Suitability of some promising soot combustion catalysts for application in diesel exhaust treatment

Applied Catalysis B: Environmental 18 (1998) 137±150

Suitability of some promising soot combustion catalysts for application in diesel exhaust treatment Claudio Badini*, Guido Saracco, Valentina Serra, Vito Specchia Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy Received 26 November 1997; received in revised form 25 January 1998; accepted 22 March 1998

Abstract In this work, the effect of thermal treatment at 3808C and 6008C, under gaseous atmospheres containing some typical components of diesel emissions (SO2 and water), was studied on some promising catalysts for diesel particulate combustion. In particular, the ageing behaviour of two novel catalysts (based on CsVO3‡KCl and KVO3‡KCl, respectively) and of a more widely studied Cu±K±V±Cl catalyst was investigated. The catalytic activities of these novel catalysts were lower than that of the Cu±K±V±Cl one, but, contrary to this last counterpart, they almost completely maintained their activity during ageing treatments in dry or humid air at 3808C and 6008C, respectively. Moreover, after prolonged thermal exposure in wet air, the activity of the Cu±K±V±Cl catalyst became comparable with that of the CsVO3‡KCl one, while remaining still slightly higher than that of the KVO3‡KCl catalyst. The thermal treatments of all the catalysts under investigation in an atmosphere containing SO2 did not cause an activity decrease. X-ray diffraction analyses showed the formation of new phases (sulphates and vanadates with a K/V ratio different from that of metavanadates) which could also improve the catalytic activity, counterbalancing the loss of active components due to evaporation at high temperatures. Furthermore, the catalyst activity was evaluated after employing repeatedly these catalysts in carbon combustion. The catalytic activities were generally slightly lowered by the repeated use, even though, from this viewpoint, that of Cu±K±V±Cl was more affected than those of the other catalysts. On the basis of the obtained results the CsVO3‡KCl catalyst was found to allow the best compromise between satisfactory catalyst activity and stability.# 1998 Elsevier Science B.V. All rights reserved. Keywords: Diesel particulate; Soot; Catalytic combustion; Catalyst stability

1. Introduction Diesel engines offer some advantages over spark ignition engines basically because they allow fuel economy and show a lower level of some noxious compounds (carbon monoxide and unburned hydro*Corresponding author. Fax: 0039 11 5644699; e-mail: [email protected] 0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-3373(98)00038-1

carbons) in their emissions. However, these engines produce speci®c pollutants that are hazardous to human health: primarily particulate, dangerous due to its potential mutagenic and carcinogenic activity [1], and nitrogen oxides, which are well-known promoters of the acid rain phenomenon and of the socalled photochemical smog [2]. On the grounds of a growing environmental concern, European legislations are becoming more and more severe concerning

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Table 1 Trends in European passenger cars diesel emission regulations [3] Legislation Euro I

Euro II

Euro III

1993

1996

2000

Euro IV

Introduction

CO (g kmÿ1) HC (g kmÿ1) NOx (g kmÿ1) HC‡NOx (g kmÿ1) Particulate (g kmÿ1)

2.72 Ð Ð 0.97 0.14

2005

a

b

c

1.0 Ð Ð 0.70 0.08

0.64 Ð 0.50 0.56 0.05

0.64 0.07 0.40 Ð 0.04

b

0.50 Ð 0.25 0.30 0.025

c

0.50 0.07 0.19 Ð 0.020

a

Diesel IDI. Initial proposition of the Commission. c Proposed Amendment from the Parliament. b

diesel exhaust emissions, as shown in Table 1 [3]. It is a general opinion, shared by most car manufacturers, that the forthcoming 2005 limits could only be met by means of catalysis and not any longer by means of simple engine or fuel modi®cations [4]. For such reason, a number of research programmes, involving both industrial and academic institutions, are currently in progress with the aim of developing catalysts for diesel particulate combustion, NO decomposition, NO reduction with hydrocarbons, etc. [5]. Focusing on diesel particulate, recent articles of ours concerned the development of catalysts showing very high diesel particulate combustion activity [6± 10]. In particular, the ®rst papers [6±9] focused on the preparation, characterisation, activity and reaction mechanism of a Cu±K±V±Cl catalyst (formerly proposed and tested by Watabe et al. [11] and Ciambelli et al. [12]), which still stands as one of the most active catalysts ever produced for diesel soot combustion purposes. At present, this catalyst seems more active than those based on noble metals or transition metal oxides and shows an activity comparable with that of the Cu±K±Mo±Cl catalyst recently developed by Neeft [13] and Mul [14]. Conversely, a very recent paper [10] considered the potential of copper-free catalysts based on mixed halides and vanadates, which showed slightly lower activity than the mentioned Cu± K±V±Cl catalyst but lower environmental problems. In fact, as opposed to this last counterpart, they do not

release CuCl2, a rather toxic compound which moreover was recently claimed to catalyse the formation of oxychlorinated compounds in diesel exhaust environments [15]. The reaction mechanism of the above catalysts was demonstrated to depend on mobility of active components. The catalytic combustion of diesel particulate is in principle a solid±solid process where the contact between the catalysts and the soot to be burned out in¯uences markedly the reaction kinetics. Recent studies performed at Delft University [13,14] pointed out how the mobility of catalyst components, due to the formation of vapour or liquid phases, is a key issue to achieve particulate conversions high enough for practical application in catalytic traps, capable of ®ltering the particulate and simultaneously promoting its combustion at the typical diesel exhaust temperatures (180±5008C) and oxygen concentrations (2±20%). In fact, despite other possible parallel reaction mechanisms being suggested (e.g. oxygen spill-over, [13]), all the most interesting catalysts formerly developed and studied at the Politecnico di Torino show a rather high activity owing to the formation of eutectic liquids among their components (mostly vanadates and halides), leading to a wetting process of diesel soot, which improves considerably the catalyst-to-carbon contact conditions and enhances the achievable soot conversion [9,10]. As the high activity and the reaction mechanism of the catalysts based on mixed vanadates or molibdates and halides have been well assessed, as discussed above, a further investigation is needed to ascertain their behaviour during the prolonged use under practical operating conditions. Despite a non-negligible catalytic activity is already shown by these catalysts at about 3008C, the mentioned reaction mechanism entails potential stability problems. In fact, the formed liquids can lose active components by evaporation into the exhaust gases; the operating temperature plays an important role in this context. Further, these active species can react with some components of the exhaust gases (SO2, H2O, etc.), possibly leading to catalyst deactivation. The present work is actually focused on the stability of three of the most interesting catalysts produced (i.e. the formerly proposed Cu±K± V±Cl catalyst, a CsCl‡KVO3 catalyst and a KCl‡KVO3 catalyst), aged at different operating temperatures and under different gaseous atmospheres: air, SO2-air mixture and wet air. The stability

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of the three catalysts to repeated reaction cycles has been tested, as well. On the basis of the obtained results, the suitability of the mentioned catalysts for application in the treatment of actual diesel exhaust will be ®nally discussed. 2. Experimental 2.1. Materials and methods All the catalysts were prepared by deposition of proper salts on an -alumina powder (alumina/catalyst weight ratioˆ1:1; alumina particle size: 60± 100 mm). The preparation of the Cu±K±V±Cl catalyst was performed using NH4VO3, KCl and CuCl2
2H2O in a suitable amount for achieving an atomic ratio Cu:K:V of 2:2:1. The other two catalysts (CsVO3‡KCl and KVO3‡KCl) were obtained starting from pure vanadates and potassium chloride in the molar ratio of 1:1. The two vanadates were prepared by melting together M2CO3 (MˆK, Cs) and V2O5. Their purity was checked by X-ray diffraction analysis. Water solutions (or suspensions) of the above listed salts were prepared and used for the impregnation of the alumina support. Afterwards, a drying treatment at 1208C and a subsequent calcination at 7008C for 4 h were carried out. The supported catalysts were submitted to X-ray diffraction (PW 1710 Philips diffractometer equipped with a monochromator, Cu Ka radiation) so as to detect the constituting phases before and after each ageing treatment. The relative intensities between the prevalent alumina peaks and those of the other compounds present in the XRD patterns allow to evaluate (unfortunately in a semi-quantitative way, only) the proportion among the phases in the catalyst samples. The catalytic activity of these samples was assessed by the temperature-programmed-oxidation technique (TPO). This method, according to which a catalyst/ carbon mixture is submitted to a programmed temperature rise permits to quickly compare the activity of samples differently aged by means of the corresponding temperature of maximum combustion rate. Each combustion peak temperature was obtained by averaging the results of three runs (maximum difference between twin experiments: 108C). The equipment used for TPO runs consisted of: a mass ¯ow meter

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delivering a gas mixture of oxygen (21 vol%) and helium (balance) at a constant rate (4 cm3 sÿ1); a quartz tube ®xed bed reactor located in a PID regulated oven; a speci®c CO2 IR analyser at the exit of the reactor connected to a computer-controlled data acquisition system (gas sampling every 0.2 s). The catalytic bed was prepared by mixing carbon, supported catalyst and silica pellets (0.3±0.7 mm in size). First catalyst and carbon were mixed in a 9:1 weight ratio in a mortar. 100 mg of this mixture was diluted by 150 mg of silica pellets and placed in the reactor. Two quartz-wool layers were placed at the top and at the bottom of the catalytic bed. TPO runs were performed, after 30 min stay at 2008C under argon ¯ow, between 2008C and 7008C with a temperature increase rate of 108C minÿ1. The outlet CO2 concentration was measured and plotted as a function of the corresponding reactor temperature, measured by a thermocouple inserted in the ®xed bed. In these activity tests amorphous carbon particles (about 45 nm in diameter, with 0.34% of ashes and 12% of adsorbed moisture) were used. The use of carbon instead of diesel soot is conservative because the presence of adsorbed hydrocarbons, typical of diesel particulate, generally favours its combustion. Based on carbon mass balances, the CO2 selectivity of the tested catalysts was found to be close to 100% in the experimental operating conditions (catalyst-to-carbon ratioˆ9:1). Catalyst ageing treatments were carried out in several different experimental conditions (using about 5 g of catalyst for each condition) in an electric oven ¯uxed with different gaseous atmospheres (50 cm3 minÿ1) at 3808C and 6008C, for periods of time up to 96 and 24 h, respectively. The atmospheres adopted were:  air;  air saturated with moisture at 508C (this gas phase shows a water content of about 12% vol., which is characteristic of diesel exhaust);  a mixture of nine parts of air and one part of nitrogen containing 2000 ppm of SO2 (i.e. overall SO2 concentration: 200 ppm, about one order of magnitude higher than the actual SO2 emission level of diesel engines employing low-sulphur fuel, [3]). A cold trap was placed downstream the oven, in order to capture any compound leaving the catalyst samples by evaporation. Atomic absorption analysis

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(Perkin-Elmer 1100B spectrophotometer) allowed qualitative analysis of the captured compounds. The overall catalyst weight loss was measured through a precision balance. In addition, each catalyst was mixed, by gentle shaking in a vessel, with carbon (catalyst/carbon weight ratio of 9:1) and kept at 4508C under calm air for 2 h; this treatment was repeated so as to carry out up to eight combustion cycles. After each combustion cycle a portion of the catalyst was used to perform a TPO run in order to detect any possible modi®cation in the catalyst activity. When necessary, X-ray analysis of the fresh and aged catalyst samples was integrated by scanning electron microscopy-energy dispersion spectroscopy analysis (SEM-EDS) and differential scanning calorimetry (DSC), the latter carried out with a 108C minÿ1 temperature rise rate.

diffraction patterns shown in Fig. 1. Particularly, the Cu±K±V±Cl catalyst contains several phases (KVO3, KxCu1ÿxVO3, Cu3(VO4)2, Cu2(OH)3Cl, KCl) arising from the reactions occurring among the precursor salts mainly during the catalyst calcination at 7008C, whereas the other two catalysts contain only the speci®c vanadate and KCl used for the preparation. All these catalysts suffer from a more or less relevant change of composition when heated under the ageing atmospheres, even though these variations are not always detrimental for the catalyst activity, as discussed later on. The phases detected by X-ray diffraction measurements on the fresh and aged catalyst samples are reported in Table 2. In order to discuss the effect of the ageing treatments on the catalyst composition and, consequently, on their activity it seems to be convenient to consider each catalyst separately.

2.2. Experimental results and discussion

2.2.1.1. Cu±K±V±Cl catalyst. The thermal treatment at 3808C for 96 h under dry or wet air causes the complete or almost complete decomposition of KxCu1ÿxVO3, probably to KVO3 and CuO (Fig. 2(a)); furthermore, while this last compound

2.2.1. Effect of ageing on chemical composition The more signi®cant phases constituting the fresh catalysts can be pointed out on the basis of the X-ray

Fig. 1. X-ray diffraction spectra of the fresh catalysts: (a) KCl‡CsVO3; (b) KCl‡KVO3; (c) Cu±K±V±Cl. Legend: () -Al2O3; (~) KVO3; (
) CsVO3; (*) KxCu1-xVO3; (&) Cu3(VO4)2; (*) Cu2(OH)3Cl; (*) KCl.

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Table 2 Phases detected in the catalysts by X-ray diffraction: Changes in catalyst composition after different ageing treatments Ageing treatment

Catalyst Cu±K±V±Cl

CsVO3‡KCl

KVO3‡KCl

As prepared

KCl Cu2(OH)3Cl Cu3(VO4)2 KxCu1-xVO3 KVO3 (traces)

KCl CsVO3

KCl KVO3

After 96 h at 3808C in dry air

KCl Cu2(OH)3Cl CuO KVO3 Cu3(VO4)2

KCl CsVO3

KCl KVO3

After 96 h at 3808C in wet air

KCl Cu2(OH)3Cl CuO Cu3(VO4)2 KxCu1-xVO3 (traces)

KCl CsVO3

KCl KVO3

After 24 h at 6008C in dry air

KCl KxCu1-xVO3 CuO KVO3

KCl CsVO3

KCl KVO3

After 24 h at 6008C in wet air

KCl CuO KVO3

KCl CsVO3

KCl KVO3

After 96 h at 3808C in SO2 atmosphere

Cu3(VO4)2 KxCu1-xVO3 Cu2V2O6 KVO3 (traces)

KCl CsVO3 K2SO4 Cs2V4O11 Cs4V2O7

KCl KVO3 K2SO4 K4V10O27

After 24 h at 6008C in SO2 atmosphere

KCl glassy phase

KCl CsVO3 K2SO4 Cs2V4O11 Cs4V2O7

KCl KVO3 K2SO4 K4V10O27

After repeated catalytic carbon combustion (eight cycles)

KCl CuO KVO3 KxCu1-xVO3 (traces)

KCl CsVO3

KCl KVO3

was always present in the X-ray diffraction patterns of the aged samples, potassium vanadate was not always detected. By the way, the poor thermal stability of the

mixed vanadate of copper and potassium was already assessed in a previous paper [8]. An interesting point to be considered is that the differences between

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Fig. 2. X-ray diffraction spectra of the Cu±K±V±Cl catalyst aged at 3808C in dry (a) or SO2-rich air (b). Legend: () -Al2O3; (~) KVO3; (
) Cu0.3K0.7VO3; (*) Cu2V2O6; (&) Cu3(VO4)2; (*) Cu2(OH)3Cl; (&) CuO; (*) KCl.

catalysts aged under dry or wet air conditions were rather marginal, despite the fact that, as discussed in the next section, more marked differences in the catalytic activity were noticed between these twin samples. When similar treatments are carried out at 6008C, but for shorter periods, or when the catalyst is repeatedly employed for carbon combustion at 4508C, the same degradation mechanism was observed; however, in these last cases a more marked change in composition was observed because of the disappearance of Cu3(VO4)2 and Cu2(OH)3Cl phases, too. In these samples copper remains mostly as CuO and vanadium is combined with potassium (Fig. 3). The comparison of XRD pattern of the fresh catalyst (Fig. 1(c)) with that resulting from 24 h of ageing at 6008C (Fig. 3(b)) stresses the formation of CuO. In line with our previous studies [6,8], it was observed by use of a cold vapour trap and by atomic adsorption analysis that all the above chemical modi®cations are in any case accompanied by an evaporative loss of copper and potassium chlorides, already at the low operating temperature (3808C), while vanadium salts show negligible tendency to leave the catalysts by evaporation in the whole temperature range tested. Further, when the catalyst underwent

the 24 h ageing treatment in dry air at 6008C its weight loss was about 4.5 wt%, referred to the initial catalyst‡support mass. The Cu±K±V±Cl catalyst shows a very different behaviour when submitted to thermal exposure (3808C or 6008C) under an atmosphere containing sulphur dioxide. Due to the formation of glassy phases X-ray diffraction can give only a little information about the chemical composition after these treatments. Fig. 2(b) shows that, at least when the treatment is performed at 3808C, copper vanadates or mixed copper and potassium vanadate are still present in the aged samples. It has to be underlined that a non-negligible amount of a new vanadate (Cu2V2O6), with copper to vanadium atomic ratio larger than that of KxCu1ÿxVO3, forms. Even if XRD pattern in Fig. 2(b) does not show the presence of crystalline phases containing sulphur, SEM-EDS put into evidence the signi®cant presence of sulphur in the sample treated at 3808C (Fig. 4). Similar results were obtained also for the sample aged at 6008C. It can be reasonably inferred that, due to the oxidising environment and the presence of vanadates that can catalyse the oxidation of sulphur dioxide to SO3 [16], sulphates form. In these conditions potassium probably reacts with sul-

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143

Fig. 3. X-ray diffraction spectra of the Cu±K±V±Cl catalyst after eight carbon combustion cycles (a), of the same catalyst aged for 24 h at 6008C in dry air (b), and of the prepared KVO3 (c). Legend: () -Al2O3; (~) KVO3; (
) Cu0.3V0.7KVO3; (&) CuO; (*) KCl.

phur oxides and copper preferentially combines with vanadium instead of giving rise to copper oxide. 2.2.1.2. KVO3‡KCl catalyst. This catalyst shows a good stability when exposed to both dry and wet air or when used for carbon combustion: these treatments do not result in any appreciable change of composition (Table 2). No significant evaporation of compounds was observed during treatments at 3808C, whereas at 6008C a slight evaporation of KCl was detected (see peaks related to KCl in Fig. 1(b) and Fig. 5(c)), leading, in the case of ageing under dry air, to a weight loss of only 0.65 wt%, i.e. one order of magnitude lower than that of the Cu±K±V±Cl catalyst (4.5 wt%). Conversely, the treatments at 3808C or 6008C under an atmosphere containing sulphur dioxide greatly affected the X-ray diffraction patterns of the catalyst (Fig. 5). A part of the potassium contained in the catalyst reacts with sulphur dioxide to form K2SO4; an appreciable amount of the K4V10O27 compound, characterised by a vanadium to potassium ratio higher than potassium metavanadate, is also obtained, probably as a consequence of the mentioned reaction which takes part of the potassium out of the KVO3 phase (Fig. 5(a)). In Section 2.2.2, it will be shown how this chemical degradation process is though not

Fig. 4. Plot of the SEM-EDS analysis carried out on the Cu±K±V± Cl catalyst aged in SO2-rich air for 96 h at 3808C.

detrimental at all from the view point of catalyst activity.

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Fig. 5. X-ray diffraction spectra of the KCl‡KVO3 catalysts after 24 h ageing in SO2-rich air at 6008C (a), eight carbon combustion cycles (b), 24 h ageing at 6008C in wet air (c). Legend: () -Al2O3; (~) KVO3; (&) K4V10O27, (*) K2SO4; (*) KCl.

2.2.1.3. CsVO3‡KCl catalyst. This catalyst gives a satisfactory behaviour from the point of view of durability because its composition does not change and its activity only suffers negligible variations after most of the ageing treatments, except for the treatment in SO2-rich air. XRD analysis does not show the disappearance or the formation of phases different from caesium metavanadate and potassium chloride after long-term treatments at 3808C and 6008C under dry or wet air (Table 2, Fig. 1(a) and Fig. 6(c)). Furthermore, its use for several carbon combustion cycles does not affect its characteristic X-ray pattern (compare Fig. 1(a) and Fig. 6(b)). These results are in line with those of a previous paper [10], which showed that caesium metavanadate is thermally stable even after treatment at 7508C for 64 h in oxidising environments. By analogy with the KVO3‡KCl catalyst, a modest evaporative loss of KCl could only be detected when operating at 6008C under either dry or wet air conditions, leading to a 1.05 wt% weight loss in the case of dry air ageing.

Conversely, the X-ray diffraction patterns of samples submitted to sulphur dioxide at 3808C or 6008C show the formation of potassium sulphate and the simultaneous decrease of the amount of potassium chloride (Fig. 1(a) and Fig. 6(a)). In addition, these spectra markedly put into evidence the formation of two caesium vanadates: Cs2V4O11 and Cs4V2O7. The newly formed vanadates show a Cs/V atomic ratio, respectively, half or twice of that of caesium metavanadate, which means that the average Cs/V atomic ratio in the catalyst does not change. In the next section it will be shown how the formation of these new vanadates and of potassium sulphate will positively affect the performance of the catalyst. 2.2.2. Effect of ageing on catalytic activity The temperatures corresponding to the maximum concentration of carbon dioxide in the outlet gas ¯owing from the TPO equipment have been considered as an index for the activity of the catalysts. The most signi®cant results of TPO runs performed on the catalysts before and after ageing are listed in Table 3,

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145

Fig. 6. X-ray diffraction spectra of the KCl‡CsVO3 catalysts after 24 h ageing in SO2-rich air at 6008C (a), eight carbon combustion cycles (b), 24 h ageing at 6008C in wet air (c). Legend: () -Al2O3; (
) CsVO3; (&) Cs2V4O11; (~) Cs4V2O7; (*) K2SO4; (*) KCl.

while the complete set of TPO results is plotted in Figs. 7±10. The exposure to dry or wet air at high temperature (3808C or 6008C) for prolonged time does not affect Table 3 Combustion peak temperatures (8C) from TPO tests for catalysts aged in different treatment conditions

the behaviour of the binary catalysts based on the coupling of a single vanadate with KCl (Figs. 7 and 8). Neither their composition, nor their activity is varied signi®cantly, which is rather encouraging for their practical application. Conversely, the performance of the Cu±K±V±Cl system is decreased (see Fig. 7 in particular). This last result is in line with the observations of Moulijn

Catalyst

As prepared After 96 h at 3808C in dry air After 24 h at 6008C in dry air After 96 h at 3808C in wet air After 24 h at 6008C in wet air After 96 h at 3808C in SO2 atmosphere After 24 h at 6008C in SO2 atmosphere After repeated catalytic combustion of carbon (eight cycles)

Cu±K± V±Cl

CsVO3‡ KCl

KVO3‡ KCl

365 408

425 430

460 460

387

423

465

433

438

465

424

422

465

380

369

422

328

398

460

446

442

482

Fig. 7. Results of TPO runs concerning the Cu±K±V±Cl catalyst (
, ~), the KCl‡CsVO3 catalyst (&, &) and the KCl‡KVO3 catalyst (*,*), aged at 3808C in dry (ÐÐÐ, open symbols) or wet (- - -, dark symbols) air.

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Fig. 8. Results of TPO runs concerning the Cu±K±V±Cl catalyst (
,~), the KCl‡CsVO3 catalyst (&,&) and the KCl‡KVO3 catalyst (*,*), aged at 6008C in dry (ÐÐÐ, open symbols) or wet (- - -, dark symbols) air.

Fig. 9. Results of TPO runs concerning the Cu±K±V±Cl catalyst (
), the KCl‡CsVO3 catalyst (&) and the KCl‡KVO3 catalyst (*), after repeated combustion cycles carried out at 4508C.

Fig. 10. TPO plots for the Cu±K±V±Cl catalyst after repeated carbon combustion cycles at 4508C.

and coworkers at Delft University (NL) concerning a similar Cu±K±Mo±Cl catalyst [13,14]. These researchers attributed the rapid activity decrease of

their catalyst to the loss of copper and potassium chlorides, owing to their comparatively high vapour pressure [17]. As enlightened in Fig. 7, the Cu±K±V± Cl activity in dry air progressively decreases with the time of exposure. When the treatment is carried out in wet air the degradation occurs more rapidly and the entire activity decrease takes place in the ®rst 24 h. Once again, the deactivating effect of water vapour has been also observed by Neeft for the Cu±K±Mo±Cl catalyst he studied [13]. However, in agreement with Watabe et al. [11], the measured loss of vanadium compounds from the catalysts was negligible in any of the operating conditions tested here. In the absence of signi®cant differences between the XRD spectra of the Cu±K±V±Cl catalyst aged in dry and in wet air at 3808C (Table 2), the loss of catalytic activity in the presence of water vapour might tentatively be attributed to an increase of volatility of chloride compounds. Deeper analyses are still needed to clarify this point, even though the enlightened deactivation and the release of toxic copper compounds in the environment could seriously reduce the chances of practical application of the mentioned catalyst. When the ageing temperature is raised to 6008C (Fig. 8) the activity loss of the Cu±K±V±Cl catalyst is less pronounced than expected and a slight improvement can even be observed after 4 h at 6008C in dry air. Chemical analysis of the aged samples does not seem to solve the problem, since in both dry and wet air environments the already mentioned tendency to decomposition of KxCu1ÿxVO3 (at 3808C and 6008C) as well as that of Cu3(VO4)2 and Cu2(OH)3Cl (at 6008C), with probable formation of KVO3 and CuO, stand clear (Table 2). As a tentative explanation of the lower activity decrease faced by the catalyst at 6008C as opposed to 3808C, it might be considered that, contrary to what happens at 3808C, most of the catalyst components are in a liquid state at 6008C either because of eutectic liquid formation or of the melting points of some compounds have been exceeded (e.g. Cu2(OH)3Cl, KVO3, K0.7Cu0.3VO3, [9]). On the one hand this occurrence should facilitate the reactions which take place among the catalyst components (Table 2), but on the other hand, on cooling down from the treatment temperature, the compounds present in the liquid phase should solidify, at least in part into rather small crystals, leading to a redistribution of the catalytic components, which

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might be bene®cial for the catalytic activity. On the contrary, prolonged ageing at 3808C may cause a growth of the crystal grains of the catalytic species with a consequent decrease of their speci®c surface. In order to strengthen this hypothesis, the catalyst aged for 96 h at 3808C in dry air was calcinated at 6008C for 4 h thereby recovering most of its original catalytic activity (TPO peak temperature: 3908C). Unfortunately, temperatures as high as 6008C are reached in diesel exhausts too seldom to rely upon this thermal-regeneration of the catalyst. Further, the aforementioned severe loss of catalytic material at 6008C should limit the effectiveness of this sort of regeneration only to the early stages of the life of the Cu±K±V± Cl catalyst. Fig. 9 shows the activity decrease observed on all the materials after they had been submitted to repeated combustion cycles. As it can be observed, all the catalysts suffer from a certain activity decrease after eight combustion cycles at 4508C. This behaviour might be attributed to localised hot spots arising from the catalysed combustion of carbon since the overall 16 h stay at 4508C entailed by eight repeated combustion cycles does not justify as marked activity decreases as those observed (Table 3), especially if the results of thermal ageing at 3808C and 6008C are considered. More likely, the formation of eutectic liquids favours the redistribution of catalyst components over the carbon surface while the combustion process takes place, thereby widening the interface through which evaporative loss of catalytically active species occurs. A detrimental in¯uence of the carbon ashes on the catalyst ef®ciency might also be hypothesised to justify the discussed activity decrease. Anyway, the loss of activity caused by this treatment was once again marked in the case of Cu±K±V±Cl catalyst but rather marginal for the binary catalysts. As concerns the CsVO3‡KCl and the KVO3‡KCl catalysts, despite no qualitative modi®cation of the chemical composition could be detected by XRD analysis, after repeated combustion runs an increase of 178C and 228C of the TPO peak temperature was respectively measured (Table 3), which is though much lower than that (818C) occurring for the Cu±K±V±Cl catalyst. As a result, the catalytic activities of the different catalysts become quite close to one another: for instance, after repeated carbon combustion, the Cu±K±V±Cl catalyst, which is the most active among the fresh

147

catalysts, exhibits an activity even slightly lower than that of the CsVO3‡KCl catalyst. In particular, for the Cu±K±V±Cl catalyst, together with the activity decrease, a modi®cation in the TPO curve also takes place, as shown in Fig. 10. In fact, the curves modify their shape when increasing the time of exposure. This means that, as long as the catalyst is progressively damaged, the carbon combustion process takes place in a more or less narrow temperature range, shifted to higher temperature values compared with the behaviour of the fresh catalyst. This behaviour can be explained because of the continuous variation of the catalyst composition: copper orthovanadate and oxychloride as well as the solid solution KxCu1ÿxVO3 leave the catalyst or decompose to KVO3 and CuO; meanwhile, evaporation of copper and potassium chlorides takes place. The progressive change of catalyst composition likely affects the formation temperature of molten phases and the amount of liquid thereby obtained at the different temperatures. It has been demonstrated elsewhere through thermal analysis [9], that in the temperature range over which the combustion process takes place, several endothermic phenomena occur in the catalyst, which can be attributed to the appearance of various eutectics having different compositions which progressively contribute to the carbon conversion as long as the percentage of molten material increases with the temperature. The occurrence of several eutectic melts among catalyst components has to be attributed to catalyst heterogeneity. In fact, as a consequence of the preparation procedure adopted (i.e. subsequent impregnation of the support with different solutions of the precursors followed by suitable heat treatments), the distribution of the crystals of the various catalyst components is not homogeneous after calcination. Moreover, when the catalyst is thermally aged, the mentioned evaporation processes are likely quicker in the external part of the catalyst mass rather than in its bulk. Getting back to the interpretation of the change of the shape of the curves in Fig. 10, a tentative explanation can be proposed: 1. In the as-prepared catalyst a suf®cient amount of low-temperature eutectic liquid forms and rapidly burns out all the available carbon in a comparatively narrow temperature range.

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2. As long as the degradation process takes place, an intermediate situation is achieved in which the number of catalyst components is increased (including residual amounts of the original compounds and the new degradation products); the residual amount of some very active compounds (likely copper chlorides and oxychlorides, if the results of Neeft [13], have to be taken as an indication) does not allow any longer a nearly complete carbon combustion below 4008C. This residual amount should be just capable of setting on the combustion process, leaving to other catalyst compounds the task of completing the combustion process as long as new eutectic liquids form at higher temperatures. As a result, the shape of the TPO combustion peaks is rather broadened. 3. Already after four and more clearly after eight combustion cycles, copper chlorides have left the sample or have been converted to CuO (Table 2), most of the low temperature activity of the catalyst is lost and a significant amount of carbon reaches 4008C unaffected. Above this temperature, eutectic liquids among the remaining catalyst components (mostly KCl, KVO3 and CuO) form and promote the combustion process in a comparatively narrow temperature range, thereby giving rise to rather sharp TPO peaks. It has to be pointed out that the described behaviour does not hold for the two binary catalysts (CsVO3‡KCl and KVO3‡KCl). These catalysts, on the grounds of a simple and constant composition (Table 2), show only a single melting point related to the eutectic liquid formed between the two major catalyst components, and a TPO peak very sharp and almost unaffected in its shape and temperature range as long as the number of combustion cycles increases. The treatment of the catalysts at 3808C or 6008C under an atmosphere containing both oxygen and sulphur dioxide gave very interesting results: in spite of the prolonged stay at high temperature, the activity either does not appreciably change or even improve for all catalysts (Table 3). This particular ®nding, also observed by others for different vanadium-based catalysts [18], will be discussed in the following on the basis of the variations in the catalyst compositions assessed by X-ray diffraction. As regards the KVO3‡KCl, the mentioned variation of catalyst composition after treatment under SO2

atmosphere (formation of K2SO4 and K4V10O27) results in an enhancement of catalyst activity after 96 h stay at 3808C. Similarly, the formation of new phases (namely K2SO4, Cs2V4O11 and Cs4V2O7) should be related to the decrease of TPO peak temperatures for the KCl‡CsVO3 samples aged at 3808C for 96 h and at 6008C for 24 h in SO2-rich air (peak temperatures lowered to 568C and 278C, respectively). Finally, as regards the Cu±K±V±Cl catalyst, two major out-comings have to be underlined: 1. The presence of SO2 mitigates the effects of the catalyst ageing process at 3808C despite a 158C increase of the TPO peak temperature can be observed compared to the fresh catalyst. However, it has to be stressed how the Cu±K±V±Cl catalyst becomes less active than the KCl‡CsVO3 one (see Table 3). 2. After 24 h treatment at 6008C a noticeable enhancement of the catalytic activity can be measured (378C lowering of the TPO peak temperature compared to the fresh catalyst). Probably, this has to be attributed to the formation of new compounds (unfortunately not detectable by XRD analysis) and to the above hypothesised redistribution process involving catalytic compounds after ageing at 6008C. In general, when adding new compounds to a solid mixture giving rise to eutectic liquids, it is likely that the temperature at which eutectic mixtures form is reduced. In this context, the formation of new compounds (sulphates, new vanadates), after treatment under SO2 containing atmosphere, might enhance the activity of the tested catalysts by lowering the temperature at which eutectic liquids appear. Some differential scanning calorimetry runs were carried out on the KVO3‡KCl system (the less aggressive for the DSC apparatus) in order to clarify this point. A ®rst DSC run, performed in the absence of carbon, allowed to ascertain that the eutectic between KVO3 and KCl is placed at 4858C, which corresponds fairly well with the combustion peak of carbon observed at 4798C during a DSC run of a mixture of carbon and catalyst [10]. A further DSC run carried out using KVO3‡KCl‡K2SO4 mixed in the 1:1:1 molar ratio allowed to detect an endothermic phenomenon having an onset at 3908C and a peak temperature at about 4728C. The formation of liquid phases at temperatures slightly lower than that for the basic KCl‡KVO3

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catalyst (DSC peak temperature: 4728C against 4858C) should then be responsible for the catalytic activity increase. Since current SO2 concentrations in diesel exhaust gases are at least one order of magnitude lower than those adopted in the present ageing treatments, the effect in actual operating conditions should be much less pronounced. However, the obtained results clearly point the way to the investigation of new modi®ed catalysts including sulphates in their basic composition. Such studies are foreseen in the near future. 3. Conclusions The comparison between the Cu±K±V±Cl and the binary KVO3‡KCl and CsVO3‡KCl catalysts allows to draw some major conclusions with respect to their activity towards carbon combustion and their durability in environments similar to that of diesel exhausts.  When comparing the activity of the fresh catalysts, the following ranking can be drawn: Cu±K±V± Cl>CsVO3‡KCl>KVO3‡KCl.  The exposition to dry or wet air at temperatures in the range 380±6008C (which can be easily experienced in practice) causes a degradation of the Cu± K±V±Cl catalyst which changes its composition through both decomposition of active components and evaporation phenomena.  This catalyst is also rapidly damaged when submitted to repeated carbon combustion runs.  The catalyst deterioration does not only result in a decrease in its activity, but also causes the release of toxic substance (containing copper) into the environment.  The binary catalysts based on caesium and potassium vanadates show a much higher durability in the same ageing conditions: in particular the activity of the CsVO3‡KCl catalyst after few combustion cycles is even slightly higher than that of its Cu±K±V±Cl counterpart in the same conditions.  The exposure of all the tested catalysts to a SO2rich oxidising atmosphere in the range 380±6008C has significant effects on both their composition and activity.

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Formation of sulphur-containing compounds was always observed: in the case of binary catalysts X-ray diffraction evidenced the formation of potassium sulphate.  At the same time, vanadates with different caesium (or potassium)/vanadium ratio form.  The presence of these new phases probably increases the mobility of the catalysts by promoting the formation of low-temperature eutectics. This phenomenon can even result in an improvement of the catalytic activity. However, as the sulphur oxide concentration adopted in this investigation was markedly higher than that typical of diesel emissions, the catalyst activity should not be markedly improved by exposure to sulphur dioxide under actual diesel exhaust conditions. On the basis of the above observations, the CsVO3‡KCl catalyst can be elected as the most promising one among those studied for practical application in the treatment of diesel exhaust gases. Experimental work is currently in progress concerning the deposition of this catalyst inside ceramic foambased traps, and the assessment of the performance of the thereby obtained catalytic devices. References [1] J.L. Mauderly, in: M. Lippman (Ed.), Environmental Toxicants: Human Exposures and Their Health Effects, 1992, pp. 119±162. [2] H. Bosch, F. Janssen, Catalytic reduction of nitrogen oxides. A review of the fundamentals and technology, Catal. Today 2 (1987) 369. [3] P. Zelenka, W. Cartellieri, P. Herzog, Worldwide diesel emission standards, current experiences and future needs, Appl. Catal. B 10 (1996) 3±28. [4] P. Zelenka, W. Kriegler, P.L. Herzog, W.P. Cartellieri, SAE Paper 900602 (1990). [5] J.C. Summers, S. Van Houtte, D. Psaras, Simultaneous control of particulate and NOx emissions from diesel engines, Appl. Catal. B 10 (1996) 139±156. [6] C. Badini, V. Serra, G. Saracco, M. Montorsi, Thermal stability of Cu±K±V catalyst for diesel soot combustion, Catal. Lett. 37 (1996) 247±254. [7] V. Serra, G. Saracco, C. Badini, V. Specchia, Cu±K±V catalysts for diesel soot combustion, Riv. Comb. 50 (10) (1996) 383±390. [8] C. Badini, G. Saracco, V. Serra, Combustion of carbonaceous materials by Cu±K±V based catalysts. I. Role of copper and potassium vanadates, Appl. Catal. B 11 (1997) 307± 328.

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