Adsorbed spectroscopic probing

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

Structural information regarding the molecular orientation of adsorbates on electrodes can be obtained using a number of Raman spectroscopic probes. While these experiments are not routine to conduct, they are beginning to approach the simplicity of the FTIRRAS experiments just described. Figure 9.13... 

There has recently been much activity in developing molecular spectroscopic probes of electrochemical interfaces, as for other types of heterogeneous systems. The ultimate objectives of these efforts include not only the characterization of adsorbate molecular structure interactions under equilibrium conditions, but also the extraction of mechanistic and kinetic information from spectral detection of reactive adsorbates. 

Methanol co-adsorbed with water displaced most of the water on the surface methanol co-adsorbed with oxygen formed surface methoxides stable to 625 K. Oxygen pretreatment of the surface did lead to the formation of a species assigned as formaldehyde, which Henderson proposed to be formed via a disproportionation reaction between two methoxides. Spectroscopic probes of the controlling intermediate were inconclusive [71,72]. 

Basicity of zeoiites IR-spectroscopic study using adsorbed molecular probes... 

Thus, the data obtained show that chloroform and acetylene and its derivatives are suitable IR-spectroscopic probe-molecules for basic centers in zeolites. These probes exhibit the following advantages as compared to the conventionally used molecules, like CO2 and pyrrole (1) the wride ranges of the frequency shifts, which allows one to differentiate the centers of different nature and strength, (2) the easiness and reversibility of adsorption/desorption of these molecules, and (3) the favorable spectral range where the spectral features attributed to adsorbed probes appear. The use of such an approach allows us to shed some light on the nature and properties of basic sites in zeolites. The similar technique will be applied in our future studies devoted to other solid superbases. 

D. A. Piasecki and M. A. Wirth, Spectroscopic probing of the interfacial roughness of sodium dodecyl sulfate adsorbed to a hydrocarbon surface, Langmuir 10 1913 (1994). 

Investigations of the surface Lewis acidity of aluminas have mainly been performed by adsorbing basic probes after previous dehydroxylation of the samples by outgassing. Based on spectroscopic results, most authors agree that at least three different types of Lewis acid sites (with weak, medium, and high acid strength) exist on transition aluminas (293). 

A qualitative study of the adsorption, in order to determine the functional group which takes part in the adsorption process, can be done by IR spectroscopy measurements. The OH stretching vibration of an hydroxylated siuface serves in prinicple as a spectroscopic probe of the molecular environment, and IR spectroscopy can be used to determine the chemical functionality and the donor acceptor properties of an oxide siuface upon adsorption of various probe molecules. Noller and Kladnig [22] found that the changes in the OH valence frequency values (Av values), upon adsorption of a variety of organic donor molecules on to acidic silica, were linearly related to the donor number of the adsorbate. 

A novel combination of SECM with mid-infrared (IR) (3-25 pm wavelength region) spectroscopy was reported by Kranz et al. [158,159]. Here, the SECM substrate is the flat top of a hemispherical ZnSe attenuated total reflection (ATR) single bounce crystal. IR radiation is directed through the crystal and focused onto a small spot at the ATR crystal-solution interface. The locally created evanescent field penetrates a few microns into solution and the radiation reflected at the interface (internally reflected) is directed toward a photodetector. Hence, IR-adsorbing molecules present in the close vicinity of the interface can be spectroscopically probed, that is, an evanescent field adsorption spectrnm is acquired. 

Another possibility for characterizing zeolite acid sites is the adsorption of basic probe molecules and subsequent spectroscopic investigation of the adsorbed species. Phosphines or phosphine oxides have been quite attractive candidates due to the high chemical shift sensitivity of 31P, when surface interactions take place [218-222]. This allows one to obtain information on the intrinsic accessibility and acidity behavior, as well as the existence of different sites in zeolite catalysts. 

Infrared spectroscopy can be considered as the first important modem spectroscopic technique that has found general acceptance in catalysis. The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. In addition, the technique is useful in identifying phases that are present in precursor stages of the catalyst during its preparation. Sometimes the infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on the adsorption sites that are present on a catalyst. 

The first electron spectroscopic study of adsorbed hydrocarbons was that reported by Eastman and Demuth (78) who used He radiation to probe the valence electrons of benzene, acetylene, and ethylene. Figure 17 shows the difference spectrum of C2H4 adsorbed on Ni(lll) at 100 and 230 K compared with the results of Clarke et al. (79) for ethylene adsorption on Pt(lOO) at 290 K, propylene adsorption on Pt(lOO), and ethylene adsorption on Pt(lll). 

Among the spectroscopic techniques, one of the most widely used to characterize the basic properties of alkaline earth metal oxides is infrared (IR) spectroscopy of adsorbed probe molecules (41,47-49) this is described below. 

In general, spectroscopic techniques and, in particular IR spectroscopy of adsorbed probe molecules such as the ones mentioned above, provide information about the nature of the basic sites on oxide surfaces. However, they do not give information about the number and strength distribution of the basic sites on a solid... 

In most spectroscopic studies, the solids to be studied are usually compressed to form pellets under pressures around 1.5-2 kbar. From an academic point of view, the stability of MTS towards pressure is very important, since most spectroscopic studies of lattice groups or adsorbed probes might be affected by a degradation of MTS during compression. For industrial applications compaction is crucial to handle the powder. Thus the mechanical properties of MTS are a very sensitive topic if we think about the future of these materials. Solids with such high porosity and small wall thickness are very likely to be crushed. Previous studies point out a very weak mechanical strength of MTS [3,4J which can jeopardize further industrial development. It has been demonstrated that these materials have the lowest mechanical stability among the... 

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