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Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures
F U E L PR O CE SS I N G TE CH N O LOG Y 89 ( 20 0 8 ) 1 1 60 – 1 1 6 8
a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures R. Jiménez a,⁎, X. García a , T. López b , A.L. Gordon a a b
Departamento Ingeniería Química, Universidad de Concepción, Casilla 160-C, Concepción, Chile Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, A.P. 55-534, 09340 México
AR TIC LE I N FO
ABS TR ACT
Article history:
The effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture, on the catalytic
Received 28 August 2007
combustion of carbon black (CB), a model compound for soot, was studied. Catalysts were
Received in revised form 9 April 2008
prepared by the Sol–Gel method and characterized by surface (BET surface area, XPS, DRIFTS)
Accepted 14 May 2008
and bulk (AAS, XRD and TPR) techniques. Samples with a 4:1 catalyst-CB ratio were subjected
Keywords:
combustion occurs at its maximum rate, was recorded for comparison of catalytic activity.
Soot
The addition of alkali metals (Li, Na, K) over the CaO–MgO mixture significantly increased
Catalytic combustion
the catalytic activity, due to the formation of surface oxygenated species that enhanced
Alkaline metals
the oxidizing properties of the catalyst surface. That activity for CB combustion increases
to catalytic oxidation in a thermo-gravimetric apparatus and the temperature Tm, at which
with the atomic number of the alkali metal contained in the catalyst. The presence of alkali metals also diminished the amount and stability of carbonates formed on the catalyst. The K-containing catalyst showed the largest activity for the catalytic CB combustion, because it shows the largest capacity to enrich its surface with α-oxygen type and promotes best the surface dissociation of that oxygen. Furthermore, surface-adsorbed OH and carbonate groups that disable the active sites and prevent the oxygen adsorption and dissociation, were less abundant and desorbed at lower temperatures, showing to be less stable on this K-containing catalyst. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Soot exhausted from diesel engines may be eliminated using impregnated filters with a catalyst capable of reducing the combustion temperature (N550 °C) to the temperature of the exhausting gases (lower than 400 °C). Previous work [1,2] has shown the successful utilization of CaO–MgO mixtures as catalyst for naphthalene gasification (naphthalene considered as a model compound for tars). Gasification, as well as combustion mechanisms are analogous, as both go through the formation of a surface C(O) complex. The activity and the synergy observed for the CaO–MgO mixtures have been attributed to the catalytic cooperation of MgO that diminishes the stability of carbonates formed over the CaO [1].
Furthermore, alkali metals have been widely used as catalysts for the combustion and gasification of carbonaceous matter [3–17]. Thus, McKee [18] studied carbon oxidation with oxygen in the presence of alkali carbonates and concluded that oxides and peroxides of the alkali metals were the active species present during the catalyzed C–O2 reaction. Studies of Cerfontain et al. [20] on coal catalytic gasification with alkali carbonates (Na, K and Cs) showed that the catalyst activity increased with the atomic number of the metal and that the oxide formed after carbonate decomposition and not the carbonate itself, is the active species for gasification. This agrees well with the work of Chen and Yang [21] who have suggested that for graphite gasification with CO2 and H2O, the catalytic activities follow the order
⁎ Corresponding author. E-mail address: [email protected] (R. Jiménez). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.05.013
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K N Na N Li, and that the active intermediates are particulate and cluster-type catalysts, and not the C–O–M (M denotes metal) complex. Mul et al [22] carried out DRIFTS assays of catalytic oxidation of carbon black impregnated with alkali nitrates. It was found a decreasing order in the series Na, K and Cs for the anhydrous carbonates generated from CO2 evolved. The absorption bands in the 1100–1150 cm− 1 region were associated to CO2 chemisorbed on the oxide alkali metal cluster. The latter result differs from the already referred results of Cerfontain et al. [20] that attributed the observed bands around 1100 cm− 1 (FTIR study) to carbon–oxygen complexes on the carbon surface, a species in which no C from CO2 is involved. Moreover, the shift of these bands to lower wave numbers in the series Na, K and Cs was related to a decrease in aromatic character of the formed carbon–oxygen complex. As indicated, most studies about alkali-based catalyst show that activity is proportional to the atomic number of the metal [7,13,14,16,17], however, assays with alkali-based La–Cr perovskites have shown that the one with Li was most active [15]. Haneda et al. [23] found that potassium was the most effective metal when evaluating the effect of addition and nature of alkali metals on the catalytic activity of cobalt oxide (Co3O4) for the direct decomposition of NO (which is also present as an undesirable compound of the diesel engine exhaust gases). Though the method to be used for catalyst preparation affects the physical properties of the catalyst, hence its activity, all catalysts in this work were prepared by the Sol– Gel method, which, has been claimed [24,25] to allow for better control of parameters, as compared with more traditional methods such as impregnation or ion exchange. Furthermore, the Sol–Gel method diminishes the undesirable catalyst's contaminants [26]. This greater parameter control translates into thermal stability and resistance to sintering, and larger surface area at low temperatures [24]. Previous papers of this research group [5,6,19] have shown the effectiveness of K/MgO and K/CaO–MgO catalysts for carbon black (CB) combustion, used as model soot. These works showed that potassium plays a key role, as the active phase during the CB oxidation, enhancing both, the catalytic activity and the selectivity towards CO2 (complete combustion). In agreement with DRIFTS bands shown by Mul et al [22], XPS measurements and DRIFTS assays carried out in those previous works referred above, point to adsorbed superficial oxygen groups, formed by reaction of alkali metal with O2, possibly peroxides and super-oxides of the general MxOy form. This surface enrichment with oxygen is thought to be the main cause of enhanced activity shown by the catalyst containing potassium. This work addresses the effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture on the activity of those catalysts for carbon black combustion. In particular, the work intends to evaluate and to compare the relative roles of alkali metals in this catalytic combustion and to contribute experimental data that may be useful to elucidate a mechanism for oxygen enrichment of the surface, that could explain the enhanced catalytic activity promoted by those alkali metals. All catalysts were characterized by surface (BET surface area, XPS, DRIFTS) and bulk (AAS, XRD and TPR) techniques.
2.
Experimental
2.1.
Catalyst preparation
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All catalysts assayed were prepared by the Sol–Gel method. The 20%CaO–80%MgO basal mixed oxide was labeled as 20-CaO; catalysts doped with 5.4% of alkali metals, i.e., 5.4% M/CaO–MgO (M corresponding to Li, Na, or K), keeping a CaO/MgO ratio of 20/80 were also prepared, and labeled as Li/20-CaO, Na/20-CaO, K/20CaO). The preparation by Sol–Gel was carried out at 70 °C by continuous stirring of magnesium ethoxide with ethanol (a pH value of 9 was kept throughout by the addition of NH4OH). The gel was obtained by dripping a Ca(NO3)2–H2O–alcohol solution for the synthesis without alkali, and MNO3 (M = Li, Na, K) was added to the solution for the synthesis of catalysts doped with alkali. The samples were dried at 70 °C and calcination was carried at 760 °C for 30 min. Carbon black (Monarch 430, BET area of 80 m2/g, mean particle diameter of 27 nm and bulk density of 460 kg/m3) was used as a model for soot. A 4:1 catalyst-CB ratio was prepared by mixing 4 mg of CB and 16 mg of catalyst, the latter being 20-CaO, Li/20CaO, Na/20-CaO or K/20-CaO. 2.2.
Catalytic activity
Catalytic oxidation of CB was carried out in a thermo-gravimetric apparatus (Netzsch 409 PC). A 4:1 catalyst:CB ratio was prepared by careful grinding, for 3 min, 4 g of CB and 16 g of catalyst in an agate mortar. This corresponds to “tight” contact, which is too intensive compared with what is achievable in a catalytic soot trap, but this contact condition is more reproducible [6,7] and thus set a good basis for activity screening studies. Catalyst-CB mixture samples of 7.5 mg were heated at 10 °C/min to 800 °C in 180 mL/min flow of air (Indura®, atmospheric composition). The combustion rate was normalized by the initial mass of the CB (that is, r = −dm / dt /mi,CB). The temperature at which combustion occurs at its maximum rate (Tm) was recorded for comparison of catalytic activity. 2.3.
Catalyst characterization
2.3.1. BET surface area The BET surface area of the catalyst samples was determined by nitrogen adsorption at 77 K, using a conventional apparatus (Micromeritics Flowsorb 2130). 2.3.2. Diffuse Reflectance FT-Infrared Spectroscopy (DRIFTS) An IF S55 Equinox (Bruker) spectrophotometer equipped with an aircooled KBr source and a MCT detector was used. Samples were located in a thermally and environmentally controlled chamber (SpectraTech 0030-103), equipped with SeZn windows and connected to the diffuse reflectance collector. The spectrum of an aluminium mirror was used as a background. The sample was heated at 10 °C/min under 30 ml/min of He (inert atmosphere) and subjected to the flow of 5%CO2/10%O2/He, as the reactive gas, chosen as a typical atmosphere at the exhaust gases of a diesel Engine. Spectra (200 scans with 4 cm− 1 resolution) at different temperatures were registered. The gases flowing out from the DRIFTS cell were analyzed by an online Balzers Thermostar GSD300T2 gas analyzer, equipped with a quadrupole mass spectrometer QMS200. 2.3.3. X-ray Photo-electronic Spectroscopy (XPS) XPS was performed with a SSI X-Probe (SSX-100/206) photoelectron spectrometer from Surface Science Instrument (Fisons) equipped with a mono-chromatized micro-focused Al–Ka X-ray source (1486.6 eV) and a hemispherical analyzer, as previously described [6].
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2.3.4. Atomic Absorption Spectroscopy (AAS) Catalyst composition was determined using a Hitachi Z-8100 spectrometer with Zeeman polarisation.
Table 1 – Bulk (AAS) and surface (XPS and BET area) characterization of the catalysts
2.3.5. Temperature-Programmed Reduction (TPR) Tests were carried out in a Micromeritics instrument. Samples (100 mg) were loaded in a quartz micro-reactor that was heated in a furnace provided with a temperature controller/programmer; a thermocouple was inserted into the catalytic bed. A gaseous mixture of 5% H2 in He was flown through the sample, which was heated at 10 °C/min up to 750 °C.
Bulk characterization Ca/Mg (atomic ratio) Alkali/Mg (weight ratio) Alkali/Mg (atomic ratio)
3.
Results
3.1.
Catalytic activity
Fig. 1 shows the TGA assays for CB combustion with alkaliadded catalyst (Li, Na and K), in tight contact with the CB (all assays were carried up to total conversion, at 760 °C). For comparison, it also includes both, the results for the noncatalyzed CB combustion, and for a catalyst without alkali metal, but identical CaO/MgO ratio Rw = 20/80. Greater catalytic activity is associated to lower values of the temperature (Tm), at which a well-defined peak of maximum rate of combustion occurs. Addition of the alkali metals to a CaO–MgO mixture with a weight ratio Rw = 20/80 resulted in a significant increase of the catalytic activity for CB combustion, as the peaks (values of Tm) were displaced more than 100 °C with respect to the peak generated by the CaO–MgO mixture (20-CaO), without the alkali metals. The K-containing catalyst was the most active with a value of Tm = 429 °C. During reactive assays using the 20-CaO catalyst, two weight-loss zones, around 400 °C and above 600 °C, were observed (Fig. 1), besides the combustion peak with Tm at
Catalyst
Surface characterization Ca/Mg Alkali/Mg (O2−) (BE) O2−/Mg O2−/Álcali Mg 2s (BE) CCarbonates1s (BE) BET surface area⁎ (m2/g)
Li/20-CaO⁎
Na/20-CaO
0.12 0.09 0.30
0.12 0.07 0.08
n.d.
0.03 0.15 530.7 2.24 14.9 88.2 290.0 17.8
5.7
K/20-CaO 0.10 0.06 0.04
0.01 0.03 529.3; 531.3 0.65 21.7 88.0 289.2 8.5
⁎The BET area of the 20-CaO catalyst (no alkali) is 20.4 m2/g. n.d.: not determined. BE: binding energy.
564 °C. These weight losses were attributed to the decomposition of hydroxyl and carbonate groups, mainly associated to the calcium [5]. In the combustion assays using Li-, Na- and Kcontaining catalysts, weight losses around 300 °C and 650– 700 °C were also observed, associated to desorption of surface hydroxyl groups and decarbonation of the catalyst. The intensity of the third peak (above 600 °C) is lower in the alkali-containing catalysts than in the 20-CaO catalyst. Moreover, this peak is found at higher temperature for the 20-CaO catalyst (Fig. 1). Considering the Tm values in CB combustion, the following order for the catalytic activity is established: K=20 Caoð429-CÞNNa=20 CaOð459-CÞNLi=20 CaOð466-CÞN20 CaOð564-CÞ:
3.2. Atomic Absorption Spectroscopy (AAS), XPS analysis and BET surface area
Fig. 1 – TGA assays for the catalytic combustion of carbon black. Heating rate: 10 °C/min; air flow: 180 ml/min.
The bulk composition by AAS, surface composition by XPS and BET characterization, are summarized in Table 1. However, catalyst Li/20-CaO could not be characterized by XPS, as the Li1s peak (binding energy-BE ~ 56 eV) and the Mg2p peak (BE ~ 50 eV) overlap. Table 1 shows the following: BET surface area decreases with the introduction of alkali metals in the 20-CaO mixture; that was also observed in previous studies of this type of catalysts [5,6]. With regard to the BET surface area of the catalysts, the order is 20-CaO N Na / 20-CaO N K / 20-CaO N Li / 20-CaO. The molar Ca/Mg ratio was similar in the three alkalicontaining catalysts, as expected from the preparation procedure. However, the alkali/Mg weight ratio was slightly different (Li / Mg N Na / Mg N K / Mg), in spite of the fact that for all cases, the same mass of alkali metal was added during the catalysts preparation. The surface Ca/Mg atomic ratio (XPS) was lower than the bulk Ca/Mg ratio for the Na- and K-catalysts. Furthermore, the surface Ca/Mg ratio for the Na/20-CaO catalyst was higher than the one for K/20-CaO.
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The Na-containing catalyst showed the highest surface alkali/Mg atomic ratio (0.15), which was greater than its atomic bulk ratio (0.08). However, for the K-containing catalysts, the opposite result was obtained (0.04 for bulk ratio N 0.03 for surface ratio). These results suggest a better dispersion of the alkali metal on the surface for the Na-containing catalyst. The Na/20-CaO catalyst showed greater surface O2−/Mg atomic ratio (by XPS characterization) and the binding energy (BE) of 530.7 eV was associated with the O2− species (Table 1). Fig. 2(a) shows the presence of a highly symmetric peak for O 1s. The binding energy for this peak (530.7 eV) also suggests the presence of oxygen from OH groups that would lead to a maximum for this peak of 531 eV. The presence of these OH groups is confirmed by DRIFT assays, as shown later. The O 1 s spectrum for the K/20-CaO catalyst shows the presence of two well-differentiated peaks, one with a binding energy of 529.3 eV, which is associated with O2− lattice species. The other peak, with similar intensity but with B.E. of 531.3 eV may be attributed to oxygen species (represented as O−) adsorbed at the catalyst surface, which are characterized as being more weakly bonded to the surface, thus showing a greater binding energy [15]. Furthermore, the XPS measured O2−/ alkali surface atomic ratio was larger for K-containing catalyst than for Na/20-CaO, results that could justify the higher activity
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observed for the K/20-CaO catalyst in the CB combustion. The BE associated with carbon linked to carbonates (CCarbonates1s) was smaller in the K/20-CaO catalyst and this could be related with a lower stability of the carbonate species in the K-catalyst surface. Moreover, the BE of Mg2s were similar for both catalysts.
3.3.
DRIFTS assays in inert and reactive atmosphere
Fig. 3 shows both, DRIFTS and gas mass spectrometry results, for the alkali-catalysts in He ambient. These spectra were obtained for the frequency range where the different hydroxyl species are observed. For the Li/20-CaO catalyst at ambient temperature, bands around 3740, 3700 and 3640 cm− 1 were observed, which are associated to different types of OH groups and that were displaced to lower frequencies at higher temperatures (Fig. 3[b]). The band which is near 3700 cm− 1 disappears between 300 and 400 °C, while the band located near 3640 cm− 1 diminishes its intensity starting above 300 °C and it is not observed at 450 °C. The mass spectra linked to this assay indicate that water and/or OH groups are evolved at around 200 °C and also show a more intensive peak that starts over 300 °C with a maximum at 400 °C. The presence of CO2 in the gases from the DRIFTS cell was observed near 200 °C, around 300 °C and over 450 °C (Fig. 3[a]).
Fig. 2 – X-ray photoelectron spectra in the O1s region of: (a) Na/20-CaO; (b) K/20-CaO.
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DRIFTS assays for the Na/20-CaO catalyst show a band at 3635 cm− 1 (associated to type III OH groups) that increases its intensity over 300 °C and then decreases over 450 °C, while the band at 3691 cm− 1 diminishes between 300 and 400 °C. These bands are slightly displaced to lower frequencies with increasing temperature. The shoulder at 3740 cm− 1 (type I OH group) intensifies with temperature. Mass spectra show peaks for water and/or OH groups at the temperature ranges of 150– 250 °C and 300–400 °C, while evolved CO2 is observed at 150, 300 °C and over 450 °C (Fig. 3[a]). For the K/20-CaO catalyst only the band at 3641 cm− 1 is observed, which is associated to type III OH groups. As in the previous cases, this band is displaced to lower frequencies with increasing temperatures, and it disappears over 400 °C (Fig. 3[b]). The mass spectra show the evolution of OH groups
and/or water around 150 °C and near 400 °C, while small CO2 peaks are observed at 150 and 250 °C and a more intensive peak between 300 and 400 °C. Fig. 4 shows the DRIFTS spectra of catalysts with Li, Na and K in a reactive atmosphere (5%CO2/10%O2/He) for frequencies between 1100 and 1200 cm− 1. In this range bands are observed corresponding to the vibration ν(O2), associated to the oxygen adsorption on the alkali metals and forming species M+(O2)− [6,27]. For each temperature, the oxygen bands associated to lithium (1164–1157 cm− 1) and sodium (1160–1151 cm− 1) appear at higher frequencies than bands associated to potassium (1132–1130 cm− 1), that is, ν(O2)Li N ν(O2)Na N ν(O2)K. As before, all bands are displaced to lower frequencies with increasing temperature.
Fig. 3 – DRIFTS assays and mass spectroscopy in He flow, for Li/20-CaO, Na/20-CaO and K/20-CaO catalysts. [a] mass spectroscopy. [b] DRIFTS in the frequency zone associated to OH groups.
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Fig. 4 – DRIFTS in reactive atmosphere (5%CO2/10%O2/He). a) Li/20-CaO; b) Na/20-CaO; c) K/20-CaO.
The tendency to, and easiness for the formation of these oxygenated species must be related to the nature of the catalytic surface and specifically with the properties of the alkali metal being present. One way to evaluate this is through thermodynamic calculations, as shown next.
3.4.
Thermodynamic calculations
Fig. 5 shows the variation of the Gibbs free energy for the formation of lithium, sodium and potassium peroxides and super oxides from their respective oxides and gaseous oxygen [28]. It is shown that formation of these species is thermodynamically favored with the increase of the atomic number and radius (K N Na N Li). For sodium, the selectivity to Na2O2 is greater than the one towards the super oxide, NaO2, while for potassium the results are opposite, that is, the variation of the Gibbs energy is more negative for the formation of the super-oxide KO2.
3.5.
Temperature Programmed Reduction (TPR) with H2
Fig. 6 shows the H2 consumption curves as a function of temperature in TPR assays. The catalyst with potassium
Fig. 6 – Temperature Programmed Reduction (TPR) in H2. Effect of the alkali metal on the redox properties of the catalyst.
generates two peaks of H2 consumption with the first one starting above 400 °C. This result shows the presence of two, qualitatively different types of oxidizing species. This agrees with TPR assays of catalysts prepared by impregnation of potassium nitrate over activated carbon, as reported by Feng et al. [29]. The catalyst with sodium shows only one peak, which starts over 450 °C while the Li/20-CaO catalyst starts consuming H2 over 600 °C. The catalyst without an alkali metal (20-CaO) shows a peak for H2 consumption that starts over 550 °C with a maximum at 670 °C.
4.
Discussion
As shown, the introduction of alkali metals (Li, Na and K) on a CaO–MgO mixture significantly increased the catalytic activity of the latter for CB combustion. This effect increases with the atomic number of the alkali metal, that is, K N Na N Li (Fig. 1). These results imply that the activity of these catalysts depends mainly upon the alkali metal itself.
4.1.
Fig. 5 – Change of Gibbs (standard) free energy for the formation reactions of Li, Na and K peroxides and superoxides (no data was available for superoxide LiO2).
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Redox properties
Combustion of carbonaceous materials and particularly CB combustion appears to take place through a redox mechanism, with the oxidizing properties of the catalyst playing a fundamental role [7,8,30]. Thus, the larger reducibility of the potassium-containing catalyst, shown by the TPR assays with H2 (Fig. 6) shows agreement with the results for K in (Fig. 1). It also explains the larger oxidizing capacity and greater activity for the catalytic combustion of CB. The present authors have previously attributed this potential redox property of potassium-containing catalysts to the greater capacity of potassium to form highly oxygenated surface species [19]. The ranking of activity in CB combustion for the alkali metal-containing catalysts (Fig. 1) agrees with the ranking of reducibility in H2 of these catalysts found in the TPR assays (Fig. 6). However, in spite of its lower activity, the 20-CaO catalyst shows a H2-consumption peak at a lower temperature
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than the Li/20-CaO catalyst, the latter being a more active catalyst for CB combustion than the mixture without alkali metal. This result is attributed to limitations in the H2 access during the TPR assays with the Li/20-CaO sample, because of the very small surface area of the latter. This small surface area is probably due to the low melting temperature of the LiNO3 that generates the plugging of solid pores during the calcinations. A similar effect of the surface area upon the H2 consumption in TPR assays has been observed, and similarly explained in other works [4,31]. However, the BET surface area for the K-containing catalyst was slightly greater than the BET area for the Li/20-CaO sample. These results suggest that peak's locations in the TPR assays are affected not only by H2 accessibility to the oxygenated species in the catalyst, but possibly also by differences in the nature of these oxygenated species, as discussed later.
4.2.
Hydroxyl groups and carbonates on the catalysts
The weight loss that was observed in the catalytic assays at around 300 °C, before the start of the CB combustion, is attributed to carbonate and hydroxyl decomposition. The DRIFTS spectra associated with mass spectroscopy shown in Fig. 3 prove this presence and decomposition of adsorbed surface groups on the alkali metal-containing catalysts. For the catalyst with potassium, just one band associated to OH groups is observed, while for catalysts with Li and Na these species seem to be more diverse. Besides, the stability of these species seem to be lower in the K/20-CaO sample, as no band is found over 400 °C, while for the Na and Li-containing samples the presence of OH groups is observed over 450 °C (Fig. 3[B]). These surface groups, adsorbed on the catalyst, disable the active sites and prevent oxygen adsorption and dissociation. Therefore, the decomposition of these adsorbed groups restores the availability of active sites, needed to initiate the CB combustion. Weight losses observed over 600 °C (Fig. 1) correspond to decomposition of the more stable carbonates. In a previous work [5] mixtures of CaO–MgO were assayed and these carbonates, labeled as “bulk carbonates”, were found to be associated to the calcium. This was confirmed by TGA assays of K/MgO catalysts [6] for which no decomposition was observed at high temperatures. Addition of alkali metals to the 20-CaO mixture decreased the stability of carbonates that decompose over 600 °C (Fig. 1). This result may be attributed to two factors that do not necessarily exclude each other. Alkali metal content inhibits formation, or enhances decomposition of carbonates formed over the catalyst surface in the presence of CO2, the latter coming either from the atmosphere or as a product of CB combustion. On the other hand, introduction of alkali metals generated a significant reduction of the BET surface area, particularly for Li and K (Table 1). These are precisely the catalysts that show smaller weight loss over 600 °C. The reduction of the BET surface area may impede the CO2 access to the calcium atoms in the catalyst bulk, thus diminishing the possibility of carbonate formation. Even at the surface, a greater calcium content may be a key factor in the degree of carbonation, as a direct relation is observed between the Ca/Mg ratio (Table 1)
and the weight loss over 600 °C, when the Na/20-CaO and K/20CaO catalysts are compared. Furthermore, the lower binding energy associated to carbonates (Ccarbonate1s) observed in the K/20-CaO (Table 1) could be linked to a lesser stability of the carbonates on this catalyst.
4.3.
Surface oxygenated species
The greater activity of the potassium-containing catalyst is partly explained by the lower electro-negativity of this metal (greater metallic character), that eases the release of its electrons and favors the surface adsorption of gaseous oxygen. Therefore, species like peroxides and super oxides with high oxygen content are formed, thus increasing the oxidizing properties of the catalyst surface. The DRIFTS assays from Fig. 4 show the formation of these peroxides and super oxides. The vibration frequency of oxygen in these species diminishes as the atomic number of the alkali metal present in the catalyst increases (ν(O2)Li N ν(O2)Na N ν(O2)K). A stronger interaction between the metallic atom and the oxygen molecule weakens the O–O bond diminishing its vibration frequency [27]. Therefore, the surface dissociation of oxygen is best promoted on the potassium-containing catalyst. The calculations presented in Fig. 5 also show that peroxide and super oxides formation is thermodynamically favored as the atomic number of the alkali metal increases (K N Na N Li). This order agrees with the catalytic activity measured for the CB combustion (Fig. 1). Thermodynamics shows that Li may have less activity, due to low formation of peroxide and super oxides. However, its catalytic activity was just slightly lower than for K or Na, which might be attributed to the better capacity of Li-containing catalysts to generate very active suprafacial oxygen, as reported by Russo et al. [15]. Surface characterization (XPS) at ambient temperature shows a greater surface concentration of O2− on the Na/20-SG catalyst than on the K/20-SG catalyst, as determined by the O2−/Mg ratio presented in Table 1. This is attributed to the larger surface atomic concentration of sodium (alkali/Mg = 0.15). However, the surface O2− content per alkali metal atom (that is the O2−/alkali ratio) is greater for the potassiumcontaining catalyst (21.7) than the same ratio in the Na/20CaO catalyst (14.9), thus confirming the better activity of potassium to promote reactions that enrich with oxygen the catalyst surface. The presence of a well-differentiated peak of O 1s at a higher BE (531.3 eV) for the catalyst K/20-CaO is attributed to a greater content of surface-adsorbed oxygen of the O− type (α-oxygen), also labeled as suprafacial oxygen, which has been identified as the main vehicle in complete oxidation processes, as soot combustion [14,15,32]. The species that are weaker-bonded to the catalyst surface should have greater mobility, hence an easier access to the contact points between soot and catalyst (probably through a spillover mechanism) thus favoring the enrichment of the CB surface with oxygenated species [15]. The previous discussion may explain why the Na/20-CaO was less active than the K/20-CaO for the catalytic combustion of CB, in spite of the greater surface concentration of oxygen shown by XPS analysis. A large percent of the oxygen species observed in the K-containing catalyst would be of the O− type,
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with high mobility and reactivity for CB combustion; this was not the case for the Na-containing catalyst. This result confirms that catalysts activity for soot combustion depends not only on oxygen content on the surface, but also on the nature of this oxygen. With regard to the alkali composition of the assayed catalysts, the latter were prepared with similar mass weight percent implying a different atomic composition of the metals. It is clear that the performance ranking of alkali metals as catalyst for this model reaction will be confirmed (and enhanced) if similar atomic composition of the added alkalis are utilized in the catalysts.
5.
Conclusions
The addition of alkali metals (Li, Na, K) over a CaO–MgO mixture, generated a significant increase of the catalytic activity, due to the formation of surface oxygenated species that enhance the oxidizing properties of the catalyst surface. The activity for the catalytic CB combustion increases with the atomic number of the alkali metal contained in the catalyst. The presence of the alkali metals also diminished the amount and stability of carbonates formed on the catalyst. It was verified that the K/20-CaO catalyst showed the largest activity for the catalytic CB combustion because it presents the greater capacity to enrich with adsorbed oxygen, in the form of O− (labeled as α-oxygen), the catalyst surface and the surface dissociation of oxygen is best promoted on this potassium-containing catalyst.
Acknowledgements The support of FONDECYT-Chile, Grants No. 11060301 and 1071016, and short stays financing from the MECESUP Program, Grant UCO0108 (for RJ) is gratefully acknowledged.
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[7] B.R. Stanmore, J.F. Brilhac, P. Gilot, The oxidation of soot: a review of experiments, mechanisms and models, Carbon 39 (2001) 2247–2268. [8] C.A. Querini, L.M. Cornaglia, M.A. Ulla, E.E. Miró, Catalytic combustion of diesel soot on Co,K/MgO catalysts. Effect of the potassium loading on activity and stability, Appl. Catál., B Environ. 20 (1999) 165–177. [9] T. Takuwa, I.S.N. Mkilaha, I. Naruse, Mechanisms of fine particulates formation with alkali metal compounds during coal combustion, Fuel 85 (5–6) (2006) 671–678. [10] H.M. An, C. Kilroy, P.J. McGinn, Combinatorial synthesis and characterization of alkali metal doped oxides for diesel soot combustion, Catal. Today 98 (3) (2004) 423–429. [11] M.L. Pisarello, V. Milt, M.A. Peralta, C.A. Querini, E.E. Miró, Simultaneous removal of soot and nitrogen oxides from diesel engine exhausts, Catal. Today 75 (2002) 465–470. [12] V. Serra, G. Saracco, C. Badini, V. Specchia, Combustion of carbonaceous materials by Cu–K–V based catalysts. 2. Reaction mechanism, Appl. Catal., B Environ. 11 (1997) 329–346. [13] J.P.A. Neeft, M. Makkee, J.A. Moulijn, Catalytic oxidation of carbon black — I. Activity of catalysts and classification of oxidation profiles, Fuel 77 (3) (1998) 111–119. [14] D. Fino, N. Russo, C. Badini, G. Saracco, V. Specchia, Effect of active species mobility on soot-combustion over Cs–V catalysts, AIChE J. 8 (2003) 2173–2180. [15] N. Russo, D. Fino, G. Saracco, V. Specchia, Studies on the redox properties of chromite perovskite catalysts for soot combustion, J. Catal. 229 (2005) 459–469. [16] G. Neri, G. Rizzo, S. Galvagno, A. Donato, M.G. Musolino, R. Pietropaolo, K– and Cs–FeV/Al2O3 soot combustion catalysts for diesel exhaust treatment, Appl. Catal., B Environ. 42 (2003) 381–391. [17] C. Badini, G. Saracco, N. Russo, V. Specchia, A screening study on the activation energy of vanadate-based catalysts for diesel soot combustion, Catal. Lett. 69 (3–4) (2000) 207–215. [18] D.W. Mckee, Mechanisms of the alkali metal catalysed gasification of carbon, Fuel 62 (1983) 170–175. [19] R. Jiménez, X. García, C. Cellier, P. Ruiz, A.L. Gordon, Soot combustion with K/MgO as catalyst: II. Effect of K-precursor, Appl. Catal., A Gen. 314 (2006) 81–88. [20] M.B. Cerfontain, J.A. Moulijn, The interaction of CO2 and CO with an alkali carbonate carbon system studied by in-situ Fourier transform infrared spectroscopy, Fuel 65 (1986) 1349–1355. [21] S.G. Chen, R.T. Yang, Mechanism of alkali and alkaline earth catalyzed gasification of graphite by CO2 and H2O studied by electron microscopy, J. Catal. 138 (1992) 12–23. [22] G. Mul, F. Kapteijn, J.A. Moulijn, A DRIFTS study of the interaction of alkali metal oxides with carbonaceous surfaces, Carbon 37 (1999) 401–410. [23] M. Haneda, Y. Kintaichi, N. Bion, H. Hamada, Alkali metal-doped cobalt oxide catalysts for NO decomposition, Appl. Catal., B Environ. 46 (2003) 473–482. [24] R.D. Gonzalez, T. López, R. Gomez, Sol–gel preparation of supported metal catalysts, Catal. Today 35 (1997) 293–317. [25] W. Zou, R.D. Gonzalez, The preparation of high-surfase-area Pt/SiO2 catalysts with well-defined pore size distributions, J. of Catal. 152 (1995) 291–305. [26] S. Katayama, N. Yamada, M. Awano, Preparation of alkaline-earth metal silicates from gels and their NOx-adsorption behavior, J. Eur. Ceram. Soc. 24 (2004) 421–425. [27] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Fourth Edition, John Wiley & Sons, New York, 1986. [28] I. Barin, O. Knacke, Thermochemical properties of inorganic substances, 1er edition, Springer-Verlag, Berlin/Heidelberg and Verlag Stahleisen m.b.H., Düsseldorf (1973).
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[29] L. Feng, X. Lee, D.B. Dadyburjor, E.L. Kugler, A temperature-programmed-reduction study on alkali-promoted, carbon-supported molybdenum catalysts, J. Catal. 190 (2000) 1–13. [30] S.J. Jelles, R.R. Krul, M. Makkee, J.A. Moulijn, The influence of NOx on the oxidation of metal activated diesel soot, Catal. Today 53 (1999) 623–630.
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Catalytic combustion of soot. Effects of added alkali metals on CaO–MgO physical mixtures R. Jiménez a,⁎, X. García a , T. López b , A.L. Gordon a a b
Departamento Ingeniería Química, Universidad de Concepción, Casilla 160-C, Concepción, Chile Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, A.P. 55-534, 09340 México
AR TIC LE I N FO
ABS TR ACT
Article history:
The effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture, on the catalytic
Received 28 August 2007
combustion of carbon black (CB), a model compound for soot, was studied. Catalysts were
Received in revised form 9 April 2008
prepared by the Sol–Gel method and characterized by surface (BET surface area, XPS, DRIFTS)
Accepted 14 May 2008
and bulk (AAS, XRD and TPR) techniques. Samples with a 4:1 catalyst-CB ratio were subjected
Keywords:
combustion occurs at its maximum rate, was recorded for comparison of catalytic activity.
Soot
The addition of alkali metals (Li, Na, K) over the CaO–MgO mixture significantly increased
Catalytic combustion
the catalytic activity, due to the formation of surface oxygenated species that enhanced
Alkaline metals
the oxidizing properties of the catalyst surface. That activity for CB combustion increases
to catalytic oxidation in a thermo-gravimetric apparatus and the temperature Tm, at which
with the atomic number of the alkali metal contained in the catalyst. The presence of alkali metals also diminished the amount and stability of carbonates formed on the catalyst. The K-containing catalyst showed the largest activity for the catalytic CB combustion, because it shows the largest capacity to enrich its surface with α-oxygen type and promotes best the surface dissociation of that oxygen. Furthermore, surface-adsorbed OH and carbonate groups that disable the active sites and prevent the oxygen adsorption and dissociation, were less abundant and desorbed at lower temperatures, showing to be less stable on this K-containing catalyst. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Soot exhausted from diesel engines may be eliminated using impregnated filters with a catalyst capable of reducing the combustion temperature (N550 °C) to the temperature of the exhausting gases (lower than 400 °C). Previous work [1,2] has shown the successful utilization of CaO–MgO mixtures as catalyst for naphthalene gasification (naphthalene considered as a model compound for tars). Gasification, as well as combustion mechanisms are analogous, as both go through the formation of a surface C(O) complex. The activity and the synergy observed for the CaO–MgO mixtures have been attributed to the catalytic cooperation of MgO that diminishes the stability of carbonates formed over the CaO [1].
Furthermore, alkali metals have been widely used as catalysts for the combustion and gasification of carbonaceous matter [3–17]. Thus, McKee [18] studied carbon oxidation with oxygen in the presence of alkali carbonates and concluded that oxides and peroxides of the alkali metals were the active species present during the catalyzed C–O2 reaction. Studies of Cerfontain et al. [20] on coal catalytic gasification with alkali carbonates (Na, K and Cs) showed that the catalyst activity increased with the atomic number of the metal and that the oxide formed after carbonate decomposition and not the carbonate itself, is the active species for gasification. This agrees well with the work of Chen and Yang [21] who have suggested that for graphite gasification with CO2 and H2O, the catalytic activities follow the order
⁎ Corresponding author. E-mail address: [email protected] (R. Jiménez). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.05.013
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K N Na N Li, and that the active intermediates are particulate and cluster-type catalysts, and not the C–O–M (M denotes metal) complex. Mul et al [22] carried out DRIFTS assays of catalytic oxidation of carbon black impregnated with alkali nitrates. It was found a decreasing order in the series Na, K and Cs for the anhydrous carbonates generated from CO2 evolved. The absorption bands in the 1100–1150 cm− 1 region were associated to CO2 chemisorbed on the oxide alkali metal cluster. The latter result differs from the already referred results of Cerfontain et al. [20] that attributed the observed bands around 1100 cm− 1 (FTIR study) to carbon–oxygen complexes on the carbon surface, a species in which no C from CO2 is involved. Moreover, the shift of these bands to lower wave numbers in the series Na, K and Cs was related to a decrease in aromatic character of the formed carbon–oxygen complex. As indicated, most studies about alkali-based catalyst show that activity is proportional to the atomic number of the metal [7,13,14,16,17], however, assays with alkali-based La–Cr perovskites have shown that the one with Li was most active [15]. Haneda et al. [23] found that potassium was the most effective metal when evaluating the effect of addition and nature of alkali metals on the catalytic activity of cobalt oxide (Co3O4) for the direct decomposition of NO (which is also present as an undesirable compound of the diesel engine exhaust gases). Though the method to be used for catalyst preparation affects the physical properties of the catalyst, hence its activity, all catalysts in this work were prepared by the Sol– Gel method, which, has been claimed [24,25] to allow for better control of parameters, as compared with more traditional methods such as impregnation or ion exchange. Furthermore, the Sol–Gel method diminishes the undesirable catalyst's contaminants [26]. This greater parameter control translates into thermal stability and resistance to sintering, and larger surface area at low temperatures [24]. Previous papers of this research group [5,6,19] have shown the effectiveness of K/MgO and K/CaO–MgO catalysts for carbon black (CB) combustion, used as model soot. These works showed that potassium plays a key role, as the active phase during the CB oxidation, enhancing both, the catalytic activity and the selectivity towards CO2 (complete combustion). In agreement with DRIFTS bands shown by Mul et al [22], XPS measurements and DRIFTS assays carried out in those previous works referred above, point to adsorbed superficial oxygen groups, formed by reaction of alkali metal with O2, possibly peroxides and super-oxides of the general MxOy form. This surface enrichment with oxygen is thought to be the main cause of enhanced activity shown by the catalyst containing potassium. This work addresses the effect of adding alkali metals (Li, Na, K) to a CaO–MgO mixture on the activity of those catalysts for carbon black combustion. In particular, the work intends to evaluate and to compare the relative roles of alkali metals in this catalytic combustion and to contribute experimental data that may be useful to elucidate a mechanism for oxygen enrichment of the surface, that could explain the enhanced catalytic activity promoted by those alkali metals. All catalysts were characterized by surface (BET surface area, XPS, DRIFTS) and bulk (AAS, XRD and TPR) techniques.
2.
Experimental
2.1.
Catalyst preparation
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All catalysts assayed were prepared by the Sol–Gel method. The 20%CaO–80%MgO basal mixed oxide was labeled as 20-CaO; catalysts doped with 5.4% of alkali metals, i.e., 5.4% M/CaO–MgO (M corresponding to Li, Na, or K), keeping a CaO/MgO ratio of 20/80 were also prepared, and labeled as Li/20-CaO, Na/20-CaO, K/20CaO). The preparation by Sol–Gel was carried out at 70 °C by continuous stirring of magnesium ethoxide with ethanol (a pH value of 9 was kept throughout by the addition of NH4OH). The gel was obtained by dripping a Ca(NO3)2–H2O–alcohol solution for the synthesis without alkali, and MNO3 (M = Li, Na, K) was added to the solution for the synthesis of catalysts doped with alkali. The samples were dried at 70 °C and calcination was carried at 760 °C for 30 min. Carbon black (Monarch 430, BET area of 80 m2/g, mean particle diameter of 27 nm and bulk density of 460 kg/m3) was used as a model for soot. A 4:1 catalyst-CB ratio was prepared by mixing 4 mg of CB and 16 mg of catalyst, the latter being 20-CaO, Li/20CaO, Na/20-CaO or K/20-CaO. 2.2.
Catalytic activity
Catalytic oxidation of CB was carried out in a thermo-gravimetric apparatus (Netzsch 409 PC). A 4:1 catalyst:CB ratio was prepared by careful grinding, for 3 min, 4 g of CB and 16 g of catalyst in an agate mortar. This corresponds to “tight” contact, which is too intensive compared with what is achievable in a catalytic soot trap, but this contact condition is more reproducible [6,7] and thus set a good basis for activity screening studies. Catalyst-CB mixture samples of 7.5 mg were heated at 10 °C/min to 800 °C in 180 mL/min flow of air (Indura®, atmospheric composition). The combustion rate was normalized by the initial mass of the CB (that is, r = −dm / dt /mi,CB). The temperature at which combustion occurs at its maximum rate (Tm) was recorded for comparison of catalytic activity. 2.3.
Catalyst characterization
2.3.1. BET surface area The BET surface area of the catalyst samples was determined by nitrogen adsorption at 77 K, using a conventional apparatus (Micromeritics Flowsorb 2130). 2.3.2. Diffuse Reflectance FT-Infrared Spectroscopy (DRIFTS) An IF S55 Equinox (Bruker) spectrophotometer equipped with an aircooled KBr source and a MCT detector was used. Samples were located in a thermally and environmentally controlled chamber (SpectraTech 0030-103), equipped with SeZn windows and connected to the diffuse reflectance collector. The spectrum of an aluminium mirror was used as a background. The sample was heated at 10 °C/min under 30 ml/min of He (inert atmosphere) and subjected to the flow of 5%CO2/10%O2/He, as the reactive gas, chosen as a typical atmosphere at the exhaust gases of a diesel Engine. Spectra (200 scans with 4 cm− 1 resolution) at different temperatures were registered. The gases flowing out from the DRIFTS cell were analyzed by an online Balzers Thermostar GSD300T2 gas analyzer, equipped with a quadrupole mass spectrometer QMS200. 2.3.3. X-ray Photo-electronic Spectroscopy (XPS) XPS was performed with a SSI X-Probe (SSX-100/206) photoelectron spectrometer from Surface Science Instrument (Fisons) equipped with a mono-chromatized micro-focused Al–Ka X-ray source (1486.6 eV) and a hemispherical analyzer, as previously described [6].
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2.3.4. Atomic Absorption Spectroscopy (AAS) Catalyst composition was determined using a Hitachi Z-8100 spectrometer with Zeeman polarisation.
Table 1 – Bulk (AAS) and surface (XPS and BET area) characterization of the catalysts
2.3.5. Temperature-Programmed Reduction (TPR) Tests were carried out in a Micromeritics instrument. Samples (100 mg) were loaded in a quartz micro-reactor that was heated in a furnace provided with a temperature controller/programmer; a thermocouple was inserted into the catalytic bed. A gaseous mixture of 5% H2 in He was flown through the sample, which was heated at 10 °C/min up to 750 °C.
Bulk characterization Ca/Mg (atomic ratio) Alkali/Mg (weight ratio) Alkali/Mg (atomic ratio)
3.
Results
3.1.
Catalytic activity
Fig. 1 shows the TGA assays for CB combustion with alkaliadded catalyst (Li, Na and K), in tight contact with the CB (all assays were carried up to total conversion, at 760 °C). For comparison, it also includes both, the results for the noncatalyzed CB combustion, and for a catalyst without alkali metal, but identical CaO/MgO ratio Rw = 20/80. Greater catalytic activity is associated to lower values of the temperature (Tm), at which a well-defined peak of maximum rate of combustion occurs. Addition of the alkali metals to a CaO–MgO mixture with a weight ratio Rw = 20/80 resulted in a significant increase of the catalytic activity for CB combustion, as the peaks (values of Tm) were displaced more than 100 °C with respect to the peak generated by the CaO–MgO mixture (20-CaO), without the alkali metals. The K-containing catalyst was the most active with a value of Tm = 429 °C. During reactive assays using the 20-CaO catalyst, two weight-loss zones, around 400 °C and above 600 °C, were observed (Fig. 1), besides the combustion peak with Tm at
Catalyst
Surface characterization Ca/Mg Alkali/Mg (O2−) (BE) O2−/Mg O2−/Álcali Mg 2s (BE) CCarbonates1s (BE) BET surface area⁎ (m2/g)
Li/20-CaO⁎
Na/20-CaO
0.12 0.09 0.30
0.12 0.07 0.08
n.d.
0.03 0.15 530.7 2.24 14.9 88.2 290.0 17.8
5.7
K/20-CaO 0.10 0.06 0.04
0.01 0.03 529.3; 531.3 0.65 21.7 88.0 289.2 8.5
⁎The BET area of the 20-CaO catalyst (no alkali) is 20.4 m2/g. n.d.: not determined. BE: binding energy.
564 °C. These weight losses were attributed to the decomposition of hydroxyl and carbonate groups, mainly associated to the calcium [5]. In the combustion assays using Li-, Na- and Kcontaining catalysts, weight losses around 300 °C and 650– 700 °C were also observed, associated to desorption of surface hydroxyl groups and decarbonation of the catalyst. The intensity of the third peak (above 600 °C) is lower in the alkali-containing catalysts than in the 20-CaO catalyst. Moreover, this peak is found at higher temperature for the 20-CaO catalyst (Fig. 1). Considering the Tm values in CB combustion, the following order for the catalytic activity is established: K=20 Caoð429-CÞNNa=20 CaOð459-CÞNLi=20 CaOð466-CÞN20 CaOð564-CÞ:
3.2. Atomic Absorption Spectroscopy (AAS), XPS analysis and BET surface area
Fig. 1 – TGA assays for the catalytic combustion of carbon black. Heating rate: 10 °C/min; air flow: 180 ml/min.
The bulk composition by AAS, surface composition by XPS and BET characterization, are summarized in Table 1. However, catalyst Li/20-CaO could not be characterized by XPS, as the Li1s peak (binding energy-BE ~ 56 eV) and the Mg2p peak (BE ~ 50 eV) overlap. Table 1 shows the following: BET surface area decreases with the introduction of alkali metals in the 20-CaO mixture; that was also observed in previous studies of this type of catalysts [5,6]. With regard to the BET surface area of the catalysts, the order is 20-CaO N Na / 20-CaO N K / 20-CaO N Li / 20-CaO. The molar Ca/Mg ratio was similar in the three alkalicontaining catalysts, as expected from the preparation procedure. However, the alkali/Mg weight ratio was slightly different (Li / Mg N Na / Mg N K / Mg), in spite of the fact that for all cases, the same mass of alkali metal was added during the catalysts preparation. The surface Ca/Mg atomic ratio (XPS) was lower than the bulk Ca/Mg ratio for the Na- and K-catalysts. Furthermore, the surface Ca/Mg ratio for the Na/20-CaO catalyst was higher than the one for K/20-CaO.
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The Na-containing catalyst showed the highest surface alkali/Mg atomic ratio (0.15), which was greater than its atomic bulk ratio (0.08). However, for the K-containing catalysts, the opposite result was obtained (0.04 for bulk ratio N 0.03 for surface ratio). These results suggest a better dispersion of the alkali metal on the surface for the Na-containing catalyst. The Na/20-CaO catalyst showed greater surface O2−/Mg atomic ratio (by XPS characterization) and the binding energy (BE) of 530.7 eV was associated with the O2− species (Table 1). Fig. 2(a) shows the presence of a highly symmetric peak for O 1s. The binding energy for this peak (530.7 eV) also suggests the presence of oxygen from OH groups that would lead to a maximum for this peak of 531 eV. The presence of these OH groups is confirmed by DRIFT assays, as shown later. The O 1 s spectrum for the K/20-CaO catalyst shows the presence of two well-differentiated peaks, one with a binding energy of 529.3 eV, which is associated with O2− lattice species. The other peak, with similar intensity but with B.E. of 531.3 eV may be attributed to oxygen species (represented as O−) adsorbed at the catalyst surface, which are characterized as being more weakly bonded to the surface, thus showing a greater binding energy [15]. Furthermore, the XPS measured O2−/ alkali surface atomic ratio was larger for K-containing catalyst than for Na/20-CaO, results that could justify the higher activity
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observed for the K/20-CaO catalyst in the CB combustion. The BE associated with carbon linked to carbonates (CCarbonates1s) was smaller in the K/20-CaO catalyst and this could be related with a lower stability of the carbonate species in the K-catalyst surface. Moreover, the BE of Mg2s were similar for both catalysts.
3.3.
DRIFTS assays in inert and reactive atmosphere
Fig. 3 shows both, DRIFTS and gas mass spectrometry results, for the alkali-catalysts in He ambient. These spectra were obtained for the frequency range where the different hydroxyl species are observed. For the Li/20-CaO catalyst at ambient temperature, bands around 3740, 3700 and 3640 cm− 1 were observed, which are associated to different types of OH groups and that were displaced to lower frequencies at higher temperatures (Fig. 3[b]). The band which is near 3700 cm− 1 disappears between 300 and 400 °C, while the band located near 3640 cm− 1 diminishes its intensity starting above 300 °C and it is not observed at 450 °C. The mass spectra linked to this assay indicate that water and/or OH groups are evolved at around 200 °C and also show a more intensive peak that starts over 300 °C with a maximum at 400 °C. The presence of CO2 in the gases from the DRIFTS cell was observed near 200 °C, around 300 °C and over 450 °C (Fig. 3[a]).
Fig. 2 – X-ray photoelectron spectra in the O1s region of: (a) Na/20-CaO; (b) K/20-CaO.
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DRIFTS assays for the Na/20-CaO catalyst show a band at 3635 cm− 1 (associated to type III OH groups) that increases its intensity over 300 °C and then decreases over 450 °C, while the band at 3691 cm− 1 diminishes between 300 and 400 °C. These bands are slightly displaced to lower frequencies with increasing temperature. The shoulder at 3740 cm− 1 (type I OH group) intensifies with temperature. Mass spectra show peaks for water and/or OH groups at the temperature ranges of 150– 250 °C and 300–400 °C, while evolved CO2 is observed at 150, 300 °C and over 450 °C (Fig. 3[a]). For the K/20-CaO catalyst only the band at 3641 cm− 1 is observed, which is associated to type III OH groups. As in the previous cases, this band is displaced to lower frequencies with increasing temperatures, and it disappears over 400 °C (Fig. 3[b]). The mass spectra show the evolution of OH groups
and/or water around 150 °C and near 400 °C, while small CO2 peaks are observed at 150 and 250 °C and a more intensive peak between 300 and 400 °C. Fig. 4 shows the DRIFTS spectra of catalysts with Li, Na and K in a reactive atmosphere (5%CO2/10%O2/He) for frequencies between 1100 and 1200 cm− 1. In this range bands are observed corresponding to the vibration ν(O2), associated to the oxygen adsorption on the alkali metals and forming species M+(O2)− [6,27]. For each temperature, the oxygen bands associated to lithium (1164–1157 cm− 1) and sodium (1160–1151 cm− 1) appear at higher frequencies than bands associated to potassium (1132–1130 cm− 1), that is, ν(O2)Li N ν(O2)Na N ν(O2)K. As before, all bands are displaced to lower frequencies with increasing temperature.
Fig. 3 – DRIFTS assays and mass spectroscopy in He flow, for Li/20-CaO, Na/20-CaO and K/20-CaO catalysts. [a] mass spectroscopy. [b] DRIFTS in the frequency zone associated to OH groups.
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Fig. 4 – DRIFTS in reactive atmosphere (5%CO2/10%O2/He). a) Li/20-CaO; b) Na/20-CaO; c) K/20-CaO.
The tendency to, and easiness for the formation of these oxygenated species must be related to the nature of the catalytic surface and specifically with the properties of the alkali metal being present. One way to evaluate this is through thermodynamic calculations, as shown next.
3.4.
Thermodynamic calculations
Fig. 5 shows the variation of the Gibbs free energy for the formation of lithium, sodium and potassium peroxides and super oxides from their respective oxides and gaseous oxygen [28]. It is shown that formation of these species is thermodynamically favored with the increase of the atomic number and radius (K N Na N Li). For sodium, the selectivity to Na2O2 is greater than the one towards the super oxide, NaO2, while for potassium the results are opposite, that is, the variation of the Gibbs energy is more negative for the formation of the super-oxide KO2.
3.5.
Temperature Programmed Reduction (TPR) with H2
Fig. 6 shows the H2 consumption curves as a function of temperature in TPR assays. The catalyst with potassium
Fig. 6 – Temperature Programmed Reduction (TPR) in H2. Effect of the alkali metal on the redox properties of the catalyst.
generates two peaks of H2 consumption with the first one starting above 400 °C. This result shows the presence of two, qualitatively different types of oxidizing species. This agrees with TPR assays of catalysts prepared by impregnation of potassium nitrate over activated carbon, as reported by Feng et al. [29]. The catalyst with sodium shows only one peak, which starts over 450 °C while the Li/20-CaO catalyst starts consuming H2 over 600 °C. The catalyst without an alkali metal (20-CaO) shows a peak for H2 consumption that starts over 550 °C with a maximum at 670 °C.
4.
Discussion
As shown, the introduction of alkali metals (Li, Na and K) on a CaO–MgO mixture significantly increased the catalytic activity of the latter for CB combustion. This effect increases with the atomic number of the alkali metal, that is, K N Na N Li (Fig. 1). These results imply that the activity of these catalysts depends mainly upon the alkali metal itself.
4.1.
Fig. 5 – Change of Gibbs (standard) free energy for the formation reactions of Li, Na and K peroxides and superoxides (no data was available for superoxide LiO2).
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Redox properties
Combustion of carbonaceous materials and particularly CB combustion appears to take place through a redox mechanism, with the oxidizing properties of the catalyst playing a fundamental role [7,8,30]. Thus, the larger reducibility of the potassium-containing catalyst, shown by the TPR assays with H2 (Fig. 6) shows agreement with the results for K in (Fig. 1). It also explains the larger oxidizing capacity and greater activity for the catalytic combustion of CB. The present authors have previously attributed this potential redox property of potassium-containing catalysts to the greater capacity of potassium to form highly oxygenated surface species [19]. The ranking of activity in CB combustion for the alkali metal-containing catalysts (Fig. 1) agrees with the ranking of reducibility in H2 of these catalysts found in the TPR assays (Fig. 6). However, in spite of its lower activity, the 20-CaO catalyst shows a H2-consumption peak at a lower temperature
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than the Li/20-CaO catalyst, the latter being a more active catalyst for CB combustion than the mixture without alkali metal. This result is attributed to limitations in the H2 access during the TPR assays with the Li/20-CaO sample, because of the very small surface area of the latter. This small surface area is probably due to the low melting temperature of the LiNO3 that generates the plugging of solid pores during the calcinations. A similar effect of the surface area upon the H2 consumption in TPR assays has been observed, and similarly explained in other works [4,31]. However, the BET surface area for the K-containing catalyst was slightly greater than the BET area for the Li/20-CaO sample. These results suggest that peak's locations in the TPR assays are affected not only by H2 accessibility to the oxygenated species in the catalyst, but possibly also by differences in the nature of these oxygenated species, as discussed later.
4.2.
Hydroxyl groups and carbonates on the catalysts
The weight loss that was observed in the catalytic assays at around 300 °C, before the start of the CB combustion, is attributed to carbonate and hydroxyl decomposition. The DRIFTS spectra associated with mass spectroscopy shown in Fig. 3 prove this presence and decomposition of adsorbed surface groups on the alkali metal-containing catalysts. For the catalyst with potassium, just one band associated to OH groups is observed, while for catalysts with Li and Na these species seem to be more diverse. Besides, the stability of these species seem to be lower in the K/20-CaO sample, as no band is found over 400 °C, while for the Na and Li-containing samples the presence of OH groups is observed over 450 °C (Fig. 3[B]). These surface groups, adsorbed on the catalyst, disable the active sites and prevent oxygen adsorption and dissociation. Therefore, the decomposition of these adsorbed groups restores the availability of active sites, needed to initiate the CB combustion. Weight losses observed over 600 °C (Fig. 1) correspond to decomposition of the more stable carbonates. In a previous work [5] mixtures of CaO–MgO were assayed and these carbonates, labeled as “bulk carbonates”, were found to be associated to the calcium. This was confirmed by TGA assays of K/MgO catalysts [6] for which no decomposition was observed at high temperatures. Addition of alkali metals to the 20-CaO mixture decreased the stability of carbonates that decompose over 600 °C (Fig. 1). This result may be attributed to two factors that do not necessarily exclude each other. Alkali metal content inhibits formation, or enhances decomposition of carbonates formed over the catalyst surface in the presence of CO2, the latter coming either from the atmosphere or as a product of CB combustion. On the other hand, introduction of alkali metals generated a significant reduction of the BET surface area, particularly for Li and K (Table 1). These are precisely the catalysts that show smaller weight loss over 600 °C. The reduction of the BET surface area may impede the CO2 access to the calcium atoms in the catalyst bulk, thus diminishing the possibility of carbonate formation. Even at the surface, a greater calcium content may be a key factor in the degree of carbonation, as a direct relation is observed between the Ca/Mg ratio (Table 1)
and the weight loss over 600 °C, when the Na/20-CaO and K/20CaO catalysts are compared. Furthermore, the lower binding energy associated to carbonates (Ccarbonate1s) observed in the K/20-CaO (Table 1) could be linked to a lesser stability of the carbonates on this catalyst.
4.3.
Surface oxygenated species
The greater activity of the potassium-containing catalyst is partly explained by the lower electro-negativity of this metal (greater metallic character), that eases the release of its electrons and favors the surface adsorption of gaseous oxygen. Therefore, species like peroxides and super oxides with high oxygen content are formed, thus increasing the oxidizing properties of the catalyst surface. The DRIFTS assays from Fig. 4 show the formation of these peroxides and super oxides. The vibration frequency of oxygen in these species diminishes as the atomic number of the alkali metal present in the catalyst increases (ν(O2)Li N ν(O2)Na N ν(O2)K). A stronger interaction between the metallic atom and the oxygen molecule weakens the O–O bond diminishing its vibration frequency [27]. Therefore, the surface dissociation of oxygen is best promoted on the potassium-containing catalyst. The calculations presented in Fig. 5 also show that peroxide and super oxides formation is thermodynamically favored as the atomic number of the alkali metal increases (K N Na N Li). This order agrees with the catalytic activity measured for the CB combustion (Fig. 1). Thermodynamics shows that Li may have less activity, due to low formation of peroxide and super oxides. However, its catalytic activity was just slightly lower than for K or Na, which might be attributed to the better capacity of Li-containing catalysts to generate very active suprafacial oxygen, as reported by Russo et al. [15]. Surface characterization (XPS) at ambient temperature shows a greater surface concentration of O2− on the Na/20-SG catalyst than on the K/20-SG catalyst, as determined by the O2−/Mg ratio presented in Table 1. This is attributed to the larger surface atomic concentration of sodium (alkali/Mg = 0.15). However, the surface O2− content per alkali metal atom (that is the O2−/alkali ratio) is greater for the potassiumcontaining catalyst (21.7) than the same ratio in the Na/20CaO catalyst (14.9), thus confirming the better activity of potassium to promote reactions that enrich with oxygen the catalyst surface. The presence of a well-differentiated peak of O 1s at a higher BE (531.3 eV) for the catalyst K/20-CaO is attributed to a greater content of surface-adsorbed oxygen of the O− type (α-oxygen), also labeled as suprafacial oxygen, which has been identified as the main vehicle in complete oxidation processes, as soot combustion [14,15,32]. The species that are weaker-bonded to the catalyst surface should have greater mobility, hence an easier access to the contact points between soot and catalyst (probably through a spillover mechanism) thus favoring the enrichment of the CB surface with oxygenated species [15]. The previous discussion may explain why the Na/20-CaO was less active than the K/20-CaO for the catalytic combustion of CB, in spite of the greater surface concentration of oxygen shown by XPS analysis. A large percent of the oxygen species observed in the K-containing catalyst would be of the O− type,
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with high mobility and reactivity for CB combustion; this was not the case for the Na-containing catalyst. This result confirms that catalysts activity for soot combustion depends not only on oxygen content on the surface, but also on the nature of this oxygen. With regard to the alkali composition of the assayed catalysts, the latter were prepared with similar mass weight percent implying a different atomic composition of the metals. It is clear that the performance ranking of alkali metals as catalyst for this model reaction will be confirmed (and enhanced) if similar atomic composition of the added alkalis are utilized in the catalysts.
5.
Conclusions
The addition of alkali metals (Li, Na, K) over a CaO–MgO mixture, generated a significant increase of the catalytic activity, due to the formation of surface oxygenated species that enhance the oxidizing properties of the catalyst surface. The activity for the catalytic CB combustion increases with the atomic number of the alkali metal contained in the catalyst. The presence of the alkali metals also diminished the amount and stability of carbonates formed on the catalyst. It was verified that the K/20-CaO catalyst showed the largest activity for the catalytic CB combustion because it presents the greater capacity to enrich with adsorbed oxygen, in the form of O− (labeled as α-oxygen), the catalyst surface and the surface dissociation of oxygen is best promoted on this potassium-containing catalyst.
Acknowledgements The support of FONDECYT-Chile, Grants No. 11060301 and 1071016, and short stays financing from the MECESUP Program, Grant UCO0108 (for RJ) is gratefully acknowledged.
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