Lean reduction of NO by C3H6 over Ag/alumina derived from Al2O3, AlOOH and Al(OH)3

The effectiveness of Ag/Al2O3 catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al2O3, and Al(OH)3 was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, O2, NO + O2-temperature programmed desorption (TPD), H2-temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and activity test, with a focus on the correlation between their redox properties and catalytic behavior towards C3H6-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C3H6 conversion was achieved over Ag/Al2O3 (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al2O3 (I) is deemed to be crucial for NO selective reduction into N2. By contrast, a high C3H6 conversion simultaneously with a moderate N2 yield was observed over Ag/Al2O3 (II) prepared from a γ-Al2O3 source. The larger particles of AgmO (m > 2) crystallites were believed to facilitate the propene oxidation therefore leading to a scarcity of reductant for SCR of NO. An amorphous Ag/Al2O3 (III) was obtained via employing a Al(OH)3 source and 500 °C calcination exhibiting a poor SCR performance similar to that for Ag-free Al2O3 (I). A subsequent calcination of Ag/Al2O3 (III) at 800 °C led to the generation of Ag/Al2O3 (IV) catalyst yielding a significant enhancement in both N2 yield and C3H6 conversion, which was attributed to the appearance of γ-phase structure and an increase in surface area. Further thermo treatment at 950 °C for the preparation of Ag/Al2O3 (V) accelerated the sintering of Ag clusters resulting in a severe unselective combustion, which competes with SCR of NO reaction. In view of the transient studies, the redox properties of the prepared catalysts were investigated showing an oxidation capability of Ag/Al2O3 (II and V) > Ag/Al2O3 (IV) > Ag/Al2O3 (I) > Ag/Al2O3 (III) and Al2O3 (I). The formation of nitrate species is an important step for the deNOx process, which can be promoted by increasing O2 feed concentration as evidenced by NO + O2-TPD study for Ag/Al2O3 (I), achieving a better catalytic performance.