Metal-surface-induced activation

Metal Surface Induced Activation of Dioxygen for Oxygenase Chemistry... 

Metal-surface-induced activation of dioxygen for oxygenase chemistry... 

Very rapid and highly localised pitting is sometimes observed on components exposed to very turbulent flow conditions leading to cavitation in the stream. In general, these conditions appear to induce corrosion rather than erosion on cast iron surfaces, in contradistinction to what usually happens with other metals, apparently because the erosive component of the liquid flow scours away corrosion-stifling films and allows the development of very active electrochemical cells on the exposed metal surfaces . 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

Mercury is a classical test to identify heterogeneous catalysts (bulk metal or colloids) due to its ability to poison metal(O) heterogeneous catalysts by formation of amalgam or adsorption on the metal surface [23]. If the catalytic activity remains unaffected when mercury is present, this fact represents an evidence for a homogeneous catalyst. But mercury can induce side reactions [23c] and also react with some molecular complexes [23c,24]. Consequently, the results obtained with mercury are not enough to conclude about the catalyst nature. From a practical point of view, it is important to use a large excess of Hg(0) with respect to the catalyst to favour the contact with it. 

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

Carbon may bind to a metal surface and induce a surface reconstruction whereby a more active metal plane is transformed to one with a lower activity.28... 

Inelastic tunnelling electrons can also be used to selectively induce either the translation over a metallic surface or desorption from the metallic surface of individual molecules, as has been shown for NH3 on Cu(lOO) surfaces (Pascual et al, 2003). Activation of either the stretching vibration of ammonia ( 408 meV) leading to lateral translation on the surface, or the inversion of its pyramidal structure (umbrella mode s(NH3) 139 meV) leading to desorption, can be achieved by adjusting 7t and Vt. 

For obvious reasons related to the necessary reduction of the amount of catalyst used in fuel cells, ORR has been studied on thin films of platinum deposited onto glassy carbon or titanium [73, 79] and on small metal particles on carbon [80-82]. The reduction of the Pt film thickness (<1 nm), as well as of the size of the particles (diam. < 3 nm), induces a moderate loss of activity attributed to differences in the adsorption of O2 on the metal surface. 

A convenient approximation in many applications is to assume that a region of interest with a RDOp p is in contact with a medium at thermal equilibrium. The system and medium are chosen so that the latter can be assumed to remain at equilibrium at all times, with a density operator yeq. In this case it is possible to search for solutions of the equations starting from a factorized density operator for the whole system, r = p 69 A/,q, in a procedure also called a Fano-factorization. [6] This however is not acceptable when the total system is subject to excitations which induce transitions among states of the medium. An example is a molecule adsorbed on a metal surface, excited by visible light which first creates electronic excitations in the substrate. In this case the active medium is described by a DOp evolving in time, and some of the common developments in the literature must be generalized. 

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