Analysis of molecular hydrogen formation on low-temperature surfaces in temperature programmed desorption experiments

The study of the formation of molecular hydrogen on low-temperature surfaces is of interest both because it enables the exploration of elementary steps in the heterogeneous catalysis of a simple molecule and because of its applications in astrochemistry. Here, we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments, beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations, and the energy barriers of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results are used in order to evaluate the formation rate of H2 on dust grains under the actual conditions present in interstellar clouds. It is found that, under typical conditions in diffuse interstellar clouds, amorphous silicate grains are efficient catalysts of H2 formation when the grain temperatures are between 9 and 14 K. This temperature window is within the typical range of grain temperatures in diffuse clouds. It is thus concluded that amorphous silicates are good candidates to be efficient catalysts of H2 formation in diffuse clouds.     

Short range order at the amorphous TiO(2)-water interface probed by silicic acid adsorption and interfacial oligomerization: an ATR-IR and 29Si MAS-NMR study

Adsorption and oligomerization of H(4)SiO(4) at the amorphous TiO(2)-aqueous interface were studied using in situ Attenuated Total Reflectance Infrared (ATR-IR) and ex situ solid state (29)Si nuclear magnetic resonance (NMR). The ATR-IR spectra indicate that a monomeric silicate species is present at low silicate surface concentration (Γ(Si)). Above a threshold Γ(Si) linear silicate oligomers are formed and these oligomers dominate the surface at high Γ(Si). Interestingly the ATR-IR spectra of H(4)SiO(4) on the TiO(2) surface are very similar to those previously observed on the poorly ordered iron oxide phase ferrihydrite. The (29)Si NMR spectrum of silicate on the TiO(2) surface shows the presence of Si in three states with chemical shifts corresponding to isolated monomers (Q(0)), the ends of linear oligomers (Q(1)) and the middle of linear oligomers (Q(2)). The ratio of the area of the Q(1) and Q(2) peaks was ≈2:1 which is consistent with the proposed formation of linear silicate trimers by insertion of a solution H(4)SiO(4) between adjacent suitably orientated adsorbed silicate monomers. A structural interpretation indicates that the observed interfacial silicate oligomerization behavior is a general phenomenon whereby bidentate silicate monomers on oxide surfaces are disposed towards forming linear oligomers by condensation reactions involving their two terminal Si-OH groups. The high surface curvature of nanometer sized spheres inhibits the formation of interfacial silicates with a higher degree of polymerization.     

Zum Aufbau schlecht geordneter Calciumhydrogensilicate. IV. Anionenzusammensetzung der Hydratationsprodukte des Tricalciumsilicats

On the Structure of III-crystallized Calcium Hydrogen Silicates. IV. Anion Composition of the Hydration Products of Tricalcium Silicate On the hydration of tricalcium silicate at 2–80°C calcium hydrogen silicates (C? S? H-phases) are formed. The mean degree of condensation of which increases with increasing temperature. The primary product of hydration is monosilicate. It condenses at 2°C to disilicate. At higher temperatures, especially at 80°C, a C? S? H-phase is formed, consisting of di- and polysilicate. These phases are compared with the former described C? S? H(di) [1] and C? S? H(di, poly) [2, 3]. The amount of low molecular cyclic silicate anions in the C? S? H-phases is lower than 1%.     

Adsorption, desorption, and diffusion of nitrogen in a model nanoporous material. I. Surface limited desorption kinetics in amorphous solid water

The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programed desorption (TPD), and reflection-absorption infrared spectroscopy. The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness indicating that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage-dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, approximately 0.5 microm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.     

Evolution of Cu-Zn-Si oxide catalysts in the course of reduction and reoxidation as studied by in situ X-ray diffraction analysis, transmission electron microscopy, and magnetic susceptibility methods

The reduced and reoxidized Cu-Zn-Si oxide catalysts as layered copper-zinc hydroxo silicates with the zincsilite structure were studied using in situ and ex situ X-ray diffraction analysis, transmission electron microscopy, and the temperature dependence of magnetic susceptibility. The catalysts were prepared by homogeneous deposition-precipitation. It was found that Cu⁰ particles were formed on the surface of a layered hydrosilicate with the zincsilite structure upon reduction with hydrogen. The reoxidation of the reduced samples with a mixture of oxygen and an inert gas, which contained no more than 0.05 vol % O₂, resulted in the formation of individual Cu₂O and CuO phases; copper ions did not return to the hydrosilicate structure. Catalytic tests of Cu-Zn-Si catalysts in methanol synthesis indicate that the specific catalytic activity of copper metal particles grows linearly with increasing zinc loading. This fact suggests that copper metal particles, which were obtained by the reduction of Cu²⁺ ions from the copper-zinc hydroxo silicate with the zincsilite structure, were responsible for activity in methanol synthesis. Consequently, the ability to return copper ions to a precursor compound in reoxidation with oxygen at low concentrations, which is known for reduced Cu/ZnO catalysts (these catalysts are highly active in methanol synthesis), is not related to the catalytic activity in methanol synthesis.     

Crystallization kinetics and excess free energy of H2O and D2O nanoscale films of amorphous solid water

Temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) are used to investigate the crystallization kinetics and measure the excess free energy of metastable amorphous solid water films (ASW) of H(2)O and D(2)O grown using molecular beams. The desorption rates from the amorphous and crystalline phases of ASW are distinct, and as such, crystallization manifests can be observed in the TPD spectrum. The crystallization kinetics were studied by varying the TPD heating rate from 0.001 to 3 K/s. A coupled desorption-crystallization kinetic model accurately simulates the desorption spectra and accurately predicts the observed temperature shifts in the crystallization. Isothermal crystallization studies using RAIRS are in agreement with the TPD results. Furthermore, highly sensitive measurements of the desorption rates were used to determine the excess free energy of ASW near 150 K. The excess entropy obtained from these data is consistent with there being a thermodynamic continuity between ASW and supercooled liquid water.     

Structural, microstructural and vibrational characterization of apatite-type lanthanum silicates prepared by mechanical milling

Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A-La₂O₃ and either amorphous or low cristobalite SiO₂. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating disc speed (350 rpm), allows the formation of the target phase after only 3 h although longer milling times are needed to eliminate all SiO₂ and La₂O₃ traces. Thus, the mechanically activated chemical reaction proceeds faster when using amorphous silica instead of low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h, respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate. The domain size of the as-prepared phases varies gradually with the temperature of post-milling thermal treatment with activation energies of about 26(8) and 52(10) kJ mol⁻¹ K⁻¹ for the apatites obtained from amorphous silica and low-cristobalite, respectively. These values suggest crystallite growth to be favored when using amorphous silica as reactant.     

Morphology and rheology of polystyrene nanocomposites based upon organoclay

A correlation between morphology development and rheology of polystyrene nanocomposites based upon organophilic layered silicates (organoclay) such as fluoromicas was found as a function of the silicate modification. Organoclay was obtained by means of ion exchange of clay with protonated amine‐terminated polystyrenes with molar mass of M_n = 121 and 5 800 g/mol. Only when applying shear forces during melt compounding of organoclay modified with high molar mass polystyrene (PS), individual silicate platelets of 1 nm diameter and 600 nm length were obtained. Dispersions of such in‐situ formed nanoparticles with aspect ratio of 600 accounted for unique elastic properties observed in the low frequency range of the dynamic modulus, whereas organoclay modified with low molecular PS did not exfoliate and exhibited rheological behavior very similar to that of conventional fillers.     

Structural and kinetic effects on a simple catalytic reaction: oxygen reduction on Ni(110)

Oxygen hydrogenation at 100 K by gas phase atomic hydrogen on Ni(110) has been studied under ultrahigh vacuum conditions by temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS). Formation of adsorbed water and hydroxyl species was observed and characterized. The coverage of the reaction products was monitored as a function of both temperature and initial oxygen precoverage. On the contrary, when high coverage oxygen overlayers were exposed to gas phase molecular hydrogen, no hydrogenation reaction took place. The results are compared to the inverse process, exposing the hydrogen covered surface to molecular oxygen. In this case, at 100 K, simple Langmuir-Hinshelwood modeling yields an initial sticking coefficient for oxygen adsorption equal to 0.26, considerably lower than for the clean surface. Moreover, formation of hydroxyl groups is found to be twice as fast as the final hydrogenation of OH groups to water. Assuming a preexponential factor of 10(13) s(-1), an activation barrier of 6.7 kcal/mol is obtained for OH formation, thus confirming the high hydrogenating activity of nickel with respect to other transition metals, for which higher activation energies are reported. However, oxygen is hardly removed by hydrogen on nickel: this is explained on the basis of the strong Ni-O chemical bond. The hydrogen residual coverage is well described including a contribution from the adsorption-induced H desorption process which takes place during the oxygen uptake and which is clearly visible from the TPD data.     

In Situ X-Ray Diffraction Study of the Initial Stages of Formation of MCM-41 in a Tubular Reactor

A continuous process for the synthesis of mesoporous silicates such as MCM-41 at room temperature has been achieved by employing a tubular reactor in which tetraethoxysilane was used as the silicate source (see schematic representation below). The formation of the hexagonal mesophase, which was monitored by using in situ XRD at different points of the tube, was completed in three minutes after mixing of the reactants. The product was stable to calcination, and N₂-sorption measurements confirmed the high surface area and narrow pore-size distribution.     

Study of the reaction of tetramethyltin with Hβ zeolite

The grafting reaction of SnMe₄ on the surface of Hβ zeolite (Si/Al = 25.0) was studied under high vacuum conditions. The chemical compositions and structure of the resulting solid were characterized by in situ FTIR, ICP, XRD, XPS, ¹³C and ¹¹⁹Sn MAS NMR, UV-Vis DRS, gas phase chromatogram (GC), chemical analyses, temperature programmed decomposition (TPD), temperature programmed reduction (TPR), and N₂ adsorption at low temperature. The results showed that SnMe₄ reacts with Hβ zeolite quantitatively at temperature as low as 183 K and the reaction occurred on the surface of the zeolite without destroying the framework of zeolite. The BET surface area and the pore volume of the zeolite decreased and the surface properties changed, however the microporous structure was retained upon the reaction and post treatment.     

Synthese sowie Kristall- und Molekülstruktur von Tetrafluoro[2-(pyrrolidinio)ethyl]silicat

Synthesis and Crystal and Molecular Structure of Tetrafluoro[2-(pyrrolidinio)ethyl]silicate The zwitterionic tetrafluoro[2-(pyrrolidinio)ethyl]silicate ( 4 ) was synthesized by reaction of trimethoxy(2-pyrrolidinoethyl)silane ( 5 ) with hydrogen fluoride in ethanol/hydrofluoric acid at 0°C. The crystal and molecular structure of 4 was studied at ?100°C by single-crystal X-ray diffraction. In addition, 4 was characterized by solution-state NMR studies (CD₃CN: ¹H, ¹³C).     

Dianionic Tris[oxalato(2—)]silicate and Tris[oxalato(2—)]germanate Complexes: Synthesis,Properties, and Structural Characterization in the Solid State and in Solution

Reaction of tetramethoxysilane with three molar equivalents of oxalic acid and two molar equivalents of 1‐(2‐hydroxyethyl)‐pyrrolidine or 1‐(2‐hydroxyethyl)piperidine in tetrahydrofuran yielded the λ⁶Si‐silicates 1‐(2‐hydroxyethyl)pyrrolidinium tris[oxalato(2—)]silicate ( 4 ) and 1‐(2‐hydroxyethyl)piperidinium tris[oxalato(2—)]silicate ( 5 ). The related germanium compounds 1‐(2‐hydroxyethyl)piperidinium tris[oxalato(2—)]germanate ( 6 ) and triethylammonium tris[oxalato(2—)]germanate ( 7 ) were synthesized analogously, starting from tetramethoxygermane and using three molar equivalents of oxalic acid and two molar equivalents of 1‐(2‐hydroxyethyl)piperidine or triethylamine. Compounds 4 — 7 were characterized by elemental analyses (C, H, N), single‐crystal X‐ray diffraction, solid‐state VACP/MAS NMR spectroscopy (²⁹Si), and solution NMR spectroscopy (¹H, ¹³C, ²⁹Si). The structural characterization was complemented by computational studies of the tris[oxalato(2—)]silicate dianion and the tris[oxalato(2—)]germanate dianion. In addition, the stability of compounds 4 — 7 in aqueous solution was studied by ¹³C NMR spectroscopy.     

Variable temperature 1H, 23Na,and 29Si MAS NMR studies on sodium silicate hydrates of composition Na2O · SiO2 · nH2O (n = 9, 6, 5): Local structure in crystals,melts, supercooled melts,and glasses

The local structure in crystals, melts, supercooled melts, and glasses of sodium silicate hydrates of composition Na₂O · SiO₂ · nH₂O (n = 9, 6, 5) is studied by variable temperature ¹H, ²³Na, and ²⁹Si MAS NMR spectroscopy. Detailed in situ investigations on the melting process of the crystalline materials reveal the importance of H₂O motion in the melting mechanism. Depending on the local coordination, crystallographically distinct Na sites show different behaviour during the melting process. Upon melting, the monomer silicate anions present in the crystalline hydrates undergo condensation reactions to oligomeric silicate anions. No recrystallization but glass formation occurs at low temperature if the melts were heated initially about 10 K above the melting point. In the glasses also oligomeric silicate anions are present with a preference for cyclotrimer species. In situ MAS NMR investigations and electric conductivity measurements of the melts, supercooled melts, and glasses suggest the distinction of three temperature ranges characterized by different local structure and dynamics of the sodium cations, water and silicate anions. These ranges comprise a glass and glass transition range A at low temperatures, an aggregation region B at intermediate temperatures, and a solution or electrolyte region C at high temperatures. In region B aggregation of sodium water complexes to hydrated polycation clusters is suggested, the dynamic behaviour of which is clearly different to that of the silicate anions, indicating that no long-lived contact ion pairs between sodium cations and silicate anions are formed.     

Isotope tracer study of hydrogen spillover on carbon-based adsorbents for hydrogen storage

A composite material comprising platinum nanoparticles supported on molecular sieve templated carbon was synthesized and found to adsorb 1.35 wt % hydrogen at 298 K and 100 atm. The isosteric heat of adsorption for the material at low coverage was approximately 14 kJ/mol, and it approached a value of 10.6 kJ/mol as coverage increased for pressures at and above 1 atm. The increase in capacity is attributed to spillover, which is observed with the use of isotopic tracer TPD. IRMOF-8 bridged to Pt/C, a material known to exhibit hydrogen spillover at room temperature, was also studied with the hydrogen-deuterium scrambling reaction for comparison. The isotherms were reversible. For desorption, sequential doses of H2 and D2 at room temperature and subsequent TPD yield product distributions that are strong indicators of the surface diffusion controlled reverse spillover process.     

Influence of compatibilizer degradation on formation and properties of PA6/organoclay nanocomposites

Nanocomposites based on polyamide 6 (PA6) and commercial layered silicates have been prepared by both in situ polymerization and melt compounding. The main aim of the present work has been centred on compatibilizer degradation, caused by the preparation conditions, in terms of nanocomposite end features. Two montmorillonite (MMT)-type, organically-modified clays (OMLS), namely Cloisite 30B^® and Nanofil 784^®, and a sodium MMT (Cloisite Na^®) have been studied. Thermal properties of the layered silicates have been evaluated by TGA, IR, WAXD and pyrolysis-gas-mass. In order to better assess the influence of high temperature processes on clay modifications, a thermal treatment which mimics the conditions used during the in situ polymerization (4 h at 250 °C) has been applied on layered silicates. The above treatment, besides the elimination of absorbed water from all the clays, turned out to prove noteworthy differences in compatibilizer modification for the two organoclays. Indeed, in the case of Closite 30B^® only a removal of organic molecules outside the silicate galleries and a likely reorganization of those present inside the galleries have been detected, while a relevant chemical modification of Nanofil 784^® compatibilizer has been conversely found.As far as nanocomposite characteristics are concerned, the latter have been found to depend on both the preparation method and clay type. In the case of in situ polymerization, also thermally-treated layered silicates, coded (T), have been used, in order to put more clearly in evidence the role of compatibilizer decomposition on nanocomposite formation and properties. Indeed, nanocomposite samples containing Closite 30B^®(T) have been found to be completely exfoliated, while the same thermal treatment seems to make worse the properties of those based on Nanofil 784^®(T). Furthermore, with respect to nanocomposites based on pristine clays, samples containing thermally-treated silicates turned out to be different in terms of both molecular mass and crystal structure of the polymer matrix. Namely, PA6 γ-form seems to be promoted for all nanocomposites prepared in such a way, probably because of water removal at high temperature, which makes -OH groups of the layered silicate more free to interact with polyamide chains, thus causing a restriction of their mobility.     

Zum Aufbau schlecht geordneter Calciumhydrogensilicate. I. Bildung und Eigenschaften einer schlecht geordneten Calciumhydrogendisilicatphase

On the Structure of Ill-crystallized Calcium Hydrogen Silicates. I. Formation and Properties of an Ill-crystallized Calcium Hydrogen Disilicate Phase Investigations on ill-crystallized calcium hydrogen silicates (C? S? H phases) prepared by precipitation from sodium silicate solutions with calcium chloride have shown that at temperatures 0°C primarily hydrogen silicates with such an anion composition will be formed, which had been present before in the sodium silicate solution. They are not stable. On storing the precipitate in the mother liquid at 0°C they are converted into disilicate with a composition of 1.1–1.5 CaO/SiO₂, if the concentration of Ca(OH)₂ is more than 0.8 g/l. After drying the thermally very instable solid at ?10°C the calcium hydrogen disilicate phase can be isolated. A structure proposal for this phase is given.     

Atomic hydrogen interaction with Ru(1010)

The interaction of atomic hydrogen with clean and deuterium precovered Ru(1010) was studied by means of temperature-programmed desorption (TPD) spectroscopy. Compared to molecular hydrogen experiments, after exposure of the clean surface to gas-phase atomic hydrogen at 90 K, two additional peaks grow in the desorption spectra at 115 and 150 K. The surface saturation coverage, determined by equilibrium between abstraction and adsorption reactions, is 2.5 monolayers. Preadsorbed deuterium abstraction experiments with gas-phase atomic hydrogen show that a pure Eley-Rideal mechanism is not involved in the process, while a hot atom (HA) kinetics describes well the reaction. By least-squares fitting of the experimental data, a simplified HA kinetic model yields an abstraction cross section value of 0.5 +/- 0.2 angstroms2. The atomic hydrogen interaction with an oxygen precovered surface was also studied by means of both TPD and x-ray photoelectron spectroscopy: oxygen hydrogenation and water production take place already at very low temperature (90 K).     

Molecular aggregation of rhodamine dyes in dispersions of layered silicates: influence of dye molecular structure and silicate properties

The molecular aggregation of six rhodamine dyes (rhodamine 560, B, 3B, 19, 6G, 123) in layered silicate (saponite and fluorohectorite) dispersions was investigated by using visible (vis) spectroscopy. The dye molecular aggregation was influenced by the properties of both the silicates and the dyes themselves. The layer charge of the silicates enhanced the molecular aggregation of the hydrophilic, cationic dyes. The presence of a carboxyl acid group in the dye molecules inhibited adsorption of the dyes on the surface of fluorohectorite, a silicate with a high charge density. A lower or no adsorption could be observed by vis spectroscopy. Strong association of the dyes to the silicate surface led to remarkable changes in the dye spectra, mainly due to the molecular aggregation. Dye assemblies initially formed after mixing the dye solutions with silicate dispersions were unstable. Decomposition of the dye molecular assemblies, and the formation of new species or molecular aggregate rearrangements, were studied on the bases of time-difference spectra. The reaction pathways were specific, not only for the dyes, depending upon their molecular structure and properties, but also on the silicate substrates.