On the reaction mechanism of desulfurisation

Data discussed in Sec. 3.1 seem to indicate that HDS activity is associated with sulfur vacancies, as exchange requires sulfur attachment to the surface. Exchange mechanism via surface SH groups was also assumed by Massoth and Zeuthen.I 1 

It has been found in HDS of H-labelled thiophene that the radioactivity of hydrogen sulfide formed from the first pulses, was much lower than that of the C-4 hydrocarbons produced. A measurable radioactivity appeared only upon treating the catalyst with H-labelled thiophene for a 

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. 

Data for sulfur uptake and HDS activity of precious metals promoted 

The high activity of monometallic catalysts in HDS supports the afore given explanation that the absence of surface zero valent Co and Ni results in a decreased Scat, Sexc and HDS activity. 

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