ACTIVATION OF NITRIC-OXIDE BY HETEROPOLY COMPOUNDS - STRUCTURE OF NITRIC-OXIDE LINKAGES IN TUNGSTOPHOSPHORIC ACID WITH KEGGIN UNITS

The heteropoly compound H3PW12O40.6H(2)O is composed Of anions of the Keggin structure, (PW12O40)(-3), linked by H5O2+ to form a body-centered cubic (bcc) structure. The water linkages can be substituted readily by NO linkages at 50-230 degrees C at low NO concentrations (i.e., under flue gas conditions) to form H3PW12O40.3NO. A substantial fraction of the absorbed NO is decomposed into N-2 upon rapid heating of the NO-linked compound. The bond energy for the NO linkages is on the order of 25 kcal/mol, based on the TPD results. From the XRD data, the bcc structure is preserved in the NO-saturated compound, except the bcc lattice constant is decreased by 4.2% upon substitution. The IR spectrum of H3PW12O40.3NO shows a single band for NO at 2270 cm(-1). Based on the IR results, the TPD data, and the literature information on the nitrosonium ion (NO+), the linkage in the NO saturated compound is an ionic form of protonated NO, (NOH)(+). Bond length calculations yield a value of 11.74 Angstrom for the ionic linkage, which is in close agreement with the bcc lattice constant of 11.68 Angstrom obtained from XRD. The structure closely resembles that of Cs3PW12O40. The NO molecule is activated upon absorption in the heteropoly compound by protonation, resulting in weakening of the N-O bond. The activation of NO, combined with rapid heating (so desorption can occur at high temperatures), results in N-2 production. The result that the sodium salt Na3PW12O40 cannot absorb NH3 or NO suggests that the ability to interact with H+ is a prerequisite to form linkages in heteropoly compounds. (C) 1995 Academic Pres, Inc.