Surface Heterogeneity in Amorphous Silica Nanoparticles Evidenced from Tapping AFM-IR Nanospectroscopy.

In this work, we propose to evaluate and validate an emerging spectroscopic space-resolved technique: atomic force microscopy coupled with infrared spectroscopy (AFM-IR) for inorganic materials in tapping mode at the nanoscale. For this aim, a preliminary investigation of sample preparation techniques was done and the stability of tapping AFM-IR spectra was evaluated on reference samples [poly(methyl methacrylate) and silica]. It was concluded that for a homogeneous polymer, it is possible to compare AFM-IR spectra with conventional Fourier-transform infrared (FTIR) spectra obtained in transmission. When an inorganic solid is considered, AFM-IR spectra are different from the global FTIR spectrum which indicates that the AFM-IR technique probes a volume which is not representative of global composition, that is, the external surface layer. Moreover, local infrared spectra recorded in the tapping mode of the external surface are significantly different depending on the analyzed regions of the same particle and between particles of the amorphous silica, implying surface heterogeneity. The AFM-IR technique allows surface description of amorphous inorganic materials at the nanoscale and opens new frontiers in the characterization of functional nanoscale and crystalline materials.

Effective bulk and surface temperatures of the catalyst bed of FT-IR cells used for in situ and operando studies.

The temperature prevailing in the catalyst bed of three different IR spectroscopic reaction cells was assessed by means of thermocouples, an optical pyrometer and reaction rate measurements. One of the cells was a custom-made transmission FT-IR cell for use with thin wafers and the two others were commercial Harrick and Spectra-Tech diffuse reflectance FT-IR (DRIFTS) cells used for the analysis of powdered samples. The rate of CO methanation measured over a 16 wt% Ni/alumina catalyst was used as a means to derive the effective temperature prevailing in the IR cells from that existing in a traditional (non-spectroscopic) reactor having a well-controlled temperature. The sample bed of these three IR cells exhibited a significantly lower temperature than that of the corresponding measure thermocouple, which was yet located in or close to the sample bed. The comparison of Arrhenius plots enabled us to determine a temperature correction valid over a large temperature range. The use of an optical pyrometer was assessed with a view to determining the temperature of the surface of the powdered beds and that at the centre of the wafer. The optical pyrometer proved useful in the case of the catalyst powder, which behaved as a black non-reflecting body. In contrast, the temperature reading was inaccurate in the case of the pressed wafer, probably due to the shiny surface and minute thickness of the wafer, which led to a significant portion of the IR radiation of the surroundings being reflected by and transmitted through the wafer. The optical pyrometer data showed that the temperature of the surface of the powdered beds was significantly lower than that of the bulk of the bed, and that the total flow rate and composition did not affect this value. This work emphasises that the effective bed temperature in spectroscopic cells can be significantly different from that given by measure thermocouples, even when located in the vicinity of the sample, but that the calibration curves derived from rate measurements can be used to overcome this problem.

In situ FT-IR investigation of etravirine speciation in pores of SBA-15 ordered mesoporous silica material upon contact with water.

Ordered mesoporous silica (OMS) has been recognized as promising adsorbent material for drug molecules with low aqueous solubility. The release of drug molecules from OMS upon contact with aqueous environment enhances their oral bioavailability. The release is governed by a complex interplay of adsorption, diffusion, and intermolecular interaction inside OMS pores. The presence of water hampers in situ FT-IR investigation of the behavior of the drug molecules upon release. The poorly water-soluble etravirine molecule having two nitrile functions was selected for an in situ FT-IR spectroscopic investigation of the release process. The stretching vibration of the nitrile organic function (υ(CN)) is a spectral feature that is accessible to FT-IR even in the presence of water. Etravirine depending on the loading was found to be present in SBA-15 pores as isolated adsorbed molecules, solvated molecules, and aggregates with intermolecular interaction similar to the crystalline state, each with a different spectroscopic fingerprint. Etravirine evacuation from the SBA-15 pores was shown to proceed in the solvated state. Surprisingly, the etravirine clusters inside pores were converted more readily into solvated molecules compared to individually adsorbed molecules.

Microsecond time-resolved Fourier transform infrared analytics in a low pressure glow discharge reactor.

A low pressure glow discharge reactor has been designed to allow time-resolved infrared spectroscopic investigation of the discharge zone in practical conditions. The benefits of such reactor are demonstrated through the study of the evolution in the infrared spectra of air/CO(2) gas mixture at the microsecond time-scale. It has been shown that the spectra are greatly affected by the electrical discharge in the 2400-2200 cm(-1) region, where the asymmetric stretch mode of CO(2) falls. The CO(2) molecules are excited through a collision with excited N(2) molecules, where the transfer of energy occurs by a resonant effect. The mechanisms involved are reversible and following plasma pulses.

Characterization of Fe sites in Fe-zeolites by FTIR spectroscopy of adsorbed NO: are the spectra obtained in static vacuum and dynamic flow set-ups comparable?

The present paper shows a direct comparison of IR spectra of adsorbed NO on two catalyst systems (Fe-silicalite and Fe-ZSM-5), recorded in a flow-through cell (in operando, where NO is carried by an excess of inert gas) and static cell (in situ, where a given pure NO equilibrium pressure is dosed). A progressive NO poly-adsorption is observed in the static cell upon increasing the NO equilibrium pressure (from Fe(2+)...(NO)(2) to Fe(2+)...(NO)(3)), while predominantly Fe(2+)...NO adducts are observed in the flow-through cell. By comparing literature spectra, it is shown that these spectral differences are intrinsically inherent to the two different experimental approaches. The two experimental set-ups are able to observe preferentially only a part of the total Fe species present on Fe-zeolites. Water contamination experiments employing different experimental conditions (order of dosage, co-dosage, different NO/H(2)O ratios) did not reproduce in the static environment the IR spectra collected with the dynamic set-up. The spectral differences could have a thermodynamic origin and be related to different adsorption enthalpies of mono- and di-nitrosyl complexes of the two Fe(2+) families and to the different NO partial pressure adopted in the two experimental configurations. These considerations have important consequences for the conclusions reached by different research groups on the structure of active Fe sites.

Structure and nuclearity of active sites in Fe-zeolites: comparison with iron sites in enzymes and homogeneous catalysts.

Fe-ZSM-5 and Fe-silicalite zeolites efficiently catalyse several oxidation reactions which find close analogues in the oxidation reactions catalyzed by homogeneous and enzymatic compounds. The iron centres are highly dispersed in the crystalline matrix and on highly diluted samples, mononuclear and dinuclear structures are expected to become predominant. The crystalline and robust character of the MFI framework has allowed to hypothesize that the catalytic sites are located in well defined crystallographic positions. For this reason these catalysts have been considered as the closest and best defined heterogeneous counterparts of heme and non heme iron complexes and of Fenton type Fe(2+) homogeneous counterparts. On this basis, an analogy with the methane monooxygenase has been advanced several times. In this review we have examined the abundant literature on the subject and summarized the most widely accepted views on the structure, nuclearity and catalytic activity of the iron species. By comparing the results obtained with the various characterization techniques, we conclude that Fe-ZSM-5 and Fe-silicalite are not the ideal samples conceived before and that many types of species are present, some active and some other silent from adsorptive and catalytic point of view. The relative concentration of these species changes with thermal treatments, preparation procedures and loading. Only at lowest loadings the catalytically active species become the dominant fraction of the iron species. On the basis of the spectroscopic titration of the active sites by using NO as a probe, we conclude that the active species on very diluted samples are isolated and highly coordinatively unsaturated Fe(2+) grafted to the crystalline matrix. Indication of the constant presence of a smaller fraction of Fe(2+) presumably located on small clusters is also obtained. The nitrosyl species formed upon dosing NO from the gas phase on activated Fe-ZSM-5 and Fe-silicalite, have been analyzed in detail and the similarities and differences with the cationic, heme and non heme homogeneous counterparts have been evidenced. The same has been done for the oxygen species formed by N(2)O decomposition on isolated sites, whose properties are more similar to those of the (FeO)(2+) in cationic complexes (included the [(H(2)O)(5)FeO](2+)"brown ring" complex active in Fenton reaction) than to those of ferryl groups in heme and non heme counterparts.

Behavior of extraframework Fe sites in MFI and MCM-22 zeolites upon interaction with N2O and NO.

We report on the characterization of an isomorphously substituted Fe-MCM-22 sample containing both Fe and Al in framework positions (Si/Fe = 44, Si/Al = 25). XANES spectroscopy was used to study the evolution of Fe sites as a consequence of thermal activation at high temperature (1073 K) and subsequent oxidation with N2O. The results were compared to those obtained in the same conditions on a well-known Fe-silicalite sample (Si/Fe = 68, Si/Al = infinity). In both samples, thermal activation causes migration of a fraction of Fe ions from framework to extraframework positions, this migration being accompanied by a reduction of Fe3+ to Fe2+. Upon oxidation with N2O at 523 K, the two samples show a different behavior. While in Fe-silicalite practically all of the Fe2+ sites formed by thermal activation are reoxidized to Fe3+, in Fe-MCM-22 only a fraction of the extraframework iron sites is involved in the reoxidation process. The accessibility of the extraframework Fe sites was also investigated by using the NO molecule as a surface probe. Upon NO dosage on the sample, the modification of the pre-edge peak and of the edge position suggests an important charge release from the extraframework Fe2+ ions to the adsorbed molecules. This could be formalized with the formation of Fe3+(NO-) complexes, compatible (on the basis of the simple molecular orbital theory) with a bent NO geometry. The formation of a complex family of Fe2+ mono-, di-, and trinitrosyl complexes was also confirmed by FTIR spectroscopy. Similarly to what was observed in the oxidation experiments, the fraction of extraframework Fe sites able to interact with NO in Fe-MCM-22 sample is smaller than that in Fe-silicalite treated in the same conditions. This trend is explained with a major clustering of extraframework Fe sites in Fe-MCM-22 sample, as was also suggested by FTIR experiments. These results suggest that the dispersion of iron in zeolitic matrixes prepared by isomorphous substitution could also depend on the zeolitic structure.