摘要:

The mechanism of heterogeneous catalytic reactions is much more difficult to elucidate than that of homogeneous systems. Despite the facilities provided by physical methods for investigating the surface of solids, obtaining detailed information on the structure of the active component in real heterogeneous catalysts presents difficulties due to the nonuniform chemical composition of the surface species. Some of these surface species are totally inactive in catalysis, and others can catalyze the given chemical reaction by different pathways and according to different mechanisms. This results in a change of selectivity to the desired product and the appearance of intermediates and reaction by-products. Furthermore, the effect of the reaction medium on the catalyst gains importance during a catalytic process when, at high temperature and pressure, one type of surface species is transformed into another, thus changing the mechanism and direction of the catalyzed reaction.

ISSN:0161-4940    

DOI:10.1080/01614949508006446

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Selective ammoxidation of paraffins into corresponding nitriles is of a prime practical significance as it could add to the sources for synthesis of a number of valuable organic products currently produced from oil. On the whole these processes seem to be more promising for commercial synthesis than partial oxidation of paraffins, as nitriles are more stable than corresponding oxy products and a higher yield of the desirable product may potentially be achieved.

ISSN:0161-4940    

DOI:10.1080/01614949508006447

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The basic objective of mechanistic studies of real catalytic processes is to dissect the course of the reaction into individual steps; ascertain their sequence; and determine the stoichiometry, structure, and electronic states of active sites and intermediates. The electron paramagnetic resonance (EPR) technique is at present widely used to explore many of these principal aspects of heterogeneous catalysis and surface chemistry. The extreme sensitivity compared to the usual spectroscopic methods is perhaps its most acknowledged advantage and makes EPR best suited to investigate and characterize low-abundance active sites and intermediates appearing during catalytic reaction. Additional information can be drawn from the theoretical analysis of the experimental spin Hamiltonian parameters within the ligand field and from angular overlap or Newman's superposition models as well as by more sophisticated quantum chemical calculations. The purpose of this paper is to show how catalysis benefits from EPR spectroscopy and to identify the issues and areas explored by this method. A comprehensive literature review is not attempted in this article; instead, attention is directed toward application of EPR for elucidation of the molecular reaction mechanism that can provide a scientific background for understanding many fundamental aspects of catalytic activity. The major events of mechanistic studies which involve the identification of active sites, activation of reagents, and determination of the reaction pathways are illustrated by selected examples and discussed. An approach that is complementary to mechanistic catalytic test studies is also presented. It consists of spectroscopic investigations of a set of partial reactions, driven by external creation of the supposed active sites and intermediates, with the aim of reproducing and verifying the feasibility of the postulated catalytic cycle. Moreover, to assure some consistency of the subject, basic characteristics of EPR spectroscopy related to surface studies and chemical theories of reactivity are concisely reviewed.

ISSN:0161-4940    

DOI:10.1080/01614949508006448

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:0161-4940    

DOI:10.1080/01614949508006449

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor