Zbigniew Kaszkur

Powder diffraction beyond the Bragg law: study of palladium nanocrystals

An experimental method of measurement of the subtle changes of structure of metal nanocrystals occurring on chemisorption of oxygen, interaction with inert gas and hydrogen, etc., is proposed. The measured patterns and their evolution are interpreted via atomistic simulations. Described are quantitative observations of the changes in peak position, intensity and half width of the 111 diffraction peak of a palladium catalyst caused by modifying the gaseous environment. The results of the measurements are in line with an atomistic model proposed earlier and prove that the measured average lattice constant of palladium clusters evolves according to their surface relaxation. The evolution of the measured peak intensity suggests surface ordering effects and was used to propose a detailed structural model of nanocrystalline metal particles. The transition of palladium into β-Pd-H in hydrogen under normal conditions was used as a structure probe and provided evidence for the presence of icosahedral clusters in a highly dispersed catalyst. The icosahedral phase is not significantly modified under hydrogen atmosphere and does not transform into the β hydride.

Cinchona Alkaloid-Metal Complexes: Noncovalent Porous Materials with Unique Gas Separation Properties**

The most common andeffective approach to design and prepare metal–organicframeworks (MOFs) or porous coordination polymers(PCPs) of desired topology and functionality is based oncoordination-driven self-assembly, and both the correctchoice of metal centers and the engineering of the ligandsfeatures, such as size, flexibility, and directionality of bindingcenters, play a decisive role.

Nanopowder diffraction analysis beyond the Bragg law applied to palladium

A method of powder diffraction analysis is proposed that is applicable to transition metal nanoparticles and which has been applied to palladium. From results of simulations it is shown that: (a) the overall average lattice constant of small transition metal clusters decreases with decreasing size of the cluster as a result of surface contraction; (b) the shift of a diffraction peak position driven by chemisorption increases monotonically with the decrease of cluster size and can be used for the estimation of cluster size; (c) the peak broadening of small clusters may be larger than that resulting from their size; the additional contribution originates from a high density of defects; (d) the extent of the microstrain may be evaluated from the peak broadening and the rate of attenuation of peak intensity; (e) the intensity scattered by small palladium clusters indicates significant surface disorder that may vary during a change of gas atmosphere and during chemisorption of oxygen at the surface.

Construction of a Porous Homochiral Coordination Polymer with Two Types of CunIn Alternating Units Linked by Quinine: A Solvothermal and a Mechanochemical Approach

The polymer network: The reaction of quinine (QN) with CuI under solvothermal, as well as liquid-assisted grinding, conditions afforded a unique 1D homochiral coordination polymer {[Cu(4)(μ(3)-I)(4)(QN)(2)][Cu(3)(μ(3)-I)(2)(μ(2)-I)(QN)(2)](2)}(n), containing both triangular Cu(3)I(3) and cubane Cu(4)I(4) clusters as connecting nodes (see scheme). Van der Waals interactions between the adjacent 1D polymer chains lead to an extended quasi-honeycomb homochiral pillared 3D network with solvent-free 1D channels.

Segregation in model palladium—cobalt clusters

Abstract N-body Sutton—Chen potentials have been developed for fcc Co and the interaction of Co and Pd atoms. These potentials were used to describe the effect of Pd segregation within Pd—Co relaxed clusters. Monte Carlo configurational minimization at room temperature was used to determine the morphology of clusters. For the diffraction patterns averaging over configurations is achieved by molecular dynamics runs. Variation in the position of the 111 peak of the average pattern with cluster alloy composition was compared with X-ray diffraction data for (Pd—Co) silica catalysts. The good fit obtained strongly supports the applicability of the model segregation profile. A key role in the modelling is played by the precise tuning of the Pd—Co potential.

X-ray diffraction study of the palladium-carbon system

Abstract The Pd-C solid solution was studied by wide angle X-ray diffraction. The Debye-Waller parameter and lattice strains were estimated for both carbon free and carburized palladium. Nearly equal values of the mean displacement of about 0.093 A was found for palladium atoms in these two cases, a value of 0.17 A was estimated for the carbon atoms in the carburized sample. An increase of lattice strains was found after decarbonization. The location of carbon atoms in the octahedral voids was confirmed.

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

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.

Direct observation of chemisorption induced changes in concentration profile in Pd–Au alloy nanosystems via in situ X-ray powder diffraction

Pd–Au nanocrystalline systems supported on silica were exposed to different gas environments and studied by in-situ XRD. Subtle changes were detected in the XRD patterns when after heating in dry air and flushing with argon a short pulse of H2 was injected into the stream of argon. These changes were interpreted in terms of an inversion of concentration profile in Pd–Au particles from Pd segregation induced by oxygen chemisorption to Au segregation for the clean surface. Full atomistic models of the alloy nanoparticles for sizes of up to 5 nm were computed using Sutton–Chen potentials, configurational minimization and molecular dynamics. Thus it was possible to simulate the inversion of the concentration profile and its effect on the XRD pattern. The results of the simulation allowed qualitative analysis of the experimental XRD evolution. A specially designed in situ camera together with a new method of process monitoring and analysis allowed direct structural observation of the dynamics of change of the concentration profile within the nanoparticles. It is demonstrated that XRD, when aided with atomistic simulations, can be suitable to follow in situ evolution of the surface structure of nanopowders.

In situ EXAFS study of the alloy catalyst Pd-Co (50%/50%)/SiO2

In situ extended X-ray absorption fine structure (EXAFS) data for nanocrystalline bimetallic Pd-Co catalyst supported on silica, for Pd K and Co K absorption edges, are presented and analysed confirming previously developed atomistic model of segregation within the alloy metal particle. Supporting evidence is presented from dynamical X-ray diffraction (DXRD) data. The confirmed segregation model concerns energy minimised partial surface segregation of Pd in contrary to a complete segregation model that does not fit well the EXAFS data.

Ab initio test of the Warren-Averbach analysis on model palladium nanocrystals

Model powder diffraction patterns were calculated via the Debye formula from atom positions of a range of energy-relaxed closed-shell cubooctahedral clusters. The energy relaxation employed the Sutton–Chen potential scheme with parameters for palladium. The assumed cluster size distribution followed lognormal distribution of a crystallite volume centred with the diameter of 5 nm, as well as two bimodal lognormal distributions centred around 4 nm and 7 nm. These models allowed an in-depth analysis of the Warren–Averbach method of separating strain and size effects in a peak shape Fourier analysis. The atom-displacement distribution in the relaxed clusters could be directly computed, as well as the strain Fourier coefficients. The results showed that in the case of the unimodal size distribution, the method can still be successfully used for obtaining the column length distribution. However, the strain Fourier coefficients obtained from three reflections (002, 004 and 008) cannot be reliably estimated with the Warren–Averbach method. The primary cause is a non-Gaussian strain distribution and a shift of the diffraction maximum, inherent to the nanoparticles, differing for every constituent cluster in the size distribution. For the bimodal size distributions, the obtained column length distributions tend to be shifted towards the centres of the modes and are less sensitive to the larger size mode.