Surface structure and surface states in magnesium oxide powders

Abstract

The photoluminescent and reflectance spectra of MgO smoke have been compared with the spectra of MgO ex-hydroxide and excarbonate. Parallel studies by electron microscopy have shown that a strong correlation exists between the optical spectra and the different morphology of the particles obtained by the different routes. The spectra observed are associated with the surface of the oxide particles and the excitation and absorption bands are associated with oxygen ions in 3and 4-fold coordination. The same kinds of sites are present on all samples but the ions of lowest coordination are much less abundant on the nearly perfect cubes of MgO smoke. The number of 3-fold coordinated sites on the smoke particles is considerably increased after exposure to water vapour for several h. This picture is consistent with the electron micrographs which show erosion of the edges and corners of the MgO smoke after treatment with water vapour. A model involving the removal of ion pairs from the comers and edges of the cube by water is shown to give the observed increase in the sites of lowest coordination.

Surface features of TiO2 nanoparticles: combination modes of adsorbed CO probe the stepping of (101) facets

“…To this aim, two other types of TiO 2 samples were considered, one constituted by nanoparticles (NP) with a well-defined truncated bipyramidal shape (TiO 2 HT), with lateral faces corresponding to (101) surfaces, 13 and the second resulting from the etching of such nanoparticles with HF (TiO 2 HT-HF), following the approach adopted in a seminal work on the erosion of well-shaped MgO nanoparticles by treatment with H 2 O vapour. 52 From a morphological point of view (see HRTEM images in Figure 2 and Figure S1 in the ESI), this corresponded to move from titania NP with rough borders as those of TiO 2 P25 (panel A), to TiO 2 HT with regular (101) terminations (parallel to lattice fringes 0.352 nm apart, corresponding to the d (101) spacing) 53 exhibiting few steps (panel B), and to the heavily stepped (101) surface profiles of TiO 2 HT-HF (panel C), directly derived from the previous material. The whole series of spectra of TiO 2 HT in the presence of decreasing amount of adsorbed CO is reported in Figure S2 in the ESI.…”

Activation of Methane to C2Hydrocarbons over Unpromoted Calcium Oxide Catalysts

“…The photoluminescence spectra of Figure 6 indicate that unsaturated oxygen sites exist on the catalyst surface as suggested by other investigators. [13][14][15][16][17] In this work, BET surface areas measured by nitrogen adsorption were 18.8 m 2 /g and 8.5 m 2 /g for the CaO powder the CaO fully-coated silica particles, respectively, which enables us to consider that the catalytic activity of CaO is more sensitive to surface geometry rather than surface area. Namely, the activity of CaO seems to be strongly dependent on some structural difference in CaO crystallites.…”

“…The excitation spectra observed at lower energy region are attributed to the surface unsaturated sites having coordination number of less than 5. [13][14][15][16][17] That is, the lower excitation energy of photoluminescence is due to the surface absorption bands associated with electronic transitions in surface oxygen ions in states of low coordination. The photoluminescence spectra of Figure 6 indicate that unsaturated oxygen sites exist on the catalyst surface as suggested by other investigators.…”

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

“…Photoluminescence (PL) spectroscopy is, therefore, complementarily used to sensitively probe defects and low-coordinated edge or corner sites (structural defects) [25]. However, the interpretation of the emission spectra is not straightforward since several recombination and energy transfer phenomena may occur after the excitation [6,25,26]. The emission spectra depend on the energy of the light used for excitation.…”