Irena Efremenko

Implication of palladium geometric and electronic structures to hydrogen activation on bulk surfaces and clusters

A great body of theoretical results obtained preferentially with density-functional methods in the last decade is analyzed with emphasis on the relationship between the orientation of the surface for semiinfinite mono-metallic bulk palladium and the cluster-size for finite palladium particles. It is demonstrated that the crystallographic orientation of a surface influences the local electronic structure and the geometric configuration of catalytic centers and this way changes their catalytic properties. Weakly bound molecularly adsorbed states and atomic adsorbates in the most reactive low-coordinated positions are especially sensitive to such changes. The unique feature of small palladium clusters, their ground state magnetism affects mainly the dynamics of dissociative adsorption and leads to the stabilization of pre-dissociated forms of adsorbate.

Quantum chemical study of neutral and single charged palladium clusters

Abstract The extended Huckel (EH) method with an electrostatic two-body correction, has been used in order to determine the structures of small single charged Pdn clusters with n=2–13 and to compare them with the neutral ones. The results for Pd2 and Pd3 are compared with density functional (DFT) calculations. Both cation and anion formations were found to strengthen the clusters due to the bonding character of their HOMO and antibonding nature of LUMO. The twin formation with bond lengths significantly smaller than those in the bulk palladium and in the corresponding neutral particles was found to be the preferential way of growth for anionic clusters; cationic clusters show a more complicated behavior. The promotion of occupation of Pd 5s AOs is suggested to be responsible for the formation of 3D structures, whereas the stability of the planar configurations is attributed to the appearance of the vacancy in the valence 4d-shell. As a result of stronger intermetallic interaction in charged clusters, both excess and deficit of electron density were found to cause the significant broadness of the d-zone.

Hydrodenitrification with PdCu catalysts : Catalyst optimization by experimental and quantum chemical approaches

A continuous process for nitrate and nitrite abatement from drinking water by catalytic hydrogenation has been developed in our lab. We describe the experimental process development procedure, and support it with semiempirical quantum chemical methods. Comparisons of activated carbon (ACC) and silica glass fiber (GFC) cloths as supports for mono- and bimetallic Pd-Cu catalysts show the former to be 45-fold and 15-fold more active for nitrite and nitrate hydrogenation, respectively, than the latter. Catalysts prepared by selective deposition of Cu on Pd/ACC led to better activity for nitrate hydrogenation than catalysts prepared by co-impregnation or ion exchange methods. The optimal Cu:Pd atomic ratio was found to be 1:2. The computational results show the following: (i) The dispersion of Pd catalysts supported on ACC is much higher than that on GFC due to the larger surface area and higher density of adsorption sites, and that accounts for the higher activity of PdCu/ACC; (ii) Nanosized Pd particles supported on ACC have a semispherical shape and possess preferentially close-packed triangular surfaces, while Pd/GFC particles are extended in the direction parallel to the support surface and show both fcc (100) and (111) planes; (iii) The interaction of Cu atoms with both supports is stronger than that of Pd; adsorbed Cu atoms show a greater ability to form monometallic than bimetallic bonds and that should result in poor mixing of the metal upon co-impregnation, as was observed experimentally; (iv) Cu atoms in bimetallic PdCu particles admit a significant positive charge; the experimentally measured solubility of metal atoms correlates with their calculated charges. The best catalyst (2 wt%Pd-0.6 wt%Cu/ACC) was employed in a novel continuous flow reactor for nitrate hydrogenation in distilled and tap water. The advantages of the reactor investigated over a conventional packed bed reactor are discussed, suggesting a potential for further process intensification.