Mixed metal oxides , defined

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

High surface area oxides are attractive materials for numerous applications in catalysis and sorption [1], There are many techniques to manually prepare these materials, such as precipitation, sol-gel pathways, templating routes and so on [2,3,4,5]. We have developed a novel versatile route which offers a simple and straightforward manner to prepare a great variety of different oxides with even higher surface areas. This method avoids filtering steps and handling of suspensions which enables simple pipette robotic systems to prepare these materials. The method is suitable for the preparation of defined phases, such as spinels or perowskites, but also for the synthesis of amorphous or multiphase mixed metal oxides and can easily be parallelized. 

Recent sol-gel methods have been recognized as promising procedures to prepare catalysts [12-14]. The sol-gel methods allows a unique way of catalyst design, because they represent an ab initio synthesis of the final solid from well defined molecular compounds [13]. By suitable choice of reagents, reaction and drying conditions, such technique allows to predefine pore structure, porosity, composition, surface polarity and crystallinity or amorphicity of metal oxides [12]. In principle, any metal that forms stable oxides can be forced to copolymerise with other metals in sol-gel procedures to provide mixed metal oxides [13]. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

Reducible mixed metal oxides (MMOs) catalysts are currently the most promising catalytic materials for the selective oxidation of propane to acrylic acid. They consist of two or more mixed oxides of transition metals, most typically based on Mo and V oxides. In general, MMO do not have well-defined structure as HPCs in fact, they are mixture of multiple crystallized and amorphous phases. Mixed oxides are typically prepared via calcination at high temperature, which gives them an excellent thermal stability. 

The examples introduced above refer to the characterization of the most common types of catalysts, usually supported metals or single, mixed, or supported metal oxides. Many other materials such as alloys [199,200], carbides [201-203], nitrides [204,205], and sulfides [206] are also frequently used in catalysis. Moreover, although modem surface science studies with model catalysts were only mentioned briefly toward the end of the review, this in no way suggests that these are of less significance. In fact, as the ultimate goal of catalyst characterization is to understand catalytic processes at a molecular level, surface studies on well-defined model catalysts is poised to be central in the future of the field [155,174], The reader is referred to the Chapter 10 in this book for more details on this topic. 

Polyphosphate is defined in this work as any linear, condensed phosphate, in which the phosphate, but not necessarily the system containing the phosphate exhibits the conditions 1 < M2O/P2O5 < 2, where M is any single or mixed metals with a total equivalency of unity. The definition is required to differentiate the total composition of a system from the polyphosphates crystallizing in a melt. An example is a polyphosphate crystallizing from a melt of ultraphosphate composition where several metal oxides may be involved. 

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