Direct measurement of enthalpy and entropy changes in NH 3 promoted O 2 activation over Cu−CHA at low temperature

Oxygen activation is a key step in the selective catalytic reduction of nitrogen oxides with ammonia (NH 3 ‐SCR) over Cu‐chabazite. We present direct measurements of oxygen adsorption at low temperatures over [NH 3 −Cu−NH 3 ] + 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 [NH 3 −Cu−NH 3 ] + complexes at 200 °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 [NH 3 −Cu−NH 3 ] + 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 [NH 3 −Cu−NH 3 ] + complexes as compared to the framework‐bound Cu + species.