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Managing Editor  | October 2017

Customized catalysts boost product yields and reduce unwanted byproducts


Two studies from the U.S. Department of Energy Oak Ridge National Laboratory (Oak Ridge, Tenn.) discovered that treating a complex oxide crystal with heat or chemicals causes different atoms to segregate on the surface of the crystals (surface reconstruction), which changes the properties of the catalysts and yields different end products.

 

oak_ridge_600

How perovskite catalysts are made and treated changes their surface compositions and ultimate product yields. (Oak Ridge National Laboratory, U.S. Dept. of Energy; illustrator Adam Malin)

 

According to a report from Oak Ridge, the studies demonstrated that catalysts designers could drive reactions that are important to specific industries, improve yields of desired products, and limit the amount of unwanted byproduct that results from the reactions. This will also reduce the need for costly (in money and energy) downstream chemical processes.

 

“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,” the article explained.

 

Using heat on the perovskite produced a chemical with more A atoms on the surface, while chemically treating the perovskite led to a catalyst with more B atoms on the surface.

 

“To test the acid-base performance of the treated perovskite catalysts, the researchers studied a model reaction, the conversion of isopropanol—basically, rubbing alcohol,” the article said. “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 proved that a high amount of tunability was possible depending on the type of treatment method. According to the article, “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 percent.”

 

Researchers used X-ray diffraction to characterize the bulk of the catalyst and scanning transmission electron microscopy, adsorption microcalorimetry, infrared spectroscopy, and low-energy ion scattering were all used to study the effect of treatments on the surface of the catalyst.

 

The research on chemical treatment of the catalysts was published in Angewandte Chemie International Edition.

 

The research on heat treatment of the catalyst was published in ACS Catalysis. The abstract stated:

 

“Although perovskite catalysts are well-known for their excellent redox property, their acid–base reactivity remains largely unknown.

 

“To explore the potential of perovskites in acid–base catalysis, we made a comprehensive investigation in this work on the acid–base properties and reactivity of a series of selected perovskites, SrTiO3, BaTiO3, SrZrO3, and BaZrO3, via a combination of various approaches including adsorption microcalorimetry, in situ FTIR spectroscopy, steady state kinetic measurements, and density functional theory (DFT) modeling. The perovskite surfaces are shown to be dominated with intermediate and strong basic sites with the presence of some weak Lewis acid sites, due to the preferred exposure of SrO/BaO on the perovskite surfaces as evidenced by low energy ion scattering (LEIS) measurements.

 

“Using the conversion of 2-propanol as a probe reaction, we found that the reaction is more selective to dehydrogenation over dehydration due to the dominant surface basicity of the perovskites. Furthermore, the adsorption energy of 2-propanol (ΔHads,2–propanol) is found to be related to both a bulk property (tolerance factor) and the synergy between surface acid and base sites.

 

“The results from in situ FTIR and DFT calculations suggest that both dehydration and dehydrogenation reactions occur mainly through the E1cB pathway, which involves the deprotonation of the alcohol group to form a common alkoxy intermediate on the perovskite surfaces.

 

“The results obtained in this work pave a path for further exploration and understanding of acid–base catalysis over perovskite catalysts.”

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