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Operando Investigations of Structure-Activity Relationships in Pd-based Model Catalysts for Methane Oxidation
No full textThe high global warming potential of CH4 makes the catalytic conversion of residual CH4 in exhaust gases vital for applications such as CH4 combus tion engines and turbines, power-to-gas or biomass plants. The most active heterogeneous catalyst system for complete CH4 oxidation at low temper atures (< 650K) in lean reaction gas mixtures (overstoichiometric oxygen content) is the class of Pd-based catalysts supported by (mixed) metal oxide supports. The activity of these catalysts is closely linked to PdO content at low temperatures, as well as structure and morphology of the nanoparticles. The catalytic conversion proceeds with the Mars-van-Krevelen mechanism through numerous elementary steps which consumes lattice oxygen of PdO and produces H2O. A persistent challenge in heterogeneous catalysis in gen eral is catalyst deactivation, which can occur through sintering or poisoning, and these processes also affect Pd-based catalysts for CH4 oxidation. Specif ically, the inhibition by H2O, deactivation by PdO reduction, and sintering are three of the core causes for deactivation. Strategies to address these issues are the use of oxide supports with different redox properties and the addi tion of other noble metals to the catalyst nanoparticles, most prominently Pt, which can alter the redox properties of the catalyst, inhibit sintering, and manage catalyst passivation by H2O. This work aims to elucidate the structural and morphological properties of Pd based catalysts under transient conditions during light-off. The objective is to improve the understanding of the mechanisms behind the enhance ment of catalyst performance by nanoparticle support interactions and the addition of Pt. Model catalysts that combine geometric simplicity and mor phological complexity were used to address these questions. This approach enables direct correlation of structural and morphological properties on the atomic scale to the catalyst’s activity. The experiments were carried out in industrially relevant temperature and pressure regimes, thereby bridging the pressure and material gap between single crystal studies and conventional packed-bed or monolith reactor experiments. α-Al2O3(0001) was selected as an inert representative and CeO2 as a redox active support with high oxygen mobility. The CeO2(001) model catalyst support surface was prepared by reactive physical vapor deposition of Ce in atomic oxygen atmosphere on YSZ(001), as commercially available CeO2 substrates are unsuitable for grazing incidence X-ray scattering. The re sulting CeO2 films were thoroughly characterized by a comprehensive set of complementary techniques to ensure tight control over its properties. The CeO2 thin films used as catalyst supports exhibited a dislocation lattice at the CeO2/YSZ interface which enabled full coverage of the film despite the iii considerable lattice mismatch. The bulk of the film was fully oxidized with a bulk-like lattice, while the surface was fully hydroxylated and covered with a molecular water level, even after annealing in ultra high vacuum under oxygen atmosphere. Two aspects of the structure and morphology were investigated: (i) the evolution under transient light-off conditions and (ii) the temporal evolution after each increment in a step-wise heating experi ment. For both studies, epitaxial Pd and PdPt nanoparticles were grown by physical vapor deposition, and catalytic testing was conducted in a custom, X-ray compatible operando flow cell equipped with inline mass spectrometry. The light-off experiments conducted with Pd/Al2O3 and Pd/CeO2 showed strong dependence of catalytic activity, structure and morphology from the support material. Notably, the reaction intermediates CO and CH2O, asso ciated with the Mars-van-Krevelen mechanism on PdO(101), were observed in the exhaust gas by mass spectrometry for the first time. These observa tions suggest that the migration and adsorption/desorption of surface species such as OH and CH2O are slower than desorption of gas-phase intermedi ates. Consequently, complete oxidation under conventional conditions may proceed through multiple adsorption–desorption cycles. Structural and mor phological data recorded in parallel by high-energy X-ray diffraction (75keV) revealed distinct oxidation mechanisms depending on the support. The com parative analysis showed that CeO2 inhibited sintering and stabilized the PdO phase more effectively than Al2O3. Furthermore, the detection of ther modynamically unstable phases under reaction conditions provided evidence for strong variations in local chemical potential at the catalyst surface. The CeO2 thin films used in the light-off experiments contained a small amount of rectangular holes, which are associated with oxygen vacancy condensation. A statistical evaluation of SEM images revealed a pronounced size difference between NPs located on the CeO2 film and those located in the holes of the f ilm. These results provide evidence that Ostwald ripening is the dominant sintering mechanism on CeO2(001) supports and that the holes act as dif fusion traps, locally enhancing sintering. In the kinetic investigation, the influence of Pt on catalytic behavior was studied by comparing the activity, structure, and morphological dynamics of Pd/Al2O3 and PdPt/Al2O3. For the first time, HEGISAXS and HEGIXRD were combined to characterize catalysts under operando conditions. Regardless of Pt content, the epitax ial relationship between Al2O3 and the nanoparticles significantly inhibited their oxidation compared to the larger NPs studied in the light-off experi ments. Pronounced morphological changes upon initial H2O desorption were observed only for Pd/Al2O3, accompanied by significant changes in the lat tice constant, ultimately leading to an overall relaxation of the lattice. In contrast, the morphology of PdPt/Al2O3 remained largely unaffected by the iv initial H2O. HEGISAXS indicated significant vertical material transport in Pd/Al2O3, pointing to strong reaction-induced reshaping which is consistent with the trends seen in HEGIXRD. In contrast, this was not observed for PdPt/Al2O3. Changes in the NP morphology in this system are instead at tributed to the formation of PdO bulk and surface phases and slow, thermally driven sintering over the course of the experiment. While the correlation be tween PdO content and catalytic activity was weak, a clear relationship was observed between activity and the strain state of the metal phase. The lattice of Pd/Al2O3 gradually relaxed over the course of the experiment, coinciding with declining activity, whereas PdPt/Al2O3 retained a highly strained lat tice, which correlated with improved performance. In summary, these findings provide direct insight into the mechanisms of cat alyst deactivation and the effects of the nanoparticle-support interactions, Pt alloying, and strain in enhancing resistance to H2O inhibition, PdO reduc tion, and sintering. The use of model catalysts, combinatory X-ray tech niques, and relevant pressure and temperature regimes established detailed structure-activity correlations and highlighted the role of surface diffusion, and desorption of products and intermediates of CH4 oxidation. Overall, these results bridge the material, pressure, and complexity gap between ide alized single crystal investigations and reactor studies
Defect-driven relaxation of nanostructured Cu examined by in situ heating high-energy synchrotron X-ray microbeam diffraction
No full textBulk nanostructured metals introduced by severe plastic deformation contain an excess of lattice defects. A nanostructured copper (Cu) processed by a high-pressure torsion technique was examined during in situ heating to investigate microstructural relaxation and quantify the evolution of microstructural parameters using high- energy synchrotron microbeam X-ray diffraction. While general microstructural relaxations, such as recovery, recrystallization, and subsequent grain growth, were observed, the key microstructural parameters, including grain size, microstrain, dislocation density, and thermal expansion coefficient, and their changes at critical temperatures were uniquely described and quantified through diffraction data. Based on this analysis, the stored energies driving thermally activated microstructural changes were estimated for individual defect types — grain boundaries, dislocations, and vacancies — that are expected to significantly influence the relaxation behavior of nanostructured Cu. This study demonstrates the effectiveness of diffraction characterization techniques for gaining a comprehensive understanding of the thermal stability of bulk nanostructured materials
About the origin of Mn-related modes in Raman scattering spectra of ZnO:Mn ceramics
No full textZnO and ZnMnO ceramics were made by sintering in air at 1150 °C for 3 h. Two types of doped samples – furnace cooled and quenched from the sintering temperature in air – were prepared. Raman scattering, UV-vis optical absorption, photoluminescence and electron paramagnetic resonance (EPR) study of obtained ceramics testify to the formation of Mn centers upon Mn doping, and the EPR spectra confirmed the effect of cooling rate on their number. In accordance with available literature data, three Raman modes at about 480, 530 and 580 cm were revealed in Raman spectra of doped ceramics in addition to those of undoped ones. Based on the analysis of obtained and reported earlier experimental data, the conclusion has been made that the appearance of these additional Raman modes in ZnO:Mn materials is caused by the photoionization of Mn centers and the interaction of photoelectrons with phonons
Influence of europium ion doping on photoinduced properties of 2D cobalt hydroxide: photocatalytic degradation and negative photoconductivity studies
No full textTransition metal hydroxides have been extensively used in the electrocatalytic oxygen evolution reaction; however, their use in photocatalytic processes remains less common. In this study, we report the successful synthesis of europium-doped (2.8 wt%) cobalt hydroxide nanoplates (Co(OH)2:Eu) via one-pot hydrothermal synthesis for photocatalytic and photoconductive applications. The structural and electronic properties of europium-doped cobalt hydroxide were studied and compared with undoped Co(OH)2 prepared via a similar procedure. Co(OH)2:Eu exhibited increased stability compared to undoped Co(OH)2. Potential applications of Co(OH)2:Eu for photocatalytic pollutant degradation were evaluated on the basis of methylene blue degradation. The band structure of Co(OH)2:Eu was proposed based on the photoconductivity behaviour. The mechanism of the driving forces of the photocatalytic degradation reaction was studied using band structure analysis and radical scavenger experiments. Furthermore, density functional theory (DFT) calculations provided a deeper understanding of the reason for the improved photocatalytic efficiency of Co(OH)2:Eu. We established that Co(OH)2:Eu possesses the local state above the conduction band edge, which induced a marked increase in the negative photoconductivity of Co(OH)2:Eu due to the binding of electrons, highlighting the potential use of rare-earth doping in optoelectronic switches
Open Strings and Heterotic Instantons
No full textMotivated by closed string perturbation theory arguments by S. Shenker, we consider non-perturbative effects of characteristic strength , with the closed string coupling constant, in supersymmetric critical heterotic string theories. We argue that in 10D such effects arise from heterotic 'D-instantons,' i.e. heterotic disk diagrams, whose existence relies on a non-trivial interplay between worldsheet and spacetime degrees of freedom. In compactifications of the heterotic string, we argue that similar effects can arise from wrapped Euclidean non-BPS 'D-strings.' Two general principles arise: The first is that the consistency of those heterotic branes on which the fundamental string can end relies on an inflow mechanism for spacetime degrees of freedom. The second is that Shenker's argument, taken to its logical conclusion, implies that all closed string theories must exhibit open strings as well
X-ray Scattering Investigations of the Structure of Water and Ice in Periodic Mesoporous Organosilicas
No full textWater is one of the most common materials on Earth and in everyday life. However, with its many anomalies which in many cases get further amplified when supercooled, it is still poorly understood. A particularly interesting case is water under confinement. When water is subject to spatial restrictions, it has been found that the structure of its network is influenced and its equilibrium and dynamical properties vary. An interesting confining matrix for water is found in the form of periodic mesoporous organosilicas (PMOs). These materials enable mesoporous confinement in a broad range of pore diameters with the possibility of tuning the pore wall-water interaction. This is achieved by organic moieties in the pore wall, which can house additional functional groups. These groups can for example have hydrophobic or hydrophilic properties. This thesis investigates the structure of water and ice under confinement in a broad range of PMOs with different pore diameters and pore wall functionalizations, by use of X-ray scattering. A strong dependence of the structure of confined water on the pore functionalization, as well as the pore diameter is found. Pores with smaller diameters and hydrophilic functionalizations lead to a decrease in density when compared to bulk water. Furthermore, a stronger tetrahedral water network is observed in these pores. At lower temperatures, an ice structure with diffuse cubic-like, hexagonal and amorphous contributions is observed. The hexagonal component also exhibits a shift in its lattice parameters when compared to bulk hexagonal ice. In smaller, hydrophilic pores, the ice crystallites are furthermore oriented in specific, preferred angles compared to the pore axis. It is also observed that the PMO host materials undergo a deformation when water is being adsorbed. Specifically, the periodicity of their organic moieties changes in dependence on their interaction with water
Tailoring biodegradable polymer mechanics for biomedical use: in situ nanobeam WAXS analysis of poly(ester amide) and poly(ester urea)
No full textBiodegradable pseudo-proteins (PPs), a novel class of synthetic polymers mimicking natural proteins, offer unique advantages for biomedical applications, including superior biocompatibility, tuneable degradation, and structural versatility. In this study, we investigate the mechanical and microstructural behavior of two representative PPs—poly(ester amide) (PEA) and poly(ester urea) (PEU)—using a novel in situ stretching technique combined with nanobeam wide-angle X-ray scattering (WAXS). This advanced methodology enables real-time, submicron-resolution analysis of molecular ordering during mechanical deformation. For both PEA and PEU, the WAXS patterns two distinguished (001) and (010) peaks associated with lamellar stacking and chain–chain packing of aliphatic segments, correspondingly. Our results reveal that PEA, characterized by higher crystallinity of the (010), exhibits brittle fracture under stress, whereas PEU demonstrates elastic behavior due to its lower (010) crystalline content and greater chain mobility. These findings establish a direct correlation between sectional crystallinity and mechanical performance, providing valuable insights for the rational design of PPs with tailored properties for biomedical use
Insights into the operational stability of wide-bandgap perovskite and tandem solar cells under rapid thermal cycling
No full textTemperature variations can induce phase transformations and strain in perovskite solar cells (PSCs), undermining their structural stability and device performance. Despite growing interest, the operational stability of triple-cation wide-bandgap (WBG) PSCs and tandem solar cells (TSCs) under rapid solar-thermal cycling remains poorly understood. Here, we investigate the operational stability of WBG PSCs (~1.68 eV) with a champion power conversion efficiency (PCE) of 24.31% and extend the study to TSCs. We find that degradation during device operation under rapid solar-thermal cycling (temperature change rate of 10 °C/min) is independent of passivation and occurs in two distinct regimes: an initial burn-in phase, which accounts for a rapid 60% relative loss in performance, followed by a steady degradation characterized by temperature-dependent fluctuations in photovoltaic parameters. By operando grazing-incidence wide-angle X-ray scattering and photoluminescence measurements, we reveal that temperature-induced strain, phase transition, and the increased non-radiative recombination collectively contribute to the degradation of PSCs. This work advances the understanding of the degradation mechanisms of WBG PSCs and TSCs, providing insights toward improving their operational thermal stability for real-world applications
Dark Vector Boson Bremsstrahlung: New Form Factors for a Broader Class of Models
No full textWe explore the sensitivity of collider experiments to a broad class of GeV-scale dark vector models of new physics via production in proton and neutron bremsstrahlung and initial state radiation. This is achieved using a new physically motivated model for timelike vector form factors with generic charges for both protons and neutrons, which is fit to a variety of timelike and spacelike data with quantified uncertainties. The production model for both proton and neutron bremsstrahlung is applied to re-cast and extend the reach of existing FASER data to GeV-mass dark photons, U(1)_B, U(1)_{B-L}, and photophobic vectors, as well as forecasts for millicharged particles at FORMOSA