Surface species, spectroscopic probes

In 1992 Tripp and Hair78 unified the two chlorosilane approaches by reacting OTS with a high surface area amorphous silica gel, in order to probe spectroscopically the different surface species. 

FIG. 3 Cation surface species on the basal planes of 2 1 layer type clay minerals. Inset indicates the spectroscopic methods used to quantify counterion surface species, their intrinsic time scales over which molecular structure is probed, and the residence time of surface species. 

Other detection methods. Besides XPS, other chemically sensitive techniques are available to probe the reaction. Surface reflection absorption infrared spectroscopy [130] and electron-energy loss spectroscopy [131] give detailed information on the vibrational states and thus the bonds of surface species. Gas-phase mass spectroscopic techniques provide information about the desorbing species. 

The use of optical methods which probe interface electronic and vibrational resonances offers significant advantages over conventional surface spectroscopic methods in which, e.g. beams of charged particles are used as a probe, or charged particles emitted from the surface/interface after photon absorption are detected. Recently, three-wave mixing techniques such as second-harmonic generation (SHG) have become important tools to study reaction processes at interfaces. SHG is potentially surface-sensitive at nondestructive power densities, and its application is not restricted to ultrahigh vacuum (UHV) conditions.However, SHG suffers from a serious drawback, namely from its lack of molecular selectivity. As a consequence, SHG cannot be used for the identification of unknown surface-species. 

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]. 

For the IR (Perkin-Elmer 1730 FTIR) spectroscopic studies self-supported wafers were pressed from the zeolite powder with a thickness of 15 mg/cm . The wafers were placed into a sample holder inside a pyrex glass cell with KBr windows, which allowed pretreatment (773 K, 1 hr, vacuum <10 Torr), introduction of probes, after cooling to the desired temperature, and registration of the spectra at 295 K. Three types of IR experiments were run (i) stepwise loading (0.1-6 Torr) the wafers with adsorbates at 295 K, (ii) generation of surface species with increasing temperature in the presence of adsorbate (closed cell), and (iii) loading adsorbate at 453 K followed by detection of surface species formed with time (1-240 min) at the same temperature. After each set of IR measurement the same sample was analyzed by UV-VIS spectroscopy. 

The experimental detection and quantification of surface species on in situ soil particles and other natural colloids is a difficult area of research because of the sample heterogeneity, low surface concentrations, and the necessity to investigate the solid adsorbents in the presence of water. Unambiguous information can be obtamed only with in situ surface spectroscopy, such as x-ray photoelectron (XPS), extended x-ray absorption hne structure (EXFAS), x-ray absorption near-edge structure (XANES), melastic electron tunneling (lETS), and electron energy loss (EELS) spectroscopies. Recent advances in the development of nonevasive, in situ spectroscopic scarmed-probe and microscopic techniques have been applied successfully to study mineral particles in aqueous suspensions (Hawthorne, 1988 Hochella and White, 1990). 

The chemical reactivity of MgO is usually related to surface defects (comers, edges, steps, etc.) with low-coordinated Mg and O sites [1-3]. These sites have raised significant interest since they can play an important role in many catalytic processes on MgO. The hydrogen molecule is considered to be a very efficient probe for IR spectroscopic characterization of these sites. H2 molecules dissociate at the defect sites of MgO producing several surface species whose stracture cannot be imuambiguously determined only on the basis of their spectroscopic features. 

Spectroscopic methods have recently been developed that are able to probe the surface species at the water/air interface. The use for this purpose of photoelectron spectroscopy depends on the short penetration depths of electrons and the electron binding energy specificity. Bohm et al. (1994) applied photoelectron spectroscopy... 

Consider a metal working electrode first. The ejected electrons from a metal electrode surface will travel a few A into the electrolyte phase and then become solvated. These solvated species display interesting chemistry and electrochemistry if suitable electron scavengers are available to interact with them. The resultant free radical species can be probed spectroscopically in these photoelectrochemical or spectrophotoelectro-chemical experiments. The irradiation can be either continuous or pulsed and with the advent of powerful laser sources a whole slew of experimental strategies open up [9,10]. Of course with continuous irradiation the spectroscopic probe will have to be orthogonally placed to avoid interference with the incoming radiation [11]. 

If a surface, typically a metal surface, is irradiated with a probe beam of photons, electrons, or ions (usually positive ions), one generally finds that photons, electrons, and ions are produced in various combinations. A particular method consists of using a particular type of probe beam and detecting a particular type of produced species. The method becomes a spectroscopic one if the intensity or efficiency of the phenomenon is studied as a function of the energy of the produced species at constant probe beam energy, or vice versa. Quite a few combinations are possible, as is evident from the listing in Table VIII-1, and only a few are considered here. 

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. 

An added difficulty that arises in the in-situ spectroscopic study of electrocatalytic systems in solution is that the active species will be located in the vicinity of the electrode so that the material in solution will generally represent a large background signal making the detection and identification of related species difficult. Thus, it would be ideal to be able to probe only that region proximal to the electrode surface and furthermore to be able to obtain structural information of the species involved. 

Surface spectroscopy offers the best opportunity to elucidate the structures of chemical species at the mineral-water interface (see Sposito, Chapter 11). The application of spectroscopic methods to probe the molecular environment of the interface is still a relatively new field. Chapters 16-19 present reviews and some recent advances in investigations of molecular structure at the mineral-water interface. A recent review of spectroscopic methods applied to soil and clay mineral systems is given in Stucki and Banwart (72). 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

---