The Active Phase

In addition to the requirements with respect to size, shape, and mechanical stability, the nature of the active phase also has to be adopted when the same catalyst is applied in different reactor concepts mainly due to differing process conditions. Vanadium phosphorous oxide composed of the vanadyl pyrophosphate phase (VO)2P207 is an excellent catalyst for selective oxidation of H-butane to maleic anhydride [44-47]. This type of catalyst has been operated in, for example, fixed-bed reactors and fluidized-bed-riser reactors [48]. In the different reactor types, different feedstock is applied, the feed being more rich in //-butane (i.e. more reducible) in the riser-reactor technology, which requires different catalyst characteristics [49]. 

The entire spectrum of tools in synthetic inorganic chemistry, including high temperature methods, precipitation, solvothermal synthesis, sol-gel chemistry, chemical vapor deposition (CVD), and soft matter techniques adopted in part from organo-metallic chemistry, have been applied to synthesize the active mass of catalysts. Table 4.2.1 summarizes basic techniques. The preparation methods differ whether 

Precipitation and coprecipitation Synthesis of defect-rich materials, easy to perform Homogeneity difficult to achieve 

Microemulsions Particle-size control High consumption of solvents 

Sol-gel techniques Homogeneity of the product Carbon-containing impurities from metal precursors 

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An additional effect of the use of an organic medium in the catalyst preparation is creation of mote defects in the crystalline lattice when compared to a catalyst made by the aqueous route (123). These defects persist in the active phase and are thought to result in creation of strong Lewis acid sites on the surface of the catalysts (123,127). These sites ate viewed as being responsible for the activation of butane on the catalyst surface by means of abstraction of a hydrogen atom. 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

In service, supported catalysts frequentiy undergo loss of activity over a period of time. In many cases, such catalyst deactivation is accompanied by the loss of accessible surface area of the active phase by sintering, by the accumulation of poisons, or by conversion of active sites to inactive species. 

The active phase, which is soHd at room temperature, is comprised of mixed potassium and sodium vanadates and pyrosulfates, whereas the support is macroporous siUca, usually in the form of 6—12 mm diameter rings or pellets. The patent Hterature describes a number of ways to prepare the catalyst a typical example contains 7 wt % vanadium pentoxide, 8% potassium added as potassium hydroxide or carbonate, 1% sodium, and 78 wt % siUca, added as diatomaceous earth or siUca gel, formed into rings, and calcined in the presence of sulfur dioxide or sulfur trioxide to convert a portion of the alkah metal salts into various pyrosulfates (81,82). 

The second phenomenon, i.e. the change in catalytic activity or selectivity of the active phase with varying catalyst support, is usually termed metal-support interaction. It manifests itself even when the active phase has the same dispersion or average crystallite size on different... 

In all these cases the support has a dramatic effect on the activity and selectivity of the active phase. In classical terminology all these are Schwab effects of the second kind where an oxide affects the properties of a metal. Schwab effects of the first kind , where a metal affects the catalytic properties of a catalytic oxide, are less common although in the case of the Au/Sn02 oxidation catalysts9,10 it appears that most of the catalytic action takes place at the metal-oxide-gas three phase boundaries. 

Most of the electrochemical promotion studies surveyed in this book have been carried out with active catalyst films deposited on solid electrolytes. These films, typically 1 to 10 pm in thickness, consist of catalyst grains (crystallites) typically 0.1 to 1 pm in diameter. Even a diameter of 0.1 pm corresponds to many (-300) atom diameters, assuming an atomic diameter of 3-10 10 m. This means that the active phase dispersion, Dc, as already discussed in Chapter 11, which expresses the fraction of the active phase atoms which are on the surface, and which for spherical particles can be approximated by ... 

This problem can in many cases be overcome by dispersing the active phase on an electronically conductive material (Fig. 12.1). There have been already at least three experimental studies,7 1215 surveyed here and demonstrating this concept. 

Two cases of electrochemical promotion of commercial catalysts have been very recently reported in the literature and, not too surprisingly, in both cases the active phase was conductive, electronically or ionically. 

But, the increasing of the yields, in this case, shows that the catalytic cycle does not involve any radical species which can be trapped. Therefore the hydroquinone inhibition is probably connected with a sensitive redox process in the activation phase. 

These are a elass of lymphoeytes thought to be distinet from helper and eytolytic T eells. Their funetion is to inhibit the activation phase of the immune responses. Their existenee as a distinet population of cells has been doubted by many investigators, but they may be lymphoeytes that can inhibit immune responses in different ways. 

In surface Nl content, a linear correlation between rate and surface Nl concentration Is not necessarily obtained. This can be easily visualized If one takes Into account that most often the active sites are only a small fraction of the active phase exposed and that some reactions are strongly affected by the size of the active metal particle. 

As a consequence it is unlikely to be the intercalated state per se which is the active phase. 

Support materials are commonly u.sed in heterogeneous catalysis. Their major function is to maximize the dispersion of the active phase by providing a large surface area over which the active phase can be distributed. In this way the cataly.st material is shaped into a form suitable for use in technical reactors. Supports are not always chemically inert they can also show certain catalytic activity and often they act as a stabilizer for the actual active phase. A number of materials are u.sed as catalyst supports. Table 3.2 gives an overview. 

Impregnation. Two methods are used in adding the active phase dry and wet impregnation. Dry impregnation is also referred to as pore volume impregnation , because... 

Depending on the process conditions, different profiles of the active phase over the particle will be obtained. A completely uniform distribution of the active material over the particle is not always the optimum profile for impregnated catalysts. It is possible to purposely generate profiles in order to improve the catalyst performance. Fig. 3.28 shows four major types of active phase distribution in catalyst spheres. 

Drying. The drying step, which follows the impregnation step, also affects the distribution of the active phase. In drying, the solution in the pores will become supersaturated, so precipitation takes place. Because of the large surface area, and associated herewith the large number of nuclei, a high dispersion can be obtained. In principle, rapid evaporation is... 

In the first step the solution enters the pores. The driving forces for the flow are capillary forces. During the flow, adsorption by ion exchange occurs. Due to the high rate of adsorption an uneven distribution of Pt ions results. Subsequently, a situation exists in which the diffusion through the pore mouth becomes rate determining. The active phase is pre.sent as a shell, which moves towards the interior of the particle as shown in Fig. 3.30. 

Figures 3.48 shows an artist impression of some typical particle shapes of the active phase.

Figure 3.48. An artist impression of possible shapes of catalyst particles present on a support a. spherical particle with only one point contact to support, b. hemispherical particle, strongly bonded to support and partially poisoned, c. metal crystallite, strongly bonded to and partially encapsulated in support, d. complete wetting of the support by the active phase. After Scholten et al, 1985 and Ba.stein cr a/., 1987.

Note that the particle shape is affected by the interaction between the active phase and the support and by the surface free energy. The former tends to lead to spreading of a particle, whereas the latter tends to form spherical particles (Scholten et al., 1985). When particles are partially poisoned (Fig. 3.48.b), chemisorption data can be interpreted wrongly the average particle size is overestimated. The same applies to particles encapsulated in the support. 

The catalyst composition has a role in the control of selectivity. The rutile-type V/Sb/(Nb) mixed oxide activates the hydrocarbon and ammonia. However, most of the ammonia is burnt to N2, rather than being inserted on the hydrocarbons this likely occurs because the catalyst is not veiy efficient in the generation of the selective Me=NH species when reaction temperatures lower than 400°C are used (11). In fact, with all catalysts the selectivity to A -containing compounds increased when the reaction temperature was increased, and the selectivity to N2 correspondingly decreased (Figure 40.6). The dilution of the active phase with tin... 

The industrial catalyst for n-butane oxidation to maleic anhydride (MA) is a vanadium/phosphoras mixed oxide, in which bulk vanadyl pyrophosphate (VPP) (VO)2P207 is the main component. The nature of the active surface in VPP has been studied by several authors, often with the use of in situ techniques (1-3). While in all cases bulk VPP is assumed to constitute the core of the active phase, the different hypotheses concern the nature of the first atomic layers that are in direct contact with the gas phase. Either the development of surface amorphous layers, which play a direct role in the reaction, is invoked (4), or the participation of specific planes contributing to the reaction pattern is assumed (2,5), the redox process occurring reversibly between VPP and VOPO4. 

Schizoaffective and mood disorder exclusion Schizoaffective disorder and mood disorder with psychotic features have been ruled out because either (1) no major depressive, manic, or mixed episodes have occurred concurrently with the active-phase symptoms or (2) if mood episodes have occurred during active-phase symptoms, their total duration has been brief relative to the duration of the active and residual periods. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

The results presented in this paper therefore show that V and Mo species supported on alumina can give rise to a catalyst which has a high selectivity for the oxidation of propane to propene and a reasonable selectivity to acrolein and that both species are essential to give the optimal behaviour. Contrary to our previous observations and what observed for bulk catalysts [5], the presence of Nb and W seem to have little effect, perhaps because the methods used here restrict the active phase to a monolayer whereas previously prepared materials may have contained multilayer oxidic species. 

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