Electronegative Promoters

There are, however, numerous cases where electronegative additives can act as promoters for catalytic reactions. Typical examples are the use of Cl to enhance the selectivity of Ag epoxidation catalysts and the plethora of electrochemical promotion studies utilizing O2 as the promoting ion, surveyed in Chapters 4 and 8 of this book. The use of O, O8 or O2 as a promoter on metal catalyst surfaces is a new development which surfaced after the discovery of electrochemical promotion where a solid O2 conductor interfaced with the metal catalyst acts as a constant source of promoting O8 ions under the influence of an applied voltage. Without such a constant supply of O2 onto the catalyst surface, the promoting O8 species would soon be consumed via desorption or side reactions. This is why promotion with O2 was not possible in classical promotion, i.e. before the discovery of electrochemical promotion. 

Pt(lll).37 The increase in metal work function induced by electronegative adsorbates (Cl, O, S, P, C) on metal surfaces is at most up to 1 eV and in 

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ADSORPTION ON SURFACES MODIFIED BY ELECTROPOSITIVE OR ELECTRONEGATIVE PROMOTERS... 

The key of the promotional action is the effect of electropositive and electronegative promoters on the chemisorptive bond of the reactants, intermediates and, sometimes, products of catalytic reactions. Despite the polymorphic and frequently complex nature of this effect, there are two simple rules always obeyed which can guide us in the phenomenological survey which follows in this chapter. 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

In section 2.5 we have examined the effect of promoters and poisons on the chemisorption of some key reactants on catalyst surfaces.We saw that despite the individual geometric and electronic complexities of each system there are some simple rules, presented at the beginning of section 2.5 which are always obeyed. These rules enable us to make some predictions on the effect of electropositive or electronegative promoters on the coverage of catalytic reactants during a catalytic reaction. 

In this sense subsurface oxygen is also acting as a promoter. The role of the alkali promoter is then to stabilize Cl and anionically bonded O (or nitrate ions) on the catalyst surface, so they can exert their promotional action. Thus alkalis in this system, which requires electronegative promoters according to the mles of section 2.5, are not really promoters but rather promoter stabilizers. This is proven by their inability to promote selectivity in absence of Cl. 

These rules are not limited to electrochemical promotion only. To the best of our knowledge they are also in good qualitative agreement with the results of classical chemical promotion (electropositive or electronegative promoters) on the rates of catalytic reactions. Several examples are shown in this chapter. 

This enables one to formulate the above promotional mles G1 to G4 (eqs. 6.1 to 6.4) also in terms of promoter coverage by simply replacing dQ> by 50,-(for electronegative promoters i) and by -59i (for electropositive promoters i). 

Rule PI If a catalyst surface is predominantly covered by an electron acceptor adsorbate, then an electron acceptor (electronegative) promoter is to be recommended. 

FI. Increasing work function 0 (e.g. via addition of electronegative promoters) strengthens the chemisorptive bond of electron donor adsorbates (D) and weakens the chemisorptive bond of electron acceptor adsorbates (A). 

Consequently the proven functional identity of classical promotion, electrochemical promotion and metal-support interactions should not lead the reader to pessimistic conclusions regarding the practical usefulness of electrochemical promotion. Operational differences exist between the three phenomena and it is very difficult to imagine how one can use metal-support interactions with conventional supports to promote an electrophilic reaction or how one can use classical promotion to generate the strongest electronegative promoter, O2, on a catalyst surface. Furthermore there is no reason to expect that a metal-support-interaction-promoted catalyst is at its best electrochemically promoted state. Thus the experimental problem of inducing electrochemical promotion on fully-dispersed catalysts remains an important one, as discussed in the next Chapter. 

This is a direct consequence of global promotional rule Gl An electronegative promoter (e.g. O2) will enhance the catalytic rate only when the catalyst surface is predominantly covered by an electron acceptor reactant (e.g. O). If there is little or no O on the surface then 02 will act as a reactant (Faradaic behaviour) and not as a promoter. 

Work Function of Surfaces, Electropositive and Electronegative Promoters... 

Action Addition of electronegative promoter Increase in Cwr Decrease in Ep... 

Figure 45 Schematic of the promotional mechanism for electrophobic reactions A via electronegative promoter addition of long lifetime (classical promotion), B via potential- or current controlled 0 backspillover (electrochemical promotion), C via self-driven backspillover (metal-support interactions).

These rules have recently been shown to apply to supported heterogeneous catalysts as well, owing to the thermally induced spillover of ions from the support to the metal/gas interface [23] and concomitant establishment of an EDL at the metal-gas interface [23, 27]. According to these rules, a reaction is electrophobic (dr/d< > > 0) when dr/dpD > 0 and dr/dpA < 0 and electrophilic (dr/d< > < 0) when dr/dpu < 0 and dr/dpA > 0. In the former case the rate, is enhanced with electronegative promoters, whereas in the latter case it is enhanced with electropositive promoters. 

To summarise, for butadiene hydrogenation over metal catalysts, the selectivity for but-l-ene formation is governed by the electronegativity of the catalyst. Hence, selectivity may be effected either by tailoring the supported metal and/or by the addition of promoter species. If electronegative promoters are utilised, these function by generating positively polarised adsorption sites that favour /.ran.s-isomer formation, by 1 4 addition. It is important to note that if the yield of but-l-ene is plotted relative to the trans cis ratio of but-2-ene, the data points for evaporated films (upon which much of the systematic research has been done), alumina supported metals and promoted metal catalysts fall on the same curve indicating the universality of the proposed mechanism. 

In the case of platinum catalysts the addition of some metal oxides has also resulted in enhanced catalytic activity, which has been attributed to the modification of platinum crystallite size, and especially to the modification of the oxidation state of platinum. It has been reported that promoters with large electronegativities, such as molybdenum, vanadium, tungsten and niobium, enhance the catalytic activity compared with the unpromoted Pt/Al203, since more electronegative promoters present a higher resistance to oxidation, and platinum remains less oxidised than those with electropositive promoters, such as alkaline and alkaline-earth metals, which are even less reactive than the unpromoted Pt/Al203 catalyst. ... 

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