Composite Titanium Dioxide Nanomaterials

Publisher Summary This chapter discusses the interaction of small metal particles with reducible oxide supports, and focuses on Group VIII (or Group 8-10) metals supported on TiO 2 . The strong suppression in catalytic activity and chemisorption capacity observed on TiO 2 -supported catalysts after reduction at high temperatures can be, at least partially, explained by a geometric blocking of sites by TiO 2 species. The dominant mechanism of formation of TiO 2 species on metal particles is migration during the high-temperature reduction, but some deposition of metal precursors during preparation may also occur. This picture finds an analog in the structure of the bimetallic clusters observed in Group VIII-Group Ib catalysts, in the sense that the structure sensitive reactions, e.g., alkane hydrogenolysis, are most affected whereas only modest effects are observed for structureinsensitive reactions, e.g., hydrogenation-dehydrogenation. The migration of reduced species from the support is accompanied by the formation of metal-Ti bonds, which provide the thermodynamic driving force for the migration. The metal-support bonding is not identical to that in the intermetallic compounds because of the associated oxygen that imparts cationic character to the Ti and Group VIII metal partner; in other words, it may be analogous to the Group VIII-Ti bonding that persists on intermetallic compounds surfaces after oxygen chemisorption.

Metal–Support Interaction: Group VIII Metals and Reducible Oxides

In 1973 the oil crisis prompted considerable world interest in the production of synfuels which, as a result, led to the development of the Mobil Methanol to Gasoline Process [1] and more recently to the Shell Middle Distillate Process [2]. Synfuels have been a subject of both scientific and political interest for most of the 20th century. With the recent Gulf crisis, which has resulted in a surge in crude oil prices, it is clear that the search to find economic alternatives to the use of petroleum as an energy source will continue. This observation is based on the fact that the world reserves of coal and natural gas far exceed those of oil, and this fact alone implies that the conversion of coal and natural gas to transportation fuels is almost certain to become more widespread within the next 20 years.

Mechanistic Aspects of the Formation of Hydrocarbons and Alcohols from CO Hydrogenation

This paper describes the synthesis of CeO2-promoted Ni/SBA-15 catalysts (denoted as CeNi/SBA-15) via a surfactant-assisted iso-volumetric impregnation method and their application in ethanol steam reforming. Various techniques including N2 adsorption–desorption, X-ray diffraction, H2 temperature-programmed reduction, temperature-programmed oxidation, transmission electron microscopy and thermogravimetric analysis were employed to characterize the prepared and spent catalysts. The results showed that the incorporation of CeO2 could effectively control the particle size of Ni via strong metal-support interaction and promote the homogeneous distribution of Ni and Ce to achieve large Ni–CeO2 interface. Consequently, the CeNi/SBA-15 catalysts exhibited superior activity with respect to the reference Ni/SBA-15 catalyst without ceria addition. The effect of ceria loading on the properties of the catalysts was also investigated, and the catalyst with Ce/Ni atomic ratio of 1 was the optimized composition owing to its strong metal-support interaction and high nickel active surface area. The ceria-promoted catalysts showed enhanced long-term stability in ethanol steam reforming. The strong interaction between Ni and CeO2, as well as the confinement of SBA-15 support restricted the nickel particle growth under harsh reaction conditions. Additionally, the ceria promoter contributed to suppressing coke deposition effectively, which led to the enhanced coking-resistance as observed on CeNi/SBA-15 catalysts.

Ceria-promoted Ni/SBA-15 catalysts for ethanol steam reforming with enhanced activity and resistance to deactivation

We review synthesis, structural characterization, and catalysis by supported bimetallic clusters. Initial steps in the preparations include (a) adsorption of molecular metal clusters with preformed metal−metal bonds; (b) synthesis in zeolite cages; and (c) adsorption of a metal complex on a supported complex or cluster of another metal. Chemisorption capacities of supported oxophilic metal−noble metal clusters for hydrogen and CO are typically less than those of supported clusters of just the noble metal. Catalytic properties distinguishing supported bimetallic clusters from monometallics include high selectivities for oxygenates in CO hydrogenation and different selectivities for CO vs CC bond hydrogenation. Advantages of precise synthesis with organometallic precursors (smallness and uniformity of the supported clusters with bonds between the two metals) are offset by the expensive preparations and difficulty of catalyst regeneration. Applications of supported bimetallic clusters are most likely in spec...

R. Burch

Papers

Strong metal-support interactions in nickel/titania catalysts: The importance of interfacial phenomena