A simple analytical equation to assess the probe effect on Spaci-MS data: Computational and experimental validation for the redox kinetics of low-T NH3-SCR over a Cu-CHA monolith catalyst

Spatially resolved Mass Spectrometry (Spaci-MS) is a powerful tool for the study of chemical processes over honeycomb monolith catalysts, the commercial configuration of catalytic converters for pollutant abatement from combustion exhausts. A limit of this technology lies in the intrinsic perturbation of the measurements caused by the mass spectrometry probe: the insertion of the capillary affects the flow distribution in the probed monolith channel due to the incremented flow resistance, altering the catalyst contact time and thus the measured catalytic activity. In this work, we study the effect of the probe on the spatially resolved low temperature DeNOx activity of a model Cu-CHA NH3-SCR catalyst. Transient tests were performed to calibrate a redox model of the investigated catalyst, incorporating Cu site reduction (RHC, CuII -> CuI) and oxidation (OHC, CuI -> CuII) kinetics. Then, the simulated axial evolution of NO conversion (validated by tests performed in a conventional flow reactor at increasing space velocity) was compared to spatially resolved data obtained in a Spaci-MS rig to capture the effect of the probe. Based on this comparison, a simple analytical formula was developed to directly assess a priori the effect of the diameter, sampling flowrate and eccentricity of the Spaci-MS capillary probe. The equation was successfully validated against both CFD simulations and the experimental NH3-SCR activity data.