IN-SITU INFRARED STUDY OF THE ABSORPTION OF NITRIC-OXIDE BY 12-TUNGSTOPHOSPHORIC ACID

An extensive diffuse reflectance infrared investigation of the interac tion of 12- tungstophosphoric acid (HPW) and its potassium salt with N Ox (x = 1 or 2) was performed. In the presence of KBr, HPW yielded a s pectrum identical to that of the potassium salt. Using pure powders, s pectra of HPW . 6H(2)O showing the (O-H) bands of H5O2+ and the correc t 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 potass ium salt; however, NO2(g) adsorbed onto the surface as loosely bound N O with v(NO) at 1870 cm(-1). The hexahydrate did not sorb NO in the ab sence 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 NOxy- intermediate. This species can be absorbed into the HPW structure as a form of NOH+, displacing water, or leave a s NO2(g). Not all structural water is displaced by NO, and water plays a role in holding the new structures together by hydrogen bonding. Th e v(NO) band of absorbed NOH+ consists of two overlapping bands, one a t 2267 cm(-1), assigned to NOH+. H2O, and a second broader band at 226 0 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 H2O to gi ve the NOH+. H2O moiety, causing rearrangement of the hydrogen bound s econdary structure. This structural contraction squeezes water out of adjacent voids, leading to formation of the sorbed anhydrous NOH+ moie ty. In HPW, NO displaces water at a particular temperature until a sta ble configuration in the secondary structure is reached. The temperatu re and amount of water present determine the relative amounts of H5O2, H3O+, NOH+. H2O, and NOH+ present.