Structure and Defect Chemistry

To improve the efficiency of photocatalysts, developments in the future must be based on an understanding of the sophisticated factors that determine the photoactivity of the water-splitting reaction (i) molecular reaction mechanisms involved in the oxidation and reduction of water on photocatalyst surfaces, (ii) structure and defect chemistry of photocatalyst surfaces, and (iii) charge transfer mechanisms between... 

The preceding sections have focused on Che properties of the bulk oxide. However, computer simulation techniques are also well established tools in the study of the structural and defect chemistry of oxide surfaces, which are often difficult to characterize by experiment alone. 

The morphology, electronic conductivity, exposed specific surface area, crystal structure, and defect chemistry (cation distribution and oxidation states) of the manganese oxides considerably influence the performance of pseudocapacitor (capacitance, cycle life, and charge/discharge rate).153,156, l57 173,iso, 186,188 ese fea-... 

In this chapter we have selected a number of case studies to show how atomistic lattice simulations can be used to investigate the structural and defect chemistry of a wide range of high-7 cuprates. Space limitations have necessarily restricted the number of examples to a small range of compounds, chiefly from the La Cu-O, Nd-Cu-O, Y Ba Cu O and Sr-Cu-O systems. 

Among the several transition alumina phases, y-Al203 is the most important and most studied phase for catalysis [57, 58]. However, even nowadays, several aspects of its structural and surface chemistry are still not well understood, since y-Al203 is a poorly crystalline solid, showing some variation in its structural stoichiometry and a wide range of defects. In the last 50 years, several empirical models for y-AI2O3 surface have been reported, trying to explain the complexity of this surface... 

All these considerations referred to the perfect structure of a single crystal which is normally the domain of structural chemistry. It refers, as it were, to the virtually defect-free crystal structure. The chemically excited crystal structure resulting from a superposition of perfect structure and defect structure is of prime significance in the context of this text (cf. Chapters 5, 6 and 7) . ... 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

As in the previous chapter, most work has been carried out on oxides, and these figure prominently here. As the literature on oxides alone is not only vast but is also rapidly increasing, this chapter focuses upon a number of representative structure types to explain the broad principles upon which the defect chemistry depends. However, despite considerable research, the defect chemistry and physics of doped crystals is still open to considerable uncertainty, and even well-investigated simple oxides such as lithium-doped nickel oxide, Li Nij- O, appear to have more complex defect structures than thought some years ago. 

The electron microscopy studies of the superconductive cuprates show that the different families differ from each other by the nature of their defect chemistry, in spite of their great structural similarities. For example, the La2Cu04-type oxides and the bismuth cuprates rarely exhibit extended defects, contrary to YBa2Cu307 and to the thallium cuprates. The latter compounds are characterized by quite different phenomena. 

In an ideal world, crystals would be perfect or stoichiometric with constant composition. But like people crystals are not exempt from imperfections or defects. Crystals with variable composition are termed non-stoichiometric crystals. The defect chemistry of oxides is enormously complex and is extremely vital to their properties. It has involved extensive research in many laboratories and is providing extraordinary insights into structural variations, the stability of structures and the formation of new structures. Here, we first define order-disorder phenomena that are commonly associated with oxides and describe our current understanding of them. The disorder or non-stoichiometry plays a crucial role in oxide applications including catalysis and it is therefore of paramount importance. 

The structure of activated carbon is best described as a twisted network of defective carbon layer planes, cross-linked by aliphatic bridging groups (6). X-ray diffraction patterns of activated carbon reveal that it is nongraphitic, remaining amorphous because the randomly cross-linked network inhibits reordering of the structure even when heated to 3000°C (7). This property of activated carbon contributes to its most unique feature, namely, the highly developed and accessible internal pore structure. The surface area, dimensions, and distribution of the pores depend on the precursor and on the conditions of carbonization and activation. Pore sizes are classified (8) by the International Union of Pure and Applied Chemistry (IUPAC) as micropores (pore width <2 nm), mesopores (pore width 2—50 nm), and macropores (pore width >50 nm) (see Adsorption). 

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