Ap­ply­ing Strain makes for bet­ter cat­a­lysts- New The­ory

Chemical Industry Digest - - New Developments -

Brown Univer­sity re­searchers have de­vel­oped a new the­ory to ex­plain why stretch­ing or com­press­ing metal cat­a­lysts can make them per­form bet­ter, as pub­lished in the jour­nal Na­ture Catal­y­sis.

Cat­a­lysts are sub­stances that speed up chem­i­cal re­ac­tions. The vast ma­jor­ity of in­dus­trial catal­y­sis in­volves solid sur­faces, of­ten met­als that cat­alyze re­ac­tions in liq­uids or gases. Re­searchers are also in­ter­ested in us­ing metal cat­a­lysts to con­vert car­bon diox­ide into fu­els, make fer­til­iz­ers from at­mo­spheric ni­tro­gen and drive re­ac­tions in fuel-cell cars.

It’s been shown in re­cent years that ap­ply­ing a strain to a cat­a­lyst can tune its re­ac­tiv­ity. The the­ory pre­dicts that ten­sile strain should in- crease re­ac­tiv­ity, while com­pres­sion should re­duce it. How­ever, Peter­son and his group kept en­coun­ter­ing sys­tems that aren’t eas­ily ex­plained by the the­ory. That got the re­searchers think­ing about a new way to view the prob­lem. The new the­ory fo­cuses on the me­chan­ics of how mol­e­cules in­ter­act with a cat­a­lyst’s atomic lat­tice.

Peter­son and his team showed that mol­e­cules bound to a cat­a­lyst’s sur­face will tend to ei­ther push atoms in the lat­tice apart or pull them closer to­gether, de­pend­ing upon the char­ac­ter­is­tics of the mol­e­cules

and the bind­ing sites. The dif­fer­ent forces pro­duced by mol­e­cules have in­ter­est­ing im­pli­ca­tions for how ex­ter­nal strain should af­fect a cat­a­lyst’s re­ac­tiv­ity. It sug­gests that ten­sion should make a cat­a­lyst more re­ac­tive to mol­e­cules that nat­u­rally want to push the lat­tice apart. At the same time, ten­sion should de­crease re­ac­tiv­ity for mol­e­cules that want to pull the lat­tice to­gether. Com­pres­sion - squeez­ing the lat­tice has an in­verse ef­fect.

The new the­ory pre­dicts a way to break tra­di­tional scal­ing re­la­tions be­tween cat­a­lysts and dif­fer­ent types of mol­e­cules.

This new the­ory sug­gests that strain can break those scal­ing re­la­tions by en­abling a cat­a­lyst to si­mul­ta­ne­ously bind one chem­i­cal more tightly and an­other more loosely, de­pend­ing on the chem­i­cal’s nat­u­ral in­ter­ac­tion with the cat­a­lyst’s atomic lat­tice and the way that the strain field is engi­neered on the cat­a­lyst sur­face.

Peter­son’s team has started putting to­gether a data­base of com­mon re­ac­tion chem­i­cals and their in­ter­ac­tions with dif­fer­ent cat­a­lyst sur­faces. That data­base could serve as a guide for find­ing re­ac­tions that could ben­e­fit from strain and the break­ing of scal­ing re­la­tions.

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