(168f) Nanostructured Catalysts for Diesel Soot Combustion

This paper deals with the preparation, by the innovative solution combustion synthesis, the characterization (by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses), the catalytic activity testing (in a temperature-programmed combustion microreactor), and the assessment of the reaction mechanism of a series of nanostructured soot combustion catalysts based on La?Co substoichiometric or alkali-metal-substituted perovskites (La_0.9CoO₃, La_0.8CoO₃, La_0.9Na_0.1CoO₃, La_0.9K_0.1CoO₃, La_0.9Rb_0.1CoO₃), whose performance is compared with that of the standard LaCoO₃. Significant catalytic activities were measured in the range 350-450°C, which paves the way towards practical use in catalytic traps for diesel particulate occasionally heated by fuel post-injection and catalytic combustion.

Keywords: Perovskite-type alkali-substituted cobaltites; Catalytic combustion; Diesel particulate;

Introduction

An intensive research has been carried out in the last decade to find catalysts active for the abatement of diesel exhaust pollutants. The main contaminants emitted by this type of engine are nitrogen oxides and soot particles. The current tendency is to minimize NO_xproduction by high exhaust gas recirculation (EGR) rates and tackle the problem of high diesel particulate emissions with the combined use of traps and oxidation catalysts [1]

Diesel particulate filters (DPFs) based on wall-flow-type monoliths are generally recognized as the most viable solution to the related pollution problem [2]. Any catalyst to be placed over the trap should possess high thermochemical stability and intrinsic activity. It should also possess a microstructure capable of maximizing the contact points with the trapped soot without increasing the pressure drop too much.

Experimental

A series of perovskite samples (La_0.9CoO₃, La_0.8CoO₃, La_0.9Na_0.1CoO₃, La_0.9K_0.1CoO₃, La_0.9Rb_0.1CoO₃) were prepared via a highly exothermic and self-sustaining reaction, the so-called ?solution combustion synthesis? method [3]. This technique is particularly suited to produce nanosized particles of catalyst. Characterization by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses techniques was then accomplished. The catalytic activity of the prepared catalysts was tested in a temperature-programmed combustion (TPC) apparatus according to standard operating procedures: an air flow was fed to the catalytic fixed-bed reactor enclosed in a quartz tube placed in an electric oven. The fixed bed was prepared by mixing 50 mg of a 1:9 by weight mixture of carbon and powdered catalyst with 150 mg of silica pellets (0.3?0.7 mm in size); this inert material was adopted to reduce the specific pressure drops across the reactor and to prevent thermal runaway. The reaction temperature was controlled through a PID-regulated oven and varied from room temperature to 800°C at a 5 °C∙min^-1 rate. The outlet gas conversion was monitored with a CO/CO₂ NDIR analyser (ABB). A detailed description of the TPC equipment has appeared in a previous paper of ours [4].

Results and discussion

Table 1 lists the Specific Surface Area (SSA), both the on set temperature and the peak temperature obtained with each catalyst tested. As expected, all of the catalysts significantly lower the combustion peak temperature compared with that of the noncatalytic combustion. An activity order can be outlined as: the La_0.9Rb_0.1CoO₃ shows the best activity by far (Tp = 377 °C); the other perovskite catalysts characterized by lanthanum deficiency exhibit quite similar activities (Tp ranging from 405 to 439 °C); the unsubstituted LaCoO₃ is by far the least active catalyst (Tp = 476 ◦C). In line with earlier papers of ours [3-5], taking into account XPS data, it can be deduced that replacement of some of the lanthanum with a lower valence alkali metal brings about the formation of high-valence cobalt (CoHV) to maintain electroneutrality and possibly to obtain more active or more concentrated oxygen species over the catalyst surface.

Conclusions

Several cobaltite catalysts (La_0.9CoO₃, La_0.8CoO₃, La_0.9Na_0.1CoO₃, La_0.9K_0.1CoO₃, La_0.9Rb_0.1CoO₃) were prepared by combustion synthesis, characterized, and tested as catalysts for diesel soot combustion. The Rb-substituted cobaltite catalyst (La_0.9Rb_0.1CoO₃) exhibits the highest activity as a consequence of its greater amount of weakly chemisorbed O^-species (α-oxygen), which were pointed out as the key players in the soot oxidation state. This catalyst is currently being tested on real flue gases in innovative catalytic wall-flow traps. Preliminary results are encouraging and will be presented at the congress. In a parallel effort, catalytic materials characterized by the highest possible α-oxygen type concentration are currently being developed, with attention to their compatibility with either trap material or the poisoning components present in diesel exhaust gas (e.g., sulfur oxides).

References

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[2]   R.M. Herck, S. Gulati, R.J. Farrauto, Chem. Eng. J. 82 (2001) 149.

[3]   N. Russo, D. Fino, G. Saracco, V. Specchia, J. Catal. 229 (2005) 459.

[4]   D. Fino, N. Russo, G. Saracco, V. Specchia, J. Catal. 217 (2003) 367.

[5]   G. Saracco, G. Scibilia, A. Iannibello, G. Baldi, Appl. Catal. B 8 (1996) 229.