Designing and Probing Photovoltaic and Photocatalytic Materials

Meeting Program — November 2012

Jason B. Bax­ter
Depart­ment of Chem­i­cal and Bio­log­i­cal Engi­neer­ing
Drex­el Uni­ver­si­ty
Philadel­phia, PA

Abstract — The sun­light inci­dent on the earth pro­vides 10,000 times more pow­er than is need­ed to meet glob­al demand. How­ev­er, con­vert­ing this ener­gy into elec­tric­i­ty or fuels effi­cient­ly and cost effec­tive­ly remains a great chal­lenge. Nanos­truc­tured solar cells present oppor­tu­ni­ties to inex­pen­sive­ly con­vert sun­light to elec­tric­i­ty through the use of archi­tec­tures tai­lored on the nanome­ter to microm­e­ter length scale. Pla­nar solar cells are sub­ject to oppos­ing con­straints where thick films are required for light absorp­tion while thin­ner films are desir­able for effi­cient charge sep­a­ra­tion. Extreme­ly thin absorber (ETA) solar cells can decou­ple these con­straints by using a thin absorber at the inter­face between high­ly struc­tured p- and n-type lay­ers. In this talk, I will describe our work on ETA solar cells that use a thin CdSe coat­ing on a ZnO nanowire array to absorb light and inject elec­trons into the oxide. Ratio­nal design of these archi­tec­tures requires con­trol over mor­phol­o­gy and microstruc­ture of the mate­ri­als, as well as knowl­edge of mate­r­i­al prop­er­ties such as pho­toex­cit­ed car­ri­er life­times and mobil­i­ties. Our approach uti­lizes a com­bi­na­tion of solar cell mea­sure­ments and ultra­fast tran­sient absorp­tion spec­troscopy to under­stand the effects of CdSe thick­ness, anneal­ing con­di­tions, and inter­fa­cial treat­ments on the dynam­ics and effi­cien­cy of charge car­ri­er sep­a­ra­tion, and ulti­mate­ly on the solar-to-elec­tric ener­gy con­ver­sion effi­cien­cy. These stud­ies pro­vide guide­lines for archi­tec­ture design and mate­ri­als selec­tion for ETA solar cells.

Jason B. Baxter

Jason B. Bax­ter

Biog­ra­phy — Dr. Jason B. Bax­ter is an Assis­tant Pro­fes­sor in the Depart­ment of Chem­i­cal and Bio­log­i­cal Engi­neer­ing at Drex­el Uni­ver­si­ty in Philadel­phia, PA, where he began in 2007. He received his B.Ch.E. from the Uni­ver­si­ty of Delaware in 2000, where he did under­grad­u­ate research on dye sen­si­tized solar cells at the Insti­tute of Ener­gy Con­ver­sion under the guid­ance of Prof. T.W. Fras­er Rus­sell. He earned his Ph.D. in chem­i­cal engi­neer­ing from the Uni­ver­si­ty of Cal­i­for­nia San­ta Bar­bara in 2005. Advised by Prof. Eray S. Aydil and fund­ed by an NSF Grad­u­ate Research Fel­low­ship, he inves­ti­gat­ed growth and char­ac­ter­i­za­tion of ZnO nanowires and their appli­ca­tion in dye sen­si­tized solar cells. From 2005–2007, Dr. Bax­ter was an ACS Petro­le­um Research Fund Alter­na­tive Ener­gy Post­doc­tor­al Fel­low at Yale Uni­ver­si­ty. There he worked with Prof. Charles A. Schmut­ten­maer in the Chem­istry Depart­ment on the appli­ca­tion of time-resolved ter­a­hertz spec­troscopy to probe tran­sient pho­to­con­duc­tiv­i­ty in oxide thin films, nanopar­ti­cles, nanowires, and bulk crys­tals.

Dr. Baxter’s cur­rent research inter­ests are in design­ing, fab­ri­cat­ing, and prob­ing semi­con­duc­tor nano­ma­te­ri­als and thin films for solar ener­gy con­ver­sion. Most cur­rent efforts focus on solar-to-elec­tric ener­gy con­ver­sion, but the group has grow­ing inter­est in pho­to­cat­alyt­ic water split­ting for clean and renew­able hydro­gen pro­duc­tion. Var­i­ous projects in the group include extreme­ly thin absorber solar cells, organ­ic solar cells, microre­ac­tor depo­si­tion of grad­ed thin films for high-through­put char­ac­ter­i­za­tion, and ultra­fast pump-probe spec­troscopy to mea­sure charge car­ri­er dynam­ics. The gen­er­al focus of the group is on striv­ing to under­stand how mate­ri­als and inter­faces affect device per­for­mance, and how these mate­ri­als and inter­faces can be con­trolled dur­ing the fab­ri­ca­tion process. Low-tem­per­a­ture solu­tion pro­cess­ing meth­ods are used when­ev­er pos­si­ble to pro­vide a path­way to low-cost, scal­able man­u­fac­tur­ing.
Dr. Bax­ter advis­es a group of 4 PhD stu­dents, 2 MS stu­dents, and 8 BS stu­dents. He has pub­lished near­ly 25 papers, which have col­lec­tive­ly gar­nered well over 1000 cita­tions. He has been award­ed over $1 mil­lion in fund­ing as lead inves­ti­ga­tor and anoth­er $3 mil­lion as co-inves­ti­ga­tor. He received the NSF CAREER Award in 2009.

Pervasiveness of Surface Metal Oxide Phases In Mixed Oxide Catalysts

Meeting Program — October 2012

Israel E. Wachs
Operan­do Mol­e­c­u­lar Spec­troscopy & Catal­y­sis Lab­o­ra­to­ry
Depart­ment of Chem­i­cal Engi­neer­ing
Lehigh Uni­ver­si­ty
Beth­le­hem, PA 18015 USA

Abstract — Mixed oxide cat­alyt­ic mate­ri­als pos­sess two or more met­al oxide com­po­nents as found in bulk mixed met­al oxides (sto­i­chio­met­ric oxides as well as sol­id solu­tions), poly­oxo meta­lates (POMs), mol­e­c­u­lar sieves, zeo­lites, clays, hydro­tal­cites and sup­port­ed met­al oxides. Although it is now well estab­lished that two-dimen­sion­al sur­face met­al oxide phas­es are present for sup­port­ed met­al oxides on tra­di­tion­al sup­ports (e.g., Al2O3, TiO2, ZrO2, SiO2, etc.), it is not cur­rent­ly appre­ci­at­ed that such sur­face met­al oxide species or phas­es are also present for oth­er types of mixed oxides. For exam­ple, recent sur­face analy­ses have demon­strat­ed that sto­i­chio­met­ric bulk mixed met­al oxides also pos­sess sur­face met­al oxide phas­es that con­trol their cat­alyt­ic activ­i­ty. For exam­ple, the cat­alyt­ic active sites for methanol oxi­da­tion to formalde­hyde over the bulk Fe2(MoO4)3 mixed oxide cat­a­lyst are sur­face MoOx species and not the bulk Fe2(MoO4)3 phase as pre­vi­ous­ly thought in the catal­y­sis lit­er­a­ture. The nanome­ter sized clus­ters in POMs also pos­sess sur­face species when a sec­ond met­al oxide com­po­nent is intro­duced (e.g., H3+xPW12-xMxO40). Depo­si­tion of met­al oxides into mol­e­c­u­lar sieves, zeo­lites, clays and hydro­tal­cites also results in the met­al oxide addi­tive usu­al­ly being present as sur­face met­al oxide species that are the cat­alyt­ic active sites for many redox and acid reac­tions. The for­ma­tion of these sur­face met­al oxide phas­es is dri­ven by their low sur­face free ener­gy and low Tam­mann tem­per­a­ture for many met­al oxides of inter­est in catal­y­sis (e.g., VOx, MoOx, CrOx, ReOx, WOx, etc.).
Biog­ra­phy — Israel E. Wachs received his under­grad­u­ate edu­ca­tion at The City Col­lege of The City Uni­ver­si­ty of New York where he grad­u­at­ed with a B.E. (ChE) in June, 1973. He received sev­er­al recog­ni­tions upon grad­u­a­tion (AIChE Award for Out­stand­ing Senior, Heller Memo­r­i­al Award for Out­stand­ing Scholas­tic Achieve­ment, and White ChE Alum­ni Award). He con­tin­ued his grad­u­ate ChE edu­ca­tion at Stan­ford Uni­ver­si­ty under the men­tor­ship of Pro­fes­sor Robert J. Madix in the area of sur­face sci­ence, and grad­u­at­ed with a PhD (ChE) in 1978. His research find­ings are con­sid­ered the first appli­ca­tion of sur­face sci­ence to catal­y­sis, and his the­sis pub­li­ca­tions are exten­sive­ly cit­ed in the sur­face sci­ence and catal­y­sis lit­er­a­ture.
INDUSTRIAL YEARS (1977–1986) — Israel joined Exxon Research & Engi­neer­ing Com­pa­ny in their Cor­po­rate Research Labs towards the end of 1977. At Exxon, he was involved with many dif­fer­ent cat­alyt­ic tech­nolo­gies over the years (selec­tive oxi­da­tion, acid catal­y­sis, syn­the­sis of syn­thet­ic fuels, hydrodesul­fu­r­iza­tion (HDS) and hydro­car­bon con­ver­sion). He obtained 100 USA and inter­na­tion­al patents dur­ing his indus­tri­al career. One of his inven­tions on the selec­tive oxi­da­tion of o-xylene to phthal­ic anhy­dride became the lead­ing inter­na­tion­al indus­tri­al cat­a­lyst for this tech­nol­o­gy and is still used around the world. At Exxon, he received the Research Incen­tive Award for one of his inven­tions on the syn­the­sis of syn­thet­ic fuels and was also select­ed to be an Exxon Fel­low for the spring semes­ter of 1986 at Cal­i­for­nia Insti­tute of Tech­nol­o­gy (Cal­Tech). He depart­ed for acad­e­mia at the end of 1986.
ACADEMIC YEARS (1987-present) — He joined the Chem­i­cal Engi­neer­ing Depart­ment of Lehigh Uni­ver­si­ty in Jan­u­ary 1987. At Lehigh, he taught many dif­fer­ent cours­es over the years: Het­ero­ge­neous Catal­y­sis, Reac­tor Engi­neer­ing, Flu­id Mechan­ics, Pro­fes­sion­al Devel­op­ment, Unit Oper­a­tions, Envi­ron­men­tal Catal­y­sis, and Air Pol­lu­tion Con­trol. He set up a world-class catal­y­sis research lab­o­ra­to­ry focus­ing on mixed met­al oxide cat­alyt­ic mate­ri­als and their char­ac­ter­i­za­tion under reac­tion con­di­tions (in situ and operan­do spec­troscopy). These stud­ies have estab­lished the foun­da­tion for the molecular/electronic struc­ture – activity/selectivity rela­tion­ships and the mol­e­c­u­lar engi­neer­ing of mixed met­al oxide cat­a­lysts. The research per­formed by Wachs and his stu­dents is well known around the world. This is reflect­ed in the many nation­al and inter­na­tion­al hon­ors he has received over the years as well as ~17,000 cita­tions to his pub­li­ca­tions with an H-index of 70 (one of the high­est among het­ero­ge­neous catal­y­sis researchers).

The cur­rent focus of Wachs’ catal­y­sis lab­o­ra­to­ry is to devel­op cat­a­lyst char­ac­ter­i­za­tion tech­niques under reac­tion con­di­tions, referred to as operan­do spec­troscopy in the recent lit­er­a­ture. The term operan­do spec­troscopy implies that the cat­a­lyst char­ac­ter­i­za­tion infor­ma­tion is being con­duct­ed simul­ta­ne­ous­ly with online prod­uct analy­sis. Along these lines, Pro­fes­sor Wachs has devel­oped instru­men­ta­tion that can simul­ta­ne­ous­ly obtain Raman, IR and UV-vis spec­tro­scop­ic infor­ma­tion and prod­uct analy­sis with an online mass spectrometer/GC sys­tem. This cut­ting-edge instru­ment is allow­ing Pro­fes­sor Wachs’ catal­y­sis research group to rapid­ly devel­op molecular/electronic struc­ture – cat­alyt­ic activity/selectivity rela­tion­ships for many dif­fer­ent cat­alyt­ic mate­ri­als and reac­tions (selec­tive hydro­car­bon oxi­da­tion, hydro­car­bon con­ver­sion with sol­id acid cat­a­lysts, gas-to-liq­uids, pho­to­cat­alyt­ic split­ting of water, enzyme catal­y­sis, CO2 cap­ture, WGS, nanocatal­y­sis, ratio­nal cat­a­lyst design, etc.).

Water Gas Shift over Industrial Cu Catalysts: A Mechanistic and Microkinetic Investigation

Meetimg Program — September 2012

Ros­tam J. Madon
BASF Cor­po­ra­tion
25 Middlesex/Essex Turn­pike
Iselin, NJ, USA 08830

Abstract — Low tem­per­a­ture water gas shift (LTS) is a com­mer­cial­ly impor­tant reac­tion that takes place over a Cu-ZnO-Al2O3 cat­a­lyst. A large num­ber of fun­da­men­tal stud­ies have been car­ried out for this reac­tion includ­ing inves­ti­ga­tions of the reac­tion mech­a­nism as typ­i­fied by Refs. [1–4]. In short, dis­cus­sions have cen­tered around (a) the redox mech­a­nism in which adsorbed H2O is dis­so­ci­at­ed to O* and OH* and the O* is removed via CO* to form CO2 – where * is an active site, and (b) for­mate as a reac­tive inter­me­di­ate. Recent­ly, Gokhale et al. [5] using a DFT inves­ti­ga­tion of the LTS reac­tion on Cu(111) pro­posed a new mech­a­nism that involves a reac­tive sur­face car­boxyl. Our study is aimed at resolv­ing which ele­men­tary steps best describe the cat­alyt­ic cycle for the LTS reac­tion. To achieve this, we used the micro­ki­net­ic mod­el­ing method­ol­o­gy pio­neered by Dumesic [6], and ana­lyzed our reac­tiv­i­ty data using all ele­men­tary steps, includ­ing those that described the redox mech­a­nism, the for­mate mech­a­nism, and the car­boxyl mech­a­nism. Thus, we ensured that there was no bias towards any par­tic­u­lar reac­tions to fit our data. We found the closed cat­alyt­ic cycle for LTS on Cu con­sists of eight ele­men­tary steps that include the for­ma­tion of COOH*, and its reac­tion with OH* to form CO2* and H2O*. The cycle does not include the reac­tion of CO2* and H* to form sur­face for­mate. How­ev­er, this is an impor­tant side reac­tion, which ensures sig­nif­i­cant cov­er­age of biden­tate for­mate species on the Cu sur­face. Biden­tate for­mate is a spec­ta­tor species whose cov­er­age increas­es with increas­ing pres­sure and decreas­es with increas­ing tem­per­a­ture. In sum­ma­ry, our inves­ti­ga­tion demon­strates that the redox and for­mate mech­a­nisms are not rel­e­vant, and that the LTS cat­alyt­ic cycle involves the for­ma­tion and reac­tion of sur­face car­boxyl. Sev­er­al relat­ed aspects of the LTS reac­tion on Cu will also be dis­cussed.


  1. Ovesen, C. V., et al. J. Catal. 158, (1996), 170.
  2. Koryabki­na, N. A. et al. J. Catal. 217, (2003), 233.
  3. Rhodes, C., Hutch­ings G.J., and Ward A.M. Catal. Today 23, (1995), 43.
  4. Her­wi­j­nen, T.V., and de Jong, W. A. J. Catal. 63, (1980), 83 and 94.
  5. Gokhale, A. A., Dumesic, J. A., and Mavrikakis, M. J. Am. Chem. Soc. 130, (2008), 1402.
  6. Dumesic, J. A., et al. “The Micro­ki­net­ics of Het­ero­ge­neous Catal­y­sis”, Amer­i­can Chem­i­cal Soci­ety, Wash­ing­ton, D. C., 1993.

Speak­er Bio — Ross com­plet­ed his under­grad­u­ate stud­ies in chem­i­cal engi­neer­ing at the Uni­ver­si­ty Depart­ment of Chem­i­cal Tech­nol­o­gy, Mum­bai, India. He did his grad­u­ate work at Stan­ford Uni­ver­si­ty, obtain­ing his Ph.D. under the guid­ance of Pro­fes­sor Michel Boudart. After com­plet­ing his post-doc­tor­al work with Pro­fes­sor W. Kei­th Hall at the Uni­ver­si­ty of Wis­con­sin — Mil­wau­kee, Ross joined Exxon Research and Engi­neer­ing Com­pa­ny. After 12 years with Exxon’s Cor­po­rate Research Lab­o­ra­to­ries, Ross joined Engel­hard Cor­po­ra­tion. Ross recent­ly com­plet­ed 25 years at Engelhard/BASF Cor­po­ra­tion where he is cur­rent­ly a Senior Research Asso­ciate.

Ross has made pio­neer­ing con­tri­bu­tions to the chem­istry and engi­neer­ing of cat­alyt­ic process­es. Ear­ly in his career with his advi­sor Michel Boudart, he devel­oped an exper­i­men­tal method to address arti­facts in kinet­ic data; a test accept­ed today as being defin­i­tive for kinet­ic con­trol in catal­y­sis. At Exxon, Ross’ stud­ies in Fis­ch­er-Trop­sch syn­the­sis demon­strat­ed the cru­cial role intra­parti­cle dif­fu­sion played in enhanc­ing hydro­car­bon chain length and in chang­ing selec­tiv­i­ty. At Engel­hard, he devel­oped impor­tant con­cepts in flu­id cat­alyt­ic crack­ing to help design com­mer­cial cat­a­lysts. He elu­ci­dat­ed the mech­a­nism by which vana­di­um caus­es struc­tur­al degra­da­tion of Y zeo­lite in FCC cat­a­lysts, and used this under­stand­ing to min­i­mize its dele­te­ri­ous effect. His stud­ies pro­vid­ed a def­i­nite assess­ment of the role of ZSM-5 addi­tives in FCC to form light olefins and high octane gaso­line. And, he defined the crit­i­cal role rare earth cations play in Y-based FCC cat­a­lysts, demon­strat­ing how the pres­ence of rare earth influ­ences hydride trans­fer reac­tions and prod­uct selec­tiv­i­ty in FCC. Most recent­ly, at BASF, Ross, togeth­er with col­leagues in acad­e­mia, elu­ci­dat­ed the mech­a­nism of the water gas shift reac­tion on cop­per, evinc­ing para­me­ters that could sig­nif­i­cant­ly improve cat­alyt­ic activ­i­ty. Impor­tant­ly, though, Ross has used his con­cep­tu­al and mech­a­nis­tic approach to cat­a­lyst research to design com­mer­cial cat­a­lysts. He is the coin­ven­tor and devel­op­er of the Redux­ion – Max­ol® fam­i­ly of FCC cat­a­lysts and of the Iso­Plus® and Ultri­um® fam­i­lies; all of which have been used com­mer­cial­ly world­wide. He coin­vent­ed the Flex-Tec® resid crack­ing cat­a­lyst which has been wide­ly and suc­cess­ful­ly deployed in demand­ing resid cat-crack­ing process­es. And most recent­ly he has devel­oped sev­er­al cop­per based cat­a­lysts for the petro­chem­i­cal indus­try.

Ross chaired the 1996 Gor­don Research Con­fer­ence on Catal­y­sis, and in 2009 was award­ed the AIChE Catal­y­sis and Reac­tion Engi­neer­ing Divi­sion Prac­tice Award.