Surface area, oxides

It has been reported that below about 370°C, sulfur oxides reversibly inhibit CO conversion activity. This inhibition is greater at lower temperatures. CO conversion activity returns to normal shortly after removal of the sulfur from the exhaust (44). Above about 315°C, sulfur oxides react with the high surface area oxides to disperse the precious-metal catalytic agents and irreversibly poison CO conversion activity. 

Other explanations have been offered for the unusually high spectral background encountered in high surface area oxide materials. Buechler... 

The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

It must be noted that short contact time reactors are typically operated under adiabatic conditions with outlet temperatures of the order of 700-1000 °C. Under such conditions, with respect to other noble metals, Rh is believed to be especially stable due to a low vapor pressure and an increased resistance to carbon formation even under severe operating conditions. The use of low surface area oxides such as (Z-AbO j and ZrO2 as support materials has been reported to improve the catalyst stability by limiting the coarsening of Rh particles while avoiding incorporation of Rh within the oxide structure [148]. 

Transmission Infrared Spectroscopy for High Surface Area Oxides... 

Washcoat High surface area oxide impregnated with catalytic species and bound to the walls in the channel of a monolithic structure. 

Refractory high surface area oxides are deposited from slurries onto the walls of the channels that make up monoliths in order to provide an adequate surface area to support the active catalytic species. Washcoats such as AI2O3 and TiC>2 are commonly used for pollution abatement applications (auto exhaust, stationary NO abatement, etc.) where the monolith is usually a ceramic. Metal monoliths are finding increasing use however, they represent only a small percentage of the total monoliths used. Optical microscopy enables one to see that the catalyzed washcoat follows the contour of the ceramic surface. Figure 7 shows the AI2O3 washcoat-ceramic interface for a typical auto exhaust catalyst. In this case, no evidence of loss of adhesion between washcoat and ceramic can be seen. 

The analysis of smoke and soot formation from polymers during combustion has been extensively studied 50,51 however, less is understood on how hydrated fillers influence this mechanism. It is likely that smoke reduction results from the deposition of carbon onto the high surface area oxide surface, produced on the decomposition of the filler.38 The volatilization of carbonaceous residue as carbon oxides then occurs, reducing obscuration effects from the smoke. 

Summarizing, Raman spectra can be recorded of single crystals, powders, glasses, nanocrystalline, and amorphous materials, and of surface species such as transition metal compounds on high-surface-area oxide supports or surface adsorbates on some metals. Thus, Raman spectroscopy is a potentially valuable tool for the characterization of a broad range of catalytic materials and surfaces. 

Keggin-type heteropoly compounds have attractive and important characteristics in terms of catalysis. They consist of heteropolyanions and counter-cations such as H, Cs or NHT When the counter-cations are protons, they are called heteropolyacids (HPA). An important characteristic of HPAs, such as 12-tungstophos-phoric acid (H3PW12O40), is the presence of very strong Bronsted acid sites. But the characteristics of HPAs strongly depend on temperature and relative humidity. When they are used in heterogeneous catalysis, it is often necessary to support them on high-surface-area oxides or activated carbons, in order to increase the surface contact with the reactants. 

Supported metal oxide catalysts consist of dispersed surface metal oxide species, the catalytic active sites, which are supported on high-surface-area oxides [1-3]. The... 

Supported M0S2 and WS2 No Promoter. Supports include high surface area oxides especially 7-AI2O3 and Si02, and carbon. The role of the support is to disperse the active components so increasing their effective surface area and catalytic activity. Oxide supports may also participate in isomerization and cracking. Interaction of an active component with a support during... 

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. 

For some compounds like titanium and zirconium, alkoholates are the best compounds, but for most of the metals we found that very good results can be achieved by using highly concentrated nitrate solutions. This is very important because most of the metal species are available as nitrate compounds, which are normally quite stable and can easily be handled in air. Experiments with halogenides and acetates showed a decreased surface area compared to the nitrates. As can be seen in Table 3, the surface area is higher for higher concentrated metal salt solutions. In some cases, like iron oxides, even molten nitrate compounds can be used and successfully turned into high surface area oxides. 

All BET surface areas are reported in Fig. 5. Whatever the series, introduction of Pr results in a decrease of the BET surface area. For series 1 samples, this decrease is even more pronounced for x > 0.5. It seems that the presence of Pr60u could be responsible for this decrease of surface area. Furthermore, the presence of zirconium remains crucial to stabilize cerium-praseodymium oxides BET surface areas of series 1 samples are much higher than those of series 2, except for x = 1. Praseodymium can stabilize the texture of ceria but its influence in obtaining high surface area oxides appears to be much weaker than in the case of zirconium addition. [2-4, 16]... 

Above lOOD C, monoclinic d-Al Oj forms, transforming to hexagonal (ABABAB) a-AljO] at 12(X) C. These are anhydrous, low surface area oxides and are not suitable for porous supports. They are used tn applications where mechanical strength, and not high surface area, is required. 

In practice, the TPR profiles of high-surface-area oxides may lead to additional features compared to the low-surface-area samples. In the case of Ce02, when no appropriate pre-treatment of the sample is adopted before the TPR, the reduction profiles are indeed affected both by the presence of reducible surface species and/or bulk carbonates whose... 

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