Abstract The development of improved zeolite materials for applications in separation and catalysis requires understanding of mass transport. Herein, diffusion of single molecules is tracked in the straight and sinusoidal channels of the industrially relevant ZSM‐5 zeolites using a combination of single‐molecule localization microscopy and uniformly oriented zeolite thin films. Distinct motion behaviors are observed in zeolite channels with the same geometry, suggesting heterogeneous guest–host interactions. Quantification of the diffusion heterogeneities in the sinusoidal and straight channels suggests that the geometry of zeolite channels dictates the mobility and motion behavior of the guest molecules, resulting in diffusion anisotropy. The study of hierarchical zeolites shows that the addition of secondary pore networks primarily enhances the diffusivity of sinusoidal zeolite channels, and thus alleviating the diffusion limitations of microporous zeolites.
Abstract To understand how hierarchically structured functional materials operate, analytical tools are needed that can reveal small structural and chemical details in large sample volumes. Often, a single method alone is not sufficient to get a complete picture of processes happening at multiple length scales. Here we present a correlative approach combining three-dimensional X-ray imaging techniques at different length scales for the analysis of metal poisoning of an individual catalyst particle. The correlative nature of the data allowed establishing a macro-pore network model that interprets metal accumulations as a resistance to mass transport and can, by tuning the effect of metal deposition, simulate the response of the network to a virtual ageing of the catalyst particle. The developed approach is generally applicable and provides an unprecedented view on dynamic changes in a material’s pore space, which is an essential factor in the rational design of functional porous materials.
Metallocenes can catalyze the polymerization reaction of olefins. Prior to reaction, they require activation on the surface of solid activator particles. The active component in solid activators is methylaluminoxane (MAO). When MAO is heterogeneously dispersed over the particles, some particles have a homogeneous MAO distribution, whereas others only have an MAO-shell. Only the homogeneous particles are capable of activating the metallocenes. Using Energy-Dispersive X-Ray spectroscopy, the interparticle heterogeneity can be assessed and allowing for prediction of the catalytic activity. More information can be found in the Full Paper by B. M. Weckhuysen, et al. on page 11944.
Abstract Millimeter- and micrometer-sized plastics are well-documented in aquatic ecosystems, but little is known about sub-micrometer particles because conventional analytical techniques lack sufficient spatial resolution or the spectroscopic means to unambiguously identify individual nanometer-sized plastic particles. We combined the spatial resolution of atomic force microscopy with chemical information from infrared spectroscopy to detect, identify, and count nanoplastics down to 20 nm in diameter in samples from different depths in the South Atlantic Ocean. We present evidence for the presence of polyethylene terephthalate (PET) nanoplastics in different states of degradation at 5000 m. Using lab-based ageing of PET, we demonstrate that nanoplastics can form even without light or interaction with the plastisphere, and that macroscopic PET items are a plausible source of PET nanoplastics in the ocean.
The nature and evolution of the hydrocarbon pool (HP) species during the Methanol-to-Olefins (MTO) process for three small-pore zeolite catalysts, with a different framework consisting of large cages interconnected by small eight-ring windows (CHA, DDR, and LEV) was studied at reaction temperatures between 350 and 450 °C using a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that small differences in cage size, shape, and pore structure of the zeolite frameworks result in the generation of different hydrocarbon pool species. More specifically, it was found that the large cage of CHA results in the formation of a wide variety of hydrocarbon pool species, mostly alkylated benzenes and naphthalenes. In the DDR cage, 1-methylnaphthalene is preferentially formed, while the small LEV cage generally contains fewer hydrocarbon pool species. The nature and evolution of these hydrocarbon pool species was linked with the stage of the reaction using a multivariate analysis of the operando UV-vis spectra. In the 3-D pore network of CHA, the reaction temperature has only a minor effect on the performance of the MTO catalyst. However, for the 2-D pore networks of DDR and LEV, an increase in the applied reaction temperature resulted in a dramatic increase in catalytic activity. For all zeolites in this study, the role of the hydrocarbon species changes with reaction temperature. This effect is most clear in DDR, in which diamantane and 1-methylnaphthalene are deactivating species at a reaction temperature of 350 °C, whereas at higher temperatures diamantane formation is not observed and 1-methylnaphthalene is an active species. This results in a different amount and nature of coke species in the deactivated catalyst, depending on zeolite framework and reaction temperature.
Operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) were performed on a Co/TiO2 Fischer–Tropsch synthesis (FTS) catalyst at 16 bar for (at least) 48 h time-on-stream in both a synchrotron facility and a laboratory-based X-ray diffractometer. Cobalt carbide formation was observed earlier during FTS with operando XAS than with XRD. This apparent discrepancy is due to the higher sensitivity of XAS to a short-range order. Interestingly, in both cases, the product formation does not noticeably change when cobalt carbide formation is detected. This suggests that cobalt carbide formation is not a major deactivation mechanism, as is often suggested for FTS. Moreover, no cobalt oxide formation was detected by XAS or XRD. In other words, one of the classical proposals invoked to explain Co/TiO2 catalyst deactivation could not be supported by our operando X-ray characterization data obtained at close to industrially relevant reaction conditions. Furthermore, a bimodal cobalt particle distribution was observed by high-angle annular dark-field scanning transmission electron microscopy and energy-dispersive X-ray analysis, while product formation remained relatively stable. The bimodal distribution is most probably due to the mobility and migration of the cobalt nanoparticles during FTS conditions.
In this work, three industrially relevant zeolites with framework topologies of MOR, FAU and FER have been explored on their ability to form an AlPO4 phase by reaction of a phosphate precursor with expelled framework aluminum. A detailed study was performed on zeolite H-mordenite, using in situ STXM and soft X-ray absorption tomography, complemented with (27)Al and (31)P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, XRD, FT-IR spectroscopy, and N2 physisorption. Extraframework aluminum was extracted from steam-dealuminated H-mordenite and shown to dominantly consist of amorphous AlO(OH). It was found that phosphoric acid readily reacts with the AlO(OH) phase in dealuminated H-mordenite and forms an extraframework amorphous AlPO4 phase. It was found that while AlPO4 crystallizes outside of the zeolitic channel system forming AlPO4 islands, AlPO4 that remains inside tends to stay more amorphous. In the case of ultrastable zeolite Y the FAU framework collapsed during phosphatation, due to extraction of framework aluminum from the lattice. However, using milder phosphatation conditions an extraframework AlPO4 α-cristobalite/tridymite phase could also be produced within the FAU framework. Finally, in steamed zeolite ferrierite with FER topology the extraframework aluminum species were trapped and therefore not accessible for phosphoric acid; hence, no AlPO4 phase could be formed within the structure. Therefore, the parameters to be taken into account in AlPO4 synthesis are the framework Si/Al ratio, stability of framework aluminum, pore dimensionality and accessibility of extraframework aluminum species.
We demonstrate an on-chip particle activity sorter, focused on iron concentration and based on magnetophoresis. This device was used for fast sorting of stepwise homogenously distributed [Fe]s. The preliminary results are very encouraging. We show that we can sort particles on magnetic moment, with a spread of a factor of four. XRF measurements confirm that the spread in magnetic moment is due to an increase in [Fe] concentration. This results fits well with particle trajectory simulations.
Dopant depth profiling and dose determination are essential for ultrashallow junction technology development. However they pose a challenge to the widely used dynamic secondary ion mass spectroscopy (SIMS) technique that suffers uncertainties due to an initial transient width comparable to the dopant depth distribution. In this work the authors report on the application of grazing incidence x-ray fluorescence (GIXRF) for arsenic in silicon dose and profile determination and its combination with SIMS in order to try to overcome the limitations of the latter in the topmost few nanometers. A polynomial variation of the sputtering rate is supposed in the first sputtering stage of the SIMS analysis and the parameters that regulate the magnitude of such correction are determined by a least square fitting of the angle dependent fluorescence signal. The total retained fluence was also measured by instrumental neutron activation analysis and synchrotron radiation soft x-ray GIXRF. The comparison among the total retained fluence determinations shows a good agreement among the techniques. Furthermore, from this first set of measurements it was clearly shown that the GIXRF profile correction is very sensitive to the SIMS profile in the very first nanometers. Therefore if matrix effects are present in the SIMS analysis beside the sputtering rate change, the tested sputtering rate correction can produce nonreliable profiles.
