Vapor phase nucleation on neutral and charged nanoparticles: Condensation of supersaturated trifluoroethanol on Mg nanoparticles

A new technique is described to study the condensation of supersaturated vapors on nanoparticles under well-defined conditions of vapor supersaturation, temperature, and carrier gas pressure. The method is applied to the condensation of supersaturated trifluoroethanol (TFE) vapor on Mg nanoparticles. The nanoparticles can be activated to act as condensation nuclei at supersaturations significantly lower than those required for homogeneous nucleation. The number of activated nanoparticles increases with increasing the vapor supersaturation. The small difference observed in the number of droplets formed on positively and negatively charged nanoparticles is attributed to the difference in the mobilities of these nanoparticles. Therefore, no significant charge preference is observed for the condensation of TFE vapor on the Mg nanoparticles

Direct observation of metal nanoparticles as heterogeneous nuclei for the condensation of supersaturated organic vapors: Nucleation of size-selected aluminum nanoparticles in acetonitrile and n-hexane vapors

This work reports the direct observation and separation of size-selected aluminum nanoparticlesacting as heterogeneous nuclei for the condensation of supersaturated vapors of both polar and nonpolar molecules. In the experiment, we study the condensation of supersaturated acetonitrile and n-hexane vapors on charged and neutral Al nanoparticles by activation of the metalnanoparticles to act as heterogeneous nuclei for the condensation of the organic vapor.Aluminum seed nanoparticles with diameters of 1 and 2 nm are capable of acting as heterogeneous nuclei for the condensation of supersaturated acetonitrile and hexane vapors. The comparison between the Kelvin and Fletcher diameters indicates that for theheterogeneous nucleation of both acetonitrile and hexane vapors, particles are activated at significantly smaller sizes than predicted by the Kelvin equation. The activation of the Alnanoparticles occurs at nearly 40% and 65% of the onset of homogeneous nucleation of acetonitrile and hexane supersaturated vapors, respectively. The lower activation of the chargedAl nanoparticles in acetonitrile vapor is due to the charge-dipole interaction which results in rapid condensation of the highly polar acetonitrile molecules on the charged Al nanoparticles.The charge-dipole interaction decreases with increasing the size of the Al nanoparticles and therefore at low supersaturations, most of the heterogeneous nucleation events are occurring on neutral nanoparticles. No sign effect has been observed for the condensation of the organic vapors on the positively and negatively charged Al nanoparticles. The present approach of generating metal nanoparticles by pulsed laser vaporization within a supersaturated organic vapor allows for efficient separation between nucleation and growth of the metal nanoparticlesand, consequently controls the average particle size, particle density, and particle size distribution within the liquid droplets of the condensing vapor. Strong correlation is found between the seed nanoparticle\u27s size and the degree of the supersaturation of the condensing vapor. This result and the agreement among the calculated Kelvin diameters and the size of the nucleating Al nanoparticles determined by transmission electron microscopy provide strong proof for the development of a new approach for the separation and characterization of heterogeneous nuclei formed in organic vapors. These processes can take place in the atmosphere by a combination of several organic species including polar compounds which could be very efficient in activating charged nanoparticles and cluster ions of atmospheric relevance

Monte Carlo simulation of acetonitrile clusters [CH3CN] N , N=2–256: Melting transitions and even/odd character of small clusters (N=2–9), heat capacities, density profiles, fractal dimension, intracluster dimerization, and dipole orientation

The thermodynamic and structural properties of acetonitrile clusters [CH3CN] N , N=2–15, 20, 30, 60, 128, and 256 have been investigated using Monte Carlo simulation. Interactions in the small clusters (N≤9) are dominated by antiparallel pairing of the molecular dipoles. The simulations reveal rigid ↔ fluid (melting) transitions with a remarkable even–odd alternation in the transition temperatures for the N=2–9 clusters. The higher melting temperatures of the even‐N clusters arise as consequences of the antiparallel paired dipoles which provide favorable electrostatic interactions. Even–odd alternation has also been observed in the configurational energies and heat capacities and the percentage of molecules possessing an antiparallel nearest neighbor. These observations are consistent with the fact that Coulomb potential terms dominate the interaction energies in clusters with NN≳60 is fairly well represented by the bulk liquid density. Order parameters characterizing dipole orientation indicate that the molecular dipoles tend to lie flat on the cluster surface for N≥30. Significant dimerization within the clusters suggests evaporation of molecules via dimers and an enhancement of evaporative loss over condensation and this may explain the slower nucleation rates observed for acetonitrile compared to the predictions of the classical nucleation theory

Gas phase clustering of N2 molecules on to TiO+: Comparison with Ti+ and evidence for the octahedral structure of TiO+(N2)5

A striking difference in the clustering efficiency of Ti+ and TiO+ toward N2 or O2 has been observed in a laser vaporizationhigh pressure mass spectrometric source. Evidence for the magic number n=5 within the sequence TiO+(N2) n is presented. The results are consistent with an octahedral structure for TiO+(N2)5

Palladium Nanoparticles Supported on Ce-Metal–Organic Framework for Efficient CO Oxidation and Low-Temperature CO2 Capture

In this article, we report the lowest-temperature CO oxidation catalyst supported on metal–organic frameworks (MOFs). We have developed a facile, general, and effective approach based on microwave irradiation for the incorporation of Pd nanoparticle catalyst within Ce-MOF. The resulting Pd/Ce-MOF material is a unique catalyst that is capable of CO oxidation at modest temperatures and also of efficient uptake of the product CO2 gas at low temperatures. The observed catalytic activity of this material toward CO oxidation is significantly higher than those of other reported metal nanoparticles supported on MOFs. The high activity of the Pd/Ce-MOF catalyst is due to the presence of Ce(III) and Ce(IV) ions within the metal–organic framework support. The Pd nanoparticles supported on the Ce-MOF store oxygen in the form of a thin palladium oxide layer at the particle–support interface, in addition to the oxygen stored on the Ce(III)/Ce(IV) centers. Oxygen from these reservoirs can be released during CO oxidation at 373 K. At lower temperatures (273 K), the Pd/Ce-MOF has a significant CO2 uptake of 3.5 mmol/g

Experimental and theoretical study of benzene (acetonitrile)(n) clusters, n=1-4

Well-resolved spectra of benzene–acetonitrile binary clustersBAn, with n=1–4 have been obtained by the (one-color) resonant two-photonionization technique using the benzene’s B2u←A1g 000 and 610 resonances. The spectra reveal a rapid increase in complexity with the number of acetonitrile molecules in the cluster, associated with van der Waal modes and isomeric forms. While only single cluster origins are found for the benzene–acetonitrile (BA) and the BA2clusters, two and four distinct isomers are identified for the BA3 and BA4clusters, respectively. The origins of the BA and BA2clusters are blueshifted with respect to the free benzene molecule by 38 cm−1 and 26 cm−1, respectively. Monte Carlo(MC) simulations reveal two types of isomeric structures of the BAnclusters. The clusters containing an even number of the acetonitrile molecules (BA2, BA4, and BA6) are dominated by acetonitrile anti-parallel paired dimers. The BA3cluster consists of a cyclic acetonitrile trimer parallel to the benzene ring. In the BA5clusters, the acetonitrile molecules are assembled in a cyclic trimer + a paired dimer configuration or in two paired dimers + a single monomer structure. The R2PI spectra, in conjunction with the MC structural models and simple energetic arguments, provide a reasonably compelling picture of the spectroscopic and dynamical phenomena associated with dipole pairing molecular cluster systems

Kinetics of ion-induced nucleation in a vapor-gas mixture

A general solution for the steady-state ion-induced nucleation kinetics has been derived, considering the differences between ion-induced nucleation and homogeneous nucleation. This solution includes a new effect for nucleation kinetics, the interaction of charged clusters with vapor molecules. Analytical expressions for the ion-induced nucleation rate have been obtained for the limiting cases of high and low thermodynamic barriers. The physical explanation of the so-called sign effect is proposed based on multipole expansion of an electric field of the cluster ion. This theory gives good agreement with experiments and is used to elucidate experimentally observed phenomena

Structure and hydration of the C4H4 •+ ion formed by electron impact ionization of acetylene clusters

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C4H4 •+ ion in the cluster beam. The measured average collision cross section of the C4H4 •+ isomers in helium (38.9 ± 1 Å2) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C4H4 •+ ion [methylenecyclopropene (39.9 Å2), 1,2,3-butatriene (41.1 Å2), cyclobutadiene (38.6 Å2), and vinyl acetylene (41.1 Å2)]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C4H4 •+ ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C4H4 •+ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C2H2)2 •+[R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys.131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C4H4 •+(H2O)nclusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured bindingenergy of the C4H4 •+H2O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene•+·H2O cluster (41 kJ/mol). The binding energies of the C4H4 •+(H2O)n clusters change little from n = 1 to 5 (39–48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water–C4H4 •+ and water–waterinteractions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C4H4 •+(H2O)5 cluster. Consequently, a drop in the binding energy of the sixth watermolecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C4H4 •+(H2O)5 cluster presumably consisting of a cyclobutadiene•+ cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C2H2)2 •+ ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation