Catalytic Promoters and Poisons

Wimmer E, Fu C L and Freeman A J 1985 Catalytic promotion and poisoning all-electron local-density-functional theory of CO on Ni(001) surfaces coadsorbed with K or S Phys. Rev. Lett. 55 2618-21... 

E. Wimmer, C.L. Fu, and A.J. Freeman. Catalytic Promotion and Poisoning All-Electron Local-Density-Functional Theory of CO on Ni(OOl) Surfaces Coadsorbed with K or S. Phys. Rev. Lett. 55 2618 (1985). 

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. 

Early higher pressure reaction smdies over Pt-Sn model catalysts by Paffett [62,63] and Somorjai [64, 65] and their coworkers revealed new insights into hydrocarbon catalysis in such systems. Szanyi et al. [62] showed that n-butane hydrogenolysis under moderate pressures (1-200 Torr H3/butane=20) and temperatures (up to 650 K) could be carried out without disruption of the ordered Sn/Pt(lll) surface alloys. This established that such catalytic reactions could be studied while maintaining the composition and geometric structure of these alloys under reducing reaction conditions (but not catalytic oxidation due to the aggressive interaction of O3 with Sn). These ordered Sn/Pt surfaces are qualitatively different from those in many studies of promoters and poisons, or disordered alloys, e.g., Au-Pt, in which the quantitative information on ensemble sizes available for reactions is difficult to determine. 

Very extensive laboratory work on all phases of the use of catalytic reduction to produce both aliphatic and aromatic amines has been reported in the literature. Studies have been carried out on many types of catalysts, catalyst supports, promoters and poisons, solvents, temperatures, pressures, and equipment. Considerable pilot-plant work and engineering studies have been undertaken and a number of commercial installations built for batch catalytic reduction and for continuous catalytic reduction. Commercial installations are now in operation for the catalytic reduction of nitro compounds and nitriles. 

In addition to the potential technological applications of electrochemical modification of catalytic activity, the ability of solid electrolytes to dose reversibly, precisely, and in situ catalyst surfaces with promoters, by "knob-turn" variation of the catalyst potential and work function, provides a unique opportunity for the systematic study of the role of promoters and poisons in Heterogeneous Catalysis. 

So X-ray absorption spectroscopy is applicable to a wide range of catalytic systems. Observing the edges due to common catalytic feedstocks (generally organics) is applicable only to surface science experiments. Some promoters and poisons (P, S and Cl) may be investigated in well chosen systems, but the entire transition element block is observable for single-crystal, metal film and supported catalyst samples. 

Co and H2S on Nb control the H permeation rate, probably by blocking active surface sites and structural defects induce H traps (Sherman and Birnbaum, 1985). C adsorption on Ni(lOO) was shown to freeze out a Ni surface phonon which results in a surface reconstrucjtion (Rahman and Ibach, 1985). From coadsorption experiments of Ko and Madix (1981) it appears that poisoning is more than simply blocking active sites. Self consistent calculations of the electronic structure perturbation induced by a catalytic poison, S on Rh (001), reveal a substantial reduction of the local density of states at the Fermi level (Fig. 4, Feibelman and Hamann, 1984). Njz rskov et al. (1984) proposed a model based on the effective medium theory, which is able to describe promotion and poisoning effects of co-absorbed electropositive and electronegative species. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

A map of how the electron density is distributed around these atoms provides important information. It tells us to what distance from the adatom the surface is perturbed or, in catalytic terms, how many adsorption sites are promoted or poisoned by the adatom. The charge density contours in Figure A. 12 are lines of constant electron density. Note that these contours follow the shape of the adsorbed atom closely, and that the electrons are very much confined to the adsorbed atom and the adsorption site. 

Electrochemical and surface spectroscopic techniques [iii, v] have shown that the NEMCA effect is due to electro chemically controlled (via the applied current or potential) migration of ionic species (e.g., Os, NalS+) from the support to the catalyst surface (catalyst-gas interface). These ionic species serve as promoters or poisons for the catalytic reaction by changing the catalyst work function O [ii, v] and directly or indirectly interacting with coadsorbed catalytic reactants and intermediates [iii—v]. 

The goal of catalyst development is to understand how the chemical and physical properties of the catalyst affect its activity and selectivity for a desired reaction. For a supported metal, the variables affecting its function are the metal composition, the metal particle size, the particle shape, the structure of the metal surface, the oxidation state of the metal, the composition of the support, and the presence of promoters or poisons. These variables influence catalytic activity by altering both the structure and electronic state of the metal. The relative importance of the structure effect versus the electronic effect has been a question that catalyst researchers have long sought to answer. 

Electrochemical promotion (EP) provides a novel in situ and high controllable means of catalyst promotion. In brief, it has been found that solid electrolytes can be used as reversible in situ promoter donors or poison acceptors. These active supports affect the catalytic activity and/or selectivity of metals deposited on them in a very pronounced, reversible and, to some extent, predictable manner. During the last ten years, EP has been studied for over forty catalytic reactions on Pt, Ph, Pd, Ag, Ni and I1O2 catalyst films using a variety of solid electrolytes i.e., ... 

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