Dual-Site RHC and OHC Transient Kinetics Predict Low-T Standard SCR Steady-State Rates over a Cu-CHA Catalyst

Abstract

The reduction (RHC: Cu II → Cu I ) and oxidation (OHC: Cu I → Cu II ) half-cycles of low-temperature (150−200 °C) NH 3 -selective catalytic reduction (SCR) were investigated by transient response methods over a model Cu-CHA catalyst (Cu loading = 1.8% w/w; silica to alumina ratio = 25). In line with previous findings: i) RHC proceeded via second-order kinetics in Cu II and with an equimolar stoichiometry between Cu II reduced and NO consumed; ii) complete reoxidation of reduced NH 3solvated Cu sites was achieved by exposing the catalyst to O 2 + H 2 O only, no other species (e.g., NO) were found to be relevant to the OHC pathway. Dedicated kinetic tests highlighted a secondorder dependence of the OHC rate on Cu I and a first-order dependence on O 2 . Coupling the RHC and OHC kinetic models, independently developed from transient tests, resulted in an accurate description of Standard SCR turnovers, predicting steady-state NO conversion, N 2 formation, and bed-average Cu-oxidation states, as well as light-off and extinction transients closely consistent with experimental measurements. These results demonstrate the rational dissection of the complex steady-state SCR reaction network into two redox half-cycles and provide a stoichiometrically and kinetically consistent closure of the Standard SCR redox cycle over Cu-CHA catalysts.

Effect of Framework Composition and NH3 on the Diffusion of Cu+ in Cu-CHA Catalysts Predicted by Machine-Learning Accelerated Molecular Dynamics

“…The NH 3 –SCR–NO x reaction is currently employed for the removal of nitrogen oxides (NOx) from exhaust gases in diesel vehicles and stationary plants through a redox catalytic cycle in which Cu + is oxidized to Cu 2+ by O 2 , NO 2 , or NO + O 2 and then reduced to Cu + by the reaction of NH 3 and NO forming harmless N 2 + H 2 O (Scheme ). − This understanding of the reaction mechanism has enabled the development of optimized catalysts by tuning the framework topology, composition, and copper speciation. In the as-prepared catalysts, Cu + and Cu 2+ cations are directly coordinated to the zeolite framework forming heterogeneous active sites, while under reaction conditions NH 3 solvates the Cu + cations forming mobile [Cu­(NH 3 ) 2 ] + complexes that act as dynamic active sites, resembling homogeneous catalysts but within the confinement of the zeolite pores.…”

Investigation of Low-Temperature OHC and RHC in NH3–SCR over Cu-CHA Catalysts: Effects of H2O and SAR

“…Prior to each test, a preoxidation step (1 h at 550 °C in 8% v/v O 2 ) was always performed, thus ensuring the complete oxidation of Cu sites, consistent with literature . In line with our previous publications, ,− a transient response approach was applied to gather mechanistic knowledge on the standard SCR process. Only a brief description is given in the following, recalling the experimental protocols already described in our previous publications: more detailed information is available in ref .…”

“…In line with our previous publications, ,− a transient response approach was applied to gather mechanistic knowledge on the standard SCR process. Only a brief description is given in the following, recalling the experimental protocols already described in our previous publications: more detailed information is available in ref .…”

“…Once stable conditions were achieved, the remaining O 2 was removed from the feed, and a second reduction phase started. As discussed in Section , by integration of the NO and N 2 transient profiles collected in the second RHC step, and applying eq , we were able to indirectly estimate the bed-average Cu I –Cu II partitioning achieved at the STD-SCR steady-state. , …”

“…The authors reported the apparent oxidation rate constants to significantly increase with higher Al density, while the reduction rate constants displayed a volcano-shaped trend with the Al content. However, it should be mentioned that their Langmuirian model is based on first-order kinetics of RHC and OHC in Cu II and Cu I , respectively, which is apparently in contrast with their proposal of a Cu/redox cycle involving a single-site Cu II reduction and a dual-site Cu I oxidation mechanism and is at variance with recent reports of both RHC and OHC rate obeying second-order kinetics in the relevant Cu sites. ,,, In any case, despite varying the oxygen content, oxidation and reduction kinetic information was still indirectly derived from NH 3 –SCR conditions.…”