Chemistry of NO2 on CeO2 and MgO: Experimental and theoretical studies on the formation of NO3

In environmental catalysis the destruction or removal of nitrogen oxides (D eNOx process) is receiving a lot of attention. Synchrotron-based x-ray abso rption near-edge spectroscopy, high-resolution photoemission, and first-pri nciples density-functional calculations (DFT-GGA) were used to study the in teraction of nitrogen dioxide with CeO2 and MgO. The only product of the re action of NO2 with pure CeO2 at 300 K is adsorbed nitrate. The NO3 is a the rmally stable species which mostly decomposes at temperatures between 450 a nd 600 K. For the adsorption of NO2 on partially reduced ceria (CeO2-x), th ere is full decomposition of the adsorbate and a mixture of N, NO, and NO3 coexists on the surface of the oxide at room temperature. Ce3+ cations can assist in the transformation of NO and NO2 in DeNOx operations. Adsorbed NO 3 (main product) and NO2 are detected after exposing MgO to NO2 gas. A part ial NO2,ads-->NO3,ads transformation is observed on MgO(100) from 150 to 30 0 K. DFT-GGA calculations show strong bonding interactions for NO2 on Mg si tes of this surface, and dicoordination via O, O is more favorable energeti cally than monocoordination via N. The NO2,ads species disappears from magn esium oxide at temperatures below 600 K, whereas part of the NO3,ads is sta ble up to temperatures near 800 K. MgO can be very useful as a sorbent for trapping NO2. A general trend is found after comparing the chemical behavio r of NO2 on different types of oxides (CeO2, MgO, TiO2, Fe2O3, CuO, ZnO). O n all these systems, the main product after adsorbing NO2 at 300 K is nitra te with minor amounts of chemisorbed NO2 and no signs of full decomposition of the adsorbate. This trend and the results of DFT-GGA calculations indic ate that NO2 is very efficient for the nitration (i.e., formation of NO3 as a ligand) of metal centers that are missing O neighbors in oxide surfaces. (C) 2000 American Institute of Physics. [S0021-9606(00)71222-0].