DFT Investigation of Hydrogenation and Dehydrogenation Reactions on Binary Metal Alloys: Effect of Surface Ensembles and Composition

Meeting Program — March 2015

Fuat E Celik
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Rut­gers, The State Uni­ver­si­ty of New Jer­sey

Fuat Celik
In sup­port­ed met­al cat­a­lysts, the trade­off between activ­i­ty and selec­tiv­i­ty presents an impor­tant chal­lenge for cat­a­lyst design. By allow­ing two dis­sim­i­lar met­als, we can attempt to tune the selec­tiv­i­ty of the cat­a­lyst by enhanc­ing bond-for­ma­tion and des­orp­tion rates through the addi­tion of a less-reac­tive ele­ment, while main­tain high bond dis­so­ci­a­tion activ­i­ty from the more active met­al. The result­ing cat­a­lyst prop­er­ties depend strong­ly on the cat­a­lyst com­po­si­tion and ratio of the two met­als (elec­tron­ic effect), but may also depend on the local struc­ture of sur­face ensem­bles of the alloy com­po­nents (geo­met­ric effect). In this talk we will explore two exam­ples of bina­ry alloys where sur­face com­po­si­tion and geom­e­try play an impor­tant role in deter­min­ing the selec­tiv­i­ty of the cat­a­lyst through den­si­ty func­tion­al the­o­ry (DFT).

In the first exam­ple, we have exam­ined the effect of plat­inum tin alloy struc­ture and com­po­si­tion on the kinet­ics and ther­mo­dy­nam­ics of dehy­dro­gena­tion and coke for­ma­tion path­ways dur­ing light alka­ne dehy­dro­gena­tion. Light alka­ne dehy­dro­gena­tion to olefins can add sig­nif­i­cant val­ue to hydro­car­bon process­es that gen­er­ate ethane and propane by con­vert­ing low val­ue com­mod­i­ty fuels to high-val­ue chem­i­cal and poly­mer pre­cur­sors. Sup­port­ed Pt cat­a­lysts are known to be active but show sig­nif­i­cant coke for­ma­tion and deac­ti­va­tion, which can be alle­vi­at­ed by alloy­ing with Sn and oth­er main group ele­ments. We aim to under­stand how the struc­ture and com­po­si­tion of these alloys affect their abil­i­ty to sup­press coke for­ma­tion. We inves­ti­gate the poten­tial ener­gy sur­faces from ethane along the desired path­way to ethene, and along the unde­sired path­ways towards sur­face carbon/coke. The effect of Pt/Sn ratio and sur­face geom­e­try is inves­ti­gat­ed. As com­pared to pure Pt, bond scis­sion is more dif­fi­cult on the alloys and des­orp­tion is more facile, and both effects are enhanced as three-fold hol­low sites con­sist­ing of only Pt atoms are elim­i­nat­ed.

In the sec­ond exam­ple, we eval­u­ate Au/Ni near-sur­face alloys as poten­tial oxy­gen reduc­tion cat­a­lysts for the direct syn­the­sis of hydro­gen per­ox­ide from O2 and H2, there­by avoid­ing the cur­rent anthraquinone process. While Au may have high­er O-H bond for­ma­tion activ­i­ty, it is a poor O2-dis­so­ci­a­tion cat­a­lyst, and like­wise Ni is very effec­tive at O2-dis­so­ci­a­tion but not oxy­gen hydro­gena­tion. Alloy­ing Au with Ni(111) low­ers H2 dis­so­ci­a­tion bar­ri­er while keep­ing the O2 dis­so­ci­a­tion bar­ri­er large rel­a­tive to O2 hydro­gena­tion. Des­orp­tion of H2O2 is sim­i­lar­ly com­pet­i­tive with H2O2 dis­so­ci­a­tion on alloy sur­faces. How­ev­er, the selec­tiv­i­ty for the OOH rad­i­cal remains a chal­lenge, with bar­ri­er­less O-O bond dis­so­ci­a­tion and large (1.3 eV) hydro­gena­tion bar­ri­ers. We fur­ther inves­ti­gate how the Au/Ni sur­face may rearrange itself to regen­er­ate three-fold hol­lows of Ni atoms in the pres­ence of strong­ly adsorb­ing sur­face species.