Transient Kinetic Analysis of the Reduction Half Cycle in the Standard SCR Redox Mechanism
Enrico Tronconi, Politecnico di Milano, Italy
Cu-CHA systems are currently the state-of-the-art catalysts for the catalytic reduction (SCR) of NOx from lean-burn vehicles [1,2]. Great attention has been paid to the low-temperature reaction mechanism on Cu-CHA under Standard SCR conditions. While there is a general consensus about its redox nature, with the oxidation state of the active Cu cations changing from Cu2+ to Cu+ in the reduction half cycle (RHC) and from Cu+ to Cu2+ in the oxidation half cycle (OHC), the detailed catalytic scheme is still debated [1-5]. Herein, we will present and discuss novel kinetic and mechanistic features of the RHC investigated by transient response methods.
The RHC of the Standard SCR reaction was studied over a Cu-CHA catalyst (2% Cu w/w) with a Si/Al2O3 ratio of 25. Transient kinetic tests were run in a lab rig suitable to test powdered catalyst samples . The typical experimental protocol consisted in an isothermal (150-175-200-220°C) stepwise exposure of preoxidized catalyst samples (16 – 32 mg) to NO and NH3. More specifically, each experiment started with a preoxidation of Cu-CHA with 8% O2 at 550°C for 1h. Then, the catalyst was progressively cooled to the desired test temperature and then saturated with NH3, still in flowing O2 to prevent Cu reduction. The transient reduction tests were performed by stepwise feeding to the reactor 500/1000 ppm of NO and NH3 in He after removing O2 from the feed, while monitoring the outlet species concentrations by MS and UV analyzers. The protocol was replicated in dry and wet conditions (0-2% H2O v/v) and at different GHSV (266250-450000 Ncm3/h/gcat). The data were fitted according to a transient integral model of the test flow reactor (cascade of 20 CSTRs). The adopted RHC rate equation was 1st order in the NO concentration, while either a first or a second order rate laws in the fraction of oxidized Cu sites was assumed in the data fit.
NO and NH3 were able to fully reduce the Cu-CHA catalyst at low temperature under different conditions. In addition, the integral balance calculations of converted NO and NH3 and of produced N2 were found in line with the Cu2+ reduction stoichiometry proposed in the literature, namely: reduced Cu2+: NO converted : NH3 converted : N2 produced = 1:1:1:1 mol/mol . The best fit of the Cu-CHA reduction transients under both dry and wet conditions was clearly obtained assuming a 2nd order dependence of the Cu reduction rate on the Cu2+ fraction: this seems hardly compatible with single-site RHC mechanisms, as commonly proposed so far, rather suggesting a key role of binuclear Cu2+ active centers. Similar conclusions apply to the fit of other transient RHC runs covering the effects of NO concentration and space velocity. H2O significatly inhibited the RHC rate . The estimated RHC activation energies were about 60 kJ/mol (dry conditions) and 30 kJ/mol (wet conditions), against apparent activation energies of the global low-T Standard SCR of ~ 59 and 53 kJ/mol, respectively.
By transient kinetic analysis we reveal a second order kinetic dependence of the RHC on the oxidized Cu fraction, which may have important implications for the NH3-SCR catalytic mechanism over Cu-CHA. The companion transient kinetic analysis of the oxidation half cycle (OHC) is ongoing.
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