Identification of surface states on finely divided supported palladium catalysts by means of inelastic incoherent neutron scattering

Palladium clusters, free and supported on surfaces, and their applications in hydrogen storage

Palladium is a late transition metal element in the 4d row of the periodic table. Palladium nanoparticles show efficient catalytic activity and selectivity in a number of chemical reactions. In this paper, we review the structural and electronic properties of palladium nanoclusters, both isolated and deposited on the surface of different substrates. Careful experiments and extensive calculations have been performed for small Pd clusters which provide ample information on their properties. Work on large Pd clusters is less abundant and more difficult to perform and interpret. Cluster deposition is a method to modify material surfaces for different applications, and we report the known results for the deposition of Pd clusters on the surfaces of a number of interesting substrates: carbonaceous substrates like graphene and some layered novel materials related to graphene, metal oxide substrates, silicon and silicon-related substrates and metallic alloy substrates. Emphasis is placed on revealing how the structural, electronic and magnetic properties change when the clusters are deposited on the substrate surfaces. Some examples of chemical reactions catalyzed by supported Pd clusters and nanoparticles are reported. An issue discussed in detail is the influence of Pd on the storage of hydrogen in porous materials. Experimental work shows that the amount of stored hydrogen increases when the absorbing material is doped with Pd atoms, clusters and nanoparticles, and a spillover mechanism from the metal particle to the substrate is usually accepted as the explanation. To shed light on this issue, a critical analysis based on density functional simulations of the mechanisms of hydrogen spillover in perfect and defective graphene doped with palladium clusters is presented.

Neutron Scattering Investigations of Hydride Species in Heterogeneous Catalysis

In heterogeneous catalysis, hydrides on the surface or in the bulk play a critical role as either active components or reaction intermediates in many hydrogen-involving reactions, but characterization of the nature and structure of these hydride species remains challenging. Neutron scattering, which is extremely sensitive to light elements, such as hydrogen, has shown great potential in meeting this challenge. In this Minireview, recent advances in neutron studies of hydride species, mainly over the two most typical classes of catalysts-metals and oxides-are surveyed. Findings on catalysts outside these categories are raised if they are considered to be relevant for contextualization in the present Minireview. The adsorption, dissociation, spillover, and reactivity of hydrogen, especially hydride species over supported metal and oxide catalysts, have been successfully investigated, mostly by means of neutron vibrational spectroscopy. Insights from these neutron studies, which are otherwise not possible with other techniques, shed light on the interaction mechanism of hydrogen with solid surfaces and reaction mechanisms in which hydrogen is involved. Future research challenges on neutron scattering studies of hydrides, as well as catalysis in general, are also highlighted, and more operando-type neutron studies need be conducted to advance the field.

Looking for the active hydrogen species in a 5 wt% Pt/C catalyst: a challenge for inelastic neutron scattering

We looked at the active hydrogen species in a highly dispersed and very homogeneous 5 wt% Pt/C industrial catalyst (Pt particle mean diameter of 2.0 ± 0.5 nm) for hydrogenation reactions, by coupling H₂ adsorption measurements with Inelastic Neutron Scattering (INS).

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.