Mobile sulfur

LEED patterns at 0 = 1 /4, but was identified at lower coverages in islands surrounded by mobile sulfur atoms at platinum, rhodium and rhenium surfaces. Sautet and co-workers42 have analysed the statistical correlations between the intensities of sulfur features in p(2 x 2) islands on rhenium surfaces and also of streaks in areas between islands, which they attribute to sulfur atoms diffusing under the tip (Figure 10.12). 

Figure 2. SNPA s mobile sulfur and asphalt blending equipment...

The most simple way to determine the amount of mobile sulfur is to expose the sulfided catalyst to H2 S... 

Comparison of thiophene HDS conversion with the ratio of mobile (exchangeable) sulfur indicated that the higher ratio of mobile sulfur was paired with higher HDS activity of the C0M0X/AI2O3 catalysts. 

The fraction of Smob/Soat and the amounts of mobile sulfur for C0M0S2 were 8—43% and 2.8—7.5 x lO S atoms/mg respectively in the 533—673 K interval,similar to the values obtained with H2 S experiments. 

On alumina supported Pd and Pt, the Smob/Scat was 0.2 and 0.12 the amount of mobile sulfur was 3—3.9 x 10 S atoms/mg at 533K.I 1 According to the authors opinion, the difference between the molybdena based and the noble metal catalysts, is a consequence of the attachment of a part of sulfur to the alumina support of the noble metal catalysts. The data indicate that almost all sulfur accommodated on the noble metals was mobile, i.e., exchangeable and participated in the HDS reaction. This was different from the case with molybdena-based catalysts. This observation was confirmed recently that Smob/Pd(Pd- -Pt) ratio was 0.2, both for alumina supported Pd and Pd-Pt, irrespective of the H2S partial pressure and the amount of the total sulfur uptake. 

An observation is made recently, comparing the mobile sulfur data of a presulfided with H2 S RU-CS/AI2O3 catalyst, with those determined for a non-presulfided sample described before by HDS of S-DBT. Smob was found to be much lower for the presulfided samples, indicating a lower density of active sites (vacancies) in samples sulfided by hydrogen sulfide. 

The effect of methyl substituents is a widely discussed issue in HDS studies.Kabe et al. foundl different S-exchange values for 4-methyl-DBT, 4,6-dimethyl-DBT and DBT. The amount of exchangeable sulfur was higher in the case of DBT than that in the case of the methyl-substituted DBT-s. In agreement with this, the HDS rate of DBT was higher than those of the methyl substituted ones. The lower amount of mobile sulfur is presumably the consequence of steric hindrance caused by methyl groups, as it follows from the sequence of the HDS rates... 

Definite correlation was found between the amount of exchangeable or mobile sulfur and the HDS activity. A deeper insight can be gained into the mechanism of the sulfur-catalyst interaction and of the catalytic conversion, on the basis of these data. 

The most general observation is to be made on the basis of the tracer studies that only a part of surface sulfur is mobile or exchangeable on catalysts with sulfides and oxides. Supported Co and Ni contain only mobile sulfur, if sulfided by thiophene and contain both mobile and immobile sulfur, if they are sulfided by H2S/H2.[ 1 This is interpreted with the differences in the metal-sulfur bond strengths, as the edge S-atoms are held more weakly than the top S-atoms of the slab.I This follows from the results of some radiosulfur tracer studies of metal single crystals that different surface S-species are formed at different surface sulfur concentrations, and on different crystal faces of Pt, and... 

Sulfur uptake and exchange data 1 referred in Sec. 2.3 indicated substantial differences of alumina supported Pd and Pt, in comparison with the molybdena based catalysts. The mobile sulfur amounts were lower [0.25 Smob/Pd (or /(Pt)] than those experienced with the Mo and W based catalysts. Near to all sulfur, accommodated on the noble metal was mobile and — different from the Mo- and W-based catalysts — participated in the reaction the sulfur mobility was substantially higher due to the much lower S-Pd and S-Pt bond strengths. This indicates that the HDS mechanism, detailed above for Mo based catalysts cannot be valid for the noble metals. Again, the number of vacancies for bonding sulfur-organic compounds is not limiting, as in the case with monometallic Co and Ni. 

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