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Chemisorption bond strength

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. [Pg.19]

Increasing catalyst surface work function causes an increase in the heat of adsorption (thus chemisorptive bond strength) of electropositive (electron donor) adsorbates and a decrease in the heat of adsorption (thus chemisorptive bond strength) of electronegative (electron acceptor) adsorbates. [Pg.30]

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... [Pg.268]

The double layer is described by its effective thickness, d, and by its field strength E (Fig. 6.15). The adsorbed moleculeJias a dipole moment P. It is well documented100 that the local field strength E can affect strongly not only the chemisorptive bond strength but also the preferred orientation of the adsorbate (Fig. 6.16). [Pg.306]

This destabilization of surface Rh oxide formation with increasing catalyst potential or work function has been shown to be due to strong lateral repulsive interactions of the backpspillover O2 species and normally chemisorbed oxygen33 which causes a pronounced, up to leV, decrease in the chemisorptive bond strength of normally chemisorbed o.35,36... [Pg.497]

Thus, it is not strictly correct to interpret the frequency difference between adsorbed and gas phase C-O in terms of chemisorption bond strength only. [Pg.157]

Based on insight gained by Fig. 6.33 we can start to design surfaces that are fine tuned towards the desired chemisorption bond strength and reactivity, as we shall see in the following. [Pg.254]

The techniques described in the foregoing do not easily provide information about the chemisorption bond strength. The Clausius-Clapeyron equation is not applicable in the range of irreversible adsorption. Only by measurements of the desorption rates during the thermal desorption processes at two slightly different temperatures can the activation energy of desorption be estimated. This method has been used by Kubokawa (55). Desorption rates can be calculated from the evolution curves obtained during isothermal desorption as shown, for example, by Czanderna (54). [Pg.197]

A massive electron transfer between the metal particles and the supports (or promoters) and the penetration of an electric field into the metal are thus not realistic ideas on the through-the-metal interaction. However, there is one mechanism for such an interaction which is well supported by the quantum theory of chemisorption when a covalent chemisorption bond is formed, it causes periodic variation (with the distance) in the chemisorption bond strength in its environment. At the nearest site a repulsion is felt, on the next-nearest an attraction, etc. [46a]. However, it is important to realize how strong this interaction is. A realistic estimate, based on observations of the field ion emission images, shows that these interactions are comparable in their strength to the physical (condensation) van der Waals forces [46b]. [Pg.171]

Broadly speaking, promoters can be divided into structural promoters and electronic promoters. In the former case, they enhance and stabilize the dispersion of the nanoparticle-dispersed active phase on the catalyst support. In the latter case, they enhance the catalytic properties of the active phase itself. This stems from their ability to modify the chemisorptive properties of the catalyst surface and to significantly affect the chemisorptive bond strength of reactants and intermediates. At the molecular level this is the result of direct ( through the vacuum ) and indirect ( through the metal ) interactions. The term through the vacuum denotes direct electrostatic, Stark type, attractive or repulsive interactions between the adsorbed... [Pg.684]

Since energies of adsorption are closely related to the chemisorption bond strength, one frequently uses the metal-adsorbate bond strength as the abscissa of volcano plots (Fig. 24). [Pg.52]

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... [Pg.74]

A very frequent feature of electrochemical promotion studies is the observed linear variation of catalytic activation energies with varying catalyst work function [9,14,15]. It had been proposed that this is due to a linear variation in chemisorptive bond strengths with catalyst work function [9,14], a proposition recently supported by TPD studies for oxygen chemisorption on IVVSZ [26]. [Pg.90]

The activity and selectivity of catalysts are determined by the properties of surface complexes formed by chemisorption. In this respect we may compare the thermodynamics of these two reactions and then analyse the influence of alloying on the chemisorption bond strength. [Pg.251]


See other pages where Chemisorption bond strength is mentioned: [Pg.15]    [Pg.34]    [Pg.60]    [Pg.63]    [Pg.181]    [Pg.248]    [Pg.267]    [Pg.316]    [Pg.590]    [Pg.45]    [Pg.126]    [Pg.149]    [Pg.156]    [Pg.157]    [Pg.48]    [Pg.119]    [Pg.137]    [Pg.201]    [Pg.224]    [Pg.385]    [Pg.625]    [Pg.703]    [Pg.705]    [Pg.119]    [Pg.62]    [Pg.222]   
See also in sourсe #XX -- [ Pg.156 , Pg.157 , Pg.158 ]

See also in sourсe #XX -- [ Pg.197 , Pg.198 ]




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