Stabilization of Small Platinum Nanoparticles on Pt–CeO₂Thin Film Electrocatalysts During Methanol Oxidation

Abstract

Pt-doped CeO x thin film electrocatalysts have recently been shown to exhibit high activity and stability at the anode of proton exchange membrane fuel cells (PEM-FC). To identify the role of the Pt dopant and the origin of the high stability of Pt–CeO x films, we applied electrochemical in situ IR spectroscopy on Pt–CeO x model thin film catalysts during methanol (1 M methanol) oxidation. The model catalysts were prepared by magnetron cosputtering of Pt (9–21 atom %) and CeO2 onto clean and carbon-coated Au supports. All samples were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) before and after reaction. At pH 1 (0.1 M HClO4) the Pt–CeO x dissolves partially during potential cycling, whereas the films are largely stable at pH 6 (0.1 M phosphate buffer). Electrochemical IR spectroscopy of the adsorbed CO shows that metallic Pt is formed on all Pt–CeO x samples during methanol oxidation. In comparison to Pt(111), Pt aggregates on Pt–CeO x show a CO on-top signal, which is red shifted by at least 25 cm–1 and suppression of the bridging CO signals. Whereas the Pt particles on Pt–CeO x films with high Pt concentration (>20 atom %) undergo rapid sintering during the potential cycling, small metallic Pt aggregates are stable under the same conditions on films with low Pt concentration (<15 atom % Pt). By means of density functional theory (DFT) calculations we analyzed the spectral shifts of adsorbed CO as a function of nanoparticle size both on free and ceria-supported Pt particles. Comparison with the experiment suggests the formation of “subnano”-particles, i.e., particles with up to 30 atoms (<1 nm particle diameter), which do not expose regular (111) facet sites. At sufficiently low Pt loading, these subnano-Pt particles are efficiently stabilized by the interaction with the ceria support under conditions of the dynamically changing electrode potential.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

“…The effect was demonstrated for Pt (111) and unsupported Pt particles as well as for ceria-supported Pt particles of different sizes 109,110 (Fig. 15).…”

“…The stability of Pt-CeO 2 catalyst films was investigated as a function of Pt loading by means of electrochemical infrared spectroscopy. 109 In these experiments CO formed during electro-oxidation of methanol was employed as a probe molecule. The corresponding IR spectra obtained from Pt-CeO x films are presented in Fig.…”

“…Density functional modeling reinforces the correlation between the stretching frequency and the particle size, on both unsupported and supported Pt particles. 109,110 In particular, the offset of the CO vibrational frequencies by 26 cm −1 with respect to Pt (111) indicates the formation of sub-nanometer-sized particles containing about 30 Pt atoms per particle. The findings both in UHV and under electrochemical conditions suggest that the conversion of atomically dispersed Pt 2+ to sub-nanometer sized Pt particles transforms the PtCeO x film into an electrocatalytically active state.…”

Structural transformations and adsorption properties of PtNi nanoalloy thin film electrocatalysts prepared by magnetron co-sputtering

“…The layer mimics the carbon support in a fuel cell, prevents alloy formation of the NiPt layer with the underlying Au substrate, and it is sufficiently transparent to preserve the IR reflectivity of the Au substrate. [26] To ensure the comparability of the results, we used the same substrate for CV. Instead of the large Au samples for EC-IRRAS, here we used a polycrystalline Au bead-type electrode, however, on which we deposited a carbon layer.…”

“…During CO oxidation, the ligand-effect of the subsurface Ni atoms together with the exposed low-coordinated Pt sites (see Ref. 26) then leads to the strong red-shift and broadening. When cycling to potentials of 1.3 VRHE and above, Pt is completely oxidized, dissolved and redeposited.…”