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Comparison of alkali-promoted ZrO2 catalysts towards carbon black oxidation
Colloids and Surfaces A: Physicochem. Eng. Aspects 330 (2008) 193–200
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Comparison of alkali-promoted ZrO2 catalysts towards carbon black oxidation D. Hleis a , M. Labaki b , H. Laversin a , D. Courcot a,∗ , A. Aboukaïs a a b
Laboratoire de Catalyse et Environnement EA 2598, Université du Littoral – Côte d’Opale, 145, avenue Maurice Schumann, 59140 Dunkerque, France Chemistry Department, Faculty of Sciences II, Lebanese University, Fanar, El-Metn, Lebanon
a r t i c l e
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Article history: Received 18 February 2008 Received in revised form 24 July 2008 Accepted 26 July 2008 Available online 5 August 2008 Keywords: Soot oxidation Zirconia Alkali promoters XRD TPR FTIR
a b s t r a c t The effect of alkali metals deposition on zirconia has been studied in the oxidation of carbon black, considered as a model of diesel soot. The study of the influence of alkali content and alkali precursor was undertaken for K/ZrO2 , evidencing a better activity for a catalyst prepared with an atomic ratio K/Zr = 0.14 and from a nitrate precursor. Using the latter preparation conditions, alkali/ZrO2 are found to be active in the oxidation of carbon black according the sequence ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . Alkali metals have an influence on the tetragonal–monoclinic crystalline modification. Alkali metals ions with low size tend to stabilize the tetragonal ZrO2 phase whereas those with higher ionic radius favour the tetragonal–monoclinic modification. Fourier transform-infrared spectroscopy (FTIR) and temperature programmed reduction (TPR) measurements show that the catalytic activity partially depends on the presence of nitrate species stabilized in alkali/ZrO2 even after calcination treatment at 600 ◦ C. Nitrate species are more stable in the presence of alkali with high ionic radius than those of low size. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Soot emitted in large amounts by diesel engines is hazardous to human health. To meet stringent environmental regulations fixed by European legislation, automobile manufacturers have to develop technologies to reduce soot emissions. Post-treatment of emissions using a particulate filter is one of the suggested solutions. However, particulate matter combustion temperature is about 600 ◦ C, much higher than the operational temperature found in exhaust gases of engines (150–400 ◦ C). The use of a catalyst impregnated on the filter enables to oxidize soot particulate at lower temperatures (300–400 ◦ C) [1–4]. Alkali metals are introduced as promoter in catalysts developed for the oxidation of soot [5–15]. Different positive effects obtained in the presence of alkali metals are now well recognized. Firstly, alkali metals facilitate the reaction by favouring a good contact between the catalyst surface and soot particles [8,11,14,16]. In fact, alkali metals generally form compounds with low melting temperature or eutectics with other phases of the catalyst [13–14,17–18]. Secondly, alkali metals increase the redox properties of the active phase by enhancing electron transfers in the catalyst [11,14]. In previous works, we have studied the role of potassium in Cu–K/ZrO2 catalysts tested in the oxidation of carbon black in loose
∗ Corresponding author. Tel.: +33 3 28 65 82 61; fax: +33 3 28 65 82 39. E-mail address: [email protected] (D. Courcot). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.07.052
contact conditions [11,19]. We showed that potassium favours the contact between the catalyst and carbon black and enhances the ability of the catalysts to release active oxygen species. Such species are issued from Cu(II) species via a redox mechanism. K deposited on ZrO2 was also found to interact with the carrier surface and K/ZrO2 also appeared to be active in the oxidation of black carbon. Up to now, few works have attempted to compare the effect of the different alkali metals supported on oxide carrier in the oxidation of soot or carbon black [13,20–21]. The aim of this work has been focused on the effect of alkali metals (Li, Na, K, Rb and Cs) on the properties alkali/ZrO2 catalysts. First, we have investigated the influence of different potassium/ZrO2 synthesis parameters such as (i) nature of the potassium precursor, (ii) potassium content. The comparison of alkali/ZrO2 properties has been undertaken using alkali nitrates as precursor. A characterization of alkali/ZrO2 from X-ray diffraction (XRD), temperature programmed reduction (TPR) and Fourier transform infrared spectroscopy (FTIR) has been carried out to evidence effects following the alkali nature. 2. Experimental part 2.1. Catalysts synthesis The zirconium oxide carrier was prepared by a precipitation method, adding dropwise an aqueous solution of zirconyl (IV) chloride to an ammonia solution under continuous stirring. The
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precipitate was filtered and washed to remove remaining chloride ions. This solid was dried at 100 ◦ C for 24 h and subsequently calcined under airflow (2 L h−1 ) at 300 or 600 ◦ C for 4 h. The obtained solid will be denoted by ZrO2 300 or ZrO2 600, the value given after the carrier name indicates the treatment temperature. The deposition of alkali metal ions was performed by impregnation of an aqueous solution containing given amounts of an alkali (Li, Na, K, Rb or Cs) nitrates onto the ZrO2 300 solid. After drying, samples were calcined for 4 h at 600 ◦ C under airflow. The asobtained solids will be denoted by Mx /ZrO2 where M is the chemical symbol of the impregnated alkali metal, and x = M/Zr atomic ratio. In the case of K/ZrO2 , the preparation method has been compared depending on the precursor salt used: KNO3 , KHCO3 or KOH, keeping otherwise all other preparation parameters unchanged.
Measurements were recorded on a Bruker Equinox 55 FTIR spectrometer at ambient temperature in the 4000–400 cm−1 range. Spectra were recorded with a resolution of 2 cm−1 and by coaddition of 32 scans. Powder X-ray diffraction (XRD) measurements were performed by a BRUKER D8 Advance diffractometer using Cu K␣ radiation ( = 1.5406 Å) in the 2 range 10◦ –80◦ , the step size was 0.02◦ and the integration time 6 s. Temperature programmed reduction (TPR) experiments were carried out in a ZETON ALTAMIRA, AMI 200 instrument. The catalyst samples calcined at 600 ◦ C (∼70 mg) were pre-treated under O2 diluted in argon flow (30 mL min−1 ) at 450 ◦ C for 1 h. After cooling under argon flow, reduction treatment (5% hydrogen in argon, 30 mL min−1 ) was performed from 30 to 600 ◦ C using a heating rate of 5 ◦ C min−1 .
2.2. Preparation and testing of (CB–catalyst) mixtures 3. Results and discussion Commercially available carbon black (CB) N330, DEGUSSA was used as a model soot in the catalytic tests. The surface area of CB is 80 m2 g−1 and the elementary analysis revealed the following composition, expressed in wt% [C: 97.23%; H: 0.73%; O: 1.16%; N: 0.19% and S: 0.45%]. (CB – catalysts) mixtures were prepared in loose contact conditions [6–12,22]. CB (6 wt%) and catalyst (94 wt%) were introduced in a small flask and were simply shaken for 30 min. The catalytic test towards CB oxidation was studied by simultaneous differential thermal and gravimetric analysis (DTA–TGA) with a NETZSCH STA 409 apparatus. About 30 mg of CB–catalyst mixture were loaded in an alumina crucible and heated from room temperature to 650 ◦ C (heating rate: 5 ◦ C min−1 ) in airflow (75 mL min−1 ). By processing the experimental data, the onset temperature (Ti ) and the final temperature (Tf ) related to CB oxidation could be derived from the TG curve. The Tm value could be detected at the maximum of the exothermic DTA peak and basically corresponds to temperature at which the CB oxidation rate is maximum. Illustrations are given in Fig. 1 showing the TGA–DTA curves recorded during oxidation of CB in the presence of ZrO2 and K0.14 /ZrO2 prepared from KNO3 precursor. In each case, the mass loss corresponds to the amount of CB initially introduced into the CB–catalyst mixture. TGA–DTA curves reveal that the oxidation of CB occurred regularly without any detection of rapid oxidation phenomena ascribable to a thermal runaway reaction.
3.1. Catalytic oxidation of CB 3.1.1. Preliminary study in the presence of Kx /ZrO2 catalysts The influence of alkali precursor and alkali content in Kx /ZrO2 was investigated prior to the study of other alkali/ZrO2 catalysts. Fig. 2 gives the temperature range in which the oxidation of CB occurred for the test under air of (CB–catalysts) mixtures. Note
2.3. Catalysts characterization FTIR experiments were performed with pellets prepared by pressing about 50 mg of catalyst powder (1 wt%) diluted in KBr.
Fig. 1. Examples of TGA–DTA measurements for the oxidation of carbon black in the presence of ZrO2 and K0.14 /ZrO2 prepared from KNO3 .
Fig. 2. Temperature range of carbon black oxidation in the presence of ZrO2 and Kx /ZrO2 catalysts prepared from different potassium precursors.
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that the non-catalytic oxidation of CB occurred between Ti = 515 ◦ C, and Tf = 641 ◦ C and gave a Tm of 627 ◦ C. The results obtained in the presence of ZrO2 indicated the low reactivity of this oxide carrier. The presence of potassium in Kx /ZrO2 improved the catalytic activity with differences following the potassium content. When K/Zr atomic ratio increases from 0.08 to 0.14, catalyst reactivity is improved but in the presence of catalysts with higher K/Zr atomic ratio, (K/Zr = 0.21), the reactivity seems to be slightly lower. Milt et al. [23] have studied K/La2 O3 systems with K contents varying between 4.5 and 10 wt% in soot oxidation reaction. A similar trend as in our case was found since the catalyst reactivity increases as the K content increases between 4.5 and 7.5 wt% whereas for 10 wt% of K, the maximum soot combustion temperature is shifted to higher value. They concluded that not only is the K content important but also the ratio of K to La2 O3 . The promoting effect of potassium was observed by other authors [8,10–11,16–19,24–26] and was mainly attributed to the fact that potassium enhances the contact between catalyst and carbon black. This contact enhancement is due to potassium species mobility at temperatures slightly lower than that of carbon black oxidation [8,17,19,25]. By this mobility, interaction between catalyst and carbon black will be increased [25]. Furthermore, potassium was found to favour oxygen release from the catalyst and to enhance dissociative adsorption of molecular oxygen from the phase gas [14,19]. Nevertheless, it appears that an optimal amount of potassium is needed to maximise potassium effect on CB oxidation. In our experimental conditions, it seems that a K/Zr atomic ratio of 0.14 is an optimal content for catalyst reactivity enhancement. To study if the nature of alkali precursor could have an influence on the catalyst reactivity, potassium was impregnated using different precursors: KOH, KHCO3 and KNO3 onto ZrO2 (K/Zr atomic ratio = 0.14). Fig. 2 indicates that the catalyst prepared using KNO3 shows the higher reactivity compared to catalysts prepared using KOH or KHCO3 . To gain more insight into the effect of alkali precursor, the physicochemical characterization of these K/ZrO2 catalysts has been undertaken (Section 3.2). 3.1.2. Catalytic behaviour of alkali/ZrO2 Fig. 3 illustrates the properties of Mx /ZrO2 catalysts prepared from alkali nitrates (x = 0.14) in the oxidation of carbon black. All alkali-based solids show a better reactivity than pure ZrO2 . Furthermore, differences are clearly observed depending on the nature of alkali impregnated on ZrO2 . Indeed the shift toward lower temperatures is nearly 80 ◦ C on Ti and 117 ◦ C on Tm between the tests performed with Li0.14 /ZrO2 and ZrO2 . A significant shift of oxidation temperature ranges is also observed when heavier alkali ions are supported on ZrO2 . The following sequence could be stated for the catalytic reactivity improvement of alkali metal/ZrO2 systems: ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . In the presence of Cs/ZrO2 catalyst, Ti and Tm decrease by more than 160 ◦ C on Ti and 200 ◦ C on Tm . It can be deduced that the activity is largely dependent on the nature of the alkali precursor impregnated on ZrO2 . Similar trends have been reported by other authors [13,21]. Neri et al. [13] found a higher activity in the presence of Cs versus K in alkali—FeV/Al2 O3 catalysts. According to Galdeano et al. [21], the activity of alkali metals containing catalysts increases with the electropositive character of the metal, so that a cesium-containing solid was found to be the most active catalyst. In order to know whether the reactivity improvement observed relies on the alkali metal itself or/and on other phenom-
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Fig. 3. Temperature range of carbon black oxidation in the presence of ZrO2 and M0.14 /ZrO2 (M = Li, Na, K, Rb and Cs) catalysts.
ena, a physico-chemical characterization of these compounds has been undertaken. It is also observed from Figs. 2 and 3 that CB oxidation occurred in a temperature range, which is more or less large following the catalysts tested. The difference between Tf and Ti , (Tf − Ti ), gives qualitative information on the reaction rate of carbon black oxidation [27]. Indeed, the tests were done with the same content of CB and with progressive increase of temperature versus time. Thus, a low difference (Tf − Ti ) indicates that the reaction occurred in a short time interval and hence a relatively high reaction rate. The precursor KNO3 displays a (Tf − Ti ) of 108◦ , KHCO3 of 170◦ and KOH of 201◦ . It is inferred that the oxidation rate is higher with KNO3 precursor. The same observation can be mentioned in the presence of other alkali promoted catalysts prepared using alkali nitrate precursors. To sum up, the effect of alkali promoters related to the catalyst reactivity depends on the nature of alkali metals: heavier alkali metals give better catalytic properties in the oxidation of CB. The use of alkali nitrate as precursor appeared to be a parameter favouring the reactivity compared to the use of alkali hydroxide or alkali hydrogen carbonate. To gain more insight into the effect of alkali nature and alkali precursor form towards the catalytic behaviour of alkali/ZrO2 , we investigated the physicochemical features of catalysts.
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D. Hleis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 330 (2008) 193–200 Table 1 Chemical composition of Mx /ZrO2 catalysts and ionic radius value of M+ ion from [30] Catalyst
M weight ratio (%)
M+ ionic radius (Å)
Li0.14 /ZrO2 Na0.14 /ZrO2 K0.08 /ZrO2 (KNO3 ) K0.14 /ZrO2 (KNO3 ) K0.21 /ZrO2 (KNO3 ) K0.14 /ZrO2 (KHCO3 ) K0.14 /ZrO2 (KOH) Rb0.14 /ZrO2 Cs0.14 /ZrO2
0.79 2.61 2.53 4.43 6.65 4.43 4.43 9.65 15.11
0.76 1.02 1.51
1.61 1.74
nia crystalline structure modification. According to these authors, alkali metals ions as Li and Na are able to diffuse inside the bulk of particles and stabilize the zirconia tetragonal phase. In contrast, when alkali metals are predominantly localized close to the surface, the monoclinic is favoured. These phenomena could occur in our case and are dependent of the ionic radius value of alkali metals [30] (Table 1). Furthermore, it is important to note the presence of additional lines on the diffractograms for Rb/ZrO2 (2 = 29.5◦ ) and Cs/ZrO2 (2 = 19.9◦ ). These lines could be ascribed to the presence of RbNO3 (JCPDS 79.1696) and CsNO3 structure (JCPDS 81.1138) in the corresponding alkali-based solids. On the contrary, no additional lines were observed for Li/ZrO2 , Na/ZrO2 and K/ZrO2 . These observations seem to show that the precursors do not decompose similarly during the calcination treatment. In other words, some alkali nitrates as RbNO3 and CsNO3 are more stable than others. These aspects have been more widely investigated using FTIR and TPR techniques.
Fig. 4. XRD patterns of the different M0.14 /ZrO2 catalysts. M: lines of zirconia monoclinic phase; T: lines of zirconia tetragonal phase; *line of CsNO3 . + line of RbNO3 .
3.2. Catalysts characterization 3.2.1. XRD measurements An XRD study of the different Mx /ZrO2 catalysts calcined at 600 ◦ C was carried out and the obtained diffractograms are displayed in Fig. 4. After calcination at 600 ◦ C, ZrO2 carrier is a mixture of tetragonal (JCPDS 50.1089) and monoclinic (JCPDS 65.1023) phases. XRD patterns for Li0.14 /ZrO2 and Na0.14 /ZrO2 reveal a higher relative amount of the tetragonal phase versus monoclinic one in comparison with ZrO2 . These alkali ions have an influence on the tetragonal–monoclinic transformation of ZrO2 . Particularly, Na species impregnated on ZrO2 300 strongly favour the stabilization of the tetragonal phase of ZrO2 so that the proportion of monoclinic phase is extremely low after calcination at 600 ◦ C. On the contrary, diffractograms for K/ZrO2 , Rb/ZrO2 and Cs/ZrO2 solids possess more similarities and show predominantly the monoclinic phase. The intensities of XRD lines relative to the tetragonal phase are lower in the case of Cs/ZrO2 . Our observations tend to show that the alkali metals have an influence on the crystallization of ZrO2 that occurred during the calcination treatment until 600 ◦ C. These effects depend on the nature of alkali metals impregnated on ZrO2 carrier, as previously reported by other authors [28,29]. Marote et al. [29] have studied the effect of alkali metals on zirco-
3.2.2. FTIR measurements Infrared spectra were first recorded for the K/ZrO2 solids prepared with different precursors and for pure ZrO2 support (Fig. 5). ZrO2 solid presents two broad lines with low intensity at 1620 and 1330 cm−1 , respectively attributed to water and CO3 2− groups adsorbed on the surface [8,31,32]. The presence of these two bands is explained by water physisorption and CO2 adsorption on ZrO2 surface by exposure to air. K0.14 /ZrO2 (KHCO3 ) and K0.14 /ZrO2 (KOH) infrared spectra are very similar showing the bands of adsorbed water (1620 cm−1 ) and of adsorbed CO3 2− (1330–1390 cm−1 ) with higher intensities than in pure ZrO2 carrier. Two additional bands at 1385 and 1350 cm−1 were detected for K0.14 /ZrO2 prepared from KNO3 . These bands are attributed to free NO3 − groups (1385 cm−1 ) and to monodentate NO3 − groups (1335–1350 cm−1 ), probably bonded to potassium [33–35]. Presence of bridged NO3 − groups could not be excluded; their characteristic band (∼1610 cm−1 ) could be masked by adsorbed water band. The above data reveal that potassium supported on zirconia solids are subject to surface hydration and carbonation. In the case of K0.14 /ZrO2 (KHCO3 ), hydrogen carbonate groups appeared to be mainly decomposed after its impregnation and the subsequent calcination of the solid. On the contrary, a part of nitrates species remains stable in the case of K0.14 /ZrO2 prepared from KNO3 even after calcination at 600 ◦ C. According to Westerberg and Fridell [36], monodentate NO3 − groups are due to the presence of well-structured KNO3 compound whereas the bridged groups are bonded to zirconia. FTIR measurements performed for the different alkali/ZrO2 (Fig. 6) revealed first that Li/ZrO2 solid exhibits two weak broad bands at 1430 and 1500 cm−1 due to NO3 − in interaction with the support [36]. The other alkali/ZrO2 solids give a band at 1335–1350 cm−1 assigned to monodentate NO3 − [33] as well as a narrow band at 1385 cm−1 due to free NO3 − . Their intensities
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Fig. 5. FTIR transmittance spectra of ZrO2 and Kx /ZrO2 catalysts prepared from different potassium precursors.
increase according the following sequence: Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . These observations tend to show that the stabilization degree of monodentate NO3 − and free NO3 − is higher in the presence of heavier alkali metals. 3.2.3. TPR measurements Fig. 7 gives the H2 -TPR profiles obtained for the Mx /ZrO2 catalysts series. First, it can be mentioned that no H2 consumption was recorded for the pure ZrO2 carrier in our experimental conditions. The carrier reducibility seems to have been slightly modified in the presence of potassium in K0.14 /ZrO2 prepared from KHCO3 or KOH, because a weak H2 consumption was observed from 420 ◦ C for these catalysts. Mx /ZrO2 catalysts from corresponding alkali nitrates give one reduction peak with maximum reduction temperature pointed out between 475 and 520 ◦ C. The amount of hydrogen consumed (131 mol g−1 for Li/ZrO2 and 1235 mol g−1 for Cs/ZrO2 ) depends on the alkali nature impregnated on ZrO2 and increases following the sequence: Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . The H2 consumptions can be ascribed considering FTIR data which showed different NO3 − species stabilized in Mx /ZrO2 . Moreover, the detection by XRD of RbNO3 and CsNO3 in Rb/ZrO2 and Cs/ZrO2 respectively let suggest the presence of relatively high fraction of alkali nitrate remained stable in these solids even after the
Fig. 6. FTIR transmittance spectra of M0.14 /ZrO2 catalysts.
calcination treatment at 600 ◦ C. On the contrary, alkali precursors are largely decomposed in the case of K/ZrO2 prepared from KOH or KHCO3 (Fig. 5). It is then proposed to ascribe the H2 consumption in Mx /ZrO2 catalysts prepared from alkali nitrates to the reduction of NO3 − species. Hence it clearly appears that higher amount of NO3 − species is stabilized in Cs0.14 /ZrO2 than other Mx /ZrO2 catalysts. The alkali metal is responsible for NO3 − stabilization on ZrO2 carrier and this effect is stronger in the presence of heavier alkali metals. These observations are in good agreement with the stability sequence of alkali nitrates deduced from formation free enthalpy values [30]. 3.3. Aging tests The effect of successive tests is an important parameter in order to verify the stability degree of catalysts. Experiments were carried out with Kx /ZrO2 catalyst prepared from potassium nitrate. CB oxidation cycles were repeated in a fixed bed reactor under airflow from 25 to 500 ◦ C and with a plateau at 500 ◦ C for 1 h. After four oxidation runs, the K0.14 /ZrO2 sample showed a slight shift of Tm from 396 to 410 ◦ C but the CB oxidation occurred nearly in the same temperature range between 355 and 470 ◦ C. The shape of TDA–TGA profiles is not significantly modified after these oxidation runs. It appeared that the catalyst prepared using KNO3 and
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Fig. 8. TPR profiles of K0.14 /ZrO2 prepared from KNO3 after different carbon black oxidation cycles.
Fig. 7. TPR profiles of the Mx /ZrO2 catalysts.
tested for four runs, shows a reactivity similar to the one of catalysts prepared using KOH or KHCO3. The evolution of the catalyst features after each run was investigated by TPR and FTIR. TPR profiles of K/ZrO2 (Fig. 8) show a single reduction peak as in the case of the sample before testing. An important decrease in H2 consumption is observed after successive runs revealing that at least a part of NO3 − species are irreversibly reduced during the oxidation of CB. FTIR spectra obtained after the successive runs under air are displayed in Fig. 9. After the first run, we observed an important intensity decrease of the bands due to bridged NO3 − at 1620 cm−1 , free NO3 − at 1385 cm−1 and monodentate NO3 − at 1335–1350 cm−1 . The decrease is less significant after the subsequent runs. Two bands at 1120 and 1160 cm−1 appeared after the first run and increased in intensity after the other runs. A literature survey [37,38] led us to assign the band at 1120 cm−1 to S O groups of sulphate type. Sulphated compounds formation could be explained by the sulphur presence in carbon black (0.45 wt%). In the experimental conditions used for catalytic tests, sulphur impurities could be oxidized into sulphate species. The presence of the band at 1160 cm−1 , after runs 2, 3 and 4 could be assigned to NO2 − species adsorbed on the solid [39].
Carrascull et al. [16] studied K/ZrO2 systems prepared from KNO3 and calcined at 600 ◦ C. These authors proposed that the catalytic performances of the solids are due to NO3 − species following a redox mechanism where nitrate is reduced to nitrite by reaction with carbon and where gaseous oxygen oxidizes nitrite species to nitrate species. Such a mechanism was also reported by van Gulijk et al. [40] and Zhang and Zou [41]. From our results, we found that the amount of NO3 − strongly decreases after the first run and then less significantly after other runs. In parallel, NO2 − species were detected in catalysts (Fig. 9). For this reason a contribution of the mechanism suggested by Carrascull et al. [16] can be suggested in our case. However, it is important to underline that a major part of NO3 − species are irreversibly decomposed from the first run (Figs. 8 and 9). As this loss of NO3 − species is observed, the intensity of both free NO3 − and monodentate NO3 − bands decreases. It demonstrates that such species are able to oxidize carbon easily but are irreversibly consumed. Nevertheless, bridged NO3 − and a part of monodentate NO3 − seem to be more stable even after the four runs and in parallel, NO2 − are detected. It led us to suggest that a part of NO3 − could be strongly bonded to the Mx /ZrO2 catalyst surface. This kind of NO3 − species could be involved in the mechanism involving reversibly NO3 − and NO2 − species. In addition, we found that the stabilization of nitrate species in Mx /ZrO2 catalysts depends on the nature of the alkali metal. A
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The lower activity of Li/ZrO2 could be linked with the size of the alkali, which is able to penetrate into the bulk of ZrO2 during the calcination treatment so that a lower proportion of alkali metals create the contact with carbon particles. On the contrary, alkali metals with high ionic radius remain on the ZrO2 surface. Moreover, alkali metals were found to stabilize nitrates more or less depending on the alkali nature. The amount of NO3 − stable after calcination at 600 ◦ C is higher in the presence of alkali with high ionic radius and these species are involved in the carbon particles oxidation. NO3 − with lower interaction with the catalyst are able to oxidize carbon easily but are irreversibly consumed. Bridged NO3 − and a part of monodentate NO3 − could be involved in a reversible mechanism. Acknowledgements The authors wish to express their thanks to the Region Nord-Pas de Calais and the European Community (EDRF) for financial supports. M. Labaki is grateful to the University of Littoral-Côte d’Opale for research fellowship. References
Fig. 9. FTIR transmittance spectra of K0.14 /ZrO2 prepared from KNO3 after different carbon black combustion cycles.
low proportion of nitrates remain stable after calcination at 600 ◦ C in the presence of Li and Na. This phenomenon has to be mainly explained considering the penetration of low size alkali inside the ZrO2 solid. The homogeneous distribution of Li and Na in the solid could explain a lower amount of alkali on the surface and consequently a lower stabilization of nitrates species, which are decomposed during the calcination treatment. In the case of heavier alkali (K, Rb and Cs), the high value of ionic radius could be the main parameter explaining the presence of alkali rather on the ZrO2 surface. Moreover, the stability of alkali nitrates is known to be higher in the case of heavier alkali [30]. Our observations are in agreement with these trends and the higher amount of NO3 − in Cs-based catalyst could explain the higher activity of this solid. Finally, this evidence of interaction between nitrogen oxide species and alkali-based ZrO2 may be responsible for the high activity of some alkali-based catalysts in the simultaneous elimination of NOx and soot [42,43]. 4. Conclusion The catalytic oxidation of carbon black has been investigated considering alkali containing ZrO2 systems. It has been shown that K/ZrO2 prepared from KHCO3 , KOH or KNO3 are active catalysts. The preparation of K/ZrO2 with K/Zr = 0.14 from alkali nitrates provides the higher activity. Comparing the catalytic behaviour of alkali/ZrO2 reveals an increasing activity in the oxidation of carbon black according to the sequence ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 .
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Comparison of alkali-promoted ZrO2 catalysts towards carbon black oxidation D. Hleis a , M. Labaki b , H. Laversin a , D. Courcot a,∗ , A. Aboukaïs a a b
Laboratoire de Catalyse et Environnement EA 2598, Université du Littoral – Côte d’Opale, 145, avenue Maurice Schumann, 59140 Dunkerque, France Chemistry Department, Faculty of Sciences II, Lebanese University, Fanar, El-Metn, Lebanon
a r t i c l e
i n f o
Article history: Received 18 February 2008 Received in revised form 24 July 2008 Accepted 26 July 2008 Available online 5 August 2008 Keywords: Soot oxidation Zirconia Alkali promoters XRD TPR FTIR
a b s t r a c t The effect of alkali metals deposition on zirconia has been studied in the oxidation of carbon black, considered as a model of diesel soot. The study of the influence of alkali content and alkali precursor was undertaken for K/ZrO2 , evidencing a better activity for a catalyst prepared with an atomic ratio K/Zr = 0.14 and from a nitrate precursor. Using the latter preparation conditions, alkali/ZrO2 are found to be active in the oxidation of carbon black according the sequence ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . Alkali metals have an influence on the tetragonal–monoclinic crystalline modification. Alkali metals ions with low size tend to stabilize the tetragonal ZrO2 phase whereas those with higher ionic radius favour the tetragonal–monoclinic modification. Fourier transform-infrared spectroscopy (FTIR) and temperature programmed reduction (TPR) measurements show that the catalytic activity partially depends on the presence of nitrate species stabilized in alkali/ZrO2 even after calcination treatment at 600 ◦ C. Nitrate species are more stable in the presence of alkali with high ionic radius than those of low size. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Soot emitted in large amounts by diesel engines is hazardous to human health. To meet stringent environmental regulations fixed by European legislation, automobile manufacturers have to develop technologies to reduce soot emissions. Post-treatment of emissions using a particulate filter is one of the suggested solutions. However, particulate matter combustion temperature is about 600 ◦ C, much higher than the operational temperature found in exhaust gases of engines (150–400 ◦ C). The use of a catalyst impregnated on the filter enables to oxidize soot particulate at lower temperatures (300–400 ◦ C) [1–4]. Alkali metals are introduced as promoter in catalysts developed for the oxidation of soot [5–15]. Different positive effects obtained in the presence of alkali metals are now well recognized. Firstly, alkali metals facilitate the reaction by favouring a good contact between the catalyst surface and soot particles [8,11,14,16]. In fact, alkali metals generally form compounds with low melting temperature or eutectics with other phases of the catalyst [13–14,17–18]. Secondly, alkali metals increase the redox properties of the active phase by enhancing electron transfers in the catalyst [11,14]. In previous works, we have studied the role of potassium in Cu–K/ZrO2 catalysts tested in the oxidation of carbon black in loose
∗ Corresponding author. Tel.: +33 3 28 65 82 61; fax: +33 3 28 65 82 39. E-mail address: [email protected] (D. Courcot). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.07.052
contact conditions [11,19]. We showed that potassium favours the contact between the catalyst and carbon black and enhances the ability of the catalysts to release active oxygen species. Such species are issued from Cu(II) species via a redox mechanism. K deposited on ZrO2 was also found to interact with the carrier surface and K/ZrO2 also appeared to be active in the oxidation of black carbon. Up to now, few works have attempted to compare the effect of the different alkali metals supported on oxide carrier in the oxidation of soot or carbon black [13,20–21]. The aim of this work has been focused on the effect of alkali metals (Li, Na, K, Rb and Cs) on the properties alkali/ZrO2 catalysts. First, we have investigated the influence of different potassium/ZrO2 synthesis parameters such as (i) nature of the potassium precursor, (ii) potassium content. The comparison of alkali/ZrO2 properties has been undertaken using alkali nitrates as precursor. A characterization of alkali/ZrO2 from X-ray diffraction (XRD), temperature programmed reduction (TPR) and Fourier transform infrared spectroscopy (FTIR) has been carried out to evidence effects following the alkali nature. 2. Experimental part 2.1. Catalysts synthesis The zirconium oxide carrier was prepared by a precipitation method, adding dropwise an aqueous solution of zirconyl (IV) chloride to an ammonia solution under continuous stirring. The
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precipitate was filtered and washed to remove remaining chloride ions. This solid was dried at 100 ◦ C for 24 h and subsequently calcined under airflow (2 L h−1 ) at 300 or 600 ◦ C for 4 h. The obtained solid will be denoted by ZrO2 300 or ZrO2 600, the value given after the carrier name indicates the treatment temperature. The deposition of alkali metal ions was performed by impregnation of an aqueous solution containing given amounts of an alkali (Li, Na, K, Rb or Cs) nitrates onto the ZrO2 300 solid. After drying, samples were calcined for 4 h at 600 ◦ C under airflow. The asobtained solids will be denoted by Mx /ZrO2 where M is the chemical symbol of the impregnated alkali metal, and x = M/Zr atomic ratio. In the case of K/ZrO2 , the preparation method has been compared depending on the precursor salt used: KNO3 , KHCO3 or KOH, keeping otherwise all other preparation parameters unchanged.
Measurements were recorded on a Bruker Equinox 55 FTIR spectrometer at ambient temperature in the 4000–400 cm−1 range. Spectra were recorded with a resolution of 2 cm−1 and by coaddition of 32 scans. Powder X-ray diffraction (XRD) measurements were performed by a BRUKER D8 Advance diffractometer using Cu K␣ radiation ( = 1.5406 Å) in the 2 range 10◦ –80◦ , the step size was 0.02◦ and the integration time 6 s. Temperature programmed reduction (TPR) experiments were carried out in a ZETON ALTAMIRA, AMI 200 instrument. The catalyst samples calcined at 600 ◦ C (∼70 mg) were pre-treated under O2 diluted in argon flow (30 mL min−1 ) at 450 ◦ C for 1 h. After cooling under argon flow, reduction treatment (5% hydrogen in argon, 30 mL min−1 ) was performed from 30 to 600 ◦ C using a heating rate of 5 ◦ C min−1 .
2.2. Preparation and testing of (CB–catalyst) mixtures 3. Results and discussion Commercially available carbon black (CB) N330, DEGUSSA was used as a model soot in the catalytic tests. The surface area of CB is 80 m2 g−1 and the elementary analysis revealed the following composition, expressed in wt% [C: 97.23%; H: 0.73%; O: 1.16%; N: 0.19% and S: 0.45%]. (CB – catalysts) mixtures were prepared in loose contact conditions [6–12,22]. CB (6 wt%) and catalyst (94 wt%) were introduced in a small flask and were simply shaken for 30 min. The catalytic test towards CB oxidation was studied by simultaneous differential thermal and gravimetric analysis (DTA–TGA) with a NETZSCH STA 409 apparatus. About 30 mg of CB–catalyst mixture were loaded in an alumina crucible and heated from room temperature to 650 ◦ C (heating rate: 5 ◦ C min−1 ) in airflow (75 mL min−1 ). By processing the experimental data, the onset temperature (Ti ) and the final temperature (Tf ) related to CB oxidation could be derived from the TG curve. The Tm value could be detected at the maximum of the exothermic DTA peak and basically corresponds to temperature at which the CB oxidation rate is maximum. Illustrations are given in Fig. 1 showing the TGA–DTA curves recorded during oxidation of CB in the presence of ZrO2 and K0.14 /ZrO2 prepared from KNO3 precursor. In each case, the mass loss corresponds to the amount of CB initially introduced into the CB–catalyst mixture. TGA–DTA curves reveal that the oxidation of CB occurred regularly without any detection of rapid oxidation phenomena ascribable to a thermal runaway reaction.
3.1. Catalytic oxidation of CB 3.1.1. Preliminary study in the presence of Kx /ZrO2 catalysts The influence of alkali precursor and alkali content in Kx /ZrO2 was investigated prior to the study of other alkali/ZrO2 catalysts. Fig. 2 gives the temperature range in which the oxidation of CB occurred for the test under air of (CB–catalysts) mixtures. Note
2.3. Catalysts characterization FTIR experiments were performed with pellets prepared by pressing about 50 mg of catalyst powder (1 wt%) diluted in KBr.
Fig. 1. Examples of TGA–DTA measurements for the oxidation of carbon black in the presence of ZrO2 and K0.14 /ZrO2 prepared from KNO3 .
Fig. 2. Temperature range of carbon black oxidation in the presence of ZrO2 and Kx /ZrO2 catalysts prepared from different potassium precursors.
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that the non-catalytic oxidation of CB occurred between Ti = 515 ◦ C, and Tf = 641 ◦ C and gave a Tm of 627 ◦ C. The results obtained in the presence of ZrO2 indicated the low reactivity of this oxide carrier. The presence of potassium in Kx /ZrO2 improved the catalytic activity with differences following the potassium content. When K/Zr atomic ratio increases from 0.08 to 0.14, catalyst reactivity is improved but in the presence of catalysts with higher K/Zr atomic ratio, (K/Zr = 0.21), the reactivity seems to be slightly lower. Milt et al. [23] have studied K/La2 O3 systems with K contents varying between 4.5 and 10 wt% in soot oxidation reaction. A similar trend as in our case was found since the catalyst reactivity increases as the K content increases between 4.5 and 7.5 wt% whereas for 10 wt% of K, the maximum soot combustion temperature is shifted to higher value. They concluded that not only is the K content important but also the ratio of K to La2 O3 . The promoting effect of potassium was observed by other authors [8,10–11,16–19,24–26] and was mainly attributed to the fact that potassium enhances the contact between catalyst and carbon black. This contact enhancement is due to potassium species mobility at temperatures slightly lower than that of carbon black oxidation [8,17,19,25]. By this mobility, interaction between catalyst and carbon black will be increased [25]. Furthermore, potassium was found to favour oxygen release from the catalyst and to enhance dissociative adsorption of molecular oxygen from the phase gas [14,19]. Nevertheless, it appears that an optimal amount of potassium is needed to maximise potassium effect on CB oxidation. In our experimental conditions, it seems that a K/Zr atomic ratio of 0.14 is an optimal content for catalyst reactivity enhancement. To study if the nature of alkali precursor could have an influence on the catalyst reactivity, potassium was impregnated using different precursors: KOH, KHCO3 and KNO3 onto ZrO2 (K/Zr atomic ratio = 0.14). Fig. 2 indicates that the catalyst prepared using KNO3 shows the higher reactivity compared to catalysts prepared using KOH or KHCO3 . To gain more insight into the effect of alkali precursor, the physicochemical characterization of these K/ZrO2 catalysts has been undertaken (Section 3.2). 3.1.2. Catalytic behaviour of alkali/ZrO2 Fig. 3 illustrates the properties of Mx /ZrO2 catalysts prepared from alkali nitrates (x = 0.14) in the oxidation of carbon black. All alkali-based solids show a better reactivity than pure ZrO2 . Furthermore, differences are clearly observed depending on the nature of alkali impregnated on ZrO2 . Indeed the shift toward lower temperatures is nearly 80 ◦ C on Ti and 117 ◦ C on Tm between the tests performed with Li0.14 /ZrO2 and ZrO2 . A significant shift of oxidation temperature ranges is also observed when heavier alkali ions are supported on ZrO2 . The following sequence could be stated for the catalytic reactivity improvement of alkali metal/ZrO2 systems: ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . In the presence of Cs/ZrO2 catalyst, Ti and Tm decrease by more than 160 ◦ C on Ti and 200 ◦ C on Tm . It can be deduced that the activity is largely dependent on the nature of the alkali precursor impregnated on ZrO2 . Similar trends have been reported by other authors [13,21]. Neri et al. [13] found a higher activity in the presence of Cs versus K in alkali—FeV/Al2 O3 catalysts. According to Galdeano et al. [21], the activity of alkali metals containing catalysts increases with the electropositive character of the metal, so that a cesium-containing solid was found to be the most active catalyst. In order to know whether the reactivity improvement observed relies on the alkali metal itself or/and on other phenom-
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Fig. 3. Temperature range of carbon black oxidation in the presence of ZrO2 and M0.14 /ZrO2 (M = Li, Na, K, Rb and Cs) catalysts.
ena, a physico-chemical characterization of these compounds has been undertaken. It is also observed from Figs. 2 and 3 that CB oxidation occurred in a temperature range, which is more or less large following the catalysts tested. The difference between Tf and Ti , (Tf − Ti ), gives qualitative information on the reaction rate of carbon black oxidation [27]. Indeed, the tests were done with the same content of CB and with progressive increase of temperature versus time. Thus, a low difference (Tf − Ti ) indicates that the reaction occurred in a short time interval and hence a relatively high reaction rate. The precursor KNO3 displays a (Tf − Ti ) of 108◦ , KHCO3 of 170◦ and KOH of 201◦ . It is inferred that the oxidation rate is higher with KNO3 precursor. The same observation can be mentioned in the presence of other alkali promoted catalysts prepared using alkali nitrate precursors. To sum up, the effect of alkali promoters related to the catalyst reactivity depends on the nature of alkali metals: heavier alkali metals give better catalytic properties in the oxidation of CB. The use of alkali nitrate as precursor appeared to be a parameter favouring the reactivity compared to the use of alkali hydroxide or alkali hydrogen carbonate. To gain more insight into the effect of alkali nature and alkali precursor form towards the catalytic behaviour of alkali/ZrO2 , we investigated the physicochemical features of catalysts.
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D. Hleis et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 330 (2008) 193–200 Table 1 Chemical composition of Mx /ZrO2 catalysts and ionic radius value of M+ ion from [30] Catalyst
M weight ratio (%)
M+ ionic radius (Å)
Li0.14 /ZrO2 Na0.14 /ZrO2 K0.08 /ZrO2 (KNO3 ) K0.14 /ZrO2 (KNO3 ) K0.21 /ZrO2 (KNO3 ) K0.14 /ZrO2 (KHCO3 ) K0.14 /ZrO2 (KOH) Rb0.14 /ZrO2 Cs0.14 /ZrO2
0.79 2.61 2.53 4.43 6.65 4.43 4.43 9.65 15.11
0.76 1.02 1.51
1.61 1.74
nia crystalline structure modification. According to these authors, alkali metals ions as Li and Na are able to diffuse inside the bulk of particles and stabilize the zirconia tetragonal phase. In contrast, when alkali metals are predominantly localized close to the surface, the monoclinic is favoured. These phenomena could occur in our case and are dependent of the ionic radius value of alkali metals [30] (Table 1). Furthermore, it is important to note the presence of additional lines on the diffractograms for Rb/ZrO2 (2 = 29.5◦ ) and Cs/ZrO2 (2 = 19.9◦ ). These lines could be ascribed to the presence of RbNO3 (JCPDS 79.1696) and CsNO3 structure (JCPDS 81.1138) in the corresponding alkali-based solids. On the contrary, no additional lines were observed for Li/ZrO2 , Na/ZrO2 and K/ZrO2 . These observations seem to show that the precursors do not decompose similarly during the calcination treatment. In other words, some alkali nitrates as RbNO3 and CsNO3 are more stable than others. These aspects have been more widely investigated using FTIR and TPR techniques.
Fig. 4. XRD patterns of the different M0.14 /ZrO2 catalysts. M: lines of zirconia monoclinic phase; T: lines of zirconia tetragonal phase; *line of CsNO3 . + line of RbNO3 .
3.2. Catalysts characterization 3.2.1. XRD measurements An XRD study of the different Mx /ZrO2 catalysts calcined at 600 ◦ C was carried out and the obtained diffractograms are displayed in Fig. 4. After calcination at 600 ◦ C, ZrO2 carrier is a mixture of tetragonal (JCPDS 50.1089) and monoclinic (JCPDS 65.1023) phases. XRD patterns for Li0.14 /ZrO2 and Na0.14 /ZrO2 reveal a higher relative amount of the tetragonal phase versus monoclinic one in comparison with ZrO2 . These alkali ions have an influence on the tetragonal–monoclinic transformation of ZrO2 . Particularly, Na species impregnated on ZrO2 300 strongly favour the stabilization of the tetragonal phase of ZrO2 so that the proportion of monoclinic phase is extremely low after calcination at 600 ◦ C. On the contrary, diffractograms for K/ZrO2 , Rb/ZrO2 and Cs/ZrO2 solids possess more similarities and show predominantly the monoclinic phase. The intensities of XRD lines relative to the tetragonal phase are lower in the case of Cs/ZrO2 . Our observations tend to show that the alkali metals have an influence on the crystallization of ZrO2 that occurred during the calcination treatment until 600 ◦ C. These effects depend on the nature of alkali metals impregnated on ZrO2 carrier, as previously reported by other authors [28,29]. Marote et al. [29] have studied the effect of alkali metals on zirco-
3.2.2. FTIR measurements Infrared spectra were first recorded for the K/ZrO2 solids prepared with different precursors and for pure ZrO2 support (Fig. 5). ZrO2 solid presents two broad lines with low intensity at 1620 and 1330 cm−1 , respectively attributed to water and CO3 2− groups adsorbed on the surface [8,31,32]. The presence of these two bands is explained by water physisorption and CO2 adsorption on ZrO2 surface by exposure to air. K0.14 /ZrO2 (KHCO3 ) and K0.14 /ZrO2 (KOH) infrared spectra are very similar showing the bands of adsorbed water (1620 cm−1 ) and of adsorbed CO3 2− (1330–1390 cm−1 ) with higher intensities than in pure ZrO2 carrier. Two additional bands at 1385 and 1350 cm−1 were detected for K0.14 /ZrO2 prepared from KNO3 . These bands are attributed to free NO3 − groups (1385 cm−1 ) and to monodentate NO3 − groups (1335–1350 cm−1 ), probably bonded to potassium [33–35]. Presence of bridged NO3 − groups could not be excluded; their characteristic band (∼1610 cm−1 ) could be masked by adsorbed water band. The above data reveal that potassium supported on zirconia solids are subject to surface hydration and carbonation. In the case of K0.14 /ZrO2 (KHCO3 ), hydrogen carbonate groups appeared to be mainly decomposed after its impregnation and the subsequent calcination of the solid. On the contrary, a part of nitrates species remains stable in the case of K0.14 /ZrO2 prepared from KNO3 even after calcination at 600 ◦ C. According to Westerberg and Fridell [36], monodentate NO3 − groups are due to the presence of well-structured KNO3 compound whereas the bridged groups are bonded to zirconia. FTIR measurements performed for the different alkali/ZrO2 (Fig. 6) revealed first that Li/ZrO2 solid exhibits two weak broad bands at 1430 and 1500 cm−1 due to NO3 − in interaction with the support [36]. The other alkali/ZrO2 solids give a band at 1335–1350 cm−1 assigned to monodentate NO3 − [33] as well as a narrow band at 1385 cm−1 due to free NO3 − . Their intensities
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Fig. 5. FTIR transmittance spectra of ZrO2 and Kx /ZrO2 catalysts prepared from different potassium precursors.
increase according the following sequence: Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . These observations tend to show that the stabilization degree of monodentate NO3 − and free NO3 − is higher in the presence of heavier alkali metals. 3.2.3. TPR measurements Fig. 7 gives the H2 -TPR profiles obtained for the Mx /ZrO2 catalysts series. First, it can be mentioned that no H2 consumption was recorded for the pure ZrO2 carrier in our experimental conditions. The carrier reducibility seems to have been slightly modified in the presence of potassium in K0.14 /ZrO2 prepared from KHCO3 or KOH, because a weak H2 consumption was observed from 420 ◦ C for these catalysts. Mx /ZrO2 catalysts from corresponding alkali nitrates give one reduction peak with maximum reduction temperature pointed out between 475 and 520 ◦ C. The amount of hydrogen consumed (131 mol g−1 for Li/ZrO2 and 1235 mol g−1 for Cs/ZrO2 ) depends on the alkali nature impregnated on ZrO2 and increases following the sequence: Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 . The H2 consumptions can be ascribed considering FTIR data which showed different NO3 − species stabilized in Mx /ZrO2 . Moreover, the detection by XRD of RbNO3 and CsNO3 in Rb/ZrO2 and Cs/ZrO2 respectively let suggest the presence of relatively high fraction of alkali nitrate remained stable in these solids even after the
Fig. 6. FTIR transmittance spectra of M0.14 /ZrO2 catalysts.
calcination treatment at 600 ◦ C. On the contrary, alkali precursors are largely decomposed in the case of K/ZrO2 prepared from KOH or KHCO3 (Fig. 5). It is then proposed to ascribe the H2 consumption in Mx /ZrO2 catalysts prepared from alkali nitrates to the reduction of NO3 − species. Hence it clearly appears that higher amount of NO3 − species is stabilized in Cs0.14 /ZrO2 than other Mx /ZrO2 catalysts. The alkali metal is responsible for NO3 − stabilization on ZrO2 carrier and this effect is stronger in the presence of heavier alkali metals. These observations are in good agreement with the stability sequence of alkali nitrates deduced from formation free enthalpy values [30]. 3.3. Aging tests The effect of successive tests is an important parameter in order to verify the stability degree of catalysts. Experiments were carried out with Kx /ZrO2 catalyst prepared from potassium nitrate. CB oxidation cycles were repeated in a fixed bed reactor under airflow from 25 to 500 ◦ C and with a plateau at 500 ◦ C for 1 h. After four oxidation runs, the K0.14 /ZrO2 sample showed a slight shift of Tm from 396 to 410 ◦ C but the CB oxidation occurred nearly in the same temperature range between 355 and 470 ◦ C. The shape of TDA–TGA profiles is not significantly modified after these oxidation runs. It appeared that the catalyst prepared using KNO3 and
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Fig. 8. TPR profiles of K0.14 /ZrO2 prepared from KNO3 after different carbon black oxidation cycles.
Fig. 7. TPR profiles of the Mx /ZrO2 catalysts.
tested for four runs, shows a reactivity similar to the one of catalysts prepared using KOH or KHCO3. The evolution of the catalyst features after each run was investigated by TPR and FTIR. TPR profiles of K/ZrO2 (Fig. 8) show a single reduction peak as in the case of the sample before testing. An important decrease in H2 consumption is observed after successive runs revealing that at least a part of NO3 − species are irreversibly reduced during the oxidation of CB. FTIR spectra obtained after the successive runs under air are displayed in Fig. 9. After the first run, we observed an important intensity decrease of the bands due to bridged NO3 − at 1620 cm−1 , free NO3 − at 1385 cm−1 and monodentate NO3 − at 1335–1350 cm−1 . The decrease is less significant after the subsequent runs. Two bands at 1120 and 1160 cm−1 appeared after the first run and increased in intensity after the other runs. A literature survey [37,38] led us to assign the band at 1120 cm−1 to S O groups of sulphate type. Sulphated compounds formation could be explained by the sulphur presence in carbon black (0.45 wt%). In the experimental conditions used for catalytic tests, sulphur impurities could be oxidized into sulphate species. The presence of the band at 1160 cm−1 , after runs 2, 3 and 4 could be assigned to NO2 − species adsorbed on the solid [39].
Carrascull et al. [16] studied K/ZrO2 systems prepared from KNO3 and calcined at 600 ◦ C. These authors proposed that the catalytic performances of the solids are due to NO3 − species following a redox mechanism where nitrate is reduced to nitrite by reaction with carbon and where gaseous oxygen oxidizes nitrite species to nitrate species. Such a mechanism was also reported by van Gulijk et al. [40] and Zhang and Zou [41]. From our results, we found that the amount of NO3 − strongly decreases after the first run and then less significantly after other runs. In parallel, NO2 − species were detected in catalysts (Fig. 9). For this reason a contribution of the mechanism suggested by Carrascull et al. [16] can be suggested in our case. However, it is important to underline that a major part of NO3 − species are irreversibly decomposed from the first run (Figs. 8 and 9). As this loss of NO3 − species is observed, the intensity of both free NO3 − and monodentate NO3 − bands decreases. It demonstrates that such species are able to oxidize carbon easily but are irreversibly consumed. Nevertheless, bridged NO3 − and a part of monodentate NO3 − seem to be more stable even after the four runs and in parallel, NO2 − are detected. It led us to suggest that a part of NO3 − could be strongly bonded to the Mx /ZrO2 catalyst surface. This kind of NO3 − species could be involved in the mechanism involving reversibly NO3 − and NO2 − species. In addition, we found that the stabilization of nitrate species in Mx /ZrO2 catalysts depends on the nature of the alkali metal. A
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The lower activity of Li/ZrO2 could be linked with the size of the alkali, which is able to penetrate into the bulk of ZrO2 during the calcination treatment so that a lower proportion of alkali metals create the contact with carbon particles. On the contrary, alkali metals with high ionic radius remain on the ZrO2 surface. Moreover, alkali metals were found to stabilize nitrates more or less depending on the alkali nature. The amount of NO3 − stable after calcination at 600 ◦ C is higher in the presence of alkali with high ionic radius and these species are involved in the carbon particles oxidation. NO3 − with lower interaction with the catalyst are able to oxidize carbon easily but are irreversibly consumed. Bridged NO3 − and a part of monodentate NO3 − could be involved in a reversible mechanism. Acknowledgements The authors wish to express their thanks to the Region Nord-Pas de Calais and the European Community (EDRF) for financial supports. M. Labaki is grateful to the University of Littoral-Côte d’Opale for research fellowship. References
Fig. 9. FTIR transmittance spectra of K0.14 /ZrO2 prepared from KNO3 after different carbon black combustion cycles.
low proportion of nitrates remain stable after calcination at 600 ◦ C in the presence of Li and Na. This phenomenon has to be mainly explained considering the penetration of low size alkali inside the ZrO2 solid. The homogeneous distribution of Li and Na in the solid could explain a lower amount of alkali on the surface and consequently a lower stabilization of nitrates species, which are decomposed during the calcination treatment. In the case of heavier alkali (K, Rb and Cs), the high value of ionic radius could be the main parameter explaining the presence of alkali rather on the ZrO2 surface. Moreover, the stability of alkali nitrates is known to be higher in the case of heavier alkali [30]. Our observations are in agreement with these trends and the higher amount of NO3 − in Cs-based catalyst could explain the higher activity of this solid. Finally, this evidence of interaction between nitrogen oxide species and alkali-based ZrO2 may be responsible for the high activity of some alkali-based catalysts in the simultaneous elimination of NOx and soot [42,43]. 4. Conclusion The catalytic oxidation of carbon black has been investigated considering alkali containing ZrO2 systems. It has been shown that K/ZrO2 prepared from KHCO3 , KOH or KNO3 are active catalysts. The preparation of K/ZrO2 with K/Zr = 0.14 from alkali nitrates provides the higher activity. Comparing the catalytic behaviour of alkali/ZrO2 reveals an increasing activity in the oxidation of carbon black according to the sequence ZrO2 < Li/ZrO2 < Na/ZrO2 < K/ZrO2 < Rb/ZrO2 < Cs/ZrO2 .
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