Cyclohexane Oxidative Dehydrogenation on Graphene-Oxide-Supported Cobalt Ferrite Nanohybrids: Effect of Dynamic Nature of Active Sites on Reaction Selectivity.

In this work, we investigated cyclohexane oxidative dehydrogenation (ODH) catalyzed by cobalt ferrite nanoparticles supported on reduced graphene oxide (RGO). We aim to identify the active sites that are specifically responsible for full and partial dehydrogenation using advanced spectroscopic techniques such as X-ray photoelectron emission microscopy (XPEEM) and X-ray photoelectron spectroscopy (XPS) along with kinetic analysis. Spectroscopically, we propose that Fe3+/Td sites could exclusively produce benzene through full cyclohexane dehydrogenation, while kinetic analysis shows that oxygen-derived species (O*) are responsible for partial dehydrogenation to form cyclohexene in a single catalytic sojourn. We unravel the dynamic cooperativity between octahedral and tetrahedral sites and the unique role of the support in masking undesired active (Fe3+/Td) sites. This phenomenon was strategically used to control the abundance of these species on the catalyst surface by varying the particle size and the wt % content of the nanoparticles on the RGO support in order to control the reaction selectivity without compromising reaction rates which are otherwise extremely challenging due to the much favorable thermodynamics for complete dehydrogenation and complete combustion under oxidative conditions.

Study of the diffusion properties of zeolite mixtures by combined gravimetric analysis, IR spectroscopy and inversion methods (IRIS).

We report the development of a new method of investigation of the mass transport properties of acidic zeolite-based materials aiming to overcome the limitations of classical approaches. It consists in hyphenating gravimetric analysis and infrared spectroscopy. The former allows assessing the diffusion from the gas phase to all the porosity, while IR allows for selective assessment of diffusion to the zeolite active sites located in the micropores. Furthermore, the data are processed by an original methodology allowing the recovery of the distribution of diffusion domains by inversion of the integral equations describing the uptake curves or the evolution of the infrared spectra. The combination of gravimetric analysis and IR spectroscopy makes it possible to monitor and distinguish diffusion within the various components of the material. The methodology has been applied to the isooctane uptake in the mechanical mixture of FAU and MFI zeolites. Analysis of both gravimetric uptake curves and evolving infrared spectra allows distinguishing and assigning diffusion domains to the H-FAU and H-MFI components of the mixture, with high and low effective diffusion rate constants, respectively. The advantages and limits of the methodology are discussed.

Impact of Zeolite Structure on Entropic-Enthalpic Contributions to Alkane Monomolecular Cracking: An IR Operando Study.

The monomolecular cracking rates of propane and n-butane over MFI, CHA, FER and TON zeolites were determined simultaneously with the coverage of active sites at reaction condition using IR operando spectroscopy. This allowed direct determination of adsorption thermodynamics and intrinsic rate parameters. The results show that the zeolite confinement mediates enthalpy-entropy trade-offs only at the adsorbed state, leaving the true activation energy insensitive to the zeolite or alkane structure while the activation entropy was found to increase with the confinement. Hence, relative cracking rates of alkanes within zeolite pores are mostly governed by activation entropy.

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.

Speciation of adsorbed CO2 on metal oxides by a new 2-dimensional approach: 2D infrared inversion spectroscopy (2D IRIS).

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.

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.

Combined XRD and infrared studies of pyridinium species in (PyH)(3)[PW(12)O(40)] single crystals.

The pyridinium salts of 12-tungstophosphoric acid (PyH)(3)[PW(12)O(40)]·2CH(3)CN (1) and (PyH)(3)[PW(12)O(40)] (2) have been prepared and studied by single crystal and powder X-ray diffraction (XRD) in order to characterize the crystallographic sites occupied by the pyridinium species. The three PyH(+) species are located on two unequivalent sites. Two species are linearly H-bonded to the oxygen atoms of the Keggins unit (α species), whereas the third one (ß) forms a bent H-bond. In order to determine the infrared bands characterizing each type of pyridinium species in the 1650-1300 cm(-1) range, infrared spectra have been recorded from room temperature to 100 K. They reveal that only α pyridinium species give rise to the unusual splitting of the PyH(+)ν8b and ν19b modes, whereas ß pyridinium species lead to a classical pyridinium spectrum.

Evidence of CO(2) molecule acting as an electron acceptor on a nanoporous metal-organic-framework MIL-53 or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2)).

The adsorption mode of CO(2) at low coverage in the nanoporous metal benzenedicarboxylate MIL-53(Cr) or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2)) has been identified using IR spectroscopy; the red shift of the nu(3) band and the splitting of the nu(2) mode of CO(2) in addition to the shifts of the nu(OH) and delta(OH) bands of the MIL-53(Cr) hydroxyl groups provide evidence that CO(2) interacts with the oxygen atoms of framework OH groups as an electron-acceptor via its carbon atom; this is the first example of such an interaction between CO(2) and bridged OH groups in a solid.

Hydrogen activation on Mo-based sulfide catalysts, a periodic DFT study.

Hydrogen adsorption on Mo[bond]S, Co[bond]Mo[bond]S, and Ni[bond]Mo[bond]S (10 1 macro 0) surfaces has been modeled by means of periodic DFT calculations taking into account the gaseous surrounding of these catalysts in working conditions. On the stable Mo[bond]S surface, only six-fold coordinated Mo cations are present, whereas substitution by Co or Ni leads to the creation of stable coordinatively unsaturated sites. On the stable MoS(2) surface, hydrogen dissociation is always endothermic and presents a high activation barrier. On Co[bond]Mo[bond]S surfaces, the ability to dissociate H(2) depends on the nature of the metal atom and the sulfur coordination environment. As an adsorption center, Co strongly favors molecular hydrogen activation as compared to the Mo atoms. Co also increases the ability of its sulfur atom ligands to bind hydrogen. Investigation of surface acidity using ammonia as a probe molecule confirms the crucial role of sulfur basicity on hydrogen activation on these surfaces. As a result, Co[bond]Mo[bond]S surfaces present Co[bond]S sites for which the dissociation of hydrogen is exothermic and weakly activated. On Ni[bond]Mo[bond]S surfaces, Ni[bond]S pairs are not stable and do not provide for an efficient way for hydrogen activation. These theoretical results are in good agreement with recent experimental studies of H(2)[bond]D(2) exchange reactions.