Interaction of nanostructured metal overlayers with oxide surfaces

Selectivity, that is, to produce one molecule out of many other thermodynamically feasible product molecules, is the key concept to develop 'clean manufacturing' processes that do not produce byproducts (green chemistry). Small differences in potential energy barriers for elementary reaction steps control which reaction channel is more likely to yield the desired product molecule (selectivity), instead of the overall activation energy for the reaction that controls turnover rates (activity). Recent studies have demonstrated the atomic- or molecular-level tailoring of parameters such as the surface structures of active sites that give rise to nanoparticle size and shape dependence of turnover rates and reaction selectivities. Here, we highlight seven molecular components that influence reaction selectivities. These include: surface structure, adsorbate-induced restructuring, adsorbate mobility, reaction intermediates, surface composition, charge transport, and oxidation states for model metal single crystal and colloid nanoparticle catalysts. We show examples of their functioning and describe in-situ instruments that permit us to investigate their roles in surface reactions.

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Molecular factors of catalytic selectivity

Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts, part I: Oxidation state and surface species on Pt/CeO2 under reaction conditions

Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria-based catalysts have also been developed for different applications in organic chemistry. The redox and acid-base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria-supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring-opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium-based catalysts very effective for a broad range of organic reactions.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201000054

Ceria‐Based Solid Catalysts for Organic Chemistry

Molecular beam experiments on model catalysts

Two types of model catalysts are compared: thin film catalysts consisting of polyhedral noble metal nanocrystals (Rh and Pt) supported by reducible and non‐reducible oxides, and their inverted pendants, submonolayers of titania and vanadia deposited under UHV conditions on the respective metal surfaces (Pd and Rh(111) and Rh (polycrystalline)). The structure and composition of the inverse catalysts were examined in situ by LEED and AES and the nanoparticles were characterized by HRTEM. The activity of thin film and inverse catalysts was studied in a series of reactions, such as the ring opening of methylcyclopentane and methylcyclobutane, the dissociation of CO and the CO methanation. Reaction conditions comprise atmospheric pressure but also molecular beam experiments. The reaction rates are related to the oxidation state of the supporting oxide, to the free metal surface area and to the number of sites at the interface between metal and support.

Studies of metal–support interactions with “real” and “inverted” model systems: reactions of CO and small hydrocarbons with hydrogen on noble metals in contact with oxides

The catalytic properties of a Pt (4%) and a Rh (2.5%) catalyst on a low-surface area ceria support were determined as a function of hydrogen reduction in the temperature range between 373 and 723 K and compared to those of silica-supported Rh (3%) subjected to equivalent treatments. Two reactions were studied: the ring opening of methylcyclobutane (MCB) as representative for structure-sensitive hydrocarbon reactions and the hydrogenation of CO as an example for C O bond activation. After reduction in the low- and mid-temperature range the rates of either reaction decrease significantly on ceria-supported Pt and Rh whereas they are hardly affected on Rh–silica. Annealing in vacuum at 723 K before the reduction step has a beneficial effect on the reaction rates but annealing after reduction leads to a general activity decrease. Accompanying ex situ high-resolution electron microscopy (HREM) investigations largely exclude particle decoration and formation of noble metal–Ce intermetallic bonds as possible effects of metal-support interaction at low and medium reduction temperatures. Taking also into account parallel surface science studies it is concluded that the observed activity decrease originates mainly from electronic perturbations at the interface between the metal nanoparticles and the increasingly reduced ceria support.

Interaction of Pt and Rh nanoparticles with ceria supports: Ring opening of methylcyclobutane and CO hydrogenation after reduction at 373 – 723 K

Surface structural changes of supported platinum nanocrystals upon various annealing treatments in oxygen and hydrogen were studied by high resolution transmission electron microscopy (HRTEM) combined with image contast simulation. Up to temperatures of 773 K changes in the number of surface steps were observed. Metal decoration was detected on Pt/CeO2 catalysts upon reduction at 973 K. The different catalytic behaviour of Pt/Al2O3 and Pt/CeO2 in the hydrogenolysis of methylcyclobutane is tentatively attributed to the reducibility of ceria since the surface structure of the Pt particles was identical in both catalysts.

Surface structure and methylcyclobutane hydrogenolysis activity of Pt/Al2O3 and Pt/CeO2 after reduction at increasing temperature (373 to 973 K)

Studies of Metal Support Interactions with “Real” and “Inverted” Model Systems: Reactions of CO and Small Hydrocarbons with Hydrogen on Noble Metals in Contact with Oxides

M. Fuchs

Papers

Studies of metal–support interactions with “real” and “inverted” model systems: reactions of CO and small hydrocarbons with hydrogen on noble metals in contact with oxides

Interaction of Pt and Rh nanoparticles with ceria supports: Ring opening of methylcyclobutane and CO hydrogenation after reduction at 373 – 723 K

Surface structure and methylcyclobutane hydrogenolysis activity of Pt/Al2O3 and Pt/CeO2 after reduction at increasing temperature (373 to 973 K)

Studies of Metal Support Interactions with “Real” and “Inverted” Model Systems: Reactions of CO and Small Hydrocarbons with Hydrogen on Noble Metals in Contact with Oxides