Low-temperature reduction of NO2 on oxidized Mo(110)

The reactions of nitrogen dioxide (NO2) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO2 reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N-2, and trace amounts of N2O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO2 to yield N(a) and O(a). N-2 forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO2 reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO2. The selectivity for N2 formation over N2O is greater for NO2 as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO2 reaction than by O-2 reaction, indicating that NO2 is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N2O4, NO2, and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N2O4 desorbs by 200 K, leaving only NO2 and NO on the surface. Infrared spectroscopy of NO2 on O-18-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.