(327c) DFT and Microkinetic Comparison of Pt, Pd and Rh(111) for Catalytic Ammonia Oxidation
N2 is the desired product over NO/N2O for NH3 oxidation on ammonia slip catalysts (ASC).  While the thermodynamic N2 selectivity is larger than 99% at ASC conditions, the N2 selectivity of highly active platinum group metals (PGMs) are often lower than 80%,  insufficient to meet the emission regulations. To date, there is no self-consistent comparison of intrinsic selectivity over PGMs. Here, we perform DFT computations of relevant reaction steps of ammonia oxidation on Pt, Pd and Rh (111). We show BrÃ¸nstedâEvansâPolanyi relationships for NHx* activations by both O* and OH* and the barriers for NHx* activation follows Pt < Pd < Rh. We build microkinetic models to predict product selectivity at reaction conditions typical of ASC and observe that the N2 selectivity follows Pt > Rh > Pd. We observe the dependence of selectivity on T and pressure follows the same trend for the three metals: low T and P(O2) : P(NH3) result in higher N* coverage and N2 selectivity. The predicted selectivity of N2, NO and N2O agree well with experimental observations. We further perform sensitivity analysis and demonstrate the high selectivity observed for Pt originates from its smaller barriers to activate NHx*. Higher N2 selectivity on Rh than on Pd, however, results from the much larger barrier to form NO than N2 on Rh, indicating Ea,NHx+OHx and Ea,N2(g) formation/ Ea,NxO(g) formation are two important factors determining the N2 selectivity.
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