Byline: FATHI HASSAN BAWA AND ALI HASSAN BAWA
Summary: Vibrational spectra have been computed by means of the B3LYP hybrid density functional in order to interpret the experiments [J. Phys. Chem. B 2002, 106, 6358], particularly regarding a transient 1225 cm[?]1 absorption during accumulation of NO2 in MgO supported BaO. The degree of generality of results is tested by comparing (NO2)x(MO)9 for x=1, 2 and M=Mg, Ca. Finger prints are produced for the single bidentate M2+[?][ONO][?][?]M2+ surface nitrite ion, a novel single monodentate [Oclus[?]NO2]2[?] ion, and the chemisorbed nitrite/nitrate ion pair, i.e., M2+ [?] [ONO][?][?]M2++ [Oclus[?]NO2][?]. The results suggest the novel monodentate [Oclus[?] NO2]2[?] to be responsible for the experimentally observed 1225 cm-1 absorption, being a transient towards surface nitrate rather than nitrite formation. This result is consistent with a mechanistic periodic DFT study concerning the initial loading of NO2 in BaO.
Keywords: Density functional theory calculations; (MgO)9 and (CaO)9 clusters; Chemisorption; NO2; Nitrite; Nitrate; Frequency.
The cluster concept is at the heart of environmental catalysis technology, as well as its importance in many applied fields of research, such as environmental catalysis [1-3], while providing an efficient means to understand, e.g. deterioration processes. Nitrogen dioxide is one of the most environmental pollutants, primary pollutants are those released directly into the atmosphere from the source in harmful form. Secondary pollutants, by contrast, are modified to hazardous form and then formed by chemical reactions as components of the air mix and interact. Solar radiation often provides the energy for these reactions. Nitrogen oxides are highly reactive gases formed when nitrogen in fuel or combustion air is heated to temperatures above 650 oC in the presence of oxygen. The initial product, nitric oxide (NO), oxidizes further in the atmosphere to nitrogen dioxide (NO2) [4, 5].
The interest originates mainly from the use of MgO and CaO in ''NOx storage/reduction'' catalysts consumption, which has the engine working under lean (oxygen excess) conditions . Under lean conditions NO is oxidized to NO2 over the MgO and stored in the form of nitrate or nitrite in the NOx storing material . The emissions of NO and NO2 have proved to be very much influenced by operating conditions [8, 9], and many heterogeneous and homogeneous reactions are important for the formation and destruction of NO and NO2. Vehicle exhaust and other combustion emissions are the main source of nitrogen oxides NOx, which affect the environment negatively.
The present work attempts to address this issue by examining the NOx storage mechanism over MgO and CaO clusters by means of quantum chemistry. The setting is provided by experimental and theoretical studies. Experimental input is taken from two investigations on the vibrational spectra of NOx species observed during calcination of barium nitrate Ba(NO3)2(s) , and during deposition of NO2 on MgO supported BaO , while very relevant theoretical aspects are found in investigations, which concern the actual NOx loading mechanism. Key observations are (a) the enhancement in stability observed when molecular NO2 is added pair-wise to an MgO (001)...