Heterogeneous catalysis surfaces and interactions with adsorbates

NH3 synthesis (Haber process) Fe on Si02 and AI2O3 support 

Catalytic cracking of heavy petroleum distillates Zeolites (see Section 27.8) 

Catalytic reforming of hydrocarbons to improve octane number Pt, Pt-Ir and other Pt-group metals on acidic alumina support 

HNO3 manufacture (Haber-Bosch process) Pt-Rh gauzes 

The majority of industrial catalytic processes involve heterogeneous catalysis and Table 27.6 gives selected examples. Conditions are generally harsh, with high temperatures and pressures. Before describing specific industrial applications, we introduce some terminology and discuss the properties of metal surfaces and zeolites that render them useful as heterogeneous catalysts. 

7 Heterogeneous catalysis surfaces and interactions with adsorbates 

We shall mainly be concerned with reactions of gases over heterogeneous catalysts. Molecules of reactants are adsorbed on to the catalyst surface, undergo reaction and the products are desorbed. Interaction between the adsorbed species and surface atoms may be of two types physi-sorption or chemisorption. 

Physisorption involves weak van der Waals interactions between the surface and the adsorbate. 

Chemisorption involves the formation of chemical bonds between siuface atoms and the adsorbed species. 

Although the biphasic catalysts described above appear analogous to those discussed in Section 26.4, it does not follow that the mechanisms by which the catalysts operate for a given reaction are similar. 

Give an example of how PPh3 can be converted into a hydrophilic catalyst. 

Over the past 25 years, much effort has been put into investigating the use of r/-block organometallic clusters as homogeneous catalysts, and equations 26.23-26.25 give examples of small-scale catalytic reactions. Note that in reaction 26.23, insertion of CO is into the O—H bond in the Monsanto process using [Rh(CO)2l2] catalyst, CO insertion is into the C—OH bond (equation 26.12). 

Despite the large of amount of work that has been carried out in the area and the wide range of examples now known, it would appear that no industrial applications of cluster catalysts have yet been found to be viable. 

---

Chapter 26 Heterogeneous catalysis surfaces and interactions with adsorbates 799... 

In real catalysis the actual situation will even be far more complex. Energetic heterogeneity due to the participation of various structural elements of the surface and interactions between adsorbed species are just a few of the complicating factors coming into play. Nevertheless it is concluded that adequate description of the kinetics may be achieved on the basis of the outlined strategy as long as the analysis is restricted to a limited range of parameters, which condition will frequently be full-filled with practical reaction situations. 

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

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... 

The work function plays an important role in catalysis. It determines how easily an electron may leave the metal to do something useful for the activation of reacting molecules. However, strictly speaking, the work function is a macroscopic property, whereas chemisorption and catalysis are locally determined phenomena. They need to be described in terms of short-range interactions between adsorbed molecules and one or more atoms at the surface. The point we want to make is that, particularly for heterogeneous surfaces, the concept of a macroscopic work function, which is the average over the entire surface, is not very useful. It is more meaningful to define the work function as a local quantity on a scale with atomic dimensions. 

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]. 

In some cases, adsorption of analyte can be followed by a chemical reaction. The Langmuir-Hinshelwood (LH) and power-law models have been used successfully in describing the kinetics of a broad range of gas-solid reaction systems [105,106]. The LH model, developed to describe interactions between dissimilar adsorbates in the context of heterogeneous catalysis [107], assumes that gas adsorption follows a Langmuir isotherm and that the adsorbates are sufficiently mobile so that they equilibrate with one another on the surface on a time scale that is rapid compared to desorpticm. The power-law model assumes a Fre-undlich adsorption isotherm. Bodi models assume that the surface reaction is first-order with respect to the reactant gas, and that surface coverage asymptotically approaches a mmiolayer widi increasing gas concentration. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

---