Identification of the active site in Cu/SSZ-13 catalysts for Selective Catalytic Reduction of NOx by NH3 by combining Operando X-ray Absorption Spectroscopy and DFT Modeling

Fabio  Ribeiro, Purdue University

Removing NOx from vehicular exhaust continues to be a longstanding challenge in environmental catalysis, as the community tries to meet ever stricter environmental regulations. Recent development of Cu-chabazite (CHA) based catalysts for the selective catalytic reduction (SCR) of NOx by NH3, have shown not only high activity in reducing NOx to the desired products but also great hydrothermal stability. We have investigated the Cu/SSZ-13 catalytic system in both operando x-ray absorption spectroscopy (XAS) as well as through density functional theory (DFT) and first principles thermodynamics modeling.
The XAS data analysis found that under standard SCR gas conditions (300 ppm NO, 300 ppm NH3, 5% O2, 5% CO2, 5% H2O and balance He) that roughly 15% of the Cu on the catalyst was present in the reduced Cu(I) oxidation state, while the remaining Cu was fully oxidized in the Cu(II) state. To further investigate the redox nature of the Cu site, transient cut-off experiments were performed during which one of the components of the reaction was replaced with inert gas as we followed the transient change in the Cu oxidation state by XAS. In this way the oxidizing or reducing portion of the reaction could be isolated and the catalyst was driven into the fully oxidized (i.e. Cu(II)) or reduced (i.e. Cu(I)) state. Removing oxygen from the reaction drove almost all of the Cu into the Cu(I) state, while removing NO (but leaving the ‘reductant’ NH3 in the mixture) drove the catalyst to the fully oxidized Cu(II) state.
The redox picture was further investigated with the addition of DFT and thermodynamics based models of the active site. Fully periodic DFT calculations of the Cu/SSZ-13 unit cell in the presence of various adsorbates showed that the Cu could be present in both the Cu(I) and Cu(II) oxidation states, depending on the gas environment. We found that the two-fold coordinate ZCu(H2O) and the four-fold coordinate ZCu(OH)2 were the most thermodynamically stable species. Similar to the XAS experiments above the ZCu(H2O) the Cu was in the reduced Cu(I) oxidation state while for the ZCu(OH)2 the Cu was in the Cu(II) oxidation state. Further analysis showed that the structures are inter-related and as the gas environment passes from the more reducing to the more oxidizing state the Cu(II) species overtakes the Cu(I) species as the more thermodynamically stable configuration. Combining the experimental and computational results provides a strong picture of how the SCR reaction progressed through a Cu redox cycle where the Cu switches between the Cu(I) and Cu(II) oxidation states.