Gas phase probes

Figure 7.4 Collection of commercial Raman probes designed for different installations (a) laboratory scale probe with interchangeable immersion or noncontact optics, shown with immersion option (b) probe shown in (a) installed in laboratory fermentation reactor (c) production scale immersion probe (d) probe shown in (c) installed in a glass reactor (e) gas phase probe with flow through cell (f) probe shown in (e) installed in process piping (g) wide area illumination (WAI) noncontact probe after completion of a pharmaceutical tablet coating operation. Adapted, with permission. Copyright 2004 Kaiser Optical Systems, Inc.

The interaction of light with matter provides some of the most important tools for studying structure and dynamics on the microscopic scale. Atomic and molecular spectroscopy in the low pressure gas phase probes this interaction essentially on the single particle level and yields information about energy levels, state symmetries, and intramolecular potential surfaces. Understanding enviromnental effects in spectroscopy is important both as a fundamental problem in quantum statistical mechanics and as a prerequisite to the intelligent use of spectroscopic tools to probe and analyze molecular interactions and processes in condensed phases. 

IGC is a gas phase technique for characterizing surface and bulk properties of solid materials. The principles of IGC are very simple, being the reverse of a conventional gas chromatographic (GC) experiment. A cylindrical column is uniformly packed with the solid material of interest, typically a powder, fiber, or film. A pulse or constant concentration of gas is then injected down the column at a fixed carrier gas flow rate, and the time taken for the pulse or concentration front to elute down the column is measured by a detector. A series of IGC measurements with different gas phase probe molecules then allows access to a wide range of physicochemical properties of the solid sample. The flow and retention of gas is shown in Figure 3. 

A thermodynamic scale of surface acidity and basicity can be constructed by exploring the acid-base properties of numerous solids and comparing the heats of adsorption and the adsorption uptakes of gas-phase probe molecules (NH3, CO2, SO2). These solids, varying in their physical and chemical properties, have been selected in order to cover a wide range of acid-base behaviours representative of acidic, amphoteric and basic solids. They can be divided into three main groups according to their adsorption properties towards acidic probes (which interact with basic solids) or basic probe molecules (which adsorb on acidic solids). Amphoteric solids display an adsorption edacity towards both acidic and basic probe molecules. 

Intrusion of gas phase probes metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Large phosphine molecule cannot diffuse through zeolite aperture into the zeolite cages to react with encaged carbonyl dusters effective for highly reactive carbonyl clusters. 

Breuker, K. McLafferty, E. W., The thermal unfolding of native cytochrome c in the transition from solution to gas phase probed by native electron capture dissociation. Angewandte Chemie International Edition England 2005,44, (31), 4911-4914. 

The strategy which is employed in order to get the above mentioned features of catalysts active sites is the adsorption of appropriate gas phase probe, under the specific experimental conditions (that are chosen in a way to be similar to those applied in the particular catalytic reaction), followed by subsequent desorption, monitored with appropriate detector. One experiment, in which the characterisation of acid/base properties of solid material is performed, is designed as follows ... 

It should be emphasized that the choice of a probe molecule should be done by taking into account all relevant parameters, and having in mind the features of solid material at which surface this probe should be adsorbed. In fact, the solid surface and the gas which is chosen as a probe for the characterization of its active sites should be considered as a pair. Very often, the separate adsorption-desorption experiments of more then one gas-phase probe is necessary in order to obtain reliable information concerning all active sites for particular solid material. The adsorption-desorption of more than one probe molecule should complete the picture about the catalysts active sites, particularly in the case of complex systems, where different types of active sites and energetic heterogeneity could be expected. 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

Figure 4.12 Time-resolved transmission (thick line) of HBT in the gas phase probed at 560 nm [77]. The excitation was performed at 325 nm. The fit (thin line) gives an exponential time constant of 2.6 ps for the IC.

Our group has made use of the surface acoustic wave (SAW) device in combination with gas chromatography as a tool for studying surface-gas phase probe interactions (5). The results of these experiments have been compared with calculated interaction energies for the same probes with highly simplified "model" surfaces. A successful preliminary study involving this type of approach was that... 

Leone S R 1989 Laser probing of ion collisions in drift fields state excitation, velocity distributions, and alignment effects Gas Phase Bimolecular Collisions ed M N R Ashford and J E Baggett (London Royal Society of Chemistry)... 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

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