N2 is the desired product over NO/N2O for NH3 oxidation on ammonia slip catalysts (ASC). The selectivity to N2 of highly active platinum group metals (PGMs), however, are often lower than 80%, insufficient to meet the emission regulations. To date, there is no self-consistent examination of mechanism and selectivity over well-defined PGM catalysts. Here, we perform DFT calculations of all relevant reaction steps in ammonia oxidation on Pt, Pd and Rh (111) and build microkinetic models to predict selectivities at reaction conditions typical of ASC. We show Brønsted–Evans–Polanyi relationships for NH3 activations by both O and OH. The barriers to oxidative dehydrogenate NH3 follow Pt &lt; Pd &lt; Rh while they are comparable for surface N2, NO and N2O formation. We demonstrate, with microkinetic modeling, that low temperature and deficient oxygen facilitate the formation of N2 and the selectivity to N2 follows Pt &gt; Rh &gt; Pd at ASC conditions. The temperature dependence of selectivity is the result of compensation effect, leading to N2 selective at low temperature and NO selective at high temperature. We demonstrate that the predicted selectivity and overall activation activities agree well with experimental observations. We further perform sensitivity analysis and show that high N* coverage and a lower barrier for N2 formation than NO/N2O are key factors to high N2 selectivity.

Understanding selectivity in ammonia oxidation: DFT and microkinetic modeling over Pt, Pd and Rh (111)

256th National Meeting (2018)

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