Integrated operando X-ray absorption and DFT characterization of Cu–SSZ-13 exchange sites during the selective catalytic reduction of NOx with NH3

Abstract

We investigate the Cu oxidation state and coordination environment in copper-exchanged chabazite (Cu–SSZ-13) under operando conditions representative of NOx selective catalytic reduction (SCR) with ammonia, using a combination of X-ray absorption spectroscopy (XAS) experiments, density functional theory (DFT), and first-principles thermodynamics models. Four-fold-coordinated Cu(II) are found to dominate the as-prepared Cu–SSZ-13 and Cu–SSZ-13 under so-called Fast and Slow SCR conditions, in which the NO2/NOx ratios are 0.5 and 1, respectively. Under Standard SCR conditions, containing no NO2 in the feed, mixed Cu(I) and Cu(II) oxidation states are observed along with a reduction in the average Cu coordination. The rate per mole was found to be about equal for Fast and Slow SCR conditions and a factor of two less for Standard conditions. Periodic DFT calculations are used to determine the structure, oxidation state, and relative stability of Cu ions charge compensated with one or two Al and with extralattice oxy, hydroxy, and water ligands. Two-fold Cu(I) and 4-fold Cu(II) species bound with H2O or OH are found to be most stable over a wide range of conditions. Using the NO2/NO ratio as a surrogate for the reaction oxidation strength provides good agreement with the observed appearance of Cu(I) species under Standard SCR conditions. The results highlight the role of Cu redox chemistry in SCR catalysis.