Simultaneous removal of soot and nitrogen oxides from diesel engine exhausts
Introduction
Particulate matter (soot) and NOx are the main pollutants in diesel engine emissions. The combination of a filter with oxidation catalysts appears to be the most plausible after-treatment technique to eliminate soot particles [1]. Since the temperature in the exhaust gases may be as low as 200 °C for small engines, and can exceed 600 °C at full load for heavy engines, a useful catalyst has to operate efficiently at low temperatures and be thermally stable. The reaction mechanism and therefore a rational catalyst design for this system has been studied only recently. Studies with a large number of formulations have been reported during the last few years, and the soot–catalyst contact appears to be one of the most important problems to be overcome [2]. In this paper, the reaction mechanisms of soot combustion with oxygen are compared using different supports (MgO, La2O3 and CeO2) promoted with potassium and/or cobalt. The discussion is largely based on our previous results [3], [5], [6], [7], [9], [10] and also the new results presented in this paper. The feasibility of the simultaneous removal of soot and nitric oxide is studied using Ba as a promoter. The contact problem between the soot and the catalyst is also addressed.
Section snippets
Soot and catalyst preparation
The soot was obtained by burning commercial diesel fuel (Repsol-YPF, Argentina) in a glass vessel as described in [3]. After collecting from the vessel walls, it was dried in a stove for 24 h at 120 °C.
The catalysts were prepared from a support (MgO, La2O3 and CeO2) suspension in water, to which Co(NO3)2 and/or KOH solution was added to obtain 12 wt.% of Co and/or 4.5 wt.% of K. Barium acetate was used for the preparation of Ba/CeO2 catalysts. The mixture was evaporated while being vigorously
Catalytic soot combustion with oxygen (TPO experiments and high frequency CO2 pulses)
The catalysts that contain potassium display good activity for soot combustion with a maximum in the TPO profile between 350 and 400 °C (Fig. 1). Isothermal experiments (not shown) indicate that the soot can be completely removed with diluted oxygen at 350 °C. When lanthanum is the support, cobalt decreases the reaction rate due to the formation of a mixed oxide with a perovskite structure as seen by XRD [6]. This takes both the support and the CoOx out of the reaction. This effect is more
Conclusions
K/La2O3 and K/CeO2 proved to be very active and stable catalysts for the combustion of soot with oxygen. A reaction mechanism involving the redox sites and the surface-carbonate species takes place on these catalysts. The support plays an important role in the combustion reaction. La2O3 contributes to this with a reaction path leading to the formation of carbonate-type intermediates, which decomposes at the reaction temperature, in a similar way to the effect introduced by potassium in all the
Acknowledgements
The authors are grateful to Agencia de Promoción a la Investigación (proy. 14-6971) and UNL (CAI+D ’96 Program). Thanks to Elsa Grimaldi for the revision of the English manuscript.
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