Selective oxidation of propylamine on oxygen-covered Au(111): a DFT study

The reaction mechanism for selective oxidation of propylamine on oxygen-covered gold has been studied by the density functional theory (DFT) and generalized gradient approximation (GGA) with slab model. Our calculation results indicated that the adsorption energy of propylamine decreases with the increasing oxygen coverage, that is -0.38, -0.20 and -0.10 eV on clean, 2/9 monolayer (ML) and 2/3 monolayer (ML) oxygen, respectively. The adsorption energies of the intermediates also have the trend of the gradual lower. The present work also indicated that the final product distribution depends on the oxygen coverage: propylamine undergoes N-H bond and C-H bond cleavage to produce propionitrile and water at low-oxygen-coverage (θ(o) = 2/9 ML), and to yield propionitrile, propionaldehyde and water at high-oxygen-coverage (θ(o) = 2/3 ML). The energy barrier of the first step of propyamine oxidation (CH(3)CH(2)CH(2)NH(2) → CH(3)CH(2)CH(2)NH) is 0.16 eV (θ(o) = 2/9 ML) and 0.38 eV (θ(o) = 2/3 ML). On the second step, the barrier energy is 0.16 (θ(o) = 2/9 ML) and 0.25 (θ(o) = 2/3 ML) eV of CH(3)CH(2)CH(2)NH → CH(3)CH(2)CH(2)N, next both C-H breakage and the barrier energy is 0.20 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.25 eV (CH(3)CH(2)CHN → CH(3)CH(2)CN) on low oxygen coverage, and 0.15 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.26 eV(CH(3)CH(2)CHN → CH(3)CH(2)CN) on the high oxygen coverage. The additional reaction step of CH(3)CH(2)CHN → CH(3)CH(2)CHO occurs on the high oxygen coverage, and the associated barrier is 0.41 eV. The calculation results show that the oxidation of propylamine can occur at room temperature due to the lower energy barrier. Furthermore, it was found that the energy barrier for the possible reaction steps at the low oxygen coverage is generally smaller than that on high oxygen coverage, which agrees with the experimental results.