In Situ Infrared Study of the Absorption of Nitric Oxide by 12-Tungstophosphoric Acid

An extensive diffuse reflectance infrared investigation of the interaction of 12-tungstophosphoric acid (HPW) and its potassium salt with NO x ( x = 1 or 2) was performed. In the presence of KBr, HPW yielded a spectrum identical to that of the potassium salt. Using pure powders, spectra of HPW·6H 2 O showing the (O−H) bands of H 5 O 2 + and the correct appearance of the Keggin anion bands are presented. At 423 K, NO (g) with or without O 2(g) interacted weakly or not at all with the potassium salt; however, NO 2(g) adsorbed onto the surface as loosely bound NO with ν(NO) at 1870 cm -1 . The hexahydrate did not sorb NO in the absence of oxygen at 423 K or higher. In the presence of oxygen at 423 K, the surface of HPW is involved in catalytic oxidation of NO (g) to NO 2(g) via an adsorbed NO x y - intermediate. This species can be absorbed into the HPW structure as a form of NOH +, displacing water, or leave as NO 2(g) . Not all structural water is displaced by NO, and water plays a role in holding the new structures together by hydrogen bonding. The ν(NO) band of absorbed NOH + consists of two overlapping bands, one at 2267 cm -1, assigned to NOH + ·H 2 O, and a second broader band at 2260 cm -1, assigned to NOH + . Both forms bridge between the O t and O c of two Keggin ions. It is proposed that NO first displaces a H 2 O to give the NOH + ·H 2 O moiety, causing rearrangement of the hydrogen bound secondary structure. This structural contraction squeezes water out of adjacent voids, leading to formation of the sorbed anhydrous NOH + moiety. In HPW, NO displaces water at a particular temperature until a stable configuration in the secondary structure is reached. The temperature and amount of water present determine the relative amounts of H 5 O 2 +, H 3 O +, NOH + ·H 2 O, and NOH + present.