7 research outputs found
Investigation of Low-Temperature OHC and RHC in NH<sub>3</sub>–SCR over Cu-CHA Catalysts: Effects of H<sub>2</sub>O and SAR
A transient kinetic approach was
applied to independently
investigate
the oxidation half-cycle (OHC) and reduction half-cycle (RHC) of NH3–selective catalytic reduction (SCR) at low temperature
(150–200 °C). Three model Cu-exchanged chabazite (Cu-CHA)
samples with fixed Cu loading (∼1.8% w/w) and different silica-to-alumina
ratios (SARs = 10-17-25) were investigated under dry and wet conditions.
We confirmed the following: (i) OHC proceeds via second- and first-order
kinetics in CuI and O2, respectively, with O2 (+H2O) alone able to completely reoxidize CuI sites; (ii) RHC proceeds via second- and first-order kinetics
in CuII and NO, respectively, according to a Cu/NO = 1:1
stoichiometry. Notably, coupling RHC and OHC rates resulted in an
accurate prediction of the steady-state standard SCR conditions, providing
the consistent closure of the SCR redox chemistry. Unexpectedly, we
revealed the impact of H2O to vary depending on the catalyst
formulation. At high SAR, water inhibits the RHC and promotes the
OHC. As a result, a limited impact was observed on steady-state deNOx activity, while the Cu-oxidation state was significantly
enhanced by H2O. With decreasing SAR, however, the H2O effect on RHC gradually shifts from inhibition to promotion,
while the OHC is always promoted. At fixed water content, we revealed
the RHC rate to decrease with decreasing Al density, with minor influence
observed on the OHC; as a result, a lower deNOx activity
was observed upon increasing SAR. Remarkably, the application of transient
kinetic analysis to decouple RHC and the OHC greatly facilitated the
identification of complex H2O and SAR effects on the global
SCR redox chemistry