Surface interactions, heterogeneous

Gas-surface interactions and reactions on surfaces play a crucial role in many technologically important areas such as corrosion, adhesion, synthesis of new materials, electrochemistry and heterogeneous catalysis. This chapter aims to describe the interaction of gases with metal surfaces in terms of chemical bonding. Molecular orbital and band structure theory are the basic tools for this. We limit ourselves to metals. 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

General Observations About x. its Relationship to the Overall Partitioning Coefficient and to the Concept of Surface-Site Heterogeneity. One approach to metal/particle surface interactions which has been developed, historically, in a variety of forms, is a conceptual model that assumes only two conditions for surface sites occupied by an adsorbate or unoccupied. In applying this approach to the solid/aqueous solution interface, the adsorption... 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

Macroscopic Coefficients and Surface-Site Heterogeneity. Beniamin and heckles model (5) of heterogeneous metal oxide surface sites includes two observations of metal ion/surface site Interactions. 

Surface interactions between water and polymer networks have a profound effect on the water structure. The properties of water in these and other heterogeneous systems are sensitive to the size of the network pores and have been described by the two-phase model which assumes partition of the water between the "bulk and the "bound water phases" Evidence for this partition has been obtained in several proton NMR studies and also in ESR studies of paramagnetic probes in zeolites, silica gels and in water containing polymers. ... 

Farin, D. Avnir, D. (1989) The fractal nature of molecule-surface interactions and reactions. In Avnir, D. (ed.) The fractal approach to heterogenous chemistry. Wiley, New York, 271-294... 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

The measurement of heats of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis in the past few decades to gain more insight into the nature of gas-surface interactions and the catalytic properties of solid surfaces. Specific attention will be focused on group IIIA containing samples in this section. 

We are concerned with both homogeneous gas-phase chemistry and heterogeneous surface chemistry. Certainly in combustion, gas-phase chemistry is usually dominant. However, there may be good reason to be concerned with heterogeneous chemistry, for example, on the relatively cool walls of a combustion chamber. Moreover there are emerging materials-synthesis and surface-modification techniques that depend on flame-surface interactions. 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

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