Loss of NOx storage capacity of Pt–Ba/Al2O3 catalysts due to incorporation of phosphorous

This work aims at determining the effect of the incorporation of P on NOx storage capacity by NOx storage-reduction (NSR) catalysts. Different amounts of phosphorous were deposited on a Pt–Ba/Al2O3 catalyst (1 wt% Pt and 17 wt% Ba) by impregnation of a phosphate salt. Samples were calcined at 723 K and characterized using X-ray diffraction (XRD), N2 adsorption isotherms and X-ray photoelectron spectroscopy (XPS). Their NOx storage capacity was also measured. It was observed that storage capacity decreased almost linearly with the P/Ba ratio and besides at a phosphorous concentration P/Ba ratio of <0.1 deterioration was low. At higher P concentrations (P/Ba ratio = 0.7) the NOx storage was significantly reduced. It is proposed that the cause of the decline in NOx retention capacity would be the formation of Ba–P phases (very likely Ba phosphates) at the expense of Ba carbonate or Ba oxide. These new phases would be unable to exchange NOx.