Reduced surface sites

A significant steady-state population of reduced surface sites is generated by the cathodic overpotential, as set by the fuel cell load. 

Production of reduced surface sites is directly proportional to [>Mell A]. Thus, the distribution of reduced surface sites and rates of subsequent metal ion release both increase as reductanf surface coverage is increased. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

The formation of formaldehyde from formic acid is not expected to occur on stoichiometric TiO2(110) because the surface cations are exclusively five-coordinate, and formaldehyde formation in the absence of reduced surface sites has been shown to be dependent on the presence of low-coordinate metal cations on titanium dioxide surfaces. Iwasawa et al. noted the absence of formaldehyde from the product slate of formic acid reaction products in their studies on the TiO2(110) surface [43]. 

Waste products are formed from propane on this catalyst primarily via the second formed acrolein. It is postulated that the acrolein, once formed, readily readsorbs on the catalyst surface, and presumably interacts with a second site which might be either a highly acidic surface site (Mo-0 H" ) or a reduced surface site ( Mo ) or simply by interaction, before desorption, with an adjacent overactive surface site. This scenario is particularly strongly implied by the observed Langmuir-Hinshelwood dependence of waste formation from propylene. 

The same can be said about re-oxidation of reduced surface sites (e.g., oxygen vacancies [..]s) if the addition of an oxygen molecule to vacancies... 

Equation 6 shows that the adsorption of component 1 at a partial pressureis reduced in the presence of component 2 as a result of competition for the available surface sites. There ate only a few systems for which this expression (with 5 1 = q 2 = 5 ) provides an accurate quantitative representation, but it provides useful quaUtative or semiquantitative guidance for many systems. In particular, it has the correct asymptotic behavior and provides expHcit recognition of the effect of competitive adsorption. For example, if component 2 is either strongly adsorbed or present at much higher concentration than component 1, the isotherm for component 1 is reduced to a simple linear form in which the apparent Henry s law constant depends onp. ... 

It can be seen from these two factors, ie, particle charge and van der Waals forces, that the charge must be reduced or the double layer must be compressed to aUow the particles to approach each other closely enough so that the van der Waals forces can hold them together. There are two approaches to the accomplishment of this goal reaction of the charged surface sites with an opposite charge on an insoluble material and neutralization of... 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

Adzlc et al (25) have shown that partial coverage of the electrode by adsorbed Pb can substantially reduce the effects of poisoning, presumably by blocking the surface sites required by the adsorbed CO. This is nicely confirmed by spectroscopic measurements. Figure 8... 

The nature of surface sites on the reduced 0.5 wt.% RU/AI2O3 and 0.5 wt.% Ru/Ti02 catalysts was probed by FTIR spectroscopy of adsorbed CO. Four adsorbed CO bands were... 

When a calcined Cr(VI)/Si02 catalyst is fed with ethylene at 373-423 K, an induction time is observed prior to the onset of the polymerization. This is attributed to a reduction phase, during which chromium is reduced and ethylene is oxidized [4]. Baker and Garrick obtained a conversion of 85-96% to Cr(II) for a catalyst exposed to ethylene at 400 K formaldehyde was the main by-product [44]. Water and other oxidation products have been also observed in the gas phase. These reduction products are very reactive and consequently can partially cover the surface. The same can occur for reduced chromium sites. Consequently, the state of sihca surface and of chromium after this reduction step is not well known. Besides the reduction with ethylene of Cr(Vl) precursors (adopted in the industrial process), four alternative approaches have been used to produce supported chromium in a reduced state ... 

In more recent applications, several types of ET cover designs also have incorporated synthetic materials, such as geomembranes, which are used to enhance the function of minimizing water into the waste. For example, the Operating Industries Inc. Landfill in California has incorporated a soil layer with a geosynthetic clay liner in the design. The cover system for this site will reduce surface gas emissions, prevent oxygen intrusion and percolation, and provide for erosion control.68... 

The common idea on the mechanisms governing the reduction of NO adsorbed species over LNT catalysts is that the regeneration process includes at first the release of NO, from the catalyst surface (i.e. from the alkali- or alkali-earth metal compound), followed by the reduction of the released NO to N2 or other products [11]. The reduction of the released NO in a rich environment is thought to occur according to the TWC chemistry and mechanisms in particular, it was suggested that NO is decomposed on reduced Pt sites [38], or that a direct reaction occurs between released NO species and the HC reductant molecules on the precious metal sites [39],... 

Also, a mechanism involving the surface diffusion of NO ad-species towards reduced Pt sites cannot be excluded (Figure 6.14b) [11], In this case, NO spills over the surface and is decomposed at reduced Pt sites. The role of the reductant in this mechanism is... 

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