Reactant coadsorbed

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

It is clear that in case (a) the rate, r, of the catalytic reaction (e.g. CO oxidation) will not be affected while in case (b) the rate increase, Ar, will at most equal I/nF (e.g. direct reaction of O2 with CO). In case (c), however, the new species introduced electrochemically onto the catalyst surface will interact with coadsorbed reactants and will change the catalytic properties of the catalyst surface in an a priori unpredictable manner, which is nevertheless not subject to Faraday s law. Thus in cases (a) and (b) there will be no NEMCA but in case (c) it is entirely logical to anticipate it. Even in case (b) one may anticipate NEMCA, if the product remains on the surface and has some catalytic or promotional properties. 

These two complementary mles are intuitively obvious, e.g. can be simply derived by considering the lateral attractive and repulsive interactions of coadsorbed reactants and promoters as already shown in section 4.5.9.2. They can explain all the observed promotionally induced kinetics for more than sixty different catalytic systems (Table 6.1). As an example these two rules can explain all the observed changes in kinetics orders with [Pg.299]

The WGS reaction is a reversible reaction that is, the WGS reaction attains equilibrium with the reverse WGS reaction. Thus, the fact that the WGS reaction is promoted by H20 (a reactant), in turn implies that the reverse WGS reaction may also be promoted by a reactant, H2 or C02. In fact, the decomposition of the surface formates produced from H2+C02 was promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility. The activation energy for the decomposition of the formates (produced from H20+CO) in vacuum is 155 kJ/mol, and the activation energy for the decomposition of the formates (produced from H2+C02) in vacuum is 171 kJ/mol. The selectivity for the decomposition of the formates produced from H20+ CO at 533 K is 74% for H20 + CO and 26% for H2+C02, while the selectivity for the decomposition of the formates produced from H2+C02 at 533 K is 71% for H2+C02 and 29% for H20+C0 as shown in Scheme 8.3. The drastic difference in selectivity is not presently understood. It is clear, however, that this should not be ascribed to the difference of the bonding feature in the zinc formate species because v(CH), vav(OCO), and v/OCO) for both bidentate formates produced from H20+C0 and H2+C02 show nearly the same frequencies. Note that the origin (HzO+CO or H2+C02) from which the formate is produced is remembered as a main decomposition path under vacuum, while the origin is forgotten by coadsorbed H20. 

The interaction of adsorbed reactants (phenol and methanol adsorbed separately and coadsorbed) and possible reaction products of phenol methylation with the Cul-xCoxFe204 system has been studied at temperatures between lOOoC and 350oC and probed by in situ FTIR spectroscopy. The spectra of adsorbed methanol, phenol and methylated products on catalyst surface, at lOOoC, did not possess much changes compared to the spectra of pure components that indicated the molecular adsorption of species on catalyst surface. The remarkable changes in the spectra occur, above 100°C due to the chemisorption of substrates, were observed and correlated with the observed reaction trend. 

An effect due to lateral interactions between adsorbed sulfur and a coadsorbed molecule. Such interactions can enhance or reduce the adsorption free energy of the reactant under consideration. 

In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

Coadsorption of reactants and subsequent thermal decomposition of the surface complexes formed have been used to resolve the mechanisms in several studies (81-84). Mutual enhancement of the adsorbed amounts of the reactants is indicative of their interaction, and if the adsorption of separately admitted components is negligible, the stoichiometry of the adsorbed complex can be determined. Further evidence for the formation of an adsorbed complex, employed in a mechanistic study of methanol synthesis over ZnO (84), is obtained by thermal decomposition of the adsorbed complex if the reactants appear simultaneously at one temperature upon thermal desorption from a coadsorbed layer, but if each reactant adsorbed separately gives a thermal desorption peak at a different temperature, the existence, although not necessarily the structure or com-... 

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]. 

Although there have been several studies of chemisorption of certain molecules of interest, there are fewer studies of co-adsorption. This possibly arises from the increasing complexities of controlling coverage in situations where competitive adsorption may exclude or alter the coverage of the reactants. The goal of co-adsorption studies is to learn about interactions between surface coadsorbates, a subject of obvious importance to their catalytic reaction. It is especially of interest from the view of emission control catalysis to study coadsorption of an oxidant and a reductant. 

When both reactants were coadsorbed on Rb-X at 308 K, indications for the formation of reaction products or bimolecular complexes were not found in the IR spectra. The spectra rather suggest that toluene and methanol are independently sorbed. It should be noted, however, that after equilibration with equal partial pressures of both reactants, toluene was the main sorbed species. Note that only part of the sites can be covered by toluene molecules due to steric reasons (theoretically 2/3 of the cations are accessible) and pore filling, while methanol achieved a coverage of approximately one molecule/cation at elevated partial pressures (p = 1 mbar). Coadsorption of toluene onto a surface preequilibrated with methanol resulted in the displacement of the main fraction of the methanol molecules (80 %) from the sorption sites [24].The same coadsorbed state was reached irrespective of the sequence of adsorption of the two reactants. If toluene was adsorbed first, coadsorption of methanol did not change the coverage of toluene. 

In contrast, toluene and methanol coadsorbed on Rb-X do not form a bimolecular precursor complex and both reactants seem to be independently adsorbed at the surface. It should be noted, however, that after equilibration of the catalyst with equal partial pressures of both reactants, toluene was the main adsorptive. During toluene methylation, sorbed toluene was again the main surface species, the reaction rate, however, was proportional to the surface concentrations of both chemisorbed species (toluene, formaldehyde). The onset of the reaction was observed at much higher temperatures than in the ring alkylation which is at large ascribed to the indispensable conversion of methanol to a formaldehyde (or formate) species. 

In the same way catalytic reactant adsorbates, and thus catalytic reactants, can be divided into electron donor and electron acceptor adsorbates or reactants. Hydrocarbons, and in particular unsaturated ones, always behave as electron donors (D), while O, Cl, and in most cases CO and NO, behave as electron acceptors (A). Adsorbates, such as H, CO, and NO, which, depending on the catalyst surface and the nature of the other coadsorbates, can change their behavior between electron donor and electron acceptor are called amphotheric adsorbates [13,14,129,130]. 

Past kinetic studies show that coadsorbed reactants on the silicon surface, methylchloride, methyl radicals and chlorine radicals compete with products for surface sites, but much is still unknown about these surface chemical processes. This reaction is carried out at elevated pressures so conventional high vacuum surface science and techniques cannot be used to uncover details of the reaction path. To circumvent this problem. Bent [1] and co-workers have developed low temperature techniques which permit direct observation of the interactions of reactants and products with the silicon surface. This type of understanding is key to continuous improvement of the Direct Process. 

As mentioned above no adsorption of CO is observed at around room temperature. When both reactants, O2 and CO, are introduced into the ion trap, the reaction kinetics of Au2 changes drastically, as seen by the offset in the Au2 signal. This offset increases when the partial pressure of CO is augmented. In addition, at temperatures below 200 K the intermediate with the stoichiometry Au2(C0)02 could be isolated (Figure 17.2A-b). The ion stoichiometry clearly shows that CO and O2 are able to coadsorb onto an Au2 dimer. From the kinetics of all observable ions, Au2, Au202, and Au2(C0)02, measured under a multitude of different reaction conditions the catalytic conversion of CO to CO2 could unambiguously be detected. The reaction mechanism that fulfills all the prerequisites and fits all kinetic data measured under all the different reaction conditions could be described by the reaction equations below ... 

The molecular origin of electrochemical promotion is currently understood on the basis of the sacrificial promoter mechanism [23]. NEMCA results from the Faradaic (i.e., at a rate I jnF) introduction of promoting species (Os in the case of O2- conductors, H+ in the case of H+ conductors) on the catalyst surface. This electrochemically introduced O2- species acts as a promoter for the catalytic reaction (by changing the catalyst work function and affecting the chemisorptive bond strengths of coadsorbed reactants and intermediates) and is eventually consumed at a rate equal, at steady state, to its rate of supply (I/2F) which is A times... 

Fig. 1.73. LDOS of Au8/O2/MgO(100)(FC). For these studies, the O2 molecule was adsorbed on the periphery and no CO molecules were coadsorbed in order to separate the interaction of O2 with the cluster from coupling effects between the reactants. The prominent peaks of the oxygen LDOS are labeled following the conventional nomenclature for the molecular orbitals of the gas-phase O2 molecule, with L and 11 meaning perpendicular and parallel to the MgO surface, respectively. The Fermi energy Fp is at 0 eV...

Sulfur is known to block the reactive sites or compete for them with a reactant the catalytic path may be further altered by chemical interaction with coadsorbed molecules of reactants. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

Micellar rate enhancement of thermal and light-initiated biomolecular reactions often occurs via preconcentration of the two reactants in the micelles [3]. In electrochemical catalysis (Scheme 2), the analogous situation can occur for participants in biomolecular reactions coadsorbed into surfactant films on electrodes. Interfacial... 

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