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 O(2) 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.

Adsorbed oxygen-induced cluster reconstruction on core–shell Ni@Pt and Pt clusters

“…With the advent of new technical methods in XRD and XAFS experiments [21,22,52], it is now becomes possible to monitor real-time surface reconstruction on Pt nanoparticles when they are interacting with oxygen. In those experimental studies, Allen et al [21] and Imai et al [22] demonstrate how the amplitude of first nearest neighbor (1NN) Pt-Pt bond distribution that plotted in Fourier transform (FT) graph of EXAFS is decreasing while at the same time another amplitude that refers to Pt-O bonds is gradually increasing as the oxidation time passes.…”

Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

“…Samples of about 60 mg were spread over a thin porous sintered glass disc and mounted vertically on the stainless steel heating block of our vacuum-proof XRD camera. The environmental camera of 150 ml volume was similar to that described previously (Rzeszotarski & Kaszkur, 2009), with a removable cap, having an X-ray window, secured by 12.5 mm of self-adhesive Capton foil. It was connected to the gas line via Swagelok vacuum fittings and stainless steel flexible tubings, enabling XRD measurements in Bragg-Brentano geometry.…”

“…Between the camera and the flex-tubing, a union tee fitting (Swagelok) provided splitting of the stream between output and the probing capillary of a mass spectrometer (Hiden Analytical, HAL VII). A detailed description of the gas feeding system has already been published (Rzeszotarski & Kaszkur, 2009). The system enabled a clean switch of a supplied gas by pumping off the manifolds, as well as a quick exchange of gases in the camera by evacuating it and flushing it twice with the selected gas.…”

“…The number of data points (the range of fitting) was adjusted to obtain the minimum random scatter of repeated measurements. A detailed description of the analytical routine was published previously (Rzeszotarski & Kaszkur, 2009). The purpose of fitting is not to fully describe the peak analytically but to find a statistically reliable measure of its position.…”

“…Using this technique, we have shown time evolution of surface reconstruction of Pt during desorption of H 2 . The magnitude of the measured effect could be scaled to the particle size (Rzeszotarski & Kaszkur, 2009). Also, new phenomena like moderatetemperature coalescence in NO atmosphere can be tracked and linked to reconstruction that occurs above 353 K at the Pt nanocrystals' surface at close to atmospheric pressure (Kaszkur et al, 2014).…”