3A1
Thermal desorption of oxygen from palladium oxide cluster ions
(The University of Tokyo) MIYAJIMA, Ken, MAFUNÉ, Fumitaka
Gas-phase metal clusters are ideal model systems to gain molecular level insight into energetics and
kinetics of catalytic reactions [1]. Among the precious metals, palladium is an important component
used in commercial three-way catalysts to reduce automobile emission. Although many studies have
been performed for the CO oxidation on Pd catalysts, detailed analysis on the forms of oxygen bound
to Pd metal is rare. The interaction of oxygen with the Pd surface is known to be quite complex as it
involves not only dissociative adsorption of O2 to form atomic oxygen but also surface oxide and bulk
oxide formation depending on
temperature and oxygen pressure. In
this study, temperature-programmed
desorption (TPD or Thermal
desorption spectrometry TDS) has
been applied to the gas-phase
palladium oxide clusters. in order to
obtain information on the nature of
the bonding of the oxygen atoms in
the palladium oxide cluster ions (see
Scheme 1) [2]. This method enables
to obtain the activation energy for
desorption of oxygen molecule from
the clusters. Activation energies, Ea,
for O2 desorption of PdnOm+ clusters
are estimated by least-squares
curve-fitting of the TPD profile
using a model equation derived from
Scheme 1 TPD experiment for the gas-phase clusters
a series of Arrhenius
equations
for
oxygen
releases. A correlation of
oxygen atom number and
proposed
geometric
structure and comparison
with
the
theoretical
calculations were discussed.
The palladium oxide
clusters, PdnOm+ (n = 2–17),
were generated by laser
ablation of a palladium
metal rod in 0.1% oxygen
doped helium carrier gas in
the gas phase. The cluster
ions were heated to
Figure 1. Two-dimensional contour representation of temperature
300–1000 K downstream
dependent mass spectra of PdnOm+ (n = 2–6).
The rate of cooling of the extension tube was ~10 K min-1.
from the cluster source (post-heating), and the abundance of PdnOm+
was examined using mass spectrometry.
Composition of abundantly formed clusters was m/n ≈ 0.6 at the
room temperature. The mechanism of the composition change of the
clusters by heat was investigated using a TPD experiment, in which
we measured the abundances of clusters as a function of the
temperature of the extension tube. A two-dimensional contour
representation of the temperature-dependent mass spectra of PdnOm+
(n = 2‒6) is shown in Figure 1. As the temperature increases, the
group of ion signals shifts toward smaller mass number at a certain
transition temperature. In more details, an oxygen molecule is
released from oxygen-rich PdnOm+, forming oxygen-deficient
PdnOm−2+. Figure 2 shows the TPD curves for each PdnOm+ clusters
(n = 4‒6). For n = 5, two series of sequential desorption of oxygen
molecules were observed: m = 7→5→3→1 and m = 6→4→2. The
curve shapes for a pair of clusters (m+2 and m) change
concomitantly. This fact supports the assertion that the major
reaction in this TPD experiment is O2 release.
Activation energies, Ea, for O2 desorption of PdnOm+
+
clusters are summarized in Figure 3. In the case of Pd6O6+ Figure 2. TPD profiles of PdnOm (n =4−6),
exhibiting
the
intensity
ratios
of
each
cluster
→ Pd6O4+ + O2, Ea were estimated to be 0.4±0.1 eV. This ion as a function of the temperature of the
energy is in good agreement with the previous binding extension tube. The intensity was
energy ΔE = 0.33 eV, calculated by Lang et al [3,4]. normalized+ such that the sum of intensities,
Σm(PdnOm ), equals one for each n.
Additionally, it was found that Pd6O4+ was
thermally stable up to 1000 K. Considering
the geometrical structure of Pd6O4+, a metal
framework takes octahedron, each oxygen
atom bonds to three neighboring Pd atoms
and they are located on four separate faces
[3,4]. Therefore, it is likely that stable clusters
take on oxygen atoms up to half the number
of faces. This rule is also valid for other
thermally stable clusters, Pd5O3+ and Pd7O6+.
Larger activation energy probably originates
from the absence of weak Pd-O bond in a
cluster. Obtained experimental results provide
activation energy of oxygen release for
information that may be helpful in designing Figure 3. Estimated
each PdnOm+ (n = 2−7) cluster. Labels indicate the oxygen
nano-sized metal-oxide catalysts.
atom numbers for each cluster. Open diamonds are the
theoretical values [3,4,5].
References
(1)
(2)
(3)
(4)
Lang, S. M.; Bernhardt, T. M. Phys. Chem. Chem. Phys. 2012, 14 (26), 9255–9269.
Miyajima, K.; Mafuné, F. J. Phys. Chem. A 2015, 119 (29), 8055–8061.
Lang, S. M.; Fleischer, I.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. J. Phys. Chem. A 2014, 118 (37), 8572–8582.
Lang, S. M.; Fleischer, I.; Bernhardt, T. M.; Barnett, R. N.; Landman, U. J. Am. Chem. Soc. 2012, 134 (51),
20654–20659.
(5) Kalita, B.; Deka, R. C. Eur. Phys. J. D-Atomic, Mol. Opt. Plasma Phys. 2009, 53 (1), 51–58.