High surface-applied oxides

Many synthetic routes for preparing transition metal oxide catalysts produce a supported metal oxide structure consisting of an active metal oxide phase (the surface oxide) dispersed on a second, high surface area oxide (the support oxide) [1-3]. A key metric in characterizing SMOs is surface density. International Union of Pure and Applied Chemistry (lUPAC) defines surface density as mass per unit area [4]. For supported metal oxides, this is vaguely interpreted as the amount of supported metal oxide active phase per surface area of the underlying oxide support. This broad definition allows considerable latitude in whether total or exposed surface oxide content is considered and whether the surface area is of the uncovered support or final catalyst. Furthermore, absence of standardized methods to measure these parameters introduces additional variability into the determination of surface density. 

This section covers environmental applications of nanomaterials insofar as they are directly applied to the pollutant of interest. The photocatalytic degradation of organic pollutants and remediation of polluted soils and water are discussed here. The high surface areas and photocatalytic activities of semiconductor nanomaterials have attracted many researchers. Semiconductor nanomaterials are commercially available, stable, and relatively nontoxic and cheap. Prominent examples that are discussed are metal oxides such as Ti02 and ZnO and a variety of Fe-based nanomaterials. 

Present catalysts are developed for process plant service where transient conditions are not a concern. Typical shift catalysts, such as copper-zinc oxide, are reduced in place and must be isolated from air. There is a need for smaller, high surface area catalyst beads on low-density monolith substrate to be developed without reducing activity. This need applies to all fuel processor catalyst, not just the shift catalysts. There is also a need to demonstrate that the low-temperature, PROX catalysts have high selectivity toward CO and long term stability under operating conditions. 

Although this type of transformation can take place in solution, usually under hydrothermal conditions, it has been most intensively investigated in the dry state. A precise separation of a transformation in the dry state from that in the presence of vater is, ho vever, often difficult because the minimum amount of water with which a via-solution transformation is still possible may be very small (see 14.3.5). This applies especially to poorly ordered and nano-sized oxides, such as ferrihydrite, with high surface areas and, therefore, high amounts of adsorbed water. 

It must not, however, be forgotten that conventional techniques (e.g., 13C Fourier transform NMR) can be applied to certain solids of catalytic significance, such as sheet silicates since in many of these systems rapid motion of intercalated or otherwise sorbed organic species secures sharp absorption lines which provide much information about the individual atomic environments. Organic species attached to high surface area solids (such as zeolites, silica, alumina, magnesia, as well as other oxides and their mixtures) are specific examples (6). 

The catalysts are high surface area ruthenium pyrochlore oxides having the general formula A2+xRu2-x 7-y (A=Pb>Bi 0selective oxidation of cydohexane-1,2-diol to adipate (reaction 20) under mild conditions (25-95 13). 

These catalysts are composed of one or several metallic active components, deposited on a high surface area support, whose purpose is the dispersion of the catalytically active component or components and their stabilization [23-27], The most important metallic catalysts are transition metals, since they possess a relatively high reactivity, exhibit different oxidation states, and have different crystalline structures. In this regard, highly dispersed transition clusters of metals, such as Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some alloys, and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn, normally dispersed on high surface area supports are applied as catalysts. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

The TPR method has now been applied to TlN O [52], VN [23], and NbC [53]. Although the TPR method produces high surface area materials, the pore structure of these is usually not controllable. Often, the pores are in the micropore regime (< 3 nm). However, a number of the solid state transformations that lead to carbides and nitrides are topotactic and exhibit pseudomorphism (retention of external particle size and shape) This provides a possible means of engineering the pore structure by preparing oxide precursors with the desired external morphology. 

---