FTIR investigation of the co-adsorption of acetonitrile and carbon monoxide on Cu-ZSM-5

Abstract

The adsorption of acetonitrile and its co-adsorption with CO on a solid-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 resulting in the formation of [Cu(CD₃CN)_{n⩽ 3}]⁺ complexes. Complexes with n⩽ 2 are stable at room temperature while those with n > 2 exist only in the presence of excess CD₃CN in the gas phase. [Cu(CD₃CN)_{n⩽ 2}]⁺ complexes readily adsorb CO forming [Cu(CD₃CN)_{n⩽ 2}(CO)_{m⩽ 2}]⁺ species in which the Cu⁺⋯CO bonds are significantly weakened by the presence of CD₃CN ligands compared to the 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⩽ 2}(CO)_{m⩽ 2}]⁺ complexes. The weakening of the Cu⁺⋯CO bond in the CD₃CN-containing complexes can primarily be attributed to the increased electron density around the Cu⁺ sites, brought about by charge donation from the lone pair electron 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 adsorption and decomposition of C₂H₅NO₂ and Cu-ZSM-5 are very similar to those seen upon CH₃CN adsorption, suggesting that adsorbed nitrile species are indeed formed in the selective catalytic reduction of NO_x.