Catalysts metal location

The Phillips Steam Active Reforming (STAR) process catalyticaHy converts isobutane to isobutylene. The reaction is carried out with steam in tubes that are packed with catalyst and located in a furnace. The catalyst is a soHd, particulate noble metal. The presence of steam diluent reduces the partial pressure of the hydrocarbons and hydrogen present, thus shifting the equHibrium conditions for this system toward greater conversions. 

The present model deals with a supported transition metal cation which is highly dispersed, at the molecular scale, on an oxide, or exchanged in a zeolite. In the case of zeolite-supported cations, the formation of different metal species in metal/zeolite catalysts (metal oxides, metal oxocations, besides cationic species) has been considered by different authors who have suggested these species to play key roles in SCR catalysis [14,15], This supported cation can also be considered as located at a metal oxide/support interface. 

Cinnamaldehyde on Pt/Carbon Catalysts The Effects of Metal Location, and Dispersion on... 

This technology has broad applicability. For instance, using the same carbon support, test results show that a new Pt/C catalyst with edge-metal location and low dispersion resulted in 36% more activity than ESCAT 20 in a standard nitrobenzene (SNB) test (Figure 6). Using the same technology with a different carbon support yielded a catalyst with 57% more activity than ESCAT 20 in SNB test (16,17). 

Catalyst Condition/ Metal Location Dispersion, (%) / metal crystallite Size(nm) Reaction Time (hours) Product Distribution% ... 

More recently, the scope of using hyperbranched polymers as soluble supports in catalysis has been extended by the synthesis of amphiphilic star polymers bearing a hyperbranched core and amphiphilic diblock graft arms. This approach is based on previous work, where the authors reported the synthesis of a hyperbranched macroinitiator and its successful application in a cationic grafting-from reaction of 2-methyl-2-oxazoline to obtain water-soluble, amphiphilic star polymers [73]. Based on this approach, Nuyken et al. prepared catalyticaUy active star polymers where the transition metal catalysts are located at the core-shell interface. The synthesis is outlined in Scheme 6.10. 

Metal location is but one of a number of applications for scanning electron microscope studies in catalysis. Other applications are the study of the morphology of platinum-rhodium gauzes used in the oxidation of ammonia and the poisoning of catalysts, in which the scanning electron microscope results show the location of poisons such as compounds containing sulfur, phosphorus, heavy metals, or coke relative to the location of the catalytic components. 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

Figure 9 Hydrocarbon emissions in the ECE/EUDC test from a 1.2-liter-enginc vehicle fitted with catalysts containing different platinum-group metals located under the floor.

In certain cases the surface of the support may be pre-modified to either increase or decrease its absorptive capacity. Techniques for the former have been thoroughly explored in the area of anchoring homogeneous catalyst complexes and metal clusters. Since this subject has been amply reviewed we will not discuss it further. These techniques primarily involve pre-treatment of the support surface with a compound that can serve as a bridging ligand. Techniques for decreasing the absorptive capacity are also of importance and these will be covered later in greater detail when we come to consider metal location on a catalyst support. 

Certain factors are analyzed to determine their effects on automotive catalyst activity. At operating gas velocities, spherical catalysts were more active than monolithic catalysts at comparable catalyst volumes and metals loadings. Palladium was the most active catalyst metal. Platinum in a mixed platinum palladium catalyst stabilizes against the effects of lead poisoning. An optimum activity particulate catalyst would contain about 0.05 wt % total metals on a gamma-alumina base with a platinum content of 0.03-0.04 wt % and a palladium content of 0.01-0.02 wt %. A somewhat thick shell of metals located near the outer surface of the particle provides better catalyst activity than a shell type distribution of metals. 

The reaction of [Bu3Sb] in heptane solution with a reduced Pd/Al203 catalyst yields a supported alloy. " When these final solids are reduced at 573 or 773 K, the second metal locates preferentially at the outer layer of the bimetallic aggregates. After reduction at 773 K, large metallic aggregates are obtained (particle size around 15 nm). The specific activity of the Pd surface atoms for isoprene hydrogenation is then lowered and the selectivity increased. 

The effect of the additive on the catalytic activity can be attributed to two kinds of effects. One is a ligand effect (an electronic effect) and the other is an ensemble effect (a steric effect). Let us consider the case in which the original metal works as the catalyst and provides a catalytic site(s). If the additive has the former effect on the catalysis of the original metal, the additive can influence the electronic density and/or electronic structure of the original metal. In this case the additive atom must be located near the catalyst metal. Since the catalytic site is located on the original metal, it is not necessary for the additive to be located on the surface. Nevertheless the additive must be located near the active site of the catalyst metal. The effect is the most effective when the additive atom is adjacent to the catalyst metal. 

In the case of an ensemble effect the catalytic sites of the catalyst metal are located on the surface and the additive metal atom must be attacked by the substrate together with the catalyst metal. Thus, both catalyst and additive metals must be located on the surface of the metal particles. Although it is not necessary for the atom of the additive metal to be adjacent to the catalyst metal, it is better for the additive atom to be adjacent to the catalytic site. Then both additive atom... 

A film is deposited in a conventional chemical vapor deposition (CVD) process when the gaseous reactants are presented with a large hot support surface. Supported growth of whiskers occurs also when the gaseous reactants are presented with discrete hot metal catalyst particles located on the surface of a suitable substrate. Unsupported whisker growth occurs when hot metal catalyst particles are freely interspersed with the gaseous reactants in the vapor phase. The most common mechanism for whisker growth is a vapor-liquid-solid transformation, and the most versatile VLS process is a metal particle catalyzed chemical vapor deposition. 

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