Ordered Mesoporous Platinum@Graphitic Carbon Embedded Nanophase as a Highly Active, Stable, and Methanol-Tolerant Oxygen Reduction Electrocatalyst

Highly ordered mesoporous platinum@graphitic carbon (Pt@GC) composites with well-graphitized carbon frameworks and uniformly dispersed Pt nanoparticles embedded within the carbon pore walls have been rationally designed and synthesized. In this facile method, ordered mesoporous silica impregnated with a variable amount of Pt precursor is adopted as the hard template, followed by carbon deposition through a chemical vapor deposition (CVD) process with methane as a carbon precursor. During the CVD process, in situ reduction of Pt precursor, deposition of carbon, and graphitization can be integrated into a single step. The mesostructure, porosity and Pt content in the final mesoporous Pt@GC composites can be conveniently adjusted over a wide range by controlling the initial loading amount of Pt precursor and the CVD temperature and duration. The integration of high surface area, regular mesopores, graphitic nature of the carbon walls as well as highly dispersed and spatially embedded Pt nanoparticles in the mesoporous Pt@GC composites make them excellent as highly active, extremely stable, and methanol-tolerant electrocatalysts toward the oxygen reduction reaction (ORR). A systematic study by comparing the ORR performance among several carbon supported Pt electrocatalysts suggests the overwhelmingly better performance of the mesoporous Pt@GC composites. The structural, textural, and framework properties of the mesoporous Pt@GC composites are extensively studied and strongly related to their excellent ORR performance. These materials are highly promising for fuel cell applications and the synthesis method is quite applicable for constructing mesoporous graphitized carbon materials with various embedded nanophases

CO₂ Capture by Temperature Swing Adsorption: Use of Hot CO₂‑Rich Gas for Regeneration

Temperature swing adsorption (TSA) is an attractive technology for CO₂ removal from gas streams. CO₂ capture by a TSA process in which the recovered CO₂ product is heated and used as regeneration purge gas has been examined. Our study is based on cyclic experiments performed on a single adsorption column packed with the commercially available zeolite NaUSY adsorbent. The commercial Aspen adsorption simulator was used to simulate the experimental system, where the model predictions agreed quite well with experimental results in terms of breakthrough and results for cycle designs based on indirect heating followed by hot product gas purge. The validated model was used to simulate the case of regeneration using only hot product gas purge, which was difficult to examine experimentally due to constraints of the experimental system used. With a three-step cycle of (1) adsorption, (2) hot gas purge, and (3) cooling, this case yielded product purities of >91% CO₂ and maximum recoveries of 55.5, 76.2, and 83.6% at specific (thermal) energy consumptions of 3.4, 3.8, and 4.5 MJ/kg of CO₂ for regeneration temperatures of 150, 200, and 250 °C, respectively. Calculated productivities also varied from 0.024, 0.037, and 0.047 kg_CO₂/kg_ads·h for the various regeneration temperatures. Incorporation of a product CO₂ purge prior to desorption with hot CO₂ purge gas increased the purity to 96% at a recovery of 90.8%these conditions are suitable for CO₂ sequestration

Determination of Composition Range for “Molecular Trapdoor” Effect in Chabazite Zeolite

Highly selective separation of small molecules, such as CO₂, N₂, and CH₄, is difficult to achieve if all of the molecules can access the internal surface so that the selectivity depends only on differences in interaction of these molecules with the surface. Recently, we reported on a “molecular trapdoor” mechanism (Shang, J.; et al. <i>J. Am. Chem. Soc.</i> <b>2012</b>, <i>134</i>, 19246–19253), which provides a record high selectivity through a guest-induced cation deviation process where the adsorbent exclusively admits “strong” molecules (e.g., CO₂ and CO) but excludes “weak” ones (e.g., N₂ and CH₄). In this study, we have investigated the range of zeolite compositions (varying Si/Al and cation type) for which a trapdoor effect is present and summarize this composition range with a simple “rule of thumb”. Cation density and cation type are the controlling factors in achieving the molecular trapdoor effect on chabazites. Specifically, the “rule” requires every pore aperture connecting the supercages to accommodate one door-keeping cation of an appropriate type. This “rule” will help guide the synthesis of “trapdoor” chabazite adsorbents for the deployment of carbon capture as well as help the development of molecular trapdoor adsorbents/membranes for other small-pore zeolites, such as RHO, LTA, and other porous materials

K-CHA_273 K_rhombohedral

Crystallographic Information File (CIF) file of K-CHA in rhombohedral structure at 273

KCHA2.2_supercell_GCMC_cubic

Crystallographic Information File (CIF) for a potassium exchange chabazite with a Si/Al ratio of 2.2, in a 3 x 3 x 3 supercell for GCMC calculations, in cubic structure

Discriminative Separation of Gases by a “Molecular Trapdoor” Mechanism in Chabazite Zeolites

Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of “molecular sieves” and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be “molecular sieving” based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits “size-inverse” separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N₂). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a “molecular trapdoor”. Our materials show the highest selectivity of CO₂ over CH₄ reported to date with important application to natural gas purification