Infrared measurement techniques, adsorbed

The objective of this study is to investigate the mechanism of propylene oxidation by a transient infrared spectroscopic technique over Rh/Al203. This technique allows simultaneous measurement of the dynamics of adsorbed species by in situ infrared spectroscopy and the product formation profile by mass spectrometry. 

The adsorption of anions on solid surfaces is of considerable interest, mainly because of its effect on the kinetics of electrochemical reactions. Several in-situ techniques have been applied toward this purpose. Infrared measurements were used to identify adsorbed species, estimate anion adsorption isotherms, and to gain information on anion interaction with electrode surfaces. " Sulfuric acid anions are possibly the conunonest anion adsorbates because of their specific adsorption on metal surfaces. Depending on the metal, its surface orientation, and the concentration of anion, either sulfate or bisulfate can be specifically adsorbed on the surface. Identifying the predominant adsorbate on platinum-group metals has engendered some controversy. While STM studies show that... 

In the last ten years the exploration of the kinetics and the mechanism of these reactions has become very fruitful, owing to the continuous effort of research workers in different countries all over the world, who made use of all the modern tools now available in catalytic research. Especially the simultaneous application of different measuring techniques, for instance the infrared study of adsorbed reaction intermediates combined with the study of kinetics and kinetic isotope effects, appeared to be very elucidating. 

In the field of heterogeneous catalysis, pyridine is frequently employed as a molecule probe of the surface acid sites of zeolites and other oxide catalysts, with which the amount and the strength of the acid sites are determined, for instance by infrared measurements of the intensity and the peah position of pyridine adsorbed on zeolite surfaces [9]. Despite its importance in this respect, details of the structure and the molecule orientation within the adsorbed pyridine layer on these nonconductive substrates are not known, due to the lack of techniques to yield such data before the advent of AFM, The series of our present work constitutes the first report on the determination, by means of. A.FM. of the array structure of the pyridine base adlayers on zeolite surfaces, and the estimation of the molecular orientation within the adsorbed layer. 

More recently, the work of Lipkowski and coworkers has sought to quantify the application of the SSR in the study of adsorbed species. Lipkowski and coworkers have built up a quantitative treatment of the infrared spectroscopy of adsorbed species based upon the SSR in an elegant series of papers [16, 83, 85, 165, 170, 171] using the combination of SNIFTIRS and conventional electrochemical techniques such as differential capacitance measurements. Thus, Li, Roscoe, and Lipkowski [165] published a study in 1999 on the adsorption of the benzoate anion at... 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Summarizing, infrared spectroscopy measures, in principle, force constants of chemical bonds. It is a powerful tool in the identification of adsorbed species and their bonding mode. Infrared spectroscopy is an in situ technique, which is applicable in transmission or diffuse reflection mode on real catalysts, and in reflection-absorption mode on single crystal surfaces. Sum frequency generation is a speciality... 

Although by now a large number of electrochemical systems have been examined using both SERS and IRRAS, including some common to both techniques (2b), the conditions employed are usually sufficiently different (e.g. disparate surface state, adsorbate concentrations) so to preclude a quantitative comparison of the spectral responses. One further hindrance to such comparisons is that it usually is difficult to remove entirely the contribution to the infrared spectra from solution-phase species. Two types of approaches are commonly used in IRRAS with this objective in mind. Firstly, modulating the infrared beam between s- and p-polarization can achieve a measure of demarcation between surface and bulk-phase components since considerably greater infrared absorption will occur for the former, but not the latter, species for p- versus s-polarized light (2.81. However, a complication is that the "surface... 

While microscopic techniques like PFG NMR and QENS measure diffusion paths that are no longer than dimensions of individual crystallites, macroscopic measurements like zero length column (ZLC) and Fourrier Transform infrared (FTIR) cover beds of zeolite crystals [18, 23]. In the case of the popular ZLC technique, desorption rate is measured from a small sample (thin layer, placed between two porous sinter discs) of previously equilibrated adsorbent subjected to a step change in the partial pressure of the sorbate. The slope of the semi-log plot of sorbate concentration versus time under an inert carrier stream then gives D/R. Provided micropore resistance dominates all other mass transfer resistances, D becomes equal to intracrystalline diffusivity while R is the crystal radius. It has been reported that the presence of other mass transfer resistances have been the most common cause of the discrepancies among intracrystaUine diffusivities measured by various techniques [18]. 

The reaction mechanism of and active sites for the reaction have been studied using various techniques combined with an isotopic tracer method. Infrared spectra were measured for detection of surface adsorbates on the Cu-ZSM5 zeolites ESR, XPS, phosphorescence, diffuse reflectance UV, and Cu-MASNMR have been used to reveal the states of the copper ions in the catalysts. CO adsorption and TPD experiments have been employed to measure quantitativdy the amounts of Cu ions, NO adsorbed, and O2 remaining on the surface. Based on these investigations, we can propose a reasonable reaction mechanism which includes Cu ions as active sites and NO" species as reaction intermediates. The reaction cycle is suggested to be as foDows elevated temp. + 2NO... 

In 1985 Allara and Nuzzo [354, 355] published the results of an extensive investigation in which adsorption took place on to an aluminium oxide layer formed on a film of aluminium deposited in vacuo on to a silicon wafer. Various carboxylic acids were dissolved in high purity hexadecane and allowed to adsorb from this solution on to the prepared aluminium oxide surface. The monolayers so formed were examined by ellipsometry and infrared spectroscopy. Contact angle measurements were made on the monolayer surfaces and radioactive labelled (tritiated) compounds were employed to study the interchange of adsorbed molecules with those in solution. Various other techniques of less immediate relevance to our present interests were also employed and reference to these two papers should be made for further particulars. Aluminium... 

The surface forces technique measures the force between molecules (eg. surfactants, polymers) adsorbed on mica sheets. In the case of large molecules such as polymers, the measurement is most sensitive to the regions closest to the solution and provides little direct information about the region adjacent to the surface. As it is a measurement between macroscopic surfaces, it is unable to provide information on microscopic chemical differences at the interface. Infrared spectroscopy could provide additional information about the quantity of adsorbed material on the mica surface, the identity and orientation of the adsorbed species, and possibly the nature of the surface linkage. 

Infrared spectroscopy has been a common tool for the study of solid surfaces (3). As in any surface spectroscopy, the number of adsorbed molecules and the surface area of the solid determines the sensitivity needed for IR studies. For low area surfaces, reflection techniques have been used to measure IR spectra of adsorbed monolayers on metal surfaces (7). However, for nonmetallic surfaces such as mica, the low reflectivity of mica makes reflection techniques less suitable for IR measurements. At the same time, the biaxial properties of mica, the parallel nature of the surfaces, and the absorbance of the mica itself present difficulties in IR spectroscopy (8). 

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