Zn Adsorption on Pd(111):  ZnO and PdZn Alloy Formation

Abstract

The adsorption and thermal desorption of Zn and ZnO on Pd(111) was studied in the temperature range between 300 and 1300 K with TDS, LEED, and CO adsorption measurements. At temperatures below 400 K, multilayer growth of Zn metal on the Pd(111) surface takes place. At a coverage of 0.75 ML of Zn, a p(2 x 2)-3Zn LEED structure is observed. Increasing the coverage to 3 ML results in a (1 x 1) LEED pattern arising from an ordered Zn multilayer on Pd(111). Thermal desorption of the Zn multilayer state leads to two distinct Zn desorption peaks: a low-temperature desorption peak (400-650 K) arising from upper Zn layers and a second peak (800-1300 K) originating from the residual 1 ML Zn overlayer, which is more strongly bound to the Pd(111) surface and blocks CO adsorption completely. Above 650 K, this Zn adlayer diffuses into the subsurface region and the surface is depleted in Zn, as can be deduced from an increased amount of CO adsorption sites. Deposition of >3 ML of Zn at 750 K leads to the formation of a well-ordered Pd-Zn alloy exhibiting a (6 x 4 square root 3/3)rect. LEED structure. CO adsorption measurements on this surface alloy indicate a high Pd surface concentration and a strong reduction of the CO adsorption energy. Deposition of Zn at T > 373 K in 10(-6) mbar of O2 leads to the formation of an epitaxial (6 x 6) ZnO overlayer on Pd(111). Dissociative desorption of ZnO from this overlayer occurs quantitatively both with respect to Zn and O2 above 750 K, providing a reliable calibration for both ZnO, Zn, and oxygen coverage.

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

“…Prior to catalytic measurements, the preparation of a suitable "multilayer" PdZn alloy both on the surface of Pd foil and Pd(111) was examined, building upon knowledge obtained by previous structural investigations of Zn deposition and alloy formation on Pd(111), by Bayer et al [5], Gabasch et al [6], Weirum et al [34], Kratzer et al [35] and Stadlmayr et al [12,36]. To avoid sticking coefficients of Zn smaller than 1 on the quartz microbalance, which would lead to underestimation of Zn thickness as reported in [35], the microbalance was cleaned by Ar + sputtering and cooled to below 270 K during evaporation.…”

“…Related experimental studies of the Zn/Pd(111) system using XPS reported different stages of the alloying process as a function of temperature, among them the formation of the theoretically predicted 1:1 surface composition [5,11,12]. Studies performed by some of us revealed tetragonal AuCu-type PdZn nanoparticles [2] and a tetragonal PdZn nearsurface alloy formed on Pd(111) at elevated temperatures and high Zn coverages beyond 5 ML, which closely resembled the active tetragonal bulk alloy state [6]. Since the electronic structure of the 1:1 PdZn alloy is similar to that of metallic copper, a phenomenological interpretation of the observed analogous catalytic performance of both Cu and PdZn was suggested [5,[11][12][13][14][15].…”

“…Reduction of Pd supported on ZnO in H 2 at T~523 K leads to the formation of particularly stable 1:1 PdZn nanoparticles crystallizing in the tetragonal AuCu-type lattice [1,2,4]. Both experimental and theoretical studies have already addressed the important issue of surface composition and segregation trends of the Pd-Zn intermetallic system (near-surface alloy on a single crystal and 1:1 PdZn nanoparticles on ZnO [5][6][7][8][9][10][11]) as a function of the Zn:Pd ratio (Zn coverage) and the temperature. For the specific 1:1 stoichiometry, DFT calculations identified a 1:1 ratio of Pd and Zn as the most stable configuration also in the surface layer, at least from the viewpoint of surface/bulk energetics that were not affected by high gas pressures or elevated temperature [8,10].…”

Growth and structural stability of well-ordered PdZn alloy nanoparticles

“…A key point for understanding the catalytic pecularities of the above-mentioned Pd-Zn alloy particles is also to determine their surface composition. Recent experiments in our laboratory 33 show that a well-ordered (6x4/√3) Pd-Zn layer forms on Pd(111) under UHV conditions at elevated temperature.Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac) …”

“…In particular, the striking differences between the Pd/SiO 2 and Pd/ZnO/SiO 2 systems demonstrated in this work may lead to a better understanding of the drastic activity and selectivity differences between Pd/SiO 2 and Pd/ZnO reported in the literature 13,23 . According to recent single crystal experiments 33,38 and theory studies 38,39 it seems that this very stable PdZn alloy phase of stoichiometric surface composition is stable over a wide range of temperatures.…”