Credit: © 2008 AAAS

Ethylene feedstocks for polymer synthesis must contain less than a few parts per million of acetylene, but those produced by steam cracking are generally contaminated with much more. This alkyne impurity can be removed by hydrogenation, but the reaction must be catalytically selective to stop at ethylene and not react further to produce ethane. Silver-modified palladium is the most commonly used catalyst for this, but its high cost has instigated a search to find possible replacements.

Now, Jens Nørskov and co-workers1 from the Technical University of Denmark have developed a method to guide the selection of efficient and selective hydrogenation catalysts. The catalyst must be able to adsorb and stabilize acetylene on its surface and destabilize the hydrogenated ethylene product, encouraging it to desorb before it can react further to form ethane. These properties are related to adsorption energies, and for quick and selective catalysis the energy of adsorption must be as negative as possible for acetylene and as positive as possible for ethylene. Because these values are positively correlated, a compromise must be found.

Using density functional theory, both adsorption energies can be predicted for a given metal catalyst, but this is computationally expensive. Nørskov and co-workers have simplified the computations by developing a scaling law that relates both energies to the adsorption energy of a methyl group for a given metal. Calculating the value for over 70 bimetallic compounds resulted in a host of possible catalysts. After considering cost and metal stability, the list was narrowed further and nickel–zinc compounds were chosen for additional investigation. By varying the zinc content, several alloys were made and their catalytic properties tested, one of which was found to have greater catalytic selectivity than that of the commonly used silver–palladium catalyst.