Higher Coverages

Graphite was tised as substrate for the deposition of carbon vapor. Prior to the tube and cone studies, this substrate was studied by us carefully by STM because it may exhibit anomalotis behavior w ith unusual periodic surface structures[9,10]. In particular, the cluster-substrate interaction w as investigated IJ. At low submonolayer coverages, small clusters and islands are observed. These tend to have linear struc-tures[12j. Much higher coverages are required for the synthesis of nanotubes and nanocones. In addition, the carbon vapor has to be very hot, typically >3000°C. We note that the production of nanotubes by arc discharge occurs also at an intense heat (of the plasma in the arc) of >3000°C. 

A plot of the adatom density versus T is shown in Fig. 4. An anomalous increase in the density is observed at high temperatures. The dashed line represents the adatom population that would be predicted if there were no lateral interactions. However, the LJ potential between adatoms tends to stabilize them at the higher coverages, and it is this effect that causes the deviation from Arrhenius behavior at high temperatures. A similar temperature dependence is observed in the rate of mass transport on some metal surfaces (8,9), and it is possible that it is caused by the enhanced population of the superlayer at high temperatures. 

However, our discussion so far applies to low coverages, as is usually the case in kinetic modeling. With highly covered surfaces another mechanism prevails, which offers an alternative to the energetically unfavorable three-centered transition state of Fig. 6.39(a). At higher coverages, ethylene and hydrogen are forced closer and... 

The second case in Fig. 7.7 corresponds to first-order desorption of CO from a stepped Pt(112) surface. This surface consists of (111) terraces and (100) steps. At coverages below one-third of a monolayer, CO only occupies the step sites, while at higher coverage the terraces are also populated, resulting in two clearly distinguish-... 

Figure 9.16. Ethylene hardly adsorbs on clean silver, but it does interact with preadsorbed oxygen atoms. At low coverages, the O atoms preferably interact with the C-H bond of ethylene, leading to its decomposition into fragments that oxidize to CO2 and H2O but at higher coverages the oxygen atoms become electrophilic and interact with the n-system of ethylene to form the epoxide. [After R.A. van Santen and H.P.C.E. Kuipers, Ac/v, Catal. 35 (1987) 265.]...

STM studies of the Au(llO) surface indicated that only the Se (2x3) structure was formed at coverages much below one monolayer and that it was formed homogeneously. At monolayer and higher coverages, a honeycomb structure composed of chains of Se atoms was observed, which at still higher coverages filled in to complete a second Se layer. 

The rate constants in table 4 for Ru/AlaOs should be considered as initial rate constants since it was not possible to achieve a higher coverage of N— than 0.25. Furthennorc, it was not possible to detect TPA peaks for Ru/AlaOs within the experimental detection limit of about 20 ppm. Ru/MgO is a heterogeneous system with respect to the adsorption and desorption of Na due to the presence of promoted active sites which dominate under NH3 synthesis conditions. The rate constant of desorption given in table 4 for Ru/MgO refers to the unpromoted sites [19]. The Na TPD, Na TPA and lER results thus demonstrate the enhancing influence of the alkali promoter on the rate of N3 dissociation and recombination as expected based on the principle of microscopic reversibility. Adding alkali renders the Ru metal surfaces more uniform towards the interaction with Na. 

The adsorption of sulfur at platinum,40 rhodium,41 rhenium42 and ruthenium43 has been studied predominantly at fcc(lll) and hcp(0001) surfaces and shows many similar characteristics. Adsorption is initially into fee hollow sites of the fee metals and hep sites of the hep metals at higher coverages, mixed site occupancy occurs. A (2 x 2) structure is the first to be recorded appearing in the... 

At higher coverages, (2 x 2) and ( 3 x J3)R30° structures have been observed, which have been ascribed as before [108, 109] to Fe-terminated Fe304(l 1 1) and a-Fe2O3(0 0 0 1) phases, respectively. 

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