Oxidation of small gas phase Pd clusters: A density functional study (original) (raw)
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Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O−O dissociation barrier in O 2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O 2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O 2 adsorption and dissociation on Au 8 , Pd 8 , and Au 8−n Pd n (n = 1−7) nanoclusters in different spin states. The density functional calculations point out that the O 2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au 5 Pd 3 and Au 1 Pd 7 show very large O 2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O 2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O 2 dissociation. Furthermore, the computed O 2 dissociation barriers are significantly low as compared to the already reported barriers (1.95−3.65 eV) on monometallic and bimetallic Au−Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au−Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.
Pd 6 O 4 + : An Oxidation Resistant yet Highly Catalytically Active Nano-oxide Cluster
Journal of the American Chemical Society, 2012
The palladium oxide cluster Pd 6 O 4 + is formed as the sole product upon reaction of a bare palladium cluster Pd 6 + with molecular oxygen in an octopole ion trap under multicollision conditions. This oxide cluster is found to be resistant to further oxidation over a large temperature range, and further O 2 molecules merely physisorb on it at cryogenic temperatures. The particular stability of Pd 6 O 4 + is confirmed by the observation that the reaction of Pd 7 + with O 2 leads to fragmentation resulting in the formation of Pd 6 O 4 + . However, in an oxygen-rich O 2 /CO mixture, Pd 6 O 4 + is identified as the catalytically active species that effectively facilitates the low-temperature oxidation of CO. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide detailed molecular level insight into the nano-oxide cluster chemistry and are able to reveal the full catalytic combustion reaction cycle.
The CO chemisorption on some active sites of Pd clusters: A DFT study
Journal of Molecular Structure: THEOCHEM, 2006
The CO adsorption on Pd, Pd 2 and Pd 4 small clusters was studied using density functional methods; the theoretical results obtained here were compared with experimental vibration frequencies of CO adsorbed on supported Pd-nanoparticles. The results indicate that the CO adsorption over Pd 2 and Pd 4 clusters give CO vibration frequencies values closed to the experiment values. Also, the theoretical results for the threefold hollow sites were compared with other nanoclusters, with nO50 atoms, where the Pd 4 cluster also give values closed to the experiment. According with the results, two and four atoms small clusters could be used to model the active interaction sites between a gas molecule like CO and Pd nanoparticle. q
The Effect of Gamma-Al 2 O 3 Support on the NO Adsorption on Pd 4 Cluster
Journal of the Brazilian Chemical Society, 2016
The effect of γ-Al 2 O 3 support on the NO adsorption on Pd 4 clusters was investigated by means of density functional theory (DFT) calculations. Pd 4 adsorbed on γ-Al 2 O 3 (represented by a Al 14 O 24 H 6 cluster) changes its preferential geometry from tetrahedral to a distorted planar structure. The alumina support promotes a higher dispersion in the palladium catalyst and reduces the NO adsorption energy to-25.6 kcal mol-1 (computed at B3LYP/LANL2DZ/6-311+G(d)), in close agreement with the experimental value of-27.2 kcal mol-1. On the bare planar Pd 4 cluster the NO molecule adsorbs in a bridge arrangement, with adsorption energy of-41.2 kcal mol-1. Adsorption on the tetrahedral Pd 4 cluster occur preferentially in an atop mode, with adsorption energy of-30.6 kcal mol-1. Charge density analysis show that the electron flux between the NO molecule and Pd 4 depends on the adsorption form, with back-donation being stronger in the bridge adsorption mode.
Density Functional Study of the Interactions between Dihydrogen and Pdn (n = 1−4) Clusters
Journal of Physical Chemistry A, 2000
The dihydrogen interactions with Pd n (n) 1-4) clusters was investigated using hybrid density functional Becke3LYP method and two ECP basis sets. The local minima configurations for a number of H 2 molecule approach modes toward Pd n clusters are presented. Some of these states may be interpreted as a physical adsorption and others as dissociative interaction of the H 2 molecule with the palladium cluster. Both geometric and energetic characteristics of weakly bonded pre-dissociated complexes on Pd 3-4 clusters are very close to those on a bulk Pd (111) surface, while the stable states with dissociated hydrogen molecules show significant differences. In contrast to the bulk surface, 2-fold coordination positions exhibit slightly higher stability of hydrogen bonding in Pd 3 and Pd 4 clusters than 3-fold ones. The binding energy is significantly higher for small clusters than for the bulk surface. The Pd 2 cluster was found to be the most active toward hydrogen capture in accordance with the experimental results.
Theoretical insight of nitric oxide adsorption on neutral and charged Pd n (n = 1-5) clusters
International Journal of Quantum Chemistry, 2015
Density functional theory (DFT) calculations within the framework of generalized gradient approximation have been used to systematically investigate the adsorption of nitric oxide (NO) molecule on neutral, cationic, and anionic Pd n (n 5 1-5) clusters. NO coordinate to one Pd atom of the cluster by the end-on mode, where the tilted end-on structure is more favorable due to the additional electron in the p* orbital. On the contrary, in the neutral and cationic Pd 2 system, NO coordinates to the bridge site of cluster preferably by the side-on mode. Charge transfer between Pd clusters and NO molecule and the corresponding weakening of NAO bond is an essential factor for the adsorption. The NAO stretching frequency follow the order of cationic > neutral > anionic. Binding energy of NO on anionic clusters is found to be greater than those of neutral and cationic clusters. V
A DFT study of the NO adsorption on Pdn (n = 1–4) clusters
Journal of Molecular Catalysis a Chemical, 2011
We report a density-functional study of some properties of the adsorption process of the NO molecule on small palladium clusters (n = 1-4). The interaction between NO and the Pd n clusters is studied on various adsorption sites. Both, NO and Pd n geometrical relaxations are taken into account. The significant conformational reconstruction of the metallic cluster upon NO adsorption induces a large decrease of the NO adsorption energy. Nevertheless, the N-O binding energy is strongly weakened when the molecule is adsorbed on the small Pd n clusters due essentially to an electrostatic repulsion between both N and O atoms. The possible dissociation process of NO on Pd 4 cluster is then investigated within two processes: the NO molecule does not dissociate on Pd 4 with process (i) (dissociation of the isolated gas phase NO molecule followed by the adsorption of both nitrogen and oxygen atoms on the cluster). Process (ii) which presents three successive steps (adsorption of the NO molecule, dissociation of the NO molecule adsorbed on Pd 4 , adsorption of the O atom on the cluster) is studied in details and we propose a reaction pathway locating transition states and intermediate species. The activation energy for process (ii) is high and the dissociation of the NO molecule on the Pd 4 cluster is thus highly improbable.
Journal of Cluster Science, 2000
This paper presents an overview of the optical, photophysical, and photochemical properties including UV-visible and luminescence spectra in solution at 298 and 77 K, along with electrochemical, and catalytic behavior under reduction conditions (for both thermally and electrochemically assisted systems) of the tri-and tetranuclear Pd 3 (dppm) 3 (CO) 2+ and Pd 4 (dppm) 4 (H) 2+ 2 clusters (dppm= bis(diphenylphosphino)methane). This review is also complemented with relevant information about their syntheses, molecular and electronic structures supported from computer modeling, EHMO and DFT calculations, and their host-guest behavior with anions and neutral molecules, in relation with their observed reactivity.