Catalyst research points the way to clean fuels

Paul Boughton

Dutch researchers believe they have solved the mystery of why the chemical catalysts used to clean fuels gradually become less active -- loss of sulphur atoms.

Crude oil contains sulphur compounds that 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.
Working at the Industrial Catalysts Group at Utrecht University in the Netherlands, researcher Bas Vogelaar established the most important causes for the deactivation of hydroprocessing catalysts in order to increase their lifetimes.
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.
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.
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. Vogelaar's work refutes 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 these vacant positions.
Vogelaar has used the results to produce a detailed model for the structure of the active phase. He has also developed a model to describe the desulphurisation reaction mechanism.
Meanwhile, research at the Technical University of Denmark (DTU) and Haldor Topsøe has found that by applying quantum theory it is possible to calculate the performance of catalysts used in everything from cars to the production of hydrogen.
Catalyts form the basis of more than 20 per cent of the world's industrial production, as well as a whole range of technologies that work towards creating a safer environment. One example is the catalytic converters that remove most of the pollution from today's cars.