PNNL

Abstract

Modern surface science techniques have been commonly applied to understand issues arising from practical catalytic systems.[1-4] However, the applicability of most of the results obtained from model systems has been limited, due, primarily, to the vastly different conditions studies on model and practical systems are carried out (catalyst composition, reaction conditions etc.).[5, 6] Therefore, the need to conduct experiments on compositionally similar systems (model and practical) is necessary to obtain valuable information on the workings of real catalysts. In this communication we demonstrate the utility of surface science studies on model catalysts in understanding the properties of high surface area, BaO-based NOx storage-reduction (NSR) catalysts.[7] We present evidence for the facile formation of surface barium aluminate-like species even at very low coverages of BaO. This Ba-aluminate layer, however, can react with NO2 resulting in the formation of a bulk-like Ba(NO3)2 phase. In order to construct model catalysts that are representative of the practical NOx storage systems, we first needed to estimate the BaO covareges on the high surface area catalysts. Since the publication of the work by Fanson et al.[8], BaO loadings of 8 – 10 wt.% on a ?-alumina support (200 m2/g) have been regarded as corresponding to one monolayer (ML) coverage, based on the unit cell size of bulk BaO. The coverage equivalent of one ML, however, was significantly underestimated. Assuming complete spreading of the BaO layer and using a Ba–O distance of ~ 2.77 Å (one unit of BaO occupies 1.53 × 10-19 m2), 10 wt.% loading of BaO would cover only about 1/3 of the alumina surface. Table 1 shows our calculated estimates of two-dimensional BaO coverages as a function of loading on a ?-Al2O3 surface (200 m2/g) based on the lattice parameters of bulk BaO[9] (5.54 Å). Based on these values, for our model system studies we prepared BaO/Al2O3/NiAl(110) materials in which the BaO coverages were very close to those of 4, 8, and 20 wt.% BaO/?-Al2O3 high surface area catalysts used in prior studies.