Reactions on Single-Crystal Surfaces

It is convenient to collect information on the interactions of alkenes with hydrogen or deuterium on single crystal surfaces into a separate sub-section because the application of methods unsuited to supported metals and powders isolates and identifies species and elementary steps, the existence of which can only be surmised by conventional kinetic studies this makes an invaluable contribution to a final discussion of mechanism. 

The ethene-deuterium reaction has been studied over Pt( 111) between 300 and 370 ethane-di was the chief product and the mean deuterium number of 

The propene-deuterium reaction has been examined more recently on the same surface using TPD spectroscopy the extensive results obtained are described in a lengthy paper. Above 230 K, all deuteropropenes and propanes were found, 

Finally, as concerns Pt(lll), the reaction of co-adsorbed ethene-d4 and hydrogen has been studied using a combination of laser-induced thermal desorption, mass-spectrometry and RAIRS exchange occurred above 215 K with an activation energy of 46 kJ mol this being below the point at which conversion to 

70 kJ mor As is often found, pre-adsorbed hydrogen favoured the formation of the 7T-state of ethene both forms reacted to give an ethyl radical, through which exchange took place, but the r-form reacted faster. 

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R. Imbhil. Oscillatory reactions on single crystal surfaces. Prog Surf Sci 44 185-343, 1993. 

The studies of well defined systems consists of spectroscopic studies of individual molecules and measurements of the rate of catalytic reactions on single crystal surfaces, as well as structure and reactivity of well-defined catalyst models. 

The reactions of small organic molecules on titanium dioxide surfaces provide the central focus of these examples. Comparisons with reactions on single crystal surfaces of other oxides illustrate some of the key structural and electronic issues in oxide surface reactivity and the importance of local interactions between surface sites and adsorbed molecules. 

There have been correspondingly few papers describing this reaction on single crystal surfaces of metals. ° Four concern the reaction on various surfaces of platinum, but their value is unfortunately somewhat limited two of them used both Pt(lOO) and Pt(lll), but TOFs were given at quite different temperatures with another pair it is only possible to compare results on Pt(l 10). While activation energies (43 10 kJ mol ) and order of reaction where determined were broadly comparable (Table 8.4), values of TOF sometimes showed a marked variation between different surfaces, and it is not possible to define a unique hierarchy of activity. Most disappointingly there is little information on product selectivi-ties apart from Siot (Table 8.4) in one publication only partial information was... 

Applications. In virtually all basic work on interactions with, and reactions on, single-crystal surfaces over the last 50 years LEED has been employed to provide essential structural information, The structures proposed would in general be ambiguous, in the sense of the existence of several possibilities, if LEED were used on its own, but when the LEED data are taken in conjunction with those from other techniques applied at the same time, they have proved invaluable. The number of publications that have included LEED is probably greater than for any of the other techniques described here. 

A number of theories have been put forth to explain the mechanism of polytype formation (30—36), such as the generation of steps by screw dislocations on single-crystal surfaces that could account for the large number of polytypes formed (30,35,36). The growth of crystals via the vapor phase is beheved to occur by surface nucleation and ledge movement by face specific reactions (37). The soHd-state transformation from one polytype to another is beheved to occur by a layer-displacement mechanism (38) caused by nucleation and expansion of stacking faults in close-packed double layers of Si and C. 

To gain an impression of the structures and reactions that occur in electrochemical systems, we consider the interface between a metal and an electrolyte solution. Figure 1.1 shows a schematic diagram of its structure. Nowadays most structural investigations are carried out on single crystal surfaces so the metal atoms, indicated by the hatched circles... 

Surface science studies dealing with the adsorption and reaction of gases on single crystal surfaces. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

Fig. 16. Schematic of the experimental apparatus to carry out catalytic reaction rate studies on single-crystal surfaces at low and high pressures in the range 10 7-104 Torr.

Methanol decomposes on titanium dioxide surfaces by mechanisms that are similar to those by which formic acid decomposes. Methanol can reversibly adsorb on single crystal surfaces of titania (reaction 16) in a molecular state, or it may dissociatively adsorb by interaction with surface lattice oxygen anions, forming a surface methoxide (reaction 17). Reaction (18) represents the disproportionation reaction of hydroxyl groups on the surface of the metal oxide. 

The following describes results of three, relatively simple chemical reactions involving hydrocarbons on model single crystal metal catalysts that illustrate this general approach, namely, acetylene cyclotrimerization and the hydrogenation of acetylene and ethylene, all catalyzed by palladium. The selected reactions fulfdl the above conditions since they occur in ultrahigh vacuum, while the measured catalytic reaction kinetics on single crystal surfaces mimic those on reahstic supported catalysts. While these are all chemically relatively simple reactions, their apparent simplicity belies rather complex surface chemistry. 

Rupprechter G (2001) Surface vibrational spectroscopy from ultrahigh vacuum to atmospheric pressure Adsorption and reactions on single crystals and nanoparticle model catalysts monitored by sum frequency generation spectroscopy. Phys Chem Chem Phys 3 4621... 

The writer is convinced that the techniques of surface physics are now making considerable impact on our understanding of catalysis. This is true of oxide catalysis and reactions on single crystals, but in this latter case some qualification is required. Success has come in the study of the reactions of small molecules where the surface stays clean. The same cannot be said of hydrocarbon reactions where the information obtained is interesting but perhaps misleading because of the appearance of carbonaceous overlayers. 

H2 electrode reactions Despite extensive studies of the H2 electrode reactions, the pathways remain controversial for many electrode surfaces, and reliable data on single-crystal surfaces are lacking. As the prime example of a relatively simple electrocatalytic... 

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