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•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.
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•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.
The presence of a core/shell behavior in Pd nanoparticles (NPs) during the formation of the metal-hydride phase has recently been highlighted combining X-ray absorption and scattering experiments J. ...Phys. Chem. C 2017, 121, 18202. In this work, we focus on the formation of the carbide phase in the bulk region and on the surface of supported palladium NPs because it affects the catalytic activity and selectivity in hydrogenation reactions. We present in situ X-ray absorption spectroscopy study of carbide formation and decomposition in 2.6 nm palladium nanoparticles supported on carbon during exposure to acetylene, hydrogen, and their mixtures at 100 °C, taken as a representative temperature for hydrogenation reactions. Fourier analysis of extended X-ray absorption fine structure (EXAFS) spectra was used to determine the average Pd–Pd bond distance in the NPs, reflecting the formation of bulk palladium carbide, while theoretical calculation of X-ray absorption near-edge structure (XANES) using the finite difference method allowed us to determine the PdC y stoichiometry in the bulk region and at the surface of the nanoparticles. The difference in the XANES and EXAFS results indicated different behavior of bulk and surface carbide formation. In particular, exposure to pure acetylene leads to the immediate formation of surface Pd–C bonds and much slower growth of bulk carbide, resulting in the increase of Pd–Pd bond distance with respect to pure metallic palladium nanoparticles by only ∼0.6% after 1 h of exposure. Vacuum conditions at 100 °C did not affect the carbide structure of both the bulk and surface of the NPs. However, exposure to H2 at 100 °C cleans the surface of palladium, removing surface Pd–C bonds, without decomposing bulk carbide. After second exposure to acetylene, this fraction of lost Pd–C bonds is immediately restored, and the bulk carbide phase continues growing. Thus, we showed how the combination of near-edge and extended structures of the absorption spectra can be utilized to determine the properties of surface and bulk regions of palladium nanoparticles, which showed different behavior in formation of the Pd–C bonds.
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•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.
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.
We describe the synthesis and corresponding full characterization of the set of UiO-66 metal-organic frameworks (MOFs) with 1,4-benzenedicarboxylate (C
H
(COOH)
, hereafter H
BDC) and ...1,4-naphthalenedicarboxylate (C
H
(COOH)
, hereafter H
NDC) mixed linkers with NDC contents of 0, 25, 50, and 100%. Their structural (powder X-ray diffraction, PXRD), adsorptive (N
, H
, and CO
), vibrational (IR/Raman), and thermal stability (thermogravimetric analysis, TGA) properties quantitatively correlate with the NDC content in the material. The UiO-66 phase topology is conserved at all relative fractions of BDC/NDC. The comparison between the synchrotron radiation PXRD and 77 K N
-adsorption isotherms obtained on the 50:50 BDC/NDC sample and on a mechanical mixture of the pure BDC and NDC samples univocally proves that in the mixed linkers of the MOFs the BDC and NDC linkers are shared in each MOF crystal, discarding the hypothesis of two independent phases, where each crystal contains only BDC or NDC linkers. The careful tuning of the NDC content opens a way for controlled alteration of the sorption properties of the resulting material as testified by the H
-adsorption experiments, showing that the relative ranking of the materials in H
adsorption is different in different equilibrium-pressure ranges: at low pressures, 100NDC is the most efficient sample, while with increasing pressure, its relative performance progressively declines; at high pressures, the ranking follows the BDC content, reflecting the larger internal pore volume available in the MOFs with a higher fraction of smaller linkers. The H
-adsorption isotherms normalized by the sample Brunauer-Emmett-Teller specific surface area show, in the whole pressure range, that the surface-area-specific H
-adsorption capabilities in UiO-66 MOFs increase progressively with increasing NDC content. Density functional theory calculations, using the hybrid B3LYP exchange correlation functional and quadruple-ζ with four polarization functions (QZ4P) basis set, show that the interaction of H
with the H
NDC linker results in an adsorption energy larger by about 15% with respect to that calculated for adsorption on the H
BDC linker.
Functionalization of metal–organic frameworks (MOFs) with noble metal nanoparticles (NPs) is a challenging task. Conventional impregnation by metals often leads to agglomerates on the surface of MOF ...crystals. Functional groups on linkers interact with metal precursors and promote the homogeneous distribution of NPs in the pores of MOFs, but their uncontrolled localization can block channels and thus hinder mass transport. To overcome this problem, we created nucleation centers only in the defective pores of the UiO-66 MOF via the postsynthesis exchange. First, we have introduced defects into UiO-66 using benzoic acid as a modulator. Second, the modulator was exchanged for amino-benzoic acid. As a result, amino groups have decorated mainly the defective pores and attracted the Pd precursor after impregnation. The interaction of the metal precursor with amino groups and the growth of NPs were monitored by in situ infrared spectroscopy. Three processes were distinguished: the gaseous HCl release, NH2 reactivation, and growth of extended Pd surfaces. Uniform Pd NPs were located in the pores because of the homogeneous distribution of the precursor and pore diffusion-limited nucleation rate. Our work demonstrates an alternative approach of controlled Pd incorporation into UiO-66 that is of great importance for the rational design of heterogeneous catalysts.
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