Publisher Summary Metals constitute a wide class of catalysts, and because catalysis occurs on the surface, there is an economic incentive, especially for precious metals, to obtain catalysts in the form of small metal particles. This, however, raises two main problems. One is fundamental in nature and addresses the question as to below which particle size the metallic properties are lost. The other is more practical and concerns the preparation and characterization of very small particles and their catalytic activity. The other problem concerns the practical ways of producing metals with particle sizes around the critical value calculated above. The easiest and most practical way to achieve this goal is to deposit the metal in low concentration on a high-surface-area support. The main purpose of using a carrier is, of course, to achieve a high dispersion of the metal and to stabilize it against sintering. In a number of reactions, however, the metal support is not inert and the overall process is actually a combination of two functions: that of the metal and that of the catalytically active support.

The Influence of Particle Size on the Catalytic Properties of Supported Metals

Temperature-programmed isotopic exchange of 18O2 with 16O of several oxides was carried out in the 200−900 °C temperature range. The oxides can be ranked according to their maximal rates of exchange obtained at the following temperatures:  CeO2, 410 °C ≫ CeO2-Al2O3, 480 °C ≈ MgO, 490 °C > ZrO2, 530 °C ≫ γ-Al2O3, 620 °C ≫ SiO2, 850 °C. Except on CeO2 and on CeO2-Al2O3 a simple exchange yielding initially 18O16O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16O2 which is indicative of the presence of binuclear species (O2, O2-, or O22-) at the ceria surface. Chlorine-free rhodium catalysts supported on these oxides were prepared with metal dispersions between 32 and 89%. The presence of rhodium accelerates considerably the oxygen exchange with the support:  the maximal rates of the exchange can be observed at much lower temperatures, by about 200−300 °C with respect to the bare oxides. This is attributed to a spillover of oxyg...

Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18O2 with 16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity†

Production of hydrogen by aqueous-phase reforming of glycerol

Various experimental results on hydrogen spillover which have been obtained since the first evidence for this phenomenon are discussed concerning the nature of the activated hydrogen species. It will be demonstrated that the physical nature of the spilt-over species, especially their charge, can only be described by considering their interaction with the solid. A new model is proposed which is based on the description of spilt-over hydrogen as electron donor located at the surface. Consequently, H+ ions and H atoms coexist on the surface of the catalyst as also stated experimentally. Their ratio is determined by the electronic properties of the adsorbate/solid system. Considering atomic and recombinative desorption of spilt-over hydrogen, the adsorption isotherms are calculated exemplary for titania. The model is applied to interpret some experimental results related to hydrogen spillover, especially the partial electron transfer from the hydrogen species to the solid.

On the nature of spilt-over hydrogen

This review paper concerns allylic oxidations and oxidative dehydrogenations. The corresponding catalysts often contain two or several oxide phases. The objective is to discuss the possible reasons why these phases cooperate (act synergetically). Different interpretations are reviewed. The authors discuss in detail the evidence showing that the synergy is often due to remote control. In such a mechanism, one phase, the donor, dissociates oxygen to form a surface mobile species which spills over to the other phase, or acceptor. The acceptor is the potentially active phase. This phase needs to be irrigated by spill-over oxygen to exhibit maximum activity and selectivity. The various modifications of the acceptor brought about by spill-over oxygen are discussed: maintaining the acceptor at a high oxidation state, preventing the destruction of the structure of the acceptor, and inhibiting the formation of carbonaceous deposits or coke precursors. Parallel experiments with the same two-phase catalysts catalysing an oxygen aided dehydration suggest that the role of spill-over oxygen is to protect some Bronsted acidity of the acceptor. This interpretation of the cooperation between phases permits definite roles to be attributed to the oxide phases present in multicomponent catalysts and to measure approximately their ability to act as donors or acceptors.

Phase Cooperation and Remote-control Effects in Selective Oxidation Catalysts

Publisher Summary This chapter focuses on spillover of sorbed species. The exchange of species from one position to another, either on the surface or through the bulk, has been well established. More unique is the mobilization of a sorbed species from one phase onto another phase where it does not directly adsorb. This has been defined as “spillover. Spillover may result in a spectrum of changes in the nonmetallic phase onto which it occurs. In the weakest sense, the spiltover species is transported across the surface of this phase as a two-dimensional gas. It may exchange with similar surface species.The spiltover species may react with the surface, which can result in the creation of surface defects and/or active sites. Further, the bulk of the solid may be transformed into a different structure. In each of these cases, the second phase is no longer an inert. It is not serving to promote the inherent activity on the first phase. The second phase is participating directly in the transport, exchange, and reaction with the spiltover species. In some cases it is able to become catalytically active on its own and thereby to participate directly in subsequent catalysis. The chapter discusses the types of phenomena associated with spillover, the spillover of species other than hydrogen, the aspect of spillover with the most significant catalytic implications, the implication of spillover to catalysis and other heterogeneous processes, the mechanism of spillover, and the nature of the surface and spiltover species.

S. J. Teichner

Papers

Spillover of sorbed species

Spillover of Sorbed Species