Reducible supports

As already indicated, the difficulty of reducing supported iron in hydrogen is well-known [6,8,11]. It probably arises from a combination of causes, the two most important of which are a strong interaction with the support [6,8] and reoxidation or inhibition by water vapour in the pores of the oxide [14]. With MgO as support, there is undoubtedly a strong tendency for iron, especially at the Fe2+ stage of reduction, to be present at least in part as FeO-MgO (Fe Mgi.jjO) solid solution [6,8]. This need not be deleterious to the ultimate formation of finely-divided iron, provided the method of preparation has led to a solid solution in which the Fe2+ ions are well-distributed. The iron particles are limited in size... 

Oxygen uptake stoichiometries of pre-reduced supported polyoxometalates... 

On reducible supports typically when Pd is deposited on LaCoOj then reduced in H2 at 450°C for obtaining Pd0/CoOx/La2O3, an alternative mechanism would likely occur, which accounts for steps involving the creation of active sites at the metal/support interface. These active sites would be composed of metallic Pd in interaction with anionic vacancies from the support potentially active for the dissociation of NO according to step (26) [54],... 

DOD is interested in new or novel advanced power and propulsion systems that will reduce fuel consumption, improve performance, extend vehicle range, reduce emissions, and reduce support costs. The Navy and Army are considering hybrids for ships, land vehicles, helicopters, and battlefield power requirements. 

Commonly, transition metals on non-reducible supports are used for the selective hydrogenation of aromatic ketones, with most research to date having been done on Pt-based catalysts. In particular, we will discuss here some interesting results published on this subject. 

A number of studies have highlighted the possibility of the use of monometallic Pd catalyst for effective nitrate/nitrite reduction if a reducible support is selected. Epron et proposed a reaction mechanism for Pd/Ce02, (Fig. 1) in which oxygen vacancies at the Pd-support interface were active sites for the reduction of nitrate. Although the selectivity to nitrogen was poor, the... 

Other reagents that have been used to reduce support-bound aromatic nitro compounds include phenylhydrazine at high temperatures (Entry 5, Table 10.12), sodium borohydride in the presence of copper(II) acetylacetonate [100], chromium(II) chloride [196], Mn(0)/TMSCl/CrCl2 [197], lithium aluminum hydride (Entry 3, Table... 

In this work the term SMSI is not restricted only to the cases when the reduced support creeps over the metal particle, so that the latter becomes partially covered with metal cations and anions of the support (e.g., Ti3+ and O2 from Ti02), but confines also the cases when the interaction leads to the formation of intermetallic compounds (e.g., Pd3Si). 

A preliminary heat treatment will remove many if not all hydroxyl groups, and bands in the 1200-1700 cm-1 then do not appear conversely when water is present they are seen more strongly, and with a reducible support such as zinc oxide the reaction... 

From this, we may conclude that while reducible supports perform best, gold on irreducible ceramic oxides still shows a modicum of activity providing that the particles are small enough the reaction then proceeds solely on the gold without any assistance from the support. 

Catalytic reaction conditions or the exposure to reducing environments may lead to the formation of reduced surface metal oxide species. It is generally difficult to obtain good Raman signals for reduced supported metal oxide species because of their low Raman cross-sections. On the other hand, many reduced transition metal ions have electronic absorption bands in the visible regime. Hence, the laser frequency may be tuned to these absorption bands, and resonantly enhanced Raman spectra should be obtained. 

Payen et al. (1986) investigated the reduction of alumina-supported molybdena and ascribed a Raman band at 760 cm-1 to reduced supported molybdenum oxide. The transformations could be reversed by reoxidization (Payen et al., 1986). Mestl and Srinivasan (1998) described some reduced phases formed from bulk molybdena, whereas reduced dispersed vanadia and chromia catalysts do not show Raman bands (Airaksinen et al., 2005 Banares et al., 2000a Gasior et al., 1988 Weckhuysen and Wachs, 1996). 

Pretreatment is extremely important in determining surface chemistry. Samples must be carefully prepared, dried, oxidized, reduced, and kept in the desired final state until admission of probe molecules and recording of spectra. Seemingly minor differences in preparation or pretreatment can produce major differences in the surface chemistry of many catalysts. Adsorbable impurities in conventional high vacuum systems do not usually seriously contaminate typical high-area catalyst samples. If, however, samples stand for many hours significant changes can often be observed, particularly with reduced supported metals. 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

Currently, PC hardware has a life cycle of less than 2 years. Operating software typically has a support hfe of 5 years, then a further 2 years of reduced support before the operating system is no longer supported by the supplier. Management of the IPC upgrade protects the system investment from threats to the system s validated status, serious maintenance problems, and potentially costly fast-track projects. 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

Figure 9.6. A comparison of CO TPD from (a) oxidized and (b) reduced ceria loaded with Rh. Oxidation of CO to CO2 is decreased for a reduced support. Reduction of ceria opens a new channel for desorption at high tempera lures. Isoiope experiments (c) show that CO is recomb i native at higher temperatures. Adapted from ref. 94,...

Measurements of the hydrophobic attraction between solid surfaces using an atomic force microscope have also been performed in solutions of NaCl and NaClOa. The attraction was found to be equivalent to that in water in 0.2M NaClOs, whilst in 0.2M NaCl the attractive force was much reduced, supporting expectations. (NaClOa, has no effect on bubble coalescence, NaCl does, cf. Table 3.1.) It has also been observed previously that the range of the attractive force acting between bubbles is substantially reduced from about lOOnm to 40nm on addition of KCl above the transition concentration. 

---