Pt adsorption

Naturally, the number of surface sites limits the amount of Pt complexes that can be adsorbed. Typically, the.se catalysts contain up to 1 wt% Pt. Adsorption is a fast process and in general an eggshell catalyst will be formed. Figure 3.29 shows the processes that occur. 

Figure 24. Iodine on Pt adsorption isotherm obtained in situ by following the edge-jump intensity at the iodine K-edge as a function of potential (A) and comparison with data obtained by Hubbard et al. via Auger intensities (B). (Adapted from Ref. 95.)...

Figure 6.6 Demonstration of the retardation of Pt adsorption on alumina by high ionic strength (a) 1 1 electrolytes ...

After 1 h on an orbital shaker, pH is recorded and 5 mL aliquots of each thin slurry are filtered for ICP analysis. Over powders, Pt adsorption is complete within minutes [19,23], and 1 h contact is sufficient to ensure equilibration. Samples of the parent solution (before contact) are also filtered and analyzed with ICP, which can be used to determine not only metal uptake, but also dissolution of the support. The metal uptake, in terms of surface density (mol/m2), is calculated by dividing the concentration difference by the SL ... 

Figure 6.27 RPA prediction of Pt adsorption over materials of various PZCs (a) CPA, (b) PTA. PZCs representative of silica and alumina are indicated. (From Schreier, M., Ph.D. Dissertation, University of Illinois, Chicago, 2004.)...

Regalbuto JR, Navada A, Shadid S, Bricker ML, Chen Q. An experimental verification of the physical nature of Pt adsorption onto alumina. J Catal. 1999 184 335-48. 

J.R. Regalbuto et al., A Scientific Description of Pt Adsorption onto Alumina, Stud. Surf. Sci. Catal, 118 (1998) 147-156. 

After the execution of a single Monte Carlo event, events of type (ii) are executed. Hydrogen atoms adsorbed on the metal crystallites are allowed to diffuse by hopping between adjacent Pt adsorption sites. The number of these diffusion events is given by the ratio of the rate of hydrogen diffusion to the rate of the next most rapid event on the lists. Typically this value is typically around 10", i.e. 10 hydrogen diffusion events occur per lattice site between each Monte Carlo step. The next event of type (i) is then executed and the process repeated until the rate of hydrogenation reaches a steady state. 

Catalysts were prepared by impregnation of Pt inside the pore structure of carbon fibers. Care was taken to eliminate the active metal from the external surface of the support. A very high dispersion of Pt was measured. Four reactions were carried out in a fixed-bed reactor competitive hydrogenation of cyclohexene and 1-hexene, cyclization of 1-hexene, n-heptane conversion and dehydrogenation of cyclohexanol. Three types of carbon fibers with a different pore size and Pt-adsorption capacity along with a Pt on activated carbon commercial catalyst were tested. The data indicate a significant effect of the pore size dimension on the selectivity in each system. The ability to tailor the pore structure to achieve results drastically different from those obtained with established supports is demonstrated with heptane conversion. Pt on open pore carbon fibers show higher activity with the same selectivity as compared with Pt on activated carbon catalysts. 

Pt adsorption of the various samples measured as a function of time is plotted in Figure 1. Three groups were identified - strong, intermediate and weak adsorption. Pt concentration in the samples and the rate of adsorption are determined from this plot. All samples were reduced at 350°C with hydrogen at a space velocity of 1000 h for 2 hrs. Their physical properties are listed in Table 1. According to XRD data, Pt in all die samples was amorphous, i.e., particle size was less than 4 nm. H to Pt ratio, measured by hydrogen chemisorption in samples based on CF-1 and CF-i, was near unity. 

Many studies on carbon monoxide adsorbed on polycrystalline and single crystal Pt, Pd, and Rh electrodes have been carried out during recent years by means of electrochemical methods and IR spectroscopy (EMIRS, SNIFTIRS, IRRAS, etc.), potential-modulated reflectance spectroscopy and other methods.Electrochemical results show that the number of Pt adsorption sites per CO molecule is changing from 2 to 1 with increasing coverage in acidic solution. There is, however, a discussion in the literature about the evaluation of absolute saturation coverage on ordered low-index platinum (and rhodium) electrodes with particular reference to Pt(l 1... 

Moreover adsorption has other important roles in electrochemical technology. In electrocatalysis, e.g. reaction (1.1) at Pt, adsorption of intermediates is a key step since their presence on the surface provides alternative lower energy pathways... 

STEM analysis of a physical mixture synthesized at a final pH of 6 is shown in Figure 3.10c. Adsorption is seen to be essentially 100% selective onto the niobia phase, and the resulting Pt particles formed on niobia after a 250 °C reduction are seen to be on the order of 1.1 nm. More details and extensive characterization of selective Pt adsorption over alumina-supported niobia are found elsewhere [48, 49]. The same principles have been used to steer Pt onto the alumina support of a siKca-bound alumina extrudate [50]. 

The physical nature of Pt adsorption on silica has been further confirmed by a cutting-edge combination of x-ray reflectivity and resonant anomalous x-ray... 

From the equilibrium constant and Henry coefficients we know, that Pt adsorption on spinels is weaker than on alumina. At the same time, the maximum specific adsorption on spinels is somewhat higher (see Table 2). 

Fig. 1 shows the typical kinetics of Pt adsorption on inels. The sorption attains equilibrium not less than in 4 days. The crashing of support granules and the circulation of impregnation solution do not change significantly the adsorption rate. Evidently, neither external nor inner diffusion limit the process. Pt is known... 

The mutual influence of components in Pt-In solutions is less pronounced. It becomes noticeable only at the large excess of one component with respect to another and is an ordinary competitiAW adsorption of components with the similar adsorption properties. Note, that In has no effect on the initial Pt adsorption within a wide concentration range. It only rather decreases the equilibrium concentration of adsorbed Pt. Here, we also can explain In effect on Pt sorption by a competitive adsorption of NOg ions, while Pt effect results from the decrease of equilibrium pH in the binary solutions. 

Moreover, adsorption has other important roles in electrochemical technology. In clectrocatalytic reactions (e.g. the evolution of at Pt) adsorption of intermediates is a key step since their presence on the surface provides alternative lower energy pathways (page 39). Also, adsorption of species not directly involved in the electron transfer process is used to modify the net electrode reaction (e.g. additives used for the modification of electroplates, in the inhibition of corrosion and for the phosphating of steel). 

This chapter will focus on the modeling of MEA and its polymer electrolyte membrane. First, 3D modeling of PEMFC and its MEA will be discussed, and an example will be put forward. Then, dynamic modeling of PEM will be introduced. Further, this chapter will move on to the fault-embedded modeling of PEM. As an extension, application of membranes in other cases will be recommended, such as in lithium battery, vanadium redox flow battery (VRFB), chlor-alkali electrolysis, water electrolysis, and solar cell. Finally, several typical examples will be given, including Pt and Pt alloy simulation with density functional theory (DFT), water formation and Pt adsorption on carbon reactive force field (ReaxFF) simulation, and coarse-grained simulations. 

Water Formation and Pt Adsorption ON Carbon ReaxFF Simulation... 

Configurations 21-23 represent the environment of the three kinds of kink sites that an atom can find on the surface. By comparing configurations 20 and 22 for all systems, one notices that for the systems Ag/Au, Ag/Pt, and Au/Pt adsorption onto kink sites (configuration 22) is favored as compared with step decoration (configuration 20), whereas for the systems Au/Ag, Pt/Ag, Pt/Au, Au/Pd, and Pd/Au the growth of a monodimensional phase at step sites should be preferred (configuration 20). 

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