Thermal Decomposition Of Azomethane Research Articles

Growth of graphene layers on HOPG via exposure to methyl radicals

The interaction of methyl radicals with hot HOPG (highly oriented pyrolytic graphite) surfaces under single-collision conditions has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Methyl radicals were generated by thermal decomposition of azomethane N 2(CH 3) 2. Significant carbon deposition under elevated surface temperature conditions was observed if substrates were used which had been decorated by nanometer sized defects prior to methyl radical exposure. Graphene layers as well as protrusions were observed to be formed depending on the defect. No carbon deposition was observed for surface temperature below 800 °C. Largest sticking probabilities of up to 10 −6 were observed for HOPG (highly oriented pyrolytic graphite) surfaces prestructured with hexagonal nanometer sized etch pits. Here, the initially resulting mono-atomic layers are pinned by the hole periphery and exhibit a densely packed hexagonal atomic structure corresponding to the graphite basal plane. For surfaces held at 1000 °C, the lateral growth rate of a graphene layer around a single hole can exceed 230 Å 2/s at a CH 3 flux of 3 × 10 17 molecules/cm 2 s. The deposition kinetics switches from 2D to 3D growth prior to completion of the first graphene layer. A growth mechanism based on CH 3 decomposition and hydrogen desorption is proposed.

Adsorption of azomethane on oxides and on silica-supported noble metals

The adsorption and thermal decomposition of azomethane (AM) were investigated on oxides (SiO 2, TiO 2 and Al 2O 3) and on silica-supported noble metal (Pt/SiO 2, Pd/SiO 2 and Rh/SiO 2) catalysts by Fourier transform infrared (FT-IR) spectroscopy with concomitant mass spectroscopic monitoring of the gas phase. AM reacts readily with the surfaces of the catalysts to form chemisorbed species. It was demonstrated that AM chemisorbed in trans-form on SiO 2, and Pd/SiO 2, while on TiO 2, Al 2O 3, Pt/SiO 2 and Rh/SiO 2, the chemisorbed cis-AM was dominantly detected. OH groups of the oxides were also involved in binding of AM on the surfaces with the formation of H-bridged bonded species. Chemisorbed cis-AM suffered surface rearrangements, as its tautomerisation into formaldehyde methylhydrazone (CH 2NNHCH 3) was observed on catalysts adsorbing AM in cis-form. Depending on the forms of chemisorbed AM and their surface rearrangements, different products were detected in the gas phase during thermal annealing.

Control of Reaction Selectivity via Surface Oxygen Coverage: Thermal Decomposition of Azomethane on Rh(111)

The thermal decomposition of azomethane on clean and oxygen-covered Rh(111) has been investigated using a combination of temperature-programmed reaction, high-resolution electron energy loss and Fourier transform infrared spectroscopies. On clean Rh(111), azomethane adsorbs molecularly in the trans conformation and is stable on the surface up to 250 K. Above 300 K azomethane reacts exclusively by nitrogen−nitrogen bond scission, yielding gaseous HCN, H2, N2, and C2N2. HCN is evolved in four peaks between 300 and 600 K. Adsorbed CN reacts via two competing pathways: recombination to cyanogen and dissociation to adsorbed N, which recombines to gaseous N2 above 600 K. Adsorbed oxygen inhibits NN bond breaking, leading to the carbon−nitrogen bond dissociation products CO, CO2, and formaldehyde. On Rh(111)-p(2×1)-O (θO = 0.5 monolayers), ∼77% of the azomethane reacts via carbon−nitrogen bond scission. The C−N bond breaking is proposed to occur via adsorbed CH2NNCH3, based on vibrational and temperature-programmed reaction data. Subsequent carbon−nitrogen bond cleavage yields gaseous N2 and is proposed to be accompanied by formation of transient CH2 and CH3, which add to surface oxygen. Methyl would add to oxygen to form methoxy, which rapidly dehydrogenates to produce CO, CO2, and H2O. Gaseous formaldehyde is proposed to form from addition of CH2 to oxygen followed by rapid desorption. The variation in the product distributions with oxygen coverage indicates a subtle interplay between the inhibition of nitrogen−nitrogen and C−H bond cleavage by oxygen and is cast in a general framework for hydrocarbon oxidation on Rh surfaces.

Conversion of Methyl Radicals to Methanol and Formaldehyde over Vanadium Oxide Catalysts

Methyl radicals, formed by the thermal decomposition of azomethane, react with V2O5to form methoxide ions. Infrared results confirm the presence of methoxide ions, and ESR spectra demonstrate that the reaction involves the reduction of V5+to V4+. The methoxide ions may either decompose to formaldehyde or react with water or surface protons to form methanol. In temperature-programmed reaction experiments, both methanol and formaldehyde reached a maximum concentration in the gas phase at 200–250°C over V2O5, but over V2O5/SiO2, the methanol and formaldehyde maxima were shifted to ca. 300 and 400°C, respectively. The production of CH3OH at the lower temperature over V2O5/SiO2suggests that methoxide ions may react with water or surface protons to form methanol more quickly than they decompose to formaldehyde. Nevertheless, in continuous flow experiments at 300 and 500°C, formaldehyde was the principal oxygenated product because of the rapid oxidation of CH3OH at this temperature, and, in the case of pure V2O5, the direct conversion of methoxide ions to formaldehyde appears to compete favorably with their reaction with water.

The surface chemistry of CN and H on Pt(111)

Through a series of studies using reflection absorption infrared spectroscopy (RAIRS), we have observed some common trends in the surface chemistry of CN-containing adsorbates on Pt(111). The molecules that have been studied include azomethane (CH 3N 2CH 3), methylamine (CH 3NH 2), hydrogen cyanide (HCN), cyanogen (C 2N 2), and acetonitrile (CH 3CN). We find that HCN decomposes to yield adsorbed CN, which subsequently hydrogenates to form aminomethylidyne, CNH 2. The aminomethylidyne species also forms from the thermal decomposition of azomethane and methylamine, but not from acetonitrile. Molecularly adsorbed cyanogen, C 2N 2, hydrogenates to form H 2NCCNH 2. Thus, the CNH 2 functionality seems to have high stability on the Pt(111) surface. Detailed interpretation of the various RAIRS results required extensive use of isotopic substitution, normal-mode analyses, and an understanding of the effects of adsorbate-adsorbate interactions on the spectra.

The thermal decomposition of azomethane on Pt(111)

The thermal decomposition of azomethane, CH 3N=NCH 3, and methylamine, CH 3NH 2,on Pt(111) has been investigated by reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction spectroscopy (TPRS). Using the surface dipole selection rule of RAIRS and comparing with the IR and Raman spectra of solid trans-and cis-azomethane, we find that trans-azomethane converts to cis-azomethane upon adsorbing onto the Pt(111) surface at 84 K. Additionally, we propose that the cis-azomethane tautomerizes to formaldehyde methylhydrazone, CH 3NHN=CH 2, which subsequently undergoes N-N bond cleavage between 200 and 275 K. Above 300 K aminomethylidyne, CNH 2, is formed. Aminomethylidyne is also formed on the methylamine dosed surface. Cyanogen, C 2N 2(g), formed from the combination of CN ads was also observed by TPRS as a minor reaction product. A comparison of the IR spectra of azomethane and methylamine dosed surfaces indicates the formation of chemisorbed methylamine as one intermediate in the decomposition of azomethane.

Quantitative Studies of Methyl Radicals Reacting with Metal Oxides

The reactive sticking coefficients of CH{sup {center_dot}}{sub 3} radicals, produced by the thermal decomposition of azomethane, were determined over ZnO, CeO{sub 2}, La{sub 2}O{sub 3}, Sr/La{sub 2}O{sub 3}, and Li/MgO at 650{degree}C. The intrinsic sticking coefficients ranged from 6 x 10{sup -8} for La{sub 2}O{sub 3} activated at 900{degree}C to 3 x 10{sup -5} over ZnO. In the presence of even 0.1 mTorr of O{sub 2} the sticking coefficient for the La{sub 2}O{sub 3} sample increased to 3 x 10{sup -6}. Although the other nonreducible oxides exhibited similar behaviors, the magnitude of the effect was not as great. Methyl radicals are believed to react via electron transfer at the surface. The electron transfer may be either to metal ions, in the case of reducible oxides, or to O{sub 2}, in the case of nonreducible oxides. Reactions of CH{sub 3}{sup {center_dot}} radicals with several high surface area, porous silicas and zeolites were also studied. The sticking coefficient on a high-purity silica was found to be 1 x 10{sup -8} at 200{degree}C. An analysis of the stable products indicated that the reaction of CH{sub 3}{sup {center_dot}} radicals with these surfaces exceeded the coupling reactions that might have occurred at the surfaces. Thatmore » is, the surfaces did not function as an effective `third body` in the coupling reaction. 22 refs., 6 figs., 3 tabs.« less

Conversion of Methyl Radicals to Methanol and Formaldehyde over Molybdenum Oxide Catalysts

Methyl radicals, formed by the thermal decomposition of azomethane, were allowed to react over both MoO[sub 3] and MoO[sub 3]/SiO[sub 2] catalysts. In the continuous flow mode, the dominant reaction was the homogeneous coupling of CH[sub 3] radicals to form C[sub 2]H[sub 6]; however, significant amounts of HCHO and CH[sub 3]OH were also observed. Over MoO[sub 3] at 300-[degree]C, CH[sub 3]OH was the principal oxygenate, but at 500[degree]C the CH[sub 3]OH was converted mainly to HCHO. At both temperatures, HCHO was the favored product over MoO[sub 3]/SiO[sub 2]. The addition of H[sub 2]O to the reactants enhanced the formation of CH[sub 3]OH at 300[degree]C. During temperature-programmed reaction studies, Ch[sub 3]-radicals were first allowed to react with MoO[sub 3]/SiO[sub 2] at 150[degree]C. Methanol began to appear in the gas phase at about 170[degree]C and was the main product up to 400[degree]C. As a result of adding H[sub 2]O continuously during the TPR experiment, CH[sub 3]OH was produced at 80[degree]C, and the integral amount of CH[sub 3]OH formed over the temperature range from 80 to 500[degree]C increased. These results are consistent with the view that CH[sub 3]-radicals react with MoO[sub 3] to form surface methoxide ions. The methoxide ions may either decomposemore » to form HCHO or they may react with surface protons or water to form CH[sub 3]OH. 24 refs., 3 figs., 4 tabs.« less

Development of a methyl radical source for use in reaction dynamics studies

A source for methyl radicals based on the thermal decomposition of azomethane was subjected to an analysis of products by mass spectrometry. This source was used previously by Schwanke et al. to study the reaction with F atoms and to probe the product mixture by IR chemiluminescence. It was found that the CH 3 yield was less than 18%, the exact number depending on the temperature of the tungsten oven. Other products were CH 4, C 2H 6, C 2H 4, C 2H 2 and C 2H 3 radicals. A computer study including 32 elementary reactions suggests that the composition of the source beam can only be explained by heterogeneous processes occurring on the oven walls.

A search for a gas-phase free-radical inversion displacement reaction at a saturated carbon atom

Five thermal and photochemical reaction systems have been examined in an attempt to detect or to place upper limits on the rates of the simple inversion reaction, CH3 + CH4 → C2H6 + H, and its reverse. No positive evidence was obtained for the occurrence of these reactions. An upper limit for the rate constant for the forward reaction of 0.063 M−1s−1 at 802 K was obtained from the thermal decomposition of methane, and upper limits of 0.044 and 0.14 at 823 and 983 K respectively were estimated from the thermal decomposition of ethane. Other systems examined were the thermal decomposition of azomethane in the presence of methane, and the mercury-photosensitized decompositions of methane and ethane, which were complicated by unexpected side reactions.

Kinetische Untersuchungen der Reaktionen von Methylradikalen mit Allen

Kinetic Studies of the Reactions of Methyl Radicals with AlleneThe addition of methyl radicals to allene has been studied in the temperature range 573…595 K. The methyl radicals were produced by thermal decomposition of azomethane. It has been shown that the additions take place with a very high selectivity at the terminal carbon atoms in allene. The rate constant k2 of the addition can be expressed by the following Arrhenius equation:

On the gas‐phase free radical displacement reaction CH3 + CD3COCD3 → CD3 + CH3COCD3

AbstractThe gas‐phase free radical displacement reaction equation image has been studied in the temperature range of 240–290°C and at 140°C with the thermal decomposition of azomethane (AM) and di‐tert‐butylperoxide (DTBP), respectively, as methyl radical sources. The reaction products of the CD3 radicals were analyzed by mass spectrometry. Assuming negligible isotope effects, Arrhenius parameters for the elementary radical addition reaction equation image were derived: The data are discussed with respect to the back reaction and general features of elementary addition reactions.

Radical equilibrium studies. II. The thermodynamic parameters of s−butyl

AbstractThe thermal decomposition of azomethane in the presence of propene has been investigated in the range of 568–638°K at reactant partial pressures of up to about 7 torr in the presence of at least 170 torr of argon. The elementary reactions taking place in such conditions are discussed in detail. It is shown that a situation approaching equilibrium is attained between the processes equation image Values are obtained for the equilibrium constant K2, which can be described by the equation Using the best available thermodynamic parameters for methyl and propene, it is concluded that those for s−butyl are in need of adjustment. We recommend the values ΔHf0 (300°K) = 13.8 ± 1.0 kcal/mol and S3000 (1 atm standard state) = 75.6 ± 3.0 cal/mol°K together with group additivity values of the heat capacity. A comparison of measured values of K2 with values calculated from independent measurements of k2 and k−2 shows a discrepancy of about a factor of 3.5.

Radical equilibrium studies; the thermodynamic parameters of n‐propyl

AbstractA novel method for the investigation of the thermodynamic properties of free radicals is described. It involves the establishment of an equilibrium of the form where R1 and R2 are free radicals, and the measurement of the recombination products of R1 and R2. The method is applied to the case where R1 is n‐propyl, R2 is methyl, and the olefin is ethene, using the thermal decomposition of azomethane (a source of methyl) in the presence of ethene in the temperature range of 581–649°K. Using the best available thermodynamic parameters for methyl, it is concluded that those for n‐propyl are in need of adjustment. We recommend the values ΔHf°(300°K) = 22.6 ± 1 kcal/mol and S300° = 67.4 ± 3 cal/mol · K together with either the heat capacity data of O'Neal and Benson or the essentially identical data derivable from the results of Purnell and Quinn.

The thermal decomposition of azomethane. III. Kinetics of the noninhibited reaction

AbstractThe thermal decomposition of azomethane (A) has been studied in a static system at temperatures between 250° and 320°C and at pressures between 5 and 402 torr, with particular attention to identification of products. Major products, in decreasing order of importance, were nitrogen, methane, ethane, methylethyldiimide, dimethylhydrazone, propane, tetramethylhydrazine, ethylene, methylpropyldiimide, and methylethylhydrazone. Carbon balance at the lowest pressure and highest temperature was 92%, but decreased with increasing pressure and decreasing temperature owing to the formation of a polymer. A fairly simple mechanism accounts reasonably well for a short chain in the decomposition, propagated by the radical CH3N2CH2 (B), and for the five most abundant products, except ethane. It turns out that there is a second source of ethane, arising by C2H5 + A → C2H6 + B; this explains an anomalously high apparent activation energy for the reaction CH3 + A → CH4 + B. Ethyl radicals are also shown to be responsible for the formation of propane, ethylene, methylethylhydrazone, and methylpropyldiimide. The radical B decomposes to CH3 + CH2 + N2, and the methylene radical (probably both singlet and triplet) is shown to yield C2H5 at low pressure and high temperature, and mostly polymer at high pressure and low temperature.

THE GAS PHASE REACTION OF METHYL RADICALS WITH HEXAFLUOROACETONE

The photolytic and thermal decomposition of azomethane in the presence of hexafluoroacetone produces small amounts of fluorinated products, mainly fluoroform. The mechanism of this and related reactions is discussed. It is concluded that the proposed reaction.[Formula: see text]has an activation energy of about 6 kcal./mole, with a steric factor of about 10−5.

The Thermal Decomposition of Azomethane in Presence of Nitric Oxide

In presence of large amounts of nitric oxide, azomethane decomposes with only a small pressure increase yielding insignificant quantities of hydrocarbons in the volatile products. This is interpreted to indicate an initial dissociation of azomethane into nitrogen and free methyls which for the most part combine with nitric oxide to produce formaldoxime. The latter is partially oxidized by nitric oxide. The upper limit of the ratio of nitric oxide to azomethane reacting appears to be two. It is probable that the addition of methyls to azomethane is not completely suppressed in presence of nitric oxide. The energy of activation of azomethane decomposition in presence of nitric oxide is the same as that for pure azomethane.

The Thermal Decomposition of Azomethane

The decomposition of azomethane has been studied in the temperature range 290–340°C by an analytical method. For 50 to 90 percent reaction the ratio of moles of product to moles of azomethane reacted is constant at 1.95. The energy of activation in terms of azomethane decomposed is 52,500 cal. Evidence is presented for the formation of tetramethylhydrazine as an intermediate during the reaction and material balances calculated from the analyses substantiate this.

The Thermal Decomposition of Azomethane

The Thermal Decomposition of Azomethane

The Effect of Ethyl Radicals on the Thermal Decomposition of Azomethane. The Kinetics of the Thermal Decomposition of Tetraethyllead. Ethyl Radicals and Hydrogen

The Effect of Ethyl Radicals on the Thermal Decomposition of Azomethane. The Kinetics of the Thermal Decomposition of Tetraethyllead. Ethyl Radicals and Hydrogen