Sulfur-Resistant Pd-Alloy Membranes for H2 Purification

Meeting Program — March 2013

James B. Miller
Depart­ment of Chem­i­cal Engi­neer­ing
Carnegie Mel­lon Uni­ver­si­ty

Abstract — Sep­a­ra­tion of hydro­gen from mixed gas streams is a key unit oper­a­tion in the gen­er­a­tion of car­bon-neu­tral fuels and elec­tric­i­ty from fos­sil- and bio-derived feed­stocks. Dense Pd mem­branes have received sig­nif­i­cant atten­tion for the sep­a­ra­tion appli­ca­tion in advanced gasi­fi­ca­tion process­es. Pd’s near-per­fect selec­tiv­i­ty reflects its unique inter­ac­tions with H2: mol­e­c­u­lar H2 dis­so­ci­ates on the cat­alyt­ic Pd sur­face to cre­ate H-atoms, which dis­solve into and dif­fuse through the Pd bulk, to even­tu­al­ly recom­bine on the down­stream side of the mem­brane. In prac­tice, Pd suf­fers from sev­er­al lim­i­ta­tions, includ­ing high cost, struc­tur­al insta­bil­i­ty, and deac­ti­va­tion by minor com­po­nents of the mixed gas, most notably H2S. Alloy­ing with minor com­po­nents, such as Cu, can be an effec­tive strat­e­gy for improv­ing mem­brane per­for­mance.

In col­lab­o­ra­tion with sci­en­tists at the Nation­al Ener­gy Tech­nol­o­gy Lab­o­ra­to­ry, we have com­bined mem­brane per­for­mance test­ing, com­pu­ta­tion­al mod­el­ing, and H2 dis­so­ci­a­tion activ­i­ty char­ac­ter­i­za­tion to pro­vide fun­da­men­tal under­stand­ing of the inter­ac­tions of H2 and H2S with Pd and PdCu alloys. We have shown that H2S influ­ences mem­brane per­for­mance by two dis­tinct mech­a­nisms: sur­face deac­ti­va­tion, which inhibits the dis­so­cia­tive adsorp­tion of H2, and reac­tion with the met­al to form a low-per­me­abil­i­ty sul­fide scale. The mech­a­nism that dom­i­nates depends on both alloy com­po­si­tion and oper­at­ing con­di­tions. Sig­nif­i­cant­ly, the sur­face of the sul­fide scale is itself active for H2 dis­so­ci­a­tion. Atom­istic mod­el­ing of the dis­so­ci­a­tion process pro­vides con­text for this obser­va­tion, show­ing that while the ener­getic bar­ri­er for H2 dis­so­ci­a­tion is high­er on Pd4S than on Pd, there exist reac­tion tra­jec­to­ries with rel­a­tive­ly low bar­ri­ers that can sus­tain the sep­a­ra­tion sequence at accept­able rates. Microkinec­tic analy­sis of H2-D2 exchange con­duct­ed over Pd and a series of PdCu alloys, both in the pres­ence and absence if H2S, con­firms this find­ing and pro­vides insight into the role of the Cu minor com­po­nent in impart­ing S-tol­er­ance to the alloy.

Final­ly, we have devel­oped a high through­put capa­bil­i­ty to explore alloy prop­er­ties over broad, con­tin­u­ous com­po­si­tion space, based on Com­po­si­tion Spread Alloy Film (CSAF) libraries of mod­el sep­a­ra­tion alloys. CSAFs are thin (~100 nm) films with com­po­si­tions that vary con­tin­u­ous­ly across the sur­face of a com­pact (~1cm2) sub­strate. Using a unique mul­ti­chan­nel microre­ac­tor for spa­tial­ly resolved mea­sure­ment of reac­tion kinet­ics across CSAF sur­faces, we have char­ac­ter­ized the kinet­ics of H2-D2 exchange across con­tin­u­ous Pd1-xCux and Pd1-x-yCuxAuy com­po­si­tion space.

James B. Miller

James B. Miller

Biog­ra­phy — Jim Miller is Asso­ciate Research Pro­fes­sor of Chem­i­cal Engi­neer­ing at Carnegie Mel­lon Uni­ver­si­ty, where he stud­ies advanced mate­ri­als for ener­gy-relat­ed appli­ca­tions in sep­a­ra­tions, catal­y­sis and chem­i­cal sens­ing. Jim earned BS, MS and PhD degrees at Carnegie Mel­lon and an MS at the Uni­ver­si­ty of Pitts­burgh. Before join­ing the fac­ul­ty in 2006, he worked in indus­try as a devel­op­er of cat­a­lysts, cat­alyt­ic process­es and chem­i­cal sen­sors for over 25 years. Jim is a two-time past pres­i­dent of the Pitts­burgh-Cleve­land Catal­y­sis Soci­ety; he recent­ly led the Society’s suc­cess­ful efforts to obtain tax exempt sta­tus in antic­i­pa­tion of NAM 2015. He is a win­ner of AIChE’s 2010 “Shin­ing Star” in recog­ni­tion of his vol­un­teer work in the Pitts­burgh Local Sec­tion.