Desorbing Reaction Products Research Articles

МАТЕМАТИЧЕСКАЯ МОДЕЛЬ БИФУНКЦИОНАЛЬНОГО КАТАЛИЗАТОРА

A mathematical model of a bifunctional catalyst for the process of hydrocarbon isomerization is proposed, taking into account the composition of the reaction mixture, as well as the ratio of metallic and acid active centers. The elementary surface processes are considered: adsorption/desorption of reagents, dehydrogenation of paraffin into olefin, isomerization of olefins, cracking, hydrogenation of isoolefins and unsaturated low molecular weight cracking products, desorption of reaction products, coke formation, diffusion of adsorbed particles. The given amounts of desorbed reaction products are calculated. Snapshots of the catalyst surface were obtained.

CO2 + CH4 Chemistry over Pd: Results of Kinetic Simulations Relevant to Environmental Issues

We have determined the Arrhenius rate constants for 99 chemical reactions on palladium and solved the tightly coupled differential equations describing the chemical kinetics at a number of temperatures ranging from 350 to 700 K. The rate equations were integrated to a week of reactor run time. In this work, we discuss the valuable insights that can be gained by closely examining the chemistry ongoing on the first differential slice of the plug flow reactor. The two-component feed gas consisted of CO2 and CH4 with a total pressure of 1 bar. The CO2−CH4 partial pressures employed ranged from 20−80% to 80−20%. In these temperature and pressure ranges, the system performs in the low-coverage regime. In addition to the feed gas, formaldehyde, methanol, molecular hydrogen, C2 hydrocarbons, formic acid, acetic acid, ketene, water, and carbon monoxide evolve from the catalyst surface in the first differential slice of the plug flow reactor. The relative amounts of the desorbing reaction products are dependent on ...

Surface Structural Information Carried by Desorbing Reaction Products

Recent progress in angle-resolved measurements of desorbing surface reaction products is reviewed. The angular and velocity distributions of desorbing products deliver the most direct structural information of the reaction site when the products are repulsively desorbed. These distributions yield the symmetry and slope of the product formation site as well as the orientation of the intermediate species emitting the product. This method works well even when the overall reaction rate is controlled by the reactant adsorption and the interaction between adsorbed species is obscured in kinetic studies at the steady-state conditions. However, information about the reaction mechanism is requisite for its application because the method is linked directly to the reaction itself. Analysis of the product formation site and its switchover is exemplified in a steady-state CO + O2 reaction on platinum, and the product emission in the NO reduction on palladium is described.

Thermal reaction of polycrystalline AlN with XeF2

Detailed studies on the thermal reaction behavior of polycrystalline aluminum nitride (AlN) with effusive xenon difluoride (XeF2) have been carried out over the sample temperature (Ts) range from 300to920K using molecular beam mass spectrometry combined with a time-of-flight technique and ex situ surface analyses, i.e., X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy (SEM). The species desorbed from the AlN∕XeF2 system were monitored using molecular beam mass spectrometry, as a function of sample temperature. Above Ts=800K, the desorbed reaction products were identified as N2 and AlF3, and their flux intensities increase monotonically as the sample temperature is increased. The flux intensity of XeF2 desorbed after physisorption to the AlN surface is found to decrease as Ts is raised above Ts=800K, and approximately one half of the incoming XeF2 is consumed by the thermal reaction at 920K. The results of surface analyses show that the thermal reaction of AlN with XeF2 starts at approximately Ts=700K, forming a reaction layer composed of AlF3. The AlF3 layer becomes thick as Ts is increased from Ts=700–800K. Above Ts=800K, however, as a result of fast desorption of AlF3 and F atoms from the AlF3 layer, only partially fluorinated AlFx (x=1 and/or 2) layers are formed and the bulk AlN is revealed again. The SEM photographs indicate that the surfaces exposed above Ts=850K are strongly etched but a slight change is observed at Ts⩽800K. On the basis of these results, three reaction stages are proposed for the AlN∕XeF2 reaction depending on the sample temperature range: Stage 1 (300⩽Ts<700K); no reaction, stage 2 (700⩽Ts<800K); surface fluorination, and stage 3 (800⩽Ts); etching. At stage 3, AlF3 formed on the surface starts to evaporate and fast etching proceeds, since the vapor pressure of AlF3 is high enough in this temperature range.

Femtosecond laser-induced reactions with O 2 on Pt{1 1 1}

A study of femtosecond laser-induced reactions of carbon-containing species coadsorbed with molecular oxygen on Pt{1 1 1} with gas-phase detection of the photoproducts is presented. After excitation at 400 nm, oxygen reacts with carbon to CO, with CO to CO 2, while in the reaction with ethene a range of combustion products are formed. The velocity distributions of the desorbing reaction products reflect the different reaction barriers.

In situ investigation of the catalytic reaction H 2 + $\frac{1}{2}$]] O 2 →H 2 O with second-harmonic generation

+ O2→H2 O in the pressure range 0.2 Torr≤ptot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θo is determined as a function of hydrogen partial pressure α. The experimentally obtained θo is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to Ef H2O≥0.7 eV.

Interaction of Propene with Pure and Hydrogen-Precovered Nickel Films. Studied by Means of Isothermal Reaction Mass Spectrometry and Thermal Desorption Spectroscopy

A special experimental setup was used to study the equilibrium gas composition after interaction of propene with pure thin polycrystalline nickel films and nickel films precovered with hydrogen under ultrahigh vacuum conditions. Isothermal reaction mass spectrometry allows the analysis of the gas phase composition under equilibrium conditions. The analyzed gas phase consists of desorbed reaction products formed in the adsorbate phase after adsorption of propene on nickel films at 273 K. This technique was combined with thermal desorption spectroscopy and measurements of adsorption-induced changes in the electrical resistance of the polycrystalline films. After interaction with pure nickel films, in addition to propene, hydrogen, methane, propane, and butane could be detected as reaction products. The interaction of propene with nickel films precovered with hydrogen leads to a higher yield of butane and additionally to the formation of pentane. Thermal desorption experiments carried out after adsorption of...

Methanol conversion to hydrocarbons over zeolite H-ZSM-5: Comments on the formation of C4 hydrocarbons at low reaction temperatures

Abstract Pulsed flow microreactor studies are described for the methanol conversion reaction. The reactor is incorporated into a standard gas Chromatograph injector and this system enables complete analysis of the products exiting the reactor to be made for each pulse. It is found that the carbon mass balance for the pulse studies are low for microporous zeolites and that it is unsafe to use such a technique to investigate the nature of the initially desorbed reaction products. Experiments at 112°C using 1 μl pulses of liquid methanol demonstrate that a C 4 hydrocarbon is formed in trace amounts for the initial two pulses (equivalent to 1 methanol per aluminium atom). However, this effect is not observed for subsequent pulses and it is considered to be the product of a non-catalytic reaction. Pretreating the zeolite with ethanol is found not to enhance this effect and no hydrocarbons are observed in similar experiments using SAPO-34. Possible mechanisms for the low-temperature formation of C 4 hydrocarbon products are discussed.

Laser-enhanced gas surface reaction of chlorine with aluminum polycrystalline surface

The supersonic molecular beam technique is used to investigate the dynamics of photochemical gas-surface reactions of Cl 2 with polycrystalline Al surface irradiated by 355- and 1064-nm laser photons. The mass and velocity distributions of desorbed reaction products are measured by time-resolved mass spectrometry. Results show that the main products are Al + and AlCl x (x=1,2,3) produced with the irradiation at 355 nm; only AlCl 3 is produced under 1064-nm laser irradiation

Design considerations and performance characteristics of a dual mode time-of-flight mass spectrometer system for surface reactivity studies

We have designed a reflectron type time-of-flight mass spectrometer (TOFMS) system with both SIMS and electron impact (EI) ion optics. The system was designed to study the interaction of solid surfaces with reactive species initially in the gas phase. In the EI mode, the main TOFMS assembly, which consists of the flight tubes, reflectron, and a channel plate detector, is fitted with the EI ion source. The entire TOFMS is mounted in a rotatable cage permitting 270° of rotation about the sample center. Reactive species are supplied by a pulsed molecular beam source. The EI source has an unobstructed view of the target throughout most of the 2π solid angle defined by the plane of the sample. Angular and time-resolved detection of scattered beam species and desorbed reaction products to a limiting partial pressure of ∼10−13 Pa is possible in this configuration. In the SIMS detection configuration, the main TOFMS assembly is refitted with SIMS extraction optics and repositioned on the sample axis. The pulsed molecular beam source is again used to supply reactive species. By controlling the timing relationship between the arrival of the pulsed molecular beam and the pulsed ion beam used to sputter the target, time-resolved detection of transient surface species is possible. A 50% mass resolution of ∼1400 has been demonstrated in both operating modes. The feasibility of time-resolved detection in both operating modes has also been demonstrated.

Molecular beam studies of a gas-surface reaction: dynamics of laser-induced chemical etching of silicon (111) by chlorine

Laser-induced chemical etching of Si(111) surface with molecular chlorine is investigated by use of a CW supersonic molecular beam under 355- and 560-nm pulsed laser irradiation. Two major desorbed reaction products, SiCl and SiCl{sub 2}, are observed by time-resolved mass spectrometry. The time-of-flight (TOF) spectra of the desorbed species are measured as a function of laser fluence and the translational energy of the incident chlorine molecules. The measured TOF spectra can be fitted to a Maxwell-Boltzmann distribution at laser fluences lower than 120 mJ/cm{sup 2}. The enhancement of the laser-induced gas-surface reaction on raising the normal component of the translational energy of the chlorine molecules is obvious for both laser wavelength cases. A reaction mechanism that mainly involves the dissociative chemisorption of Cl{sub 2}, the reaction of the adsorbed species, and the laser-induced desorption on the surface is proposed.

Infrared study of the adsorption of N,N-diethylacetamide and three N,N-diethylaminoketones on silica immersed in carbon tetrachloride

Infrared spectra are reported of silica immersed in carbon tetrachloride containing an amide and three diethylaminoketones with general formulae Et2N(CH2)nCOCH3(n= 0, 1, 2 and 3). Adsorptive interactions involved either proton transfer from isolated surface silanol groups to diethylamino groups or the formation of hydrogen bonds between silanol groups and carbonyl or diethylamino groups. Adsorbate molecules each interacted either through one functional group (CO or NEt2) with single silanol group adsorption sites, or simultaneously through both functional groups with a pair of vicinal isolated silanol groups. Single silanol groups formed hydrogen bonds with the acceptor oxygen atoms in adsorbed N,N-diethylacetamide (n= 0) molecules. For the diethylaminoketones (n= 1, 2 and 3) hydrogen bonding or proton transfer interactions involving the amino group were more favourable than hydrogen bonding interactions involving the carbonyl group. However, increasing the value of n enhanced the formation of surface complexes in which both functional groups were simultaneously liganded to the surface. Modes of adsorption involving a single ligand became more significant as the coverage of adsorption sites was increased. Surface catalysed condensation or fission reactions of N,N-diethylaminoketone adsorbates led to partially characterised desorbed reaction products.