Periodic density functional theory calculations of bulk and the (010) surface of goethite

<p>Abstract</p> <p>Background</p> <p>Goethite is a common and reactive mineral in the environment. The transport of contaminants and anaerobic respiration of microbes are significantly affected by adsorption and reduction reactions involving goethite. An understanding of the mineral-water interface of goethite is critical for determining the molecular-scale mechanisms of adsorption and reduction reactions. In this study, periodic density functional theory (DFT) calculations were performed on the mineral goethite and its (010) surface, using the Vienna <it>Ab Initio </it>Simulation Package (VASP).</p> <p>Results</p> <p>Calculations of the bulk mineral structure accurately reproduced the observed crystal structure and vibrational frequencies, suggesting that this computational methodology was suitable for modeling the goethite-water interface. Energy-minimized structures of bare, hydrated (one H₂O layer) and solvated (three H₂O layers) (010) surfaces were calculated for 1 × 1 and 3 × 3 unit cell slabs. A good correlation between the calculated and observed vibrational frequencies was found for the 1 × 1 solvated surface. However, differences between the 1 × 1 and 3 × 3 slab calculations indicated that larger models may be necessary to simulate the relaxation of water at the interface. Comparison of two hydrated surfaces with molecularly and dissociatively adsorbed H₂O showed a significantly lower potential energy for the former.</p> <p>Conclusion</p> <p>Surface Fe-O and (Fe)O-H bond lengths are reported that may be useful in surface complexation models (SCM) of the goethite (010) surface. These bond lengths were found to change significantly as a function of solvation (i.e., addition of two extra H₂O layers above the surface), indicating that this parameter should be carefully considered in future SCM studies of metal oxide-water interfaces.</p

IR Operando Study of the Acetylene Effect on the NO Reduction Mechanism on Pd–Rh/CeO2–ZrO2 Three-Way Catalyst

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Correlations between Acidity, Surface Structure, and Catalytic Activity of Niobium Oxide Supported on Zirconia

International audienceThe development of the acidity and the relationship between acidity, catalytic activity, and the surface structure for niobium oxide supported on zirconia were investigated for a series of solids. The catalysts were active for 2-propanol dehydration only above a threshold in Nb loading. The acidity was studied by infrared spectroscopy of adsorbed 2,6-dimethylpyridine as a probe molecule, and the onset of activity was correlated with that of the formation of relatively strong Brnsted acid sites. The variation in the abundance of these sites also correlated with the catalytic activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric Nb surface species. These results were compared to those previously reported for the WOx/ZrO2 catalysts

Quantitative IR characterization of the acidity of various oxide catalysts

International audienceThe integrated molar absorption coefficients of the infrared bands characteristic of adsorbed lutidine (2,6-dimethylpyridine) were determined for the purpose of quantifying the acid sites of solid catalysts. The integrated molar absorption coefficients were measured for lutidine adsorbed through H-bonding, coordination to Lewis sites and protonation on Brønsted acid sites. The solids were chosen to present all possible bondings with lutidine and to cover a wide range of common types of catalysts or supports: silica, phosphated silica, HY zeolite, alumina, zirconia, WOx supported on zirconia and NbOx supported on zirconia

The Role of the Synthesis Method in the Structure Formation of Cobalt Aluminate

66 FIELD Section Title:Surface Chemistry and Colloids Department Chemie, Physikalische Chemie, Ludwig Maximilians Universitaet Muenchen, Munich, Germany. FIELD URL: written in EnglishCobalt aluminate (spinel) was prepd. via two synthesis routes firstly a co-pptn. method and secondly a nanocasting method. The surface chem. of these materials was characterized by IR-spectroscopy of the surface hydroxyl groups and of coordinatively unsatd. (cus) cations (Al and Co) by carbon monoxide CO at low temp. The goal was to investigate whether or not the prepn. of the spinel phase had an effect on this structural characteristics, namely on the inversion degree. The hydroxyl (deuteroxyl) spectra were characterized by six types of O-H stretching bands. While the O-H stretching frequencies were identical for both materials, the relative intensities of the bands were clearly different indicating a distinct distribution of the different types of hydroxyl groups which most likely originates from a distinct distribution of the cations in the two differently synthesized materials. The prepn. method obviously yields spinel structures having variable degrees of inversion. Coordinatively unsatd. cus cations are exposed on the surface during the removal of O-H groups by thermal dehydroxylation. Those Lewis acid centers show an acid strength distribution which is again an indication for formation of two distinct degrees of inversion. [on SciFinder(R)

Optimization of ammonium fluoride route to hierarchical ZSM-5 zeolites

International audiencePreparation of hierarchical zeolites via a chemical etching is a hot topic as it strongly improves microporous material's diffusive properties and thus their catalytic activity. However, an issue that is often neglected is the softening the post-treatment condition. Herein we we report the optimization of NH4F etching, using less concentrated solutions and milder treatment conditions. Following characterization by ICP-AES, XRD, 27 Al MAS NMR, TEM, and N2 physisorption, it has been shown that it is possible to soften the post-treatment conditions by playing with three synthesis parameters: (1) Decreasing the NH4F/ZSM-5 wt/wt ratio allows the generation of mesopores of different diameter (from 27 to 55 nm); (2) Decreasing the etching time provides precise control over the textural properties of the hierarchical ZSM-5; (3) Decreasing the initial NH4F weight concentration also allows to tune the pore diameter from large (55 nm) to a small mesopore (5 nm)

Acidity of titania-supported tungsten or niobium oxide catalysts Correlation with catalytic activity

International audienceThe acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration. At a reaction temperature of 403 K, WOx/TiO2 catalysts were inactive for a surface density ofW 1.2 Watom nm2. Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Brønsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbOx/TiO2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Brønsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Brønsted acid sites was observed

Acidity of titania-supported tungsten or niobium oxide catalystsCorrelation with catalytic activity

International audienceThe acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm-2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration.At a reaction temperature of 403 K, WOx/TiO2 catalysts were inactive for a surface density of W 1.2 W atom nm-2. Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Brønsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbOx/TiO2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Brønsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Brønsted acid sites was observed