The dynamic structural evolution of Rh species in mordenite (MOR) zeolite was investigated using in situ spectroscopic techniques and DFT calculations. In situ X-ray absorption spectroscopy and operando infrared (IR) revealed that metallic Rh species were oxidized to afford isolated [Rh(NO)2]+ species under NO flow at 200°C, whereas small Rh metal clusters is formed under the subsequent H2 flow. Ab initio thermodynamics analysis shows that the plausible structures under NO and H2 at 200°C are [Rh(NO)2]+ and Rh clusters in MOR, which is consistent with the experimental observations. A comparative study of Rh-loaded Al2O3 suggests that Al sites in MOR increase the thermodynamic stability of isolated Rh+ species and thus prevent their overoxidation to Rh2O3 under NO. NO capture in the form of [Rh(NO)2]+ and its selective reduction toward NH3 under H2 flow were observed by in situ IR measurements. The RhMOR catalyst exhibited ~60% of NOx conversion above 200°C under periodic lean/rich conditions. Transition state calculations showed that the activation barrier for NO reduction to NH3 on [Rh(NO)2]+ (178 kJ/mol) is higher than that for Rh13 (156 kJ/mol), suggesting that Rh metal clusters are preferable NH3 formation sites, where the Rh13-catalyzed NO reduction into N2 and N2O was less preferable than NH3 formation, which is consistent with the experimental results. Combined with operando IR experiments under lean (NO + O2) and rich (NO + H2) conditions, we show that the reversible dynamic structural evolution of Rh species ([Rh(NO)2]+ Rh metal clusters under lean and rich conditions) is a key mechanistic feature for unsteady-state de-NOx via the capture of NO, its selective reduction to NH3, and the selective reduction of NO with NH3 formed in situ.
Additional experimental details, materials, and methods, including shematic view of experimental setup.
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