Adsorption Of Ammonia Molecules Research Articles

Silanol-Related and Unspecific Adsorption of Molecular Ammonia on Highly Dehydrated Silica

The interaction of molecular ammonia with the highly dehydrated surface of a common nonporous amorphous silica (Aerosil 300 from Degussa) comprises two phenomena: (i) the widely studied H-bonding interaction with isolated surface silanols, with typical IR features, and (ii) an unspecific interaction that escaped detection so far, being not conspicuous in the IR spectra and only clearly revealed by quantitative measurements. These two adsorption processes occur simultaneously, notwithstanding a marked difference in their corresponding interaction energy. The former process is well described by a Langmuir model, the latter by a Henry-type adsorption isotherm. Coupling of adsorption microcalorimetry with IR spectroscopy at a controlled temperature showed that the silanol-related interaction has ΔH0 = −58.4 kJ mol–1 and ΔS0 = −218 J mol–1 K–1 (reference: 303 K, 1 mbar), whereas the unspecific interaction has ΔH0 = −26.9 kJ mol–1 and ΔS0 = −135 J mol–1 K–1. The perturbation induced on the IR modes of adsorbed ammonia molecules is small in both cases, and the corresponding frequencies close to those of the gaseous species. Experimentally determined energy values and vibrational features were compared with corresponding results of ab initio calculations (comprising dispersive contributions) on the interaction of ammonia molecules with a slab model of amorphous silica recently investigated by using large-scale periodic B3LYP calculations. All features, both energetic and vibrational, of the silanol-related ammonia adsorption process are accounted for very satisfactorily. Computational results suggest that unspecific adsorption takes place on dehydrated patches of the surface and that such adsorption is dominated by dispersion interactions, while no features of H-bonding are present. The calculated ΔH0 value (−17.5 kJ mol–1) is remarkably smaller than the corresponding experimental value (−26.9 kJ mol–1), probably because of some inadequacy of the model to represent the actual structure of the dehydrated patches. The simultaneous occurrence of a weak and a relatively strong ammonia–silica interaction, as well as the absence of other possible adsorption modes, is discussed.

Room-temperature Fano resonance tunable by chemical doping in few-layer graphene synthesized by chemical-vapor deposition

We report the observation of Fano-type phonon resonance using infrared Fourier transform spectroscopy at room temperature in few-layer $(\ensuremath{\sim}3)$ graphene synthesized by chemical-vapor deposition on copper substrate. We subsequently demonstrate a continuous tuning of Fano line shape from antiresonance to phonon dominated by ammonia chemical doping. The few-layer Fano resonance exhibits a strong asymmetric characteristic between $n$-doped and $p$-doped graphene, different from that observed in double-layer graphene gated by external electrodes. This asymmetry probably arises from nonuniform charge-distribution among the layers of graphene due to the substrate effect on the bottom layer and adsorbed ammonia molecules on the top layer. Simultaneous measurements of Fano line shape and infrared signature of adsorbed ammonia molecules have also revealed the orientation of ammonia molecules on graphene and confirmed the charge-transfer mechanism of chemical doping.

Charge-Trapping Effects Caused by Ammonia in Carbon Nanotubes

Hysteretic behaviors caused by low-concentration ammonia gas are found in single-walled carbon nanotube based field-effect transistors. The transfer curves are found to shift towards negative gate voltage when the gate voltage is swept forwardly upon introducing ammonia. In contrast, no significant change in the transfer curves is observed for the backward sweeping of the gate voltage. This phenomenon is repeatable even after the devices are annealed in dry air at 200 degrees C for 2 hrs. Our findings can be interpreted in terms of additional charge traps induced by the adsorbed ammonia molecules. The observed hysteretic behavior enables the devices to work as memory cells, in which the carbon nanotube field-effect transistors act as readout and ammonia molecules play roles of charge storage.

15N Solid-State NMR Characterization of Ammonia Adsorption Environments in 3A Zeolite Molecular Sieves

The quantitative analysis of ammonia binding sites in 3A zeolite molecular sieves using solid-state 15N MAS NMR spectroscopy is reported. By utilizing 15N-enriched ammonia (15NH3) gas, the different adsorption/binding sites within the zeolite were characterized as a function of NH3 loading. Using 15N MAS NMR, multiple sites were resolved that have distinct cross-polarization dynamics, relaxation, and chemical shift behavior. A combination of 15N/23Na and 15N/27Al TRAPDOR NMR methods was used to demonstrate significant dipolar coupling between adsorbed ammonia molecules and both Na cations and Al framework species in the zeolite cage. An estimate of the 23Na−15N and 27Al−15N distance was obtained from simulation of the TRAPDOR results. Two-dimensional 1H → 15N CP-MAS NMR exchange spectroscopy was implemented to probe dynamics between the two primary adsorbed ammonia environments. In the 3A zeolite, the observed exchange process between the ammonia sites does not display a significant temperature dependence...

Application of CuBr Ion Conductor Thin-Films for Ammonia Gas Detection

ABSTRACTCuBr films are used to design an ammonia gas microsensor operating at ambient temperature, with high sensitivity and much improved selectivity in comparison with usual conductometric microsensors. The specific detection mechanism relies on adsorption of ammonia molecules and interactions with mobile Cu+ ions in the ionic conductor film. The CuBr films can be prepared by r.f. sputtering, but more simply and cost efficiently by chemical reaction of copper metal with bromine ions. Photolithographic techniques are applied to miniaturize the sensor.

Charge transfer from ammonia physisorbed on nanotubes.

We report the use of nanotube field-effect transistor devices for chemical sensing in a conducting liquid environment. Detection of ammonia occurs through the shift of the gate voltage dependence of the source-drain current. We attribute this shift to charge transfer from adsorbed ammonia molecules, with the amount of charge estimated to be as small as 40 electrons for the smallest shift detected. Using the concentration dependence of the response as an adsorption isotherm, we are able to measure the amount of charge transfer to be 0.04 electron per ammonia molecule.

Effect of wall adsorption on the photoacoustic detection of ammonia with a 1.53-μm DFB diode laser

The adsorption of ammonia molecules (and other polar molecules such as nitrogendioxide, water, etc.), on the walls of the photoacoustic (PA) detector influences the accuracy of the concentration measurement. This effect becomes larger for small concentrations and depends on the direction of the concentration change. This means that a hysteresis of the PA signal is observed between measurements carried out by decreasing and by increasing the concentration. The adsorption problem was reduced considerably by performing the measurements in a continuous flow. For measurements with high flow rate a new photoacoustic detector was developed. The differential design provides good flow and electronic noise suppression. The PA signal has been generated by tuning a 40-mW, 1.53-μm GEC-Marconi DFB laser to the absorption peak of ammonia at 1527 nm. The dependence of the PA signal on the flow rate of ammonia–nitrogen mixtures with different concentrations was investigated. In another experiment the hysteresis of the PA signal was investigated by increasing and decreasing the concentration in several steps, but maintaining the same total flow rate. It was found that a higher flow rate results in a smaller error of the concentration measurement and a shorter response time of the PA detector.

Influence of Silicon Content and Nature of the Template Molecule on Physico-Chemical and Catalytic Properties of SAPO-5

Two series of SAPO-5 molecular sieves were synthesized: (i) Using tripropylamine as a template and varying the Si content in the gel, and (ii) varying the template molecule at constant silicon content in the gel. The products were characterized physico-chemically and tested in toluene disproportionation. It was found that the rising silica content in the synthesis gel has only little influence on the SiO2 incorporation into the SAPO. Investigation of the acid character revealed that the concentrations of the adsorbed ammonia molecules are nearly constant and independent of the total silicon content of the crystals. The catalytic activity showed no correlation either with the silicon content of the catalysts or with the acidity measured by temperature-programmed desorption of ammonia. It is concluded that the incorporated silicon exists not only in a monomeric form and that the TPD of ammonia reflects not only the strong Bronsted acidity. The SAPO-5 sample synthesized with triethylamine possessed the strongest acidity corresponding to the highest catalytic activity. It is supposed that in the presence of TEA molecules silicon replaces mainly P atoms in the AlPO4-5 framework.

Adsorbate–metal interactions: An optical second harmonic generation study

The interaction of ammonia with clean and CO‐covered Re(0001) surface was investigated, utilizing optical second harmonic generation (SHG), temperature programmed desorption (TPD), and work function change (Δφ) techniques. The surface coverage of ammonia was continuously monitored during both isothermal desorption and equilibrium measurements as a function of crystal temperature and ambient NH3 pressure. A fast decrease of the activation energy for desorption from 21±1 to 9±1 kcal/mol as coverage increases from 0.2 to 0.9 of the saturation coverage was found. This indicates the existence of very strong repulsive interactions among adsorbed ammonia molecules. The pre‐exponent factor was also determined as a function of coverage, for the first time for any ammonia–metal system. Its fast decrease with increasing coverage, reveals an example for a compensation effect in desorption. The coadsorption system of CO and NH3 was also studied. It was found that while ammonia effectively blocks the adsorption of CO on Re(0001) when it is preadsorbed, the opposite order of adsorption, namely preadsorption of CO, hardly affects the adsorption–desorption kinetics of NH3. It was also found that decrease of work function caused by ammonia of 2.4 eV at saturation coverage, shifts upward exactly by the increase of Δφ by CO on the clean surface if the latter is preadsorbed. These minor effects of CO if preadsorbed, are in sharp contrast to the dramatic decrease of the second harmonic (SH) response due to ammonia on Re(0001), if only small amounts of CO are present on the surface. It is suggested that submonolayer coverage of CO are enough to remove any electronic interaction which otherwise leads to a strong SH response. A large SHG quenching cross section of CO is introduced, implying a nonlocal effect of CO on the adsorbed NH3 on Re(0001). If ammonia is preadsorbed, much of the quenching effect due to the coadsorbed CO is removed. This is discussed in terms of NH3 islands formation.

Infrared study of the adsorption of water and ammonia on calcium carbonate

Infrared spectra are reported of calcite as a function of heat treatment temperature in the range 293–583 K and after the adsorption of water, deuterium oxide and ammonia. Isolated and hydrogen-bonded surface hydroxyl groups and associatively adsorbed water were desorbed from the calcite surface at 373 K. Bulk hydroxyl groups in synthetic calcite were destroyed at 583 K. The dissociative adsorption of water leads to isolated hydroxyl groups at Ca2+ sites and surface bicarbonate anions. Physisorption of water either involves hydrogen-bonding interactions or occurs at Lewis-acidic Ca2+ sites which can also ligate weakly with adsorbed ammonia molecules.

Characterization of zeolite acidity. II. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTi.r. spectroscopy techniques

The Ammonia Temperature Programmed Desorption (NH 3— TPD) technique has been used to study the acidity of six zeolite samples. The results indicate that, in general, three desorption peaks are observed. The three peaks fall in the temperature regions of; less than 473 K, 473–673 K, and higher than 673 K. The first desorption peak is believed to be associated with physically adsorbed or weakly chemically adsorbed ammonia molecules; the second peak is NH 3 molecules adsorbed on zeolite hydroxyl groups, and the third peak is associated mainly with dehydroxylation, strong Brönsted acid sites, and/or Lewis acid sites. The additional data provided by the FTi.r. enables interpretation of the NH 3— TPD chromatogram obtained by the electronic microbalance.

Basic molecules as probes for the study of mo-alumina catalysts

Ammonia adsorption studies at 513/sup 0/-573/sup 0/K showed that the deposition of molybdenum oxide on alumina, followed by calcining, decreased the number and strength of the acid sites; that sulfiding alumina decreased the number and increased the strength of the acid sites; and that sulfiding MoO/sub 3//Al/sub 2/O/sub 3/ decreased the number of acid sites but increased their strength considerably. This last increase was attributed to the generation of anionic vacancies on the MoS/sub 2/ species during the reduction and sulfidation process. The adsorbed ammonia molecules on calcined MoO/sub 3//Al/sub 2/O/sub 3/ were mobile and on sulfided MoO/sub 3//Al/sub 2/O/sub 3/, they were localized at fixed adsorption sites.

Intrinsic surface phonons in amorphous silica

We have detected intrinsic surface phonons in amorphous silica by performing Raman scattering and infrared-reflectivity measurements on samples of porous Vycor glass. Qualitative agreement of our experimental spectra with theoretical calculations provides an identification of the surface features in terms of microscopic atomic motions. We find a sharp Raman-active peak at 980 ${\mathrm{cm}}^{\ensuremath{-}1}$ associated with the stretching vibration of a hydroxyl group against the surface silicon atom to which it is bonded. The corresponding wagging motion of the hydroxyl group appears in the infrared spectrum as a sharp feature at 380 ${\mathrm{cm}}^{\ensuremath{-}1}$. Both the appearance of these surface features as sharp peaks in the spectra and their relative amplitudes in the Raman and infrared spectra can be explained only if the sympathetic vibrations of underlying substrate layers are taken into account. Experimental confirmation of our assignment is provided by the effects of deuteration and adsorbed ammonia molecules on the Raman-active surface modes. Our experiment is the first in which the perturbations of these surface modes due to adsorbed molecules are studied. In our examination of the effects of water adsorption on the Raman spectrum of porous Vycor, we have observed a decrease in the intensity of the bulk defect mode near 600 ${\mathrm{cm}}^{\ensuremath{-}1}$ with increasing water coverage. We also present evidence that the intense scattering background observed in Raman experiments on silica and other oxide surfaces is primarily an intrinsic electronic surface effect.

Kinetics of decomposition of ammonia at low pressures on metal surfaces

The catalytic decomposition of ammonia at 10–5-10–4 torr and in an ultra-high vacuum was studied on nickel, rhodium, tantalum and tungsten wires. The promoting and poisoning effect of carbon when present in wires in limited and excess amounts, respectively, generally confirmed previously reported observations for platinum. A decay of activity occurred after flashing a wire and was attributed to a depletion of carbon in the surface region of the wire as a result of reaction with ammonia (or with a surface nitride). With rhodium, ammonia was observed mass-spectrometrically to react with a wire poisoned by excess carbon to give hydrogen cyanide. Absolute rate theory calculations indicate that at low temperatures where the activity of a rhodium wire decreases rapidly with decreasing temperature, the rate-limiting step in ammonia decomposition is the desorption of nitrogen from the catalyst surface. Such kinetic behaviour is inconsistent with decomposition rates observed at temperatures in excess of about 650°K. The activation energies on tantalum (14 kcal mole–1) and tungsten (25 kcal mole–1) are thought to contain terms for migration of the adsorbed ammonia molecules, formation of active sites and evaporation from the surface.

AN INFRARED STUDY OF THE ADSORPTION OF AMMONIA ON POROUS VYCOR GLASS

The infrared spectrum of ammonia adsorbed on porous glass at 20 °C and 150 °C has been studied in the region 1450–4000 cm−1. No absorption band due to the asymmetric bending mode of ammonia was observed but in the NH stretching region, bands occurred at 3280 cm−1, 3320 cm−1, 3365 cm−1, and 3400 cm−1. The bands at 3320 cm−1 and 3400 cm−1 were easily removed by evacuation and are due to ammonia molecules hydrogen bonded through the nitrogen atom to surface hydroxyl groups. The bands at 3280 cm−1 and 3365 cm−1 were not removed by evacuation even at 150 °C and are due to ammonia molecules held to surface Lewis acid sites by the nitrogen lone-pair electrons. The site for this adsorption is not a surface hydroxyl group. These results are further evidence for the existence of the two adsorption sites proposed by Folman and Yates. Deuteration of the surface OH groups was easily accomplished with D2O vapor at 300 °C and the rate of hydrogen exchange between adsorbed ammonia molecules and surface OD groups was found to be rapid.

The nature of silica-alumina surfaces III. Statistical interpretation of the adsorption of ammonia on alumina

Statistical calculations have been made for the adsorption of ammonia on alumina in the region of fixed adsorption. There is a broad distribution of adsorption energies, and an expression for this distribution has been derived. Adsorption data fit the integral form of the Langmuir equation. Configurational entropies have been shown to be several-fold less than those for uniform sites. Evidence is given which indicates that the changes in vibrational entropies of adsorbent and adsorbate as a result of perturbations caused by adsorption are small. Below a coverage of 0.36 × 10 14 molecules/cm 2, adsorbed ammonia molecules have lost all translation and rotation. Above this value, it is possible that one degree of rotational freedom may have been regained.

Kinetics of isotope exchange between hydrogen and liquid ammonia on heterogeneous catalysts

The isotope exchange between hydrogen and liquid ammonia has been studied in the temperature range between −60 ° and +25 °C at pressures up to 150 atm in the presence of various catalysts, in particular supported platinum catalysts. Comparative exchange experiments were carried out with gaseous ammonia at pressures from 25 Torr up to the saturation pressure (8.7 atm at 20 °C). (1) Group eight metals catalyze the H D exchange with liquid ammonia even at temperatures below −60 °C. The activity decreases in the order: Pt, Pd, Ni, and Fe. (2) For a platinum-carbon catalyst suspended in liquid ammonia the rate of exchange increases proportionally to the amount of catalyst in the range from 10 to 100 g/liter. (3) Platinum catalysts supported on carbon and on silica gave the same exchange constants per unit weight and resulted in the same activation energy irrespective of the widely different surface areas. (4) The following apparent activation energies were obtained: Pt C , 10.0 ± 0.5; Pt SiO 2 , 9.6 ± 0.6; Pd C , 11.3 ± 0.7; Raney-Ni, 11.5 ± 0.8 kcal/mole. (5) Comparative measurements with potassium amide indicated that this homogeneous catalyst has a considerably higher activity and a lower activation energy (5.4 ± 0.6 kcal/mole) than the heterogeneous catalysts. Contrary to exchange on these catalysts transport processes are rate determining in the potassium amide system under the conditions investigated. (6) While the rate of exchange in liquid ammonia increases proportionally to the hydrogen pressure in the case of potassium amide a dependence on the square root of the hydrogen pressure has been found for the platinum/carbon catalyst. (7) The experimental results support the following mechanism: Isotope exchange occurs between chemisorbed hydrogen atoms and a chemisorbed ammonia molecule, i.e. according to a Langmuir-Bonhoeffer-Hinshelwood mechanism. In principle the mechanism is the same in the gas as well as in the liquid phase. The number of atoms exchanged per unit time is, however, considerably smaller in the liquid, since a large portion of the surface is blocked up for hydrogen chemisorption by adsorbed ammonia molecules. If any, there is only a very small contribution from an ionic mechanism to the total exchange in liquid ammonia.