Properties Of Surface Sites Research Articles

Density functional theory investigation of the interaction of water withα−Al2O3andα−Fe2O3 (11̲02) surfaces: Implications for surface reactivity

Density functional theory calculations were performed in conjunction with ab initio thermodynamics, bond valence calculations, and density of states studies to investigate the chemical reactivity of \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$ and \ensuremath{\alpha}-Fe${}_{2}$O${}_{3}$ $(1\overline{1}02)$ surfaces in a humid environment. Isostructural \ensuremath{\alpha}-Fe${}_{2}$O${}_{3}$ $(1\overline{1}02)$ displays a much higher degree of surface reactivity with respect to water adsorption and aqueous heavy metal ions than \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$. The reason for these differences has not been fully explained. We have found that, while both metal oxides exhibit a similar stable $(1\overline{1}02)$ surface at and below room temperature, corresponding to a stoichiometric surface with the first layer of metal ions missing, the degree of hydroxylation of the surface oxygen atoms leads to differences in the atomic layer relaxation in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ $(1\overline{1}02)$ compared to \ensuremath{\alpha}-Fe${}_{2}$O${}_{3}$ $(1\overline{1}02)$, which has also been confirmed previously by crystal truncation rod x-ray diffraction studies. Also in agreement with these experimental studies, we find the atomic layer spacing of the most energetically stable (1$\stackrel{\underline{}}{1}$02) ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surface is relatively insensitive to the inclusion of multiple layers of physisorbed water. This is in contrast to previously reported density functional theory studies of hydrated (1$\stackrel{\underline{}}{1}$02) $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ surfaces, the results of which are confirmed in this work, that a monolayer of water is required for good agreement with experimental measurements of the atomic layer relaxation. These changes in atomic spacing result in changes in electron charge distributions and in Lewis and Br\o{}nsted acid/base properties of surface sites, which influence the relative reactivities of the two surfaces. However, the higher reactivity of the hydrated $(1\overline{1}02)$ surface of \ensuremath{\alpha}-Fe${}_{2}$O${}_{3}$ can be attributed mainly to the empty $d$ states of the surface Fe atoms, which exhibit a first peak at \ensuremath{\sim}1 eV above the Fermi level and act as very strong Lewis acid sites. In comparison, the empty $p$ states of Al in the hydrated $(1\overline{1}02)$ \ensuremath{\alpha}-Al${}_{2}$O${}_{3}$ surface, which are \ensuremath{\sim}5 eV above the Fermi level, should be much less reactive to potential adsorbates.

Inverse gas chromatography, surface properties, and interactions among components of paint formulations

Inverse gas chromatography (IGC) was applied to the surface characterization of polymers and pigments used in the formulation of protective coatings. IGC measurements over a significant temperature range provided surface energy and acid-base interaction parameters for these materials. Two sets of IGC data were obtained: (1) the quantity of vapor used to probe solid surfaces was extremely small, with results describing the properties of the most energetic surface sites; and (2) finite concentrations of vapor probes were used, with results describing the average properties of surface sites. A comparison of the two sets gave information on the heterogeneity of sites on the polymer and pigment surfaces. Heterogeneity parameters were defined for sites interacting through dispersion forces as well as for those able to interact as acids and bases. The present work reinforces the usefulness of surface characterizations by IGC by showing that the stability of pigments dispersed in the polymer vehicles was a function of the acid-base interaction between polymer-pigment pairs.

Dynamic scaling of a reaction-limited decay process

A model of surface deconstruction in 1+1 dimensions is presented in this paper. The process of decay goes on only at surface sites in a reaction-limited regime and the statistical properties of surface sites are studied in comparison with the well-known results of the Eden growth. In particular, the dynamic behaviour of the short-time exponent β is investigated for large strips and differences between Eden growth and the present decay model are pointed out. It is shown how a simple scheme of site extraction can produce a multiple cluster decay whose secondary mechanical effects often arise from and overlap to a generic process of chemical erosion. The analysis of the particles released during deconstruction is related to experimental results on highly ordered pyrolythic graphite. Such an analysis can explain some discrepancies between continuum theory and a specific chemical disaggregation.

Surface coordination number and surface redox couples on catalyst oxides, a new approach of the interpretation of activity and selectivity II. Experimental and theoretical bases

Thermal-desorption and adsorption microcalorimetry data concerning the oxygen adsorption-desorption process on catalyst oxides reveal a “quantification” of oxygen adsorption heats. As discussed previously, this “quantification” implies the surface coordination number n, taking into account the formation enthalpy of the oxide. For symmetrical structures such as NiO, TiO 2, Cr 2O 3, the adsorption heats Q depend mainly on the three parameters. Δ t H°, c the bulk and n the surface coordination numbers of oxygen, so that: Q( n) = Δ tH° × n/c . This simple relationship, which is verified in many cases, is used firstly to define the surface states and the properties of surface sites. Its quantitative exploitation allows the calculation, knowing the enthalpy of formation and the structure of the oxide, of the free enthalpy of adsorption ΔG of oxygen, i.e. the chemical potential of the surface in each of the discrete states defined by n. The activity depends on the lowest value of n present on the surface. This theoretical approach constitutes a generalization of the classical theory of oxidation-reduction processes. It can be applied to all heterogeneous reactions, and particularly, it allows one to explain oxygen (or other reactants) adsorption-desorption features. In the case of catalyst oxides, it leads to a better understanding of their activity and selectivity, and to a rationalization of the interpretation of kinetical data.

Influence of TiO2 on surface properties of Al2O3

PASCA and ESCA were utilized to investigate the TiO2−Al2O3 and Mo−Co/TiO2−Al2O3 systems. It was revealed that introduction of titania onto Al2O3 with the proper content (4 wt.%) significantly influences the electronic properties of surface sites, particularly the kink sites (i.e. strong electron acceptor, or CUS Al3+) and hence markedly improves the HDS activity of the Mo−Co/TiO2−Al2O3 catalysts.