A comprehensive model for the supported vanadium oxide catalyst: The umbrella model

Supported vanadium oxide catalysts are widely used in industry. However, the molecular structure of the active species, responsible for the actual catalysis, is for a large part still unknown. This thesis describes four years study on the elucidation of this molecular structure. It mainly focuses on the umbrella model (Figure 8.1). This model was introduced by Dr. Gijzeman as an alternative to models already present in the literature. In this model the vanadium atom is linked to the surface via only one Si-Os-V bond and further consists of a V=O and a perturbed O2 molecule linked to the central vanadium atom. Spectroscopic characteristics (Raman, IR) for the umbrella model have been calculated for two types of support, alumina and silica, and compare very well with experimental data found in the literature. From this comparison it could be concluded that the umbrella model is able to assign vibrations to the observed peaks at 900, 980 and 1040 cm?1 and can also explain the width of the peaks. A comparison between several model structures, the umbrella models and three alternative models from literature, has been made against experimental Raman and IR spectra. It could be concluded that the method of synthesis has an influence on the type of active species on the support. Regarding the conversion of methanol to formaldehyde, one of the processes for which this catalyst is used, a plausible reaction mechanism has been presented based on the umbrella model. Basically it involves three steps. In the first step a methanol molecule is converted into formaldehyde and water, with the removal of one oxygen atom from the -VO3 species. In the second step this process is repeated, which leaves a -VO species that can readily reactivated by the attachment of a whole oxygen molecule. For this last step to be possible spin orbit coupling, a relativistic property, is needed. A first study into the relativistic properties of the vibrations of molecules has been done. keywords: supported vanadium oxide catalyst model, theoretical chemistry, Raman, I

Note on the Calculation of Analytical Hessians in the Zeroth-Order Regular Approximation (ZORA)

The previously proposed atomic zeroth-order regular approximation (ZORA) approch, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimization, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer

On the umbrella model for supported vanadium oxide catalysts

An earlier-proposed model for the molecular structure of VO₄ species on alumina was tested and compared with the pyramid model on a silica support. The model can be described as a chemisorbed Osupport-V=O(O₂) species with a similar geometry as on alumina. Results of DFT calculations on this model are consistent with the experimental Raman and EXAFS data collected on low-loaded silica-supported vanadium oxide catalysts. The band observed at 915 cm ¯¹ is assigned to an O-O stretch vibration. The thermal motion of the bound O₂ molecule can explain the broadness of this band. These findings, in combination with our previous work, demonstrate that the umbrella model is a viable, internally consistent model for supported vanadium oxide catalysts at least at low loadings

Note on the Calculation of Analytical Hessians in the Zeroth-Order Regular Approximation (ZORA)

The previously proposed atomic zeroth-order regular approximation (ZORA) approch, which was shown to eliminate the gauge dependent effect on gradients and to be remarkably accurate for geometry optimization, is tested for the calculation of analytical second derivatives. It is shown that the resulting analytic second derivatives are indeed exact within this approximation. The method proves to yield frequencies that are remarkably close to the experimental frequency for uranium hexafluoride but less satisfactory for the gold dimer

Determining the Structure of Silica-Supported Monomeric Vanadium Oxide Catalysts Based on Synthesis Method and Spectral Data from Theoretical Calculations

Infrared and Raman spectra have been calculated for several molecular cluster models for the silica-supported monomeric vanadium oxide catalysts that are proposed in the literature: the pyramid model, the umbrella model, and a model containing two bonds to the support, a V=O group and an OH group. A related model with one bond to the support, a V=O and two OH groups, will also be discussed. From the comparison with literature, it is concluded that two models, the umbrella model and the model with two bonds to the support, are realistic descriptions of actual systems. The presence of a particular compound depends on the method of preparation. The internal V-O distances by themselves are not enough to distinguish between the presented models

Optical countermeasures against CLOS weapon systems

There are many weapon systems in which a human operator acquires a target, tracks it and designates it. Optical countermeasures against this type of systems deny the operator the possibility to fulfill this visual task. We describe the different effects that result from stimulation of the human visual system with high intensity (visible) light, and the associated potential operational impact. Of practical use are flash blindness, where an intense flash of light produces a temporary “blind-spot” in (part of) the visual field, flicker distraction, where strong intensity and/or color changes at a discomfortable frequency are produced, and disability glare where a source of light leads to contrast reduction. Hence there are three possibilities to disrupt the visual task of an operator with optical countermeasures such as flares or lasers or a combination of these; namely, by an intense flash of light, by an annoying light flicker or by a glare source. A variety of flares for this purpose is now available or under development: high intensity flash flares, continuous burning flares or strobe flares which have an oscillating intensity. The use of flare arrays seems particularly promising as an optical countermeasure. Lasers are particularly suited to interfere with human vision, because they can easily be varied in intensity, color and size, but they have to be directed at the (human) target, and issues like pointing and eye-safety have to be taken into account. Here we discuss the design issues and the operational impact of optical countermeasures against human operators

Proof of concept novel low toxicity obscurant (SERDP WP2405)

A significant health and possibly environmental problem is associated with military smokes and obscurants. Besides the fact that these obscuration munitions pose a significant toxicity hazard to the user, the environmental impact of these munitions can also be significant (e.g. due to the use of hexachlorethane). The objective of the SERDP project WP-2405, Proof of Concept Novel Low-toxicity Obscurant, is to investigate the technical feasibility to develop a new effective and environmentally benign obscurant. The project is a collaboration between TNO and Edgewood Chemical and Biological Center. As starting point the result of SERDP project WP-2148 [1] will be used, a sea salt based composition. The focus of the new project will be on the additional steps needed to gain a performance of approximately 70% (or better) of that of traditional HC smoke. The newly developed compositions vary in different additives, as to improve the overall performance of the obscurant while maintaining the low toxicity. Mass extinction coefficients were determined for different compositions by igniting 5 grams of pressed compositions in sub scaled grenades in a smokebox (V~2m3), at different relative humidity. The best five performing compositions were then studied to determine the acute toxicity for human lung cells in culture with an air-liquid interface system, VITROCELL®. This was done by igniting 10 grams in sub scaled grenades in a bio aerosol chamber (V~12m3)

Strobes: Pyrotechnic Compositions That Show a Curious Oscillatory Combustion

Strobes are pyrotechnic compositions which show an oscillatory combustion; a dark phase and a flash phase alternate periodically. The strobe effect has applications in various fields, most notably in the fireworks industry and in the military area. All strobe compositions mentioned in the literature were discovered by trial and error methods and the mechanisms involved remain unclear. Many oscillatory systems such as Belousov–Zhabotinsky reactions, cool flames, self-propagating high-temperature synthesis have been observed and theories developed to elucidate their unstable behavior based on chemical interactions or based on physical processes. These systems are compared to experimental observations made on strobe mixtures

Strobes: An Oscillatory Combustion

Strobe compositions belong to the class of solid combustions. They are mixtures of powdered ingredients. When ignited, the combustion front evolves in an oscillatory fashion, and flashes of light are produced by intermittence. They have fascinated many scientists since their discovery at the beginning of the 20th century. However, the chemical and physical processes involved in this curious oscillatory combustion remain unknown. Several theories have been proposed: One claims that two different reactions occur: one during the slow dark phase and another during the fast flash phase. The alternation between the phases is ascribed to heat variations. Other theories suggest that the formation of intermediate species during the dark phase and the change of phase are caused by variations in their concentration. A ternary strobe composition with ammonium perchlorate, magnalium, and barium sulfate is analyzed. The role of barium sulfate is studied by replacing it by other metal sulfates that have different physical properties (melting points), and the burning of the compositions is recorded with a high-speed camera and a spectrometer coupled with a charge-coupled device (CCD) camera. Experimental results show noticeable differences in the physical and chemical processes involved in the strobe reaction