Insight into the active phase of CO oxidation on Ni/Pt and NiO1 − x/Pt model catalysts from a first principles investigation

Abstract

CO oxidation on bimetallic and metal oxide has drawn much attention in the past years due to its importance both technologically and theoretically, but the active phase as well as the detailed reaction mechanism on the bimetallic surface oxide (i.e., a sandwich-like surface structure) are still unclear. In this work, the CO oxidation on the various Pt–Ni model catalysts [including Pt(111), Pt/Ni/Pt(111), Ni/Pt(111), NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111)] was studied by performing the density functional theory calculations. It was found that the CO oxidation reaction would process with a higher reaction barrier on metals at lower oxygen coverage via the Langmuir-Hinshelwood (L-H) mechanism, whereas CO oxidation reaction would take place with a lower barrier at higher oxygen coverage on metals or in the presence of molecular oxygen/CO (on NiO1−x-like systems) via the Eley-Rideal mechanism. The calculation results show that the activation energy of CO oxidation follows the order: Pt(111) (0.75eV)>Pt/Ni/Pt(111) (0.69eV)>Ni/Pt(111) (0.47eV at 1ML oxygen), which is in general agreement with the experimental observations. On the surface oxide NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111) systems, it was found that the molecular CO can subtract the surface lattice oxygen to form CO2 spontaneously through the Eley-Rideal mechanism on NiO1−x/Pt/Ni/Pt(111), whereas such kinetic behavior cannot occur on the NiO1−x/Pt(111) system, suggesting the high reactivity of CO oxidation on NiO1−x/Pt/Ni/Pt(111). The possible reason was analyzed by the magnitude of surface oxygen vacancy formation energy, namely NiO1−x/Pt/M/Pt(111) with relatively low vacancy formation energy as compared to that of NiO1−x/Pt(111) (3.46 vs 4.51eV). Moreover, we extend the above study to a more general case in which the subsurface metals in NiO1−x/Pt/M/Pt(111) system including VIII group metals like Fe/Co/Ni and the IB group metals like Cu, and it was found that the molecular CO can subtract the surface lattice oxygen atom to form CO2 spontaneously via the E-R reaction mechanism for all these NiO1−x/Pt/M/Pt(111) systems.