Surface sites of differing activity

Surface defects, such as steps, kinks, and pits, establish surface sites of different activation energy, with different rates of reaction ... 

A typical example of the FIAT method applied to the determination of differential enthalpies of adsorption is illustrated in Figure 8 for adsorption of potassium chloroplatinate on a graphitised carbon black. The first injection of 0.2 pmol of K2PtCl6 produced a molar heat of adsorption of 128 kJmoland the fourth injection yielded 31 kJmol k The individual adsorptions occur on surface sites of different activity and reflect the heterogeneity of the surface. On the other hand, similar adsorption experiments on alumina produced relatively low heats of adsorptions emphasising the high affinity for transition metal salts of the polar sites present in graphitised carbons. 

Table 5.2 compares the dissolution rate of various Al-minerals. The differences are remarkable. At pH = 3, the half life of surface sites of different aluminum (hydroxides varies from 2 years (corundum) to 20 hours (bayerite). The large difference in rates must be due to different coordinative arrangements of the active surface groups. Although no detailed theory is available, it is perhaps reasonable to assume, that the dissolution rate increases with the frequency of surface groups which be present as endstanding =AI-OH groups. 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

The main findings have been connected orientational effects in the surface films formed on heterogeneous adsorbents of various distributions of adsorption sites. In the case of the surface composed of different active centers placed alternately, the perpendicular of orientation of dimers is predominant in the whole concentration region. For all of the remaining systems, however, the parallel orientation is more probable. 

Recenl work has defined more carefully ihe nature of active sites. Metal surfaces are thought to contain three main types of sites terraces, ledges (or steps) and kinks, which correspond to one, two. and three coordinatively unsaturated sites of organometallic chemistry. These sites display differing activities toward saturation, isomerization, and CKChiingQ 7 J0,68 JO 1.103,104,105). 

Let us consider a surface on which particles are adsorbed on sites with different activation energy of desorption, and the distribution of these energies over the surface is discrete so that ni0 particles are initially in a state with an activation energy of desorption Edt, n particles with an energy Ed/, etc. Such a model corresponds to a concept of adsorption on different crystal planes each of which is homogeneous, or to a concept of different adsorption states of the particles adsorbed on a single crystal (26, 88). 

SZ/S102 is usually prepared by a sol-gel process, and its catalytic activity depends on the reactivity of afkoxide precursors, amount of absorbed sulfuric acid, thermal treatment conditions, and number of active SZ sites on the surface. Because of different... 

Adsorption and reaction of C2H4. Ethylene reversibly adsorbed on clean Au(lll) at 95 K. Desorption of ethylene in TPD from the clean Au(lll) surface was observed from approximately 100 - 250 K, with a large peak at 104 K. The reason for the broad desorption peak is uncertain. Outka and Madix (8) have shown that a broad range of desorption is typical of C2 hydrocarbons on Au(llO). They propose that these molecules are weakly bound and therefore occupy a variety of binding sites with different activation energies, consequently they desorb at different temperatures. We observed no H2 evolution from the surface up to temperatures of 700 K and AES showed no residual carbon on the surface after heating. Thus, C2H4 is reversibly, and most likely molecularly, adsorbed on clean Au(lll). 

The idea that catalyst surfaces possess a distribution of sites of different energies has been around since the 1920s, but it has not been possible until fairly recently to show that adsorption sites on terraces, steps, and kinks differ in energy. For example, hydrogen shows stronger bonding to steps and kinks on platinum than on the 111 terraces. In addition, the activation energy for H2 dissociation is about zero on the step face and about 8.4 kJ mole-1 on the terrace plane. In addition, carbon monoxide is adsorbed with dissociation on the kinks of Pt, but in the molecular form on the steps and terraces. 

Fig. 1. Geometry of different active sites on the Pd surface and reactions of olefins catalyzed by these sites. (Taken from Ref. 22.)...

There are two possible explanations for the higher BE compared with the pure (a- and (1-) A1F3 reference samples. Either, the activation of -/-alumina with a fluorocarbon results in a very active surface state which differs from the pure reference fluorides, possibly due to the presence of chemisorbed HF molecules at the surface, or that the difference is the result of partial hydrolysis of the ex situ prepared reference samples. However, the surface of A1F3 samples, prepared by classical crystallization from HF solution, is not modified in such an extreme manner after exposure to air. Only small changes in the BE can be detected and this suggests that there are in fact real differences in the activity/states of the surface sites of fluorinated y-alumina catalysts. 

Mechanical activation ofkaolinite was investigated in [14-19], It was stated that at the initial stages of activation kaolinite undergoes a substantial increase of the number of active surface sites of acidic and basic character. Their concentration increases from 0.9 mmol/g in the initial sample to 1.3 mmol/g in the samples activated for 0.5 h then it decreases with increasing activation time. These sites are of different nature, e.g., silanole groups Si-OH, exhibiting weakly acidic character (with pK, 6.5-9.5), or strongly acidic sites of Lewis type arose due to coordination-unsaturated metal cations (mainly aluminium) on the surface [17]. 

Thus, the adsorption of CO on active ZrOa catalysts led to the formation of various types of adsorption species of CO having different reactivities toward Ha, and these species were found to play a significant role in the hydrogenation of CO. Moreover, it is likely that CO is adsorbed on the active surface sites of low coordination and that an electron transfer from the other surface sites to this CO species leads to the formation of the dimeric adsorbed species (CO)a. These dimeric species react, step by step, with CO molecules from the gas phase to from a relatively stable cyclic polymer species of (CO)s and then (CO) ", Such adsorbed CO species easily react with hydrogen and are also activated through the dissociative adsorption of hydrogen on surface sites of low coordination or the coordina-tively unsaturated surface sites on the catalyst. 

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