Function of Platinum

From flow-reactor experiments with stacked beds of supported platinum catalyst and Printex-U, discussed in [16], it was concluded that the platinum catalyst does not need to be mixed with the soot to be effective. The combination of platinum catalyst and gas phase NO is effective through the gas phase. 

In engine experiments, the platinum is particularly effective after some period of use or in combination with a platinum treated filter. This leads to the assumption that the role of the platinum in the soot is small and that the platinum collected on the filter is more important for the oxidation of soot. This is confirmed by flow-reactor experiments, that showed that platinum present in the soot did not influence the oxidation rate in any case (not shown). The platinum particles in the soot are too small (atomic dispersion) to contribute to the oxidation of NO and the concentration of platinum is very low. 

The results from both types of experiments lead to the same conclusion platinum agglomerates on the filter act as an NO oxidation catalyst. The resulting NO2 subsequently oxidises the soot. The observation that platinum treated filters are more effective after a high-temperature treatment is supported by studies rejtorted by Xue etal. [17]. There is a direct relation between the size of platinum particles and the effectiveness as an NO-oxidation catalyst. 

Platinum present in the soot does not influence the oxidation rate of soot, either in presence or absence of NO. The influence of the platinum additive on the engine-out emissions cannot be neglected and platinum may contribute to the soot oxidation on other places than on the filter, such as the exhaust manifold and in the exhaust pipes. 

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FIGURE 12 Average coordination number as a function of platinum cluster diameter for cuboctahedral clusters. 

Figure 1. Compilation of platinum mass activities as a function of platinum B.E.T. surface area [ ] Watanabe et alJ [0] Buchanan et al.s [ ] Buchanan et al. and [0] Bregoli6. The solid line is 0.6A.m 2 constant specific activity platinum. The broad arrow on the abscissa denotes the maximum surface area for a platinum crystallite when all of the atoms are located at the surface (275 m2 g 1 Pt). Phosphoric acid at 190 °C and 0.9 V vs. hydrogen in the same electrolyte, (a) Data up 210 m2g" Pt. (b) Data below 100m2g Pt m Bregoli6 results on unsupported platinum black.

Figure 68. The function of platinum and rhodium on a fully formulated three-way catalyst washcoat in the conversion of CO, HC and NO, (monolith catalyst with 62 cells cm" three-way formulation, aged on an engine bench for 20 h engine bench test at a space velocity of 60000NIC h exhaust gas temperature 673K exhaust gas composition lambda 0.999, dynamic frequency I Hz amplitude 1 A/F). Reprinted with permission from ref. [34], (C 1991 Society of Automotive Engineers, Inc.

We begin by considering the iridium EXAFS of a reference material such as metallic iridium or a catalyst containing pure iridium clusters. An EXAFS function for the iridium in the platinum-iridium catalyst is then generated from the function for the reference material by introducing adjustments for differences in interatomic distances, amplitude functions, and phase shifts. In making such adjustments, we are aided by the fact that the amplitude functions and phase shift functions of platinum are not very different from those of iridium, as shown in Figures 4.27 and 4.28. 

Because of the similarity rn the amplitude functions of platinum and iridium, we do not separate the backscattering contributions of platinum and iridium atoms in the analysis of EXAFS data on platinum-iridium clusters or alloys. In our quantitative treatment of EXAFS arising from nearest neighbor atoms of platinum and iridium, the EXAFS function of Eq. 4.3 consists of only one term, as will be seen in the following discussion. 

To obtain structural information on platinum-iridium clusters from EXAFS data, we concentrate primarily on the determination of interatomic distances. To obtain accurate values of interatomic distances, we need to have precise information on phase shifts. In this regard, we are fortunate that the phase shift functions of platinum and iridium are not very different. 

SV40 DNA replication in CV-1 cells as a function of platinum concentration in the medium (panel A) or DIN (panel C). In panel B, DIN is plotted as a function of platinum concentration in the medium. SV40-infected cells were treated with cA-DDP ( ) or trans-DDV (o) at the indicated concentrations for 40 h. SV40 DNA replication relative to control (untreated) cells was measured by incorporation of [ HJthymidine, added after the first 24 h of platinum treatment, and Pt in isolated SV40 chromosomes was measured by A AS. The data shown are from a representative experiment. Experiments were carried out in quadruplicate. Reproduced by permission from Reference 85. 

Figure 11 (left) Number of carbon atoms, C270> deposited per platinum surface site, Pt j,j., and activity in cyclopentane hydrogenolysis, a, as a function of platinum dispersion. 

Fractals refer to broken, nonnatural number dimensions. If one studies the selectivity of ethylene epoxidation as a function of silver particle size (section 3.7) or hydrogenolysis (section 3.2) as a function of platinum particle size, significant changes are found. 

A quinone/hydroquinone redox couple is used to test the functioning of platinum electrode. Thereductionreactionofthiscoupleisquinone + 2H+ + 2e = hydroquinone ° = 0.699V. Assume the ratio of the activities to be [quinone]/hydroquinone] = 1. The electrodes are tested in buffer solutions with pH of 7 or 4 containing the redox couple at 25°C. 

Fig. 14. Ratio of surface area from X ray diffraction to area from the electrochemical determination as a function of platinum loading of the carbon support...

An interesting study of platinum dispersion in NaY zeolite as a function of platinum concentration (2,5, and 10 wt.% Pt) was reported by Ryoo et al. [174]. The platinum clusters were prepared by conventional ion exchange with Pt(NH3), calcination at 583 K and reduction at 673 K. The Pt-Pt coordination numbers measured by EXAFS were constant in the three samples (5.0,5.9 and 5.1, respectively), and the cluster nuclearity measured by Xe adsorption isotherms was also constant (ca. 50 atoms). Similar results were also obtained with... 

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