ACS Engineering Au最新文献

Porous structures can be favorably used in solar thermochemical reactors for the volumetric absorption of concentrated solar radiation. In contrast to isotropic porous topologies, hierarchically ordered porous topologies with stepwise optical thickness enable more homogeneous radiative absorption within the entire volume, leading to a higher and more uniform temperature distribution and, consequently, a higher solar fuel yield. However, their design and optimization require fast and accurate numerical tools for solving the radiative exchange at the pore level within their complex architectures. Here, we present a novel voxel-based Monte Carlo ray-tracing algorithm that discretizes the pore-level domain into a 3D binary digital representation of solid/void voxels. These are exposed to stochastic rays undergoing reflection, absorption, and re-emission at the ray-solid intersection found by querying the voxel value along the ray path. Temperature distributions are found at radiative equilibrium. The algorithm’s fast execution allows its use in a gradient-free optimization scheme. Three hierarchically ordered topologies with parametrized shapes (square grids, Voronoi cells, and sphere lattices) exposed to 1000 suns radiative flux are optimized for maximum solar fuel production based on the thermodynamics of a ceria-based thermochemical redox cycle for splitting H₂O and CO₂. The optimized graded-channeled structure with square grids achieves a 4-fold increase in the volume-specific fuel yield compared to the value obtained for an isotropic reticulated porous structure.

Chemoselective hydrogenation of α-β unsaturated hydrocarbons is a widely studied chemical transformation. In this study, hydrogenation of cinnamaldehyde (CAL) to the corresponding products, viz hydrocinnamaldehyde (HCAL) and hydrocinnamyl alcohol (HCOL) and cinnamyl alcohol (COL), over the different exposed facets of a Pd-based catalyst is studied. The Pd octahedra having (111) facet shows 90% selectivity toward HCAL with 100% conversion in a short duration (45 min). Pd cube having (100) facet shows selectivity (55%) toward HCOL, while Pd spheres show initial selectivity toward HCAL but to HCOL over a prolonged reaction period. The experimental results are corroborated by density functional theory (DFT) calculations, wherein we observe a lower activation barrier E_a = 51 kJ/mol for HCAL formation on the Pd(111) surface. However, an alternative route through the COL intermediate is more prominent on the Pd(100) surface.

Computational fluid dynamics (CFD) modeling plays a pivotal role in optimizing fixed bed catalytic chemical reactors to enhance performance but must accurately capture the various length- and time-scales that underpin the complex particle–fluid interactions. Within catalytic particles, a range of pore sizes exist, with micro-pore scales enhancing the active surface area for increased reactivity and macro-pore scales enhancing intraparticle heat and mass transfer through intraparticle convection. Existing particle-resolved CFD models primarily approach such dual-scale particles with low intraparticle macro-porosities as purely solid. Consequently, intraparticle phenomena associated with intraparticle convection are neglected, and their impact in the full bed scale is not understood. This study presents a porous particle CFD model, whereby individual particles are defined through two distinct porosity terms, a macro-porosity term responsible for the particle’s hydrodynamic profile and a micro-porosity term responsible for diffusion and reaction. By comparing the flow profiles through full beds formed by porous and solid particles, the impact of intraparticle convection on mass and heat transfer, as well as on diffusion and reaction, was investigated.

The activity and stability of bimetallic Pt–Ir nanoparticles supported on an Al₂O₃/ZSM-5 mixture were investigated as a function of pretreatment and regeneration conditions for butane hydrogenolysis to ethane. Catalyst characterization by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy before and after aging under butane hydrogenolysis conditions for 12 weeks confirmed that the bimetallic nanoparticles were resistant to sintering, coking, and bulk metal segregation. However, for catalysts that were pretreated through an initial H₂ reduction, n-butane conversion decreased from 68 to 34% after 12 days on stream while maintaining ∼76% selectivity to ethane. A specific regeneration (or pretreatment) protocol was identified, involving the exposure of the oxidized catalyst to a butane and hydrogen mixture followed by post-reduction, which recovered the catalyst activity and enhanced catalyst stability such that n-butane conversion decreased <5% after 6 days on stream. The influence of various treatments on the structure and surface composition of the bimetallic nanoparticles was hypothesized based on analysis of in situ and cryogenic CO probe-molecule diffuse reflectance infrared Fourier transform spectroscopy measurements. Based on this analysis, it was inferred that high-temperature H₂ treatment of oxidized catalysts resulted in intraparticle segregation into a Pt shell and Ir core that was detrimental to long-term catalyst performance. The core–shell structure was reversible upon catalyst oxidation in O₂, forming an oxidized Ir (IrO_x) shell and Pt core. Treatment of the oxidized catalyst with a butane and H₂ mixture deposited CO and hydrocarbon adsorbates on the IrO_x shell, which stabilized Ir on the nanoparticle surface, even under reductive conditions. Post-reduction in H₂ restored the initial n-butane conversion with improved catalyst stability due to the adsorbate-stabilized, Ir-enriched surface. Therefore, carefully designed pretreatment protocols that deposit stable spectator adsorbates are presented as a valuable tool for controlling the surface composition of bimetallic nanoparticles under reaction conditions to improve their catalytic performance.

Hydrogen is the key component in terms of energy economy, and electrochemical water splitting is one of the most important strategies to replace the widely used fossil fuels. The search for efficient electrocatalysts toward water splitting for hydrogen generation is very important. Transition metal-based chalcogenides have great attraction as efficient electrocatalysts due to their high conductivity, distinct valence electron configuration, and different surface morphological nano/microstructures. In this Review, recently developed transition metal-based chalcogenides (S, Se, and Te) as electrocatalysts toward hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting have been discussed.

Catalytic NH₃ synthesis is a well-studied reaction, but its use in renewable energy storage is difficult due to the need for small-scale production, requiring greatly reduced operating temperatures and pressures. NH₃ inhibition on supported Ru catalysts becomes more prevalent at low temperatures, decreasing the reaction rates. In addition, promoter species are prone to oxidation at lower temperatures, further depressing the reaction rate. In situ NH₃ removal techniques have the potential to enhance NH₃ synthesis under milder conditions to combat both NH₃ inhibition and thermodynamic limitations, while the regeneration of the adsorber can potentially reactivate promoter species. The deactivation event of 5 wt % Ru/CeO₂ (3.9 nm average Ru particle size) was first explored in detail, and it was found that slight oxidation of Ce³⁺ promoter species is the major cause of deactivation at lower temperatures, which is easily restored by high-temperature H₂ treatment. Ru/CeO₂ was then mixed with zeolite 4A, a substance showing favorable NH₃ capacity under mild reaction conditions. In situ adsorption of NH₃ significantly increased the reaction rate of Ru/CeO₂ at 200 °C with 5 kPa H₂ and 75 kPa N₂, where the reaction rate increased from 128 to 565 μmol g^–1 h^–1 even at low H₂ conversions of 0.25% (average NH₃ yield of 0.01%). The temperature swings that were utilized to measure NH₃ uptake on zeolite 4A were also found to provide a reactivation event for Ru/CeO₂. In situ NH₃ removal went beyond equilibrium limitations, achieving H₂ conversions up to 98%. This study sheds light on the kinetics of the use of in situ NH₃ removal techniques and provides insight into future designs utilizing similar techniques.

The effects of co-feeding CO₂ and methane on the performance of La_0.8Sr_0.2FeO₃ (LSF) were studied with different CO₂ concentrations. The reaction was conducted in chemical looping mode at 900 °C and a weight hourly space velocity (WHSV; g methane/g catalyst/h) of 3 h^–1 during 15 min reduction (10 mol % methane with 0–1.8% CO₂ in nitrogen) and 10 min oxidation (10 mol % oxygen in nitrogen) cycles. Analyses of X-ray diffraction and X-ray photoelectron spectroscopy data of spent materials indicated that CO₂ reacts with the oxygen vacancies on the LSF surface during methane reduction, increasing CO selectivity in POM. As the CO₂ feed concentration increased to an optimal value (1.6% CO₂), the CO selectivity increased to 94%. Under those conditions, the EOR (extent of reduction) of LSF, defined as the amount of oxygen depleted from the lattice, was 0.18–0.15 mmol/min·g_cat. Reducing the EOR to 0.09–0.08 mmol/min·g_cat (1.8% CO₂) led to partial methane combustion. These results were confirmed by altering the operating conditions (WHSV = 2 and 1 h^–1, T = 950 °C) and CO₂ feed concentrations while extending the reduction time. Operation in an optimal EOR range (0.17–0.10 mmol/min·gcat) that enabled optimal CO selectivity (>90%) was obtained without oxidative regeneration for the 18 h reduction time.

An experimental study of the impact breakage of a single tapioca grain using an air jet mill is carried out. High-velocity jets at the circumference of the cylindrical grinding chamber propel the grain tangentially, resulting in numerous collisions with the cylinder walls prior to breakage. Videography and image analysis are used to obtain the trajectory of the particle and the sizes of the fragments. Each experiment is repeated 25 times at three different grinding jet pressures (1, 1.5, and 2 bar). The average collision rate and the average breakage times are nearly constant for the higher pressures at 1000 1/s and 0.18 s, respectively. The size distribution at the end of the experiment, obtained using a laser particle size analyzer, is trimodal. The probability of first breakage versus the cumulative specific kinetic energy of impacts is shown to follow the Vogel–Peukert equation (Powder Technology 2003, 129, 101–110).

Stable foams that can resist disproportionation for extended periods of time have important applications in a wide range of technological and consumer materials. Yet, legislative initiatives limit the range of surface active materials that can be used for environmental impact reasons. There is a need for technologies to efficiently produce multiphase materials using more eco-friendly components, such as particles, and for which traditional thermodynamics-based processing routes are not necessarily efficient enough. This work describes an innovative foaming technology that can produce ultrastable Pickering-Ramsden foams, with bubbles of micrometer-sized dimensions, through pressure-induced particle densification. Specifically, aqueous nanosilica-stabilized foams are produced by foaming a suspension at subatmospheric pressures, allowing for adsorption of the particles onto large bubbles. This is followed by an increase back to atmospheric pressure, which induces bubble shrinkage and compresses the adsorbed particle interface, forming a strong elastoplastic network that provides mechanical resistance against disproportionation. The foam’s interfacial mechanical properties are quantified to predict the range of processing conditions needed to produce permanently stable foams, and a general stability criterion is derived by considering the interfacial rheological properties under slow, unidirectional compression. Foams that are stable against disproportionation are characterized by interfaces whose mechanical resistance to compressive deformations can withstand their tendency to minimize the interfacial stress by reducing their surface area. Our ultrastable nanosilica foams are tested in real-life applications by introducing them into concrete. In comparison to other commercial air entrainers, our microfoam improves concrete’s freeze–thaw resistance while supplying higher material strength, providing an economically attractive, industrially scalable, and durable alternative for use in real-life applications involving cementitious materials. The applicability of our stability criterion to other rheologically complex interfaces and the versatile nature of our foaming technology enables usage for a broad class of materials, beyond the construction industry.

In the efforts to corroborate safer environmental CO₂ mitigation strategies, herein, we elucidate engineered practices that convert the absorbed CO₂ in a solid material and its utilization in the path of product synthesis. In this way, the cheaper lime material, the primary calcium resource, when exposed to CO₂ capture, and the storage material (CO₂CSM) prepared by using 1,2-ethylenediamine and 1, 4-butanediol resulted in the formation of controlled vaterite and aragonite CaCO₃ polymorphs in their respective pure forms mediated by the functionalized CO₂CSM. The investigation studies demonstrated that the obtained CO₂CSM under the supercritical CO₂ state has a higher uptake and release efficiency of CO₂ equivalent to 3.730 and 3.17 mmol/g, respectively. Therefore, the conversion of raw materials depended on the amount of CO₂CSM availed in the reaction and would be complete at the expense of supercritical CO₂CSM in the solid-type reaction. The mechanism study explains the fundamental formation of products correlating to the amount of CO₂CSM supplied in the reaction which would initiate the reaction, while the amine functional group of the material could stabilize and effectively control the transition of vaterite to aragonite phases of CaCO₃. The so-obtained CaCO₃ phases were tested for their antiwear and friction stability of the lubricant 500SN; vaterite and aragonite demonstrated good reinforcement of the mechanical properties of lubricants compared to the calcite type. Therefore, this system proposes a validation platform of using sequestrated CO₂ to generate products with industrial commercialization benefits in the reinforcement of organic-based lubricants.