58 Trends in activity and stability
will also be introduced: alloys of Pt and some alkaline earths exhibit promising
The XRD characterization shows that all these materials have similar bulk
structures and it will allow to give an interpretation of these trends on the basis of
the nearest neighbour Pt-Pt distance in the bulk, although strain relaxation effects
are expected to occur in the Pt overlayer. Investigating the nanoscale properties of
these Pt overlayers, (e.g. their crystal structure, orientation and level of ordering)
is a fundamental step towards a full understanding of their catalytic properties.
The thickness of the overlayer in particular is an important parameter defining the
level of strain relaxation and providing details about the dealloying mechanism. A
model for estimating these thicknesses on the basis of the AR-XPS measurements
will be introduced.
This work enters in the context of various other studies that in the last two
decades tried to establish experimental trends in the ORR activity of Pt alloys.
Mukerjee et al. studied the activity of carbon supported alloyed nanoparticles
(PtM/C with M = Cr, Mn, Fe, Co, Ni): they related it to the nearest neighbour
Pt-Pt distance and to the level of Pt d-band vacancy 72. Instead, Stamenkovic et
al. reported trends in activity of various polycrystalline extended surfaces (namely
Pt3Ti, Pt3V, Pt3Fe, Pt3Co and Pt3Ni) more similar to those studied here; they chose
the experimental (or theoretical) d-band center position as a descriptor of the activity
38, 44. More recently, Hwang and coworkers also correlated the position
of the d-band center with the activities of various sputter-deposited thin films, including
some alloys of Pt and early transition metals as Pt3Y and Pt3Zr 151. We
anticipate that, in agreement with the d-band model 53–55 and with the d-band
shift observed by Strasser et al. on strained Pt overlayers 31, an analogous trend
could be also observed for the alloys tested in this chapter. However, it will be experimentally
challenging to perform accurate measurements of the d-band on these
alloys. Indeed, these should be carried out after ORR testing, when the Pt-skeleton
structures are formed as a consequence of surface dealloying. The presence of C
and O contaminations and of the 4f states of the lanthanides overlapping to the
d-band would highly complicate the analysis of the XPS-UPS spectra. A number of
trends based on the DFT calculated binding energies to one of the ORR intermediates
(HOO, HO and O) have also been reported 16, 38, 47, 56, 152.
On the basis of the results of this chapter a manuscript about the trends in
activity and stability of these alloys is currently in preparation: the abstract can be
found in the end of this thesis (included Paper 6). All electrochemical measurements
have been performed by María Escudero-Escribano. DFT calculations were
performed by Vladimir Tripkovi´c.