Molybdenum adsorption kinetics

In the previous sections of this review, it has been shown that most effective catalysts for the selective oxidation of propylene contain at least two types of metal oxides—an amphoteric or low-valence oxide, such as bismuth, tin, iron, or cobalt, and an oxide of a high valence metal, such as molybdenum or antimony. Moreover, it has been suggested several times that each of these metal oxide components may give rise to an active site for example, propylene may adsorb on an active site associated with one of the metal oxide components, and oxygen may adsorb on an active site associated with another metal oxide component. This problem has been studied using spectroscopic, adsorption, and kinetic techniques. It now seems appropriate to consider some of these studies in detail, attempting to relate the solid structure of the catalyst to the active sites wherever possible. 

An explanation might be found if monomer chemisorption takes place instead. Indeed boehmite dissolution occurs by means of its lateral surfaces [19] monomer chemisorption on lateral boehmite surface would block surface functional sites and thus inhibit at least partially boehmite dissolution. Therefore, at intermediate molybdenum loading, as monomer concentration in solution is not negligible compared to AHM concentration, monomer adsorption on boehmite lateral surfaces should compete with aluminum extraction. Fig. 8 reports boehmite dissolution kinetics carried out at pH=4.8, for two [AHM] concentrations (intermediate and high molybdate loading). This result could support the fact that aluminum dissolution is slowed down at intermediate molybdenum loading and that no substantial moly-bdate adsorption could take place a plateau is reached in the adsorption isotherm (Fig. 1). 

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

The type of anode catalyst has a strong effect on the severity of CO poisoning, since the catalyst affects the kinetics of CO adsorption (equation (2.12) and equation (2.13)) and CO oxidation (equation (2.18) and equation (2.19)). Based on these mechanisms, many CO-tolerant electrocatalysts have been developed by Pt alloying, such as PtRu (platinum/ruthenium) [24,38], PtSn (platinum/tin) [39-41], and PtMo (platinum/molybdenum) [42-44]. Generally, alloying Pt with a second element can enhance the catalytic activity of the Pt through one or more of the following effects ... 

The authors determined the adsorption and the surface reaction kinetic parameters of methanol reaction over monolayer supported vanadium, molybdenum, chromium, rhenium catalysts, and bulk iron molybdate. 

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