Adsorption of acetylene

Weinelt M, Huber W, Zebisch P, Steinruck H-P, Ulbricht P, Birkenheuer U, Boettger J C and Rdsch N 1995 The adsorption of acetylene on Ni(110) an experimental and theoretical study J. Chem. Phys. 102 9709... 

Figure 2.43. Charge density differences induced by adsorption of acetylene in the two different sites on Cu(110) showing (top) the HBE species and (bottom) the LBE species. The induced changes in the charge density are compared with those generated for the gas phase molecule by a singlet to triplet - it excitation. From Ref. [92].

Fig. 21. Adsorption isotherm and composition of the gas phase on equilibrium with the surface for the adsorption of acetylene on rhodium—silica at 20°C. °, Total molecules adsorbed , gas phase acetylene , ethane.

In a recent study of the adsorption of acetylene on platinum single crystals by low energy electron diffraction [160], it has been shown that acetylene adsorbs on the (111) planes. These results show that, on a clean Pt (111) surface, acetylene adsorbs at a distance of 1.95 A above the topmost plane of platinum atoms, either in the C2 or, less likely, the Bl mode shown in Fig. 23. No evidence was found for adsorption in the A or A2 modes, which corresponds to a 7r-complex structure or for the B2 mode corresponding to a di-o-complex, although it was stated that such structures may be possible with a less stable overlayer which had been observed. 

The reported rate equations for the hydrohalogenation of acetylene, ethylene, propene and vinyl chloride are summarised in Table 15. Of special interest is the last entry it is based on a model which assumes two types of active centres, the first one for the adsorption of acetylene, the second for the adsorption of hydrogen chloride and vinyl chloride. 

Selective hydrogenation of dienes or acetylene caused by competitive adsorption of acetylene or dienes with olefins. 

The first two examples both involved the creation of cationic species on an acidic zeolite. In both cases we did not need to model the interaction of the cation with the zeolite framework good agreement was obtained with just calculation of the isolated cation. Apparently, the cation is not strongly perturbed by the presence of the zeolite. Such fortunate circumstances are rare. Here we show an example of how theoretical NMR calculations can help elucidate chemistry on a basic metal oxide surface, in particular, the adsorption of acetylene on MgO (26). For this study we needed to model the active sites of the catalyst, for which there are many possibilities. It is assumed the reactive sites are those in which Mg and O are substantially less coordinated than in the bulk. Comer sites are those in which Mg or O are three-coordinate, whereas Edge sites have four-fold coordination. These sites are where the strongest binding of the adsorbates are obtained. 

Experimentally, we used in situ 13C NMR with magic angle spinning (MAS) to measure spectra of the species formed by adsorption of acetylene on MgO powders. Three signals were observed following the initial adsorption of acetylene, 77, 96, and 121 ppm. Two additional peaks at 27 and 58 ppm became apparent after the catalyst was stored at 298 K for 12 hours. 

Fig. 11. Adsorption of acetylene on Si(OOl). The experimental STM scans show three different adsorption configurations, labeled I-III (A). They are due to the possibility of restructuring of the carbon bond to either a double bond (configurations (1) and (2) in frame (B)), or to a single bond (configurations (3) and (4)) in frame (B)). The resulting STM images (frame (C)) in the simulation agree quite well with three of the configurations found in the experiments ((A), features I, II, and III). The experimental images were taken from [58].

ABB Lummus Global Acetylene extraction C2S Selective adsorption of acetylene with dimethyl form amine (DMF) 4 1991... 

THE ADSORPTION OF ACETYLENE AND ETHYLENE ON TRANSITION METAL SURFACES... 

The density functional theory and the cluster model approach enable the quantitative computational analysis of the adsorption of small chemical species on metal surfaces. Two studies are presented, one concerning the adsorption of acetylene on copper (100) surfaces, the other concerning the adsorption of ethylene on the (1(X)) surfaces of nickel, palladium and platinum. These studies support the usefulness of the cluster model approach in studies of heterogeneous catalysis involving transition metal catalysts. 

The experimental study of the low temperature adsorption of acetylene on transition metal surfaces has lead to the identification of two main types of structures (A and B) shown in Fig. 1. 

FIGURE 1. Structures resulting from adsorption of acetylene on metal surfaces... 

The adsorption of acetylene on metal surfaces is commonly described using a frontier orbital approach, developed by Dewar [1] and Chatt and Duncanson [2]. In this model the bonding to the metal surface is described as a donation of acetylene 7i-electrons into the metal and back-donation from the metal into the acetylene antibonding 71 " orbital. 

The contradictory available experimental results regarding the C2H2 / Cu(lOO) adsorption system served as a motivation for the first of the theoretical studies presented in this paper. The aim of this study was to compile theoretical results for the adsorption of acetylene on four different adsorption sites on the (100) surface of... 

However, a study of the comparative kinetics of the transformation of C,H.> and QD2 over this catalyst showed that the rate of adsorption of acetylene is very much higher than the rate of adsorption of C2D2 iknlkn = 2-3). This effect implies the rupture of the H(D)—C bond in the process of isomerization, and confirms the first step of Meriwether s scheme ... 

E are equal to 2.5% and are matched by deviations of the calculated values of fillings, 6, (and accordingly of the adsorption values) by 9.5% from the value 6 = 0.368 for the characteristic point. At lov er temperatures, considerable deviations are observed. On the other hand, for the adsorption of acetylene on active centers of zeolite NaA, the value of E remains practically constant (E = 8090 cal/mole) over the temperature range from 273° to 423 °K. 

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