Poisoning bifunctional reaction

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

The situation is much different when an acidic support is used. First, the C8 aromatic products have a distribution that approaches an equilibrium composition. The Pt catalyst on an acidic support is both more active and produces aromatics more selectively than Pt on a nonacidic support (84). It is concluded that the bifunctional mechanism involving cyclization by the acid site followed by a bifunctional ring expansion/dehydrogenation reactions is much more rapid than the monofunctional metal catalyzed dehydrocyclization reaction. For the catalyst based on an acidic support, the tin added initially acts as a catalyst poison (Figure 5), at least during the initial 1-2 weeks of usage. 

With the aid of selective pyridine-poisoning experiments, we will show that isomerization of alkanes over NiSMM is a bifunction-ally catalyzed reaction. 

Selective acid-base poisoning on bifunctional allQ lation reaction... 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

In the studies regarding the influence of various oxidants, chiral auxiliaries, solvents, and temperatures on enantioselectivity in the AE reaction, the use of tert-butyl hydroperoxide (TBHP), DET, and toluene at -20 °C gave the optimum results. However, in the bifunctional single-pot CSC followed by AE reaction (Table 5.14, Method B), the ee s and conversions are lower than in the independent AE reactions. This is due to the poisoning effect of water adsorbed on to the catalyst, which is formed in sim in the CSC reaction. When the catalyst was dried under a nitrogen flow for 1 h at 250 °C immediately after the CSC, similar enantioseleetivity in the AE reaction was obtained, supporting the above theory. 

The latter feature renders the Ap XANES a suitable technique to unravel complex reaction mechanisms and synergetic actions, as they are often active in bimetalhc catalysts. One example might be the bifunctional mechanism in CO poisoning of Pt-Ru fuel cell anodes, which will be discussed in detail below. 

The major problem arising from the use of HPj s in acid catalysis is their deactivation over time and the requited regeneration. It is obvious that HPAs cannot be regenerated by combustion, since their stability does not go beyond —300"C. However, there are two ways to tackle the problem. One is to decrease the rate of decay by using, when possible, bifunctional catalysts in the presence of llj and/or to work under supercritical conditions for which the prepolymer-polymer type of products responsible for deactivation could be washed out. The other possibility deals with the regeneration of the catalyst. For example, since at low reaction temperatures one should not obtain coke products and polymer species should predominate, it should also be possible to remove the poisonous products by washing with adequate solvent under the optimum conditions, or by mild hydrogenation. 

Rare earth oxides have been studied to a lesser extent than alkaline earth oxides. However, they show characteristic selectivity in the dehydration of alcohols. Secondary alcohols form 1-olefins, whereas the same reaction over an acid catalyst produces the thermodynamically more stable 2-olefin (312). An example of an industrially important rare earth oxide catalyst is Zr02. It has several applications, including the reduction of aromatic carboxylic acids with hydrogen to aldehydes (314), the dehydration of 1-cyclohexyl ethanol to vinyl cyclohexane (315), and the production of diisobutyl ketone from isobutyraldehyde (316). The extensive use of Zr02 is mainly due to its resistance to poisoning by H2O and CO2, and its inherent catalytic activity is a result of its bifunctional acid-base properties. It contains both weakly acidic and basic sites, neither of which is susceptible to poisoning. The acid-base functionality of Zr02 is displayed in the reaction of alkylamine to nitrile (278) (Fig. 33). To form nitriles from both secondary and tertiary amines, both acid and base sites are required. 

Kinetics and Mechanism. The dehydrocyclization of alkanes may occur by two different mechanism involving only the metallic function, or by a bifunctional mechanism where the dehydrogenation-hydrogenation steps occurs on the metal, and the cyclization occurs on the acid sites. The metallic mechanism is very sensitive to poisons such as sulfur and coke (18), and because of this in the commercial process the paraffins dehydrocyclization occurs mainly by the bifunctional mechanism (8). Details about the paraffin dehydrocyclization reaction on metals can be found elsewhere (9,32-34). 

In order to remove such poisoning species from the catalytic surface, extra oxygen atoms have to be provided to complete the oxidation reaction to CO2 these extra oxygen atoms, from water, have to be activated at the catalytic surface. This phenomenon is called the bifunctional theory of electrocatalysis or, more simply, the bifunctional mechanism. 

From a general point of view, at ambient pressure and temperature, all electrooxidations of short-chain aliphatic alcohols (in acid) require the presence of the expensive precious metal Pt. However, as is well known, Pt is readily poisoned by CO-like intermediate species formed during methanol oxidation at low temperatures. It has been found that Pt-based binary or ternary catalysts, Pt-Mi, Pt-Mi-M2 (M = Ru, Sn, etc.) can improve the reaction kinetics of methanol electrooxidation based on the bifunctional effect (promoted mechanism by the second metal) [17,19,22,24,26] and/or on the tuned electronic properties of Pt (the intrinsic mechanism) [28,29,36]. The bifunctional effect is illustrated in the following equations ... 

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