Revealing the Dynamic Structure of Complex Solid Catalysts Using Modulated Excitation X‐ray Diffraction

X‐ray diffraction (XRD) is typically silent towards information on low loadings of precious metals on solid catalysts because of their finely dispersed nature. When combined with a concentration modulation approach, time‐resolved high‐energy XRD is able to provide the detailed redox dynamics of palladium nanoparticles with a diameter of 2 nm in 2 wt % Pd/CZ (CZ=ceria–zirconia), which is a difficult sample for extended X‐ray absorption fine structure (EXAFS) measurements because of the cerium component. The temporal evolution of the Pd(111) and Ce(111) reflections together with surface information from synchronous diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements reveals that Ce maintains Pd oxidized in the CO pulse, whereas reduction is detected at the beginning of the O₂ pulse. Oxygen is likely transferred from Pd to Ce³⁺ before the onset of Pd re‐oxidation. In this context, adsorbed carbonates appear to be the rate‐limiting species for re‐oxidation.     

Bismuth Tri‐ and Tetraarylcarboxylates: Crystal Structures,In Situ X‐ray Diffraction,Intermediates and Luminescence

A systematic investigation of the systems Bi³⁺/carboxylic acid/HNO₃ for the tri‐ and tetracarboxylic acids pyromellitic acid (H₄Pyr), trimellitic acid (H₃Tri) and trimesic acid (H₃BTC) acid led to the discovery of five new bismuth carboxylates. Structural characterisation allowed the influence of the linker geometry and the Bi³⁺:linker molar ratio in the starting solution on the crystal structure to be determined. The crystallisation of three selected compounds was investigated by in situ energy‐dispersive X‐ray diffraction. Three new crystalline intermediates were observed within minutes, and two of them could be isolated by quenching of the reaction mixture. Their crystal structures were determined from laboratory and synchrotron X‐ray powder diffraction data and allowed a possible reaction pathway to be established. In depth characterisation of the luminescence properties of the three bismuth pyromellate compounds was carried out. Fluorescence and phosphorescence could be assigned to (mainly) ligand‐ and metal‐based transitions. The polymorphs of Bi(HPyr) exhibit different luminescence properties, although their structures are very similar. Surprisingly, doping of the three host structures with Eu³⁺ and Tb³⁺ ions was only successful for one of the polymorphs.     

Direct In Situ Investigation of Milling Reactions Using Combined X‐ray Diffraction and Raman Spectroscopy

The combination of two analytical methods including time‐resolved in situ X‐ray diffraction (XRD) and Raman spectroscopy provides a new opportunity for a detailed analysis of the key mechanisms of milling reactions. To prove the general applicability of our setup, we investigated the mechanochemical synthesis of four archetypical model compounds, ranging from 3D frameworks through layered structures to organic molecular compounds. The reaction mechanism for each model compound could be elucidated. The results clearly show the unique advantage of the combination of XRD and Raman spectroscopy because of the different information content and dynamic range of both individual methods. The specific combination allows to study milling processes comprehensively on the level of the molecular and crystalline structures and thus obtaining reliable data for mechanistic studies.     

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO₃ nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO₆ polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO₃.     

Mesoscale Characterization of Nanoparticles Distribution Using X‐ray Scattering

The properties of many functional materials depend critically on the spatial distribution of an active phase within a support. In the case of solid catalysts, controlling the spatial distribution of metal (oxide) nanoparticles at the mesoscopic scale offers new strategies to tune their performance and enhance their lifetimes. However, such advanced control requires suitable characterization methods, which are currently scarce. Here, we show how the background in small‐angle X‐ray scattering patterns can be analyzed to quantitatively access the mesoscale distribution of nanoparticles within supports displaying hierarchical porosity. This is illustrated for copper catalysts supported on meso‐ and microporous silica displaying distinctly different metal distributions. Results derived from X‐ray scattering are in excellent agreement with electron tomography. Our strategy opens unprecedented prospects for understanding the properties and to guide the synthesis of a wide array of functional nanomaterials.     

Controlling Nucleation and Crystal Growth of a Distinct Polyoxovanadate Cluster: An In Situ Energy Dispersive X‐ray Diffraction Study under Solvothermal Conditions

The formation of the antimonato polyoxovanadates [V₁₄Sb₈(C₆H₁₅N₃)₄O₄₂(H₂O)] ? 4H₂O ( 1 ), (C₆H₁₇N₃)₂[V₁₅Sb₆(C₆H₁₅N₃)₂O₄₂(H₂O)] ? 2.5H₂O ( 2 ), {C₆H₁₅N₃}₄[V₁₆Sb₄O₄₂] 2H₂O ( 3 ) (C₆H₁₅N₃=1‐(2‐aminoethyl)piperazine, AEP) has been studied under solvothermal conditions by using in situ energy dispersive X‐ray diffraction (EDRXD). The syntheses were performed with an identical ratio for Sb₂O₃ and NH₄VO₃. If the reactions slurries are not stirred during the solvothermal reaction and by applying 70–75 % amine concentration, the products contain all three compounds, whereas 3 is observed at 80 %. Under stirring conditions, variation of the concentration of AEP led to crystallization of the three different compounds at distinct concentrations, that is, 1 is formed at 75 %, 1 and 2 between 75 and 80 % and 3 at 80 %. At an amine concentration of 77.5 %, first reflections of 2 occurred and at later stages, compound 1 started to crystallize. The sample with the lowest number of V^IV species was formed at the lowest amine concentration, whereas crystallization of 3 required the highest concentration. The formation of the compounds occurred without crystalline intermediates and/or precursors. With increasing reaction temperature, the incubation time was significantly reduced.     

Watching Nanoparticles Form: An In Situ (Small‐/Wide‐Angle X‐ray Scattering/Total Scattering) Study of the Growth of Yttria‐Stabilised Zirconia in Supercritical Fluids

Understanding nanoparticle‐formation reactions requires multi‐technique in situ characterisation, since no single characterisation technique provides adequate information. Here, the first combined small‐angle X‐ray scattering (SAXS)/wide‐angle X‐ray scattering (WAXS)/total‐scattering study of nanoparticle formation is presented. We report on the formation and growth of yttria‐stabilised zirconia (YSZ) under the extreme conditions of supercritical methanol for particles with Y₂O₃ equivalent molar fractions of 0, 4, 8, 12 and 25 %. Simultaneous in situ SAXS and WAXS reveals a quick formation (seconds) of sub‐nanometre amorphous material forming larger agglomerates with subsequent slow crystallisation (minutes) into nanocrystallites. The amount of yttria dopant is shown to strongly affect the crystallite size and unit‐cell dimensions. At yttria‐doping levels larger than 8 %, which is known to be the stoichiometry with maximum ionic conductivity, the strain on the crystal lattice is significantly increased. Time‐resolved nanoparticle size distributions are calculated based on whole‐powder‐pattern modelling of the WAXS data, which reveals that concurrent with increasing average particle sizes, a broadening of the particle‐size distributions occur. In situ total scattering provides structural insight into the sub‐nanometre amorphous phase prior to crystallite growth, and the data reveal an atomic rearrangement from six‐coordinated zirconium atoms in the initial amorphous clusters to eight‐coordinated zirconia atoms in stable crystallites. Representative samples prepared ex situ and investigated by transmission electron microscopy confirm a transformation from an amorphous material to crystalline nanoparticles upon increased synthesis duration.     

Design of a Full‐Profile‐Matching Solution for High‐Throughput Analysis of Multiphase Samples Through Powder X‐ray Diffraction

Finding a clear route to new structures : The design of an adaptable time warping (ATW) methodology (see figure) for automatically, quickly, and reliably deciphering X‐ray diffraction patterns is described.

     

Time‐Resolved In Situ Synchrotron X‐ray Study and Large‐Scale Production of Magnetite Nanoparticles in Supercritical Water

A general solution : In situ synchrotron X‐ray scattering in a high‐pressure pulsed injection reactor (see picture) shows that magnetite nucleation and growth are temporally separated. Gram‐scale crystalline, pure phase, superparamagnetic magnetite nanoparticles were synthesized without surfactants in supercritical water in less than one hour using a laboratory‐scale continuous‐flow reactor.

     

Thermotropic Liquid Crystals Based on Extended 2,5‐Disubstituted‐1,3,4‐Oxadiazoles: Structure–Property Relationships,Variable‐Temperature Powder X‐ray Diffraction,and Small‐Angle X‐ray Scattering Studies

A class of extended 2,5‐disubstituted‐1,3,4‐oxadiazoles R¹‐C₆H₄‐{OC₂N₂}‐C₆H₄‐R² (R¹=R²=C₁₀H₂₁O 1 a , p‐C₁₀H₂₁O‐C₆H₄‐C?C 3 a , p‐CH₃O‐C₆H₄‐C?C 3 b ; R¹=C₁₀H₂₁O, R²=CH₃O 1 b , (CH₃)₂N 1 c ; F 1 d ; R¹=C₁₀H₂₁O‐C₆H₄‐C?C, R²=C₁₀H₂₁O 2 a , CH₃O 2 b , (CH₃)₂N 2 c , F 2 d ) were prepared, and their liquid‐crystalline properties were examined. In CH₂Cl₂ solution, these compounds displayed a room‐temperature emission with λ_max at 340–471 nm and quantum yields of 0.73–0.97. Compounds 1 d , 2 a – 2 d , and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized‐light optical microscopy and differential scanning calorimetry. Structure determination by a direct‐space approach using simulated annealing or parallel tempering of the powder X‐ray diffraction data revealed distinctive crystal‐packing arrangements for mesogenic molecules 2 b and 3 a , leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200–210 °C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable‐temperature X‐ray diffractometry. Nondestructive phase transitions (crystal‐to‐crystal, crystal‐to‐mesophase, mesophase‐to‐liquid) were observed in the diffractograms of 1 b, 1 d , 2 b, 2 d , and 3 a measured at 25–200 °C. Powder X‐ray diffraction and small‐angle X‐ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure–property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.     

The Crystal Structure of Trimethyl Borate by Neutron and X‐ray Powder Diffraction

The crystal structure of one of the simplest organoboron compounds, trimethyl borate does not appear to have been determined hitherto. The compound is of interest for the study of π‐donor ligands and their interaction with the π‐acceptor behavior of trigonal boron and the consequences of such interactions on molecular structure. We used powder neutron (with isotopically labeled material) and X‐ray diffraction to determine the crystal structure of trimethyl borate at 15 K and 200 K (neutron) and 200 K (X‐ray). The material is hexagonal (Z = 2) with a = b = 6.950(8) Å and c = 6.501(3) Å at 15 K. The unit cell volume is 272.00(1) ų. The space group is P6₃/m (SG 176) at 15 K and 200 K. This is the first crystal structure solved on the Neutron Powder Diffractometer (NPDF) at the Lujan Center.     

In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X‐ray Absorption Spectroscopy (GIXAS)

The surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe₂O₃ and from b.c.c. Cr to corundum type Cr₂O₃. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in.     

In situ X‐ray Diffraction Studies of Cation and Anion Inter­calation into Graphitic Carbons for Electrochemical Energy Storage Applications

Graphite is a redox‐amphoteric intercalation host and thus capable to incorporate various types of cations and anions between its planar graphene sheets to form so‐called donor‐type or acceptor‐type graphite intercalation compounds (GICs) by electrochemical intercalation at specific potentials. While the LiC_x/C_x donor‐type redox couple is the major active compound for state‐of‐the‐art negative electrodes in lithium‐ion batteries, acceptor‐type GICs were proposed for positive electrodes in the “dual‐ion” and “dual‐graphite” cell, another type of electrochemical energy storage system. In this contribution, we analyze the electrochemical intercalation of different anions, such as bis(trifluoromethanesulfonyl) imide or hexafluorophosphate, into graphitic carbons by means of in situ X‐ray diffraction (XRD). In general, the characterization of battery electrode materials by in situ XRD is an important technique to study structural and compositional changes upon insertion and de‐insertion processes during charge/discharge cycling. We discuss anion (X^–) and cation (M⁺) intercalation/de‐intercalation into graphites on a comparative basis with respect to the M_x⁺C_n^– and C_n⁺X_n^– stoichiometry, discharge capacity, the intercalant gallery height/gallery expansion and the M–M or X–X in‐plane distances.     

Effects of Adsorbate Coverage and Bond‐Length Disorder on the d‐Band Center of Carbon‐Supported Pt Catalysts

Determination of the factors that affect the d‐band center of catalysts is required to explain their catalytic properties. Resonant inelastic X‐ray scattering (RIXS) enables direct imaging of electronic transitions in the d‐band of Pt catalysts in real time and in realistic environmental conditions. Through a combination of in situ, temperature‐resolved RIXS measurements and theoretical simulations we isolated and quantified the effects of bond‐length disorder and adsorbate coverage (CO and H₂) on the d‐band center of 1.25 nm size Pt catalysts supported on carbon. We found that the decrease in adsorbate coverage at elevated temperatures is responsible for the d band shifts towards higher energies relative to the Fermi level, whereas the effect of the increase in bond‐length disorder on the d‐band center is negligible. Although these results were obtained for a specific case of non‐interacting support and weak temperature dependence of the metal–metal bond length in a model catalyst, this work can be extended to a broad range of real catalysts.     

Aspherical‐Atom Modeling of Coordination Compounds by Single‐Crystal X‐ray Diffraction Allows the Correct Metal Atom To Be Identified

Single‐crystal X‐ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid‐state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical‐atom least‐squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld‐atom refinement (Acta Crystallogr. Sect. A­ 2008 , 64, 383–393; IUCrJ. 2014 , 1,61–79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B­ 2013 , 69, 91–104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear‐coordinate 3d metal complexes, for which the wrong element is found if standard independent‐atom model scattering factors are relied upon, are studied, and it is shown that only aspherical‐atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed.     

X‐Ray Diffraction as a Local Probe Tool

For the structural characterization of nanoscale objects, X‐ray diffraction is widely used as a technique complementing local probe analysis methods such as scanning electron microscopy and transmission electron microscopy. Details on strain distributions, chemical composition, or size and shape of nanostructures are addressed. X‐ray diffraction traditionally obtains very good statistically averaged properties over large ensembles—provided this averaging is meaningful for ensembles with sufficiently small dispersion of properties. In many cases, however, it is desirable to combine different analysis techniques on exactly the same nano‐object, for example, to gain a more detailed insight into the interdependence of properties. X‐ray beams focused to diameters in the sub‐micron range, which are available at third‐generation synchrotron sources, allow for such X‐ray diffraction studies of individual nano‐objects.     

In Situ High‐Energy Synchrotron Radiation Study of Boehmite Formation,Growth, and Phase Transformation to Alumina in Sub‐ and Supercritical Water

Boehmite (AlOOH) nanoparticles have been synthesized in subcritical (300 bar, 350 °C) and supercritical (300 bar, 400 °C) water. The formation and growth of AlOOH nanoparticles were studied in situ by small‐ and wide‐angle X‐ray scattering (SAXS and WAXS) using 80 keV synchrotron radiation. The SAXS/WAXS data were measured simultaneously with a time resolution greater than 10 s and revealed the initial nucleation of amorphous particles takes place within 10 s with subsequent crystallization after 30 s. No diffraction signals were observed from Al(OH)₃ within the time resolution of the experiment, which shows that the dehydration step of the reaction is fast and the hydrolysis step rate‐determining. The sizes of the crystalline particles were determined as a function of time. The overall size evolution patterns are similar in sub‐ and supercritical water, but the growth is faster and the final particle size larger under supercritical conditions. After approximately 5 min, the rate of particle growth decreases in both sub‐ and supercritical water. Heating of the boehmite nanoparticle suspension allowed an in situ X‐ray investigation of the phase transformation of boehmite to aluminium oxide. Under the wet conditions used in this work, the transition starts at 530 °C and gives a two‐phase product of hydrated and non‐hydrated aluminium oxide.