Formation and stability of barium aluminate and cerate in NOx storage-reduction catalysts

Abstract

The formation and stability of BaAl 2 O 4 and BaCeO 3 in Pt-Ba/Al 2 O 3 and Pt-Ba/CeO 2 based NO x storage-reduction (NSR) catalyst has been investigated using kinetic measurements, X-ray diffraction, thermal analysis and X-ray absorption spectroscopy. In as-prepared state, the Ba-component in the NSR catalysts was made up of amorphous BaO and BaCO 3 . The formation of BaAl 2 O 4 started above 850°C, whereas the formation of BaCeO 3 was already observed at 800°C and was faster than that of BaAl 2 O 4 . The stability of BaAl 2 O 4 and BaCeO 3 in various liquid and gaseous atmospheres was different. BaAl 2 O 4 was rapidly hydrated at room temperature in the presence of water and transformed to Ba(NO 3 ) 2 and γ-alumina in the presence of HNO 3 , whereas BaCeO 3 was decomposed to much lower extent under these conditions. Interestingly, BaCeO 3 was transformed to Ba(NO 3 ) 2 /CeO 2 in the presence of NO 2 /H 2 O at 300 -500 °C. Also the presence of CO 2 led to decomposition of barium cerate, which has important consequences for the catalyst ageing under NO x -storage conditions and can be exploited for regeneration of thermally aged NSR-catalysts.

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

“…[38] The formation of such aluminate phases is well known, also for real catalyst materials. [47][48][49] On annealing in O 2 to 800 K, the Ba aluminate nanoparticle system is preserved, and the particles show slow ripening, restructuring, and diffusion into the support.…”

Performance Improvement of NO x -Storage BaO/Al2O3 by Using Barium Peroxide as the Precursor of BaO

“…The amount of stored NO x A NO x ð Þobtained from the NO x breakthrough curves was always larger than that of the desorbed NO x D NO x ð Þ from NO x -TPD, which is due to the desorption of some weakly adsorbed NO x species before the commencement of temperature ramp in NO x -TPD. It appeared that the different nature of barium [5]. Thus the present results should provide a technically viable way for improving the thermal stability of NSR catalysts.…”

“…The high temperature during the periodic desulfation (in excess of 650°C) and/or the oxidation of hydrocarbon, CO or H 2 in rich engine operation would lead to particle growth of the noble metal, and the depletion of the NO x storage materials via the reaction with Al 2 O 3 support to form the mixed metal oxides (i.e., barium aluminates) that are inert for NO x storage [3][4][5][6]. While the particle size of the noble metal is more crucial to the coldstart performance of the NSR catalysts [7], the decreased efficiency of the NO x storage components due to the formation of inert mixed metal oxides affects severely the performance under steady state driving conditions [6].…”

Effects of temperature and reductant type on the process of NOx storage reduction over Pt/Ba/CeO2 catalysts

“…To avoid such influence, a pre-reduction treatment at mild conditions was suggested to reactivate Pt on Pt/Ba/CeO 2 , leading to significant enhancement in NSR performance. The thermal stability of Pt/Ba/CeO 2 was also compared with conventional Pt/Ba/Al 2 O 3 [21,22]. After thermally aging the two catalysts above 800 • C, BaCeO 3 and BaAl 2 O 4 were formed, which are believed to be detrimental for NO x storage.…”