FTIR INVESTIGATION OF THE COADSORPTION OF ACETONITRILE AND CARBON-MONOXIDE ON CU-ZSM-5

The adsorption of acetonitrile and its co-adsorption with CO on a soli d-state ion-exchanged Cu-ZSM-5 catalyst have been studied. Acetontrile is adsorbed on both Cu+ and Cu2+ centres of Cu-ZSM-5. The adsorption of acetonitrile on Cu+ sites proceeds in a sequential fashion resultin g in the formation of [Cu(CD3CN)(n less than or equal to 3)](+) comple xes. Complexes with n less than or equal to 2 are stable at room tempe rature while those with n>2 exist only in the presence of excess CD3CN in the gas phase. [Cu(CD3CN)(n less than or equal to 2)](+) complexes readily adsorb CO forming [Cu(CD3CN)(n less than or equal to 2)(CO)(m less than or equal to 2)](+) species in which the Cu+... CO bonds are significantly weakened by the presence of CD3CN ligands compared to t he CD3CN-free Cu+... CO complexes. On [Cu(CD3CN)(n>2)](+) complexes CO is observed to displace one or two CD3CN ligands forming [Cu(CD3CN)(n less than or equal to 2)(CO)(m less than or equal to 2)](+) complexes . The weakening of the Cu+... CO bond in the CD3CN-containing complexe s can primarily be attributed to the increased electron density around the Cu+ sites, brought about by charge donation from the lone pair el ectron on the nitrogen atom of the adsorbed CD3CN toward Cu+. With two or more CO ligands the CD3CN Cu+ bond weakens owing to the Cu+... CO interaction and probably to steric crowding around the Cu+ adsorption site. The characteristics of the IR absorption bands observed in the a dsorption and decomposition of C2H5NO2 and Cu-ZSM-5 are very similar t o those seen upon CH3CN adsorption, suggesting that adsorbed nitrile s pecies are indeed formed in the selective catalytic reduction of NOx.