A transient FTIR study of species formed during NOx storage in the Pt/BaO/Al2O3 system

FTIR spectroscopy has been used to study samples of Al2O3, Pt/Al2O3, BaO/Al2O3 and Pt/BaO/Al2O3 under exposure to either NO2 and O-2 or NO and O-2, between 100 and 400 degreesC. The resulting spectra are fitted to various species, mainly nitrites and nitrates, formed on Al2O3 and BaO. The exposure of NO2 and O-2 on the Al2O3 sample results in the formation of monodentate, bridged and bidentate nitrates. The relative abundance of these nitrate species change with temperature. The obtained enthalpy changes for transition from monodentate to bridged nitrate and from monodentate to bidentate nitrate are -7.7 and 7.4 kJ/mol, respectively. The corresponding entropy changes are -7.1 and 30.2 J/mol K, respectively. For NO and O-2. lower concentrations of nitrates are detected as well as both linear and bridged nitrites. When Pt is present in the catalyst, NO is oxidised to NO2 at temperatures above 150 degreesC resulting in the formation of large amounts of nitrates. It seems as if the formation of nitrites proceed the formation of nitrates. For the samples with both BaO and Al2O3, similar species as outlined above are observed on Al2O3 while mainly nitrites and hyponitrites are observed on Ba when exposed to NO + O-2 below 200 degreesC. When exposed to NO2 + O-2, nitrates are formed on BaO as well as two features attributed to bidentate and bridged nitrate on alumina at the alumina-barium oxide interface. Formation of bulk Ba(NO3)(2) was not observed. It is concluded that Al2O3 plays an important role as a storage site at 100 degreesC less than or equal to T less than or equal to 300 degreesC and by providing a large surface area for BaO. There are also indications that nitrates formed on Al2O3 may spill over to BaO. (C) 2001 Elsevier Science B.V. All rights reserved.