Structure heterogeneous catalysis

The plan of this chapter is as follows. We discuss chemisorption as a distinct topic, first from the molecular and then from the phenomenological points of view. Heterogeneous catalysis is then taken up, but now first from the phenomenological (and technologically important) viewpoint and then in terms of current knowledge about surface structures at the molecular level. Section XVIII-9F takes note of the current interest in photodriven surface processes. 

Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. 

Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27,1990 edited by R.K. Grasselli and A.W. Sleight... 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

In heterogeneous catalysis, solids catalyze reactions of molecules in gas or solution. As solids - unless they are porous - are commonly impenetrable, catalytic reactions occur at the surface. To use the often expensive materials (e.g. platinum) in an economical way, catalysts are usually nanometer-sized particles, supported on an inert, porous structure (see Fig. 1.4). Heterogeneous catalysts are the workhorses of the chemical and petrochemical industry and we will discuss many applications of heterogeneous catalysis throughout this book. 

The next level is that of small catalytically active particles, with typical dimensions of between 1 and 10 nm, and inside the pores of support particles (pm range). The questions of interest are the size, shape, structure and composition of the active particles, in particular of their surfaces, and how these properties relate to catalytic reactivity. Although we will deal with heterogeneous catalysis, the anchoring of catalytic... 

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. 

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

The introduction of new synthetic techniques has led to the discoveries of many new electronic materials with improved properties [20-22]. However, similar progress has not been forthcoming in the area of heterogeneous catalysis, despite the accumulation of considerable information regarding structure-reactivity correlations for such catalysts [14-19]. The synthetic challenge in this area stems from the complex and metastable nature of the most desirable catalytic structures. Thus, in order to minimize phase separation and destruction of the most efficient catalytic centers, low-temperature methods and complicated synthetic procedures are often required [1-4]. Similar challenges are faced in many other aspects of materials research and, in general, more practical synthetic methods are required to achieve controlled, facile assembly of complex nanostructured materials [5-11]. 

One key aspect of SOMC is the determination of the structure of surface complexes at a molecular level one of the reasons being that our goal is to assess structure-activity relationships in heterogeneous catalysis, which requires a firm characterization of active sites or more exactly active site precursors. While elemental analysis is an essential first step to understand how the organometallic complex reacts with the support, it is necessary to gather spectroscopic data in order to understand what are the ligands and... 

Structured catalysts, including monoliths, are very promising as far as pressure drop and high performance for selective reactions are concerned. The perspectives for the use of monolithic catalysts in heterogeneous catalysis have been analysed by Cybulski and Moulijn (1994) and are further discus.sed in Section 5.4.7.7. 

In spite of the importance of having an accurate description of the real electrochemical environment for obtaining absolute values, it seems that for these systems many trends and relative features can be obtained within a somewhat simpler framework. To make use of the wide range of theoretical tools and models developed within the fields of surface science and heterogeneous catalysis, we will concentrate on the effect of the surface and the electronic structure of the catalyst material. Importantly, we will extend the analysis by introducing a simple technique to account for the electrode potential. Hence, the aim of this chapter is to link the successful theoretical surface science framework with the complicated electrochemical environment in a model simple enough to allow for the development of both trends and general conclusions. 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution into the present and future of the industrial world. It illustrates the transition of electrochemical sciences from a solid chapter of physical electrochemistry (covering mainly electron transfer reactions, concepts of electrode potentials and stmcture of the electrical double layer) to the field in which electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is supported by high-level theory, connects to other areas of science, and includes focus on electrode surface structure, reaction environment, and interfacial spectroscopy. 

Jan C.J. Bart (PhD Structural Chemistry, University of Amsterdam) is a senior scientist with broad interest in materials characterisation, heterogeneous catalysis and product development who spent an industrial carrier in R D with Monsanto, Montedison and DSM Research in various countries. The author has held several teaching assignments and researched extensively in both academic and industrial areas he authored over 250 scientific papers, including chapters in books. Dr Bart has acted as a Ramsay Memorial Fellow at the... 

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