X-Ray Diffraction is widely considered as a major contributor to the development of the XX century chemistry. This status has been achieved in spite of the fact that the technique was used mostly for bulk phase analysis and crystal structure solution – neglecting principal processes of chemical reactions that have their origin e.g. at the solid-gas interface – at the surface . Up to now observation of surface interactions was possible only for internal surface of crystalline porous materials e.g. zeolites. Nowadays with availability of nanopowdered materials, the in-situ diffraction is able to offer a major breakthrough – when number of atoms at the surface is comparable to that of bulk atoms, we can directly observe elementary chemical processes at the surface. The challenging task is to interpret these observations. To this end a large scale atomistic simulations may offer a significant help. I believe that in close future a direct observation of the solid-gas interface via in-situ nanopowder diffraction will provide many surprises in the established picture of elementary chemical reactions.
PROJECT – to make possible structural observation (in-situ) of the state of interface (metal-gas) during slow physico-chemical process.
- This general goal comprises of two principal tasks: be able to detect the signal originating from the interface
- understand this signal and its evolution in terms of structure.
The structural information can be accessed via powder diffraction of X-rays when applied to nanopowdered metal. The scattered intensity due to the interface region is only noticeable when the number of atoms on the metal surface contributes significantly to the overall number of metal atoms. This may be the case for nanopowders. Such small lumps of matter even when apparently crystalline, scatter X-rays in the way not obeying strictly the Bragg law. The terms coined by me are: “nanopowder diffraction” and “diffraction beyond the Bragg law”. One thing is to detect the powder diffraction evolution on some surface process (chemical reaction, chemisorption, disordering etc.), the other is to interpret the observed changes in terms of structure. To this end already for some years I attempt to develop numerical modeling tools addressing experimentally observed processes and designed to be easily interfaced to the experiment.
On the other hand it appears that if designing a right scenario for the in situ structural process to study (employing such parameters as gas composition, temperature, pressure) the nanopowder diffraction alone may suffice in providing satisfactory structural model.
The principal tool developed to complement the in situ nanopowder diffraction experiment is CLUSTER – the building and simulation program with graphical interface.
Some illustration of the program potential is presented graphically on the following slide show. The slide resolution is 1024×768.
CLUSTER can be downloaded together with its environment in gzipped tar archive. Following guidelines in deployment.txt and correcting few symlinks one should form application working under Linux OS in graphical Xwindow environment. For the moment there is no manual for it and help is limited to scant hints. Works the best with resolution at least 1024×768.
The scope of the project was broadened to interpret the evolution of the structure of bimetallic nanoparticles in some chemical reaction conditions.
The importance of the project may be well understood bearing in mind that most processes of heterogeneous catalysis occur at the surface of metal nanoparticles finely dispersed on some more chemically inert support. Although many physical techniques has already been employed in catalysis to study such surface states and species, these are mostly spectroscopic and do not allow a direct structural insight in reaction conditions.
Some general philosophy of the project can be found in a popular article published in journal of Polish Academy of Science –“Akademia”.
Au-Pd alloy 3871 atom (magic number 10) model cluster after configurational and spatial energy minimisation. All the atoms were subjected to configurational (Monte-Carlo, Metropolis) and spatial energy minimisation (relaxation of the coordinates). The configurational Monte-Carlo minimisation was supplemented with spatial relaxation at
every unlike atom exchange. The resulting segregation profile is presented above.
The same 3871 atom Au-Pd alloy cluster was constrained to have its surface build up solely of Pd atoms. The analogous minimization leads now to a reversal of the segregation radial profile. Both energy relaxations used semi-empirical N-body Sutton-Chen potentials applicable to fcc metals.
Powder X-Ray Diffraction appears to be a sufficiently sensitive technique to register subtle structural changes at the nanocrystal surface provided the nanocrystalline peaks are measured with good statistics. The principal publications paving the way to understand surface restructuring due to chemical reactions are listed below:
- Kaszkur Z.,
Nanopowder diffraction analysis beyond the Bragg law applied to Palladium.
Journal of Applied Crystallography, 33, 87-94(2000).
- Kaszkur Z.,
Powder Diffraction beyond the Bragg law: study of palladium nanocrystals.
Journal of Applied Crystallography, 33, 1262-1270(2000).
- Kaszkur Z.,
Direct observation of chemisorption induced changes in concentration profile in Pd-Au alloy nanosystems via in situ X-ray powder diffraction,
Physical Chemistry Chemical Physics, 6, 193-199(2004).
- Kaszkur Z., Mierzwa B., Pielaszek J.,
Ab initio test of the Warren-Averbach analysis on model palladium nanocrystals.
Journal of Applied Crystallography, 38, 266–273 (2005).
- Kaszkur Z.,
Test of applicability of some powder diffraction tools to nanocrystals.
Zeitschrift für Kristallographie, 23, 147-154 (2006).
- Rzeszotarski P., Kaszkur Z.,
Surface reconstruction of Pt nanocrystals interacting with gas atmosphere. Bridging the pressure gap with in situ diffraction.
Phys.Chem.Chem.Phys., 11, 5416 – 5421 (2009).
- Kaszkur Z., Mierzwa B., Juszczyk W., Rzeszotarski P., Łomot D.,
Quick low temperature coalescence of Pt nanocrystals on silica exposed to NO- the case of reconstruction driven growth?
RSC Adv., 4 (28), 14758 – 14765 (2014).
- Kaszkur Z., Rzeszotarski P., Juszczyk W.,
Powder Diffraction in studies of nanocrystal surfaces – chemisorption on Pt.
Journal of Applied Crystallography, 47, 2069-2077 (2014).
- Kaszkur Z., Juszczyk W., Łomot D.,
Self diffusion in nanocrystalline alloys.
Physical Chemistry Chemical Physics, 17, 28250-28255 (2015).
- Kaszkur Z., Zieliński M., Juszczyk Z.,
The real background and peak asymmetry in diffraction on nanocrystalline metals.
Journal of Applied Crystallography, 50, 585-593 (2017).