Catalytic solid-liquid interface

In recent years IR spectroscopy has become a powerful method to probe catalytic solid-gas interfaces in situ (7). In contrast, catalytic solid-liquid interfaces have hardly been investigated in situ by vibrational spectroscopy or other techniques. An exception to this statement is represented by extensive work in electrochemistry, whereby vibrational spectroscopy has been used for many years to probe solid-liquid interfaces at electrodes (2). 

In our experience, the principal challenges in the application of ATR IR spectroscopy for investigations of functioning solid catalysts are associated with the sensitivity of the measurement and the complexity of the samples. The former is an issue common to most surface spectroscopies. The latter has to do with the simultaneous presence of many species at a catalytic solid-liquid interface these species include dissolved reactants, adsorbed intermediates, spectators, and products. The spectra are a superposition of the spectra of the individual species. The question of whether a species is a spectator or instead involved in the catalytic cycle is not easily answered and represents a challenge for in situ spectroscopy in general. Thus, there is a need for specialized techniques to be used in combination with ATR spectroscopy to enhance sensitivity and introduce selectivity. 

C. Investigation of Complex Reaction Network Occurring at Catalytic Solid-Liquid Interface... 

IX. Other Methods for IR Spectroscopy of Catalytic Solid-Liquid Interfaces... 

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. 

Overall, this chapter aimed to emphasize and demonstrate the great potential of utilizing a multidisciplinary approach to bimetallic systems that combines computational methods with a number of highly sophisticated in situ and ex situ surface-sensitive techniques at electrified solid-liquid interfaces. Advances in the understanding of fundamental properties that govern catalytic processes at well-defined multimetallic... 

Many important heterogeneous catalytic reactions occur at the interface between a solid catalyst and liquid or liquid-gas reactants. Notwithstanding the importance of solid-catalyzed reactions in the presence of liquid reactants, relatively little attention has been paid to spectroscopic methods that allow researchers to follow the processes occurring at the solid-liquid interface during reaction. This lack can be explained in part by the fact that there are only a few techniques that give access to information about solid-liquid interfaces, the most prominent of them being attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray absorption fine structure (XAFS) spectroscopy. 

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

The present chapter is concerned only with catalysis at the solid/liquid interface and will not deal with microheterogeneous catalysis by enzymes, micelles and polyelectrolytes even though the resulting kinetics are closely similar [4], Moreover, little reference will be made to catalytic processes involving gases as these have been the subject of Vols. 19-21 of this series, nor to catalytic polymerisations which have been treated in Vols. 14, 14A, and 15. 

In the analysis of heterogeneous solubilization, the role of the solid-phase reaction in influencing the overall reaction is different from that for the usual gas-solid catalytic reaction. The most important situation is that the film and internal diffusion effects within the solid and at the solid-liquid interface are significant. 

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