Well-defined, highly uniform metallic nano-structures as selective heterogeneous catalysts, photo-electro-catalysts, and platforms for chemical characterization

2009 Spring Symposium

 
Suljo Lin­ic
Depart­ment of Chem­i­cal Engi­neer­ing
Uni­ver­si­ty of Michi­gan
Ann Arbor, MI


Abstract — The cen­tral objec­tive of our research effort is to employ com­bined experimental/theoretical approach­es to devel­op pre­dic­tive the­o­ries of het­ero­ge­neous catal­y­sis and to apply these the­o­ries to for­mu­late ener­gy-effi­cient, selec­tive, and sta­ble cat­a­lysts. We are moti­vat­ed by a real­iza­tion that recent sci­en­tif­ic advance­ments, main­ly in the area of mol­e­c­u­lar sci­ence, have poten­tial to bring a rev­o­lu­tion­ary trans­for­ma­tion to the field of dis­cov­ery in het­ero­ge­neous cat­a­lysts.

I will present our recent work where we explored poten­tial uti­liza­tion of high­ly uni­form metal­lic nano-struc­tured mate­ri­als as selec­tive het­ero­ge­neous cat­a­lysts. The advan­tage of these mate­ri­als com­pared to con­ven­tion­al cat­alyt­ic mate­ri­als is that their struc­ture can be con­trolled with almost atom­ic pre­ci­sion, and that it is pos­si­ble to syn­the­size high­ly homo­ge­neous struc­tures. We demon­strat­ed some of these advan­tages recent­ly when we showed that well-defined, tai­lored Ag nano-struc­tures are much more selec­tive in het­ero­ge­neous epox­i­da­tion of eth­yl­ene to form eth­yl­ene oxide (EO) (Eth­yl­ene + ½O2 → EO) than con­ven­tion­al indus­tri­al cat­a­lysts.

We showed using quan­tum chem­i­cal Den­si­ty Func­tion­al The­o­ry (DFT) cal­cu­la­tions, where we stud­ied crit­i­cal ele­men­tary chem­i­cal steps that gov­ern the selec­tiv­i­ty to EO in the process, that the Ag(100) sur­face should be inher­ent­ly more selec­tive than the Ag(111) sur­face. We note that cat­alyt­ic par­ti­cles, syn­the­sized using con­ven­tion­al syn­the­sis pro­ce­dure and cur­rent­ly used in com­mer­cial eth­yl­ene epox­i­da­tion process, are dom­i­nat­ed by the (111) sur­face. To syn­the­size Ag nano-struc­tures which are dom­i­nat­ed with the Ag(100) faces, we employed a syn­the­sis pro­ce­dure which uses organ­ic sta­bi­liz­er mol­e­cules to direct the growth of the nano-struc­ture in a par­tic­u­lar direc­tion and to con­trol the sur­face facets that ter­mi­nate the nano-struc­ture. This syn­thet­ics strat­e­gy allowed us to syn­the­size well-defined and high­ly uni­form Ag nano-wires and nano-cubes which are dom­i­nat­ed by the (100) facet. Sub­se­quent exper­i­ments showed that Ag nano-wires and nano-cube cat­a­lysts can achieve selec­tiv­i­ty to EO, which is, at dif­fer­en­tial con­ver­sion, by ~ 40 % high­er than for con­ven­tion­al Ag cat­a­lysts.

We have also recent­ly start­ed explor­ing these metal­lic nano-struc­tures as pos­si­ble plat­forms for chem­i­cal char­ac­ter­i­za­tion. The fea­tures of these nano-struc­tures that are par­tic­u­lar­ly appeal­ing are: (i) the nanos­truc­tures are well defined on atom­ic lev­el, and their sur­face to vol­ume ratio is fair­ly high, which makes these struc­tures inher­ent­ly bet­ter suit­ed for the stud­ies of sur­face chem­i­cal process­es com­pared to tra­di­tion­al sin­gle crys­tal mod­el sys­tems, which are while very well defined, char­ac­ter­ized by low sur­face to vol­ume ratio. (ii) we can syn­the­size the nanos­truc­tures with high degree of uni­for­mi­ty in size and shape, which rules out pos­si­ble effects due to diver­si­ty in size and shape, i.e. these, (iii) the nanos­truc­tures are effec­tive scat­ter­ers of elec­tro­mag­net­ic radi­a­tion which make them suit­able as plat­forms for a num­ber of chem­i­cal char­ac­ter­i­za­tion tech­niques includ­ing sur­face enhanced Raman (SERS) or IR spec­tro­scopies. We will demon­strate the util­i­ty of the nano-struc­tures for chem­i­cal char­ac­ter­i­za­tion by a way of an exam­ple, where we mon­i­tored in-situ eth­yl­ene epox­i­da­tion.

We will also show that the well-defined metal­lic nano-struc­tures exhib­it inter­est­ing prop­er­ties when exposed to UV and vis­i­ble light. We will show how these char­ac­ter­is­tics can be used to design nov­el pho­to-elec­tro-cat­alyt­ic mate­ri­als and process­es.

Speaker’s Biog­ra­phy — Suljo Linc came to the Unit­ed States from Bosnia under the aus­pices of a Soros Foun­da­tion Fel­low­ship, here he received a BS degree in Physics from West Chester Uni­ver­si­ty (1998) , and a Ph.D. in Chem­i­cal Engi­neer­ing under Pro­fes­sor Mark Barteau (2003) where he inves­ti­gat­ed the the­o­ret­i­cal and exper­i­men­tal aspects of alkene par­tial oxi­da­tion on sil­ver. He accept­ed a post­doc­tor­al posi­tion in Matthias Scheffler’s The­o­ry Group at the Fritz Haber Insti­tute of the Max Planck Soci­ety in Berlin, and in 2004 took a posi­tion in the Depart­ment of Chem­i­cal Engi­neer­ing at the Uni­ver­si­ty of Michi­gan. Suljo has received a num­ber of awards, includ­ing NSF Career Award in 2006, and Young Sci­en­tist Prize from the Coun­cil of the Inter­na­tion­al Asso­ci­a­tion of Catal­y­sis Soci­eties, Paris, France, July 2004. Suljo’s research inter­ests include fuel cells, chi­ral syn­the­sis, car­bon catal­y­sis, catal­y­sis at nano-scales, and the fun­da­men­tals of sur­face activ­i­ty and selec­tiv­i­ty.