Potential of Mixed Halides and Vanadates as Catalysts for Soot

Reproducibility was quite good, the maximum difference between twin Tp values ..... The two peaks observed for the CsCl + KVO3 catalyst could be due t...
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Ind. Eng. Chem. Res. 1997, 36, 2051-2058

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Potential of Mixed Halides and Vanadates as Catalysts for Soot Combustion Guido Saracco,* Valentina Serra, Claudio Badini, and Vito Specchia Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy

Several soot combustion catalysts, based on a single halide and potassium vanadate supported on R-Al2O3, have been prepared, characterized, and tested. Three catalysts were selected on the basis of their prevalent activity and thermal stability: KCl + KVO3, RbCl + KVO3, and CsCl + KVO3. These compositions are more environmentally acceptable than previously studied Cu-based catalysts. The CsCl + KVO3 catalyst showed the highest catalytic activity and appreciable resistance to thermal degradation. The catalyst based on RbCl + KVO3 showed the best thermal stability (comparatively low loss of chlorides at high temperatures). The KCl + KVO3 catalyst, despite its slightly lower activity and thermal stability, is to be particularly appreciated for its low cost. The reaction mechanism of the catalysts is primarily linked to the formation of eutectic liquids between halides and metavanadates. Such liquids wet the carbon and promote its combustion according to redox processes in which metavanadates should play a major role. Introduction Diesel engines are facing a considerable success for both stationary (e.g. power plants) and mobile (i.e. lightand heavy-duty vehicles) applications, owing to the combination of their high efficiency, reliability, and durability with rather low operating costs. Tschoke and Walz (1994), while reviewing the growing perspectives of diesel engines, stated that their sales are expected to increase considerably over the next years, in Asia and Europe particularly. However, along with the continuous growth of diesel market, serious concerns are arising with regard to the prevalent pollutants associated with this type of engine: nitrogen oxides and particulate. A possible way of reducing diesel particulate emissions is the use of a proper filtering trap, capable of separating the particulate matter from the exhaust gases. Once particulate is collected, it is necessary to burn it off, thereby regenerating the laden trap. Since the diesel soot combustion temperature is not regularly achieved at vehicle exhaust pipes for sufficient periods of time to allow self-regeneration, two alternative routes appear feasible: (1) periodic thermal combustion of the trapped soot by temperature rise above 600 °C; (2) continuous trap regeneration, at the exhaust gas temperature, by catalytic combustion promoted by depositing suitable catalysts within the trap itself. Route 1 entails serious stability problems for the trap materials, since temperatures as high as 1200 °C can be locally reached when the soot is burned (Badini et al., 1996a). Conversely, the major challenge of route 2 lies in finding catalysts capable of carrying out particulate combustion at the temperatures normally reached at diesel exhausts (180-600 °C). A considerable amount of research has been carried out in recent years to develop suitable catalysts and catalytically coated traps for diesel soot removal (Neeft, 1995). In this context, perhaps the most promising catalyst for diesel particulate combustion is based on Cu/K/V/Cl compounds (Watabe et al., 1983; Ciambelli * Corresponding author. Phone: 39+11-5644699. Fax: 39+11-5644699. E-mail: [email protected]. S0888-5885(96)00654-9 CCC: $14.00

et al., 1994; Badini et al., 1996a,b; Serra et al., 1996), which can ignite particulate at temperatures well below 400 °C. Previous studies on this catalytic system elucidated its reaction mechanism (Serra et al., 1996): chlorides and vanadates form eutectic mixtures having rather low melting points. The liquids thereby obtained improve the contact between soot and catalytic species, allowing rapid particulate combustion through catalytically-promoted redox mechanisms. However, owing to this particular reaction mechanism, the catalyst suffers from significant deterioration at high temperatures (above 700 °C), primarily due to evaporation of chlorides, rather than to chemical interaction of the catalytic species with other components of diesel flue gases such as SO2, NOx, etc. (Badini et al., 1996a). Among such chlorides, copper chloride (CuCl2) raises a special concern due to its intrinsic toxicity. Further, this compound is also a well-known oxychlorination catalyst which can lead, in contact with diesel exhausts, to the formation of toxic chlorinated compounds, such as dioxins (Luijk et al., 1994). The aim of the present work is the development of various catalytic systems which could possibly combine a high particulate combustion activity with a comparatively low volatility of the active species and a negligible release of toxic compounds. A number of catalysts, based on a single halide and KVO3, were prepared and tested for such a purpose, according to the following guidelines: (a) copper was removed from the set of the basic elements employed; (b) potassium was substituted, at least in part, with different base or transition metals (Ca, Cs, Rb, Na, Sn, etc.); (c) chlorine was substituted, in a couple of cases, with different halides (Br, F). The most promising catalysts thereby obtained were submitted to chemical characterization and checked for their thermal stability and reaction mechanism. On the basis of such investigations, somewhat more positive conclusions about their suitability to diesel-exhaust treatment applications could be drawn as opposed to the well-known Cu-K-V-Cl catalyst. © 1997 American Chemical Society

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Materials and Methods Carbonaceous Material. An amorphous carbon (average particle size, 45 nm; 12.2 wt % of moisture lost after drying at 110 °C; 0.34 wt % ashes after calcination at 800 °C) was used in the activity tests to simulate diesel particulate. The almost complete absence of SOF characterizing such carbon has to be regarded as conservative, since the higher the SOF content of diesel particulate, the lower its combustion temperature (Serra et al., 1996). Catalyst Preparation. The binary catalysts were prepared by incipient-wetness impregnation of an R-Al2O3 powder grain size (38-63 µm) with aqueous solutions of potassium metavanadate and a single halide (KF, KCl, KBr, NaCl, LiCl, RbCl, CsCl, MgCl2, CaCl2, SnCl2, BaCl2) in a 1:1 molecular ratio (for more details about the impregnation procedure and apparatus, see Badini et al., 1996b). The impregnated powder was then dried at 120 °C for 1 h and calcinated at 700 °C for 4 h. For all the catalysts the weight ratio between the precursor salts and the R-Al2O3 support was set equal to 1 after the drying step. For such a purpose, several impregnation/drying cycles had to be performed prior to calcination. All heat treatments were performed in static air. A binary catalyst based on CuCl2 and KVO3 was also prepared for comparison purposes. As counterparts to the above binary catalysts, monocomponent catalysts based on single vanadates (KVO3, CsVO3, RbVO3) or halides (KCl, KBr) were also prepared following an analogous procedure. The CsVO3 and RbVO3 used in this last context were prepared by melting proper mixtures of the corresponding carbonates and vanadium pentoxide. The purity of such vanadates was checked by XRD analysis. All the other compounds employed in the preparations were highpurity commercial products (Alfa, Fluka, Aldrich). The interest in Rb and Cs vanadates, not directly employed in any preparation of bicomponent catalysts, arose as a consequence of the later described characterization results, which showed how these vanadates form in considerable amounts in the RbCl + KVO3 and CsCl + KVO3 catalysts, respectively. Apart from their actual composition after the drying and the calcination step of their preparation procedure, the binary catalysts prepared will be named hereafter on the basis of their precursors (i.e. a specific halide and KVO3). Finally, some standard Cu-K-V-Cl catalyst, supported on R-Al2O3 (50 wt %), was also prepared, as a reference, according to the procedures described by Badini et al. (1996b). Cu, K, and V are present in such catalyst in a 2:2:1 atomic ratio. Catalyst Characterization. The composition and microstructure of the investigated catalysts were studied by X-ray diffraction (XRD) using a Philips PW1710 apparatus equipped with a monochromator (Cu KR radiation). On some particular samples, mostly related to the below described thermal stability tests, specific elemental analyses were also carried out by atomic absorption spectroscopy (K, Cs, Rb, and V) or by potentiometric titration with a specific Orion electrode (Cl-). Activity Tests. A preliminary screening of the activity of the different catalysts toward carbon combustion was performed by thermogravimetric analysis (TGA). Each catalyst was accurately mixed in a mortar with the carbon in a 1:1 weight ratio. Afterward the mixture was submitted to a linear rise of temperature (scanning rate: 40 °C/min) under an air flow in the TGA

Figure 1. Typical TGA (thick lines) and DTG (thin lines) plots for the noncatalytic and the KCl + KVO3 catalyzed combustion of carbon.

equipment (Perkin-Elmer Series 7; sample amount: 3 mg). The peak temperature Tp of the derivative DTG curve (i.e. the temperature corresponding to the maximum combustion rate) was chosen as the key parameter in the catalyst screening tests. Such activity test was not only carried out on the calcinated catalysts but also on the simply dried ones, for a comparison. Three tests were carried out on each sample so as to determine an average Tp value. Reproducibility was quite good, the maximum difference between twin Tp values being about 10 °C. Activity tests were also performed on the most active catalysts by means of differential scanning calorimetry (Dupont DSC apparatus; overall sample amount: 3 mg), by employing the same heating rate and air flow conditions of the above TGA runs. In this last case, specific runs were also performed in the absence of amorphous carbon so as to check whether endothermic DSC peaks, attributable to the likely formation of eutectic liquids among catalyst components, were present. Unsupported catalysts were used in this context so as to maximize the endothermic peak per unit mass of loaded sample (total amount: 10 mg). These last analyses turned out to be crucial in assessing the reaction mechanism of the catalysts. Catalyst Thermal Stability. Some aging tests were performed, by isothermal treatment at 750 °C in static air, on the most active catalysts. Such operating temperature was conservatively chosen well above the range of temperatures normally reached at the silencer of diesel vehicles. The total weight loss of the studied catalysts was monitored after different aging periods, and meanwhile the chemical composition of most samples was checked. Results and Discussion Typical TGA-DTG plots are shown in Figure 1 for both the noncatalytic combustion of carbon (i.e. blank run performed with pure R-Al2O3 instead of a catalyst) and the calcinated KCl + KVO3 catalyst, one of the most promising catalysts, as later discussed. The effect of the catalyst in lowering the combustion temperature appears quite clearly. The peak temperatures (Tp) of the DTG curves obtained for any prepared catalyst, in both dried and calcinated form, are listed in Table 1. The Tp values of the reference Cu-K-V-Cl catalyst and of the noncatalytic combustion are also reported. Such data are plotted in Figure 2 for an easier comparison, as well. None of the prepared bicomponent catalysts attained, in its calcinated form, the activity of the standard Cu-

Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2053 Table 1. Peak Temperatures of the DTG Curves for the Prepared Binary Catalysts (Heating Rate ) 40 K/min; Air Flow Rate ) 50 cc/min; Catalyst-to-Carbon Weight Ratio ) 1:1) catalyst KVO3 + RbCl KVO3 + CsCl KVO3 + LiCl KVO3 + NaCl KVO3 + SnCl2 KVO3 + CuCl2 KVO3 + BaCl2 KVO3 + CaCl2 KVO3 + MgCl2 KVO3 + KCl KVO3 + KF KVO3 + KBr Cu-K-V-Cl catalyst no catalyst

Tp after drying (°C)

Tp after calcination (°C)

470 480 580 610 510 505 610 525 545 480 540 490

475 455 590 570 550 475 650 610 585 480 545 485 435 725

K-V-Cl catalyst (Tp ) 435 °C). However, at least for some of them, the activity loss with respect to this last counterpart was not severe, especially if the noncatalytic combustion peak temperature is considered (725 °C). Five catalysts showed particularly promising activity, namely those based on KVO3 coupled with CuCl2, KCl, KBr, CsCl, and RbCl, respectively. The other catalysts showed rather poor catalytic activity and will not be considered further. With the only exception of the KCl + KVO3 system, the Tp values of the five selected binary catalysts after calcination were somewhat different from those after simple drying. This variation was of the same order of magnitude of the experimental error for the KBr- and the RbCl-based catalysts, while a rather pronounced lowering of Tp was noticed after calcination for the catalysts based on CsCl and CuCl2. This suggests that perhaps some reactions or transformations have taken place during calcination at high temperatures between the two basic catalyst compounds (halide and vanadate) at least in these last catalytic systems. This point will be further addressed later on. Thermal stability tests were then carried out on the four Cu-free bicomponent catalysts by measuring the catalyst weight (R-Al2O3 support included) vs time, during prolonged heating at 750 °C in static air. The results obtained are plotted in Figure 3, together with those regarding the Cu-K-V-Cl catalyst, as derived from Badini et al. (1996a). As a common feature of all selected catalysts an asymptotic behavior was observed with only a marginal variation of the residual weight after about 40 h. However, for the KBr + KVO3 catalyst such asymptotic condition was reached for a weight loss much higher (close to 40 wt %) than that of the other three catalysts (well in the range between 10 and 20 wt %). Besides, as reported in Table 2, this superior weight loss was coupled with a considerable loss of activity compared with the other catalysts. In particular, the Tp value of the KBr + KVO3 catalyst was increased by about 40 °C, while that of the other catalytic counterparts suffered from an enhancement of just 10-15 °C. The following parameter can be defined in order to better enlighten this point:

R ) 100

(Tp)c,a - (Tp)c (Tp)nc - (Tp)c

(1)

where the nc and c subscripts stand for noncatalytic/

Figure 2. Results of the activity tests on the dried and the calcinated catalysts (catalyst-to-carbon ratio ) 1:1).

Figure 3. Weight loss (R-Al2O3 support included) of four selected catalysts during the thermal stability tests (750 °C in static air). Table 2. Peak Temperatures of the DTG Curves of Four Selected Binary Catalysts before and after Aging for 64 h at 750 °C catalyst

Tp before aging (°C)

Tp after aging (°C)

KVO3 + KCl KVO3 + RbCl KVO3 + CsCl KVO3 + KBr

480 475 455 485

490 488 470 525

catalytic and the subscript a refers to the 64-h aged catalyst. The KBr + KVO3 catalytic system was characterized by an R value of 17%, a value about three times higher than those of the other catalysts, which remained in any case well inside the range 4-6%. Besides, the above catalyst did not show a particularly high catalytic activity, which remained appreciably lower than that of the CsCl + KVO3 catalyst. For such reasons no further investigations were carried out on the KBr + KVO3 catalyst. On the other hand, the behavior of the other catalytic systems appeared promising. The RbCl + KVO3 catalyst showed a particularly good thermal stability, allowing only about 12% weight decrease after prolonged stay at 750 °C. The CsCl + KVO3 catalyst exhibited a particularly high catalytic activity: even after the aging treatment its Tp value was slightly lower than those of the other two just-calcinated catalysts (see Table 2). Both the above catalysts suffer from evaporative losses of active components less than the reference Cu-KV-Cl catalyst. Finally, the KCl + KVO3 catalyst, which showed only reasonable stability and activity, remains

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Figure 5. XRD spectra of (a) the aged KCl + KVO3 catalyst (64 h at 750 °C in static air); (b) the calcinated KCl + KVO3 catalyst; (c) pure KVO3. Symbols: R ) R-Al2O3; 4 ) KVO3; O ) KCl; * ) not identified.

Figure 4. Variation of the percent residue of each constituting element vs the aging time (750 °C in static air) for the following catalysts: (a) KCl + KVO3; (b) RbCl + KVO3; (c) CsCl + KVO3.

nonetheless interesting for its cost, much lower than that of both Rb- and Cs-based catalysts. For the reasons stated, these last three catalysts were further studied so as to assess whether changes in their chemical composition arise during the preparation and the aging treatments and in order to determine which reaction mechanism governs their catalytic activity. At first, the loss of the constituting elements of each single catalyst was measured during the aging treatment. The percent residue of each constituting element, referred to the amount of such elements present in the original catalyst, is plotted vs the aging time in Figure 4. Some general considerations, valid for any studied catalyst, can be drawn from these data. (1) Chlorine-containing compounds are very prone to leave the catalysts at 750 °C. (2) On the basis of simple material balances involving also the base metals, it can be argued that chlorine leaves the catalyst at high temperatures in the form of alkali metal chlorides. In this context, it should be noted that in both the RbCl + KVO3 and the CsCl + KVO3 catalysts, together with a certain loss of the base metals Rb and Cs, a comparable decrease of potassium content takes places. This can be interpreted assuming that RbCl or CsCl, respectively, tends to react with

KVO3 so as to form cesium or rubidium vanadates and KCl. This last compound evaporates easily at high temperature. Confirmation of such occurrence will be derived by XRD analysis later on. (3) Vanadium-containing compounds show a much lower tendency to volatilization than chlorides. This is a common feature of the three binary catalysts considered and of the Cu-K-V-Cl reference catalyst (Badini et al., 1996a). Coming to more specific considerations about each single catalytic system, it can be stated that the KCl + KVO3 catalyst was characterized by a more marked high-temperature evaporation of chlorides (KCl) than the RbCl + KVO3 and the CsCl + KVO3 catalysts. Further, contrary to the other two selected catalysts, an appreciable vanadate loss occurred. Both these considerations explain the inferior overall behavior displayed by this catalyst in Figure 3 compared with its two counterparts. The Rb-containing catalyst, which showed the lowest overall weight loss after prolonged aging, seems to derive this positive property from a limited loss of chlorides. Further, it is worthwhile to notice that rubidium leaves this catalyst at a rate slower than that of potassium. Finally, the Cs-based catalyst showed a low loss of vanadates comparable to that of the RbCl + KVO3 catalyst, even though a somewhat higher loss of chlorides was noticed. This last feature, coupled with the comparatively high atomic weight of cesium, resulted in the slightly higher aging percent weight loss of the CsCl + KVO3 catalyst compared with the Rb-based one (Figure 3). Some materials were investigated through X-ray diffraction so as to understand better the chemical composition of the catalyst and to provide explanations of the changes observed during heating. In particular, Figures 5, 6, and 7 refer to the KCl + KVO3, the RbCl + KVO3, and the CsCl + KVO3 catalysts, respectively. The XRD spectra were obtained after calcination at 700 °C and after 64-h heating at 750 °C in calm air. Further, the X-ray patterns derived for pure alkali metal vanadates (KVO3, RbVO3, CsVO3) are also reported for a better evaluation of the above spectra. Some general issues have to be taken into account for a proper analysis of the XRD spectra. First, the

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Figure 6. XRD spectra of (a) the aged RbCl + KVO3 catalyst (64 h at 750 °C in static air); (b) the calcinated RbCl + KVO3 catalyst; (c) pure RbVO3. Symbols: R ) R-Al2O3; 4 ) RbVO3; O ) KCl; 0 ) RbCl; * ) not identified.

Figure 7. XRD spectra of (a) the aged CsCl + KVO3 catalyst (64 h at 750 °C in static air); (b) the calcinated CsCl + KVO3 catalyst; (c) pure CsVO3. Symbols: R ) R-Al2O3; 4 ) CsVO3; O ) KCl; b ) CsCl; 0 ) KVO3; * ) not identified.

intensity scale of each single XRD pattern in these figures was chosen in order to put into evidence also the weakest peaks in each spectra. Since the intensity range is not the same for the different spectra, no direct quantitative information about the variation in composition of the samples can be attained on the basis of the peak intensities. The elemental loss analyses were definitely more helpful for this purpose. However, semiquantitative indications about the loss of active species can be drawn by comparing the intensities of their peaks with those of the inert alumina in the same spectrum. Further, it was noticed that the relative peak intensities of vanadates are very sensitive to the preparation procedure. In fact, two cards are reported in the database of JCPDS-ICDD for KVO3 (26-1342 and 331052) and three for CsVO3 (33-381, 33-382, and 37-165), which appreciably differ from one another with regard to the relative peak intensities. Finally, the last step of catalyst preparation involves in any case a calcination at 700 °C, at which temperature the vanadates and the halides melt at least in part, forming a liquid that is then driven to the complete solidification during rapid cooling to room temperature. In this last context, the

formation of not well-crystallized or even amorphous phases (these last not detectable by XRD analysis) cannot be excluded. For the KCl + KVO3 based catalyst, Figure 5 shows that no other components from those employed in the catalyst preparation could be detected in either the calcinated or the aged sample. However, three rather weak peaks, present in the spectra in Figure 5a,b (see asterisk markers), insufficient to allow any precise identification, could possibly be related to some degradation products. The comparison between the intensities of R-Al2O3 and KCl peaks in the two above spectra showed that this last compound was lost, in part, during aging at 750 °C, which confirms the results of the elemental analyses (Figure 4a). With the RbCl + KVO3 based catalyst (see Figure 6), no peaks ascribable to RbCl were found in the calcinated catalyst spectra. Conversely, KCl and RbVO3 were detected, indicating the occurrence of an exchange reaction between KVO3 and RbCl. The spectrum was quite similar to an unreported one concerning the simply dried catalyst. This suggested that the above exchange reaction could take place already at low temperatures during the impregnation or the drying step. Slight changes can be noticed in the aged catalyst spectrum: the peaks of KCl were no longer present, whereas traces of RbCl and of unidentified products were present. Further, the evaporative loss of catalyst components became evident on comparing the intensities of the R-Al2O3 peaks with those of the other catalyst components in Figure 6a,b. Finally, the XRD pattern of the calcinated CsCl + KVO3 based catalyst showed the clear presence of CsVO3 and KCl deposited on the alumina support. Contrary to the previous case, the exchange reaction between KVO3 and CsCl might be incomplete. In fact, even if the CsCl and KVO3 strongest peaks were overlapped with some peaks of the other catalyst components, some weak peaks attributable to KVO3 alone were noticeable in Figure 7b. Further, an unreported XRD spectrum of the simply dried catalyst showed also the weaker peaks of the JCPDS-ICDD spectra of CsCl and KVO3, which implies that the above mentioned double-exchange reaction occurs only partially during the drying step of the preparation procedure. After aging, the catalyst XRD pattern showed the presence of KVO3 and some very weak new unidentified peaks in addition to those of R-Al2O3, CsVO3, and KCl. Once more, as a consequence of the evaporative loss of catalyst components, the R-Al2O3 peak intensities generally increased with respect to those of the catalytically active species. On the basis of the above characterization results, a more confident analysis of the reaction mechanism could be attempted. As a first investigation, four monocomponent catalysts (50 wt % on R-Al2O3) were prepared on the basis of the four major components detected in the three selected catalysts in their calcinated form: KCl, KVO3, CsVO3, and RbVO3. TGA-DTG runs were then performed on such catalysts according to the same operating procedures employed for the binary catalysts. The derived Tp values, listed in Table 3, are in all cases higher than those of the binary catalysts. This suggests a sort of synergism in the action of the different components of the binary catalysts, which leads to particularly high catalytic activity. Twin DSC runs were then performed on the three selected catalysts in order to better elucidate this

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Figure 8. Results of DSC runs on (a) KCl + KVO3 catalyst; (b) RbCl + KVO3 catalyst; (c) CsCl + KVO3 catalyst. Thick lines are carbon catalytic combustion (1:1 catalyst-to-carbon weight ratio); thin lines ) eutectic melting experiments (pure unsupported catalytic compounds). Endothermic and exothermic peak temperatures are reported. Table 3. Peak Temperatures of the DTG Curves of Some Monocomponent Catalysts, Supported on r-Al2O3 (50 wt %), and Their Melting Points Measured by DSC catalyst

Tp (°C)

Tm (°C)

KVO3 CsVO3 RbVO3 KCl

620 572 562 585

520 641 567 790

point: the first run was carried out in the presence of carbon to analyze its catalytic combustion; the second was performed in the absence of any carbonaceous material in order to detect any peak attributable to phase transitions and/or reactions among the catalyst components. The results obtained are plotted in Figure 8. The DSC peak temperatures of carbon combustion runs (KCl + KVO3 ) 479 °C; RbCl + KVO3 ) 449 °C; CsCl + KVO3 ) 420 °C) are in all cases slightly lower than those measured by TGA-DTG, which is likely an intrinsic feature of the two different measuring apparatuses (different operating principles, different crucible geometry, different temperature detection systems, etc.). However, the activity order remains the same. Conversely, the DSC runs performed in the absence of carbon clearly showed endothermic peaks at the same operating temperatures at which the catalytic combustion took place. Such endothermic peaks were single in case of the KCl + KVO3 and the RbCl + KVO3 catalysts, whereas for the CsCl + KVO3 catalyst two different endothermic peaks could be noticed. Exothermic peaks of comparable size were measured at the same temperatures, during downward DSC runs from high to low temperatures. This suggests that the

considered peaks are related to reversible transformations. On the basis of direct observations and in line with the previous work on the Cu-K-V-Cl catalyst (Serra et al., 1996), such transformations could be attributed to eutectic liquid formation. The two peaks observed for the CsCl + KVO3 catalyst could be due to the rather complex composition of the catalyst in its calcinated form and to local uneven composition: two different eutectic liquids involving different catalyst components are likely formed at different temperatures. Table 3 also lists the melting points (Tm) of the compounds detected in appreciable amount in the calcinated binary catalysts. All the Tm values are higher than either the corresponding Tp values for the binary catalysts or the temperatures at which the endothermic peaks in Figures 8a,b,c were detected. This is further evidence of the eutectic nature of the liquids formed. The formation of such liquid phases appears to be a key factor in determining the catalytic activity of the studied catalysts, since it takes place almost at the same operating temperature range at which carbon combustion occurs at its highest rate. It can be stressed that liquid formation enables mobility of the catalytically active species, thereby providing a deeper contact with the carbon through a wetting process. However, it should be noted that, for the CsCl + KVO3 catalyst, the temperatures of formation of the two eutectic liquids mentioned above were respectively below and above the DSC Tp value (Figure 8c). It can be argued that the first eutectic liquid contained components which were not particularly active and capable only of initiating the combustion process, which had its peak about 30 °C later under the spur of increased temperature levels. When the second eutectic liquid forms, an additional increase of the catalytic activity took place, as demonstrated by the shoulder of the DSC combustion peak above 430 °C. This conclusion agrees with that of Neeft et al. (1996), who recently stated that in many catalysts for diesel soot combustion a clear relationship between their activity and either their melting point or their vapor pressure can be observed. Some other papers in the field were recently focused on the importance of catalyst mobility, acquired by either liquid or vapor formation, for diesel soot combustion (Ahlstro¨m and Odenbrand, 1990; Mul et al., 1995; Neeft, 1995; Serra et al., 1996). Further, McKee and co-workers described the superior performance of low-melting binary and ternary eutectic catalysts in promoting carbon gasification (McKee et al., 1985) and combustion (McKee, 1987). In this latter case, in particular, eutectic mixtures of alkali and transition metal oxides were studied. Liquid-phase catalysts are rather frequently encountered in the process industry (Villadsen and Ljvbjerg, 1978; Mross, 1983). For instance, certain oxidation reactions, such as the synthesis of phtalic anhydride from naphthalene and p-xylene or that of sulfur trioxide from sulfur dioxide, are carried out by use of V2O5 catalysts doped with considerable amounts of alkali metal sulfate, generally potassium sulfate. Conversely, catalysts based on copper chloride and potassium chloride are often employed in oxychlorination processes. In both cases, the alkali metal is used to bring the active component, i.e. vanadium pentoxide or copper chloride, into a low-melting phase. In the catalysts studied in the present work, the main role of alkali metal chlorides, and primarily of KCl owing to the above discussed double-exchange reactions between the other base metal

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chlorides and KVO3, should be the same: they form eutectic liquids with other compounds (vanadates) which, once the carbon surface has been wetted, catalyze its oxidation according to redox processes. Other synergetic effects might be ascribable to the presence of alkali metal chlorides in oxidizing catalysts and can also be active in the catalysts studied in the present work. For instance, Akimoto and Echigoya (1974) showed how in oxide catalysts the electrondonating effect of the alkali metal ions increases the reactivity of the oxygen in the MdO bond (where M is a transition metal ion in the catalyst, such as vanadium). In this context, the promoting effect was found to be inversely proportional to the electronegativity of the alkali metal, which results in the following activity order: Cs > Rb > K. This is exactly the same order measured for the selected binary catalysts for carbon combustion. This feature is only in apparent discrepancy with the results of the monocomponent metavanadate catalysts listed in Table 3, which shows a combustion activity order Rb > Cs > K. In fact, as opposed to its Cs-based counterpart, the RbVO3 catalyst melted at a temperature very close to its Tp value. It is very likely that the superior combustion rate of the Rb-based monocomponent catalyst is not merely due to its intrinsic activity but also to the improved catalyst-carbon contact enabled by liquid formation. Besides, measures performed using 18O showed, for transition-metal-oxide catalysts, a much higher rate of oxygen exchange with the gaseous phase, due to their doping with KCl (Mross et al., 1983). Finally, a certain role of the alkali metals as electron donors/acceptors has been postulated in their interaction with carbon: they should weaken the C-C bonds and favor the formation of C-O bonds during catalytic oxidations (Wen, 1980; McKee, 1983). The VdO bonds present in the structure of the vanadates detected in the three studied catalysts (i.e. KVO3, RbVO3, CsVO3), should be the major feature responsible for the catalytic oxidation of soot, by acting as “oxygen pumps” through repeated reduction (oxygen is given to carbon)-reoxidation (oxygen is back from the gas phase) cycle (McKee, 1970). According to Ciambelli et al. (1990, 1993), the Cu-K-V-Cl catalyst, used as a reference in this study, is capable of delivering the oxygen needed for the oxidation of carbonaceous materials, even if the reaction is performed in inert atmosphere, and the oxygen content in the catalyst can be subsequently restored by exposition to air. The variable content of oxygen in metavanadates can likely explain this effect. Ahlstro¨m and Odenbrand (1990) explain the redox activity of V2O5 with the vanadium capability of existing in several oxidation states with small energy differences between one another. In particular, according to McKee (1970), the catalytic effect of V2O5 resulted from the interaction of carbon and V2O5 to give V6O13, which was then reoxidized back to V2O5 by ambient oxygen. Similar conclusions were drawn by Nakamura et al. (1994) for the R-CuVO3 vanadate. The vanadium contained in the above metavanadates should thus oscillate between at least two oxidation states, thereby taking part in oxidation-reduction cycles at the carbon surface. As a final investigation, performed with the aim of strengthening this last hypothesis concerning the prevalent role of metavanadates in carbon catalytic combustion, some DSC combustion runs were performed employing pure unsupported metavanadates (KVO3, RbVO3,

CsVO3) as catalysts, by employing the same overall amount of carbon of the runs performed with the binary catalysts (about 1.5 mg), but increasing the catalyst-tocarbon weight ratio up to 10. Such high ratio was chosen in order to possibly enhance the number of catalyst-carbon contact points, thereby allowing an intimate contact despite the absence of alkali metal chlorides as liquid-formation promoters. The obtained peak temperatures were well inside the range of the DSC peak temperatures (420-470 °C) observed for the binary catalyst, thereby showing that metavanadates are intrinsically active at such temperatures. It can be thus concluded that the carbon combustion activity of the binary catalysts studied is primarily linked to the formation of eutectic liquids, between the alkali metals and the metavanadates, which rapidly wet the carbon particles. Wetting of the carbon by such salt phases is a necessary but insufficient condition for high catalytic activity, since the components of the eutectic mixtures should possess intrinsic redox activity. In this context, metavanadates should play an important, but probably not exclusive, role. Conclusions Previous studies on a Cu-K-V-Cl catalyst for diesel soot combustion (Badini et al., 1996a,b; Serra et al., 1996) indicated a very high activity but also some drawbacks of this catalyst concerning either its stability (mainly affected by evaporation of active components at high temperatures) or its environmental impact (loss of toxic CuCl2, which moreover catalyzes the formation of mutagenic compounds such as dioxins). A set of binary catalysts based on R-Al2O3-supported alkali halides and potassium vanadate have been prepared, characterized, and tested as potential alternative candidates for diesel exhaust treatment. Among them, three catalysts (KCl + KVO3, RbCl + KVO3, and CsCl + KVO3) were selected for their prevalent activity and thermal stability. The CsCl + KVO3 catalyst shows superior catalytic activity, whereas the RbCl + KVO3 catalyst is characterized by the best thermal stability (lower loss of chlorides in calm air at 750 °C). The KCl + KVO3 catalyst is more attractive from a cost standpoint. Despite their slightly lower activity compared to the Cu-K-V-Cl catalyst (the CsCl + KVO3 catalyst entails an increase of the DTG combustion peak temperature of just 20 °C), the above catalysts appear to be more promising candidates for practical application, particularly for their lower environmental impact. In order to assess this potential, some pilot plant tests are in progress by treating diesel exhaust gases with traps, on which the selected catalysts have been deposited. Literature Cited Ahlstro¨m, A. F.; Odenbrand, C. U. I. Catalytic combustion of soot deposits from diesel engines. Appl. Catal. 1990, 60, 143-156. Akimoto, M.; Echigoya, E. Participation of double-bond-type lattice oxygen in vapour phase catalytic oxidation of olefins. J. Catal. 1974, 35, 278-288. Badini, C.; Serra, V.; Saracco, G.; Montorsi, M. Thermal stability of Cu-K-V catalyst for diesel soot combustion. Catal. Lett. 1996a, 37, 247-254. Badini, C.; Saracco, G.; Serra, V., Combustion of carbonaceous materials by Cu-K-V based catalysts. I. Role of copper and potassium vanadates. Appl. Catal. B 1997, 11, 307-328. Ciambelli, P.; Corbo, P.; Parrella, P.; Scialo`, M.; Vaccaro, S. Catalytic oxidation of soot from diesel exhaust gases. 1. Screen-

2058 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 ing of metal oxide catalysts by TG-DTG-DTA analysis. Thermochim. Acta 1990, 162, 83-89. Ciambelli, P.; Palma, V.; Vaccaro, S. Low temperature carbon particulate oxidation on a supported Cu/V/K catalyst. Catal. Today 1993, 17, 71-78. Ciambelli, P.; D’Amore, M.; Palma, V.; Vaccaro, S. Catalytic oxidation of amorphous carbon black. Combust. Flame 1994, 99, 413-421. Luijk, R.; Akkerman, A. M.; Slot, P.; Olie, K.; Kapteijn, F. Mechanism of formation of polychlorinated dibenzo-p-dioxins and dibenzofurans in the catalytic combustion of carbon. Environ. Sci. Technol. 1994, 28, 312-321. McKee, D. W. Metal oxides as catalysts for the oxidation of graphite. Carbon 1970, 8, 623-635. McKee, D. W. Mechanisms of the alkali metal catalysed gasification of carbon. Fuel 1983, 62, 170-175. McKee, D. W. Carbon oxidation catalysed by low melting point oxide phases. Carbon 1987, 25, 587-588. McKee, D. W.; Spiro C. L., Kosky P. G.; Lamby E. J. Eutectic salt catalysts for graphite and coal char gasification. Fuel 1985, 64, 805-809. Mross, W. D. Alkali doping in heterogeneous catalysis. Catal. Rev.-Sci. Eng. 1983, 25, 591-637. Mul, G.; Neeft, J. P. A.; Kapteijn, F.; Makkee, M.; Moulijn, J. A. Soot oxidation catalysed by a Cu/K/Mo/Cl catalyst. Evaluation of the chemistry and performance of the catalyst. Appl. Catal. B 1995, 6, 339-352. Nakamura, M.; Namatame, H.; Fujimori, A.; Miso, A.; Okatabe, S.; Onoda, M.; Nagasawa H. Valence fluctuation in R-CuVO3 studied by photoemission spectroscopy. J. Solid State Chem. 1994, 112, 100-105.

Neeft, J. P. A. Catalytic oxidation of soot. Potential for the reduction of diesel particulate emissions. Ph.D. Thesis, Delft University of Technology, The Netherlands, 1995. Neeft, J. P. A.; Makkee, M.; Moulijn, J. A.; Catalysts for the oxidation of soot from diesel exhaust gases. I. An exploratory study. Appl. Catal. B 1996, 8, 57-78. Serra, V.; Saracco, G.; Badini, C.; Specchia, V. Combustion of carbonaceous materials by Cu-K-V based catalysts. II. Reaction mechanism. Appl. Catal. B 1997, 11, 329-346. Tschoke H. E. and Walz L., Bosch diesel distributor injection pump systems - modular concept and further development. SAE Technical Paper Series 1994, Nr. 945015. Villadsen, J.; Ljvbjerg, H. Supported liquid-phase catalysts. Catal. Rev.-Sci. Eng. 1978, 17, 203-272. Watabe, Y.; Yrako, K.; Miyajima, T.; Yoshimoto, T.; Murakami, T. “Trapless trap”-A catalytic combustion system for diesel particulates using ceramic foam. SAE Technical Paper Series 1983, Nr. 830082. Wen, W.-Y. Mechanisms of alkali metal catalysis in the gasification of coal, char or graphite. Catal. Rev.-Sci. Eng. 1980, 22, 1-28.

Received for review October 14, 1996 Revised manuscript received January 12, 1997 Accepted January 23, 1997X IE960654+

X Abstract published in Advance ACS Abstracts, March 1, 1997.