Supports platinum experiment

N.M.R. STUDIES of ADSORBED ETHYLENE We have also investigated the reaction of C ethylene with colloidal palladium. Our initial intent was to attempt to observe the formation of ethylidyne from ethylene on the surface of the colloidal palladium particles, a reaction which is known to occur readily on the surface of supported palladium and on palladium single crystals (17). Such a reaction has been identified for ethylene on supported platinum by magnetic resonance experiments in which spin echo double resonance techniques were used to characterize the organic species (18,19), but direct observation of resonances for adsorbed ethylene or ethylidyne was not possible in the highly inhomogeneous solid samples used. The chemical shift differences... 

From Figures I2.2 a and 12.2.b it is dear that in f1ow>reactor experiments NO does hardly influence the soot oxidation rate in absence of a supported platinum catalyst. From results not shown here it is clear that the same holds for the effect of a supported platinum catalyst the oxidation rate in the absence of NO is the same, with or without supported platinum catalyst. This leads to the conclusion that only... 

What, if any, relevance do such results have when predicting the influence of adsorbed bismuth on the CO of supported platinum nanoparticle catalysts In order to test the transferrability of results obtained on single crystals to practical fuel-cell anode catalysts, a series of experiments was performed [77] on a gas diffusion electrode of carbon-supported platinum (0.22 mg cm ) catalyst (Johnson Matthey). Figure 10 shows the results of polarization measurements for hydrogen oxidation at clean and bismuth-modified (0.65-ML) catalysts. In order to establish the CO tolerance of the electrodes, in addition to experiments involving pure H2,... 

During the oxidation experiments with NO in the gas phase and a supported platinum catalyst mixed with the soot, the outlet NO concentration did not decrease very much. A typical example of the NO outlet concentration is shown in Figure 12.3. Up to 60% soot conversion the NO concentration decreased with typically 25 ppm (approximately 10%). At higher soot conversion levels, the NO concentration decreases more and stabilises around 200 ppm. This coincides with the thermodynamic conversion of NO to NO2, which is approximately 30% imder these conditions. Because at that point no soot is present in the reactor, it can be... 

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 flow-reactor experiments, the effect of NO in the gas phase and a supported platinum catalyst mixed with the soot is twice as large for cerium when compared to that of copper, iron, and Printex-U. All other conditions are similar and, therefore, it is concluded that cerium catalyses the oxidation of soot with NO2. Because there is... 

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

Later experiments by McNicol (66) and by a group of French workers (67-72), in which the water formed during reduction was removed by a trap at 78°K, were consistent with the results of Webb in showing a change in oxidation state of rhenium from +7 to 0. These workers indicated also that the properties of alumina-supported platinum-rhenium catalysts depend on the method of preparation. 

In these experiments oxidation of ethanol with a gas phase concentration of 200 ppm was studied. Figure 2 shows the light-ofF temperatures (temperature at which 50% conversion of the compound in question occurs) for ethanol oxidation over eight supported platinum and palladiiun catalysts as measured with a continuous flame ionization detector. Depending on by-product formation the signal from the instrument changes. The sensitivity for ethanol is for example about twice as high as for acetaldehyde. A marked difference can be observed for the activities of the different supported Pt catalysts. 

The main soluble intermediates could be readsorbed and oxidized to form CO2 or extracted from the surface under configuration of continuous flow rate. The last situation represents a loss of energy due to an incomplete methanol oxidation. This is well elucidated in the experiments where the extraction of solution in front of the electrode results in lower current than in experiments without sample collection [9]. For supported platinum, Jusys et al. [10] observed that an increasing conversion to CO2 would be attained with increasing Pt load by the cost of faster consumption of formaldehyde facts that are attributed to an increased readsoption rate on electrodes with enlarged electrochemical surface area. 

Reversed-flow gas chromatography (RF-GC) has been used to study the kinetics of surface-catalyzed reactions and the nature of the active sites. RF-GC is technically very simple and it is combined with a mathematical analysis that gives the possibility for the estimation of various physicochemical parameters related to catalyst characterization in a simple experiment under conditions compatible with the operation of real catalysts. The experimental findings of RF-GC for the oxidation of CO over well-studied silica-supported platinum-rhodium bimetallic catalysts are in agreement with the results of other workers using different techniques ascertaining that RF-GC methodologies can be used for the characterization of various solids with simplicity and accuracy. 

EC-NMR has made considerable progress during the past few years. It is now possible to investigate in detail metal-liquid interfaces under potential control, to deduce electronic properties of electrodes (platinum) and of adsorbates (CO), and to study the surface diffusion of adsorbates. The method can also provide information on the dispersion of commercial carbon-supported platinum fuel cell electrocatalysts and on electrochem-ically generated sintering effects. Such progress has opened up many new research opportunities since we are now in the position to harness the wealth of electronic, Sp-LDOS as well as dynamic and thermodynamic information that can be obtained from NMR experiments. As such, it is to be expected that EC-NMR will continue to thrive and may eventually become a major characterization technique in the field of interfacial electrochemistry. 

The first move toward substantial lowering of Pt loading employed in PEFCs, while maintaining comparable cell performance, was described by Raistrick [7]. Raistrick experimented with gas-diffusion electrodes developed for PAFCs. In such electrodes, the catalyst layer is a mixture of carbon-supported platinum (Pt/C) and PTFF, deposited onto a carbon cloth or paper. This is done usually by filling the macropores first with a mixture of (uncatalyzed) carbon powder and PTFF, with... 

FIGURE 3.10 Illustration of a typical transient experiment of COad electro-oxidation at nanoparticle catalysts. The current (grey) is measured in response to an applied voltage step at a catalyst surface initially fully covered with a monolayer of CO. Snapshots at the bottom illustrate the evolution of the surface state in the 2D model of the heterogeneous particle surface. (Reprinted with permission from Andreaus, B.et al. 2006. Kinetic modeling of COad monolayer oxidation on carbon-supported platinum nanoparticles. J. Phys. Chem. B, 110, 21028-21040, Figures 2,7,8,9, American Institute of Physics.)... 

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