The Remarkable Enhancement of CO-Pretreated CuO-Mn2O3/γ-Al2O3 Supported Catalyst for the Reduction of NO with CO: The Formation of Surface Synergetic Oxygen Vacancy

NO reduction by CO was investigated over CuO/gamma-Al2O3, Mn2O3/gamma-Al2O3, and CuO-Mn2O3/gamma-Al2O3 model catalysts before and after CO pretreatment at 300 degrees C. The CO-pretreated CuO-Mn2O3/gamma-Al2O3 catalyst exhibited higher catalytic activity than did the other catalysts. Based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse reflectance spectroscopy (DRS), Raman, and H-2-temperature-programmed reduction (TPR) results, as well as our previous studies, the possible interaction model between dispersed copper and manganese oxide species as well as gamma-Al2O3 surface has been proposed. In this model, Cu and Mn ions occupied the octahedral vacant sites of gamma-Al2O3, with the capping oxygen on top of the metal ions to keep the charge conservation. For the fresh CuO/gamma-Al2O3 and Mn2O3/gamma-Al2O3 catalysts, the -Cu-O-Cu- and -Mn-O-Mn- species were formed on the surface of g-Al2O3, respectively; but for the fresh CuO-Mn2O3/gamma-Al2O3 catalyst, -Cu-O-Mn- species existed on the surface of gamma-Al2O3. After CO pretreatment, -Cu-square-Cu- and -Mn-square-Mn- (square represents surface oxygen vacancy (SOV)) species would be formed in CO-pretreated CuO/gamma-Al2O3 and CO-pretreated Mn2O3/gamma-Al2O3 catalysts, respectively; whereas -Cu-square-Mn- species existed in CO-pretreated CuO-Mn2O3/gamma-Al2O3. Herein, a new concept, surface synergetic oxygen vacancy (SSOV), which describes the oxygen vacancy formed between the individual Mn and Cu ions, is proposed for CO-pretreated CuO-Mn2O3/gamma-Al2O3 catalyst. In addition, the role of SSOV has also been approached by NO temperature-programmed desorption (TPD) and in situ FTIR experiments. The FTIR results of competitive adsorption between NO and CO on all the CO-pretreated CuO/gamma-Al2O3, Mn2O3/gamma-Al2O3, and CuO-Mn2O3/gamma-Al2O3 samples demonstrated that NO molecules mainly were adsorbed on Mn2+ and CO mainly on Cu+ sites. The current study suggests that the properties of the SSOVs in CO-pretreated CuO-Mn2O3/gamma-Al2O3 catalyst were significantly different to SOVs formed in CO-pretreated CuO/gamma-Al2O3 and Mn2O3/gamma-Al2O3 catalysts, and the SSOVs played an important role in NO reduction by CO.