In this work, by combining FTIR results with periodic DFT calculations, we highlight the role of different anatase surface sites in the CO2 adsorption and in the subsequent reactions. It is shown that CO2 is mainly adsorbed in linear form at the (101) surface, while the formation of a variety of surface carbonates is occurring at the (001) one. The role of coadsorbed water in the formation of surface bicarbonates is also investigated. All the structures identified by FTIR spectroscopy were modeled by DFT, and the adsorption geometries and the energy of formation for the surface species were carefully analyzed. For the most stable structures we also performed the calculation of the vibrational frequencies that were compared with the FTIR data.

By combining electron microscopy, FTIR spectroscopy of different CO isotopic mixtures, and DFT calculations, a complete assignment of the IR spectrum of CO adsorbed on P25 (a mixture of 85% anatase and 15% rutile) at various dehydration states, on its pure rutile component, and on nanoanatase was obtained. It is shown that the measurements at 60 K provide IR spectra of unprecedented quality and that the spectroscopic method is extremely powerful not only to study the surface Lewis and Brønsted acidity but also to investigate the particles morphology. Indeed, as CO adsorbed on different faces is characterized by different stretching frequencies, the IR spectrum contains information on the exposed faces and hence on particles morphology. This information, combined with the study of the IR spectrum of CO adsorbed on defects, of dipole−dipole interactions between parallel oscillators adsorbed on extended faces and with HRTEM results allowed us to fully explore the relations between particles morphology and surface properties. By comparing the spectra of CO obtained on P25, on the pure rutile fraction of P25, and on nanoanatase, the high crystalline character of P25 is inferred, which is likely the key of its outstanding photocatalytic activity. It is also found that the IR spectrum of CO on P25 is the sum of the rutile and anatase contributions and that no additional surface features ascribable to anatase− rutile junctions are noticeable, thus contributing to the strong debate present in the literature.

We present three methods of the synthesis of zirconium metal–organic framework UiO-67 functionalized with platinum bipyridine coordination complexes (bpydcPtIICl2 and bpydcPtIVCl4) acting as linkers in the MOF framework. These Pt complexes can be reduced to bpydcPt0 under flow of H2 gas in the 600–700 K range, as probed by a sophisticated parametric refinement of in situ EXAFS data. IR spectroscopy testifies the high coordinative unsaturation of the reduced centers, able to form bpydcPt0(CO)2 dicarbonyl complexes upon CO adsorption. The large pore size of UiO-67 allows for ligand exchange between 2 Cl– and even bulky ligands such as toluene-3,4-dithiol. Framework bpydcPtIICl2 complexes can also be oxidized at room temperature to bpydcPtIVBr4 through oxidative addition of liquid Br2. XANES spectroscopy was used to monitor the changes in the Pt oxidation state along the observed reactions. Platinum bipyridine-functionalized UiO-67-Pt displays the same exceptional stability as the parent material as testified on both long and local range by in situ XRPD and Pt L3-edge EXAFS data.

Titanium dioxide is one of the most important metal oxides because of its applications as a white pigment, as an important component in solar cells, and as a photocatalyst. 1 In the last two applications, the relevant phenomena are occurring at the surface of anatase nanoparticles, which are generally considered to be more active than rutile ones. 2 Therefore, it is relevant for both technological and fundamental motivations to study the structure of the different surfaces terminating the anatase nanocrystals. This study can be performed both experimentally and theoretically.Concerning the experimental approach, the use of Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules has emerged as the leading method. 3 In particular, carbon monoxide, a weak Lewis base, is usually chosen to probe the Lewis acid sites of TiO 2 . 4À8 The stretching frequency of the adsorbed CO is related to the electrophilicity and polarizing power of the surface Lewis acid sites: the greater the electrophilicity of the metal cation, the higher the blue shift with respect to the value in the gas phase (2143 cm À1 ). 4 The variation of the stretching frequency originates from the combination of different mechanisms: (1) the interaction between the CO dipole moment and the surface electric field (Stark effect), (2) the repulsive potential due to the vibration of the CO molecule against a rigid surface (wall effect), and 9 (3) the dipoleÀdipole interactions between the adsorbed molecules. 10 However, when highly dispersed phases are concerned, the particles expose a variety of faces, and consequently, the IR spectra of adsorbed CO can be constituted by the superposition of several components whose unambiguous assignment is troublesome. For oxides characterized by a rock salt structure, such as MgO, a satisfactory interpretation of the spectra of adsorbed CO has been obtained by comparing the IR spectra with high-resolution transmission electron microscopy (HRTEM) results. 11 This has been made possible by the simple cubic morphology of the MgO particles, which definitely exposes a predominant family of faces, a fact which makes the interpretation of the HRTEM images straightforward. In the case of highsurface-area TiO 2 , the determination of the nanoparticles' morphology by HRTEM is much more difficult, and consequently, the combined use of electron microscopy and IR spectroscopy of adsorbed CO is not so fruitful. This is the case where the help of a computational study on the structure of the most probably exposed surfaces and on the vibrational properties of CO adlayers adsorbed on them can be of invaluable utility. This approach has dual importance: in fact, while on one side, it helps the interpretation of IR results, on the other side, it allows one to ABSTRACT: Periodic DFT calculations of the structure of (101), (100), (001), and (112) anatase faces and of the vibrational properties of CO adsorbed on them at two coverages allow assigning the main features of FTIR spectra of CO adsorbed at 60 K on highly dehydroxyl...

Microporous metal-organic frameworks are a class of materials being vigorously investigated for mobile hydrogen storage applications. For high-pressure storage at ambient temperatures, the M(3)[(M(4)Cl)(3)(BTT)(8)](2) (M-BTT; BTT(3-) = 1,3,5-benzenetristetrazolate) series of frameworks are of particular interest due to the high density of exposed metal cation sites on the pore surface. These sites give enhanced zero-coverage isosteric heats of adsorption (Q(st)) approaching the optimal value for ambient storage applications. However, the Q(st) parameter provides only a limited insight into the thermodynamics of the individual adsorption sites, the tuning of which is paramount for optimizing the storage performance. Here, we begin by performing variable-temperature infrared spectroscopy studies of Mn-, Fe-, and Cu-BTT, allowing the thermodynamics of H(2) adsorption to be probed experimentally. This is complemented by a detailed DFT study, in which molecular fragments representing the metal clusters within the extended solid are simulated to obtain a more thorough description of the structural and thermodynamic aspects of H(2) adsorption at the strongest binding sites. Then, the effect of substitutions at the metal cluster (metal ion and anion within the tetranuclear cluster) is discussed, showing that the configuration of this unit indeed plays an important role in determining the affinity of the framework toward H(2). Interestingly, the theoretical study has identified that the Zn-based analogs would be expected to facilitate enhanced adsorption profiles over the compounds synthesized experimentally, highlighting the importance of a combined experimental and theoretical approach to the design and synthesis of new frameworks for H(2) storage applications.

TiO2 anatase nanoparticles are among the relevant players in the field of light-responsive semiconductor nanomaterials used to face environmental and energy issues. In particular, shape-engineered TiO2 anatase nanosheets with dominant {001} basal facets gained momentum because of the possibility to exploit different and/or improved functional behaviors with respect to usual bipyramidal TiO2 anatase nanoparticles, mainly exposing {101} facets. Nevertheless, such behavior depends in a significant extent on the physicochemical features of surfaces exposed by nanosheets. They can vary in dependence on the presence or removal degree of capping agents, namely, fluorides, used for shape-engineering, and experimental investigations in this respect are still a few. Here we report on the evolution of interfacial/surface features of TiO2 anatase nanosheets with dominant {001} facets from pristine nanoparticles fluorinated both in the bulk and at their surface to nanoparticles with F– free surfaces by treatment in a basic solution and to totally F– free nanoparticles by calcination at 873 K. The nanoparticles fluorine content and its subsequent evolution is determined by complementary techniques (ion chromatography, TOF-SIMS, XPS, AES, SEM-EDX), probing different depths. In parallel, the evolution of the electronic properties and the Ti valence state is monitored by UV–vis spectroscopy and XPS. The calcination treatment results in {001} facets poorly hydroxylated, hydrated, and hydrophilic, which appear as surface features consequent to the expected (1 × 4) reconstruction. Moreover, IR spectroscopy of CO adsorbed as probe molecule indicates that the Lewis acidity of Ti4+ sites exposed on (1 × 4) reconstructed {001} facets of calcined TiO2 nanosheets is weaker than that of cationic centers on {101} facets of bipyramidal TiO2 anatase nanoparticles. The samples have also been tested in phenol photodegradation highlighting that differences in surface hydration, hydroxylation, and Lewis acidity between TiO2 nanoparticles with nanosheet (freed by F– by calcination at 873 K) and bipyramidal shape have a strong impact on the photocatalytic activity that is found to be quite limited for the nanoparticles mainly exposing (1 × 4) reconstructed {001} facets.

During the last three decades low dimensional systems attracted increasing interest both from the fundamental and the technological point of view due to their unique physical and chemical properties. X-ray Absorption Spectroscopy (XAS) is a powerful tool for the characterization of such kind of systems, owing to its chemical selectivity and high sensitivity in interatomic distances determination. Moreover this technique can simultaneously provide information on electronic and local structural properties of the nanomaterials, significantly contributing to clarify the relation between their atomic structure and their peculiar physical properties. This review provides a general introduction to XAS, discussing the basic theory of the technique, the most used detection modes, the related experimental setups and some complementary relevant characterization techniques (DAFS, EXELFS, PDF, XES, HERFD XAS, XRS). Subsequently a selection of significant applications of XAS spectroscopy to 2D, 1D and 0D systems will be presented. The selected low dimensional systems include IV and III-V semiconductor films, quantum wells, quantum wires and quantum dots; carbon based nanomaterials (epitaxial graphene and carbon nanotubes); metal oxide films, nanowires, nanorods and nanocrystals; metal nanoparticles. Finally, the future perspectives for the application of XAS to nanostructures are discussed.

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