Activation of nitric oxide by heteropoly compounds: Structure of Nitric Oxide Linkages in tungstophosphoric acid with keggin units

The heteropoly compound H₃PW₁₂O₄₀ · 6H₂O is composed of anions of the Keggin structure, (PW₁₂O₄₀)^-3, linked by H₅O⁺₂ to form a body-centered cubic (bcc) structure. The water linkages can be substituted readily by NO linkages at 50-230°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 N₂ 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 Å for the ionic linkage, which is in close agreement with the bcc lattice constant of 11.68 Å 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 N₂ 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.