Carbon monoxide surface structure

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

We have already considered two reactions on the H2CO potential energy surface. In doing so, we studied five stationary points three minima—formaldehyde, trans hydroxycarbene, and carbon monoxide plus hydrogen molecule—and the two transition structures connecting formaldehyde with the two sets of products. One obvious remaining step is to find a path between the two sets of products. 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

PEMFC)/direct methanol fuel cell (DMFC) cathode limit the available sites for reduction of molecular oxygen. Alternatively, at the anode of a PEMFC or DMFC, the oxidation of water is necessary to produce hydroxyl or oxygen species that participate in oxidation of strongly bound carbon monoxide species. Taylor and co-workers [Taylor et ah, 2007b] have recently reported on a systematic study that examined the potential dependence of water redox reactions over a series of different metal electrode surfaces. For comparison purposes, we will start with a brief discussion of electronic structure studies of water activity with consideration of UHV model systems. 

Gasteiger HA, Markovic NM, Ross PN Jr. 1996. Structural effects in electrocatalysis Electrooxidation of carbon monoxide on Pt3Sn single-crystal surfaces. Catal Lett 36 1-8. 

Lucas CA, Markovic NM, Ross NM. 1999. The adsorption and oxidation of carbon monoxide at the Pt(lll)/electiol3de interface atontic structure and surface relaxation. Surf Sci 425 L381-L386. 

Oda I, Inukai J, Ito M. 1993. Compression structures of carbon monoxide on a Pt(lll) electrode surface studied by in situ scanning tunnehng microscopy. Chem Phys Lett 203 99 103. 

Williams ED, Weinberg WH. 1979. The geometric structure of carbon monoxide chemisorbed on the ruthenium (001) surface at low temperatures. Surf Sci 82 93. 

Since much of the impetus for our STM studies stems from earlier spectroscopic investigations of alkali metals and alkali metal-modified surfaces,6 we consider first what was learnt from the caesiated Cu(l 10) surface concerning the role of different oxygen states, transient and final states, in the oxidation of carbon monoxide, and then examine how structural information from STM can relate to the chemical reactivity of the modified Cu(110) surface. 

Park PW, Ledford JS (1998) The influence of surface structure on the catalytic activity of cerium promoted copper oxide catalysts on alumina oxidation of carbon monoxide and methane. Catal Lett 50(1—2) 41 48... 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

Carbon monoxide adsorbed on sufficiently small palladium particles disproportionates to surface carbon and carbon dioxide. This does not occur on large particles. The CO-O2 reaction is shown to be structure-insensitive provided the metal surface available for the reaction is estimated correctly. 

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

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