Surface phases bimetallic systems

The relative inertness of the carbon surface is of paramount importance when carbon materials are going to be used as supports for hydrogenation catalysts. These systems usually consist of more than one metallic phase (bimetallic systems) and even by metals promoted by metal oxides. The carbon inertness facilitates interaction between the metals and/or between the metals and the promoters, yielding more active and selective catalysts than those supported on other common supports. These aspects will be illustrated by examples of the application of carbon-supported catalysts to the hydrogenation of carbon oxides. 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

The well characterized and stable surface phases observed on the Sn-Pt(l 11) have provided researchers in the chemisorption and catalysis field with a substrate of great interest for studying the properties of bimetallic interfaces. Simple probe gases such as CO have been studied after adsorption on this system [45] as well as a variety of organic molecules such as acetylene [46], cyclohexane and benzene [47, 48], butane and isobutane [49], methanol, ethanol and water [50]. Several surface reactions of the above gases were also studied. 

Among ordered bimetallic systems, the Pt-Sn one can be considered at present as the most in-depth studied not only for its surface structural properties, but also for its reactivity and catalytic properties. A comparable detailed knowledge exists only for a few other cases, among platinum alloys we can cite the Ni-Pt and Co-Pt systems, examined for their catalytic properties and the Pt-Ti system studied for their electrocatalytic properties [5]. Sparse data relative to the surface properties of several other Pt alloys exist (e.g. FeaPt and CuaPt -[3] and PtaMn [51]. All these data available pertain to fee phases either random substitutional or ordered compounds. Data exist also for other cubic ordered alloys which are isostructural with the PtaSn compound, e.g. NiaAl [52, 53] and AuaPd [28] and finally the Au-Cu system, which has been object of interest as the prototypical LI2 or Pm3m ordered system in the CuaAu composition [54, 55]. 

Although simple, flat surface phases are observed, the Pt-Sn system is also remarkable for the complexity of mesoscopic phenomena observed, such as the pyramids formed on the Pt3Sn(100) surface. These phenomena are obviously related to the high surface energy of the system, which is possibily the inter-metallic compound with the largest enthalpy of formation studied so far for its surface properties. No comparable phenomena have been observed in other bimetallic systems so far. 

The methodology of the addition of tin into supported platinum or rhodium seems to play an important role in the behaviour of the active phase obtained. Controlled surface reactions of organometallic compounds with metal surfaces result in the formation bimetallic systems with specific properties in the hydrogenation of different unsaturated compounds. ° However, the nature of the Sn-Pt or Sn-Rh bimetallic phase formed, and its influence on the final properties of the catalyst, have not been yet well determined and this is still a subject to be investigated. 

Studies of small particles by Sinfelt [29] and his co-workers have shown that when the particles sizes become very small and dispersions tend toward unity (that is, when virtually every atom is at the surface), alloy systems exhibit phase diagrams very different from those that characterize bulk systems. For example, microclusters containing Cu and Ru, Cu and Os, or Au and Ni can be produced in any ratio of the two elements, indicating complete miscibility or solid solution behavior. In the bulk phase these elements are completely immiscible. This very different behavior of the surface phases of bimetallic systems finds important applications in the design of catalysts to carry out selective chemical reactions. Moran-Ldpez and Falicov [30] developed a theory—using pairwise interactions—of alloy surface segregation that explains this effect. Bimetallic systems remain miscible at lower temperatures in two dimensions than in three dimensions. 

Different SPM systems were developed to study the thermal properties. Thus a tiny thermocouple can be used to measure the heat flow from the surface and to test the local thermo conductivity of polymer surfaces [161]. Recently, a bimetallic cantilever has been used as temperature sensor to investigate phase transitions of n-alkanes with a heat sensitivity of 500 pj for a sample mass as low as to... 

A rapid survey of the methods utilized for the study of binary alloys, and specifically for the Pt-Sn system will be reported here. In the present review, we consider only studies performed in conditions of ultra high vacuum (UHV), where bimetallic Pt-Sn surfaces are stable. It is known that in air and in general in the presence of oxygen at pressures larger than ca. 10 Torr, tin alloyed with platinum tends to oxidize and de-alloy to form oxide phases, a phenomenon that will not be treated here. 

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