Catalysts, bifunctional reforming mechanisms

This review is concerned with a discussion of the reactions of hydrocarbons over bifunctional catalysts, primarily from the viewpoint of mechanism and kinetics. Some discussion will also be given of the structure and properties of typical bifunctional reforming catalysts, since this is somewhat helpful in understanding how the catalyst functions in promoting the various reactions. In addition, at appropriate places in the article, the practical application of the principles of bifunctional catalysis in commercial reforming processes will be considered. 

Figure S Reaction mechanism for MCP conversion and coke formation on bifunctional reforming catalysts.

A separate problem arises when the surface of the support bears non-metallic active centres. Such bifunctional catalysts are widely used in naphtha reforming and it has been suggested that a bifunctional mechanism may also operate in syngas reactions [35]. 

The reformation of lower paraffins to aromatics has been studied for about 20 yr by using zeolite catalysts. Recently, an excellent review was published of lower alkane transformation to aromatics on ZSM-5 zeolites [2]. From the studies of the mechanism of this reaction, it has been suggested that the bifunctional catalysts, having solid acidity and dehydrogenation activity, can effectively promote the aromatization of lower paraffins[3-6]. It has been reported that ZSM-5 and ZSM-11 are excellent solid acid catalysts [7] and the transition metals [8], Ga [9] and Zn [9] show high dehydrogenation activity in this reaction. In the case of bifunctional... 

The bifunctional catalytic n-hexane isomerization was performed in a microflow reactor under atmospheric pressure, using a mechanical mixture (Iw/w) of catalyst and a standard Pt/A Os reforming catalyst (0.35 wt% Pt). The latter was previously reduced at 723 K for 4 h. The catalytic test was performed as follows The mixture Pt/A Os + catalyst was pretreated first under He flow at 723K for 2 hours and then under H2 for 30 min at 493K. 

In discussing the mechanism of hydrocarbon transformations on bifunctional catalysts, it is useful to refer to a reaction scheme originally proposed by Mills et al. (16) to describe the reforming of C6 hydrocarbons. The scheme is shown in Figure 5.2. The vertical reaction paths in the figure take place on the hydrogenation-dehydrogenation centers of the catalyst and the horizontal reaction paths on the acidic centers. 

Four nonmetallic additives are to be mentioned here sulfur and chlorine are added intentionally to influence catalyst activity, hydrogen is always present in commercial reforming, and carbon accumulates as a result of a side reaction producing polymeric coke on metallic and acidic sites (72). Chlorine ( 1%) is added to alumina to ensure proper acidic properties to promote bifunctional mechanism (2,4,72). The presence of Cl slowed down catalyst deactivation and accentuated metal-support interaction (73). It could also suppress hydrogenolysis activity of commercial reforming catalyst. At the same time, carbon accumulated from CCI4 added to the reformer stream to ensure the steady Cl content of reforming... 

Cobalt is used to promote CO oxidation in reformers [284, 285], suggesting PtCo alloys may be useful catalysts for H2 oxidation in the presence of CO. PtCo alloys have been proposed as improved methanol oxidation catalysts [286] because cobalt may assist with CO removal (CO is an intermediate in meflianol electrooxidation) through a mechanism analogous to the PtRu bifunctional mechanism. PtCo alloys have also been studied as improved ORR catalysts [200, 287, 288]. In addition to their improved ORR kinetics, these alloys have been shown to be more tolerant to methanol crossover in direct methanol fuel cells (DMFCs), again possibly through improved CO removal kinetics [289]. However, Stevens et al. [235] observed no impact on CO-stripping with the addition of eobalt to Pt, and explained this as due to surface cobalt dissolving away. 

Can support influence the kinetic rates and reaction mechanism that means, is it a bifunctional site system O Coimor [19] demonstrated the involvement of the oxygen of the zirconia network in the dry reforming of methane on Pt/Zr02 catalysts. DRIFTS results showed that the presence of carbonate during the reaction, due to the oxygen in the network of zirconia, was replaced by CO2 [20]. 

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