Adsorption and Decomposition of NO on Magnesium Oxide: A Quantum Chemical Study

SLIDE AND ROLLING MECHANISMS OF Pt CLUSTERS OUT OF OXYGEN VACANCY REGION ON MgO(100) SURFACE

We present the theoretical DFT/B3LYP investigations on the cluster nucleation process and moving mechanism for the Pt n (n = 1, 2, 3, 4, 8, and 12) clusters deposited on MgO surface with an embedded cluster model. The structures and energies of the Pt n clusters adsorbed on the perfect and oxygen vacancy MgO surfaces have been calculated. Based on the nucleation of the Pt n clusters on the oxygen vacancy sites of MgO surfaces, the two moving mechanisms of slide and rolling out of the vacancy region have been energetically investigated. The results show that the needed energies per atom moving out of the vacancy region decrease as the cluster sizes increase. Consequently, the big clusters move likely out of the vacancy sites by either slide or rolling model under a certain temperature condition.

THEORETICAL STUDY OF N2O ADSORPTION AND DECOMPOSITION AT REGULAR AND DEFECT SITES OF MgO (001) SURFACE

The adsorption of N 2 O at regular and defect sites of MgO (001) surface has been studied systematically using cluster models embedded in a large array of point charges by density functional method. The calculated results show that the MgO (001) surface with oxygen vacancies exhibits high catalytic reactivity toward N 2 O adsorptive-decomposition at variance with the regular MgO surface or the surface with magnesium vacancies. Much elongation of O–N bond of N 2 O after adsorption at oxygen vacancy site with O end of N 2 O down indicates that O–N bond has been broken with concurrent production of N 2, leaving a regular site instead of the original oxygen vacancy site (F center). Besides, the MgO (001) surface with magnesium vacancies hardly exhibits catalytic reactivity. It can be concluded that N 2 O dissociation is likely occurred at MgO (001) surface oxygen vacancy sites, which is consistent with the generally accepted viewpoint in the experiment.

A Biomimetic Approach to the Chemical Inactivation of Chrysotile Fibres by Lichen Metabolites

Some lichens were recently reported to modify the surface state of asbestos. Here we report some new insight on the physico-chemical modifications induced by natural chelators (lichen metabolites) on two asbestos samples collected in two different locations. A biomimetic approach was followed by reproducing in the laboratory the weathering effect of lichen metabolites. Norstictic, pulvinic and oxalic acid (0.005, 0.5 and 50 mM) were put in contact with chrysotile fibres, either in pure form (A) or intergrown with balangeroite, an iron-rich asbestiform phase (B). Mg and Si removal, measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), reveals an incongruent dissolution for pure chrysotile (A), with Mg removal always exceeding that of Si, while chrysotile-balangeroite (B) follows a congruent dissolution pattern in all cases except in the presence of 50 mM oxalic acid. A much larger removal of Mg than Si in the solutions of 0.5 and 50 mM oxalic acid with chrysotile (A) suggests a structural collapse, which in the case of chrysotile-balangeroite (B) only occurs with 50 mM oxalic acid; in these cases both samples are converted into amorphous silica (as detected by X-ray diffraction (XRD)). Subsequent to incubation, some new phases (Fe(2)O(3), CaMg(CO(3))(2), Ca(C(2)O(4)) x H(2)O and Mg(C(2)O(4))2 x H(2)O), similar to those observed in the field, were detected by XRD and micro-Raman spectroscopy. The leaching effect of lichen metabolites also modifies the Fenton activity, a process widely correlated with asbestos pathogenicity: pure chrysotile (A) activity is reduced by 50 mM oxalic acid, while all lichen metabolites reduce the activity of chrysotile-balangeroite (B). The selective removal of poorly coordinated, highly reactive iron ions, evidenced by NO adsorption, accounts for the loss in Fenton activity. Such fibres were chemically close to the ones observed in the field. Chrysotile-rich rocks, colonised by lichens, could be exposed to a natural bioattenuation and considered as a transient environmental hazard.