Spontaneous Phase Segregation in Bare Palladium-Platinum Nanoparticles Evidenced By Superior Hydrogen Absorption

Palladium Platinum bimetallic alloys are widely used in heterogeneous catalysis as active components for oxygen reduction reaction, anodes in low temperature fuel cells and hydrogenation of aromatic compounds in fuels. Pd-Pt catalysts often operate at elevated temperatures, for example in catalytic converters for automobile exhausts. Under such conditions segregation/alloying is a particularly important issue. However, in most experimental studies on Pd-Pt nanoparticles the homogeneous alloying is consistently reported or a priori assumed.

Specific interactions of surfaces with monolayers of foreign adatoms (metals and nonmetals) are generally known as a particularly sensitive surface probes [1], among which copper and hydrogen underpotential deposition (UPD) are the most widely used. However, by using this approach little is known about the nanoparticle interior. In contrast, chemical potential of absorbed hydrogen does provide deep insight into the homogeneity on the atomic scale since it is determined by near interactions between metal atoms and absorbed hydrogen occupying interstitials.

In this study we have shown that Pd-Pt nanoalloys are much more susceptible to segregation than their bulk counterparts. The spontaneous segregation and chemical ordering of Pd-Pt nanoalloys was evidenced by hydrogen absorption enhanced nearly by three orders of magnitude with respect to random Pd-Pt alloys [2].

Chemical surface and bulk compositions of the nanoalloys were established by using XPS and ICP-MS, respectively. Carbon monoxide adsorption/oxidation was used as a surface probe and as an indicator of catalytic properties of investigated bimetallic nanoparticles. Hydrogen sorption properties of these alloys were investigated by means of hydrogen UPD and Hydrogen insertion in acidic solutions by using classical transient electrochemical methods. Hydrogen absorption isotherms exhibited a markedly enhanced hydrogen solubility and distinct phase transition in the Pd-Pt-H systems that is absent in Pd-Pt alloys with random atom distribution.

We also report the highest hydrogen sorption in Pd₈₀Pt₂₀ alloy exceeding H/(Pt+Pd)=0.5 at the ambient temperature and hydrogen pressure, which additionally emphasizes the importance of both surface purity and segregation on catalysis and hydrogen storage. Our results clearly show that phase segregation is an extremely important factor that is frequently overlooked in experimental studies yet quite crucial for practical applications. 

Literature

[1] Solla-Gullón, J.; Rodríguez, P.; Herrero, E.; Aldaz, A.; Feliu, J. M. Phys. Chem. Chem. Phys. 2008, 10, 1359-1373.

[2] Januszewska, A.; Dercz, G.; Lewera, A.; Jurczakowski, R. J. Phys. Chem. C, Submitted.