Sulfur preadsorbed

Variation of the amounts of I CO and CO formed in CH OH dehydrogenation as a function of preadsorbed sulfur. 

Bonzel and Ku performed a detailed study on the influence of sulfur on the adsorption of CO (208) as well as on the catalytic C02 formation (209) on a Pt(lll) surface. It was found that preadsorbed sulfur affected both the adsorption energy of CO as well as the total amount of CO adsorbed. A Pt surface saturated with sulfur (6S x 0.75) was no longer able to adsorb any CO. Consequently, the rate of C02 formation also decreased continuously with increasing sulfur content of the surface and became practically zero for 6S x 0.28. These results demonstrate the role of sulfur as a rather effective catalyst poison. 

In a separate series of experiments, the influence of sulfur on the decomposition of a mixture consisting of CO/C2H4/H2 over iron was investigated. Previous work [17] had shown that while iron did not catalyze the decomposition of ethylene, even in the presence of hydrogen, when a small fraction of CO was added to the reactant, a dramatic increase in the rate of decomposition of the olefin was observed. This behavior was rationalized according to a model in which the presence of coadsorbed CO resulted in what is believed to be reconstruction of the iron to form a surface, which favors dissociative chemisorption of ethylene. In the current study, we have extended this study to include the case where sulfur is preadsorbed on the metal surface in an attempt to determine how such adatoms modify the coadsorption characteristics of CO and C2H4 on iron. 

Results of previous investigations 23,110, 111, 113, 141, 157-165) show that hydrogen adsorption on nickel at room temperature is lowered by preadsorbed sulfur. Moreover, the fraction by which hydrogen adsorption is reduced in polycrystalline and supported nickel catalysts is generally proportional to the mean fractional coverage of sulfur. This is illustrated by data in Fig. 16 from Bartholomew and co-workers 112, 113, 141, 157-162). 

A few previous studies (83a, 181,183,185,187) have examined the effects of preadsorbed sulfur on the nature of adsorbed CO species or CO adsorption states on metals. Rhodin and Brucker (181) investigated variations in CO bonding on clean and partially sulfur-deactivated oc-Fe(lOO) surfaces. 

Since preadsorbed sulfur generally blocks the adsorption of other molecules, it would only be logical to expect that it would also prevent the adsorption of H2S or S2. Previously discussed studies of sticking coefficients (73, 83, 92, 99, 101) and H2S adsorption on metals (57, 106, 112-115) provide evidence that the sticking coefficient and heat of adsorption for H2S or S2 decrease with increasing coverage. Thus, rates and strengths of sulfur adsorption on sulfur-saturated metal surfaces are clearly lower than those on a clean metal surface. 

To summarize, previous studies have established the qualitative effects and to a lesser extent quantitative effects of preadsorbed sulfur on the adsorption of other molecules, particularly CO and H2. Unfortunately, in most of the previous work quantitative relationships between the coverage of sulfur and decrease in adsorption or changes in adsorption states for a given adsorbate were not obtained. Thus, determination of the quantitative effects... 

That sulfur is responsible for suppressing hydrocracking of organic molecules on Pt is consistent with the work of Fischer and Kelemen (88) showing that bonding of benzene on Pt(100) is sufficiently modified by preadsorbed sulfur to enable an increasing fraction of the adsorbed benzene to desorb at elevated temperatures rather than to dehydrogenate. 

Differences are also noticed in the values of the surface pH and amounts of preadsorbed water. The pH values for the exhausted samples after subsequent SOj and HjS adsorption runs are much lower than those after HjS adsorption followed by SOj adsorption. This suggests differences in the surface reaction products. These differences are also reflected in the amount of water adsorbed after the first runs in the breakthrough tests. After SOj adsorption much more water is preadsorbed before the next run than after adsorption of HjS. This once again indicates differences in the chemistry of inorganic phase. After SO2 adsorption it is likely that still some oxides able to adsorb water are present (hydrophilic surface) whereas reactions with HjS and deposition of sulfur [12,14] almost totally "screen" active centers for water adsorption. 

Figure 14 Correlation between the rate of the HOR (at 0.05 V versus RHE) and the stripping charge for the oxidative removal of COad preadsorbed (at 0.05 V versus RHE) on Pt(lOO) and (111) surfaces in 0.5-M sulfuric acid.

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