(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). [1] 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%, [2] 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 Brønsted–Evans–Polanyi relationships for NH_x* activations by both O* and OH* and the barriers for NH_x* 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 NH_x*. Higher N₂ selectivity on Rh than on Pd, however, results from the much larger barrier to form NO than N₂ on Rh, indicating E_a,NHx+OHx and E_{a,N2(g) formation}/ E_{a,NxO(g) formation} are two important factors determining the N₂ selectivity.

REFERENCES

1. S. Shrestha, M. P. Harold, K. Kamasamudram, A. Kumar, L. Olsson, K. Leistner, Catal. Today 267(2016) 130.
2. L. Chmielarz, M. Jabłonska, A. Struminski, Z. Piwowarska, A. Wegrzyn, S. Witkowski, M. Michalik, Appl. Catal. B 130-131(2013) 152.