Surface reconstruction of Pt nanocrystals interacting with gas atmosphere. Bridging the pressure gap with in situ diffraction

Abstract

A carefully designed in situ XRD experiment when guided by atomistic simulations can provide data on the atomistic structure of a surface layer of platinum nanoclusters. Even the adsorption process for a strongly bonded adsorbate can be monitored and interpreted, providing data that are not available from other techniques. The data reported here present the first observation of surface reconstruction of nanocrystals by X-ray diffraction known to the authors. We were able to observe repeatable in situ evolution of Pt nanocrystal diffraction peak positions on exchange of gas atmosphere from hydrogen to helium. Experiments at room temperature and at 373 K shows various hydrogen desorption rate in He atmosphere but a very similar rate of an average lattice constant change with clearly separated desorption and reconstruction phases. The hydrogen desorption rate has been shown to be controlled by a slower process—hydrogen spillover and its activation energy was estimated. Diffraction peaks of Pt on exposition to O₂ shift at various degree to lower angles due to surface relaxation—the effect being particle-size-dependent and illustrating elongation of surface Pt–Pt bonds caused by adsorption. The results show the possibility for XRD to become a nanosurface science tool enabling the combination of structure analysis with adsorption/desorption measurements within the pressure gap and material gap.