Adsorbed species identification

Since the early 1980s, cyclic voltammetry has been used to characterize single-crystal electrodes in terms of surface order, presence or absence of defects contaminations, and so on. In some cases the voltammo-grams have also been used for identification of adsorbates, mostly on the basis of electric charge calculation. However, since the double-layer charging and surface reactions such as UPD and anion adsorption may occur in parallel, it is difficult to break down the total voltammetric charge into all of its individual components. Therefore, interpretation from voltammetry alone, used as a tool for the adsorbed species identification, may be ambiguous. This has been, for instance, the case with the interpretation of the unusual adsorption states on the Pt(lll) electrode (73-82). 

HREELS High-resolution electron energy-loss spectroscopy [129, 130] Same as EELS Identification of adsorbed species through their vibrational energy spectrum... 

The identification of particles adsorbed on solid surfaces and recognition of their properties is one of the fundamental problems in research on adsorption and heterogeneous catalysis. Desorption of the adsorbed species from a surface and its subsequent analysis is an important method for solv-... 

It is only since 1980 that in situ spectroscopic techniques have been developed to obtain identification of the adsorbed intermediates and hence of reliable reaction mechanisms. These new infrared spectroscopic in situ techniques, such as electrochemically modulated infrared reflectance spectroscopy (EMIRS), which uses a dispersive spectrometer, Fourier transform infrared reflectance spectroscopy, or a subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy (SNIFTIRS), have provided definitive proof for the presence of strongly adsorbed species (mainly adsorbed carbon monoxide) acting as catalytic poisons. " " Even though this chapter is not devoted to the description of in situ infrared techniques, it is useful to briefly note the advantages and limitations of such spectroscopic methods. 

The other vibrational spectroscopies, laser Raman and magic angle spinning NMR, have also been useful. Despite its low resolution, high resolution EELS has been usefiil in UHV work for assessment of surface cleanliness and for the identification of adsorbed species. 

Currently there are four major lines of approach towards gas-phase epoxidation of propylene (1) mechanistic studies of Au/Ti02 catalysts through kinetics, spectroscopic identification of adsorbed species and... 

The identification of surface adsorbed species has been carried out with FT-IR [69] and Raman spectroscopy [70] during reaction and with GC-MS after epoxidation reaction [72]. The aggregation of gold NPs is not appreciable during reaction at temperatures below 473 K [69,72]. Catalyst deactivation, which happens within a few hours causing a decrease in C3FI6 conversion by about 50%, can be accounted for by the accumulation of successively oxidized compounds after isomerization and cracking of... 

Sexton BA. 1981. Identification of adsorbed species at metal-surfaces by electron-energy loss spectroscopy (EELS). Appl Phys A 26 1-18. 

When considering libraries of spectra for identification purposes, the effect of sample preparation on spectral characteristics is also important. Two FUR sampling methods have been adopted for IR analysis of TLC eluates in the presence of a stationary phase, namely DRIFTS [741] and PAS [742], of comparable sensitivity. It is to be noted that in situ TLC-PA-FTIR and TLC-DRIFT spectra bear little resemblance to KBr disc or DR spectra [743,744]. This hinders spectral interpretation by fingerprinting. For unambiguous identification, the use of a reference library consisting of TLC-FTIR spectra of adsorbed species is necessary. 

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... 

Figure 1.12 Transmission IR spectra obtained during the oxidation of 2-propanol on a Ni/Al203 catalyst as a function of reaction temperature [90], A change in the nature of the adsorbed species from molecular 2-propanol to acetone is seen above 440 K. Experiments such as these allow for the identification of potential reaction intermediates during catalysis. (Reproduced with permission from Elsevier.)...

Li, C Domen, K Maruya K Onishi, T. Spectroscopic identification of adsorbed species derived from adsorption and decomposition of formic acid, methanol, and formaldehyde on cerium oxide, J. Catal, 1990, Volume 125, Issue 2, 445-455. 

The two bands at around 1700 cm may be reasonably attributed to Vc=o two different adsorbed species, probably acrolein and acrylic acid. In this compound, in fact, the vc=o band is found at a frequency about 20 cm higher than in acrolein (9). In these adsorbed compounds [for example, on V-Mo oxides (9)], the vc=c band is expected at a nearly the same value as in 7C-bonded propylene (around 1625 cm ), whereas other IR active bands are covered by the stronger bands due to physisorbed propane. A more clear identification of the above species, therefore, is not possible. The shoulder at about 1425 cm" may be attributed to VsCOO in adsorbed acrylate, but the VasCOO band expected at around 1550 cm is absent. A more reasonable interpretation is the formation of alkene oligomers. In fact, propene adsorbed on HNaY gives rise to the formation of a main band at about 1460 cm (9), apart from vch 5ch bands that, in our case, are covered by the band of physisorbed propane. However, all adsorbed species are removed by evacuation, indicating their weak interaction with the surface. 

A significant advantage of the methodology of NMR spectroscopy is that it allows application of pulse sequences for the discrimination of nuclei in specific local structures, if these nuclei are characterized by a coupling with other nuclei. Examples are dipolar-dephasing techniques such as those used in TRAPDOR experiments (Section II.C). CP experiments can be applied for the discrimination of nuclei in various structures, for example, for the identification and investigation of strongly adsorbed species with low mobility. 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

We were interested in the change in the oxidation state of Pd (II), incorporated in the zeolite, during heat treatment in oxygen or in vacuo. Hydrogen and carbon monoxide interactions were also studied. The experiments involved two techniques ESR, which provides direct identification of palladium in an ionic state, and IR spectroscopy, which gives information on the superficial structure of the exchanged zeolite and on the adsorbed species. 

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