The investigators discovered that treating a complex oxide crystal with either heat or chemicals caused different atoms to segregate on the surface, i.e., surface reconstruction. Those differences created catalysts with dissimilar behaviors, which encouraged different reaction pathways and ultimately yielded distinct products.
By using thermal and chemical treatments, catalyst designers may be able to drive industrially important chemical reactions to improve yields of desired products and reduce unwanted products so post-reaction separation costs can be significantly lowered.
“The surface of a catalyst is a playground for the molecules to do the chemical reaction,” said ORNL chemist Zili Wu, the senior author of two recent papers about the effect of the atomic composition of a catalyst surface on acid-base chemistry. “If you can tune your catalyst to obtain the desired product, i.e., achieve high selectivity, you will reduce the side products. Then you don’t need costly and energy-intensive downstream chemical separation as much.”
The researchers surveyed four catalysts of perovskite, a mixed oxide crystal made of cubic unit cells of the atomic composition ABO3, with A as a rare-earth metal cation (positively charged ion), B as a transition-metal cation and O as oxygen.
Treating a perovskite with heat resulted in a catalyst with more A atoms on its surface, scientists including first co-authors Guo Shiou Foo and Felipe Polo-Garzon reported in ACS Catalysis. Treating the same perovskite with chemicals instead produced more B atoms on the surface, scientists including first author Polo-Garzon subsequently reported in Angewandte Chemie International Edition.
To test the acid-base performance of the treated perovskite catalysts, the researchers studied a model reaction, the conversion of isopropanol—basically, rubbing alcohol. Depending on the pre-treatment conditions, the perovskite could selectively turn the alcohol into propylene, a building block of plastics, through a dehydration reaction, or acetone, an industrial solvent, through a dehydrogenation reaction.
The experiments showed a wide range of tunability was possible with different treatments. The same perovskite starting material, subjected to different treatments, could yield a desired product, such as acetone or propylene, in a wide range, from 25% to 90%.
The title of the Angewandte Chemie International Edition paper is “Controlling Reaction Selectivity through the Surface Termination of Perovskite Catalysts.”
The title of the ACS Catalysis paper is “Acid−Base Reactivity of Perovskite Catalysts Probed via Conversion of 2-Propanol over Titanates and Zirconates.”