Expansion of interatomic distances in platinum catalyst nanoparticles
IFF-News February 1, 2010
by Kui Du, Frank Ernst, Michael C. Pelsozy, Juri Barthel, and Karsten Tillmann
The atomistic structure of Pt catalyst nanoparticles has been studied using quantitative high-resolution transmission electron microscopy. The particles are found to exhibit a faceted, cubo-octahedral shape, extended planar defects, and mono-atomic surface steps. High-resolution imaging with negative spherical aberration yielded atomic-resolution images with a minimum of artifacts. Combining digital image processing, quantitative image analysis, and HRTEM image simulations to determine local variations of the spacing between neighboring Pt atom columns, an expansion of the lattice parameter in the particle core and even larger, locally varying expansion of Pt-Pt next-neighbour distances at the particle surface was observed. The latter likely originates from an amorphous oxide on the nanoparticle surface and/or dissolution of oxygen on subsurface sites. These structural features may significantly impact the catalytic activity of Pt nanoparticles.
The observation of expanded inter-atomicspacings at the particle surface and surface corrugationcaused by stacking faults, twin boundaries, and surfacesteps suggests that the impact of strain and extended structuraldefects on catalytic activity warrants further investigation.The complex correlation observed between thespatial variation of inter-atomic spacings, surface scales,particle shape, and contact to other particles demonstratesthat techniques that average over a large ensemble ofparticles may lead to non-realistic conclusions. A clear disadvantage of HRTEM is its limitation to ex situ studies, i.e.studies performed not in the same environment and at thesame temperature at which respective catalysts operate. Apossible route to a deeper understanding could be to combineTEM studies with in situ techniques, such as XRD andXAS, in order to enable more detailed interpretation ofXRD and XAS in situ data in terms of structural detailsof individual particles.
Kui Du, Frank Ernst, Michael C. Pelsozy, Juri Barthel, and Karsten Tillmann: Expansion of interatomic distances in platinum catalyst nanoparticles, Acta Materialia 58 (2010) 836-845