No abstract

No abstract

No abstract

Hydrazine (HZ) and monomethylhydrazine (MMH) in air were monitored continuously using a hand-held ion mobility spectrometer equipped with membrane inlet, 63Ni ion source, acetone reagent gas, and ambient temperature drift tube. Response characteristics included detection limit, 6 ppb; linear range, 10-600 ppb; saturated response, >2 ppm; and stable response after 15-30 min. Ammonia interfered in hydrazines detection through a product ion with the same drift time as that for MMH and HZ. Acetone reagent gas was replaced with 5-nonanone to alter drift times of product ions and separate ammonia from MMH and HZ. Patterns in mobility spectra, ion identifications from mass spectra, and fragmentation cross-sections from collisional-induced dissociations suggest that drift times are governed by ion-cluster equilibria in the drift region of the mobility spectrometer. Practical aspects including calibration, stability, and reproducibility are reported from the use of a hand-held mobility spectrometer on the space shuttle Atlantis during mission STS-37.

Here, we report ab-initio calculations developed with a twofold purpose: understand how adsorbed water molecules alter the infrared spectrum of the metal-organic framework MIL-53(Al) and to investigate which are the associated physico-chemical processes. The analyzed structures are the two anhydrous narrow (np⊘) and large (lp⊘) pore forms and the hydrated narrow pore form (np-H2O) of the MIL-53(Al). For these structures, we determined their corresponding infrared spectra (FTIR) and we identified the vibrational modes associated to the dominant spectral lines. We show that wagging and scissoring modes of CO2 give flexibility to the structure for facilitating the lp⊘- np⊘ transition. In our studies, this transition is identified by eight vibrational modes including the δCH(18a) vibrational mode currently used to identify the mentioned transition. We report an exhaustive band identification of the infrared spectra associated to the analyzed structures. Moreover, the FTIR for the np-H2O structure allowed us to identify four types of water molecules linked to the host structure by one to three hydrogen bonds.

We investigate the oxidation of aluminum low-index surfaces ͓͑100͒, ͑110͒, and ͑111͔͒ at low temperatures ͑300-600 K͒ and three different gas pressure values. We use molecular dynamics ͑MD͒ simulations with dynamic charge transfer between atoms where the interaction between atoms is described by the Es+ potential composed of the embedded atom method ͑EAM͒ potential and an electrostatic contribution. In the considered temperature range and under different gas pressure conditions, the growth kinetics follow a direct logarithmic law where the oxide thickness is limited to a value of ϳ3 nm. The fitted curves allow us to determine the temperature and the pressure dependencies of the parameters involved in the growth law. During the adsorption stage, we observe a rotation of the oxygen pair as a precursor process to its dissociation. In most cases, the rotation aligns the molecule vertically to the Al surface. The separation distance after dissociation ranges from 3 to 9 Å. Atomistic observations revealed that the oxide presents a dominant tetrahedral ͑AlO 4 ͒ environment in the inner layer and mixed tetrahedral and octahedral ͑AlO 6 ͒ environments in the outer oxide region when the oxide thickness reaches values beyond ϳ2 nm.

In this paper we report Grand Canonical Monte Carlo simulations to characterize the competitive trapping of CO and N 2 molecules into clathrates, for various gas compositions in the temperature range from 50 to 150 K. The simulations evidence a preferential trapping of CO with respect to N 2 . This leads to the formation of clathrates that are preferentially filled with CO at equilibrium. This is irrespective of the composition of the gas phase, the fugacity and the temperature. Moreover, the results of the simulations show that the small cages of the clathrate structure are always filled in first place, and this is independent of either the guest structure or the temperature. This issue has been associated to the rather significant differences in the calculated heats of encapsulation (∼2-3 kJ/mol) between the small and the larges cages. In addition, calculations with the simplified ideal adsorbed solution theory (IAST) are developed for allowing a comparison with the results arising from the GCMC simulations. Interestingly, this shows that the occupancy isotherms of the mixed N 2 -CO clathrates can be perfectly represented by knowing the occupancy isotherms of the corresponding singleguest clathrates. This suggests that experiments performed with the single guest CO and N 2 clathrates might be sufficient to get information concerning the corresponding mixed clathrates by using the IAST approach.

No abstract