Chemisorbed intermediates steps

Chemical system, 32 278-283 Chemisorbed intermediates, 38 1-135 see also Oxide electrocatalysts cathodic hydrogen evolution, 38 58-66 chemical identity, 38 16-23 species from dissociative or associative chemisorption, 38 20-23 species from electrochemical discharge steps, 38 16-20... 

A heterogeneous catalytic reaction, by definition, necessitates the participation of at least one chemisorbed intermediate (54) and involves a sequence of interlinked and interdependent (55,56) steps, which include the adsorption of reactant(s), one or more surface rearrangements, and the desorption of product(s). More than one area of the solid may be active in promoting reaction the activity of such regions may vary from one crystallographic... 

The unique features of the VPO catalysts in carrying out the reaction steps shown in Table 1 are (i) the ability to selectively activate n-butane during the rate-determining step, (ii) rapid oxidation of chemisorbed intermediates to maleic anhydride with high selectivity, and (iii) the lack of desorption of any intermediate contributing to high selectivity to maleic anhydride. Other cata-... 

The involvement of chemisorbed intermediates in electrocatalytic reactions is manifested in various and complementary ways which may be summarized as follows (i) in the value of the Tafel slope dK/d In i related to the mechanism of the reaction and the rate-determining step (ii) in the value of reaction order of the process (iii) in the pseudocapacitance behavior of the electrode interface (see below), for a given reaction (iv) in the frequency-response behavior in ac impedance spectroscopy (see below) (v) in the response of the reaction to pulse and linear perturbations or in its spontaneous relaxation after polarization (see below) (vi) in certain suitable cases, also to the optical reflectivity behavior, for example, in reflection IR spectroscopy or ellipso-metry (applicable only for processes or conditions where bubble formation is avoided). It should be emphasized that, for any full mechanistic understanding of an electrode process, a number of the above factors should be evaluated complementarily, especially (i), (ii), and (iii) with determination, from (iii), whether the steady-state coverage by the kinetically involved intermediate is small or large. Unfortunately, in many mechanistic works in the literature, the required complementary information has not usually been evaluated, especially (iii) with 6(V) information, so conclusions remained ambiguous. 

We have indicated above that for a simple electron-transfer reaction, not involving a chemisorbed intermediate, or for such a step in a more complex process where the coverage, 9, by intermediates is small (say, <1%, when the discharge step producing the intermediate is rate controlling) the Tafel slope dVjd ni is simply... 

Selective NOx removal is an area where the boundary between sorbent and catalyst tends to disappear. Many different types of sorbents have been investigated for NOx sorption for both cold (near ambient temperature) and hot (200-400 °C) applications. Transition metal oxides appear to be the best. In both temperature ranges, a significant amount of adsorption is achieved usually when assisted by catalytic oxidation of NO to NO2 as an intermediate step. In air near ambient temperature, about 5% of the NO is in the form of NO2. NO2 adsorbs more easily than NO for both chemisorption and physical adsorption. The normal boiling point of NO is -152°C and that of NO2 is 21 °C. Thus, NO2 adsorbs readily near ambient temperature in micropores and mesopores by pore filling. In chemisorption, NO2 readily forms nitrite and nitrate on transition metal oxides. These chemisorbed species can be decomposed or desorbed only at elevated temperatures, e.g., 200-300 °C. Other surface species are also formed, such as NO+ (nitrosyl). These species could be desorbed at near-ambient temperature. The complex chemistry of NO is due to the fact that there is one electron occupying the antibonding orbital of NO, which is empty in most other molecules. 

At low temperature, the dissociation of adsorbed NO species occmring over reduced perovskite and yielding N2O and N2 was recognized as the rate determining step for catalytie reduction of NO by CO. The dimeric species of NO, such as N2O2, can be an intermediate, the formation of which involves the N-N bond formation and N-0 bond eleavage[40]. Two parallel reaetions for chemisorbed NO dissociation occurring over a redueed surfaee with N2O and N2 as the respective products were assumed ... 

It was found that the electrocatalytic activity strongly depends on the nature of the electrode it decreases in the order Rh > Ru > Ir > Pd and Pt for the transition-metal electrodes and in the order Cu > Ag > Au for the coinage metals. It was concluded that the rate-determining step on Ru, Rh, Ir, Pt, Cu, and Ag is the reduction of nitrate to nitrite. It was assumed that chemisorbed nitric oxide is the key surface intermediate in the nitrate reduction. It was suggested that ammonia and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO reduction, which forms N2O or N2 only if NO is present in the solution. On Cu the production of gaseous NO was found, which was explained by the weaker binding of NO to Cu as compared to the transition metals. 

Table VIII lists the total bond energies of reactants and products as well as conceivable intermediates in the gas-phase (D) and chemisorbed (D + Q) states on Pt(lll), Pd(lll), Ni(lll). Tables IX and X summarize the activation barriers of the conceivable elementary steps leading to CH4 and CH3OH, respectively.

FT synthesis kinetics are similar on Co and Ru catalysts and reflect similar CO activation and chain growth pathways on these two metals. These kinetic expressions are consistent with the stepwise hydrogenation of surface carbon formed in fast CO dissociation steps (26). Chemisorbed CO and CH t species are the most abundant reactive intermediates, as expected from the high binding energy of CO and carbonaceous deposits on Co and Ru surfaces (7, 76-80). As a result, reaction orders in CO remain negative at the usual inlet pressures but can become positive as CO reactants are depleted by transport limitations within pellets or by high levels of conversion within the catalyst bed. 

In cases where two bonds are cleaved, it is often suggested that the cleavage occurs in one step. This assumption is based on the fact that a stepwise reaction would lead to the formation of either cyclopropyl or cyclobutyl derivatives which are usually inert under the conditions employed. However, it is well established that the pervasive hydrogen species chemisorbed on transition metal surfaces is the hydrogen atom . Thus the mechanism could still be stepwise since the cyclic intermediates are obtained as highly reactive radicals which can further add hydrogen in a series of successive steps. 

Several reactions of principal interest in eiectrocatalysis involve a first step in which discharge of an ion or electron transfer to or from a molecule takes place, resulting in formation of a chemisorbed radical intermediate. In most cases, the species thus produced is not strictly a free radical since strong electronic interaction with surface states, often unpaired d electrons, on/in the electrode surface (cf Fig. S) results in formation of a surface molecular compound, the chemisorbed species, usually distributed in a two-dimensional array. 

Evidently, however, another species arises in a side, self-poisoning, reaction and extensively covers the surface, inhibiting the progress of the above main reaction in the sequence of steps shown (89-91) In situ IR spectroscopy shows that this species is principally chemisorbed CO, bridged or linearly bonded to surface metal atoms. Its behavior is similar to that observed with CO directly chemisorbed at a Pt electrode from the gas phase. However, the mechanism of its catalytic formation from HCOOH is unclear. It is well known that CO can be formed from HCOOH by dehydration, but such conditions do not obtain at a Pt electrode in excess liquid water. Hence a catalytic pathway for adsorbed CO formation has to be considered. The species C=0 or C—OH are not to be regarded as the kinetically involved intermediates in the main reaction sequence (Section IV). Because the poisoning species seems to be formed in the presence of coadsorbed, H steps such as... 

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