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We report on the solvent-free synthesis of the perfluorinated analogue of the well-known breathing metal-organic framework MIL-53(Al), i.e., F4-MIL-53(Al), featuring tetrafluoroterephthalic acid as linker. The crystal structure was solved and refined from powder X-ray diffraction data and confirmed to be isoreticular with MIL-53(Al). In contrast to the MIL-53(Al) analogue, F4-MIL-53(Al) displays a peculiar breathing effect solely induced by the temperature and not by the desorption of molecules at increasing temperature. This unique behaviour is experimentally reported here for the first time.

A new fluorinated member of the MIL-53(Al) family, displaying a unique temperature-induced phase transition, was synthesised via a solvent-free route.

The reaction mechanism of CO2 and methanol toward dimethyl carbonate over ZrO2 in the liquid phase is elucidated by means of ATR-IR augmented by MCR-ALS analysis.

The hierarchization of zeolites to overcome the major drawbacks related to molecular diffusion limitation in micropores is a popular concept in heterogeneous catalysis. Despite the constant increase of new synthesis strategies to produce such hierarchical systems, the deep knowledge of their structural arrangement and how the zeolitic lattice is organized in a multilevel porous system is often missing. This information is essential to design a structure, tuning the porosity and the distribution of easily accessible active sites, and successively controlling the catalytic properties. In the present work, the synthesis of one of the most sophisticated forms of the hierarchical ZSM-5 zeolite has been reproduced, obtaining two multilevel porous materials with different crystallinity degrees, with the final aim of investigating and clarifying the finest features of their active sites. For this purpose, an extended characterization step by means of a unique multitechnique approach has been performed, thus revealing the active site nature, abundance, and distribution. IR spectroscopy with different molecular probes and a targeted catalytic test based on the hydroconversion reaction of n-decane were the toolbox for disclosing how the MFI lattice takes part in the hierarchical structure and how it, working in synergy with the mesoporous system, confers to this material a totally new shape-size selectivity. Merging the information obtained for the synthesized hierarchical zeolite with the characterization results of two reference materials (a mesoporous aluminum-containing MCM-41 and a microporous commercial ZSM-5), it was possible to define an internal and external map of the pore network of this complex and unique molecular sieve, where strong Brønsted acidic sites are located at the mouth of the MFI micropores and, at the same time, exposed at the surface of the mesoporous channels. Hence, the possibility of easily releasing bulky products is ensured and the application possibilities of the MFI lattice are expanded beyond cracking reactions. ispartof: Acs Applied Materials & Interfaces vol:13 issue:41 pages:49114-49127 ispartof: location:United States status: published

This contribution aims at analysing the current understanding about the influence of Al distribution, zeolite topology, ligands/reagents and oxidation state on ions mobility in Cu-zeolites, and its relevance toward reactivity of the metal sites. The concept of Cu mobilization has been originally observed in the presence of ammonia, favouring the activation of oxygen by formation of NH3 oxo-bridged complexes in zeolites and opening a new perspective about the chemistry in single-site zeolite-based catalysts, in particular in the context of the NH3-mediated Selective Catalytic Reduction of NOx (NH3-SCR) processes. A different mobility of bare Cu+/Cu2+ ions has been documented too, showing for Cu+ a better mobilization than for Cu2+ also in absence of ligands. These concepts can have important consequences for the formation of Cu-oxo species, active and selective in other relevant reactions, such as the direct conversion of methane to methanol. Here, assessing the structure, the formation pathways and reactivity of Cu-oxo mono- or multimeric moieties still represents a challenging playground for chemical scientists. Translating the knowledge about Cu ions mobility and redox properties acquired in the context of NH3-SCR reaction into the field of direct conversion of methane to methanol can have important implications for a better understanding of transition metal ions redox properties in zeolites and for an improved design of catalysts and catalytic processes.

The effect of differently substituted 2,2'-bipyridine ligands (

Ti silicates, and in particular, titanium silicalite-1 (TS-1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H2 O2 as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS-1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.

This work presents the synthesis of nearly defect-free ZSM-5 nanosized crystals, prepared in fluoride medium by seeding with silicalite-1. This material was carefully characterized and its catalytic performances in the methanol to hydrocarbons (MTH) reaction were assessed. Such fluoride-based material was compared to a reference ZSM-5, produced through a conventional alkaline synthesis but from the same seeding. Despite both the materials show closely identical morphology and they have a comparable acid site population, the catalyst prepared using the fluoride route showed significantly longer lifetime in MTH compared to the catalyst prepared using conventional synthesis at high pH. The slower deactivation for the samples prepared using the fluoride route was ascribed, thanks to a thorough in situ IR spectroscopy study, to its lower density of internal defects. According to the UV-Raman characterization of coke on the spent catalyst, the fluoride-based ZSM-5 catalyst produces less molecular coke species, most probably because of the absence of enlarged cavities/channels as due to the presence of internal defects. On the basis of these observations, the deactivation mechanism in the ZSM-5 synthesized by fluoride medium could be mostly related to the deposition of an external layer of bulk coke, whereas in the alkali-synthesized catalyst an additional effect from molecular coke accumulating within the porous network accelerates the deactivation process.

Cu-exchanged zeolites are widely studied materials because of their importance in industrial energetic and environmental processes. Cu redox speciation lies at the center of many of these processes but is experimentally difficult to investigate in a quantitative manner with regular laboratory equipment. This work presents a novel technique for this purpose that exploits the selective adsorption of CO over accessible Cu(I) sites to quantify them. In particular, isothermal volumetric adsorption measurements are performed at 50 °C on a series of opportunely pre-reduced Cu-ZSM-5 to assess the relative fraction of Cu(I); the setup is fairly simple and only requires a regular volumetric adsorption apparatus to perform the actual measurement. Repeatability tests are carried out on the measurement and activation protocols to assess the precision of the technique, and the relative standard deviation (RSD) obtained is less than 5%. Based on the results obtained for these materials, the same CO adsorption protocol is studied for the sample using infrared spectroscopy, and a good correlation is found between the results of the volumetric measurements and the absorbance of the peak assigned to the Cu(I)–CO adducts. A linear model is built for this correlation, and the molar attenuation coefficient is obtained, allowing for spectrophotometric quantification. The good sensitivity of the spectrophotometric approach and the precision and simplicity of the volumetric approach form a complementary set of tools to quantitatively study Cu redox speciation in these materials at the laboratory scale, allowing for a wide range of Cu compositions to be accurately investigated.

HCN in the gas form is considered as a primary nitrogen source for the synthesis of prebiotic molecules in extraterrestrial environments. Nevertheless, the research mainly focused on the reactivity of HCN and its derivatives in aqueous systems, often using external high-energy supply in the form of cosmic rays or high energy photons. Very few studies have been devoted to the chemistry of HCN in the gas phase or at the gas/solid interphase, although they represent the more common scenarios in the outer space. In this paper we report about the reactivity of highly pure HCN in the 150-300 K range at the surface of amorphous and crystalline Mg

The reaction of CuI(bpy)2 complexes with tBuOOH was followed by a multi-technique approach (DFT-supported lab-scale or synchrotron spectroscopies and electrochemistry). The findings evidence the formation of a CuIIOH species as reactive intermediate.

A Design of Experiments approach allowed studying the parameters affecting TS-1 synthesis. Few of them drive the key properties of the TS-1 (synthesis yield, Ti content and speciation), namely hydrolysis time and crystallization time and temperature.

The production of carbon-rich hydrocarbons via CO2 valorization is essential for the transition to renewable, non-fossil-fuel-based energy sources. However, most of the recent works in the state of the art are devoted to the formation of olefins and aromatics, ignoring the rest of the hydrocarbon commodities that, like propane, are essential to our economy. Hence, in this work, we have developed a highly active and selective PdZn/ZrO2+SAPO-34 multifunctional catalyst for the direct conversion of CO2 to propane. Our multifunctional system displays a total selectivity to propane higher than 50% (with 20% CO, 6% C1, 13% C2, 10% C4, and 1% C5) and a CO2 conversion close to 40% at 350 °C, 50 bar, and 1500 mL g–1 h–1. We attribute these results to the synergy between the intimately mixed PdZn/ZrO2 and SAPO-34 components that shifts the overall reaction equilibrium, boosting CO2 conversion and minimizing CO selectivity. Comparison to a PdZn/ZrO2+ZSM-5 system showed that propane selectivity is further boosted by the topology of SAPO-34. The presence of Pd in the catalyst drives paraffin production via hydrogenation, with more than 99.9% of the products being saturated hydrocarbons, offering very important advantages for the purification of the products., JACS Au, 1 (10), ISSN:2691-3704

In this work, we characterize the surface sites of Mg2SiO4, a main constituent of dust grains from the interstellar medium/protoplanetary disks, by using infrared (IR) spectroscopy of adsorbed CO, ...

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective 17 O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

The local structural and electronic properties of Cu sites during the direct conversion of methane to methanol with molecular oxygen was followed by X-ray absorption spectroscopy (XAS) on a set of Cu-zeolites with different topology (large pore MOR and small pore CHA frameworks) and similar chemical composition (Si/Al ∼ 11 and 12, respectively). Two low loading (LL) and high loading (HL) samples were prepared by ion exchange, and their methanol productivity measured in laboratory fixed bed reactor tests, resulting in selectivity higher than 85%. Both Cu-MOR samples outperformed Cu-CHA ones, resulting in 0.28 and 0.26 molCH3OH/molCu for the LL (Cu/Al = 0.13) and HL (Cu/Al = 0.36) samples, vs 0.10 and 0.17 molCH3OH/molCu for LL and HL Cu-CHA, respectively (Cu/Al = 0.16 and 0.49). The evolution of Cu oxidation state and local coordination environment was followed in all the reaction steps (O2 activation, flush with He, CH4 dosage and subsequent CH3OH desorption by applying a wet stream) both during temperature changes (ramp in O2 up to 500 °C, cooling to 200 °C for subsequent steps, etc.) and in steady-state conditions. Strong similarities were observed in the local structure of Cu sites in all samples, which showed similar redox dynamics during O2 activation and subsequent cooling to 200 °C. Even though only qualitative considerations can be made on the overall spectral evolution, XANES indicates that not all the Cu(I) formed during CH4 loading belongs to active sites.

Forsterite is a nesosilicate with Mg2SiO4 composition, representing the iron-free endmember of the family of olivines. Interestingly, aside its finding as mineral component of the Earth’s crust and mantle, it has been observed in extraterrestrial environments where have been found as principal inorganic constituents of meteorites, comets and in the core of dust grains found in the interstellar medium and in protoplanetary disks [1]. It has often been indicated that these grains could be possible sites for the occurrence of astrochemical reactions involving simple prebiotic molecules (SPM, e.g. CO, CO2, H2O, HCN, etc.). In order to assess the role of Mg2SiO4 in this prebiotic scenario, the understanding of the surface properties of grain-core phases is of primary importance. We thoroughly characterized two Mg2SiO4 model samples, namely an amorphous magnesium silicate (AMS) and a crystalline forsterite (Forst), and we compared their surface acid-base properties as a function of their structure (amorphous vs crystalline) [2]. The AMS sample was synthesized via thermal plasma route, as proposed by Koike et al. [3], whereas Forst was obtained by thermal annealing of the former at 1073 K. The acid-base properties of the two systems were investigated by monitoring the adsorption of selected probe molecules (CO, CO2 and CD3CN) by IR spectroscopy and then by studying the perturbation of their vibrational fingerprints upon interaction with specific surface functionalities. As an example, the IR spectra of CD3CN adsorbed on AMS and Forst in the υ(CN) frequency range are shown in Figure 1. The spectra for both materials are characterized by three main bands at 2300, 2266 and 2215 cm-1. The 2300 cm-1 component is assigned to CD3CN interacting with Lewis acid sites, i.e. surface Mg2+ cations, whereas the 2266 cm-1 band is assigned to the features of CD3CN interacting with weak Bronsted acid sites (surface silanols) and of not interacting CD3CN forming multilayers. These components are closely similar among both samples. Instead, they remarkably differ with respect to the 2215 cm-1 band, which is much more intense in AMS than in Forst. We assigned this signal to anionic species formed by deprotonation of acetonitrile in presence of strong surface basic sites. These results, confirmed also by the adsorption of other probe molecules, showed as AMS and Forst have a similar population of surface acid sites, whereas only the former shows a significant surface basicity. Finally, we investigated HCN adsorption on both samples. HCN is a molecule of particular interest since its adsorption and reactivity on silicates in interstellar media is supposed to play a role in the synthesis of biomolecules contributing to the development of life [4]. The HCN reaction leads to the formation of a variety of oligomers (at least up to 12 terms) and the surface basicity, already highlighted by the other probe molecules, seems to play a key role in the process. Figure 1. IR spectra of AMS and Forst outgassed for 2 hours at 673 K in vacuum and contacted with 40 mbar of CD3CN at r.t. (curves a) and outgassed for 30 min at r.t. after CD3CN contact (curves b). Spectra collected at decreasing CD3CN pressures are reported as thin lines for both materials. The contribution from the bare activated material has been subtracted. The spectra have been normalized to the specific surface area of the samples. References [1] T. Henning, Cosmic Silicates. Annu. Rev. Astron. Astrophys. 2010, 48, 21–46. [2] M. Signorile, L. Zamirri, A. Tsuchiyama, P. Ugliengo, F. Bonino, G. Martra, ACS Earth Sp. Chem. 2020, 4, 345–354. [3] C. Koike, Y. Imai, H. Chihara, H. Suto, K. Murata, A. Tsuchiyama, S. Tachibana, S. Ohara, Astrophys. J. 2010, 709, 983–992 [4] J. P. Ferris, and W. J. Hagan Jr, Tetrahedron. 1984, 40, 1093-1120.

THORONDOR is a data treatment software with a graphical user interface (GUI) accessible via the browser-based Jupyter notebook framework. It aims to provide an interactive and user-friendly tool for the analysis of NEXAFS spectra collected during in situ experiments. The program allows on-the-fly representation and quick correction of large datasets from single or multiple experiments. In particular, it provides the possibility to align in energy several spectral profiles on the basis of user-defined references. Various techniques to calculate background subtraction and signal normalization have been made available. In this context, an innovation of this GUI involves the usage of a slider-based approach that provides the ability to instantly manipulate and visualize processed data for the user. Finally, the program is characterized by an advanced fitting toolbox based on the lmfit package. It offers a large selection of fitting routines as well as different peak distributions and empirical ionization potential step edges, which can be used for the fit of the NEXAFS rising-edge peaks. Statistical parameters describing the goodness of a fit such as χ2 or the R-factor together with the parameter uncertainty distributions and the related correlations can be extracted for each chosen model.

Cu-exchanged zeolites have been shown to possess Cu-oxo species active towards the direct methane to methanol (DMTM) conversion, carried out through a chemical-looping approach. Different Cu-zeolites have been investigated for the DMTM process, with Cu-mordenite (Cu-MOR) being among the most active. In this context, an accurate determination of the local structure and nuclearity of selective Cu-oxo species responsible for an efficient DMTM conversion still represents an ongoing challenge for characterization methods, including synchrotron-based X-ray absorption spectroscopy (XAS). Herein, we explore the potential of an alternative analysis of Extended X-ray Absorption Fine Structure (EXAFS) data using wavelet transform (WT) to enhance the technique sensitivity to multimeric Cu species hosted in the MOR framework. Combining ex situ XAS measurements under model red-ox conditions with in situ data collected after the key steps of the DMTM process, we demonstrate how EXAFS-WT enables unambiguous detection of Cu–Cu scattering contributions from multimeric Cu-species. As also confirmed by complementary in situ IR spectroscopy results, these are observed to dynamically respond to the chemical environment over the different conditions probed. We finally report a proof-of-concept EXAFS fit using the WT representation, applied to the structural refinement of O2-activated Cu-MOR. The fitting results reveal a Cu local coordination environment consistent with mono-(μ-oxo) di-copper cores, with Cu–Cu separation of ∼3.1 A, paving the way to future applications and developments of the method in the field of Cu-zeolite research and beyond.

Formamide is abundant in the interstellar medium and was also present during the formation of the Solar system through the accretion process of interstellar dust. Under the physicochemical conditions of primordial Earth, formamide could have undergone decomposition, either via dehydration (HCN + H2O) or via decarbonylation (CO + NH3). The first reactive channel provides HCN, which is an essential molecular building block for the formation of RNA/DNA bases, crucial for the emergence of life on Earth. In this work, we studied, at the CCSD(T)/cc-pVTZ level, the two competitive routes of formamide decomposition, i.e. dehydration and decarbonylation, either in liquid formamide (by using the polarization continuum model technique) or at the interface between liquid formamide and amorphous silica. Amorphous silica was adopted as a convenient model of the crystalline silica phases ubiquitously present in the primordial (and actual) Earth's crust, and also due to its relevance in catalysis, adsorption and chromatography. Results show that: (i) silica surface sites catalyse both decomposition channels by reducing the activation barriers by about 100 kJ mol−1 with respect to the reactions in homogeneous medium, and (ii) the dehydration channel, giving rise to HCN, is strongly favoured from a kinetic standpoint over decarbonylation, the latter being, instead, slightly favoured from a thermodynamic point of view.

The Raman characterization of catalysts (as for any material) is generally a difficult task, since the numerous drawbacks affecting this technique, e.g. overlapping with photoemission phenomena and excitation induced sample damaging. These side effects are detrimental toward advanced applications of Raman spectroscopy, as in situ and/or operando ones. If photoemission can be often avoided by changing the excitation wavelength, the sample damaging requires to take advantage of specific tools, developed ad hoc. The aim of this work is to show the potentialities of a novel Raman setup, specifically designed for the in situ study of catalytic materials, where the sample is moved by means of a magnetic-driven sample-holder, reducing the excitation induced damaging to a negligible extent. Various examples related to catalysis field (involving inorganic, organic and mixed systems) are reported to support this statement.

Hydrogen peroxide to propylene oxide (HPPO) reaction is an attractive process exploiting titanium silicalite-1 (TS-1) as a catalyst in combination with aqueous hydrogen peroxide as an oxidizing agent. Beyond the industrial interest, TS-1 represents one of the most widely characterized catalysts due to its unique properties. However, a unified description on the speciation of the different Ti species and their correlation to catalytic performances is missing in the literature. This work aims to exploit spectroscopic techniques (namely, diffuse reflectance UV–vis, Raman, FT-IR, and Ti K-edge XANES) in a qualitative and quantitative way to thoroughly characterize Ti sites in a selected set of industrially relevant TS-1 samples, each one owning a peculiar Ti speciation. The outcomes of this study have been then related to the activity of each catalyst in HPPO reaction, showing its linear correlation with the content of perfect Ti sites (i.e., isomorphously substituting Si in the zeolitic framework). Other Ti ...

Choline/amino acid-based ionic liquids were synthetized via ionic metathesis and their CO2 absorption performances evaluated by employing different experimental approaches. In order to overcome any viscosity-related problem, dimethyl sulfoxide (DMSO) was employed as solvent. IL-DMSO solutions with different IL concentrations were evaluated as absorbents for CO2, also investigating their good cyclability as desirable for real industrial CO2 capture technologies. 1H-NMR and in-situ ATR-IR experiments were the toolbox to study the CO2 chemical fixation mechanism under different experimental conditions, proving the formation of distinct chemical species (carbamic acid and/or ammonium carbamate). In general, these ILs demonstrated molar uptakes higher than classical 0.5 mol CO2/mol IL and the capacity to release CO2 in extremely mild conditions. The possible biological adverse effects were also analyzed, for the first time, in zebrafish (Danio rerio) during the development, by assessing for different toxicological endpoints, proving the non-toxicity and high biocompatibility of these bio-inspired ILs.

Formamide has been recognized in the literature as a key species in the formation of the complex molecules of life, such as nucleobases. Furthermore, several studies reported the impact of mineral phases as catalysts for its decomposition/polymerization processes, increasing the conversion and also favoring the formation of specific products. Despite the progresses in the field, in situ studies on these mineral-catalyzed processes are missing. In this work, we present an in situ UV-Raman characterization of the chemical evolution of formamide over amorphous SiO2 samples, selected as a prototype of silicate minerals. The experiments were carried out after reaction of formamide at 160 &deg, C on amorphous SiO2 (Aerosil OX50) either pristine or pre-calcined at 450 &deg, C, to remove a large fraction of surface silanol groups. Our measurements, interpreted on the basis of density functional B3LYP-D3 calculations, allow to assign the spectra bands in terms of specific complex organic molecules, namely, diaminomaleonitrile (DAMN), 5-aminoimidazole (AI), and purine, showing the role of the mineral surface on the formation of relevant prebiotic molecules.

The effect of water on the kinetics of oxymethylene dimethyl ether (OME) synthesis from dimethoxymethane (OME1) and trioxane (TRI) has been investigated in a combined kinetic and in situ infrared s...

The role of anatase TiO2 on the performance of VOx-TiO2 is studied by in situ spectroscopies on a series of catalysts with 1.9–9.7 V at/nm2. At low coverages, VOx is preferentially located at defect sites, spreading on the main TiO2 {101} and {100} facets at intermediate coverages. Bronsted sites concentration is maximum around 6 V atoms/nm2, and EPR shows a significant amount of V4+ at intermediate VOx densities, indicating the influence of TiO2 on their reducibility. The activity is proportional to the square of the VOx density up to 5.8 V atoms/nm2. All catalysts show the same activation energy for NH3-SCR of 56.9 ± 1.0 kJ/mol in the temperature range 160−280 °C, indicating that the reaction path does not change in the studied V coverage range. The activity does not follow the observed trends for Bronsted acid sites or reducibility, and it seems that the ability to form V-pairs is the only requirement for catalytic activity.

Copper exchanged CHA zeolites were investigated by in situ Diffuse Reflectance UV–vis-NIR spectroscopy in order to study the evolution of the copper environment in respect to the sample composition (2 samples differing in Si/Al and Cu content) and the activation procedure (in O2 or N2 from room temperature to 400 °C). In situ measurements allowed showing the different behavior of the two samples, both in the d-d and Ligand to Metal Charge Transfer regions, confirming a strong effect of the lattice composition to determine the copper environment and its reactivity. One of the two sample (Si/Al = 15 and Cu/Al = 0.5) was further investigated, in order to clarify the nature of the Cu-oxo species, formed along activation in oxygen. To shed light on this point, in situ measurements starting from a reduced form of the sample, obtained by activation in inert atmosphere at high temperature, were also considered. Additional suggestions on the nature of these species were obtained by resonant Raman measurements conducted in controlled atmosphere.

A molecular understanding of coke formation during the methanol to olefins (MTO) reaction is of great importance in order to adopt strategies that can improve the lifetime of the catalyst. UV-Raman has proven to be a valuable characterization tool, because of the favorable resonance conditions achieved towards key coke components, e.g. aromatic and olefinic compounds. Still, the applicability of UV-Raman in MTO is limited by the high risk of inducing sample damage under the exposure to the intense UV excitation laser. This drawback has been addressed by specific experimental setups, exploiting the concept of sample movement under the laser beam during the measurement, so that the laser-sample interaction is minimized by averaging over a wider sample surface. In this work, the impact of two different experimental setups, both relying on sample movement, are evaluated for the operando investigation of the MTO reaction over a SAPO-34. While in one case the whole catalytic bed is moved (fluidized bed), in the other case the sample is pressed into a pellet and rotated. Due to the peculiar spatial evolution of the reaction zone during MTO in a fixed-bed reactor, each method provides valuable information, but at very different spatiotemporal scales.

The synthesis of CO2 sorbents capable of working on combustion flue gases is a challenging topic in the field of carbon capture and sequestration. Indeed, the presence of moisture in combustion exh...

Mineral surfaces have been demonstrated to play a central role in prebiotic reactions, which are understood to be at the basis of the origin of life. Among the various molecules proposed as precursors for these reactions, one of the most interesting is formamide. Formamide has been shown to be a pluripotent molecule, generating a wide distribution of relevant prebiotic products. In particular, the outcomes of its reactivity are strongly related to the presence of mineral phases acting as catalysts toward specific reaction pathways. While the mineral&ndash, products relationship has been deeply studied for a large pool of materials, the fundamental description of formamide reactivity over mineral surfaces at a microscopic level is missing in the literature. In particular, a key step of formamide chemistry at surfaces is adsorption on available interaction sites. This report aims to investigate the adsorption of formamide over a well-defined amorphous silica, chosen as a model mineral surface. An experimental IR investigation of formamide adsorption was carried out and its outcomes were interpreted on the basis of first principles simulation of the process, adopting a realistic model of amorphous silica.

This perspective article aims to underline how cutting-edge synchrotron radiation spectroscopies such as extended X-ray absorption spectroscopy (EXAFS), X-ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti-based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV-Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band-gap and shape engineered TiO2 materials and microporous titanosilicates (ETS-10, TS-1 and Ti-AlPO-5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.

In this work, we investigate two activation procedures for a MoS2/TiO2 model catalyst. When the activation is performed in two steps, by sequentially dosing first H2S followed by reduction in H2 ,a n intermediate Mo oxysulfide phase forms, leading finally to a 3–4 layers thick nano-MoS2 phase. Conversely, when the activation is performed in a single step by dosing H2S and H2 simultaneously, the activation proceeds via a layer-by-layer growth, leading to a MoS2 structure remarkably different from conventional MoS2. To achieve this structural understanding, a well-defined model system was synthesized and a multitechnique approach has been applied to characterize the growth and surface properties of MoS2 on TiO2. The results evidence the complexity of the structural evolution during the activation procedure and the sensitivity of the final product to subtle changes in the activation procedure. Catalytic tests determined that the one-step activation of MoS2 on TiO2 results in a more active HDS, HDN, and HYD catalyst, as expected due to smaller MoS2 nanoparticles in terms of both average diameter of the plates and number of stacking layers.

Titanium Silicalite-1 (TS-1), because of its crystalline structure and its well-defined Ti sites, represents the prototype of a single site catalyst. According to this fundamental aspect and to the relevant role of TS-1 as selective catalyst in important industrial partial oxidation reactions, TS-1 has been widely characterized through both experimental and computational techniques. Still, several fundamental aspects of its structural and catalytic properties have to be addressed. Among these, an intriguing topic is the Ti location in the various sites of the MFI framework. The independent sites are generally considered to be 12, following the Pnma space group of TS-1 at high Ti loading. However, when Ti loading is lower than 2 atoms per unit cell, diffraction showed that the system must be described by the P21/n space group, thereby allowing 24 independent sites. With respect to previous studies, this work aims to exploit this datum to give a more accurate description of the TS-1 system at low Ti loadings, adopting a state of the art methodology (all electron periodic B3LYP-D2 calculations). The relative stabilities of the 24 Ti sites have been evaluated, showing a good agreement with previous studies. The simulation of adsorption energies for ammonia (present as reactants in some of the most important industrial reactions catalyzed by TS-1) over the most stable sites have been computed as well, in order to validate the obtained models. Additionally to binding energies, adsorption enthalpies and Gibbs free energies have been obtained through an approximate reduced Hessian scheme. The improved local description of the Ti sites (in combination with the adducts stabilities given by the energetic data) allowed the deep understanding of subtle effects, such as the number of molecular ligands each Ti atom can actually host upon adsorption. These results, showing only few sites can efficiently host two ligands in the neighborhoods of STP conditions, allowed for the first time the heterogeneity in the experimental outcomes reported over the last two decades to be rationalized.

The methanol-to-hydrocarbons (MTH) reaction represents a versatile, industrially viable alternative to crude-oil based processes for the production of chemicals and fuels. In the MTH reaction, the shape selectivity of acidic zeolites is exploited to direct the synthesis towards the desired product. However, due to unavoidable side reactions occurring under processing conditions, all MTH catalysts suffer deactivation due to coke formation. Though it is likely that some common characteristics for carbon formation exist for all zeolite topologies, it has been proposed that the differences in shape selectivity among the different catalysts will also influence the individual deactivation mechanisms. As deactivating species are mostly aromatic compounds, highly methylated benzenes and/or polycyclic aromatic hydrocarbons (PAHs) have been discussed. In some cases, these can further grow to extended carbon structures. Here, we have investigated the hydrocarbon reactivities and carbon formation for five topologically different zeolite catalysts through an operando UV-Raman approach, taking advantage of the high sensitivity of this technique towards aromatic and other carbonaceous species. The combination of the spectroscopic tool with activity measurements allowed us to obtain valuable details and some general trends on the deactivation paths during MTH. This approach made accessible unique insight on the complex chemistry of MTH by allowing the real-time observation of hydrocarbon transformations typical for the peculiar topology of each catalyst, usually inaccessible by ex situ techniques.

In the last decade the appearance of progressively more sophisticated codes, together with the increased computational capabilities, has made XANES a spectroscopic technique able to quantitatively confirm (or discard) a structural model, thus becoming a new fundamental diagnostic tool in catalysis, where the active species are often diluted metal centers supported on a matrix. After providing a brief historical introduction and the basic insights on the technique, in this review article, we provide a selection of four examples where operando XANES technique has been able to provide capital information on the structure of the active site in catalysts of industrial relevance: (i) Phillips catalyst for ethylene polymerization reaction; (ii) TS-1 catalyst for selective hydrogenation reactions; (iii) carbon supported Pd nanoparticles for hydrogenation reactions; (iv) Cu-CHA zeolite for NH3-assisted selective reduction of NOx and for partial oxidation of methane to methanol. The last example testifies how the multivariate curve resolution supported by the alternating least-squares algorithm applied to a high number of XANES spectra collected under operando conditions allows to quantitatively determine different species in mutual transformation. This approach is particularly powerful in the analysis of experiments where a large number of spectra has been collected, typical of time- or space-resolved experiments. Finally, machine learning approaches (both indirect and direct) have been applied to determine, from the XANES spectra, the structure of CO, CO2 and NO adsorbed on Ni2+ sites of activated CPO-27-Ni metal-organic framework.

We report on the creation and characterization of Pb-related color centers in diamond upon ion implantation and subsequent thermal annealing. Their optical emission in the photoluminescence (PL) regime consists of an articulated spectrum with intense emission peaks at 552.1 and 556.8 nm, accompanied by a set of additional lines in the 535–700 nm range. The attribution of the PL emission to stable Pb-based defects is corroborated by the correlation of its intensity with the implantation fluence of Pb ions. PL measurements performed as a function of sample temperature (in the 143–300 K range) and under different excitation wavelengths (i.e., 532, 514, 405 nm) suggest that the complex spectral features observed in Pb- implanted diamond might be related to a variety of different defects and/or charge states. The emission of the 552.1 and 556.8 nm lines is reported at the single-photon emitter level, demonstrating that they originate from the same individual defect. This work follows from previous reports on optically active centers in diamond based on group-IV impurities, such as Si, Ge, and Sn. In perspective, a comprehensive study of this set of defect complexes could bring significant insight on the common features involved in their formation and opto-physical properties, thus offering a basis for the development of a new generation of quantum-optical devices.