Heterogeneous catalysis metallic catalysts

However, in heterogeneous catalysis, metals are usually deposited on nonconducting supports such as alumina or silica. For such conditions electrochemical techniques cannot be used and the potential of the metallic particles is controlled by means of a supplementary redox system [8, 33]. Each particle behaves like a microelectrode and assumes the reversible equilibrium potential of the supplementary redox system in use. For example, with a platinum catalyst deposited on silica in an aqueous solution and in the presence of hydrogen, each particle of platinum takes the reversible potential of the equilibrium 2H+ + 2e H2, given by Nemst s law as... 

Most often, in the heterogeneous catalysis the catalysts are soUds — usually metals, very often precious metals — while the reactants are liquid or gaseous. The catalysts are usually composed of a number of (metal) components in various combinations. The tests are performed in parallel under the otherwise equal temperature and pressure conditicms in a large number of microreactors. fii each microreactor the combination of the tested catalyst components is different and this results in a different yield or selectivity of the test reactirm. This is usually evaluated by composition analysis of the reaction products. 

In theory, almost all metal oxides can be synthesized by sol-gel process. Among others, an abundant literature can be found on SiOj, AljOj, TiOj, ZrOj as well as on the corresponding mixed oxides [19, 67,71-73]. Thus all porous oxide materials used in heterogeneous catalysis as catalyst support or precursors of catalyst supports can be prepared by sol-gel process. In the field of inorganic gels, the present review will focus mainly on silica gels. 

Keywords Heterogeneous catalysis, carbon catalyst, graphene, metal nanoparticles... 

The proposed mechanism by which chlorinated dioxins and furans form has shifted from one of incomplete destmction of the waste to one of low temperature, downstream formation on fly ash particles (33). Two mechanisms are proposed, a de novo synthesis, in which PCDD and PCDF are formed from organic carbon sources and Cl in the presence of metal catalysts, and a more direct synthesis from chlorinated organic precursors, again involving heterogeneous catalysis. Bench-scale tests suggest that the optimum temperature for PCDD and PCDF formation in the presence of fly ash is roughly 300°C. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

One problem with heterogeneous catalysis is that the solid catalyst is easily poisoned. Foreign materials deposited on the catalytic surface during the reaction reduce or even destroy its effectiveness. A major reason for using unleaded gasoline is that lead metal poisons the Pt-Rh mixture in the catalytic converter. 

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

Heterogeneous catalysis by metals has been of long-standing interest, with bimetallic catalysts a particular focus.Transition metal carbonyls have... 

---