Nitrides and Oxynitrides

Only a few nominally ionic nitride perovskites are known, of which the best characterised is ThTaNj. This has the ideal cubic stmcture  

ThTaNj cubic a = 0.40205nm, Z= 1 space group, Pm3m (221)  

The Th i ions lie at the corners of the unit cell. The Ta ions lie at the cell centre and are surrounded by a regular octahedron of ions. 

Many more perovskite phases are known in which both oxygen and nitrogen occupy the anion positions to form the oxynitride perovskites, with a formula ABO N, typified by NdTiO N (Table 2.3). These materials are generally approximated as ionic compounds, so that 

Ensuring charge balance is maintained gives a number of possibilities  

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Progress in the design and fabrication of high-quality optical microresonators is closely related to the development of novel optical materials and technologies. The key material systems used for microresonator fabrication include silica, silica on silicon, silicon, silicon on insulator, silicon nitride and oxynitride, polymers, semiconductors such as GaAs, InP, GalnAsP, GaN, etc, and crystalline materials such as lithium niobate and calcium fluoride. Table 2 smnmarises the optical characteristics of these materials (see Eldada, 2000, 2001 Hillmer, 2003 Poulsen, 2003 for more detail). 

Nitrides and Oxynitrides. Crystalline films of NbN and Nb Nj have been prepared. Electron-diffraction studies showed that the former has a NaCl-type structure and the latter is isomorphous with Ti405 and Ta Nj. " The phase 0-TaN, 0 has been prepared by nitriding Ta metal powder with ammonia gas at 850—1200°C. Although it could not be obtained pure, the phase was characterized by X-ray diffraction studies. The free energy of formation of TajNj has been determined as -272 42 kJ mol and that of TaO, 05N0.95 as -396 21 kJ moP. ... 

Nitrides and oxynitrides represent a relatively new class of catalytic material. Justin Hargreaves and D. McKay (University of Glasgow, UK) show that these materials have only recently been explored for reactions (e.g., photocatalysis) beyond those that take advantage of their acid-base properties and their ability to mimic Pt-based catalysts. Tuning the acid-base properties of nitrides is possible by incorporating oxygen within their structure. 

Generally, metal nitride and oxynitride catalysts have been prepared by the temperature programmed ammonolysis method. This technique was originally applied by Volpe and Boudart to M0O3 precursors to produce y-Mo2N which was pseudomOrphic with the precursor, with surface areas up to 225 m g The resultant surface area of materials is known to be a strong function of the synthesis parameters as demonstrated in Table 1. ... 

In another example, the high temperature reaction of calcium and strontium sub-nitrides with molybdenum foil has also recently been claimed to produce novel molybdenum ternary nitride oxides which contain isolated [MoN4] tetrahedra in an ordered sub-lattice of and anions. High temperature-high pressure syntheses are also yielding novel metastable structures, such as the spinel analogues mentioned previously. There is therefore an ever-increasing base of nitrides and oxynitrides with diverse structures and potential catalytic interest. 

In this section, the application of nitrides and oxynitrides as supports or catalysts themselves is outlined. The section is sub-divided by reaction/reaction type or application. Although emphasis is given to the more recent developments, where appropriate the historical context is given. 

Hydrotreating and Hydrogenation. - Nitride and oxynitride catalysts have been extensively investigated as catalysts for hydrotreating (in particular hydronitrogenation and hydrodesulfurisation) and hydrogenation (for CO, aromatics, crotonaldehyde and alkenes ° the reduction of NO with H2 has been covered in the previous section). 

Extensive studies describe the development of new materials, such as transition-metal nitrides and oxynitrides, as new potential catalysts for hydroprocessing operations. Studies were performed to acquire information on the effect of high levels of ZSM-5 additive on light olefins and gasoline composition in FCC.289 It was found that ZSM-5 can substantially increase propylene and butenes, and... 

Shen Z, Nygren M (2000) Nd-Doped a-Sialon and Related Phases Stability and Compatibility. In Hampshire S, Pomeroy MJ (eds) Nitrides and Oxynitrides. Trans Tech Publications Ltd, Switzerland, p 191... 

Table 4.6. A selection of chemical shifts for silicon nitride, silicon oxynitride and SiAION compounds and related metal silicon nitrides and oxynitrides, with respect to TMS.

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