Mechanistic Studies of the Steam Reforming of Methanol on Pd/ZnO Catalysts

2007 Spring Symposium

Eseoghene Jero­ro and John M. Vohs
Depart­ment of Chem­i­cal and Bio­mol­e­c­u­lar Engi­neer­ing
Uni­ver­si­ty of Penn­syl­va­nia
Philadel­phia, PA 19104–6393 USA

Abstract — Methanol and oth­er alco­hols are poten­tial bio-renew­able sources of hydro­gen. The use of alco­hols, how­ev­er, as a source of H2 or for H2 stor­age requires sta­ble reform­ing cat­a­lysts that have high activ­i­ty at low tem­per­a­tures. One such cat­a­lyst that has received much atten­tion for steam reform­ing of CH3OH (SRM) [CH3OH + H2O → CO2 + 3H2] is Pd sup­port­ed on ZnO. Pd/ZnO cat­a­lysts have unusu­al­ly high selec­tiv­i­ty (>;95%) for the pro­duc­tion of CO2 and H2 from methanol, in spite of the fact that bulk Pd exhibits near­ly 100 % selec­tiv­i­ty for the dehy­dro­gena­tion of CH3OH to CO and H2 under typ­i­cal SRM con­di­tions. Iwasa and oth­ers have demon­strat­ed that par­tial alloy­ing of the Pd with Zn is required to obtain a high­ly selec­tive Pd/ZnO SRM cat­a­lyst. While the impor­tance of alloy for­ma­tion has been estab­lished, the mech­a­nism by which Zn alters the selec­tiv­i­ty to pro­duce CO2 rather than CO is not under­stood. Iwasa et al. have pro­posed, how­ev­er, that the dra­mat­ic change in selec­tiv­i­ty may result from the desta­bi­liza­tion of η2-CH2O inter­me­di­ates in which the car­bonyl group is par­al­lel to the sur­face and bond­ing is by both the C and O atoms in favor of η1-CH2O in which the car­bonyl is per­pen­dic­u­lar to the sur­face and only the oxy­gen inter­acts with the met­al. Pre­sum­ably this lat­er species is more resis­tant to dehy­dro­gena­tion and reacts with hydrox­yl groups to pro­duce CO2 and H2.

In order to elu­ci­date how alloy­ing with Zn affects the CH3OH dehy­dro­gena­tion activ­i­ty of Pd, the struc­ture and reac­tiv­i­ty of mod­el cat­a­lysts con­sist­ing of sub­mono­lay­er amounts of Zn sup­port­ed on a Pd(111) sin­gle crys­tal have been inves­ti­gat­ed. I will present tem­per­a­ture pro­grammed des­orp­tion (TPD) data for the reac­tion of methanol and formalde­hyde on Pd(111) as a func­tion of Zn cov­er­age as well as results of a high res­o­lu­tion elec­tron ener­gy loss spec­troscopy (HREELS) study of the bond­ing con­fig­u­ra­tions of CO, CH2O, and CH3OH on Zn/Pd(111) sur­faces. TPD data for the reac­tion of methanol on Pd sup­port­ed on ZnO(0001) sin­gle crys­tal sur­faces will also be pre­sent­ed. The TPD data shows that the for­ma­tion of the Pd Zn alloy severe­ly shuts down the dehy­dro­gena­tion of CH3OH and CH2O which cor­re­sponds to the increase in η1-CH2O seen in the HREELS. Over­all, these results pro­vide fun­da­men­tal insight into how Zn alters the reac­tiv­i­ty of Pd for the dehy­dro­gena­tion of CH3OH and this insight is use­ful in elu­ci­dat­ing the mech­a­nism of the steam reform­ing of methanol on Pd/ZnO cat­a­lysts.