CHARACTERIZATION OF PDCU(110) SINGLE-CRYSTAL SURFACE COMPOSITIONS DURING CO CHEMISORPTION

The PdCu(110) single crystal alloy surfaces (bulk Pd/Cu = 1:1) were pr epared by sputtering and annealing at different temperatures. Surfaces with Cu/Pd contents ranging from similar to 100% Pd to similar to 100 % Cu at the top surface layer could be obtained. AES with surface regi on (SR) analysis of similar to 4 layers, LEED and TDS of CO with top s urface layer (TL) analysis were the methods used for characterization. LEED of clean surfaces prepared at low temperatures (< 550 K) showed the Pd(1 x 2) structure. By treating the alloy at higher temperatures (> 550 K), Cu segregation occurs and the structure changes to (? x I) and then to Cu(1 x 2). For a Pd-rich surface with Cu/Pd(SR) less than or equal to 0.1, CO TDS showed five desorption states, three of them a ppearing at 450, 400 and 340 K respectively, thus resembling those fro m CO/pure Pd(110). The other two new CO-Pd binding states appear at 25 0 and 190 K, which are characteristic for the alloy surface. The appea rance of the 250 K maximum indicates a ligand effect in CO-desorption. At Cu/Pd (SR) of greater than or equal to 0.3 the CO desorption from Pd bridge-sites is absent in the spectra. The other two high temperatu re desorption sites shift to lower temperatures (by similar to 50 K) i ndicating the ensemble effect. With increasing Cu concentration in the TL, TDS show an additional peak at 160 K. Another fascinating propert y of this alloy is the possibility of preparing the TL with similar to 100% Pd, while in the next three subsurface layers (3SSL) the Cu/Pd r atio ranges from 0.1 to 2. For the Pd-rich TL, the CO initial sticking coefficient increases with increasing Cu/Pd ratio in the 3SSL. Detail ed analysis of all these results revealed that the bcc lattice model i s essential for the PdCu (110) alloy surface at Cu/Pd(SR) > 0.2.