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

Authors:
  • JSJános Szanyi
    János Szanyi
  • MPMark T. Paffett
    Mark T. Paffett
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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 NOx.