Energy Trends in Catalysis

In this chapter, we will discuss how adsorption energies and activation energies for surface chemical processes are often correlated. As discussed in Chapter 2, there are two distinct types of surface-adsorbate interactions physisorption and chemisorption (see Fig. 6.1). We will treat different kinds of correlations in the two types of interactions separately in the following. 

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For the study of trends in catalysis, one often looks at a given reaction and reaction pathway and varies the catalytic surface. Here, the variations in entropy from system to system are usually very small and give rise to variations in the rate by less than an order of magnitude. This should be compared to the changes in rates induced by varying the energy barriers of different reactions. A change in barrier of, for... 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

Alloy stability is always of concern in heterogeneous catalysis, but in electrocatalysis there are new mechanisms for destabilizing alloys, namely electrochemical dissolution or corrosion. Greeley and Norskov developed an intuitive and simple thermodynamic framework for estimating the stability of alloy surfaces in electrochemical environments. " Their scheme is essentially an extension of an atomistic thermodynamic approach that uses chemical potentials to determine stability to one that uses electrochemical potentials to determine stability. They estimate the electrochemical potentials using total energies calculated within DFT and ideal solution behavior of the ions to consider concentration and pH effects. Within this formalism they are able to estimate the dissolution potential of metals in alloys. They further compared the trends in dissolution behavior to trends in segregation behavior and... 

The introduction of solid catalysts into a traditionally non-catalytic free-radical process like combustion occurred in recent years under the influence of two pressures, the energy crisis and the increased awareness of atmospheric emissions. The major applications of catalytic combustion are twofold at low temperatures to eliminate VOC s and at high temperatures (>1000 C) to reduce NOx emission from gas turbines, jet motors, etc. Both these applications are briefly reviewed here. Some recent developments in high-temperature catalytic combustion are trend-setters in catalysis and hence of particular interest. For instance, novel materials are being developed for catalytic applications above 1000 C for sustained operation for over one year. Where material/catalyst developments are still inadequate, systems engineering is coming to the rescue by developing multiple-monolith catalyst systems and the so-called hybrid reactors. 

We have made a concerted effort to present state-of-the-art developments in the area of nanocatalysis. The variety of topics from synthesis, characterization, and modeling to applications to a range of energy conversion processes provides an overview of modem catalysis and current trends in nanocatalysis research. We would like to thank all the contributors for providing a scientifically stimulating discussion in their respective chapters. 

The trends in zeolite chemistry and catalysis were governed by a few general trends in chemistry and applied catalysis over the last 10-20 years, including the shifts in the paradigms related to raw materials, energy, environmental concerns, product demand, and the like. 

The mutual stimulation of scientists coming from different disciplines, viz. photochemistry, electrochemistry, coordination chemistry, anal ical chemistry, biology, medicine, material chemistry, smfface science, electronics, and catalysis, resulted in the extremely rapid development of photocatalysis (265). This review shows the fast progress made in our understanding of the processes occmring in nature that are catalyzed by metal complexes and driven by solar energy, and demonstrates their general trends and fine details. 

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