Richard B. Hall

Multiple IR photon laser induced reactions of cyclopropane

The multiple photon IR laser induced reactions of cyclopropane resulting from excitation of either of two fundamental vibrational modes has been demonstrated. In each case the products are typical of those formed with thermal excitation but the relative yields are not. The yield of the lowest energy barrier channel, isomerization, relative to higher channels, fragmentation, is dependent on the mode excited. Excitation of the C–H asymmetric stretch at 3.22 μm produces propylene with almost no fragmentation; excitation of the CH2 wag at 9.50 μm produces roughly equal yields of propylene and fragmentation products. Collisions play a critical role in this process. At 3.22 μm, the effect of collisions with argon is to increase yields of the fragmentation channel. As these collisions cannot add any significant amount of energy to the cyclopropane, it is concluded that the upper channel is opened due to intermode vibrational energy redistribution. At 9.50 μm, argon tends to favor the lower energy pathway. These ...

Isotope Selectivity of Infrared Laser-Driven Unimolecular Dissociation of a Volatile Uranyl Compound

Isotope-selective photodissociation of the volatile complex uranyl hexafluoroacetylacetonate � tetrahydrofuran [UO2(hfacac)2 � THF] has been achieved with both a continuous-wave and a pulsed carbon dioxide laser. The photodissociation was carried out in a low-density molecular beam under collisionless conditions. Transitions of the laser are in resonance with the asymmetric O-U-O stretch of the uranyl moiety, a vibrational mode whose frequency is sensitive to the masses of the uranium and oxygen isotopes. Unimolecular dissociation is observed mass spectrometrically at an extremely low energy fluence, with no evidence of an energy fluence or intensity threshold. The dissociation yield increases nearly linearly with increasing energy fluence. At constant fluence the dissociation yield is independent of contact time between the radiation field and the molecule, indicating that the decomposition is driven by laser energy fluence and not laser intensity. The oxygen and uranium isotope selectivities measured in these experiments are nearly those predicted by the ratio of the linear absorption cross sections for the respective isotopes. Thus, essentially complete selectivity is observed for oxygen isotopes, while a selectivity of only about 1.25 is measured for the uranium isotopes. A model presented to describe these results is based on rapid intramolecular vibrational energy flow from the pumped mode into a limited number of closely coupled modes.

Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Isotope tracer experiments prove the role of methyl in the oxidative coupling of methane and disprove a recently proposed mechanism involving methylene. The ethane product from oxidative coupling of CH4/CD4 mixtures over a Li/MgO catalyst consists of C2H6, C2D6 and CH3CD3 thus proving that ethane is formed by combination of methyl intermediates. The co-reaction of labelled CH4 (D and13C) with C2H4 produces propylene labelled predominantly at the methyl (3) position, thus proving C3 formation by terminal addition of methyl to ethylene rather than via a cyclic intermediate as has been proposed.

Direct differentiation of surface and bulk compositions of powder catalysts: application of electron-yield and fluorescence-yield NEXAFS to LixNi1−xO

We have applied near-edge X-ray absorption fine structure (NEXAFS) to characterize the surface and bulk properties of LixNi1−xO catalysts. In our experimental set-up, NEXAFS spectra of powder materials could be obtained by measuring the intensity of either electron-yield or fluorescence-yield. While the electron-yield method is sensitive only to the top few atomic layers, the fluorescence-yield method can detect species up to a few thousands angstroms deep into the bulk structure. The ability to distinguish surface and bulk compositions is demonstrated in studies of a number of Li0.5Ni0.5O samples, of which the surface compositions vary as a function of preparation procedures. In addition, NEXAFS investigations following the reaction of LixNi1−xO with CH4 have also been carried out and the results indicate that the initial surface reaction intermediates are Li2CO3.

A vibrational investigation of the stability, morphology and surface reactivity of NiO on Ni(100)

Abstract The formation and thermal stability of NiO on Ni(100) have been investigated using high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). Our results indicate that the saturated NiO/Ni(100) layer prepared at 300 K is rather poorly ordered and is thermally unstable at higher temperatures. Heating this NiO/Ni(100) layer to 800 K produces a surface with mixtures of crystalline NiO(100) clusters and c(2 × 2)−O chemisorbed local structures. The long range order of the NiO(100) clusters could be improved by repeated cycles of oxygen adsorption at 300 K followed by heating to 800 K. The NiO(100) clusters obtained after 9 cycles of such dosing-annealing exhibit bulk-like properties, as suggested both by the off-specular EELS measurements and by the experimental observation that the intensities of the multiple loss features follow the expected Poisson distribution. The NiO bond strength of the NiO(100) clusters, estimated from the overtone spectra, is

Isotopic selectivity in the laser induced dissociation of molecules with overlapping absorption bands

3.6 eV. In addition, the reduction of NiO(100) clusters by H2 at 800 K has also been investigated. The NiO(100) clusters are reduced preferentially with respect to the c(2 × 2)−O overlayer, resulting in a reduction sequence of NiO(100) → c(2 × 2)−O → p(2 × 2)−O → Ni(100).

Evidence for the potassium‐promoted activation of methane on a K‐doped NiO/Ni(100) surface

Measurement of the isotopic selectivity as a function of temperature are described for IR laser induced dissociation of UO2(hfacac)2⋅THF in a low density molecular beam. The measured selectivity was found to increase from 1.2 at 120 °C to 1.9 at 65 °C. This behavior is consistent with a model which takes into account both the initial thermal energy content of the molecule and the energy barrier to dissociation. Indeed, such behavior is expected for any complex molecule which has isotopically shifted, but overlapping absorption spectra as is the case for the 235U and 238U containing molecules used in this study.

Surface-structure-dependent reaction pathways of methyl groups on NI(100) and NI(111) surfaces

The formation and reactivity of multilayer NiO films, both with and without added potassium, have been investigated using high‐resolution electron‐energy‐loss spectroscopy, low‐energy electron diffraction, and x‐ray photoelectron spectroscopy. We have found a strong interaction between K and NiO, as evidenced by the formation of K–O bonds, by a high‐temperature potassium thermal‐desorption peak and by an activated rate of NiO formation on K/Ni(100). We have also investigated the reactivities of the K‐doped NiO surface toward CH4 and H2 by monitoring the removal rate of oxygen from the surface. Our results demonstrate that the presence of K significantly enhances the reactivity of NiO with CH4, while the presence of K does not increase the reactivity of NiO toward H2.

Direct Abstraction of Surface-Bound Hydrogen on Ni(111) by Free Methyl Radicals

Publisher Summary Reactions of hydrocarbon fragments such as methyl groups on transition metal surfaces play an important role in a number of catalytic processes, including oxidative coupling of methane, methanation of syngas, steam reforming, and partial oxidation of methane to syngas. Although important, knowledge of the fundamental mechanisms and kinetics of their reactions is limited. Because of the significance of these reactions, there are an increasing number of investigations of the fundamental reaction steps of small alkyl fragments on well characterized surfaces. Several methods have been used for the preparation of adsorbed alkyl radicals and for the study of their reactions. Most frequently, surface-bound alkyl fragments are produced by decomposing alkyl halides on the metal surface. Recently, Stair and coworkers developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces and on molybdenum oxide thin films. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor located inside an ultrahigh vacuum chamber. This method avoids the complications of co-adsorbed halide atoms, it allows higher coverage to be reached, and it allows the study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. This chapter discusses some of the mechanistic trends and describes an unexpected dependence of the reaction pathway on metal surface structure.

Production of light olefins

Methane (CH3D) is formed directly by abstraction by methyl radicals of adsorbed D on a Ni(111) surface. This reaction occurs at a surface temperature of 120 K and in the presence of open metal binding sites. A direct reaction by unaccommodated methyl radicals is involved, since accommodated methyl radicals on Ni(111) react only above 200 K. Modeling efforts show that the cross section for accommodation of methyl radicals to open metal sites is approximately five times larger than that for D abstraction.