The mechanism of CO2 reduction over Pd/Al2O3: a combined steady state isotope transient kinetic analysis (SSITKA) and operando FTIR investigation

2017 Spring Symposium

Xiang Wang, Hui Shi and János Szanyi, Insti­tute for Inte­grat­ed Catal­y­sis, Pacif­ic North­west Nation­al Lab­o­ra­to­ry, Rich­land, WA

Abstract — Under­stand­ing the crit­i­cal steps involved in the het­ero­ge­neous cat­alyt­ic CO2 reduc­tion has attract­ed a lot of atten­tion recent­ly. In order to ful­ly under­stand the mech­a­nism of this reac­tion the deter­mi­na­tion of both the rate-deter­min­ing steps and reac­tion inter­me­di­ates are vital. Steady-State Iso­topic Tran­sient Kinet­ic Analy­sis (SSITKA) is one of the most pow­er­ful tech­niques used to inves­ti­gate the ele­men­tary steps under steady-state reac­tion con­di­tions. This tech­nique pro­vides valu­able infor­ma­tion on mean res­i­dent life­time of sur­face inter­me­di­ates, sur­face con­cen­tra­tions of adsorbed reac­tant species and an upper bound of the turnover fre­quen­cy. Cou­pling SSITKA with operan­do-FTIR spec­troscopy allows us to dis­crim­i­nate between active and spec­ta­tor species present on the cat­alyt­ic sur­face under steady state reac­tion con­di­tions.  In the present work operan­do SSITKA exper­i­ments cou­pled with trans­mis­sion FTIR, mass spec­trom­e­try (MS) and gas chro­matog­ra­phy (GC) were per­formed to probe both the chem­i­cal nature and kinet­ics of reac­tive inter­me­di­ates over a Pd-Al2O3 cat­a­lysts and pro­vide a clear mech­a­nis­tic pic­ture of the CO2 hydro­gena­tion reac­tion by reveal­ing the rate-deter­min­ing steps for CH4 and CO pro­duc­tion.

Fig­ure 1 shows nor­mal­ized real-time sig­nals for the decay and increase of methane (a) and car­bon-monox­ide (b) in the efflu­ent at 533 K reac­tion tem­per­a­ture after the feed gas was switched at 0 s from CO2/H2/Ar mix­ture to 13CO2/H2 mix­ture.  With increas­ing tem­per­a­ture, the decay of CH4 and CO get faster.  By inte­gra­tion under the decay curves , the mean sur­face-res­i­dence times CH4 and  CO), the abun­dance of adsorbed sur­face inter­me­di­ates lead­ing to CH4 and CO prod­ucts  CH4 and  CO) at 533–573 K were cal­cu­lat­ed. At low tem­per­a­ture, CO2 metha­na­tion is slow­er than the reverse water-gas shift reac­tion, but became faster as the tem­per­a­ture was increased over 563 K.  The sim­i­lar appar­ent acti­va­tion ener­gies obtained for the hydro­gena­tion of adsorbed CO and for the for­ma­tion of CH4 indi­cates that the hydro­gena­tion of CO is the rate-deter­min­ing step dur­ing the CO2 metha­na­tion reac­tion. More­over, the sim­i­lar appar­ent acti­va­tion ener­gies esti­mat­ed for the con­sump­tion of adsorbed for­mates (FTIR) and for the for­ma­tion of CO (MS), indi­cates that the H-assist­ed decom­po­si­tion of for­mates is the rate deter­min­ing step in the reverse water gas shift reac­tion.  The rate-deter­min­ing step for CO for­ma­tion is the con­ver­sion of adsorbed for­mate, while that for CH4 for­ma­tion is the hydro­gena­tion of adsorbed car­bonyl. The bal­ance of the hydro­gena­tion kinet­ics between adsorbed for­mates and car­bonyls gov­erns the selec­tiv­i­ties to CH4 and CO. We applied this knowl­edge to design cat­a­lysts and achieved high selec­tiv­i­ties to desired prod­ucts. 

Fig­ure 1. Nor­mal­ized response of (a) CH4 and 13CH4 prod­ucts and (b) CO and 13CO prod­ucts as func­tions of time.

Biog­ra­phy — Dr. Szanyi‘s research is focused on sur­face sci­ence, spec­troscopy and kinet­ic stud­ies on het­ero­ge­neous cat­alyt­ic reac­tion sys­tems aimed at under­stand­ing struc­ture-reac­tiv­i­ty rela­tion­ships. In par­tic­u­lar, he is inter­est­ed in under­stand­ing the mech­a­nis­tic con­se­quences of very high (atom­ic) met­al dis­per­sion on dif­fer­ent sup­port mate­ri­als. Using a series of ensem­ble aver­aged spec­troscopy meth­ods he inves­ti­gates the fun­da­men­tal prop­er­ties of met­al atoms and small met­al clus­ters pre­pared under well con­trolled UHV con­di­tions. These results pro­vide infor­ma­tion on the ener­get­ics of the inter­ac­tions between high­ly dis­persed met­als and select­ed probe mol­e­cules. Apply­ing in situ RAIR spec­troscopy they study the bind­ing con­fig­u­ra­tions of adsor­bates to met­als, and iden­ti­fy sur­face species present on the met­al and sup­port mate­ri­als under ele­vat­ed reac­tant pres­sures. Simul­ta­ne­ous­ly, they are con­duct­ing detailed kinet­ics and operan­do spec­troscopy mea­sure­ments on mod­el high sur­face area sup­port­ed met­al cat­a­lysts using flow reac­tors and SSITKA/FTIR/MS tech­niques. These mea­sure­ments pro­vide detailed kinet­ic infor­ma­tion togeth­er with sur­face spe­ci­a­tion that allow them to great­ly enhance our mech­a­nis­tic under­stand­ing of het­ero­ge­neous cat­alyt­ic sys­tems, in par­tic­u­lar the reduc­tion of CO2. Dr Szanyi is also involved in research relat­ed to the fun­da­men­tal under­stand­ing of auto­mo­tive emis­sion con­trol catal­y­sis, con­duct­ing research in selec­tive cat­alyt­ic reduc­tion of NOx on zeo­lite-based cat­a­lysts, low tem­per­a­ture NO and CO oxi­da­tion on met­al oxides, and low tem­per­a­tures NOx and HC stor­age in zeo­lites.