In Situ Spectroscopic Studies on the Redox Cycle of NH3−SCR over Cu−CHA Zeolites

Abstract The selective catalytic reduction of NO with ammonia (NH 3 −SCR) catalyzed by Cu−CHA zeolites is thoroughly investigated using in situ spectroscopic experiments combined with on‐line mass spectroscopy (MS) under steady‐state NH 3 −SCR conditions and transient conditions for Cu(II)/Cu(I) redox cycles. Quantitative analysis of the in situ XANES spectra of Cu−CHA under steady‐state conditions of NH 3 −SCR show that NH 3 ‐coordinated Cu(II) species is the dominant Cu species at low temperatures (100–150 °C). At higher temperatures, Cu(II) species and [Cu(NH 3 ) 2 ] + complex coexist, possibly because the rate of the Cu(II)→Cu(I) reduction step is comparable to that of the Cu(I)→Cu(II) oxidation step. In situ XANES, IR/MS, and UV‐vis/MS experiments on the reduction half cycle demonstrate that the reduction of Cu(II) species occurs via the reaction of NH 3 ‐liganded Cu(II) with NO to yield N 2 and H 2 O. For the oxidation half cycle, in situ XANES experiments of Cu(I) oxidation in 10 % O 2 at 200 °C indicate that an increased density in CHA zeolite exhibits a higher oxidation rate. In situ UV‐vis experiments of Cu(I) reoxidation using different mixtures of oxidant feed gas demonstrate the key role of O 2 in the oxidation cycle. It is suggested that the reoxidation of Cu(I) to Cu(II) species occurs with only O 2 as the oxidant, and a high Cu density in CHA zeolite promotes SCR activity by enhancing the oxidative activation of Cu(I) to Cu(II) during the catalytic cycle.