Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 539

experimental conditions (in particular, the nature of the supporting electrolyte and the
electrode potential), methanol dissociative adsorption may follow an O22H bond
cleavage pathway, resulting in a reactive methoxy adsorbate and avoiding surface poi-
soning with CO ads . There is also spectroscopic evidence for reaction intermediates
such as CHO ads and COOH ads , which demonstrates that some methanol molecules
are partially dehydrogenated and yield formaldehyde or formic acid [Iwasita and
Vielstich, 1986; Beden et al., 1987a, b; Wasmus et al., 1995; Zhu et al., 1999].
The MOR is a surface-sensitive reaction [Clavilier et al., 1981a]. The influence of
surface crystallography on the partitioning between the “direct” and the “indirect” path-
ways was investigated by Housmans and co-workers using electrochemistry, mass
spectrometry, and modeling [Housmans et al., 2006; Housmans and Koper, 2003].
The first observation of PSEs for the MOR on Pt was reported by Attwood and
co-workers in 1980. They found a bell-shaped curve for the dependence of SA on
particle size, with the maximum at about 2 nm [Attwood et al., 1980]. Takasu and
co-workers reported a decrease in SA with decreasing particle size for Pt particles
deposited on GC by vacuum deposition, as depicted in Fig. 15.11 [Yahikozawa
et al., 1991]. This PSE was detected in both HClO 4 and H 2 SO 4 and at various electro-
lyte concentrations [Yahikozawa et al., 1991]. The PSE was confirmed by Kabbabi
Figure 15.11 Specific activities for the MOR on Pt/Vulcan XC-72 (1) [Guerin et al., 2004]
and Pt/GC (2) [Takasu et al., 2000] in aqueous electrolytes as functions of particle size. For
curve 1, currents were measured after 1 minute at 0.6 V vs. RHE in 1 M CH 3 OH þ 0.5 M
H 2 SO 4 at room temperature; for curve 2, they were measured after 10 minutes at 0.6 V vs.
RHE in 0.1 M CH 3 OH þ 0.02 N HClO 4 at 298K. (Curves have been replotted from Guerin
et al. [2004], Copyright 2004 American Chemical Society; and from Takasu et al. [2000],
Copyright 2000, with permission from Elsevier.)