Theoretical investigation of the structures of unsupported 38-atom CuPt clusters

A genetic algorithm has been used to perform a global sampling of the potential energy surface in the search for the lowest-energy structures of unsupported 38-atom Cu–Pt clusters. Structural details of bimetallic Cu–Pt nanoparticles are analyzed as a function of their chemical composition and the parameters of the Gupta potential, which is used to mimic the interatomic interactions. The symmetrical weighting of all parameters used in this work strongly influences the chemical ordering patterns and, consequently, cluster morphologies. The most stable structures are those corresponding to potentials weighted toward Pt characteristics, leading to Cu–Pt mixing for a weighting factor of 0.7. This reproduces density functional theory (DFT) results for Cu–Pt clusters of this size. For several weighting factor values, the Cu30Pt8 cluster exhibits slightly higher relative stability. The copper-rich Cu32Pt6 cluster was reoptimized at the DFT level to validate the reliability of the empirical approach, which predicts a Pt@Cu core-shell segregated cluster. A general increase of interatomic distances is observed in the DFT calculations, which is greater in the Pt core. After cluster relaxation, structural changes are identified through the pair distribution function. For the majority of weighting factors and compositions, the truncated octahedron geometry is energetically preferred at the Gupta potential level of theory

Theoretical Investigation of Photoinduced Processes in Subnanometer Oxide-Supported Metal Catalysts

We report a computational study and analysis of the optical absorption and photodecay processes in two subnanometer metal complexes deposited on an oxide support, the regular MgO(100) surface: (i) Ag3(HCO3)(C2H4)2(O) and (ii) Ag3(CO2F)(C2H4)2(O). These aggregates are chosen as derivatives of a Ag3(CO3)(C2H4)2(O) ligand/metal-cluster/support complex, previously singled out as a key intermediate in the path of ethylene partial oxidation to ethylene epoxide catalyzed by Ag3/MgO(100), and serve as model systems to investigate photochemical phenomena in ligand/metal-cluster/support complexes by subnanometer metal catalysts, an appealing field for future research. After generating optimized initial configurations and building cluster models that take properly into account the effect of the charge-separated oxide support, we use time-dependent density-functional theory (TDDFT) to determine first the photoabsorption spectra of the two aggregates and then to follow the evolution of their excited states in the optical region. We show that complexes containing such bicarbonate and fluorocarbonate groups are sensitive to optical adsorption, often leading to ligand detachment and/or cluster disaggregation, thus pointing to an "optical frailty"of these subnanometer cluster species, possibly rationalizing previous experimental observations. Additionally, we correlate the nature of the given excitations and of the corresponding photoinduced reaction products via an analysis of overlap population-density of states (OP-DOS), geometric parameters, and spatial distribution of the molecular orbitals involved in the excitation, thus providing the set of methodological tools needed to explore this novel field

2D-3D structural transition in sub-nanometer Pt-N clusters supported on CeO2(111)

Transition metal particles dispersed on oxide supports are used as heterogeneous catalysts in numerous applications. One example is platinum clusters supported on ceria which is used in automotive catalysis. Although control at the nm-scale is desirable to open new technological possibilities, there is limited knowledge both experimentally and theoretically regarding the geometrical structure and stability of sub-nanometer platinum clusters supported on ceria. Here we report a systematic, Density Functional Theory (DFT) study on the growth trends of CeO 2 (111) supported Pt N clusters (N = 1-10). Using a global optimization methodology as a guidance tool to locate putative global minima, our results show a clear preference for 2D planar structures up to size Pt 8 . It is followed by a structural transition to 3D configurations at larger sizes. This remarkable trend is explained by the subtle competition between the formation of strong Pt-O bonds and the cluster internal Pt-Pt bonds. Our calculations show that the reducibility of CeO 2 (111) provides a mechanism to anchor Pt N clusters where they become oxidized in a two-way charge transfer mechanism: (a) an oxidation process, where O surface atoms withdraw charge from Pt atoms forming Pt-O bonds, (b) surface Ce 4+ atoms are reduced, leading to Ce 3+ . The active role of the CeO 2 (111) support in modifying the structural and eventually the chemical properties of sub-nanometer Pt N clusters is computationally demonstrated

Tetrahelix conformations and transformation pathways in Pt1Pd12 clusters

The threshold method is used to explore the potential energy surface of the Pt1Pd12 bimetallic cluster, defined by the Gupta semiempirical potential. A set of helical structures, which follow a Bernal tetrahelix pattern, correspond to local minima for the Pt1Pd12 cluster, characterizing the region of the energy landscape where these structures are present. Both right-handed and left-handed chiral forms were discovered in our searches. Energetic and structural details of each of the tetrahelices are reported as well as the corresponding transition probabilities between these structures and with respect to the icosahedron-shaped global minimum structure via a disconnectivity graph analysis

Determination of the energy landscape of Pd12Pt1 using a combined genetic algorithm and threshold energy method

In this work we present a thorough exploration of the potential energy surface (PES) of Pd-Pt bimetallic nanoparticles at the specific composition Pd12Pt1, using the combination of a genetic algorithm and the threshold method for global optimization and exploration of the barrier structure, employing the semi-empirical Gupta many-body potential for modeling the interatomic interactions. The structural and energetic analysis of Pd12Pt1 nanoparticles, including binding energies (E-b), symmetries and common-neighbor analysis (CNA) allowed us to identify a large set of representative structures of local minima, with an icosahedral motif found to be the putative global minimum for Pd12Pt1. A detailed study of the icosahedral motif was carried out by an exhaustive exploration of low energy isomers, in order to understand qualitatively structural interconversion. 2-D tree (disconnectivity) graphs are plotted to map the structures of minima on the PES of Pd12Pt1. DFT calculations were performed on representative structures to establish the energetic hierarchy and structural stability

Global Minimum Pt₁₃M₂₀ (M = Ag, Au, Cu, Pd) Dodecahedral Core–Shell Clusters

In this work, we report finding dodecahedral core–shell structures as the putative global minima of Pt₁₃M₂₀ (M = Ag, Au, Cu, Pd) clusters by using the basin hopping method and the many-body Gupta model potential to model interatomic interactions. These nanoparticles consist of an icosahedral 13-atom platinum core encapsulated by a 20 metal-atom shell exhibiting a dodecahedral geometry (and <i>I</i>_<i>h</i> symmetry). The interaction between the icosahedral platinum core and the dodecahedral shell is analyzed in terms of the increase in volume of the icosahedral core, and the strength and stickiness of M–Pt and M–M interactions. Low-lying metastable isomers are also obtained. Local relaxations at the DFT level are performed to verify the energetic ordering and stability of the structures predicted by the Gupta potential finding that dodecahedral core–shell structures are indeed the putative global minima for Pt₁₃Ag₂₀ and Pt₁₃Pd₂₀, whereas decahedral structures are obtained as the minimum energy configurations for Pt₁₃Au₂₀ and Pt₁₃Cu₂₀ clusters

Comparative kinetic characterization of the activity of glycosylated and non-glycosylated trypsin-like serine protease isolated from adults of Rhyzopertha dominica (Coleoptera: Bostrichidae) reared on the grain of three different cultivars of wheat

Rhyzopertha dominica is a pest that uses trypsin-like serine protease enzymes to hydrolyse the proteins in the cereal grains on which it feeds. The present study reveals for the first time that that there are both glycosylated and non-glycosylated serine proteases. The progeny of R. dominica reared on the grain three varieties of wheat were used to fractionate their trypsin-like serine proteases using Concanavalin A affinity chromatography. The albumin fractions from the wheat cultivars used in this study were subjected to size exclusion chromatography to fractionate the albumin inhibitors that are highly specific for the serine protease activity of R. dominica. Kinetic and thermodynamic assays were used to differentiate both types of enzymes. In general, the catalytic efficiency values Vmax/Km for glycosylated proteases were higher, indicating that glycosylation increases the affinity for the substrate. Inhibition assays using wheat albumins revealed that the glycosylated enzymes had higher Ki values, indicating a low affinity for the inhibitors than the non-glycosylated enzymes. Thermodynamic analysis indicates that glycosylation increases the activation energy Ea improving the serine proteases' catalysis. Thus it is likely that R. dominica uses glycosylated proteases in order to optimize the hydrolysis of cereal proteins and nullify the action of wheat grain protease inhibitors and increase its chances of survival