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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.50 no.4 Concepción Dec. 2005

http://dx.doi.org/10.4067/S0717-97072005000400004 

 

J. Chil. Chem. Soc., 50, N° 4 (2005), págs: 651-665

 

CaO-MgO CATALYSTS FOR SOOT COMBUSTION: KNO3 AS SOURCE FOR DOPING WITH POTASSIUM.

 

ROMEL JIMÉNEZ, XIMENA GARCÍA, ALFREDO L. GORDON*

Depto. de Ingeniería Química, Universidad de Concepción, Casilla 160-C, Correo-3, Concepción, Chile. Fax (56) (41) 247491. *e-mail: algordon@udec.cl


ABSTRACT

MgO, CaO, an equal ratio mixture of the oxides (50-CaO) and a sample with 10% potassium added to the previous mixture, showed increasing activity for the catalytic combustion of carbon black, a model compound for soot. These catalysts were characterized by XPS, BET, DRIFTS and AAS. Activity was measured by TGA and by TPO; CO and CO2 concentration in the outlet gas flow were measured by NDIR.

Pure MgO was barely active. The 50-CaO catalyst was as active as the pure CaO in spite of a lower content of calcium. This was attributed to greater contact area and better dispersion of calcium, lesser bulk and surface carbonation, cooperative effect of magnesium that diminishes the stability of adsorbed surface compounds, and a greater accessibility of O2(g) to calcium sites.

Potassium introduction over the 50-CaO produced high activity, related to a increase of the surface concentration of oxygen on the catalyst and larger CO2/CO ratio in the exit gases. Moreover, potassium addition resulted in a lower content of calcium on the surface, hence less carbonation of the catalyst.

Keywords: soot, catalytic combustion, CaO-MgO mixtures, potassium


1. INTRODUCTION

Soot in the exhaust gases of Diesel engines, mostly made up of solid carbonaceous particles smaller than 10mm (PM10), is very damaging to human health. Soot elimination may be achieved by catalytic filters located at the exit of the exhaust gases, where the soot particles could be trapped and subjected to immediate catalytic combustion [1,2]. Catalysts should promote the combustion in the filter at the exhaust gas temperatures (180 - 400ºC), as the uncatalyzed combustion occurs above 550ºC.

Catalysts for soot combustion should exhibit high activity (capacity to lower the combustion temperature), and to burn the soot particles at the same rate of their deposition on the filter; otherwise, the pressure drop in the filter may build up to unbearable values. The catalysts must also exhibit high thermal stability, an extended period of activity and resilience to poisoning by usually present steam, CO2 and SO2 [3,4]. Besides, it is desirable that the catalyst may enhance the reduction of NOx, which is also present in the exhaust gases.

Several materials have been assayed for the catalytic combustion of soot [4, 5], particularly alkali and alkaline-earth metals [2,5], V and Co [1,6,7], and noble metals [8,9]. The present work deals with the utilization of CaO- and MgO-based catalysts of proven activity for combustion and gasification of carbonaceous materials. The catalytic activity of highly dispersed calcium has been shown for coal and char gasification [3,10-12]. The use of labeled O2 and secondary ion mass spectrometer (SIMS) showed the formation of surface species CaO(O), generated by dissociative chemisorption of O2(g) on CaO [13]; the active oxygen from CaO migrates to the carbon surface to form C(O) which desorbs as CO(g). As vacant sites (Cf) around CaO are replaced by C(O), most of active oxygen cannot migrate, but they react only with the C(O) on the interface between carbon and CaO, followed by CO2 evolution [11]. This latter reaction is faster than C(O) decomposition and accounts for the greater CO2 generation in the catalyzed reaction; moreover, Ca addition does not change the nature of the C(O) sites, but it increases its surface concentration [11,12].

TPD and mass spectroscopy of Ca-carbon samples have been used to determine the mechanism of calcium as a catalyst and its deactivation in the CO2 gasification of carbon [10,14]. The several peaks from the TPD spectra were interpreted as different Ca-carbon contact zones and a proposed model considers that only catalyst-carbon interfaces are responsible of the activity observed during the gasification assays, while the external surface of the CaO (without contact with the carbon) is inactive.

Querini et al. [2,6,7] have studied catalysts based on Co+K/MgO in which the MgO is a suitable support but has low catalytic activity; these catalysts have shown fair activity in soot combustion. The authors attribute significant effects to the addition of potassium to the Co/MgO catalyst, such as: i) to increase the reducibility of Co, thus weakening the Co-O bond and facilitating soot oxidation by a redox mechanism, ii) to favor the catalyst-soot contact by the formation of low-melting compounds, iii) to facilitate the formation of surface intermediate carbonates during soot combustion.

Neeft et al. [5] reported that CaO shows a moderate activity for soot combustion but it is affected by carbonation. In relation to this carbonation, CaO-MgO mixtures have been used for the catalytic gasification of tars and naphthalene, showing a synergic effect that is attributed to the cooperation provided by the MgO to the CaO in promoting the formation of unidentate carbonates instead of more stable bidentate species; this facilitates the above mentioned gasification [15,16].

As in oxidation, gasification of carbon takes place through the formation of C-O surface complexes that, subsequently, desorb completing the catalytic cycle; that leads to consider these catalysts also for soot oxidation. In fact, the literature supports the reasonable expectation that catalytic oxidation of soot should occur by a oxidation-reduction mechanism, with the formation of intermediate surface complexes C(O) [1,2,4,14,17].

This work addresses the activity evaluation of a CaO-MgO mixture in the catalytic combustion of carbon black (CB) used as a model compound for soot. The work compares the results from this mixture and the results from the pure oxides. As in catalytic gasification of tars, it was expected that the MgO in the CaO-MgO mixtures would cooperate by freeing the CaO surface of the more stable bidentate carbonates which are formed. This is the main subject of this work, as these and other catalysts have been tried before, but no such mixture has been assayed for the catalytic combustion of soot; additionally, the effect of potassium loading over a relatively successful CaO-MgO mixture was also assayed. The catalysts were evaluated by XPS and DRIFTS; evidence is sought in these analyses for the confirmation, or rejection, of the mechanisms outlined above.

2. EXPERIMENTAL

2.1. Catalyst preparation

For the MgO and CaO catalysts, distilled water and commercial oxides (Merck, 98% purity) were mixed to incipient wetness. The slurries thus formed were dried at 105 ºC for 24 h and calcined for 30 min at 760 ºC.

Two catalyst mixtures were prepared by impregnation, namely a sample with 50% each of CaO and MgO (labeled 50%) and a sample with 10%K added to a CaO-MgO mixture with the same weight ratio CaO/MgO = 50/50 (labeled K/50-CaO). Calcium acetate (Merck) was dissolved at 60°C in distilled water in a rotavapor (Laborota 4002) and pure MgO powder (Merck) was added to the solution. Potassium was added from aqueous solutions of KNO3; this mixture was stirred for 30 min at 60°C. Samples were dried at 105 ºC for 24 h and calcined for 30 min at 760ºC.

2.2. Catalytic activity

TPO experiments. Catalytic activity was measured by temperature-programmed oxidation (TPO) in a fixed bed reactor. Carbon black (CB, Monarch 430, BET area: 80 m2/g, mean particle diameter: 27 nm and density: 460 kg/m3) was used as a model for soot. This choice of model compound is mainly based on the low volatile content of carbon black. Therefore the results of its catalytic combustion must be viewed as conservative as compared with the combustion of Diesel soot, as the latter has a higher volatile content, hence a lower combustion temperature [18]. A 4:1 catalyst/CB ratio was used and 'tight contact' was achieved by accurately mixing 60 mg of catalyst and 15 mg of CB in an agate mortar during 5 minutes prior to each combustion assay. The catalyst-CB powder was mixed with 800 mg of inert SiO2-grains (250 - 400 mm) to prevent thermal runaways and reduce the pressure drop of the fixed bed reactor. A constant flow of 800 mL/min of air plus He with 10% O2 was used as the reacting gas. The temperature was increased at 10ºC/min to 700ºC. The CO and CO2 concentration in the outlet gas flow were analysed by a HORIBA NDIR analyser.

TGA experiments: The catalytic tests were also performed in a thermogravimetric apparatus (Netzsch 409 PC). A 4:1 catalyst/CB ratio was used and 'tight contact' was achieved by mixing 4 mg of CB with 16 mg of catalyst in an agate mortar during 2 minutes prior to the combustion assay. This corresponds to contact conditions that are more 'intimate' compared with what is achievable in a catalytic soot trap, but they were used because they ensure a much higher degree of reproducibility and thus provide a good basis for activity screening studies. Samples with 7.5 mg of catalyst-CB mixture (with 1.5 mg of CB) were heated in 180 mL/min of air (Indura) at 10 ºC/min to 800 ºC.

2.3. Catalyst characterisation

XPS: XPS was performed with a SSI X-Probe (SSX-100/206) spectrometer from Surface Science Instruments (Fisons) equipped with a monochromatized microfocused 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 approximately 1000 mm in diameter. Under these conditions, the energy resolution estimated by the Au 4f7/2 full width at half maximum (FWHM) of standard gold sample was 1.1 eV. 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 baseline (Shirley type) (performed with CASA-XPS program).

BET surface area: The BET surface area of the catalyst samples was determined using a conventional flow apparatus (Micromeritics Flowsorb 2130) by nitrogen adsorption at 77 K.

DRIFTS: An IF S55 Equinox (Bruker) spectrophotometer equipped with an air cooled KBr source and a MCT detector was used. Samples were located in a thermally and environmentally controlled chamber (SpectraTech 0030-103). The sample (catalyst/CB mixture) was heated at 10 °C/min under 30 mL/min of reactive gas (20%O2/He) and spectra (200 scans with 4 cm-1 resolution) at different temperatures were registered ("in situ" reaction). The gases flowing out from the DRIFTS cell were analyzed in a Balzers Thermostar GSD300T2 quadrupole mass spectrometer QMS200, connected on line to the DRIFTS apparatus.

Atomic Absorption Spectroscopy (AAS): A Hitachi Z-8100 spectrometer with Zeeman polarization was used to determinate catalyst composition.

3. RESULTS

3.1. Catalytic activity.

3.1.1. TPO experiments. Figure 1 shows the evolution curves for CO and CO2 from the TPO assays. The temperature at which combustion of carbon black occurs at the maximum rate (Tm) was taken for comparison of catalytic activities (CO2 and CO peaks). Thus, lower values of Tm are associated with a higher catalytic activity.


Figure 1. CO and CO2 evolution during temperature programmed oxidation (TPO) of CB.
(A) Uncatalyzed reaction, catalyzed combustion with (B) MgO, (C) CaO, (D) 50-CaO and (E) K/50-CaO.

Table 1 shows values of Tm and the total molar ratios of CO2 to CO generated, calculated from the area under the curves of Fig. 1. The uncatalyzed reaction of the CB starts around 500ºC, but its maximum takes place above 650ºC (Figure 1-A). The uncatalyzed reaction has a selectivity of 50%, which is in the order of previously reported values for the combustion of other carbon blacks (39%), obtained by Serra et al. [18].


Table 1. Temperature at maximum reaction rate (Tm) and CO2/CO ratios from TPO assays.

Catalyst

Tm (ºC)

Molar ratio CO2/CO


Carbon black (CB)

(Uncatalyzed reaction)

679

1.0

MgO

656

1.7

CaO

561

8.4

50-CaO

563

8.3

K/50-CaO

430

9.0


As shown in Fig. 1-B, the catalytic activity of MgO is modest, as previously found by other authors [5,6]. However, MgO presence improves the selectivity towards CO2 by 13% (Table 1). On the other hand, when using pure CaO as catalyst, Tm is displaced to lower temperatures (Fig. 1-C), showing a significant activity increase and a huge increase in the CO2/CO ratio. Furthermore, a second CO2 peak is observed above 600ºC, without a companion CO peak. This is attributed to decarbonation of the catalyst, as will be shown in Section 4.1.

The 50-CaO catalyst (Fig. 1-D) exhibited similar activity to that of CaO, as shown by Tm values and CO2/CO ratios in Table 1. This catalyst also showed a double peak for CO2; however, the area under the second peak is smaller than for CaO, confirming bulk carbonate formation and decomposition related to Ca content, as will be discussed in section 4.1 (Figures 1-C and D).

Addition of 10% K to the 50-CaO catalyst (labeled as K/50-CaO with equal CaO/MgO mass ratio as the 50-CaO catalyst) increased markedly the activity, as shown by the decrease of Tm by more than 120 ºC, and a rise in the CO2/CO selectivity (Table 1).

3.1.2. TGA experiments: Figure 2-A,B shows the TGA curves for the catalytic combustion when CB and the catalyst are mixed in 'tight contact'. The curves were normalized with respect to the initial mass of the CB; hence, all areas under the CB consumption curves are the same as the area under the uncatalyzed curve (Fig. 2-B). Temperature Tm in these TGA curves has the same meaning as in the TPO assays, and it corresponds to the temperature of maximum weight loss.


Figure 2. Rate of weight loss in TGA of carbon black (CB) combustion.
[A] MgO, CaO and 50-CaO catalyzed combustion; [B] K/50-CaO catalyzed combustion and uncatalyzed reaction of CB.

Figure 2-A summarizes the results for the catalyzed reactions with MgO, CaO and the 50-CaO mixture. The curve for MgO shows a slight weight loss around 300ºC that may be due to decomposition of surface groups formed while exposed to the atmosphere, such as hydroxyls and carbonates of low stability [15,16,18]. The activity of the MgO for the catalytic combustion of CB is weak, as the combustion peak is only displaced by approximately 30ºC.

The curves corresponding to the catalytic combustion using CaO and the 50-CaO catalyst show a weight loss (which is greater for the CaO) in the 350-400ºC range. This loss is attributed to the decomposition of surface hydroxyl and carboxyl groups [15,16,18] that may be formed by the contact of the catalyst with atmospheric CO2 and humidity. Moreover, these same curves show a weight increase between 400 and 500ºC (also greater for CaO) that is attributed to adsorption of the CO2 produced when the CB combustion starts. This result is in agreement with the widely reported tendency of CaO to carbonation [12,14-16], a phenomenon that increases with the calcium content of the catalyst. This will be further described and discussed in Section 4.1, in conjunction with the results of the DRIFTS assays.

A second zone of weight loss is observed in the 500-600ºC range (this time greater for the 50-CaO catalyst) and this is attributed to the catalyzed combustion of the CB. Finally a third zone with a peak above 700ºC was recorded (far more intense for the CaO); this is attributed to desorption of the more stable carbonates formed on the CaO-containing catalysts (Fig. 2). This interpretation will be further supported by the results discussed in Section 4.1.

Figure 2-B shows curves for the uncatalyzed reaction and the catalytic combustion with K/50-CaO. The uncatalyzed combustion starts above 500ºC with a peak of maximum rate at 657ºC while the catalyst with potassium was very active, displacing the combustion peak to 437ºC. Also, in this latter case a second peak (600-750ºC) is observed in the same range of temperatures as for the CaO and 50-CaO catalysts, although this peak is less intense.

Table 2 summarizes the temperatures of all peaks observed in the TGA experiments and the mass variation corresponding to those peaks, calculated from the area under the curves.


Table 2. TGA experiments (Figure 2). Temperatures of peaks (T#) and corresponding masses (M#) (from area calculation of peaks).

 

Peak N°1

Peak N°2

Peak N°3

Peak N°4

Sample

T1 (ºC)

M1 (mg)

T2 (ºC)

M2 (mg)

T3 (ºC)

M3 (mg)

T4 (ºC)

M4 (mg)


CB
(Uncatalyzed)

657

1.455

-

-

-

-

-

-

MgO

317

0.183

627

1.639

-

-

-

-

CaO

380

0.165

474

0.443

545

0.470

724

1.892

50-CaO

397

0.086

437

0.107

546

0.891

710

1.242

K/50-CaO

437

1.291

727

0.500

-

-

-

-


With CaO as catalyst, decomposition at about 400°C (peak Nº1) nearly doubles the weight loss with respect to the 50-CaO catalyst. This suggests that the formation of compounds on the catalyst at ambient temperature (which decompose around 400ºC) occurs proportionally to the CaO content in the catalyst.

A similar comparison of peak Nº2, which corresponds to adsorption of CO2 (Table 2 and Fig. 2-A) for CaO and 50-CaO shows a much greater mass increase for the CaO-catalyzed reaction. This also suggests a relation between the CaO content of the catalyst and its tendency toward carbonation in the presence of CO2.

The temperatures corresponding to peak Nº3 for CaO and 50-CaO were similar (545 y 546°C, respectively) and they correspond to the Tm values for these catalysts, exhibing almost identical activity. The weight loss observed above 600ºC (peak Nº4), which is related to decomposition of the more stable carbonates, was larger for the CaO-catalyzed reaction (1.892 mg) confirming that carbonate formation increases with the CaO content of the catalyst. Weight loss occurred above 600ºC for the reaction catalyzed by the K/50-CaO, and it was much lower than for CaO or 50-CaO (Fig. 2-B and Table 2), evidencing less carbonation of the K-containing catalyst.

3.2. BET surface area and XPS.

As observed in Table 3, the BET surface area for the catalysts without K decreases with the CaO content, that is, MgO > 50-CaO > CaO, a result that agrees with previous results for similar catalysts by Alarcon et al. [16]. Potassium introduction diminished drastically the BET surface area of the catalyst, when compared with the 50-CaO catalyst (equal CaO/MgO ratio), a result that was also reported by Querini et al. [6,7] for K/MgO catalysts.

Table 3. Surface characterization by XPS and BET surface area.

Catalyst

MgO

CaO

50-CaO

K/50-CaO

50-CaO (used)

K/50-CaO (used)


C1s (CO3-2) (% at.)

5.78

12.2

9.71

7.35

6.07

4.08

O2-  (% at.)

12.3

0

0

25.46

0

0

Ca/Mg

-

-

0.57

0.12

0.91

0.50

(CO3-2)/ Mg

0.20

-

0.80

0.42

0.91

0.43

O2-/Mg

0.43

-

0

1.44

0

0

K/Mg

-

-

-

0.56

-

0.25

K/Ca

-

-

-

4.61

-

0.49

BET area (m2/g)

61.7

37.0

42.5

6.9

n.d.

n.d.


n.d.: Not determined

The XPS surface characterization for fresh catalysts is summarized in Table 3, as well as the XPS analysis for 50-CaO and K/50-CaO samples after being used in catalytic combustion of CB in the fixed reactor (TPO assays).

For fresh catalysts (without alkali metal), the surface content of carbon linked to carbonates (binding energy for C1s ª 289 eV) increases as the calcium content increases, being the highest for pure CaO (Table 3).

With regard to the surface concentration of oxygen species O2- (associated with a b.e. O1s < 531 eV) a surface atomic percentage of 12.3% is observed for MgO, while none was found in the CaO and 50-CaO catalysts. The introduction of the alkali metal (K/50-CaO) increased the surface oxygen concentration (of O2-) to 25.46% and it increased the O2-/Mg ratio (Table 2).

The Mg2s binding energy in the MgO (88.4 eV) is slightly lower than in the 50-CaO mixture (88.9 eV), while the binding energies of the Ca2s in the CaO and the 50-CaO are almost identical (438.4 and 438.5 eV, respectively). Moreover, the surface (XPS measured) Ca/Mg atomic ratio of 0.57 for the 50-CaO catalyst was lower than the bulk ratio (0.72), which was calculated from the CaO/MgO mass ratio of 1 in the catalyst. The implications of this finding are discussed in Section 4.1.

Values in Table 3 also show that potassium introduction in the CaO-MgO mixture resulted in a lower surface content of Ca atoms (2.16 %) and a lower Ca/Mg ratio (0.12) compared with the 50-CaO sample (6.87 and 0.57, respectively).

The bulk atomic ratio Ca/Mg was 0.73 in the K/50-CaO catalyst (as determined by AAS), a value that is similar to the theoretically expected value of 0.72, while the bulk atomic K/Mg ratio was 0.246. Moreover, potassium introduction reduced slightly the binding energy of the calcium to 438.1 eV, while the binding energy of the Mg2s was, again, the same value as in pure MgO (Table 2).

The last two columns of Table 3 show the surface characterization (XPS measured) of the 50-CaO and K/50-CaO samples after a TPO assay. When compared with the results of the fresh samples, it is found that the surface content of carbonates is lower for the used catalyst. Furthermore, for both catalysts the binding energies of Ca2s and Mg2s, as well as the Ca/Mg ratio, increase in the used catalysts with respect to the fresh one. For the 50-CaO catalyst the surface Ca/Mg ratio was higher than the bulk ratio (0.91 vs. 0.72), and for the K/50-CaO catalyst this ratio also increased (0.5 vs. 0.12) but still was lower than the bulk ratio of 0.72. For this K/50-CaO catalyst, a significant reduction of the surface content of potassium was observed after combustion.

3.3. 'In situ' DRIFTS of carbon black combustion

The 'in situ' DRIFTS assays of the combustion reaction using four catalysts are shown in Figures 3 - 6, and Table 4 lists the species assigned to each IR band.


Figure 3. [A] 'In situ' DRIFTS assays during the catalytic combustion of CB with MgO. [B] Mass spectroscopy of gases from the DRIFT cell.

The spectra obtained at ambient temperature show similar bands in the 1300-1600 cm-1 range for all catalysts, corresponding to different species of carbonates formed due to the basic nature of the samples and their affinity for ambient CO2 [15,16,18]. The wide band at 2800-3600 cm-1 and the shoulder at 1645 cm-1 are characteristic of adsorbed water and the bands are more intense in the presence of MgO; bands in the 3600-3750 cm-1 correspond to several surface hydroxyl groups. Furthermore, a wide band in the 1100 - 1200 cm-1 range was found for all samples, with the maximum peak displaced to higher frequencies in the order MgO > CaO > 50-CaO > K/50-CaO.

A band at 3585 cm-1 associated to carboxyl groups is observed for the 'in situ' reaction with MgO at 300°C (Fig. 3-A) and the band at 920 cm-1 corresponds to the lattice oxide. The weak band that appears at 3730 cm-1 from 400°C corresponds to OH- type I, which has a strong nucleophilic character, masked at lower temperatures by the wide band of adsorbed water. The intensity of the unidentate carbonate bands (1410 - 1540 cm-1) also starts to diminish above 400°C and the band at 1080 cm-1 is displaced to 1035 cm-1. Bands at 1200 and 920 cm-1 did not change during this assay.

The mass spectroscopy of gases from the DRIFTS cell (Fig. 3-B) shows the presence of water around 300°C and a CO2 peak. A second increase in the CO2 concentration above 450°C corresponds to the onset of carbon black combustion.

The 'in situ' reaction with CaO (Fig. 4-A) at ambient temperature shows a shoulder-type band associated with type II OH- groups (3695 cm-1) and a second very intense band associated with type III OH- (3633 cm-1) which, together with the band at 3540 cm-1, start to disappear above 400°C.


Figure 4. [A] 'In situ' DRIFTS assays during the catalytic combustion of carbon black with CaO. [B] Mass spectroscopy of gases from the DRIFT cell.

The band at 3633 cm-1 completely disappears at about 500°C. The widening of the band located at 1400-1600 cm-1 starts above 400°C, indicating the formation of bidentate carbonates [1]. A band at 877 cm-1 is observed (not found in the assay with MgO), which corresponds to calcium carbonate (calcite); this band becomes more intense above 400°C. At ambient temperature, bands at 923 cm-1 (lattice oxide), 1787 cm-1 (bridge-type carbonates) and 1070 cm-1 (unidentate carbonates) are present and remain above 550°C. A set of bands in the range 2800 - 2900 cm-1 may be related to aldehyde and formate groups; they become stronger at higher temperatures. The band at 2500 cm-1 may be associated either with linearly adsorbed CO2 or to carboxyl groups with a H-bridge.

The mass spectrometer signal (Fig. 4-B) indicates the release of water and CO2 above 350°C. The CO2 concentration increases slightly between 350 and 450°C; it increases abruptly above 450°C, indicating the onset and development of the CB combustion reaction.

Figure 5-A shows the DRIFTS results for the 'in situ' reaction with the 50-CaO mixture. The 1789 cm-1 (bridge-type carbonates), 1070 cm-1 (unidentate carbonates), 920 cm-1 shoulder (lattice oxide) and 877 cm-1 (calcite) bands are observed over the whole range of assayed temperature. The strong band observed at 3633 cm-1 (OH groups) completely disappears in the 400 - 450°C range, while the type II-OH- band (3693 cm-1) that was so strong at ambient temperature becomes negligible at 350°C. The width of the band associated with the various carbonate species (1400-1600 cm-1) increases slightly over 450°C. As with the CaO sample, the bands close to 2500, 2800 and 2900 cm-1 are associated with different species of carboxylic, formate and aldehyde groups, respectively.


Figure 5. [A] 'In situ' DRIFTS assays during the catalytic combustion of carbon black with 50-CaO. [B] Mass spectroscopy of gases from the DRIFT cell.

Mass spectroscopy assays with 50-CaO (Fig. 5-B) show water peaks between 300 and 500°C, a temperature zone where coincidentally with the DRIFTS assays the bands associated to OH groups disappear. The CO2 curve abruptly increases above 450°C, indicating the development of carbon black combustion.

Figures 6-A and 6-B show the DRIFTS results for the 'in situ' reaction with K/50-CaO. A small band at 3740 cm-1, which corresponds to OH- type I groups, is observed at ambient temperature and remains throughout the assayed temperatures, while the 3633 cm-1 band (OH- type III groups) completely disappears above 450°C.


Figure 6. [A] 'In situ' DRIFTS assays during the catalytic combustion of carbon black with K/50-CaO. [B] Mass spectroscopy of gases from the DRIFT cell.

In the region where the various carbonates species absorb (1300-1600 cm-1), a typical strong split band for unidentate carbonates, with maxima at 1396 and 1456 cm-1, is observed. This band becomes a single and even stronger signal at 200°C. The 875 cm-1 band (calcite), adjacent to the 920 cm-1 shoulder associated with lattice oxide, is also observed. Moreover, bands at 1740 and 1770 cm-1 were also recorded. The band at 1740 cm-1 might correspond to n(CO) vibration from aldehydes and/or H-bridge carboxylic acids; this is confirmed by the n(CH) and n(OH) at 2900 and 2790 cm-1, respectively. The 1770 cm-1 band (corresponding to bridge-type carbonates) becomes stronger above 450°C.

Mass spectroscopy for the K/50-CaO catalyst (Fig. 6-B) indicates two ranges of water liberation. The first from 100 to 350°C, with small amounts of CO2 liberated at around 150°C and 250 to 350°C, and the second that starts around 350°C might be related to the decomposition of type III OH- groups observed in DRIFTS assays (Fig. 6-A). At the latter temperature, a strong CO2 release begins, with a maximum around 450°C. A new increment in CO2 concentration begins at 500°C.

It is worthwhile to point out that the increase in intensity above 400°C in all these DRIFTS assays must be cautiously considered as the sample absorbs less radiation because of its lower carbon black content subsequent to the 'in situ' combustion. Although the latter does not necessarily imply an increase in the associated species, a decrease in band intensity can be interpreted as a disappearance of the corresponding surface compound.


Table 4. IR bands assignments for DRIFTS analyses presented in Figures 3 to 6.

Wavenumber (cm-1)

Species

Reference

3750 – 3630

OH groups

[20]

3600 – 2800

Adsorbed water

[15,16]

3650 – 3500

n(OH)  carboxylic groups

[16]

3000 – 2900

n(CH)  aldehydic groups

[16]

2840, 2811, 2750

Different formates

[15,16]

2700 – 2500

n(CO)  carboxylic groups with H-bridge

[15]

2500 – 2250

Linearly adsorbed CO2

[19]

1789 – 1760, 1035

Bridged-type carbonate

[15]

1760 - 1700

n(CO)  carboxylic groups
n(CO)  aldehydic groups

[11]

1630 – 1660 shoulder

Adsorbed water
Bicarbonate

[15]
[19]

~1600, ~1300

Bidentated carbonate

[15,16]

1540 – 1480, 1410, 1070

Unidentated carbonate

[15,19]

1480 – 1400

Potassium carbonate

[21]

1588, 1247 – 1234

Potassium nitrate

[24]

~ 1220, ~ 3600

1150 – 1259

Bicarbonate
Bridged-type carbonate

[19,20]
[19]

1122 – 1107

SOCs* over potassium

[4,8]

930 – 920

Oxide lattice vibrations

[19]

880 – 870

Calcium and potassium carbonates

[21]


*Surface oxygen complexes

4. DISCUSSION

4.1. CaO-MgO mixture compared to pure oxides.

TPO and TGA assays show that the catalytic activity of CaO-based catalysts is higher than the one exhibited by pure MgO (Figs. 1 and 2). This is in agreement with literature reports [5,11,12] about carbon gasification. However, a similar catalytic activity was observed for pure CaO and the 50-CaO mixture (50%CaO - 50%MgO), evidenced by the Tm values in TPO assays (Fig. 1 and Table 1). Furthermore, the CO2 generation selectivity was also similar for these two catalysts.

Important differences between the CaO and 50-CaO catalysts were observed in the thermogravimetric assays. First, weight losses around 400°C indicate that a greater CaO content favors the formation of superficial species in the presence of ambient CO2 and moisture, which were mainly hydroxyl and carboxyl groups. This is reflected by the fact that M1CaO > M150-CaO (Table 2) and is confirmed by the 'in situ' DRIFTS assays where the bands between 3500 and 3700 cm-1 (OH- groups) start to disappear at 400°C, while the mass spectroscopy reflects mainly water release (Figs. 4 and 5). CO2 release seems scarce in this temperature zone for the CaO and 50-CaO catalysts as reflected by the mass spectroscopy and the TPO assays where no CO2 peaks around 400°C were observed (Figs. 1C and 1D).

However, for MgO a CO2 peak was observed near 350°C in both assays (TPO, Fig. 1-B; mass spectroscopy, Fig. 3-B), in agreement with a decrease in intensity of the band associated with unidentate carbonates in the DRIFTS spectra (Fig. 3-A). These results suggest that for the MgO catalyst the superficial compounds decomposed before the onset of combustion (» 400°C) are mainly unidentate carbonates, while the decomposition of OH- group bonded to the surface metallic atom prevails for the CaO-based catalysts (CaO and 50-CaO) [20]. Thus, the calcium atoms are free to participate in the activation of gaseous oxygen by formation of the CaO(O) complex, propitiating a catalytic oxidation of carbon through the formation of C(O) surface complexes. This mechanism is based on the redox cycle CaO ¤ CaO(O) [15,22] and, according to other authors, the presence of calcium does not influence the nature of C(O) but the amount of C(O) [11].

Peak N°2 in Fig. 2-A, which represents an increase in weight for CaO and the 50-CaO mixture, might be attributed to CO2 adsorption produced during CB combustion, as indicated in Section 3.1.2. This was verified by the 'in situ' DRIFTS assays (Figs. 4 and 5), where it was observed that the combustion reaction starts once the surface OH- groups have decomposed as shown by a remarkable increase in the intensity of the CO2 peak in mass spectroscopy assays. A fraction of the CO2 produced is adsorbed on the calcium catalysts (CaO and 50-CaO), as evidenced by the widening and increased intensity of the bands associated with carbonates (1300 - 1600 cm-1) and aldehyde and formate species (2800 - 2900 cm-1) that are produced as combustion takes place with increasing temperature (Figs. 4 and 5-B).

In contrast to what is observed with CaO samples, TGA assays with MgO as catalyst showed that although CO2 is released above 450°C, no CO2 was adsorbed (trough). This was evident as the bands associated with carbonates increasingly disappear as the reaction takes place at increasing temperatures (Fig. 3).

The CB consumption by reaction, CO2 reaction with CaO and CO2 adsorption on Ca take place simultaneously for the CaO and 50-CaO catalysts. The reactions may be represented as follows:

At the beginning of the oxidative reaction, i.e., when the rate is still low, the consumed carbon mass is smaller than the adsorbed CO2 mass, which could explain the increase in weight of the sample (trough) observed in the TGA assays for CaO and the 50-CaO mixture; the net gain of weight clearly is related to the 4:1 ratio of catalyst to CB. Oxygen chemisorption may be taking place as well, thus contribuiting to the observed weight increase.

An increase in CB consumption rate and a decrease in carbonate adsorption rate take place with a rise in temperature, as expected; the maximum weight loss rate was observed near 550°C for CaO and 50-CaO, and it corresponds to the zone of catalytic oxidation of CB (Fig. 2-A and peak N°3 in Table 2). On the other hand, the maximum reaction rate with MgO was observed only above 600°C (Fig. 2-A).

Another weight loss that starts at 600°C and was more intense for CaO was observed in the TGA assays with CaO and 50-CaO (Fig. 2-A). This was attributed to bulk calcium carbonate decomposition, which was mainly formed during CB combustion because of the CaO reaction with the CO2 produced. This formation and decomposition of bulk calcium carbonate was observed by Cazorla-Amorós et al. [10,14] during CaO-catalyzed carbon gasification. A second peak for CO2 above 600°C was also observed in the TPO assays (Figs. 1C and 1D); this was also related with the same phenomenon of carbonate decomposition. The possibility that this peak includes the oxidation of some CB residue that does not react catalytically was ruled out. In fact, no CO generation was observed in the second peak of Figs. 1C and 1D, while the uncatalized reaction (Fig. 1A) clearly shows the presence of CO in this particular temperature range. Similar results have been previously reported in the literature for the uncatalyzed reaction [18].

There is general agreement between the observed results in TPO and TGA assays, as expected; however, some differences were observed: (a) in the peak temperatures associated with CB combustion and carbonate decomposition, and (b) in the intensity of the peaks associated with carbonate decomposition. It might be that the gas flow through the reactive bed in the TPO assays is the main distinguishing factor, by increasing the contact of gas with the solid and propitiating a more efficient sweep of the CO2 produced, thus diminishing the CO2 partial pressure above the catalyst surface and suppressing its carbonation. On the other hand, the catalyst-CB sample in the TGA assays is within a cylindrical crucible, with a sole opening in its top; the reactive gas does not flow through the catalyst-CB sample and this hinders the CO2 sweep.

Figure 2-A and Table 2 clearly show that catalyst carbonation increases with the calcium content of the sample (CaO > 50-CaO > MgO), as also observed in the XPS analyses (Table 3). This phenomenon would not affect the catalytic activity, at least not directly, since the TPO and TGA assays show that the activity in CB combustion was CaO »50-CaO > MgO.

A lack of O2- (ª 530 eV) species in the CaO and 50-CaO samples was observed in the XPS characterization (Table 3), probably due to the greater abundance of superficial hydroxyl and carbonate groups, while the O2-observed in MgO is lattice oxygen, firmly bound to Mg and of small activity (O2-/Mg = 0.43 < 1, the stoichiometric ratio, Table 3).

Although MgO was almost inactive in CB combustion, its activity increased upon the addition of calcium (50-CaO), suggesting that the observed activity is mainly related to the presence of calcium. However, pure CaO exhibited an activity similar to the 50-CaO mixture, which has half of the CaO content. A plausible explanation for such behavior might have been that the calcium catalytically active sites are more active in the mixture than in pure CaO but the XPS assays cast a doubt on this possibility, as the observed bond energies were similar for Ca2s in the CaO and 50-CaO catalysts (Table 3). Indeed, the activity of the catalytic sites depends on the bond energies and if these are not modified for the species participating in the rate-determining step, no change of activation energy should be expected. The determination of intrinsic kinetic parameters might contribute to further elucidate this effect; however, the interaction of CO2 with both catalysts in CB combustion at low temperatures might also complicate data interpretation.

Another way to explain these results can be found in studies of carbon gasification in the presence of CaO [10,14], where the existence of two types of sites in CaO was postulated: active sites in the carbon-catalyst contact region, which participate in the oxidation, and inactive sites, which are further away from the contact region and react only with CO2(g) to form carbonates. In this work, the 50-CaO catalyst exhibited similar activity to that of CaO, and this is attributed to a similar number of active sites in contact with CB for both samples. In spite of a lesser Ca content, the BET surface area for the 50-CaO mixture was larger than for pure CaO, and this is expected to lead to a larger catalyst-CB contact area.

Moreover, it is well known [12] that dispersion of the catalytically active species plays a fundamental role in regard to the catalyst-substrate contact. The XPS characterization (Table 3) indicates that the Ca/Mg ratio for the fresh 50-CaO sample (0.57) was less than in the bulk (0.72) at ambient temperature. At this condition, the presence of adsorbed superficial groups (OH, carboxylic and different carbonate species) on calcium was proved (DRIFTS) and perturbs the XPS measurement of all surface Ca. However, the Ca/Mg ratio for the 50-CaO mixture after being used in the reaction was a higher value of 0.91. This value is thought to reflect more accurately the catalyst dispersion on the 50-CaO catalyst, because it was measured when the surface was already freed of the superficial compounds, due to decomposition at the highest temperatures reached in the assay (700°C).

On the other hand, MgO presence might diminish the stability of the compounds superficially adsorbed on CaO, thus providing more active sites for CB combustion. A similar phenomenon was observed in tar gasification with CaO-MgO mixtures [15,16], where this was attributed to a cooperative effect of MgO and CaO that favors the formation of unidented carbonates which are easier to remove from the surface than bidentate carbonates [15,16]. This is thought to be confirmed in Table 3 as a slight increase in the binding energy of the Mg2s in the 50-CaO mixture (88.9 eV) with respect to the pure MgO (88.4 eV). Although this is a small change, it shows a tendency which agrees with other results in this work. This in turn suggests that Mg in the 50-CaO sample shows a stronger interaction with the absorbed surface groups, thus diminishing the interaction of these groups with calcium.

Furthermore, the 'in situ' DRIFTS assays (Figs. 4 and 5) show that the band at 3633 cm-1 in the 50-CaO sample is not observed at 450°C while for the CaO this band disappears completely at 500°C. The latter indicates that decomposition of OH groups takes place at a lower temperature in the 50-CaO mixture than in the pure CaO.

Another relevant factor is the strong carbonation found for the CaO catalyst (TGA and XPS assays) that may cause a steric obstacle for oxygen access to the calcium sites, thus presumably decreasing the actual number of catalytically active sites during combustion. This is similar to the CO2 coal gasification catalyzed by calcium [10,14], where the loss of activity of calcium was found to be a function of the catalyst surface area exposed to the reaction atmosphere and this is associated with the drop in the available surface area of the CaO catalyst and therefore presumably with a decrease in calcium-carbon contact area [14].

As shown above, MgO showed little activity towards the catalytic combustion of CB. The activities of CaO and of the mixture 50-CaO were similar but the latter sample was less prone to carbonation. These similar activities, in spite of the lower content of CaO in the 50-CaO sample, may be attributed to (i) a higher BET surface area for the 50-CaO sample which is expected to increase the catalyst-CB contact area, (ii) good dispersion of calcium on the catalyst surface which increases the number of active sites in contact with the CB, thus diminishing the concentration of inactive sites (which are thought to be responsible for bulk carbonation), (iii) a possible cooperative effect, by virtue of which magnesium diminishes the stability of the adsorbed surface compounds on calcium, thus facilitating the turnover of these sites for the combustion of CB, (iv) a reduced access of O2(g) to the calcium sites in the pure CaO catalyst due to the high concentration of surface groups such as carbonates.

In spite of this improvement, the activity of the 50-CaO catalyst is not high enough to be successfully utilized for the catalytic combustion of soot emitted by Diesel engines, as the temperature of the exhaust gases rarely exceeds 450 ºC. That is why potassium addition to the 50-CaO catalyst with the same CaO/MgO ratio was tried as a feasible and, as proven, a successful alternative.

4.2 Potassium addition to a CaO-MgO mixture.

As shown in Fig. 1 and Table 1, potassium introduction (sample K/50-CaO) significantly increases the catalytic activity as evidenced by a Tm decrease of more than 130°C with respect to the same mixture without potassium, and more than 240°C with respect to the uncatalyzed CB combustion. Moreover, the potassium-containing catalyst increased the selectivity to CO2 generation (a greater CO2/CO ratio) during the combustion (Table 1). This is consistent with the notion that K/50-CaO was a more effective catalyst, both in terms of activity (diminishing temperature) and, in this case, selectivity [17].

A similar conclusion may be derived from TGA assays (Fig. 2 and Table 2) where the degree of catalyst carbonation is seen to be lower with potassium present. (compare the weight loss peaks associated with carbonate decomposition starting at 600°C). In fact, the mass related to peak N°4 corresponding to the mixture 50-CaO (M4 = 1.242 mg) is far greater than M2 corresponding to the sample K/50-CaO (M2 = 0.500 mg) which has the same CaO/MgO ratio. This quantitative comparison is valid as the TGA results are normalized to the initial CB mass and the catalyst/CB mass ratio was 4/1 for all cases. Potassium introduction resulted in less carbonation both in absolute terms and with respect to the CaO composition in the catalyst. Thus the M4/initial mass of CaO ratio is 0.621 while for the K/50-CaO catalysts the M2/initial mass of CaO is 0.278.

As shown before for the catalysts without potassium (CaO and 50-CaO), the amount of bulk carbonates that decompose above 600ºC increases with calcium content in the sample. However, potassium introduction resulted in less carbonation. As shown by XPS characterization, this is probably due to a smaller content of surface CaO when potassium is present.

The XPS characterization also shows a reduction in the surface content of carbonates and a slight decrease of the binding energies of Ca2s and Mg2s in the potassium-containing catalyst, in comparison with the 50-CaO mixture. Moreover, the Ca/Mg surface atomic ratio was much smaller for the K/50-CaO sample (0.12 for the fresh catalyst, 0,50 for the used one) than for the 50-CaO sample (0.57 and 0,91 for the fresh and used catalyst, respectively), and also the BET surface area diminished drastically with potassium introduction. These results suggest that in the K/50-CaO sample the calcium, which enhances carbonation (see Section 4.1), is less exposed to the surface of the K/50-CaO catalyst than the 50-CaO sample, thus reducing the CO2 adsorption by the catalyst.

Even for a used K/50-CaO catalyst, which is assumed freer of surface compounds than a fresh catalyst (having smaller values of CO3-2/ Mg and CO3-2/Ca ratios), the XPS Ca/Mg surface atomic ratio of 0.5 is smaller than the bulk atomic Ca/Mg ratio (0.73). This result confirms again a lesser presence of calcium on the surface of the potassium-containing catalyst (Table 3).

Potassium introduction also increased considerably the XPS-measured content of oxygen on the surface (a greater O2-/Mg ratio as seen in Table 3). DRIFTS assays confirm this by the presence of a band at 1122 cm-1, which was also observed by Mul et al. [21], and is associated with adsorbed superficial oxygen groups, specifically to O2- species, observed in a triangular K+O2- complex formed by reaction of alkali metal with O2 [21,22]. This surface enrichment with oxygen is thought to be the main cause of the enhanced activity shown by the catalyst containing potassium.

As in Ca-catalyzed gasification [11], the activated oxygen on the alkaline metal reacts with the carbon atoms of the CB located at the catalyst/carbon interface, eventually producing C(O) complexes that desorb directly as CO(g) or react with an adjacent activated oxygen to generate a new complex of the form C(O-O) that decomposes to CO2(g). In this mechanism, O2(g) is essential for the formation of new activated oxygen on the catalyst surface.

In the 50-CaO catalyst (without alkali) the calcium participates in the generation of active oxygen through the CaO(O) complex [11,13] by a mechanism which is similar to the one described in the previous paragraph. However, CaO(O) formation is thermodynamically less favored than a complex K-especies, which is catalytically more active [23]. This explains the previously observed increase of surface oxygen when potassium is introduced [21] and it suggests that oxygen activation will take place mainly on the sites associated to the alkali metal in the catalyst K/50-CaO that contains both potassium and calcium.

The K/50-CaO catalyst showed a somewhat larger selectivity towards CO2 than 50-CaO (Table 1), which is explained by the higher surface concentration of activated oxygen in the potassium-containing catalyst. For high concentration of the complex C(O) on the CB, a larger content of activated oxygen favors the formation of complexes of the form C(OO) that finally decompose as CO2(g) [11].

However the difference in selectivity provided by these catalysts is small, probably because in the oxidation catalyzed by 50-CaO the reaction of the activated oxygen with the surface complex C(O) (Reaction 1 of the mechanism below) is faster than the desorption of C(O) (Reaction 2). This is in agreement with the proposal of Kyotani et al. [11]:

CaO(O) + C(O) ® CaO + C(OO) (1)
C(O) ®CO(g) (2)

Thus, the catalysts CaO and 50-CaO showed selectivity to CO2 that was similar to the catalyst with potassium, in spite of a lower concentration of surface activated oxygen.

5. CONCLUSIONS

Catalysts with magnesium (as MgO), calcium (as CaO), a mixture with equal ratio of CaO and MgO, and with potassium showed increasing activity for the catalytic combustion of carbon black, a model compound for soot emitted by Diesel engines.

Pure MgO was barely active in the catalytic combustion of carbon black while calcium, both in the pure CaO and in the 50-CaO samples, showed higher activity. However, CO2 from the combustion that is adsorbed on the Ca-containing catalyst formed carbonates that reduced the activity.

The 50-CaO catalyst was as active as the pure CaO in spite of a lower content of calcium, which is the more active component. This is attributed to (i) greater contact area of catalyst-carbon black and better dispersion of calcium over the catalyst, (ii) a lesser tendency toward bulk and surface carbonation of the catalyst, (iii) the cooperative effect of magnesium that diminishes the stability of surface compounds adsorbed on the calcium, and (iv) a greater accessibility of O2(g) to the calcium sites.

It was concluded that the activity of the 50-CaO mixture was not high enough for the removal of soot emitted by Diesel engines. Potassium introduction over the previous mixture, keeping the CaO/MgO ratio, produced a catalyst with very high activity for the combustion of carbon black. This increase was related to a significant increase of the surface concentration of oxygen on the catalyst that increased the CO2/CO ratio in the exit gases. Moreover, potassium addition resulted in a lower content of calcium on the surface, hence less carbonation of the catalyst.

ACKNOWLEDGEMENTS

The support of FONDECYT-Chile, Grant Nº 1030296 and financing for short stays abroad from MECESUP, Grant UCO0108 (for RJ) are gratefully acknowledged. The authors thank Professor Patricio Ruiz (U. of Lovaina) for helpful discussions, for hosting stays at Lovaina and facilitating the use of XPS and DRIFTS equipments, used during those stays.

 

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