Anchoring oxidation catalyst, promotion

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Stable anchoring of an oxidation catalyst can be promoted by reaction conditions, including the choice of solvents, catalytic species, and bases. For example, Fe- or Cu-exchanged materials have improved stability if they are used under alkaline conditions (138,181). On the other hand, the presence of strong or complexating acids must be avoided since they induce metal leaching (162). 

For oxides such as TiOj or ZrOj, the strong metal-support interaction and the anchor effect between the metal oxides and the adjacent Pt atoms are major reasons for the increased electrochemical stability (Liu et al., 2006,2010 Bauer et al., 2010). In a report by Huang et al. (2010), after being subjected to continuous voltammetric cycles in the range of 0.6-1.4 V vs. RHE in an RRDE, the Pt/Nb Ti,. )Oj catalyst showed nearly 10-fold higher ORR activity than the Pt/C catalyst. The main disadvantages of such catalysts are the relatively lower electronic conductivity and lower surface area. Under these conditions, the metal oxide is more like a support promoter than a catalyst promoter. 

Besides the use of vanadium-based catalysts, a wide variety of other catalyst compositions were reported. A recent review focussed on FeSbO based catalysts promoted by appropriate additives as suitable for the ammoxidation of alkyl-substituted aromatics and hetero aromatic compounds. A unique preparation method of a fluid-bed catalyst is presented using nitric acid oxidation of antimony trioxide catalyzed with iron ions. The catalysts thus prepared have superior catalytic and physical properties. [78]. In addition, some unique compositions were reported by different research groups. For instance, new ammoxidation catalysts based on rhenium carbonyl cluster complexes containing antimony and bismuth ligands were reported by Adam et al. [79]. Single-site multifunctional catalysts based on [Cu RUj C ] nanocluster anchored to inner walls of mesoporous silica were also used in the ammoxidation of 3P [80]. 

Similarly, Au NPs have also been supported on r-GO an N-doped r-GO and the materials have been tested as catalysts for the aerobic oxidation of benzyl alcohol [40-41]. It was found that the presence of nitrogen as dopant is beneficial to obtain Au NPs with small size, therefore exhibiting enhanced catalytic activity [41]. The use of Au NPs and Au-Pd NPs supported on r-GO has also been reported to promote the aerobic oxidation of alcohols and oxidation of methanol to methyl formate [42]. In one of these examples, r-GO has been functionalized with imidazolium ionic liquid covalently anchored to the r-GO in order to increase the affinity of the support for Au NPs. 

In situ generated NHC-Cu-TEMPO complex 34 [eqn (12.3)] was explored in the aerobic oxidation of primary alcohols to the corresponding aldehydes. " Surprisingly, the addition of a base (potassium tert-butoxide, triethylamine) had a detrimental effect on the catalytic reaction using 34 as catalyst. This fact contrasted with other reported Cu-N-based ligands/TEMPO catalytic systems, in which the addition of base favored the reaction. The reaction proceeded more efficiently in chlorobenzene than in other solvents. Unexpectedly, [(NHC)CuX] complexes, prepared in situ from Cu powder and the corresponding imidazolium salt, or their simple combination with TEMPO, were completely inactive in the oxidation of alcohols. It was proposed that TEMPO anchored to Cu-NHC complexes facilitated the intramolecular proton abstraction, promoting the oxidation of alcohols. However, the mechanism was not further explored, and the relation between structure and activity of the catalyst remained unclear. 

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