Adsorption and Reactivity of Olefins

The discussion of the interaction of C2 molecules is separated in two parts, probing TCE and ethene—each molecule on surfaces and supported size-selected Pf clusters. 

The adsorption properties of TCE on surfaces are smdied with a combination of EES and TPD measurements and act as a model system of a weakly interacting molecule on surfaces. With the confirmation of physisorption behavior on the studied surfaces, the adsorption on different sizes of supported Pt clusters were probed and evaluated in comparison to the surfaces. 

Second, the well known chemisorption behavior of ethene is characterized with the same combination of EES and TPD on surfaces and serves as a future comparison for the study of the chemisorption behavior of ethene on size-selected Pt clusters by means of EES. The reactivity of ethene towards the hydrogenation reaction is probed by TPR and also further preliminary experiments (AES and IRRAS) are shown in order to investigate the mechanism of the ethene hydrogenation reaction on size-selected clusters. 

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The main focus of the UHV experiments was the investigation of the electronic structure as well as the adsorption properties and reactivity of olefins adsorbed on surfaces and supported Pt clusters. 

Dittmeyer et al. [9] reviewed the applications of a palladium membrane reactor to catalytic dehydrogenation of paraffins. Based on simplified simulation, it was concluded that a beneficial effect of hydrogen removal through the membrane could be offset by strong adsorption of the product olefin on the catalyst surface that would eliminate the active surface sites and effectively slow down the forward reaction rates. Also, it was observed that the long time stability and reactivity of the palladium membranes would require more research. 

The first results part is dedicated to studies under UHV conditions and begins with the determination of the sensitivity of the MIES/UPS setup. Next, the measurements of the electronic structure of two different cluster sizes is presented. The capabilities of the setup are further elucidated, using CO as probe molecule and its adsorption properties probed on Pf(lll). Last and most important, the adsorption properties and reactivity of the olefins TCE and ethene are studied as a function of coverage on surfaces and also on supported Pt clusters by means of EES, TPD/TPR as well as IRRAS and AES. 

The chapter is divided into four major parts, illustrating the capabilities of the MIES/ UPS setup towards application for adsorption studies and cluster science as well as investigation of the reactivity of olefins on Pt clusters. For comparison, the experiments on the clusters are repeated on inert MgO support and a Pf(l 11) single crystal. 

From the results of other authors should be mentioned the observation of a similar effect, e.g. in the oxidation of olefins on nickel oxide (118), where the retardation of the reaction of 1-butene by cis-2-butene was greater than the effect of 1-butene on the reaction of m-2-butene the ratio of the adsorption coefficients Kcia h/Kwas 1.45. In a study on hydrogenation over C03O4 it was reported (109) that the reactivities of ethylene and propylene were nearly the same (1.17 in favor of propylene), when measured separately, whereas the ratio of adsorption coefficients was 8.4 in favor of ethylene. This led in the competitive arrangement to preferential hydrogenation of ethylene. A similar phenomenon occurs in the catalytic reduction of nitric oxide and sulfur dioxide by carbon monoxide (120a). 

The 02 ion on MgO does not react with CO or alkanes at 77 K but the EPR signal disappears slowly at room temperature (361). Similarly, on ZnO (390) it reacts only slowly with propylene at room temperature and not with CO, H2, or ethylene. A slow reaction with propylene is also observed for 02 on V2Os/MgO at room temperature (391). Yoshida et al. (392) have studied the reactivity of adsorbed oxygen with olefins on the V20j/Si02 system. Adsorption of propylene destroyed the signal from 02 slowly at room temperature and the reaction products, aldehydes with some acrolein, were desorbed as the temperature was raised to 150°C. More quantitative... 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

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