A Small Paramagnetic Platinum Cluster in an NaY Zeolite: Characterization and Hydrogen Adsorption and Desorption

Tuning the Catalytic Activity of Pd x Ni y (x + y = 6) Bimetallic Clusters for Hydrogen Dissociative Chemisorption and Desorption

Density functional theory was used to study dissociative chemisorption and desorption on Pd _x Ni _y (x + y = 6) bimetallic clusters. The H₂ dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd₃Ni₃ and Pd₂Ni₄ clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd₃Ni₃ and Pd₂Ni₄ are also lower than that of the Pd₆ cluster and comparable to that of the Ni₆ cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific Pd _x Ni _y bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

Superatom chemistry: promising properties of near-spherical noble metal clusters

Atomic angular moments are nearly quenched in bonded structures, but superatoms in cylindrical environments develop molecular orbital moments.

Structural stability and uniformity of magnetic Pt13 nanoparticles in NaY zeolite

Based on first-principles calculations, the structural stability and magnetic variety of Pt₁₃ nanoparticles encapsulated in a NaY zeolite are investigated. Among 50 stable isomers in the gas phase, due to geometrical constraints, only about 1/3 of those clusters can be inserted in the zeolite pores. Severe structural rearrangements occur depending on whether the solid angle at the Pt vertex bound to the super-cage is larger than 2 sr (i.e., icosahedron). The most relevant example is the structural instability of the icosahedron and, when including van der Waals dispersion forces the opening of the gas phase global minimum moves towards a new L-shaped cubic wire, otherwise unstable. The total magnetisation of the encapsulated Pt₁₃ decreases due to the stabilisation of less coordinated isomers, with the majority of clusters characterised by a total magnetisation of 2μ_B, while the majority of free clusters exhibit a threefold value. This analysis allows the understanding of the magnetic behaviour observed in recent experiments through the variety of the isomers which can be accommodated in the zeolite pore.

Reactivity of diatomics and of ethylene on zeolite-supported 13-atom platinum nanoclusters

CO and NO react on hydrogen-covered 13-atom Pt clusters, O₂ does not, and the hydrogenation of ethene shows structure sensitivity.

Single-molecule sensing using carbon nanotubes decorated with magnetic clusters

First-principles and nonequilibrium Green's function techniques are used to investigate magnetism and spin-polarized quantum transport in metallic carbon nanotubes (CNT) decorated with transition metal (Ni(13), Pt(13)) magnetic nanoclusters (NC). For small cluster sizes, the strong CNT-NC interaction induces spin-polarization in the CNT. The adsorption of a benzene molecule is found to drastically modify the CNT-NC magnetization. Such a magnetization change should be large enough to be detected via magnetic-AFM or SQUID magnetometry, hence suggesting a novel approach for single-molecule gas detection.

Size effect of Pd clusters on hydrogen adsorption

The size effect of Pd clusters on hydrogen adsorption was investigated by density functional theory calculations. From molecular dynamics simulations, we found that the hydrogen molecules were dissociatively adsorbed on the Pd clusters; the most stable adsorption site was the hollow site. We also found that the adsorption energy increased as the size of Pd clusters decreased. These results were in good agreement with experimental findings. By analyzing the electronic structure, we found that the d-band center shifted toward the Fermi energy as the size of the Pd cluster decreased. This shift resulted in stronger hydrogen bonding.

A mechanistic study of hydrogen spillover in MoO(3) and carbon-based graphitic materials

We present a systematic study of the mechanisms of the hydrogen spillover process from a Pt(6) cluster onto a well-known hydrogen bronze material, MoO(3), and several carbon-based materials, including a graphene sheet and single walled carbon nanotubes, using density functional theory (DFT). We show that initially hydrogen undergoes a sequential dissociative chemisorption upon interacting with the Pt(6) cluster. The threshold desorption energy of H atoms was identified. We then mapped out the energetics required for hydrogen atoms to flow onto the surfaces of the selected materials in the vicinity of the subnanometer Pt(6) particle and to diffuse to other sites of the substrates. Our results indicate that while the spillover of H atoms onto the MoO(3) lattice can be greatly facilitated by the abundant H-bonding network, the process becomes energetically difficult on carbon-based materials via chemisorption since it requires C-H bond breaking. Spillover in the selected carbon-based materials could only become possible if 'cold' H atoms could come out of the saturated catalyst.

EPR and DFT study on the stabilization of radiation-generated methyl radicals in dehydrated Na-A zeolite

Electron paramagnetic resonance (EPR) spectroscopy was applied to study paramagnetic species stabilized in Na-A zeolite exposed to gaseous methane and gamma-irradiated at 77 K. Two types of EPR spectra were recorded during thermal annealing of zeolite up to room temperature. Owing to the results for the zeolite exposed to (13)CH(4) the multiplet observed at 110 K was assigned to a (.-)CH(3)...Na(+) complex. After decay of the multiplet, the isotropic quartet of methyl radical was recorded in the temperature range of 170-280 K. On the basis of the EPR parameters it is postulated that (.-)CH(3) radicals in this temperature region are able to freely rotate inside the zeolite cage. The structures of the (.-)CH(3)...Na(+) adsorption complex and respective hyperfine coupling constants were calculated by applying DFT quantum chemical methods. Two different models were applied to represent the zeolite framework: the 6T structure of one six-membered ring and the 3T cluster. The hyperfine coupling constants calculated for the (.-)CH(3)...Na(+) adsorption complex for both applied models show very good agreement with those obtained experimentally.

Structure and magnetization of small monodisperse platinum clusters

Magnetization measurements of well-characterized monodisperse Pt clusters consisting of 13+/-2 atoms in a zeolite confirm the predicted extraordinary magnetic polarization with up to 8 unpaired electrons on a cluster, corresponding to a magnetic moment of 0.65(5) microB per atom. The effect is partly quenched by hydrogen chemisorption. The study provides insight into the electronic structure of the cluster and is fundamental for an understanding of how magnetism develops in small clusters.

Size matters: why nanomaterials are different

Gold is known as a shiny, yellow noble metal that does not tarnish, has a face centred cubic structure, is non-magnetic and melts at 1336 K. However, a small sample of the same gold is quite different, providing it is tiny enough: 10 nm particles absorb green light and thus appear red. The melting temperature decreases dramatically as the size goes down. Moreover, gold ceases to be noble, and 2-3 nm nanoparticles are excellent catalysts which also exhibit considerable magnetism. At this size they are still metallic, but smaller ones turn into insulators. Their equilibrium structure changes to icosahedral symmetry, or they are even hollow or planar, depending on size. The present tutorial review intends to explain the origin of this special behaviour of nanomaterials.