Experiments on the Artificial Disturbance Evolution in 2D and 3D Spanwise Modulated Boundary Layers at Mach 2 and 2.5

AbstractExperimental data on the nonlinear wave train development in 3D supersonic boundary layer over swept wing and experimental investigation of wave train development in spanwise modulated boundary layer on flat plate and swept wing at Mach number 2 and 2.5 are presented. Artificial disturbances in the boundary layer were excited by periodical glow discharge mainly at 10 and 20kHz. It was found, that some disturbance excitation downstream cannot be explained by linear stability theory. It was obtained that receptivity of supersonic boundary layer to the surface excited controlled pulsations and the wave train development essentially depends on the Mach number

Al Promotion of In2O3 for CO2 Hydrogenation to Methanol

In2O3 is a promising catalyst for the hydrogenation of CO2 to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In2O3 using flame spray pyrolysis to prepare a series of In2O3−Al2O3 samples in a single step (0−20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In2O3 lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO2 adsorption and methanol formation. The rest of Al is present as small Al2O3 domains at the In2O3 surface, blocking the active sites for CO2 hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol %Al), the particle size of In2O3 decreases due to the stabilizing effect ofAl2O3. Nevertheless, these smaller particles are prone to sintering duringCO2 hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such asAl on the reducibility and texture of In2O3 as a CO2 hydrogenation catalyst

Ni and ZrO₂ promotion of In₂O₃ for CO₂ hydrogenation to methanol

Transition metals, such as Ni, Pd, and Pt, and ZrO2 are known as efficient promoters in M-In2O3-ZrO2 catalysts for CO2 hydrogenation to methanol. Herein, we systematically investigated the role of Ni and ZrO2 promoters by preparing ternary NiO-In2O3-ZrO2 catalysts and binary counterparts by flame spray pyrolysis. The highest methanol rate was obtained for the Ni(6 wt%)-In2O3(31 wt%)-ZrO2(63 wt%) composition. DRIFTS-SSITKA shows that formate is the key intermediate in the hydrogenation of CO2 to methanol. Kinetic analysis shows the competition between methanol and CO formation. The rate-limiting step in methanol formation is likely the hydrogenation of surface methoxy species. Ni and ZrO2 play different promoting roles without showing synergy with respect to each other. Ni promotes hydrogenation of surface formate and methoxy species, while ZrO2 maintains a high In2O3 dispersion, the smaller In2O3 size likely stabilizing formate and other intermediates during their conversion to methanol.</p

Isomorphously Substituted [Fe,Al]ZSM-5 Catalysts for Methane Dehydroaromatization

Dehydroaromatization of methane (MDA) under non-oxidative conditions is a promising reaction for direct valorization of natural gas and biogas. Typically, Fe-modified ZSM-5 catalysts display low aromatic productivity and high coke selectivity in the MDA reaction. Herein, we show the benefit of starting from isomorphously substituted Fe-sites in [Fe,Al]ZSM-5 zeolites prepared by direct hydrothermal synthesis. Upon calcination, these samples contain predominantly isolated Fe3+ species, either atomically dispersed within the zeolite framework or anchored at exchange sites inside zeolite channels. In terms of the integral hydrocarbon productivity, [Fe,Al]ZSM-5 catalysts outperform Fe/ZSM-5, prepared by impregnation, as well as Mo/ZSM-5 catalysts with the same Si/Al ratio and molar metal loading. Operando X-ray absorption spectroscopy coupled with mass spectrometry (XANES-MS) demonstrates that the initial tetrahedral Fe3+ within the zeolite framework or at exchange sites are transformed into octahedral extraframework Fe2+ active sites during the MDA reaction and form small Fe2O3 clusters during oxidative regeneration. Combining activity measurements and operando thermogravimetry shows that the duration of the induction period, related to the formation of active hydrocarbon pool intermediates, strongly depends on the Fe dispersion and loading and can be used as a suitable descriptor for the MDA activity of [Fe,Al]ZSM-5. The shorter induction period of [Fe,Al]ZSM-5 in comparison to impregnated Fe/ZSM-5 can be linked to the higher methane conversion rate over highly dispersed Fe-sites and faster formation of active hydrocarbon pool intermediates.</p

Direct conversion of methane to zeolite-templated carbons, light hydrocarbons, and hydrogen

Efficient direct valorization of methane to value-added chemicals and materials remains an unsolved challenge for modern chemistry and materials science. In this work, we explored direct non-oxidative conversion of methane to hydrogen, hydrocarbons, and valuable zeolite-templated carbon materials. First, using a set of spectroscopy and microscopy characterization tools, we investigated how the reaction conditions influence the process of carbon formation inside the typical zeolite-based catalyst Mo/ZSM-5. Then, we explored the effect of the zeolite topology on the growth of carbon materials from methane decomposition over Mo/MFI, Mo/MOR, Mo/BEA, and Mo/FAU templates. Finally, we applied the obtained insights to prepare high-quality zeolite-templated carbons directly from methane using Fe/BEA and Fe/FAU templates. Altogether, our results represent a carbon-economic method of methane conversion to COx-free hydrogen gas, a mixture of light aliphatic and aromatic hydrocarbons, and valuable carbon materials

Structure sensitivity of CO2 hydrogenation on Ni revisited

Despite the large number of studies on the catalytic hydrogenation of CO2 to CO and hydrocarbons by metal nanoparticles, the nature of the active sites and the reaction mechanism have remained unresolved. This hampers the development of effective catalysts relevant to energy storage. By investigating the structure sensitivity of CO2 hydrogenation on a set of silica-supported Ni nanoparticle catalysts (2–12 nm), we found that the active sites responsible for the conversion of CO2 to CO are different from those for the subsequent hydrogenation of CO to CH4. While the former reaction step is weakly dependent on the nanoparticle size, the latter is strongly structure sensitive with particles below 5 nm losing their methanation activity. Operando X-ray diffraction and X-ray absorption spectroscopy results showed that significant oxidation or restructuring, which could be responsible for the observed differences in CO2 hydrogenation rates, was absent. Instead, the decreased methanation activity and the related higher CO selectivity on small nanoparticles was linked to a lower availability of step edges that are active for CO dissociation. Operando infrared spectroscopy coupled with (isotopic) transient experiments revealed the dynamics of surface species on the Ni surface during CO2 hydrogenation and demonstrated that direct dissociation of CO2 to CO is followed by the conversion of strongly bonded carbonyls to CH4. These findings provide essential insights into the much debated structure sensitivity of CO2 hydrogenation reactions and are key for the knowledge-driven design of highly active and selective catalysts

Ni-In Synergy in CO2Hydrogenation to Methanol

Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In2O3. NiO-In2O3was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3and NiO end members have similar high specific surface areas and morphology. The main products of CO2hydrogenation are CH3OH and CO with CH4being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcath), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2is mainly due to low-barrier H2dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2on Ov © 2021 The Authors. Published by American Chemical Societ

Pt/CeO2 as Catalyst for Non-Oxidative Coupling of Methane:Oxidative Regeneration

Direct non-oxidative coupling is a promising route for methane upgrading, yet its commercialization is hindered by the lack of efficient catalysts. Pt/CeO2 catalysts with isolated Pt species have attracted increasing interest in recent years. Herein, we studied the catalytic role and evolution of isolated Pt centers on CeO2 prepared by flame spray pyrolysis under the harsh reaction conditions of non-oxidative methane coupling. During the reaction at 800 °C, the isolated Pt sites sinter leading to a loss of the ethylene and ethane yield. The agglomerated Pt can be redispersed by using an in situ regeneration strategy in oxygen. We found that isolated Pt centers are only able to activate methane at the initial reaction stage, and the CePt5 alloy acts as the active phase in the prolonged reaction