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 E-f(O-vac) (the formation energy of O-bri vacancy) or E-ads(O@M-5c) (the adsorption energy of O at the metallic M-5c, 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 E-f(O-vac) and E-ads(O@M-5c) to obtain the optimum activity, which requires the bifunctional cooperation of the metallic M-5c and lattice O-bri site: M-5c can efficiently adsorb NO, and the O-bri site can provide the reactive O species. MnO2 is close to the active region, accounting for its good catalytic activity. Second, E-f(O-vac) and E-ads(O@M-5c) show a linear-scaling limitation for the pure rutile-type oxides, yielding that their catalytic activity can be solely described by E-f(O-vac), 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 E-ads(NO@M-Sc) (E-f(O-vac)) 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 E-f(O-vac) 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 O-bri 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.