Formation Of Gas-phase Products Research Articles

Formation of carbonyls and hydroperoxyenals (HPALDs) from the OH radical reaction of isoprene for low-NOx conditions: influence of temperature and water vapour content

The formation of gas-phase products from the reaction of OH radicals with isoprene for low-NOx conditions ([NOx] ≤ 1010 molecule cm−3) has been studied in an atmospheric pressure flow tube (Institute for Tropospheric Research-Laminar Flow Tube, IfT-LFT) operating in the temperature range of 293–343 K and a relative humidity of < 0.5 % up to 50 %. The photolysis of H2O2 or ozone photolysis in the presence of water vapour served as the NOx-free OH radical sources. For dry conditions at 293 K, the measured yields of methyl vinyl ketone (MVK), 0.07 ± 0.02, and methacrolein (MACR), 0.12 ± 0.04, were in reasonable agreement with literature data. Beside the C4-carbonyls, further product signals have been attributed tentatively to glycolaldehyde, methylglyoxal, hydroxyacetone, 3-methylfuran, C5-hydroperoxyenals (HPALDs) and C5-hydroxy-hydroperoxides. A simplified, “classical” reaction mechanism without efficient HPALD production describes well the observed yield for MVK and MACR. Unexpected high MVK and MACR yields of up to 0.65 in total were measured under conditions of a relative humidity of 50 % using both OH radical sources and two different measurement techniques for organics (proton transfer reaction mass spectrometry and gas chromatography with flame ionization detector). The reaction mechanism applied is not able to describe the strong increase of MVK and MACR yields with increasing water vapour content. The signal attributed to the HPALDs showed a distinct rise of about one order of magnitude increasing the temperature from 293 K to 343 K. A rough estimate leads to a HPALD yield of 0.32 at 343 K with an uncertainty of a factor of two. The results of this study do not support a predominant formation of HPALDs under atmospheric conditions in low-NOx areas. The surprisingly high MVK and MACR yields measured for a relative humidity of 50 % and the formation of glycolaldehyde, methylglyoxal and hydroxyacetone necessitate further research.

Sulfur dioxide oxidation induced mechanistic branching and particle formation during the ozonolysis of β-pinene and 2-butene

Recent studies have suggested that the reaction of stabilised Criegee Intermediates (CIs) with sulfur dioxide (SO(2)), leading to the formation of a carbonyl compound and sulfur trioxide, is a relevant atmospheric source of sulfuric acid. Here, the significance of this pathway has been examined by studying the formation of gas phase products and aerosol during the ozonolysis of β-pinene and 2-butene in the presence of SO(2) in the pressure range of 10 to 1000 mbar. For β-pinene at atmospheric pressure, the addition of SO(2) suppresses the formation of the secondary ozonide and leads to highly increased nopinone yields. A complete consumption of SO(2) is observed at initial SO(2) concentrations below the yield of stabilised CIs. In experiments using 2-butene a significant consumption of SO(2) and additional formation of acetaldehyde are observed at 1 bar. A consistent kinetic simulation of the experimental findings is possible when a fast CI + SO(2) reaction rate in the range of recent direct measurements [Welz et al., Science, 2012, 335, 204] is used. For 2-butene the addition of SO(2) drastically increases the observed aerosol yields at higher pressures. Below 60 mbar the SO(2) oxidation induced particle formation becomes inefficient pointing to the critical role of collisional stabilisation for sulfuric acid controlled nucleation at low pressures.

Infrared and near-infrared spectroscopic probing of atomic layer deposition processes

The focus of this paper is the application of Fourier transform infrared spectroscopy (FTIR) and near infrared-tuneable diode laser spectroscopy (NIR-TDLAS) as in situ tools to monitor the formation of gas phase products during atomic layer deposition (ALD). Two studies are chosen to highlight the importance of monitoring the gas phase, in this surface driven process, for both mechanistic and reactor dynamics considerations. The first study is the FTIR monitoring of the ALD of HfO 2, from Hf[N(CH 3) 2] 4 and H 2O where it was found that, in addition to the expected HN(CH 3) 2, gas phase species were generated including CO, CH 4 and HCN. The second ALD system investigated was the growth of Al 2O 3 from Al(CH 3) 3 and H 2O employing NIR-TDLAS as a real time in situ probe to monitor the evolution of CH 4.

Photooxidation of α-pinene at high relative humidity in the presence of increasing concentrations of NO x

The photooxidation of ∼1 ppm α-pinene in the presence of increasing concentrations of NO 2 was studied in a Teflon ® chamber at 72–88% relative humidity and 296–304 K. The loss of α-pinene and formation of gas-phase products were followed using proton-transfer reaction mass spectrometry (PTR-MS). Gas-phase reaction products (and their yields) include formaldehyde (5±1%), formic acid (2.5±1.4%), methanol (0.6±0.3%), acetaldehyde (3.9±1.7%), acetic acid (8.6±1.9%), acetone (12±3%), pinonaldehyde (22±6%), and pinene oxide (0.9±0.1%). There was evidence of organic nitrates; and small peaks were tentatively assigned to norpinonaldehyde, 4-oxopinonaldehyde, propanedial, 2,3-dioxobutanal and 3,5,6-trioxoheptanal or 3-hydroxymethyl-2,2-dimethylcyclobutylethanone. The formation and growth of new particles were followed using a scanning mobility particle sizer (SMPS), and their chemical composition and density probed using single particle mass spectrometry (SPLAT II). SPLAT II showed that the suspended SOA consisted of a complex mixture of organic nitrates and oxygenates having a density of 1.21±0.02 g cm −3, 20% larger than often assumed in calculating SOA yields. Three-wavelength light scattering measurements were consistent with particles having a refractive index characteristic of organic compounds, but the data could not be well matched at all three wavelengths with a single refractive index. The effect of addition of cyclohexane or NO on particle formation showed that ozonolysis was the major mechanism of SOA formation in this system. However, unlike simple ozonolysis, organic nitrates are formed in both the gas and particle phases. Identifying and measuring specific organic nitrates in both the gas and particle phases in air may help to elucidate why SOA formation has been reported in field studies to be associated with polluted urban areas, yet the carbon in these particles is largely contemporary, i.e., non-fossil fuel carbon.

Catalytic effect of Fe/C powder on the formation of gas-phase products of vacuum pyrolysis of N,N′-ethylenebisstearamide

Abstract In the present work N,N′-ethylenebisstearamide (EBS) in its free form and mixed with iron/carbon powder was thermolyzed in a closed system under vacuum to produce gas-phase products (GPP). A catalytic effect of the iron/carbon surface on the gasification of thermolyzed EBS is observed over the 573–973 K temperature range. The overall yield of GPP measured after one hour of heating at a constant temperature increased with temperature (T The total yield, and the yields of the main identified GPP (CO, CH4, CO2 and C2H4), measured for free EBS are significantly lower (∼30 times) than those observed for mixture of EBS with iron/carbon powder. The catalytic formation of H2/N2, H2O and small amounts of other volatile 1 organic products is assumed at temperatures below 973 K. The formation of GPP from low volatility products of EBS decomposition is assumed to proceed via at least two competing mechanisms (EBS → Primary products → 2CO (1) or CO2 (2)). This mechanistical model can be used as a basis for understanding the delubrication mechanism of ferrous compacts and for the catalytic gasification of heavy amides (EBS) or polyamides (Nylon) widely used in industry.

FT-IR and mass spectrometric studies on the interaction of acetaldehyde with TiO 2-supported noble metal catalysts

Surface species formed during the adsorption of acetaldehyde at 300–673 K on TiO 2-supported Pt, Rh and Au catalysts were investigated by Fourier transform infrared spectroscopy. Two forms of molecularly adsorbed acetaldehyde–H-bridge bonded on surface OH groups and adsorbed on Lewis sites through one of the oxygen lone pairs–were identified. β-aldolization of acetaldehyde led to the formation of crotonaldehyde, which adsorbs on Lewis sites of TiO 2 through one of the oxygen lone pairs and on metallic sites via the C atom of the aldehyde group. Adsorbed acetaldehyde can be oxidized into surface acetate and it can be reduced resulting in adsorbed ethoxy. Mass spectroscopic analysis of the gas phase composition revealed that the formation of gas phase products (crotonaldehyde, water, benzene, hydrogen, ethylene, acetylene and methane) depends on the nature of the metals and the reaction temperature. An attempt was made to find a possible link between the surface species and the formation of the primary gas phase products.

HONO decomposition on borosilicate glass surfaces: implications for environmental chamber studies and field experiments

Nitrous acid (HONO) is the major source of OH in polluted urban atmospheres, so an understanding of its formation and loss processes both in urban atmospheres and in laboratory systems is important. Earlier studies over a limited range of conditions showed that HONO is taken up and undergoes reaction on surfaces. We report here a comprehensive set of studies of the decay of HONO and the formation of gas phase products over a range of initial HONO concentrations (0.1–11 ppm) at 1 atm pressure in N2 at 296 K and 0, 20 and 50% relative humidity (RH), respectively. The loss of HONO and increase in gas phase products were measured over time using long path FTIR spectroscopy. Studies were carried out in an unconditioned borosilicate glass cell and in the same cell after pretreatment with dry gaseous nitric acid. In the HNO3-conditioned cell, the loss of HONO was first order at all values of relative humidity (RH), and NO2 was the only significant gas phase product. For the unconditioned cell, the reaction order increased from first order at 0% RH to second order at 50% RH. The gas phase products at 0% RH were equal amounts of NO and NO2. The yield of NO increased to >90% at 50% RH while the yield of NO2 decreased to ≤10%. For both the unconditioned and the HNO3-treated cell, the rate of loss of HONO decreased with increasing RH. These results suggest that there is a competition between water, HONO and HNO3 for surface sites. Displacement of HONO from the cell walls by water was observed in separate experiments. Possible mechanisms, and the implications for HONO formation in environmental chambers and in air, are discussed.

Spectroscopic studies of the adsorption and reactions of chlorofluorocarbons (CFC-11 and CFC-12) and hydrochlorofluorocarbon (HCFC-22) on oxide surfaces

Raman and Fourier transform infrared (FT-IR) spectroscopy have been applied to a systematic investigation of the adsorption and decomposition of dichlorodifluoromethane (CCl 2F 2, CFC-12), fluorotrichloromethane (CCl 3F, CFC-11), chlorodifluoromethane (CHClF 2, HCFC-22) and molecular chlorine on oxide surfaces. Additionally, the effects of heating and ultraviolet photolysis of the CFC and HCFCs adsorbed on the oxide surfaces have been investigated. Spectral features for these species indicated a small wavenumber shift (1−6 cm −1) associated with the adsorbed phase. Some evidence, specifically the appearance of the Raman band at 507 cm −1, is presented to show that chlorine decomposition species are associated with these oxide surfaces. It was concluded that the new spectral feature (at ca. 507 cm −1) related with the decomposition of the CFC and HCFC molecules was an important indicator of the extent to which the reaction between the adsorbed CFC and HCFC and oxide surface has taken place. The extent of CFC-surface interaction has been quantified in terms of a maximum (Raman) frequency shift parameter ( A M). Wavenumber shifts suggest both cation-adsorbate and non-specific adsorption interactions are occurring in the internal channels of the zeolites. Slow decomposition of the adsorbed CFCs under ultraviolet-visible photolysis (at λ > 300 nm) and/or thermal treatment was observed spectroscopically. Using FT-IR spectroscopy, the formation of gas-phase products (CO, CO 2, HCl) both onyn photolysis and heating was evident. Results of these measurements are compared with the observed atmospheric reactivity of these compounds.

Carbon dioxide hydrogenation over nickel/zirconia catalysts from amorphous precursors: on the mechanism of methane formation

An amorphous Ni{sub 64}Zr{sub 36} alloy is used as a precursor for a carbon dioxide hydrogenation catalyst. Upon exposure to CO{sub 2} hydrogenation conditions, the glassy metal is partly transformed into crystalline metallic nickel particles and less well-ordered zirconium dioxide. A conventional Ni/ZrO{sub 2} catalyst, which was synthesized by coprecipitation and calcination of the amorphous precipitate, served as a reference. The as-prepared catalyst exhibits a very similar catalytic behavior to the alloy-derived catalyst. The structural and chemical changes are characterized by gas adsorption, X-ray diffraction, and thermal analysis. In CO{sub 2} hydrogenations over these catalysts, methane is almost exclusively produced, and traces of ethane are formed besides, as evidenced by gas chromatography. To study the origins of this selectivity behavior, in situ diffuse reflectance FTIR spectroscopy has been applied. Surface species in carbon dioxide as well as carbon monoxide hydrogenation reactions are correlated with the formation of gas-phase products. CO{sub 2}/H{sub 2} mixtures rapidly yield surface formate as the immediate precursor of the methane product on the coprecipitated catalyst. Doubly and singly bound adsorbed CO is detected besides; doubly bound CO is observed to originate from surface formate. In CO/H{sub 2} reactions, higher saturated hydrocarbons are produced besides methane. Themore » formation of these products proceeds from singly bound adsorbed CO via a dissociative mechanism. The respective hydrogenation steps are discussed in detail in a corresponding reaction scheme.« less

Carbon dioxide hydrogenation over Au/ZrO2 catalysts from amorphous precursors: catalytic reaction mechanism

An active catalyst for carbon dioxide hydrogenation is obtained by exposing an amorphous Au25Zr75 alloy to CO2 hydrogenation conditions. During this in situ activation, metallic gold particles of 8.5 nm mean size are formed, and the zirconium component of the catalyst is oxidized to ZrO2. For comparison, a further Au/ZrO2 catalyst was synthesized by coprecipitation, followed by calcination of the amorphous precipitate. The calcination step strongly enhances the activity of the catalyst; gold segregation and zirconia crystallization are found to occur in this process. The structural and chemical changes are characterized by gas adsorption, X-ray diffraction and thermal analysis.The main products of CO2 hydrogenation over these catalysts, as identified by gas chromatography, are methanol and CO. To investigate the reaction mechanism, diffuse reflectance FTIR spectroscopy has been used. Observed surface species are correlated with the formation of gas-phase products. Adsorption of CO2–H2 results in rapid formation of formate as the primary surface intermediate; two types of formate species are clearly detected on the coprecipitated catalyst, and are assigned by means of formic acid adsorption experiments. CO formation from CO2 appears to proceed via surface carbonate, in a surface reaction that corresponds to a ‘basic variant’ of the reverse water-gas-shift reaction. The CO formed in this process is, in turn, the starting point for a series of surface hydrogenation steps that yield π-bonded formaldehyde, surface-bound methylate and finally methanol. This sequence of reactions is confirmed by separate CO–H2 adsorption experiments.

Solid phase equilibria in the Au-Ga-As, Au-Ga-Sb, Au-In-As, and Au-In-Sb ternaries

The Au-Ga-As, Au-Ga-Sb, Au-In-As, and Au-In-Sb ternaries were surveyed using x-ray powder diffraction to determine which metallic phases exist at equilibrium with the III-V compound semiconductors. In closed, small-volume systems (i.e., formation of gas-phase products was prevented), Au does not react with GaAs but does react with the other III-V's investigated to produce Au-group III intermetallic compounds and another solid phase containing the group V element. However, each semiconductor formed pseudobinary systems with at least two different intermetallic compounds. The bulk phase diagrams determined in this study provide frameworks within which much of the experimental data in the literature concerning the products of reactions at Au/III-V interfaces can be understood.