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Chen agreed. “Without any one of these techniques, we would not have reached our conclusions. And we can only really do this in a national laboratory setting where there are facilities and expertise across all these areas,” he said.
So, what were those conclusions? The discovery of two key principles, or descriptors, that determine whether and how the metal atoms move, and how those shifts drive reaction selectivity.
The key principles are: 1) the “formation energy” of the bimetallic catalyst—how tightly bound together the two metals are, and 2) the binding energy between the catalyst and oxygen released from the CO2 during the reaction.
If the two metals are bound together strongly (e.g., when palladium is paired with cobalt), the catalyst won’t bind with the freed oxygen and will remain an alloy, as shown in the top half of the illustration. This catalytic arrangement favors the breaking of carbon-carbon bonds, selectively transforming CO2 and ethane into carbon monoxide and hydrogen gas—the components of syngas.
But if the catalyst’s affinity for freed oxygen is strong enough to overcome the formation energy of the alloy—as is the case for palladium paired with indium—the paired metal will move to the surface of the catalyst to form an oxide shell. That configuration favors breaking carbon-hydrogen bonds, driving the pathway that produces ethylene.
The other metals the scientists paired with palladium fell somewhere between these two extremes. Scientists used the full dataset to extract the two key principles.
“By using the descriptors we’ve identified, now we can help guide the design of catalysts for either pathway—to make either syngas or ethylene,” Chen said.
In addition, as Liu pointed out, “these are generalized descriptors, which means they are not only applicable for one or two specific catalysts. Our experiments and theory prove that this approach works for the palladium-based catalysts. We think that could be extended for other bimetallic catalysts, which is something we will be looking at in the future.”
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