Activity Trend for Low-Concentration NO Oxidation at Room Temperature on Rutile-Type Metal Oxides

The catalytic oxidation of low-concentration NO at room temperature has drawn increasing attention to eliminate NO in the large semiclosed spaces. However, the location of efficient catalysts is a challenging task. Herein, to rationalize the activity trend of NO oxidation and facilitate the catalyst screening/design, we computationally investigate the low-concentration NO oxidation processes on an important rutile-type of metal oxides (MO2, M = Mn, Ru, Ir, Rh) at room temperature. Some key scaling relations for the elementary steps following either the Mars-van Krevelen (MvK) mechanism or Langmuir-Hinshelwood (LH) mechanism, are revealed as a function of Ef(Ovac) (the formation energy of Obri vacancy) or Eads(O@M5c) (the adsorption energy of O at the metallic M5c site), and a 3D activity map following the MvK mechanism at room temperature is quantitatively constructed by combining the DFT results with microkinetic analyses. First, we identified the active region in terms of Ef(Ovac) and Eads(O@M5c) to obtain the optimum activity, which requires the bifunctional cooperation of the metallic M5c and lattice Obri site: M5c can efficiently adsorb NO, and the Obri site can provide the reactive O species. MnO2 is close to the active region, accounting for its good catalytic activity. Second, Ef(Ovac) and Eads(O@M5c) show a linear-scaling limitation for the pure rutile-type oxides, yielding that their catalytic activity can be solely described by Ef(Ovac), that is, giving a 2D volcano-typed curve, meaning that pure MnO2 cannot give rise to the optimum activity. To break this limitation, it requires an increase (decrease) in Eads(NO@M5c) (Ef(Ovac)) for enhancing the catalytic activity of MnO2, which could be achieved by doping Ti into MnO2(110) from our calculation results. Third, we examined the activity with the LH mechanism for a comparison, which indicates an oxide-specific mechanism: from MnO2-based oxides to RhO2, the MvK mechanism is favored, but switches to the LH mechanism on the RuO2 and IrO2 surfaces as Ef(Ovac) increases. Equally importantly, the MvK mechanism is found to be favored compared with the LH one on the whole, implying that the participation of lattice Obri is necessary for achieving room-temperature oxidation of low-concentration NO. This work could provide a significant insight into low-concentration NO oxidation at room temperature. © 2018 American Chemical Society.