Foreword to the special virtual issue on X-ray spectroscopy to understand functional materials: instrumentation, applications, data analysis.

Foreword to the virtual focused issue of Journal of Synchrotron Radiation on X-ray spectroscopy to understand functional materials: instrumentation, applications, data analysis.

Deciphering Local Structural Complexity in Zn/Ga-ZrO CO Hydrogenation Catalysts.

In the last few decades, massive effort has been expended in heterogeneous catalysis to develop new materials presenting high conversion, selectivity, and stability even under high-temperature and high-pressure conditions. In this context, CO hydrogenation is an interesting reaction where the catalyst local structure is strongly related to the development of an active and stable material under hydrothermal conditions at / > 300 °C/30 bar. In order to clarify the relationship between catalyst local ordering and its activity/stability, we herein report a combined laboratory and synchrotron investigation of aliovalent element (Ce/Zn/Ga)-containing ZrO matrixes.

Enabling a bioinspired ,,-copper coordination motif through spatial control in UiO-67: synthesis and reactivity.

Metal-organic frameworks (MOFs) featuring zirconium-based clusters are widely used for the development of functionalized materials due to their exceptional stability. In this study, we report the synthesis of a novel N,N,N-ligand compatible with a biphenyl dicarboxylic acid-based MOF. However, the resulting copper(I) complex exhibited unexpected coordination behaviour, lacking the intended trifold coordination motif.

Zn Redistribution and Volatility in ZnZrO Catalysts for CO Hydrogenation.

ZnO-ZrO mixed oxide (ZnZrO) catalysts are widely studied as selective catalysts for CO hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of and characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn species are mobile between the solid solution phase with ZrO and segregated and/or embedded ZnO clusters.

Elucidating the reaction mechanism of SO with Cu-CHA catalysts for NH-SCR by X-ray absorption spectroscopy.

The application of Cu-CHA catalysts for the selective catalytic reduction of NO by ammonia (NH-SCR) in exhaust systems of diesel vehicles requires the use of fuel with low sulfur content, because the Cu-CHA catalysts are poisoned by higher concentrations of SO. Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO is crucial for elucidating the SO poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO reacts with the [Cu(NH)O] complex that is formed in the pores of Cu-CHA upon activation of O in the NH-SCR cycle.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH, CH and CO.

Full Spectroscopic Characterization of the Molecular Oxygen-Based Methane to Methanol Conversion over Open Fe(II) Sites in a Metal-Organic Framework.

Iron-based enzymes efficiently activate molecular oxygen to perform the oxidation of methane to methanol (MTM), a reaction central to the contemporary chemical industry. Conversely, a very limited number of artificial catalysts have been devised to mimic this process. Herein, we employ the MIL-100(Fe) metal-organic framework (MOF), a material that exhibits isolated Fe sites, to accomplish the MTM conversion using O as the oxidant under mild conditions.

Cu-loaded zeolites enable the selective activation of ethane to ethylene at low temperatures and pressure.

Cu-zeolites are found to activate the C-H bond of ethane already at 150 °C in a cyclic protocol and form ethylene with a high selectivity. Both the zeolite topology and Cu content are found to impact the ethylene yield. Ethylene adsorption studies with FT-IR, demonstrate that oligomerization of ethylene occurs over protonic zeolites, while this reaction does not occur over Cu-zeolites.

Ionic Nickel Embedded in Ceria with High Specific CO Methanation Activity.

CO hydrogenation to methane is gaining increasing interest as one of the most promising ways to store intermittent renewable energy in the form of chemical fuels. Ni particles supported on CeO represent a highly efficient, stable and inexpensive catalyst for this reaction. Herein, Ni-doped CeO nanoparticles were tested for CO methanation showing an extremely high Ni mass-specific activity and CH selectivity.

From Lab to Technical CO Hydrogenation Catalysts: Understanding PdZn Decomposition.

The valorization of CO to produce high-value chemicals, such as methanol and hydrocarbons, represents key technology in the future net-zero society. Herein, we report further investigation of a PdZn/ZrO + SAPO-34 catalyst for conversion of CO and H into propane, already presented in a previous work. The focus of this contribution is on the scale up of this catalyst.

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites.

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 NH 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 NH-mediated Selective Catalytic Reduction of NO (NH-SCR) processes. A different mobility of bare Cu/Cu ions has been documented too, showing for Cu a better mobilization than for Cu also in absence of ligands.

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNO Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy.

Cu-exchanged chabazite is the catalyst of choice for NO abatement in diesel vehicles aftertreatment systems via ammonia-assisted selective catalytic reduction (NH-SCR). Herein, we exploit X-ray absorption spectroscopy powered by wavelet transform analysis and machine learning-assisted fitting to assess the impact of the zeolite composition on NH-mobilized Cu-complexes formed during the reduction and oxidation half-cycles in NH-SCR at 200 °C. Comparatively analyzing well-characterized Cu-CHA catalysts, we show that the Si/Al ratio of the zeolite host affects the structure of mobile dicopper(II) complexes formed during the oxidation of the [Cu(NH)] complexes by O.

SO Poisoning of Cu-CHA deNO Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy.

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH-assisted selective catalytic reduction of NO (NH-SCR) for the abatement of NO emission from diesel vehicles. However, the presence of a small amount of SO in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO using X-ray absorption spectroscopy.

Multifunctional Catalyst Combination for the Direct Conversion of CO to Propane.

The production of carbon-rich hydrocarbons via CO 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/ZrO+SAPO-34 multifunctional catalyst for the direct conversion of CO to propane.

Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts.

The speciation of framework-interacting Cu sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NO with NH is studied, to investigate the influence of the Al content on the copper structure and their reactivity towards a NO/O mixture. To this aim, three samples with similar Cu densities and different Si/Al ratios (5, 15 and 29) were studied using in situ X-ray absorption spectroscopy (XAS), FTIR and diffuse reflectance UV-Vis during pretreatment in O followed by the reaction. XAS and UV-Vis data clearly show the main presence of ZCu sites (with Z representing a framework negative charge) at a low Si/Al ratio, as predicted.

Copper Pairing in the Mordenite Framework as a Function of the Cu /Cu Speciation.

A series of gas-phase reactants is used to treat a Cu-exchanged mordenite zeolite with the aim of studying the influence of the reaction environment on the formation of Cu pairs. The rearrangement of Cu ions to form multimeric sites as a function of their oxidation state was probed by X-ray absorption spectroscopy (XAS) and also by applying advanced analysis through wavelet transform, a method able to specifically locate Cu-Cu interactions also in the presence of overlapping contributions from other scattering paths. The nature of the Cu-oxo species formed upon oxidation was further crosschecked by DFT-assisted fitting of the EXAFS data and by resonant Raman spectroscopy.

THORONDOR: a software for fast treatment and analysis of low-energy XAS data.

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.

Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH-Mediated Selective Catalytic Reduction.

The NH-mediated selective catalytic reduction (NH-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH-SCR, oxygen only reacts with Cu ions, which are present as mobile Cu diamine complexes [Cu(NH)].

EXAFS wavelet transform analysis of Cu-MOR zeolites for the direct methane to methanol conversion.

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).

Bridging Solution and Solid-State Chemistry of Dicyanoaurate: The Case Study of Zn-Au Nucleation Units.

The behavior in solution of the dicyanoaurate anion in the presence of other metal centers has so far been little explored, despite its importance in material science. The design and synthesis of systems with controlled coordination behavior, using chelating ligands and Zn, has allowed us to detect self-assembly and oligomerization in solution. This phenomenon has been studied with C and H NMR, absorption and emission UV-vis spectroscopy, ESI-MS, and XAS at both the Au L-edge and Zn K-edge: all of these techniques confirm the presence of Au-Zn aggregation products.

Glutathione activation of an organometallic half-sandwich anticancer drug candidate by ligand attack.

In contrast to the clinical drug cisplatin, the anticancer complex [Os(η6-p-cymene)(4-(2-pyridylazo)-N,N-dimethylaniline)I]+ [1-I] is inert towards hydrolysis and targets cancer cell metabolism rather than DNA. A combination of DFT calculations and X-ray absorption spectroscopy (XAS) suggests that hydrolytic activation of 1-I involves catalytic attack by the intracellular tripeptide glutathione (GSH) on the azo bond of the chelating ligand in the complex.

? Divergent coordination behavior of early-transition metals towards MOF-5.

Treatment of MOF-5 with NbCl(THF) in acetonitrile leads to incorporation of Nb(iv) centers in a fashion that diverges from the established cation metathesis reactivity of this iconic material. A combination of X-ray absorption spectroscopy analysis and reactivity studies altogether supported by density functional theory computational studies document an unprecedented binding mode for the ZnO(OC-) secondary building units (SBUs), which in Nb(iv)-MOF-5 function as -chelating ligands for NbCl moieties, with no exchange of Zn observed. This unusual reactivity expands the portfolio of post-synthetic modification techniques available for MOFs, exemplified here by MOF-5, and underscores the diverse coordination environments offered by this and potentially other MOFs towards heterometal species.

Molecular Niobium Precursors in Various Oxidation States: An XAS Case Study.

Although X-ray absorption spectroscopy (XAS) has become an indispensable tool in characterization of solid-state materials, it is less of a staple in molecular chemistry of niobium. Scattering X-ray techniques remain relatively unexplored for the systematic study of molecular niobium compounds. Here, we use XAS to probe the niobium environment in commonly used Nb precursors in +V, +IV, and +III oxidation states.

The Nuclearity of the Active Site for Methane to Methanol Conversion in Cu-Mordenite: A Quantitative Assessment.

The direct conversion of methane to methanol (MTM) is a reaction that has the potential to disrupt a great part of the synthesis gas-derived chemical industry. However, despite many decades of research, active enough catalysts and suitable processes for industrial application are still not available. Recently, several copper-exchanged zeolites have shown considerable activity and selectivity in the direct MTM reaction.

Cu-CHA - a model system for applied selective redox catalysis.

We review the structural chemistry and reactivity of copper-exchanged molecular sieves with chabazite (CHA) topology, as an industrially applied catalyst in ammonia mediated reduction of harmful nitrogen oxides (NH3-SCR) and as a general model system for red-ox active materials (also the recent results in the direct conversion of methane to methanol are considered). Notwithstanding the apparent structural simplicity of the material, a crystalline zeolite with only one crystallographically independent T site, the Cu-SSZ-13 catalyst reveals a high degree of complexity that has been decrypted by state of the art characterization tools. From the reviewed data, the following important aspects in the understanding of the Cu-SSZ-13 catalyst clearly emerged: (i) the structural dynamics of the Cu-species require precise control of the environmental conditions during activation and characterization; (ii) the availability of a large library of well-defined catalysts with different Si/Al and Cu/Al compositional ratios is key in unravelling the red-ox properties of the active Cu sites; (iii) a multi-technique approach is required, combining complementary techniques able to provide independent structural, electronic and vibrational information; (iv) synchrotron radiation based techniques (EXAFS, XANES, XES and time-resolved powder XRD) played a relevant role; (v) operando methodology (possibly supported by advanced chemometric approaches) is essential in obtaining structure-reactivity relations; (vi) the support of theoretical studies has been indispensable for the interpretation of the experimental output from characterization and for a critical assessment of mechanistic models.