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Electrochemical reduction of carbon dioxide (CO2) to useful products is an emerging power-to-X concept, which aims to produce chemicals and fuels with renewable electricity instead of fossil fuels. Depending on the catalyst, a range of chemicals can be produced from CO2 electrolysis at industrial-scale current densities, high Faraday efficiencies, and relatively low cell voltages. One of the main challenges for up-scaling the process is related to (bi)carbonate formation (carbonation), which is a consequence of performing the reaction in alkaline media to suppress the competing hydrogen evolution reaction. The parasitic reactions of CO2 with the alkaline electrolytes result in (bi)carbonate precipitation and flooding in gas diffusion electrodes, CO2 crossover to the anode, low carbon utilization efficiencies, electrolyte carbonation, pH-drift in time, and additional cost for CO2 and electrolyte recycling. We present a critical review of the causes, consequences, and possible solutions for the carbonation effect in CO2 electrolyzers. The mechanism of (bi)carbonate crossover in different cell configurations, its effect on the overall process design, and the economics of CO2 and electrolyte recovery are presented. The aim is to provide a better understanding of the (bi)carbonate problem and guide research directions to overcome the challenges related to low-temperature CO2 electrolysis in alkaline media.

The following sections are included:IntroductionBenefits of computational experimentsNanoporous materialsPore system of a materialStructure modelsThe unit cellStructure file formatsWhere can you find zeolite and MOF structural information?Periodic boundary conditions and the minimum-image conventionForce fieldsRunning a simulationUnits and conversionsOutlineStructural PropertiesEnergy landscapesCharacterization of the pore system of a materialSurface areas, pore volume and pore-size distributionsMolecular Mechanics (MM)Monte Carlo (MC) and AdsorptionBasicsNVT ensembleAdsorption: Gibbs and grand-canonical ensembleEnthalpy of adsorptionMixture adsorption simulationsIdeal Adsorbed Solution Theory (IAST)Molecular Dynamics (MD) and DiffusionNewton’s equations of motionThermo- and barostatsRadial distribution functionsDynamic correlation functionsMean-squared displacement and single component diffusionActivation energyFree energy analysis and dc-TSTSelectivityBreakthrough Simulation of Fixed-bed Separation DevicesIntroductionModeling the fixed-bedConclusionAcknowledgementsReferencesAppendix: RASPA Input for Computing Adsorption and Diffusion PropertiesIntroductionForce field files for CO2 and CH4 in zeolitespseudo atoms.def atom definition fileCO2.def molecule filemethane.def molecule filenitrogen.def molecule fileITQ-29.cif framework definition fileforce field mixing rules.def force field definition fileBlocked pocket definitionsStructural properties of ITQ-29Surface areaVoid-fractionPore-size distributionMinimization of CO2 in ITQ-29Gibbs single component adsorption of CO2 in ITQ-29 using CBMCCO2 single component adsorption using CBMCCH4 single component adsorption using CBMCMixture adsorption of CO2/CH4 in ITQ-29Mixture adsorption of CO2/CH4 in ITQ-29 using CBMCDynamical properties of CO2 and CH4 in ITQ-29MSD, VACF, and RDF of CH4 in ITQ-29 using MDFree energy profiles of CH4 in ITQ-29 at 300 K and 4 molecules per cavity

Computational chemistry uses computer simulation to assist in solving chemical problems. Typical workflows of computational chemists include the use of dozens of utilities. 3D modeling programs are powerful tools that help researchers visualize their work and create illustrative graphics. In this review, we describe and highlight tools and visualization packages that are commonly used in the field of (bio-)chemistry and material science.

Force Field based Monte Carlo (MC) simulations are conducted to predict the performance of an absorption heat pump cycle involving NH3/ionic liquid (IL) (refrigerant/absorbent) as working pair. To investigate the thermodynamic performance of the cycle, various properties such as the enthalpy of absorption, heat capacity, and solubility of refrigerant in the absorbent are required. As an alternative to experiments, MC simulations are used to predict the required properties. The simulations are performed at temperatures ranging from 303 K to 373 K and pressures ranging from 4 to 16 bar. The thermodynamic performance parameters such as the coefficient of performance, COP, and the circulation ratio, f, of NH3 paired with [emim][Tf2N] are investigated using MC simulations and compared to results obtained from correlated experimental data, showing a reasonable agreement. MC simulations could be used as an inexpensive alternative for preliminary design considerations involving potential working pairs for absorption heat pump cycles in the absence of available experimental data.

Chemical absorption of CO2 using aqueous amine-based solvents is one of the common approaches to control acidic gases emissions to the atmosphere. Improvement in the efficiency of industrial processes requires precise monitoring tools that fit with the specific application. Process monitoring using in-line multivariate measurement methods provides access to time resolved data of the reaction progress and the composition of the reaction mixture. Fast acquisition of valuable information about the process on site can be used for automated monitoring and process control, saving operational costs and reducing waste products. Therefore, different analytical techniques are being explored for the purpose of their practical application in process analysis. This work compares three vibrational spectroscopy techniques for monitoring CO2 absorption by aqueous monoethanolamine (MEA) solutions: Raman spectroscopy, near infra-red (NIR) spectroscopy, and attenuated total reflectance Fourier transform infra-red (ATR FTIR) spectroscopy. The spectroscopic information has been used to estimate the concentration of CO2 captured by the chemical solvent. The study aims to determine the potential applicability of the spectroscopic methods to the in-line and real time monitoring of a post-combustion capture process. Partial least squares (PLS) regression models were built based on the spectroscopic data before and after spectra pretreatment procedures. All three spectroscopic methods are shown to be well suited. The estimation model constructed using NIR data provides the highest accuracy for estimation of the CO2 loading, with an average deviation of about 0.01 molCO2/molMEA. The models based on Raman and ATR FTIR measurements show deviations of around 0.02 molCO2/molMEA.

We develop an equation of state (EoS) for describing isotropic-nematic (IN) phase equilibria of Lennard-Jones (LJ) chain fluids. The EoS is developed by applying a second order Barker-Henderson perturbation theory to a reference fluid of hard chain molecules. The chain molecules consist of tangentially bonded spherical segments and are allowed to be fully flexible, partially flexible (rod-coil), or rigid linear. The hard-chain reference contribution to the EoS is obtained from a Vega-Lago rescaled Onsager theory. For the description of the (attractive) dispersion interactions between molecules, we adopt a segment-segment approach. We show that the perturbation contribution for describing these interactions can be divided into an “isotropic” part, which depends only implicitly on orientational ordering of molecules (through density), and an “anisotropic” part, for which an explicit dependence on orientational ordering is included (through an expansion in the nematic order parameter). The perturbation theory is used to study the effect of chain length, molecular flexibility, and attractive interactions on IN phase equilibria of pure LJ chain fluids. Theoretical results for the IN phase equilibrium of rigid linear LJ 10-mers are compared to results obtained from Monte Carlo simulations in the isobaric-isothermal (NPT) ensemble, and an expanded formulation of the Gibbs-ensemble. Our results show that the anisotropic contribution to the dispersion attractions is irrelevant for LJ chain fluids. Using the isotropic (density-dependent) contribution only (i.e., using a zeroth order expansion of the attractive Helmholtz energy contribution in the nematic order parameter), excellent agreement between theory and simulations is observed. These results suggest that an EoS contribution for describing the attractive part of the dispersion interactions in real LCs can be obtained from conventional theoretical approaches designed for isotropic fluids, such as a Perturbed-Chain Statistical Associating Fluid Theory approach.

Corrosion in amine treating plants is known to cause integrity failures, plant shutdown, costly repairs, and so forth. The use of an amine treatment system for postcombustion CO2 capture brings additional challenges in terms of the flue gas quality, flue gas composition, operating conditions, scale of operation, and so forth. These differences are expected to have a significant impact on the overall well-being of the plant and the maintenance strategy over its lifetime. The degradation of solvent, by oxidative and thermal degradation pathways, leads to the formation of various degradation products which are known to be corrosive. The oxidative degradation of amine leads to the formation of ammonia which can be emitted to the atmosphere in the treated flue gas stream. This study aims at verifying the interrelation between (1) solvent degradation, (2) corrosion, and (3) NH3 emissions, based on two test campaigns of over 1500 operating hours, in a CO2 capture pilot plant. An online tool for real-time corrosion monitoring, SmartCET, is presented and compared with offline corrosion coupon. The different process and operating conditions such as the change in the flue gas composition, start-up and shutdown, and solvent replacement were correlated with the variations in the corrosion parameters (general corrosion rate, pitting factor, and corrosion mechanism indicator (CMI)), for both campaigns. The three parameters showed only a small increase for most part of both the campaigns (>75 days), and followed a similar trend. However, during both the campaigns, a rapid increase of all the three parameters was observed. In one of the campaigns, the three parameters increased by about 2 orders of magnitude in a time period of only about 20 days, which points toward an autocatalytic out of control solvent chemistry. The common characteristic before the onset of autocatalytic solvent chemistry, is the re-start-up of the pilot plant after an extended period of shutdown. On the basis of these tests, the use of an online corrosion monitoring tool will enable long-term steady state operation of a full scale postcombustion CO2 capture plant.

Industrial scale implementation of post combustion CO2 capture (PCCC) can be hindered by solvent emissions due to its impact on the environment and the operating costs. The issue of aerosol based emissions has only been recently reported for a PCCC process and very little fundamental knowledge is available in the scientific community on this topic. Therefore, it is important to understand the mechanism of aerosol formation and growth so that appropriate countermeasures can be applied in reducing the total emissions. In this study, a simplified methodology is presented for predicting aerosol based emissions from a CO2 capture column of a PCCC process. The basis of this methodology is to split the counter-current gas-liquid interaction from the cocurrent gas-aerosol interaction. The absorption column is discretised into multiple alternating gas-liquid and gas-aerosol sections in Aspen Plus with an assumption that aerosols behave as a continuous phase rather droplets. The degree of supersaturation, which is important for aerosol formation and growth, is calculated along the column. The effect of the changes in parameters of the PCCC plant, such as the CO2 content of the inlet flue gas, the lean solvent temperature and the lean solvent loading on aerosol based emissions are investigated. The aerosol based emissions follows the trend of the supersaturation ratio in the absorber column.

A great variety of single- and multi-component nanocrystals (NCs) can now be synthesized and integrated into nanocrystal superlattices. However, the thermal and temporal stability of these superstructures and their components can be a limiting factor for their application as functional devices. On the other hand, temperature induced reconstructions can also reveal opportunities to manipulate properties and access new types of nanostructures. In situ atomically resolved monitoring of nanomaterials provides insight into the temperature induced evolution of the individual NC constituents within these superstructures at the atomic level. Here, we investigate the effect of temperature annealing on 2D square and hexagonal arrays of FexO/CoFe2O4 core/shell NCs by in situ heating in a transmission electron microscope (TEM). Both cubic and spherical NCs undergo a core–shell reconfiguration at a temperature of approximately 300 ° C, whereby the FexO core material segregates at the exterior of the CoFe2O4 shell, forming asymmetric dumbbells ('snowman-type' particles) with a small FexO domain attached to a larger CoFe2O4 domain. Upon continued annealing, the segregated FexO domains form bridges between the CoFe2O4 domains, followed by coalescence of all domains, resulting in loss of ordering in the 2D arrays.

Precipitating amino acid solvents are an alternative to conventional amine scrubbing for CO2 capture from flue gas. Process operation with these solvents leads to the formation of precipitates during absorption that need to be re-dissolved prior to desorption of CO2. The process configuration is crucial for the successful application of these solvents. Different process configurations have been analyzed in this work, including a full analysis of the baseline operating conditions (based on potassium taurate), the addition of lean vapor compression, multiple absorber feeds, and the use of different amino acids as alternative solvents to the baseline based on potassium taurate. The analysis is carried out with an equilibrium model of the process that approximates the thermodynamics of the solvents considered. The results show that the precipitating amino acid solvents can reduce the reboiler duty needed to regenerate the solvent with respect to a conventional MEA process. However, this reduction is accompanied by an expenditure in lower grade energy needed to dissolve the precipitates. To successfully implement these processes into power plants, an internal recycle of the rich stream is necessary. This configuration, known as DECAB Plus, can lower the overall energy use of the capture process, which includes the energy needed to regenerate the solvent, the energy needed to dissolve the precipitates, and the energy needed to compress the CO2 to 110 bar. With respect to the energy efficiency, the DECAB Plus with lean vapor compression configuration is the best configuration based on potassium taurate, which reduces the reboiler duty for regeneration by 45% with respect to conventional MEA. Retrofitting this process into a coal fired power plant will result in overall energy savings of 15% with respect to the conventional MEA process, including compression of the CO2 stream to 110 bar. Potassium alanate was found to reduce the energy use with respect to potassium taurate under similar process configurations. Therefore, the investigation of potassium alanate in a DECAB Plus configuration is highly recommended, since it can reduce the energy requirements of the best process configuration based on potassium taurate.

CO2 readily reacts in aqueous amine solutions. The properties of free CO2 in amine solutions are therefore difficult to obtain directly and are often predicted from the nonreacting molecule N2O due to the similarities in mass and structure. This often-used empirical "CO2/N2O analogy" is verified in this work using molecular simulation. Continuous fractional component Monte Carlo (CFCMC) simulations in the osmotic ensemble were used to compute the Henry coefficients of CO2 and N2O in the solvents water, ethanol, n-heptane, and a 30% aqueous MEA solution at a temperature of 303 K. Molecular dynamics (MD) simulations were performed to compute the self-diffusivities of CO2 and N2O in the aforementioned solvents at 303 K. Different force fields for CO2 and water were used. The computed Henry coefficients and self-diffusivities of CO2 and N2O in the solvents are in good agreement with available experimental data. The simulation results indicate that the CO2/N2O analogy is valid for aqueous MEA solution at 303 K. The Henry coefficient and self-diffusivity ratios of CO2 to N2O in water and 30% MEA solution are approximately 0.77 and 1.1, respectively. Additional simulations where all the amines have reacted with CO2 confirm that reactions have little impact on the physical absorption and diffusion properties of CO2.

A great variety of single- and multi-component nanocrystals (NCs) can now be synthesized and integrated into nanocrystal superlattices. However, the thermal and temporal stability of these superstructures and their components can be a limiting factor for their application as functional devices. On the other hand, temperature induced reconstructions can also reveal opportunities to manipulate properties and access new types of nanostructures. In situ atomically resolved monitoring of nanomaterials provides insight into the temperature induced evolution of the individual NC constituents within these superstructures at the atomic level. Here, we investigate the effect of temperature annealing on 2D square and hexagonal arrays of FexO/CoFe2O4 core/shell NCs by in situ heating in a transmission electron microscope (TEM). Both cubic and spherical NCs undergo a core–shell reconfiguration at a temperature of approximately 300 ° C, whereby the FexO core material segregates at the exterior of the CoFe2O4 shell, forming asymmetric dumbbells ('snowman-type' particles) with a small FexO domain attached to a larger CoFe2O4 domain. Upon continued annealing, the segregated FexO domains form bridges between the CoFe2O4 domains, followed by coalescence of all domains, resulting in loss of ordering in the 2D arrays.

Emission of solvent and its degradation products from a typical absorption-desorption based Post Combustion CO2 Capture (PCCC) process is inevitable and thus, an area of growing concern. Recently, it has been pointed out that emissions can also occur by means of aerosol droplets. Conventional counter-measures such as a water wash and a demister are highly effective in reducing vapour based solvent emissions (97%). The MEA emission levels consisted of both vapour and aerosol droplets. Thus, the water wash is capable of lowering vapour based emissions while, the BDU removes only aerosol based emissions. The measured concentrations of nitrosamines (N-nitrosodimethylamine, NDMA; N-morpholine, NMOR; N-nitrosodiethanolamine, NDELA) in the treated flue gas stream from the absorber are very low and in the order of 10ng/Nm3. Ammonia emission levels were in the range of 10-70mg/Nm3. A major disadvantage of the BDU is its high pressure drop. The BDU led to a pressure drop increase of about 50mbar which is considered to be quite significant considering the total volume of gas to be processed. Based on a rough estimate, the BDU would lead to an increase of 26-52% in the electricity consumption and 1€/tonne of CO2 in operating cost, for a full scale CO2 capture plant.

We performed classical molecular dynamics (MD) simulation to understand the mechanism of CO2 adsorption and transport on graphite surface. The temperature of the system in our simulation was in the range 300-500K. The simulation data show that there are two layers of CO2 molecules absorbed on the surface. These two layers have a different behavior. The first CO2 layer is isolated as it does not exchange molecules with the second layer and is liquid-like, while the second layer exchanges molecules with the gas phase. The layers are separate thermodynamic systems. We use the simple Langmuir model to fit the adsorption isotherm for the second layer. The enthalpy of adsorption is calculated ?H0 = -16 kJ/mol. This value is in good agreement with experimental data of adsorption of CO2 on activated carbon. Along the graphite surface, the diffusion coefficient of CO2 in the first layer and the second layer are roughly of 10-11 m2/s, 10-10 m2/s respectively. These values are much smaller than for H2.

Amino acid salt based solvents can be used for CO2 removal from flue gas in a conventional absorption-thermal desorption process. Recently, new process concepts have been developed based on the precipitation of the amino acid zwitterion species during the absorption of CO2. In this work, a new concept is introduced which requires the precipitation of the pure amino acid species and the partial recycle of the remaining supernatant to the absorption column. This induces a shift in the pH of the rich solution treated in the stripper column that has substantial energy benefits during CO 2 desorption. To describe and evaluate this concept, this work provides the conceptual design of a new process (DECAB Plus) based on a 4 M aqueous solution of potassium taurate. The design is supported by experimental data such as amino acid speciation, vapor-liquid equilibria of CO2 on potassium taurate solutions, and solid-liquid partition. The same conceptual design method has been used to evaluate a baseline case based on 5 M MEA. After thorough evaluation of the significant variables, the new DECAB Plus process can lower the specific reboiler energy for solvent regeneration by 35% compared to the MEA baseline. The specific reboiler energy is reduced from 3.7 GJ/tCO 2, which corresponds to the MEA baseline, to 2.4 GJ/tCO2, which corresponds to the DECAB Plus process described in this work, excluding the low-grade energy required to redissolve the precipitates formed during absorption. Although this low-grade energy will eventually reduce the overall energy savings, the evaluation of DECAB Plus has indicated the potential of this concept for postcombustion CO2 capture. © 2013 American Chemical Society.

Many process schemes have been proposed in literature to decrease the energy demand of amine based carbon dioxide processes. These process schemes are generally analysed in terms of energy demand savings and compared to a common baseline based on the solvent monoethanolamine (MEA). In this work, the application of one of these process schemes (lean vapour compression or LVC) has been evaluated and optimised based on maximising the net present value (NPV) of the process scheme savings (including capital investment), rather than minimising energy demand in the form of equivalent work. Two scenarios have been analysed. In the first scenario, the capture plant was fully adapted to the effect of LVC. In the second scenario, LVC is retrofitted to a basic capture plant design. For both scenarios the net present value (expressed in MD ) of the process scheme over the whole plant life was calculated as a function of the LVC operating conditions. It was found that the NPV of the LVC process scheme is always positive and attractive from a financial point of view. The first scenario has been identified as the most attractive scenario for LVC application. Although the extent of the savings depends on design conditions and financial assumptions, this approach shows that the optimisation based on minimising equivalent work does not necessarily match the optimisation based on maximising net present value.

Downconversion of one visible photon into two infrared photons has been reported for the lanthanide ion couple ( Pr3+ , Yb3+ ) in a variety of host lattices. The mechanism responsible for downconversion is controversial and has been reported to be either a two-step energy transfer process (via two first-order transfer steps, the first being cross relaxation) or cooperative energy transfer from Pr3+ to two Yb3+ ions (a second-order process). Here we report experiments on downconversion for the ( Pr3+ , Yb3+ ) in LiYF4 . Luminescence decay curves of the Pr3+ emission are recorded as a function of the Yb3+ concentration and analyzed using Monte Carlo simulations for both cooperative energy transfer and energy transfer through cross relaxation. We obtain a good agreement between experiment and simulations for energy transfer by cross relaxation but not for cooperative energy transfer. The observation that cross relaxation is more efficient than cooperative energy transfer is consistent with Judd-Ofelt calculations for the transition probabilities involved in the two energy transfer processes and the lower probability for the second-order cooperative transfer

Energy losses inherent to the conversion of sunlight to electricity in solar cells are mainly due to the so-called spectral mismatch: low energy photons are not absorbed while the energy of high energy photons is only partly used by the solar cell. The losses can be significantly reduced by adapting the solar spectrum. A promising avenue is the use of a downconversion material where one higher energy visible (blue-green) photon is “cut” into two lower-energy near-infrared photons that both can be used by the solar cell. Here the efficiency of downconversion for the (Nd3+,Yb3+) couple in YF3 is studied to investigate if efficient two-step energy transfer occurs from the 4G9/2 level of Nd3+ (situated around 21?000?cm?1 or 470 nm) exciting two neighboring Yb3+ ions to the 2F5/2 level (around 10?000?cm?1 or 1000 nm). Optical measurements of YF3 doped with Nd3+ and Yb3+ show that there is efficient energy transfer from Nd3+ to Yb3+, but downconversion from the 4G9/2 level does not occur due to fast multiphonon relaxation. Relaxation from this level to lower-energy levels populates the 4F3/2 level of Nd3+ from which efficient one-step energy transfer to Yb3+ occurs. Analysis of the luminescence decay curves for different Yb3+-concentrations using Monte Carlo simulations reveals a high nearest neighbor transfer rate (3.3×105?s?1) through a dipole-dipole interaction mechanism. Downconversion is observed from the 4D3/2 level (situated in the UV, around 28?000?cm?1 or 360 nm) with an estimated quantum efficiency up to 140%. For application in solar cells this UV to 2 NIR downconversion will only result in a marginal reduction of spectral mismatch losses.

Downconversion of one visible photon into two infrared photons has been reported for the lanthanide ion couple ( Pr3+ , Yb3+ ) in a variety of host lattices. The mechanism responsible for downconversion is controversial and has been reported to be either a two-step energy transfer process (via two first-order transfer steps, the first being cross relaxation) or cooperative energy transfer from Pr3+ to two Yb3+ ions (a second-order process). Here we report experiments on downconversion for the ( Pr3+ , Yb3+ ) in LiYF4 . Luminescence decay curves of the Pr3+ emission are recorded as a function of the Yb3+ concentration and analyzed using Monte Carlo simulations for both cooperative energy transfer and energy transfer through cross relaxation. We obtain a good agreement between experiment and simulations for energy transfer by cross relaxation but not for cooperative energy transfer. The observation that cross relaxation is more efficient than cooperative energy transfer is consistent with Judd-Ofelt calculations for the transition probabilities involved in the two energy transfer processes and the lower probability for the second-order cooperative transfer

Molecular dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this “oriented attachment” process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb- or Se-terminated.

Energy losses inherent to the conversion of sunlight to electricity in solar cells are mainly due to the so-called spectral mismatch: low energy photons are not absorbed while the energy of high energy photons is only partly used by the solar cell. The losses can be significantly reduced by adapting the solar spectrum. A promising avenue is the use of a downconversion material where one higher energy visible (blue-green) photon is “cut” into two lower-energy near-infrared photons that both can be used by the solar cell. Here the efficiency of downconversion for the (Nd3+,Yb3+) couple in YF3 is studied to investigate if efficient two-step energy transfer occurs from the 4G9/2 level of Nd3+ (situated around 21 000 cm−1 or 470 nm) exciting two neighboring Yb3+ ions to the 2F5/2 level (around 10 000 cm−1 or 1000 nm). Optical measurements of YF3 doped with Nd3+ and Yb3+ show that there is efficient energy transfer from Nd3+ to Yb3+, but downconversion from the 4G9/2 level does not occur due to fast multiphonon relaxation. Relaxation from this level to lower-energy levels populates the 4F3/2 level of Nd3+ from which efficient one-step energy transfer to Yb3+ occurs. Analysis of the luminescence decay curves for different Yb3+-concentrations using Monte Carlo simulations reveals a high nearest neighbor transfer rate (3.3×105 s−1) through a dipole-dipole interaction mechanism. Downconversion is observed from the 4D3/2 level (situated in the UV, around 28 000 cm−1 or 360 nm) with an estimated quantum efficiency up to 140%. For application in solar cells this UV to 2 NIR downconversion will only result in a marginal reduction of spectral mismatch losses

We investigated the adsorption and the diffusion of alkanes in the sodium-exchanged zeolite Mordenite (Na-MOR) using molecular simulations. MOR-type zeolite consists of main channels (6.5 × 7 Å) oriented along the z crystallographic axis that are connected to small side pockets (3.4 × 4.8 Å). It is well-known that the adsorption of alkanes in Na-MOR strongly depends on the precise location of the framework Al atoms either in the main channel or the side pockets (Calero and co-workers, Angew. Chem. Int. Ed. 2007, 46, 276). We found that this effect can be characterized by a single-order parameter: the number of framework Al in the main channel divided by the number of framework Al in the side pocket (M/S ratio). For any M/S ratio, the adsorption isotherm follows from a linear interpolation between the reference isotherms. This enabled us to predict adsorption isotherms for any distribution of the Al framework atoms or estimate the M/S ratio for a given isotherm. We found that the same model can predict the effect of the M/S ratio on the self-diffusion coefficient, the Maxwell−Stefan diffusion coefficient, and the accessible micropore volume.

A simulation method is presented to calculate efficiently the pore size distribution of microporous materials. The microporosity of several typical zeolites is analysed. The effects of the inaccessible pores and non-framework cations on the pore size distribution of zeolites have also been studied. Molecular simulations of Ar adsorption are used to investigate the correlations between the adsorption isotherms and the pore size distribution. The feasibility to derive the pore size distribution of zeolite micropores directly from adsorption isotherms is discussed.

A simulation method is presented to calculate efficiently the pore size distribution of microporous materials. The microporosity of several typical zeolites is analysed. The effects of the inaccessible pores and non-framework cations on the pore size distribution of zeolites have also been studied. Molecular simulations of Ar adsorption are used to investigate the correlations between the adsorption isotherms and the pore size distribution. The feasibility to derive the pore size distribution of zeolite micropores directly from adsorption isotherms is discussed.

We investigated the adsorption and the diffusion of alkanes in the sodium-exchanged zeolite Mordenite (Na-MOR) using molecular simulations. MOR-type zeolite consists of main channels (6.5 × 7 Å) oriented along the z crystallographic axis that are connected to small side pockets (3.4 × 4.8 Å). It is well-known that the adsorption of alkanes in Na-MOR strongly depends on the precise location of the framework Al atoms either in the main channel or the side pockets (Calero and co-workers, Angew. Chem. Int. Ed. 2007, 46, 276). We found that this effect can be characterized by a single-order parameter: the number of framework Al in the main channel divided by the number of framework Al in the side pocket (M/S ratio). For any M/S ratio, the adsorption isotherm follows from a linear interpolation between the reference isotherms. This enabled us to predict adsorption isotherms for any distribution of the Al framework atoms or estimate the M/S ratio for a given isotherm. We found that the same model can predict the effect of the M/S ratio on the self-diffusion coefficient, the Maxwell−Stefan diffusion coefficient, and the accessible micropore volume.

The exchange kinetics of native ligands that passivate CdSe quantum dots (hexadecylamine (HDA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP)) by thiols is followed in situ. This is realized by measuring, in real-time, the decrease in emission intensity of the QDs upon addition of hexanethiol (HT) which quenches the emission. The effect of adding an excess of native ligands prior to thiol addition on the capping exchange is studied to provide insight in the bond strength and exchange kinetics of the individual surfactants. Temperature-dependent measurements reveal faster kinetics with increasing temperature. A kinetic model to describe the time-dependent measurements is introduced, taking into account the equilibrium between native ligands before thiol addition and describing the evolution of surface coverage by all ligands over time. The model allows us to extract the quenching rate for a single thiol ligand (0.004 ns−1) as well as exchange rates, equilibrium constants, activation energies, and changes in Gibbs free energy for replacement of the different native surfactants by HT. The analysis reveals that the substitution half-time of HDA by HT (72 s) is much shorter than for TOP (5 h) or TOPO (2.5 h) under the same conditions. The temperature dependence of the kinetics shows that the activation energy for exchange of HDA/TOPO by hexanethiol (1.6 kJ/mol) is much smaller than for TOP (20.9 kJ/mol).

We carried out Monte Carlo simulations of gold nanocrystals (NCs) and (111) slabs covered with alkyl thiols, with and without explicit solvent (n-hexane), at T ) 300 K. Adsorption isotherms for propane- and octanethiol showed a phase behavior measured previously in experiments. Comparison of the adsorption isotherm of octanethiol in hexane on a (111) slab with experimental data suggests that, in this system, no thiolate bond was formed. The geometry of a gold surface strongly influences the formation and structure of the capping monolayer. On a (111) surface, attractive interactions between carbon chains are more pronounced than on a NC. This leads to a stronger penetration of the capping layer by the solvent. Adsorption selectivity for binary alkyl thiol mixtures is stronger in vacuum than in solution. The convex shape of the NCs also reduces the adsorption selectivity of binary thiol mixtures. This result shows that the solvent cannot be ignored in simulations.

The configurational-bias Monte Carlo (CBMC) technique in the grand-canonical (μVT) ensemble has been used to calculate the pure and binary adsorption equilibrium isotherms of propane and propylene in zeolite 4A (zeolite Na-LTA) at 298.15, 373, 423, and 473 K in the 0-500 kPa pressure range. The influence of sodium cations on the adsorption properties of zeolite 4A was investigated using the united atom (UA) force field in order to establish effective Lennard-Jones (LJ) parameters for interactions between the cations and the sp2-hybridized bond of propylene as well as the other cation-carbon interactions present in this system. A cation-free zeolite Linde type A (LTA) framework and the inclusion of sodium cations into the zeolite framework have been studied in this work. The quality of the parameters describing the carbon-sodium interactions was investigated by comparing with published experimental results for the cation-free LTA and by using other published LJ interaction parameters. Surprisingly, it was found that the sodium force field developed by Calero et al. [J. Am. Chem. Soc. 2004, 126, 11377] which is suitable for studying adsorption in other Na-contained zeolite frameworks, namely, FAU and MFI, does not work well for the Na-LTA system. It turns out that this is due to the interaction with sodium, as the simulations without cations agree very well with adsorption experiments on cation-free LTA. Our force field for zeolite 4A is in good agreement with recent experiments which show that the adsorbed amount of propane is much higher than reported previously. Good agreement of simulations with experiments was also obtained for adsorption of propane/propylene mixtures in zeolite 4A.

The cross-link chemistry of CdTe quantum dots (QDs) in solution is studied for different types of aliphatic (flexible) and aromatic (rigid) dithiol linker molecules. A remarkable difference in the cross-linking efficiency is observed: the rigid dithiols are shown to form aggregates at much lower concentrations. Qualitative and quantitative information on the formation of aggregates is obtained from cryogenic transmission electron microscopy (cryo-TEM) images and photoluminescence decay measurements. The luminescence decay curves are analyzed with a model for energy transfer to neighboring QDs in aggregates. The analysis shows that the cross-linking efficiency is 4 times higher for the rigid dithiols than for the flexible dithiols. The difference is attributed to the formation of loops for the flexible dithiols by attaching with both thiol groups to the same nanocrystal surface (preventing cross-linking), whereas the rigid aromatic dithiols cannot form loops and the second thiol group is oriented away from the surface (enabling cross-linking). The difference in conformation between flexible and rigid dithiols is confirmed by studies on the red-shift in the optical absorption spectra due to capping exchange of amines by monothiols or dithiols and by molecular simulations.

The nine-site benzene model of Zhao et al. (J. Phys. Chem. B 2005, 109, 5368-5374) has been used to systematically study the adsorption of benzene, propene, and benzene-propene mixtures in zeolites mordenite, Y, , silicalite, and MCM22. Interaction parameters for the benzene-zeolite interactions have been fitted to available adsorption experiments from literature. As an independent check of our force field, we have performed additional adsorption experiments using the TEOM technique and excellent agreement between simulations and TEOM experiments was found. High adsorption selectivities for benzene in benzene-propene mixtures were found in all zeolites, except for silicalite.

We numerically study the distribution P(f) of contact forces in frictionless bead packs, by averaging over the ensemble of all possible force network configurations. We resort to umbrella sampling to resolve the asymptotic decay of P(f) for large f, and determine P(f) down to values of order 10-45 for ordered and disordered systems in two (2D) and three dimensions (3D). Our findings unambiguously show that, in the ensemble approach, the force distributions decay much faster than exponentially: P(f)~ exp(-cfα), with a = 2.0 for 2D systems, and α = 1.7 for 3D systems.

The adsorption of mixtures of alkyl thiol surfactants on the Au(111) surface as well as on icosahedral gold nanocrystals (NCs) was investigated by molecular simulation. We compared the molfraction of each surfactant type on the gold structure with the molfraction of each surfactant type in the surrounding bulk solvent. For alkyl thiol surfactants with 1 to 4 carbon atoms difference, we found that, in contrast to the (111) surface, the adsorption selectivity on a NC is almost identical to that in the solvent.

The nine-site benzene model of Zhao et al. (J. Phys. Chem. B 2005, 109, 5368-5374) has been used to systematically study the adsorption of benzene, propene, and benzene-propene mixtures in zeolites mordenite, Y, , silicalite, and MCM22. Interaction parameters for the benzene-zeolite interactions have been fitted to available adsorption experiments from literature. As an independent check of our force field, we have performed additional adsorption experiments using the TEOM technique and excellent agreement between simulations and TEOM experiments was found. High adsorption selectivities for benzene in benzene-propene mixtures were found in all zeolites, except for silicalite.

In the present work, a recently developed force field for molecular simulation of the adsorption properties of linear alkanes in the sodium form of faujasite (FAU), MFI, and MOR-type zeolites is applied to reproduce experimental results of propane adsorption properties in zeolite 13X. The Lennard-Jones (LJ) parameters for interactions between the sp²-hybridized bond of propylene and the other atoms of this system are successfully determined by fitting against experimental isotherms. This new set of parameters allows the calculation of adsorption properties of propane-propylene mixtures, as well as the isosteric heat of single-component adsorption of propane and propylene in zeolite 13X. Good agreement between simulation results and experimental data for propane and propylene adsorption loadings at temperatures of 303, 323, 343, 373, 423, and 473 K and pressures in the range of 0.2-110 kPa confirms the applicability of the force field. In addition, molecular simulation will be used to guide the desorbent choice for propane/propylene separation by cyclic adsorptive processes.

In the present work, a recently developed force field for molecular simulation of the adsorption properties of linear alkanes in the sodium form of faujasite (FAU), MFI, and MOR-type zeolites is applied to reproduce experimental results of propane adsorption properties in zeolite 13X. The Lennard-Jones (LJ) parameters for interactions between the sp²-hybridized bond of propylene and the other atoms of this system are successfully determined by fitting against experimental isotherms. This new set of parameters allows the calculation of adsorption properties of propane-propylene mixtures, as well as the isosteric heat of single-component adsorption of propane and propylene in zeolite 13X. Good agreement between simulation results and experimental data for propane and propylene adsorption loadings at temperatures of 303, 323, 343, 373, 423, and 473 K and pressures in the range of 0.2-110 kPa confirms the applicability of the force field. In addition, molecular simulation will be used to guide the desorbent choice for propane/propylene separation by cyclic adsorptive processes.

To create porous scaffolds releasing in a controlled and independent fashion two different proteins, a novel approach based on protein-loaded polymeric coatings was evaluated. In this process, two water-in-oil emulsions are forced successively through a prefabricated scaffold to create coatings, containing each a different protein and having different release characteristics. In a first step, a simplified three-layered system was designed with model proteins (myoglobin and lysozyme). Poly(ether-ester) multiblock copolymers were chosen as polymer matrix, to allow the diffusion of proteins through the coatings. The model system showed the independent release of the two proteins. The myoglobin release was tailored from a burst to a linear release still on-going after 60 days, while the lysozyme release rate was kept constant. Macro-porous scaffolds, with a porosity of 59 vol.%, showed the same ability to control the release rate of the model proteins independently. The relation between the coatings properties and their release characteristics were investigated with the use of a mathematical diffusion model based on Fick's second law. It confirmed that the multiple coated scaffolds are biphasic system, where each coating controls the release of the protein that it contains. This approach could be of value for tissue engineering applications.

Granulaire materialen zoals zand en graankorrels hebben eigenschappen overeenkomstig met zowel vloeisto en als vaste sto en. Zo kan droog zand in een zandloper net als water stromen, terwijl als we op het strand lopen zand juist meer op een vas- te stof lijkt. Door deze bijzondere eigenschappen staan granulaire materialen in de belangstelling bij natuurkundigen. Een deel van het onderzoek naar deze materialen richt zich op zogenoemde krachtennetwerken.