Oxides high surface area

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... 

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]... 

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... 

Metal oxide catalysts are extensively employed in the chemical, petroleum and pollution control industries as oxidation catalysts (e.g., oxidation of methanol to formaldehyde, oxidation of o-xylene to phthalic anhydride, ammoxidation of propylene/propane to acrylonitrile, selective oxidation of HjS to elemental sulfur (SuperClaus) or SO2/SO3, selective catalytic reduction (SCR) of NO, with NHj, catalytic combustion of VOCs, etc.)- A special class of metal oxide catalysts consists of supported metal oxide catalysts, where an active phase (e.g., vanadium oxide) is deposited on a high surface area oxide support (e.g., alumina, titania, ziiconia, niobia, ceria, etc.). Supported metal oxide catalysts provide several advantages over bulk mixed metal oxide catalysts for fundamental studies since (1) the number of surface active sites can be controlled because the active metal oxide is 100% dispersed on the oxide support below monolayer coverage,... 

M. L Hair. Transmission infrared spectroscopy for high surface area oxides. In A. T. Bell and M. L. Hair, editors. Vibrational Spectroscopies for Adsorbed Species, ACS Symposium Series 137, pages 1-11. American Chemical Society, Washington, D.C., 1980. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

Catidyst Preparation Nickel is used most frequently as a catalyst to produce methane. It is usually deposited on high-surface-area oxides such as y-Al203 and Ti02- Potassium is used frequently as a promoter in the catalyst formulation. Copper, copper oxide, and zinc oxide together are used to produce methanol selectively. Small particles of the mixed oxides are used usually without support. 

Most high surface area materials are oxides formed by calcining hydroxide that was in nanoscale colloid or sol form in aqueous solution. High surface area oxides (pellets) are usually composed of secondary particles that are composed of nanoscale primary particles, as shown in Figure 14.1. The primary particles are said to maintain their... 

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