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New Insights into Industrial Catalysts
August 1, 2013   
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Warsaw researchers say they have discovered previously unknown behavior in catalysts used in industry to increase the rate of chemical reactions.

Specifically, palladium-copper catalysts tend to change their structure during the activation process before the reaction, as a result of which the reaction is catalyzed by a different catalyst than that originally prepared for it, say the researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw.

Catalysts are used in important environmental protection operations such as the removal of nitrates from ground water and the removal of chlorine from dry cleaning waste.

Popular catalysts include well-known silica-supported palladium-copper catalysts. A team of researchers led by Prof. Zbigniew Karpiński from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw has shown that these catalysts behave differently than researchers have assumed up to now.

A catalyst is a chemical that speeds up a chemical reaction by participating in it and is regenerated after the reaction is completed. Apart from speeding up the reaction time, using a catalyst may result in increased reaction selectivity, i.e. a higher yield of the target product as compared with the by-products.

Selective catalysts are usually systems composed of more than one metal. Palladium catalysts are often modified with copper. Active catalyst nanoparticles are deposited on a silica (SiO2) support. Before the reaction, the palladium-copper (Pd-Cu) catalyst prepared in such a way is heated at a high temperature in the presence of hydrogen. The purpose of the operation is to activate the catalyst, which means to provide the catalyst’s atoms with energy allowing them to take part in the final reaction.

The proportion of the metals used in the catalyst has a substantial effect on the operational efficiency of a bimetallic catalyst. “While taking X-ray measurements we discovered something researchers were not aware of so far,” says Magdalena Bonarowska, Ph.D., from the Institute of Physical Chemistry. The results indicated that, during the activation process in the hydrogen atmosphere at temperatures above 400 degrees Celsius, palladium interacts with silica in the support and thus escapes from the active catalyst nanoparticles, according to Bonarowska.

“A catalyst that was originally composed of, say, 75-percent palladium and 25-percent copper can have a strongly disturbed ratio of these metals—for instance fifty-fifty,” Bonarowska says. “Moreover, its crystal structure changes. This means that the reaction will be catalyzed by a catalyst that is different from the one originally prepared.”

Palladium losses from the catalyst’s active nanoparticles lead to faster catalyst deactivation. Practically speaking, this translates into additional, substantial costs related to the unloading of a chemical reactor and regeneration or even replacement of the catalyst inside the reactor, Bonarowska says.

Karpiński, the research team leader, said, “It’s not uncommon that silica-supported palladium-copper catalysts must be activated at temperatures as high as 500°C. The operation aims at mixing both metals dispersed on the surface of the support to the highest degree possible. It is, however, worth considering if—provided the target reaction allows for that—the activation of the catalyst at lower temperatures, but for instance for a longer time, wouldn’t be a better solution.”

Palladium-copper catalysts on various supports, including silica, are used for the removal of nitrates from ground water and for selective reduction of numerous organic chemicals, including the reduction of nitro compounds to amines, and unsaturated hydrocarbons (e.g., acetylene to ethylene or butadiene to butene). They are also used for electrocatalytic oxidation of methanol and hydrogen-assisted dechlorination, i.e. the removal of chlorine from harmful organic chemicals with the use of hydrogen.
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