Promoter atoms

The catalytic properties of a surface are determined by its composition and structure on the atomic scale. Hence, it is not sufficient to know that a surface consists of a metal and a promoter, say iron and potassium, but it is essential to know the exact structure of the iron surface, including defects, steps, etc., as well as the exact locations of the promoter atoms. Thus, from a fundamental point of view, the ultimate goal of catalyst characterization should be to look at the surface atom by atom, and under reaction conditions. The well-defined surfaces of single crystals offer the best likelihood of atom-by-atom characterization, although occasionally atomic scale information can be obtained from real catalysts under in situ conditions as well, as the examples in Chapter 9 show. 

Janssens et al. [38, 40] used photoemission of adsorbed noble gases to measure the electrostatic surface potential on the potassium-promoted (111) surface of rhodium, to estimate the range that is influenced by the promoter. As explained in Chapter 3, UPS of adsorbed Xe measures the local work function, or, equivalently, the electrostatic potential of adsorption sites. The idea of using Kr and Ar in addition to Xe was that by using probe atoms of different sizes one could vary the distance between the potassium and the noble gas atom. Provided the interpretation in terms of Expression (3-13) is permitted, and this is a point the authors checked [38], one thus obtains information about the variation of the electrostatic potential around potassium promoter atoms. 

Irrespective of the exact configuration around the promoter atom, we have a detailed picture of the Co-Mo-S phase on the atomic scale. Figure 9.23 summarizes schematically what a working Co-Mo/A1203 hydrodesulfurization catalyst looks like. It contains MoS2 particles with dimensions of a few nanometers, decorated with cobalt to form the catalytically highly active Co-Mo-S phase. It also contains cobalt ions firmly bound to the lattice of the alumina support, and it may contain crystallites of the stable bulk sulfide Co9S8, which has a low activity for the HDS reaction [49]. 

The MES investigations showed that part of the promoter atoms in Co-Mo catalysts is generally present in a structure also containing molybdenum and sulfur atoms (6j. This structure was termed the Co-Mo-S structure ( 3) and since the promotion of the HDS activity was found to be associated with this structure (9., 11) much work has been initiated in order to characterize further the properties of this Co-Mo-S structure. 

Chiral Lewis acid promoted atom transfer reaction (Kharasch reaction) of a-halo oxazolidinone imide 90 and 1-octene 92 has been reported by Porter et al. (Scheme 23) [78]. The enantioselective atom transfer utilizing Zn(OTf)2 and phenyl bisoxazoline ligand 93 as a chiral Lewis acid. The yields of the products, however, were quite low ranging from 5-15% and only moderate enantioselectivities were achieved (up to 40%). 

Silver salts are well established to promote atom transfer reactions.1-5 In 1946 Bachmann and coworkers reported that silver oxide facilitated the Wolff rearrangement6 of a-diazocarbonyl compounds.7 After this initial report, several other silver(I) reagents (including AgN03 and Ag02CPh)8,9 were identified to provide higher yields,... 

The valence electron of a promoted atom readily interacts with other activated species in its vicinity to form chemical bonds. The mechanism is the same for all atoms, since the valence state always consists of a monopositive core, loosely associated with a valence electron, free to form new liaisons. Should the resulting bond be of the electron-pair covalent type, its properties, such as bond length and dissociation energy can be calculated directly by standard Heitler-London procedures, using valence-state wave functions (section 5.3.4). 

The HDS and HDN activities of a M0S2/AI2O3 catalyst both increase substantially on addition of cobalt or nickel. Several explanations for the promoter function of cobalt and nickel have been proposed (3-6). One model is based on the assumption that the promoter atoms induce a surface reconstruction of the edges of the M0S2 layers, leading to a greater number of exposed molybdenum atoms and, thus, to enhanced activity (24). On the other hand, infrared (19) and EXAFS (16, 20) investigations demonstrated that the molybdenum atoms are covered by the sulfur and the promoter atoms. Thus, if only the molybdenum atoms were active, the activity should decrease upon addition of cobalt or nickel atoms. 

The most stable position for the cobalt promoter atoms was calculated to be at the edge, substituting as it were for the molybdenum atoms (38). The promoter decreased the equilibrium sulfur coverage of the edge from 50 to 0-17% for Co/Mo(edge) = 1, and it weakened the sulfur-metal bond... 

The catalyst particles are built of crystallites, the interior of which is extremely pure iron, whereas the surface layers have a high content of promoter atoms. The total crystallite surface area as will be discussed later, is some two to three times greater than that found by low-temperature surface area measurements by the BET-method. It is concluded that interfacial layers join the crystallites, and only where the distance between two crystallites is too great or their orientation too divergent is the interfacial layer split up into two separate surface layers, thus producing a rift network through the catalyst particles. It is through this rift network that the inner surface is reached by the reactants, and this inner surface is utilized as effectively as is the outer surface. 

Extensive information concerning distribution of the promoters, penetration below the promoters of adsorbed atoms, and chemical behavior of the promoters was obtained by Brunauer and Emmett (25,26). They used chemisorption of carbon monoxide, carbon dioxide, nitrogen, hydrogen, and oxygen, individually and successively measuring the influence of one type of chemisorption upon another type. It was concluded that CO and C02 were chemisorbed as molecules, H2 and N2 as atoms, and 02 probably as ions. C02 is chemisorbed on the alkali molecules located at the surface, whereas H2, N 2, CO, and 02 are chemisorbed on the iron atoms. From the effect of presorbed CO upon the chemisorption of C02 and vice versa it was concluded that the promoters are concentrated on the surface and are distributed so effectively that most surface iron atoms are near to a promoter atom. Strong indication... 

Alumina-supported Co- and Ni-promoted molybdenum sulphide hydrotreating catalysts are the main workhorses in many refineries and have, therefore, attracted a lot of attention from catalytic chemists. They are usually prepared via co-impregnation, i.e. pore-volume impregnation with both Mo and the promoter atom present in solution. After drying and calcining, the catalyst manufacture is complete, but it has to be sulphided before use. Traditionally, this is done in situ... 

Next, imagine that the promoted atom is one of many, all similarly activated by a static field of applied pressure. All atoms are in the same valence state and interact non-locally through quantum torque and the quantum-potential field, which becomes a function of all particle coordinates. This... 

Chemical reaction occurs between reactants in their valence state, which is different from the ground state. It requires excitation by the environment, to the point where a valence electron is decoupled from the atomic or molecular core and set free to establish new liaisons, particularly with other itinerant electrons, likewise decoupled from their cores [114]. The energy required to promote atoms into their valence state has been studied before [24] in terms of the simplest conceivable model of environmental pressure, namely uniform isotropic compression. This was simulated by an atomic Hartree-Fock procedure, subject to the boundary condition that confines all electron density to within an impenetrable sphere of adjustable finite radius. 

Scheme 13 Lewis-acid promoted atom transfer cascades...

One important component of practical epoxidation catalysis for which many questions remain open is the role of promoters. Industrially, both Cs and Cl are used as promoters.76 Saravanan et al. used cluster DFT calculations to examine the interactions between adsorbed Cs and oxametallacycles on Ag(lll).77 These calculations suggested that both neutral Cs and Cs+ gave similar outcomes and that the promoter atom made the formation of a surface oxametallacycle less energetically favorable than on the bare Ag surface. More recently, Linic and Barteau used plane wave DFT calculations to probe the effect of adsorbed Cs on the transition states controlling the formation of EO and acetaldehyde from oxametallacycles on Ag(lll).78 The role of Cs was... 

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