Hydrogen thermal desorption spectra: insights from molecular simulation

Thermal desorption spectra of a number of metal-organic frameworks were studied using grand canonical Monte Carlo simulation. Our simulation results are in qualitative agreement with experimental results but also show that great care must be taken when choosing the force field to describe the hydrogen/framework interaction. As the simulations additionally yield the positions and potential energies of the adsorbed molecules it is straightforward to assign the peaks and features in the thermal desorption spectra to specific adsorption sites. We show that the location of the peaks is directly related to the hydrogen-framework interaction which is a complex function of the chemical and topological environment of the pore space, the pore size and the presence of specific interaction sites such as open metal sites. Finally, we demonstrate that an IRMOF-8 sample used to obtain an experimental thermal desorption spectrum must have indeed been catenated as previously suspected. Overall, molecular simulation is a useful tool to complement the interpretation of experimental thermal desorption spectra.     

Regularities of oxygen and water thermal desorption from barium cerate doped by neodymium,samarium, and gadolinium

Processes of thermal desorption of oxygen molecules and water from BaCe_1–x M _x O_3–δ, where M= Nd, Sm, and Gd, presintered in air at the temperature of 650°C are studied. It is found that oxygen is desorbed only from neodymium–doped barium cerate and is almost not evolved from barium cerate doped by samarium and gadolinium. The amount of desorbed oxygen features a square dependence on cationic doping by neodymium. At similar degrees of cationic doping, the amount of water desorbed from neodymium–doped barium cerate is always lower than that from the cerate doped by samarium and gadolinium. The obtained experimental data on thermal desorption and analysis of literature data served as a basis for the conclusion as to the mixed valency of neodymium Nd(III)–Nd(IV) in BaCe_1–x Nd _x O_3–δ. In this case, at similar doping degrees x, the hydration degree of BaCe_1–x Nd _x O_3–δ is lower and the oxygen index is higher than in BaCe1_–x (Sm,Gd) _x O_3–δ. The differences become more pronounced at high degrees of cationic doping and must decrease at an increase in temperature.     

Water desorption from an oxygen covered Pt(111) surface: multichannel desorption

Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.     

Field desorption mass spectra of phthalocyanines

Mass spectr of phthalocyanines are readily obtainable by field desorption mass spectrometry. Molecular ions are normally the base peaks in the spectra and minimal fragmentation is observed. The technique provides a rapid method for the qualitative exmination of mixtures of phthalocyanines.     

Analysis of airborne substances by means of microwave thermal desorption

A method has been developed to enable thermal desorption of airborne substances from solid adsorbents based on active charcoal by means of microwave energy, followed by on-line GC-FID analysis. Ceramic traps were filled with 50 mg of commercially available graphitized carbon black. Adsorption efficiency and desorption conditions for three substances covering a broad range of vapor pressure were investigated. The recovery rate, breakthrough, and linearity of the system were determined by direct fortification with standard solutions, followed by microwave thermal desorption and gas chromatographic separation. The recovery efficiencies were 80–84 %, with a standard deviation of 3–6 %. The correlation coefficient of the linear range between 1 (10 for one of the compounds) and 1000 ng was > 0.999. Initial experiments were performed to investigate the enrichment of the standards from the gaseous state. The recovery rates of the most volatile compound was increased by optimizing the density of the packing.     

The thermal desorption mass spectra of polytetrafluoroethylene modified under the action of radiation at elevated temperatures

The products of the pyrolysis of polytetrafluoroethylene (PTFE) in a vacuum modified under the action of gamma irradiation in the molten state were studied mass spectrometrically at various temperatures. Radiation modification was found to cause the appearance of an additional stage of the thermal decomposition of PTFE at decreased temperatures compared with not irradiated PTFE. The chemical composition of the gas components of thermal decomposition substantially changes after the irradiation of PTFE, which increases the molecular weight of decomposition products. A comparison with the results of an additional study of the thermal decomposition of tetrafluoroethylene copolymer with hexafluoropropylene is evidence of the formation of side CF₃ groups in PTFE under irradiation. Data on the composition of gaseous products of PTFE radiolysis under gamma irradiation were obtained.     

The thermal behaviour of some rhodium complexes

The thermal behaviour of several complexes of rhodium has been investigated. Complexes containing nitrogen ligands readily decompose to the oxide, Rh₂O_3^? Complexes with phosphorus and arsenic ligands decompose to the same oxide of rhodium, although difficulty is encountered in removing all the phosphorus and arsenic. The suitability of the decomposition to Rh₂O₃ as an analytical technique for rhodium is discussed.     

Effects of adsorbate islanding on reaction kinetics and thermal desorption spectra: a monte carlo-quasichemical model

The role of adsorbate lateral interactions in surface reaction kinetics is studied for bimolecular reactions A + B → AB. One reactant (A) tends to aggregate into islands, B is mobile and AB is a rapidly desorbing product. A model combining Monte Carlo dynamics for the slow processes (A-aggregation and AB-formation) and an appropriately modified quasichemical approximation for the fast ones (B-redistribution) is used to calculate reaction rates. Reaction desorption spectra and activation energies are calculated for different initial conditions and adsorbate lateral interactions.     

Rapid thermal hydrogen production from 2,3-butanediol catalyzed by homogeneous rhodium catalysis

Thermal production of hydrogen from butane-2,3-diol is achieved with rates of up to 125 catalyst turnovers per hour using the homogeneous rhodium catalysts, [Rh(bipy)₂]Cl, [RhH(PⁱPr₃)₃] and [RhCl(PPh₃)₃]. (bipy = 2,2′-bipyridyl).     

Field desorption mass spectra of stereoisomeric glycosides

Field desorption mass spectrometry (FD-MS) yields differentiated spectra for unshielded α- and β-stereoisomeric glycosides. The small but characteristic differences enable a distinction to be made between the two isomers. Valuable structural information can be deduced from the simple cleavage between the sugar moiety of the molecule and the aglycon. A short comparison with electron-impact (EI) and field ionisation (FI) spectra is given.     

Angle-resolved thermal desorption of atoms from solid surfaces: one-phonon mechanism

A model for one-phonon thermal desorption is presented in which the structure of the substrate phonons, expressed as a projection on a surface atom of the phonon density of states, appears as a separate factor in the angle- and energy-resolved desorption rate. Desorption from both localized, and delocalized initioladatom states is considered. Under certain circumstances one can obtain the cosine-distribution of the equilibrium theory, but in general, the desorption flux from delocalized states deviates from the cosine law by being peaked away from the surface normal, whereas for localized initiol states, the flux is concentrated more in the normal direction.     

Oxygen interaction with rhodium at low pressures II. Oxygen desorption

Oxygen desorption on polycrystalline Rh and single crystal Rh(100) at 400–1600 K has been studied using thermal desorption and numerical calculation methods. With low surface coverages (<0.3) the adsorbed oxygen particles pentrate the metal and the diffuse back to the surface (peak ₂). At >0.3 the processes of formation/decomposition of surface oxide Rh₂O₃ takes place (peak ₁).

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- O₂ Rh Rh(100) 400–1600 . (<0,3) O ë ( ₂), >0,3 - - (Rh₂O₃) ( ₁).

     

Low energy electron stimulated desorption from DNA films dosed with oxygen

Desorption of anions stimulated by 1-18 eV electron impact on self-assembled monolayer (SAM) films of single DNA strands is measured as a function of film temperature (50-250 K). The SAMs, composed of 10 nucleotides, are dosed with O(2). The OH(-) desorption yields increase markedly with exposure to O(2) at 50 K and are further enhanced upon heating. In contrast, the desorption yields of O(-), attributable to dissociative electron attachment to trapped O(2) molecules decrease with heating. Irradiation of the DNA films prior to the deposition of O(2) shows that this surprising increase in OH(-) desorption, at elevated temperatures, arises from the reaction of O(2) with damaged DNA sites. These results thus appear to be a manifestation of the so-called "oxygen fixation" effect, well known in radiobiology.     

Rotational cooling of rigid rotors desorbed from flat surfaces

A simple model is presented for predicting the final rotational state distribution of an initially physisorbed rigid rotor. Based on the assumptions that the adsorbed rotor is freely rotating and that desorption occurs by a weak coupling between the rotational and desorbing degrees of freedom, a significant rotational “cooling” is predicted.