Oxidation intermediates surface states

An indication of the substantial difference in the adsorption behavior of the intermediate surface states that are involved in the OER on oxide films at Pt in alkaline and acid solutions is given (257,258) by the curves of log C o versus V in Fig. 25. In alkaline solution, logQ o versus V plots show the higher and the lower negative dK/dlogC. o values indicated in Fig. 25,... 

Like CO oxidation on Ru, the understanding for ethylene epoxidation on Ag has continued to evolve. Many questions remain open, including the reaction mechanism on the Ag structures, and the role of intercalated oxygen atoms. Another dimension that is little explored so far is the surface states in a combined oxygen-ethylene atmosphere. Greeley et al. have reported recently that an ethylenedioxy intermediate may be present at appreciable coverage under industrial reaction conditions, the effect of which on the structure of the surface is unknown. More importantly, the implication of a dynamic co-existence of various surface oxides under reaction conditions for the reaction mechanism needs to be explored and understood at greater depth. 

As discussed previously, the surface states responsible for the reduction peak could be intrinsic surface states or states associated with a surface-attached intermediate in the series of reactions leading to O-evolution. The latter possibility was deemed to be more likely since no change in voltage across the Helmholtz layer (no change in capacitance) was observed when these states are in the oxidized form. 

Where there is direct overlap with the valence band edge, the electron transfer process may be so facile as to give rise to the Hofer-Moest reaction (.2), in which the intermediate alkyl radical is itself oxidized (while it is still adsorbed to the electrode surface) to give a carbonium ion. The reaction of this carbonium ion with the aqueous electrolyte would then yield water-soluble products such as methanol, in keeping with our observation that anodic gas evolution is suppressed under these conditions. In acidic solutions, where the Kolbe reaction is energetically allowed, its kinetic competition with the other reactions on SrTiC>3 thus depends on the absence of defect surface states which are present in some electrode crystals and not in others. 

It should be noted that more complex molecules than CO (e.g., methanol) produce many kinds of intermediates in the course of the catalytic oxidation, and they will chemisorb to form surface states. If the energy of the surface states formed by chemisorption of these intermediates are shallow enough from the delocalized band (conduction band and valence band) edges in the... 

In the course of catalytic oxidation, the production rate of intermediates that finally generate CO2 and H2O is also limited by rx. Some intermediates are the chemisorbed species that form surface donors and acceptors, and the other intermediates are the excited species. Both the generation rate of carriers originated from the surface states and the production rate of the excited species is governed by rx- Thus the dependence of CTL intensity on flow velocity should agree with that of rx in the diffusion-controlled region. 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

He found that below 573 K, the rate of surface reduction of C03O4 is much slower than the rate of CO. oxidation. Thus, participation of Ol in CO oxidation at 488 K is unlikely. If Ol were consumed in CO oxidation at steady state, one would expect regeneration of 0 to be fast. Experimental evidence suggests the opposite, l.e. that catalyst regeneration is slow. Formation of mono-dentate carbonate ([Ol] C02 in step 2) along with bidentate carbonate had been spectroscopically observed on C03O4 surfaces upon CO adsorption by Hertl (6) and Goodsel (7). Formation of these species rather than carbonyl type species indicate the roles played by O and 0qo. Step 2 could proceed via an intermediate step in which bidentate carbonate is formed as shown ... 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

The band structure of oxides is very important for their behavior as electrocatalysts through the role of surface states and chemisorption of intermediates at their surfaces. When a crystalline solid is terminated by a surface, a new set of electronic states appears associated with the surface which are a continuation from the bulk band structure of the solid. These surface states are d-band surface states on transition metal oxides, which play a vital role... 

Adsorbed species other than hydrogen and hydroxyl ions that are able to give up or accept electrons are also surface states. The reaction intermediates that are able to act as donors or acceptors through charge transfer reactions can be viewed as surface states. As will be described in more detail in the section on anodic behavior, partially oxidized sihcon atoms. Si " (n < 4), i.e., the reaction intermediates, act as transient surface states and play an important role in a range of electrode processes. 

The results of the large number of studies on surface states indicate that those on the surface of silicon tend to be associated with oxide when the surface is covered with an oxide or to reaction intermediates when the surface is not covered with an oxide. A high density of surface states is generally observed when the electrode is illuminated or a current is passed through. Reaction intermediates can behave like surface states. 

The Si intermediate complexes generated from the oxidation of a part of the excess Si bonds in this transition region are responsible for fixed charge and fast surface states. They may be formed according to the reaction... 

One problem in meaningful application of the basic electrochemical theories is related to surface states that may be associated with surface defects, interface states at silicon/oxide interface, adsorbed species, or reaction intermediates. They are often conveniently considered to be responsible for the results that are inconsistent with the basic theories. However, the understanding on the nature of surface states at the silicon/elec-trolyte interface is still poor one is aware of some consequences of surface states but knows little of their origins and their specific roles in various electrode processes. 

Heterogeneous catalysis has, until recently, been exclusively the preserve of the surface chemist. Detailed study of the bulk structural features has become in oartant with the advent of shape selective catalysts, notably zeolites, where the distinction between external and internal surface is difficult to make, but surface studies have been considered most appropriate for other systems. However, in many real catalysts, where the catalytic action undoubtedly occurs on the external surface, it does so by means of intermediate structural states, and the catalytic efficiency is then dependent upon the relative stability and interactions of such intermediate states with the bulk material. Consequently an understanding of the structural chemistry and structural modification possible in the parent catalyst phase is still essential to understanding the catalytic action. This is especially true in the case of oxidation catalysts, where it can be shown (l) that lattice oxygen plays a part in the catalytic process. 

Platinum ions reduce to metallic Pt by injecting holes into the Si valence band. Thus Pt ions act as an oxidizing agent for silicon, and result in the simultaneous formation of photoluminescent porous silicon under certain conditions. Nickel ions may exchange charge with both the conduction and the valence band. The reduction of Ni ions competes with hydrogen evolution, and the deposition of Ni can only be achieved at high pH where it is kinetically faster. The role of silicon surface states as reaction intermediates is discussed. 

In Fig. 13, we depict the P-hydride elimination surface reaction for the CH2(CHO)-CH2- surface species to give acrolein. This microscopic reverse of this step involves the selective hydrogenation of acrolein, a valuable selective oxidation intermediate. Acrolein is a structural moiety for maleic anhydride, and therefore an ideal model for the hydrogenation of maleic anhydride to succinic a ydride. The predicted transition state is shown in the center of Fig. 13. The corresponding barrier for addition of hydrogen to adsorbed acrolein (the reverse reaction) is +82 kJ/mol. 

Finally, Satterfield (p. 185) states that highly mobile" oxygen should result in a highly active, nonselective catalyst. Germain in fact says that for simple metal oxides there is a direct, but limited, correlation between activity and oxygen mobility. Satterfield also points out, however, that oxygen mobility as a sole criterion for catalyst activity and selectivity is somewhat limited. For instance, this concept does not account for the effect of partially oxidized intermediate adsorption on selectivity in series-type reactions nor for the effects of mixed oxide composition and catalyst surface defects on catalytic reactivity, especially in partial oxidation reactions. 

If the ion is specifically adsorbed, the clear distinction between reduced and oxidized states is no longer possible as a consequence of the formation of bonding orbitals and the fast electron fluctuation between the metal surface and the adsorbed ion. The solvation shell cannot achieve a stable configuration for oxidized and reduced states of the adsorbed ion. Instead, an intermediate solvent distribution develops for a mean residence probability of an electron on the adsorbed ion. 

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