Supported mixed oxides

Chemical vapor deposition might be an attractive route to produce supported mixed oxides. To achieve this, suitably volatile precursors are required. 

The state of the art of nitrogen oxides reduction over both massive and supported mixed oxides may be summarized as follows ... 

Recent studies using supported mixed oxides show encouraging results. Better characterizations of the solids are needed to ascertain the loci of catalytic activity on the surface of these complex systems. 

On the preparation side, the development of more economical synthesis methods apt for the commercial-scale production of high-surface-area solids is required. Methods such as atomic layer epitaxy offer a good route to obtain supported mixed oxides. However, this method in its present version is expensive and restricts the potential applications of these materials. The key factor in these methods is to achieve a good spreading of the active material on the support surface. New characterization procedures are needed to ascertain whether or not the supported nanoparticles of the desired compound are indeed formed. 

The preparation of supported mixed oxides is also relevant in the processing of one type of membrane materials in which the active solid is deposited on top of catalytically inactive porous structure such as alumina. However, the synthesis of membranes entirely made of mixed oxides should not be overlooked, although this is a matter that needs further refinement to produce adequate materials. 

The use of supported mixed oxides in both catalytic combustion and NO reduction are challenging applications. In the case of high-temperature catalytic combustion there is an unsatisfied need to obtain solids able to maintain high combustion activity during extended operation, i.e., high-surface-area structurally stable solids. Another hot subject that deserves exploration is the use of washcoated highly dispersed mixed oxides for the selective reduction of NO with CO and hydrocarbons. These studies should be conducted in the presence of both SO2 and H2O to evaluate the potential practical application of these catalysts. 

Bulk and supported mixed oxide compositions, from binary metal oxides to quaternary metal oxides, consist in general of large crystalline phases possessing low surface area values (typically from 1 to 10m g ). Examples of oxides of this type of catalytic relevance are V-Nb-0, Mo-Nb-O, Co-Ti-0, Ni-Ti-0, etc. The acid-base properties of mixed metal oxides have been found to change with the nature of the constituents and their relative concentrations, preparation (co-precipitation and sol-gel synthesis among are the most popular methods used), and pre-treatments procedures. Appropriately choosing the mentioned variables, mixed oxides can be used to prepare catalysts with the desired-acid-base characteristics. 

Supports are often prepared first and the catalyst and promoter components added later (60). Metal oxide supports are usually prepared by precipitation from aqueous solutions. Nitrates are commonly used anions alkaUes and ammonium are commonly used cations. Metal oxide supports, eg, sihca and alumina, are prepared in the form of hydrogels. Mixed oxides such as siUca—alumina are made by cogelation. Carefiil control of conditions such as pH is important to give uniform products. 

Performance of an Anode-supported Solid Oxide Fuel Cell in a Mixed-gas Configuration... 

An iron phosphate catalyst with a P/Fe atomic ratio of 1.2 used in this study was prepared according to the procedures described in the previous studies [6-8]. On the other hand, a V-P oxide catalyst with a P/V atomic ratio of 1.06 and pumice supported 12-molybdophosphoric acid (H3PM012O40) and its cesium salt (CS2HPM012O40) catalysts were the same as used in a previous study [9]. Pumice supported W03-based mixed oxide catalysts were the same as used in a previous study [10]. 

The possible strategies are coprecipitation to prepare mixed hydroxides or carbonates [5], cosputtering of gold and the metal components of the supports by Ar containing O2 to prepare mixed oxides [23], and amorphous alloying to prepare metallic mixed precursors [24]. These... 

Table 13.3. Hydrogenation of Aldehydes a,p-Unsaturated at 363 K and at H2 Pressure of 0.62 MPa Over Ir Catalysts Supported on Mixed Oxides. Initial Activity and TOP at 10 % of Conversion.

Figure 13.1. Hydrogenation of FFAL over Ir catalysts supported over mixed oxides, (a.) Ir/Ti-Si, (b.) Ir/Nb-Si.

Takahashi, N., Suda, K.A., Hashisuka, I. et al. (2007) Sulfur durability of NO, storage and reduction catalyst with supports of Ti02, Zr02 and Zr02-Ti02 mixed oxides, Appl. Catal. B 72, 187. 

Not only has the Ti precursor been investigated, but also the structure of the molecular sieve has been heavily investigated. Thus we now have an array of silica and silica-alumina molecular sieve supported Ti catalysts. These include Ti on amorphous Si02,10,11 Ti on a variety of Si02 mixed oxides,12 Ti-0 (titanium-beta),13"17 Ti-MCM-48,18 Ti-MCM-41,19 Ti-HMS,18 titanium-... 

It is a matter of speculation as to whether or not the activity would pass through a significant maximum at a surface composition between 0 and 30% Rh. It is interesting to note in this connection that the magnetic susceptibility (156, 157) and the electronic specific heat coefficient (156) increase from low values at 60% Ag-Pd through pure palladium and reach a maximum at - 5% Rh-Pd, thereafter decreasing smoothly to pure rhodium. Activity maxima have also been reported for reduced mixed oxides and supported alloys of group VIII metal pairs. For example, in the... 

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