摘要:

The effect of additives on metal oxide and silver catalysts to improve the activity and selectivity of various processes is considered. Additives influence the kinetic parameters of a process (rate and activation energy) and the reaction mechanism (structure of surface compounds, energy of their bond with the catalyst). Modification causes the formation of new compounds in the bulk and at the surface of the catalyst, thereby altering the composition of active sites and inducing formation of other sites. The effects of species added to multicomponent oxide systems, as well as various aspects of modification by acid-base additives are considered. Specific features of the influence of additives and the role of modified multifunctional catalytic systems are also discussed.

ISSN:0144-235X    

DOI:10.1080/01442358909353220

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The formation and main structural properties of superoxo and peroxo complexes are briefly described. These complexes are involved in catalytic oxidations (oxygenations) by dioxygen occurring under mild conditions in the presence of iron, cobalt, manganese, rhodium and other transition metal complexes. Examples of catalytic systems are taken from cytochrome P-450 models with specific reference to the mechanisms involved. Metalloporphyrin-catalysed oxidations of hydrocarbons involve free-radical chain processes in most cases. Added reducing agents modify the mechanism of oxidation by pumping electrons into key intermediates. They help the formation of oxometal species, which may transfer oxygen atoms to substrates, thereby improving the selectivity. Olefin oxidation is a difficult task for the homogeneous catalysts described: only a few working systems are available, based mainly on rhodium phosphine complexes. Peroxometalacycles are presumably formed, whose decomposition involves co-oxidation of a phosphine molecule. The oxidation of hindered phenols catalysed by cobalt (II) Schiff-base complexes points to the involvement of superoxocobalt species abstracting a H-atom from the phenol. Complex reaction patterns emerge from the oxidation of o-disubstituted phenols. The synthetic potential of homogeneous catalytic oxidation is illustrated on examples including various types of organic compounds.

ISSN:0144-235X    

DOI:10.1080/01442358909353221

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

In gases molecular rearrangements are stochastic processes, that is, the relative amount of each configuration is determined by the difference in free energy between the molecular configurations. In normal liquids the same relationships apply, although AG is perturbed by differences in nearest-neighbour interactions with each configuration. In a strongly bound solid it may be that no molecule can transform until they all do, in this case the process involves a first-order phase transition. In weakly bound molecular crystals, polymers and similar media, the range of interaction may lie between the nearest-neighbour and the macroscopic limit, that is, the process may be neither first order nor stochastic.

ISSN:0144-235X    

DOI:10.1080/01442358909353222

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Starting from a general quantum-mechanical perturbation theory on the nonlinear optical (NLO) effect in crystals, this review gives a systematic presentation of the basic concepts and calculation methods of the ‘anionic group theory for the NLO effect of crystals’ and a brief discussion of the approximations involved. Calculations have been made for the second harmonic generation (SHG) coefficients of a few typical NLO crystals. Comparisions between these theoretical values and the experimental values made both on powdered crystals and on single crystals suffice to show the feasibility of the theoretical treatment and calculation methods. On this basis, borate ions of various structure types are classified and systematic calculations are carried out for the NLO susceptibilities of some typical borate crystals with good prospects of applications in opto-electronics.

ISSN:0144-235X    

DOI:10.1080/01442358909353223

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor