Pt-based

In 1993, UOP commercialized an improved Pt-based catalyst, 1-210. This catalyst is based on a molecular sieve, but not an alurninosihcate zeoHte. UOP claims that yields ate about 10% better than those for 1-9 catalyst. EB to xylenes conversion is about 22—25% with a Cg aromatics per pass loss of about 1.2—1.5%. As discussed below, UOP s Isomar process can also use zeoHte catalysts which convert EB to benzene rather than to xylenes. UOP has hcensed over 40 Isomar units. 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

Discuss to what extent the Phillips ethylene polymerization catalyst satisfies all of the criteria that define a catalyst. Compare this polymerization catalyst with the FCC catalyst and Pt-based reforming catalysts. 

Figure 12. Electrocatalytic oxidation of CO (from a CO-saturated 0.1 M HCIO4 solution) on different Pt-based electrodes (sweep rate 5 mV/s, 25 "C) ( ) smooth Pt, (0) 0.1 mg cm Pt dispersed in a polyaniline film,(A) 0.1 mg cm" R-Sn dispersed in a polyaniline film.

Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation [9] was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-0 mixed oxide [10] was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor). 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

Au/C was established to be a good candidate for selective oxidation carried out in liquid phase showing a higher resistance to poisoning with respect to classical Pd-or Pt-based catalysts [40]. The reaction pathway for glycerol oxidation (Scheme 1) is complicated as consecutive or parallel reactions could take place. Moreover, in the presence of a base interconversion between different products through keto-enolic equilibria could be possible. 

Jacob T. 2006b. Water formation on Pt and Pt-based alloys a theoretical description of a cata-l3dic reaction. ChemPhysChem 7 992-1005. 

Dunietz BD, Markovic NM, Ross Jr PN, Head-Gordon M. 2004. Initiation of electro-oxidation of CO on Pt based electrodes at full coverage conditions simulated by ah initio electronic structure calculations. J Phys Chem B 108 9888-9892. 

Teliska M, Murthi VS, Mukeqee S, Ramaker DE. 2007. Site-specific vs specific adsorption of anions on Pt and Pt-based alloys. J Phys Chem C 111 9267-9274. 

In order to establish a clear strategy, we have examined the properties of Pt-based catalysts for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) systematically and comprehensively using various techniques, the results of which complement each other. We have also developed a standard method to evaluate the real activity. In this chapter, we summarize our recent research results. 

To evaluate the catalytic activity or to investigate the reaction mechanism, planar electrodes with well-defined characteristics such as surface area, surface and bulk compositions, and crystalline structure have often been examined in acidic electrolyte solutions. An appreciable improvement in CO tolerance has been found at Pt with adatoms such as Ru, Sn, and As [Watanabe and Motoo, 1975a, 1976 Motoo and Watanabe, 1980 Motoo et al., 1980 Watanabe et al., 1985], Pt-based alloys Pt-M (M = Ru, Rh, Os, Sn, etc.) [Ross et al., 1975a, b Gasteiger et al., 1994, 1995 Grgur et al., 1997 Ley et al., 1997 Mukeijee et al., 2004], and Pt with oxides (RuO cHy) [Gonzalez and Ticianelli, 2005 Sughnoto et al., 2006]. 

Figure 10.4 Area-normalized CL spectra of Pt4/7/2 for the pure Pt (dotted Une), Pt5gCo42 (solid line), and PtgoRu4o (dashed line) alloys with respect to p (a) as-prepared (h) after electrochemical stabilization. The samples were thin film pure Pt or Pt-based alloys (diameter 8 mm and thickness 80 nm) prepared on Au disks by DC sputtering. Electrochemical stabilization of Pt58 C042 was performed by repeated potential cycling between 0.075 and 1.00 V at a sweep rate of 0.10 V s in 0.1 M HCIO4 under ultrapure N2 (99.9999%) until CV showed a steady state. PtgoRu4o was stabilized by several potential cycling between 0.075 and 0.80 V at 0.10 V s in 0.05 M H2SO4 under ultrapure N2. (From Wakisaka et al. [2006], reproduced by permission of the American Chemical Society.)...

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

It is very important to develop a high performance cathode catalyst, because a sluggish ORR causes a large overpotential at low temperatures. With respect to the total performance of activity and stability, the cathode catalyst material is limited to Pt or its alloys at present. In acidic media such as Nation electrolyte or aqueous acid solutions, four-electron reduction is dominant at Pt-based electrodes ... 

Min M, Cho J, Cho K, Kim H. 2000. Particle size and alloying effects of Pt-based alloy catalysts for fuel cell applications. Electrochim Acta 45 4211-4217. 

Finally, trimetallic compounds have been developed to enhance the electroactivity of Pt-based catalysts, for either methanol or ethanol electro-oxidation. A long time ago, it was reported that adsorption of molybdates (Na2Mo04) at a Pt black electrode... 

Numerous studies have shown that Pt-based binary alloy electrocatalysts such as Pt-Fe, Pt-Co, Pt-Ni, and Pt-Cr exhibit a higher catalytic activity for the ORR in an... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Zhou WJ, Li WZ, Song SQ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, Poulianitis K, Kontou S, Tsiakaras P. 2004a. Bi- and tri-metaUic Pt-based anode catalysts for direct ethanol fuel cells. J Power Sources 131 217-223. 

Nappom WT, Laborde H, Leger J-M, Lamy C. 1995. Flectro-oxidation of Ci molecules at Pt-based catalysts highly dispersed into a polymer matrix Effect of the method of preparation. J Electroanal Chem 404 153-159. 

The reductive alkylation of DAP with acetone led to high conversions and selectivity to the dialkylated product over Al, Bl, and BS2 catalysts. The ASl catalyst, which typically has lower activity than the Al or Pt-based catalysts showed greater formation of heterocycles. These results indicate that a more active catalyst, a shorter reaction time, a higher operating temperature, or sterically hindered amines/ketones will help minimize the formation of the heterocycles. Similar high selectivities were obtained with DAP-MIBK reaction over BSl and BS2 catalysts with no heterocycles being formed. However, over Al, the undesired heterocyclic compound was over 15%. This indicates that the reaction between diamines and ketones has a significant potential to form heterocyclic compounds unless the interaction between these is kept to a minimum by the use of a continuous flow reactor as proposed by Speranza et al. (16) or by other methods. 

A particular attention on the mechanisms for the formation of N20 over noble metals has been paid in our laboratory [37-40]. It was previously found that an enhancement in the initial selectivity towards the production of N2 (Table 10.1) during the CO + NO reaction can be related to an increase in the relative rate of step (13) over supported Pt-based catalysts [33], Unexpectedly, Rh exhibits a poor selectivity towards the formation of N2 at low conversion and low temperature, which has been mainly related to a stronger NO adsorption on Rh than on Pt and Pd. 

Table 10.1. Selectivity towards the formation of nitrogen on supported Pt-based catalysts at 300°C (initial partial pressure of NO and CO equal to 5 x 10 3 atm [37])...

Burch, R., Fornasiero, P. and Southward, B.W.F. (1999) An investigation into the reactivity, deactivation and in situ regeneration of Pt-based catalysts for the selective reduction of NOx under lean burn conditions, J. Catal. 182, 234. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

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