Surface Species Formed after NO Adsorption and NO + O₂ Coadsorption on ZrO₂ and Sulfated ZrO₂:  An FTIR Spectroscopic Study

Abstract

Adsorption of NO and its coadsorption with oxygen on ZrO 2 and sulfated ZrO 2 have been studied by FTIR spectroscopy. It has been found that NO adsorption on ZrO 2 results in formation of N 2 O (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 1042−1004 cm - 1 ) develop and enhance the Lewis acidity of nearby Zr 4+ 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 Zr 4+ 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−ZrO 2 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.