Molecular simulation evidence for solidlike adsorbate in complex carbonaceous micropore structures

Adsorption of a model nitrogen vapor on a range of complex nanoporous carbon structures is simulated by grand canonical Monte Carlo simulation for a single subcritical temperature above the bulk freezing point. Adsorption and desorption isotherms, heats of adsorption, and three-dimensional singlet distribution functions (SDFs) were generated. Inspection of the SDFs reveals significant levels of solidlike adsorbate at saturation even in the most complex of the microporous solids considered. This strongly suggests that solidlike adsorbate will also occur for simple subcritical vapors adsorbed on real noncrystalline solids such as microporous carbons at temperatures above the bulk freezing point, supporting indirect experimental observations. The presence of significant levels of solidlike adsorbate has implications for characterization of microporous solids where adsorbate density is used (e.g., determination of pore volume from loading). Detailed consideration of the SDF at different loadings for a model microporous solid indicates solidlike adsorbate forms at distributed points throughout the pore space at pressures dependent on the nature of the local porosity. The nature of the local porosity also dictates the freezing mechanism. A local freezing/ melting/refreezing process is also observed. Introduction of mesoporosity into the model causes hysteresis between the adsorption and desorption isotherms. Adsorption in the hysteresis loop occurs by a series of local condensation events. It appears as if the presence of adjacent microporosity and/or adsorbate within it affects the pressure at which these events occur. Reversal of the condensation during desorption occurs throughout the mesoporosity at a single pressure; this pressure is unaffected by the presence of adjacent microporosity or the adsorbate within it. It is also shown that the empirical concept of "pore size" is not consistent for describing adsorption in the complex solids considered here. A new concept is, therefore, proposed that seeks to account for the factors that affect local adsorption energy: local geometry, microtexture, surface atom density, and surface chemistry.     

Homogeneous and heterogeneous triplet — triplet annihilation in anthracene-doped phenanthrene crystals

Delayed and prompt fluorescence spectra and delayed fluorescence decay data for 10^?5?10^?6 M/M crystals of anthracene in phenanthrene are reported. It is shown that these data indicate that a heterogeneous (host-dopant) annihilation process takes place which produces the excited singlet state of the host molecule.     

Homogeneous and heterogeneous metallocene/MAO-catalyzed polymerization of trialkylsilyl-protected alcohols

We investigated the ethylene copolymerization by utilizing Me₂Si(Ind)₂ZrCl₂/MAO and Me₂Si(Ind)₂ZrCl₂/MAO/SiO₂ with 10-undecene-1-oxytrimethylsilane and 10-undecene-1-oxytriisopropylsilane and the ethylene copolymerization by using ⁱPr(CpInd)ZrCl₂/MAO and ⁱPr(CpInd)ZrCl₂/MAO/SiO₂ with 5-norbornene-2-methyleneoxytrimethylsilane and 5-norbornene-2-methyleneoxytriisopropylsilane. The trimethylsilyl (TMS) protecting group could not prevent the catalyst deactivation caused by the addition of the polar comonomer. In contrast to that, good catalyst activities and comonomer contents were obtained with the triisopropylsilyl (TIPS) protected monomer. The homopolymerization of 10-undecene-1-OTIPS was carried out with Me₂Si(Ind)₂ZrCl₂/MAO.     

Comments on correct determination of structural parameters for adsorbents of heterogeneous micropore systems

Summary Different forms for functions determining the micropores volume after the structural parameter are given. It is stated that the accepted range of the variability of the structural parameterk can influence considerably the values of parameters determining the structural heterogeneity of microporous adsorbents. The lack of normalization can cause great errors in the micropore volume determination.

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Über die korrekte Bestimmung von Strukturparametern für Adsorbentien auf heterogenen, mikroporösen Systemen

Zusammenfassung Es werden verschiedene Formen für Funktionen, die das Mikroporenvolumen bestimmen, gegenüber dem Strukturparameter angenommen. Man stellte fest, daß im angenommenen Bereich die Änderungen des Strukturparametersk die Werte der die Strukturheterogenität der mikroporösen Adsorbenten bestimmenden Parameter bedeutend beeinflussen. Der Mangel an Normalisierung kann große Fehler in der Bestimmung des Mikroporenvolumens verursachen.

     

Homogeneous and heterogeneous free radical polymerizations in environmentally responsible supercritical carbon dioxide

Fluoropolymers are used in many technologically demanding applications because of their balance of high-performance properties. A significant impediment to the synthesis of variants of commercially available amorphous fluoropolymers is their general insolubility in most solvents except chlorofluorocarbons (CFCs). The environmental concerns about CFCs can be circumvented by preparing these technologically important materials in supercritical fluids (SCFs). The homogeneous solution homo- and copolymerization of highly fluorinated acrylic, styrenic and olefinic monomers in supercritical carbon dioxide using free radical methods will be discussed [Science, 257 , 945 (1992)]. Detailed decomposition rates and efficiency factors will be presented for azobisisobutyronitrile (AIBN) in supercritical carbon dioxide and will be compared to conventional liquid solvents [Macromolecules, 26 , 2663 (1993)]. Additionally, viscosities of polymer solutions in supercritical CO₂ have been measured using a high pressure, falling cylinder viscometer. The results show that the polymer solution viscosities in supercritical CO₂ are an order of magnitude lower than with the same polymers in conventional organic solvents. The results from these homogeneous solution polymerization studies has allowed us to also consider heterogeneous polymerizations in a carbon dioxide continuous phase. Conventional emulsion polymerizations of unsaturated monomers are performed in either aqueous or organic dispersion media with addition of surface active agents (surfactants) to stabilize the colloidal dispersion that forms. With free radical initiators that are preferentially soluble in the continuous phase, high rates of polymerization and high molar mass polymers can be obtained simultaneously. Herein we describe an environmentally responsible alternative to aqueous and organic dispersing media for emulsion polymerizations which utilizes supercritical carbon dioxide, in conjunction with molecularly engineered free radical initiators and amphiphilic molecules that are specifically designed to be interfacially active in CO₂. Conventional lipophilic monomers, exemplified by methyl methacrylate and styrene, can be polymerized heterogeneously using a fluorinated azo-initiator in supercritical CO₂ in the presence of added surfactant to form stable emulsions that result in submicron size particles. Detailed surfactant and initiator syntheses and phase behavior will also be discussed.     

On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy

The applicability of Raman spectroscopy to characterize disordered and heterogeneous carbonaceous materials (CM) is discussed, by considering both natural and synthetic CM. First, different analytical mismatches during the measurement are discussed and technical indications are provided in order to eliminate them. Second, the accuracy and relevance of the different parameters obtained by the decomposition of spectra by conventional fitting procedure, is reviewed. Lastly, a new Raman technique (Raman area mode microspectroscopy) giving an homogeneous repartition of power within a large laser beam is presented, this technique being powerful to study strongly heterogeneous CM and/or photosensitive samples.     

Homogeneous and heterogeneous systems : Flow injection analysis today and tomorrow

Chemical conversions by means of soluble and immobilized reagents including catalysts (enzymes) are reviewed, and estimates are made towards development of miniaturized devices which besides handling homogeneous samples will have the capability to process heterogeneous sample materials such as blood. Reference covering available FIA literature published in 1987 are listed in an Appendix.     

Electrorheological effect of carbonaceous materials with hierarchical porous structures

Carbonaceous materials with different hierarchical porous structures for electrorheological (ER) dispersed phase have been synthesized by carbonization of as-prepared starch/silica precurser at different temperatures. The N₂ adsorption isotherms show that Cmeso-700 and Cmeso-500 particles have the BET surface areas of 1028 and 603 m² g⁻¹, respectively. They both have the mesoporous pores with size of about 4.6 nm and the microporous pores (1.1 and 1.5 nm, respectively). The BET surface areas and C/O atomic ratio of porous carbon materials can be increased with the carbonization temperatures. The rheological measurements indicate that the Cmeso-700 and Cmeso-500 ERF have the better ER effect resulted from their hierarchical porous structures. The shear stress of Cmeso-700 ERF is 900 Pa at 1000 s⁻¹ under 3 kV mm⁻¹, which is almost 4.5 times larger than that of Cmicro-350 ERF. The mesoporous carbon ERFs also show the better sedimentation stability than microporous carbon ERFs. The different ER effect of carbonaceous particles may derive from their different dielectric polarization property induced by the hierarchical porous structures.     

Homogeneous vs heterogeneous polymerization catalysis revealed by single-particle fluorescence microscopy

A high-sensitivity and high-resolution single-particle fluorescence microscopy technique differentiated between homogeneous and heterogeneous metathesis polymerization catalysis by imaging the location of the early stages of polymerization. By imaging single polymers and single crystals of Grubbs II, polymerization catalysis was revealed to be solely homogeneous rather than heterogeneous or both.     

Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts.By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).     

Homogeneous and heterogeneous binary colloidal clusters formed by evaporation-induced self-assembly inside droplets

In this paper, we report the preparation of binary clusters of colloidal particles with different sizes or species into complex structures using oil-in-water emulsion droplets as confining geometries. First, polystyrene or silica particles with bimodal size distribution were packed densely by evaporation-induced self-assembly inside oil-in-water emulsion droplets. The configurations of larger particles inside the droplets minimize the second moment of the particle locations for the ratio of large to small particle sizes less than 3. Also, the configurations of bimodal clusters were predicted by using a surface evolver simulation, and the simulation predictions were compared with the experimental results. In addition, heterogeneous colloidal clusters were produced by emulsifying the binary mixture suspension of polystyrene and silica particles in aqueous medium followed by evaporating the oil phase. A density gradient centrifugation was applied to fractionate the asymmetric binary dimers comprised of PS and silica microspheres.     

Multiple bonds to silicon: 20 years later

The discovery of stable compounds containing SiC and SiSi bonds in 1981 led to the overturning of the ‘double bond rule’, and paved the way for the recent explosive growth in multiple bond chemistry of the heavier elements. This paper traces the events leading up to the 1981 discoveries, summarizes some of the developments in multiply-bonded silicon chemistry since that date, and outlines challenges for further research in this area.     

Low-background Germanium spectrometry the bottom line three years later

Three years ago, state-of-the-art low-background germanium spectrometry was discussed, and speculations were advanced as to the origin of the remaining background. Some of those speculations have been shown to the incorrect. Contemporary lead shielding contains 100 Bq/kg of²¹⁰Pb. Our 450-year-old lead was shown to contain <100 mBq/kg A high purity electroformed copper Marinelli shield was placed around the detector with no efffect on the background, which implied that the source is other than the 450-year-old shield. A new limit on the²¹⁰Pb in this old lead shield is <9 mBq/kg. Electroformed copper components were found to contain²²⁶Ra and²²⁸Th contaminations at levels of 3500 and 110 Bq/kg, respectively. High purity H₂SO₄, recrystallized CuSO₄, and a BaSO₄ scavenge in the electroforming bath have reduced these contaminations to <25 and 9 Bq/kg, respectively. In copper, cosmic ray induced nuclear reactions are now the dominant source of raioactivity. For example,⁵⁸Co can be readily measured after only a 24-hour exposure at sea level. A new germanium spectrometer containing 2150 grams of 87.44% enriched⁷⁶Ge has been fabricated to mitigate the effect of cosmogenic⁶⁸Ge in the background. Current background spectra are presented, and potential sources identified.     

碳质表面异相还原NO2的反应机理

基于量子化学密度泛函理论（DFT），研究了碳质表面异相还原NO₂的反应机理，针对Zigzag与Armchair两种碳质表面，采用M06-2X方法与6-311G （d）基组联用，优化得到了不同反应路径下所有驻点的几何构型与能量，并对各路径进行了热力学与动力学分析，重点探究了CO在NO₂异相还原反应中的作用规律，同时考察了碳质表面与反应温度对异相反应的影响。计算结果表明，NO₂在碳质表面的异相还原过程主要分为两个阶段，即NO₂还原阶段与碳氧化物释放阶段。通过对比无CO分子参与的反应可知，参与反应的CO分子可以降低各阶段的反应能垒并且加快各阶段的反应速率；CO分子存在时，NO₂还原阶段的反应能垒被降低，促进了NO₂还原成NO的异相反应过程，同时参与反应的CO分子与碳质表面剩余氧原子结合，形成CO₂分子并释放，使碳氧化物释放阶段的反应能垒降低，从而促进了整体还原反应的进行。此外，与Armchair型相比，基于Zigzag型碳质表面的NO₂异相还原反应能垒更低且反应速率更快，说明NO₂异相还原反应更容易在Zigzag型碳质表面进行。最后，由反应动力学分析可知，随着温度上升，各阶段的反应速率均增大，说明提高温度对碳质表面的NO₂异相还原能够起到促进作用。     

Self-assembly of heterogeneous supramolecular structures with uniaxial anisotropy

Uniaxial anisotropy in two-dimensional self-assembled supramolecular structures is achieved by the coadsorption of two different linear molecules with complementary amine and imide functionalization. The two-dimensional monolayer is defined by a one-dimensional stack of binary chains, which can be forced to line up along steps in vicinal surfaces. The competing driving forces in the self-organization process are discussed in light of the structures observed during single molecule adsorption and coadsorption on flat and vicinal surfaces and the corresponding theoretical calculations.