Hydrocarbon overlayers

We simply point out that the stable hydrocarbon overlayer formed from the chemisorption of either ethylene or acetylene and hydrogen on both Pt(lll) and Rh(lll) yield identical vibrational spectra (Figure 7). A more complete discussion of the similarities between the chemisorption of ethylene on Rh(lll) and Pt(lll) is presented elsewhere (24). 

The catalytic partial pressures and temperatures used were thus simiLar to those employed by Boudart and were therefore expected to result in structure insensitivity- However, the hydrocarbon overlayer on the catalyst surfaces was also expected to exceed monolayer coverage In the light of previous results and it was recognised that this might be amportant ... 

This conclusion is supported by the data presented in Figure 4(b), where the rate of reaction is shown for a series of perfect square particles of various side lengths. Smaller particles reduce the ethene pressure required to form the hydrocarbon overlayer which blocks further hydrogen adsorption and greatly reduces the reaction rate. The chemical structure of the deposited hydrocarbon overlayer is of no consequence, the only important consideration is the geometric structure of the overlayer. Once there is enough hydrocarbon on the surface to block all adjacent pairs of vacant sites (i.e. potential hydrogen adsorption sites) the... 

Where PFPE is perfluoropolyether, Fe is ferrocene, D is deuter-ated, and SS is a stainless steel surfece with an adventitious hydrocarbon overlayer. 

Combination tra/ s—sedimentary trap features that result from both stratigraphic and structural mechanisms. There can be many combinations for stratigraphic and structural traps. An example of such a trap would be a reef feature overlaying a porous and permeable sandstone, but in which the sequence has been faulted (see Figure 2-54). Without the fault, which has provided an impregnable barrier, the hydrocarbons would have migrated further up dip within the sandstone. 

As an analyst you understand the meaning of the scientific data you produce. However, it must be remembered that laymen often do not and so the data need to be documented in a form that is easily understood. For example, the chromatographic analysis of hydrocarbon oil from an oil spill can produce a chromatogram with over 300 components. Explaining the significance of such data to a jury will be of little benefit. However, overlaying it with a standard trace can demonstrate pictorially whether there is a similarity or not. The customer requires information from the analyst to prove a point. If the data are not fully documented, then the point cannot be proven. A customer who has confidence in a laboratory will always return. 

VII. The Role of the Carbonaceous Overlayer in Hydrocarbon Reactions on Platinum Surfaces... 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

A breakdown of the structural results by type of surface shows results for nearly 50 clean, unreconstructed metal surfaces and about 10 alloys and reconstructed metal surfaces. The structures of about 65 atomic overlayers on metal surfaces have been determined, some 40 of these involving chalcogen atoms. Just over 20 molecular structures have been determined for metal surfaces, half of these being overlayers of undissociated carbon monoxide and the others various hydrocarbons. Turning to semiconductors, some 13 clean, usually reconstructed structures were determined, against nearly 10 atomic overlayer structures. In addition, about 15 insulator surface structures have been investigated. 

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