Adsorption spectral features

In the investigations of molecular adsorption reported here our philosophy has been to first determine the orientation of the adsorbed molecule or molecular fragment using NEXAFS and/or photoelectron diffraction. Using photoemission selection rules we then assign the observed spectral features in the photoelectron spectrum. On the basis of Koopmans theorem a comparison with a quantum chemical cluster calculation is then possible, should this be available. All three types of measurement can be performed with the same angle-resolving photoelectron spectrometer, but on different monochromators. In the next Section we briefly discuss the techniques. The third Section is devoted to three examples of the combined application of NEXAFS and photoemission, whereby the first - C0/Ni(100) - is chosen mainly for didactic reasons. The results for the systems CN/Pd(111) and HCOO/Cu(110) show, however, the power of this approach in situations where no a priori predictions of structure are possible. 

Further, adsorption of methylated phenols (o-cresol and 2,6-xylenol) has given very weak and broad spectral features that were correlated to their weak interaction with the catalyst. Among the ortho-methylated phenols, 2,6-xylenol desorbs fast from the surface at 200 C than o-cresol. These methylated phenols, unlike phenol, desorbed from the catalyst above 200°C that was well below the actual reaction temperature (350°C). Hence, the desorption susceptible nature of the methylated phenols above 200°C facilitated the efficient methylation at 350°C [74]. In contrast to the Cu-containing ferrospinels, CoFc204 shows little interaction [74] of phenol with methanol, when they are co-adsorbed and this might be a limiting factor to the overall reaction. 

FT-IR spectra [126] showed that the presence of CO2 does not significantly modify the spectral features of the NO surface species originating upon the adsorption of NO2. In particular, both in the presence and in the absence of CO2, nitrates are mainly formed at the catalyst surface (1410, 1320 and 1020 cm and 1550 cm ). Accordingly, it is concluded that the adsorption of N O2-CO2 mixtures strictly parallels that of NO2 in the absence of CO2, that is, the occurrence of the nitrate route is not greatly affected by the presence of CO2. 

The differences in catalytic reactivity between Ti-HMS, Ti-MCM-41, and Ti-SBA-3 cannot be attributed to differences in Ti siting. XANES and EXAFS studies showed that the titanium center adopts primarily tetrahedral coordination in all three catalysts12. Also, the coordination environment is very similar for the three catalysts, as judged from the similarities in the EXAFS features. UV-VIS adsorption spectra showed no phase segregation of titania, the spectral features being consistent with site-isolated titanium centers. Because the framework walls of HMS tend to be thicker than MCM-41, the superior reactivity of Ti-HMS cannot be due to an enhancement in the fraction of Ti available for reaction on the pore walls. Thicker walls should bury more titanium at inaccessible sites within the walls. The most distinguishing feature is the greater textural mesoporosity for Ti-HMS. This complementary textural mesoporosity facilitates substrate transport and access to the active sites in the framework walls. 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

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. 

Specific adsorption of sulfate-bisulfate generally displays spectral features that are blue shifted (i.e., to higher frequencies) with higher electrode potentials, as observed for both polyciystal-line and the Pt(lll) surface. The shift is explained in... 

The transmission FTIR studies of aqueous protein solutions indicate how structural and conformational differences in a protein can be related to spectral changes, and that spectral features can be used to identify proteins in mixtures. However, these studies involve static systems, and our goal was to study flowing systems and the adsorption of proteins onto various surfaces. 

In the presence of excess CO (at 1(X) Torr CO and 40 Torr O, for example) the vibrational spectrum became considerably simpler. The spectral features we assign to CO adsorption at an oxidized Pt site and to CO both disappeared. Only the peaks associated with the presence of incommensurate and terminal CO species are detectable in this circumstance. This is shown in Fig. 10b. 

The interaction of deuterated and chlorinated acetonitrile, CD3CN and CCI3CN, respectively, with Bronsted acid sites of H-ZSM-5 and H,D-ZSM-5 and the thus-induced changes in the IR spectra were interpreted by Pehnenschikov et al. [657] in the frame of the resonance theory of the A-B-C triplet, developed for molecular H-complexes. The approach was similar to that of water adsorption (vide supra). Kotrla and Kubelkova [729] discussed in great detail the spectral features observed on the adsorption of deuterated acetonitrile, designated as... 

Adsorption-desorption processes This process is the interconversion of molecular species between the soluble and the surface-confined state. This may be detectable when the two states exhibit a difference in light absorption. The soluble state gives rise to an absorption spectral feature, while the adsorbed... 

State exhibits a reflection spectral feature. These two features are distinguishable in many cases. Otherwise, the adsorption-desorption process can stiU be detectable when an ER signal from the underlying electrode surface varies with this process or when this process also interconverts the orientation of the molecules between ordered and random. We have actually succeeded in detecting adsorption-desorption on an HOPG electrode for MB [25] and rho-damine dyes [82]. 

These spectral features have received controversial assignments. In early reports (414), the new v(OH) bands formed upon water adsorption were assigned to new types of silanol or aluminol groups. In more recent publi-... 

In comparing the spectral features of all three optical experiments dealing with the Cu(llO) surface it is obvious that the resonance at 2.1 eV is present and dominant in all data sets. The sensitivity of electronic surface states (of clean Cu(llO)) to adsorbed oxygen (and probably other adsorbates) causes in this case the optical techniques to be quite sensitive to adsorbates, enabling their use to monitor the kinetics of adsorption/desorption kinetics, for example. Since electronic surface states are quite common for a number of semiconductor surfaces, it is understandable that optical response investigations are sensitive to adsorbates especially on these surfaces. Hence they are frequently employed to study kinetic phenomena involving adsorption or thin film growth. 

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