Solid state mechanism, selective

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Hulanicki and A. Lewenstam, Interpretation of the Response Mechanism of Solid-State Ion-Selective Electrodes Regarding Diffusion Processes, in Ion-Selective Electrodes (ed. E. Pungor and I. Buz s), International Conference held in Budapest 1977, Akad6miai Kiad6, Budapest and Elsevier, Amsterdam (1978). 

The active site on the surface of selective propylene anmioxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an CC-H abstraction component such as Bi3+, Sb3+, or Te4+ an olefin chemisorption and oxygen or nitrogen insertion component such as Mo6+ or Sb5+ and a redox couple such as Fe2+/Fe3+ or Ce3+/ Ce4+ to enhance transfer of lattice oxygen between the bulk and surface of the catalyst. The surface and solid-state mechanisms of propylene ammoxidation catalysis have been determined using Raman spectroscopy (40,41), neutron diffraction (42—44), x-ray absorption spectroscopy (45,46), x-ray diffraction (47—49), pulse kinetic studies (36), and probe molecule investigations (50). 

De Marco, R. Jiang, Z. T. Becker, T. Clarke, G. Murgatroyd, G. Prince, K., Response mechanisms and new approaches with solid-state ion-selective electrodes A powerful multitechnique materials characterization approach, Electroanalysis 2006, 18, 1273-1281... 

Much of the understanding of the solid state mechanism of heterogeneous catalysis stems from fundamental studies of single phase model compounds (1-5). In many cases, the role of a metal component in a catalytic process has been discerned through its incorporation into solid solutions of relatively inert host matrices (O. In the case of the selective oxidation and... 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

It can be concluded in general, that electrochemical properties of copper(II) sulphide are influenced by chloride ions and consequently that copper(II) sulphide plays an active role in build up of chloride interference whenever this substance is present in the membrane. The redox reaction responsible for the interference discussed above shows the importance of consideration and inclusion of such mechanism in the general picture of interfering pathways on solid-state ion-selective electrodes. Explanation of the mechanism has so far dominated by traditional reasoning restricted only to simple ion-exchange reactions at the membrane surface. 

A general approach to fabricating solid-state ion-selective microelectrodes has been described whereby a conducting electroactive polymer, which is both an electronic and an ionic conductor (e.g., polypyrrole, polythiophene, or polyaniline), is used to mediate charge exchange between an ion-selective membrane (an ion conductor) and a metal substrate (an electronic conductor) [28]. These electrodes are reported to be robust and mechanically flexible while exhibiting good potential stability with no redox sensitivity. While applications to potentiometric SECM have been described [28], these electrodes have yet to find use in corrosion studies. 

Alloy crystal and thermal data symbols. A number of tables show, for selected alloys, the highest melting points observed in the systems considered, as well as the mechanism of formation (p = peritectic melting, syn = synthetic reaction, s.s.r. = solid-state reaction, est. = estimated melting point, etc.), the value of the Raynor Index (<1, =1 or>l). The question mark means that no reliable data are available. 

According to the aggregation state of the component elements and the method selected for starting and performing their reaction, several preparative procedures can be considered, such as melting (direct reaction in the liquid state), solid-state synthesis (mechanical alloying), combustion synthesis, etc. 

In this section, we discuss the high performance of the Rejo cluster/HZSM-5 catalyst, its active structure and dynamic structural transformation during the selechve catalysis, and the reaction mechanism for direct phenol synthesis from benzene and O2 on this novel catalyst [73, 107]. Detailed characterization and determination of active Re species have been conducted by XRD, Al solid-state MAS NMR, conventional XAFS and in situ time-resolved energy dispersive XAFS, which revealed the origin and prospects of high phenol selectivity on the novel Re/HZSM-5 catalyst [73]. 

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