Electrochemistry at Well-Characterized Bimetallic Surfaces - Fuel Cell Catalysis: A Surface Science Approach - page 262

create novel, stable, and more efficient multimetallic catalysts. A number of procedures
can be employed for thin film (0.1 - 1 monatomic layers: 0.1 - 1 ML) depositions; how-
ever, the major division is between deposition in vacuum and in electrochemical
environments [Sieradzki et al., 1999; Tomellini and Attard, 1991].
In our recent work, we have used Pd deposited on Pt(hkl) in order to characterize
thin metal films [Arenz et al., 2002] and to test their catalytic activity (Fig. 8.16). We
have employed both methodologies: thermal evaporation in UHV and electrochemical
deposition. For the Pd/Pt(111) system, in situ SXS measurements have been used to
show that after the formation of 1 ML of pseudomorphic Pd film, three-dimensional
pseudomorphic crystalline islands of pure Pd begin to aggregate.
For the purpose of demonstrating the effects of surface coverage by Pd, u Pd , on the
rate of electro-oxidation of formic acid and the ORR, Fig. 8.17 reveals that the i versus
u Pd relationship again has a volcano-like form, with the maximum catalytic activity
being exhibited for 1 ML of Pd. The examples that we have given indicate that volcano
relationships are the rule rather than the exception, emphasizing the importance of a
systematic evaluation of the catalyst factors that control catalytic activity. A thorough
Figure 8.16 (a) Cyclic voltammetry for Pt(111)-nPd deposited in UHV and in 0.05 M H 2 SO 4
with 50 mV/s. (b - e) corresponding LIES spectra after nML Pd deposition (0 , n , 1); He þ
energy ¼ 1 keV. (e) Ball models assessed from the in situ SXS analysis.