Surface areas supports

The primary determinant of catalyst surface area is the support surface area, except in the case of certain catalysts where extremely fine dispersions of active material are obtained. As a rule, catalysts intended for catalytic conversions utilizing hydrogen, eg, hydrogenation, hydrodesulfurization, and hydrodenitrogenation, can utilize high surface area supports, whereas those intended for selective oxidation, eg, olefin epoxidation, require low surface area supports to avoid troublesome side reactions. 

The SHB concept was expanded to chiral phosphine catalysts by de Rege et al., who reacted the trifluoromethanesulfonate (triflate) counter anion of the cationic complex [Rh(COD)((R,Rj-MeDuPhos)] with the surface hydroxyl groups of the silaceous mesoporous material MCM-41 [122]. The complex was loaded to a level of 1.03 wt% Rh. A decrease in support surface area and pore volume is consistent with the complex being located within the support pores. The counterion is very important in this process if the anion of the homogeneous catalyst precursor is altered to BArp no adsorption of the catalyst is observed. It is postulated that the mechanism of triflate binding is hydrogen bonding with the support, and that the... 

A related problem is the reduction in support surface area. This is especially a problem in the case of titania, where the anatase polymorph is only stable under oxidative regeneration conditions from about 400°C to 750°C. The addition of Si, Zr and Ta as promoter elements may avoid or diminish surface collapse of the support oxide. 

There are 4 jjimol of R groups per square meter of support surface area, with little bleeding of the stationary phase from the column during chromatography. For separating optical isomers (Figure 25-8), many optically active R groups, such as the one Exercise 25-B, are commercially available.5... 

Catalyst/Support Surface area (m2 g ) Pore volume (m3 g I)... 

For a "co-impregnated" catalyst all, or the dominant fraction of, both Pt and Sn are located on the surface alumina support. For the following discussion we consider the role of only the support surface area, the metal concentration and the Sn/Pt ratio. First consider the case of a series of catalysts with a constant Pt loading but with variable Sn/Pt ratios. 

The interaction of cupric ions with alumina supports has subsequently been studied more extensively as a function of the support surface area, metal loading, and calcination temperature (93,279) by means of EXAFS and X-ray absorption-edge shifts, in conjunction with XRD, EPR, XPS, and optical reflectance spectroscopy. These techniques, each sensitive to certain structural and electronic aspects, allow a unified picture of the phases present and the cation site location. Four Cu2 + ion sites are distinguished in the catalysts. In low concentrations (typically below about 4 wt. % Cu/100 m2/g support surface area) Cu2 + ions enter the defect spinel lattice of the A1203 support. The well-dispersed surface copper aluminate has Cu2+ ions predominantly occupying tetragonally (Jahn-Teller) distorted octahedral sites, although... 

The objective in the preparation of supported catalysts is to have the catalytically active crystallites separated. When this is the case the only way sintering can occur is if the catalyst particulates migrate across the support surface or if there is a vapor phase sintering promoter present in the reaction medium. The lower the catalyst load and the higher the support surface area, the less likely that sintering will take place. The migration ability of the catalytically active species depends primarily on the strength with which it is bonded to the support. If there is a weak interaction the catalyst particles can move across the support... 

Support Surface Area (m2 / g) Chlorine Content (wt%) Sulfur Content (wt%)... 

Shortly after the initial foray into the use of microporous titanosilicates as the highly dispersed Ti-supports for propylene epoxidation, interest shifted to meso-porous titanosilicates. Mesoporous Ti-containing materials are similar to micro-porous materials in that they offer highly dispersed Ti centers and reasonably well-defined tetrahedral Ti sites incorporated in a silicious framework. Moreover, the existence of a mesoporous pore system of sufficient dimensions to incorporate Au species in the range of 2 nm allows for Au entities to access essentially the entirety of the support surface area and enhances transport of reactants and products to and from the sites. 

The Pd/CNF catalyst displays a catalytic activity as high as that obtained on a commercial catalyst supported on activated charcoal despite the large difference between the two supports surface area, i.e. 100 m /g for the CNFs instead of 1000 m /g for the activated charcoal. The high hydrogenation activity observed on the CNF-based catalyst was attributed to the high external surface area of the support and to the peculiar interaction existing between the prismatic planes and the metallic particles. Recently, a significant improvement was introduced via a new synthesis route which allows the possibility for these nanoscopic materials, to be supported on a macroscopic host structure [17]. 

The procedure above is particularly useful for preparing supported noble metal (NM Pd, Pt, Rh) catalysts. Though obviously sensitive to the support surface area, metal loading, and the specific experimental protocol, this procedure, at the laboratory scale, often leads to well dispersed metal systems with relatively narrow metal particle size distributions (97,117,183,235). 

Figure 4.7. TPD-MS study of the H2 desorption from a Pt(7%)/Ce02 catalyst reduced at A) 473 K B) 773 K. After reduction, the samples were evacuated at 773 K (Ih) in a flow of He, and treated with flowing H2 (Ih) at 298 K (A1 and Bl) 473 K (A2 and B2) and 773 K (B3). Then, they were cooled to 191 K (solid/liquid acetone cold trap), and finally the TPD-MS diagrams were recorded in two steps from 191 K-298 K (free heating of the sample), and from 298 K upwards The reported diagrams correspond to the latter step. Trace C corresponds to the bare support reduced at 773 K (Ih) and further cooled to 191 K, always in a flow of H2. Catalyst prepared by impregnation from an aqueous solution of [Pt(NH3)4](OH)2 Support surface area 34 m g". Experimental TPD conditions amount of catalyst 200 mg He flow rate 60 cm. min Heating rate 10 K.min . Diagrams taken from (117).

Support Surface Area BET (m2/g) Catalyst %V205 (wt %) Surface Area BET (m2/g) Vatoms per nm2 V=0 band (cm-i)... 

Sample Sample weight fmgl Pt weight [mg] Pt content fmg/g support] Surface area fm /g] Pt/surface area [10- g/m ]... 

Catalyst. A Pt/Al203 Cl catalyst containing 0.375% Pt and 0.87% Cl was used. It was prepared impregnating with I PtClg samples of CK 300 alumina from Ketjen following the method of Castro et al. ( 1). The support surface area was 200 m /g and the catalyst had 90% metal dispersion. 

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