Effect of Molecular Guest Binding on the d-d Transitions of Ni2+ of CPO-27-Ni: A Combined UV-Vis, Resonant-Valence-to-Core X-ray Emission Spectroscopy, and Theoretical Study.

We used Ni K-edge resonant-valence-to-core X-ray emission spectroscopy (RVtC-XES, also referred to as direct RIXS), an element-selective bulk-sensitive synchrotron-based technique, to investigate the electronic structure of the CPO-27-Ni metal-organic framework (MOF) upon molecular adsorption of significant molecular probes: H2O, CO, H2S, and NO. We compare RVtC-XES with UV-vis spectroscopy, and we show that the element selectivity of RVtC-XES is of strategic significance to observe the full set of d-d excitations in Ni2+, which are partially overshadowed by the low-energy π-π* transitions of the Ni ligands in standard diffuse-reflectance UV-vis experiments. Our combined RVtC-XES/UV-vis approach provides access to the whole set of d-d excitations, allowing us a complete discussion of the changes undergone by the electronic configuration of the Ni2+ sites hosted within the MOF upon molecular adsorption. The experimental data have been interpreted by multiplet ligand-field theory calculations based on Wannier orbitals. This study represents a step further in understanding the ability of the CPO-27-Ni MOFs in molecular sorption and separation applications.

Selective catalytic reduction of NO over Fe-ZSM-5: mechanistic insights by operando HERFD-XANES and valence-to-core X-ray emission spectroscopy.

An in-depth understanding of the active site requires advanced operando techniques and the preparation of defined catalysts. We elucidate here the mechanism of the selective catalytic reduction of NO by NH3 (NH3-SCR) over a Fe-ZSM-5 zeolite catalyst. 1.3 wt % Fe-ZSM-5 with low nuclearity Fe sites was synthesized, tested in the SCR reaction and characterized by UV-vis, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. Next, this defined Fe-zeolite catalyst was studied by complementary high-energy-resolution fluorescence-detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (V2C XES) under different model in situ and realistic working (operando) conditions identical to the catalyst test bench including the presence of water vapor. HERFD-XANES uncovered that the coordination (between 4 and 5), geometry (tetrahedral, partly 5-fold), and oxidation state of the Fe centers (reduced in NH3, partly in SCR mixture, slight reduction in NO) strongly changed. V2C XES supported by DFT calculations provided important insight into the chemical nature of the species adsorbed on Fe sites. The unique combination of techniques applied under realistic reaction conditions and the corresponding catalytic data unraveled the adsorption of ammonia via oxygen on the iron site. The derived reaction model supports a mechanism where adsorbed NOx reacts with ammonia coordinated to the Fe(3+) site yielding Fe(2+) whose reoxidation is slow.

dd excitations in CPO-27-Ni metal-organic framework: comparison between resonant inelastic X-ray scattering and UV-vis spectroscopy.

We identify the dd excitations in the metal-organic framework CPO-27-Ni by coupling resonant inelastic X-ray scattering (RIXS) and UV-vis spectroscopy, and we show that the element selectivity of RIXS is crucial to observing the full dd multiplet structure, which is not visible in UV-vis. The combination of calculations using crystal-field multiplet theory and density functional theory can reproduce the RIXS spectral features, crucially improving interpretation of the experimental data. We obtain the crystal-field splitting and magnitude of the electron-electron interactions and correct previously reported values. RIXS instruments at synchrotron radiation sources are accessible to all researchers, and the technique can be applied to a broad range of systems.

High energy resolution core-level X-ray spectroscopy for electronic and structural characterization of osmium compounds.

A comprehensive study of the bulk solid OsCl3 and the molecular ion [Os(bpy)2(CO)Cl](+) is presented illustrating the application of RIXS and HERFD XANES spectroscopies to the investigation of both bulk materials and molecular complexes. In order to analyze the experimental results, DFT simulations were performed taking into account spin-orbit interaction. Calculations for both compounds resulted in good agreement with the experimental RIXS and HERFD XANES data, shedding light on the details of their local atomic and electronic structure. In particular, the spatial distribution of molecular orbitals was obtained, which allowed the determination of the origin of the absorption peaks. It was shown that for materials containing heavy atoms, only the application of advanced RIXS and HERFD XANES spectroscopies makes it possible to extract the information on local atomic and electronic structure details from XANES data.

Resonant X-ray emission spectroscopy reveals d-d ligand-field states involved in the self-assembly of a square-planar platinum complex.

Resonant X-ray Emission Spectroscopy (RXES) is used to characterize the ligand field states of the prototypic self-assembled square-planar complex, [Pt(tpy)Cl]Cl (tpy=2,2':6',2''-terpyridine), and determine the effect of weak metal-metal and π-π interactions on their energy.

Investigation of the valence electronic states of Ti(IV) in Ti silicalite-1 coupling X-ray emission spectroscopy and density functional calculations.

We present an application of valence to core X-ray emission spectroscopy to understand the electronic structure of the industrially relevant catalyst titanium silicalite-1. The experimental spectrum was modelled within density functional theory, adopting a one electron approach, investigating the effects of different basis sets, density functionals and cluster sizes. The description of titanium silicalite-1 valence states follows the Kohn-Sham evaluation of the molecular orbitals involved in the computed transitions.

Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2.

Theoretical predictions show that depending on the populations of the Fe 3d xy , 3d xz , and 3d yz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe-ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)-high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.

Valence to core X-ray emission spectroscopy.

This Progress Report discusses the chemical sensitivity of Kß valence to core X-ray emission spectroscopy (vtc-XES) and its applications for investigating 3d-transition-metal based materials. Vtc-XES can be used for ligand identification and for the characterization of the valence electronic levels. The technique provides information that is similar to valence band photoemission spectroscopy but the sample environment can be chosen freely and thus allows measurements in presence of gases and liquids and it can be applied for measurements under in situ/operando or extreme conditions. The theoretical basis of the technique is presented using a one-electron approach and the vtc-XES spectral features are interpreted using ground state density functional theory calculations. Some recent results obtained by vtc-XES in various scientific fields are discussed to demonstrate the potential and future applications of this technique. Resonant X-ray emission spectroscopy is briefly introduced with some applications for the study of 3d and 5d-transition-metal based systems.

Interaction of NH3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study.

In the typical NH3-SCR temperature range (100-500 °C), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. Therefore, the study of adsorbed ammonia at high temperature is a key step for the understanding of its role in the NH3-SCR catalytic cycle. We employed different spectroscopic techniques to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, that is, (i) NH3 bonded to copper centers, (ii) NH3 bonded to Brønsted sites, and (iii) NH4(+)·nNH3 associations. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu(+) species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.

Taking snapshots of photosynthetic water oxidation using femtosecond X-ray diffraction and spectroscopy.

The dioxygen we breathe is formed by light-induced oxidation of water in photosystem II. O2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction centre is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2-flash (2F) and 3-flash (3F) photosystem II samples, and of a transient 3F' state (250 µs after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O-O bond formation, coupled to Mn reduction, does not yet occur within 250 µs after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 Å. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.

Silica-supported Ti chloride tetrahydrofuranates, precursors of Ziegler-Natta catalysts.

The structural and electronic properties of silica-supported titanium chloride tetrahydrofuranates samples, obtained by impregnating a polymer-grade dehydroxylated silica with TiCl4(thf)2 and TiCl3(thf)3 complexes, precursors of Ziegler-Natta catalysts, are investigated by means of FT-IR, XAS, XES and diffuse reflectance UV-Vis spectroscopy, coupled with DFT calculations. The properties of the two silica-supported samples are very similar, irrespective of the starting precursor. In both cases, most of the chlorine ligands originally surrounding the Ti sites are substituted by oxygen ligands upon grafting on silica. As a consequence, the electronic properties of silica-supported Ti sites are largely different from those of the corresponding precursors, and in both cases most of the grafted Ti sites have a formal oxidation state of +4. The whole set of experimental data provide evidence that mono-nuclear Ti species are mainly present at the silica surface.

Spectroscopic and adsorptive studies of a thermally robust pyrazolato-based PCP.

The pyrazolato-based PCP [Ni(8)(µ(4)-OH)(4)(µ(4)-OH(2))(2)(µ(4)-PBP)(6)] (NiPBP, H(2)PBP = 4,4'-bis(1H-pyrazol-4-yl)biphenyl), whose 3-D architecture is built upon octametallic hydroxo clusters reciprocally connected by organic spacers, is a very promising candidate for gas adsorption applications, owing to its remarkable thermal stability (up to 400 °C in air) and its high void volume (70%). As such, NiPBP was selected as a proof-of-concept material to demonstrate how an optimized set of solid state techniques can concur to create a comprehensive and coherent picture, relating (average and local) structural features to adsorptive properties. To this aim, the response of NiPBP toward different gases, retrieved by gas adsorption measurements (N(2) at 77 K, in the low pressure region; H(2) at 77 K, in the high pressure region), was explained in terms of local-level details, as emerged by coupling electronic, X-ray (absorption and emission), and variable temperature IR spectroscopy.