Step effects, dissociative chemisorption

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. 

For atomic chemisorption, similar structural effects are found (see the middle panel of Figure 4.10). As for molecular chemisorption, low-coordinated atoms at steps bind adsorbates stronger and have lower barriers for dissociation than surfaces with high coordination numbers and lower d band centers. The d band model thus explains the many observations that steps form stronger chemisorption bonds than flat surfaces [1,20,24-28]. The finding that the correlation with the d band center is independent of the adsorbate illustrates the generality of the d band model. 

E2S.15 Two observations must be explained in this catalytic deuteration of C-H bonds. The first is that ethyl groups are deuterated before methyl groups. The second observation is that a given ethyl group is completely deuterated before another one incorporates any deuterium. Let s consider the first observation first. The mechanism of deuterium exchange is probably related to the reverse of the last two reactions in Figure 26.20, which shows a schematic mechanism for the hydrogenation of an olefin by D2. The steps necessary for deuterium substitution into an alkane are shown below and include the dissociative chemisorption of an R-H bond, the dissociative chemisorption of D2, and the dissociation of R-D. This can occur many times with the same alkane molecule to effect complete deuteration. 

A semantic point should be mentioned the term electrocatalysis cannot strictly be applied to electron transfer steps or simple electron transfer reactions since there cannot be any noncatalyzed equivalent pathway in the absence of an electrode surface. However, adsorption effects specific to the electrode surface can influence the kinetics of an electron transfer step involving adsorbed products and/or reactants and in this limited sense electrocatalysis could arise in an electron transfer reaction. The term electrocatalysis, however, applies more correctly to the influence of electrode material and the state of electrode surfaces on the behavior of those types of chemical-catalyzed steps, e.g., dissociative chemisorption in some fuel cell oxidations, that are coupled with electron transfer processes. 

FIGURE 7.8 Turnover frequency (TOP) of ammonia synthesis as a function of the dissociative chemisorption energy of nitrogen. Top panel Experimental data from Aika et al. (1973). Middle panel Result of the microkinetic model for stepped metal surfaces (blue Une). Reaction conditions are 673 K, 100 bar, Hj N2 ratio of 3 1, and y = 0.1. The effect of potassium promotion has been included (red Une). Effects of promotion will be discussed in Chapter 12. Lower panel Microkinetic model using a two-site model for the adsorption of intermediates. Adapted from Vojvodic et al. (2014). 

Numerous quantum mechanic calculations have been carried out to better understand the bonding of nitrogen oxide on transition metal surfaces. For instance, the group of Sautet et al have reported a comparative density-functional theory (DFT) study of the chemisorption and dissociation of NO molecules on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium to estimate both energetics and kinetics of the reaction pathways [75], The structure sensitivity of the adsorption was found to correlate well with catalytic activity, as estimated from the calculated dissociation rate constants at 300 K. The latter were found to agree with numerous experimental observations, with (111) facets rather inactive towards NO dissociation and stepped surfaces far more active, and to follow the sequence Rh(100) > terraces in Rh(511) > steps in Rh(511) > steps in Pd(511) > Rh(lll) > Pd(100) > terraces in Pd (511) > Pd (111). The effect of the steps on activity was found to be clearly favorable on the Pd(511) surface but unfavorable on the Rh(511) surface, perhaps explaining the difference in activity between the two metals. The influence of... 

These facts obviously raise the question of what constitutes the best computational model of a small catalytic particle. As catalysis is often a local phenomenon, a cluster model of the reactive or chemisorption site may give quite a reasonable description of what happens at the real surface [1,3,30]. However, the cluster should still be large enough to eliminate cluster edge effects, and even then one must bear in mind that the cluster sizes employed in many computational studies are still much smaller than real catalytic particles (say 10-50 versus 50-1000 atoms, respectively). Hence, a slab model of a stepped surface may provide a much more realistic model of the active site of a catalytic nanoparticle. Hammer [31,32] has carried out quite extensive DFT-GGA slab calculations of N2 and NO dissociation at stepped Ru and Pd surfaces, showing how the dissociation energy is significantly lower at the low-coordination step sites compared to terrace sites. The special reactivity of step sites for the dissociation of NO and N2 has been demonstrated in several recent surface-science studies [33,34]. Also, the preferential adsorption of CO on step sites has been demonstrated in UHV [35], under electrochemical conditions [36], as well as by means of DFT-GGA slab calculations [37]. 

Iron clusters exhibit facile chemisorption toward methanol, the reaction proceeding with little or no cluster-size selectivity. An interesting feature of this system is that the chemisorption rate constants are nearly identical toward various isotopic sjjecies (CH3OH, CH30D,CD3 0H). If dissociation of a C—H or O—H bond was the initial step, then this should be manifested in an observable kinetic isotope effect. Thus the initial chemisorption step most likely involves the lone-pair orbital localized on the oxygen atom. More extensive studies of the chemistry of the Fe methanol system have been explored using infrared multiple-photon dissociation spectroscopy. These results are discussed in detail in Section Vlll. 

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

Figure 17.4. Snapshots from CPMD simulations [69] of the ORR first reduction step on Pt(l 11) for initial distances don = 2.5 A and dos = 3 A, at an intermediate degree of proton solvation (H30) (H20)3 (Scheme 2 shows the initial configuration). Left at 0.12ps, Center at 0.19 and 0.22 ps. Right at 0.30 ps. The net effect of higher proton solvation is the delay of proton transfer thus chemisorption takes place first, followed by OOH formation, chemisorption, and dissociation on the surface. Proton transfer from hydronium to a water molecule is also detected (third image from the left).

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