Simultaneous probing of bulk liquid phase and catalytic gas-liquid-solid interface under working conditions using attenuated total reflection infrared spectroscopy

Infrared spectroscopic monitoring of solid-state processes

We put a spotlight on IR spectroscopic investigations in materials science by providing a critical insight into the state of the art, covering both fundamental aspects, examples of its utilisation, and current challenges and perspectives focusing on the solid state.

Adsorption States of N2/H2 Activated on Ru Nanoparticles Uncovered by Modulation-Excitation Infrared Spectroscopy and Density Functional Theory Calculations

The adsorption states of N₂ and H₂ on MgO-supported Ru nanoparticles under conditions close to those of ammonia synthesis (AS; 1 atm, 250 °C) were uncovered by modulation-excitation infrared spectroscopy and density functional theory calculations using a nanoscale Ru particle model. The two most intense N₂ adsorption peaks corresponded to the vertical chemisorption of N₂ on the nanoparticle's top and bridge sites, while the remaining peaks were assigned to horizontally adsorbed N₂ in view of the site heterogeneity of Ru nanoparticles. Long-term observations showed that vertically adsorbed N₂ molecules gradually migrated from the top sites to the bridge sites. Compared to those adsorbed vertically, N₂ molecules adsorbed horizontally exhibited a lower dipole moment, an increased N─N bond distance, and a decreased N─N bond order (i.e., were activated), which was ascribed to enhanced Ru-to-N charge transfer. H₂ molecules were preferentially adsorbed horizontally on top sites and then rapidly dissociated to afford strongly surface-bound H atoms and thus block the active sites of Ru nanoparticles. Our results clarify the controversial adsorption/desorption behavior of N₂ and H₂ on AS catalysts and facilitate their further development.

Strong Activity Enhancement of the Photocatalytic Degradation of an Azo Dye on Au/TiO2 Doped with FeOx

The doping of Au/TiO2 with FeOx is shown to result in a strong enhancement of its photocatalytic activity in the degradation of the azo dye Orange II. In order to examine the source of this enhancement, Au-FeOx/TiO2 nanocomposites containing different molar ratios of Au:Fe were synthesized, and X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM) analyses indicated that the TiO2-supported Au nanoparticles were partially covered with an amorphous layer of FeOx species, in which the iron was present as Fe2+ and Fe3+. The metal-semiconductor system, i.e., Au/TiO2, showed only a moderate degradation rate, whereas doping with FeOx strongly enhanced the degradation activity. The bandgap energy decreased gradually from Au/TiO2 (3.13 eV) to the catalyst with the highest FeOx loading Au-FeOx (1:2)/TiO2 (2.23 eV), and this decrease was accompanied by a steady increase in the degradation activity of the catalysts. XPS analyses revealed that compared to Au/TiO2, on Au-FeOx/TiO2 a much higher population density of chemisorbed and/or dissociated oxygen species was generated, which together with the decreased bandgap resulted in the highest photocatalytic activity observed with Au-FeOx (1:2)/TiO2. The processes occurring during reaction on the catalyst surface and in the bulk liquid phase were investigated using operando attenuated total reflection IR spectroscopy (ATR-IR) combined with modulation excitation spectroscopy (MES), which showed that the doping of Au/TiO2 with FeOx weakens the interaction of the dye with the catalyst surface and strongly enhances the cleavage of the azo bond.

New Approach to Simultaneous In Situ Measurements of the Air/Liquid/Solid Interface Using PM-IRRAS

Vibrational spectroscopy techniques have evolved to measure gases, liquids, and solids at surfaces and interfaces. In the field of surface-sensitive vibrational spectroscopy, infrared spectroscopy measures the adsorption on surfaces and changes from reactions. Previous polarized modulated-infrared reflection-absorption spectroscopy (PM-IRRAS) measurements at the gas/solid interface were developed to observe catalytic reactions near reaction conditions. Other PM-IRRAS measurements use liquid cells where the sample is submerged and compressed against a prism that has traditionally been used for electrochemical reactions. This article presents a new method that is used to observe in situ adsorption of molecules using PM-IRRAS at the gas/liquid/solid interface. We demonstrate the meniscus method by measuring the adsorption of octadecanethiol on gold surfaces. Characterization of self-assembled monolayers (SAMs), the "gold standard" for PM-IRRAS calibration measurements, was measured in ethanol solutions. The condensed-phase (air/liquid) interface in addition to the liquid/solid interface was measured simultaneously in solution. These are compared with liquid attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy measurements to confirm the presence of the SAM and liquid ethanol. A model of the three-phase system is used to approximate the thickness of the liquid ethanol layer and correlate these values to signal attenuation using PM-IRRAS. This proof-of-concept study enables the measurement of reactions at the gas/liquid/solid interface that could be adapted for other reactions at the electrode and electrolyte interfaces with applications in environmental science and heterogeneous catalysis.

In Situ FTIR Reactor for Monitoring Gas-Phase Products during a (Photo)catalytic Reaction in the Liquid Phase

Various catalytic and photocatalytic reactions in the liquid phase give rise to gas products. Therefore, the identification and quantification of these products are of high importance and are even essential for some reactions. In this paper, a new in situ FTIR reactor is designed and used for analyzing the gas headspace of a (photo)catalytic reaction in solution. It allows the identification and quantification of the gas-phase products of a liquid reaction under operating conditions and in real time. The new reactor has been tested in three representative photocatalytic reactions widely studied as model reactions in the liquid phase: i.e., (i) decomposition of formic acid, (ii) oxidation of methylene blue, and (iii) reduction of CO₂. The validity of the results has been confirmed by analyzing the headspace at the end of the reaction using gas chromatography technique. The new reactor opens the possibility to follow online the (photo)catalyst activity. This is useful for ensuring the stability of the catalyst and studying the evolution of the selectivity during the reaction. The nondestructive behavior of the FTIR technique allows its coupling with other techniques for obtaining complementary results. The new reactor setup is easy to handle and to ship and is very efficient, which makes it very suitable for performing complementary, fast and/or preliminary studies.

Elucidating the reactivity of methoxyphenol positional isomers towards hydrogen-transfer reactions by ATR-IR spectroscopy of the liquid–solid interface of RANEY® Ni

In the presence of Raney® Ni and 2-propanol, guaiacol is orientated parallel to the catalyst surface, whereas 3- and 4-methoxyphenol forms a titled adsorption surface complex.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Adsorption and stability of chiral modifiers based on 1-(1-naphthyl)-ethylamine for Pt catalysed heterogeneous asymmetric hydrogenations

Catalytic and in situ ATR-IR spectroscopic investigations provide mechanistic insight relevant to heterogeneous asymmetric hydrogenation on Pt catalyst using naphthylethylamine-based modifiers.