Konrad Hayek Festschrift in Topics in Catalysis

Steam reforming of methanol on PdZn near-surface alloys on Pd(1 1 1) and Pd foil studied by in-situ XPS, LEIS and PM-IRAS

“…On the Pd surface, methanol was in part decomposed via CH 2 O to CO, producing hollow-bonded CO (characterized by the peak at 1873 cm -1 , indicative of ~0.4 ML coverage). As reported in [32,40,42] this leads, together with the parallel formation of surface CH x species, to a self-poisoning of the process at low temperature. On PdZn, no methanol dehydrogenation occurred at 300 K.…”

“…After annealing in 10 -6 mbar CO to 473 K, which in the range of the alloy stability window, the PdZn alloy was clearly established. This proves the homogeneity of the entire multilayer alloy surface, because in case of remaining Pd patches bridge-bonded CO (at 1955 cm -1 ) and on-top CO (at 2085 cm -1 ) would additionally occur at these p/T conditions [38][39][40] (note that the diameter of the IR beam is several mm). Taking into account thermal desorption data (T des of CO from PdZn is ~210 K,  sat =0.5 ML CO) [41], each Pd atom is occupied by a CO molecule.…”

“…5 c. In agreement with the LEIS/AES/XPS measurements, there is a stability window up to ~600 K. Interestingly, the intensity of the on-top CO species is highest for the corrugated "Zn-out/Pd-in" surface. Annealing to 623 and 673 K induced a decrease of the ~2070 cm -1 species, which was successively replaced by CO resonances characteristic of Zn-lean alloy (broad peak around 1930 cm -1 ) and Pd(111), the latter with bridge and on-top bonded CO at 1955 cm -1 and 2085 cm -1 , respectively, and a shoulder at 1890 cm -1 of hollow-bonded CO [38][39][40].…”

“…According to related theory work [11,16], the Cu-like electronic structure of the PdZn valence band is important for the selective control of the entire series of dehydrogenation steps, leading from methanol to CO. In particular, the PdZn 1:1 alloy was predicted to stabilize formaldehyde and suppress its further dehydrogenation toward formyl -CHO and finally CO (on Pd, CH x species induce a similar stabilization [32,40]). Our experiment strongly supports this concept, since formaldehyde formation is predominant in the stability range of the multilayer PdZn surface alloy.…”

“…up to reaction temperatures <600 K the typical 2070 cm -1 peak characterized the PdZn multilayer, whereas upon reaction at >600 K the PdZn multilayer decomposed and CO resonances typical of Zn-lean PdZn or of developing Pd patches/islands were observed after cooling. For example, after methanol dehydrogenation at 623 K, cooling to 300 K in the gas yielded a spectrum with peaks at 2070 cm -1 (still characteristic of PdZn) and at 1918 cm -1 (characteristic of bridging/hollow CO on Pd(111) [40] and/or of Zn-lean PdZn [44]). Intermediate species of methanol dehydrogenation such as CH 2 O could not be detected (the peak at 1034 cm -1 is due to the C-O stretching vibration of gas phase methanol), due to a presumably too small steady-state concentration below 565 K and its decomposition above 565 K. In addition, the Pd(111) single crystal used for PM-IRAS was <1 cm 2 so that the expected amount of CH 2 O is much smaller than for Pd foil in the allglass reactor.…”