RESUMEN
The spillover of oxygen species is fundamentally important in redox reactions, but the spillover mechanism has been less understood compared to that of hydrogen spillover. Herein Sn is doped into TiO2 to activate low-temperature (<100 °C) reverse oxygen spillover in Pt/TiO2 catalyst, leading to CO oxidation activity much higher than that of most oxide-supported Pt catalysts. A combination of near-ambient-pressure X-ray photoelectron spectroscopy, in situ Raman/Infrared spectroscopies, and ab initio molecular dynamics simulations reveal that the reverse oxygen spillover is triggered by CO adsorption at Pt2+ sites, followed by bond cleavage of Ti-O-Sn moieties nearby and the appearance of Pt4+ species. The O in the catalytically indispensable Pt-O species is energetically more favourable to be originated from Ti-O-Sn. This work clearly depicts the interfacial chemistry of reverse oxygen spillover that is triggered by CO adsorption, and the understanding is helpful for the design of platinum/titania catalysts suitable for reactions of various reactants.
RESUMEN
This paper investigates the parameters that influence the selective adsorption of phenol, toxic molecule, from a semi-model biofuel mixture containing alkanes and different proportions of aromatic compounds. The adsorption capacity, selectivity and regeneration ability of different adsorbents, i.e. zeolites, silica-based solids, alumina and activated carbon, were related to their textural properties and the nature, strength or location of their acidic sites. This work demonstrates that phenol differently adsorbs in the micropores and mesopores. In the micropores of faujasites, phenol is condensed into the supercages. Otherwise, in the mesopores of the zeolite, phenol interacts with the silanol groups. On purely siliceous adsorbents, a ratio of one phenol adsorbed on one silanol group could be established. As for selectivity, the strong acidic sites of the faujasites are necessary to favor phenol adsorption compared to toluene. By contrast, the amount of strong Brønsted and Lewis acid sites limits regeneration. Hence, a compromise has to be found and the best performances were obtained using a slightly dealuminated zeolitic adsorbent presenting both micro and mesopores.
RESUMEN
Atomic-scale images of MoS2 slabs supported on γ-Al2O3 were obtained by high-resolution scanning transmission electron microscopy equipped with a high angular annular dark field detector (HR STEM-HAADF). These observations, obtained for sulfide catalysts prepared with or without citric acid as a chelating agent, evidenced variations in morphology (shape) and size of the MoS2 nanoslabs, as detected indirectly by the adsorption of CO followed by infrared spectroscopy. Quantitative dispersion values and a morphology index (S-edge/M-edge ratio) were determined from the slabs observed. In this way, HR STEM-HAADF underlines that the addition of citric acid to Mo catalysts decreases the size of the particles and modifies the shape of the MoS2 nanoslabs from slightly truncated triangles to particles with a higher ratio of S-edge/M-edge. These finding are of great significance since tayloring the morphology of the slabs is a way to increase their catalytic activity and selectivity. Furthermore, this work demonstrated that the IR/CO method is a relevant approach to describe the MoS2 morphology of supported catalysts used in hydrotreatment processes for clean fuel production.
RESUMEN
A new methodology based on the inversion of adsorption isotherms obtained using infrared spectroscopy has been developed. It provides a description of coexisting surface species in terms of their individual IR spectra and surface affinities in a new two dimensional, 2D IR spectroscopic technique. When implemented with simultaneous gravimetric analysis, it further provides the quantification of adsorbed species. The adsorption of CO2 on monoclinic ZrO2 was investigated using this technique with temperature and pressure ranges of 353-673 K and 10(-4)-0.4 bar, respectively. The sets of spectra obtained at constant temperature and variable pressures (spectroscopic isotherms) were inverted assuming they obey a generalized Langmuir isotherm. This procedure yields a 2D map in which the IR spectra of the prominent surface species formed upon CO2 adsorption are resolved in one dimension - hydrogen carbonates, bidentate carbonates and polydentate carbonates - while these species are resolved according to their surface adsorption affinities (logarithm of adsorption equilibrium constants, ln K) on the other dimension. This technique also allows for the unambiguous determination of the thermodynamic stabilities of the various adsorbed species. The inversion of the gravimetric isotherms recorded simultaneously with the infrared spectra leads to a quantitative distribution function of CO2 adsorption sites whose components match those of the 2D infrared map and allows for a straightforward quantification of the corresponding sites, namely (i) weakly basic sites leading to bridged carbonates, hydrogen carbonates and bidentate carbonates (~0.7 µmol m(-2), Δ(ads)H = -70 to 90 kJ mol(-1)), (ii) mild basic sites leading to a second type of bidentate carbonates (~0.8 µmol m(-2), Δ(ads)H = -110 to 120 kJ mol(-1)) and (iii) strong basic sites leading to polydentate carbonate species (~0.1 µmol m(-2), Δ(ads)H < -120 kJ mol(-1)). Finally, the advantages and limitations of the present methodology are discussed. Because this technique is not limited to a particular spectroscopy or physical process, it should find other applications in the field of spectroscopic characterization of surfaces.
RESUMEN
Methanol adsorption on MgO samples with different morphologies is studied by infrared (IR) spectroscopy coupled to volumetry or thermogravimetry measurements to probe qualitatively and quantitatively the acid-base paired sites. The molar absorption coefficients of ν(OC) vibration of non-dissociated methanol, type I and type II methoxy species are determined by analyzing data obtained under specific adsorption conditions (ε(ND)=2.5 cm µmol(-1); ε(I)=ε(II)=6.1 cm µmol(-1)). Thanks to these results, the amounts of different adsorbed methanol species are evaluated. These various species are formed on surface sites presenting different topologies. Hence, the IR method is the only one allowing us to both discriminate and quantify the defects on the MgO surfaces, in terms of concentrations of convex and concave defective zones. This study reveals that sol-gel preparation leads to a MgO surface presenting a greater amount of concave defective zones than precipitated MgO.
RESUMEN
The positions of nu8a and nu*(NH) bands in the spectrum of protonated 2,6-dimethylpyridine vary with the strength of Brønsted acidity: the higher the nu*(NH) wavenumber and the lower the nu8a wavenumber, the stronger the acidity. The results obtained with 2,6-dimethylpyridine adsorption correlate with those obtained by CO adsorption experiments on a series of faujasite zeolites (LiHNaY, KHNaY, HY, HY(SA), HNaX). These relations were extended to gamma-Al2O3 having weak Brønsted acidity, not detected by pyridine and hardly detected by CO. The number (0.1 per nm2) and the strength (corresponding to delta nu (OH) by CO = 155 cm(-1)) of the most acidic OH groups of Al2O3, as well as the position of the corresponding nu (OH) band (<3700 cm(-1)) are deduced from 2,6-dimethylpyridine adsorption experiments.