Influence of Ba precursor on structural and catalytic properties of Pt–Ba/alumina NOx storage-reduction catalyst

Abstract

The effect of the choice of barium precursor (nitrate or acetate), in the preparation of Pt–Ba/γ-Al2O3 NSR catalysts, on their structure and catalytic behavior has been investigated by means of X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and catalytic tests. XRD patterns showed that the type of Ba precursor and repeated NSR cycles influence the crystallinity of Ba species. Depending on the Ba precursor, remarkable differences in Ba(NO3)2 crystallite sizes as well as availability of the Pt for hydrogen chemisorption were observed. TPD and TPR studies showed that the catalyst prepared using the Ba acetate-precursor route is more efficient in the decomposition and reduction of Ba nitrates at lower temperatures. The catalyst prepared by the Ba acetate-precursor route showed both enhanced NOx storage ability and selectivity toward nitrogen in the reduction process, which is attributed to the intimate contact of the small crystallites of the Ba-component and the Pt particles. Environmental transmission electron microscopy (ETEM) measurements revealed that introduction of H2O vapor induces morphological changes of the Ba phase leading to partial liquefaction as a result of the formation of hygroscopic barium hydroxides. The interplay of this liquid-like behavior of the Ba phase together with the chemical transformations occurring during NOx storage and reduction can lead to remarkable structural changes affecting the NSR performance.