Pt catalysts

Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

Pt/Ru Catalyst Polymer Pt Catalyst Porous Gas Layer Electrolyte Layer Diffusion Membrane Electrode... 

Organic sulfur Organic sulfur Oxidation with HNO3 in presence of Ba + Combustion in O2 (with Pt catalyst) to produce SO2 and SO3, BaCb B3S04... 

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. 

Hydrogenolysis of propylene oxide yields primary and secondary alcohols as well as the isomeri2ation products of acetone and propionaldehyde. Pd and Pt catalysts favor acetone and 2-propanol formation (83—85). Ni and Cu catalysts favor propionaldehyde and 1-propanol formation (86,87). 

Reductions of avemiectin containing five double bonds with Pd oi Pt catalysts proceed at almost comparable rates at the two disubstituted 10,11- and... 

Class (2) reactions are performed in the presence of dilute to concentrated aqueous sodium hydroxide, powdered potassium hydroxide, or, at elevated temperatures, soHd potassium carbonate, depending on the acidity of the substrate. Alkylations are possible in the presence of concentrated NaOH and a PT catalyst for substrates with conventional pX values up to - 23. This includes many C—H acidic compounds such as fiuorene, phenylacetylene, simple ketones, phenylacetonittile. Furthermore, alkylations of N—H, O—H, S—H, and P—H bonds, and ambident anions are weU known. Other basic phase-transfer reactions are hydrolyses, saponifications, isomerizations, H/D exchange, Michael-type additions, aldol, Darzens, and similar... 

In practice, 1—10 mol % of catalyst are used most of the time. Regeneration of the catalyst is often possible if deemed necessary. Some authors have advocated systems in which the catalyst is bound to a polymer matrix (triphase-catalysis). Here separation and generation of the catalyst is easy, but swelling, mixing, and diffusion problems are not always easy to solve. Furthermore, triphase-catalyst decomposition is a serious problem unless the active groups are crowns or poly(ethylene glycol)s. Commercial anion exchange resins are not useful as PT catalysts in many cases. 

Advantages of the hydrosilation system (Fig. 3) include the elimination of solvent, improved cure speed, and potential for UV or thermal cure. Drawbacks to the system include more expensive multiroll coating methods, potential poisoning of the Pt catalyst (with Sn, S, Cr, amines, etc.), poor anchorage to some films, and a need to carefully balance the hydride to vinyl ratio employed for cure to avoid detrimental interactions with acid containing adhesives [23,53]. 

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). 

NOC1 is a powerful oxidizing agent, and causes expins when mixed with reducing substances. For instance, an expln occurs when it is mixed with an equal quantity of hydrogen. When powdered As or Sb is introduced into gaseous NOG, spontaneous combustion occurs. An expl reaction was reported when it was sealed in a tube with a residue of acet in the presence of Pt catalyst (Ref 2)... 

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)... 

Figure 1.3. Rate and catalyst potential response to step changes in applied current during C2H4 oxidation on Pt deposited on YSZ, an O2 conductor. T = 370°C, p02=4.6 kPa, Pc2H4=0.36 kPa. The catalytic rate increase, Ar, is 25 times larger than the rate before current application, r0, and 74000 times larger than the rate I/2F,16 of 02 supply to the catalyst. N0 is the Pt catalyst surface area, in mol Pt, and TOF is the catalytic turnover frequency (mol O reacting per surface Pt mol per s). Reprinted with permission from Academic Press.

Figure 4.3. Scanning electron micrographs of the top side of a porous Pt catalyst film (a) and of a section perpendicular to the Pt catalyst-yttria-stabilized zirconia (YSZ) interface (b).4 Reprinted with permission from Academic Press.

Figure 4.4. Scanning tunneling micrograph of the surface of a Pt catalyst used in NEMCA studies Scan size 62 A Vbias=0.5 V, Itunnd=15 nA.1 Reprinted with permission from Elsevier Science.

For Pt catalyst-electrodes a good operating temperature is 350-380°C34,35 while lower temperatures (300-330°C) are suitable for Ag.26,27... 

Figure 4.11. Typical Tafel plots for Pt catalyst-YSZ interfaces during C2H4 oxidation on Pt the large difference in I0 values between the two Pt films (labeled R1 and R2) is due to the higher calcination temperature of Pt film R2 vs Pt film Rl.4 Reprinted with permission from Academic Press.

Setting 1=0 gradually restores Uwr to -0.3 V while r remains practically unaffected. Restoration of the initial r value requires potentiostatic setting of UWr at 0.4 V, and thus removal of Na(Pt) from the Pt catalyst surface. 

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... 

Figure 4.24. Effect of gaseous composition on the regular (open-circuit) catalytic rate of C2H4 oxidation on Pt/YSZ and on the NEMCA-induced catalytic rate on the same Pt catalyst film maintained at UWr=1V, T=370°C, pc2h4 =0.65 kPa.4 Reprinted with permission from Academic Press.

P.D. Petrolekas, S. Balomenou, and C.G. Vayenas, Electrochemical promotion of Ethylene Oxidation on Pt Catalyst Films deposited on Ce02, J. Electrochem. Soc. 

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... 

Figure 5.10. Transient response of catalyst work function O and potential Uwr upon imposition of constant currents I between the Pt catalyst (labeled26 C2) and the Pt counter electrode p"-A1203 solid electrolyte T = 240°C, p02 = 21 kPa Na ions are pumped to (I<0) or from (I>0) the catalyst surface at a rate I/F.26 Reprinted with permission from Elsevier Science.

Thus referring to Na supply onto a Pt catalyst surface with surface area Aq via a 3"-Al203 solid electrolyte (Fig. 5.10), one can use Faraday s law to obtain ... 

Figure 5.39 c shows the O Is spectmm obtained during electrochemical O2 pumping in vacuum at Uwr 1.1V. The spectmm clearly proves massive electrochemically controlled O2 backspillover onto the Pt catalyst surface with a concomitant shift of the broad Ols spectmm peak maximum to -530 eV. 

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 5.46 shows clearly how the application of potential changes the brightness and thus the workfunction O, of the grounded Pt catalyst-electrode (windows 2 and 3) and of the YSZ surface, (window 1), in accordance to the above discussed alignment (pinning) of the two Fermi levels. 

Figure 5.54. Effect of sodium coverage on the change AUWR of polycrystalline Pt catalyst potential UWr and on the catalytic rates of CO oxidation (solid lines37) and C2H4 oxidation (dashed lines36). Comparison with the theoretical Na coverage required to form the Pt(l 11)-(12xl2)-Na adlayer 0 is based on the number of surface Pt atoms 09a is based on the number of surface O atoms corresponding to the Pt(l 1 l)-(2x2)-0 adlattice. Reprinted from ref. 78 with permission from Elsevier Science,...

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