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Deep restructuring of surface of Pt nanocrystals during strong adsorbate bonding - bridging the pressure gap with in situ diffraction

Zbigniew A. Kaszkur 1,  Piotr R. Rzeszotarski 2

1. Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 52/56, Warszawa 01-224, Poland
2. Polish Academy of Sciences, Institute of Physical Chemistry, Kasprzaka 44/52, Warszawa 01-224, Poland

Abstract
Structure of a catalyst surface subjected to industrial catalytic processes may differ significantly from the surface of a single-crystal exposed to a low pressure (10-5mbar) of adsorbate, as studied via surface science techniques. Lack of a suitable experimental methods enabling in situ insight into atomistic structure of the real catalyst surface in normal conditions of reaction is usually described as a pressure gap in heterogeneous catalysis. Majority of studies concern single crystals and so the conclusions drawn from them. This illustrates another gap- so called materials gap in catalysis.

Contrary to a common belief, a carefully designed in situ XRD experiment, when guided by atomistic simulations can provide data on 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 accessible from other technique.

Chemisorption of NO, CO and O2 under pressure of 1 bar causes deep restructuring of surface of Pt nanocrystals supported on amorphous silica. The effect on the diffracted intensity is analogous to amorphisation of a few surface Pt layers. Diffraction peaks of Pt on exposition to NO, CO and O2 shift at various degree to lower angles due to surface relaxation affecting the overall lattice constant - the effect being particle size-dependent. Experiments at room temperature and at 100 deg.C shows various desorption rate in He atmosphere of preadsorbed hydrogen. A molecular model of the surface phenomena based on diffraction experiments and literature data is proposed.

 

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Presentation: Oral at 11th European Powder Diffraction Conference, Microsymposium 10, by Zbigniew A. Kaszkur
See On-line Journal of 11th European Powder Diffraction Conference

Submitted: 2008-05-27 23:05
Revised:   2009-06-07 00:48