Formic Acid Decomposition on Bulk Metal Catalysts

2012 Spring Symposium

Yadan Tang, Charles A. Roberts, Israel Wachs
Depart­ment of Chem­i­cal Engi­neer­ing
Lehigh Uni­ver­si­ty

Abstract — Mea­sured trends in cat­alyt­ic reac­tiv­i­ty over vary­ing met­al cat­a­lysts have been used to facil­i­tate the opti­miza­tion of bimetal­lic catalysts.[1] An impor­tant exam­ple of such a trend is the Sachtler-Fahren­fort vol­cano curve, in which reac­tiv­i­ty of met­al sur­faces for formic acid decom­po­si­tion is plot­ted against the sta­bil­i­ty of inter­me­di­ates, i.e. the bulk heat of for­ma­tion of the for­mate on a spe­cif­ic met­al surface.[2] It is ques­tion­able, how­ev­er, to cor­re­late a bulk prop­er­ty with cat­alyt­ic reac­tiv­i­ty, a process that occurs exclu­sive­ly at the sur­face. The cur­rent study inves­ti­gates the cor­re­la­tion between formic acid decom­po­si­tion and reac­tiv­i­ty of bulk met­al cat­a­lysts (i.e. Fe, Ru, Pd, Pt, Au, Ag, Ni, Co, and Cu) using mod­ern tech­niques such as in situ dif­fuse reflectance infrared Fouri­er trans­form spec­troscopy (DRIFTS) and tem­per­a­ture pro­grammed sur­face reac­tion (TPSR) spec­troscopy. In situ DRIFTS mon­i­tors the for­mate struc­ture on the sur­face of bulk met­al cat­a­lysts dur­ing the adsorp­tion and decom­po­si­tion of formic acid. By uti­liz­ing a tem­per­a­ture ramp­ing pro­ce­dure, in situ DRIFTS also pro­vides insights into ther­mal sta­bil­i­ty of adsorbed for­mates. TPSR spec­troscopy detects the tem­per­a­ture at which the peak activ­i­ty for decom­po­si­tion of the adsorbed for­mates occurs, there­fore pro­vid­ing a mea­sure of the reac­tiv­i­ty of the met­al sur­face. In situ DRIFTS and TPSR spec­troscopy exper­i­ments agree with the pre­vi­ous report­ed find­ing that the decom­po­si­tion of HCOOH pro­ceeds via two steps: 1) for­ma­tion of sur­face adsorbed for­mate (HCOO-M) inter­me­di­ates; and 2) decom­po­si­tion of for­mate inter­me­di­ates into gas phase prod­ucts such as CO, CO2, H2 and H2O.[3] The for­mate struc­ture on var­i­ous met­al cat­a­lysts are iden­ti­fied and assigned based on a pre­vi­ous study on formic acid via high res­o­lu­tion elec­tron ener­gy loss spec­troscopy (HREELs).[3] The cur­rent study finds that the for­mate species on Fe, Ru, Pd, Pt and Au are bridged; on Co and Ni are mon­oden­tate; and on Cu and Ag are con­vert­ed from mon­oden­tate to bridged at high­er tem­per­a­ture in agree­ment with HREE­Ls work on both Cu(100) and Ag(110).[4] The TPSR decom­po­si­tion tem­per­a­tures, Tp, were plot­ted ver­sus the bulk heat of for­ma­tion of for­mates report­ed by Sachtler and Farenfort[2]. Rather than a vol­cano trend, the plot is observed to con­tain two dis­tinct lin­ear rela­tion­ships indi­cat­ing that trends in reac­tiv­i­ty of met­als should be eval­u­at­ed based on sur­face prop­er­ties rather than bulk.

[1] Jacob­sen, Claus J. H., Dahl, S., Clausen, Bjerne S., Bahn, S., Logadot­tir, A., and Nørskov, Jens K. J. Am. Chem. Soc. 123, 8404 (2001).
[2] Sachtler, W.M.H., and Fahren­fort, J., in “Pro­ceed­ings, 2nd Inter­na­tion­al Con­gress on Catal­y­sis, Paris, 1960,” p.831. Tech­nip, Paris, 1961.
[3] Colum­bia, M.R., Thiel, P.A. J. Eelec­tro­an­a­lyt­i­cal Chem. 369, 1–14 (1994).
[4] Sexton,B.A. Surf. Sci., 88, 319 (1979).

Speaker’s Biog­ra­phy — Yadan Tang is a grad­u­ate stu­dent in Chem­istry at Lehigh Uni­ver­si­ty, advised by Pro­fes­sor Israel Wachs. She received her B.S. in Mate­r­i­al Sci­ence and Engi­neer­ing Depart­ment at East Chi­na Univ. of Sci­ence and Tech­nol­o­gy in 2006. She received her M.S. in Chem­istry Depart­ment at Lehigh Univ in 2010. Since joined in Wachs group in 2011, she has been involved in formic acid decom­po­si­tion on bulk met­al cat­a­lyst and sup­port­ed met­al oxides on zeo­lite.