Display omitted
•Reversible hydride phase observed in Pd/C catalysts with EXAFS and XRPD.•Irreversible carbide phase observed in Pd/C catalysts with EXAFS and XRPD.•EXAFS and XRPD hardly discriminate ...Pd-hydrate from Pd-carbide.•XANES simulations easily discriminate Pd-hydrate from Pd-carbide.•The active PdHx (or PdCy) phase can be determined in operando conditions.
In a number of hydrogenation reactions, palladium nanoparticles may undergo a transition to the hydride or the carbide phase, which affects the catalytic properties. In the current work, we determine the structural evolution of an industrial Pd/C catalyst in the presence of hydrogen and acetylene by means of in situ X-ray absorption spectroscopy and X-ray powder diffraction. We observe reversible hydride phase formation and irreversible formation of the carbide phase. The near-edge structure of the absorption spectra (XANES) plays the key role in distinguishing between hydride and carbide phases. We show that the presence of hydrogen and carbon atoms have a direct effect on the near-edge region which is reproduced by theoretical simulations performed in the Monte-Carlo approach.
We report an in situ, temperature and H2 pressure-dependent, characterization of (2.6 ± 0.4) nm palladium nanoparticles supported on active carbon during the process of hydride phase formation. For ...the first time the core–shell structure is highlighted in the single-component particles on the basis of a different atomic structure and electronic configurations in the inner “core” and surface “shell” regions. The atomic structure of these particles is examined by combined X-ray powder diffraction (XRPD), which is sensitive to the crystalline core region of the nanoparticles, and by first shell analysis of extended X-ray absorption fine structure (EXAFS) spectra, which reflects the averaged structure of both the core and the more disordered shell. In the whole temperature range (0–85 °C), XRPD analysis confirms the existence of two well-separated α- and β-hydride phases with the characteristic flat plateau in the phase transition region of the pressure-lattice parameter isotherms. In contrast, first shell interatomic distances obtained from EXAFS exhibit a slope in the phase transition region, typical for nanostructured palladium. Such difference is explained by distinct properties of bulk “core” which has crystalline structure and sharp phase transition, and surface “shell” which is amorphous and absorbs hydrogen gradually without forming distinguishable α- and β-phases. Combining EXAFS and XRPD we extract, for the first time, the Pd–Pd first-shell distance in the amorphous shell of the nanoparticles, that is significantly shorter than in the bulk core and relevant in catalysis. The core/shell model is supported by the EXAFS analysis of the higher shells, in the frame of the multiple scattering theory, showing that the evolution of the third shell distance (ΔR 3/R 3) is comparable to the evolution of (Δa/a) obtained from XRPD since amorphous PdH x shell gives a negligible contribution in this range of distances. This operando structural information is relevant for the understanding of structure-sensitive reactions. Additionally, we demonstrate the differences in the evolution of the thermal parameters obtained from EXAFS and XRPD along the hydride phase formation.
Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light ...stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal–organic frameworks and (ii) light induced dynamics of spin crossover in metal–organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code Phys. Rev. B, 2001, 63, 125120 we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni–CO configuration was found to be linear, while Ni–NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous Fe(bpy)32+ with a 100 ps delay we identified the Fe–N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.
Bimetallic Ce/Zr-UiO-66 metal-organic frameworks (MOFs) proved to be promising materials for various catalytic redox applications, representing, together with other bimetallic MOFs, a new generation ...of porous structures. However, no direct proof for the presence of both metals in a single cornerstone of UiO-type MOFs was reported so far. Employing element-selective X-ray absorption spectroscopy techniques herein, we demonstrate, for the first time, that our synthesis route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and bimetallic CeZr
cornerstones. Performing multiple-edge extended X-ray absorption fine structure analysis, we determine the exact stoichiometry of the cornerstones, which explains the dependence of thermal and chemical stability of the materials on Ce content.
Display omitted
•Different theoretical methods in the simulation of the XANES spectra are discussed.•An extended list of available codes for XANES spectra simulation is provided.•The potential of ...operando XANES in catalysis is described with relevant examples.•Chemometric methods in the treatment of operando XANES spectra is discussed.•Machine learning approaches are used to provide structural determination from XANES.
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.
Experimental technique for preparing of space arranged arrays of bimetallic AuAg nanoparticles in the near-surface region of glass, exhibiting surface plasmon resonance (SPR) characteristics varying ...in the wider ranges comparing to monometallic Au, Ag particles or corresponding thin films, is presented together with the structural characterization of the obtained particles. The suggested technique is based on the UV laser irradiation (λ = 193 nm) of the glass surface preliminary doped with silver ions and then sputter coated by a thin gold layer. Optical extinction spectra of the prepared AuAg/glass samples demonstrated strong dependence of SPR upon the number of laser pulses applied. The relationship “SPR characteristics – particles structure and composition – synthesis conditions” was studied using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray fluorescence (XRF) method and X-ray absorption fine structure (XAFS) spectroscopy. The latter provided the structural information on AuAg bonds, which directly evidenced on the formation of bimetallic AuAg nanoparticles in AuAg/glass samples. The XAFS derived values of structural parameters of AuAu and AuAg bonds allowed to visualize the distribution of gold and silver over the volume of representative AuAg nanoparticle and to suggest the most plausible cluster models of the architecture of such particle for the prepared AuAg/glass samples. It was revealed that the core-shell architecture of Au@Ag particles (Au-core, Ag-shell) is the most plausible in AuAg/glass samples prepared by the low number of laser pulses, while the structure of disordered solid solution is suitable for AuAg particles in samples prepared by ∼ 50 and more pulses. Calculations of contributions into experimental optical extinction spectra of AuAg/glass samples from particles with the revealed composition, size and architecture were performed to prove that the proposed structural models of nanoparticles are not in contradiction with the observed optical properties of the samples.
Display omitted
•UV laser irradiation of preliminary prepared glasses forms plasmonic AuAg NPs arrays.•SPR of AuAg/glass samples shows strong dependence upon the number of laser pulses.•Most plausible architecture of AuAg NPs at low number of pulses is Au-core@Ag-shell.•AuAg NPs prepared by ∼50 and more pulses have the structure of solid solution.•Structural models of NPs are supported by calculations of optical extinction spectra.
The formation of palladium hydride and carbide phases in palladium-based catalysts is a critical process that changes the catalytic performance and selectivity of the catalysts in important ...industrial reactions, such as the selective hydrogenation of alkynes or alkadienes. We present a comprehensive study of a 5 wt% carbon supported Pd nanoparticle (NP) catalyst in various environments by using
in situ
and
operando
X-ray absorption spectroscopy and diffraction, to determine the structure and evolution of palladium hydride and carbide phases, and their distribution throughout the NPs. We demonstrate how the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) spectra and X-ray powder diffraction (XRPD) patterns allows discrimination between the inner "core" and outer "shell" regions of the NP during hydride phase formation at different temperatures and under different hydrogen pressures, indicating that the amount of hydrogen in the shell region of the NP is lower than that in the core. For palladium carbide, advanced analysis of X-ray absorption near-edge structure (XANES) spectra allows the detection of Pd-C bonds with carbon-containing molecules adsorbed at the surface of the NPs. In addition, H/Pd and C/Pd stoichiometries of PdH
x
and PdC
y
phases were obtained by using theoretical modelling and fitting of XANES spectra. Finally, the collection of
operando
time-resolved XRPD patterns (with a time resolution of 5 s) allowed the detection, during the ethylene hydrogenation reaction, of periodic oscillations in the NPs core lattice parameter, which were in phase with the MS signal of ethane (product) and in antiphase with the MS signal of H
2
(reactant), highlighting an interesting direct structure-reactivity relationship. The presented studies show how a careful combination of X-ray absorption and diffraction can differentiate the structure of the core, shell and surface of the palladium NPs under working conditions and prove their relevant roles in catalysis.
We present a comprehensive study of a 5 wt% Pd/C catalyst in various environments by using
in situ
and
operando
X-ray absorption and diffraction.
Selective binding of ethylene and ethylene action inhibitor in nanoporous materials can provide new strategies for harvest preservation technologies. In this work, we investigate the process of ...ethylene and 1-methylcyclopropene (1-MCP), also known as ethylene action inhibitor, binding on HKUST-1, a metal–organic framework (MOF) with open copper sites. For the first time, in situ infrared spectroscopy combined with theoretical simulations was used to obtain the three-dimensional structure of ethylene and 1-MCP bound to Cu-sites of HKUST-1. Time-resolved infrared spectra were collected to selectively determine the rates of ethylene desorption from Cu-sites and estimate its binding energies which were also compared with theoretical results. Not only a successful investigation of ethylene binding sites in HKUST-1 is presented but also a methodology that can be extended to study ethylene/1-MCP storage and release in porous materials with open metal sites which is relevant to the harvest preservation technology.
Display omitted
•Three different phases of Pd are found during ethylene hydrogenation reaction.•Palladium carbide is formed under reaction conditions even in the excess of H2.•Pre-hydrogenated ...palladium nanoparticles demonstrate lower catalytic activity.
Palladium nanoparticles (NPs) find numerous applications as catalysts for hydrogenation of hydrocarbons. Under reaction conditions, formation of palladium hydrides and carbides may occur, which affects the catalytic properties of the catalyst. Unlike pure hydride phase, whose phase diagram is well-studied, conditions of formation of palladium carbides and mixed hydride/carbide structures in Pd NPs is poorly investigated. We present an operando study of a supported Pd/C catalyst during ethylene hydrogenation reaction in a variable C2H4/H2 ratio. By simultaneous analysis of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra, we demonstrated an irreversible behavior in the catalyst structure explained by carbide phase formation even in the instant excess of molecular hydrogen with respect to molecular ethylene in the gas feed. The relevant role of the carbide phase was demonstrated by correlations between the carbide phase amount and ethylene conversion. An interesting behavior of the atomic structure of the Pd NPs has been observed upon periodic variation of the amount of ethylene in the reaction mixture. The obtained results provide significant information about the structural evolution of Pd NPs during ethylene hydrogenation reaction and highlight the importance of operando XANES spectroscopy for characterization of the working catalysts.
Palladium-based catalysts are exploited on an industrial scale for the selective hydrogenation of hydrocarbons. The formation of palladium carbide and hydride phases under reaction conditions changes ...the catalytic properties of the material, which points to the importance of operando characterization for determining the relation between the relative fractions of the two phases and the catalyst performance. We present a combined time-resolved characterization by X-ray absorption spectroscopy (in both near-edge and extended regions) and X-ray diffraction of a working palladium-based catalyst during the hydrogenation of ethylene in a wide range of partial pressures of ethylene and hydrogen. Synergistic coupling of multiple techniques allowed us to follow the structural evolution of the palladium lattice as well as the transitions between the metallic, hydride and carbide phases of palladium. The nanometric dimensions of the particles resulted in the considerable contribution of both surface and bulk carbides to the X-ray absorption spectra. During the reaction, palladium carbide is formed, which does not lead to a loss of activity. Unusual contraction of the unit cell parameter of the palladium lattice in the spent catalyst was observed upon increasing hydrogen partial pressure.
PDF