Carbon coverage

For example, consider the dissociative adsorption of methane on a Ni(lOO) surface. If the experiment is performed above 350 K, methane dissociates into carbon atoms and hydrogen that desorbs instantaneously. Consequently, one determines the uptake by measuring (e.g. with Auger electron spectroscopy) how much carbon is deposited after exposure of the surface to a certain amount of methane. A plot of the resulting carbon coverage against the methane exposure represents the uptake curve. 

Figure 2. Carbon coverage as a function of C2H4 exposure at 323 K.

Exposure Dependence of SIMS. Additional insight into the relationships between emitted secondary ions and species present on the surface is obtained by examining the exposure dependence of the secondary ions and the relationship of this dependence to the change in carbon coverage with exposure as shown in Figure 2. 

Figure 8. Variation of the RuC+/Ru+ Cl), RuC /Ru2 (2), RuC2hJ/Ru+ (3) and RuCH+/Ru (4) ion ratios and the carbon coverage (- -) with C2H4 exposure at 323 K. All variables are normalized to their maximum values.

Fig. 4. Methane production rate at 62S K over a Ni(100) catalyst as a function of surface carbon coverage at various reaction conditions. (From /te/ 12.)...

Carbon contaminated surfaces were prepared by thermally decomposing benzene on the nickel surface. The carbon overlayer was ordered but the diffraction pattern was complex. It did not correspond to a graphitic ring structure. Approximate carbon coverages were estimated using Auger calibration curves based on thermally decomposed benzene (10). 

Elemental and surface analysis measurements had shown that the cokes contained significant amounts of well-dispersed iron chlorides and other contaminants. XPS measurements had shown the importance of the chemical nature of the cokes and their interactions with the catalyst components (and not simply the degree of carbon coverage of the catalyst). IINS focused on the hydrogen-containing part of the coke provided a quite different view of finely divided, highly contaminated, bulk samples of cokes from commercial processes. 

During the growth, the anatase (001) films are (1x1) terminated. The (1x1) termination has been confirmed ex-situ with FEED and XPD by Herman et al. [146]. The surface was only outgassed at 100°C and not sputtered. Hence, a carbon coverage of 0.6 monolayers was estimated based on XPS measurements. The surface was well ordered and the structure in XPD was fully consistent with the bulk-like termination depicted in Fig. 19. A two-domain (1x4) reconstruction formed on a similar sample after sputtering and annealing the (1x1) surface in UHV [147]. Based on angle-resolved mass-... 

Fig. 3 shows how the results can be described when using 0 as the sum of carbon coverage and oxygen coverage. 

The selective semi-hydrogenation of ethyne in ethene is also an industrial process of vital importance, used in both laboratory practice and relevant to fine chemicals and polymer production [445,462]. The reaction is generally performed over low loaded Pd catalysts. Systematic investigations have been performed over Pd-Ag, Pd-Cu and Pd-Au supported catalysts that are superior to mono-metallic Pd catalysts [445,461,463]. The presence of Au decreases the carbon coverage and improves the ethene selectivity [445]. 

The data disclosed by Vannice are highly valuable, as they allow a comparison on turnover basis. With the evidence now available regarding the development of extensive overlayers of surface carbon during FT synthesis (cf. Section IV), we would recommend that any forthcoming interpretation of the overall kinetics takes explicity into account the variations in surface-carbon coverage with both the reaction conditions and the nature of the transition metal. Beeck 30 already pointed out that carbon deposition may well depend on the morphology of the surface, a point substantiated in more recent surface studies (57). This may underlie the large support effects disclosed in one of Vannice s more recent kinetic studies (52). 

Carbon Layers From Ethylene. Carbon coverage as a function of ethylene exposure at 323 K has been determined previously by recording the amount of CO desorbed during TPO XU Half monolayer (M.L.) coverage by carbon requires about 2 Langmuirs (L) of C H, while 1.1 monolayer coverage corresponds to 15 L. Molecular species removed... 

CO Thermal Desorption. On a clean Ru(OOl) surface, 9 L of CO induces saturation coverage by molecular CO. When this dose of CO is applied to a surface preexposed to C H at 323 K, the CO uptake is diminished but not completely blocked by the carbon layer. The CO uptake is still 90% of the saturation value when the carbon coverage is 1/4 M.L. and falls to 1/4 of the saturation value after 15 L preexposure to ethylene. 

Analysis of the CO desorption spectra, and others obtained at lower carbon coverage, in terms of results for CO adsorption on the clean surface (2-5), leads to several conclusions. At low carbon coverages the effect of the carbon is slight. This corresponds to a CO TPD spectrum similar to that found for a clean surface but decreased in intensity and with a broadened high temperature peak. High carbon coverage blocks some sites and weakens the remainder. 

The absolute probability for the dissociative chemisorption of CH4 on Ni(lll) is plotted versus the normal component of the kinetic energy of the incident methane molecule in Fig. 2. The dissociation probability was measured by monitoring the amount of deposited carbon by Auger electron spectroscopy. Since methane does not adsorb molecularly at the surface temperature of 475 K at which these measurements were carried out (ref. 9), the carbon Auger feature results only from the methane that has dissociatively chemisorbed. The carbon Auger signal is calibrated for absolute carbon coverage and the absolute flux of the incident methane beam is determined from procedures outlined in detail previously (refs. 6b,10). The absolute dissociation probability plotted in Fig. 2 is the ratio of the absolute number of adsorbed carbon atoms per unit area to the absolute number of methane molecules per unit area incident on the surface. 

The influence of adsorbed Si, P, S, and Cl on the medium-pressure cyclotrimerization of acetylene to benzene over Pd(l 11), (100), and (110) has been studied by Logan et al. (113). Whereas both sulfur and chlorine decrease the activity, silicon increases the activity. The effect of phosphorous depends on the crystal face. According to their work function measurements, sulfur withdraws electron density from the Pd surface (as is also expected for chlorine), whereas Si donates electron density, and P has the least effect on the work function. Thus, the qualitative influences on catalytic activities correlate with the influences of the additives on the electronic character of the surface. In addition, Si decreases the carbon coverage seen in postreaction AES from —82 to —70% of a monolayer, whereas sulfur and chlorine increase the amount of carbonaceous residue. The authors interpreted these results by suggesting that the electron-donating ligands keep the Pd surface cleaner for the desirable reaction by... 

Hasenberg and Schmidt (119) studied the synthesis of HCN from CH4 + NH3 mixtures at 0.01 to 10 torr over a polycrystalline Pt surface that was initially clean. Under reaction conditions the surface was covered with nearly a monolayer of carbon. Addition of 02 to the reaction mixture reduced the carbon coverage, decreasing the HCN production rate at the expense of NO production. A kinetic model was developed that fit the data well. 

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