Effects of dioxygen pressure on rates of NOx selective catalytic reduction with NH3 on Cu-CHA zeolites

•NOx SCR rates on Cu-CHA show a Langmuirian dependence on O2 pressure.•In operando X-ray absorption quantifies Cu(I) and Cu(II) distributions during SCR.•First- and zero-order rate constants reflect the oxidation and reduction limit.•Rates increase with Cu ion density as more sites become pairable and thus active.•Transient XAS quantifies oxidation and reduction rates and Cu active sites.