Surface species formed after NO adsorption and NO+O2 coadsorption on ZrO2 and sulfated ZrO2:: An FTIR spectroscopic study

Adsorption of NO and its coadsorption with oxygen on ZrO2 and sulfated ZrO2 have been studied by FTIR spectroscopy. It has been found that NO adsorption on ZrO2 results in formation of N2O (2282, 2244, and 1233 cm(-1)) and small amounts of nitro species (1443 and 1423 cm(-1)), nitrates (1558, 1189, and 1057 cm(-1)), and nitric acid (1621 and 1142 cm(-1)). With time, the nitrate bands (1640-1560, 1290-1220, and 10421004 cm(-1)) develop and enhance the Lewis acidity of nearby Zr4+ cations. As a result, coordinated NO (1906 cm(-1))is formed. On sulfated zirconia, nitrosyls (1908 cm(-1)) are produced immediately after NO adsorption evidencing an increased acidity of the Zr4+ cations as a result of the sulfate inductive effect. In addition, nitrates and nitric acid are produced similarly to the case of the sulfate-free sample. Addition of oxygen to the NO-ZrO2 system results in formation of nitrites (1180 cm(-1), disappearing in excess oxygen) and in a strong increase in concentration of the nitrates. Besides, NO+ (2206 cm(-1)) is found. The nitrate species manifest combination modes in the 2700-2400 cm(-1) region, which facilitates their spectral identification. Coadsorption of NO and O-2 on the sulfated sample results in a similar picture, but the nitrates formed are less stable.