Tuning the Electrocatalytic Oxygen Reduction Reaction Activity of PtCo Nanocrystals by Cobalt Concentration and Phase Transformation Methods

2018 Spring Symposium

Jen­nifer D. Lee, Ph.D. Can­di­date, Christo­pher B. Mur­ray Group, Depart­ment of Chem­istry, Uni­ver­si­ty of Penn­syl­va­nia

Abstract — The pro­ton exchange mem­brane fuel cell (PEMFC) is a crit­i­cal tech­nol­o­gy to enhance the clean, sus­tain­able pro­duc­tion and usage of ener­gy, but prac­ti­cal appli­ca­tion remains chal­leng­ing because of the high cost and low dura­bil­i­ty of the cath­ode cat­a­lysts that per­form oxy­gen reduc­tion reac­tion (ORR). Efforts have been placed on the study of intro­duc­ing first-row tran­si­tion met­als in Pt-M alloys to reduce the Pt load­ing and mod­u­late geo­met­ric, struc­tur­al and elec­tron­ic effects. To fur­ther improve the ORR reac­tion rate and cat­a­lysts sta­bil­i­ty, alloys that adopt an inter­metal­lic struc­ture, espe­cial­ly the tetrag­o­nal L10-PtM phase, has been one of the most promis­ing mate­ri­als. In this con­tri­bu­tion, monodis­perse PtCo nanocrys­tals (NCs) with well-defined size and Co com­po­si­tion are syn­the­sized via solvother­mal meth­ods. The trans­for­ma­tion from face-cen­tered cubic (fcc) to ordered face-cen­tered tetrag­o­nal (fct) struc­ture was achieved via ther­mal anneal­ing. Depend­ing on the selec­tion of trans­for­ma­tion meth­ods, dif­fer­ent degrees of order­ing were intro­duced and fur­ther cor­re­lat­ed with their ORR per­for­mance. A detailed study of the anneal­ing tem­per­a­ture and com­po­si­tion depen­dent degree of order­ing is also high­light­ed. This work pro­vides the insight of dis­cov­er­ing the opti­mal spa­tial dis­tri­b­u­tions of the ele­ments at the atom­ic lev­el to achieve enhanced ORR activ­i­ty and sta­bil­i­ty.

Commercial Perspective of Alternative Routes to Acrylic Acid Monomer

2018 Spring Symposium

Jin­suo Xu, The Dow Chem­i­cal Com­pa­ny, 400 Arco­la Rd, Col­legeville, PA 19426

Abstract — Acrylic acid and cor­re­spond­ing acry­lates are major monomers for a vari­ety of func­tion­al poly­mers used broad­ly in our dai­ly life such as coat­ing, sealant, and per­son­al care. The two-stage selec­tive oxi­da­tion of propy­lene to acrolein and then to acrylic acid was first com­mer­cial­ized in ear­ly 70s and quick­ly became the dom­i­nant route to acrylic acid. Dri­ven by feed­stock cost or avail­abil­i­ty or sus­tain­abil­i­ty, sig­nif­i­cant efforts from both indus­try and acad­e­mia were devot­ed to devel­op­ing alter­na­tive routes to acrylic acid. Cat­a­lyst plays crit­i­cal role in the key step of trans­form­ing dif­fer­ent raw mate­r­i­al into prod­uct effec­tive­ly, for exam­ple, mixed met­al oxides MoVTeN­bOx in propane selec­tive oxi­da­tion, sol­id acids in dehy­dra­tion of glyc­erin or 3-HP, and var­i­ous oxides in aldol con­den­sa­tion of acetic acid and formalde­hyde. This pre­sen­ta­tion will dis­cuss the progress of these major routes, the chal­lenges towards com­mer­cial­iza­tion, and poten­tial solu­tions.

Mechanisms, active intermediates, and descriptors for epoxidation rates and selectivities on dispersed early transition metals

2018 Spring Symposium

Daniel Bre­gante, Alay­na John­son, Ami Patel, David Fla­her­ty, Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing, Uni­ver­si­ty of Illi­nois, Urbana-Cham­paign

Abstract — Ear­ly tran­si­tion met­al atoms (groups IV-VI) dis­persed on sil­i­ca and sub­sti­tut­ed into zeo­lites effec­tive­ly cat­alyze the epox­i­da­tion of alkenes with hydro­gen per­ox­ide or alkyl per­ox­ide reac­tants, yet the under­ly­ing prop­er­ties that deter­mine the selec­tiv­i­ties and turnover rates of these cat­a­lysts are unclear. Here, a com­bi­na­tion of kinet­ic, ther­mo­dy­nam­ic, and in situ spec­tro­scop­ic mea­sure­ments show that when group IV — VI tran­si­tion met­als are dis­persed on sil­i­ca or sub­sti­tut­ed into zeo­lite *BEA, the met­als that form the most elec­trophilic sites give greater selec­tiv­i­ties and rates for the desired epox­i­da­tion path­way and present small­er enthalpic bar­ri­ers for both epox­i­da­tion and H2O2 decom­po­si­tion reac­tions.

In situ UV–vis spec­troscopy shows that these group IV and V mate­ri­als acti­vate H2O2 to form pools of hydroper­ox­ide and per­ox­ide inter­me­di­ates. Time-resolved UV–vis mea­sure­ments and the iso­mer­ic dis­tri­b­u­tions of cis-stil­bene epox­i­da­tion prod­ucts sug­gest that the active species for epox­i­da­tions on group IV and V tran­si­tion met­als are only M-OOH and M-(O2)2– species, respec­tive­ly. Mech­a­nis­tic inter­pre­ta­tions of turnover rates show that these group IV and V mate­ri­als cat­alyze epox­i­da­tions (e.g., of cyclo­hex­ene, styrene, and 1-octene) and H2O2 decom­po­si­tion through sim­i­lar mech­a­nisms that involve the irre­versible acti­va­tion of coor­di­nat­ed H2O2 fol­lowed by reac­tion with an olefin or H2O2. Epox­i­da­tion rates and selec­tiv­i­ties vary over five- and two-orders of mag­ni­tude, respec­tive­ly, among these cat­a­lysts and depend expo­nen­tial­ly on both the ener­gy for lig­and-to-met­al charge trans­fer (LMCT) and chem­i­cal probes of the dif­fer­ence in Lewis acid strength between met­al cen­ters. Togeth­er, these obser­va­tions show that more elec­trophilic active-oxy­gen species (i.e., low­er-ener­gy LMCT) are more reac­tive and selec­tive for epox­i­da­tions of elec­tron-rich olefins. The micro­p­ores of zeo­lites about active sites can serve to pref­er­en­tial­ly sta­bi­lize reac­tive states that lead to epox­i­da­tions by chang­ing the mean diam­e­ter of the pore or the den­si­ty of near­by silanol groups. Con­se­quent­ly, these prop­er­ties pro­vide oppor­tu­ni­ties to increase rates of epox­ide for­ma­tion over that with­in meso­porous sil­i­cas. Con­sis­tent­ly, H2O2 decom­po­si­tion rates pos­sess a weak­er depen­dence on the elec­trophilic­i­ty of the active sites and the sur­round­ing pore envi­ron­ment, which indi­cates that cat­a­lysts with both greater rates and selec­tiv­i­ties may be designed fol­low­ing these struc­ture-func­tion rela­tion­ships.

Low temperature NOx storage on zeolite supported Pd for low temperature diesel engine emission control

2018 Spring Symposium

L. Man­taroşie1, H.Y. Chen2, J. Col­lier1, D. Liu2, D. Duran-Mar­tin1, V. Novak1, R. R. Rajaram1 and D. Thompsett1
1John­son Matthey Tech­nol­o­gy Cen­tre, Son­ning Com­mon, Read­ing, RG4 9NH, UK
2John­son Matthey Inc., Emis­sion Con­trol Tech­nol­o­gy, Wayne, PA 19087, USA

Abstract — Recent leg­is­la­tion require­ments have turned con­trol­ling NOx emis­sions into one of the biggest tech­ni­cal chal­lenges fac­ing car man­u­fac­tur­ers [1]. At present, the main tech­nolo­gies avail­able for this appli­ca­tion are NO x stor­age and reduc­tion (NSR) or urea based selec­tive cat­alyt­ic reduc­tion (SCR) [2,3]. Both tech­nolo­gies can achieve high NO x reduc­tion effi­cien­cies once they reach their oper­at­ing tem­per­a­ture (typ­i­cal­ly 200°C or high­er). Dur­ing cold start, when the exhaust tem­per­a­ture is below 200°C, both sys­tems are less effi­cient at NOx removal.

To meet the NOx emis­sion stan­dards dur­ing the cold start (200°C) a new con­cept has been intro­duced: pas­sive NOx adsor­bers (PNA) [4]. These are mate­ri­als which store NOx (main­ly as NO) at low tem­per­a­tures and then ther­mal­ly release the stored NOx once the down­stream NOx reduc­tion cat­a­lyst (NSR or SCR) reach­es its oper­at­ing tem­per­a­ture.

This con­tri­bu­tion will report the remark­able abil­i­ty of zeo­lite sup­port­ed Pd to store NO with very high trap­ping effi­cien­cy at tem­per­a­tures below 200°C and con­di­tions that sim­u­late real exhaust from diesel engines. The study will focus on the char­ac­ter­i­za­tion of the Pd stor­age sites on zeo­lites com­pared to oxide sup­ports and under­stand­ing unique nature of the active species in these mate­ri­als. The prop­er­ties of pal­la­di­um sup­port­ed on three dif­fer­ent zeo­lites of var­i­ous pore sizes (CHA, MFI and BETA) will be com­pared to clas­si­cal oxid­ic sup­ports (Al2O3 and CeO2).

Also, with the aim of pro­vid­ing insight into the behav­iour of these mate­ri­als dur­ing engine oper­a­tion, the evo­lu­tion of the NO stor­age sites under var­i­ous gas feed com­po­si­tions has been probed through com­bined “operan­do” IR and XAS exper­i­ments. The find­ings of these study will be dis­cussed in rela­tion to the real­is­tic oper­a­tion of the nov­el PNA tech­nol­o­gy.

Ref­er­ences:
[1] L. Yang, V. Fran­co, A. Campestri­ni, J. Ger­man, P. Mock, ICCT report on NOX con­trol tech­nolo­gies for Euro 6 Diesel pas­sen­ger cars, 2015
[2] W.S. Epling, L.E. Camp­bell, A. Yez­erets, N.W. Cur­ri­er, J. E. Parks II, Catal­y­sis Reviews, 163 (2004) 46.
[3] I. Nova, E. Tron­coni (Eds.) Urea-SCR Tech­nol­o­gy for deNOx After Treat­ment of Diesel Exhausts, Springer New York, 2014.
[4] E. Melville, R.J. Bris­ley, O. Keane, P.R. Phillips, and E.H. Mountstevens, US patent 8, 105, 559, 2007

Influence of Confining Environment Polarity on Ethanol Dehydration Catalysis by Lewis Acid Zeolites

2018 Spring Symposium

Jason S. Bates and Raja­mani Gounder, Charles D. David­son School of Chem­i­cal Engi­neer­ing, Pur­due Uni­ver­si­ty, West Lafayette, IN

Abstract — The dif­fer­ent reac­tiv­i­ty of Lewis acid sites (M) in zeo­lite frame­works, when con­fined with­in non-polar (hydropho­bic) or polar (hydrophilic) sec­ondary envi­ron­ments, can arise from dif­fer­ences in com­pet­i­tive inhi­bi­tion by sol­vents, sol­vent-medi­at­ed mech­a­nisms, and extend­ed sol­vent struc­tures.3 Frame­work Lewis acid cen­ters also adopt open ((HO)-M-(OSi≡)3) and closed (M-(OSi≡)4) con­fig­u­ra­tions that show dif­fer­ent reac­tiv­i­ty for Baey­er-Vil­liger oxi­da­tion, glu­cose iso­mer­iza­tion, and aldol con­den­sa­tion.5 Here, we inter­ro­gate the reac­tiv­i­ty of Sn cen­ters iso­lat­ed with­in Beta zeo­lites using bimol­e­c­u­lar ethanol dehy­dra­tion to diethyl ether (404 K). Sn sites in open and closed con­fig­u­ra­tions, quan­ti­fied from IR spec­tra of adsorbed CD3CN before and after reac­tion, con­vert to struc­tural­ly sim­i­lar inter­me­di­ates dur­ing ethanol dehy­dra­tion catal­y­sis (404 K) and revert to their ini­tial con­fig­u­ra­tions after regen­er­a­tive oxi­da­tion treat­ments (21% O2, 803 K). Dehy­dra­tion rates (404 K, 0.5–35 kPa C2H5OH, 0.1–50 kPa H2O) mea­sured on ten low-defect (Sn-Beta-F) and high-defect (Sn-Beta-OH) zeo­lites were described by a rate equa­tion derived from mech­a­nisms iden­ti­fied by DFT cal­cu­la­tions, and sim­pli­fied using micro­ki­net­ic mod­el­ing to iden­ti­fy kinet­i­cal­ly-rel­e­vant path­ways and inter­me­di­ates. Polar hydrox­yl defect groups locat­ed in con­fin­ing envi­ron­ments pref­er­en­tial­ly sta­bi­lize reac­tive (ethanol-ethanol) and inhibito­ry (ethanol-water) dimer­ic inter­me­di­ates over monomer­ic ethanol inter­me­di­ates. As a result, equi­lib­ri­um con­stants (404 K) for ethanol-water and ethanol-ethanol dimer for­ma­tion are 3–4× high­er on Sn-Beta-OH than on Sn-Beta-F, con­sis­tent with
insights from sin­gle-com­po­nent and two-com­po­nent adsorp­tion mea­sure­ments. Intrin­sic dehy­dra­tion rate con­stants (404 K) were iden­ti­cal among Sn-Beta-OH and Sn-Beta-F zeo­lites; thus, mea­sured dif­fer­ences in dehy­dra­tion turnover rates sole­ly reflect dif­fer­ences in preva­lent sur­face cov­er­ages of inhibito­ry and reac­tive dimer­ic inter­me­di­ates at active Sn sites. The con­fine­ment of Lewis acidic bind­ing sites with­in sec­ondary envi­ron­ments of dif­fer­ent defect den­si­ty con­fers the abil­i­ty to dis­crim­i­nate sur­face inter­me­di­ates on the basis of polar­i­ty, pro­vid­ing a design strat­e­gy to accel­er­ate turnover rates and sup­press inhi­bi­tion by water.

Ref­er­ences:
[1] Con­rad, S.; Wolf, P.; Müller, P.; Orsted, H., Her­mans, I. Chem­CatChem. 2017, 9, 175–182.
[2] Li, G.; Pid­ko, E.A.; Hensen, E.J.M. Catal. Sci. Tech­nol. 2014, 4, 2241–2250.
[3] Har­ris, J.W.; Cor­don, M.J.; Di Iorio, J.R.; Vega-Vila, J.C.; Ribeiro, F.H.; Gounder, R. J. Catal.
2016, 335, 141–154.
[4] Boronat, M.; Con­cep­ción, P.; Cor­ma, A.; Renz, M.; Valen­cia, S. J. Catal. 2005, 234, 111–118.
[5] Lewis, J.D.; Ha, M.; Luo, H.; Fauch­er, A.; Michaelis, V.K.; Román-Leshkov, Y. ACS Catal. 2018,
3076–3086.
[6] Bukows­ki, B.C.; Bates, J.S.; Gounder, R.; Gree­ley, J. J. Catal. 2018, under review.

Fabrication of Nano-Structured Catalyst Supports by ALD

2018 Spring Symposium

Ray­mond J. GorteChem­i­cal & Bio­mol­e­c­u­lar Engi­neer­ing, Uni­ver­si­ty of Penn­syl­va­nia, Philadel­phia, PA

Abstract — Inter­ac­tions between a tran­si­tion-met­al cat­a­lyst and its sup­port can strong­ly alter the sta­bil­i­ty and activ­i­ty of the cat­a­lyst. Impor­tant exam­ples include sup­port effects with ceria and the so-called “Intel­li­gent Cat­a­lysts” in which the met­al can be redis­persed by reversible ex-solu­tion from a per­ovskite lat­tice. How­ev­er, the sur­face areas of these func­tion­al sup­ports are often too low or unsta­ble; and, in the case of per­ovskites, the length scales for ingress and egress may be too long to take advan­tage of the effect. We are address­ing these issues by deposit­ing very thin films of var­i­ous func­tion­al oxides, ~0.5 to 2 nm thick, onto high-sur­face-area sup­ports, includ­ing Al2O3 and MgAl2O4 , by Atom­ic Lay­er Depo­si­tion. We have demon­strat­ed that a wide range of oxides can be deposit­ed as dense, uni­form, con­for­mal films on var­i­ous sup­ports. The films exhib­it sur­pris­ing­ly good ther­mal sta­bil­i­ty and pro­vide cat­alyt­ic prop­er­ties sim­i­lar to that observed with bulk oxides, but with high­er sur­face areas.

Spectroscopic Technique Development for Understanding Solvent Effects in Liquid Phase Reactions

Meeting Program — April 2018

Nicholas Gould — Stu­dent Speak­er

Advi­sor: Bingjun Xu
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Uni­ver­si­ty of Delaware
 

Abstract — Bio­mass con­ver­sion reac­tions are fre­quent­ly con­duct­ed in a sol­vent, due to the high­ly oxy­genat­ed nature of the feed­stock.1,2 Thus, het­ero­ge­neous cat­alyt­ic active sites exist at a sol­id-liq­uid inter­face, where the sol­vent can mod­i­fy sur­face and adsor­bate ener­get­ics. Even when the sol­vent does not play a direct role in the reac­tion mech­a­nism, it can sta­bi­lize or desta­bi­lize adsor­bates, inter­me­di­ates, and tran­si­tion states, often lead­ing to marked­ly dif­fer­ent rates and selec­tiv­i­ties between sol­vent choic­es.3–5 How­ev­er, sol­vent effects are poor­ly under­stood because cat­a­lyst char­ac­ter­i­za­tion tech­niques, such as probe mol­e­cule adsorp­tion in FTIR, are most often con­duct­ed under vac­u­um or in vapor phase.6,7 Fur­ther, most stud­ies on sol­vent effects focus on screen­ing sol­vents via cat­alyt­ic activ­i­ty test­ing, where mul­ti­ple fac­tors that can influ­ence reac­tiv­i­ty exist simul­ta­ne­ous­ly: com­pet­i­tive adsorp­tion, sta­bi­liza­tion of reac­tants and tran­si­tion states, and phase equi­lib­ria dif­fer­ences. Thus, there is cur­rent­ly a need for exper­i­men­tal tech­niques capa­ble of extract­ing fun­da­men­tal ther­mo­dy­nam­ic prop­er­ties of sol­vents in sim­ple sys­tems, with the end goal of decou­pling the effects of sol­vent in cat­alyt­ic activ­i­ty tests.8

Atten­u­at­ed total reflec­tion (ATR) fouri­er trans­form infrared spec­troscopy (FTIR) was used to char­ac­ter­ize zeo­lites with probe mol­e­cules in the pres­ence of sol­vent. The ATR-FTIR was fur­ther devel­oped into a quan­ti­ta­tive tech­nique, with a pro­ce­dure for deter­min­ing extinc­tion coef­fi­cients for adsorbed pyri­dine on zeo­lites in the pres­ence of sol­vent.9 This allowed for quan­ti­ta­tive com­par­isons of the effect of sol­vent on probe mol­e­cule uptake and pro­to­na­tion in zeo­lite pores. Ongo­ing appli­ca­tions of the ATR-FTIR cell include adsorp­tion isotherms, dif­fu­sion mea­sure­ments, and tem­per­a­ture pro­grammed des­orp­tion (TPD) in porous mate­ri­als in liq­uid phase. Fur­ther, the effect of sol­vent on charge sta­bi­liza­tion in zeo­lite pores was stud­ied using a home­made TPD set up under back pres­sur­ized, flow­ing sol­vent. Pre­lim­i­nary pyri­dine des­orp­tion tem­per­a­tures from an H/ZSM-5 sam­ple reveal that the abil­i­ty of a sol­vent to sta­bi­lize pyri­dini­um ions decreas­es in the order: water > ace­toni­trile > alka­ne ≈ vac­u­um.

Ref­er­ences:

  1. G. W. Huber, S. Ibor­ra and A. Cor­ma, Chem. Rev., 2006, 106, 4044–4098.
  2. D. M. Alon­so, S. G. Wettstein and J. A. Dumesic, Green Chem., 2013, 15, 584–595.
  3. M. A. Mellmer, C. Sen­er, J. M. R. Gal­lo, J. S. Luter­bach­er, D. M. Alon­so and J. A. Dumesic, Angew. Chemie — Int. Ed., 2014, 53, 11872–11875.
  4. P. J. Dyson and P. G. Jes­sop, Catal. Sci. Tech­nol., 2016, 6, 3302–3316.
  5. J. F. Haw, T. Xu, J. B. Nicholas and P. W. Goguen, Nature, 1997, 389, 832–835.
  6. F. Zaera, Chem. Rev., 2012, 112, 2920–2986.
  7. H. Shi, J. Lercher and X.-Y. Yu, Catal. Sci. Tech­nol., 2015, 5, 3035–3060.
  8. N. Gould and B. Xu, Chem. Sci., 2018, 9, 281–287.
  9. N. S. Gould and B. Xu, J. Catal., 2017, accept­ed.