Structure sensitivity of silver-catalyzed ethylene epoxidation

The influence of particle size (20-200 nm) of Ag/α-Al 2O 3 catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al 2O 3 support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups. </p

Structure sensitivity of silver-catalyzed ethylene epoxidation

The influence of particle size (20-200 nm) of Ag/α-Al 2O 3 catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al 2O 3 support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups. </p

The Physics of Star Cluster Formation and Evolution

© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe

Structure sensitivity of silver-catalyzed ethylene epoxidation

The influence of particle size (20-200 nm) of Ag/α-Al 2O 3 catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al 2O 3 support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups

Clinical outcomes following DAA therapy in patients with HCV-related cirrhosis depend on disease severity

Background & Aims: HCV-infected patients with cirrhosis achieve high sustained virological response (SVR) rates with direct-acting antivirals (DAAs) even after hepatic decompensation. We aimed to assess the clinical outcome following DAAs among patients with compensated and decompensated cirrhosis in relation to SVR and changes in model for end-stage liver disease (MELD) score. Methods: Consecutive DAA-treated chronic HCV-infected patients with cirrhosis from 4 hepatology clinics were included. The primary endpoint in survival analyses was clinical disease progression, defined as liver failure, hepatocellular carcinoma, liver transplantation or death. Results: In total, 868 patients were included with a median age of 59 (IQR 54–65) years; 719 (83%) with Child-Pugh A cirrhosis and 149 (17%) with Child-Pugh B/C cirrhosis. SVR was attained by 647 (90%) Child-Pugh A patients and 120 (81%) Child-Pugh B/C patients. During a median follow-up of 28 (IQR 20–36) months, 102 (14%) Child-Pugh A patients and 96 (64%) Child-Pugh B/C patients experienced clinical disease progression. SVR was independently associated with an improved event-free survival in patients with Child-Pugh A cirrhosis (adjusted hazard ratio [HR] 0.47; 95% CI 0.27–0.82, p = 0.007), but not in patients with Child-Pugh B/C cirrhosis (adjusted HR 1.23; 95% CI 0.67–2.26; p = 0.51). Twelve weeks post-DAAs, 28 (19%) patients with Child-Pugh B/C cirrhosis had ≥2-point MELD decline, but their 2-year event-free survival did not differ from those with a stable MELD (47.9%; 95% CI 28.7–67.1 vs. 48.9%; 95% CI 38.1–59.7, respectively, p = 0.99). Conclusions: Among patients with chronic HCV infection, DAA-induced SVR was associated with a reduced risk of clinical disease progression in patients with Child-Pugh A cirrhosis but not in patients with Child-Pugh B/C cirrhosis. In Child-Pugh B/C cirrhosis, a ≥2-point MELD decline did not translate into improved clinical outcome. Lay summary: Chronic HCV infection can be cured with antiviral therapy. In this study, we evaluated the long-term effects of antiviral therapy on liver-related complications in patients with cirrhosis. Our results suggest that patients with compensated cirrhosis who were cured of their HCV infection have a lower rate of complications. In contrast, the rate of complications was not related to virological cure among those with decompensated cirrhosis. While these patients seem to remain in need of liver transplantation, antiviral therapy may lower their priority on the liver transplantation waiting list