Inhibition of NH3-SCR over Cu-CHA at high partial pressures of water: Measurements and DFT-based kinetic modeling

Cu-exchanged chabazite (Cu-CHA) is a widely applied catalyst for selective catalytic reduction of nitrogen oxides in oxygen excess. The application of Cu-CHA to exhaust from H₂-fueled engines depends on the behavior of this material at high partial pressures of water. We have performed flow-reactor measurements, which show that the NO${}_x$ conversion at 200 °C over a Cu-CHA (3.2 wt% Cu, Si/Al=6.7) catalyst decreases with increasing partial pressures of water from 2 to 25%. Simultaneously, the apparent reaction order in water decreases from $-0.17$ to $-1.18$, following the simple empirical equation $-3.43{(p_{H_{2}O}/p^{\circ })}^{0.76}$. The experimental results are corroborated with DFT-based microkinetic modeling. The DFT calculations show that H₂O competes with NO adsorption on the Cu-sites and hinders the reaction. The kinetic model describes accurately the inhibiting effect of water after minor adjustments to the computed Gibbs free energies of water adsorption.