The Cu CHA deNOx Catalyst in Action: Temperature Dependent NH3 Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES.
The small pore Cu CHA zeolite is today the object of intensive research efforts to rationalize its outstanding performance in the NH3 assisted selective catalytic reduction (SCR) of harmful nitrogen oxides and to unveil the SCR mechanism. Herein we exploit operando X ray spectroscopies to monitor the Cu CHA catalyst in action during NH3 SCR in the 150 400 degC range, targeting Cu oxidation state, mobility, and preferential N or O ligation as a function of reaction temperature. By combining operando XANES, EXAFS, and vtc XES, we unambiguously identify two distinct regimes for the atomic scale behavior of Cu active sites. Low temperature SCR, up to ~200 degC, is characterized by balanced populations of Cu(I)/Cu(II) sites and dominated by mobile NH3 solvated Cu species. From 250 degC upward, in correspondence to the steep increase in catalytic activity, the largely dominant Cu species are framework coordinated Cu(II) sites, likely representing the active sites for high temperature SCR.
Journal of the American Chemical Society