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MgO as catalyst
Applied Catalysis A: General 314 (2006) 81–88 www.elsevier.com/locate/apcata
Soot combustion with K/MgO as catalyst II. Effect of K-precursor Romel Jime´nez a, Ximena Garcı´a a, Caroline Cellier b, Patricio Ruiz b, Alfredo L. Gordon a,* a
Departamento de Ingenierı´a Quı´mica, Universidad de Concepcio´n, Chile b Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Universite´ Catholique de Louvain, Belgium
Received 24 March 2006; received in revised form 1 August 2006; accepted 3 August 2006 Available online 20 September 2006
Abstract This paper explores the possible effect of the potassium precursor on the activity of K/MgO catalysts for soot combustion and, in particular, which of previously described potassium roles would be more affected. The catalytic activity of the prepared K-catalysts was evaluated through thermogravimetric assays using catalyst-carbon black mixtures. Selected catalyst samples were characterized by XPS, AAS, BET surface area, carbothermic reduction, DRIFTS and XRD. The effect of physical contact was also assessed. The experimental results show that the addition of potassium from different precursors (KOH, KNO3) affects the basic nature of the catalyst surface and its interaction with active species, and this is reflected in the catalytic activity. With KNO3 as the precursor, a more active catalyst was obtained, a result that is attributed to: (i) higher concentration and dispersion of potassium, (ii) greater mobility and reactivity of the activated oxygen species, and (iii) lower stability of surface carbonates and OH groups, facilitating the formation of active species and favoring the mobility of surface activated oxygen. # 2006 Elsevier B.V. All rights reserved. Keywords: Soot; Catalytic combustion; Potassium
1. Introduction Diesel engine emissions are harmful for environmental and human health, because of their high content of particulate matter (soot) and NOx. Soot can be retained in the lungs and promote cancer, while nitrogen oxides contribute to acid rain and smog, as well as the formation and depletion of the stratospheric ozone layer. Soot may be captured by filters, which preferably contain deposited catalysts whose role is to lower the soot ignition temperature (>550 8C for the uncatalyzed reaction) to typical values for diesel exhaust gases (180–400 8C), thus enabling self-regeneration of the filters. The search for efficient and cheap catalysts and the design of effective and low-cost filters constitutes a major technological challenge.
* Corresponding author. Tel.: +56 41 2204534; fax: +56 41 2247491. E-mail address: [email protected] (A.L. Gordon). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.08.002
The catalytic activity of potassium salts (e.g., KOH, KNO3, KCl, K2CO3 and K2SO4) has been widely studied [1–5] for the gasification of carbonaceous materials. Some of these salts have also been used as precursors for soot combustion catalysts [6–12]. Thus, for example, Carrascull et al. [6] studied the concentration effect of potassium nitrate on the catalytic activity of KNO3/ZrO2. A significant enhancement of the catalytic soot combustion rate was observed up to a KNO3 concentration of 11.5% remaining constant up to 20.5% and decreasing at higher concentration. This behavior could be related to surface saturation and reduction in the dispersion of the active phase with increasing concentration of KNO3. However, nitrate concentrations in the catalysts are nominal, no mention is made of the actual concentrations after calcination of the catalysts and combustion rates were not normalized with respect to the initial mass of soot or catalyst. The effect of the nitrate [6] was attributed by these authors to (i) the alkali metal-carbon intermediates formed during the gasification process, (ii) an improvement of the catalyst/soot contact by the molten nitrate, and (iii) the oxidizing role of the
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nitrate ion, being transformed into some reduced species (e.g., nitrite). In the subject of coal gasification that is closely related to soot combustion [13], Hu¨ttinger and Minges [1] had studied the kinetics of the reduction of several potassium salts in a fixed bed flow reactor. They found that the activation of KNO3 starts by its decomposition: 2KNO3 $ K2 O þ O2 þ NO þ NO2
(1)
The gaseous products react with the carbon substrate to form CO2, which in turn reacts with the oxide to give K2CO3. These authors proposed a general activation scheme [1], with KOH as the key component, from which the active species, a nonstoichiometric compound, KxOy (y < x), is formed. In an even previous work, Yuh and Wolf [5] had evaluated the catalytic effect of different potassium salts during steam gasification of coal and chars. The following order was found: KOH K2K2CO3 > KHCO3 > KNO3 > K2SO4 with almost no activity for KCl. On the other hand, Lang [14] had theorized about the anion effect of several salts from alkaline metals. It was postulated that some anions could compete with the char for the alkali cations and thus inhibit the formation of an alkali–carbon complex, which was believed to be the active gasification site. Moreover, he proposed that the alkali salts of weak acids are good catalysts, while those of strong acids are poor. However, KNO3, the salt of a strong acid showed a higher catalytic activity than KOH, considered the salt of water, a weak acid [14]. Querini et al. [7–9] studied the catalytic combustion of diesel soot using Co/MgO, K/MgO and Co, K/MgO catalysts, where potassium was added to the suspension in the form of KOH. The authors reported negligible activities for pure MgO and K/MgO. For the Co/MgO catalysts, a direct relation between catalytic activity and cobalt reducibility was found, suggesting that the reaction proceeds through a redox mechanism. From these and other experiments [7–12,15], the role of potassium was attributed to the following factors: (i) it forms low melting point compounds (such as KOH, KNO3) or eutectics with other catalyst’s components, thus enhancing catalyst-soot contact and improving activity; (ii) it acts as an electron donor, preserving the reducibility and dispersion of transition metals like Co, V, Mo or Cu; (iii) it forms an intermediate carbonate, thus providing a route for CO2 release [7–10]. In a previous article [16], we have shown that KOH/MgO is a very active catalyst for carbon black combustion, a result that does not agree with the observations of Querini et al. [7–9]. Three effects attributed to potassium explained the observed catalytic activity: (a) increase in surface oxygen concentration, (b) weakening of the Mg–O bonds with migration of oxygen species to the surface, and (c) reduction of the stability of carbonates formed that, in presence of K, decompose at lower temperatures [16]. One question that arises is whether the potassium precursor affects the activity of the K/MgO catalyst for soot combustion and if so, which of the above-mentioned roles would be the more
affected. Thus, the activity of K/MgO catalysts prepared from KNO3 and KOH as precursors was compared in carbon black combustion with air. Carbon black was selected as a model compound for soot, and air as a model gaseous feed for the exhaust gases. A more realistic gaseous feed containing also NO, which is present in diesel engine exhausts, is being currently tested to present those results in an upcoming manuscript. 2. Experimental 2.1. Catalyst preparation Catalyst impregnation was effected as follows: 0.717 g of KOH and 1.293 g of KNO3 (Merck, 99% purity) were dissolved in 70 mL of distilled water. The solutions were mixed with 9.5 g of MgO (Merck, 99% purity). The slurries were dried at 120 8C and calcined during 30 min at 760 8C (samples were labeled as KOH/MgO and KNO3/MgO, respectively). 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 KOH/MgO or KNO3/MgO. To study the effect of physical contact, CB-catalyst samples with ‘‘loose’’ and ‘‘tight’’ contact were prepared. For loose contact (LC), the mixture was shaken manually and gently during 1 min in a glass bottle, while tight contact (TC) mixtures were prepared by mixing the components in an agate mortar during 2 min, prior to the combustion assay. Reproducibility of combustion kinetics for this ‘‘tight’’ contact mixture was verified through TGA experiments. ‘‘Tight’’ conditions correspond to contact that is probably too good compared with what is typically achieved in catalytic soot traps. However, results for this type of contact achieve high reproducibility and thus set a good basis for catalytic activity screening tests. 2.2. Catalytic activity Catalytic oxidation of CB was carried in a thermogravimetric apparatus (Netzsch 409 PC). Catalyst-CB mixture samples of 7.5 mg were heated to 800 8C in 180 mL/min flow of air (Indura, atmospheric composition) at 10 8C/min. 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 comparisons of catalytic activity. The activation energies (Eact) and the frequency factors (ko) were calculated for a conversion range between 10 and 25% (determined from the area under the DTG curves), according to the methodology used by Stratakis and Stamatelos [17]. For the isothermal kinetic experiments, the catalyst-CB mixture was heated in the thermogravimetric apparatus to 380 8C under inert atmosphere (N2, 99.996% pure). This temperature was kept for 10 min and then the gaseous atmosphere was switched to 180 mL/min of air for 3 h. The sample was then heated to 800 8C in air to ensure complete combustion of CB.
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2.3. Catalyst characterization 2.3.1. Atomic absorption spectroscopy (AAS) A Hitachi Z-8100 spectrometer with Zeeman polarization was used to determine catalyst composition. 2.3.2. X-ray diffraction (XRD) The crystalline structure of the catalysts was analyzed by XRD utilizing a RIGAKU-Dmax-IIIC equipment with Cu Ka1 radiation (40 kV/20 mA), in a scan range of 3–708 and a scan speed of 18/min. Analysis of phases was determined using the JCPPDS files. 2.3.3. X-ray photoelectron spectroscopy (XPS) XPS was performed with a SSI X-Probe (SSX-100/206) photoelectron spectrometer (Surface Science Instrument, Fisons) equipped with a monochromatized micro-focused Al Ka X-ray source (1486.6 eV) and a hemispherical analyzer. The analyzer pass energy was set at 50 eV and the spot size was ca. 1000 mm. Under these conditions, the energy resolution estimated by the standard Au 4f7/2 full width at half maximum (FWHM) was 1.1 eV. Charge neutralization was achieved by using an electron flood gun adjusted at 8 eV and placing a Ni grid 3 mm above the sample. The sample powders were compressed in stainless steel cups (4 mm diameter) and placed on a ceramic carrousel. The samples were outgassed overnight to 105 Pa and then introduced into the analysis chamber where the pressure was about 107 Pa. The binding energies were calculated taking as reference the C–(C,H) component of the C 1 s adventitious carbon peak fixed at 284.8 eV. For each sample, a survey spectrum was first recorded and was followed by detailed scans of C 1s, O 1s, specific sample elements and finally C 1s again to control the charge compensation stability as a function of analysis time. The spectra were deconvoluted with the least squares fitting routine provided by the manufacturer with a Gaussian/Lorentzian ratio of 85/15 and after subtraction of a calculated (Shirley type) baseline (performed with CASA-XPS program). The atomic concentration ratios were calculated by normalizing the surface area ratio with sensivity factors based on Scoffield cross sections. Selected samples of fresh and used catalysts (after soot combustion) were characterized to check for possible changes in the surface K/Mg ratio.
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controlled chamber (SpectraTech 0030–103), equipped with SeZn windows and connected to the diffuse reflectance collector. The spectrum of an aluminum mirror was used as background. Spectra of samples (200 scans with 4 cm1 resolution) at ambient temperature in 30 mL/min He were captured first. Each sample was then heated at 10 8C/min under the desired atmosphere, i.e., inert (He) or reactive (10%O2/ 5%CO2/He mixture), and their spectra were registered at different temperatures. The CO2 presence in the reactive gas mixture is representative of the TGA atmosphere in combustion experiments. Moreover, this composition closely resembles the one found in the exhaust gases of diesel engines. The gases flowing out from the DRIFTS cell were analyzed in a Balzers Thermostar GSD300T2 gas analyzer equipped with a quadrupole mass spectrometer QMS200, connected on line with the DRIFTS apparatus. 3. Results 3.1. Catalytic activity The effects of potassium precursor and physical contact are shown in Fig. 1. Analogous curves obtained in previous work [16] for pure MgO catalyst and for the uncatalyzed CB combustion are also presented as a convenient reference. Values of Tm and the kinetic parameters are summarized in Table 1. As shown by peak positions and Tm values, the KNO3/MgO catalyst was more active than KOH/MgO, for both loose and tight contact (see Table 2). The sensitivity of the catalysts to physical contact can be assessed through the value of DTm, defined as DTm = [Tm (loose contact) Tm (tight contact)]. It is observed that the KOH/ MgO catalyst, with DTm = 105 8C, is more sensitive to the type of contact than the KNO3/MgO catalyst (DTm = 19 8C). Activation energies (Table 1) are very similar for both catalysts (slightly lower for the KNO3/MgO) and no effect of the CB-catalyst contact type over their values was observed. On the other hand, addition of potassium resulted in a reduction of about 20 kJ/mol in comparison to pure MgO (see ref. [15]).
2.3.4. Carbothermic reduction In order to evaluate the oxidation of the carbonaceous material in the absence of gas-phase oxygen, a 4:1 catalyst-CB sample of 7 mg was heated at 10 8C/min in the thermogravimetric apparatus from 25 to 800 8C, under 180 mL/min of N2 (Indura, 99.996% pure). The curves obtained were normalized in each experiment using the initial CB mass. 2.3.5. Diffuse reflectance FT-infrared spectroscopy (DRIFTS) An IFS 55 Equinox (Bruker) spectrophotometer equipped with an air-cooled KBr source and a MCT detector was used. Samples were located in a thermally and environmentally
Fig. 1. Effect of potassium precursor and physical contact on the catalytic activity of KOH/MgO and KNO3/MgO.
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Table 1 Catalytic combustion of carbon black in air with KOH/MgO and KNO3/MgO as catalysts (temperature of maximum combustion rate and kinetic parameters) Sample
Tma (8C)
Activation energyb, Eact (kJ mol1)
Frequency factorb ko (min1mgi,CB1)
Temperature ranges (10–25% of CB conversion)
KOH/MgO (loose contact) KOH/MgO (tight contact) KNO3/MgO (loose contact) KNO3/MgO (tight contact)
545 440 440 421
180.5 181.9 169.5 175.4
7.0 10 10 5.3 10 12 4.9 10 10 4.9 10 12
489–514 399–415 402–424 379–402
a b
Temperature of maximum combustion rate. The kinetic parameters were calculated in the range of 10–25% of CB conversion.
With respect to the frequency factors (see Table 1), values of the order of 1010 were obtained for loose contact and 1012 for tight contact. 3.1.1. Isothermal experiments The isothermal CB combustion runs, performed at 380 8C utilizing tight-contact catalysts (Fig. 2) demonstrate that, although both materials are catalytically active, a higher reaction rate was reached, at this temperature, for KNO3/MgO, a result that is in agreement with the TPO curves of Fig. 1. Besides, with KNO3/MgO total consumption of CB was achieved after 40 min versus >60 min for KOH/MgO. 3.1.2. Carbothermic reduction From Fig. 3, it is seen that pure MgO was catalytically inactive in the absence of gaseous oxygen; in contrast, when potassium was present, as for the KOH/MgO and KNO3/MgO samples, these catalysts were able to induce CB oxidation even in the absence of gaseous oxygen. The CB combustion peak using KOH/MgO is more pronounced and it occurs at lower temperature, indicating that this catalyst is more active than KNO3/MgO (Fig. 3). The area under the KOH/MgO curve corresponding to carbothermic reduction is equivalent to 63% of the area under the peak obtained for combustion in air (Fig. 1); in contrast, for KNO3/MgO this area corresponds to approximately 15% (see Fig. 1).
3.2. Catalyst characterization 3.2.1. Atomic absorption spectroscopy (AAS) Table 2 shows the bulk metal contents measured by AAS analysis. The Mg and K concentrations are slightly lower in the KOH/MgO catalyst. The K/Mg ratio is similar for both catalysts. 3.2.2. BET specific surface areas The BET surface area of the KOH/MgO catalyst practically duplicates the BET area of KNO3/MgO (27.7 m2/g versus 15.2 m2/g). This result confirms that potassium introduction diminishes the surface area in comparison with pure MgO [16]. It also shows that for this type of reaction (catalyst-carbon black-O2) there is no direct relation between the BET surface area of the catalyst and the combustion rate of carbon black. In fact, the combustion rate is more affected by the number of contact points between catalyst and carbon black [8]. 3.2.3. X-ray diffraction The diffraction spectra are shown in Fig. 4. When K is added from KOH, the presence of Mg(OH)2, KOH, K2O and K2CO3 is observed. When the precursor is KNO3, the observed crystalline phases are KNO3, K2CO3, MgNO3 and MgO. However, a higher K2CO3/MgO ratio is observed for the KOH/MgO as shown in Fig. 4 from the intensity ratio of their corresponding peaks (2u = 38.44 and 26.86, respectively).
Table 2 Bulk (AAS) and surface (XPS) characterization of KOH/MgO and KNO3/MgO catalysts Catalyst Bulk characterization Mg (wt%) K (wt%) Atomic ratio K/Mg Surface characterization K 2p (at%) K/Mg O2/Mg CO32/Mg Mg 2s (B.E.)b O1s (B.E.) K 2p3/2 (B.E.) BET surface area (m2/g) a b
KOH/MgO
KNO3/MgO
45.9 2.31 0.03
51.40 2.80 0.03
1.12 0.04 1.89 0.10 87.9 530.6 292.7 27.7
1.86 0.06a 0.64 0.12 87.9 529.2 292.8 15.2
Same values measured before and after the soot combustion. B.E., binding energy (eV).
Fig. 2. Isothermal catalytic combustion of carbon black.
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Fig. 3. Carbothermic reduction of carbon black.
3.2.4. XPS Results of XPS analyses are presented in Table 2. For both catalysts, the K/Mg atomic surface ratio is higher than the bulk ratio (AAS). In addition, the KNO3/MgO catalyst possesses a higher surface concentration of potassium (1.86) and higher K/ Mg surface atomic ratio (0.06) than the KOH/MgO catalyst, with 1.12 and 0.04, respectively. Furthermore, for selected used (after soot combustion) KNO3/MgO catalysts the surface K/Mg ratio was measured, and as shown in Table 2, that ratio remained constant. The atomic surface ratio of O2/Mg is higher for KOH/MgO (1.89 compared with 0.64 of KNO3/ MgO). The binding energy (BE) associated with O2 for the latter was lower than the corresponding O2 BE of KOH/MgO (529.2 eV versus 530.6 eV). On the other hand, the BE of Mg 2s and K 2p3/2 were similar for both catalysts (87.9 eV). 3.2.5. DRIFTS experiments Results for KNO3/MgO in the presence of He are shown in Fig. 5A and B, while Fig. 6 shows the DRIFTS results in a reactive atmosphere. The results of similar experiments for the
Fig. 4. XRD of KOH/MgO and KNO3/MgO samples.
Fig. 5. DRIFTS and mass spectroscopy assays with KNO3/MgO in He at different temperatures: (A) DRIFTS; (B) mass spectrometry.
KOH/MgO catalyst, as well as the band assignments for the species associated with each IR band, were previously reported [16]. At room temperature, the presence of bridge-type (1762 cm1) and unidentate (1415, 1498 and 1060 cm1)
Fig. 6. DRIFTS assays with KNO3/MgO in reactive atmosphere (5%CO2/ 10%O2/He) at different temperatures.
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carbonates is observed (Fig. 5A). The disappearance of the wide band (2800–3600 cm1) and the shoulder at 1645 cm1 is evidence for water desorption and superficial bicarbonate decomposition. This is confirmed by CO2 and water formation at temperatures above 150 8C, as shown in Fig. 5B. With an increase in temperature, the 1498 cm1 band diminishes and, simultaneously, a band at 1035 cm1 appears. This is interpreted as decomposition of unidentate carbonates and their transformation to bridge-type carbonates. Decomposition starts at 200–300 8C, i.e., at lower temperatures than those where similar changes were observed for KOH/MgO catalyst [16], and is confirmed by the appearance of a CO2 peak close to 300 8C (Fig. 5B). For the same temperature interval, the intensity of the 3697 cm1 band (type I OH) diminished. The 3629 cm1 band, associated with type II OH, is also reduced with a rise in temperature. Generally, the presence of OH groups and their stability as temperature increases seems smaller for the KNO3/MgO catalyst. The bridge-type carbonate (1760, 1035 cm1), potassium carbonate (1415–1420 cm1) and the 1234 cm1 bands, associated most probably with potassium nitrate [18], are still present at 500 8C. DRIFTS experiments under reactive atmosphere for the KNO3/MgO catalyst (Fig. 6) show that the 1247 cm1 band starts to disappear above 450 8C. Moreover, the band associated with adsorbed superficial oxygen complexes interacting with alkaline metal (1107 cm1) starts to appear [18]. The unidentate carbonates seem to decompose and/or transform to bridge-type carbonates above 400 8C, as suggested by the reduction in the 1520 cm1 band and the displacement of the 1047 cm1 band to 1035 cm1. The wavelength zone where normally the OH groups are observed shows very low intensity bands. A 3626 cm1 band with a low intensity is observed even at 540 8C. 4. Discussion Catalysts generated by addition of potassium from two different precursors (KNO3 and KOH) showed qualitative differences in their surface properties. Different crystalline phases present in these catalysts are evidenced by their XRD patterns (see Fig. 4). Magnesium and potassium hydroxides are predominant in the KOH/MgO catalyst, while the nitrates of the same metals are predominant in the KNO3/MgO sample. The results presented in the foregoing sections showed that the already shown [16] beneficial effects of potassium addition using KOH are even better if KNO3 is selected instead Kprecursor. The details of such behavior are discussed below. 4.1. Catalytic activity and active sites The results of this work agree with those reported previously [16]. Addition of potassium clearly increases the catalytic activity, through a decrease of the activation energy, with respect to both pure MgO (Ea = 204 kJ/mol) and the uncatalyzed reaction (Ea = 254 kJ/mol). This confirms that potassium plays an important role in the rate-determining step of CB combustion, which is likely the dissociative O2
adsorption or the transfer of adsorbed oxygen from the catalyst surface to the CB surface [16]. For the KNO3/MgO catalyzed reaction the activation energy was slightly lower, suggesting the presence of catalytic sites that are even more active (Table 1). The higher activity of KNO3/MgO was observed in greater detail during isothermal experiments (Fig. 2), as manifested by the higher values of the maximum rate (ca. 50 mg/gi,CB/min) and the short time for complete CB conversion (40 min), although both catalysts showed significant activity at 380 8C. The activity of the KNO3/MgO catalyst is attributed to a higher concentration and an enhanced surface dispersion of the active species (Table 2). For both catalysts, the K/Mg surface ratio was higher than the ‘bulk’ atomic ratio, indicating that the alkaline metal is preferably deposited on the MgO surface. Nevertheless, both the surface atomic percent and the surface K/Mg mass ratio were higher in KNO3/MgO than in KOH/ MgO, as shown by the values in Table 2. Possible changes in the surface K/Mg ratio of the catalyst after soot combustion, as reported in the literature [7], were checked, and no change was found as shown in the values of Table 2. This should be the expected result if the samples have been calcinated, prior to soot combustion, to a significantly higher temperature than the reaction temperature. In fact, the possible effect of calcination temperature and time on catalyst activity was discussed in a previous paper [16]. 4.2. Surface active species In a previous paper [16], it was shown that potassium presence increases the abundance of oxygen on the catalyst surface and enhances the reactivity of this superficial oxygen; this occurs in both, the KOH/MgO and KNO3/MgO catalysts. DRIFTS assays, both in He and in reactive atmosphere, for KOH/MgO [16] and KNO3/MgO (Figs. 5 and 6) show the presence of adsorbed superficial oxygen complexes (bands at 1100–1120 cm1) and more specifically O2 species observed in MO2 complex formed by reaction of the alkali metal with O2 [16,18]. However, it was suggested that the reaction rate was affected not only by the abundance of surface oxygen (better for the KOH/MgO catalyst, as shown by XPS measurement of 1.89 versus 0.64 for the KNO3/MgO catalyst), but also by the reactivity and mobility of the surface oxygenated species (better for the KNO3/MgO catalyst). Thus, in the presence of gaseous oxygen, the reaction rate is controlled to a greater extent by the reactivity and mobility of the superficial oxygenated species than by their concentration. The previous suggestions are also in agreement with the proposed mechanism [16], where the ratedetermining step of CB combustion was associated with the transfer of adsorbed oxygen from the catalyst surface to the CB surface. The oxygenated surface species are thus more mobile and more reactive in the KNO3/MgO catalyst than in KOH/MgO. As to why the adsorbed oxygen species are more reactive and mobile on the KNO3/MgO catalyst than in the KOH/MgO
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catalyst, this is attributed to the K presence on the surface, which is higher on KNO3/MgO than in KOH/MgO (see Table 2). As is known, the good electron donor property of K favors the surface adsorption of gaseous oxygen and its dissociation, enhancing at the same time the reactivity of the lattice oxygen due to the weakening of the Mg–O bonds on the catalyst surface. The catalyst with the higher surface concentration of potassium (KNO3/MgO in our case) will contain Mg–O bonds that are weaker and thus oxygenated surface species that are more reactive and probably more mobile as well. In the absence of gaseous oxygen (carbothermic reduction assays), MgO was inactive [16], while the catalyst with a higher O2 surface concentration (KOH/MgO) showed higher catalytic activity (Fig. 3), although complete conversion of the initially loaded CB was not reached. The total CB consumption ratio (63%/15%) turns out to be proportional to the atomic O2/Mg surface ratios (1.89/0.64); this implies, as intuitively expected, that the presence of surface oxygen species is rate-limiting in CB oxidation in the absence of O2(g). Summing up, in the absence of gaseous oxygen the abundance of surface oxygen is the main factor affecting the reactivity and rate of reaction, while in gaseous oxygen presence, the reactivity and mobility of the surface oxygenated species are the predominant factors. A relevant matter to this discussion is that surface nitrates could directly oxidize soot particles. However, as shown below, the amount of KNO3 that is present in the catalyst during the TGA experiments (the catalyst/CB ratio was 4/1) does not explain the total CB consumption that was measured. The influence of catalyst precursor anion was studied in catalysis of water vapor gasification of carbon by potassium [1]. In particular, several reactions were determined as possible for the oxidation of graphite with KNO3; simultaneous thermogravimetry and differential thermoanalysis in argon were performed using a mixture composed of 120.7 mg of potassium nitrate and 126.9 mg of graphite. A weight loss of 50 mg was measured between 450 and 500 8C (Fig. 6), that is a 39.4% of the carbon content. In the TGA assays of this work, the KNO3/CB ratio is four times smaller than in those experiments [1]; however, all the carbon black is combusted below 600 8C. Thus, it is improbable a key role of KNO3 in the CB oxidation. In fact, from the included calculations it follows that the maximum available amount of nitrate in the catalytic assays would account for less than 10% of the total carbon black consumption, if the entire available nitrate were to be fully utilized in the oxidation of carbon black. The comparison among these results has been summarized in the following calculations. For the latter, all K is assumed in the form of KNO3, which obviously is not true. In spite of that, and with only 1/3 of the KNO3 amount assayed in the referred work [1], the present experiments showed 100% of soot conversion. That proves that KNO3 does not play a significant role in the direct soot oxidation and the role of K as a catalyst is validated.
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Calculation of possible soot oxidation by surface KNO3 KNO3/MgO catalyst (this work) K (wt%) C (weight) Catalyst/C = 4/1, for combustion KNO3 KNO3/C
After Hu¨tinnger and Minges [1] 2.80 (by AAS) 1.50 mg
0.435 mg 0.290
CB conversion in TGA 100%
KNO3 KNO3/C
1.427 mg 0.951 (between 450 and 500˚ C)
CB conversion in TGA 39.4%
4.3. Surface carbonates and OH groups The results of DRIFTS assays coupled with mass spectroscopy (Figs. 5 and 6), both in this work and in our previous report [16], show that carbonate decomposition occurs at a lower temperature for the KNO3/MgO catalyst, that is, over this catalyst surface carbonates are less stable. This experimental fact indicates that KNO3 generates a less basic surface than KOH. The anion nature of the potassium precursor may cause this difference. In fact, for coal gasification the anion may have more effect on catalytic activity than the alkaline metal cation, as previously discussed [14]. Our results show a greater activity of the KNO3/MgO with respect to the KOH/MgO in the catalytic combustion of carbon black. As described before, Lang’s [14] study of the ‘‘anion effect’’ in the catalytic gasification of coal also found that, KNO3 was more active than KOH, a result that contradicted his hypothesis about the competition between the anion and the char to form an alkaline–carbon complex. Therefore, other aspects, probably related to the basicity of the catalytic surface and the mobility of the active phases seem to play a relevant role. The greater stability of carbonates and hydroxylic groups on the surface of the KOH/MgO catalyst may limit the ease of formation of the (re)active sites during combustion. Furthermore, the CO2(g) formed during combustion may interact more strongly with the KOH/MgO surface due to its higher basicity, thus further reducing the number of available catalytic sites. The higher concentration and stability of OH groups and carbonates over KOH/MgO may reduce the mobility of surface oxygen. Thus, a lower basicity of KNO3 is an important factor responsible for higher catalytic activity. Conversely, the bonding and adsorption of oxygen is greater over a more basic (high electronic density) surface such as KOH/MgO, as evidenced by the higher surface concentration of O2 (greater ratio O2/Mg) and the higher binding energy associated with this species (530.6 eV) in the KOH/MgO catalyst. This stronger interaction hinders the mobility of surface oxygen. 4.4. Effect of contact type Fig. 1 and Table 1 allow us to estimate the sensitivity of both assayed catalysts to the type of contact with CB. This is a
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relevant factor as the conditions to be found in an actual catalytic filter will be probably much closer to a ‘‘loose’’ than to a ‘‘tight’’ contact. The KOH/MgO catalyst is seen to be more sensitive than KNO3/MgO. This is attributed both to a higher surface concentration of potassium in the KNO3/MgO catalyst and a higher mobility of the surface oxygen activated on this catalyst. As the combustion reaction progresses, it is increasingly difficult to maintain constant the number of contact sites at the catalyst-CB interface. Thus, the activity of a catalyst having a higher concentration of active sites and/or a higher mobility of its active phase(s) will be less affected by such an expected decrease in the number of its contact points with CB. This appears to be the case of the KNO3/MgO, whose higher measured activity is attributed to a greater mobility of the O2, as discussed in Section 4.2.
complete conversion, at least for the assayed mass ratio of catalyst to CB of 4:1. These surface oxygen species, activated on the catalyst surface, are the main oxidizing agents in the presence of gaseous oxygen and their mobility appears to be essential for effective catalysis; this explains why KNO3/MgO was more active in air.
Acknowledgements The support of FONDECYT-Chile Grant No. 1030296 and short stays financing from the MECESUP Program Grant UCO0108 (for RJ) are gratefully acknowledged. The ‘‘Fonds National de la Recherche Scientifique’’ of Belgium and the ‘‘Re´gion Wallonne’’ are also thanked for the acquisition of the XPS and the DRIFTS equipments.
5. Conclusions The addition of the same amount of potassium from different precursors (KOH, KNO3) generates catalysts that are qualitatively different, which is in agreement with several studies carried out about the ‘‘anion effect’’ on the catalytic gasification of coal. The catalysts have surfaces of different basicity. This affects the interaction of the active species with the support surface and is reflected in the different behavior of these catalysts in carbon black combustion. With KNO3 as potassium precursor, a more active catalyst was obtained for the combustion of CB with air. This result is attributed to several factors: (i) a higher concentration and surface dispersion of the potassium that is associated with the active phase of the catalyst, (ii) a higher mobility of the oxygen species activated on this catalyst, and (iii) a lower stability of surface carbonates and OH groups, which facilitates the formation of active species during combustion and favors the mobility of the activated surface oxygen species. These findings highlight the following mechanistic aspects of potassium-catalyzed soot oxidation: (i) potassium from both precursors activates the lattice oxygen on the catalyst surface, thus enhancing its combustion reactivity. Surface oxygen is more abundant on the surface of KOH/MgO explaining its higher activity in the absence of O2(g). (ii) The presence, adsorption and dissociation of gaseous oxygen on the catalyst surface are indispensable steps for combustion to proceed until
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K.J. Hu¨ttinger, R. Minges, Fuel 65 (1986) 1112–1121. M.B. Cerfontain, A. Moulijn, Fuel 65 (1986) 1349–1355. B.C. Gorrini, L.R. Radovic, A.L. Gordon, Fuel 69 (1990) 789. S.G. Chen, R.T. Yang, J. Catal. 138 (1992) 12–23. Sh.J. Yuh, E.E. Wolf, Fuel 62 (1983) 252–255. A.L. Carrascull, M.I. Ponzi, E.N. Ponzi, Ind. Eng. Chem. Res. 42 (2003) 692–697. C.A. Querini, M.A. Ulla, F. Requejo, J. Soria, U.A. Sedra´n, E.E. Miro´, Appl. Catal. B: Environ. 20 (1999) 165–177. C.A. Querini, M.A. Ulla, F. Requejo, J. Soria, U.A. Sedra´n, E.E. Miro´, Appl. Catal. B: Environ. 15 (1998) 5–19. E.E. Miro´, F. Ravelli, M.A. Ulla, L.M. Cornaglia, C.A. Querini, Catal. Today 53 (1999) 631–638. M.L. Pisarello, V. Milt, M.A. Peralta, C.A. Querini, E.E. Miro´, Catal. Today 75 (2002) 465–470. V. Serra, G. Saracco, C. Badini, V. Specchia, Appl. Catal. B: Environ. 11 (1997) 329–346. G. Saracco, C. Badini, N. Russo, V. Specchia, Appl. Catal. B: Environ. 21 (1999) 233–242. R. Jime´nez, X. Garcı´a, A.L. Gordon, J. Chil. Chem. Soc. 50 (4) (2005) 651–665. R.J. Lang, Fuel 65 (1986) 1324–1329. B.R. Stanmore, J.F. Brilhac, P. Gilot, Carbon 39 (2001) 2247–2268. R. Jime´nez, X. Garcı´a, C. Cellier, P. Ruiz, A.L. Gordon, Appl. Catal. A: Gen. 297 (2006) 125–134. G.A. Stratakis, A.M. Stamatelos, Combust. Flame 132 (2003) 157. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, fourth ed., John Wiley & Sons, New York, 1986.
Soot combustion with K/MgO as catalyst II. Effect of K-precursor Romel Jime´nez a, Ximena Garcı´a a, Caroline Cellier b, Patricio Ruiz b, Alfredo L. Gordon a,* a
Departamento de Ingenierı´a Quı´mica, Universidad de Concepcio´n, Chile b Unite´ de Catalyse et Chimie des Mate´riaux Divise´s, Universite´ Catholique de Louvain, Belgium
Received 24 March 2006; received in revised form 1 August 2006; accepted 3 August 2006 Available online 20 September 2006
Abstract This paper explores the possible effect of the potassium precursor on the activity of K/MgO catalysts for soot combustion and, in particular, which of previously described potassium roles would be more affected. The catalytic activity of the prepared K-catalysts was evaluated through thermogravimetric assays using catalyst-carbon black mixtures. Selected catalyst samples were characterized by XPS, AAS, BET surface area, carbothermic reduction, DRIFTS and XRD. The effect of physical contact was also assessed. The experimental results show that the addition of potassium from different precursors (KOH, KNO3) affects the basic nature of the catalyst surface and its interaction with active species, and this is reflected in the catalytic activity. With KNO3 as the precursor, a more active catalyst was obtained, a result that is attributed to: (i) higher concentration and dispersion of potassium, (ii) greater mobility and reactivity of the activated oxygen species, and (iii) lower stability of surface carbonates and OH groups, facilitating the formation of active species and favoring the mobility of surface activated oxygen. # 2006 Elsevier B.V. All rights reserved. Keywords: Soot; Catalytic combustion; Potassium
1. Introduction Diesel engine emissions are harmful for environmental and human health, because of their high content of particulate matter (soot) and NOx. Soot can be retained in the lungs and promote cancer, while nitrogen oxides contribute to acid rain and smog, as well as the formation and depletion of the stratospheric ozone layer. Soot may be captured by filters, which preferably contain deposited catalysts whose role is to lower the soot ignition temperature (>550 8C for the uncatalyzed reaction) to typical values for diesel exhaust gases (180–400 8C), thus enabling self-regeneration of the filters. The search for efficient and cheap catalysts and the design of effective and low-cost filters constitutes a major technological challenge.
* Corresponding author. Tel.: +56 41 2204534; fax: +56 41 2247491. E-mail address: [email protected] (A.L. Gordon). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.08.002
The catalytic activity of potassium salts (e.g., KOH, KNO3, KCl, K2CO3 and K2SO4) has been widely studied [1–5] for the gasification of carbonaceous materials. Some of these salts have also been used as precursors for soot combustion catalysts [6–12]. Thus, for example, Carrascull et al. [6] studied the concentration effect of potassium nitrate on the catalytic activity of KNO3/ZrO2. A significant enhancement of the catalytic soot combustion rate was observed up to a KNO3 concentration of 11.5% remaining constant up to 20.5% and decreasing at higher concentration. This behavior could be related to surface saturation and reduction in the dispersion of the active phase with increasing concentration of KNO3. However, nitrate concentrations in the catalysts are nominal, no mention is made of the actual concentrations after calcination of the catalysts and combustion rates were not normalized with respect to the initial mass of soot or catalyst. The effect of the nitrate [6] was attributed by these authors to (i) the alkali metal-carbon intermediates formed during the gasification process, (ii) an improvement of the catalyst/soot contact by the molten nitrate, and (iii) the oxidizing role of the
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nitrate ion, being transformed into some reduced species (e.g., nitrite). In the subject of coal gasification that is closely related to soot combustion [13], Hu¨ttinger and Minges [1] had studied the kinetics of the reduction of several potassium salts in a fixed bed flow reactor. They found that the activation of KNO3 starts by its decomposition: 2KNO3 $ K2 O þ O2 þ NO þ NO2
(1)
The gaseous products react with the carbon substrate to form CO2, which in turn reacts with the oxide to give K2CO3. These authors proposed a general activation scheme [1], with KOH as the key component, from which the active species, a nonstoichiometric compound, KxOy (y < x), is formed. In an even previous work, Yuh and Wolf [5] had evaluated the catalytic effect of different potassium salts during steam gasification of coal and chars. The following order was found: KOH K2K2CO3 > KHCO3 > KNO3 > K2SO4 with almost no activity for KCl. On the other hand, Lang [14] had theorized about the anion effect of several salts from alkaline metals. It was postulated that some anions could compete with the char for the alkali cations and thus inhibit the formation of an alkali–carbon complex, which was believed to be the active gasification site. Moreover, he proposed that the alkali salts of weak acids are good catalysts, while those of strong acids are poor. However, KNO3, the salt of a strong acid showed a higher catalytic activity than KOH, considered the salt of water, a weak acid [14]. Querini et al. [7–9] studied the catalytic combustion of diesel soot using Co/MgO, K/MgO and Co, K/MgO catalysts, where potassium was added to the suspension in the form of KOH. The authors reported negligible activities for pure MgO and K/MgO. For the Co/MgO catalysts, a direct relation between catalytic activity and cobalt reducibility was found, suggesting that the reaction proceeds through a redox mechanism. From these and other experiments [7–12,15], the role of potassium was attributed to the following factors: (i) it forms low melting point compounds (such as KOH, KNO3) or eutectics with other catalyst’s components, thus enhancing catalyst-soot contact and improving activity; (ii) it acts as an electron donor, preserving the reducibility and dispersion of transition metals like Co, V, Mo or Cu; (iii) it forms an intermediate carbonate, thus providing a route for CO2 release [7–10]. In a previous article [16], we have shown that KOH/MgO is a very active catalyst for carbon black combustion, a result that does not agree with the observations of Querini et al. [7–9]. Three effects attributed to potassium explained the observed catalytic activity: (a) increase in surface oxygen concentration, (b) weakening of the Mg–O bonds with migration of oxygen species to the surface, and (c) reduction of the stability of carbonates formed that, in presence of K, decompose at lower temperatures [16]. One question that arises is whether the potassium precursor affects the activity of the K/MgO catalyst for soot combustion and if so, which of the above-mentioned roles would be the more
affected. Thus, the activity of K/MgO catalysts prepared from KNO3 and KOH as precursors was compared in carbon black combustion with air. Carbon black was selected as a model compound for soot, and air as a model gaseous feed for the exhaust gases. A more realistic gaseous feed containing also NO, which is present in diesel engine exhausts, is being currently tested to present those results in an upcoming manuscript. 2. Experimental 2.1. Catalyst preparation Catalyst impregnation was effected as follows: 0.717 g of KOH and 1.293 g of KNO3 (Merck, 99% purity) were dissolved in 70 mL of distilled water. The solutions were mixed with 9.5 g of MgO (Merck, 99% purity). The slurries were dried at 120 8C and calcined during 30 min at 760 8C (samples were labeled as KOH/MgO and KNO3/MgO, respectively). 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 KOH/MgO or KNO3/MgO. To study the effect of physical contact, CB-catalyst samples with ‘‘loose’’ and ‘‘tight’’ contact were prepared. For loose contact (LC), the mixture was shaken manually and gently during 1 min in a glass bottle, while tight contact (TC) mixtures were prepared by mixing the components in an agate mortar during 2 min, prior to the combustion assay. Reproducibility of combustion kinetics for this ‘‘tight’’ contact mixture was verified through TGA experiments. ‘‘Tight’’ conditions correspond to contact that is probably too good compared with what is typically achieved in catalytic soot traps. However, results for this type of contact achieve high reproducibility and thus set a good basis for catalytic activity screening tests. 2.2. Catalytic activity Catalytic oxidation of CB was carried in a thermogravimetric apparatus (Netzsch 409 PC). Catalyst-CB mixture samples of 7.5 mg were heated to 800 8C in 180 mL/min flow of air (Indura, atmospheric composition) at 10 8C/min. 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 comparisons of catalytic activity. The activation energies (Eact) and the frequency factors (ko) were calculated for a conversion range between 10 and 25% (determined from the area under the DTG curves), according to the methodology used by Stratakis and Stamatelos [17]. For the isothermal kinetic experiments, the catalyst-CB mixture was heated in the thermogravimetric apparatus to 380 8C under inert atmosphere (N2, 99.996% pure). This temperature was kept for 10 min and then the gaseous atmosphere was switched to 180 mL/min of air for 3 h. The sample was then heated to 800 8C in air to ensure complete combustion of CB.
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2.3. Catalyst characterization 2.3.1. Atomic absorption spectroscopy (AAS) A Hitachi Z-8100 spectrometer with Zeeman polarization was used to determine catalyst composition. 2.3.2. X-ray diffraction (XRD) The crystalline structure of the catalysts was analyzed by XRD utilizing a RIGAKU-Dmax-IIIC equipment with Cu Ka1 radiation (40 kV/20 mA), in a scan range of 3–708 and a scan speed of 18/min. Analysis of phases was determined using the JCPPDS files. 2.3.3. X-ray photoelectron spectroscopy (XPS) XPS was performed with a SSI X-Probe (SSX-100/206) photoelectron spectrometer (Surface Science Instrument, Fisons) equipped with a monochromatized micro-focused Al Ka X-ray source (1486.6 eV) and a hemispherical analyzer. The analyzer pass energy was set at 50 eV and the spot size was ca. 1000 mm. Under these conditions, the energy resolution estimated by the standard Au 4f7/2 full width at half maximum (FWHM) was 1.1 eV. Charge neutralization was achieved by using an electron flood gun adjusted at 8 eV and placing a Ni grid 3 mm above the sample. The sample powders were compressed in stainless steel cups (4 mm diameter) and placed on a ceramic carrousel. The samples were outgassed overnight to 105 Pa and then introduced into the analysis chamber where the pressure was about 107 Pa. The binding energies were calculated taking as reference the C–(C,H) component of the C 1 s adventitious carbon peak fixed at 284.8 eV. For each sample, a survey spectrum was first recorded and was followed by detailed scans of C 1s, O 1s, specific sample elements and finally C 1s again to control the charge compensation stability as a function of analysis time. The spectra were deconvoluted with the least squares fitting routine provided by the manufacturer with a Gaussian/Lorentzian ratio of 85/15 and after subtraction of a calculated (Shirley type) baseline (performed with CASA-XPS program). The atomic concentration ratios were calculated by normalizing the surface area ratio with sensivity factors based on Scoffield cross sections. Selected samples of fresh and used catalysts (after soot combustion) were characterized to check for possible changes in the surface K/Mg ratio.
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controlled chamber (SpectraTech 0030–103), equipped with SeZn windows and connected to the diffuse reflectance collector. The spectrum of an aluminum mirror was used as background. Spectra of samples (200 scans with 4 cm1 resolution) at ambient temperature in 30 mL/min He were captured first. Each sample was then heated at 10 8C/min under the desired atmosphere, i.e., inert (He) or reactive (10%O2/ 5%CO2/He mixture), and their spectra were registered at different temperatures. The CO2 presence in the reactive gas mixture is representative of the TGA atmosphere in combustion experiments. Moreover, this composition closely resembles the one found in the exhaust gases of diesel engines. The gases flowing out from the DRIFTS cell were analyzed in a Balzers Thermostar GSD300T2 gas analyzer equipped with a quadrupole mass spectrometer QMS200, connected on line with the DRIFTS apparatus. 3. Results 3.1. Catalytic activity The effects of potassium precursor and physical contact are shown in Fig. 1. Analogous curves obtained in previous work [16] for pure MgO catalyst and for the uncatalyzed CB combustion are also presented as a convenient reference. Values of Tm and the kinetic parameters are summarized in Table 1. As shown by peak positions and Tm values, the KNO3/MgO catalyst was more active than KOH/MgO, for both loose and tight contact (see Table 2). The sensitivity of the catalysts to physical contact can be assessed through the value of DTm, defined as DTm = [Tm (loose contact) Tm (tight contact)]. It is observed that the KOH/ MgO catalyst, with DTm = 105 8C, is more sensitive to the type of contact than the KNO3/MgO catalyst (DTm = 19 8C). Activation energies (Table 1) are very similar for both catalysts (slightly lower for the KNO3/MgO) and no effect of the CB-catalyst contact type over their values was observed. On the other hand, addition of potassium resulted in a reduction of about 20 kJ/mol in comparison to pure MgO (see ref. [15]).
2.3.4. Carbothermic reduction In order to evaluate the oxidation of the carbonaceous material in the absence of gas-phase oxygen, a 4:1 catalyst-CB sample of 7 mg was heated at 10 8C/min in the thermogravimetric apparatus from 25 to 800 8C, under 180 mL/min of N2 (Indura, 99.996% pure). The curves obtained were normalized in each experiment using the initial CB mass. 2.3.5. Diffuse reflectance FT-infrared spectroscopy (DRIFTS) An IFS 55 Equinox (Bruker) spectrophotometer equipped with an air-cooled KBr source and a MCT detector was used. Samples were located in a thermally and environmentally
Fig. 1. Effect of potassium precursor and physical contact on the catalytic activity of KOH/MgO and KNO3/MgO.
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Table 1 Catalytic combustion of carbon black in air with KOH/MgO and KNO3/MgO as catalysts (temperature of maximum combustion rate and kinetic parameters) Sample
Tma (8C)
Activation energyb, Eact (kJ mol1)
Frequency factorb ko (min1mgi,CB1)
Temperature ranges (10–25% of CB conversion)
KOH/MgO (loose contact) KOH/MgO (tight contact) KNO3/MgO (loose contact) KNO3/MgO (tight contact)
545 440 440 421
180.5 181.9 169.5 175.4
7.0 10 10 5.3 10 12 4.9 10 10 4.9 10 12
489–514 399–415 402–424 379–402
a b
Temperature of maximum combustion rate. The kinetic parameters were calculated in the range of 10–25% of CB conversion.
With respect to the frequency factors (see Table 1), values of the order of 1010 were obtained for loose contact and 1012 for tight contact. 3.1.1. Isothermal experiments The isothermal CB combustion runs, performed at 380 8C utilizing tight-contact catalysts (Fig. 2) demonstrate that, although both materials are catalytically active, a higher reaction rate was reached, at this temperature, for KNO3/MgO, a result that is in agreement with the TPO curves of Fig. 1. Besides, with KNO3/MgO total consumption of CB was achieved after 40 min versus >60 min for KOH/MgO. 3.1.2. Carbothermic reduction From Fig. 3, it is seen that pure MgO was catalytically inactive in the absence of gaseous oxygen; in contrast, when potassium was present, as for the KOH/MgO and KNO3/MgO samples, these catalysts were able to induce CB oxidation even in the absence of gaseous oxygen. The CB combustion peak using KOH/MgO is more pronounced and it occurs at lower temperature, indicating that this catalyst is more active than KNO3/MgO (Fig. 3). The area under the KOH/MgO curve corresponding to carbothermic reduction is equivalent to 63% of the area under the peak obtained for combustion in air (Fig. 1); in contrast, for KNO3/MgO this area corresponds to approximately 15% (see Fig. 1).
3.2. Catalyst characterization 3.2.1. Atomic absorption spectroscopy (AAS) Table 2 shows the bulk metal contents measured by AAS analysis. The Mg and K concentrations are slightly lower in the KOH/MgO catalyst. The K/Mg ratio is similar for both catalysts. 3.2.2. BET specific surface areas The BET surface area of the KOH/MgO catalyst practically duplicates the BET area of KNO3/MgO (27.7 m2/g versus 15.2 m2/g). This result confirms that potassium introduction diminishes the surface area in comparison with pure MgO [16]. It also shows that for this type of reaction (catalyst-carbon black-O2) there is no direct relation between the BET surface area of the catalyst and the combustion rate of carbon black. In fact, the combustion rate is more affected by the number of contact points between catalyst and carbon black [8]. 3.2.3. X-ray diffraction The diffraction spectra are shown in Fig. 4. When K is added from KOH, the presence of Mg(OH)2, KOH, K2O and K2CO3 is observed. When the precursor is KNO3, the observed crystalline phases are KNO3, K2CO3, MgNO3 and MgO. However, a higher K2CO3/MgO ratio is observed for the KOH/MgO as shown in Fig. 4 from the intensity ratio of their corresponding peaks (2u = 38.44 and 26.86, respectively).
Table 2 Bulk (AAS) and surface (XPS) characterization of KOH/MgO and KNO3/MgO catalysts Catalyst Bulk characterization Mg (wt%) K (wt%) Atomic ratio K/Mg Surface characterization K 2p (at%) K/Mg O2/Mg CO32/Mg Mg 2s (B.E.)b O1s (B.E.) K 2p3/2 (B.E.) BET surface area (m2/g) a b
KOH/MgO
KNO3/MgO
45.9 2.31 0.03
51.40 2.80 0.03
1.12 0.04 1.89 0.10 87.9 530.6 292.7 27.7
1.86 0.06a 0.64 0.12 87.9 529.2 292.8 15.2
Same values measured before and after the soot combustion. B.E., binding energy (eV).
Fig. 2. Isothermal catalytic combustion of carbon black.
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Fig. 3. Carbothermic reduction of carbon black.
3.2.4. XPS Results of XPS analyses are presented in Table 2. For both catalysts, the K/Mg atomic surface ratio is higher than the bulk ratio (AAS). In addition, the KNO3/MgO catalyst possesses a higher surface concentration of potassium (1.86) and higher K/ Mg surface atomic ratio (0.06) than the KOH/MgO catalyst, with 1.12 and 0.04, respectively. Furthermore, for selected used (after soot combustion) KNO3/MgO catalysts the surface K/Mg ratio was measured, and as shown in Table 2, that ratio remained constant. The atomic surface ratio of O2/Mg is higher for KOH/MgO (1.89 compared with 0.64 of KNO3/ MgO). The binding energy (BE) associated with O2 for the latter was lower than the corresponding O2 BE of KOH/MgO (529.2 eV versus 530.6 eV). On the other hand, the BE of Mg 2s and K 2p3/2 were similar for both catalysts (87.9 eV). 3.2.5. DRIFTS experiments Results for KNO3/MgO in the presence of He are shown in Fig. 5A and B, while Fig. 6 shows the DRIFTS results in a reactive atmosphere. The results of similar experiments for the
Fig. 4. XRD of KOH/MgO and KNO3/MgO samples.
Fig. 5. DRIFTS and mass spectroscopy assays with KNO3/MgO in He at different temperatures: (A) DRIFTS; (B) mass spectrometry.
KOH/MgO catalyst, as well as the band assignments for the species associated with each IR band, were previously reported [16]. At room temperature, the presence of bridge-type (1762 cm1) and unidentate (1415, 1498 and 1060 cm1)
Fig. 6. DRIFTS assays with KNO3/MgO in reactive atmosphere (5%CO2/ 10%O2/He) at different temperatures.
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carbonates is observed (Fig. 5A). The disappearance of the wide band (2800–3600 cm1) and the shoulder at 1645 cm1 is evidence for water desorption and superficial bicarbonate decomposition. This is confirmed by CO2 and water formation at temperatures above 150 8C, as shown in Fig. 5B. With an increase in temperature, the 1498 cm1 band diminishes and, simultaneously, a band at 1035 cm1 appears. This is interpreted as decomposition of unidentate carbonates and their transformation to bridge-type carbonates. Decomposition starts at 200–300 8C, i.e., at lower temperatures than those where similar changes were observed for KOH/MgO catalyst [16], and is confirmed by the appearance of a CO2 peak close to 300 8C (Fig. 5B). For the same temperature interval, the intensity of the 3697 cm1 band (type I OH) diminished. The 3629 cm1 band, associated with type II OH, is also reduced with a rise in temperature. Generally, the presence of OH groups and their stability as temperature increases seems smaller for the KNO3/MgO catalyst. The bridge-type carbonate (1760, 1035 cm1), potassium carbonate (1415–1420 cm1) and the 1234 cm1 bands, associated most probably with potassium nitrate [18], are still present at 500 8C. DRIFTS experiments under reactive atmosphere for the KNO3/MgO catalyst (Fig. 6) show that the 1247 cm1 band starts to disappear above 450 8C. Moreover, the band associated with adsorbed superficial oxygen complexes interacting with alkaline metal (1107 cm1) starts to appear [18]. The unidentate carbonates seem to decompose and/or transform to bridge-type carbonates above 400 8C, as suggested by the reduction in the 1520 cm1 band and the displacement of the 1047 cm1 band to 1035 cm1. The wavelength zone where normally the OH groups are observed shows very low intensity bands. A 3626 cm1 band with a low intensity is observed even at 540 8C. 4. Discussion Catalysts generated by addition of potassium from two different precursors (KNO3 and KOH) showed qualitative differences in their surface properties. Different crystalline phases present in these catalysts are evidenced by their XRD patterns (see Fig. 4). Magnesium and potassium hydroxides are predominant in the KOH/MgO catalyst, while the nitrates of the same metals are predominant in the KNO3/MgO sample. The results presented in the foregoing sections showed that the already shown [16] beneficial effects of potassium addition using KOH are even better if KNO3 is selected instead Kprecursor. The details of such behavior are discussed below. 4.1. Catalytic activity and active sites The results of this work agree with those reported previously [16]. Addition of potassium clearly increases the catalytic activity, through a decrease of the activation energy, with respect to both pure MgO (Ea = 204 kJ/mol) and the uncatalyzed reaction (Ea = 254 kJ/mol). This confirms that potassium plays an important role in the rate-determining step of CB combustion, which is likely the dissociative O2
adsorption or the transfer of adsorbed oxygen from the catalyst surface to the CB surface [16]. For the KNO3/MgO catalyzed reaction the activation energy was slightly lower, suggesting the presence of catalytic sites that are even more active (Table 1). The higher activity of KNO3/MgO was observed in greater detail during isothermal experiments (Fig. 2), as manifested by the higher values of the maximum rate (ca. 50 mg/gi,CB/min) and the short time for complete CB conversion (40 min), although both catalysts showed significant activity at 380 8C. The activity of the KNO3/MgO catalyst is attributed to a higher concentration and an enhanced surface dispersion of the active species (Table 2). For both catalysts, the K/Mg surface ratio was higher than the ‘bulk’ atomic ratio, indicating that the alkaline metal is preferably deposited on the MgO surface. Nevertheless, both the surface atomic percent and the surface K/Mg mass ratio were higher in KNO3/MgO than in KOH/ MgO, as shown by the values in Table 2. Possible changes in the surface K/Mg ratio of the catalyst after soot combustion, as reported in the literature [7], were checked, and no change was found as shown in the values of Table 2. This should be the expected result if the samples have been calcinated, prior to soot combustion, to a significantly higher temperature than the reaction temperature. In fact, the possible effect of calcination temperature and time on catalyst activity was discussed in a previous paper [16]. 4.2. Surface active species In a previous paper [16], it was shown that potassium presence increases the abundance of oxygen on the catalyst surface and enhances the reactivity of this superficial oxygen; this occurs in both, the KOH/MgO and KNO3/MgO catalysts. DRIFTS assays, both in He and in reactive atmosphere, for KOH/MgO [16] and KNO3/MgO (Figs. 5 and 6) show the presence of adsorbed superficial oxygen complexes (bands at 1100–1120 cm1) and more specifically O2 species observed in MO2 complex formed by reaction of the alkali metal with O2 [16,18]. However, it was suggested that the reaction rate was affected not only by the abundance of surface oxygen (better for the KOH/MgO catalyst, as shown by XPS measurement of 1.89 versus 0.64 for the KNO3/MgO catalyst), but also by the reactivity and mobility of the surface oxygenated species (better for the KNO3/MgO catalyst). Thus, in the presence of gaseous oxygen, the reaction rate is controlled to a greater extent by the reactivity and mobility of the superficial oxygenated species than by their concentration. The previous suggestions are also in agreement with the proposed mechanism [16], where the ratedetermining step of CB combustion was associated with the transfer of adsorbed oxygen from the catalyst surface to the CB surface. The oxygenated surface species are thus more mobile and more reactive in the KNO3/MgO catalyst than in KOH/MgO. As to why the adsorbed oxygen species are more reactive and mobile on the KNO3/MgO catalyst than in the KOH/MgO
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catalyst, this is attributed to the K presence on the surface, which is higher on KNO3/MgO than in KOH/MgO (see Table 2). As is known, the good electron donor property of K favors the surface adsorption of gaseous oxygen and its dissociation, enhancing at the same time the reactivity of the lattice oxygen due to the weakening of the Mg–O bonds on the catalyst surface. The catalyst with the higher surface concentration of potassium (KNO3/MgO in our case) will contain Mg–O bonds that are weaker and thus oxygenated surface species that are more reactive and probably more mobile as well. In the absence of gaseous oxygen (carbothermic reduction assays), MgO was inactive [16], while the catalyst with a higher O2 surface concentration (KOH/MgO) showed higher catalytic activity (Fig. 3), although complete conversion of the initially loaded CB was not reached. The total CB consumption ratio (63%/15%) turns out to be proportional to the atomic O2/Mg surface ratios (1.89/0.64); this implies, as intuitively expected, that the presence of surface oxygen species is rate-limiting in CB oxidation in the absence of O2(g). Summing up, in the absence of gaseous oxygen the abundance of surface oxygen is the main factor affecting the reactivity and rate of reaction, while in gaseous oxygen presence, the reactivity and mobility of the surface oxygenated species are the predominant factors. A relevant matter to this discussion is that surface nitrates could directly oxidize soot particles. However, as shown below, the amount of KNO3 that is present in the catalyst during the TGA experiments (the catalyst/CB ratio was 4/1) does not explain the total CB consumption that was measured. The influence of catalyst precursor anion was studied in catalysis of water vapor gasification of carbon by potassium [1]. In particular, several reactions were determined as possible for the oxidation of graphite with KNO3; simultaneous thermogravimetry and differential thermoanalysis in argon were performed using a mixture composed of 120.7 mg of potassium nitrate and 126.9 mg of graphite. A weight loss of 50 mg was measured between 450 and 500 8C (Fig. 6), that is a 39.4% of the carbon content. In the TGA assays of this work, the KNO3/CB ratio is four times smaller than in those experiments [1]; however, all the carbon black is combusted below 600 8C. Thus, it is improbable a key role of KNO3 in the CB oxidation. In fact, from the included calculations it follows that the maximum available amount of nitrate in the catalytic assays would account for less than 10% of the total carbon black consumption, if the entire available nitrate were to be fully utilized in the oxidation of carbon black. The comparison among these results has been summarized in the following calculations. For the latter, all K is assumed in the form of KNO3, which obviously is not true. In spite of that, and with only 1/3 of the KNO3 amount assayed in the referred work [1], the present experiments showed 100% of soot conversion. That proves that KNO3 does not play a significant role in the direct soot oxidation and the role of K as a catalyst is validated.
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Calculation of possible soot oxidation by surface KNO3 KNO3/MgO catalyst (this work) K (wt%) C (weight) Catalyst/C = 4/1, for combustion KNO3 KNO3/C
After Hu¨tinnger and Minges [1] 2.80 (by AAS) 1.50 mg
0.435 mg 0.290
CB conversion in TGA 100%
KNO3 KNO3/C
1.427 mg 0.951 (between 450 and 500˚ C)
CB conversion in TGA 39.4%
4.3. Surface carbonates and OH groups The results of DRIFTS assays coupled with mass spectroscopy (Figs. 5 and 6), both in this work and in our previous report [16], show that carbonate decomposition occurs at a lower temperature for the KNO3/MgO catalyst, that is, over this catalyst surface carbonates are less stable. This experimental fact indicates that KNO3 generates a less basic surface than KOH. The anion nature of the potassium precursor may cause this difference. In fact, for coal gasification the anion may have more effect on catalytic activity than the alkaline metal cation, as previously discussed [14]. Our results show a greater activity of the KNO3/MgO with respect to the KOH/MgO in the catalytic combustion of carbon black. As described before, Lang’s [14] study of the ‘‘anion effect’’ in the catalytic gasification of coal also found that, KNO3 was more active than KOH, a result that contradicted his hypothesis about the competition between the anion and the char to form an alkaline–carbon complex. Therefore, other aspects, probably related to the basicity of the catalytic surface and the mobility of the active phases seem to play a relevant role. The greater stability of carbonates and hydroxylic groups on the surface of the KOH/MgO catalyst may limit the ease of formation of the (re)active sites during combustion. Furthermore, the CO2(g) formed during combustion may interact more strongly with the KOH/MgO surface due to its higher basicity, thus further reducing the number of available catalytic sites. The higher concentration and stability of OH groups and carbonates over KOH/MgO may reduce the mobility of surface oxygen. Thus, a lower basicity of KNO3 is an important factor responsible for higher catalytic activity. Conversely, the bonding and adsorption of oxygen is greater over a more basic (high electronic density) surface such as KOH/MgO, as evidenced by the higher surface concentration of O2 (greater ratio O2/Mg) and the higher binding energy associated with this species (530.6 eV) in the KOH/MgO catalyst. This stronger interaction hinders the mobility of surface oxygen. 4.4. Effect of contact type Fig. 1 and Table 1 allow us to estimate the sensitivity of both assayed catalysts to the type of contact with CB. This is a
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relevant factor as the conditions to be found in an actual catalytic filter will be probably much closer to a ‘‘loose’’ than to a ‘‘tight’’ contact. The KOH/MgO catalyst is seen to be more sensitive than KNO3/MgO. This is attributed both to a higher surface concentration of potassium in the KNO3/MgO catalyst and a higher mobility of the surface oxygen activated on this catalyst. As the combustion reaction progresses, it is increasingly difficult to maintain constant the number of contact sites at the catalyst-CB interface. Thus, the activity of a catalyst having a higher concentration of active sites and/or a higher mobility of its active phase(s) will be less affected by such an expected decrease in the number of its contact points with CB. This appears to be the case of the KNO3/MgO, whose higher measured activity is attributed to a greater mobility of the O2, as discussed in Section 4.2.
complete conversion, at least for the assayed mass ratio of catalyst to CB of 4:1. These surface oxygen species, activated on the catalyst surface, are the main oxidizing agents in the presence of gaseous oxygen and their mobility appears to be essential for effective catalysis; this explains why KNO3/MgO was more active in air.
Acknowledgements The support of FONDECYT-Chile Grant No. 1030296 and short stays financing from the MECESUP Program Grant UCO0108 (for RJ) are gratefully acknowledged. The ‘‘Fonds National de la Recherche Scientifique’’ of Belgium and the ‘‘Re´gion Wallonne’’ are also thanked for the acquisition of the XPS and the DRIFTS equipments.
5. Conclusions The addition of the same amount of potassium from different precursors (KOH, KNO3) generates catalysts that are qualitatively different, which is in agreement with several studies carried out about the ‘‘anion effect’’ on the catalytic gasification of coal. The catalysts have surfaces of different basicity. This affects the interaction of the active species with the support surface and is reflected in the different behavior of these catalysts in carbon black combustion. With KNO3 as potassium precursor, a more active catalyst was obtained for the combustion of CB with air. This result is attributed to several factors: (i) a higher concentration and surface dispersion of the potassium that is associated with the active phase of the catalyst, (ii) a higher mobility of the oxygen species activated on this catalyst, and (iii) a lower stability of surface carbonates and OH groups, which facilitates the formation of active species during combustion and favors the mobility of the activated surface oxygen species. These findings highlight the following mechanistic aspects of potassium-catalyzed soot oxidation: (i) potassium from both precursors activates the lattice oxygen on the catalyst surface, thus enhancing its combustion reactivity. Surface oxygen is more abundant on the surface of KOH/MgO explaining its higher activity in the absence of O2(g). (ii) The presence, adsorption and dissociation of gaseous oxygen on the catalyst surface are indispensable steps for combustion to proceed until
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