Very low pressure pyrolysis of phenylacetic acid

A Kinetic study of the very low pressure thermolysis of phenylacetic acid above 900 K reveals that its decomposition occurs in a concerted manner, yielding toluene and carbon dioxide, probably via a four-centre transition state [reaction (1)] [graphic omitted] with log[k_∞(1)/s^(–1)]=[(13 ± 0.3)–(12 200 ± 500)]/T

Mass spectrometric direct measurements of the vibrational relaxation of hot n-butane by SF_6 and toluene in crossed molecular beams

Vibrational temperatures of hot n-butane beams before (T_(vi)) and after (T_(vf)) being deflected by nearly collinear beams of SF_6 and toluene at 300 K have been measured by mass spectrometry using low-energy electron impact ionization. For n-butane initially at 605 K a net decrease of ΔT_v = -21 ± 7 K was observed corresponding to (ΔE_v) = -230 ± 75 cm^(-1) and to a vibrational accommodation coefficient α_v = 0.18 relative to collisions in which the pool of oscillators becomes thermalized

Reactive Vs. Adsorbed Oxygen in the Heterogeneous Oxidation of Methane Over Li/MgO

The kinetics of O_2(g) adsorption, desorption, and CH_4(g) or H_2(g) oxidation on 7% Li-doped MgO were investigated by on-line, modulated beam mass spectrometry in a low-pressure, continuous flow reactor between 930–1130 K. Under these conditions, which suppress gas-phase reactions, only purely heterogenous processes take place in this system. Dosage of the amounts of bound oxygen removed at several stages of evacuation, or consumed by reaction with methane or hydrogen, indicates that oxygen adsorption is a reversible process leading to at least two major types of chemisorbed species. The kinetics of O_2(g) exchange is a slow process in the time scale of the experiments, that can not be described by Langmuir-type equilibrium isotherms involving either dissociative or nondissociative adsorption on one or two types of active sites. Fast adsorption on a precursor state, followed by slow conversion into a more strongly bound species seem essential to model oxygen take up by the solid. On an oxygen saturated oxide, but in the absence of O_2(g), methane reacts rapidly with about 14% of the pool of exchangeable oxygen, after which a slower oxidation reaction sets in. Hydrogen behaves similarly. During the slow oxidation stage methane is oxidized into carbon oxides and ethane, but not water, and partially decomposes into unvolatile carbon-containing species without releasing H_2(g)

Low-level vibrational relaxation in single gas-surface encounters. Molecular structure, surface, and temperature effects

Direct measurement of the vibrational temperatures attained by different gases after single encounters with hot silica or platinum surfaces at T_S have been obtained by modulated-beam mass spectrometry. Vibrational energy accommodation coefficients a, for n-butane, n-octane, and 1-chlorobutane are smaller than one over both surfaces, slightly increase with molecular size or polarity, and are nearly independent of T_S between 350 and 700 K. Relaxation is more efficient on platinum

Dynamics of gas-surface interactions. Temperature dependence of the translational relaxation of polyatomic gases on silica and platinum

Single-collision translational energy excitation of n-butane, 1 -chlorobutane, and n-octane on hot silica and platinum surfaces at T_s has been studied by modulated beam mass spectrometry in the range T_s = 350-750 K. The linear decline of the derived translational accommodation coefficient with T_s contrasts with previous results on vibrational relaxation for the same systems and is taken as evidence of (1) the inverse dependence of nonactivated sticking on incident kinetic energy via detailed balance and (2) the weak coupling of translational and vibrational modes in gas-surface collisions

Gas-phase free radicals in the catalytic decomposition of hexane over tungsten. A modulated-beam mass spectrometric study

Desorption of alkyl and alkenyl (methyl, ethyl, allyl, and propyl) radicals from hot surfaces in the low-pressure (1 x 10^(12) - 8 x 10^(12) molecules cm^(-3) catalytic decomposition of hexane over W (1100-1800 K) and Pt (850-1400 K) has been studied by modulated-beam mass spectrometry. Time-of-flight analysis (phase spectrometry) of reaction products provided direct evidence about their identity. The effects of temperature, pressure, and surface treatment upon radical yields have also been investigated. Trapping of reactive species with iodine in a tandem reactor confirmed the presence of alkyl radicals. A mechanism is proposed in which free radicals, once formed, either desorb from the surface or decompose further into smaller fragments and olefins

Optimum Yield of the Purely Heterogeneous Oxidative Dimerization of Methane

The optimal C_2-yield per pass for the purely heterogeneous catalytic oxidation of methane by O_2 on Sm_2O_3 under continuous cofeed operation: η_(opt) ≤ 0.22 ± 0.03, is computed from actual kinetic data for all the pseudoelementary chemical reactions involved. The optimization dispenses with adjustable kinetic parameters, and explores the combined effects of CH_4 and O_2 flow rates, catalyst mass, residence time, and temperature variations on the objective function η = methane conversion × C_2-selectivity for the scheme: CH_4 (+O_2) ⇒ CH_3(→) 1/2 C_2H_6⇒ 1/2 C_2H_4⇒ CO. The fact that the derived η_(opt) value essentially coincides with the largest C_2-yields reported on a myriad catalysts under vastly different conditions: (1) proves that the ceiling to selectivity is solely determined by the competition among heterogeneous processes and (2) implies a substrate-independent selectivity pattern for the species formed in O_2 dissociative chemisorption. We briefly analyze the physical basis of these propositions and of the prospects of improving C_2-yields beyond 25%

Kinetics and Mechanism of the Heterogeneous Oxidation of Ethane and Ethylene on Samarium(III) Oxide

The rates and products of the purely heterogeneous oxidations of C_2H_6(g) and C_2H_4(g) on Sm_2O_3 in the presence of O_2(g) were investigated in a very low-pressure flow reactor by on-line molecular beam mass spectrometry, about 1000 ± 100 K. Ethane is oxidized to ethyl radicals, which undergo unimolecular decomposition into (C_2H_4 + H) or further oxidation to CO. C_2H_4 oxidation leads to CO as initial product, that is subsequently converted into CO_2. Steady state rates are proportional to k_i‘([O_2]) × [C_2H_n], with k_i‘([O_2]) = k_i × (K_i[O_2])^(1/2)/{1+(K_i[O_2])^(1/2)} (i = 3, 4 for n = 6, 4, respectively), which is consistent with the direct oxidation of hydrocarbons on surface oxygen species in dissociative equilibrium with O_2(g). Alternate or simultaneous measurement of the oxidation rates for C_2H_6, C_2H_4, and CH_4, the latter proportional to k_1‘[CH_4], on the same Sm_2O_3 sample as function of [O2] and temperature, led to the following expressions:  log (k_3/k_1) = −(0.14 ± 0.30) + (663 ± 300)/T (I), log(k_4/k_1) = (1.08 ± 0.35) − (646 ± 365)/T (II), log (K_1/nM^(-1)) = (2.76 ± 0.46) − (4363 ± 468)/T (III), log (K_3/nM^(-1)) = (1.85 ± 0.22) − (4123 ± 260)/T (IV), log(K_4/nM^(-1)) = (5.31 ± 0.65) − (6480 ± 647)/T (V) (nM = 10^(-9)M), that are independent of catalyst mass, active area, or morphology. Equations I−V imply that ethane and ethylene are oxidized faster than methane at all relevant temperatures. Although the activation energies, E_4 > E_1 > E_3, correlate with the corresponding BDE(C−H) energies suggesting a common H-atom abstraction mechanism, the A-factor for the oxidation of ethylene is about tenfold larger. Oxidations occur on distinguishable O_s species generated by endothermic, exentropic O_2 chemisorption involving cooperative participation of the solid

Direct evidence of inefficient vibrational energy transfer in gas-surface collisions

Vibrational energy equilibration between n-octane and hot seasoned silica surfaces at T, has been studied at low pressures (≤10^-4) torr) by modulated beam mass spectrometry in the range 350 ≤ T_S ≤ 800 K. The sensitive temperature dependence of the dissociative ionization of n-octane at 15 eV was used to monitor the temperatures attained by the gas after undergoing a few nonreactive collisions with the surface. In this system, the low-lying vibrational levels of n-octane, whose relative populations account for the mass spectral changes, never reach T_S in a single collision: the vibrational energy accommodation coefficient has a value of 0.66 at 350 K and decreases with temperature

Accuracy of semiclassical partition functions for an oscillator in a finite well

Semiclassical techniques are used to evaluate the partition function Q of a Morse oscillator. The empirical Pitzer—Gwinn quantization rule of the classical partition is found to be highly accurate even for shallow potential wells