Another degree of freedom in literature reports is the

At the higher space velocity (285 k h−1) shown in Fig. 3b it LY2109761 is clear that the model predictions capture all of the trends although the quantitative agreement is not as good as in Fig. 3a. Specifically, the model over-predicts the NH3 conversion in the 250–300 °C temperature range, and in turn over-predicts the N2 and N2O yields. At the higher space velocity the reaction system is impacted by external mass transport limitations at high temperature. This is indicated by the incomplete ammonia conversion asymptote at high temperature. This is indeed captured well by the model which underscores the importance of having estimated the diffusivity independently (Fig. 1). On the other hand, the poorer predictability in the intermediate temperature range of 250–300 °C is similar to the over-prediction in the inert γ-Al2O3 top layer experiments. One cannot rule out the impact of washcoat diffusion even for the washcoat thickness of only 15 μm. Colombo et al. [21] showed that diffusion limitations could be neglected for washcoat thickness only below 10 μm in their study of ammonia oxidation. We attribute the over-prediction to two factors. First, it is known that there is an uneven washcoat thickness around the periphery of the channel [31]. That is, more severe limitations are expected in the channel corners where the washcoat is thicker. Not accounting for this may lead to an over prediction of the conversion. Another possibility follows from our earlier point that the estimated diffusivity in the inert layer corresponds to diffusion in macropores while the diffusion limitation in the active layer is likely in smaller mesopores. This would mean that the effective diffusivity is over-estimated which in turn leads to an over prediction of the conversion in this regime.