Probing zeolites by vibrational spectroscopies.

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Speciation of adsorbates on surface of solids by infrared spectroscopy and chemometrics.

Speciation, i.e. identification and quantification, of surface species on heterogeneous surfaces by infrared spectroscopy is important in many fields but remains a challenging task when facing strongly overlapped spectra of multiple adspecies. Here, we propose a new methodology, combining state of the art instrumental developments for quantitative infrared spectroscopy of adspecies and chemometrics tools, mainly a novel data processing algorithm, called SORB-MCR (SOft modeling by Recursive Based-Multivariate Curve Resolution) and multivariate calibration. After formal transposition of the general linear mixture model to adsorption spectral data, the main issues, i.e. validity of Beer-Lambert law and rank deficiency problems, are theoretically discussed. Then, the methodology is exposed through application to two case studies, each of them characterized by a specific type of rank deficiency: (i) speciation of physisorbed water species over a hydrated silica surface, and (ii) speciation (chemisorption and physisorption) of a silane probe molecule over a dehydrated silica surface. In both cases, we demonstrate the relevance of this approach which leads to a thorough surface speciation based on comprehensive and fully interpretable multivariate quantitative models. Limitations and drawbacks of the methodology are also underlined.

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study.

This work is a mechanistic study of total and partial methanol photooxidation using operando FTIR coupled to gas phase analysis techniques (gas-IR and MS). Methoxy and formate/formyl species play a key role in the reaction. Methoxy species are formed by thermal and photochemical dissociation of methanol. The formation of methylformate is favored by a high surface coverage by methoxy species. Surface and/or bridged oxygen atoms are also important actors. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments showed that the limiting step is the conversion of chemisorbed formyl/formate and that methylformate is a secondary product from a reaction between methoxy and neighboring formyl species. Methanol concentration, among other reaction parameters, influences greatly the selectivity of photooxidation.

Incorporation of clusters of titanium oxide in Beta zeolite structure by a new cold TiCl4-plasma process: physicochemical properties and photocatalytic activity.

A new post-synthetic approach, involving cold plasma treatment, was employed for the preparation of TiO2-Beta zeolite. Zeolite Beta nanoparticles were first subjected to plasma induced deposition of TiCly (with y≤ 3), which were further converted into TiOx (with x≤ 2) upon O2-plasma treatment. Different steps of the new elaborated plasma approach were monitored using in situ FTIR spectroscopy. D2O isotopic exchange was used in order to shed light on the formation of Si-O-Ti bonds induced by TiCl4-plasma followed by O2-plasma treatments. The obtained TiO2-Beta materials were studied by a set of complementary characterization techniques including FTIR, TEM, SEM-EDS, XRD, N2 sorption, NMR and UV-Vis. The silanol content and the acidic properties of TiO2-Beta composites were also studied. The elaborated materials were tested as photocatalysts for methanol photooxidation in the gas phase. TiO2-Beta presents a methanol photooxidation rate 8 times higher than a conventional P25-TiO2 catalyst under UV irradiation.

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.

The porosity, acidity, and reactivity of dealuminated zeolite ZSM-5 at the single particle level: the influence of the zeolite architecture.

A combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), focused-ion-beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), confocal fluorescence microscopy (CFM), and UV/Vis and synchrotron-based IR microspectroscopy was used to investigate the dealumination processes of zeolite ZSM-5 at the individual crystal level. It was shown that steaming has a significant impact on the porosity, acidity, and reactivity of the zeolite materials. The catalytic performance, tested by the styrene oligomerization and methanol-to-olefin reactions, led to the conclusion that mild steaming conditions resulted in greatly enhanced acidity and reactivity of dealuminated zeolite ZSM-5. Interestingly, only residual surface mesoporosity was generated in the mildly steamed ZSM-5 zeolite, leading to rapid crystal coloration and coking upon catalytic testing and indicating an enhanced deactivation of the zeolites. In contrast, harsh steaming conditions generated 5-50 nm mesopores, extensively improving the accessibility of the zeolites. However, severe dealumination decreased the strength of the Brønsted acid sites, causing a depletion of the overall acidity, which resulted in a major drop in catalytic activity.

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.

Analysing and understanding the active site by IR spectroscopy.

IR spectroscopy is a technique particularly adapted for understanding the mechanism of catalytic reactions, being able to probe the surface mechanisms at the molecular level. In this critical review the main advances in the field are presented, both under the aspects of the in situ and operando approaches. A broad view of the most authoritative literature of the domain is given, based largely on the experience built up at the LCS laboratory in the last decades. After having presented the general methodology to observe a potential active site directly or by probe molecule adsorption, several examples illustrate the qualitative and quantitative analysis of the physical-chemical properties of the surface entities. The last part of the review is dedicated to the discrimination of the role of the active site and its links with the catalytic steps; the hot problem of the reaction intermediates and their visibility via spectroscopic techniques is critically addressed (138 references).

Interaction of water and ammonium in NaHY zeolite as detected by combined IR and gravimetric analysis (AGIR).

Qualitative as well as quantitative information is obtained simultaneously by combining thermogravimetry and operando IR spectroscopy with on line mass spectrometry. A microbalance has been adapted to an infrared reactor cell. The weight and IR spectra of a solid sample can be measured together in real time in operando conditions, in a gas flow at temperature between room temperature and 773 K. Molar absorbance coefficients epsilons can be obtained directly. The new technique was first applied to adsorbed water and ammonia on a solid acid catalyst, HY zeolite. Epsilon coefficients for water bands were measured. They are influence by the hydration level, and the nu+delta combination band is shown to be a much better option to determine the amount of adsorbed water on zeolites. Epsilon has also been measured for ammonium ions, and water was shown to greatly influence the spectral response of these ions.

Real-time infrared detection of cyanide flip on silver-alumina NOx removal catalyst.

Spectroscopic studies of the mechanistic steps that occur on supported precious metal catalysts used in industrial and automotive applications are hampered by a dearth of suitable experimental methods. We used femtosecond laser excitation followed by nanosecond time-resolved in situ Fourier-transform infrared spectroscopy to initiate a catalytic reaction on alumina-supported silver catalysts, which are of interest in minimizing nitrogen oxide emissions from fuel-efficient lean-burn engines. We found that the key intermediate step in the reaction between carbon monoxide and nitric oxide is the flip of a cyanide group from a silver nanoparticle to the alumina support (with a lifetime of 2 microseconds), which indicates the central role played by the interface between the metal particle and the oxide support.

Infrared and microwaves at 5.8 GHz in a catalytic reactor.

An improved micro-reactor cell for IR spectroscopic studies of heterogeneous catalysis was built around a 5.8 GHz microwave cavity. The reactor can operate at 20 bars and with conventional heating up to 720 K, with reactant gas flows velocities (GHSV) from 25,000 to 50,000 h(-1). The temperature of the sample under microwave irradiation was measured by time resolved IR emission spectroscopy. The first experiment performed was the IR monitoring of the desorption of carbonates induced by irradiating an alumina sample by microwaves at 5.8 GHz.

Probing the Lewis acidity and catalytic activity of the metal-organic framework [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate).

An optimized procedure was designed for the preparation of the microporous metal-organic framework (MOF) [Cu3(btc)2] (BTC=benzene-1,3,5-tricarboxylate). The crystalline material was characterized by X-ray diffraction, optical microscopy, SEM, X-ray photoelectron spectroscopy, N2 sorption, thermogravimetry, and IR spectroscopy of adsorbed CO. CO adsorbs on a small number of Cu2O impurities, and particularly on the free CuII coordination sites in the framework. [Cu3(btc)2] is a highly selective Lewis acid catalyst for the isomerization of terpene derivatives, such as the rearrangement of alpha-pinene oxide to campholenic aldehyde and the cyclization of citronellal to isopulegol. By using the ethylene ketal of 2-bromopropiophenone as a test substrate, it was demonstrated that the active sites in [Cu3(btc)2] are hard Lewis acids. Catalyst stability, re-usability, and heterogeneity are critically assessed.

Multiplex MQMAS NMR of quadrupolar nuclei.

A multiplex phase cycling method (N. Ivchenko et al., J. Magn. Reson. 160 (2003) 52-58) has been used to record two-dimensional MQMAS spectra with a very short phase cycling. A straightforward procedure has been developed to easily process the data. Combining this Multiplex approach and the new Soft-Pulse-Adding-Mixing (SPAM) method considerably increases the signal-to-noise ratio of the conventional MQMAS experiment. The Multiplex acquisition procedure is much simpler than the echo/anti-echo method recently proposed, and has been applied with success to record (87)Rb spectra of RbNO(3) and (27)Al 3Q and 5Q MQMAS NMR of microporous aluminophosphate AlPO(4)-14.

NO2 disproportionation for the IR characterisation of basic zeolites.

NO2 disproportionation on alkaline zeolites is used to generate nitrosonium (NO+) and nitrate ions on the surface, and the infrared vibrations observed are very sensitive to the cation chemical hardness and to the basicity of zeolitic oxygen atoms.