Catalyst-surface

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The available surface area of the catalyst gready affects the rate of a hydrogenation reaction. The surface area is dependent on both the amount of catalyst used and the surface characteristics of the catalyst. Generally, a large surface area is desired to minimize the amount of catalyst needed. This can be accomphshed by using either a catalyst with a small particle size or one with a porous surface. Catalysts with a small particle size, however, can be difficult to recover from the material being reduced. Therefore, larger particle size catalyst with a porous surface is often preferred. A common example of such a catalyst is Raney nickel. 

The earliest attempts to prepare deuterated steroids were carried out by exchange reactions of aliphatic hydrogens with deuterium in the presence of a surface catalyst. Cholesterol, for example, has been treated with platinum in a mixture of deuterium oxide and acetic acid-OD, and was found to yield... 

There are three methods which are commonly used in the steroid field to replace a halogen atom by deuterium. These methods involve treatment of the halides— generally chloride, bromide or iodide—(a) with lithium aluminum deuteride, (b) with deuterium gas and a surface catalyst or (c) with zinc in O-deuterated acids or alcohols. 

Replacement of halides with deuterium gas in the presence of a surface catalyst is a less useful reaction, due mainly to the poor isotopic purity of the products. This reaction has been used, however, for the insertion of a deuterium atom at C-7 in various esters of 3j -hydroxy-A -steroids, since it gives less side products resulting from double bond migration. Thus, treatment of the 7a- or 7j5-bromo derivatives (206) with deuterium gas in the presence of 5% palladium-on-calcium carbonate, or Raney nickel catalyst, followed by alkaline hydrolysis, gives the corresponding 3j3-hydroxy-7( -di derivatives (207), the isotope content of which varies from 0.64 to 1.18 atoms of deuterium per mole. The isotope composition and the stereochemistry of the deuterium have not been rigorously established. 

Dc=(number of surface catalyst atoms/total number of catalyst atoms) 100(11.1)... 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

The first example of acid catalysis appeared in a 1934 patent in which it is claimed that surface catalysts, particularly hydrosilicates of large surface area , known at that time under the trade name Tonsil, Franconit, Granisol, etc. lead to a smooth addition of the olefine to the molecule of the primary aromatic amine . Aniline and cyclohexene were reacted over Tonsil at 230-240°C to give, inter alia, the hydroamination product, N-cyclohexylaniline [47]. 

Exfoliated graphite - amorphous carbon (EG-AC) composites are widely used in a broad range of applications like adsorption of gases and liquids, separation, and high-surface catalyst supports. They also seem to be attractive for energy- and gas- storage systems. 

The synthesis of catalytic photocathodes for H2 evolution provides evidence that deliberate surface modification can significantly improve the overall efficiency. However, the synthesis of rugged, very active catalytic surfaces remains a challenge. The results so far establish that it is possible, by rational means, to synthesize a desired photosensitive interface and to prove the gross structure. Continued improvements in photoelectrochemical H2 evolution efficiently can be expected, while new surface catalysts are needed for N2 and CO2 reduction processes. 

Diem, D., and W. Stumm (1984), "Is Dissolved Mn2+ being oxidized by Oj in the Absence of Mn-Bacteria or Surface Catalysts ", Geochim. Cosmochim. Acta 48,1571-1573. 

Aside from thermal interactions, chemical interactions between flames and surfaces are also important. Chemical interactions are typically manifested either by radical recombination on cold (relatively inert) walls [4, 5] or by heterogeneous combustion on chemically active surfaces (catalysts). Among the available technologies, catalysts have the best potential for NO, reduction. A review on recent advances in catalytic combustion is given elsewhere [6]. The focus here is on combustion near inert surfaces. 

Before 1970 there was very little unleaded gasoline on the market, but by 1974 all gas stations were offering it. In 1974, unleaded fuel had become a necessity for most new cars because of their catalytic converters placed in the exhaust system. These contain platinum or palladium compounds that act as a surface catalyst to bum the hydrocarbons more completely. But lead coats the platinum and palladium and deactivates the converters, so unleaded gas must be used. Up to 4 g/gal of lead could be used in the 1970s, but this was decreased to 0.1 g by 1986. Since 1995 no leaded gas could be used in the U.S. Fig. 7.6 shows the dramatic shift from leaded to unleaded gas between 1975 and 1992. 

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

The use of specially designed particles with controlled size, morphology, surface catalysts, and dopants can surely influence the selectivity which can ultimately be attained in photoelectrochemical conversions. Judicious control of light flux will also influence the course of photostimulated redox reactions. Only very few tests of these effects in organic systems have yet been reported, however. 

Because ammonia may replace active oxygen anions on the catalyst surface, catalysts exist which are selective for ammoxidation but not for oxidation. An example of this kind is the Bi—Sb—O catalyst studied by Barannik and Ven yaminov [39]. 

The frequently observed preference for anti-elimination over syn-elimi-nation on alumina (for a summary of earlier results see ref. 7, later especially ref. 96) has been a cause of much controversy. However, as has been explained in Sect. 2.1.2, it is a natural reaction course for concerted elimination, provided that suitably spaced acidic and basic sites are available on the surface. Catalysts which operate by means of the El-like mechanism... 

Deuterioammonia, 188, 190 Deuteriobor ation, 191 Deuteriobromic acid, 214 Deuteriodiborane, 191 Deuterio-Raney nickel, 215 Deuterium gas and a surface catalyst, 199 Deuterium and tritium gas, 179 3,17/8-Diacetoxyestra-3,5-diene, 486 3,20-Diacetoxypregna-3,5,20-triene, 411 (20S)-2/3,3j3-Diacetoxy-5a-pregn-7-en-6 one-20-carboxylic acid methylester, 301 Diborane, 89, 100... 

Steric approach control, 67 Steric strains, 71 Steric strain control, 69 Steroid hydrogenation, 111 5/3-Stigmast-22-en-3-one, 130 Stigmasterol, 266 Sulfur dichloride, 459 Sulfur tetrafluoride, 459, 472 Sulfur tetrafluoride fluorination, 471 Surface catalysts, 157... 

First, it is frequently found that when two molecules of different kinds react under the influence of a surface catalyst, both must be actually adsorbed—it is not enough for one to be adsorbed and for the other to strike it. The evidence for this is that excess of one reactant can bring about an actual decrease in the reaction rate due to displacement of the other reactant. 

Whilst NOx emissions from petrol vehicles can be controlled by catalytic reduction, this is not very effective under the oxygen-rich conditions of diesel combustion. A diesel oxidation catalyst (DOC) is similar to a TWC in terms of structure and configuration but is only capable of oxidation. As the exhaust gases pass through the catalyst CO, unbumt HC and volatile PM are oxidised. The conversion efficiency is a function of cell size, reactive surface, catalyst load and catalyst temperature, although emissions of CO and HC are typically reduced with an efficiency of more that 95%. 

Fe2+. Because peroxide decomposition was slowest under the same conditions at which benzoic acid decomposition was highest, it is important to consider the efficiency of hydroxyl radical formation from peroxide decomposition. With the surface catalyst, either hydroxyl radical is not readily available to benzoic acid and is scavenged by other species, or the mineral-catalyzed decomposition of hydrogen peroxide involves additional, nonhydroxyl radical-forming pathways for peroxide decomposition. 

---