Pore Size Tailoring in Large-Pore SBA-15 Silica Synthesized in the Presence of Hexane

Large-pore SBA-15 silicas were synthesized using poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer Pluronic P123 as a template and hexane as a micelle expander. The reaction was initially carried out at 15 °C, followed by the heating of the synthesis gel at temperatures from 40 to 130 °C. Small-angle X-ray scattering data indicate that highly ordered two-dimensional hexagonal material (SBA-15 structure) formed at 15 °C and was preserved even after 5 days of heating at 130 °C. The unit-cell parameter for as-synthesized SBA-15 silicas was about 16.5 nm and increased only slightly after the heat treatment, whereas the unit-cell parameter after calcination was appreciably larger (16 vs 14 nm) for materials that were subjected to the thermal treatment. The pore size distribution of SBA-15 formed at 15 °C was narrow and centered at ∼9.5 nm, which is close to the upper limit of pore diameters typically reported for SBA-15. The presence of constrictions in the pores of this material was evident. The heat treatment led to the elimination of the constrictions and to the pore diameter increase to 15 nm or more, tailored by the selection of appropriate treatment temperature and time. The pore size increase was the fastest during the first day of treatment, but it continued for at least 5 days. The pore size distribution broadened as the time of the treatment increased beyond 1 day. The pore size increase appears to be primarily related to the decrease in the degree of shrinkage during the calcination (removal of the template) and the decrease in the pore wall thickness

Pore Size Tailoring in Large-Pore SBA-15 Silica Synthesized in the Presence of Hexane

Large-pore SBA-15 silicas were synthesized using poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer Pluronic P123 as a template and hexane as a micelle expander. The reaction was initially carried out at 15 °C, followed by the heating of the synthesis gel at temperatures from 40 to 130 °C. Small-angle X-ray scattering data indicate that highly ordered two-dimensional hexagonal material (SBA-15 structure) formed at 15 °C and was preserved even after 5 days of heating at 130 °C. The unit-cell parameter for as-synthesized SBA-15 silicas was about 16.5 nm and increased only slightly after the heat treatment, whereas the unit-cell parameter after calcination was appreciably larger (16 vs 14 nm) for materials that were subjected to the thermal treatment. The pore size distribution of SBA-15 formed at 15 °C was narrow and centered at ∼9.5 nm, which is close to the upper limit of pore diameters typically reported for SBA-15. The presence of constrictions in the pores of this material was evident. The heat treatment led to the elimination of the constrictions and to the pore diameter increase to 15 nm or more, tailored by the selection of appropriate treatment temperature and time. The pore size increase was the fastest during the first day of treatment, but it continued for at least 5 days. The pore size distribution broadened as the time of the treatment increased beyond 1 day. The pore size increase appears to be primarily related to the decrease in the degree of shrinkage during the calcination (removal of the template) and the decrease in the pore wall thickness

Catalytic Activity Maps for Alloy Nanoparticles

To enable rational design of alloy nanoparticle catalysts, we develop an approach to generate catalytic activity maps of alloy nanoparticles on a grid of particle size and composition. The catalytic activity maps are created by using a quaternary cluster expansion to explicitly predict adsorbate binding energies on alloy nanoparticles of varying shape, size, and atomic order while accounting for interactions among the adsorbates. This cluster expansion is used in kinetic Monte Carlo simulations to predict activated nanoparticle structures and turnover frequencies on all surface sites. We demonstrate our approach on Pt–Ni octahedral nanoparticle catalysts for the oxygen reduction reaction (ORR), revealing that the specific activity is predicted to be optimized at an edge length of larger than 5.5 nm and a composition of about Pt0.85Ni0.15 and the mass activity is predicted to be optimized at an edge length of 3.3–3.8 nm and a composition of about Pt0.8Ni0.2

Rational Design of Pt₃Ni Surface Structures for the Oxygen Reduction Reaction

A cluster expansion approach has been used to investigate the relationship between surface structures of Pt₃Ni alloy catalysts and their catalytic activity. With the help of this approach, we have constructed a direct bridge between the atomic structure and catalytic properties of Pt–Ni catalysts at a variety of compositions and chemical environments. We predict that Pt₃Ni­(111) surfaces have substantial subsurface disorder, and as a result, the ORR activity of different surface sites varies by approximately 3 orders of magnitude. Using this model, we identify a Pt₃Ni­(111) surface with a multilayer Pt skin that is predicted to maximize catalytic activity and predict the conditions under which a Pt₃Ni surface should be synthesized to realize high catalytic activity

Theoretical Insights into the Effects of Oxidation and Mo-Doping on the Structure and Stability of Pt–Ni Nanoparticles

Pt–Ni nanoparticles are promising catalysts for the oxygen reduction reaction but they suffer from Ni dissolution in oxidizing conditions. It has recently been shown that it is possible to stabilize octahedral Pt–Ni nanoparticles by doping them with a small amount of Mo. Using ab initio calculations and a quaternary cluster expansion, we provide atomic-scale explanations for the enhanced stability of Mo-doped Pt–Ni nanoparticles. We predict that for Mo-doped Pt₃Ni nanoparticles with only a small amount of Mo doping (around 1.6% mole fraction) the equilibrium concentration of Ni atoms on the particle surface is greatly reduced, limiting the rate at which Ni atoms dissolve from the particles. Mo doping also increases Pt/Ni vacancy formation energies in the surface layer, which further stabilizes the nanoparticles against Ni dissolution and helps preserve the nanoparticle shape. Our calculations also reveal insights into the shape evolution of Pt–Ni nanoparticles: the preferential oxidation of edges can make (111) face sites more vulnerable to dissolution than edge sites, which may contribute to the observed formation of Pt–Ni nanoframes and nanoparticles with concave surfaces

Synthesis of Ultra-Large-Pore SBA-15 Silica with Two-Dimensional Hexagonal Structure Using Triisopropylbenzene As Micelle Expander

It is proposed herein that in order to obtain ultralarge-pore ordered mesoporous silicas using surfactant-templated synthesis with micelle expanders, one should select a micelle swelling agent with a moderate swelling ability to achieve an appreciable pore diameter enlargement while avoiding the formation of heterogeneous and/or poorly defined nanostructure. It is suggested to identify viable swelling agents based on the extent of solubilization of swelling agents in micellar solutions. On the basis of this reasoning, cyclohexane, 1,3,5-triethylbenzene and 1,3,5-triisopropylbenzene (TIPB) were selected for the synthesis of large-pore SBA-15 silicas with two-dimensional (2-D) hexagonal structures of cylindrical mesopores. SBA-15 with pore diameter tunable from 10 to 26 nm was obtained at initial synthesis temperature 12.25−20 °C using Pluronic P123 triblock copolymer as a micellar template and triisopropylbenzene as a micelle expander. Structures of the materials were characterized using small-angle X-ray scattering, TEM, and gas adsorption. The lowering of the initial synthesis temperature with adjustment of the amount of TIPB afforded pore diameters up to 26 nm (BJH pore diameters up to 34 nm) and (100) interplanar spacings up to 26 nm for 2-D hexagonal structure. As the initial synthesis temperature was lowered further, the pore diameter increased further (to ∼50 nm) with appreciable retention of cylindrical pore shape, but the pore structure became heterogeneous. The present approach makes silicas with 2D hexagonally ordered cylindrical pores of diameter up to 26 nm readily available and opens new opportunities in the synthesis of materials with other pore geometries and framework types

Measured Ba concentrations (mean±SD) in water (Ba:Ca_Water, A-D) and otoliths (Ba:Ca_Otolith, E-H) across treatments (two-way ANOVA, P<0.05).

Measured Ba concentrations (mean±SD) in water (Ba:CaWater, A-D) and otoliths (Ba:CaOtolith, E-H) across treatments (two-way ANOVA, P<0.05).</p

Results of ANOVA to evaluate the effects of water elemental concentrations (Me:Ca_Water) on otolith elemental concentrations (Me:Ca_Otolith) for Sr and Ba.

Results of ANOVA to evaluate the effects of water elemental concentrations (Me:CaWater) on otolith elemental concentrations (Me:CaOtolith) for Sr and Ba.</p

Relationship between Me:Ca_Otolith and Me:Ca_Water for Sr (A) and Ba (B).

Lines were fitted by linear regression analysis at individual elemental concentrations. Data represent the mean values of Me:CaOtolith or Me:CaWater for each treatment.</p

Measured Sr concentrations (mean±SD) in water (Sr:Ca_Water, A-D) and otoliths (Sr:Ca_Otolith, E-H) across treatments (two-way ANOVA, <i>P</i><0.05).

Measured Sr concentrations (mean±SD) in water (Sr:CaWater, A-D) and otoliths (Sr:CaOtolith, E-H) across treatments (two-way ANOVA, P<0.05).</p