By Pt catalyst

Normally, ORR catalyzed by Pt catalyst occurs predominately through a four-electron transfer pathway to water (or to... 

Dicyclohexylarnine may be selectively generated by reductive alkylation of cyclohexylamine by cyclohexanone (15). Stated batch reaction conditions are specifically 0.05—2.0% Pd or Pt catalyst, which is reusable, pressures of 400—700 kPa (55—100 psi), and temperatures of 75—100°C to give complete reduction in 4 h. Continuous vapor-phase amination selective to dicyclohexylarnine is claimed for cyclohexanone (16) or mixed cyclohexanone plus cyclohexanol (17) feeds. Conditions are 5—15 s contact time of <1 1 ammonia ketone, - 3 1 hydrogen ketone at 260°C over nickel on kieselguhr. With mixed feed the preferred conditions over a mixed copper chromite plus nickel catalyst are 18-s contact time at 250 °C with ammonia alkyl = 0.6 1 and hydrogen alkyl = 1 1. 

A few industrial catalysts have simple compositions, but the typical catalyst is a complex composite made up of several components, illustrated schematically in Figure 9 by a catalyst for ethylene oxidation. Often it consists largely of a porous support or carrier, with the catalyticaHy active components dispersed on the support surface. For example, petroleum refining catalysts used for reforming of naphtha have about 1 wt% Pt and Re on the surface of a transition alumina such as y-Al203 that has a surface area of several hundred square meters per gram. The expensive metal is dispersed as minute particles or clusters so that a large fraction of the atoms are exposed at the surface and accessible to reactants (see Catalysts, supported). 

Similar routes are available for the production of HBr and HI, The catalysed combination of H2 and Br2 at elevated temperatures (200-400°C in the presence of Pt/asbestos, etc.) is the principal industrial route for HBr, and is also used, though on a relatively small scale, for the energetically less-favoured combination of H2 and I2 (Pt catalyst above 300°C). Commercially HI is more often prepared by the reaction of I2 with H2S or hydrazine, e.g. ... 

Nitric Add by the Oxidation of Ammonia. Here, the catalytic oxidation of ammonia under press using a Pt catalyst maintained at a temp of 900—1000° is the process used. The reaction press is the rate determining step, being directly proportional to the product nitric acid concn (Refs 6, 22, 26, 30, 34, 36, 37 41). 

Pt is attacked by bromine trifluoride at 280° in the presence of K fluoride (Ref 5). Finely divided Pt and some other metals will cause a mixt of Hj and 02 to explu at ordinary temps (Ref 1). A little Pt black dropped into a hydrogen peroxide soln can cause an expln (Ref 2). Pt and molten Ii react violently at 540° 20° (Ref 7), and an incandescent reaction occurs when it is wanned gently in gaseous oxygen difluoride (Ref 6). The decompn of 92% per-monosulfuric acid is expl in the presence of smooth or finely divided Pt (Ref 3). The re-. acting mass formed by the mixt of P and Pt can become incandescent when heated (Ref 8). Dry, used Pt catalyst has exWd while being screened (Ref 4)... 

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t < - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a... 

Thus the picture which emerges is quite clear (Fig. 5.4) At steady state, before potential (or current) application, the Pt catalyst surface is covered, to a significant extent, by chemisorbed O and C2H4. Then upon current (and thus also potential) application O2 ions arriving from the solid electrolyte at the tpb at a rate I/2F react at the tpb to form a backspillover ionically strongly bonded species... 

The backspillover O species on the Pt surface have an O Is binding energy 1.1 eV lower than on the same surface under open-circuit conditions. The Pt catalyst-electrode is surrounded by isoenergetic oxygen species both at the Pt/YSZ and at the Pt/vacuum interfaces.67... 

Figure 8.2. Plot of the effect of gaseous composition and of n=( )/kbT during C2H4 oxidation on two Pt catalyst films, labeled R1 and R2, showing that the rate expression given by (Eq. 8.1) is valid both under open-circuit conditions (open symbols) and also under NEMCA conditions (filled symbols).1 Reprinted with permission from Academic Press.

Recent NEMCA investigations have shown that J3"-A1203, a Na+ conductor, can be used as an active catalyst support to dramatically enhance the rate and selectivity of several enviromentally important reactions such as NO reduction by CO, H2 and C3H6, all catalyzed by Pt. Sodium supply to the catalyst has been found to enhance not only the catalytic activity, but also product selectivity to nitrogen. 

The reduction of NO by CO on Pt/p"-Al203 is another system exhibiting spectacular electrochemical promotion behaviour.7,23,24 Electrochemical supply of Na+ on the Pt catalyst surface can cause the rates of C02 and N2 formation (rCo2 and rN2)to increase by 48% and 1300%, respectively, over their values on a clean surface.23... 

Subsequently the film was impregnated with an aqueous H2PtCl6 solution followed by drying, calcination in air at 450°C and reduction with H2 at 250°C. The resulting finely dispersed Pt catalyst was characterized using H2 and CO chemisorption. The dispersion of the Pt catalyst was found to be 20-100% in different samples. 

In the field of materials synthesis, T8[CH = CH2]8 has been used to prepare three-dimensional (meso)porous polymers with high surface area via reactions with TgHg or T8[OSiMe2H]8 in the presence of a Pt catalyst as described in Section Xu et al. prepared a POSS-based monomer by reaction... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

The reduction of this ester over Pd differed from the corresponding reaction over Pi in every important particular. Enantiomeric excess was low (high over Pt) and in the reverse sense (e g cinchonidine modification provided an S-excess in the product over Pd but an R-excess over Pt) Enantioselective reactions underwent self-poisoning over Pd (proceeded to completion over Pt), were of non-integral order (integral over Pt) and proceeded more slowly than reaction over unmodified catalyst (enhanced rate over Pt) Enantioselective reaction was solvent-specific over Pd (not over Pt) and was favoured by low catalyst reduction temperature (high reduction temperature for Pt)... 

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