Direct measurement of enthalpy and entropy changes in NH3 promoted O2 activation over Cu‐CHA at low temperature

Oxygen activation is a key step in the selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR) over Cu-chabazite. We present direct measurements of oxygen adsorption at low temperatures over [NH3-Cu-NH3](+) complexes and framework-bound Cu+ species in Cu-chabazite with Si/Al=14 using isothermal microcalorimetry combined with mass spectrometry. The enthalpy change for O-2 adsorption over [NH3-Cu-NH3](+) complexes at 200 degrees C is determined to be -79 kJ/mol. By fitting a Langmuir isotherm, the corresponding entropy change is determined to be -142 J/(mol*K) at 10 % O-2. The results show that O-2 adsorption at low temperatures over [NH3-Cu-NH3](+) complexes is more facile than on framework-bound Cu+ species. The experimental results are in agreement with density functional theory calculations showing a lower barrier for O-2 activation over the [NH3-Cu-NH3](+) complexes as compared to the framework-bound Cu+ species.