Pair Distribution Function Analysis Research Articles

Intercalated Water Drives Anomalous Thermal Expansion in the Tetragonal Zircon Structured Bismuth Vanadate BiVO4 Photocatalyst.

The thermal transformation of zircon to scheelite in BiVO4 was studied by in-situ Synchrotron X-ray diffraction and TGA/FTIR analysis. Upon heating, the tetragonal zircon polymorph of BiVO4 (tz-BiVO4) transitioned to the tetragonal scheelite (ts-)polymorph between 693-773 K, then to monoclinic fergusonite (mf) polymorph upon cooling. An anomaly in thermal expansion was observed between 400-500 K, associated with loss of H2O/NH4+. Heating tz-BiVO4 resulted in contraction of the V-O bond distance and VO4 polyhedra volume due to rotation of the VO4. Attempts to study this by neutron diffraction failed due to the large incoherent scatter from the hydrogenous species. Efforts to remove these species while maintaining the tz-BiVO4 structure were unsuccessful, suggesting they play a role in stabilizing the zircon polymorph. The local structure of both mf-BiVO4 and tz-BiVO4 were investigated by X-ray Pair Distribution Function analysis.

Behavior of Microstrain in Nd3+-Sensitized Near-Infrared Upconverting Core-Shell Nanocrystals for Defect-Induced Tailoring of Luminescence Intensity.

In an attempt to optimize the upconversion luminescence (UCL) output of a Nd3+-sensitized near-infrared (808 nm) upconverting core-shell (CS) nanocrystal through deliberate incorporation of lattice defects, a comprehensive analysis of microstrain both at the CS interface and within the core layer was performed using integral breadth calculation of high-energy synchrotron X-ray (λ = 0.568551 Å) diffraction. An atomic level interpretation of such microstrain was performed using pair distribution function analysis of the high-energy total scattering. The core NC developed compressive microstrain, which gradually transformed into tensile microstrain with the growth of the epitaxial shell. Such a reversal was rationalized in terms of a consistent negative lattice mismatch. Upon introduction of lattice defects into the CS systems upon incorporation of Li+, the corresponding UCL intensity was maximized at some specific Li+ incorporation, where the tensile microstrain of CS, compressive microstrain of the core, and atomic level disorders exhibited their respective extreme values irrespective of the activator ions.

Exploring the Effects of the Photochromic Response and Crystallization on the Local Structure of Noncrystalline Niobium Oxide.

Niobium oxide (Nb2O5) is a versatile semiconductor material with photochromic properties. This study investigates the local structure of noncrystalline, short-range-ordered niobium oxide synthesized via a sol-gel method. X-ray atomic pair distribution function analysis unravels the structural arrangements within the noncrystalline materials at a local scale. In the following, in situ scattering and diffraction experiments elucidate the heat-induced structure transformation of the amorphous material into crystalline TT-Nb2O5 at 550 °C. In addition, the effect of photocatalytic conditions on the structure of the material was investigated by exposing the short-range-ordered and crystalline materials to ultraviolet light, resulting in a reversible color change from white to dark brown or blue. This photochromic response is due to the reversible elongation of the nearest Nb-O neighbors, as shown by local structure analysis based on in situ PDF analyses. Optical band gap calculations based on the ultraviolet-visible spectra collected for both the short-range-ordered and crystalline materials show that the band gap values reduced for the darkened materials return to their initial state after bleaching. Furthermore, electron energy loss spectroscopy reveals the reduction of Nb5+ to Nb4+ centers as a persistent effect. The study establishes a correlation between the band gap and the structure of niobium oxide, providing insights into the structure-performance relation at the atomic level.

A bicarbonate-rich liquid condensed phase in non-saturated solutions in the absence of divalent cations.

Bicarbonate (HCO3 -) and sodium (Na+)-containing solutions contain droplets of a separate, bicarbonate-rich liquid condensed phase (LCP) that have higher concentrations of HCO3 - relative to the bulk solution in which they reside. The existence and composition of the LCP droplets has been investigated by nanoparticle tracking analysis, nuclear magnetic resonance spectroscopy, refractive index measurements and X-ray pair distribution function analysis. The bicarbonate-rich LCP species is a previously unaccounted-for, ionic phenomenon which occurs even in solutions with solely monovalent cations. Its existence requires re-evaluation of models used to describe and model aqueous solution physicochemistry, especially those used to describe and model carbonate mineral formation.

Characterization of Surfactant Spheroidal Micelle Structure for Pharmaceutical Applications: A Novel Analytical Framework

We introduce an innovative theoretical framework tailored for the analysis of Pair Distribution Function (PDF) data derived from Small-Angle X-ray Scattering (SAXS) measurements of core-shell micelles. The new approach involves the exploitation of the first derivative of the PDF and the derivation of analytical equations to solve the core-shell micelle structure under the hypothesis of a spheroidal shape. These analytical equations enable us to determine the micelle’s aggregation number, degree of ellipticity, and contrast in electron density between the core-shell and shell-buffer regions after having determined the whole micelle size and its shell size from the analysis of the first derivative of the PDF. We have formulated an overdetermined system of analytical equations based on the unknowns that characterize the micelle structure. This allows us to establish a Figure of Merit, which is utilized to identify the most reliable solution within the system of equations.

Formation of a zirconium oxide crystal nucleus in the initial nucleation stage in aluminosilicate glass investigated by X-ray multiscale analysis

Understanding the nucleation mechanism in glass is crucial for the development of new glass-ceramic materials. Herein, we report the structure of a commercially important glass-ceramic ZrO2-doped lithium aluminosilicate system during its initial nucleation stage. We conducted an X-ray multiscale analysis, and this analysis was used to observe the structure from the atomic to the nanometer scale by using diffraction, small-angle scattering, absorption, and anomalous scattering techniques. The inherent phase separation between the Zr-rich and Zr-poor regions in the pristine glass was enhanced by thermal treatment without changing the spatial geometry at the nanoscale. Element-specific pair distribution function analysis using anomalous X-ray scattering data showed the formation of a liquid ZrO2-like local structural motif and edge sharing between the ZrOx polyhedra and (Si/Al)O4 tetrahedra during the initial nucleation stage. Furthermore, the local structure of the Zr4+ ions resembled a cubic or tetragonal ZrO2 crystalline phase and formed after 2 h of annealing the pristine glass. Therefore, the Zr-centric periodic structure formed in the early stage of nucleation was potentially the initial crystal nucleus for the Zr-doped lithium aluminosilicate glass-ceramic.

Quantifying the cooperative evolution of microphase segregation and nanostructural order in annealed polyurethanes of MDI‐BDO‐PTHF

AbstractHere, we quantify the relationship between microphase segregation and local structure development in thermoplastic polyurethanes. Samples were prepared with varying ratios of 4,4′‐methylene diphenyl diisocyanate (MDI) and 1,4‐butanediol (BDO) to polytetrahydrofuran (PTHF) and annealed at temperatures between RT–140 °C for 20 h each. Distinct populations of smaller and larger ordered domains are distinguished by pair distribution function analysis. The intermediate‐range order of nanoscale hard domains in the paracrystalline state (form I) is directly extracted and characterized. The results suggest that form I of the MDI‐BDO:PTHF system presents a conformational and packing order similar to the form III structure, typically obtained from stretching/annealing, but with limited spatial coherence of 3–7 nm. Heating above of the hard segments is necessary to achieve a substantial structural response from the annealing treatment. Increasing promotes purification of the soft phase via microphase segregation, coalescence of the hard blocks, and growth of paracrystalline phase, while a minority fraction of longer‐range ordered domains left over from production remains relatively constant. We observe a composition‐dependent decrease in the average nearest‐neighbor distance that can be correlated with the greater number of CC, CN, and CO bonds with increased hard segment content. A non‐linear deviation in the trend is observed to correlate with sample densification. The mechanical and thermal behavior of the samples is intimately tied to the segregation state.

Evolution and Interplay of Lithium Metal Interphase Components Revealed by Experimental and Theoretical Studies.

Lithium metal batteries (LMB) have high energy densities and are crucial for clean energy solutions. The characterization of the lithium metal interphase is fundamentally and practically important but technically challenging. Taking advantage of synchrotron X-ray, which has the unique capability of analyzing crystalline/amorphous phases quantitatively with statistical significance, we study the composition and dynamics of the LMB interphase for a newly developed important LMB electrolyte that is based on fluorinated ether. Pair distribution function analysis revealed the sequential roles of the anion and solvent in interphase formation during cycling. The relative ratio between Li2O and LiF first increases and then decreases during cycling, suggesting suppressed Li2O formation in both initial and long extended cycles. Theoretical studies revealed that in initial cycles, this is due to the energy barriers in many-electron transfer. In long extended cycles, the anion decomposition product Li2O encourages solvent decomposition by facilitating solvent adsorption on Li2O which is followed by concurrent depletion of both. This work highlights the important role of Li2O in transitioning from an anion-derived interphase to a solvent-derived one.

Suppressed Lone Pair Electrons Explain Unconventional Rise of Lattice Thermal Conductivity in Defective Crystalline Solids

AbstractManipulating thermal properties of materials can be interpreted as the control of how vibrations of atoms (known as phonons) scatter in a crystal lattice. Compared to a perfect crystal, crystalline solids with defects are expected to have shorter phonon mean free paths caused by point defect scattering, leading to lower lattice thermal conductivities than those without defects. While this is true in many cases, alloying can increase the phonon mean free path in the Cd‐doped AgSnSbSe3 system to increase the lattice thermal conductivity from 0.65 to 1.05 W m−1 K−1 by replacing 18% of the Sb sites with Cd. It is found that the presence of lone pair electrons leads to the off‐centering of cations from the centrosymmetric position of a cubic lattice. X‐ray pair distribution function analysis reveals that this structural distortion is relieved when the electronic configuration of the dopant element cannot produce lone pair electrons. Furthermore, a decrease in the Grüneisen parameter with doping is experimentally confirmed, establishing a relationship between the stereochemical activity of lone pair electrons and the lattice anharmonicity. The observed “harmonic” behavior with doping suggests that lone pair electrons must be preserved to effectively suppress phonon transport in these systems.

Molecular Insights into Sequence-Specific Protein Hydrolysis by a Soluble Zirconium-Oxo Cluster Catalyst.

The development of catalysts for controlled fragmentation of proteins is a critical undertaking in modern proteomics and biotechnology. {Zr6O8}-based metal-organic frameworks (MOFs) have emerged as promising candidates for catalysis of peptide bond hydrolysis due to their high reactivity, stability, and recyclability. However, emerging evidence suggests that protein hydrolysis mainly occurs on the MOF surface, thereby questioning the need for their highly porous 3D nature. In this work, we show that the discrete and water-soluble [Zr6O4(OH)4(CH3CO2)8(H2O)2Cl3]+ (Zr6) metal-oxo cluster (MOC), which is based on the same hexamer motif found in various {Zr6O8}-based MOFs, shows excellent activity toward selective hydrolysis of equine skeletal muscle myoglobin. Compared to related Zr-MOFs, Zr6 exhibits superior reactivity, with near-complete protein hydrolysis after 24 h of incubation at 60 °C, producing seven selective fragments with a molecular weight in the range of 3-15 kDa, which are of ideal size for middle-down proteomics. The high solubility and molecular nature of Zr6 allow detailed solution-based mechanistic/interaction studies, which revealed that cluster-induced protein unfolding is a key step that facilitates hydrolysis. A combination of multinuclear nuclear magnetic resonance spectroscopy and pair distribution function analysis provided insight into the speciation of Zr6 and the ligand exchange processes occurring on the surface of the cluster, which results in the dimerization of two Zr6 clusters via bridging oxygen atoms. Considering the relevance of discrete Zr-oxo clusters as building blocks of MOFs, the molecular-level understanding reported in this work contributes to the further development of novel catalysts based on Zr-MOFs.

Nanoscale Stabilization Mechanism of Sodium Sulfate Decahydrate at Polyelectrolyte Interfaces.

Sodium sulfate decahydrate (SSD) is a low-cost phase-change material (PCM) for thermal energy storage applications that offers substantial melting enthalpy and a suitable temperature range for near-ambient applications. However, SSD's consistent phase separation with decreased melting enthalpy over repeated thermal cycles limits its application as a PCM. Sulfonated polyelectrolytes, such as dextran sulfate sodium (DSS), have shown great effectiveness in preventing phase separation in SSD. However, there is limited understanding of the stabilization mechanism of SSD by DSS at the atomic length and time scales. In this work, we investigate SSD stabilization via DSS using neutron scattering and molecular dynamics (MD) simulations. Neutron scattering and pair distribution function analysis revealed the structural evolution of the PCM samples below and above the phase change temperatures. MD simulations revealed that water from the hydrate structure migrates from the hydrate crystal to the SSD-DSS interfacial region upon melting. The water is stabilized at this interface by aggregation around the hydrophilic sulfonic acid groups attached to the backbone of the polyelectrolyte. This architecture retains water near the dehydrated sodium sulfate, preventing phase separation and, consequently, stabilizing SSD rehydration. This work provides atomistic insight into selecting and designing stable and high-performance PCMs for heating and cooling applications in building technologies.

Structure and phase transitions in niobium and tantalum derived nanoscale transition metal perovskites, Ba(Ti,MV)O3, M=Nb,Ta.

The prospect of creating ferroelectric or high permittivity nanomaterials provides motivation for investigating complex transition metal oxides of the form Ba(Ti, MV)O3, where M = Nb or Ta. Solid state processing typically produces mixtures of crystalline phases, rarely beyond minimally doped Nb/Ta. Using a modified sol-gel method, we prepared single phase nanocrystals of Ba(Ti, M)O3. Compositional and elemental analysis puts the empirical formulas close to BaTi0.5Nb0.5O3-δ and BaTi0.5Ta0.5O3-δ. For both materials, a reversible temperature dependent phase transition (non-centrosymmetric to symmetric) is observed in the Raman spectrum in the region 533-583K (260-310°C); for Ba(Ti, Nb)O3, the onset is at 543K (270°C); and for Ba(Ti, Ta)O3, the onset is at 533K (260°C), which are comparable with 390-393K (117-120°C) for bulk BaTiO3. The crystal structure was resolved by examination of the powder x-ray diffraction and atomic pair distribution function (PDF) analysis of synchrotron total scattering data. It was postulated whether the structure adopted at the nanoscale was single or double perovskite. Double perovskites (A2B'B″O6) are characterized by the type and extent of cation ordering, which gives rise to higher symmetry crystal structures. PDF analysis was used to examine all likely candidate structures and to look for evidence of higher symmetry. The feasible phase space that evolves includes the ordered double perovskite structure Ba2(Ti, MV)O6 (M = Nb, Ta) Fm-3m, a disordered cubic structure, as a suitable high temperature analog, Ba(Ti, MV)O3Pm-3m, and an orthorhombic Ba(Ti, MV)O3Amm2, a room temperature structure that presents an unusually high level of lattice displacement, possibly due to octahedral tilting, and indication of a highly polarized crystal.

Improved Structural Description of Different γ-Al2O3 Materials Using Disordered δ5-Al2O3 Phase via X-ray Pair Distribution Function Analysis

Extensive research has been conducted in the past on the crystallographic characteristics of γ-Al2O3 support materials due to their advantageous properties in heterogeneous catalysis. While their structure is most commonly described as spinel, their intrinsic disorder and nanostructure have prompted alternative models involving tetragonal space groups, supercells, or occupancy of non-spinel positions. X-ray pair distribution function (PDF) analysis has further postulated the existence of short-range order domains with structural remnants from boehmite precursors from which γ-Al2O3 is commonly prepared via calcination. In this PDF study, we now show that a recently theoretically found monoclinic δ5-Al2O3 phase is, in fact, best suited for describing the structure of different commercial Al2O3 supports, as well as a self-prepared and an industrial Ni/Al2O3 methanation catalyst. Furthermore, in situ experiments under catalytic cycling in the methanation reaction demonstrate that the nanoscale structure of this δ5 phase is preserved during cycling, pointing towards the high stability of the therein-represented disorder. A complete description of the disordered Al2O3 support structure is crucial in the field of heterogeneous catalysis in order to distinguish disorder within the bulk support from additional interfacial restructuring processes such as surface oxidation or spinel formation due to nanoparticle–support interactions during catalytic cycling in in situ scattering experiments.

Toward an Understanding of the Anisotropy in Hcp Zinc Metal: Total Scattering Structural Study Using Synchrotron-Based, Temperature-Dependent, X-Ray Pair Distribution Function Analysis

Abstract: Among hexagonal close packing (hcp) metals, zinc has a non-ideal hcp structure due to a significantly increased axial ratio c/a. We studied the behavior of the ratio of lattice constants c/a and the ratio of thermal displacement parameters (U_33 / U_11) in the temperature range of 100-300 K, using the X-ray total scattering atomic pair distribution function (PDF) analysis over different refinement r-ranges. The temperature-dependent PDF analysis confirmed that the crystal structure of zinc has significant static distortion along the c-direction. The measured value of atomic displacement parameters of zinc showed a notable anisotropy as expressed by the ratio U_33 / U_11 of ≈ 2.2 at 100 K. The lattice parameter ratio c/a was slightly reduced at low temperatures but remained unusually large. The extrapolated value of the ratio c/a reached 1.832 at 0 K. The PDF refinements using the crystallographic model revealed that there are some local structure features in zinc that are not captured by the crystallographic model. On the other hand, our local structure refinement indicated a presence of local bond distortion along the z-direction that averaged out over wider r-range refinements with a large U_33 value. Keywords: Hcp anisotropy, Zinc metal, Total scattering, X-ray scattering, Pair distribution function. PACs numbers: 61.46.Df, 61.10.-i, 78.66.Hf, 61.46.-w.

The Chemistry of Spinel Ferrite Nanoparticle Nucleation, Crystallization, and Growth.

The nucleation, crystallization, and growth mechanisms of MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 nanocrystallites prepared from coprecipitated transition metal (TM) hydroxide precursors treated at sub-, near-, and supercritical hydrothermal conditions have been studied by in situ X-ray total scattering (TS) with pair distribution function (PDF) analysis, and in situ synchrotron powder X-ray diffraction (PXRD) with Rietveld analysis. The in situ TS experiments were carried out on 0.6 M TM hydroxide precursors prepared from aqueous metal chloride solutions using 24.5% NH4OH as the precipitating base. The PDF analysis reveals equivalent nucleation processes for the four spinel ferrite compounds under the studied hydrothermal conditions, where the TMs form edge-sharing octahedrally coordinated hydroxide units (monomers/dimers and in some cases trimers) in the aqueous precursor, which upon hydrothermal treatment nucleate through linking by tetrahedrally coordinated TMs. The in situ PXRD experiments were carried out on 1.2 M TM hydroxide precursors prepared from aqueous metal nitrate solutions using 16 M NaOH as the precipitating base. The crystallization and growth of the nanocrystallites were found to progress via different processes depending on the specific TMs and synthesis temperatures. The PXRD data show that MnFe2O4 and CoFe2O4 nanocrystallites rapidly grow (typically <1 min) to equilibrium sizes of 20-25 nm and 10-12 nm, respectively, regardless of applied temperature in the 170-420 °C range, indicating limited possibility of targeted size control. However, varying the reaction time (0-30 min) and temperature (150-400 °C) allows different sizes to be obtained for NiFe2O4 (3-30 nm) and ZnFe2O4 (3-12 nm) nanocrystallites. The mechanisms controlling the crystallization and growth (nucleation, growth by diffusion, Ostwald ripening, etc.) were examined by qualitative analysis of the evolution in refined scale factor (proportional to extent of crystallization) and mean crystallite volume (proportional to extent of growth). Interestingly, lower kinetic barriers are observed for the formation of the mixed spinels (MnFe2O4 and CoFe2O4) compared to the inverse (NiFe2O4) and normal (ZnFe2O4) spinel structured compounds, suggesting that the energy barrier for formation may be lowered when the TMs have no site preference.

Direct Synthesis of CuPd Icosahedra Supercrystals Studied by In Situ X-Ray Scattering.

Nanocrystal self-assembly into supercrystals provides a versatile platform for creating novel materials and devices with tailored properties. While common self-assembly strategies imply the use of purified nanoparticles after synthesis, conversion of chemical precursors directly into nanocrystals and then supercrystals in simple procedures has been rarely reported. Here, the nucleation and growth of CuPd icosahedra and their consecutive assembly into large closed-packed face-centered cubic (fcc) supercrystals are studied. To this end, the study simultaneously and in situ measures X-ray total scattering with pair distribution function analysis (TS-PDF) and small-angle X-ray scattering (SAXS). It is found that the supercrystals' formation is preceded by an intermediate dense phase of nanocrystals displaying short-range order (SRO). It is further shown that the organization of oleic acid/oleylamine surfactants into lamellar structures likely drives the emergence of the SRO phase and later of the supercrystals by reducing the volume accessible to particle diffusion. The supercrystals' formation as well as their disassembly are triggered by temperature. The study demonstrates that ordering of solvent molecules can be crucial in the direct synthesis of supercrystals. The study also provides a general approach to investigate novel preparation routes of supercrystals in situ and across several length scales via X-ray scattering.

Investigation of O/N Ordering in Perovskite-Type Oxynitrides La1−xYxTa(O,N)3 on Long Range and Short Scale

Oxynitrides such as LaTa(O,N)3 are attractive materials as photoelectrodes for photoelectrocatalytic solar water splitting. The potential anionic ordering in their perovskite-type structure has been shown to impact the materials’ properties. Given the importance attributed to it, the present study reports a detailed experimental analysis supported by simulations of the anionic ordering of La1−xYxTa(O,N)3. The influence of O/N and yttrium content on the anionic order was assessed. Neutron diffraction analysis was performed on four different nominal compositions—LaTaON2, LaTaO2N, La0.9Y0.1TaON2, and La0.9Y0.1TaO2N—at 10 K and 300 K to study potential long-range ordering. Neutron pair distribution function (PDF) analysis was performed on all samples at 10 K and on non-Y-substituted samples at 300 K to evaluate short-range ordering. There was no evidence of long-range O/N order in any of the compounds. In contrast, at a short range (1.5 Å ≤ r &lt; 6 Å), a Pnma (a−b+a−) tilting pattern and local cis-ordering of the anions were seen. The latter faded rapidly, leaving the Pnma tilting pattern in a 6 Å ≤ r ≤ 11 Å range. At higher distances, the PDF analysis agreed with the Imma (a−b0a−) O/N disordered long-range structure. As the O/N content changed, not much difference in behavior was observed. Yttrium substitution introduced some disorder in the structure; nonetheless, it showed marginal influence on octahedral tilting and anionic ordering.

Atomically Precise Single-Site Catalysts via Exsolution in a Polyoxometalate-Metal-Organic-Framework Architecture.

Single-site catalysts (SSCs) achieve a high catalytic performance through atomically dispersed active sites. A challenge facing the development of SSCs is aggregation of active catalytic species. Reducing the loading of these sites to very low levels is a common strategy to mitigate aggregation and sintering; however, this limits the tools that can be used to characterize the SSCs. Here we report a sintering-resistant SSC with high loading that is achieved by incorporating Anderson-Evans polyoxometalate clusters (POMs, MMo6O24, M = Rh/Pt) within NU-1000, a Zr-based metal-organic framework (MOF). The dual confinement provided by isolating the active site within the POM, then isolating the POMs within the MOF, facilitates the formation of isolated noble metal sites with low coordination numbers via exsolution from the POM during activation. The high loading (up to 3.2 wt %) that can be achieved without sintering allowed the local structure transformation in the POM cluster and the surrounding MOF to be evaluated using in situ X-ray scattering with pair distribution function (PDF) analysis. Notably, the Rh/Pt···Mo distance in the active catalyst is shorter than the M···M bond lengths in the respective bulk metals. Models of the active cluster structure were identified based on the PDF data with complementary computation and X-ray absorption spectroscopy analysis.

Tailored (La0.2 Pr0.2 Nd0.2 Tb0.2 Dy0.2 )2 Ce2 O7 as a Highly Active and Stable Nanocatalyst for the Oxygen Evolution Reaction.

Designing highly active and robust catalysts for the oxygen evolution reaction is key to improving the overall efficiency of the water splitting reaction. It has been previously demonstrated that evaporation induced self-assembly (EISA) can be used to synthesize highly porous and high surface area cerate-based fluorite nanocatalysts, and that substitution of Ce with 50% rare earth (RE) cations significantly improves electrocatalyst activity. Herein, the defect structure of the best performing nanocatalyst in the series are further explored, Nd2 Ce2 O7 , with a combination of neutron diffraction and neutron pair distribution function analysis. It is found that Nd3 + cation substitution for Ce in the CeO2 fluorite lattice introduces higher levels of oxygen Frenkel defects and induces a partially reduced RE1.5 Ce1.5 O5 + x phase with oxygen vacancy ordering. Significantly, it is demonstrated that the concentration of oxygen Frenkel defects and improved electrocatalytic activity can be further enhanced by increasing the compositional complexity (number of RE cations involved) in the substitution. The resulting novel compositionally-complex fluorite- (La0.2 Pr0.2 Nd0.2 Tb0.2 Dy0.2 )2 Ce2 O7 is shown to display a low OER overpotential of 210 mV at a current density of 10 mAcm-2 in 1M KOH, and excellent cycling stability. It is suggested that increasing the compositional complexity of fluorite nanocatalysts expands the ability to tailor catalystdesign.

Ex situ and in situ studies on structural features of Pt/Ce0.75Zr0.25O2 catalyst for water gas shift reaction

Pt/Ce1-xZrxO2 catalysts have shown high activity in water gas shift (WGS) reaction, but their structural organization has been studied insufficiently. This work represents a detailed structural study on the supported Pt species and the metal/support interface in a 5 wt% Pt/Ce0.75Zr0.25O2 catalyst in the initial state, after reductive activation, and after WGS reaction in H2-enriched reformate-simulating mixture. A wide range of methods was used: ex situ and in situ X-ray diffraction (XRD) studies, high resolution transmission electron microscopy (HRTEM), X-ray atomic pair distribution function (PDF) method, pseudo in situ X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR). The Ce0.75Zr0.25O2 mixed oxide was shown to provide a highly dispersed state of Pt species. XPS revealed that the initial catalyst contains Pt2+species. PDF analysis allowed us to propose the structure of ultrafine PtO particles and to elucidate the metal-support interaction with fixation of Pt ions on the support surface. In situ XRD, pseudo in situ XPS, and H2-TPR studies revealed the reduction of ultrafine PtO particles in H2 atmosphere at low temperatures (20–90°С) with the formation of metallic Pt0 particles and simultaneous partial reduction of the support surface due to the hydrogen spillover. Catalytic tests of the as-prepared and poisoned with chlorine 5wt% Pt/Ce0.75Zr0.25O2 catalysts in the WGS reaction showed an important role of the oxygen vacancies in the oxide support as active sites. These findings contribute to the understanding catalytic performance of Pt/Ce1-xZrxO2 systems.