On Pt catalysts

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

The CH4 oxidation on Pd31 exhibits a very pronounced NEMCA behavior at much lower temperatures (380-440°C) compared with those on Pt catalysts (650-750°C). In this temperature range the reaction exhibits inverted volcano behavior.31 For positive overpotentials the p values are as high as 89, with A values up to 105.31 Negative overpotentials also enhance the rate31 with p values up to 8. 

Figure 3.27 Conversion of NHj (open symbols) and selectivity to N2O (closed symbols) for the ammonia oxidation process on Pt catalyst. Micro reactors A1 (A), A2 ( ), A3 ( ), B ( ) and C ( ) were used (see Table 3.1) [98].

Catalyst layer architecture As a consequence of the diminishing remrns from ever higher dispersion, the effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years primarily on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm. High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst... 

We earlier found (ref. 5) that the hydrogenolysis of methyloxirane on Pt catalysts takes place via an associative mechanism (Scheme 2). 

Hydrogen oxidation on Pt catalyst in adiabatic recirculating reactor Horak and Jirafcek (45) X X... 

Such electronic transfer induced by sulfur adsorption was also pointed out by using cinnamic acid as a probe molecule (48). The UV-visible reflexion spectra of adsorbed cinnamic acid on nonpoisoned and partly poisoned platinum catalysts shows that adsorption on pure platinum induces a shift of the peaks toward the higher wavelengths and an appearance of fine structure. Sulfurization of platinum induces a further enhancement of higher wavelength peaks. Binding energy of cinnamic acid is thus increased by sulfur adsorption on Pt catalysts. 

A stability analysis for such a system was performed by Wicke et al. (98), who modeled the H2/O2 reaction on Pt catalysts. The reaction was simplified to two differential equations that are easily treated analytically ... 

According to a different theoretical approach, the kinetics of oxygen on Pt catalysts were interpreted on the basis of the separation distance between metal nanoparticles. Pt crystallites, separated by less than about 20 nm, were observed to exhibit lower electrocatalytic activity because of either diffusion or a mutual... 

Effects of Promoters and Supports on Coke Formation on Pt Catalysts During CH4 Reforming with CO2... 

Having examined interactions on Ir we now turn to adsorptions on Pt. Catalyst chemists must wonder what AES, UPS, LEED, and flash desorption have revealed about acetylene and benzene adsorption on Pt(lOO). Fischer et al. have established the following facts ... 

The PROX reaction has been extensively investigated on Pt catalysts supported on AI2O3 [341-343] and zeolites [344], Oh and Sinkevitch [341] compared the efEciency of several noble metals over alumina the CO conversion was found to decrease in the following order Ru > Rh > Pt > Pd (metal loading 0,5 wt% for all catalysts). Other systems, which have been investigated for the PROX reaction include metal oxides [345], oxide-supported noble metal catalysts [346] and bimetallic [347] catalysts. 

Bewick et al." identified CO as the species that acts as a catalytic poison and inhibits further oxidation of methanol on Pt electrodes. The reactive intermediate is a formate species, HCOO that generates asynunetric COO vibration around 1300 cm, leading to an increase in the methanol oxidation current after CO oxidation. "Recently, water molecules were detected adsorbed on the Ru sites on Ru and Pt-Ru (but not on Pt) catalysts, and were assigned as the oxy gen donor to the methanol adsorbates that promote methanol oxidation."" This was considered as directly supporting the bi-functional mechanism of Pt-Ru catalysts for the methanol-oxidation reaction. ... 

Keil and Wiecke (1980) investigated oscillations in the CO oxidation on Pt catalysts in a tubular reactor under isothermal conditions. At lower and higher CO contents the kinetics of the reaction could be described uniformly by a Langmuir-Hinshelwood relationship. 

Wicke et al. (1980) presented a study of oscillations during oxidation of hydrogen on Pt catalyst. Oscillations of catalyst temperature at different 02 concentrations were shown to differ from each other. Following a discussion of the mechanism of the oscillations, they also compared the H2 oxidation experiments with CO oxidation. 

From Table 2, we can observe that on S-[Pt-Zn] catalysts more NO is transformed and less N2O and NO2 are formed than on Pt catalyst, except when 200mg of S-[Pt-Zn] is used. The temperatures at which these maxima are observed are about 20°C higher on S-[Pt-Zn] than on Pt but the summits of these volcano cwves are more flat for NO conversion on S-[Pt-Zn] which explains why in a range of 20°C we have the same NO conversion. Moreover at 35% NO conversion a larger domain of temperatures is observed as mentioned in Table 3. 

The conversion of NO increases with oxygen concentration (Figure 4) but this increase is more marked between 0% and 2% O2 The initial slope from the NO conversion curve versus the percentage of Oj is 1.5 higher on [Pt-Zn] than on Pt catalyst, when the reaction is performed at 250°C. 

Comparative sulfur storage on Pt catalysts effect of the support (Ce02, Zr02 and Ce02-Zr02)... 

The purpose of this work is to compare the nature, amount and H2 reducibility of the sulfate species formed from SO2 oxidation on Pt catalysts supported on Ce02, Zr02 and a Ce02-Zr02 solid solution. Two techniques have essentially been used thermogravimetry and IR spectroscopy, both in static conditions. 

Figure 5. Effect of heating treatments in a static vacuum on CO adsorbed on Pt catalysts.

Effective promotion on Pt catalysts with additives such as Re, Rh, Pd and Ir... 

A number of structure-sensitive reactions take place in the catalytic converter which, as expected, will be influenced by the noble metal dispersion. These include first of all the very important oxidation of saturated hydrocarbons. It is an accepted view that whenever the surface reaction involves the scission of a C-C bond structure-sensitivity is to be expected. There is ample evidence that oxidation of saturated hydrocarbons, especially those of short chain length, does not proceed readily on Pt catalysts with very high dispersion. 

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