Luminescence of nanocrystalline ZnS:Cu2+

\u3cp\u3eTemperature dependent luminescence and luminescence lifetime measurements are reported for nanocrystalline ZnS:Cu\u3csup\u3e2+\u3c/sup\u3e particles. Based on the variation of the emission wavelength as a function of particle size (between 3.1 and 7.4 nm) and the low quenching temperature (T\u3csub\u3eq\u3c/sub\u3e = 135 K), the green emission band is assigned to recombination of an electron in a shallow trap and Cu\u3csup\u3e2+\u3c/sup\u3e. The reduction in lifetime of the green emission (from 20 μs at 4 K to 0.5 μs at 300 K) follows the temperature quenching of the emission. In addition to the green luminescence, a red emission band, previously only reported for bulk ZnS:Cu\u3csup\u3e2+\u3c/sup\u3e, is observed. The red emission is assigned to recombination of a deeply trapped electron and Cu\u3csup\u3e2+\u3c/sup\u3e. The lifetime of the red emission is longer (about 40 μs at 4 K) and the quenching temperature is higher.\u3c/p\u3

Synthesis of highly-uniform titania overcoats on a mesoporous alumina catalyst support by atomic layer deposition and their application in hydroprocessing

The feasibility of gas phase deposition using a Ti alkoxide precursor for precise surface modification of catalysts was demonstrated by modifying a mesoporous alumina support with a Ti oxide overcoat. Titanium tetra-isopropoxide yields a Ti oxide layer that covers homogeneously the alumina surface. Uniformity of the deposited TiO2 was verified by SEM-EDX, on both intra-particle and inter-particle levels. Only a few atomic layer deposition (ALD) cycles were required in order to obtain Ti contents with a relevance for industrial application. The pore size distribution of the overcoated catalyst support was barely affected by the coating process. Synthesized CoMo catalysts based on the Ti-alumina carrier showed up to 40% higher activity compared to a catalyst supported on pristine alumina, in hydroprocessing under industrial testing conditions. The TiO2 coating appeared to be stable, showing no agglomeration characteristics after reaction as corroborated by TEM-EDX. ALD provides a scalable route with low waste generation for the production of precisely structured TiO2-Al2O3 hydroprocessing catalyst supports.</p

Catalytic activity in individual cracking catalyst particles imaged throughout different life stages by selective staining

Fluid catalytic cracking (FCC) is the major conversion process used in oil refineries to produce valuable hydrocarbons from crude oil fractions. Because the demand for oil-based products is ever increasing, research has been ongoing to improve the performance of FCC catalyst particles, which are complex mixtures of zeolite and binder materials. Unfortunately, there is limited insight into the distribution and activity of individual zeolitic domains at different life stages. Here we introduce a staining method to visualize the structure of zeolite particulates and other FCC components. Brønsted acidity maps have been constructed at the single particle level from fluorescence microscopy images. By applying a statistical methodology to a series of catalysts deactivated via industrial protocols, a correlation is established between Brønsted acidity and cracking activity. The generally applicable method has clear potential for catalyst diagnostics, as it determines intra- and interparticle Brønsted acidity distributions for industrial FCC materials

Electron Tomography Reveals the Active Phase–Support Interaction in Sulfidic Hydroprocessing Catalysts

Conventional two-dimensional (2D) transmission electron microscopy of sulfidic hydroprocessing catalysts can be deceiving and give the impression that parts of the support are overloaded with active phase. High-angle annular dark field scanning transmission electron microscopy tomography reveals details on the morphology of MoS₂ crystallites and their interaction with the Al₂O₃ and TiO₂ support particles. The three-dimensional (3D) reconstruction shows that the active phase is mainly present as MoS₂ single slabs of various shapes aligned with the support. It becomes clear that the surface of the support particles is, in fact, only partly covered by the active phase and the pores remain accessible for reactant molecules

Microspectroscopic insight into the deactivation process of individual cracking catalyst particles with basic sulfur components

The effect and localization of thiophene-like poisons were studied on fluid catalytic cracking (FCC) catalyst at the individual particle level. The thiophene-like poisons interact on the Brønsted acid sites of the catalytic materials, forming oligomeric carbocations and coke species, which absorb and emit light in the visible region. The matrix components are not active in the formation of those light absorbing species. In contrast, zeolite Y and ZSM-5 were very active in inducing oligomer formation and the product distribution was different depending on the zeolite pore structure. Comparison of thiophene results with alkane and alkene catalytic cracking studies reveal that FCC particles have more affinity to react with thiophene molecules compared to n-hexane, but 1-hexene may compete with thiophene in the formation of carbocationic species on Brønsted acid sites. Moreover, a different reactivity was observed in thiophenes with distinct electron withdrawing/releasing substituents and molecular sizes. Our results demonstrate that the carbocations are coke intermediates, and the FCC particles containing zeolite Y promote to a higher extent coke formation: the large supercages allow the accommodation of more bulky coke species. On the other hand, FCC particles containing ZSM-5 stabilize the carbocations within the narrower cylindrical pores, diminishing coke formation. Confocal fluorescence microscopy can resolve the location of sulfur components at the single particle level with submicron resolution. Fluorescence microscopy images reveal heterogeneous domains with highly bright fluorescence across the FCC particles, which are attributed to the selective formation of oligomeric carbocations and coke species on the zeolitic material. The presence of thiophenes with different substituents and sizes was also studied by this approach. This demonstrates the potential of confocal fluorescence microscopy to identify reactivity differences of thiophene-like molecules on FCC catalyst particles in a spatially-resolved manner. © 2012 Elsevier B.V. All rights reserved

Microspectroscopic insight into the deactivation process of individual cracking catalyst particles with basic sulfur components

The effect and localization of thiophene-like poisons were studied on fluid catalytic cracking (FCC) catalyst at the individual particle level. The thiophene-like poisons interact on the Brønsted acid sites of the catalytic materials, forming oligomeric carbocations and coke species, which absorb and emit light in the visible region. The matrix components are not active in the formation of those light absorbing species. In contrast, zeolite Y and ZSM-5 were very active in inducing oligomer formation and the product distribution was different depending on the zeolite pore structure. Comparison of thiophene results with alkane and alkene catalytic cracking studies reveal that FCC particles have more affinity to react with thiophene molecules compared to n-hexane, but 1-hexene may compete with thiophene in the formation of carbocationic species on Brønsted acid sites. Moreover, a different reactivity was observed in thiophenes with distinct electron withdrawing/releasing substituents and molecular sizes. Our results demonstrate that the carbocations are coke intermediates, and the FCC particles containing zeolite Y promote to a higher extent coke formation: the large supercages allow the accommodation of more bulky coke species. On the other hand, FCC particles containing ZSM-5 stabilize the carbocations within the narrower cylindrical pores, diminishing coke formation. Confocal fluorescence microscopy can resolve the location of sulfur components at the single particle level with submicron resolution. Fluorescence microscopy images reveal heterogeneous domains with highly bright fluorescence across the FCC particles, which are attributed to the selective formation of oligomeric carbocations and coke species on the zeolitic material. The presence of thiophenes with different substituents and sizes was also studied by this approach. This demonstrates the potential of confocal fluorescence microscopy to identify reactivity differences of thiophene-like molecules on FCC catalyst particles in a spatially-resolved manner. © 2012 Elsevier B.V. All rights reserved