstructure) diffractometer using a Cu K␣ radiation. Diffraction peaks of crystalline phases were compared with those of standard compounds reported in the JCPDS Data File.
S. Scir`e et al. / Applied Catalysis B: Environmental 40 (2003) 43–49
Fig. 1. Conversion of 2-propanol and products distribution on CeO2 (A), AuCeCP (B) and AuCeDP (C) catalysts.
3. Results and discussion Fig. 1 shows the conversion of 2-propanol and the yields to CO2 and acetone as a function of reaction temperature, on Au/cerium oxide catalysts prepared by coprecipitation (AuCeCP) or deposition–precipitation
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(AuCeDP) and for comparison on the Au-free cerium oxide (CeO2 ). It is possible to observe that on the CeO2 sample (Fig. 1A), the oxidation of 2-propanol starts at 160 ◦ C, reaching a total conversion at about 340 ◦ C. It must be reminded that CO2 , water and acetone were the only products formed under the experimental conditions used. Acetone is formed at low temperature with a nearly 100% selectivity, which falls quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . According to the literature [2,12], it seems likely that acetone is the first oxidation product and it is intermediate in 2-propanol combustion. However, our experimental results do not allow to rule out that CO2 is also formed by direct oxidation of the alcohol. It must be noted that the formation of other products (acetaldehyde, acetic acid and propene) has been reported in the 2-propanol oxidation on several metal oxides (Mn3 O4 , TiO2 , V2 O5 ) [12–14]. Analogously to that reported in the case of the Au/Fe2 O3 system [2,4], also on the cerium oxide based catalysts considered in the present work, the lack of formation of these compounds is probably due to the basic character of the oxide used. The activity of the AuCeCP catalyst (Fig. 1B) is slightly higher than that exhibited by CeO2 (Fig. 1A). In fact, on AuCeCP, both the light-off temperature and the temperature at which the CO2 formation arises are shifted about 40 ◦ C lower than on the pure ceria. In the case of the AuCeDP sample (Fig. 1C), the light-off temperature is sensibly lower (about 80 ◦ C). Moreover, on this sample CO2 begins to be formed at 100 ◦ C, i.e. 80 and 120 ◦ C lower compared to AuCeCP and CeO2 , respectively. The conversion of methanol, as a function of reaction temperature, on cerium oxide based catalysts is reported in Fig. 2. It must be underlined that CO2 is the only product revealed. The absence of intermediate compounds in the case of methanol oxidation can be probably explained considering that formic aldehyde (which should be the intermediate oxidation product) is a very reactive species which evolves easily to CO2 [2,14]. The figure shows that the presence of gold improves the catalytic activity of pure cerium oxide towards the combustion of methanol. Analogously to that found in the 2-propanol oxidation, also in the oxidation of methanol, the sample prepared by DP is more active than that prepared by CP. The same trend is also observed in the toluene oxidation (Fig. 3). In
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Fig. 2. Conversion of methanol on Au/CeO2 catalysts.
this latter case, the difference between the catalytic behaviour of AuCeDP and that of AuCeCP is more relevant than that observed in the oxidation of alcohols. In fact, on the AuCeDP catalyst, toluene conversion starts at about 200 ◦ C approaching 100% at ca. 360 ◦ C, whereas on AuCeCP and CeO2 catalysts the light-off temperatures are, respectively, 200 and 300 ◦ C higher. The catalytic activity results previously discussed clearly indicate that gold enhances the combustion
activity of ceria. The presence of gold is more effective in improving the catalytic behaviour of cerium oxide based catalysts when gold is added to ceria by deposition–precipitation. In order to understand the influence of the preparation method on the activity of Au/CeO2 catalysts towards VOC oxidation several characterisations (XRD, XPS, H2 -TPR and surface area measurements) have been carried out.
Fig. 3. Conversion of toluene on Au/CeO2 catalysts.
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Fig. 4. XRD profiles of Au/CeO2 catalysts.
XRD spectra of CeO2 , AuCeCP and AuCeDP are reported in Fig. 4. The XRD profile of the Au-free cerium oxide sample reveals the presence of diffraction peaks related to CeO2 phase. On both gold containing catalysts, together with the characteristic pattern of CeO2 , a small broad peak at about 38◦ , attributed to metallic gold, can be also observed. The average size of gold particles, estimated from the line-width of this peak by using the Sherrer equation, was 6 nm for AuCeDP and 8 nm for AuCeCP. The formation of smaller gold particles by deposition– precipitation compared to coprecipitation has been reported on several Au/metal oxides catalysts [11, 15,16]. Fig. 5 reports XPS spectra, in the Au 4f region, of the AuCeDP and AuCeCP samples. In this region, each gold species shows two peaks due to the Au 4f7/2 and Au 4f5/2 transitions. On both samples, these peaks are centred at 83.5 ± 0.2 and 87.0 ± 0.2 eV, respectively, indicating the presence of gold in the metallic state [3,15,17]. This is quite reasonable considering that gold samples have been calcined at 450 ◦ C. It has been in fact reported that at calcination temperatures higher than 300 ◦ C gold is present just as Au0 [3,18]. The quantitative XPS analysis of AuCeCP and AuCeDP samples evidences that the atomic content of gold on the surface of both samples is less than 1%. However, as shown in Fig. 5, the intensity ratio of Au 4f peaks between DP and CP samples is about 3, indicating that AuCeDP contains a higher amount
of gold on the surface compared to AuCeCP. This is in accordance with literature data which report that deposition–precipitation has the advantage over coprecipitation to favour the location of gold on the surface, thus avoiding that active species are buried within the support [16]. Fig. 6 reports temperature programmed reduction (H2 -TPR) profiles carried out on cerium oxide based catalysts. In the examined temperature range (30–600 ◦ C), the CeO2 sample showed one reduction
Fig. 5. XPS spectra of Au/CeO2 catalysts.
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Fig. 6. H2 -TPR profiles of Au/CeO2 catalysts.
peak with a maximum at 520 ◦ C, which can be attributed to the reduction of the surface capping oxygen of ceria [19,20]. It must be reminded that the reduction of bulk oxygen of ceria has been reported to occur at T > 700 ◦ C [19,20] and therefore it does not appear in TPR profiles of Fig. 6. On gold/cerium oxide catalysts, the peak related to the reduction of surface oxygen of ceria resulted to be shifted down to 240 and 140 ◦ C on AuCeCP and AuCeDP, respectively. This indicates that the presence of gold facilitates the reduction of ceria surface oxygen species, with an effect which is more relevant in the case of the DP sample. Similar promotion effects on the CeO2 reducibility are well known for noble metals (Pt, Rh, Pd) supported on ceria [19,20]. A higher oxygen reducibility of ceria has been also observed by Fu et al. on Au/CeO2 catalysts and related to a weakening of the Ce–O bond induced by gold [11]. An analogous effect of gold in enhancing the reducibility of a metal oxide has been reported on Au/Fe2 O3 catalysts [2]. Considering that the reducibility of the lattice oxygen of a metal oxide reflects the reactivity of this oxygen [2,21], H2 -TPR results of Fig. 6 clearly indicates that the surface capping oxygen reactivity of investigated catalysts is in
the order: AuCeDP > AuCeCP > CeO2 . From Fig. 6 it can be also inferred that H2 consumption measured on the AuCeDP sample is considerably higher than that of AuCeCP and CeO2 catalysts. According to the literature [22] this result can be explained considering that AuCeDP presents a higher surface area (105 m2 /g) compared to AuCeCP (50 m2 /g) and CeO2 (45 m2 /g). This is confirmed by the fact that a cerium oxide sample (HSACeO2 ) with a higher surface area (95 m2 /g) showed an H2 -TPR profile (Fig. 6) with the same reduction temperature of surface oxygens, but with a higher H2 consumption compared to the CeO2 sample (45 m2 /g) used as catalyst in this work. On the basis of these H2 -TPR results, it can be inferred that the amount of reducible surface oxygen species in the Au/CeO2 system is mainly controlled by the specific area of ceria. The role of gold appears to be that of modifying the properties of ceria, enhancing the reducibility of CeO2 surface oxygen. It is highly probable that the presence of gold causes a decrease in the strength of the surface Ce–O bonds adjacent to gold atoms [11,23], thus leading to a higher surface lattice oxygen mobility and therefore to a higher reactivity of these oxygens. In this context, in order that Au can perform its positive function, it is essential that gold particles are located on the surface of ceria. This accounts for the higher oxygen reducibility observed in the case of the DP sample (Fig. 6), for which a higher amount of gold on the surface has been found (Fig. 5). Accordingly, catalytic activity data showed that the sample prepared by deposition–precipitation is sensibly more active than that prepared by coprecipitation, thus pointing out that there is a direct relationship between surface oxygen mobility of the gold/oxide system and its catalytic activity towards the oxidation of VOCs. This is quite reasonable considering that the VOCs oxidation occurs through a Mars-van Krevelen mechanism implying surface lattice oxide ions as the active oxygen species [2,12]. Nevertheless, it must be reminded that DP and CP samples exhibited slightly different gold particle sizes, and this could somehow affect the catalytic activity of these samples. It has been, in fact, reported that the activity of gold catalysts towards several reactions increases as the Au particle size decreases [16]. Even though, with our data, it is not possible to rule out this hypothesis, however, it must be noted that, in the present case, the difference in the particle dimensions of DP and CP samples is
S. Scir`e et al. / Applied Catalysis B: Environmental 40 (2003) 43–49
too small and, in our opinion, not sufficient to justify the remarkably different catalytic behaviour observed. Moreover, it is plausible that Au particle size is not a crucial factor in the combustion of VOCs, since the reactive oxygen for this reaction is provided by ceria. A similar conclusion has been also reported in the water gas shift reaction on Au/CeO2 catalysts [11]. A detailed investigation on the effect of gold size on the catalytic combustion of VOCs on several Au/metal oxide catalysts is currently in progress. 4. Conclusions Au/CeO2 catalysts appear to be promising for applications in the field of the combustion of volatile organic compounds. The high activity of the Au/CeO2 system might be related to the capacity of gold nanoparticles to weaken the Ce–O bond, thus increasing the mobility/reactivity of the surface lattice oxygen which is involved in the volatile organic compounds oxidation through a Mars-van Krevelen reaction mechanism. In this context, deposition–precipitation has been found to be more suitable than coprecipitation to obtain highly active Au/CeO2 catalysts, because deposition–precipitation leads to gold nanoparticles which are preferentially located on the surface of ceria. References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] S. Minicò, S. Scirè, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B: Environ. 28 (2000) 245.
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[3] S. Minicò, S. Scirè, C. Crisafulli, S. Galvagno, Appl. Catal. B: Environ. 34 (2001) 277. [4] S. Scirè, S. Minicò, C. Crisafulli, S. Galvagno, Catal. Commun. 2 (2001) 229. [5] A. Trovarelli, Catal. Rev. Sci. Eng. 38 (1996) 439. [6] A. Trovarelli, C. de Leitenburg, M. Boaro, G. Dolcetti, Catal. Today 50 (1999) 353. [7] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environ. 27 (2000) 179. [8] T. Bunluesin, R.J. Gorte, Appl. Catal. B: Environ. 15 (1998) 107. [9] S.D. Gardner, G.B. Hoflund, D. Schryer, J. Schryer, B. Upchurch, E. Kielin, Langmuir 7 (1991) 2135. [10] W. Liu, M. Flytzani-Stephanopoulos, J. Catal. 153 (1995) 304. [11] Q. Fu, A. Weber, M. Flytzani-Stephanopoulos, Catal. Lett. 77 (2001) 87. [12] M. Baldi, E. Finocchio, F. Milella, G. Busca, Appl. Catal. B: Environ. 16 (1998) 43. [13] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, Appl. Catal. B: Environ. 22 (1998) 249. [14] E.M. Cordi, P.J. O’Neill, J.L. Falconer, Appl. Catal. B: Environ. 14 (1997) 23. [15] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [16] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [17] A.M. Visco, F. Neri, G. Neri, A. Donato, C. Milone, S. Galvagno, Phys. Chem. Chem. Phys. 1 (1999) 2869. [18] E.D. Park, J.S. Lee, J. Catal. 186 (1999) 1. [19] A. Trovarelli, G. Dolcetti, C. De Leitenburg, J. Kaspar, P. Finetti, A. Santoni, J. Chem. Soc., Faraday Trans. 88 (1992) 1311. [20] H.C. Yao, Y.F. Yao, J. Catal. 86 (1984) 254. [21] V.R. Choudhary, B.S. Uphade, S.J. Pataskar, Appl. Catal. A: Gen. 227 (2002) 29. [22] V. Perrichon, A. Laachir, G. Bergeret, R. Frety, L. Tournayan, J. Chem. Soc., Faraday Trans. 90 (1994) 773. [23] C. Serre, F. Garin, G. Belot, G. Maire, J. Catal. 141 (1993) 1.
Catalytic combustion of volatile organic compounds on gold/cerium oxide catalysts Salvatore Scirè a,∗ , Simona Minicò a , Carmelo Crisafulli a , Cristina Satriano a , Alessandro Pistone b a
Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy b Dipartimento di Chimica Industriale ed Ingegneria dei Materiali, Università di Messina, Salita Sperone 31, 98166 Messina, Italy Received 3 March 2002; received in revised form 29 May 2002; accepted 29 May 2002
Abstract Catalytic combustion of some representative volatile organic compounds (VOCs) (2-propanol, methanol and toluene) was investigated on gold/cerium oxide catalysts prepared by coprecipitation (CP) and deposition–precipitation (DP). The presence of gold has been found to enhance the activity of cerium oxide towards the oxidation of the selected volatile organic compounds, the extent of this effect depending on the preparation method of gold catalysts. On the basis of characterisation data (H2 -TPR, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), BET surface area) it has been suggested that the catalytic activity of the Au/CeO2 system is related to the capacity of gold nanoparticles to weaken the surface Ce–O bonds adjacent to Au atoms, thus enhancing the reactivity of the CeO2 surface capping oxygen which is involved in the volatile organic compounds oxidation through a Mars-van Krevelen reaction mechanism. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Volatile organic compounds; Catalytic combustion; Gold catalysts; Cerium oxide; Methanol; 2-Propanol; Toluene
1. Introduction Volatile organic compounds (VOCs), which are emitted from many industrial processes and transportation activities, are considered as an important class of air pollutants [1]. Catalytic combustion is one of the most promising technology for VOCs abatement, due to its definitive character and save of energy [1]. The major advantages of this approach are that it can operate with dilute effluent streams (<1% VOCs) and at lower temperatures than thermal oxidation. Supported noble metals (Pt, Pd, Rh) or metal ∗ Corresponding author. E-mail address: [email protected] (S. Scir`e).
oxides (Cu, Cr, Mn) are typical catalysts for such an application [1]. Recently, some of us have shown that Au/iron oxide catalysts present a high activity towards the catalytic oxidation of VOCs [2]. The catalytic behaviour of this system has been found to be dependent on preparation method and pre-treatment conditions used [3]. The high activity of Au/Fe2 O3 catalysts has been explained on the basis of the capacity of gold to increase the mobility of the iron oxide lattice oxygen which is involved in the VOCs oxidation through a Mars-van Krevelen mechanism [2,4]. Ceria (CeO2 ) is a crucial component in the automobile three-way catalysts primarily for its role in oxygen
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 2 7 - 3
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storage, taking up oxygen under oxidising conditions and releasing it under reducing ones [5]. Nowadays, the importance of ceria in catalysis is rapidly growing with new applications in different fields [6]. Potential uses of CeO2 for the removal of soot from diesel engine exhaust, for the removal of organics from wastewaters and as an additive for combustion processes have been described [6]. Noble metal-modified (Pt, Pd, Rh) cerium oxide catalysts have been reported [5–8] to exhibit a higher oxygen storage capacity and reducibility than pure ceria, thus resulting in better catalytic performance. Gold supported on ceria has been also described as a very good catalyst for redox reactions, such as CO [9,10] and CH4 oxidation [10]. More recently, Au/CeO2 was found to be very active for low temperature water–gas shift reaction [11]. Following these considerations in this paper, we report a study on the catalytic oxidation of some representative VOCs (2-propanol, methanol and toluene) on Au/cerium oxide catalysts. The influence of two different preparation methods (coprecipitation (CP) and deposition–precipitation (DP)) on the performance of Au/CeO2 catalysts has been investigated.
2. Experimental Au/cerium oxide catalysts were prepared by coprecipitation or deposition–precipitation, using HAuCl4 (Fluka) and Ce(NO3 )3 ·6H2 O (Aldrich) as precursors. In the case of the CP method, an aqueous mixture of the precursors was poured at 7.5 ml/min rate into an aqueous solution of Na2 CO3 (1 M and pH = 11.9) maintained at 70 ◦ C under vigorous stirring (500 rpm). In the DP method, after the pH of the aqueous solution of HAuCl4 was adjusted to the value of 8 using 0.1 M NaOH, cerium oxide, prepared as later described, was added under vigorous stirring (500 rpm) to the gold solution, keeping the slurry at 70 ◦ C for 2 h. Both in CP and DP methods the obtained slurries were kept digesting for 24 h, washed several times (until disappearance of nitrates and chlorides), then dried under vacuum at 70 ◦ C and finally ground before use. Cerium oxide was prepared by precipitation from Ce(NO3 )3 ·6H2 O following preparation conditions similar to those employed for the coprecipitated Au/CeO2 catalyst.
Before catalytic activity tests and characterisation measurements, all samples were calcined at 450 ◦ C for 1 h by flowing a 10% O2 stream diluted in He. Samples were coded as follows: AuCeCP for the catalyst prepared by coprecipitation, AuCeDP for that prepared by deposition–precipitation and CeO2 for the pure cerium oxide. The gold content of the Au/cerium oxide catalysts used in this work, measured by atomic absorption, was 5.0 wt.% for AuCeDP and 4.7 wt.% for AuCeCP. Catalytic activity tests were performed in a continuous-flow fixed-bed microreactor, using 0.1 g of catalyst (80–140 mesh) diluted with an inert glass powder. The reactant mixture was fed to the reactor by flowing a part of the He stream through a saturator containing the VOC and then mixing with O2 and He before reaching the catalyst. The reactant mixture was 0.7 vol.% VOC and 10 vol.% O2 , the remainder being helium. A space velocity (GHSV) of 7.6 × 10−3 mol/(h gcat ) was always used. The effluent gases were analysed on-line by a gas chromatograph, equipped with a packed column with 10% FFAP on Chromosorb W and FID detector, and by a quadrupole mass spectrometer (VG quadrupoles). The carbon balance was always higher than 95%. Surface area measurements were carried out using the BET nitrogen adsorption method with a Sorptomatic series 1990 (Thermo Quest). Before the tests all samples were outgassed (10−3 Torr) at 120 ◦ C. X-ray photoelectron spectroscopy (XPS) measurements were performed with a Perkin-Elmer PHI 5600 ESCA/SAM spectrophotometer equipped with a hemispherical analyser both a dual Al/Mg anode and monochromatic Al source. The operating conditions were kept constant at 13 keV and 300 W. Spectra were obtained by using the Mg K␣1,2 radiation (1253.6 eV) with a pass energy of 23.5 eV. The pressure in the analysis chamber was about 10−8 Pa during the analysis. In order to subtract the surface charging effect, the C 1s hydrocarbon peak has been fixed, in agreement with the literature, at a binding energy of 285 eV. X-ray powder diffraction (XRD) analysis of the samples was performed with an APD 2000 (Italstructure) diffractometer using a Cu K␣ radiation. Diffraction peaks of crystalline phases were compared with those of standard compounds reported in the JCPDS Data File.
S. Scir`e et al. / Applied Catalysis B: Environmental 40 (2003) 43–49
Fig. 1. Conversion of 2-propanol and products distribution on CeO2 (A), AuCeCP (B) and AuCeDP (C) catalysts.
3. Results and discussion Fig. 1 shows the conversion of 2-propanol and the yields to CO2 and acetone as a function of reaction temperature, on Au/cerium oxide catalysts prepared by coprecipitation (AuCeCP) or deposition–precipitation
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(AuCeDP) and for comparison on the Au-free cerium oxide (CeO2 ). It is possible to observe that on the CeO2 sample (Fig. 1A), the oxidation of 2-propanol starts at 160 ◦ C, reaching a total conversion at about 340 ◦ C. It must be reminded that CO2 , water and acetone were the only products formed under the experimental conditions used. Acetone is formed at low temperature with a nearly 100% selectivity, which falls quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . According to the literature [2,12], it seems likely that acetone is the first oxidation product and it is intermediate in 2-propanol combustion. However, our experimental results do not allow to rule out that CO2 is also formed by direct oxidation of the alcohol. It must be noted that the formation of other products (acetaldehyde, acetic acid and propene) has been reported in the 2-propanol oxidation on several metal oxides (Mn3 O4 , TiO2 , V2 O5 ) [12–14]. Analogously to that reported in the case of the Au/Fe2 O3 system [2,4], also on the cerium oxide based catalysts considered in the present work, the lack of formation of these compounds is probably due to the basic character of the oxide used. The activity of the AuCeCP catalyst (Fig. 1B) is slightly higher than that exhibited by CeO2 (Fig. 1A). In fact, on AuCeCP, both the light-off temperature and the temperature at which the CO2 formation arises are shifted about 40 ◦ C lower than on the pure ceria. In the case of the AuCeDP sample (Fig. 1C), the light-off temperature is sensibly lower (about 80 ◦ C). Moreover, on this sample CO2 begins to be formed at 100 ◦ C, i.e. 80 and 120 ◦ C lower compared to AuCeCP and CeO2 , respectively. The conversion of methanol, as a function of reaction temperature, on cerium oxide based catalysts is reported in Fig. 2. It must be underlined that CO2 is the only product revealed. The absence of intermediate compounds in the case of methanol oxidation can be probably explained considering that formic aldehyde (which should be the intermediate oxidation product) is a very reactive species which evolves easily to CO2 [2,14]. The figure shows that the presence of gold improves the catalytic activity of pure cerium oxide towards the combustion of methanol. Analogously to that found in the 2-propanol oxidation, also in the oxidation of methanol, the sample prepared by DP is more active than that prepared by CP. The same trend is also observed in the toluene oxidation (Fig. 3). In
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Fig. 2. Conversion of methanol on Au/CeO2 catalysts.
this latter case, the difference between the catalytic behaviour of AuCeDP and that of AuCeCP is more relevant than that observed in the oxidation of alcohols. In fact, on the AuCeDP catalyst, toluene conversion starts at about 200 ◦ C approaching 100% at ca. 360 ◦ C, whereas on AuCeCP and CeO2 catalysts the light-off temperatures are, respectively, 200 and 300 ◦ C higher. The catalytic activity results previously discussed clearly indicate that gold enhances the combustion
activity of ceria. The presence of gold is more effective in improving the catalytic behaviour of cerium oxide based catalysts when gold is added to ceria by deposition–precipitation. In order to understand the influence of the preparation method on the activity of Au/CeO2 catalysts towards VOC oxidation several characterisations (XRD, XPS, H2 -TPR and surface area measurements) have been carried out.
Fig. 3. Conversion of toluene on Au/CeO2 catalysts.
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Fig. 4. XRD profiles of Au/CeO2 catalysts.
XRD spectra of CeO2 , AuCeCP and AuCeDP are reported in Fig. 4. The XRD profile of the Au-free cerium oxide sample reveals the presence of diffraction peaks related to CeO2 phase. On both gold containing catalysts, together with the characteristic pattern of CeO2 , a small broad peak at about 38◦ , attributed to metallic gold, can be also observed. The average size of gold particles, estimated from the line-width of this peak by using the Sherrer equation, was 6 nm for AuCeDP and 8 nm for AuCeCP. The formation of smaller gold particles by deposition– precipitation compared to coprecipitation has been reported on several Au/metal oxides catalysts [11, 15,16]. Fig. 5 reports XPS spectra, in the Au 4f region, of the AuCeDP and AuCeCP samples. In this region, each gold species shows two peaks due to the Au 4f7/2 and Au 4f5/2 transitions. On both samples, these peaks are centred at 83.5 ± 0.2 and 87.0 ± 0.2 eV, respectively, indicating the presence of gold in the metallic state [3,15,17]. This is quite reasonable considering that gold samples have been calcined at 450 ◦ C. It has been in fact reported that at calcination temperatures higher than 300 ◦ C gold is present just as Au0 [3,18]. The quantitative XPS analysis of AuCeCP and AuCeDP samples evidences that the atomic content of gold on the surface of both samples is less than 1%. However, as shown in Fig. 5, the intensity ratio of Au 4f peaks between DP and CP samples is about 3, indicating that AuCeDP contains a higher amount
of gold on the surface compared to AuCeCP. This is in accordance with literature data which report that deposition–precipitation has the advantage over coprecipitation to favour the location of gold on the surface, thus avoiding that active species are buried within the support [16]. Fig. 6 reports temperature programmed reduction (H2 -TPR) profiles carried out on cerium oxide based catalysts. In the examined temperature range (30–600 ◦ C), the CeO2 sample showed one reduction
Fig. 5. XPS spectra of Au/CeO2 catalysts.
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Fig. 6. H2 -TPR profiles of Au/CeO2 catalysts.
peak with a maximum at 520 ◦ C, which can be attributed to the reduction of the surface capping oxygen of ceria [19,20]. It must be reminded that the reduction of bulk oxygen of ceria has been reported to occur at T > 700 ◦ C [19,20] and therefore it does not appear in TPR profiles of Fig. 6. On gold/cerium oxide catalysts, the peak related to the reduction of surface oxygen of ceria resulted to be shifted down to 240 and 140 ◦ C on AuCeCP and AuCeDP, respectively. This indicates that the presence of gold facilitates the reduction of ceria surface oxygen species, with an effect which is more relevant in the case of the DP sample. Similar promotion effects on the CeO2 reducibility are well known for noble metals (Pt, Rh, Pd) supported on ceria [19,20]. A higher oxygen reducibility of ceria has been also observed by Fu et al. on Au/CeO2 catalysts and related to a weakening of the Ce–O bond induced by gold [11]. An analogous effect of gold in enhancing the reducibility of a metal oxide has been reported on Au/Fe2 O3 catalysts [2]. Considering that the reducibility of the lattice oxygen of a metal oxide reflects the reactivity of this oxygen [2,21], H2 -TPR results of Fig. 6 clearly indicates that the surface capping oxygen reactivity of investigated catalysts is in
the order: AuCeDP > AuCeCP > CeO2 . From Fig. 6 it can be also inferred that H2 consumption measured on the AuCeDP sample is considerably higher than that of AuCeCP and CeO2 catalysts. According to the literature [22] this result can be explained considering that AuCeDP presents a higher surface area (105 m2 /g) compared to AuCeCP (50 m2 /g) and CeO2 (45 m2 /g). This is confirmed by the fact that a cerium oxide sample (HSACeO2 ) with a higher surface area (95 m2 /g) showed an H2 -TPR profile (Fig. 6) with the same reduction temperature of surface oxygens, but with a higher H2 consumption compared to the CeO2 sample (45 m2 /g) used as catalyst in this work. On the basis of these H2 -TPR results, it can be inferred that the amount of reducible surface oxygen species in the Au/CeO2 system is mainly controlled by the specific area of ceria. The role of gold appears to be that of modifying the properties of ceria, enhancing the reducibility of CeO2 surface oxygen. It is highly probable that the presence of gold causes a decrease in the strength of the surface Ce–O bonds adjacent to gold atoms [11,23], thus leading to a higher surface lattice oxygen mobility and therefore to a higher reactivity of these oxygens. In this context, in order that Au can perform its positive function, it is essential that gold particles are located on the surface of ceria. This accounts for the higher oxygen reducibility observed in the case of the DP sample (Fig. 6), for which a higher amount of gold on the surface has been found (Fig. 5). Accordingly, catalytic activity data showed that the sample prepared by deposition–precipitation is sensibly more active than that prepared by coprecipitation, thus pointing out that there is a direct relationship between surface oxygen mobility of the gold/oxide system and its catalytic activity towards the oxidation of VOCs. This is quite reasonable considering that the VOCs oxidation occurs through a Mars-van Krevelen mechanism implying surface lattice oxide ions as the active oxygen species [2,12]. Nevertheless, it must be reminded that DP and CP samples exhibited slightly different gold particle sizes, and this could somehow affect the catalytic activity of these samples. It has been, in fact, reported that the activity of gold catalysts towards several reactions increases as the Au particle size decreases [16]. Even though, with our data, it is not possible to rule out this hypothesis, however, it must be noted that, in the present case, the difference in the particle dimensions of DP and CP samples is
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too small and, in our opinion, not sufficient to justify the remarkably different catalytic behaviour observed. Moreover, it is plausible that Au particle size is not a crucial factor in the combustion of VOCs, since the reactive oxygen for this reaction is provided by ceria. A similar conclusion has been also reported in the water gas shift reaction on Au/CeO2 catalysts [11]. A detailed investigation on the effect of gold size on the catalytic combustion of VOCs on several Au/metal oxide catalysts is currently in progress. 4. Conclusions Au/CeO2 catalysts appear to be promising for applications in the field of the combustion of volatile organic compounds. The high activity of the Au/CeO2 system might be related to the capacity of gold nanoparticles to weaken the Ce–O bond, thus increasing the mobility/reactivity of the surface lattice oxygen which is involved in the volatile organic compounds oxidation through a Mars-van Krevelen reaction mechanism. In this context, deposition–precipitation has been found to be more suitable than coprecipitation to obtain highly active Au/CeO2 catalysts, because deposition–precipitation leads to gold nanoparticles which are preferentially located on the surface of ceria. References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] S. Minicò, S. Scirè, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B: Environ. 28 (2000) 245.
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