Role Of Gas-phase Reactions Research Articles

Formation of complex organic molecules in cold objects: the role of gas-phase reactions

Abstract While astrochemical models are successful in reproducing many of the observed interstellar species, they have been struggling to explain the observed abundances of complex organic molecules. Current models tend to privilege grain surface over gas-phase chemistry in their formation. One key assumption of those models is that radicals trapped in the grain mantles gain mobility and react on lukewarm ( ≳ 30 K) dust grains. Thus, the recent detections of methyl formate (MF) and dimethyl ether (DME) in cold objects represent a challenge and may clarify the respective role of grain-surface and gas-phase chemistry. We propose here a new model to form DME and MF with gas-phase reactions in cold environments, where DME is the precursor of MF via an efficient reaction overlooked by previous models. Furthermore, methoxy, a precursor of DME, is also synthesized in the gas phase from methanol, which is desorbed by a non-thermal process from the ices. Our new model reproduces fairly well the observations towards L1544. It also explains, in a natural way, the observed correlation between DME and MF. We conclude that gas-phase reactions are major actors in the formation of MF, DME and methoxy in cold gas. This challenges the exclusive role of grain-surface chemistry and favours a combined grain-gas chemistry.

Role of Gas-phase Reactions and Thermal Gradient Control in Carbon Nanotube Synthesis

ABSTRACTWe investigate the role of precursor thermal rearrangement and surface catalytic reactions in the synthesis of vertically aligned carbon nanotubes (VA-CNTs) by acetylene-based, chemical vapor deposition (CVD) and demonstrate a millimeter-long growth of single-walled CNT (SWNT) without water assistance. A substrate heater was used to create an ascending temperature gradient from gas injection to catalyst substrate. Whereas temperature of catalyst substrates primarily determines their catalytic activity, it is a thermal condition of a gaseous mixture in the CVD chamber that also influence growth yield and structural features of as-grown CNTs. Employing Egloff’s characterization, [1] we discuss the importance of various gas thermal zones in producing high-quality nanotubes with augmented growth efficiency. We continue to report production of millimeter-long, VA-SWNT having a mean diameter of 1.7 ± 0.7 nm, catalyzed by iron on an alumina support. Important finding is that a million of aspect ratio of SWNT arrays can be produced, without water assistance, via combined action of an ascending temperature gradient toward catalyst substrate and low partial pressures of acetylene carbon feedstock. Our results do not only emphasize the role of precursor thermal rearrangement in CNT synthesis, but also offer a practical route to the modulation of such complex phenomena for an ultrahigh-yield growth of narrow VA-SWNT.

Comment to M. W. Beckstead’s paper “Recent progress in modeling solid propellant combustion”

Publishing the discussion of two famous researchers in the field of solid propellant combustion, the Editorial Board has to mention that the issues discussed have a long story. The ideas on the leading role of solid-phase reactions in combustion of double-base and composite propellants were formulated by P. F. Pokhil approximately 40 years ago (Combustion, Explosion, and Shock Waves, 1969, Vol. 5, No. 3). Simultaneously, he emphasized that the burning rate in the general case depends on interaction of all stages in the combustion wave. Up to now, it was only an experiment that could offer reliable proof of the existence of some particular mechanism of combustion, and critical discussions of available experimental information are of crucial importance. In this connection, it should be noted that the considerations of the leading role of gas-phase reactions at elevated pressures presented in this discussion can also be revised, because, with appropriate kinetics of solid-phase chemical reactions, their role can become dominating with increasing pressure (and surface temperature).

Photocatalytic degradation of trichloroethylene in dry and humid atmospheres: role of gas-phase reactions

Photocatalytic degradation of trichloroethylene in dry and humid atmospheres: role of gas-phase reactions

Photocatalytic degradation of trichloroethylene in dry and humid atmospheres: role of gas-phase reactions

This study reports the role of the homogeneous photochemical oxidation (PCHO) reaction occurring in the gas-phase during heterogeneous photocatalytic degradation (PCTD) reaction of trichloroethylene (TCE) in humid and dry atmospheres. PCTD reaction was carried out in the presence of TiO2 glass fiber cloth (TiO2-GFC) while chlorine radicals generated from the exposure of chlorine gas to UV-illumination induced the PCHO reaction. The reaction profiles of the PCHO and PCTD reactions monitored independently in dry and humid atmospheres by in situ FTIR spectroscopy reveal the difference between the reaction pathways of phosgene formation and DCAC oxidation. Degradation of TCE to CO2 increased from 52% at 0% relative humidity (RH) to 71% at 100% RH during PCTD reactions while during PCHO reaction it decreased from 27% at 0% RH to 2% at 100% RH. Reactivity of both PCHO and PCTD reactions decreased with increasing RH. RH corresponding to ∼25% RH was found to be the most favorable condition for PCTD reactions because reasonably high reactivity and mineralization of TCE can be accomplished simultaneously.

04/00688 New insight into the role of gas phase reactions in the partial oxidation of butane: Marengo, S. et al. Catalysis Today, 2003, 81, (2), 205–213

The commercial development of gas-charged coal reservoirs as sources of natural gas, primarily in the United States, is a recent milestone in the world hydrocarbon industry. A key factor that led to the commercial development of coalbed methane in the United States was documentation of the magnitude and extent of this gas resource. Through various studies over the last 17 years, the coalbed methane resource in 13 distinct coal basins and regions in the United States has been estimated at 9,720 to 11,720 billion m3. Using methods developed in the US and elsewhere, the coalbed methane potential of most major coal-bearing regions in countries of the world have been estimated to be from of 89 to 269 trillion m3 of gas in-place. However, unique characteristics of coalbed methane often preclude a direct application of previously established resource estimation methods for conventional hydrocarbons. The estimation of the volume of gas that is contained within a coal seam(s) and the estimation of the volume of gas that could be recovered from the seam(s) is dependent upon numerous physical variables. Two methods have been suggested for the estimation of the volume of gas that is contained within a coal seam: Volumetric and material balance. The more common volumetric method employs the simple, yet accurate equation that combines the volume of gas stored per unit volume of reservoir with total reservoir volume. The less commonly used material balance method employs mass-balance equations combined with reservoir pressure/porosity functions, requiring extensive knowledge and understanding of the coal seam reservoir. The assessment methodology for large basins or small prospects is similar. However, basinal or regional assessments often must employ data reduction or data simplification techniques to optimize the assessment operation and results.

New insight into the role of gas phase reactions in the partial oxidation of butane

The partial oxidation of n-butane at high alkane to oxygen ratio was studied in the presence of a Pt–Rh gauze and in an empty tubular reactor under identical conditions. Temperature-programmed reaction (TPR) experiments with the metal gauze showed dramatic changes in the product distribution in the range 25–500 °C. Total oxidation of butane at low temperature is followed by selective conversion to olefins and oxygenates around 400 °C; a further increase in the oven temperature enhances selectively olefins formation. In situ infrared and visible imaging of the reacting gauze revealed remarkable ignition/extinction phenomena of the surface reactions. Fast ignition on the metal catalyst was observed around 200 °C; upon further raising the oven temperature, suppression of these reactions occurred at a temperature level corresponding to the transition to high selectivity conditions. These results indicate a shift from heterogeneous to homogeneous reaction mechanism, the latter being responsible for high selectivity to partial oxidation products (84% olefins+oxygenates). With the empty reactor, conversion and selectivity were similar to those obtained in the presence of the catalyst. The study shows that the gauze plays no significant role in butane partial oxidation, as reactions take place in the void upstream of the catalyst, presumably via an alkylperoxy intermediate.

Effect of fuel binder and oxidiser particle diameter on the combustion of ammonium perchlorate based propellants

The decomposition and combustion behaviour of ammonium perchlorate (AP) and simple bi-propellant systems were studied. Three types of fuel binders were used in order to examine the role of binder on the combustion process. Paraffin wax was chosen as a simple model fuel, and AP/3–9% (mass%) paraffin wax mixtures were used. It was observed that the paraffin melted and volatilized to react with AP decomposition products in the gas phase. The same behaviour was observed for AP/polyvinyl chloride propellant. In contrast, hydroxy terminated polybutadiene (HTPB) undergoes condensed phase reactions before the reaction with AP occurs. In agreement with the kinetic data, the condensed-phase heat release was higher for AP/paraffin combustion than that of AP/HTPB propellant. However, a lower burning rate was observed for the paraffin propellant and evidence is given that the role of gas-phase reactions is more pronounced even at atmospheric pressure.

Thermochemistry and Kinetics of Gas-Phase Reactions Relevant to the CVD of Coatings: New Data for Process Models

AbstractUnderstanding the role of gas-phase reactions is an important step in the development of useful CVD process models. In this article, we review the general types of gas-phase reactions that can occur and discuss quantum-chemistry techniques for predicting their thermochemistry and kinetics. We also describe the use of high-temperature flow reactors to measure gas-phase reaction kinetics. Coupling these theoretical and experimental methods is a powerful approach to the characterization of CVD precursor chemistry. We illustrate this in a discussion of the reaction between BC13and NH3, which is important in the deposition of hexagonal boron nitride coatings.

Spectroscopic studies during pulsed laser ablation deposition of C–N films

Mass and optical emission spectra were recorded during XeCl laser ablation of graphite targets in vacuo and in low pressure (up to 2.5 mbar) N2 and NH3 atmospheres to study the role of gas phase reactions in C–N compound formation. Mass spectra were recorded during ablation in vacuo (10−6 mbar) and in N2 at 1×10−4 mbar. In vacuo, during the first stage of ablation, the mass spectrum was characterized by the peaks at 12 and 26 amu (C and CN). The CN peak intensity was slowly decreasing with the laser pulse number, to disappear after ∼3000 laser pulses. In contrast, the peak at 26 amu is permanent during the whole ablation when N2 is introduced in the chamber. The optical emission spectra recorded during ablation in vacuo are dominated by the bands of the C2 Swan system. Weak bands from CN can also be distinguished. Spectra recorded during ablation in NH3 are also dominated by the bands of the C2 Swan system. During ablation in N2 (10−4–2.5 mbar), strong bands of the CN violet system are detected. Their intensities increase with increasing N2 pressure and, at the same pressure, with increasing laser fluence. It was observed that the N atom density recorded in the deposited C–N films is increasing with the intensity of the CN emission.

Selective Catalytic Reduction (SCR) of NO with Methane over CoZSM-5 and HZSM-5 Zeolites: On the Role of Free Radicals and Competitive Oxidation Reactions

The reactions of CH4with NO, NO2, and O2, and with the mixtures of NOx+O2, were studied over CoZSM-5 and HZSM-5 zeolites. Based on the comparison of the “light-off” temperatures as well as the activation energies of these reactions, a conclusion is drawn that over both catalysts at temperatures below 500°C NO reduction into N2and CH4oxidation into COxare coupled and initiated by the reaction of CH4with NO2. The results demonstrate that this reaction may be considered as a limiting reaction step of NO2reduction into N2, and of the SCR reaction for the mixture of CH4+NO+O2when O2is present in large amounts. With the latter mixture when O2concentration is relatively low (up to about 1.5%) another reaction step, namely, the catalytic oxidation of NO by O2into NO2appears to control the rate of the SCR process. Thus, an important role of O2is to convert NO into NO2. At low temperatures (below 400°C) O2does not compete with NOxfor CH4, but even at these temperatures O2participates in oxidation of the intermediate reaction species decreasing in this way the selectivity of the SCR process. At higher temperatures (above 450°C) a strong competition between O2and NOxfor CH4is observed and this results in a further decrease in the process selectivity. It is shown that the higher SCR selectivity of HZSM-5 catalyst in comparison with CoZSM-5 is due to the stronger competition between O2and NO2for CH4over the latter catalyst. The effects of space velocity and reactor size were briefly explored. These results show that the reaction pathways are the same for both catalysts studied in this work. Based on these, and literature results, the formation of CH3·free radical is postulated and possible reactions of this radical with NOxand O2, that form the pathways of the SCR process, are considered. Finally, the possible role of gas phase reactions in the SCR process is discussed.

Role of gas phase reactions in subatmospheric chemical-vapor deposition ozone/TEOS processes for oxide deposition

Deposition rates, wet etch rates, and thickness uniformity experiments were performed using O3/TEOS thermal chemical-vapor deposition. Our results for oxide deposition show optimum process window around 200 Torr for producing films of good quality (uniformity and material properties). This is in excellent agreement with the modeling predictions over a broad range of pressure (100–600 Torr) and temperature (370–470 °C). The model invokes both gas phase and surface reaction mechanisms. The former is needed to produce deposition precursors and leads to an observed increase-maximum-decrease dependence on the deposition pressure; this decrease is associated with a competing (parasitic) role of gas phase reactions. Our experiments identify particle formation at higher pressures which is consistent with the expected dual role of gas phase reaction in generating (1) required deposition precursors and (2) particulates in the gas phase under some conditions.

On the role of gas-phase reactions in the mechanism of the selective reduction of NO x

The selective reduction of NO over Cu-, Co- and H-mordenite was studied using CH 4, C 2H 4 and CH 3OH, and compared with the same reaction in the gas phase. Co- and H-mordenite proved to be active and selective for the three reducing agents, while Cu-mordenite was so only when C 2H 4 and CH 3OH were used. Besides being reduced in the presence of O 2, NO acted as a homogeneous catalyst in the oxidation of CH 4 and C 2H 4. A free-radical mechanism similar to the one occurring in the methane partial oxidation reaction could explain that behavior. Interestingly, on Co-mordenite, the conversion of nitric oxide to nitrogen began to occur at similar temperatures for both CH 4 and CH 3OH. For both Co-M and Cu-M the NO to N 2 conversion started after the CH 3OH conversion reached 100%. In the gas-phase reaction no NO conversion was observed when CH 4 or C 2H 4 were used as reducing agents, but a 30% of the nitric oxide conversion was obtained when CH 3OH was the reducing reactant at 600°C. Very similar results were observed with or without O 2 in the feed stream. When catalysts were present, the presence of oxygen noticeably increased the rate of NO to N 2 conversion.

Dual-bed strategies to improve hydrocarbon yields in the oxidative coupling of methane

The performances of Li-containing catalysts have been compared in the oxidative coupling of methane (OCM), Li/Sn/MgO was found to be superior in activity almost equal to Li 2CO 3/ MgO in selectivity. Using these catalysts, attempts were made to further increase the C 2+ yield of the OCM process. Blank reactor experiments on the oxidation of ethane, and catalytic OCM experiments with SiC in the post-catalyst volume showed that the role of gas phase reactions in the formation of carbon oxides is typically minimal. However, the use of a presumably ‘inert’ second bed of SiC, while not increasing selectivities, can increase C 2+ yields. Ga-doped MFI zeolites are known to be good ethane dehydrogenation catalysts. The possibility of OCM using a 5 wt.-% Ga-doped ZSM-5 zeolite (MFI-5) was examined, and it was found that MFI-5 showed some OCM activity. The addition of some oxygen was necessary to convert ethane to aromatics at higher temperatures. An attempt was then made to integrate the OCM aromatization functions in a single reactor. Intimate mixtures of Li/MgO MFI-5 gave low activity selectivity no aromatization. However, using a Ga/ MFI catalyst bed on top of a Li/Sn/MgO catalyst again increased overall C 2+ yields, this time accompanied by aromatics formation. Possible explanations for these phenomena are offered.

The role of gas-phase reactions in boron nitride growth by chemical vapor deposition

Abstract The rate of growth of thin solid films by chemical vapor deposition (CVD), as well as the stoichiometry of the deposited solid, is explored by assuming a chemical mechanism for the deposition reactions and selecting a dynamic model for the reactor. The specific case of boron nitride growth from diborabe is illustrated. Competition among gas phase and surface reactions will give rise to complex interrelationships of growth rate and stoichiometry to the flow rates of reactants. The specific model illustrated explains qualitatively, and to some degree quantitatively, the observations in the literature on the boron nitride/diborane system.

Role of gas phase reactions in silicon chemical vapor deposition from monosilane

A simple flow reactor experiment is described which measures the gas phase reaction contribution to silicon chemical vapor deposition from SiH4. The approach uses the fact that the rate constant for SiH4 homogeneous decomposition exhibits a linear total pressure dependence in the low-pressure chemical vapor deposition (LPCVD) regime. Gas phase reaction channels are found to be significant even under silicon LPCVD conditions.

The role of gas phase reactions, electron impact, and collisional energy transfer processes relevant to plasma etching of polysilicon with H2 and Cl2

Plasma etching of silicon with H2 and Cl2 was simulated with mixtures of H2, Cl2, and SiCl4. The gas phase chemistry was elucidated by varying power and pressure. The cross sections for production of H (3p2P) atoms were probed through the emission response to pressure variations. Addition of rare gases to a hydrogen plasma identified qualitatively major energy transfer paths between the rare gas metastables and the H2/H system. We conclude that dissociation of hydrogen molecules and excitation of hydrogen atoms are separate electron impact collisional events that constitute the main excitation route in a hydrogen plasma. We find further that the role of metastable H atoms in the 2s2S state at 10.19 eV is minor, and that the direct dissociative excitation in a single electron impact at 16.57 eV is insignificant. The occurrence of state specific collisional energy exchange has limiting consequences for any actinometric method.

Role of Gasphase Reactions During Radiant Ignition of Solid Propellants

Abstract —A homogeneous propellant subjected to monochromatic radiant heating at the surface and in contact with an oxidizing gas is considered. Differential equations describing the conservation of species and energy in one dimension are solved numerically on a computer. The results indicate that though the gasphase reactions are important in establishing a gasphase flame, their influence in evaluating the ignition behaviour of the propellant can be conveniently neglected, at least for low heating rates and high pressures. Only at very low pressures and extremely high heating rates do the gasphase processess become important.