Surface steps dissociative adsorption

Hinrichsen, Muhler, and co workers—micro kinetic analysis parameterized by redox model. Hinrichsen et al.317 investigated the elementary steps by micro kinetic analysis by applying temperature and concentration-programmed experiments over Cu/Zn0/Al203, and modeling the data with the simplified redox mechanism in the spirit of Ovesen, Topsoe, and coworkers.303 This included 3 steps (1) dissociative adsorption of H2 on Cu metallic surface (2) dissociative adsorption of H20 leading to an adsorbed H2 molecule and an O adatom and a reduction step by CO to form gas phase C02 and a free active site (see Scheme 71). 

Figure C2.7.6. STM images of an Ru(OOOl) surface after dissociative adsorption of NO at 315 K. (A) Image (38 nmx33 nm) showing two terraces separated by a monatomic step (blaek stripe). (B) Close-up (6 nmx4 nm) showing an O island and individual N atoms. Individual O atoms are imaged as dashes (arrow) [9]...

We will start by analyzing the reaction on the surface of a catalyst that bonds intermediates too strongly (to the left of the maximum in the activity map, Fig. 7.2). The surface coverage will be high (0 1) and desorption of AB will be the rate-determining step. This means that the first reaction step (dissociative adsorption of A, Eq. 7.2) is in equilibrium and hence that /j 1 and V/ The TOP, can now be approximated via the second reaction step, r ... 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

Nozaki, Matsukawa, and Mano (81) suggested for Example 31 that the rate-determining step is CO reaction with a surface that has been oxidized by NO thus. Step 1 or 6 for CO can provide a lower limit (since the reaction is 0.4 order in NO) for logL. But logL = 18 is rather large for a lower limit, and other possibilities may have to be considered. Dissociative adsorption of NO (Step 2 for NO) could be the rate-determining step. Example 32 is for the same reaction, but using a different catalyst and the log L calculations are similar. 

Examples 28 and 29 are for deuterium-hydrogen exchange reactions. The reaction in Example 28 was carried out at a temperature low enough so that the only reaction after adsorption was that of CgHj (ads) with D (ads), followed by desorption. Sarkany, Guczi, and Tetenyi (54) suggested for their reaction that the rate-determining step was cyclohexane adsorption. The log L values indicate that dissociative adsorption of cyclohexane (Step 2, cyclohexane), for which log L = 19, is possible. But some surface freedom of cyclohexane is required. Dissociative adsorption of D2, for which log L = 15, seems more likely. 

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