Energetics of NH3 storage on zeolite SCR catalysts
Josh Pihl, Oak Ridge National Laboratory
Copper-exchanged zeolite catalysts active in the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by NH3 have played a key role in making it possible for high efficiency lean-burn engines (such as those running on diesel fuel) to comply with emissions regulations. However, further tightening of emissions standards is requiring still more advances in catalyst performance. Achieving high NOx conversion while minimizing NH3 slip and efficiently utilizing the reductant (such as urea) requires careful management of the NH3 inventory on the surface of the SCR catalyst. On-board control strategies for NH3 dosing rely heavily on SCR device models that accurately predict transient NH3 storage levels. These models must account for a wide range of operating parameters, including temperature, gas composition, and catalyst age, as well as drive-cycle hysteresis. We have found that the widely used laboratory strategies for measuring NH3 storage characteristics tend to convolute the effects of adsorption energetics, reaction rates, and mass transport, thereby greatly complicating the calibration of model parameters. The resulting models typically do not accurately represent NH3 storage behavior over a realistic range of operating conditions. To circumvent these challenges, we have measured NH3 desorption isotherms that isolate just the energetics of the adsorption/desorption process. We have also used a standard thermodynamic relation to estimate adsorption enthalpy as a function of NH3 coverage from the isotherm data. This approach has been applied to measure the energetics of NH3 adsorption over two different commercial catalyst formulations.