Abstract
The evolution of Ba sites in two distinct NOx storage and reduction (NSR) catalysts that are based on alumina or zirconia-titania mixed oxide (ZrO2-TiO2 or ZT) during NOx adsorption/desorption was investigated by in situ and operando IR spectroscopy. Based on various evidences from the in situ study, medium sized Ba sites on alumina supported fresh catalysts are proposed to experience sintering under NOx adsorption to form bigger particles, while for ZT, initially possessing smaller sized Ba particles, the formation of a thin layer or very fine particles of Ba would proceed under the same condition. This evolution can also be affirmed by observations from the operando IR study showing that NOx adsorption on ZT supported catalyst is initially faster than on alumina supported catalyst (time on stream lower than 300s), but after long adsorption time (about 50min) the two catalysts show similar storage capacity. This new mechanistic insight suggests that NOx ad/desorption rate, which is critical for optimizing NSR performance, needs to be controlled by support materials whose interaction with the Ba particles not only determines their initial size (fresh catalyst) but also their resistance towards sintering during the NOx adsorption
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