Conference Agenda

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X-ray absorption spectroscopy (XAS) has imposed as a powerful method to track structural and chemical dynamics of metal ions hosted in nanoporous frameworks, such as zeolites and metal-organic frameworks (MOFs), for selective redox catalysis applications [1]. Analysis of the XANES and EXAFS regions offers a highly complementary view with respect to diffraction-based methods guaranteeing a unique sensitivity to the local electronic and structural properties of metal centers. These are often disorderly distributed in the crystalline matrix, and occur as dynamic mixtures of different species, responding to the physico-chemical environment while undergoing a rich redox chemistry mediated by host-guest interactions. Continuous instrumental developments at synchrotron sources today enable in situ/operando XAS studies at high time and energy resolution, allowing to monitor such dynamic systems with unprecedented accuracy [1]. In this contribution, the potential of these methods, empowered by advanced data analysis strategies and synergic integration with multi-technique laboratory characterization and computational modelling, will be exemplified by selected research results.

A first example will focus on the Cu-exchanged chabazite (Cu-CHA) zeolite, currently representing the catalyst of choice for deNO_x applications in the automotive sector via NH₃-assisted Selective Catalytic Reduction [2]. Here, the potential of Multivariate Curve Resolution (MCR) of time-resolved XANES datasets, quasi-simultaneous XANES/PXRD, and EXAFS Wavelet Transform analysis will be highlighted, to accurately quantify condition/composition-dependent Cu-speciation in CHA zeolites and therein establish robust structure-activity relationships, essential to design improved catalysts. A second case study will consider local structural and chemical transformations of Pt ions in Pt-functionalized UiO-67 MOFs [3], tracked by parametric refinement of time-resolved operando EXAFS under conditions yielding either isolated Pt^II sites anchored to the MOF framework (potentially interesting for C−H bond activation) or very small Pt⁰ nanoparticles inside the MOF cavities (potentially interesting for hydrogenation reactions).

[1] S. Bordiga et al., Chem. Rev. 2013, 113, 1736. C. Garino et al., Coord. Chem. Rev. 2014, 277-278, 130. E. Borfecchia et al., Chem. Soc. Rev. 2018, 47, 8097.

[2] A. Martini et al, Chem. Sci. 2017, 8, 6836. C. W. Andersen, et al., Angew. Chem. Int. Edit. 2017, 56, 10367. K. A. Lomachenko et al., J. Am. Chem. Soc. 2016, 138, 12025. C. Negri et al., J. Am. Chem. Soc. 2020, 142, 15884.

[3] S. Øien, et al., Chem. Mater. 2015, 27, 1042. L. Braglia, et al., Phys. Chem. Chem. Phys. 2017, 19, 27489. L. Braglia, et al., Faraday Discuss. 2017, 201, 265.

This investigation is devoted to metal-organic frameworks (MOFs) with UiO-67 topology, the materials with a three-dimensional porous structure and high surface area. Due to the diversity of species, MOFs are used in such areas as luminescent sensors, catalysts, filters, storage and transportation of light gases, and many others [1]. Using noble metals to functionalize metal-organic frameworks is a promising way for constructing new materials for catalytic applications [2, 3]. Although numerous successful synthesis of MOFs functionalized by metal ions and metal nanoparticles were reported, the exact mechanisms of structural evolution of the metal sites in many cases are still unknown. Determination of these mechanisms as well as investigation of the intermediate active sites formed during the synthesis is important for tailoring the specific catalytic properties of materials. In this work, we investigate structural changes in UiO-67 functionalized by Pd and Pt depending on the activation conditions by a combination of theoretical and experimental techniques.

Functionalization of UiO-67 by Pd and Pt was achieved via substitution of 10% standard biphenyl dicarboxylate linkers by MCl₂-2,2-bipyridine-5,5-dicarboxylic acid (MCl₂bpydc, M = Pd, Pt) [4, 5]. The obtained materials were further activated by heating to 300 °С in inert (He) and reducing (H₂/He) atmospheres. Evolution of the atomic and electronic structure was monitored by in situ extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES) spectroscopies and X-ray powder diffraction (XRPD). All spectroscopic data for Pd K- and Pt L₃-edges were analysed simultaneously by MCR-ALS approach [6] to determine the number of pure species formed during the activation and their spectra.

To interpret the experimental data, we have performed DFT-calculations and XANES simulation by FDMNES code for different potential intermediates. The atomic models included the initial MCl₂bpydc linker and a number of possible reaction pathways in presence of H₂ substitution of both chlorine atoms by hydrogen atoms with formation of Cl₂ molecule, substitution of one chlorine by hydrogen atom with formation of HCl molecule; detachment of one or two chlorines with formation of HCl molecules, detachment of MCl₂ fragment from the linker with its substitution by two hydrogens bonded to nitrogen atoms of the linker; and simulating inert conditions: simple detachment of MCl₂ fragment, detachment of chlorines with formation of Cl₂ molecule. All reaction pathways were ranged according to the calculated reaction enthalpies and XANES spectra were calculated for the most probable ones.

The reaction pathways with the lowest reaction enthalpies were verified by good agreement between calculated and experimentally observed XANES spectra. For UiO-67-Pd, detachment of PdCl₂ is the most probable pathway in both inert and H₂ atmospheres which correlate with experimental results. For UiO-67-Pt, four different structures have been identified. In the presence of hydrogen, detachment of one chlorine atom should occur first. The second possible transition in the same environment is the detachment of PtCl₂ from the linker with the addition of two hydrogen atoms to nitrogen atoms with the further formation of Pt nanoparticles at temperatures above 200 °C. While formation of bare Pt-sites occurs from 200 to 300 °C in the inert flow [7, 8]. Thus, the XANES spectroscopy supported by theoretical calculations allowed verifying and describing intermediate states from the experimental spectra.

[1] Evans J. D., et al., Coord Chem Rev. (2019) 380 378-418. [2] Tanabe K. K., Cohen S. M., Chem Soc Rev. (2011) 40 (2) 498-519. [3] Wang Z., Cohen S. M., Chem Soc Rev. (2009) 38 (5) 1315-29. [4] Braglia L., et al., Phys Chem Chem Phys. (2017) 19 (40) 27489-27507. [5] Bugaev A. L., et al., Faraday Discuss. (2018) 208 287-306. [6] Jaumot J., et al., Chemom Intell Lab Syst. 140 (2015) 1-12. [7] Bugaev A. L., Skorynina A.A., et al., Catal. Today. (2019), 336, 33-39. [8] Bugaev A. L., Skorynina A.A., et al., Data Brief. (2019) 25, 104208.

Keywords: XANES; XRD; MOFs; MCR; DFT

This research was supported by the Russian Science Foundation, project № 20-43-01015.

Many catalysts for energy related applications, in particular metallic nanoalloys, readily undergo atomic-level changes during the electrochemical reactions driving the applications. The origin, dynamics and impact of the changes on the performance of the catalysts under actual operating conditions are, however, not well understood. This is largely because they are studied on model nanocatalysts under controlled laboratory conditions. We will present results from recent studies [1, 2, 3] on the dynamic behavior of metallic nanoalloy catalysts inside an operating proton exchange membrane fuel cell. Results show that their atomic structure changes profoundly, from the initial state to the active form and further along the cell operation. The electrocatalytic activity of the nanoalloys also changes. The rate and magnitude of the changes may be rationalized when the limits of traditional relationships used to connect the composition and structure of nanoalloys with their electrocatalytic activity and stability, such as Vegard’s law, are recognized. In particular, deviations from the law can well explain the behaviour for Pt-3d metal nanoalloy catalysts under operating conditions. Moreover, it appears that factors behind their remarkable electrocatalytic activity, such as the large surface to volume ratio and “misfit” between the size of constituent atoms, are indeed detrimental to their stability inside fuel cells. The new insight into the atomic-level evolution of nanoalloy electrocatalysts during their usage is likely to inspire new efforts to stabilize transient structure states beneficial to their activity and stability under operating conditions, if not synthesize them directly.

1. V. Petkov et al. Nanoscale 11, 5512 (2019).
2. Zh. Kong et al. J. Am. Chem. Soc. 142, 1287 (2020).
3. Z.-P. Wu et al. Nature Commun. 12, 8597 (2021)

The topic of renewable energies is vastly received as an x-factor towards our energy problems in this modern world times. This notion holds tight following the current global outcry of dilapidating fossil fuels. The science fraternity continually aims at tabling innovative strategies towards engineering renewable energy sources. This transcends to efforts of making chemical fuels which are easily storable using solar energy.

Water reduction catalysts (WRCs), as widely known, are currently used for the splitting of water to H₂ and O₂. The hydrogen (H₂) generated, is eyed as a prominent potential fuel source. These catalysts are often tailored with different transition metal elements somewhat coupled with effective macro-cyclic organic scaffolds [1]; as suitable approach to store energy at times of reduced power supply by harnessing solar energy [2,3].This renowned WRC science was preceded by the prominent scientific stun of the ruthenium(III) complex {[Ru^III(byp)₃]³⁺}, as a photo-synthesizer [4]. Interestingly, different other light harvesting moieties are lately being considered, exhibiting greater photo-stability, longevity and rigidity as elemental prerequisites [5], many of whom are based on polypyridyl entities.

In this presentation we discuss aspects of the ligand design strategy and cobalt coordination chemistry as exhibited in the value chain in the scheme just above. X-ray crystallographically characterised structures will be used to discuss different relative geometric preferences and characteristics of the respective analogues.

Alternative fuel sources are needed to replace fossil fuels to reduce the emission of greenhouse gases contributing to global warming. Hydrogen gas is one popular choice to replace fossil fuels [1] as an energy storage medium, due to its high energy density per unit weight. Hydrogen can be generated renewably by sunlight driven, photocatalytic water-splitting. Metal oxides, including those with a Ruddlesden-Popper layered perovskite structures are being studied as potential photocatalysts [2]. The structure contains multiple cationic sites, which allows for different combinations of metal cations for tuning the bandgap. The layered structuring also allows for the intercalation of different cations within the structure that allows for modifications post synthesis, therefore further optimising the photocatalyst [3].

KLaTiO₄ is a n=1 Ruddlesden-Popper that can be used as a Hydrogen Evolution Catalyst (HEC), producing 9.540 μmol of H₂ gas per hour from 20 mg of catalyst, when using methanol as sacrificial electron donor and platinum co-catalyst, and illuminated by a Hg lamp with a 305 nm cut-off filter. The main disadvantage of KLaTiO₄ is its high bandgap (4.09 eV) that is above the visible light region, which makes it a poor choice for a HEC that uses solar energy. Reduction of the bandgap of KLaTiO₄ for sunlight driven hydrogen evolution was attempted by cationic and anionic doping. The crystal structures, and sample purity, was determined using synchrotron X-ray powder diffraction (PXRD) and Rietveld refinement.

Cationic doping of KLaTiO4 was achieved by partially replacing lanthanum with praseodymium or ytterbium, yielding two solid solution series: KLa_xPr_1-xTiO₄ and KLa_xYb_1-xTiO₄ (x = 0.005, 0.01 and 0.03). While none of the samples from KLa_xPr_1-xTiO4 series produced hydrogen, all the KLa_xYb_1-xTiO₄ were able to produce H₂. In comparison to KLaTiO4, ytterbium-doped samples have reduced catalytic activity compared to KLaTiO₄, as seen in figure 1.

Anionic doping of KLaTiO₄ was attempted with nitrogen. Attempts to synthesise KLaTiO₃N were done by using TiN as a reagent in place of TiO2 with annealing the sample under N₂ flow at 800 °C. PXRD patterns of initial samples show good crystallinity, and no observable structural difference to KLaTiO₄. When tested as HEC in identical testing condition stated above all nitrogenated samples had similar rates of hydrogen evolution.