Loss of sulphur atom reduces activity of catalyst

March 16, 2005

Chemical catalysts used to produce clean fuels gradually become less active. Dutch researcher Bas Vogelaar believes that the loss of sulphur atoms might be an important cause of this. He investigated hydroprocessing catalysts which remove sulphur compounds from petrol and diesel.

Crude oil contains sulphur compounds which form sulphur oxides during the combustion process. These sulphur oxides are an important source of acid rain. Hydroprocessing catalysts are used in oil refineries to remove these sulphur compounds in order to produce clean fuels. After two years of use, the hydroprocessing catalysts have lost so much of their activity that they need to be replaced. This is an intensive and expensive operation. Vogelaar established the most important causes for the deactivation of hydroprocessing catalysts in order to increase the lifetime of these.

A hydroprocessing catalyst consists of a carrier of aluminium oxide, to which various active metals are added. A so-called active phase is created on the surface of the catalyst. The chemical reactions during which the sulphur is removed take place here. The active phase consists of a combination of molybdenum, sulphur and nickel or cobalt.

Self-cleaning

Small graphite-like particles accumulating on the hydroprocessing catalysts are one of the causes of the decrease in activity. Vogelaar discovered that this 'coke' mainly precipitates on the carrier. He believes that the active phase has a 'self-cleaning' effect, which can counteract the precipitation of coke. The researcher discovered that under model conditions, the activity of the catalysts mainly decreases due to the loss of sulphur from the catalyst. This process might also play a role in the deactivation of these catalysts during the production of clean fuels.

Desulphurisation

Catalysts can convert sulphur compounds in two ways. The sulphur atom is directly removed from the compound or a chemical reaction (hydrogenation) takes place after which the atom is removed. The results of Vogelaar refute the generally accepted theory that for both mechanisms a sulphur atom must first of all be removed from the active phase. For the direct removal of sulphur a so-called 'vacant position' is indeed necessary. The hydrogenation step however takes place on sulphur atoms at the edge of the active phase and not on the 'vacant positions'. Vogelaar has used these results to produce a detailed model for the structure of the active phase. He has also developed a model which describes the desulphurisation reaction mechanism.
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Netherlands Organization for Scientific Research

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