Redox surface

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

Pt loading occurs via ion exchange, while Bi loading involves a redox surface reaction of BiO+ with hydrogen covered platinum particles (1-2 nm), according to the following stoichiometry [74] (Pd-H)a(js + BiO+ —> (Pd-Bi)surf + H30+ ... 

Attaching the enzyme directly on the electrode surface is expected to improve elec-trocatalytic efficiency and response and improve the reproducibility of immobilization (147). Metallic (122, 144, 145) and carbonaceous (146) enzyme electrodes develop potentiometric responses to H2O2 produced by the enzymatic reaction. Unfortunately, the signal is markedly dependent on the redox surface of the electrode and thus on the electrode pretreatments (which are quite difficult to reproduce). 

Supported Nb205 [54], Ta20s [55] and WO3 [26] catalysts typically possess almost no redox potential and exclusively behave as surface acid sites. Other than their acidic properties, these supported metal oxides possess similar molecular and electronic structural characteristics as the redox surface sites discussed above. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Bimetallic (Pt-Bi) or trimetallic (Pt-Pd-Bi) catalysts are available commercially from Degussa [85]. Metal promoters such as bismuth or lead, were added to platinum metals by co-impregnation, by impregnation of the supported noble metal catalyst with an aqueous promoter salt solution, or by redox surface reaction. A very simple and efficient way of loading Pt/C or Pd/C catalysts with bismuth is to add the required amount of aqueous Bi0N03 solution to a suspension... 

Gold was. deposited by the refilling method (12,13), which consists In a Redox surface reaction between chemisorbed hydrogen on platinum and the cation of the second metal (Au) according to the following scheme ... 

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