Abstract: Nanocrystalline zeolite particles are applied in a wide range of catalytic reactions. Their size, shape, and the distribution of catalytically active sites significantly vary throughout zeolite batches and within individual particles. This variability leads to a heterogeneous distribution of catalyst performance. Directly investigating the structure–activity relationship at the nanoscale is essential for the rational improvement of catalyst materials. In this work, integrated fluorescence and electron microscopy is employed to correlatively study the structure and performance of individual H-ZSM-22 particles. The needle-shaped morphology of these zeolite particles originates from the lateral fusion of multiple elementary nanorods. Indirect bulk scale experiments have suggested that this process converts catalytically inactive external Al into catalytically active internal Al. The correlative investigation performed in this research provides direct evidence that this conversion takes place, as an inactive shell of 20–40 nm thickness is observed and reactivity is confined to the crystal core. Furthermore, within the catalyst particles nanometer scale catalytic hotspots have been revealed and they are assumed to result from the presence of structural imperfections that locally increase accessibility into the microporous structure. Linear polarized excitation light experiments confirmed that catalytic transformations exclusively occurred on acid sites confined within the microporous structure.
The direct synthesis of hierarchically intergrown silicalite-1 can be achieved using a specific diquaternary ammonium agent. However, the location of these molecules in the zeolite framework, which is critical to understand the formation of the material, remains unclear. Where traditional characterization tools have previously failed, herein we use polarized stimulated Raman scattering (SRS) microscopy to resolve molecular organization inside few-micron-sized crystals. Through a combination of experiment and first-principles calculations, our investigation reveals the preferential location of the templating agent inside the linear pores of the MFI framework. Besides illustrating the attractiveness of SRS microscopy in the field of material science to study and spatially resolve local molecular distribution as well as orientation, these results can be exploited in the design of new templating agents for the preparation of hierarchical zeolites
The impressive optoelectronic performance and low production cost of metal halide perovskites have inspired applications well beyond efficient solar cells. Herein, we widen the materials engineering options available for the efficient and selective photocatalytic oxidation of benzylic alcohols, an industrially significant reaction, using formamidinium lead bromide (FAPbBr3) and other perovskite-based materials. The best performance was obtained using a FAPbBr3/TiO2 hybrid photocatalyst under simulated solar illumination. Detailed optical studies reveal the synergetic photophysical pathways arising in FAPbBr3/TiO2 composites. An experimentally supported model rationalizing the large conversion enhancement over the pure constituents shows that this strategy offers new prospects for metal halide perovskites in photocatalytic applications.
We used single-molecule fluorescence microscopy to study self-diffusion of a feedstock-like probe molecule with nanometer accuracy in the macropores of a micrometer-sized, real-life fluid catalytic cracking (FCC) particle. Movies of single fluorescent molecules allowed their movement through the pore network to be reconstructed. The observed tracks were classified into three different states by machine learning and all found to be distributed homogeneously over the particle. Most probe molecules (88%) were immobile, with the molecule most likely being physisorbed or trapped; the remainder was either mobile (8%), with the molecule moving inside the macropores, or showed hybrid behavior (4%). Mobile tracks had an average diffusion coefficient of D = 8 × 10–14 ± 1 × 10–13 m2 s–1, with the standard deviation thought to be related to the large range of pore sizes found in FCC particles. The developed methodology can be used to evaluate, quantify and map heterogeneities in diffusional properties within complex hierarchically porous materials.
Rationale: Ambient air pollution, including black carbon, entails a serious public health risk because of its carcinogenic potential and as climate pollutant. To date, an internal exposure marker to black carbon particles having cleared from the circulation into the urine does not exist. We developed and validated a novel method to measure black carbon particles in a label-free way in urine. Methods: We detected urinary carbon load in 289 children (aged 9-12 years) using white-light generation under femtosecond pulsed laser illumination. Children’s residential black carbon concentrations were estimated based on a high-resolution spatial temporal interpolation method. Measurements and Main Results: We were able to detect urinary black carbon in all children, with an overall average (SD) of 98.2 x 105 (29.8 x 105) particles/mL . The urinary black carbon load was positively associated with medium-term up to chronic (one month or more) residential black carbon exposure, i.e. +5.33 x 105 particles/mL higher carbon load (95% CI: 1.56 x 105 to 9.10 x 105particles/mL) for an interquartile range (IQR) increment in annual residential black carbon exposure. Consistently, children who lived closer to a major road (≤ 160 m) had higher urinary black carbon load (6.93 x 105 particles/mL; 95% CI: 0.77 x 105 to 13.1 x 105 )). Conclusions: Urinary black carbon mirrors the accumulation of medium-term up to chronic exposure to combustion-related air pollution. This specific biomarker reflects internal systemic black carbon particles, cleared from the circulation into the urine, providing its utility to unravel the complexity of particulate-related health effects.
The performance of zeolites as solid acid catalysts is strongly influenced by the accessibility of active sites. However, synthetic zeolites typically grow as complex aggregates of small nanocrystallites rather than perfect single crystals. The structural complexity must therefore play a decisive role in zeolite catalyst applicability. Traditional tools for the characterization of heterogeneous catalysts are unable to directly relate nanometer-scale structural properties to the corresponding catalytic performance. In this work, an innovative correlative super-resolution fluorescence and scanning electron microscope is applied, and the appropriate analysis procedures are developed to investigate the effect of small-port H-mordenite (H-MOR) morphology on the catalytic performance, along with the effects of extensive acid leaching. These correlative measurements revealed catalytic activity at the interface between intergrown H-MOR crystallites that was assumed inaccessible, without compromising the shape selective properties. Furthermore, it was found that extensive acid leaching led to an etching of the originally accessible microporous structure, rather than the formation of an extended mesoporous structure. The associated transition of small-port to large-port H-MOR therefore did not render the full catalyst particle functional for catalysis. The applied characterization technique allows a straightforward investigation of the zeolite structure–activity relationship beyond the single-particle level. We conclude that such information will ultimately lead to an accurate understanding of the relationship between the bulk scale catalyst behavior and the nanoscale structural features, enabling a rationalization of catalyst design.
In this report we reveal the presence of significant variations in Brønsted catalytic activity within and between individual H-ZSM-5 zeolite crystals. Fluorescence microscopy in combination with a fluorogenic probe was used to resolve the catalytic activity at the nanoscale. The observed variations in catalytic activity could be directly linked to structural parameters and crystal morphology observed in scanning electron microscopy and by specifically staining crystal defects. The obtained results are directly compared with ensemble averaged information from techniques such as pyridine IR spectroscopy and nitrogen physisorption, typically used to characterize acid zeolites. The inter- and intra-particle heterogeneities resolved by the employed fluorescence approach remain unaddressed by bulk characterization. Our experimental results relate the heterogeneous catalytic activity to variation in both the Si/Al ratio and mesoporosity induced during the zeolite synthesis.
Understanding the role of the hierarchical pore architecture of SSZ-13 zeolites on catalytic performance in the Methanol-to-Olefins (MTO) reaction is crucial for guide the design of better catalysts. We investigated the influence of the space velocity on the performance of a microporous SSZ-13 zeolite, and several hierarchically structured SSZ-13 zeolites. Single catalytic turnovers, as recorded with fluorescence microscopy verified that the hierarchical zeolites contain pores larger than the 0.38 nm apertures native to SSZ-13 zeolite. The amount of fluorescent events correlated well with the additional pore volume available due to hierarchical structuring of the zeolite. Positron Emission Tomography (PET) using 11C-labelled methanol was used to map the 2D spatial distribution of the deposits formed during the MTO reaction in the catalyst bed. PET imaging demonstrates that hierarchical structuring not only improves the utilization of the available microporous cages of SSZ-13 but also that the aromatic hydrocarbon pool species are involved in more turnovers before they condense into larger multi-ring structures that deactivate the catalyst.
Controlling the morphology of organolead halide perovskite crystals is crucial to a fundamental understanding of the materials and to tune their properties for device applications. Here, we report a facile solution-based method for morphology-controlled synthesis of rod-like and plate-like organolead halide perovskite nanocrystals using binary capping agents. The morphology control is likely due to an interplay between surface binding kinetics of the two capping agents at different crystal facets. By high-resolution scanning transmission electron microscopy, we show that the obtained nanocrystals are monocrystalline. Moreover, long photoluminescence decay times of the nanocrystals indicate long charge diffusion lengths and low trap/defect densities. Our results pave the way for large-scale solution synthesis of organolead halide perovskite nanocrystals with controlled morphology for future device applications.
See also highlight in ChemistryViews