Infrared Study of a Novel Acid−Base Site on ZrO2 by Adsorbed Probe Molecules. I. Pyridine, Carbon Dioxide, and Formic Acid Adsorption

The adsorption of pyridine, carbon dioxide (CO 2 ), and formic acid (HCOOH) on various types of surface sites of ZrO 2, vacant sites (for example: bare surface Zr 4+ O 2- ions), and those modified by terminal ( t -) and bridged ( b -) OH as well as t - and b -methoxy (OCH 3 ) groups, has been studied by infrared (IR) spectroscopy. The selective reaction of CO 2 with the t -OH group at 213 K and the adsorption of pyridine to replace the t -OH group on ZrO 2 at 373−573 K confirms that t -HO−Zr site possesses acidic and basic properties. Moreover, the nucleophilic substitution adsorption of pyridine occurs more actively at the t -HO−Zr site than that at the vacant t -site on ZrO 2 . At 523 K, HCOOH reacts with basic t -OH or t -OCH 3 species before the b -OH or b -OCH 3 species. The selective reaction of HCOOH with t - and b -OCH 3 groups on ZrO 2 disappears at 573 K. The high activity of t -HO−Zr sites is attributed to relatively stronger Lewis acidic sites generated by releasing t -OH groups in the adsorption, and is also related to the high reactivity of t -OH groups. The Zr ions or t -OH groups at t -HO−Zr sites can play different roles in the adsorption of HCOOH, CO 2, and pyridine, leading to its distinguishable features from those on vacant and b -OH sites. On the other hand, the OCH 3 species at 573 K is supposed to be movable during the adsorption, resulting in the disappearance of the difference in reactivity of t - and b -OCH 3 groups in the adsorption of HCOOH to replace OCH 3 on ZrO 2 .