Promoter synergism, role

Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

When acid chelators are combined with one of the phenolic-type antioxidants, they can act synergistically. This type of synergism has been referred to as acid synergism (S). In this case, the primary role of a chelator is to bind metals, or metalloproteins that promote oxidation and thus, allow the antioxidant to perform its function and capture free radicals. In this manner, acid synergism is different than the BHA/BHT system. 

Crystal structure plays a secondary role in catalysis by the Transition Metal Sulfides. As the periodic trends for HDS of the binary sulfides shows the dominant effect is which transition metal is present in the reaction, this transition metal takes on the structure and stoichiometry of the phase which is most stable in the sulfur containing catalytic environment. The unsupported promoted catalyst systems can be grouped into "synergic" pairs of sulfides. Because these pairs are related to the basic periodic trends of the binary Transition Metal Sulfides through average heats of formation. 

The role of a seeond element in methanol oxidation on Pt alloy catalysts depends on how to lower the potential of water dissociation, to weaken the CO ehemisorption, and to direetly enhanee the electrooxidation of CO on Pt. It is obvious that none of the binary alloy catalyst, even the most used Pt-Ru catalyst, contains all of flic promoting effects simultaneously. The purpose of exploring ternary and quaternary Pt alloy catalysts is to combine all of the promoting effects into one alloy system to maximize the catalytic MOR activity. For example, Kim et al. [61] ascribed the enhancement of MOR activity on the Pt-Ru-Sn ternary alloy to the synergic effects of Ru as a water activator and Sn as an electronic modifier. Samjeske et al. [62] explained the synergic effect of Mo and Ru in the Pt-Ru-Mo ternary alloy by the way that Mo can shift the oxidation of weakly adsorbed CO on the Ru sites to lower potentials, facilitating the bi-functional effect. 

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