Preparation and characterization of cobalt and copper ion-exchanged ZSM-5 zeolite as catalysts for nitric oxide decomposition

ZSM-5 zeolites exchanged by Co(II) ions (Co-ZSM-5) or Cu(II) ions (Cu-ZSM-5) accomplish selective catalytic reduction of nitric oxide (NO). Reactions of NO with copper ions in Cu-ZSM-5 and with cobalt ions in Co-ZSM-5 have been studied by electron spin resonance (ESR), electron spin echo modulation (ESEM) and diffuse reflectance ultraviolet-visible spectroscopy. The ESEM results indicate that Cu(II) ions are present in hydrated Cu-ZSM-5 as Cu(II)-(OH –)(H₂O)₅. Upon thermal dehydration to 450°C, about 85% of the paramagnetic Cu(II) ions in ZSM-5 change to diamagnetic species such as Cu(I) and Cu(II)-O–. ESR studies of Co(II) ions in dehydrated Co-ZSM-5 suggest formation of Co(II)(H₂O)(OL) ₃ where OL indicates lattice oxygen.; After NO adsorption onto dehydrated Cu-ZSM-5, NO interactions with Cu(I), Cu(II)-O– and Cu(II) have been studied by ESR and Fourier transform infrared (FTIR) spectroscopy. Based on observation of Cu(I)-NO, Cu(I)-(NO)₂, Cu(I)-NO₂, Cu(II)-NO, Cu(II)-(NO ₂)(NO–) and Cu(II)-(N₂O)O– , a NO decomposition cycle over Cu-ZSM-5 has been proposed. In contrast, NO adsorption onto Co(II) in ZSM-5 leads to the formation of Co(0)-(NO) ₂ based on ESR, diffuse reflectance ultraviolet-visible and FTIR spectroscopy.; FTIR studies of CO adsorption on dehydrated Cu-ZSM-5 and Co-ZSM-5 suggest the formation of Cu(I)-CO and Cu(II)O–(CO)(H₂O) at room temperature and Co(II)-CO below 200 K. In Co(II)-CO complexes, hyperfine analysis indicates that 91% of the unpaired electron spin density is localized on Co(II).; Selective catalytic reduction of NO with propene (C₃H ₆-SCR) and propane (C₃H₈-SCR) over Cu-ZSM-5 and Co-ZSM-5 has been evaluated. A reaction pathway at 450°C for C₃H ₆-SCR over Cu-ZSM-5 is proposed based on Cu-allyl, Cu-allyl oxime, Cu-ethenyl isocyanate, Cu-primary amine and Cu-ethenyl reaction intermediates identified by IR absorption. Different pathways are proposed for C₃H ₈-SCR over Cu-ZSM-5 at 250 to 340°C and for C₃H ₈-SCR over Co-ZSM-5 at 230 to 550°C based on observation of CH ₃NO₂, HNCO, CO₂, Al-NCO, Si-NCO and Co-NCO as reaction intermediates.; FTIR spectroscopy was used to interpret the interaction between the gases produced from nitromethane decomposition and Cu(II)-ZSM-5 or Co(II)-ZSM-5. Si-NCO and Cu(I)-NCO are observed at 300°C during nitromethane decomposition over Cu-ZSM-5. Co(II)-NCO is observed at 200 to 450°C over Co-ZSM-5. During the formation of Co(II)-NCO, NCO substitutes for water in Co-ZSM-5.