Mechanistic study of the abatement of nitrogen oxides over metal-ion exchanged ZSM-5 catalysts

Oxygen-bridged copper dimers, Cu-O-Cu{dollar}sp{lcub}rceil 2+{rcub}{dollar}, that form in ZSM-5 at high Cu loading were found to be EPR "silent". They are detectable by their perturbation of the lattice vibration as observed by an FTIR band at 918-923 cm{dollar}sp{lcub}-1{rcub}{dollar}. Upon reduction by H{dollar}sb2{dollar}, CO or thermal treatment, formation of cuprous centers is detected by an FTIR band at 966 cm{dollar}sp{lcub}-1{rcub}{dollar}. FTIR data show that the oxocation is reformed after NO exposure and thermal desorption; the evolution of N{dollar}sb2{dollar}O is assumed. These Cu complexes may act as reactive sites for NO decomposition.; Upon NO + O{dollar}sb2{dollar} exposure, Cu/ZSM-5 forms nitro and nitrate groups that are stable at 200{dollar}spcirc{dollar}C. These complexes are chemically reduced to N{dollar}sb2{dollar} upon C{dollar}sb3{dollar}H{dollar}sb8{dollar} exposure while remaining inert to CH{dollar}sb4{dollar}. Conversely, nitrito complexes are most prominent on Co/ZSM-5 and are stable at 150{dollar}spcirc{dollar}C. This complex is reduced to N{dollar}sb2{dollar} upon exposure to C{dollar}sb3{dollar}H{dollar}sb8{dollar} or CH{dollar}sb4{dollar}. The reactivity of these complexes matches the known hydrocarbon specificity for NO{dollar}sb{lcub}rm x{rcub}{dollar} reduction in the continuous flow mode. Therefore, the documented steady state hydrocarbon specificity can be traced back to the nature of the NO{dollar}sb{lcub}rm y{rcub}{dollar} groups formed upon exposure NO + O{dollar}sb2{dollar}.; The reaction of NO with adsorbed acetone oxime has been studied over Cu/ZSM-5 by employing FTIR and MS. It is found that {dollar}sp{lcub}15{rcub}{dollar}NO{dollar}sb{lcub}rm gas{rcub}{dollar} reacts with {dollar}sp{lcub}14{rcub}{dollar}N-labelled acetone oxime to form {dollar}sp{lcub}14{rcub}{dollar}N{dollar}sp{lcub}15{rcub}{dollar}N and {dollar}sp{lcub}14{rcub}{dollar}N{dollar}sp{lcub}15{rcub}{dollar}NO at temperatures below which the acetone oxime decomposes or isomerizes. From these results, a possible reaction pathway for SCR is proposed in which an adsorbed complex such as acetone oxime reacts with a gas phase NO molecule.; Formation of PdO particles was observed in ion exchanged Pd/HZSM-5 and Pd/NaZSM-5 after reduction and reoxidation at 500{dollar}spcirc{dollar}C. Formation of Pd{dollar}sp{lcub}2+{rcub}{dollar} is not observed in the absence of NO{dollar}sb{lcub}rm x{rcub}{dollar}. The rate of NO{dollar}sb{lcub}rm x{rcub}{dollar} reduction over a reduced and reoxidized Pd/HZSM-5 catalyst slowly increases with time on stream while PdO is converted to Pd{dollar}sp{lcub}2+{rcub}{dollar}. Pd/HZSM-5 is active also when Pd is initially present as PdO particles, but Pd/NaZSM-5 remains inactive when Pd is present as Pd{dollar}sp{lcub}2+{rcub}{dollar} ions. Metal sites and protons are required for catalysis; Pd/HZSM-5, therefore, is a bifunctional catalyst.