Earth Sciences & Env.
Engineering & Tech.
Information & Comm. Tech.
Life Sciences & Biotech
Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
The chemistry of polyoxometalates (POMs) remains one of the most fascinating areas of modern science because of the ability of the POMs to display a remarkable structural diversity with fascinating shapes and of their relevance to a wide variety of applications including, e.g., homogenous and heterogeneous catalysis, chemical analysis, material sciences, solid-state technology, design of molecular sensors, medicine etc.
Recently, our group successfully synthesised new polynuclear open-shell nickel or cobalt polyoxocations of the type [X@M6O12L12]+/2+ (X = Na+; X, M = Ni2+, Co2+) stabilised by chelating anionic ligands L where LH = (pyridine-2-yl)methanol. Their structures display striking similarities to the Anderson-type structures found in polyoxometalates. Furthermore, the nature of the metal plays a crucial role in the magnetic properties of these coordination clusters, ranging from ferromagnetic, antiferromagnetic to single-molecule magnet behaviour. While [Zn7O12L12]2+  is diamagnetic because of the closed-shell character of the d10 Zn2+ atoms, incorporation of a paramagnetic metal ion X (e.g. Cr3+, Fe2+/3+, Co2+/3+) into the [Mo6O24H6]n– framework to form the Anderson-type structures results in open-shell POMs.
The main goal of our project is to understand the magnetic-structural correlations as well as the relationships between the electronic and bonding situations in above-mentioned metal-based ionic aggregates which adopt the typical Anderson-type POM structures in order to assist in the further development of physical chemical approaches toward the synthesis and applications of this class of compounds.
Density functional theory (DFT) geometry optimisations of [X@M6O12L12]+/2+ (X = Na+; X, M = Ni2+, Co2+, Zn2+) were performed at the uB3LYP level in the gas phase and condensed media, in H2O and THF solutions. A comparative analysis with [TeMo6O24]6–, [CoMo6O24H6]2– and [Bi7I24]3– was carried out at the same level of theory to gain an insight into their molecular orbital behaviour. In order to obtain the correct ground-state of [X@M6O12L12]+/2+ complexes and identify their fundamental structural properties, the atomic spin densities distributions were analysed first. The energy differences between the equilibrium open- and closed-shell un- and symmetrised Anderson-type structures were estimated in order to better understand the magnetic-structural correlations. It is noteworthy that the arrangement of the complexes [X@M6O12L12]+/2+ (X = Na+; X, M = Ni2+, Co2+) in their unit cells have an impact on the aforementioned correlations. Inspection of the orbital schemes of the studied Anderson-type structures indicated e.g. that the molecular orbitals in the gas phase lie much higher in energy than those obtained from the computations in solution.
The study of the geometrical and electronic structures of herein-presented complexes, which are structurally closely related, allowed us to find insightful correlations among the considerable diversity encountered. It should be noted, however, that the results obtained represent a preliminary exploration of the problem and the further studies of the reported systems at DFT and ab initio will be performed.
 (a) Coronado, E.; Gómez-García, C. J. Chem. Rev. 1998, 98, 273; (b) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Chem. Rev. 2010, 110, 6009.
 (a) Zhang, J.; Teo, P.; Pattacini, R.; Kermagoret, A.; Welter, R.; Rogez, G.; Hor, T. S. A.; Braunstein, P. Angew. Chem. Int. Ed. 2010, 49, 4443. (b) Pattacini, R.; Teo P., Zhang, J.; Lan, Y.; Powell, A. K.; Nehrkorn, J.; Waldmann, O.; Hor, T. S. A.; Braunstein, P., Dalton Trans. 2011, 40, 10526.
 Tesmer, M.; Müller, B.; Vahrenkamp, H. Chem. Commun. 1997, 721.
 Romo, S.; de Graaf, C.; Poblet, J. M. Chem. Phys. Lett. 2008, 450, 391.
Ceria CeO2 is well known as a key component of the automotive catalysts and efficient catalysts or support for a series of other catalytic process of great industrial and environmental interest. Some of the most important applications of these systems are related to their reactivity towards CO or CO2 in relation to reduction of pollutions and greenhouse gases. In addition, it is found experimentally as appropriate support for noble metal catalysts for several reactions directed to production or deep purification of hydrogen as environmentally friendly fuel in fuel cells. Some of these reactions as water-gas shift reaction and preferential oxidation of CO in hydrogen feed, include as a key step interaction of the ceria with carbon monoxide (CO) and carbon dioxide (CO2). Two features of ceria are found very important for the activity and performance of the support/catalyst: the size of ceria particle (the nanoparticles are found by several orders of magnitude more active that large particles) and the degree of oxidation/reduction of the sample. By this reason our computational investigation was based on ceria particle (Ce21O42) designed in the host group (instead of extended ceria surface used in the other computational studies). During the research stay we performed extensive calculations along the following directions:1. Determination of preferable position for formation of oxygen vacancies on the ceria nanocluster and estimation of the energy for vacancy formation;2. Interaction of CO and CO2 with ceria nanoparticle;3. Interaction of platinum cluster (Pt8) with the ceria nanoparticle
The search for different positions of oxygen vacancies was started from the stoichiometric ceria nanocluster Ce21O42 by removal of selected oxygen atoms. Such removal of neutral oxygen atom leads to appearance of two Ce(III) ions from two Ce(IV) ions in the initial stoichiometric particle and one of the unclear issues in the preferred location of those Ce(III) centers. According to the calculated relative energies of the structures, the stability of the O vacancies decreases in the following order: sub-surface layer, surface, edge. The preferable location of the Ce(III) ions are neighbors of the O vacancy when it is on the surface or edge, or farther from it when it is sub-surface.
The interaction of the ceria nanoparticle with CO resulted in spontaneous formation of CO2 or of surface carbonate when the local structure of the particle is suitable. The energy for CO2 formation from CO (accompanied with formation of an oxygen vacancy on the cluster) is found between 0.8 and 1.5 eV. Interaction of CO2 with the catalyst’s surface also results in formation of carbonate species with different stability depending on the adsorption mode of the carbonate and its location (edge, corner or facet). The simulated vibrational frequencies of the carbonates are found similar to the experimentally measured frequencies of some of the surface species.
In order to find stable location of the platinum cluster Pt8 on the ceria nanoparticle we checked 8 different structures with variation of the shape and location of the metal cluster. The optimized structures, initially with 4 unpaired electrons, were re-optimized with 2 and with 6 unpaired electrons. In one case the state with 4 unpaired electrons (one of which on the ceria part forming Ce(III) center) was found the most stable, while for the other structures the state with lower spin multiplicity (triplet) was found with lower energy. In the most stable structure the binding energy of platinum was calculated at 5.6 eV with five platinum atoms interacting with the ceria surface.
The calculations are performed with periodic plane-wave DFT method with PW91 exchange-correlation functional as implemented in VASP program. The kinetic energy cut-off was selected at 415 eV and a cube with dimensions of 2.00 nm each side was selected as the unit cell for the calculations. This size provides ca. 1.00 nm distance between nanoparticles in two neighboring unit cells. Due to the internal deficiency of pure DFT functionals to describe localized electrons, we applied the DFT+U approach in order to provide proper localization of the extra electron on the individual Ce(III) cations.
The investigation is supported by HPC-Europa2 program at Barcelona Supercomputer Center.
The main objective of this project has been the dynamic and kinetic of atom+diatom reactions (i.e., A + BC → AB + C) where the Renner-Teller (RT) effect is present. To do this the time dependent real wave-packet (RWP) method(1) has been used. The RT rovibronic effect(2) arises when two electronic states are degenerate for collinear geometries of the A + BC system and then, for these arrangements, the system should be able to change the electronic state (non adiabatic effect). This fact makes impossible the separation of nuclear and electronic motions and breaks down the Born-Oppenheimer (BO) approximation. Moreover, the coupled-channel (CC) scheme is used to account for the Coriolis coupling that couples the K and K±1 states (K is the quantum number corresponding to the total angular momentum projection along the z axis). If the Hamiltonian operator includes both the RT and the Coriolis terms, the dynamic method is exact.When we carry out the propagation of a WP for a given value of J (total angular momentum quantum number), and for a selected initial condition specified by the v0, j0, K0 quantum numbers (where v0 and j0 are the vibrational and rotational quantum numbers, respectively), there will be several final K values for the system (e.g., for v0=1, j0=1, K0=1, J=40, the possible values of K are from K=0 to K=40, that is to say, 41 propagations are required). Thus, in this context the best way to proceed is to use a parallel computing framework, where each processor is involved in the propagation of a RWP involving a single K value.The first reaction studied is the NH(a1Δ)+H’(2S) one, where several reaction products can be obtained: NH(a1Δ)+H’(2S) → N(2D)+H2(1Σg+) (1) depletion NH(a1Δ)+H’(2S) → NH’(a1Δ)+H(2S) (2) exchange NH(a1Δ)+H’(2S) → NH(X3Σ−)+H’(2S) (3) quenching NH(a1Δ)+H’(2S) → NH’(X3Σ−)+H(2S) (4) exchange + quenchingFrom these set of reaction channels, reactions (3) and (4) can only occur if the RT effect is taken into account. Thus, the propagation of the RWP starts in the A2A1 excited potential energy surface(3) (PES), and when it reaches the H-N-H’ collinear arrangements it becomes possible to change the electronic state (jump into the X2B1 PES(3)) thanks to the RT effect. Moreover, the NH2 X2B1 state has a deep minimum that traps the RWP for long time, increasing in this way the probability of nonadiabatic electronic transition (change of the electronic state). Finally, the part of the RWP corresponding to reaction channels (1), (2), (3) and (4) is determined and the probability of each one of them is calculated, as a function of the initial conditions.The second and last reaction studied is the isotopic substitution reaction N(2D)+HD(X1Σg+), where two possible channels are possible: N(2D)+HD(X1Σg+) → NH(X3Σ-)+D(2S) (1) N(2D)+HD(X1Σg+) → ND(X3Σ-)+H(2S) (2)This study is based on the previous work of our group(4) on N(2D)+H2, where the main conclusion was that only the ground PES(3) (X2B1) plays an important role in the rate constants calculations, at least in the range of the experimental conditions available. According to this, we have used the BO approximation and the RWP propagation considering only the X2B1 ground PES. Of course, the inclusion of the CC method is very important for a good treatment of these reactions and the parallel code is the most convenient way to perform the propagation of the RWPs.
(1) S. K. Gray and G. G. Balint-Kurti, J. Chem. Phys. 108, 950 (1998)(2) C. Petrongolo, J. Chem. Phys. 89, 1297 (1988)(3) Z.-W. Qu, H. Zhu, R. Schinke, L. Adam, and W. Hack, J. Chem. Phys. 122, 204313 (2005)(4) S. Akpinar, P. Defazio, P. Gamallo and C. Petrongolo, J. Chem. Phys. 129, 174307 (2008)
The first thing we had to do was the conversion of our parallel code to an open/mpi code, due to the requirements of the CINECA machines. After this, we checked the code running some jobs and comparing the results with others obtained using other computers. The success of this step allowed us to begin the dynamic study of the reactions indicated in the previous section.At the beginning we checked the convergence of the RT-CC-RWP (NH(a1Δ)+H’(2S)) and BO-CC-RWP (N(2D)+HD(X1Σg+)) calculations verifying a large number of numerical parameters (e.g., rotational basis, mesh, number of iterations, etc.). Once the results were well converged, we performed the RWPs propagations for the two reactions selected.For the first reaction, NH(a1Δ)+H’(2S), the different initial conditions investigated have been the following: NH (v0=0, j0=2,3,4) and J=0,1,2,…,40, and K0=0,1,…,min(j,J). Because the CC method has been used, the RWP has been propagated so many times as given by the number of possible K final values (J+1), using a single processor for each propagation. A total of 80000 iteration steps are required to reach convergence and due to this the propagations of the RWPs have been very time demanding.At present, we are carrying the analysis of the data using the flux and the asymptotic methods to obtain the probability of the four reaction channels. These probabilities will be the basis to obtain both the cross section and the rate constants for all processes.For the second reaction, N(2D)+HD(X1Σg+), the method employed is simpler, since the BO approximation has been used. The convergence is reached at 40000 iterations and then the RWP propagation has not been so time demanding as in the first reaction studied. At this point, we are performing the analysis of the RWPs to obtain the reaction probabilities using the flux method for both reaction channels. After this and in the same manner as in the NH(a1Δ)+H’(2S) reaction, we will determine the cross sections and rate constants and compare the theoretical data with the experimental data of the literature.